De Prony brake

The de Prony Brake is a simple device invented by Gaspard de Prony to measure the torque produced by an engine. The term Brake horsepower is one measurement of torque obviously derived from the method of measurement.

Essentially the measurement is made by wrapping a cord or belt around the output shaft of the engine and measuring the force transferred to the belt through friction. The friction is increased by tightening the belt until the frequency of rotation of the shaft is reduced. In practice more engine power can then be applied until the limit of the engine is reached.

There are various methods of measuring the force applied to the brake. For the belt method it is common to use a pair of spring balances and applying a pretension to the belt. When the shaft is rotating one balance will observe an increased tension whilst the other a reduced tension. The difference is a measure of the torque when the diameter of the shaft is taken into account.

An alternate mechanism is to clamp a lever to the shaft and measure using a single balance. The torque is then related to the lever length, shaft diameter and measured force.

The power output can be calculated as:

horsepower = Pi × 2 × lever length × revolutions per minute × measured force [Kemp’s Engineer’s Year Book] [Reeds Applied Mechanics for Engineers]

References