Put to the Test: Using a Prony Break

Doug Wise built his own Prony brake to test engine horsepower for steam engines and tractors.

| January 2008

Ever wonder if the horsepower listed for your tractor or steamer is correct? If you had a Prony brake, you could find out in no time.

Don't try to find a Prony brake at your local farm implement store. Such devices are not produced commercially, says Doug Wise of Cedarville, Ill. But if you're mechanically inclined, you can build your own way to test engine horsepower - like he did.

Invented in 1821 by French engineer and mathematician Gaspard de Prony, the Prony brake - actually an early dynamometer - measures the torque produced by an engine. In fact, the term "brake horsepower" derives from this method of torque measurement. de Prony built the machine to test water wheels, according to Doug's mother, Judy Wise, who has delved into the history of de Prony and his invention.

With de Prony's device, measurement is made by wrapping a belt around the engine's output shaft and measuring the force transferred to the belt through friction. Friction is increased by tightening the belt until the shaft's rotation frequency is reduced. Additional engine power can be applied until the engine's limit is reached.

Various methods can be used to measure the force applied to the brake. For the belt method it is common to use a pair of spring balances and apply a pretension to the belt. When the shaft rotates, one balance will demonstrate an increased tension while the other shows reduced tension. Factor in the shaft diameter, and the difference is a measure of the torque.

In another approach, 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.