The Principles of the Joy and Walschaerts Motions
THE Stephenson, Allan, and Gooch link motions required four eccentrics on the driving axle, as explained in the chapter “Locomotive Valve Gears”. With inside-
Joy’s Valve Gear
In 1879 the late David Joy, who at one time was Locomotive Superintendent the old Oxford, Worcester and Wolverhampton Railway (known among the railwaymen as the “Old Worse and Worse”, and now part of the Great Western Railway), invented a valve gear which abolished eccentrics entirely, and left the crank axle free, so that substantial crank webs could be fitted, and a centre bearing also provided if necessary. This valve gear was especially adapted for locomotives with valves above or below the cylinders; it was tried on an old Bury type goods engine on the Furness Railway and proved, successful. In June 1880 the London and North Western Railway fitted it to a new 0-
Joy’s valve gear is very simple. A radius rod connects the valve spindle to a pair of die-
If the slide shaft is tilted forward, it is obvious that when the die-
THE PRINCIPLE OF JOY’S GEAR is illustrated in this diagram. Movement of the valve spindle is derived from the sliding of the die-
To reverse the gear, the lever is pulled back, and the guides now incline backwards, so that, when the die-
Two difficulties arose in the practical application of Joy’s idea, and he solved both problems in a simple manner. The first was the operation of the die-
As we have seen in the description of the Stephenson link motion, the valve must be advanced at the beginning of each stroke, to an amount sufficient to start opening the steam port when the crank is on dead centre. If the radius rod connecting the valve spindle to the die-
To obviate this, the vibrating link was extended above the die-
JOY’S VALVE GEAR, invented by the late David Joy in 1879, dispenses with eccentrics, the valve being operated by a dire-
This valve gear gave excellent results, proved very satisfactory in working and maintenance, and was for many years the standard gear on the London and North Western and Lancashire and Yorkshire Railways for main-
Walschaerts Valve Gear
Walschaerts valve gear is one of the most extensively used gears of modern times; yet it was invented a long time before it came into general use, for the idea was evolved as far back as 1844. The inventor of the gear was a young foreman mechanic named Egide Walschaerts, who was in charge of the locomotive shop of the Belgian State Railways, and was therefore a practical man with experience of other forms of valve gear. He had evidently been having trouble with the valve gears on the engines in his charge; and, not liking the combination of movements which operated the valves, he set to work and devised a gear in which each part did one job, and one only, as we shall see in the explanation of the working.
Walschaerts did not patent the gear himself, as the State Railways would not permit an employee to make personal profit out of an invention; but a friend made the application in his name, and the gear was patented in the year stated above. It was, however, some four years before Walschaerts applied his gear to one of his engines, and the result was most satisfactory.
LOCOMOTIVES with piston-
Improvements followed, the gear attracted the attention of other locomotive engineers, and in due course found a wide application on the continent of Europe. The ever-
THE ORIGINAL VALVE GEAR designed by the Belgian engineer, Egide Walschaerts, in 1844, employed an eccentric in conjunction with the crosshead to operate the valve. The T end of the eccentric rod carries two pins; the lower pin is shown engaged with its slot in the reversing link. On lifting the eccentric rod, so that the top pin engages with the upper slot in the link, the direction of the engine is reversed. The combination lever is pivoted on the pin of a crank attached to the reversing link shaft.
In 1890, Messrs. Beyer, Peacock & Co built some tank engines for Ireland and fitted them with Walschaerts gear; then came another lull. Meanwhile, the gear was going ahead on the Continent, had spread to America, and was being applied by private British locomotive-
Egide Walschaerts’ first design was really the Carmichael gear “turned inside out”, with a lap and lead movement added. The Carmichael gear is described in the chapter beginning on page 834. Similarly to the Carmichael gear, Walschaerts employed a single eccentric set at right angles to the crank; but instead of the gabs being attached to the eccentric rod, they were turned about, combined to form a sort of oval link, and attached to the equivalent of the upper arm of the Carmichael rocking lever, the lower arm being dispensed with altogether.
