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The driving pinion delivers its power to a bevel driving gear which is carried by the differential in the rear axle housing. From the differential gear independent' shafts or axles drive the rear wheel hubs.

The function of the clutch is to permit the engine to be run independently of the transmission gearing when desired. The engine can drive the car only when one of the sets of gears in the gearset and the clutch are engaged simultaneously. For example, if the clutch is out or released, even if the gears were in mesh in the change speed device, the rear wheels would not be turned until the clutch cone was allowed to engage the female member formed in the flywheel rim. At the other hand, when the parts are as shown with the clutch in engagement and the speed gears out of mesh the engine crankshaft will revolve without turning the rear wheels.

The most important requirement in considering clutch forms is that such devices be capable of transmitting the maximum power of the engines to which they are fitted without any power loss due to slipping. A clutch must be easy to operate and but little exertion should be required of the operator. When the clutch takes hold, the engine power should be transmitted to the gearset and driving means in a gradual and uniform manner, or the resulting shock may seriously injure the mechanism. When released it is imperative that the two portions of the clutch disengage positively so that there will be no continued rotation of the parts after the clutch is disengaged.

The design should be carefully considered with a view of providing as much friction surface as possible to prevent excessive slipping and loss of power. It is very desirable to have a clutch that will be absolutely silent whether engaged or disengaged. If the clutch parts are located in an accessible manner it may be easily removed for inspection, cleaning or repairs. It is desirable that some adjustment be provided, so a certain amount of wear can be compensated for without expensive replacement. A simple, substantial design with but few operat. ing parts as illustrated is more to be desired than a complex device which may have a few minor advantages, but which is

more likely to cause trouble. The friction clutch in its various efficient types is the one that more nearly realizes the requirements of the ideal clutch.

How Sliding Gearsets Operate.—The majority of change speed gearsets which have been generally fitted to automobile service are sliding gear arrangements. In the selective system it is possible to go into any one of the speeds or gear ratios desired without passing into other speeds and with but a limited movement of the shifting members. The sliding gear system was one of the first to receive general application in early forms of motor vehicles and in its primitive condition it was but a modification of the back gearing used on certain classes of machine tools such as lathes, drill presses, etc. One of the advantages of this type when compared to other gear transmissions, such as the planetary, is that it is possible to provide a greater number of speed changes and obtain a higher driving efficiency when on the lower ratios because but two pairs of gears are in mesh.

The usual number of gear ratios provided is three forward speeds and one reverse motion. On some of the heavier touring cars four forward speeds are provided and when this is done engineers differ as to whether the direct drive should be on the third ratio and the fourth speed obtained by gearing up and having the driving shaft revolve faster than the main shaft of the engine or have the fourth speed a direct drive, the crankshaft and the driving shaft turn at the same speed. Those who favor the former method contend that as most of the regular driving is done at a medium rather than at an extreme high speed the direct drive on the third is preferable to a direct drive on the highest ratio. If the highest speed was obtained by a direct drive the natural tendency of the motorist would be to use this most, but there would be many conditions where the ratio would be too high and one of the lower gears would have to be used. If the direct drive was obtained in the third ratio this would be employed the greater part of the time, and as there would be less wear on the gearing with the direct drive engaged it would be preferable to use this as much as possible.

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Fig. 17.-Sectional View, Showing Construction of Buick Rear Axle. The operation is extremely simple. The gears on the countershaft are not shifted as they are firmly secured thereto. The sliding members are carried on the main shaft and can engage gears of varying diameter on the lay shaft, the relation between the diameter of the gears engaged determining the gear ratio.

Frame Parts.-Among the parts of the motor car that may be considered closely related to the frame are the side members, cross pieces and braces. It is necessary to have supporting irons by which the mud guards are carried over the front and rear wheels and also to have step hangers to which the running boards are attached. Mud guard supporting irons are usually bolted to the frame or fit into suitable sockets so they may be easily removed. As there is no occasion for removing the running board supporting irons, it is common practice to rivet these to the frame side members. Barring an accident the frame of a car is not likely to be damaged, and in order to prevent loosening of this essential part of the automobile chassis all of the members comprising the frame form a permanent assembly because they are riveted together. The springs by which the weight of the car is carried and which promote easy riding are considered as frame parts. Springs of many forms have been used in automobiles, but the practice has become standardized at the present time to the use of laminated leaf springs in their various forms. Semi-elliptic springs are the most popular for suspending the front end of the car and cantilever springs seem to be the most popular equipment for rear end of 1917 cars. A number of cars use the three-quarter elliptic spring, and the employment of other types, such as full elliptic, platform and transverse springs, is so rare as to be unconventional design. The frame also supports the steering gear and in some cases the clutch and brake pedal assembly and the change speed gear controlling levers when mounted independently of the power plant. In some types of cars radius and torque rods are used to keep the axles in proper relation to the frame.

This construction is being rapidly succeeded by the simpler form in which the rear springs are used to keep the axle in its proper relation to the frame, to take the push of the drive and to resist

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Fig. 18.-At Top, Brake Assembly of Buick Automobile, Showing

Relation of Internal Expanding and External Contracting Band Brakes. At Bottom, the Spiral Bevel Driving Gears and Differential Parts are Shown.

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