D -slide Valve

  a) Steam lap or outside lap :- It is distance by which the outer edge of the D-slide valve overlaps the steam port.  Note : In actual practice, the displacement of the D-slide valve is greater than the steam lap by a distance known as lead of the valve.  b)  Exhaust lap or inside lap :- It is the distance by which the inner edge of the D-slide valve overlaps the steam port.  c)  Valve travel :- It is the distance moved by the valve from one end to the other end.  Note : The eccentricity  or throw of eccentric is equal to half of the valve travel.