Allen Cut-Off Valve Gear, Patent Model

Allen, Horatio · ca 1857

This model was filed with the application to the U.S. Patent Office for Patent Number 18,837 issued to Horatio Allen on December 15, 1857. Allen’s design was for an improved cut-off valve gear for steam engines. Cut-off refers to the action of closing the intake valve which admits high pressure steam from the boiler into the cylinder of the engine. A cut-off valve gear is a series of cams, levers, gears and/or shafts that control the point in the stroke of the piston at which cut-off occurs . For steam engines that operate at varying loads and speeds it is important to the efficiency of the engine that cut-off be timed properly and be adjustable. When the engine is operating at low speeds and heavy loads steam is fed to the piston through a large part of its travel, and the cut-off closes relatively close to the time that the exhaust valve opens. High pressure steam is needed throughout that period due to the load. When the engine is operating at higher speeds and lesser loads, it is desirable to have cut-off happen much earlier in the stroke – perhaps at 20-25% of the total stroke. This increases the efficiency of the engine and conserves fuel by having more of the work be performed by the expansion of the steam within the cylinder pushing against the piston. Allen’s design provided a means of adjusting the point of cut-off while the engine was running. This was not new, and Allen based his work on an earlier patent of his and one by Samuel Gilman . The earlier patent designs relied on a combination of cams, levers and shafts to take motion from the push rods of the engine to control the motion of the intake valve. One element of the original Allen patent was a “loose toe” lever that pushed the steam valve stem upward to open it. The loose toe lever would then lock into place until the cut-off point was reached at which time it would be unlocked allowing the valve stem to fall back into place. Timing adjustment was effected by changing the length of a second lever arm so as to close the valve earlier or later. Allen claimed as new for this patent the addition of a piston within a chamber of water or other fluid. The chamber was closed and provided with appropriate valves so that it would provide a damping resistance to the motion of the rod attached to the piston. This rod was connected to the loose toe lever that allowed the intake valve to close after cut-off, thus controlling its fall rather than having the intake valve shut abruptly. Mr. Allen was an engineer of significant accomplishment working on locomotives, steamships, bridges and tunnels. He became President of the New York Novelty Iron Works in 1842 , and his early patent design for valve gear was used in the engine built by that company for the steamship Adriatic. Novelty Iron Works continued to make engines for many steamships through the Civil War years.