A non-synchronous transmission is typically found in commercial, agricultural and heavy equipment. Unlike the transmission in a family sedan, a non-synchronous transmission is much more difficult to shift. The gears in a non-synchronous transmission need to be brought up to speed in order to mesh with each other while shifting. Double clutching and a good ear for the proper engine speed aid experienced drivers in shifting a non-synchronous transmission. Once experienced, many drivers are able to shift a non-synchronous transmission without using a clutch at all.
In a typical passenger vehicle, the transmission is fully synchronized. This is also known as a constant mesh transmission, meaning that the gears are always meshed and in motion with each other. This type of transmission uses a cone and collar system to bring a gear up to the same speed as the previous gear so it can slide into gear and lock without emitting a grinding noise. The gears in this type of transmission are meshed onto the output shaft, and the gears are changed by sliding back and forth between selected gears within the transmission.
The operation is much different in a non-meshed or non-synchronous transmission. In this type of transmission, none of the gears are moving while a selected gear is being used. In order to shift, the new gear must be brought up to the same speed as the current gear and then slid from the current gear and into the selected gear. This is accomplished by double clutching and revving the engine until the proper engine speed is matched with the correct gear speed.
When double clutching a non-synchronous transmission, the clutch pedal is first pushed half way down and the transmission is pulled out of gear. Next, the clutch pedal is released allowing the engine speed to slow down to the proper speed to shift gears. Then, the clutch pedal is again pushed only half way down and the shifter is used to push the transmission into the chosen gear. The engine speed is manipulated by feathering the throttle pedal until the gear slips smoothly into place within the transmission. With practice, an operator can hear the correct engine speed and make gear changes by manipulating the throttle and without using the clutch pedal at all.
The purpose for the non-synchronous transmission is that it is a much stronger transmission than the constant mesh version. The non-synchronous transmission is able to pull much heavier loads without damage to the gears. The extremely close ratio of the gearing also allows a machine to operate at very slow wheel speed while maintaining power and high engine speeds.