Electric Lifts and Cranes. 11

Proceedings.1 RAVENSHAW ON ELECTRIC LIFTS AND CRANES. 11

30 March, 1897. JOHN WOLBE BARRY, C.B., F.R.S., President, in the Chair.

(Paper No. 3038. )

“ Electric Lifts and Cranes.” By HENRYWILLOCK RAVENSHAW, Assoc. M. Inst. C.E. THE object of this communication is to directattention to the electrical and mechanical problems which present themselves it1 the application of electric motors to working lifts and cranes ; and todescribe the methods bywhich they have been solved in particular cases which have come under the notice of the Author.

ELECTRICLIFTS. Thesimple gearing and slowly-moving mechanism necessary wherehydraulic power is available afford many advantages. Withthe electric motor, however, thecurrent varies with the work developed, while with the usual form of hydraulic motor the quantity of water used is as great with a light as with a heavy load. Thegreater simplicity of thehydraulic lift would therefore probably be an insuficient advantage to justify its in- troduction in places where electricity was already available, and where no hydraulic installation existed. Motor.-The class of electric motor generally employed does not callfor special description. Any good commercial machine can be used, a principal necessity being that it should require little attention.The machinesshould be sparkless under wide varia- tions of load, the brushes self-adjusting for wear, and the direction of rotation reversible. The bearings should also be self-lubricating. The efficiency should be high at all loads ; but a comparatively small machine can be used, as in most cases the maximum load is of short duration, the machine standing idle for at least half the time. Series motors have been used in some cases, but they are not suited for this work, as the speed varies excessively with a varying

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. 12 RAYENSHAW ON ELECTRIC LIFTS AND CRANES. [Minutes of load. Shunt motors are remarkably self-regulating,if the potential is constant, and they are now almost always used, series coils of a few turnsbeing sometimes addedto enablethe motor to be startedquickly and with areasonable current. A distinctive characteristic of theelectric motor is the comparatively high speed of rotation of thearmature. It is possible to make a motor that will run at a very low speed, but the large size and heavy first cost of such a machinemake it more convenientto employ a mechanical method of reduction. Gearing.-In the earlier electric lifts, beltswere generally used to transmit thepower from the motor to the winding-drum, and in many cases the motor ran continuously. Reversing, starting and stopping were effected mechanically. This arrangement is, however, cumbrous, and positive gearingis now generally used- the motor beingstarted, stopped and reversed asrequired. Doublehelical spur-gearingis perhaps the simplest and most efficientmethod of reducingthe speed, butthe noise insepa- rable from it prohibitsits use in most places where lifts are required. Worm-gearingis more expensive and less efficient; but on account of its compactness andsilent running itis almost universally adopted. Thearrangement does notadmit of wide variation. A quickpitch for the worm, ball- or collar- bearings to take the thrust, and the worm and wheel running in an oil-bath, form the principal characteristics. The older form of slow-pitched worm-gear had the advantage of being self-holding ; but in an efficient worm-gear the wheel will drive the worm, so that no reliance can be placed on its holding power. When once the wheel and worm have worn themselves to a good bearing sur- face, thegear gives little trouble. The ball-bearings run well, ifthe ball-races are made of good steeland are ground true afterbeing hardened. When ball-races are made of unsuitable materials and arenot properly hardened thewearing surfaces become pitted, and much frictionand noise occur. Notwith- standing the silent running of good worm-gear, a certain amount of vibration is always transmitted to the worm-wheel shaft; it is therefore usual to drive the rope-drum throughintermediate pads of india-rubber, otherwise an unpleasant vibration is sometimes felt in the lift-cage. The power is generally transmitted from the worm-wheel shaft to the cage by wire ropes wound on a grooved barrel. A modified arrangement, used by Messrs. Easton, Anderson and Goolden, is shown in Figs. 1-3, Plate 1. In this case, one end of each cage- rope is fastened to the balance-weight. On the shaft of the worm-

