THE OF POWER FROM THE ENGINE TO THE ROAD VHEELS IN M t )TO IC VE H ICLES.

BY L. A. LEGROS, M.I.MEcH.E. (MEMBER).

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The Transmission of Power from the Engine to the Road in Motor .

By L. A. LEQROS, M.1.Mech.E. (Mern bed.

INits broader sense, transmission covers the whole of those parts of a which convey power from the engine to the road wheels. It therefore includes the , box, and the connection between the gear box and the driving wheels. The term is, however, so frequently used for the last-mentioned portion of the driving gear that its meaning may before long become restricted to this detail,

CONNECTIONBETWEEN ENGINEAND GEARBox. In some of the early in which the axis of the crank-shaft was transverse to the car the power was transmitted to the gear box by a chain or chains. This arrangement was undoubtedly very efficient, but since fashion has decreed that the engine shall be in front, and that its axis shall be longitudinal with the car, this method has ceased to be used. Details of fall outside the scope of the present paper. Under normal circumstances-that is, when the clutch is not being slipped-the driven portion is locked to the driving portion. In consequence of the frame of the car springing, the shaft, which is connected to the driven portion of the clutch, must be capable of deviation from alinement while transmitting the power ; in fact, the connection should permit of an error of alinement amounting in ordinary cases to one-eighth of an inch in the foot. (Fig. I.) Failure to pro.vide for this may result in difficulty in engaging or disengaging the clutch, and give rise to trouble in gear-changing. An ideal form for the joint between the clutch shaft and clutch is that adopted on some large turbine drives, and consists in giving B2-

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the end of the shaft the form of a section of a sphere and cutting teeth on the spherical portion. (Fig. 2.) These teeth engage in parallel ways, slotted in the cylindrical interior of the driven part. The bearing on the end of the engine shaft, which carries the clutch shaft, should be so arranged as to permit the spherical portion to move about its centre when the alinement of the shaft varies. In the case of touring cars having a base of loft. to r---

I-.i FIG.1.-Error in alineinent caused by spring of the frame.

11 ft. the frame niay, in many cases, become spruug when a heavy body is added, to as great tin extent as one-tenth of ari inch in the foot over the portions occupied by the engine and gear box suspensions, and this amount may easily be exceeded in the engine. Therefore, it is advisable, in lining up the gear box, to make the axis of the engine and gear box meet above the centre of the clutch shaft (a). It is also desirable that means be provided for adjusting the alinement of the gear box, which the adoption of a three-point

FIG,2.--Clutch Shaft.

suspension facilitates. A number of car builders, it is true, secure both the engine and the gear box rigidly to the frame, and the frame cad be stiffened vertically by the use of truss bars, but the narrowed frame has but little lateral stiffness in the portion which

(a) Just as the distance between the centres of the rods on a loco- motive is made longer than the distance between the centres of the driving and the trailing , to allow for the expansion of the frame when heated by the fire-bpx. Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENGINE TO ROAD WHEELS. 337 affects this connection. If the frame is straight and deep, or, what is better still, if the main portion of both the engine and the gear box forms one casting, the arrangement may work well, but, with the usual proportions of frames and connections, rigid fixing of the gear box to the frame may give rise to trouble.

GEARBox. When the gear is adjustable, provision should be made for niaintaining the alinement of the gear box with the side brackets (in the case of a chain-driven car), or with the brake gem (in the case of a propeller shaft-driven car, when, as is usual, the brake drum is fitted at the gear box end of the propeller shaft). In the earlier gear boxes, arranged for chain trantjmission, with four changes and a reverse, the sliding were usually mounted on the main shaft, which was permanently connected to the clutch shaft, and the auxiliary shaft drove the cross shaft and chain . Where a propeller shaft is used, the auxiliary shaft is usually gear-driven from the clutch shaft, and the sliding gears slide on the main shaft, which is coupled to the propeller shaft. When the clutch shaft is connected to the main shaft by a dog clutch, the so-called “direct drive” is obtained. The author does not propose to entcr into details of gear box design, except to state that, as a general principle, the gears should be so arranged that the various ratios form a geometrical progression. In the case of a well-known and much-copied gear box there are only three sizes of wheels used, namely, 17, 24 and 31 teeth. Three forward speeds and a reverse are obtained with these, the ratios being 1 : 17/31 : 1P/312, and the reverse equal to the lowest forward speed (b).

CONNECTIONBETWEEN GEARBox AND BACKAXLE OR WHEELS. The transmission from the gear box to the back or wheels may be divided into the following main classes :- (1) Central chain. (2) Side chains. (3) Pinions and internal gears on the driving wheels.

(6) It is compariitively recently only that the builders of tools have recognized the advantage of arranging the speeds in geometrical progression, and in some cases this has also been applied to the feedn. Many machine tools we still made with an irregular progression of ratios.

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(4) Propeller shaft and bevel gears. (5) . (6) Variable speed gears.

(1) CENTRALCHAIN.

This is now very little used j with this form the bending moment on the back axle, due to the pull of the chain when the maximum torque from the gear box is being transmitted, may be nearly equal to that which would result from applying the whole of the back axle load at its centre ; this result would be obtained, for example,

FIG. 3,-Ohain drive : unequal chain wheels. if the wheels slipped with a torque at the periphery equal to one- third of the load, and if the pitch-diameter of the chain wheel were one-third of the diameter of the road wheels. The pull of the chain tends to bend the axle in a horizontal plane, and the load on the springs tends to bend it iii a vertical plane ; to obtain the maximum bending mohzent at any section it is therefore necessary to find the resultant of the bending moments at that sectio; in the horizontal and vertical planes. With the central chain it is difficult to properly protect the chain wheels from dirt; the removal of the chain for cleaning and greasing

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF 1’OWER FROM ENGINE TO ROAD WHEELS. 339 is also more difficult than is the case with side chains. It is there- fore probable that this system will find decreasing favour with designers.

(2) SIDECHAINS. In the usual arrangement the differential gear is placed inside the gear box and the projecting shafts are each connected to ,the short overhung shafts carrying the driving chain wheels by means of Oldham’s . These short shafts, as first made, were usually subjected to heavy and unequal loads j and the bearings,

~ A - -,-I - I‘ --- - - e- l

. -.,

FIG.4.- : Equal Chain Wheels. which were plain bushss, wore rapidly, with the result that the chain-wheel shafts tended to get out of true with the diffeFntia1 shafts. When the axes became out of parallel, rapid wear took place in both the Oldham’s couplings and the chains. These difficulties were, to a great extent, overcome by dishing the chain wheels. The Oldham’s coupling gives the driven shaft the same angular velocity as that of the driver. Owing to the difference in diameter of the chain wheels, the velocity imparted to the road wheels is subjected to variation with the movement of the springs, as much chain being coiled on the

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 340 THE INCQRPORATRI) INSTITIJTION OF AUTOMORTLE ENGINEERS. driven chain wheel as is uncoiled from the driving chain wheel, Fig. 3. If the chain wheels weiw equal, the movement about the axis of the driving chain wheel would be one of translation in a circle, Fig. 4; but as the radius of the driving chain wheel r is usually smaller than that of the driven chain wheel R, the latter tends to be accelerated. If the angle between the two positions of the line of centres be a, and if e be the angle moved through by a point on the circumference of the driven chain wheel, then

Fm. 5.-Chain drive : limiting case equivalent to double radius rods. e = a (R-r) / R, when the radius rod end is coaxial with the driving chain wheel. For constructional reasons this is seldom actually the case, but is so neQrly so that we may consider it as coaxial. The result is, that in running on uneven roads or at high speeds, when the axle moves considerably relatively to the frame, the wheel is subjected to fluctuations of velocity, with attendant fluctuation8 in the pull on the chain. This tends to accentuate snatching ; but the effect is reduced by increase in the chain wheel ratio r R, which occurs on the higher-powered cars.

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(3) PINIONSAND INTERNALGEARSON THE DRIVINGWHEELS. This form of drive is very largely used for heavy traction; it is very generally employed on electric vehicles, and has been adopted as the standard for some years on a touring car (Chenard-Walcker). It has certain advantages because the gears are easily protected from risk of dtimage by stones and dirt, and the ratio of the driven wheel to the driver can be made larger than in some of the other cases named. It is essential that the axis of the should maintain a cinstant distance from the axis of the wheel, and th: question of change of velocity ratio due to flexure of the springs is dependent on the mode of suspension of the driving shaft or of the motors; as these methods vary considerably, the various cases require individual investigation. A serious objection to the use of this form of gear for traction

FI~.6.-Single Hooke’s Joint. existed prior to the introduction of gear slotting which develop the form of the tooth by means of a slotting cutter of the same shape as the pinion tooth. In these machines the feed is obtained by rotating the work and the cutter respectively through angles having the same ratio as that of the pinion to the gear wheel; teeth of a sufficient degree of accuracy for quiet running can be rapidly machined by this method if the cutter can pass right through the internal gear, or if a shallow channel can be provided for clearing the shavings.

