This model was submitted to the U.S. Patent Office with the application for the patent issued to Henry Otto and Patrick F. Bell, of Bloomington, Illinois, December 18, 1883, no. 290650.
The model represents a D-slide valve of ordinary shape, with most of the back cut away and formed in the shape of a short hollow cylinder. This cylinder is filled with a closed piston suspended on rollers on a flat bar, which, in turn, is suspended from the top of the valve chest. The bar passes through a tunnel in the piston and is of sufficient length to accommodate the valve travel. The effect of this construction is that the steam pressure ordinarily exerted on the back of a flat valve is in this case exerted on a piston that is not a part of the valve but is suspended independently.
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.
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