Synchromesh gearbox

The fundamental difference between the conventional gearbox and the synchromesh is that in the former the gear wheels are brought into mesh - by sliding - only when the actual gear-change is performed, whereas in the latter all the pairs of gear wheels are constantly in mesh.

The various transmission ratios are engaged by means of sleeves which are slid into position. Since the gear wheels themselves do not have to be moved in relation to one another, they can be provided with helical or spiral teeth for the sake of quiet running. In each pair of gear wheels one wheel is, for example, rigidly mounted on the lay-shaft, whereas the other wheel is loosely rotatable on the main shaft. To engage a particular ratio, the loose wheel of the pair of gear wheels concerned is locked to the shaft by means of dogs (locking elements).

One set of dogs is mounted on the inside of a sleeve (the dog sleeve), and the other set is on the gear wheel which is to be engaged. The dog sleeve can slide axially on the main shaft but is locked to it in so far as rotation is concerned. However, before the dogs can engage with one another so as to transmit force, the dog sleeve (which is rotating at the same speed as the main shaft) and the gear wheel to be engaged (which rotates at a different speed) have to be synchronised, i.e., their speeds must become equal. This is done by means of small cone clutches or plate clutches.

With cone clutches the gear wheel to be engaged is formed with a conical protrusion which slides into a conical socket in the dog sleeve. Plate clutches consist of small plates mounted on the shaft, which are pressed together by the pressure exerted by the sleeve, so that the two rotating parts, which are at first rotating at different speeds, are synchronised by slowing down and acceleration of the respective parts.

When a particular ratio (gear speed) is selected, the dog sleeve is shifted towards the gear wheel to be engaged.

The above-mentioned clutches thereupon come into action and synchronise the speeds of the two rotating parts. Then the dog sleeve can move farther forward and engage with the dogs on the gear wheel, just as if these two parts were stationary.

The driving shaft is thus locked to the gear wheel. When engaging the selected gear, the motorist at first feels a resistance, which continues until the two rotating parts are synchronised. Only when this has been completed can the sliding sleeve be moved into its final position and the desired ratio thus definitely engaged.

In automobile engineering a number of different kinds of synchromesh gear systems are used.

They all function on the principle described here, but differ in various technical details.