(4) PROPELLE~~SHAFTSAND BEVELGEARS. There are several forms of this transmission; of these the simplest is (a) the single Hooke’s joint(c). In this form the

(a) This has always been known among English engineers a8 the Hoolre’s joint. Cardan lived from A.D. 1501 to 1575, and Hooke from

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 342 THE INCORPORATED INSTlTUTION OF ACTOMOBILE ENGINEERS. torque of the back axle is usually carried by means of a tube fixed to the back axle casing; this contains a bearing near the front end of the propeller shaft and another near the bevel pinion ; the couple balancing the drive is transferred to the frame through the springs and the Hooke’s joint, Fig. 6; as the bearings are usually at least 2 feet apart and both are near the ends of the pro- peller shaft, the loads carried are not very large, and amount possibly to about 600 lb. as a maximum, Pig. 4. This is, however, a con- siderable addition to the lpad on the pins of the Hookds joint, and more particularly on the elid faces of the cross piece j provision is seldom made for the adequate lubrication of these surfaces. Hooke’s Joint.- When used for transmission at angles less than 5 degrees the two axes of the pins may, in practice, be placed out of plane, which permits of a somewhat more simple, but less accu- rate, construction. When the axes of the pins are in the same

FIG.7.-Double Hooke’s Joint, Shafts meeting : Uniform velocity. plane, the ratio of the angular velocities of the two shafts varies from cosg to l/cosg, where a is the inclination of the axes of the two shafts to each other. From this it follows that if two Hooke’s joints are used in such manner that the intermediate shaft forms equal angles with the first and third shafts, then, for a uniform angular velocity of the first shaft a uniform angular velocity of the third shaft can be obtained. Where the axes of the three shafts lie in

A.D. 1635 to 1703. Cardan merely described the ordinary gimbals which were known and used in the thirteenth century. The question of priority was investi- gated by Willis, who searched Cardan’s works carefully, and could only find

‘1 a diagram of three hoops joined to each other in succession by diametral axes . . . . Cardan tells us that he fiaw it in the house of a friend, and is unable to assign a use for it.” (“ Principles of ” (R. Willis), 2nd ed. p. 439 et sap.) Hooke, on the other hand, constructed the actual joints with shafts, and understood the elliptic charactcr of the motion trammitted to the second shaft, which he used practically in connection with sundials ; ho wrote some memoirs on the subject to the Royal Society.

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENUINE TO ROAD WHEELS. 343 one plane the problem is very simple; it is merely necessary to arrange the joints so that when the axis of the pin of the first shaft lies in the plane, the axis of the pin of the third shaft also lies in the plane (d). (Pigs. 7 and 8.) It is by no means generally known

FIG.8.-Double Hooke’s Joint. Shafts pardIel : uniform velocity. that the double Hookds joint can be employed to transmit uniform velocity to a shaft neither parallel to, nor meeting, the first shaft j the conditions which must be fuli2led are :- (i) that the second or intermediate shaft must be equally inclined to both the first and third shafts j and

111 FIG.9.-Double Hooke’s Joint : shafts neither meeting nor parallel (the shaft 111. is inclined at an angle a to the plane of the paper) ; uniform velocity.

(ii) that when the axis of the pin of the first shaft lies in the plane of the first and intermediate shafts, the axis of the

(d) Hankine show8 that uniform motion can thus be obtained in the third shaft when it is parallel to the first. (“ Machinery and Millwork,” 5th ed. pp. 203- 205.) Reuleaux shows that this holds for, any position of the axis of the third shaft on the envelope of a cone having the angle at its vertex equal to 2 a. (“ Der Konstrukteur,” 4th ed. p. 387.) The condition which the joints must fulfil for shafts which are neither parallel nor meeting is given by Willis. (“Principles of Mechanism,” pp, 464, 455.)

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pin of the third shaft must lie in the plane of the third and iriterinediate shafts. (Fig. 9.) It may be as well to draw attention to the fact that if the position of the pin on the third shaft (in any of the above cases) is rotated through 90 deg., it then occupies the position giving the maximum variation in angular velocity, which then varies from cos’a to l/coS’a. (Pig. 10.)

FIG.I Q

FIG.lO.--Double Hooke’s Joint : shafts either meeting or parallel : arrangemont giving maximum variation in velocity.

(a) When a single Hoolce’s joint is used the distance from the centre of the pins may vary with the movement of the springs, or with wear of the radius-rod ends ; it is therefore essential that the Fropeller shaft should be arranged to slide in the portion of the Hooke’s joint carried on it. (b) When two Hooks’s joiiats are used, the casing of the back

FIG.11 .-Single Eooke’s Joint : Ball Joint Bearing for Torque Tube.

axle contains both bearings for the third shaft; in this case it is necessary to provide means for preventing the back axle casing from rotating, and this may be done either by means of double radius rods or, frequently, by means of a torque tube fixed to the back axle casing and formed at the forward end as a portion of a sphere which is carried in a spherical bearing on the frame (Mars)

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(Fig. ll), or attached by some forni of universal joint which perniits of unequal movement of the two back wheels relatively to the frame of the car. (Delaunay-Belleville.) The torque tube must be of sufficient diameter to allow for the movement which the propeller shaft may make relatively to it. (c) Flexible Shaftts.--In January, 1903, Weller proposed a form of flexible joint for use between the clutch and gear box in which a star-shaped diaphragm of thin sheet steel was secured to the one member at its pqiphery, and to the other at its centre. This was fitted on the Weller car but was not used, so far as the Author is aware, either for transmitting a considerable horse-power, or at an angle of more than one or two degrees. The author, in con- junction with the late Mr. de Havilland and Mr. G. Knowles,

FIG.12.-Double Flexible Joint Transmission. designed a joirit (Fig. 12) which was used on some cars, the engines of which developed over 30 h.p. at 900 revolutions per minute; when the low gear was in use the torque to which the joint was subjected would amount to about 600 lb. at 1 ft. The joints, as first made, ran some 400 miles before failure took place, and it was obvious that by increasing the stiffness of the plates (and also their number to compensate for this) the joint could have been made mechanically successful. There were, however, other objections to it in practice, chiefly because it became covered with mud which was difficult to remove, and that the plates rusted and so became weakened j moreover, an increase in the number of plates beyond the form shown in the figure rendered the joint more costly than the double Rooke's joint.-It is interesting to note that a large number of very successful runs were made with these joints.

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Anyuhr Velocity of Road TVheeZs.--(a) With the single Hooke’s joint when the back axle moves relatively to the frame it turns with the torque tube about the centre of the joint ; it is necessary in this case to allow for a motion of rotation of the axle casing relatively to the frame. If the angle through which the torque tube moves is a, that through which the back wheels move is also a, and con- sequently the acceleration is as great as it would be in the limiting case of chain transmission, that is, when the pinion has no magni- tude. (Fig. 5.) It is to be noted that this acceleration is independent of the bevel ratio. Owing to these fluctuations in velocity, the stresses in the pinion teeth are increased where there is large movement of the torque tube owing to badness of the road or to the high speed at which the car is driven. When two Hooke’sjoints, or a flexible shaft, are employed the

FIG.13.-Drive transmitted through the top plate of the Bearing Spring (much exaggerated). conditions still depend on the nature of the movement which the back axle makes relatively to the frame. If radius-rods are used the conditions are as already given. (b) When the back axle casing is fixed directly to the springs its movement relatively to the frame under the action of the springs is practically the same as if it were connected by a parallel motion. The loading of the springs, however, is in this case rendered more complex by the addition of the torque necessary to balance the driving effort. In forward running this increases the loading of the front half of the top spring plate and decreases that on the back half. The spring tends to assume the curve shown exaggerated in Fig. 13, and the acceleration of the road wheels when the axle approaches the frame is negative. This arrangement, which is very frequently used on low-powered cars, probtlbly tends to increase the life of the tyres.

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(c) When the movement of the back axle is controlled by a parallel motion, as in the Lanchester and some other cars, the wheels have a movement of translation, and there is no acceleration due to the action of the springs.

a

FIG.14.-Limiting Case of Helical Gear.

(5) PROPELLERSHAFT AND WORMDRIVE. This may be arranged either-(a) with single Hooke’s joint or (b) with double Hooke’s joint. The latter is more usual. The same observations regarding the acceleration of the back wheels

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apply as in the cases of bevel drive and, as in that case, the ratio of the reduction does not affect the question. Position of the Worm.-(a) The worm may be placed below the worm-wheel, in which case perfect lubrication is ensured, but it is necessary to take precautions against loss of lubricant through the bearing of the worm shaft. In this system the clearance from thc ground is reduced by an amount equal to the diameter of the worm, which is a serious disadvantage for colonial work, and for heavy traction, where the minimum clearance below the back axle must be large. (b) It is more usual to place the worm above the worm-wheel. Driving back through the Worm.-It is necessary in the applica- tion of the worm drive to traction that the worm can be drivm by the worm-wheel when the engine is declutched; this does not depend on the reduction ratio, because a worm and worm-wheel can

be constructed to drive backwards for any ratio from CL, to 1 and from 1 to 11~).This can be easily shown by a diagram (Fig. 14) ; in the CaS0 a, the worm-wheel is represented by a nut working in a slide and forruing a segment of a wheel of a radius of infinite length ; as the wheel diametor is decreased the case 6 is reached in which both worm and uorm-wheel are equal in diameter (the simple case of helical gears), and if the diameter of the second wheel is now incieased to its limit it in turn becomes the segment of a wheel of infinite radius. If the upper gear is considered as the driver in each case, then in a the lower gear rotates (e), in 6 the lower gear rotates, and in c the lower gear has a motion of transla- tion ; if the inclination of the thread is 45 deg., it is obvious that each will drive backwards. Given the distance between the two axes the dimensions of the gears can be found when two factors are decided, viz., the reduc- tion ratio and the inclination of the helix to the tangent on either wheel (f). It is this inclination which fixes the practical limits to

(e) Cams in which a worm drive is used to obtain a largely increased speed are not very common. An example with radius of infinite length is the Archi- medean drill, and another common case is that of the projectile in a rifle-barrel ; examplev of'large ratio are the fan-governor of the swiss niusical boxes and the table of the ordinary gramophone, silence being more important than efiiciency in these latter examples. (f)This question has been dealt with by Mr. F. W. Lanchester in his paper, "Somc Problems peculiar to the Automobile." (Proc. Inst. A. E., Vol. 11, p. 205 et sep.)

