A horse engine (also called a horse power or horse-power) is a now-largely obsolete machine for using draft horses to power other machinery. It is a type of animal engine somewhat more sophisticated than a horse mill. A common design for the horse engine was a large treadmill on which one or more horses walked. The surface of the treadmill was made of wooden slats linked like a chain. Rotary motion from the treadmill was first passed to a planetary gear system, and then to a shaft or pulley that could be coupled to another machine.
Horse engines were used to power farm implements and industrial processes. Examples of machines that were powered with a horse engine include the threshing machine, the corn sheller, pumps and machines for sawing wood. Horse engines were modular in that they could be attached to whichever implement they were needed for at the time, preventing these machines from needing to incorporate power sources into their design. They could also be used interchangeably with other forms of power, such as a hand crank, line shaft, stationary engine, or the PTO shaft or flat belt pulley from a tractor, which eventually replaced them. Today there are modern versions of these being built that have been used with great success to do household chores as well as power a complete Amish-run machine shop in Kentucky.
The term "horse power" probably predates the name of the horsepower unit of measurement. (For reference the 1864 Webster's Dictionary defines horse-power as “A machine operated by one or more horses; a horse engine.")
- The Papers of John C. Calhoun - mention horse power machines
- Photograph of a horse power being used to thresh wheat in southeastern Washington State. From the Garfield County Heritage Collection.
- Photograph of a horse power in action