Gearshift Forks and Synchromesh Units

Gearshift forks and synchromesh units




The selector forks (26) (27) (28) are permanently mounted on the selector shafts (29) (30) (31), which are free to move axially within the transmission casing. A selector arm (32) on the tubular actuator (33), which is turned by an actuator disc (not shown in picture), transmits the movements, either directly to the 5th/reverse selector fork (26), or indirectly via the 3rd/4th selector fork (27) via carrier (34) and shift shaft (30) or to the 1st/2nd selector fork (28) via the lever arm (35) and shift shaft (31).
The synchromesh units are located on the transmission shafts in the gearbox as follows: 1st/2nd synchromesh unit (15) on lower layshaft, 3rd/4th unit (12) on the primary shaft and 5th/reverse unit (20) on the upper layshaft. Each unit consists of a synchromesh hub (22), synchromesh rings (not shown in picture) with spring-loaded interlock bearings (24) holding the sleeves (23) locked on the hub. There are also blocker bars (47) which transmit power from the synchromesh sleeve to the bulk rings.
Each hub is splined onto its respective shaft and rotates with it. Both sides of each hub have freely rotating gearwheels. Each gearwheel is in constant mesh with its respective train.
When a gear is engaged, the appropriate selector fork moves the synchromesh sleeve along the hub. The bulk ring slows down the freely rotating gearwheel, enabling the sleeve to mesh smoothly and quietly and lock the dog to the hub and shaft. Power is transmitted via the gearbox in the respective gears by the synchromesh units locking the appropriate gearwheels onto the shafts.
Smooth, positive gear changing was one of our major aims in developing this gearbox. We have made light work of gear changing by using an efficient synchromesh system with large diameters and 6.5 degree cone angles.