A. One in which the steam pressure on the top and bottom of the valve is nearly equalized.  This is done by protecting a portion of the top of the valve from the steam pressure.  It is usually balanced by strips held against the pressure or balance plate by one or more springs.  This is done to prevent live steam from getting on top of valve and thus relieve the valve from the top pressure which would cause excessive friction between the bottom of the valve and its seat.  The hole through the top is to allow any steam which might leak by the strips to pass into the exhaust, so pressure could not accumulate on the top of the valve, also to equalize the exhaust pressure between the top of the valve and exhaust cavity as well as to assist in lubricating the balance plate.

28.  Q. What is meant by inside and outside admission valves?

A. With an inside admission valve (usually a piston valve), the live steam comes between the piston valve heads, the outside end of the heads being connected with and exposed to exhaust pressure, it admits steam past the inside edges of the valves.  An outside admission valve has the space between the ends connected to the exhaust and a space at the ends connected with the live steam.  It admits steam past its outside edges.  A piston valve can be either inside or outside admission, while a slide valve is always outside admission.

29.  Q. What is the relative motion of the main piston and the steam valves for inside admission, and, on the other hand, for outside admission?

A. If the piston is in the front end of the cylinder, an inside admission valve must move forward in order to connect the inside of the valve with the front live steam port to admit steam against the piston.  The outside end of the valve opens the exhaust port for the back end of the cylinder.  In the same position of the piston an outside admission valve must move backwards to open the steam port or in the same direction as the steam piston when commencing its stroke.

30.  Q. What is an Allen ported valve, and what is its object?

A. An Allen ported valve is an outside admission slide valve having an extra port from one end of the valve to the other, above the exhaust cavity and through the body of the valve.  This extra port is calculated to admit steam through the valve at the same time that steam passes by the end of the valve into the same steam port, thus doubling the area of opening for live steam when the port is first opened.

31.  Q. What is the difference in the valve motion for outside admission valves and for inside admission valves?

A. An outside admission valve must be moved in the opposite direction to an inside admission valve in relation to the movement of the steam piston when beginning its stroke; therefore either the position of the eccentric or the position of the rocker arms in relation to the rocker shaft must be opposite for a change in these valves.