1. Description

The synchromesh mechanism is used to prevent “gear noise” and to make gear shifting smoother. This mechanism is called “synchromesh” because two gears which rotational speed is different are synchronized by friction force during gear shifting. Transaxles with synchromesh mechanism have the following advantages.

(1) They eliminate the need for the driver to “double clutch” (depressing the clutch pedal twice each time gears are shifted).

(2) When shifting gears, the power can be transmitted with less delay.

(3) Shifting can be carried out more smoothly without damaging the gears.

2. Key type synchromesh mechanism

(1) Construction

a. Each forward gear on the input shaft is engaged at all times with the corresponding gear on the output shaft. These gears rotate at all times even after the clutch is engaged because they are not fixed on the shaft and run idle.

b. The clutch hubs mesh with shafts by splines inside the clutch hub. Furthermore, the hub sleeve meshes with the outer circumference spline of the clutch hub and can move to the axial direction.

c. The clutch hub has three grooves in the axial direction, and the shifting keys enter the grooves. The shifting key is pushed against the hub sleeve at all times by the key spring.

d. When the gear shift lever is in the neutral position, the protrusion of each shifting key fits inside the slot in the hub sleeve.

e. A synchronizer ring is located between the clutch hub and the cone of the speed gears and is pushed against one of these cones. Small grooves are provided over the entire conical area inside of the synchronizer ring to improve friction. Furthermore, the ring also has three grooves for the shifting keys to fit into.

(2) Operation

a.  Neutral

Each speed gear is meshed with the corresponding driven gear and run idle on the shaft.

b. Start of synchronization

As the shift lever is moved, the shift fork, which is fitted in a groove in the hub sleeve, moves in the direction indicated by the arrow. Since the protrusion at the center of the shifting key is fitted to the hub sleeve groove, the shifting key also moves to the arrow direction at the same time, and pushes the synchronizer ring to the speed gear cone portion, resulting in the synchronization start.

c.  Midway through synchronization

When the shift lever is moved further, the force which is applied to the hub sleeve overcomes that of the shifting key spring and the hub sleeve rides up into the protrusion of the key.

d. End of synchronization

The force being applied to the synchronizer ring becomes even stronger and pushes the speed

gear cone portion. This causes the speed of the speed gear to be synchronized with that of the hub sleeve.

When the speeds of the hub sleeve and the speed gear become equal to each other, the synchronizer ring begins rotating slightly in the rotation direction. As a result, the hub sleeve splines mesh with the synchronizer ring splines.

e. End of shift change

After the hub sleeve spline meshes with synchronizer ring spline, the hub sleeve moves further and meshes with the speed gear spline. Then, the shift change  ends.

SERVICE HINT:

If the inner circumference of the synchronizer ring and the speed gear cone portion become worn, both speeds cannot be in sync. Abnormal noise occurs or it becomes difficult to shift gears.

3. Triple-/Double-cone type synchromesh mechanism

To increase the synchromesh capacity, recent models have adopted a triple- cone or double-cone

synchromesh mechanism particularly for the 2nd and 3rd gears.

(1) Triple-cone type synchromesh mechanism The triple-cone synchromesh mechanism divides the synchronizer ring into an outer ring, middle ring, and inner ring. When the shifting key pushes the outer ring, the outer ring and the middle ring form a single cone, then the middle ring and the inner ring become a single cone. Furthermore, the inner ring and the gear piece become a single cone portion, so friction is generated by all three cone portions. Therefore, the capacity to absorb rotational speed differences between gears is large and the synchronizing process is completed smoothly.

(2) Double-cone type synchromesh mechanism The double-cone type synchromesh mechanism is basically the same as the triple-cone type synchromesh mechanism, except that it does not provide synchronizing between the inner ring and the gear piece.

4. Keyless type synchromesh

A keyless synchromesh mechanism has the key spring serving the role of the shifting key and is used for the transaxle 5th gear in some models.

a. Hub sleeve

There are three grooved protrusions inside the hub sleeve to push the key spring during synchronization.

b.Clutch hub

Three notches are located around the clutch hub to secure the synchronizer ring and key spring.

c. Key spring

The key spring has four claws. One claw is to secure the key spring itself, while the other three claws hold the shifting keys.

d. Synchronizer ring

There are grooves in which the key spring claws enter at three points along the circumference of the ring. The groove is chamfered partially.

5. Reverse synchromesh mechanism

The synchronizer ring for the gears for forward reduces the input shaft rotation speed before the reverse gear is shifted. In this way, the reverse idler gear and the input shaft reverse gear are connected smoothly. In recent models, the use of synchromesh mechanisms for reverse gear is becoming more common.

6. Reverse pre-balk mechanism

While the gears are being shifted to the reverse gear, the shift inner lever No. 3 makes contact with the pin of shift fork shaft No. 1 and moves shift fork shaft No. 1 the distance A in the direction of the “2nd gear”. This causes the 2nd gear synchronizer ring to operate in order to decrease the rotational speed of the input shaft. When shift inner lever No. 3 separates from the pin of  shift fork shaft No. 1, the reverse gear shifting ends.