The Slide Valve
There were probably more steam engines built which used the simple slide valve to admit and distribute steam than any other type of valve. They were relatively simple to set up, easy to maintain and lasted for a long period with minimal maintenance (Assuming they were adequately lubricated), on the downside they were not the most efficient or economic method of distributing steam and could absorb a large amount of the engines power to drive, they were generally paired with throttle governors which again were not the most efficient, however the slide valve could cope with a wide range of power and speed demands where some of the more sophisticated valve mechanism were better suited to driving at a particular speed or load.
The sectional view above shows the main elements of the simple slide valve, although there were a number of variants in shape, proportion and style.
Steam from the boiler (controlled by a throttle valve and maybe a governor valve) enters down the vertical pipe into the valve chest C, the valve chest cover H is fitted over the top and bolted down to form a steam tight box. The slide valve V which is like a hollow inverted box with a flat face at the bottom contacts and is able to slide left and right over the valve port face S1 - S2 without steam leaking past the port face. B is a yoke which connects the valve with the eccentric rod D. D exits the valve chest through a steam tight gland which allows the rod to slide but does not allow steam to escape.
Glands would generally be made of several rings of graphite impregnated asbestos 'rope' clamped into the valve gland pocket from the outside, however metallic packing was used in some applications. The piston rod gland would also be made in the same way.
A reciprocating engine works by steam pushing the piston backwards and forwards along the cylinder driving the crankshaft via a piston rod, connecting rod and crank. On the crankshaft an eccentric is fitted, this is as the name suggests an disk mounted eccentrically around which a strap is fitted which allows the disk to rotate, the eccentricity is transmitted to the strap and by linking this by the eccentric rod, this displacement is transferred to the slide valve. The degree of eccentricity (the difference between the centre of the disk and the centre of the crankshaft) is doubled and this amount is the distance the valve can be moved, it is generally known as the 'throw'.
The angle between the crank and eccentric can be taken as being 90 degrees for the purposes of this description and the relative motion is shown in the diagram below.
Valve V moves to the right, uncovering port S1 which allows steam into the left hand end of the cylinder, this pushes the piston down, while this is happening valve V connects the port S2 and the exhaust port E together (while isolating them from the steam in the valve chest) as the piston moves down the cylinder the exhaust steam exits by this route and out through E1 the exhaust pipe.
At a point during the stroke (known as the point of 'cut off' the valve starts to return to the right and port S1 is covered again, no more steam enters (it is 'cut off') and the steam in the cylinder continues to expand until the piston is at the end of its stroke. Now the valve will begin to uncover port S2 and allow steam into the other end of the cylinder (and connect S1 to the exhaust port E) and the cycle repeats from the opposite end.
This diagram shows the relative movements of valve and piston for one revolution of an engine.
This is the inside of a valvechest, showing the valve port face and perched above (and reversed so it can be seen) the actual slide valve itself.
The Piston valve operates in much the same way but instead of the valve being a box it is a cylindrical 'bobbin' shaped valve working in a cylinder with ports arranged radially.
The Expansion Slide valve is a more efficient version of the slide valve.
See the Valvegear page for moving graphics.