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENGINE TO ROAD WHEELS. 349 the design of this class of transmission ; the efficiency of the drive from each wheel becomes respectively smaller and greater as the angle is increased up to the limit at which driving back is no longer possible. The end thrust is also dependent on the inclination, ad may become very heavy in driving back. Formerly it was extremely difficult to take such thrusts satisfactorily (g), but the advent of reliable ball-thrust bearings has opened a large field for the use of worm drives.

PIC+.15.--Integrating Disk.

(6) VARIABLESPEED GEARS. Many attempts have been made to obtain a uniformly variable gear. These may be broadly divided into five classes :- (a) Integrating disks. (b) Variable throw cranks operating toothless ratchets., (c) Expanding pulleys and . (d) Hydraulic gears. (e)&Electro-magnetic gears. (a) The integrating disk is a device in which a wheel driven by the engine can be made to occupy any position on an axis at right

@) The thrust blook of the marine-engine shaft is a good example. C Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 350 THE INCORPORATED INSTlTUTION OF AUTOMOBILE ENUINSERP. angles to that of the disk on which it rolls, and to which it com- municates the movement. (Pig. 15.) It has been actually usedfor traction purposes as far back as 1892, when a tramcar driven by a Weyman oil engine with this type of transmission ran for some time. It was known as the ‘‘ Connelly Motor” (A). This form of c

a Brakes for altering stroke. d. Bevel wheel ring (front). b. Bevel pinion. e. Spring. c. Bevel whccl ring (back). f. Silent ratchet. Variable Throw Crank with silent ratchet drive. By applying the brake to the front or back bevel wheel, the stroke can be shortoncd or lengthened. Only one of the ratchets has been shown.

Two Ratchets.

Four Ratchets. Velocity Diagram of Silent Ratchet Variable Speed Gear, FIG.16. transmission requires very heavy loading of the integrating disk and wheel, with the result that the loads are awkward to balance. With plain bearings the author has found the efficiency to be extremely low (less than 50 per cent.). Moreover, as most of the running is done on one part of the disk, this tends to wear (h) Eeagitzeeritzg, 1892, Vol. LIV. p. 15.

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENQINE TO ROAD WHEELI. 351 unequally, and so causes trouble. The gear has also been used on low-powered touring cars (Maurer Union). (b) Variable Throw Cranks and Toothless Ratchets.-various devices of this kind have been invented. The author has seen I

I I End View. Section of Belt.

Open. Closed. FIG.17 .-Fouillaron Expanding Pulley. some half a dozen schemes of the kind applied to traction ; none were really successful in practice, and it is simplest to consider the reason for their failure. As usually made the variable crank c2 Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 352 THE INCORPORATED INSTITUTION OF AUTOMOBILE ENGINEERS. imparts a harmonic motion to the rocking arm carrying the silent pawl. A gear of this kind (Meischke-Smith) is shown in Fig. 16. In this gear the stroke is varied by applying brakes to either one or the other of the two bevel-tooth rings engaging with the bevel pinion on the screw. The arnis are usually arranged as a pair, SO that the is kept in motion by the second crank while the first is free and making the return stroke. It is merely necessary to plot a diagram of the velocities of the pawls to see that each ratchet must produce acceleration during a small portion of its travel only, and that during the remaining portion, till the other ratchet commences to drive, the car must run by its own momentum; thus the car is propelled by a series of jerks which may be inappreciable when the car is travelling fast and the momentum large, but are disagreeable and cause heavy stresses when the car is running slowly under load, when starting, or in hill-climbing. The velocity curves for two and also for four ratchets are shown in Fig. 16. (c) Expandiny pzcllcys have been devised- of several kinds (t). Probably that which has been most used for traction is the Fouillaron. In this the pulleys are formed of two truncated corrugated cones, capable of sliding on a shaft vertex-to-vertex, and so arranged that the hollows in the one are opposite the ridges 011 the other. The ridges are continued down to the central boss ; the hollows are cut away a little below the circumference ; by this means the conical surfacos ma3 be made to intersect at a variable distance from the axis, Fig. 17. A gear effects the sliding of the ,one part of each of the two pulleys, so that if one pulley is increased in effective diameter the other is decreased. The belt consists of trapezoidal sections of leather threaded on a catgut core ; the segments are gripped by the corrugated faces of the conical pulleys. In the car built specially for this gear the engine is placed obliquely to the centre of the chassis, and the drive to the back axle is also arranged obliquely and parallel to the engine shaft. The variable speed gear occupies the space usually devoted to the gear box, the transmission from the driven shaft to the back axle being effected by means of a double aooke’s joint.

(i\ A variable belt gear with expanding drum pulleys was exhibited at a recent Machinery Exhibition at Olympia. A gear very similar to that described here is in use on some facing lathes for automatioally varying the speed to correspond to the diameter of the cutting circle.

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(a) Hydraulic Gears.--A gear of this type has been tried and run for several years on heavy waggons (j). It consisted of three hydraulic pump cylinders and three hydraulic power cylinders all contained in a casing which could rotate. The pump cylinders were of constant stroke, but the throw of the crank of the power

FIG.18.-Hall’s Hydraulic Variable Gear : Pump Delivery Diagram. cylinders could be varied so as to give any speed of the driven shaft between that of the driver (casing) and one-fourth that speed. By adopting the same method of investigation that has been applied to the toothless ratchet we obtain the diagram of pump delivery. (Fig. 18.) A recent12 invented gear dependent on the same principle

a. Regnlating hand wheel. c. Pump cylinders which revolve. b. Spheriod scrolb for varying the d. Stationary ring forming annular inclination of the disk. slide valve. FIG.1 g.-Janney-Williamd Variable Hydraulic Gear (False section). is the Janney-Williams j in this case there are nine pump cylinders all parallel to the shaft and forming one piece which rotates with the shaft; the pump plungers are connected by ball joints to a

(j)This gear (Hall) is described and illustratrd in the Proceedings of the Cycle Engineers’ Institute, 1901, Vol. 111. p. 120 et sep.

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 354 THE INCORPORATED INSTITU'SIOM OF AUTOMOBILK ERGINEERS. disk which has a ball thrust bearing between it and a fixed disk, the inclination of which to the shaft can be varied(Ic). (Fig. 19.) The fluid pumped is delivered by ports in a central block to the motor cylinders, also nine in number, the range of speed obtainable being from zero to equality. The effect of the increase in the number of cylinders is to produce a very nearly uniform delivery and con- sumption of liquid in the rctspective cylinders, as will be seen from the diagram. (Fig. 20.) In the case of three cylinders the delivery of the pumps variqs from 100 to 86.6 per cent., but with nine cylinders the variation is from 100 to 99.6 per cent. only. It is to be noted that iii these hydraulic gears the velocity is dependent on the sum of the ordinates of the delivery curves, whereas in the mechanical ratchet gears it is only the ratchet

PIG.20.-Janney-Williams' IHydraulic Variable Gear : Pump Delivery Diagram. which has the greatest velocity at any moment that propels the vehicle. (e) Electro Magnetic Gears.--A number of electric methods for performing a similar function to the hydraulic gears have been proposed, but will be better dealt with in some future paper.

EPICYCLICAND OTHER GEARS. (a) Cryptogears.-Many American and a few European cars are fitted with spicyclic gears, with a system of brakes for setting the various trains in operation. Examples of this are the Lanchester

(k) A rotary steam-engine (West) was constructed on a similar principle some thirty-fivegears ago.

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENGINE TO ROAb WHEELS. 358 and the Wilson-Pilcher; in the former a change speed lever ia used to actuate the selecting mechanism and permit the application of the particular epicyclic gear brakes for the speed required. This makes a perfectly sound mechanical job though the parts are more numerous than in the ordinary gear box ; with this method it is impossible to rub the teeth of the gears together and so cauae noise and damage. (b) The Hunaphris Gear.-In this gear the pinion is of star form, each arm,having cylindrical ends terminating in hemispheres ; the wheel takes the form of a cylindrical disk with holes, conical for a portion of. their length on the engaging side. The pinion has a small to-and-fro movement of translation on the shaft when driving.

- I FIG.21.-Trundle and Rack.

