When the piston is at the beginning of its stroke the steam port is generally open by a small amount called the lead of the valve.
The reciprocating motion of the slide valve is nearly always derived from an eccentric fixed on the crank-shaft of the engine. Slide valves are generally made of brass, bronze, or cast iron.
Exercise 59: Simple Slide Valve.—At (d), fig. 58, is shown a sectional elevation of a simple slide valve for a steam-engine, the section being taken through the centre line of the valve spindle, while at (e) is shown a cross section and elevation, and at (f) a plan of the same. Draw all these views full size, and also a sectional elevation at A B. The valve is made of brass, and the valve spindle and nuts of wrought iron.
Exercise 60: Slide Valve Casing, &c., for Steam-engine.—Draw, half size, the views shown at (a), (b), and (c), fig. 58; also a sectional plan at L M. (b) is an elevation of the valve casing with the cover and the valve removed. (a) is a sectional elevation, the section being taken through the axes of the steam cylinder and valve spindle. (c) is a sectional plan, the section being a horizontal one through the centre of the exhaust port. The inlet and outlet for the steam are clearly shown in the sectional plan: in the sectional elevation their positions are shown by dotted circles.
The stroke of the piston is in this case 12 inches, so that from the dimensions given at (a) it must come within a quarter of an inch of each end of the cylinder; this is called the cylinder clearance.
The piston has three Ramsbottom rings, a quarter of an inch wide and a quarter of an inch apart.
The steam cylinder and valve casing are made of cast iron.
Cocks.—A cock consists of a slightly conical plug which fits into a corresponding casing cast on a pipe. Through the plug is a hole which may be made by turning the plug to form a continuation of the hole in the pipe, and thus allow the fluid to pass, or it may be turned round so that the solid part of the plug lies across the hole in the pipe, and thus prevent the fluid from passing. As the student will be quite familiar with the common water cock or tap such as is used in dwelling-houses we need not illustrate it here.
Fig. 59.