MASCOT March April 2003

How Does it Work? - No 2 -The Constant- and Synchro-mesh Gearbox

The Tool Chest

In our first dip into the Tool Chest we saw how a sliding mesh gearbox works - with considerable skill and double de-clutching on the part of the driver! So how did they make things easier for the driver - and quieter? The first improvement was the use of "Constant Mesh" gears, which are connected to the mainshaft by a sliding dog clutch as shown in Diagram 1. The 2nd speed gear (or 2nd and 3rd in a 4-speed box) is in constant mesh with its layshaft gear, but rotates freely on the mainshaft. The dog clutch slides along splines on the mainshaft and engages with dog teeth on the gear, thus connecting the drive from the engine to the mainshaft.

Constant mesh allows the use of stronger helical gears which also run much quieter than the sliding, spur gears. However, as the teeth are angled across the gear, axial thrust is created, requiring the use of thrust washers. This problem was overcome by using double helical gears, which were pioneered by Citroen, who still use the inverted double "V" as their logo. Manufacturers did not generally use constant mesh for 1st and Reverse gears as they are normally engaged when the vehicle is stationary. Diagram 2 shows the three types of gearing.

There can still be some noise as the dog teeth engage, and, as with sliding spur gears, double de-clutching is still necessary to bring the two sets of dog teeth to the same speed so that they can be slid into engagement quietly.

To eliminate the need to double de-clutch, the synchromesh gearbox was introduced. The basic gearbox is laid out in the same manner as the constant mesh, but with the addition of a cone clutch fitted between the dog and gear members, as shown in Diagram 3. The male clutch member is on the gear and the female on the dog clutch hub, which has a splined outer sleeve located by a series of balls and springs. Movement of the selector moves the whole dog clutch until the cones make contact, when the friction brings them both to the same speed. Continued movement of the selector causes the spring-loaded balls to be overridden and the internal splines of the outer sleeve engage with the dog teeth on the gear. However, the change must not be rushed as it takes time for the friction at the cones to bring the rotating gears to the same speed. Change gear too quickly and there may still be some noise.

To overcome this disadvantage, the Baulk or Blocker Ring System was introduced - see Diagram 4, Radial movement of this additional ring is limited to half a spline width, which prevents the dog teeth from engaging until the speeds of the two members are equal.

If your car has a synchromesh gearbox it is recommended you do not double-de-clutch as this can cause undue wear on the synchro rings. TC