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. The greater simplicity of the hydraulic 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 machine make it more convenient to 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, how- ever, 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 through intermediate pads of india-rubber, otherwise an unpleasant vibration is some- times 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 as there 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 is a 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 at- tendant being generallyemployed, and the usualhand-rope adopted. A small reversing motor, or a pair of magnets or solenoids con- trolled by aswitch inthe cage,may be used to actuatethe regulating-gear. Thecurrent has,however, to be carried from the cage, either by hanging wires or by sliding 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 work- ing 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 current as 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.