A transmission is a mechanical device for connecting a source of torque to a load, while providing some useful conversion, such as trading rotational speed for torque, changing the axis of rotation, or balancing torque among multiple loads.
Transmissions are found in vehicles, and are broadly divided into two categories: automatic and manual. This page is about manual transmissions, which are distinguished from automatic transmissions by a number of characteristics:
- In the manual transmission, the gear selection is performed strictly the vehicle's driver or rider. More specifically, the driver's choice of gear is made only mentally. The controls of the manual transmission are used for carrying out the actual execution of the gear selection. Through direct linkage, these controls perform the required sequence of mechanical rearrangements within the transmission. In a manual transmission, there is no difference between indicating the intent to the machine, and carrying out that intent. By contrast, transmissions which have an indirect control for announcing a gear change, but which then independently perform the mechanical realization of that choice, are called semi-automatic. Fully atomatic transmissions also take on the responsibility of generating the decision to make a gear change, thereby eliminating most of the driver's interaction with the transmission.
- Manual transmissions have a small set of fixed gear ratios. The most widely deployed automatic transmission designs do also, but continuously-variable automatic transmissions provide a continuous range of gear ratios. In principle, a continously-variable transmission could be designed as a manual unit, allowing the driver to control the gear ratio by direct mechanical linkage, and perhaps also the clutch when starting and stopping. In practice, instead, some vehicles with continuously-variable transmissions provide a semi-automatic mode which emulates fixed gear ratios.
- Manual transmissions expose the driver or rider to whatever mechanism they have for addressing the interruption of power flow during starts, stops and gear changes. In automotive transmissions, there is a typically a driver-operated clutch which can disconnect the engine from the transmission. There may also be a freewheel mechanism which allows two coupled parts to appear disconnected while one spins faster than the other. Such a device is commonly found in bicycles, which allow the rider to stop pedaling without stopping the bicycle.
- Some manual transmissions expose the driver to the problem of matching the revolution speed of independently rotating gears or couplings, which is required in order that the parts can be connected together to complete the gear selection. Synchronized transmissions semi-automatically perform this speed matching, allowing the driver to be almost entirely oblivious to the problem. In modern passenger vehicles, manual transmissions are synchronized in all of their forward gears, but reverse gears with limited or no synchronization are still very common.
Manual transmissions can be found in trucks, cars and motorcycles. The derailleur or hub gear systems of bicycles are also manual transmissions.