It is more desirable, in many cases, to use a single eccentric on the engine shaft, which can be done by pivoting the segment L, [Fig. 64], to a stationary support M, and connecting one end of the segment by a link N with the single eccentric O.
In this construction the valve rod P is shifted vertically by a rod Q, operated from the reversing lever, thus providing a changeable motion through one eccentric.
Balanced Slide Valves.—In the chapter pertaining to the steam engine, a simple form of slide valve was shown, and it was stated therein that the pressure of the steam bearing on the valve would quickly grind it down. To prevent this various types of balanced valves have been made, a sample of which is shown in [Fig. 64].
The valve chest A has in its bottom two ports C, D, leading to the opposite ends of the cylinder, and within is the sliding valve E, which moves beneath an adjustable plate F connected with the top or cover G of the valve chest.
This is also modified, as shown in [Fig. 66], in which case the slide valve H bears against the cover I at two points, so that as there is steam on the upper surface to a slightly greater area than on the lower side, there is sufficient downward pressure to hold it firmly on its seat, and at the same time not cause any undue grinding. This valve also has double exhaust ports J, J.
Balanced Throttle Valve.—[Fig. 67] will give a fair idea of the construction of throttle valves, the illustration showing its connection with a simple type of governor.