Integrating Steam Engine Indicator


This device is designed to integrate the area under the indicator diagram and will total the work for several strokes of the piston. A small metal cylinder about one inch by three-quarters is geared to a counter. The cylinder turns about its horizontal axis, and a small metal disc bears against the cylinder. The plane of the disc coincides with the axis of the cylinder and is turned by the motion of the indicator piston. The motion of the engine piston moves the cylinder horizontally. This combined action results in the rotation of the cylinder. The increment to rotation is proportional to the angle of the disc.

The manufacturer of this indicator is unknown. The spring is missing, although Thompson indicator springs will fit it. Similar instruments are described in William Robinson’s book Gas and Petroleum Engines: A Manual for Students and Engineers (Spon & Chamberlain, New York: 1902).

An engine indicator is an instrument for graphically recording the pressure versus piston displacement through an engine stroke cycle. Engineers use the resulting diagram to check the design and performance of the engine.

A mechanical indicator consists of a piston, spring, stylus, and recording system. The gas pressure of the cylinder deflects the piston and pushes against the spring, creating a linear relationship between the gas pressure and the deflection of the piston against the spring. The deflection is recorded by the stylus on a rotating drum that is connected to the piston. Most indicators incorporate a mechanical linkage to amplify the movement of the piston to increase the scale of the record.

When the ratio of the frequency of the pressure variation to the natural frequency of the system is small, then the dynamic deflection is equal to the static deflection. To design a system with a high natural frequency, the mass of the piston, spring, stylus, and mechanical linkage must be small, but the stiffness of the spring must be high. The indicator is subjected to high temperatures and pressures and rapid oscillations, imposing a limitation on the reduction in mass. Too stiff a spring will result in a small displacement of the indicator piston and a record too small to measure with accuracy. Multiplication of the displacement will introduce mechanical ad dynamic errors.

The parameters of the problem for designing an accurate and trouble free recorder are such that there is no easy or simple solution. Studying the variety of indicators in the collection shows how different inventors made different compromises in their designs.

Location: Currently not on view

See more items in: Work and Industry: Mechanical and Civil Engineering, Engineering, Building, and Architecture, Work, Industry & Manufacturing, Indicators


Exhibition Location:

Credit Line: Gift of Department of Mechanical Engineering, University of Illlinois, Urbana, Illinois

Data Source: National Museum of American History

Id Number: MC.316793Catalog Number: 316793Accession Number: 228496

Object Name: indicator, steam engine

Physical Description: wood (case material)Measurements: overall: 4 7/8 in x 9 3/4 in x 10 in; 12.3825 cm x 24.765 cm x 25.4 cmoverall - from catalog card: 4 in x 7 in x 7 in; 10.16 cm x 17.78 cm x 17.78 cm


Record Id: nmah_847277

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