Indicator with Reducing Wheel

Description
This indicator, designed to meet the requirements of early 20th century high-speed engines, employs the lightest construction consistent with strength and accuracy. It is equipped with a reducing wheel, which is a self-contained device capable of reducing engine strokes of 14 to 72 inches to the proper stroke of the paper drum.
The cylinder of this indicator is supported so that its lower end is free and its longitudinal expansion or contraction is unimpeded. The annular space between the cylinder and the casing is designed to serve as a steam jacket. The piston is an extremely thin steel shell with shallow channels on its outer surface to provide steam packing and moisture lubrication. The piston rod is hollow and is connected to the pencil mechanism by means of a swivel head that can be screwed in or out of the rod to adjust the position of the diagram on the paper. The pencil mechanism is kinematically a pantograph that theoretically gives the pencil point a movement exactly parallel to the piston and the amount of the movement of the piston is multiplied six times at the pencil point. The design of the piston spring is peculiar to this indicator. It is made of a single piece of spring steel wire wound from the middle into a double coil, the ends of which are screwed into a metal head drilled helically to receive the spring.
The exact strength of spring is obtained by screwing the spring into the head more or less, when they are firmly fixed. The foot of the spring is a small steel bead firmly pinned to the straight portion of wire at the bottom of the spring. This takes the place of the heavier brass foot used in earlier indicators.*
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
Object Name
indicator, steam engine
Measurements
overall: 8 7/8 in x 6 1/2 in x 9 1/2 in; 22.5425 cm x 16.51 cm x 24.13 cm
place made
United States: Massachusetts
ID Number
MC*309833
catalog number
309833
accession number
109635
subject
Engineering, Building, and Architecture
Work
Industry & Manufacturing
Indicators
Bulletin 173
See more items in
Work and Industry: Mechanical and Civil Engineering
Indicators
Bulletin 173
Data Source
National Museum of American History, Kenneth E. Behring Center
Credit Line
Gift of Crosby Steam Gage & Valve Co., Boston, MA.
Related Publication
Frank A. Taylor. Catalog of the Mechanical Collections of the Division of Engineering United States National Museum, Bulletin 173

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