Sliding Gear Transmission with Parallel Shafts
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| Illustration 1 | g01085109 |
The sliding gear transmission can be found in small lawn and farm machinery. Some older model tractors use the sliding gear transmission.(Caterpillar D3, D4, and D6)
The sliding gear transmission contains spur gears and shafts. The sliding gear transmission provides a variety of speeds. A sliding gear transmission can be configured with input and output shafts that are parallel (Illustration 1) or input and output shafts that are in line.
When the input and output shafts are parallel, the input shaft drives the output shaft. The output shaft transmits the power. There is normally a third shaft (reverse idler shaft in Illustration 1) which reverses or varies the power flow. All three shafts are in parallel and interact through the shifting of their gears. The gear configuration (Illustration 1) provides three forward speeds and one reverse speed.
Sliding Gear Transmission with In-line Shafts
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| Illustration 2 | g01085111 |
When the input and output shafts lie in a straight line but are not connected (Illustration 2), a countershaft transmits power between them.
The power flow that is shown in Illustration 2 is for low gear or first gear. Power is flowing from gear "A" on the input shaft to gear "D" on the countershaft, which is also turning gear "F". Gear "F" on the countershaft, in mesh with sliding gear "C" on the output shaft, causes that gear to rotate and transmit power to the wheels.
A feature of this transmission is a high gear, which bypasses the countershaft. It is provided by connecting gears (A) and (B), which locks the input and output shafts.
D6C Sliding Gear Transmission
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| Illustration 3 | g01085112 |
Sliding gear transmissions have many variations. The sliding gear transmissions often provide as many as ten forward speeds and two reverse speeds.
Illustration 3 shows the D6C Sliding Gear transmission. The positions of the three parallel shafts in the transmission can be seen at the right end. The transmission upper shaft (input shaft) is connected to the engine through a flywheel clutch. The countershaft is at the right and the output shaft is at the bottom of the transmission.
The two shift levers, which the operator uses to change gears, are the forward and reverse lever (in back) and the speed shift lever.
Gear Shift Mechanism
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| Illustration 4 | g01085114 |
Illustration 4 shows a manual transmission with the shift controls removed. The shift controls are used to change gear positions. In sliding gear, collar shift, and synchromesh transmissions, the gears or collars are moved by means of a shifter fork which fits in a groove in the collar or gear. This fork is connected to the gearshift lever and moved manually by the gearshift lever. The gearshift lever reaches directly into the transmission from the operator's platform.
The forward and reverse shifting fork is the single fork (1) at the top. The other three forks move the speed gears. The input shaft and the forward/reverse sliding gear are visible inside the case (at the top.)
The shifter forks, which slide the gears or collars, are attached to the gearshift rails or shafts. The gearshift lever has a ball that fits into a socket in the top of the transmission cover. This permits swiveling of the gearshift lever into the various shift positions. Movement of the lever causes the finger on the lower end to select and move one of the rails, the forks, and a gear or collar.
For example, in a four-speed transmission, as the gearshift lever is moved from NEUTRAL (N) to the left, the finger on the lower end of the lever moves into the slot in the low and second speed rail. This action selects the rail, forks, and gears in order to be moved. Then, as the lever is pushed forward into low speed, the rail assembly is pushed toward the rear. Then the low gear is engaged.
A locking system must be used in order to hold the shift rail in position and to prevent other rails from moving. The shift rails contain spring-loaded detent balls that drop down into undercuts on the rails as the rails are moved into the various shift positions. This holds the gears in mesh.
Input Shaft
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| Illustration 5 | g01085115 |
A model will be used to illustrate how the sliding gear transmission works.
The first components of the model (Illustration 5) are an input shaft or upper shaft that are cranked by the engine and a forward and reverse sliding gear. The drive shaft and gear turn with the engine when the clutch is engaged. The forward and reverse sliding gear is splined to the input shaft. The input shaft and gear turn together and the gear can slide back and forth. The gear is painted to represent a spur gear, the kind of gear that is used in sliding gear transmissions.
Shifting Fork Moves Sliding Gear
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| Illustration 6 | g01085116 |
The forward and reverse sliding gear is pushed back and forth on the shaft by a shifting fork that fits in the fork groove on the hub of the gear (Illustration 6). The operator's forward and reverse selector lever moves this shifting fork.
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| Illustration 7 | g01085118 |
When the shifting fork is moved left toward the rear of the machine (Illustration 7), the sliding gear meshes with a driven gear that is fastened to the countershaft. The driven gear does not slide. In the D6C transmission, this gear train provides the forward drive speeds.
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| Illustration 8 | g01085119 |
When the shifting fork is moved right toward the front of the machine (Illustration 8), the forward and reverse sliding gear meshes with an idler gear. The idler gear is located on the output shaft. The idler gear rotates on bearings so the output shaft does not rotate. The idler gear is meshed with a driven gear on the the countershaft. The driven gear does not slide. In the D6C transmission, this gear train provides the reverse drive speeds.
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| Illustration 9 | g01085120 |
In Illustration 9, two gears (speed gears) have been added to the countershaft. The speed gears are the drive gears of the speed gear train. The speed gear trains are powered by the countershaft, which is driven by the forward gear train or by the reverse gear train.
Sliding Gear in Neutral Position
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| Illustration 10 | g01085123 |
In Illustration 10, the driven gears of the speed gear train have been added to the output shaft. The driven gears in the speed gear trains are free to slide back and forth on the splined output shaft. The sliding gear is in the neutral position. The operator's speed selector lever controls the shifting fork that moves them.
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| Illustration 11 | g01085126 |
In Illustration 11, the shifting fork has moved the sliding gear. The speed gear train is a speed decreasing train. The speed gear train in mesh will transfer more power to the sprockets.
The D6C transmission has three more speed gear trains that give the tractor five forward and four reverse speeds. The model in Illustration 11 has two forward speeds and two reverse speeds.