A non-synchronous transmission— also called a crash gearbox— is a form of manual transmission based on gears that do not use synchronizing mechanisms. They require the driver to manually synchronise the transmission's input speed and output speed. Non-synchronous transmissions are found primarily in various types of industrial machinery, such as tractors and semi-tractors. Prior to 1960, most cars used constant-mesh but non-synchronous transmissions.
History
Most early automobiles were rear-engined, using a single-speed transmission and belt drive to power the rear wheels. In 1891, the FrenchPanhard et Levassor automobile used a three-speed manual transmission and is considered to have set the template for multi-speed manual transmissions in motor vehicles. This transmission used a sliding-gear design without any form of speed synchronization, causing frequent grinding of the gear teeth during gear shifts. The Panhard design was refined over the years by other manufacturers to include "constant-mesh" gears. The first usage of synchromesh was by Cadillac in 1928.
Trained drivers of vehicles with non-synchronous transmissions sometimes use the techniques listed below. If improperly implemented, these techniques can cause damage to the vehicle or the loss of control of the vehicle.
Double-clutching: releasing the clutch in neutral to synchronise the speeds of the shafts within the transmission
Unlike any other type of transmission, non-synchronous transmissions often have a clutch brake mechanism, which is usually activated by the clutch pedal is pressed all the way to the floor or pressing a button on the top of the gear lever. The purpose of the clutch brake is to slow down the rotation of the transmission's input shaft, which assists in shifting the transmission into neutral or first gear when the vehicle is at a standstill. The clutch brake not only slows or stops the idle gear axis but can also prevent shifting into gear until the clutch pedal is released a few inches off the floor. In order to shift into gear, the clutch must be halfway off the floor, otherwise, the clutch brake will prevent the transmission from being shifted into or out of gear.
Comparison of transmissions
Any transmission that requires the driver to manually synchronize the engine speed with the speed of the driveshaft is non-synchronous. non-synchronous transmissions are mostly used in semi-trucks, large industrial machines and power take-offs. Most manual transmissions in modernpassenger vehicles are fitted with synchromesh to equalize the shaft speeds within the transmission, so they are synchronous transmissions. All automatic transmissions have synchronizing mechanisms, and semi-automatic transmissions that use dog clutches typically have cone-and-collar synchronizing mechanisms.
