Transmission Types

Modern manual transmissions are divided into two major categories:

• Constant mesh

• Synchromesh

Constant Mesh Transmission

The constant mesh transmission has two parallel shafts where all forward gears of the countershaft are in constant mesh with the mainshaft gears, which are free to rotate. Reverse can either be a sliding collar or a constant mesh gear. On some earlier versions, first and reverse gears were sliding gears.

To eliminate the noise developed by the spur-tooth gears used in the sliding gear transmission, automotive manufacturers developed the constant mesh transmission. The constant mesh transmission has parallel shafts with gears in constant mesh. In neutral, the gears are free running but, when shifted, they are locked to their shafts by sliding collars.

When the shift lever is moved to third, the third and fourth shifter fork moves the sliding collar toward the third speed gear. This engages the external teeth of the sliding collar with the internal teeth of the third speed gear. Since the third speed gear is meshed and rotating with the countershaft, the sliding collar must also rotate. The sliding collar is splined to the main shaft, and therefore, the main shaft rotates with the sliding collar. This principle is carried out when the shift lever moves from one speed to the next.

Synchromesh Transmission

The synchromesh transmission also has gears in constant mesh (Figure 10-11). However, gears can be selected without clashing or grinding by synchronizing the speeds of the mating part before they engage.

The construction of the synchromesh transmission is the same as that of the constant mesh transmission with the exception that a synchronizer has been added. The addition of synchronizers allows the gears to be constant mesh when the cluster gears and the synchronizing clutch mechanisms lock the gears together.

The synchronizer accelerates or slows down the rotation of the shaft and gear until both are rotating at the same speed and can be locked together without a gear clash. Since the vehicle is normally standing still when it is shifted into reverse gear, a synchronizer is not ordinarily used on the reverse gear.