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] RAVENSHAW ON ELECTRIU LIFTSAND CRANES. 13 wheel is keyeda drum having twice as many grooves asthere are ropes. Thelatter passhalf round thedrum over aguide- pulley and half round the drum again, the balance-weights being sufficient to give thenecessary friction between the pulley and the ropes. A number of ropes on a high lift can thus be used with the advantage of a comparatively narrow drum. Theprincipal exception from theusual practice of reducing the speed by worm-gearing is the Sprague lift, which is exten- sively used in America. In it ascrew is driven by the motor, directly or throughgearing. Travelling on this screw isa nut which carries a pair of sheaves, over which the ropes pass in a similar manner to the ordinary hydraulic jigger. The nut forms virtuallya ball-bearing, the balls returning through a race in such a way that they follow in a continuous stream. In theevent of a ball jambing,the nut, which is held by friction-plates,revolves with the screw, and the travel of the lift isarrested. Starting- and Regulating-Gear.-The driving-gearpresents few difficulties,seldom showing variation from auniform pattern. The requirements of the regulating-gear are :-Prompt starting, stopping,reversing, and ease of beingworked byunskilled persons; absence of suddenjerks or jumps at starting,due to rushes of current;minimum current consumption, andregular speed with varying loads. Automatic lifts are not largelyused in Great Britain, a skilled attendant being generallyemployed, and the usualhand-rope adopted. A small reversing motor, or a pair of magnets or solenoids controlled by aswitch inthe cage,may be used to actuatethe regulating-gear. Thecurrent has,however, to be carried from the cage, eitherby hanging wires or bysliding contacts; and where a rope is used the simplest arrangement is obtained. When a motor is running, the current in the armature is that resulting from the difference between the electromotive-force of supply and the back electromotive-force of motor. If the machine is switched on, while it is standing, there is no back electromotive force, and the current is that due to the full electromotive force of supply, the resistance of the armature and circuit being constant. As in a good motor the backelectromotive force isonly about 5 per cent. less than that of the supply, the current, on starting without added resistance, wouldbe nearly twenty times as greatas the working current, being practically equivalent to a short circuit. This would cause agreat fluctuation in the supplyelectromotive forcein the adjacent circuits and an enormous turning effort in the motor, causing it to start with it jerk. Indeed, the fuses would be melted,

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. 14 RAVENSHAW ON ELECTRIC LIFTS AND CRANES. [Minutes of as such a rush of current would be dangerous, and could not be tolerated. In order to start smoothly and without a great rush of current, resistance musttherefore be insertedin the motor circuit. It is usual to providea switch so arrangedthat a resistance is inserted in series with the armature. This resistance allows onlysuficient current to flow toenable the motor to start, and is afterwards cut out by a further movement of the switch-handle. The switch is sometimes worked entirely by the attendant, who switches on slowly, giving the motor time to start. With a lift in which a hand-rope is used, there is no indication of the position of the switch, and an unskilledperson might suddenly switch out all the resistance, causing almost as great a rush of currentas if there were no resistance in use. Variousarrange- ments of dashpots have been used to prevent the rope from being pulled suddenly down; it should, however, require a very small effort towork it, and an arrangement which automatically cuts out the resistance is more satisfactory. The following arrangement has been adopted by the Otis Elevator Company. When the hand- rope or other starting-gearis actuated, the currentis switched on in the right direction, and the brake is released; a switch arranged to automatically cut out the resistance is also released by a cam, but its action is retardedby a dashpotand a solenoid, which does not release theswitch until the current has fallen below a certainlimit. This has been largely used, and works well. Another form of gear designed by the Author is shown in Fig. 2, Plate 1. It consists of a centrifugal governor driven from the motor-spindle, and controlled by a spring, so that the travel of the switch which it actuates is nearly proportional to the speed of the motor. This switch is arranged to decrease the resistance in the main circuit as the speed of the motor increases, all the resistance being cut out justbefore the motor attains its maximum speed. Sufficient resistance is inserted toallow the motor to start with its maximum load; but if the motor is over-loaded or any of the gear is jammed, the resistance is not cut out, and only theordinary starting current continues. When fitted withthis starting-gear, an electric motor behaves in almost exactlythe same way as a steam-engine, and is specially applicable to winches and cranes, as it is impossible to overload the machine. A similar gear has been used by Messrs. Siemens and Ha1ske.I When lifts are supplied with energy from a central station, the authorities generally specify that current shall not be switched on

Minutes of Procecilings Inst. C.E., vol. cxxv. p. 515.

