Building a Large Prony Brake

Determining Capacity and Putting it to the Test

| March/April 2003

Part Two of a Two Part Series

When I decided to build a large Prony brake, it had to be big enough to test most steam traction engines. To estimate the capacity of my machine, I went back and looked at information I collected when I wrote 'The Design, Construction and Use of a Small Prony Brake' for the July 2000 issue of Gas Engine Magazine. In that article I listed the specifications for 18 brakes I found in old textbooks. Two of the brakes in that list were comparable to the brake I was building. The data on those two brakes, along with the corresponding data from my brake, are shown below.

Determining the Capacity of My Prony Brake

Basing my conclusions on the information above, I determined the drum of my brake might have a capacity of 100 HP at 275 rpm. My revolutions-per-minute figure in the table above was calculated using the diameters of the flywheels and the revolutions-per-minute listed in reprints of old steam engine catalogs. Knowing the diameter of the pulley on my brake, which is 40 inches, I was able to determine that the speed of the brake drum would range from 225 to 275 rpm, depending on the speed of the engine and the size of its flywheel.


The capacity of my brake may be limited to a lower value because of the width of the drive pulley. The pulley is 9-1/2 inches wide, and a wider belt may be required to transmit higher horsepower outputs. I have yet to make this determination.

I also checked to see how fast the brake would turn belted up to an old gas tractor, such as a Rumely OilPull, and I was surprised to see that the belt speeds on these machines are greater than on traction engines. For example, a 25-45 OilPull will turn my brake at 338 rpm, which is too fast for the pulley I am using.

Determining the Maximum Safe Speed of the Pulley

To determine whether or not my 40-inch drive pulley can be safely operated at 275 rpm, I referred to F.A. Halsey's 1916 Handbook for Machine Designers and Shop Men. According to Halsey's formula, a 40-inch diameter cast iron pulley can be expected to burst at a rim velocity of 193.1 feet-per-second. This would occur at a speed of 1,106 rpm. At 275 rpm the rim velocity of my pulley should be only 48 feet per second. Thus, at this speed I have a safety factor of 4 on speed. Halsey reports that in 1916 The Fidelity and Casualty Company would accept for insurance wheels with a safety factor on speed as low as 2.24. Based on my determinations using Halsey's figures, I will not operate my brake at any speed over 300 rpm. At this speed the rim velocity will be 52.3 feet-per-second and the safety factor on speed will be 3.7. According to Halsey, The Fidelity and Casualty Company guidelines would have allowed my pulley to run at a maximum speed of 494 rpm.