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| Illustration 1 | g01085127 |
The collar shift (constant mesh) transmission can be found in some older model Caterpillar D7 and D8 tractors. The collar shift transmission has parallel shafts with the gears in constant mesh. In neutral, the gears are free-running but when shifted the gears are locked to the shafts by sliding collars. The D7E Collar Shift transmission is shown in Illustration 1.
Collar Shift Transmission Helical Gears
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| Illustration 2 | g01085128 |
The collar shift transmission uses helical gears (Illustration 2) which remain in constant mesh. There are a several reasons why helical gears are used in the transmissions of the larger tractors. Helical gear teeth are stronger than spur gear teeth because the teeth of a helical gear are longer than the teeth of a spur gear of the same width. Also, helical gears can operate more smoothly and quietly than spur gears, because several teeth of a helical gear are partly meshed at one time.
Collar Shift Transmission Drive Gears
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| Illustration 3 | g01085130 |
The operation of the collar shift transmission will be explained by building up a typical constant mesh gear train. The drive gears (Illustration 3) are splined to the shafts and turn with the gears.
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| Illustration 4 | g01085133 |
Driven gears have smooth bores and spin on sleeves (Illustration 4). The sleeves are splined to shafts. The bore of the driven gear fits over the sleeve. The rim of the driven gear's hub has teeth cut on the rim. These teeth are matched with the internal teeth of the sliding collar. The drive gears and driven gears are always meshed. When the drive gears turn and the sliding collar is not being used, the driven gears turn on their sleeves. Since the gears are constantly in mesh and cannot slide on their shafts, one gear in each constant mesh gear set runs free on the shaft when the machine is in neutral.
Sliding Collar Assembly
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| Illustration 5 | g01085134 |
Each driven gear has a sliding collar assembly beside it, next to the toothed hub (Illustration 5). A sliding collar assembly has two parts, the sliding collar and the gear. The gear is splined to the shaft. The collar's bore is splined. The shifting fork fits into the groove on the outside of the collar. The shifting fork slides the collar back and forth on the teeth of the gear. The shaft and the sliding collar assembly turn together.
Sliding Collar Locks Driven Gear
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| Illustration 6 | g01085135 |
To shift gears in a sliding collar transmission, the flywheel clutch stops rotating the shaft and a shift lever moves a shifting fork. The shifting fork slides the collar partway onto the teeth of the driven gear's hub (Illustration 6). In this position, the sliding collar locks the driven gear to the sliding collar assembly. When the shift is complete, the driven gear, the sliding collar, and the shaft turn together.
Sliding Collar Gear Train
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| Illustration 7 | g01085136 |
The basic sliding collar gear train is shown in Illustration 7. The top shaft is the idler shaft. The idler shaft has been moved to the top for demonstration purposes. The center shaft is the input shaft. The power from the flywheel clutch comes in from the right side. The left end of the input shaft is the Power Take Off (PTO) shaft. The bottom shaft is the output shaft. The bevel pinion is located on the left side.
Gear Train in First Gear Forward
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| Illustration 8 | g01085137 |
The gear trains are engaged in first gear forward (Illustration 8). The driving gears (1) and the driven gears (2) are shown. The sliding collars are shown in neutral positions. The arrows indicate that the collars slide in order to lock the driven gears to the shafts. The collar of the gear on the top shaft is shifted by the shifting fork for first and second speed. The collar of the gear on the bottom shaft is shifted by the forward and reverse shifting fork. In this transmission, the forward drive train has an idler. The idler gear (3) is located on the input shaft. The idler gear rotates on bearings. The rotation of the idler gear is independent of the input shaft. When the machine is traveling in forward speeds, the input shaft and the output shaft rotate in opposite directions.
Gear Train in First Reverse
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| Illustration 9 | g01085138 |
First reverse is engaged in Illustration 9. The collar of the gear on the bottom shaft has been shifted in the opposite direction. The idler gear is not used. When the machine is traveling in reverse speeds, the input shaft and the output shaft rotate in the same direction.
Gear Train in Fifth Gear Forward
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| Illustration 10 | g01085139 |
Fifth gear forward is engaged in Illustration 10.
This transmission has five speeds forward and four reverse.
Reverse cannot be used in fifth speed, so the position of the forward and reverse shifting fork does not matter. A large gear on the input shaft drives a small gear on the output shaft. The gears on the countershaft are not used in fifth speed.