The Walschaerts valve gear is a type of steam engine valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844. The gear is sometimes named without the final "s", since it was incorrectly patented under that name. It was extensively used in steam locomotives from the late 19th century until the end of the steam era.
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The Walschaerts valve gear was slow to gain popularity. The Stephenson valve gear remained the most popularly used valve gear on 19th-century locomotives. The Walschaerts gear had the advantage that it could be mounted entirely on the outside of the locomotives, leaving the space between the frames clear; this caused adoption first among some articulated locomotives.
In 1874, New Zealand Railways ordered two types of Double Fairlie locomotives from Avonside. Both the B class and E class Double Fairlies were fitted with Walschaerts valve gear. This was certainly the first use of this technology to be used in New Zealand, and is possibly the first time a British manufacturer has supplied it.
The Mason Bogie locomotive type was the first to use the Walschaerts gear in North America.
The first application in Britain was on a Single Fairlie 0-4-4T, exhibited in Paris in 1878 and purchased by the Swindon, Marlborough and Andover Railway in 1883. According to Ahrons,[1] the locomotive saw very little service as nobody seems to have known how to set the valves and this led to enormous coal consumption.
In the 20th century, the Walschaerts valve gear[2] was the most commonly used type, especially on larger locomotives. In Europe, its use was almost universal, whilst in North America, the Walschaerts gear outnumbered its closest competitor, the Baker valve gear, by a wide margin.
In Germany, the Walschaerts gear is often named the Heusinger valve gear after Edmund Heusinger von Waldegg, who invented the mechanism independently in 1849. Heusinger's gear was closer to the form generally adopted, but most authorities accept Walschaerts' invention as sufficiently close to the final form.
The Walschaerts valve gear is an improvement on the earlier Stephenson valve gear in that it enables the engineer to operate the steam engine in a continuous range of settings from maximum economy to maximum power. At any setting, the valve gear satisfies the following two conditions:
These functions are shown in the animated diagram below.
In an economical setting, steam is admitted to the expanding space for only part of the stroke; at a point set by the engineer, the intake is cut off. Since the exhaust is also shut, during the rest of the stroke the steam that has entered the cylinder expands in isolation, and so its pressure decreases. For maximum economy, the engineer carefully sets the cutoff point so that, when the exhaust valve opens, the steam is down to exactly atmospheric pressure. Thus, all the mechanical energy available from the steam (in the absence of a condenser) is used.
The Walschaerts valve gear enables the engineer to change the cutoff point without changing the points at which intake starts and at which exhaust starts. With the Stephenson valve gear, in contrast, if the engineer shortens the intake period, the openings of the intake and exhaust are delayed, occurring after a stroke has begun.
Economy also requires that the throttle is wide open, so that no energy is wasted pushing steam through a constriction, and that the boiler pressure is at the maximum safe level to maximize thermal efficiency. For economy, a steam engine is used of a size such that the most economical settings yield the right amount of power most of the time, such as when a train is running at steady speed on level track.
When greater power is necessary, e.g. when gaining speed when pulling out of a station and when ascending a gradient, the Walschaerts valve gear enables the engineer to set the cutoff point near the end of the stroke, so that the full pressure of the boiler is exerted on the piston for almost the entire stroke. With such a setting, when the exhaust opens, the steam in the cylinder is near full boiler pressure. The pressure in the steam at that moment serves no useful purpose; it is wasted driving a sudden pulse of pressure into the atmosphere, but this waste is more than compensated by maximized economy at other times.
This sudden pulse of pressure causes the loud “choo” sound that members of the public associate with steam engines, because they mostly encounter engines at stations, where efficiency is sacrificed as trains pull away. A steam engine well adjusted for efficiency makes a soft “hhHHhh” sound that lasts throughout the exhaust stroke, with the sounds from the two cylinders overlapping to produce a nearly constant sound.
The heart of the Walschaerts valve gear is the combination lever. Its lower end moves in phase with the piston rod; its upper end moves 90° out of phase with the piston rod. The valve stem thus moves in a combination of these two motions. Through the reach rod and the radius rod lifter, the engineer adjusts the position of the die block in the expansion link, and thus adjusts the amplitude of the motion of the upper end of the combination lever (including the possibility of reversal of that motion, which reverses the engine).
