Balanced Valves

A balanced valve is one in which means are employed to relieve the back of the valve of the steam pressure, and thus prevent its being forced to its seat with unnecessary pressure.

In some of the most successful balanced valves this is accomplished by providing a cover plate, which may be set up to exclude the steam from the back of the valve which works (a sliding fit) between the valve face and the face of the cover plate. Such a method of balancing is sufficiently effective for all practical purposes, if the following conditions are observed: The valve rod must be accurately guided so as to avoid side strains; the valve must fit accurately to its seat and to the cover plate, and the adjustment so made that the valve slides freely at first, being steam tight, and yet allowing room for lubrication to enter. When the travel of a valve, balanced by a cover plate, is varied to alter the point of cut off, the construction must be such that the ends of the valve at the shortest stroke pass over the ends of the seat and cover plate faces, or otherwise the middle of the seat and cover plate faces will wear hollow.

The Buckeye, Porter-Allen, and Straight-Line Engines are examples of practically balanced valves. The first of these has a balancing device that follows up the wear; the second has an adjustment whereby the cover plate may be set up to take up the wear; and in the third the wear is reduced to a minimum, by accurately fitting and guiding the parts.

The construction of the valve in the Straight-Line Engine is shown in Fig. 3304, in which b represents the cylinder bore; the valve v rests on a parallel strip n, and on its top rests the parallel strip m, the pressure relieving plate p is set up firmly against the pieces m n, whose thicknesses are such as to leave the valve a working fit between the faces of r r and of p.

Instead of the valve sliding on a flat face, it may work upon a shaft or spindle as a centre, its face moving in an arc of a circle, and its action will be the same as a flat valve having the same proportions. Fig. 3304a represents a valve v of this construction, whose shaft is at s, a being an arm fast on s, and driven by the eccentric rod r. To find the necessary amount of travel for such a valve, we draw lines, as f, g, from the inner edges of the steam ports, through the centre of the shaft s, and also draw an arc through the centre of the eye of arm a, and where lines f g cut the arc, as at d and e, are the extremes of motion of a.