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] RAVENBHAW ON ELECTBIC LIFTS AND CRANES. 15 in steps of more than,say, 10 amperes. The reversing-switch, which is driven by a hand-rope, is therefore generally fitted with a certain number of resistance contacts, if the starting-current for the motor exceeds the specified limit. This resistance is also of use with fast-running lifts, where unskilled persons are liable to hold the rope too long and reverse the motor suddenly. The governor- gear, however, in such a case at once promptly inserts the resistance. A gear having an almost similar effect has been designed by the Author. It consists of a solenoid, wound with fine wire and connected across the brushes of the motor. This actuates a resistance-switch and is controlled by a spring or weight. When the motor is standing,only a minutecurrent flows through the solenoid; but directly the motor starts, the potential at the brushes increases and causes the solenoid togradually cut resistance out. This gear is convenient in some cases, but has the disadvantage that current is always flowing through the solenoid while the motor is running, thus diminishing theefficiency of the system. TheBrake.--It is important that, a lift should stop promptly when required, aud some form of brake is always employed. This may be worked from the hand-rope, but an automatic magnetic brake is generally used, it being important that the rope should not require much effort to work it. For this purpose the ordinary bar electro-magnet is generally adopted, as a magnet of reason- able dimensions can be made to give a pull of l ton through a distance of $ inch, The magnet can be arranged to release the brake when the current is switched on, the cage being automatically stopped when the circuit is purposely or accidentally broken. For this reason themagnetic. is far safer thsn the mechanical brake. Theshunt coils of the motor andthe magnet are controlled by the starting switch, a resistance, wound to have little self-induction, being fitted to prevent sparking when the magnet- circuits are broken. This resistance is inserted by the switch just before the magnets are cut off, and is left in circuit with them while they remain disconnected. The Author’s usual practice is to make this of the same resistance as the magnet-coils with which it is connected. Bythis arrangement the circuit of a magnet weighing several tons can be broken without an injuriousspark. Indicator.-Although the main current may be switched off, the hand-rope can be left in such a position that the brakes and field- magnets are still in circuit. In the lifts designed by the Author an electric vibrator or “buzzer ” is connected in parallel with the field magnet-coils, a large resistance beinginserted to prevent

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. l6 RAVENSHAW ON ELECTRIC LIFTS AND CRANES. [Minutes Of waste of current. A visible indicator can be used if the noise of the buzzer is objected to. An indicator should always be fitted to prevent accidents and waste of power. Tests.-By the courtesy of the 0th Elevator Company, the Author has been able to carry out tests of an electric lift erected by them, the particularsof which are given in AppendixI and in Fig. 4, which show therelation between the energy consunled and the cost of working with various loads. The height through whichthe lift travelled was 36$ feet. The cost of workingis lowest at about half-load, the cage and load being almost exactly

Fig. 4.

0 25 a a 0 I ;20 c i 16 D W L 3 111 10 z 0 0 > 04 Y z W

0 too 200 300 “00 sco 600 700 LOAD - LBS. OTIS LIFT-CURVESSHOWING ENERGYCONSUMED AT DIFFERENT LOADS.

balanced. Thislift wasnew, and would perhapsgive better results after working for some time; it may, however, be taken as a fairly representative example of its kind.

