A jackshaft is an axle-like device that is driven from one object and drives another object. Many machines use a jackshaft to operate accessories that cannot be directly run from a power source. A common jackshaft drive is found on a snowmobile. The engine runs power from its clutch to the jackshaft with a belt, the belt attaches to the jackshaft pulley and turns the shaft that turns the track with a chain and sprocket.
Early factories used this type of drive system to operate the different work stations. The plant would typically have a coal-fired steam engine that would turn a flywheel. Large belts would be run from this flywheel to a series of jackshafts which would in turn power machines in the individual work areas. This series of belts and shafts were all run off of one singular engine in most cases.
Many farm implements are operated with jackshafts. Using power from the tractor's engine, the energy is sent via the tractor's power take off or PTO shaft. The PTO sends the power to the implement and then divides the power into different areas of the implement through the use of a jackshaft. This is done primarily because a shear pin is used in each separate shaft and is able to shear the pin and stop a particular piece of the machine if a problem is encountered without breaking the entire machine.
Another benefit of the jackshaft is its ability to operate a component without being aligned with the power source. This trait is particularly noticeable on wide-tire custom motorcycles. The trend of placing very wide tires on the rear of a motorcycle is made possible by running the engine's primary drive to a jackshaft. The shaft is then extended out past the edge of the tire and wheel where a sprocket is attached that drives the wheel via a chain.
Driving the motorcycle wheel in this manner allows the motorcycle builder to mount the engine in the center of the frame. This maintains balance as well as proper component mounting and operation. Riding characteristics are much the same as a stock motorcycle and control and maneuverability is not affected. The result is a custom look with a factory stock ride quality.
Jackshafts have been in use since the time of the Romans and help to cock their catapults. They are in use today in everything from clocks to heavy machinery. They are an invaluable tool when creating a machine and have been used by every culture and society around the globe for centuries.