STEPHENSON’S VALVE GEAR DISTORTIONS IN TIMING
To watch the flailing links and interactions of Stephenson’s gear belies its very simple concept. The provision of a simple eccentric for forward gear and the addition of an opposite one to serve reversal, joined by a curved link which can be raised to the influence of the backward eccentric or depressed to the influence of the forward eccentric requires little science. The two harmonic components are already embodied in the eccentric setting itself, and there is more for the designer to do in ensuring that the subsequent driveline preserves this simplicity than in chasing distortions.
There are two obvious distortions from angularity – first the main crank and secondly the eccentric. A third and more virulent distortion is introduced by offsetting the pins of a launch-type link behind the link slot. The first two problems are met by the simple means of offsetting the link trunnions such that the link tips to push the dieblock exactly where the valve requires it for equality of distribution, rather like varying the depth in gear in a controlled way throughout the cycle.
The virulence of the launch link offset should remind the designer only to offset the minimum amount to serve mechanical clearances. It is greater than the other two angularities put together. Choosing the correct link type for the driveline automatically creates the least offset of suspension necessary for correction. Choosing the wrong arrangement in ignorance compounds the issue and requires more offset in the trunnions – never to successfully remove the distortion completely. In correctly using the launch link for a direct drive to inside admission valves the angularities of crank and eccentric counter that of the link, leaving just a small amount to be rendered harmless by the suspension offset. Choosing to use a launch link in place of a locomotive link summates the problems quite differently, necessitating a large suspension offset which cannot cope efficiently.
Where the eccentrics face the cylinder with the crank on rear dead centre the two angularities oppose each other, though they are not equal, resulting in only a small correction necessary by expansion link trunnion offset. This is the case for locomotive links driving directly to outside admission valves or indirectly to inside admission valves via a 180 degree rocker.
The adoption of a launch link and its virulent error needs the summation of both the angularity of crank and eccentrics to counter its amount. Therefore both crank and eccentrics should face the cylinder concurrently, either driving directly to inside admission valves or indirectly to outside admission valves. It was almost taboo to adopt any other arrangements in locomotive design offices. Very occasionally the ‘wrong’ driveline has been chosen and the consequent extra link slip accepted for some reason best known to the designer, but in models the lack of proper design knowledge has resulted in far too many poor layouts. Caveat emptor.
The symmetry of the central suspension is vital to maintaining the immaculate results of which Stephenson’s gear is capable. The astute designer will accept nothing less. Normally, equal leads are only available at one cut off setting but this can be corrected throughout the range by the addition to eccentric rod lengths of a very small amount that ‘re-centres’ the symmetry of its operation. This disallows the jumping of rods to set the valves – proper means of adjustment should be in place. Quite why this correction to the leads was never carried out is obscure, since it is always rigidly and automatically applied to the eccentric rods in Walschaerts’ gear in order to preserve the principle of equal leads at the dead centres.
As mentioned elsewhere, end suspension cannot support symmetry between fore and back gears and destroys the gear’s inherently good qualities. It is perhaps unfortunate that most traction engines used end suspension, though often through a dedicated pin placement. The heavy flywheel necessary for a single cylinder operation has the addition work of smoothing out irregularities arising from poor event equality. The saving grace is that the short valve travels help to minimise angularities and to admit of smaller eccentrics.
In the marine environment the use of box links or bar links and end suspension was common. Although mollified by large propellers, acting like a heavy flywheel to dampen irregularities, the design situation is very different in requiring differing performance from each end of the cylinder to accommodate gravity. These engines worked for long periods of constant speed and the steam turbine effectively coped with high speed requirements. A comparison with the steam locomotive’s working conditions serves no useful purpose.