G. H. Crosby Indicator, Patent Model

Description
This indicator was filed to the U.S. Patent Office with the application for patent no. 219149 issued to G. H. Crosby, September 2, 1879.
The model cannot be disassembled and the piston is stuck in the cylinder. It consists of a large drum with spiral spring and single record, but the linkage is different from the other Crosby indicators in the collection: a spring releases to move the drum back away from the pencil print.
The improvements claimed for this design are a jacket about the steam cylinder to prevent radiation or loss of heat from the cylinder; a method of supporting the cylinder and jacket so that each might expand freely when heated; the carrying of the rotary drum on a lever so that it could be moved up to and away from the marker; and a peculiar parallel motion for effecting the straight line motion of the marker in which “the lever is connected with the piston-rod by a join, and not indirectly by a link, as in the Richards indicator.”*
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.
*Reference:
This description comes from the 1939 Catalog of the Mechanical Collections of the Division of Engineering United States Museum Bulletin 173 by Frank A. Taylor.
Location
Currently not on view
date made
1879
patent date
1879-09-02
inventor
Crosby, George H.
place made
United States: Massachusetts
associated place
United States: Massachusetts, East Somerville
Physical Description
brass (overall material)
Measurements
overall: 12 7/8 in x 7 in; 32.7025 cm x 17.78 cm
ID Number
MC.308701
catalog number
308701
accession number
89797
patent number
219,149
See more items in
Work and Industry: Mechanical and Civil Engineering
Work
Industry & Manufacturing
Indicators
Bulletin 173
Engineering, Building, and Architecture
Data Source
National Museum of American History, Kenneth E. Behring Center

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