Tho gear is a derivative of the lanterii wheel (or trundle, as it waa called by the millwrights) which has cylindrical staves. The cylindrical pins having their axes parallel to that of the trundle would, if engaged with a straight rack, require teeth of a form which is a parallel to the cycloid, distant tho radius of the trundle- pin inside the cycloid. (Fig. 21 .) In such a gear the staves would only bear against the teeth during the period of approach. In the Humphris gear the curvature of the pitch line of the rack teeth about an axis at right angles to that of the trundle causes contact to take place over a section of arc which comprises part of the period of recess. For this repson the end-thrust on the wheel may become nil. The gear is, however, one of point contact and not of line contact, but as one surface has double convex curvature and

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 356 THE INCORPORATED INSTITUTION OF AUTOMOBILE ENQINEERS. the surface on which it slides has single concave curvature, the conditions npproacJ8 those of line contact. The same pinion can be engaged with other circles of holes in the disk, and the gear change can be effected by disengaging the pinion from one circle of holes and engaging it with another. It is worth noting that there is no special virtue in the hemispherical end except for facilitating gear- changing, and that the ends of the pins might be formed to other curves-for example, to that formed by the rotation of a circular arc about its chord-provided that the holes in the disk are of the appropriate shape to gear with the star pins. (c) Gear changed by moving a Bevel Pinion from one Bevel Wheel to another.-A bevel pinion can only roll correctly with a bevel wheel when the pitch cones have a common vertex. If the same pinion is used to gear with two or three bevel wheels of different diameters, it can, at the best, only have line contact in one case ; it is, how- ever, obvious that a car constructed with such a gear, if correctly made for the top gear, may run a very considerable distance with these wheels, because the lower gears may be but seldom, and then only little, used. This niay give rise to an impression that this barbaric uiethod is much better than is actually the case (I). (d) Combiiintion of Gear Box with the BacJc Axle Casing.--In this method a prop~llershaft connects the clutch to the back axle casing, and the rods operating the change gear are also led to the back axle. There is no nwchanical objection to this method except that it considerably increases the .%eightof a part which most automobile engineers have endeavoured to keep down to the lowest practicable limit.

Methods qf transmitting the Driving Efort from the Back Axle to the Prame.-There are four methods in general use :- (a) Radius rods, which have been mentioned under Propeller Shafts j (b) Torque tubejointed to frame, also mentioned as above;

(I) The author wishes to point out that it would be quite possible to arrange two bevel pinions, having the same number of teeth, and machined in one piece 80 that they could be made to engage in each part respectively with two bevel wheels ha&g diffe.ent diameters and numbers of teeth. It would be necessary to provide for a to and fro movement of the bevel wheels so as to disengage before shifting the pinions, and re-engage after. It would also be necessary to provide a space betu een the teeth of the bevel wheels rather greater than the width of either pinion.

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(c) Springs; if the forward end of the spring is anchored to the frame and the hack end hung in shackles, the front half of the top bpriiig plate will be in compression when running forward; if the back end of the spring is anchored and the front hung in a shackle, then the back half will he in tension. This would appear to be the better method whore this type 01 drive is adopted j (d) Combi?aation of spring and radius rod. This is generally adopted for those cars on which cantilever springs are used; the spring is then called upon to perform little beyond its proper function of weight-carrying.

EFFICIENCYOF TRANSMISSION. The author has endeavoured to obtain data of the efficiency of traiismission, but the information is both meagre and incomplete. (1) Experiments made by Mr. Henry Hess (m), in America, on a gear box with plain hearings, three speeds and reverse, and with direct dog clutch drive 011 top gear, show that the efficiency varied under different loads asfollows :-

I rrmxmitted B.H.P. 1 EEciency per cent. rnu 3rJ. 2nd. lat.

5.90 6.12 4.86 - Tj- 7.65 6.68 6.44 78.2 I - 11 9 41 5.21 7.92 72.0

13 11.19 9.G4 67.5

I

15 12.55 11.34 11.00 10.60 8.02 90.4 63.9

17 13.89 12.6,i __ -. 8.56 91.0 61.6 I 9.02 I - -I- 60-5

(111) The Botor (I’ruder, 25sh September, 1907, pp. 728-730.

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In thrse experiments the speed appears to have been kept con- stant for the engine end of the transmission, and to have been equal to about 1,200 rovcdri+ionsper minute. Some tests on a well-known car were published a little later in the Motor Trader (11). T‘lnc, accompanying diagrams showed the relation between the hor5e-power delivered at the road wheels and that applied at the engine pnd. The gear box is not described, but apparently WRS fittoti with ball bearings, three speeds and reverse, the direct drive and secwiid speed only being used in the tests. The method of measuring the power at the road wheels is not dtscribed ; consequently, in the results, the efficiency of the trans- mission to the back axle is included, and the transmission from the road wheel to tlir road is probably not included. The efficiency at 1,200 revolutions per minute is about 84 per cent. on the second gear and 86 per cent. on thc direct drive. As a combined efficiency of gear box and bevel drive these figures will bear comparison with those given in Mr. X-Tess’s experiments. In a paper read before the Engineers’ Sdciety of Western Pennsylvania (o), Mr. E. IT. Belden gives the efficiency of the ordinary propeller &aft drive as 88 per cent., or, when the bevels are eqnaI, arid the propeller shaft is nearly parallel to the frame, as much as 98 per cent. IIe does not state how the experiments were made, or whether the measurements were made at the axle or at the periphery of the road wheels. He says: “Engineers are agreed that the greater part of the power 109s of the transmission of motion in automobiles is in the propeller shaft and csrdan joints, duo altogether to the friction of tho cardan joints when transniitting power at a great angle ” The author of the present paper does not agree that this statement is applicable to the ordinary car ; if it were true, the joints, whrch are seldom adequately lubricated, would become rapidly overheated, as the mass of metal in the joints is small and the casing mually fitted impedes radiation. Some experiment\ imdc undci the author’s diroction on a gear box with ball bearings, and having three speeds and reverse, showed that the loss which occurs in the gear box on the second and first gears, obtbined fiorn the difference of torque under various conditions; amounted to between 1 and 3.5 per cent. The amount was so small that the stiffness of the electrical connections to the

(n) The Motor Trader, 8th January, 1908, p. 99. (0) Proceedinge of the Engirieera’ Society uf WeeternPennsylvania, Vol. XXIII. p. 95 et seq.

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENUIR’E TO ROAI) WHEELS. 359 dynamo, by which the power was measured, was sufficient to deprive the tests of much value. To carry out the experiments, it would be necessary to fit mercury cup contacts and take special precautions, by means of adjusting screws, so as to bring the field of the magnets into the same position under various loads, and so avoid the intro- duction of error into the total figures obtained. It would also be necessary to fit a damping device between the gear box and dynamo to absorb the small vibrations which occur, owing to slight imper- fections in tbe gear, and which are apt to synchronise with the period of parts of the measuring gear.

APPLIANCESFOR TESTINGi THE EFFICIENCY OF TRANSMISS~ON. The Automobile Club of America have installed a dynamometer plant (at a cost stated to be about .€2,000 (p)),in which the torque and speed are automatically recorded by means of two rules at right angles placed in front of a chart. The chart is marked out with a series of rectangular hyperbola having the same asymptotes. The point of intersection of the two rules enables the approximate horse- power to be ascertained immediately and without calculation. In this plant the driving wheels of the car run on two drums of about doubb the diameter of the car wheels. A similar, but less elaborate, plant has been designed by Mr. J. Delmar Morgan and installed at the Victoria Garage, Norwich (2). In this each driving wheel of the car is carried on .a pair of rollers each apparently about one-half the diameter of the road wheels of an ordinary car, or about 440 mm. diameter. Both of the above appliances enable the power at the periphery of the road wheels to be measured, but the author has been unable to find any conclusive efficiency tests made on these pieces of apparatus. It is the author’s opinion that a very large portion of the loss between the cngine and the road takes place in the pneumatic tyre, the fabric of which is subjected to repeated shearing stresses on the sides and to bending stress& in the tread : them, together with the alternate compression and expansion of the air in the tyre, cause considerable heating although the tyres run under conditions which are very favourable to the maintenance of a low tempera-

(p) The Azrlocar, 25th January, 1908, pp. 136 et seq. (4) The Autooar, 2nd May, 1908, pp. 640 et seq.

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ture ; since this occurs when the tyres are run on a fairly flat road, it is reasonable to suppose that when they are run on pulleys of a considerable curvature the loaaea in the tyre will be even greater. The author does not think that much importance can be attached to experiments on this type of apparatus uptil experiments have been made with various tyres under given conditions of load, air pressure, and under a definite torque (provided by direct' coupling the wheel to an which has been calibrated). If, as seems probable to the author, these losses are large, and. dependent on the curvature of the pulley, then a comparison should be made between the brake horse-power delivered at the axle and that delivered at the road wheels. It may be found advantageous to fit a broad belt and two drums, instead of the present arrangement.

In conclusion, the author wishes to draw the attention of the members of the Institution to the desirability of obtaining informa- tion in detail on the losses which occur in transmission, as follows :- (1) Losses due to friction in ball-bearings- (a) under radial load ; (b) under axial load. (2) Losses due to friction of wheel teeth. It is easy to calculate the pressures and speeds of the wheels engaged, and if the data (under 1 (a), above) are known the experiments can be made on an ordinary gear box, and the proper distri- bution of losses between gears and bearings arrived at. (3) Losses in the chain, bevel, or worm-drive. In the case of the chain, it is only necessary to have the data under 1 (a) to enable the loss in the chain to be separated. In the case of bevel or worm-drives both axial and radial loads must be calculated, and the losses under 1 (b), as well as 1 (a), allowed for. (4) Losses in the tyre when running under normal conditions; these could be best determined on an electrically driven vehicle both when driven and hauled. (5) Another point of great importance in carrying out any tests of this class on ordinary petrol cars is the condition of the carburator. Xf the carburator is tuned for running on

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the road, then it is very improbable that it is performing its function equally well when the car is at rest, because the conditions of vibration (affecting the level of the petrol in the float chamber) and those of the air current through the bonnet (affecting the quantity and temperature of the air drawn in), are not the same as the conditions under which the car was tuned on the road. It would be better to fit up a car with a carefully calibrated electric motor, in place of the zetrol engine, and use this for the transmission tests ; this method would eliminate a very large possible source of error in the results. The author hopes that systematic tests on lines similar to those which he has suggested may be carried out in the near future, as these would be of far greater help than any racing records in determining the lines of development by which waste may be avoided. The author desires to express his thanks to Messrs. J. H. P. Darnau and P. W. Pattinson, Graduates of this Institution, for their assiutanco in the preparation of the diagrams with which the Paper is illustrated.