When the eccentric rod was driving the upper end of the link, the arm moved in unison with it; when driving the lower end, the arm moved in the opposite direction, and the engine was thus reversed. To obtain the lap and lead movement, the valve rod was not directly connected to the pin in the rocking arm as in the Carmichael gear, but to the upper end of a rod swinging on this pin. The lower end of the hanging rod, which was slotted, was oscillated by a connecting link from the crosshead. As in the Joy gear, the angularity of this rod, when the engine was on dead centre, advanced the valve far enough to start opening the steam port.
So far, so good; but the oval link allowed for only a limited range of expansive working. Walschaerts, therefore, set to work to improve it, and after some experimenting, evolved the gear as we know it to-
The principal improvement consisted in making the eccentric oscillate a link, which was altered to the Gooch curved-
It will now be seen that each part of the gear had one job allotted to it, doing just that one job, and no more. The eccentric actuated the link; the link moved the die-
WALSCHAERTS VALVE GEAR, with direct-
While the principle of Walschaerts gear remains as above on the locomotives of to-
In American and similar types of overseas engines having bar frames it is usual to fix a heavy yoke right across the frames, and make this support both guide bar brackets and expansion links. The links themselves are of various forms; there is the single open link as used on the Southern and other lines, which consists of a plain slotted link, with a small “wing” on either side, to carry the trunnion pins by which the link is hung. Then there is the three-
The box link is common in America, and it is used also on the Great Western and LMS Railways. It consists of two curved slabs of metal, resembling the slides in a Joy gear, the adjacent sides of which have grooves cut in them to accommodate the two die-
THREE CYLINDERS of a locomotive, built by Robert Stephenson and Co, for the Buenos Ayres and Pacific railway. Steam to the inside cylinders is controlled by the Walschaerts valve gear arranged on the Stephenson-
The radius rods are lifted and lowered by three principal methods; in two of them a weighbar or reverse shaft with lifting links is provided, exactly as described for the Stephenson link motion, the connexion of the lifting links to the radius rod being either in front of or behind the expansion link. The third and most satisfactory method is to run the weighbar or reverse shaft through two bearings in the frame, on the centre line of the link trunnions. The radius rods extend beyond the links, and are slotted; the arms of the weighshaft operate them direct by means of sliding blocks attached to the arms and working in the radius rod slots. The radius rods thus move in a straight line, and the small amount of die slip existing is equal at either end of the link.
To prevent undue wear on the valve spindle bushings, most of the valve spindles are provided with a small crosshead and guides, which take the weight of the combination lever, and prevent it from coming on the spindle. On many American and overseas locomotives, where the distance between piston-
Although the Walschaerts gear is best suited to cylinders having the valves on top, it is not difficult to apply it to cylinders having valves at the side. On the Southern Railway’s “Baltic” tanks, recently converted to 4-
An Ingenious Inside Drive
The gear is well designed, very neat, compact and efficient. The locomotives themselves had the largest cylinders of any passenger tank engines in Britain, and until the advent of electric traction for main-
On an inside-
Another ingenious inside drive has been applied by Messrs. Robert Stephenson & Co., Ltd., the builders whose experience dates from the “Rocket”, to some three-
It is, however, for outside cylinders that Walschaerts valve gear has been most generally adopted. On the railways of the United States, Canada, and other countries where vast distances have to be travelled far from repair shops, the facility with which adjustments and lubrication of the working parts can be effected has helped in popularizing the gear.
ON A GREAT WESTERN TANK LOCOMOTIVE used on the Vale of Rheidol line. Walschaerts valve gear is fitted. The radius rod is attached to the combination lever below the valve spindle crosshead, as the cylinder’s steam supply is controlled by slide valves with admission over their outside edges. (The above picture shows the application of the gear illustrated in the last diagram above.)
[From part 34, published 20 September 1935]