Stephenson Valve Gear - Applications

Applications

Abner Doble considered Stephenson valve gear: "(...) the most universally suitable valve gear of all, for it can be worked out for a long engine structure or a short one. It can be a very simple valve gear and still be very accurate, but its great advantage is that its accuracy is self-contained, for the exact relationship between its points of support (eccentrics on shaft, valve crosshead, and link hanger arm) have but little effect on the motion of the valve. Its use on engines in which all the cylinders lie in one plane, represents, in the belief of the writer, the best choice." Another benefit of the Stephenson gear, intrinsic to the system, is variable lead: usually zero in full gear and increasing as cutoff is shortened.

Stephenson valve gear is a convenient arrangement for any engine that needs to reverse and was widely applied to railway locomotives, traction engines, steam car engines and to stationary engines that needed to reverse, such as rolling-mill engines. It was used on the overwhelming majority of marine engines.

Details of the gear differ principally in the arrangement of the expansion link. In early locomotive practice, the eccentric rod ends were pivoted at the ends of the link while, in marine engines, the eccentric rod pivots were set behind the link slot (or below on a vertical engine). These became known respectively as the 'locomotive link' and the 'launch link'. The launch link superseded the locomotive type as it allows more direct linear drive to the piston rod in full gear and permits a longer valve travel within a given space by reducing the size of eccentric required for a given travel. Launch-type links were pretty well universal for American locomotives right from the 1850s but, in Europe, although occurring as early as 1846, they did not become widespread until around 1900. Larger marine engines generally used the bulkier and more expensive marine double-bar link, which has greater wearing surfaces and which improved valve events by minimising geometric compromises inherent in the launch link.