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 while the rear wheels make one revolution. On low powered tour ing cars the reduc tion is usually four 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. 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 directly 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 Fig. 173.-Part Sectional View of Worm Gear Drive Assembly With Worm Placed Beneath the Axle. just as in the usual form of live axle. In fact, the average countershaft assembly is practically a live rear axle with sprockets placed at the axle ends which in turn transmit the power to rear wheels revolving independently of each other on a fixed non-rotating axle which supports the car weight. One reduction ratio is obtained between the bevel gears of the countershaft and the speed is still further reduced by making the drive sprockets on the rear wheels larger in diameter than those on the countershaft. The racing chassis at Fig. 175 shows the method of using side chain drive with enclosed chains that is used on heavy touring cars while the illustration Fig. 176 shows the usual exposed chain employed in motor trucks. |