Meccano Synchromesh Gearbox

Many Meccanomen know the crash action when two gears are engaged, especially when one of them is turning rather fast. Before the days of synchromesh gearboxes, gear engagement in cars required double-declutching, where the clutch pedal is first pushed in once to disengage the engine from the transmission. This takes the pressure off the dog teeth so that the collar can be moved into neutral. Then the clutch pedal is released and the engine revved to the "right speed." The right speed is the rpm value at which the engine should be running in the next gear. The idea is to get the next gear and the collar rotating at the same speed so that the dog teeth can engage. Then the clutch pedal is pushed in again and the collar locked into the new gear. At every gear change one has to press and release the clutch twice, hence the name "double-clutching."

Although many people choose an automatic transmission for its convenience, a manual transmission is superior in many ways. First, the manual transmission provides a direct link from the engine to the wheels, allowing for much greater efficiency and control. Also, a manual transmission allows the driver to feel as if he is part of the car for a much more enjoyable experience.

A clever improvement in the design of the manual gearbox in modern cars uses synchronizers to eliminate the need for double-clutching.

It should be noted that the gears themselves between main shafts and lay shafts are always in constant mesh. Something must be added which allows the speeds of the driving and driven members to be automatically synchronised before coupling, so as to avoid the shock and noise in gear changing.

A synchromesh purpose is to allow the collar and the gear to make frictional contact before the dog teeth make contact. This lets the collar and the gear synchronize their speeds before the teeth need to engage.

In the Meccano synchromesh gearbox, the input and output shafts are in the centre line of the gearbox. Various input gears are locked to the input shaft and constantly mesh with output gears running freely on layshafts on either side. Use is made of the rod with keyway, which is a rod with a single spine for the layshafts. In a vehicle, the rod would be thicker and with multiple splines to distribute the loads on the rod. The collar is a socket coupling, and it holds a large contrate which becomes the dog. This mechanism slides along the rod with keyway but always turns with it. The matching contrate is attached to the gear and is free to rotate on its shaft.There is a compression spring on the rod with keyway between the two contrates. As the collar (socket coupling and contrate) are forced towards the matching contrate and gear by the gear lever acting through the selector forks, increasing pressure is brought to bear on the clutch unit between the back of the gear and a flanged wheel. A rubber ring on a pulley provides the friction needed between the two.  Thus the collar and its associated contrate are brought up to speed before the teeth of the two contrates meet. It can be seen therefore, that the desired gear to be selected is in fact already engaged but is not linked to the output shaft. To do this requires first a clutch to bring the splined shaft up to speed, and then the contrates to lock before final solid engagement occurs. The final drive is from the splined shaft to the output shaft.