The Indicator


The steam engine indicator is an instrument use to set-up and maintain steam engines (see description and usage below). If you constructed a new engine or made repairs that required dismantling parts of the valve motion, you took a set of cards when the engine was back in steam. If you had an engine in service, you took indicator ‘cards’ routinely to be sure the valve motion hadn't gotten out of adjustment through wear or damage. The first indicator was invented by James Watt in the early 19th century and they are still used to tune up engines today. They were very precisely made tools that were often handed down from father to son and usually exhibit beautiful craftsmanship and finish.   The Museum of making has assembled an extensive collection of more than 500 indicators representing most of the commonly used instruments during the period 1850- 1930.


The indicator was a prized instrument. It was very precisely made, by makers in New England such as Crosby, American, Ashcroft and a host of others in both the USA and in Europe. Every indicator came in a cabinet-grade wood carrying case with a set of springs for different pressure ranges, a pad of cards, a small metal tube of leads, a scale for use when figuring the area of the diagram, and a small oiler. Indicators were treated with great care and not left laying around. Every steam engine indicator manufacturer's literature featured testimonial letters from stationary engineers, mill owners and similar as to how the indicators saved tons of coal or straightened out the engines in some plant or other.


If you worked on the engine or made repairs that required you to dismantle parts of the valve motion, you took a set of cards when the engine was back in steam. If you had an engine in service, you took cards routinely to be sure the valve motion hadn't gotten out of adjustment thru wear or some other damage.


One has to remember that the indicators were built in a time when a lot of the precision work was done by hand. Lathes and small mills were used to build those indicators. The craftsmanship on the indicators was the finest. It all went hand-in-hand with the areas of New England and the UK Midlands where fine precision machine tools were made along with the indicators.



Description and use of the Indicator -  By Joe Michaels


I never owned a Bachrach indicator. I have used the basic old time indicators like an American-Thompson or Crosby indicators on engines. The indicator tells quite a tale if you know what you are looking at. If your indicator has been mishandled so the frame is sprung a little, or the piston worn, or you do not set the indicator up correctly, or there is deflection or slop in the mechanism or mounting, you will get a horrific looking card that can send you on a goose chase.

The Steam engine indicator is a tool which records and graphs two functions: Piston position and pressure within the cylinder of a steam engine. The indicator produces a "card" or "diagram" in which the vertical axis (Y) is the pressure within the cylinder and the horizontal axis (X) is the piston position as it moves thru its stroke.


The steam engine indicator is a mechanical instrument. It consists of a small cylinder with a very precisely machined piston, perhaps 0.4"-0.750" bore. The piston in this cylinder is arranged to push against a spring. The piston rod extends up and works a parallel motion to move a scriber or pencil lead in a straight vertical line. This piston, cylinder, and spring are a pressure transducer in modern terms.


The scriber moves up and down in response to changes in cylinder pressure. The vertical line the scriber moves on is aligned with the vertical centerline of a hollow drum. This drum is mounted on a bracket or frame along with the pressure transducer cylinder. The drum is on a shaft so it can turn freely. It has a set of spring fingers on it which hold a piece of paper or "card" that is wrapped around the drum. The drum has a set of stops and a coiled spring inside it to return it to its original position. Around the base of this drum is a set of flanges forming a small pulley. String or wire is wrapped around this and turns the drum.


The spring the piston works against is sized for the working steam pressure of the engine.  Indicators came with sets of springs covering ranges of pressures. With the correct spring is in the indicator, the cylinder of the indicator is connected to the three-way cock on the cylinder of the engine under test.


The string or wire wrapped around the drum should have a loop or hook on its end. It is then pulled out and tied to a hook or some other handy point on the engine cross-head. The engine's stroke must be less than the circumference of the drum. If the engine's stroke is longer than the circumference of the indicator drum, a "reducing motion" must be used. This can be a series of pulleys, some gearing or a linkage.


Now to take the card. The indicator had been cleaned and its piston oiled before being put on the engine. The engine is warmed, started rolling, then brought up to speed and put under load. The indicator valves are opened, putting steam to the indicator 3 way cock. You take a wiping rag and take the cap off the indicator cock. You then stand wide and blow the cock clear by opening it in each position. Steam will blast out in spurts as the slide valve ports steam to each side of the piston. Once the indicator clock is blown clear, the indicator is connected to the cock. The piping should be rigid enough to hold the indicator without flexing or movement. The indicator has a union on the bottom of its cylinder which mates up with the indicator cock.


An indicator card (most indicator makers furnished pads of them) is wrapped around the indicator drum. The spring fingers on the drum hold this paper in place. All wrinkles are worked out and the paper must lay flat and smooth against the drum.

