Car Parts and Accessories

What happens when gears are engaged

The gear wheels in a constant-mesh gearbox cannot all be fixed to their shafts or no movement would be possible; so there has to be a system which permits all the gear wheels except those required for a particular ratio to run freely. Usually all the gear wheels on one shaft are fixed to it and the wheels on the other shafts can revolve around their shaft until a ratio is selected. Then one of the free-running wheels is locked to the shaft, and that pair of wheels can transmit power.

The locking of the gear wheels to a shaft is done by collars, which are splined to the shaft. This method of fixing means that the collar must revolve with the shaft but it can slide along to lock on to the gear wheels on either side, or remain between them, allowing both to spin freely.

Around each collar is a groove engaged by a two-pronged fork which is fixed to a sliding rod mounted in the gearbox housing. One, two or three of these selector rods are linked to the gear lever. Moving the gear lever causes a selector rod to slide to or fro. As it slides, the collar gripped by the selector fork is slid along the shaft to engage with, or move away from, a gear.

Dog clutch helps to mesh gears

Collars are locked on to constant-mesh gear wheels by a mechanism called a dog clutch. The face of the collar carries a ring of projections, called dogs, which look rather like gear teeth.

There is a corresponding set of dogs on one face of the gear wheel next to the collar. When the collar is slid along the splined shaft to engage a gear, the two sets of dogs mesh and the wheel revolves with the collar.

A collar usually has a dog clutch on each face, so that when slid one way it engages one gear and when slid in the opposite direction it engages another gear.

At an intermediate point the collar is not engaged with either gear wheel, and both can run freely without power being transmitted. In a direct-top gearbox a dog clutch is also used to link the input shaft and the output shaft to provide direct drive to the wheels in top gear.

Synchromesh for smooth gear changes

In the simplest type of constant-mesh gearbox, which is now obsolete, gears could be engaged simply by snatching the gear lever from one position to the next as fast as possible. This was noisy and jerky.

To do the job more quietly and smoothly, the two sets of dogs had to be allowed to reach the same speed, so that they would slide together without clashing. This synchronisation was achieved in changing up by pausing slightly in neutral. The pause allowed friction and oil drag to slow down the free-running input component to the speed of the output one—the gear connected to the road wheels through the remainder of the transmission.

In changing down, the gears were synchronised by double de-clutching — engaging the clutch with the gearbox in neutral and revving the engine to speed up the input component, then disengaging the clutch again and engaging the appropriate gear.

Drivers today are relieved from the need for double de-clutching by a synchronising device built into the sliding collars in the gearbox. This synchromesh device is usually fitted to all forward gears, but in some cars it is not provided in bottom gear.

Synchromesh works like a friction clutch. As the collar is pushed towards the gear wheel with which it is to mesh, a conical ring on the gear wheel in front of the dogs comes into contact with the surface of a matching conical hole in the collar. The friction between the conical surfaces brings the free-running gear wheel up or down to the speed of the input shaft.

The collar continues to move along and the two sets of dogs slide smoothly into mesh. However, if the gear lever is moved too fast on some cars, the gears will clash.

Baulking system prevents engagement until gears are synchronised

Modern synchromesh mechanisms incorporate a baulking system which obstructs the movement of the collar and will not allow the dogs to engage until perfect synchronisation has been achieved.

If the revolving parts do not turn at the same speed, perhaps because the clutch is not properly disengaged, the gear lever will resist the driver’s efforts to push it into position.

There are at least three different kinds of baulking systems in general use today, but all produce the same effect. One common type uses a baulk ring to keep the two sets of dogs apart until they are revolving at the same speed.

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