Building a Large Prony Brake

Determining Capacity and Putting it to the Test

PRONY BRAKE

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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.

BRUCE BABCOCK'S (ALMOST) FINISHED PRONY BRAKE, LOADED AND READY TO GO TO ITS FIRST OUTING AT THE MAD RIVER STEAM AND GAS SHOW IN URBANA, OHIO, LAST JUNE.

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.

The Backstop

The backstop is a very small item, but a vitally important one if a brake is being used with a steam engine. Because steam engines can be started in either direction (and it is often necessary to reverse them momentarily to get them to rotate in the desired direction), it is necessary to provide a backstop on the arm of the brake. If the backstop were not in place, the arm of the brake could rotate backward away from the scale and damage the brake - or worse yet injure anyone who might be in its way.

On my brake, I installed a piece of 2-inch angle iron that holds a piece of 2x4 pine about an inch above the top of the arm. As an additional precaution, to prevent damage to the scale I connected the brake arm to a lever with a piece of 1/8-inch steel cable. This allows me to easily raise the arm against the backstop until the drum is rotating in the correct direction. Dave Smith, one of many people whose advice proved of great help, told me how the failure to do this on another Prony brake resulted in the breakage of the levers on the scale.

LEVER FOR RAISING BRAKE ARM AGAINST BACKSTOP WHEN BRAKE DRUM IS ROTATING BACKWARDS. BRUCE ADDED THIS FEATURE AS AN ADDITIONAL PRECAUTION AGAINST DAMAGING THE PRONY BRAKE'S SCALE.

Uses of the Prony Brake

One of my reasons for building a large Prony brake was to have a way to provide a load for a steam engine, a necessity if you're using an indicator to evaluate the adjustment of the slide valve. I have several indicators, but haven't had an opportunity to use one. It may seem odd, but for my purpose a large Prony brake is nothing more than an accessory for a small and intricate steam engine indicator. For a description of a steam engine indicator, see my article, 'Engine Analyzers for Steam Engines: The Story of the Steam Engine Indicator' in the July/August 2001 issue of Iron-Men Album.

It appears that some engine owners and/or operators like to use a Prony brake to determine how much power they can get out of their steam engines. For instance, in recent years a 65 HP Case engine was reported to produce 123.5 HP on a brake at an engine show. If the slide valve is properly set, there are only two ways to increase the horsepower of a steam engine. One is to increase the speed and the other is to increase the pressure. According to the late Amos Rixman, writing in the May/June 1988 issue of Iron-Men Album, it has been common practice since the days of the tests at Winnipeg to use '... some extra rpm to obtain a few additional HP.' According to Rixman, the Case engine mentioned above was running 20 to 25 rpm high.

Another approach to using a Prony brake was reported by David Schramm in the March/April 1989 issue of Iron-Men Album. According to Schramm, at the National Threshers Economy Run in 1988 the engines were run '... at about three-quarters of the rated belt horsepower, or whatever horsepower the engineer preferred.' Not one of the results he reported in the article exceeded the horsepower rating of the engine.

It is my intention that my brake will not be used with any engine operating above its rated pressure or its rated speed. I want my brake to be used in conjunction with one of my indicators to see how well an engine can be made to run: not to see how hard it can be made to run.

Putting the Brake to Use

On June 20, 2002, I decided that even though my brake was not 100 percent complete, it was close enough to take it to the Mad River Steam and Gas Engine Show at Urbana, Ohio. Over the next three days, with a great deal of help from other exhibitors and spectators, the brake was belted up to Mike and Clint Johnson's two J.I. Case traction engines, John Yowler's Huber traction engine, a Case 30 gas tractor and a Cockshutt 50 gas tractor.

The brake easily handled loads up to 70 HP, and I am sure, had there been a larger engine available, it would have handled more. The only limitation we experienced was with the Cockshutt 50 gas tractor the engine had to be throttled back to stay within the 300 rpm limit I had established for the cast-iron belt pulley on the brake. However, the brake still provided a good load for the Cockshutt and gave it a chance to get a good workout.

One concern, which I mentioned last issue, was whether the brake was adequately balanced to spin at 300 rpm, since I could only test spin it to just over 200 rpm with my International 350 tractor. There was no noticeable vibration with my International, and there was no noticeable vibration running it at 300 rpm with the Cockshutt 50 tractor.

MARK SCHLEPPI'S 40-60 HUBER BELTED TO THE PRONY BRAKE AT THE MIAMI VALLEY STEAM THRESH ERS SHOW IN PLAIN CITY, OHIO, LAST JULY.

