Demonstration Model of a Slide Valve for a Steam Engine

Description:

This is a demonstration model of a slide valve for a steam engine. By about 1820 such valves had evolved as the simplest devices for controlling the steam inlet and exhaust for a steam engine cylinder. This type of valve was operated by an eccentric that caused it to slide back and forth, alternately opening ports for inlet and exhaust at each end of the cylinder. Inlet steam was cut off when the inlet port closed at a fixed point in the stroke of the engine.

The speed of the engine was controlled by throttling the steam supply. No provision was made for adjusting the point of cut off while the engine was running. This feature was added in later evolutions of valve designs resulting in increased efficiency and the ability for automatic control of the speed of the engine. Several patent and demonstration models of such variable cut off valves are included in the collections.

The model was built to operate and show how the valve functioned. It shows a steam cylinder and valve case in cross section. The crankshaft of the engine is at the right of the model, and the eccentric is seen mounted on that shaft. The eccentric operates a crank arm which in turn operates the D valve. A steam pressure versus volume curve is shown below the cylinder. A label at the right of the model describes the functioning of the valve. A separate label on the top of the model case describes the limitations of the valve design.