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The New Zealand Railways Magazine, Volume 2, Issue 2 (June 1, 1927)

[section] — Slide Valves.

Slide Valves.

A plan view is shown in Fig. 1 of the simple slide valve. Shown below is a section of the same valve with a portion of the steam chest port face, as if both valve and port face had been cut through along the central line as in the plain view at A1. The
Plate II

Plate II

slide valve (which is like a shallow pie-dish in shape), slides backwards and forwards over the three ports, steam being admitted into one cylinder as shown in Plate 1. The steam forces the piston from one end of the cylinder to the other, while steam can escape from the other side of the piston, back through the port the same way that it came down, over the port bar or bridge, then under the hollow portion of the valve (called the exhaust cavity) into the exhaust port, C, and out of the funnel.
Fig. 2 shows the opposite steam port open to live steam, and the other open to exhaust steam. The slide valve does exactly the same duty as the other valve shown in Fig. 1, but it has a hollow frame let into the back of it. This frame may be in the form of a hollow square or oblong, a circular ring or piston. The frame is held up to a bearing plate (as shown) which is cast with the steam
Fig. I—Plan View of Slide Valve

Fig. I—Plan View of Slide Valve

Section of Simple Slide Valve

Section of Simple Slide Valve

chest cover. In this case small spiral springs are shown holding the frame up to its bearing. There is also, in some cases, a flange or lip formed so that the steam can assist in holding up the frame to its bearing.

In the valve shown in Fig. 1, the whole of the back of the valve is exposed to the full steam pressure. This causes the valve to press very hard on to the steam chest port face, so that it takes a good deal of power to drive it. The pressure is relieved somewhat by the upward lift of the exhaust steam when passing under the valve, and, also, by the live steam in the port that the valve has just closed—but friction even there is very great.

Coming back to Fig. 2 it will be seen that page 32 with the frame in the back of the valve, the portion of the valve exposed to the steam pressure is small compared with the total area of the valve. Therefore, providing the frame is steam-tight, the valve is not so hard to move with steam on as with the ordinary kind of slide valve. When fitted with a frame as in Fig. 2, this type of valve is said to be balanced.

Fig 2

Fig 2

A is the valve, B the frame, C the springs, D the lip, or flange, of the frame, E the small hole or port through which any steam may escape into the exhaust port which may leak by the frame. This hole is usually about⅗ in in diameter.