The Steam Engine Indicator

Case showing reducing motion

Indicator on 9 x 10 Case showing reducing motion or sometimes called ''detente.''

Content Tools

106 South Elm Street, Newkirk, Oklahoma 74647

During the early part of the 20th Century, the piston steam engine was in almost universal use driving the machinery in factories and the generators in electric light plants; Corliss engines in the larger, 'Automatics' in the smaller plants.

The operating engineers who attended the engines were generally required to hold an operator's license. To obtain it he would need to have at least an elementary knowledge of a device called the steam engine indicator.

The indicator is not a very complicated instrument. James Watt invented it. It is just about impossible to set the valves on a Corliss engine without an indicator and it is a great help in getting the valve on an 'automatic' especially a piston valve.

The indicator shows the engineer how the steam is acting in the cylinder: the 'valve events'; admission; cut-off; and release on the power stroke and back pressure and compression on the return. Any mis adjustment of the valve gear and leakage will show. In fact, the indicator card shows the engineer the same things that a cardiogram shows your doctor. The horsepower being developed can be calculated.

An indicator is especially needed in adjusting the reversing gears on traction engines and locomotives. The builders of traction engines needed them very badly but some surely didn't use them.

The writer operated a sawmill at Harrison, Arkansas, until a few years ago. It was driven by a SKINNER steam engine with cylinder diameter 14 inch, stroke 15 inch and with Shepard automatic valve gear with 'inertia' governor.

I had been using two Case traction engines to pull the mill but it got too big for them, so I set them aside to play with and acquired the larger engine.

For this more 'sophisticated' valve gear, I needed an indicator, and had the good luck to obtain a fine instrument that had never been used and had been kept in perfect condition. The trouble was that it had been made for use on large engines and the reducing motion, or 'detente' as it is sometimes called could only be used on from 36 down to 18-inch stroke. This caused me to have to make a reducing motion. The one I made can be used on 15 down to 10-inch.

If any reader has ever had actual experience with an indicator, I would be very glad to exchange cards, especially cards taken on a traction engine. I would especially like to get some of the younger steam engine lovers interested.

I strongly maintain that it is not possible to really know the steam engine unless you know the indicator.