Farm Collector

Put to the Test: Using a Prony Break

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.

Doug, a machinist by trade, built his Prony brake about 12 years ago from plans given to him by a man who frequented steam shows. With two noteworthy exceptions, he used scrap metal and stray parts. The flywheel came from a Freeport, Ill., ice cream plant compressor, and the scale is a commercially produced Toledo scale. Doug’s Prony brake even features one piece of modern technology: a digital, hand-held tachometer.

He displayed the brake last summer at the Northern Illinois Steam Power Club’s Sycamore Steam Show, one of three shows (along with Freeport, Ill., and Edgerton, Wis.) Doug and his parents, Lester and Judy, take it to each year.

In show demonstrations, Doug connects the prony brake to a steamer by a large belt, like that used on a thresher or farm implement. If a tractor is being tested, the connection is made at the tractor’s PTO.

Once the steamer is connected, the engine operator runs up its speed. As he does, Doug turns a sort of steering wheel that tightens a round clamp around the brake’s drum (or flywheel) being turned by the belt.

As the clamp is tightened, wood blocks rub against the drum, creating pressure and registering pounds on the scale. While Doug increases the pressure by turning the wheel, his dad attaches a digital tachometer to the drum shaft to measure rpms.

The pront brake test starts at minimum pressure and increases gradually. Judy takes readings of pounds on the scale and rpms on the tachometer. She converts the numbers into horsepower for each reading until they reach their peak. Once the steamer or tractor starts working harder than normal, it has hit its peak and its maximum horsepower.

“We take the pounds on the scale and the speed on the shaft,” Doug says. “Then we multiply the pounds on the scale times the speed on the shaft, move the decimal point and come up with the horsepower of the machine being tested. It’s pretty accurate.”

At the Sycamore show he tested a 1916 Minneapolis owned by Steve Green of Oregon, Ill. Unfortunately, the belt slipped off before the engine hit its peak. “We came up with a little over 80, but he probably has another 15-20 (horsepower left),” Doug says. “That engine should have 100 hp easily.”

He apologized for the belt coming off, admitting he needed a new one. “This belt could be from the 1940s,” Doug says. “Most of them around here are about that same age, but they last a long time.” A new belt could cost $1,000 or more, so Doug’s been looking for a good used one. At shows he often borrows a wide belt. “I use it with the big engines and it never slips or comes off,” he says.

A club-owned Baker fan was stationed next to Doug’s Prony brake test demonstration at the Sycamore show. Also belt-driven, the fan puts a load on an engine to see how it operates under a load, but no results or readings are generated. The faster it goes, the more friction is created, making the engine work harder. But that’s a story for another day. FC

Lyle Rolfe has been a newspaper reporter/photographer for more than 40 years. As a freelance writer and photographer, his work has been published in Classic Cars, Cars & Parts and Rural Heritage. Contact him at 2580 Wyckwood Court, Aurora, IL 60506; (630) 896-2992; e-mail:

  • Published on Jan 1, 2008
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