Building a Large Prony Brake

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 4×6
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 4×6 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.

FABRICATING END PIECES FOR BRAKE BAND. THESE ATTACH TO THE BRAKE
CONTROL WHEEL AND THE ARM THAT ACTUATES THE SCALES.

FLANGES FOR THE BRAKE DRUM INSTALLED, AND THE BRAKE BAND WRAPPED
AROUND DRUM FOR FITTING PRIOR TO FINAL MACHINING OF DRUM.

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.

THE FINISHED BRAKE CONTROL WHEEL. TURNING THE WHEEL INCREASES OR
DECREASES THE LOAD PLACED ON THE BRAKE DRUM.

THE NEARLY COMPLETED PRONY BRAKE. THE WATER TANK IS JUST VISIBLE
UNDER THE BRAKE DRUM, AS IS THE WATER PUMP

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: badcock2@gte.net