A rare and very old steam engine


John Magnuson in his shop with the engine.

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The designer of this steam engine must have reasoned that it was easier to have the cylinder oscillate than to incorporate a crosshead. Part of the thinking could have been that without a crosshead the forces applied by the piston rod to the crankshaft throw would always be at the optimum angle. By not having a crosshead, the engine could be quite a bit shorter. Maybe it had to fit in a small space?

Most other oscillating cylinder engines have a flat surface on one side of the cylinder with two steam holes in it. These holes are located on either side of a screw on which the cylinder pivots. The part that the flat surface of the cylinder matches up with is a stationary flat surface having four holes. The cylinder is usually spring-loaded to this surface by a compression spring placed on the pivot screw. This allows the cylinder to oscillate with a minimum steam loss.

When steam pressure is present at two of the holes in the stationary surface, the steam will enter the proper hole in the cylinder and cause the piston to move toward the other end of the cylinder. The spent steam on the other side of the piston will escape out of one of the exhaust holes in this surface to the atmosphere. Note that the engine will not start by itself if it stopped at top dead center or bottom dead center.

As the crank end of the piston rod travels around with the crank throw, the cylinder will oscillate which will cause the steam holes and the exhaust holes to line up at the proper times. When this is taking place, the piston rod will be alternately pushing and pulling on the crank throw causing the crankshaft to rotate. This type of engine is referred to as an oscillating-cylinder, double-acting engine.

The oscillating cylinder engine is pictured quite different. Instead of using a flat stationary surface with the cylinder spring loaded to as described above, it incorporates a steam chest in which there is a slide valve. This design has several advantages. It eliminates the need for the spring loading, which has an obvious disadvantage in that anything more than low steam pressure will tend to unseat the cylinder from the stationary flat surface. It also eliminates the shear force exerted on the screw the cylinder rotates on, which can cause structural problems.

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.

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.

The throttle is a knob-type high pressure stainless steel valve. The lever visible on the top of the cylinder is connected to two condensate drain valves on the bottom of each end of the cylinder. When the lever is moved toward the crankshaft-end of the cylinder both drain valves open.

When I got the engine, the crank end of the connecting rod had a 10-24 tapped hole in it, but no grease cup. The steamer now sports a new grease cup that I turned out of brass salvaged from a brass candlestick. The close fitting cover threads are lapped so there is no grease leakage. The other bearings are oiled through oil holes. Water soluble oil going in with the steam lubricates the piston.

At one time, the engine was owned by Northern States Power Company (now Xcel Energy) in Minneapolis, Minn., where I understand it powered something. Please contact me if you have information regarding this engine or what it may have powered.

Contact John Magnuson at 4640 Ensign Ave. N., New Hope, MN 55428 • (763) 533-5787 magnu@msn.com