We were able to use the large Toledo scale for readings up to 200 pounds, but had to switch to the hydraulic crane scale for larger loads. The Toledo scale has an internal dashpot to absorb vibrations and make the scale readable. I filled the dashpot with motor oil prior to the show, and it worked very well. The hydraulic scale, however, has no dashpot to dampen vibrations, and consequently it was much harder to read.

Following the Urbana show I made several modifications to the brake. I installed the 6-inch diameter tachometer, a mechanical lubricator and a dashpot to smooth out the operation of the hydraulic scale. The tachometer and lubricator are both driven from a jackshaft located above and running perpendicular to the axis of the brake drum. A twisted belt powers the jackshaft from the main shaft. The lubricator did not deliver enough oil I suspect I may need to speed it up and also switch to heavier oil. Both Darke County's brake and National Thresher's brake use animal fat for lubrication. I hesitate to do this for fear that by spring my brake will smell like a dead animal regardless of how well I attempted to clean it.

I made the dashpot from 3-inch pipe. The plunger is 1/4-inch thick and is 1/16-inch smaller than the inside of the pipe. I filled the dashpot with motor oil. This greatly improved the operation of the hydraulic scale when I used the brake at the Miami Valley show in Plain City, Ohio. The dash-pot is attached to the frame of the brake and the plunger is attached to the arm that runs from the drum to the scales.

When we ran tests on the steam engines and tractors we recorded the scale readings and the revolutions on a large chart I had prepared. I used a 20-inch slide rule for horsepower calculations. I feel the use of a slide rule was in keeping with the theme of my exhibit, 'Measuring Horsepower as it was Done a Hundred Years Ago.'

At both the Urbana and Plain City shows I had, in addition to the Prony brake, a display of steam engine indicators. This included my replica of the first indicator (invented in the 1790s by John Southern, an engineer who worked for James Watt), a Richards indicator from the 1860s, a Crosby indicator that was loaned to me for the exhibit by John Yowler and a Robertson-Thompson indicator.

After two years of collecting parts, working out the design and constructing the Prony brake, it was a real pleasure to see it perform satisfactorily. I wish I could list all of the names of people who helped put the brake into operation. Many of them I had never met prior to the shows, and I never even got the names of some who pitched in to help.

Now that I have a Prony brake and several indicators from which to choose, my next goal is to start taking indicator cards on steam engines. With any luck, I hope to do this by the end of the summer.

I want to thank Robert T. Rhode for his assistance in the preparation of this article,

Suggested Reading

Over the years, the late Amos Rixman wrote many excellent articles on the subject of horsepower and its measurement. These articles were published in several steam hobby magazines, and I recommend them to anyone who is interested in this topic. Of particular interest are his pointed comments regarding the use of steam engines on Baker fans. His articles were one of the most valuable resources available to me as I designed and built my Prony brakes and as I wrote my articles describing them. I deeply regret never having had the opportunity to meet Amos Rixman to learn firsthand from his experience, or to observe his renowned performance as the narrator and interpreter of horsepower tests.

The following list includes specific references to some of Amos Rixman's writings, as well as some other articles that may be of interest:

Babcock, Bruce E., 'The Design, Construction and Use of a Small Prony Brake,' Gas Engine Magazine, July 2000.

Babcock,Bruce E. 'Engine Analyzers for Steam Engines: The Story of the Steam Engine Indicator,' Iron-Men Album, July/August 2001.

BRUCE BABCOCK RECORDS THE RESULTS OF ROBBY ROLL'S 19-65 PORT HURON'S RUN ON THE PRONY BRAKE AT THE MIAMI VALLEY SHOW IN PLAIN CITY, OHIO. NOTE THE SLIDE RULE IN BRUCE'S LEFT HAND, WHICH HE USED IN MAKING HIS FINAL CALCULATIONS.

EVEN SCALE ENGINES GOT A WORK OUT ON THE PRONY BRAKE. THIS IS JOHN WATKIN'S 44 PERCENT 23-90 BAKER BELTED TO THE PRONY BRAKE AT THE MIAMI VALLEY SHOW IN PLAIN CITY, OHIO.

Halsey, F.A., Handbook for Machine Designers and Shop Men, 1916.

Kent, William, Mechanical Engineers' Pocket-Book, 1910.

Rixman, Amos, 'Horsepower Performance Plays Part in Case Heritage Celebration at Pawnee and in Mt. Pleasant Activities 1987,' Iron-Men Album, May/June 1988.

Rixman, Amos, 'Horsepower: Its Source, Definition, Measurement and Presence in Our Daily Lives,' Engineers and Engines, April/May 1989.

Rixman, Amos, 'Pawnee 1989 Show Again Scores with a Popular Hit,' Iron-Men Album, May/June 1990.

Schramm, David 'National Threshers Steam Engine Economy Run,' Iron-Men Album, March/April 1989.

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