Building a Large Prony Brake

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 2×4 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,