Steam Engine Indicators

I thought that the readers of Steam Traction might be interested in knowing more about what was happening in the photo on the back cover of the September/October 2005 issue of Steam Traction where I was photographed on a Baker 21-75 HP.

The gentleman on the ground behind me is Bill Newell, the engineer is Jason Newell, and the engine is the Newell family’s fine 21-75 A.D. Baker steam traction engine.

The reason for the smiles is that Bill, Jason and I have just succeeded in “taking a card” for the first time from the rod end of the cylinder on the Baker. The instrument attached to the cylinder hides Jason’s smile, but the twinkle in his eye is obvious. When I say we were successful in taking a card, what I mean is that we succeeded in using a 100-year-old engine analyzer, known as a steam engine indicator, to obtain a pencil and paper printout showing what was going on inside the Baker’s cylinder when the engine was running at 200 revolutions per minute and delivering 37 HP to the Prony brake. The engine is rated for 285 revolutions per minute but was governed down to 200 for the purpose of using the indicator. We did not attempt to determine the maximum horsepower.
The indicator we used was made by James L. Robertson & Sons, and attached to it was a Robertson-Thompson Improved Victor Reducing Wheel. By the time these instruments were manufactured, about 1900, the steam engine indicator had been in use for over 100 years. John Southern, an engineer who worked for James Watt, invented the indicator in 1796.

Steam Indicator Basics 

The steam engine indicator performs its function using two inputs taken from the engine: the pressure in the cylinder and the position of the piston. As the pressure in the cylinder rises and falls with each stroke, the change in pressure causes a small piston in the indicator to also rise and fall. This piston is connected to a delicate arrangement of levers and pivots that amplifies the movement so that a pencil lead on the end of one of the levers draws a line on a paper that is wrapped around a metal drum. In the photo, the white paper can be seen at the top of the indicator. The pencil lead is in the right-hand end of the small horizontal lever in front of the paper. If there were no other input, the pencil would simply draw a vertical line that would indicate nothing more than the maximum and minimum pressure in the cylinder.

Because a simple vertical line would be useless, one end of a string is wrapped around the metal drum that holds the paper, and the other end is attached to the crosshead of the engine. In the photo, you will note that a pipe nipple has been installed in the wrist pin where the grease cup is normally located. The purpose of this piece of pipe is to provide a place to attach the end of the string. As the crosshead moves back and forth, the string causes the paper to move in exact unison with it.

Once the indicator is attached to the cylinder, the pencil is rising and falling with the pressure of the steam, and the paper is rotating back and forth as the piston moves: The simultaneous movement of the pencil and the paper produces a plot showing the pressure at every point in the stroke of the engine.

Obtaining these two inputs is not quite as simple as it might sound.

Many traction engines left the factory with ports drilled in the cylinder for the specific purpose of attaching an indicator. The indicator on the Baker is installed on the port on the rod end of the cylinder, and a pipe plug can be seen in the indicator port near the head end of the cylinder. On Russell engines, the ports are located on the top of the cylinder, but on many engines, especially older ones, there are no ports. I understand, on engines that do not have ports, the indicator can be installed on the cylinder drains.

Because indicators were made so the string must travel no more than 4 inches, it is not possible to attach the string directly to the metal drum and the crosshead as I described. With the 21-75 Baker engine having a stroke of 10 inches, it was necessary to use the Robertson-Thompson Improved Victor Reducing Wheel to convert that travel to less than 4 inches.

To show the effect of “hooking up” the reverse lever, we took two cards on one piece of paper. The shorter one shows the pressure in the rod end of the engine at every point in the stroke when the reverse lever was all the way back, or, as some would say, “down in the corner.”

The taller diagram shows the plot of the pressure when the reverse lever was in the fourth notch and the valve was cutting off the steam earlier in the stroke. Because we were using a Prony brake for the load, we were able to control and measure the horsepower put out by the engine. In both instances shown in the figure, the engine was running at the same speed and doing the same amount of work. The area enclosed by the line drawn by the indicator is proportional to the power generated by the engine, provided the speed is constant. In this way, two diagrams that represent the same amount of power enclose the same area. The diagrams show that, for the engine to produce the same amount of power when hooked up as it does when it is down in the corner, the governor must open wider and allow more steam into the cylinder. This is shown by the increased pressure (greater height). But because the steam is cut off earlier, the net result is that less steam is used to do the same amount of work.

The high speed of steam traction engines affects the shape of the card. One effect is that the steam line tends to slope downward because of the high velocity of the steam as it attempts to fill the rapidly enlarging space behind the piston. On slow operating engines, the cards look quite different. When we took the cards from the engines of the Belle of Louisville we used a more elaborate piping setup than on the Baker engine. On the Belle we installed the indicator on a three-way valve that was connected by pipes to the indicator ports on both ends of the engine. With this arrangement, we were able to take a card from one end of the engine, and then, by simply switching the three-way valve, take another card from the opposite end of the engine. On the Baker engine, we had to move the instrument from one indicator port to the other.

In contrast to the 21-75 Baker, with its 9-1/4-inch bore, 10-inch stroke and running at 200 revolutions per minute, the two 110-year-old engines on the Belle of Louisville have 16-1/2-inch bores, 6-1/2-foot strokes and run at 10 to 13 revolutions per minute.
The cards from the Belle of Louisville were taken in 2005 by a crew of volunteers that included steam traction engine owners Dave Schramm; Robert T. Rhode; Joe Kramer, and his sons Jason and Andrew; Mehl Young; and Brenda and Micky Stant; a retired steam boat captain named Alan Bates; a retired steamboat chief engineer named Kenny Howe; a mechanical engineer named Keith Baylor; Tom Schiffer, the owner of a small steam boat; the current chief engineer of the Belle, Jim McCoy; Paul McConnell, who is fascinated by the history of technology; and myself. I don’t want to forget the team member who was responsible for arranging the opportunity to use the indicator on the Belle. His name is Mark Jordan. Mark, who has owned Frick traction and portable engines for many years, is also a national board certified boiler inspector for the state of Kentucky. The indicator we used was identical to the one used on the Newells’ 21-75 Baker at Wauseon, Ohio. The reducing motion was a much different device because of the need to reduce the 6-1/2-foot stroke to 4 inches.

The high cost of fuel was one of the factors that caused chief engineer Jim McCoy to be receptive to allowing a group of strangers to experiment with an indicator on the Belle’s engines. Based on information obtained from the indicator cards, Chief McCoy was able to reduce the steam consumption of the two engines by simply adjusting the cut off. According to my calculations, the pounds of steam used per hour, per indicated horsepower, might have been reduced by as much as 13 percent. The indicator cards also disclosed the need for further repairs and adjustments that will most likely be accomplished while the Belle is laid up for the winter. We have been informed that we will be invited back to use the indicator to evaluate the effectiveness of this work.

An excellent account of the history of the steam engine indicator can be found online at This site includes extensive excerpts from an upcoming book on the steam engine indicator written by John Walter of Brighton, England. Walter was our long-distance team member when we took the cards from the engines of the Belle of Louisville.

Contact steam enthusiasts Bruce Babcock at: 11155 Stout Road, Amanda, OH 43102; (740) 969-2096; e-mail:

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