Robertson-Thompson Steam Engine Indicator

Description
This Robertson-Thompson steam engine indicator, serial number 7735, consists of a brass piston with two grooves; a brass cylinder; an internal, single wound spring, which can be changed; a medium sized drum with a coil spring and a single record; and a short pencil lead for the stylus. Accompanying the indicator is a box with two extra springs, drum springs, seven wooden pulleys for the reducer, two scales, and two extra pistons.
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
Object Name
indicator, steam engine
date made
ca 1900
Measurements
box - from catalog card: 7 in x 9 1/2 in x 10 1/2 in; 17.78 cm x 24.13 cm x 26.67 cm
overall: 7 1/4 in x 6 in x 2 in; 18.415 cm x 15.24 cm x 5.08 cm
place made
United States: New York
ID Number
MC*318483
catalog number
318483
accession number
234643
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Work and Industry: Mechanical and Civil Engineering
Work
Industry & Manufacturing
Indicators
Engineering, Building, and Architecture
Data Source
National Museum of American History, Kenneth E. Behring Center

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