ELECTRICCRANES. Cranes driven by electric motors are coming into frequent use, the ease by which the power can be transmitted to the movable framework of a travelling or jib crane, and the adaptability and compactness of theelectric motor presenting many advantages over other forms of mechanism. In the case of a travelling crane the current can easily be collected from conductors stretched along thewalls or gantrysupporting the crane. Theserequire no attention when properly fixed, and are preferable to fast-running ropes or square shafts which are running continuously and waste much power. . Where electricity is psed for other purposes there

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] RAVENSHAW ON ELECTRIC LIFTS AND CRANES. 17 is little loss of power whilethe crane is standing, as in those cases whereseparate motors are used for each motion they are stopped, startedand reversed asrequired. Where mechanical methods of control are employed, the motor must naturally be running idle for a large part of the time, but even in this case the attendant can start and stopit when it is not required. The use of a separate motor for each motion appears at first sight to present,many advantages. The power can be appliedto the vork in the simplestmanner, all clutches being dispensed with, and smooth starting, regulating and reversing are obtained. On the other hand, with a single motor, the electrical details are of the simplest kind, and the rush of current, which is almost certain to occur at starting with a careless driver, is avoided. The subsidiary notionsabsorb so little power compared withlifting that the designer often !prefers to use simple friction-clutches, which are understood by an ordinary mechanic, to the more complicated and often unmechanical electrical gear. There seems little to choose between the twosystems, both of which are largelyused with good results. By the permission of the authorities atWoolwich Arsenal, and of Messrs. Easton, Anderson and Goolden, the Author is enabled to give, Appendix 11, the results of a series of tests of a 20-ton electric crane, Figs. 5-8, Plate 1, the electrical contrivance of which was designed by him. This cranewas erected as an auxiliary to the large radial steam-crane in use in the annealingshop at Woolwich Arsenal. It consists of a radial girder supported at the inner end by a cast-iron belt which encircles the upper part of the centre column supportingthe steam-crane. Thebelt is free to rotate, being carried on coned supporting wheels which run on a circular race bolted to the column. The outer end of the girder is supported by a vertical leg, built of latticegirders, carrying the chain-barrel,electric motor and driving gear. The leg rests on two two-wheeled bogies, the wheels running on a single circular linewhich is also used bythe steam-crane. The mechanical details of this crane were designed by Mr. C. H. Moberly, M. Inst. C.E.; theAuthor, however, proposes to describe hereonly the details of the electrical gear. The current is supplied from the electrical central-station at the Arsenal, at a potential of about 310 volts. The cables are carried up one of the legs of the centre column, and are attached to two collecting-rings fixed on insulatorsupon the lower part of therace which supports the revolving belt, Figs. 7. These .collecting-rings are made of lengths of copper strip, 2 inches [THE INST. C.E. VOL. cxxx.] C

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. 18 RAVENSHAW ON ELECTRIC LIFTS AND CRANES. minute8 Of by inch,and, being bent to the correctcurve, are fixed by countersunkbolts to small gun-metal blocks, screwed on to the ends of horizontalAetna insulating bolts, Thefixing for these bolts is shown to a larger scale in Figs. 8. The insulators consist of gunmetal bolts screwed at one endand covered for nearlythe whole lengthwith a hard fibrous material,giving excellentmechanical supportwith remarkably good insulation. Theinsulation-resistance of the cables insidethe crane-house, includingthe collecting-rings and collectors, wasfound at the time of theexperiments to be about 5 megohms-an excel- lentresult considering the atmosphere of steam anddirt in which the crane was working. The current is collected from the rings by gunmetalplungers, 5 inchin diameter, fixed on insulating blocks toan arm boltedto theradial girder. The plungers have a range of more than 3 inch, to allow for unevenness in the rings, and have flexible stripsfixed to their outer ends by which the current is carried to the terminals to which the cables from the motor are attached. This arrangement of collectors and rings is shown in Figs. 8. The cables are carried along the girders to the main switch at the foot of the vertical leg. Fixed to the verticalgirder is a largecircular switch of solidconstruction, having a number of contacts connected withiron-wire resist- ances, capable of carryingthe working current of the motor without over-heating. An ampere-meter and volt-meter are fixed above the switch. One motor only is used; it is shunt wound, with a drumarmature and carbonbrushes, andruns at about 900 revolutions per minute at 310 volts. There is no sparking at the brushes with varying load, and little attention is required. The shaftof the motor is coupled direct to a worm-gear by a shaft 4 feet long, large enough to take the torque, but small enough to compensate for any want of alignment between themotor and the worm-shaft. This arrangement works well, and, having no back- lash, is preferable to a flexiblecoupling. The worm-gear men- tioned consists of a treble-thread steel worm, gearinginto a worm-wheel, having fifty-one teeth of 2-inch pitch, the reduction being in the ratioof 17 to 1. The directionof rotation being constant, a simplethrust-bearing is employed, consisting of a number of hardened-steel lenses. This was considered to be more suitable than a ball-bearing for so large a power, as the motor is only used at full power for a few minutes at a time. On one end of the worm-wheel shaft is fixed a spur- wheel, which transmits the power through a train of gearing and reversing friction-clutches to the travelling and radial motions.