A sharp 3H or 4 H pencil lead is placed in the holder on the arm of the parallel motion. The indicator is now ready for use on the engine.


A typical side crank engine will be running at about 120 rpm or less, so a person with reasonably good reflexes should be able to hook the indicator cord to the cross-head. You make sure the little arm with the pencil lead is swung well clear of the drum. You then take the indicator cord and pull it taut, and hook it to the cross-head. The indicator drum will now be turning through some portion of a revolution, mirroring the stroke and position of the piston.


You then open the three-way cock to port steam from one side of the cylinder (head end or crank end) to the indicator. As soon as that is done, the indicator piston starts going up and down in response to changes in cylinder pressure. Let the indicator make some strokes to warm, swap ends with the 3 way cock and let it warm using both sides of the three way cock.


Now it's time to "take the card". You first bring the pencil lead against the card as it rolls past the pencil lead. With the cock closed, you get a straight horizontal line. This would be the "atmospheric line". Now pick an end of the cylinder to start with- say the "head end". Throw the cock to the left, and gently swing the indicator mechanism in so the pencil lead just touches the card as it moves past. With very light pressure, you hold the mechanism so the pencil lead "draws the diagram". When the diagram appears, back the lead away from the paper. Throw over the 3 way cock to the "crank end" of the cylinder and repeat the process.


You now have a "card" on the engine. Shut the three way cock. Unhook the cord from the cross-head. Remove the card, take it to the desk and "read the card".


The diagrams drawn by each side of the cylinder will look kind of like the outline of the side-view of a person’s foot. A diagram of both head end and crank end of the cylinder will look like the two feet cross at about the instep area.


The card describes the events within the engine cylinder. It also serves to show if the valves are correctly set, valves seating OK, rings not leaking by, and both ends of the cylinder making the same power. Visually, if you know what you are looking at, you can tell a lot about the engine. There will be a line at the extreme right and left of each foot that shoots almost straight up. That is the admission line when the steam valve comes open and full boiler (or steam line) pressure enters the cylinder. The line will then curve off to the right, and be relatively flat for a short ways. This is the time the eccentric has the slide valve fully opened. As the slide valve begins to close, the line will begin to slope downwards. At the point the slide valve goes fully closed, the line will take off like a ski slope, downwards. That point is "cutoff". The ski slope portion of the diagram (looking like the instep area of the imaginary foot) is the "Expansion" line. The ski slope will end with a kind of curving and will break into a horizontal line across the bottom of the diagram. This is the exhaust line. The exhaust line heads back towards the starting point of the diagram. Before it connects back to the starting point, the exhaust line will curve upwards, just like the heel on the side view of a person's foot.


That "heel" area is the "Compression" portion of the diagram. The compression occurs when, following the exhausting of steam, the slide valve goes closed. This allows the piston, which is heading towards dead center, to compress any steam trapped within the cylinder. This occurs to cushion the piston at end of stroke and to compress the remaining steam to warm the cylinder to minimize condensation when the valve opens to admit more steam.


Having finished compression, the eccentric opens the steam valve, admission occurs again and the cycle is repeated.


OK, you now have this diagram in hand and you look it over. If those two "feet" don't look mighty similar, you have one end of the cylinder doing more work than the other end. You might hear or feel it in how the engine runs.


Assuming the card looks reasonably good, you then move to the next level. You take the card and determine the area enclosed by each "foot" diagram for each side of the cylinder. You can do this by "graphical integration" (taking strips of the diagram and calculating the area of each such strip), or with a Planimeter. The planimeter is traced over the diagram and reads the area enclosed by it. Either way, you get the area enclosed by the curves on the diagram. Indicator makers furnished scales to measure the pressure side of the diagram with. Once you have the area enclosed by the diagram, you divide by the engine stroke. You now have the Mean Effective Pressure. You get the MEP for each end of the cylinder- head end & crank end. They should be mighty close, if not equal. A deduction on the crank end is expectable for the area of the piston rod.


You took the card with the engine running under steady load, on the governor. While you were taking the card, you and a helper took a Starrett Revolution Counter (or similar) and your watch and got the RPM off the crankshaft.


Now you know MEP, cylinder bore, stroke, and rpm.

You calculate the horsepower the engine is developing using the famous formula you should know in your sleep:


HP = (P x L x A x N) divided by 33000


where: HP = Horsepower

P = Mean Effective Pressure, converted to

pounds per square foot

L = length of stroke, converted to feet

A = Area of piston converted to square


N = rpm


33000 = 550 foot pounds per second (equivalent of

1 HP) x 60 seconds per minute


You run this calculation for each side of the cylinder based on the MEP you got from the card. You remember to deduct the area of the piston rod when figuring the crank end. You plug in the numbers and you now have what is known as "Indicated Horsepower" or IHP for each end of the cylinder. You add the two together and you have the IHP for the engine.



