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What is a Prony Brake?

By T. L. Childree
Updated Feb 24, 2024
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The Prony brake is a simple type of dynamometer used to measure the amount of torque produced by a motor or engine in order to determine its brake horsepower rating. The device was invented in 1821 by a French engineer named Gaspard de Prony, and continues to be widely used by tractor and heavy equipment manufacturers. A Prony brake consists of a belt that is wrapped around the output shaft of an engine or motor. A series of weights are then attached to the ends of the belt until the engine stalls under the friction created by the load. The amount of weight needed to stall the engine determines the amount of torque or pulling power that the engine is capable of producing.

A modern Prony brake usually consists of some type of brake drum along with a pulley that is connected to an engine’s output shaft on one side, and a brake lever on the opposite side. A device used to measure the amount of force being applied to the brake arm is also attached to the Prony brake as well. An additional device is utilized to measure the number of revolutions per minute (RPM) that the engine’s output shaft is turning during the test. A tension band is also attached to the pulley in order to apply braking force to the output shaft of the engine being tested.

To begin the testing procedure, the engine is first started and gradual pressure is then applied to the tension band. Measurements of the braking force being applied at the end of the brake arm are then taken, along with measurements of the engine’s RPMs. As the braking load being applied to the output shaft increases, the engine’s governor increases fuel delivery to the engine in order to keep the RPMs constant. When the governor can no longer keep the RPMs at a constant speed, the engine reaches what is known as its stall point. The final RPM and brake force measurements are then taken and recorded and the testing procedure is concluded.

Once the test has been concluded. The engine’s horsepower is then calculated using a relatively simple mathematical formula. To calculate an engine’s horsepower using a Prony brake test the final braking force measurement is multiplied by the circumference of the circle that the end of the brake arm would make if were to rotate freely. This number is then added to the final number of RPMs that the engine’s output shaft reached at its stall point.

The sum of these two numbers is then divided by 33,000 in order to determine the brake horsepower rating for the engine being tested. The brake horsepower rating of an engine is generally regarded as the maximum amount of power that the engine is capable of producing.

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Discussion Comments

By Vincenzo — On Oct 21, 2014

@Logicfest -- The author also did a great job of pointing out that horsepower and torque are not the same thing. You could have a car with 300 horsepower and that does not necessarily mean that it will have a lot of torque to match.

That has to do with gearing. That sports car isn't geared for hauling freight, so the torque rating could be less compared to a truck with a similar engine. The difference is in the way the vehicle is geared. People want a lot of torque out of a truck, but just enough to shoot a car down the road quickly in a sports car.

By Melonlity — On Oct 20, 2014

@Logicfest -- That is very true with gasoline engines, but electric motors are a different thing. You area at max torque right off the bat with one of those engines. Power curve? Forget about that. You are at the upper end of an electric engine's performance as soon as the car starts moving.

And, yes, that also applies to those electric "helper" motors in hybrid vehicles.

By Logicfest — On Oct 20, 2014

Good job pointing out that an engine has to build up to its maximum horsepower and torque. People make a big deal out of the maximum power numbers without pointing out that your vehicle will spend most of its time well below those upper limits. You don not hit max horsepower and torque as soon as you hit the gas. It takes time to build up to those levels.

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