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

Fig. 173.-Part Sectional View of Worm Gear Drive Assembly With Worm Placed Beneath the Axle.

just as in the usual orm 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.

Fig. 174.-Plan View of Light Truck Chassis Having a Double Reduction With Final Drive by Chains and Change Speed Gearing

Q. How can a compound drive be obtained without use of chains?

A. Some rear axle constructions are designed to use two sets of gears and thus a compound drive is possible without using chains. For example the drive from the gearset will be to a bevel pinion which engages a bevel gear mounted on a short countershaft inside of the rear axle housing, and this countershaft carries a small spur pinion which engages with a large spur gear on the differential casing. Other forms have bevel gear drive to the center of the axle and have a small spur pinion at the end of each live axle shaft which engages a large internal spur gear that forms part of the brake drum. In this method of construction the rear axle is virtually two members, the greater part of the car weight being carried on

Fig. 175.-Side View of Touring Car Chassis Using Chain Drive.

a non-rotating dead axle on which the wheels revolve while the power transmission is through an auxiliary live axle assembly attached to the fixed axle and used only to transmit power.

Q. What are the common forms of driving chains? A. The chains ordinarily used for power transmission purposes are the block, roller and silent chains, as outlined at Fig. 177. Q. What is chain pitch?

A. The pitch of a chain is a term used in speaking of chain sizes and it is the distance between the center of one tooth space to that of the other tooth space in the link immediately adjacent.

Q. What is the difference between a block and roller chain?

A. As will be seen by reference to Fig. 177-A the block chain is composed of the fewest parts, being made up of approximately B shaped blocks joined together by means of side plates or links of the same pattern, the links being held together by through rivets which form a bearing for the chain blocks. In the roller chain as outlined at B, the blocks are replaced by compound members formed of two side plates held together by tubular bushings on which hardened steel rollers revolve, in addition to the usual link and rivet. The advantage of the roller chain is that it rolls over the

sprocket with less friction and wear than a block chain and it is used to the entire exclusion of the simpler form in automobile power transmission systems.

Q. What is a silent chain and how is silence obtained?

A. The silent chain is the form in which toothed links are employed which engage with suitable depressions in the sprocket which is directly opposite in action to that of the block and roller