Building a Large Prony Brake

How Much Power Does That Engine Really Produce? A Working Prony Brake Provides the Answer



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Working Out Design Considerations and Initial Details: Part One of a Two Part Series

Dennis Jacowski gives Mike Johnson's 50 HP Case a work out on Bruce Babcock's Prony brake at the 2002 Mad River Steam & Gas Show in Urbana, Ohio.


About three years ago, I built a small Prony brake to take to engine shows so people could see how small hit-and-miss gas engines perform working under a load. A Prony brake belted to an engine not only supplies a uniform load on the engine, it also measures the engine's horsepower output, and it demonstrates how horsepower was measured 100 years ago. Running my small Prony brake, I noticed it wasn't just engine owners who were interested in the brake many spectators stopped to examine the device, ask questions and watch it in operation. At the first show where I exhibited the brake, I recall an elderly gentleman proclaiming, 'Now I know what they mean when they say 'brake' horsepower!' My small Prony brake is described in detail in 'The Design, Construction and Use of a Small Prony Brake' in the July 2000 issue of Gas Engine Magazine.

A Short History of the Prony Brake

The concept of rating engines, water wheels and windmills in terms of horsepower dates back to the early 1700s, but it wasn't until the late 1700s that anyone made a real effort to determine just how many foot-pounds per minute a horse was capable of producing. Fittingly, it was James Watt, the father of the modern steam engine, who was the first person to make this determination. Watt was selling steam pumping engines, and he needed a reliable way to calculate the output of his engines so potential customers would have an idea of their capacity. Watt studied mine ponies lifting coal at a coal mine, and he found that, on average, a mine pony could perform 22,000 foot-pounds of work every minute. Put another way, he found that a horse exerting one 'horsepower' could lift 220 pounds of coal 100 feet in one minute.

Watt, however, wanted to make sure customers would have no reason to complain of the power output of his engines, so he arbitrarily increased this figure by 50 percent. His final formula determined that one horsepower was equal to 33,000 foot-pounds of work a minute. This meant a five horsepower Watt engine would do significantly more work than five horses. It's interesting to note that Watt's number inflation created at least a certain level of trouble for users of gas and steam engines for over 125 years, as builders of engines continued the practice of under-rating their engines.

Watt applied his horsepower formula to his pumping engines, and by using the number of gallons pumped, the height the water was raised and the time required for a given volume he was able to calculate equivalent horsepower output. However, when Watt began building 'rotative' engines (engines with a crank and flywheel) he did not have a reliable method of calculating power output. Some time around 1800 his assistant, John Southern, invented the steam engine indicator, and Watt may have used this to calculate the input horsepower of his engines. But because of the low efficiency of his engines and the primitive construction of the indicator, he could probably only get a rough estimate of the power delivered to a rotating load. It is, however, from this exercise that we get the term 'indicated horsepower.'

How the Prony Brake Measures Horsepower

The problem of how to measure the horsepower of a rotating shaft was solved by Gaspard de Prony in France in 1826, when he invented the first friction brake. This device came to be known as the Prony brake. The following sketch, (at below) shows the simplicity of Gaspard de Prony's invention. In use, the stationary lever (D) is clamped around a rotating shaft (A) and the two bolts above the shaft are tightened until the engine is working up to full load. Weights (B) are then added to the scale pan until the lever (D) drops slightly away from the upper stop (C). To calculate horsepower the only things needed to be known are the length of the lever (D), the weight of the lever (D) at its right hand end, the additional amount of weight (B) added to the scale pan and the speed of the shaft (A) in rpm.

The calculation is:

Foot Pounds per Minute = Pi x 2 x Length of D x RPM x Weight

And because Watt's figure of 33,000 foot-pounds per minute per horsepower has survived through both the 19th and 20th centuries, we end up with:

Horsepower =

Pi x 2 x Length of D x RPM x Weight/ 33,000

Designing With the Spectator in Mind

Because of the interest spectators showed in my small Prony brake, I decided that should I build a larger brake I would build it not only to measure horsepower but also to show, as clearly as possible, how the brake is constructed and how it works. When I finally decided to build a large brake, the first decision I made was to design it with a drum as high above the ground as practical to make it clearly visible. Another decision was to use a scale with a dial large enough so people who were not close to the machine could observe changes in the force exerted on the scale. I also wanted people to be able to clearly see the speed the Prony brake is turning. A large Toledo scale found at a garage sale serves the first purpose well, and a large tachometer with a 6-inch dial fulfills the latter.

To make my brake as self-sufficient as possible, I constructed it with its own water reservoir and a recirculating water system. This frees me from having to set up near a water tap, and I don't have to worry about cooling water creating a big mud puddle around the machine. Additionally, the recirculating water system is designed so spectators can observe the water flowing into and out of the brake drum. I installed dial thermometers (salvaged from a scrap pile) in both the supply and return lines to show the rise in temperature of the water as it passes through the brake drum. The thermometers illustrate the fact that the Prony brake converts mechanical energy into heat, and in fact one way of looking at a Prony brake is to think of it as a mechanical water heater. Another important requirement was that the brake be easily portable. To accomplish this, I mounted it on a set of hard rubber wheels so I can load and unload it by myself.

Using Available Materials

My second criterion for the design was an economic one. It was important that the brake be built from scrap and/or second-hand materials. New components were limited to such things as the 4x6 treated timbers that I used to make the brake lining and two 3-7/16-inch taper-lock bushings that hold the brake drum to the shaft. I machined the tapered hubs for these bushings from pieces of scrap.

