What Are Prony Brakes?

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A Prony brake is a dynamometer for measuring engine horsepower output.
A Prony brake is a dynamometer for measuring engine horsepower output. (Image: wheel brake image by Christopher Nolan from Fotolia.com)

A Prony brake is a type of dynamometer for measuring the power output of engines. According to the website of the Buckley Old Engine Club of Michigan, the Prony brake was invented by French engineer and mathematician Gaspard de Prony in 1821 to measure the brake horsepower of engines, which translates to the amount of work an engine can do. The Prony brake is still used today to test tractor manufacturers’ engine horsepower claims.

One Horsepower

Horsepower is a power-measuring unit that originated in Britain. It is defined as the force needed to lift a weight of 550 pounds one foot in one second, or lift a weight of 33,000 pounds one foot in one minute. That force is expressed in terms of foot-pounds, hence 550 foot-pounds per second or 33,000 foot-pounds per minute (550 x 60) equals one horsepower.

James Watt

The horsepower unit was created in the early 19th Century by James Watt, inventor of the first practical steam engine, to provide prospective steam-engine buyers with a comparison of steam engine power to the power of a draft horse, says the Buckley Engine Club website. Watt measured the average pulling power of strong draft horses, then performed mathematical calculations to derive his horsepower equivalent for an engine.

Prony Brake Components

The Prony brake consists of a brake drum connected on one side to an input shaft or pulley that will connect to the engine being tested, and on the opposite side to a brake arm braced against the device’s frame. The brake arm acts like the brake arm on a bicycle coaster brake. There’s also a tension band around the brake drum to apply braking force, a device for measuring the exact amount of force being applied to the brake arm by the tension band, and a device for measuring the revolutions per minute (RPM) on the engine-input side of the Prony brake.

How Test is Run

You measure an engine’s horsepower with a Prony brake by starting the engine, tightening the tension band on the Prony brake's drum, measuring the braking force in pounds at the end of the brake arm and counting the engine’s RPM, according to the Buckley Engine Club’s website. An engine has a governor that feeds more fuel or steam to the engine as braking load increases, adding power in an effort to keep the RPM constant. The test concludes at the point where the engine’s governor no longer can keep the RPM constant, called the stall point.

Calculating Horsepower

According to the Buckley Engine Club, you calculate the horsepower of the engine by measuring the braking force in pounds being applied at the end of the brake arm at the engine’s stall point, multiplying that number by the circumference of the circle the end of the brake arm would describe if it were free to rotate, and by the RPM of the engine at the stall point. Then you divide that product by 33,000 to get the brake horsepower rating for that engine. The brake horsepower rating is equal to the maximum power the engine can put out.

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