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THE DISCUSSION.

Mr. P. W. LANCEESTER(in opening the discussion) said :- I think me may all thank Mr. Legros for his very interosting paper. In the first place, I should like to ask Mr. Legros what is the advantage in employing gears in precise geometrical pro- gression; whether he ckn state exactly the advantage to be gained, whether it is merely that such a relation gives a suitable series of gears for ordinary use, or whether it has anything to do with the cutting or other mechanical reason. I think that in low speed cars the ratio between the two highest gears can be greater than in a high speed car where the wind resistance makes a big difference for small changes of velocity. On page 340 Mr. Legros

FIG.01. mentioned the fact that two equal sized chain wheels caused no displacement of the motor shaft relatively to the road wheels. I believe I was the first to call attention to this point. The best way to demonstrate the fact is by means of Fig. 01. If two equal wheels be coupled by a chain their relative rotational positions, indicated by two arrows in the figure, will remain parallel under any oscillating motion of the suspension, and thus the inertia of the flywheel will not give rise to forces tending to '' snatch " the chain. In days when it was customary to use a small pinion, about 1896, when most of the gearing down was done by means of the chains, I patenteas the feature of using an equal, or approximately equal chain transmission, and arranging the reduction independently of the chain drive (Pig. 02), :ind as an alternative to this arrangement 1 showed the chain taken over a guide pulley (Fig. 03). I think

* Patent No. 21697 of 1896.

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENGINE TO ROAD WHEELS. 363 had it not been for the fact that, I gave up the chain in 1897 in favour of a live axle and worm drive I should very likely have been upholding that patent to-day. Instead of coming in as a revolutionary step it came in little by little as part of the process of evolution. T think Mr. Lngros pointed out one very bad feature sometimes men in live axle cars in connection with the arrangement of the

FIG.02. torque rod: the presence of a perfectly rigid single jointed “trunk,” The whole of the axle frame and wheels oscillate about the single joint. This is a bad form of transmission, it provides for no elasticity at all. In a well designed live axle, if the road wheels go over two obstructions equally, such as over two bricks at the same time, the oscillation of the suspension may be arranged not to affect the drive

FIG.03. at all, but if one wheel goes over a brick and the other sinks into a hollow, or if the rear of the frame moves angularly from any other cause, that angular motion does put a considerable strain on the gear especially when on the lower gears, because it necessitates the flywheel following suit ; unless the clutch slips the engine speed is rapidly changed by the oscillations of the axle frame. Fig. 13 is not quite clear. I think if a spring were to bend in

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 364 THE INCORPORATED INSTITUTION OF AUTOMOBILE ENUZNEERS. the peculiar manner shown it would be found to be a badly designed spring. It seems to me that the torque would merely result in the limbs of the spring being bent to different degrees of curvature as if ihe two members were unequally loaded, and would not produce a kink as drawn by Mr. Legros. I know many cars which use the method very successfully, and it seems quite unnecessary from the point of view of suspension location to use link work. On the question of worm drive I am one of the oldest advocates of this form of transmission, but I d6 not advocate the use of the worm on a motor vehicle exclusively on the ground of efficiency. I think in general it is roughly of the same efficiency as a bevel drive; I would even admit that mmetimes it is not quite so efficient ; in any case it is a small matter. I usually advocate the worm on the ground of silence, and that the propeller shaft and the whole line of shafting can be arranged some G ins. lower down than with the bevel ; this means a lower body, a lower centre of gravity, more comfortable access to the vehicle and a better hung vehicle ; briefly, it makes a more compact and safer design. To put the worm on the top of the wheel 12 ins. higher than in my own arrangement is, to my mind, most undesirable ; there is no real point in using a worm in this way unless it is because the maker cannot make a silent bevel. On the question of hydraulic and electric gearing I am sorry the author of the paper has not given some particulars of magneto- electric transmission. There does seem some probability of the electric transmission gear becoming of more general use in con- nection with public service vehicles. I do not for the moment see much future for the hydraulic “gear,” though there are some interesting electric methods which are not as widely known as ought to be the case. Mi-. A. S. HILL:I think one of the outstanding features of this paper which we have heard from Mr. Legros is that there is an absence of a definite test of the efficiency of the appliances which he has been describing to us to-night, and I think it would be a very good thing for this Institution at so1110 future date to consider whether they would fill UP to some extent the omission by getting out some definite data of the efficiency of the various means of transmission which have been detailed. I am mainly going to confine myself to the question of chain drive, and I suppose all will agree with me when I say that the chain drive as generally

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OP YOWER FROM ENGINE TO ROAD WHEELS. 365 used on motor cars has not had a proper chance of showing its efficiency. I will say straight away that there is no definite data that is recognised as authoritative as to the efficiency of chain drives ; and I am sure that all chain makers in this country would welcome any authoritative definite tests as to the efficiency of chain drives. AS far as chains are concerned we have to judge of their efficiency from those tests which we are able to make in our own works, and speaking of my own, we find that we get the best efficiency from an inverted tooth chain. We get 96 to 97 per cent. efficiency, or rather that was the case with chains of one inch pitch and two inches wide running at 1300 revolutions per minute in a horizontal position. The ratio was 2 to 1, and we always believe that the ratio should never be greater than 24 to 1. There was a professor, I do not know whether he is alive now, in the States from Cornell University who once gave out that the efficiency of a chain on a bicycle-a new chain on a bicycle-is 99 per cent. I do not say that he was correct, but this figure was quoted hefore the Cycle Engineers' Institute by Mr. Qarrard. The inverted tooth type of chain gives in our judgment and from our experiments the best efficiency at high speeds, whereas the roller chain shows a higher efficiency than the inverted tooth chain at low speeds. Of course in making experiments you always have to consider the size of the wheels upon which the chain is performing its work, and the angle of the chain. We believe that to get the best efficiency a roller chain should never be over a driving wheel of less than 12 teeth and an inverted tooth chain should never be over one of less than 20 teeth. I say that this is to get the best efficiency, but I do not say that you cannot get very good efficiency by decreasing these figures. Another point to which, I think, automobile engineers have not paid sufficient attention, is the question of the actual form of tooth, in respect to the space, shape, and section of tooth for the driving wheel in distinction to the driven wheel, though it is generally recognised that there should be a difference. In my judgment, motor car rrianui'acturers do not study this matter sufficiently to get the best results, because they have not consulted the chain malyrs as much as they might do. I think we all recognise that the , when new, is a good transmission and gives very fair efficiency, but 1doubt in my own mind whether a worn bevel gear would give equal efficiency to a chain similarly worn if it were possible to eom- u Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 366 THE INCORPORATED INSTITUTION OF AUTOMOBILE ENGINEEKS. pare them. The worm gear referred to by Mr. Lanchester, I should imagine, is generally considered a very poor form of transmission as far as efficiency is concerned ; the only reason why it @V0S the efficiency, which we know it does give, is because it almost invari- ably runs in an oil bath. That brings me to the point as to the general treatment of chains in comparison with the treatment given to bevel gears and worn1 drives. After all is said and done, the chain is nothing but n series of bearings which need proper lubri- cation to get their full efficiency, and to get the same conditions as are obtained in other kinds of mechanism. One reason why chains do not always compare so favourably is because transmission by chain on automobiles has not had proper attention so far as lubri- cation is concerned. It is impossible to properly lubricate the chain unless by some means or other it is enclosed. It is quite obvious that chains running at high speeds will throw off the lubricant by centrifugal force. They do not get the same kind of treatment as you would naturally expect to give a bearing or series of bearings which you may be using in your own machine shop. There is another point in regard to chain transmission which Mr. Legros refers to, to some extent, and that is the alinement, or rather the non-alinement, of the two chains of the wheel, which is caused to a great extent by the dirt and grit getting in the outer bearings of the short shafts which carry the pinion wheels. These bearings, by reason of exposure, soon get worn and allow plenty of side shake and rock. The result is that the chain wheels get out of line and the chain then cannot perform its work advantageously. Indeed, we often find the links of a chain, which has been used, to be very much worn on the one side, in fact we have known them to be cut in two by reason of the wheels being out of line, and this non- alinement of the wheels is very often caused by the wear which takes place on the short shaft which carries the pinion wheel, due to the exposed position in which it is placed. Mr. F. L. MARTINEAU: Mr. Lanchester has given us some interesting features with regard to chain drive ; one item of which he dad0 a great point was, that by utilising chain wheels of equal diapeter, no snatching of the chain occurs. The same result can be obtained by fitting the radius rods so as to swing from a point equivalent to the centre of an equal sized chain wheel, i.e., by making the radius rod parallel to the top part of the chain and swiuging it from a centre under the sprocket shaft (Fig. 04).

