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Fig. 169.-Four-Speed Motor Truck Gearset With Drive Pinion, Ring Gear and Differential Assembly Incorporated in Same Case and Used in Connection With Counter Shaft Drive.

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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 crankshaft if it is housed in a part of the motor case or to lose its alinement relative to the driving gearing if it is part of the rear axle or countershaft assembly.

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

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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

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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.

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