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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 chaiņ 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
chains in which the sprocket teeth fit openings in the chain. In the silent chain outlined at Fig. 177-C the individual link members are practically the same and the chain is made up of a large number of these link plates held together by side plates of the usual pattern in order to obtain greater strength. A silent chain is quieter in action than either a block or roller chain, because as it runs over the sprocket the tendency of the downwardly projecting teeth is to entirely fill the spaces made to receive them in the sprockets by spreading apart so there is no lost motion between the chain and sprockets, thus noise is reduced appreciably.
Fig. 177.-- Forms of Driving Chains That Have Received General Ap
plication. A-Block Chain. B-Roller Chain. C—Silent Chain.
Q. Why is enclosed chain drive better than open drive?
A. In order to obtain satisfactory service from driving chains it is necessary that these be kept well lubricated and free from dirt or grit that will produce wear at the bearing surfaces. If a driving chain is run without a case the particles of dirt and grit that adhere to the links not only absorb the lubricant but interpose an item of friction between the parts of the chain as well as between the chain and the sprockets that means considerable power loss. While it is possible to run block and roller chains without casing it is absolutely necessary to protect the silent chain and keep it thoroughly oiled or it will soon depreciate. The life of an enclosed chain is three or four times greater than that of an unprotected chain doing the same work and it will be quieter in operation on account of the coating of lubricant.
Q. What are the advantages of gear drive?
A. The advantage that enclosed bevel or worm gear drive offers is that these parts are maintained at a point of high efficiency because they are kept from the dirt and are always properly lubricated. The efficiency of gearing when properly meshed and kept clean and well oiled is very high and the usual arrangement of the driving gears of the rear axle produces a compact and enduring mechanism.
Q. What are universal joints and why are they needed?
A. Universal joints are flexible driving members which are not so flexible as tò fail to provide a positive drive between shafts that may not always remain in proper alinement. Universal joints are needed on all driving shafts transmitting power from one member carried on the frame to the gearing at the rear axle because the movement of the rear axle is independent of that of the frame which is supported by resilient, yielding springs. Without universal joints the driving shaft would be subjected to much strain as the parts of the chassis mounted on the springs would move at a different ratio than those carried directly by the tires. Most open propeller shafts use two universal joints, while enclosed shafts need but one, at the front end.
Q. Compare the various methods of final drive with relation to their efficiency.
A. The most efficient means of final drive is by worm gearing which is said to have an efficiency ranging from 90 to 95%. Single chain drive follows with an efficiency of about 90% when new and properly oiled. Then bevel gearing follows with an efficiency of about 85% while the various forms of compound gears are the least efficient, the power loss in transmission ranging from 20% in a combined spur and beveled gear reduction to 30 or 35% with beveled gear and side chain reduction.
Q. What is the skew bevel gear drive and what are its advantages?
A. The skew bevel or worm bevel, as it is sometimes called, differs from the regular form of bevel gear in that the teeth on pinion and ring gear are cut on an angle, as in worm gearing. The construction of typical skew bevel driving gears detached from the differential casing is shown in illustration Fig. 177-a, the view at the left being of the gears detached from the differential assembly while that at the right shows the application to the rear axle used on Packard motor cars. It is claimed that the skew bevel produces increased quietness of drive as backlash or looseness between the teeth, which is a common cause of noisy action of conventional bevel gearing, is entirely eliminated by the angular teeth. It is also claimed that whereas ordinary bevel gearing has a straight line contact, the worm bevel with its curved teeth has one set of teeth constantly meshing while the next set is becoming disengaged. This affords a continuous contact, in some degree the same as accomplished with worm gearing.
Q. Are gasoline automobiles always driven by the rear wheels?
A. Gasoline automobiles have been designed where the drive is through all four wheels which combine directive and tractive functions. A side elevation of the new Rambler four wheel drive motor truck with important parts depicted is clearly shown at Fig. 177-b. In this the power is obtained by the regular Rambler power plant driving through a dry disc clutch to a change speed gear of special construction mounted on the chassis frame about midway between the front and rear axles. One drive shaft extends from the front end to the front axle while a similar member transmits power to the rear axle which is a duplicate of that used at the front of the car.
The special form of gear box necessary is outlined at Fig. 177-c. In this the drive is through gearing at all speeds, there being no direct drive clutch as is true of conventional gear sets. The transmission provides four speeds. forward and a reverse and in ad