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One objection to this is that it makes the chain snatch worse on the reverse, but as you use the reverse so little it does not matter. The chief drawback, however, is that as you load the car and depress the springs the chain becomes looser, but a chain is very kind in this way and takes up little discrepancies such as these. It is quite essential that the chain should be in ail oil bath, and this is easily obtained if you have a central chain. Mr. Legros says it is difficult to obtain this, but I cannot see the diffioulty. With regard to efficiency tests, I quite agree with the author in the advisability of taking tests. The list he has given us is a formidable one, and requires a considerable number of very accixrate readings to be

FIG.04. taken. What I should like to see myself is the difference in efficiency measured on the same car using a pneumatic tyre and a steel tyred wheel rolling on another steel wheel, because then we should have a resistance which is known already. The tests of efficiency that the author gives are very interesting indeed, showing as much as 88 per cent. and 91 per cent., which I see is the maximum. I notice that he mentions the Janney-Williams hydraulic gear. An efficiency test of another hydraulic gear may be interesting. I have here an efficiency test o€ one which was given in (‘ The American Machinist.’’ The efficiency comes out at as much as 92 per cent. at 275 revs. per min., and comes down to 84 n2

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 368 THE INCORPOXATED INS’rlTUTION OF AUTOMOBILE ENGINEERS. at 375 revs. per min., giving a curve something like that shown in Fig. 5. The VICE-PRESIDENT: What is the gear ? Mr. MARTINEAU: It is the Manly ” gear with five cylinders. Mr. T. B. BROWNE:Mr. Legros in his paper says on the first page that fashion has decreed that the engine shall be in front. I am afraid fashion has decreed a good many other things in connection with modern motor cars, which are not really the best practice from an standpoint. The live axle is the fashion now: and a particular type of live axle is the particular fashion. He says also (( Other methods have almost ceased to be used,” but this is not the ease as regards commercial cars and

pleasure cars with very heavy bodies, where the unsprung weight of the live axle on the road is a serious disadvantage. On the point of fashion again, I think at the present time Fig. 11 is the fashionable design of live axle with the cardan-shaft enclosed in a tube, but as we have heard from the author of the paper, Fig. 8 is the better arrangement from an engineering point of view. In regard to the figures on page 357 the efficiencies seem very high. I should certainly have thought that the efficiencies given would have been less, and that the losses there shown would have been accounted for by the bearings alone in the best cases, apart from the losses at the teeth of the wheels. As regards the suggested disadvantage of the central chain pull

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENGINE TO ROAD WHEELS. 869 in the centre of the axle, the bending effort due to this could be easily supported by a light strut placed in front of the axle. As regards the best ratios for the gears, I agree with a ratio in geometrical progression, except that I think it is better practice to have a slightly bigger drop between the direct top and next gear on account of the higher efficiency of the direct drive. In coming to page 360 to the question of teshg with carburettors on the road and brake tests, I quite agree with the statement as to the enormous difference between the tests on the road and in the shop, especially as regards petrol consumption. Vibration makes a very great difference to many carburettors in this way. As regards the desirability of obtaining practical information, I should like to add that we ought to get some particulars as to the actual co-efficient of friction between the wheels and the road; I should like to ask Mr. Legros if he knows of any data on this point. In designing our shafts and transmission gear, we all allow for a certain co-efficient of friction. I always take it at 0-4myself; I do not know what Mr. Legros takes it at. I have no practical figures and no results in that way to work upon. Then, we should very much like to have Bome tests as to the actual strength of the teeth of bevel and spur wheels. Have any practical tests been made on actual wheels as used in our cars? Very seldom one finds a bevel wheel with the teeth bearing all over; more often I find only a part of the area of the working surface of the teeth is worn showing where it has been driving, even with wheels in the best makes of cars. Mr. G.. H. BAILLIE: First I should like to add my quota of thanks to Mr. Legros for his very interesting paper to-night. Secondly, I should like to ask him for more. I rather wish he had given us a little more of his own opinions and the benefit of his experience on some of the very interesting ‘devices he has described. As an instance, take the expanding pulleys and the oil transmission gears. The expanding pulleys have been at the shows for many years, and I think at the show before last there was an exceptionally good example of the oil transmission gear in the Pittler device, a gear that was built on engineering lines by a well-known man. When one examines these things at the show the showman points out how they are going to revolutionise the motor industry. This is the kind of statement that one naturally resents, and one proceeds to hunt out the defects and point them out. The showman, however, especially if he is the inventor, always wins and ends by showing

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 370 THE INCORPORATEI) INSTIT'VY'ION OF AV'I'OMOL1II,E ENGINEERS. one what a fool one is. Still, however difficult it may be to find any real defects, these devices are never seen on the road, and it would be very interesting if Mr. Legros would point out what are the weak points in the devices he has described which have prevented their success. Another point on which I shou2d like the author to give his opinion is, as to the advantage or disadvantage of using back springs for doing the work of radius rods. Radius rods as often constructed give a lot of trouble, and if there is no serious dis- advantage in using springs to do the work of a radius rod I think it is a better system altogether. At the beginning of his paper Mr. Legros referred to the connection between the gear box and the clutch. He referred also to the unit systeni, which does away with the necessity of a flexible connection to the gear box. The unit system, however, is often very difficult and troublesome to take apart, and in connection with commercial work the taking down of the parts is a very important matter indeed. It makes a great deal of difference if one has to take down an engine, clutch, and gear box to repair the engine alone or the gear box alone. As a flexible coupling between the clutch and gear box, the Oldham coupling has a great advantage over the double universal joint in that it takes up very little room indeed. There are a few cars, for instance the Metallurgique and the Belsiee, which use it, and I am surprised it is not more used, because if properly designed it gives no trouble at all. There is one system of transmission that Mr. Legros has not referred to, and that is the De Dion bystem. It is another system on which I think it would be of interest to have the author's opinion. I believe that +,he system is still protected by a valid patent, but soon the patent will have lapsed, and it will be interesting to see whether it is adopted by other makers. There is one fatal mistake MI-. Le8 ros has made ; on page 356 he spoke about the Combination of gear box with back axle casing. I think he should have known that as soon as a gear box is put on a back axle casing one cannot refer to it any more. Another thing I want to ask Mr. Legros about is his experience with the spring drive. A spring drive, one would think, would be a splendid thing, and yet it is used very little indeed; I want to know why it is very little used. Is it that there are difficulties in the construction, or is it that it is not useful when it is constructed 2 The table of transmission efficiencies gives some very interesting figures. I am surprised that the efficiency on the direct drive is only about two to three per cent. above the efficiency when there

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENGINE TO ROAD WHEELS. 371 are two pairs of gear wheels transmitting the power. That comes to about one or two per cent. loss in a pair of gear wheels, which confirms Mr. Legros’ own experiments. I myself have noticed on many cars, and I expect a good many others have noticed the same, that when the gear ratio between the direct drive and the next lower drive is not very great, dropping down to the lower speed on a hill is of very little use and one has to drop down to a still lower gear almost at once. Personally I have always attributed that to the fact that power is lost when on an indirect gear. If, however, the loss of efficiency is only two or three per cent., it would have no appreciable effect. With reference to the i‘ Humphris ” gear, it always appeared to me that there is a good deal more rubbing and less rolling than in the ordinary type of , and yet it appears to have rgther less, or at any rate, no more surface of contact than the ordinary gear has. The greater amount of rubbing appears to me to be one of the chief defects of this gear. Mr. E. A. RAINER:Most of the points I had marked have been dealt with. I note the author brings to the front a point which has been apparently overlooked by many designers, as it is not often adopted, namely, the need for two flexible joints between the gear box and the clutch. On a car I have been concerned with this has been done, but many cars are not so fitted. The author has suggested a rather expensive type of joint for this purpose, and I think many manufacturers would arrange a less costly design. The author questions the benefits of the truss bars. In experiments which have come under my notice it was found possible to use safely a much lighter frame. The method seems good and a number of makers have adopted it. The idea is mentioned of arranging the crank case and gear box as one casting. I have seen this tried with a 6 cylinder engine-the length being about 5 feet from the front of the engine to the back of the gear box-but it was found inconvenient with this length to provide sufficient strength to allow of its being supported only at the ends. It was easier to truss the frame and support the engine and gear box separately, and to fit flexible couplings between them. Obviously when arranged as one piece this must only be supported at the ends, otherwise it is distorted by any sagging of the frame. The author speaks of a coupling in the Oardan shaft of steel plates (Fig. 12). I have seen it similar idea tried on a 50 h.p. car at the back end only of the shaft, and it was found satisfactory j one

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 372 THE INCORPORATED INSTITUTION OF AUTOMOBILE ENGINEERS. benefit being that it did away with wearing parts. The iuclination of the parts was limited by the arrangement of the torque rod. Mr. J. M. STRACHAN: We are greatly indebted to Mr. Legros for his interesting paper. I certainly consider it a most interesting thing for the motor manufacturer and for the engineer engaged in the motor business. I may claim to be an old-fashioned engineer as well as an up-to-date engineer, having made all types of gears. I commenced making gear wheels when I was thirteen or fourteen years of age, and subsequently went into the motor manufacturing business. Latterly I have been making several very important types of gearing, among which I may mention the epicyclic class of gearing. I certainly think the efficiency of the epicyclic gearing is good up to a certain point, but when the speed of the gear wheels gets high, the efficiency certainly goes down very rapidly owing to the amount of oil that has to be displaced out of the teeth If you use thick oil in one of these gear boxes you will find the efficiency goes down rapidly with increase of speed. On the lower speeds and when running the engine up very high, you will find you do not get much more power out of the lower speeds than you do with the high speed owing to the fact of the oil having to be displaced out of the teeth. Certainly for easy driving of the cars, especially for a beginner, the epicyclic gear is an ideal gear. With reference to the chain drive, this type of gearing in my estimation is certainly good, and I should say the best gearing of all is the chain gear, provided always that you keep the sand out of the lubricant and out of the gears. When sand gets into the chain of a bicycle, as you will haye noticed, the efficiency goes down very rapidly, I think quite down to 25 per cent. With reference to the other systems of gearing, I have had a great deal to do with the system known as the Humphris gear. I have found by actual experience that this particular gear for a light car may be good, but for any other size of car I cannot see where the adtwhge comes in. One of the difficulties with it is that when driving a car with the engine it is all right, but when the car begins to drive the engine it is certainly all wrong, and I have found from actual experience with the gear that it was impossible to make the plate drive the pinion owing to the way in which this particular gear is made. Another point is that the teeth and the hole are made of different bizes, and it is quite impossible for the gear wheels to work on a pitch circle of constant radius. The pitch line has to move to and from the centre owing to the form of the teeth, and this

