sprocket pinion at each end of the countershaft, a chain from each pinion to a sprocket wheel on each road wheel. In motor trucks a worm gearset often takes the place of the bevel gear set, being better adapted for effecting a large reduction of the rotary speed of the shafts, while less suitable for motor cars, caus ing more resistance against coasting and other movements of the vehicle for which engine power is not applied. Other variations, known as internal gear drive and compound rear axle gear reduction, are referred to under McrroR TRUCKS. All other methods for transmitting the power are exceptional.
The clutch may be of the friction cone type, the brake type or the plate type, the latter hav ing few or many plates which may be lubricated Fig. 7 shows a plate clutch with one plate. Fig. 8 one with many, half of which are secured to rotate with the flywheel of the engine and the alternate ones with the clutch shaft. The plates in clutches operating without lubricant or dry. Types rarely employed are the hydrau lic and the pneumatic.
Fig. 6 shows a cone clutch arranged to cen tre itself automatically in the flywheel.
are generally made of other materials than steel or bronze, or alternate plates are of metal and the intervening plates of composition material. Frequently the clutch as well as the gearset is so encased as to form a rigid unit with the engine (Fig. 9), and the enlarged °unit power plane so formed is then often mounted in the automobile frame with support at only three Figs. 10, 11 and 12 show the unit power plant and the equivalent for clutch and gearset used in the most popular small motor car con struction. It operates with a planetary gear and provides one radical gear reduction for use at the starting of the car, for reversing and for conditions requiring a maximum of driving power (torque) at the road wheel rims. For all other conditions the rotary speed of the engine is continued without reduction to the points (or four points of which two are very close together), with the object of preventing any twisting or other deformation of the frame, which may be caused by the roughness of roads, from reaching and affecting the alignment of engine, clutch and gear shafts. If thi: means bevel gear on the rear axle, where it is reduced in the proportion of one to four, approximately. With this important exception the typical gear set follows more or less closely the design shown in Fig. 13, representing a gearbox (and clutch) with three forward gearspeeds and a reverse. For many large motor cars and heavy motor trucks four forward gearspeeds are used.
The methods by which gears are shifted have undergone considerable change in recent years. The shifting lever and quadrant which
were formerly on the outside of the motor car body are now inside of it as shown in Fig. 14, the change being a result of the general adop tion of gleft side drive,D or changing the driver's l' rv.1, (also serving to exclude dust) is not adopted, a universal joint is interposed between the clutch and the gearbox.
seat from the right to the left side of the vehicle. A long development in control of gear shifting, from to "selective' is described in most books on the subject but is passing into the remoter sphere of mechanical history. Similarly the gradual improvement in shifting forks and jaw clutches in the gearbox. A very recent development is a construction by which all gear wheels remain in mesh, excepting the reverse, the main object being to reduce the amount of skill required foi silent operation, all clash of gear avoided. The latest omnibuses equipped with gearboxes of this mensions are reduced in the same proportion) strong enough, the use of very strong alloy steel in gears has been necessary, calling for a specific development of metallurgical science and the exercise of painstaking judgment. For, if the gear teeth are made too small, the pressure upon lubricants becomes so large as to squeeze them out, thereby increasing friction and wear. and if the teeth are broadened to avoid this consequence the movements of the shifting forks needed for changing gear become larger than desirable, the gearbox longer and heavier, the shafts more subject to vibration.
Mainly in England the use of silent chains (q.v.) in gearboxes has been resorted to for overcoming these difficulties. Resonance of the aluminum box containing the gears is also to be avoided, so the material is thickened and the shaft bearings in its walls are made as long and large, to postpone wear and looseness, as other dimensions permit. The exterior of an To have spur gears operate silently the first requirement is the accurate involute cut of the gear teeth, perfected by grinding and polish, and great improvement in milling machines has been necessary to effect the smooth running and small wear of the gears with which the motoring public is now familiar. Another re quirement relates to the circumferential speed of the gear wheels, which should be as small as possible to reduce the humming noise of the mesh. reduce the diameters of the gears accordingly and yet have the teeth (whose di aluminum gearbox mounted separately in the frame is shown in front view in Fig. 15.