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] RAVENSHAW ON ELECTRIC LIFTS AND CRANES. 19 On the other end of the worm-wheel shaft is a bevil-wheel, which actuatesthe winding-barrel through a friction-clutchand two- speed gear. A self-acting brake is connected to the chain-barrel by pawls and a ratchet-wheel automatically holding up the load, lowering being effected by releasing the brake. Owing to the special requirements of this crane, and the fact that it was advisable for the attendantto have ready access to the motor and gear, a good deal of gearing was used, and the power wasapplied in a somewhat indirect manner. In consequence a chain-block, weighingnearly 2 tons, was used. Thisweight causes a considerable reduction in the efficiency at light loads; but the loss is easily ascertained, and the ratio of the electrical energy consumed to the work developed in lifting the load, with thefalling block, is givenin Appendix 11. Although it was not possible to test the particular electro-motor used, a Table of efficiencies is given of a machine almost identical with it in size and construction, andthe estimated power absorbed is shown. The efficiencies may certainly be taken as correct within 1 per cent. With no load on the hook, the weight on the inner race was 14 tons, and on the outer line 46h tons, the total weight being SO& tons. Duringthe tests the load wassuspended from the middle of the girder, the weight being equally distributedbetween the ends.

The Paper is illustrated by eight tracings, from which Plate 1 and the Fig. in the text have been prepared.

[APPENDIXES. c2

m I B I l4

Downloaded by [ University of Sussex] on [18/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] RAVENSHAW ON ELECTRIC LIFTS AND CRANES. 21 APPENDIX II. TESTSOF A 20-TON ELECTRIORADIAL CSANE AT WOOLWIOH ABSENAL, 17~~MARCH,1896. Hoisting.

No. of Test . . .

Eficiency of Electric Motor. HP. at shaft . . 48 12 Efficiency, per cent. : I 92 1 if 88.5 Circular Travel.

No. of Test ...... 1 53 1 54 I 55 1 56 1 57 I 68 Load on hook . . tons I Light ' Light 9 9 20 20 55.0 55.0 55.0 54.0 54.0 Speedminute at racer,. feet. per)' . 55.0 . . Potential. . . . volts ~ 308.0 312.0 307.0 300.0 310.0297.0 Current . . . amperes 15'3 15.0 19.3 19.0 18.021.0 Work . . . . E.HP. ~ 6.3 1 6.3 7.9 7.5 7'64~ Radial Crops Traverse.

No.-of Test ...... 1 79 86 --Ip --Ip -----80 1 81 ~ 82 I 85

Load on hook . . tons 1 Light Light ~ 9 1 9 20 20

Speed of carriage,feet per ~ 33.7 minute . . . . . 33.4533.45 33.45 32.27 32.4 Potential. . . . volts307.0 308.0302.0 ,l 305.0 287.0 282.0 Current . . . amperes 12'4 13.018.75, 19.3 26.8 27.0 Work . . . . E.HP. ~ 5.1 5.31 7.6 I 7'9 ~ 10.3 10.2

l=i(I : :i

U CROSSSECTION--*

DETAILS OF COLLECTOR

PARTPLAN - l ELEVATION

Fl Fl VV KAVhNSHAVV,