To make possible a period of cutoff for efficiency, the valve is designed so that both intake and exhaust are blocked when the valve rod is in a range centered at the center of its travel. This range is its lap.
When maximum power is sought, the exhaust opens almost immediately after the intake closes. Therefore, the valve must move rapidly through this middle range of cutoff during such operation. Since this rapid transition occurs when the piston passes through dead centre, the motion of the valve must be 90° out of phase with that of the piston rod. The engineer can satisfy these two conditions by moving the reach rod to an extreme position, so that the die block is near an end of the expansion link, where the motion is fastest and its amplitude is greatest. The upper end of the combination lever then dominates the motion of the valve stem; the valve is in the cutoff range only briefly, and its phase is near 90°.
For more efficient operation, the engineer moves the die block near the center of the expansion link. The amplitude of the valve stem’s motion is then lower, and its phase is shifted away from 90°. The Walschaerts valve gear, when correctly designed, provides constant valve lead irrespective of the chosen cutoff in both forward and reverse gear. With the piston in the front or rear dead centre positions, the expansion link should be vertical and moving the die block up and down should not alter the valve spindle position.
The amount of lead provided is determined by the proportions of the combination lever and the lap designed into the valve. It can only be changed by changing the proportions of these components.
The valve gear operation combines two motions; one is the lead motion (imparted at the bottom of the combination lever) and the other is the directional motion required in full gear (imparted at the top of the combination lever). To understand the operation, consider the cutoff in mid-gear. Because the die block does not move, the end of the radius rod is in effect a fixed point about which the combination lever pivots, so the valve motion is that caused directly by the cross head motion acting through the union link and combination lever. This moves the valve in each direction by a distance which should be set equal to lap plus lead of the valve, so that intake and exhaust both begin at the start of a stroke, i.e. at dead center.
If this were the only motion made by the valve then the engine would only start if the piston were close to dead centre, and even then the direction in which the engine moved would depend upon whether it was fractionally before or after the dead centre point.
Consider the case of outside admission valves such as slide valves. A second motion is superimposed on the crosshead motion to the valve; the return crank provides a motion which leads the crankpin motion by 90 degrees. By choice of position of the reach rod the die block may be moved from top to bottom which superimposes a varying portion of this motion ranging from either 90 degrees in lead ahead of the crank (full forward gear) through none in midgear to 90 degrees lagging the crank (full reverse gear). Lag and lead of these motions are always described relative to the engine moving forwards. The larger the magnitude of the superimposed component, the later the cutoff and thus the less expansion allowed for the steam in the cylinder.
For inside admission valves (most piston valves, but there are some exceptions on rebuilt locomotives) the lag and lead motions must be reversed usually by locating the eccentric crank pin 90 degrees after (following) the main crank pin (in forward motion) and the combination rod pivots on the radius rod above the linkage to the valve rods instead of below. This is the case shown in the diagrams above.
To lay out the Walschaerts gear, the proportions of the combination lever must be chosen. The total movement of the valve rod in mid gear should be symmetric between front and back dead centre. A displacement of the union link end by half the piston travel must cause a valve rod displacement of the valve lead plus the valve lap. The ratio of distance from union link end to pivot with radius rod to the distance between the valve rod end to the pivot with the radius rod should be in the same proportion as half piston travel to valve lap plus lead.
Next, the fact that in midgear the valve opening should be the same for the piston in both front and rear dead centre positions is used to find the mid gear position of the pivot between the combination rod and reach rod. In practice the valve spindle length is adjusted to ensure this as part of setting the timing. Also the dimensions of the lead and lap lever are set so that the valve moves twice the lead + lap of the valve.
With the reverser in mid gear position since there should be no superimposed return crank motion, the expansion link die slot should be an arc of a circle centred on the pivot in mid gear and of radius equal to the length of the radius rod. This condition precludes adjustment of the valve gear by altering the length of the radius rod.
The throw of the return crank must now be determined; The actual throw required from the crank is determined from the dimensions of the expansion link.
There have been many variants of Walschaerts valve gear, including:
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