The relations existing in the matters of fuel economy, i gear speeds and gear ratio, con venience n driving, vehicle weight, the special qualities of the engine which are indicated by its power and torque curves, form a subject on which the opinions of engineers differ, not so much with regard to the mechanical results of a given set of conditions as with regard to the question of what results are most desirable. The answer varies greatly with the purpose of the vehicle. And thus a great variety of en gines are in use, weighing from 15 down to five pounds per horse-power, having a maximum speed from 800 up to 4,000 revolutions per minute and from one to 12 cylinders. The gearbox arrangements vary accordingly. As a rule, the larger the range and resources of the engine, the smaller and simpler the gearbox can he, for given working conditions.
Universal joints, drive shafts, bevel gears and differential gears, of which typical examples are included in preceding illustrations, are me chanical' units known in many applications out side of motor cars, but in these they reach their most refined development. Universal joints formed of leather disks on the plan shown in Fig. 16 have been widely tried, with the object of securing silence and avoiding lubrication and wear, as well as the exuding of oil or grease liable to occur in these members if they are entirely metallic, but these substitute arrange ments have not proved durable or adapted for manufacture on a large scale and are now seldom used.
Bevel gears have been so refined that their mechanical efficiency reaches 95 to 96 per cent when new, and spiral bevel gears of the type shown in Fig. 17, which are gaining rapidly in favor for high-class construction, are almost silent in operation.
The differential (typical bevel gear with differential, Fig. 18), whose function is to allow one driving wheel to rotate more rapidly than the other, as is necessary when the motor car turns, causes an unsuitable action when one of the driving wheels is in a slippery place in the road, so that it can be spun around rapidly with less resistance than required for starting its mate. The engine then drives this wheel alone and traction is lost. A remedy used little for motor cars but widely for motor trucks — and obligatory for European army trucks — consists in a locking device by which the differential can be made inoperative. To accomplish the same purpose more completely and conveniently, differential gears have been invented in which the differentiating movement is braked by hydraulic or mechanical friction or arranged with helical gears whose diameters and pitch are so chosen that they can be oper ated in one direction but much less readily in the other. Fig. 19 shows a sketch of a differential of this nature which is not yet standard but is not uncommonly preferred to the normal type, especially for motor trucks.
The lubrication of the transmission elements, of springs and shackles, of the steering gear, the wheel bearings, etc., is not yet developed
to a system. In nearly all motor cars each casing and bearing must be watched separately and provided with lubricant separately and at irregular intervals. In the isolated instances where a system has been established, it begins with the engine. As it -is recognized that the durability of the mechanical construction de pends greatly upon having lubrication attended to unfailingly, and as constant (but in practice always irregular) attention to lubricating in volves a large share in the work of caring for a motor car, a universal development toward an almost automatic and complete system of motor car lubrication, now barely inaugurated, is widely expected. The difficulties lie mainly in rendering it unfailing, so that no false security, with the attending instances of seri ous damage to parts, may be the result.
While the worm gear sets and internal-gear drives are occasionally used for motor cars, their use for motor trucks and electric vehicles is more common.
Transmission by chain drive from a coun tershaft is typical for motor wagons and motor trucks and is referred to under these heads below, but it is also still used in a few high speed motor cars, especially in Germany, hav ing survived from the time, before 1905, when it was the most widely used transmission system for motor vehicles of all classes. It does not differ in principle from the chain drive used for bicycles.
Electric transmission from an automobile engine has been frequently tried for motor cars and motor trucks but has never gained adoption for regular and extensive manufac ture. As it admits of dispensing with the shift ing of gears and can easily be adapted to the driving of all four wheels, however, it is con stantly taken up again, in one place or another, when developments or discoveries in electric science or machinery seem to promise new and superior results. As the features of greatest special interest in these vehicles are electrical and no type of motor car or motor truck with electric transmission is yet widely established industrially, descriptions of the most recently developed systems of this order (represented in the United States by the Entz transmission, in England by the gas-electric omnibuses operated mainly in Manchester, and on the continent by the Balachowski & Caire gas-electric trucks) are given under ELECTRIC VEHICLES.
Hydraulic transmissions exist sporadically in experimental motor cars and are used in a few motor truck constructions, such as the Manly (American-La France) in the United States, the Hele-Shaw in England, the Hugo Lentz in Germany and France, and the im proved Janney-Williams (Delaunay-Belleville) in France. As they are not yet important in the automobile industry and the principles employed in them are similar to those applied more extensively and typically for hydraulic marine steering gears and other hydraulic machinery, descriptions or explanations of hydraulic motor vehicle transmissions are given under