Fig. 3297.
The construction of a common slide valve is shown in [Fig. 3297], in which the valve is shown in its mid-position. p p are the cylinder steam ports (as the openings through which the steam passes from the steam chest to the cylinder are termed), and at x is the cylinder exhaust port, through which the steam escapes from the cylinder. z is the valve exhaust port or exhaust cavity.
The lip of a valve is the width of its flange face, or the distance l, which is measured from the steam edge a to the exhaust cavity z. At the other end of the valve, h is the lip extending from the steam edge b to the exhaust cavity.
Steam lap is the distance the steam ends (or the steam edges as they are called) a, b overlap the steam ports, this distance being shown on the ends of the valve at a c. If the valve had no steam lap, its steam edges would just cover the ports, as denoted by the dimension w.
Exhaust lap is the amount the exhaust cavity z overlaps the bridges q q′, as at p, r.
Unequal steam lap is given to cause the point of cut off to occur at equal points in the piston stroke; thus in the figure there is more steam lap at the head end than at the crank end of the valve. But unequal lap could also be given in order to greatly vary the points of cut off for the two piston strokes, if such was desired.
Unequal exhaust lap may be given to equalize the point of release, or to equalize the points of compression.
The head end of the valve (or of the cylinder) is that which is furthest from the crank shaft, the other end, or that nearest to the crank shaft, being termed the crank end.