Link Valve Gears
Developments after the separate expansion valve led to more sophisticated valve gears that could achieve the same goal of varying inlet lap with a single valve. The first of these were the link valve gears, particularly the Stephenson link valve gear. This uses a pair of eccentrics with a sliding link mechanism between them that adds as a mechanical adding device. Selecting intermediate positions provides a valve actuation with the effect of increasing cut-off. As such valve gears also provided, and were first developed for, reversing, they were widely used on locomotives. Theoretically, the precise effect is that of reduced valve travel, rather than an earlier cut-off. This has the effect of reducing overall valve opening, reducing initial steam supply and so having the effect of wire-drawing rather than pure expansion. Despite this, the Stephenson gear became one of the two most-widely used gears for locomotives.
Read more about this topic: Expansion Valve (steam Engine)
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