Slide valve
The slide valve is a rectilinear valve used to control the admission of steam into and emission of exhaust from the cylinder of a steam engine.
This valve worked by "connecting the upper and lower valves so as to be worked by one rod or spindle, and in making the stem or tube which connects them hollow, so as to serve for an induction pipe to the upper end of the cylinder."
Where the cylinders are horizontal, as in a steam locomotive, the valves would be side-by-side.
The balanced slide valve was invented by the Scottish engineer Alexander Allan.
It was not much used in the UK but, at one time, had great popularity in the United States.
Steam enters via the steam port SP , and is admitted by the slide valve SV through the upper passage S to push down the piston P . At the same time, exhaust steam from below the piston passes back up the lower passage S , via the valve cavity, to exhaust E . As the piston descends, the valve moves upwards to admit steam below the piston and release exhaust from above.
