The D Slide Valve

The various events which are governed by the D slide valve of a steam engine are as follows:

The live steam period is that during which the steam is admitted from the steam chest into the cylinder and the steam admitted during this period is termed live steam.

The point of cut off is that at which the valve closes the steam port, and the admission of steam into the cylinder is stopped, hence the point of cut off is at the end of the live steam period.

The period of expansion is that during which the steam is allowed to expand in the cylinder, and therefore begins at the point of cut off, and ends at the point of release.

The point of release is that at which the valve opens the port and permits the steam to escape.

The point of compression is that at which the exhaust port is closed, which occurs before the piston has reached the end of its stroke; the steam that has not passed out of the cylinder is therefore compressed, the compression continuing until the valve opens for the lead.

The lead of the valve is the amount the port is open to the live steam when the crank is on the dead centre.

The point of admission is that at which the port opens for the live steam to enter, and it follows that the lead and compression both act as a cushion, arresting the motion of the piston when it reaches the end of the stroke.

Cushioning begins, however, at the time the exhaust port is closed enough to arrest the escape of the steam, while compression begins when the valve has closed the exhaust port.

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.