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENGINE TO ROAD WHEELS. 373 results in thrusts on the teeth which are not obtained on ordinary gearing. Ordinary gearing has a rolling and slightly sliding motion, but in the Humphris gear I ‘should say there is quite as much sliding motion as there is in the ordinary type of gear. With reference to the velocity ratios 1very much doubt whether they are what is claimed for them to be. I have tested them, but I have never seen any formula yet for making the teeth of the Humphris gear. Mr. CHAXLESWHEELER: We have not yet boxed the compms, though we have been interested in Mr. Lanchester and his worm drive, Mr. Hill and his ‘(worm0 ” chain drive, and Mr. Renouf has entertained us with his toothless ratchets. But there is one cardinal point that has not been touched : no one has mentioned belt trans- mission. First, however, I should like to deal with the point raised by Mr. Hill, who referred to some efficiency tests carried out by a professor in America. I think this was Professor Carpenter, of New York, in 1898. Several speakers have joined issue with Mr. Legros, and a number of them start with page 1 of his paper. I propose to follow their example. Without any desire to be pedantic, I should like Mr. Legros to have made his opening sentence a little more definite, and to have added a word or two so as to make it read something like this :-“ In its broader sense, transmission covers the whole of those parts of a car which convey power from the engine to the periphery of the road wheels.” It is a great point, and one of considerable importance, that tyres should be mentioned, because very great loss occurs at the wheel periphery. I am therefore in full agreement with the author later on when he treats the subject of tyres. Now it is not only true that there is a very great difference in efficiency in tyres of different construction, but there is IL considerable difference in efficiency in tyres of the same make and type. On another occasion, in dealing with the question of earburation and carburator design, I ventured to express the opinion that if as much thought and attention had been devoted to the surface type of carburator as has been given by designers to the spray type of carburator, we should hltve as efficient a carburator of the former type as of the latter ; and I am not sure whether the same observa- tion may not be applied to the subject of belt driving as compared with other forms of transmission. Belt transmission has this attraction : the cost of manufacture is very low, and consequently makers of “motor cars for the million” have something very

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 374 THB INCORPORATXD INSTITUTION OF ATTTOMORILE ENGINEERS. attractive here if they will study the subject. It is really remark- able, seeing that a relatively high efficiency conibined with economy can be obtained withbelt transmission, that manufacturers abandoned this type of drive. It is also worthy of note that the belt, after a very severe struggle with the chain, worm and other forms of trans- mission, has beaten all of these in its application to motor cycles. I do not prOpOS8 to enumerate the virtues of belt driving, but might observe, apropos of the recently-issued regulations of Scotland Yard, that belt transmission has one great virtue, namely, that of silence. In conclusion, I should be pleased to hear from Mr. Legros a few observations on the subject of belt transmission. At the same time I warn him that 1 am sensible of the fact that it has several drawbacks, or, rather, it did have when it was dropped from practical politics some eight or ten years ago. Mr. LEGROS,in replying upon the discussion, said: Mr. Lan- chester has raised the question of bolting the engine and the gear box together. I mentioned one casting myself, and thought that covered this question. In regard to his views on geometrical progression, I quite agree with him when he says that when there is a great resistance, which increases like wind resistance, it may be an advantage to adopt a different progression; but for most ordinary purposes for slow speed vehicles and cycles, it appears that the difference in loss between direct and indirect drive is small, so small, indeed, that I think in common practice, as in machine tools, geometrical progression will be found to be the best; it has also the advantage that the number of sizes of wheels can be kept very low. With regard to the parallel motion, obtained by equal chain wheels, I did not know that Mr. Lanchester had been first in the field; I know he has generally been first in most things of the kind, and I am not at all surprised that he was first in this case also. In regard to the point which Mr. Lanchester has raised as to transverse action when one wheel goes over a brick and another goes into a hole, I quite agree with what he says. Referring to Fig. 13, I quite agree with Mr. Lanchester as to the change under ordinary working conditions. The spring distortion shown in the figure is greatly exaggerated; it was obtained for the draughtsman by bending a thin plate of steel. If an ordinary steel rule is bent in accordance with the conditions, it will take the form shown by the top plate in the figure. With regard to electric gears, I left them out as being so very large a subject that I could not deal with them as they ought to be

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 TRANSMISSION OF POWER FROM ENGINE TO ROAD WHEELS. 375 dealt with. A paper on this subject should result in a discussion of great interest and value), and I hope such a paper may be sub- mitted in the near future. With regard to Mr. Hill’s remarks about the advisability of encouraging research, I have great hopes that, as this Institution becomes bigger, we shall be able to do something in that direction. But there is the question of cost to be considered, and this lies in the hands of the council and the gods. Mr. Ell gave us the number of revolutions and the reduction ratio in chain drives, but I should have preferred the speed of the chain, which would be more useful from the designer’s point of view. It is interesting to notice that the roller chain is found to be the best for slow speeds. One point has not been mentioned by Mr. Hill, but it was men- tioned a number of years ago by Mr. Hans Renold ; it is that in a roller chain the teeth of the driver wheel should not be the same pitch as that of. the chriin rollers, but the teeth should bear against the chain and take the load over a considerable arc of the wheel, and to secure this the pitch of the driver wheel should be a little greater than that of the chain, so that the first few teeth are relieved of the full pull of the chain. Thls is a point not so generally known as it should be. It is necessary to design the driver wheel on these lines, because the chain as it wears becomes longer, and if provision be not made in the first instance, the chain will always be pulling at the tops of the teeth, and snatching will begin very much earlier than it otherwise would. The same argument applies to the driven wheel, which should be of a shorter working pitch than the chain. This can be readily secured in practice, as Mr. Renold has shown, by making both equal in the first instance, the wear of the chain ensuring correct working when running. In regard to the Humphris gear, my opinions are still unshaken, although I did not know of the new Humphris gear until I saw the model of it this evening. I had only seen the form which was exhibited at the Notor Show, and this appeared to have hemi- spherical ends to the sprocket teeth. It is possible to produce a gear which has developed teeth, but the developed teeth will not, I think, gear correctly with differentpitch citcles of disk holes. As I said in the paper, there is a double convex curvature of the teeth and a single concave curvature in the hole. With these conditions the contact is point contact actually, and I do not see any reason to alter that statement ; when pressure is applied to the teeth there

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 376 THE INCORPORATED INSTITUTION OF AUTOMOBILE ENGINEERS. is obtained a small area of contact, but this occurs with gearing of every kind. With regard to the silent ratchet I have examined no less than four of these gears practically : ( 1) the Holt gear for tramcars, (2) the Hagen gear for lorries, (3) the Cairnes gear for light touring cars, (4) the Lamplough gear for ordinary touring cars. In each case the difficulty occurred when the engine was running slowly with the torque large; it must also occur when the engine speed is low. The vehicle then has to fall back on the momentum alone, which carries it forward and the jerking becomes appreciable. A bicycle depends so much less on the momentum that the conditions are much more favourable. Mr. Martineau has raised some interesting points respecting radius rods, but, as he says, the amount of driving that is done hackwards is so very small that it does not much matter. What Mr. Martineau says as to trying a pneumatic tyre and a steel tyre on the same surface would be interesting. My statements were largely based on experiments made some years ago when motor cars were first coming out. Instead of driving a car by gears and transmitting the power from the axle to the wheel, it was proposed that, on the basis that there is enough friction between the wheel and the road to drive the car along, the gearing should be eliminated and the car driven along by applying the power direct by means of a friction wheel acting on the periphery of the road wheel. This syatem failed because of the loss at the point of contact, and because of the difficulty of getting suficient adhesion between the two surfaces when covered with mud and dirt. These experiments were never published by the man who made them, though they were very interesting. Mr. Martineau has quoted Manly’s gear, and very interesting it is. It may be of interest to see what the effect is of summing the sine-curves representing the delivery from the cylinders of these hydraulic gears ; the results are somewhat unexpected. In the following table the minimum delivery is given as a

Cylinders. Delivery. Cylinders. Delivery, 1 ...... 0.0 2 ...... 0.0 3 ...... 86.60 4 ...... 70.71 5 ...... 95.01 6 ...... 86.60

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It will be seen that a four-throw single acting pump gives a delivery not nearly so uniform as a three-throw pump; that a six-throw single acting pump gives the same variation as a three- throw single acting pump; and generally that a more uniform delivery is obtained with any odd number greater than the half of an even number of plungers. Mr. Janney’s gear is not described at all in detail, as far as I know. I have referred to the patent specification, and found that this showed an odd number of cylinders because it enabled impulses to be obtained continuously throughout the revolution, instead of leaving a period at which the impulses stopped. Mr. Janney evidently realised this, but he did not show clearly that he was getting a better result by having one cylinder less than an even number. Mr. Browne raised several points about fashion, but be did not refer to heavy cars. There, of course, the chain has held its own to a great extent. It has been free from popular prejudice; it has been a question of what was the cheapest and what was likely to last. Mr. Browne asked for the co-efficient of friction between the wheel and the road : I have always taken 0.4 as being about the figure. Then, as to the strength of teeth, our knowledge is limited to that obtained from the failures met with in one’s own experience. If the members will send particulars of the failures they have investigated to the Institution we shall arrive at some reliable data. Mr. Baillie has asked for information respecting expanding pulleys. I have seen them running on a lathe under practically the same conditions as on a car, and they appear to run well. With regard to oil transmission gears, these have been dealt with as far as I could under the limited conditions of the paper. The Pittler gear was N. different kind of rotary engine to the variable crank engine, but was used to attain a similar end. With regard to the back springs used as radius rods, I do not see the slightest objection to this practice. I did not refer to the De Dion transmission, because it was only a small detail. It was not possible to deal with many interesting points in the limited time allotted for the paper, and this has resulted in the omission of many interesting matters. The spring drive is used by the Metal- lurgique cars. When cars fitted with a spring drive are run over an uneven road, the road wheels will slip, and this is probably detrimental to the tyre.

