OUTSIDE LAP.
The change referred to, which so greatly enhanced the efficiency of the slide-valve, consisted in lengthening the valve-face, so that, when the valve stood in the center of the seat, the edges of the valve extended a certain distance over the induction ports, as in [Fig. 7]. This extension of the valve is called outside lap, or simply lap. The effect of lap is to close the steam-port before the piston reaches the end of the stroke, and the point at which the steam-port is closed is known as the point of cut-off. When the steam is cut off, and confined within the cylinder, it pushes the piston along by its expansive energy, doing work with heat that would be lost were the cylinder left in communication with the steam-chest till the end of the stroke.
Fig. 7.
When a slide-valve is actuated by an eccentric connected directly with the rocker-arm or valve-stem, the point of cut-off caused by the extent of lap, remains the same till a change is made on the valve, or on the throw of the eccentric, unless an independent cut-off valve be employed. Locomotives having the old hook motion worked under this disadvantage; because the hook could not vary the travel of the valve, which is the method usually resorted to for producing a variable cut-off. The link and other simple expansion gears perform their office of varying the cut-off in this way.