Countershaft Transmission
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| Illustration 1 | g01085663 |
Countershaft transmissions use clutches to transfer power through gears. Countershaft transmissions use constant mesh spur gears. The transmission does not have sliding collars. Speed and direction shifts are accomplished by hydraulically engaging various clutch packs. Advantages of the countershaft transmission include fewer parts and less weight.
A four speed forward and three-speed reverse countershaft transmission (Illustration 1) will be used to explain the components and operation of a countershaft transmission.
Countershaft Transmission in Side View
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| Illustration 2 | g01085665 |
Illustration 2 shows some of the internal components that make up the countershaft transmission. There are three main clutch shafts. The low forward/high forward shaft and the reverse/second shaft are both in constant mesh with and driven off the input shaft. The reverse/second shaft is in constant mesh with and drives the third/first shaft. The low forward/high forward shaft is not in mesh with the third/first shaft. The third/first shaft is in constant mesh with and drives the output shaft. The output shaft drives both the front and rear drive axles.
Countershaft Transmission in End View
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| Illustration 3 | g01085667 |
Illustration 3 shows a rear view of the countershaft transmission. Notice the position of the input and output shaft relative to the speed and directional clutch shafts.
Countershaft Transmission Clutches
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| Illustration 4 | g01085669 |
The clutches (Illustration 4) are engaged hydraulically and released by spring force. The speed and direction that is chosen by the operator determines the clutches that will be engaged. The clutches are chosen in order to obtain the proper gear ratio.
Countershaft Transmission Clutch Piston
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| Illustration 5 | g01085670 |
The clutch piston (1) in Illustration 5 has an inner seal and an outer seal. Speed or directional clutch pressure performs the following functions:
- Fills the cavity behind the clutch piston
- Moves the piston to the left against the piston spring
- Engages the clutch discs and plates
When the discs have worn one half the oil groove depth, the clutch piston travels far enough to unseat (blow) the outer seal. This prevents the discs and plates from ever running metal to metal.
Countershaft Transmission Clutch Plates and Disk
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| Illustration 6 | g01085671 |
The clutch plates and discs (1) in Illustration 6 are mounted inside the clutch housing. Splines on the outside diameter of the plates are engaged with splines in the clutch housing. Both the plates and the housing rotate together.
The clutch discs are stacked between the clutch plates. The inside teeth of the discs are engaged with the outside teeth of the hub. The clutch discs have a friction material that is bonded to their surface so there is no metal to metal contact between the clutch discs and the clutch plates.
Countershaft Transmission Clutch Hub
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| Illustration 7 | g01085672 |
The hub (1) that is shown in Illustration 7 is the component in the clutch pack that the gear is splined to. The discs in the clutch pack are also splined to the hub. When the clutch piston engages the clutch, plates and discs transfer power through the hub to the gear.
Countershaft Transmission Shafts
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| Illustration 8 | g01085681 |
The transmission shafts (Illustration 8) carry the gears inside the transmission. The number of shafts and gears is determined by the transmission and the machine model.
Countershaft Transmission Shaft Oil Passages
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| Illustration 9 | g01085682 |
Each of the transmission shafts has three internal oil passages (Illustration 9). One passage is for carrying the oil for the lubrication and cooling of the clutches, bearings, and gears. The other two passages are for carrying pressure oil for the engagement of the clutches on each shaft.
Countershaft Transmission in Neutral
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| Illustration 10 | g01085683 |
When the transmission is in NEUTRAL (Illustration 10), there are no clutches that are engaged.
Torque from the engine is transferred through the torque converter shaft to the transmission. The torque converter shaft is splined to the input shft assembly. The torque converter shaft drives the transmission input shaft assembly. Since the reverse clutch is not engaged and neither forward clutch is engaged, there is no torque transfer from the input shaft assembly to any countershaft assembly or the output shaft assembly.
Countershaft Transmission in First Gear Forward
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| Illustration 11 | g01085684 |
To transmit power, one directional clutch and one speed clutch must be engaged. When the clutch engages, the clutch holds the hub that is carrying the appropriate gear. When the hub is held, power can flow through the gear.
In first gear forward (11), the low forward clutch is engaged and the first gear clutch is engaged. The low forward clutch holds the gear on the end of the shaft. Power is transmitted from a gear on the input shaft to the gear on the end of the forward shaft. The gear in the middle of the forward low/high shaft drives a gear on the reverse/second shaft. The first gear clutch holds the large gear on the third/first shaft. Power is transmitted from the gear on the end of the reverse/second shaft to the large gear on the third/first shaft.
When the first gear clutch is engaged, power is transferred from the gear to the shaft. The gear on the third/first shaft transfers power to a gear on the output shaft.
Countershaft Transmission in Second Gear Reverse
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| Illustration 12 | g01085685 |
In second gear reverse, (12) the reverse clutch is engaged and the second gear clutch is engaged. Power is transmitted from a gear on the input shaft to a gear on the reverse/second shaft.
When the second gear clutch is engaged, power flows from the gear on the reverse/second shaft to a gear that is splined to the third/first shaft. The gear on the end of the third/first shaft transfers power to the gear on the output shaft.
Countershaft Transmission in Third Gear Reverse
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| Illustration 13 | g01085686 |
In third gear reverse, (Illustration 13) the reverse clutch is engaged and the third gear clutch is engaged. Power is transmitted from a gear on the input shaft to a gear on the reverse/second shaft.
When the third gear clutch is engaged, the gear on the end of the third/first shaft is held. Power is transferred from the gear on the second/reverse shaft to the held gear.
The gear on the other end of the third/first shaft transfers power to the gear on the output shaft.
Countershaft Transmission in Fourth Gear Forward
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| Illustration 14 | g01085687 |
In fourth gear forward, (Illustration 14) the forward high directional clutch engaged and the third gear clutch is engaged. Power is transmitted from a gear on the input shaft to a gear on the forward low/high shaft. The gear in the middle of the forward low/high shaft drives a gear on the reverse/second shaft.
When the third gear clutch is engaged, the gear on the end of the third/first shaft is held. Power is transferred from the gear on the second/reverse shaft to the held gear.
The gear on the other end of the third/first shaft transfers power to the gear on the output shaft.