Tabor Steam Engine Indicator

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Ashcroft Mfg. Co. of Bridgeport, Connecticut, manufactured this Tabor steam engine indicator, serial number 2329. It consists of a steel piston with a guide below the spring; a vented brass cylinder; an internal, single wound spring, which can be changed; a large drum with a coil spring and a single record. The piston, spring, and parts of the linkage are missing.
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.
Currently not on view
Ashcroft Mfg. Co.
place made
United States: Connecticut
Physical Description
wood (case material)
overall: 5 1/4 in x 9 1/4 in x 8 3/8 in; 13.335 cm x 23.495 cm x 21.2725 cm
walnut case - from catalog card: 5 1/4 in x 7 1/2 in x 9 1/4 in; 13.335 cm x 19.05 cm x 23.495 cm
ID Number
catalog number
accession number
Credit Line
Gift of Department of Mechanical Engineering, University of Illinois, Urbana, Illinois
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Work and Industry: Mechanical and Civil Engineering
Industry & Manufacturing
Engineering, Building, and Architecture
Data Source
National Museum of American History


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