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.
Fig. 3304 a.
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.
PISTON VALVES.
Fig. 3305.
A piston valve acts the same as a flat or plain (D) valve, having the same amount of lap lead and travel. In [Fig. 3305] we have a cylinder with a flat valve on one side and a piston valve on the other, the head end ports being about to take steam, and it is seen that the eccentrics occupy the same positions for the two valves. The steam ports are, for the piston valve, annular grooves provided in the bore in which the valve fits. The piston valve is balanced because it receives its steam pressure on the ends, but it will not follow up its wear as the flat valve does, hence it is liable to leak.