LOCOMOTIVE VALVE-GEARS COMPENDIA


| November/December 1971



958 Sheridan Avenue Ypsilanti, Michigan 48197

Valve gears, their design, their operation and their relative efficiency have been an intriguing study and a serious problem that have eventuated into a wide variety of inventions. From the time of the building of the first locomotive, the method employed to actuate the valves that are required to supply and regulate steam from the boiler to the cylinders and discharge it after it had performed its work, were wildly experimental and mechanically crude.

About 1840 the double-eccentric hook motion had naturally evolved into a shifting-link action, under the name it has ever borne----The Stephenson Link Motion; and due to the ultra conservatism and a lack of a creative and a progressive incentive on the part of American locomotive builders, this ancient and somewhat misfit valve gear remained a vital part of all American locomotives even as late as several years of the early years of 1900.

However, to remain solvent and to be competitive in reference to new and accruing more modern methods of freight transportation, longer freight trains were essential as a matter of expediency and sound railway economics, which situation, in turn, required more powerful and larger locomotives for railway operation. In the more powerful locomotives, the increased diameter of the boiler barrel left no room underneath for the necessarily larger four eccentrics that would be required, including the rest of the Stephenson valve gear. This fact, in conjunction with the added inertia of the heavier required parts, make it positively impossible to continue the application of the Stephenson link-motion valve gear for any further use in large locomotive construction. Thus it was the force of circumstances that triumphed over the bullheaded and dogmatic conservatism of American locomotive builders and forced the discontinuation of further applications of the Stephenson link motion valve gears on American locomotives! Due to its many years of successful operation on European locomotives, its comparatively light weight, its outside convenience for inspection and repairs, its minimal width factor and its superior steam distribution qualities; these advantageous qualities in summation, determined the consideration and the adoption of the Walschaert valve gear as the only feasible application and practical solution for the pressing locomotive valve gear problem.

The year circa 1844 was a transitional period in the development of valve gears. At this time, Egide Walschaert was Master Mechanic for the Belgian State Railways. Egide Walschaert, it seems, was very much dissatisfied with the results obtained from the use of two eccentrics to govern the motion of one main valve. He visualized the possibilities, from the standpoint of economy, of further utilizing the expansive power of steam in the cylinders an accomplishment hitherto had been impossible with the hook motion. So with the incentive of attaining a greater operational economy as an objective, he invented the form of valve gear that bears his name.

It was in 1848 that Walschaert first applied his new valve gear to a locomotive on a trial basis, and the results turned out exceedingly favorable just as he had anticipated. His greatly improved valve gear attracted attention throughout the Continent, and in due time it came into general use on all the principal railways of Europe. The mechanism of the Walschaert valve gear is entirely different than the general run of valve gears in that the resultant motion of the valve is due to two independent component motions; one motion is produced by the eccentric-crank-arm which moves the valve to a position of no lap plus no lead, while the remainder of the valve movement, lap plus lead, is actuated by the crosshead.