A rare and very old steam engine

| February 2009


John Magnuson in his shop with the engine.

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.