Synchromesh Transmission
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| Illustration 1 | g01085645 |
The synchromesh transmission (Illustration 1) is a constant mesh, collar-shift transmission. The synchromesh transmission is equipped with synchronizers to equalize the speed of the mating parts before the mating parts engage.
The synchronizer is used in all manual automotive transmissions. The synchronizer is common in other machines where shifting while moving is required.
The following synchronizers are the four most common synchronizers:
- Block synchronizer
- Disc and Plate Synchronizer
- Plain Synchronizer
- Pin Synchronizer
Each of these synchronizers has a common function. Synchronizers match the speed of the gear to the speed of the shaft so the connection can be made. All of the synchronizers use friction to synchronize the mating parts.
Synchronizers are used in all manual automotive transmissions. Pin synchronizers are used in some Caterpillar Backhoe Loader transmissions.
Block Synchronizer
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| Illustration 2 | g01085646 |
A block synchronizer consists of the following components:
- Hub
- Shifter plates
- Lock rings
- Synchronizer sleeve
- Two blocking rings
The hub is splined to the shaft. The hub has splines on the outer diameter. The synchronizer sleeve has splines on the inside diameter. The synchronizer sleeve is slid onto the hub. Shifter plates are between the hub and the synchronizer sleeve. The shifter plates can be used to slide the synchronizer sleeve. The blocking rings are located on each side of the hub and the synchronizer sleeve. The blocking rings have slots to allow movement of the shifter plates.
The blocking rings and the shoulders of the driven gears have matched cone-shaped surfaces. The surface between the blocking ring and the shoulder of the driven gear provides the frictional force in order to synchronize the speed of the gear to the speed of the shaft.
Block Synchronizer during Synchronization
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| Illustration 3 | g01085647 |
When a shift is made, the shifting plates move the synchronizer sleeve toward the selected gear. This pushes the blocking ring onto the shoulder of the gear. The speeds begin to synchronize. When the blocking ring and the gear are rotating at the same speed, the synchronizer sleeve will be able to slide over the blocking ring teeth and onto the teeth on the hub of the driven gear. This completes the shift.
Plain Synchronizer
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| Illustration 4 | g01085647 |
The plain synchronizer is similar to the block synchronizer, but the plain synchronizer has fewer parts. A plain synchronizer consists of the following components:
- Hub
- Synchronizer sleeve
- Springs
- Balls
The hub is splined to the shaft. The hub has splines on the outer diameter. The synchronizer sleeve has splines on the inside diameter. The synchronizer sleeve is slid onto the hub. Springs and balls are located between the synchronizer sleeve and the hub. The hubs and the shoulders of the driven gears have matched cone-shaped surfaces. The surface between the hub and the shoulder of the driven gear provides the frictional force to synchronize the speed of the gear to the speed of the shaft.
When a shift is made, the shifting fork moves the synchronizer sleeve toward the selected gear. This pushes the hub onto the shoulder of the gear. The speeds begin to synchronize. The springs hold the balls in a groove in the center of the synchronizer sleeve. This prevents the synchronizer sleeve from moving off the hub and trying to engage the gear teeth too soon. When the hub and the gear are rotating at the same speed, the synchronizer sleeve will be able to slide onto the teeth on the hub of the driven gear. This completes the shift.
Disk and Plate Synchronizer
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| Illustration 5 | g01085650 |
A disk and plate synchronizer consists of the following components:
- Synchronizer gear
- Blocker plate
- Detent balls
- Springs
- Discs
- Plates
- Synchronizer drum
- Output gear
During synchronization, the blocker plate is driven by the synchronizer gear which is on the input shaft. When synchronization is not needed, the detent balls and springs hold the blocker plate in neutral. The synchronizer drum is driven by the output gear which is on the output shaft. The discs rotate with the synchronizer drum. The plates rotate with the blocker plate.
When a shift is made, the shifter fork moves the synchronizer drum forward. The discs and plates contact each other. This causes the discs and plates to rotate together. The thrust force will move the blocker plate. This causes the blocker plate to lock onto the synchronizer gear. When the clutch is completely engaged, the blocker plate and the synchronizer gear will rotate with the output shaft. This removes the thrust force from the blocker plate. The detent balls and springs will push the blocker plate back into the neutral position.
Pin Synchronizer
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| Illustration 6 | g01085652 |
The pin synchronizer consists of the following components.
- Hub
- Two stop ring
- Pin assemblies
- Two outer stop rings
The hub is splined to the shaft. The outer stop rings are spined to the driven gears. The stop ring and pin assemblies are loosely pinned to the hub.
When a shift is made, the shifter fork moves the hub. The stop ring and pin assembly prevents the actuating hub from engaging the gear. When all of the parts are rotating at the same speed, the force between the pin and the hub is reduced. The hub slides over and engages the driven gear.
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| Illustration 7 | g01085655 |
There are four main gear-carrying shafts in the synchromesh transmission shown in Illustration 7. The input gears rotate on the input shaft (at top). The input shaft has three oil passages for lubrication. The countershaft transfers torque from the forward input gear to the speed range gears. The connection is illustrated with the dash lines. The reverse idler shaft carries the reverse idler gears and connects the reverse input gear to the countershaft. The output shaft (blue) carries the speed range gears and synchronizers. The speed range gears rotate on fluted areas on the output shaft except for first gear, which is supported by needle bearings. The synchronizers are splined to the output shaft drive hub. There are two synchronizers. One synchronizer is for first and second gears and one synchronizer is for third and fourth gears.
In the forward gears, the input gear turns the countershaft which is constantly engaged with the speed range gears. In first gear, the shifter fork (not shown) slides the synchronizer to the left and first gear engages the output shaft. Only one synchronizer can be engaged at a time.
In reverse, power is transmitted to the reverse input gear and the reverse idler shaft which reverses the direction of the countershaft. The output shaft rotates in the opposite direction and the machine travels in reverse.
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| Illustration 8 | g01085656 |
Illustration 8 shows a pin synchronizer. The synchronizer consists of two cups and two cones. The cones are mounted to the sliding hub by blocker (alignment) pins and spring loaded split pins. The inner diameter of the sliding hub is splined to the output shaft drive hub. The speed range gears have matching splines. The cups are splined to the speed range gears and rotate with the speed range gears. The NEUTRAL position is shown on the left.
When the operator moves a shift lever to engage a gear, the shifter fork moves the sliding hub to the left. The sliding hub contacts the blocker pin shoulders. The force on the pins causes the pins to push the cone against the cup. When the cone contacts the cup the speed range gear rotates at the same speed as the output shaft as shown in the center diagram.
When there is torsional load or mismatch of speeds between the speed range gear and the output shaft, the blocker pin shoulders keep the sliding collar from engaging. When the speeds are synchronized there are no torsional loads on the blocker pins (right diagram). The sliding hub engages the splines on the gear. This engages the first gear to the output shaft.
The spring loaded split pins will collapse on one side. The spring acts as a locking mechanism to keep the sliding hub and the output shaft engaged. The shifter fork shaft has ball detents which also help to keep the sliding hub and the output shaft engaged.