The basic design for the brake centers on a large steel ring 36 inches in diameter, 1-inch thick and 16 inches wide that a friend, Tom (T.C.) Spires, gave me. This became the brake drum. I fabricated the hub from a piece of 6-inch schedule-80 pipe and its five spokes are made from pieces of 3-1/2-inch pipe. I flame-cut the flanges for the drum from a piece of scrap 1/2-inch plate and then welded them to the drum. Tom Creek, owner of Manifold and Phalor Machine Shop in Reynoldsburg, Ohio, let me use one of his vertical boring mills and a neighbor of mine, Larry Peters (a machinist at Manifold and Phalor), machined the brake drum. The hard rubber wheels came from a portable tank that had been scrapped by a local manufacturer.

The shaft for the brake drum is 3-7/16-inches in diameter. I found two old 2-15/16-inch ball bearing pillow blocks, so I turned the shaft down to that size on each end. One of the pillow blocks has a much heavier frame, so I placed that one next to the pulley. To minimize the bending moment in the shaft caused by the tension in the drive belt, I set one of the pillow blocks over against the hub of the pulley. This can be seen in above photograph.

I formed the large hand wheel that controls the load on the brake by wrapping a piece of 1/2-inch pipe around a 32-inch steel wheel. The hub in the hand wheel is a cross, made out of 1-inch galvanized pipe with a hole drilled through it to fit over the extension on the adjusting screw that controls brake load. The spokes are pieces of 3/4-inch and 1-inch pipe. The 1-inch pieces are screwed into the four sides of the cross. The 3/4-inch pieces connect the 1-inch pieces to the 1/2-inch hand wheel.

Another challenge was finding a way to put an 18-inch radius on the inside of the 16 4x6 wood blocks that form the brake band. This was done using a 10-inch diameter saw blade mounted at a 10-degree angle in my vertical milling machine. What I ended up with was not a true radius, but rather a segment of an ellipse.

Finding a Drive Pulley

The second biggest challenge after the brake drum was the drive pulley. I had given up on ever finding a steel or cast iron pulley and was looking for a front wheel from a large truck. I assumed that a tire with the natural crown on the tread would make a great pulley. I was about to purchase a truck wheel from a junkyard when I noticed that an old grain elevator in nearby Amanda, Ohio, was being torn down. I contacted the owners, who told me it was not very likely I would find what I was looking for, but they referred me to the demolition crew, who they thought might be able to help. I am not sure the demolition crew had any idea as to what I was looking for, but they directed me to a local scrap yard where they were taking all of the metal from the mill. When I inquired at the scrap yard about the availability of a large pulley for use with a flat belt, the proprietor simply asked how wide a pulley I wanted. He then took me to the 40inch pulley that is now on my brake. At 10 cents a pound it cost me $29.50! The only problem with the pulley is that, because of its large diameter (which was exactly what I wanted), the brake drum now turns slower than I had originally assumed. The slower speed increases the load on the scales.

Selecting a Scale

The lower rpm (caused by the larger pulley) limits the amount of power I can measure using the old 200-pound capacity Toledo scale I purchased at a garage sale. The largest steam engine I expect to be able to use with this scale is about 45 HP. I watched the auctions on eBay and eventually found a hydraulic crane scale with a 500-pound capacity. The dial is only 12 inches in diameter, which is smaller than I would like, but it is still large enough to be visible to many spectators. Theoretically, I should be able to use this scale with a 32/110 J.I. Case engine. With the 42-1/2-inch drive pulley on the engine turning at 230 rpm, the force on the scale would be 437 pounds at 110 horsepower. It will be a while before I will take on an engine of that size.



To support the hydraulic scale, I built a framework over the top of my brake. This allows me to use the hydraulic scale for high torque loads, but still use the big Toledo scale with its 24-inch dial for lower torque loads. This framework also supports the tachometer, the mechanical lubricator, some of the piping and a small canopy to protect me from the sun.

Balancing the Brake Drum

From the time I started fabricating the brake drum from miscellaneous pieces of pipe, 1/2-inch plate and the steel ring, I was concerned about how I would balance it. I took great pains to assure the ring was as concentric with the shaft as I could possibly make it, but the alignment is not perfect. Also, when I mounted the drum on the shaft and put the shaft in the pillow block bearings, I found that the ring was not exactly round. However, it appears I may have inadvertently installed the ring in such a way that the error in its roundness offset the errors in my fabrication. Good luck seems to have been on my side throughout this project!

Before installing the brake band I proceeded to balance the brake drum. To do this, I simply installed the cast-iron pulley on the shaft so that all of the rotating parts were together. Then I spun the assembly by hand and allowed it to coast to a stop. I did this five or six times, reversing the direction of rotation each time. When the drum stopped, I marked the light (top) side with soapstone. I then, with a little trial and error, selected a piece of 1/2-inch plate and welded it inside of the brake drum on the light side. I then spun the drum five or six more times. I simply repeated this process a few times until the drum no longer stopped in roughly the same location after each spin. I then belted the brake up to my 8 HP United hit-and-miss engine and spun the drum at 110 rpm. There was no perceptible vibration in the frame of the brake at this speed. Next, I belted it up to my 350 International Harvester tractor and spun the brake at 210 rpm. Again, there was no perceptible vibration.



I had dreaded trying to balance the brake drum, assuming it would be a nearly impossible task. To my relief, it ended up being a very simple, albeit rather tedious, process. As a finishing touch, I painted the inside of the drum black so the small scrap pile I constructed in there would not be too obvious.

Next issue: In Part II: Building a Large Prony Brake, Bruce Babcock puts the finishing touches on his large Prony brake, testing it out and putting it to work at shows around the Midwest.

Steam enthusiast Bruce E. Babcock is a regular contributor to Steam Traction. Contact him at: 11155 Stout Road, Amanda, OH 43102, or e-mail: