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two double end clutches and two loosely revolving members, and the countershaft to which four gears are secured. Both of these shafts are carried on suitable ball-bearings, and the driving end of the main shaft is provided with a bevel drive pinion which engages with a driven gear which forms part of a countershaft carried as part of the gearset. The main shaft is made in two pieces, one end telescoping into the other. The power of the engine is applied to the short portion of the main shaft or sleeve into which the end of the longer shaft telescopes. One of the constant mesh gears is an integral part of the sleeve. This gear is in mesh with a larger member carried by the countershaft.

To obtain a high speed or direct drive, the high and intermediate speed sliding clutch is pushed forward until it engages with the projecting teeth of the constant mesh drive gear. This serves to lock the two portions of the main shaft together and power is transmitted directly from the constant mesh drive gear sleeve which is turned by the engine to the bevel drive pinion. If, however, the high and intermediate sliding clutch is moved toward the back of the gearset in such a way that it locks the intermediate gear C to the shaft, then the engine power is transmitted from constant mesh gear H to countershaft gear B and from intermediate drive gear D to the driven gear on the main shaft with which gear D meshes. To obtain low or reverse speed the other sliding clutch is moved so that low speed driven gear F is locked to the main shaft for the slow speed and reverse gear H is locked to the main shaft for reverse motion.

The countershaft turns slower than the drive sleeve of which the constant mesh drive gear A forms a part on account of the difference in size between gears A and B. When the intermediate ratio is desired, practically the only reduction in speed between the engine and the drive pinion is obtained by the difference in size between gears A and B because gears C and D are practically the same diaameter. On low speed and reverse motion there is a reduction in speed because gear E is smaller than gear F and gear G is smaller than gear H. On the direct drive, the speed of the bevel drive pinion and the engine crankshaft is the same. On the intermediate ratio there is one reduction in speed at the constant mesh

gear. On the low and reverse speed, there are two reductions in speed, one at the constant mesh gears and the other between the driving gear on the countershaft and the driven gear on the main shaft. When both clutches are in a neutral position the master clutch may be engaged without driving the transmission main shaft. Before either sliding clutch member is engaged it is necessary to interrupt the engine drive by means of the master clutch.

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Fig. 161.-Sectional View of Coventry Silent Chain Individual Gear Box With Chains Removed from Gearing to Show Arrangement of Parts.

Q. Are gears the only speed reducing members used in gearsets of this character?

A. Forms of individual clutch gear boxes have been devised abroad where silent chains are used to connect the main shaft and countershaft instead of having gears in mesh. A gear box of this character with the chains removed is shown at Fig. 161. The various speeds are obtained by sliding clutches, just as in the previously described outfit, the only exception being the method of obtaining the reverse ratio.

Q. How is the reverse drive obtained with silent chain gear box?

A. Instead of using a sliding positive clutch a large gear is slid into mesh with a small gear on the countershaft. Where chains are used both shafts turn in the same direction and the use of a pair of gears reverses the motion of the main shaft relative to that of the countershaft which is turning in the same direction as the constant mesh sprocket driven by the engine.

Q. What are the advantages of the silent chain gear box? A. It is contended that in heavy vehicle service such as motor truck work or in omnibuses that it is necessary to use the lower gear ratios a large part of the time. The ordinary forms of gear boxes are noisy when in use and become more noisy as the parts wear. If silent chains are used the gearing is quieter and more enduring than the conventional form of spur gear construction.

LESSON EIGHTEEN

ACTION OF SLIDING GEAR TRANSMISSION

Q. What are the essential parts of a sliding gearset?

A. The sectional view of gearset outlined at Fig. 162 clearly shows the principal parts of a sliding gearset and their relation to each other. Two shafts are provided, one of which is divided in two parts and is called the main shaft or transmission shaft, while the other shaft is a solid member and is termed the "countershaft." The gears on the countershaft are keyed to it while those on the main shaft are adapted to be moved back and forth by shifting forks carried on gear shifting shafts. The shafts are supported on suitable bearings preferably of the anti-friction type and the entire assembly is housed in an oil tight aluminum casing.

Q. How does the sliding gearset differ from the individual clutch type?

A. The sliding gearset is so called because speed changes are effected by sliding one gear into engagement with a responding member on the countershaft. In the individual clutch forms, all gears are always in mesh, whereas in the sliding gear type only the gears that are used in transmitting power are engaged. In both forms of transmission there is one pair of gears that are constantly meshed and the main shaft construction is just the same in either the sliding gear or individual clutch forms. One end of the main shaft proper telescopes into a member which carries the constant mesh gear and to which the clutch shaft is attached. Instead of shifting the clutches as in the individual clutch form the gears themselves are moved along the keys on the main shaft.

Q. Name the two main types of sliding gearsets.

A. Two forms of sliding gear arrangements have received general application namely the progressive system in which one shift

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Fig. 162.-Three-Speed Sliding Gear Set of the Selective Type Used in Overland Cars Showing Principal Parts.

ing member serves for all speeds and the selective system where two or more sliding gear members are used.

Q.

Describe construction of simple three speed progressive sliding change speed gearing.

A. A typical progressive sliding gear assembly is outlined a

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