Variable Gears

A development is now taking place of this type of variable trans mission for larger powers. In a useful form variation of the distance apart of the conical sides of a driving and driven pulley is effected when much power has to be transmitted by a hand wheel that operates a combina tion of worm and worm wheel so as to turn to other worms and thereby move levers, which re spectively operate the movable cheeks of the conical pulleys either inwards or outwards. In all probability the oldest type of infinitely variable gear is that in which a friction roller is moved edgeways to and fro on the disk or circular flat plate. If, as in a certain type of motor-car the speed of the driven disk is least when the roller is nearest the edge, the greatest driving force is then obtained. On the other hand, when the roller has approached towards the centre of the disk p the greatest speed is obtained with corresponding reduction of driving force. The power transmitted with this gear is however comparatively small as it depends entirely on frictional contact of the surfaces and the wear of the frictional surfaces is considerable.

Mechanical.

Variable transmission of mechanical kind can be said to be practically confined to the employment of trains of toothed wheels. The most familiar case of mechanical gearing is that of a lathe headstock, in which by coupling up the largest step pulley to the spur wheel to the spindle of the headstock a direct drive is obtained from the speed pulleys. If this coupling is re leased and by means of a handle shown in the figure the back axle is approached to the main spindle so as to bring into engagement the two pairs of tooth wheels, the driving now takes place through the pinion driven by the step cone pulley through the axle shaft, and back to the lay spindle. In crease of driving force with cor responding reduction of speed is thus obtained which in the pres ent example is nine times as great as by the direct drive.

In a standard type of motor car change speed gear box, the engagement of different ratios of spur wheels and pinions is ob tained by sliding the teeth side ways into mesh with each other.

This is called the "clash" engagement. In another type of gear box no such sideway sliding takes place, the teeth of the respective wheels being always in mesh with each other. Change of gear in this case is effected by sliding the different jaw clutches into engagement with each other.

Pneumatic.

Many attempts have been made to introduce pneumatic variable transmission since 190o by Dunlop, Lenz, Lebach and others. The best account of these pneumatic devices as applied on a large scale to locomotives was given in La Revue Generale des Chemins de Fer, May 1923, by Brille. Up to the present these devices, though extremely interesting and ingenious, have not proved efficient in practice.

Electrical.

Various inventors have suggested continuously variable electric change speed gear. As already explained a mere reduction of effort by interposed resistance is not a true variable transmission, and all the most important examples of real variable electric transmission as on electric railways, is of essentially a step by step nature. Locomotive systems such as Crochat's mine loco

motive, the loco-tractor of Moyse, those of Westinghouse, of Sultzer Diesel and Dewa are on the same fundamental principle as the Tilling-Stevens electric transmission, namely, production of electrical current by a dynamo from an internal-combustion engine and the operation by an electric motor of some form of gearing to turn the wheels.

Hydraulic.

The last form of transmission, and in its develop ment the most recent, is hydraulic transmission. A beautiful de vice by Hastie was described as long ago as 1881. By means of two springs the stroke of the crank can be altered, when the crank pin slides in a groove.

When the effort is great the springs are compressed and the stroke is increased correspondingly, enabling a constant hydraulic pressure to overcome a variable resistance. This resistance may vary within wide limits, with the corresponding economy in the matter of the fluid employed. In the foregoing case the working fluid was water supplied by a high-pressure hydraulic main on the principle first introduced by Armstrong. Modern hydraulic variable gear in which oil is the working fluid has been brought into extensive operation chiefly to enable electrical power to overcome very variable resistance without causing a rise in the amperes transmitted. The principle of operation is exactly identical with the generation of electrical current by a dynamo, which current is transformed into mechani cal effort by an electrical motor. In the case of hydraulic trans mission, however, a variable stroke pump is employed to produce a flow of incompressible fluid, generally oil, which operates in a hydraulic motor. Thus it is only necessary to effect a change in the stroke of the pump when any required hydraulic pressure can be obtained with the great advantage of not unduly increasing the driving effort on the pump.

Hydraulic variable gear has been employed for a great variety of purposes such as cranes, lifts, winches, tipping wagons, rail cars, gun mountings, planing and broaching machines, swing bridges, etc. The chief types of these gears may be distinguished as parallel and radial. Fig. 4 shows a section of the Janney Williams gear (either pump or motor) which is an example of the former type, and it will be seen that the variation in effort is at tained by varying the angle of what is known as a swash plate, with the result of altering the stroke of the pistons. As the stroke is reduced the working pressure can be increased ; thus when the stroke is very small a very great pressure can be produced and consequently great resistance can be overcome. The necessary valve action is obtained by causing the opening at the inner end of the cylinders to pass in succession the supply and exhaust ports.