A rare and very old steam engine
(Page 2 of 3)
February 2009
John Magnuson
The slide valve on the engine pictured is similar to the slide valve used on double acting crosshead-type engines. The valve stem is actuated by a shoe attached to its end. This shoe is captivated by a slot in the actuator in which it slides up and down. The shoe is always in contact with the inner surfaces of the actuator.
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The actuator is attached to the frame of the engine and can have its angle changed with a lever. The three-position lever has a spring-loaded pawl that locks into notches. Pulling up on the knob releases it from the notch it is in so another position may be selected. The end positions cause the engine to rotate forward and backward. The center position parks the engine.
It’s very important to keep the shoe, the actuator, and the valve stem well lubricated. Failure to do this may cause the valve stem to bend. The shoe moves up and down in the slot of the actuator because the steam chest, with its slide valve, is part of the cylinder and oscillates with it as the crankshaft turns.
The cylinder oscillates on two 1/2-inch brass pipes that are free to rotate in bearings on each side of the frame. The pipes are connected to the steam chest. One of them brings steam to the chest and the other takes the exhaust steam from the chest.
The cylinder is insulated with shaped mahogany battens that completely surround it. These battens are protected by the copper cover visible in the pictures. Insulating the cylinder of a steam engine keeps it hot, thereby reducing condensation.
The engine has a 1-5/8-inch bore with a 2-11/16-inch stroke. Its piston has one piston ring. The flywheel/pulley is 7-1/4 inches in diameter with a 1-1/2-inch face. The engine develops approximately 0.8 HP with an inlet pressure of 80 pounds per square inch. It weighs 24 pounds with a wood base, and it has a crankshaft diameter 0.872 inches.
Because the engine’s cylinder oscillates, it must have a steam-tight coupling to the steam line that will allow for this rotation. The inner part of the rotary coupling on the slide valve side of the cylinder is made out of a 1/2-inch brass pipe coupling. I turned down the outside of the coupling to a smooth finish and then drilled three steam holes in the center, each 120 degrees apart. One end of this part is screwed on to the brass pipe leading to the steam chest. The outer part of the coupling is turned out of aluminum to a diameter 0.060 inches greater than the brass inner part. Two internal grooves for O-rings are bored at each end. A stainless steel 1/4-inch tube fitting is threaded into the center of one side to allow steam to enter. The clearance between the outside of the brass part and the inside of the aluminum part allows steam to get to the three holes. The O-rings act as both steam seals and bearings for the brass part inside to rotate on. The near end of the sliding coupling is sealed with a brass cap.