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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 corresponding 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 at

Fig. 163 and the arrangement of parts is clearly shown. In general construction it is the same as all sliding gear transmissions, having a main shaft which telescopes into the driving member to which the clutch cone is attached. One common shifting member, which is composed of the gears K and I and the positive jaw clutch J, is adapted to slide along the main shaft from one end to the other. This shifting member is moved by a gear shift lever which works on the segment providing five positions for the shift lever. Each of these positions is indicated by a notch cut into the guide segment and a simple form of locking member is employed to hold the lever in position in any one of the desired speed ratios. A transmission of this form is called "progressive" because the various speeds must be obtained in a constant unchanged ratio. For instance, if one is in the neutral position indicated and it is desired to go into the high speed, it is necessary to first pass through the intermediate ratio, before one can engage the high speed clutch. If one is in the reverse position and wishes to go to the high speed, the hand lever must be shifted through the slow speed and intermediate gears progressively before the direct drive is obtained.

Q. Outline method of obtaining low speed.

A. The low speed ratio is obtained by moving the hand lever back into a notch on the segment, which moves the bell crank and pushes the sliding member along the squared main shaft until gear K of the sliding member engages gear F on the countershaft. As previously stated the member of which gear A forms a part is normally independent of the squared main shaft and drives the countershaft through the medium of the gear B. There is a difference in size between the constant mesh gears and the countershaft turns slower than the gear member A. The power applied to the countershaft is transmitted back to the squared portion of the main shaft to which the universal driving joint is attached, by the small gear F which is one-half the diameter of gear K so that there is a reduction in speed of about four to one, meaning that the clutch cone will revolve four times to one revolution of the universal joint.

Q. Describe action when on intermediate speed.
A. To obtain an intermediate ratio the gear shift lever is moved

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Fig. 163.-Outlining Construction and Method of Operating Three

Speed Progressive Gearset.

from the neutral notch of the segment indicated at Fig. 163 to the intermediate notch on the segment. This pushes the sliding member forward so that the gear I on the main shaft engages the gear E. which is the same size, on the countershaft. As before, the drive is through gear A to gear B, which gives the countershaft half the speed of gear A and then from gear E to gear I on the main shaft wh ch turns that member at the same speed as the countershaft so that a gear ratio of two to one is obtained.

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