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Fig. 170.-Gearset Located at McCue Rear Axle.

A. The important advantage of combining the gearset with either the power plant or driving member or the rear axle driven member is that the parts are kept in absolute alinement because there is no possibility of the gearset getting out of line with the ankshaft if it is housed in a part of the motor case or to lose its inement relative to the driving gearing if it is part of the rear axle or countershaft assembly.

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LESSON NINETEEN

METHODS OF DRIVE TO REAR WHEELS

Q. Describe common methods of power transmission in automobiles.

A. The power of the engine is usually transmitted from the speed changing mechanism to the rear wheels by some method of positive drive mechanism which may be bevel, spur, or worm gearing, or sprocket and chains.

Q. What is the influence of power plant location on method of drive?

A. The arrangement of the power plant relative to the rear axle has material influence on the method of drive employed. If the engine is a simple form with the crankshaft at right angles to the frame side it is customary to drive directly from a simple planetary gearing on the crankshaft to a live axle by a chain. If the engine is located in such a way that the crankshaft is parallel to the frame members, then a shaft is employed to transmit the power from the gearset directly to gearing in the rear axle or from sprockets on a countershaft to similar members on the rear wheels.

Q. What is the influence of gear ratio on final drive?

A. When low gear ratios are desired, as on heavy commercial cars, it is necessary to use either a double reduction drive system or worm gearing, if a single reduction is employed.

Q. What determines the speed reduction needed between driving and driven members?

A. The ratio of rear wheel speed relative to that of the crankshaft of the power plant depends upon the amount of power available, the maximum speed it is desired to attain, and the weight of the vehicle. On high powered pleasure cars, where high speeds

are required and where powerful engines are provided, it is possible to gear the rear axle in such a way that the engine will make

two and one half to three turns

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to one, while on heavy commercial vehicles the reduction will range from eight to twelve to one and even greater than this if a small engine is required to move a heavy load. Obviously, the lower the reduction, the less speed obtained for a given number of engine revolutions.

Q. Describe cheapest method of direct drive. A. The cheap

Fig. 171.-Side View of Typical Shaft-Driven Chassis Showing Driving Shaft With Two Universal
Joints and Arrangement of Parts of Power Transmission System.

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Fig. 172.-Plan View of Shaft-Drive Chassis With Enclosed Drive Shaft and Gearset Mounted at Front End of Torque Tube.

est method of direct drive is by attaching the engine direct y to the rear axle and driving from the crankshaft to the differential case by a pair of spur gears. This method is used only on steam carriages at the present time and on some forms of electric automobiles, though in the latter application compound gearing is generally used to obtain the desired low ratio. Another simple driving means which is used at the present time only on two or three makes of cars employing friction transmissions is to join. the transmission shaft directly to the rear axle by some form of driving chain.

Q. Describe method of direct drive ordinarily used.

A. The method of direct drive ordinarily used is shown at Figs. 171 and 172. In the former the drive shaft which transmits power from the sliding gearset to the bevel driving gears at the rear axle is provided with two universal joints, one at each end and is exposed. The driving and braking strains on the axle are resisted by a pressed steel member extending from the rear axle to the rear frame across member that supports the gearset. At Fig. 172 the same method of drive is employed except that but one universal joint is used and the driving shaft is enclosed in a tubular housing that also serves as a torque member.

Q. What is an efficient method of direct drive when speed reduction ratio is low?

A. A worm drive assembly, such as outlined at Fig. 173, provides a very satisfactory method of drive where a low reduction. ratio is desired without using compound gearing or a double reduction. The worm has a tooth angle of about 45 degrees and drives the worm wheel, which is provided with angularly disposed teeth placed around its circumference

Q. Name parts of compound drive or double reduction system generally used.

A. Where a lower gear reduction is desired than is possible to obtain by the use of a direct drive connection the compound drive as shown at Fig. 174 is utilized. In this the power is transmitted from the engine to a countershaft placed across the frame; the drive from engine to the countershaft is by means o bevel gears

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