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Mr. Rainer has raised the question of using one casting for the engine and gear box. I think that, unless it is so made as to be of sufficient depth and stiffness, it will be asked to do more work than it will be able to do. Mr. Ptrachan was interesting in his remarks on epicyclic gear, and also in his preference for chain drive. I think that possibly the difficulty he raised with regard to the Humphris gear was real, that is to say, while the gear is driving the car it is all right ; but when the car is driving the gear and the engine it may not work well. At the Olympia Show Mr. Humphris showed A box which you could drive backwards, and through which you could supply power in both directions, but this was not under load. Mr. Wheeler has asked about the belt drive. Though this form of drive is 60 very ancient it still holds its own in most engineering shops ; chains and bevels have not been able to displace it there. If it were not for the one trouble of mud and dirt on the road we should probably still be driving motor cars by means of belt trans- mission at the present day. The VICE-PRESIDENT(Dr. H. 8. Hele-Shawl : By your applause you have anticipated the thanks that I was going to ask you to accord to Mr. Legros. Mr. Legros was asked and kindly consented to prepare a paper on this subject, which is such a wide one that I am afraid the discussion has suffered in consequence. We must try and avoid in the future attempting to deal with subjects of such magnitude in one paper, as there is really sufficient matter for not merely 0110, but a dozen papers, and jt is inipossible for the discussion to really tfiffectivelycentre itself on the many important points under such circnmatanees. I was obliged to allow those gentlemen whose systems had been alluded to to speak, and this prevented more general questions of interest being fully dealt with. We could, for instance, devote a single evening to tooth gearing alone. As it is, a number of points have had to be touched upon only superficially and many left out altogether. All that does not, however, detract one iota from the great ability which the author of the paper has shown in dealing with the subject in the terse way he has. I therefore ask you to give your thanks to Mr. Legros for the great pains he has taken, and for the careful manner in which he has prepared a paper so full of interesting matter, and for the valuable information he has laid before the Institution this evening.

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COMMUNICATED.

The following written contribution has been received :- Mr. F. STRICKLAND: I regret that I was unable to be in London at the reading of Mr. Legros’ very interesting paper on transmission gears. There are, however, several points I should like to make some remarks on; the pape* covers so much ground that I can only touch on a very few. In the first place, I should like to point out that Mr. Legros has not given us much information as to what his opinions are with regard to the reliability and durability of the various devices, and that to the ordinary man who buys a motor car this is the all-important consideration. No amount of theoretical ‘‘ efficiency” or perfect angular motion will compensate for the machine failing to go when it is wanted. With regard, however, to the efficiency itself, there are several very interesting points. There can be no possible doubt that Mr. Legros’ opinion, that the loss in the universal joints cannot be anything like as great as is assumed by some writers, is quite correct. In the first place a large loss in the joints would, as he points out, involve their running hot, and in the second it would involve a perfectly impossible co-efficient of friction. No doubt if a badly designed joint is run with poor lubrication, and worked at a large angle, there will be a considerable percentage of loss, and in this case if any great amount of power is transmitted the joints actually do get hot, but universal joints in a well-designed car run very nearly, or quite, in alinement. This being so it is not very apparent how the efficiencies in Mr. Hess’s experiments can be correct, unless the bevel drive was run without oil, or in some way quite differently to the gear box gear, as the loss on the top speed, when only the bevel gear would be transmitting the power, varied from 9 to 12 per cent, while the loss in the gear box on the second speed when the drive passed through two pairs of wheels never exceeded 3 per cent., or 1% per cent. per pair of wheels. On the other hand, driving through the reverse added from 6 per cent. to 20 per cent. loss for the one extra pair of wheels. In practice a good deal can be learnt as to the actual relative performances of gears on the road. If the loss in the gear box is actually as low as

Downloaded from pau.sagepub.com at UNIV NEBRASKA LIBRARIES on June 9, 2016 380 THE INCORPORATED 1NSTITVTION OF AUTOMOBILE ENGINEERS. in these experiments, and also in the ones which Mr. Legros himself quotes, there can be no perceptible difference in the performance of a car on its direct drive, or when transniitting power through gears in the gear box. So far as my own experience goes the loss is quite perceptible, e.g., a car with a shaft to shaft drive on all speeds will be inferior on its top to one with a direct drive, but superior on its lower speeds owing to the drive only pa,ssing through one pair of wheels instead of two. It would be interesting to know whether Mr. Legros’ experience is, that there is actuallj no difference. It is, of course, obvious that, as Mr. Legros says, the rolling resistance of the tyres on the ground cannot be counted as loss in the transmission gear, and that therefore the testing machines in which the road wheels run on rollers are of little use for deter- mining the efficiencyof the gear, however valuable they may be for other purposes. It would be interesting to have some more details as to the performance of the worm drive. The advantage of this in silence is obvious, but its efficiency seems doubtful as compared with the bevel under actual working conditions on the road. If worm- driven cars habitually entered for trials-such as the Scottish trials-there would be little difficulty in obtaining the efficiency from the hill climbs, but unfortunatsly they are generally absent. From theoretical considerations it would seem likely that when running fast with small pressure the worm would be very efficient, even when placed above the worm wheel, but that when the road wheels were revolving slowly, as in going up a hill, and the pressure on the worm was great, the oil would be nearly or quite squeezed out from between the surfaces, and the efficiency would be low. For this reason it has seemed to me that it might be a great advantage to drive through a worm on the top speed and a bevel on the lower ones. With the ordinary arrangement of live-axle car this would involve two propeller shafts, but there does not appear to be any very great objection to this, and there are several other arrangements in which no additional corriplication is incurred.

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REPLY TO . In reply to the written communication of Mr. F. Strickland the author wrote: No mere opinion of his on the reliability or durability of any device could be of the same value as the actual results obtained in practice and shown in the carefully summarized reports of the trials made by the Scottish Automobile Club, for example. Reliability and durability are matters which depend on a number of other factors than the class of transmission ; the stress allowed, the material employed and the protection from dirt are all highly important factors. The author is, however, of the opinion that the question of success or failure of auy transmission is very largely dependent on the uniformity of angular velocity transmitted. In the case of gear wheels new methods such as have been devised to ottain greater uniformity of angular velocity, with the resulting advantages of increased silence and reduction in the ratio of maximum to minimum stress. It will probably be safe to predict that for still higher velocities the developed surface of gear teeth will be finished by grinding so as to ensure still greater accuracy. Furthermore, it is generally recognised that fluctuationu in the velocity of the road wheels (or in the torque applied to them) result in increased wear and tear of the tyres, and as so large a percentage of the delays and ‘‘ incidents ” of the road is due to tyre troubles, it is of importance that they should be run under the best conditions. In reply to Mr. Strickland’s remarks ou the comparative efficiency of gears with shaft-to-shaft and with direct drive the author thinks that it would be very difficult to get two cars having the same gear ratio, actual horse power, weight, size of tyres, windage area and other conditions equal on which to make the tests; and unless made under exactly similar conditions by an absolutely unprejudiced driver they would be worthless. The author thinks that any opinion on the performance of a car (whether his own or any other person’s) is of much the same value as an opinion on the speed of a passing car. In these matters facts are what are required, and mere idle speculation should be eliminated from the investigations at the earliest possible moment. E

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The author has expressed his opinion that tests should be made by means of a dynamometer taking the power from the road wheels of the car, but not from the periphery. For this purpose he would suggest to whatever club or laboratory may propose making such apparatus, that two coaxial shafts driving a brake drum should be fitted up and arranged with double Hooke's joints and short shafts facing each other. These short shafts should be castellated, and on each should slide a dished dummy wheel, with its periphery fitted as in the well-known Stepney wheel ; various sizes of these could be slipped on to the shafts according to the size of the road wheels of the car to be tested, and the freedom to slide on the castellated shafts would permit the adaption to any width of car. The coaxial shafts should be equipped with large chain wheels, and hoth coupled to a lay shaft on which the brake drum mould be placed. It would then be easy to calibrate the gear for its own frictional losses by means of an electric motor, or the driven shaft could be carried by a suitable system of levers arranged as in a testing machine. Such a gear could be far more cheaply made and operated than those roller gears referred to in the paper, and it would enable measurements to be made dealing with those portions of the car with which the automobile engineer is chiefly concerned. Experiments made on such an appliance, when com- pared with results obtained on the road, could greatly help to solve the question as to where the chief losses of power occur.

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