Diagnosing an Engine  Problem -  By Joe Michaels


I was on a job with Mike Korol, the Skinner Engine erector. We were looking at three Filer & Stowell 4-valve non-releasing steam engines in a prison power plant. These were basically Corliss type engines, but had no dashpots. The Corliss type valves worked thru a quick-closing mechanism, and had cut-off governing. Anyhow, Mike had his trusty Bachrach indicator. The first engine was put on line and loaded (paralleled into the prison system and the power grid). Mike didn’t even put his indicator on the engine. He worked the indicator 3 way cock and saw and hear how the steam blew for each end of the cylinder. He put a hand on the eccentric rod. He watched how the engine ran and could tell from the valve motion where the cut-off was occurring.


Mike made a preliminary diagnosis that the engine had a cracked ring, based on the steam blow from he indicator cock. He also decided one of the admission valves had a twisted stem. We took a set of cards and confirmed his thinking. The engine was taken off line and Mike and a crew did the repairs. Mike made an offset woodruff key for the valve stem to compensate for the twist. We got the engine back on line and Mike checked the steam blast from the three way cock- he showed me how the blasts were now distinct and equal. The engine was put under load, and now Mike was going crazy. He had reset the valves, and was confident he had made a good valve setting. Under load, that engine was running at way too late a cut-off for the load. Mike took a set of indicator cards and confirmed this fact. While he was taking the cards, I went over to the control board to get the amperage and voltage. In that way, I could quickly calculate the HP, allowing for generator efficiency.


At the control board, I saw the Power Factor Meter was way over on the "leading" side. I asked the shift operator why this was. He told me he was "balancing power factors", putting the power factor meter on one engine way over on the lagging side, so this engine had to have it's power factor on the leading side. I asked the operator if I could make a few adjustments. I took the controls and adjusted the Power Factor on the engine we were working on. I got it to a 0.8 Lagging PF. I could hear Mike Korol yelling: "Joey: What the f--k did you do?!" He had his hand on the eccentric rod and was hollering the engine was now running as it should, cut-off nice and early. Mike took a great set of cards on that engine with no further adjustment of the valves.


It turned out the shift operator had come from some prison where the power plant had DC generators. He had been taught to synchronize an AC generator, but no one had explained Power Factor to him. Mike then told me he had gone on a service call to a steam laundry on Long Island. They had two Ames Uniflows- one horizontal, one vertical. They claimed the generators were running hot and they were using way too much fuel oil and were thinking of scrapping the engines. They had gotten the engines and generators used from a laundry in Brooklyn, The chief engineer at the laundry in Broklyn had told the people in the new installation what their fuel consumption ought to have been, and they were way above it.


Mike had been out there a few times to take cards and set the valves. He swore he had the valves set right, had gotten good cards when he had loaded each engine, and left. Soon enough, the owners of that laundry had called him back, claiming the engines were in need of adjustment again. After the prison plant episode, Mike went out there to Long Island and found they also had no idea about adjusting power factor. He got beautiful cards for the Ames engines with no further adjustment to the valves once he got the plant people straightened out as to power factor.



Reflections on the Engine Men - By Joe Michaels


What is somewhat amazing was that was an era of "self made men"; men who had minimal educations started off hauling ashes, wiping down the plant, moved up to firing, and eventually became stationary engineers. The examinations required them to be able to calculate heating surface, load on stay bolts, horsepower, as well as having a solid working knowledge of the theory and operation of steam plants. Men learned more math and perfected their handwriting, and learned not only to use the indicators but to "read the cards". They learned to use a planimeter or to do the "strip method" of graphical integration to calculate the MEP. These were proud men, and justly so. They had "graduated from the slice bar and coal scoop". In the plant and community they had a recognized position and took the job seriously in most cases.


Look at the bearing of the men in the old photos. Well groomed, neat work clothes, even a white shirt and waxed moustache sometimes. Nothing came easily to most of those men. It was study, ask the older men to help, maybe take a course or two at night if they were in a major city, or maybe some correspondence courses.


To be a stationary engineer meant paying ones dues, with years of dirty work and study. Being able to use an indicator and knowing how to "read the cards" as well as do the associated calculations usually meant a man was at the top of his game in a stationary plant. You can be sure the men in those photos had paid their dues.


It was a different era, for sure. Aside from the spit-shined beam engine and ornate engine room there was often a spitoon (or cuspidor) against the wall. It was a part of the era as well.


That's the anatomy, principals, use and an indicator story.


Joe Michaels