The Walschaerts valve gear is a type of valve gear invented by Belgian railway mechanical engineer Egide Walschaerts in 1844 used to regulate the flow of steam to the pistons in steam engines. 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.
The Walschaerts valve gear was slow to gain popularity. The Stephenson valve gear remained the most commonly used valve gear on 19th-century locomotives. However, the Walschaerts valve gear had the advantage that it could be mounted entirely on the outside of the locomotives, leaving the space between the frames clear and allowing easy access for service and adjustment; which resulted in it being adopted in some articulated locomotives.
The first locomotive fitted with the Walschaerts valve gear was built at the Belgian Tubize workshops, and was awarded a gold medal at the 1873 Universal Exhibition in Vienna.
In 1874 New Zealand Railways ordered two NZR B class locomotives. They were Double Fairlie locomotives, supplied by Avonside; the first use in New Zealand of Walschaerts valve gear and probably the first time that a British manufacturer had supplied it. They were Cape gauge.
The Mason Bogie, a modified Fairlie locomotive of 1874, 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 March 1882.According to Ahrons, 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 gearwas 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 derived Baker valve gear, by a wide margin.
In Germany and some neighbouring countries, like Poland and Czechoslovakia, the Walschaerts gear is generally 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 driver 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:
In an economical setting, steam is admitted to the expanding space for only part of the stroke; at a point set by the driver, 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. Thus, the most energy available from the steam (in the absence of a condenser) is used.
The Walschaerts valve gear enables the engine driver to change the cutoff point without changing the points at which intake starts.
Economy also requires that the throttle be wide open and that the boiler pressure is at the maximum safe level to maximise 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 engine driver 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 drives a sudden pulse of pressure into the atmosphere and is wasted .
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 valve gear operation combines two motions; one is the primary lead motion which is imparted at the bottom of the combination lever (12). The secondary is the directional/amplitude motion which is imparted at the top. Consider that the driver has adjusted the reversing lever such that the die block is at mid-gear. In this position the secondary motion is eliminated and the piston valve travel is shortest, giving minimal injection and exhaust of steam. The travel of the piston valve is twice the total of lap plus lead.
Contrast this to when the die block is at the bottom of the expansion link (7), giving maximum steam injection and exhaust. This is the most powerful forward setting and is used in accelerating forward from rest. Conversely when the die block is at the top of the expansion link (7), maximal power in reverse is obtained. (On some engines the die block was in the top of the link in forward gear. This type was generally used on tank engines, which worked in forward and reverse equally.)
Once the locomotive has accelerated the driver can adjust the reverser toward the mid-gear position, decreasing cut-off to give a more economical use of steam. The engine's tractive effort is then less than it was at starting, but its power is greater.
The primary lead motion is provided by the crosshead arm (9) and the union link (11). This pivoting bar gives the in phase component of motion to the bottom of the combination lever (12).
The secondary directional/amplitude motion is derived from a mechanical linkage made up of several components.
The eccentric crank (UK: return crank) (1) is rigidly attached to the con-rod pin connected to the main drive wheel. Note that this is the only suitable attachment point on any of the drive wheels that is not fouled by the passage of the coupling rod or the connecting rod. The eccentric crank is of a length such that the pin attachment to the eccentric rod (2) is 90 degrees out of phase with the lead motion.
The eccentric rod provides motion to the expansion link (7) which is pivoted in a central location back to the body of the locomotive. The expansion link holds the radius bar (8), captive by a die block which is integral with the radius bar but is free to move vertically in a constrained curved path along the expansion link.
The vertical position of the radius bar is controlled in the cab by the driver adjusting the reverser which in turn controls the mechanical linkage; reach rod (3), lifting link (4), lifting arm (5) and reverse arm and shaft (6).
In this way the secondary, out of phase, driver controlled component of motion is imparted to the top of the combination lever (12) by the radius bar (8).
The combination lever combines these two motions with the resultant acting upon the valve stem (13), suitably restrained by the valve stem guide (10), which in turn acts upon the piston valve (14).
The Walschaerts gear can be linked to inside or outside admission valves. This article has only considered inside-admission piston valves until now, but outside-admission valves (slide valves and some piston valves) can use Walschaerts valve gear. If the valves have outside admission the radius bar connects to the combination lever below the valve stem rather than 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:
A crosshead is a mechanism used as part of the slider-crank linkages of long reciprocating engines and reciprocating compressors to eliminate sideways pressure on the piston. Also, the crosshead enables the connecting rod to freely move outside the cylinder. Because of the very small bore-to-stroke ratio on such engines, the connecting rod would hit the cylinder walls and block the engine from rotating if the piston was attached directly to the connecting rod like on trunk engines. Therefore, the longitudinal dimension of the crosshead must be matched to the stroke of the engine.
The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing gear. It is sometimes referred to as the "motion".
This is a glossary of the components found on typical steam locomotives.
Under the Whyte notation for the classification of steam locomotives, 4-12-2 represents the wheel arrangement of four leading wheels, twelve coupled driving wheels, and two trailing wheels. This arrangement was named the Union Pacific type, after the only railroad to use it, the Union Pacific Railroad.
The Gresley conjugated valve gear is a valve gear for steam locomotives designed by Sir Nigel Gresley, chief mechanical engineer of the LNER, assisted by Harold Holcroft. It enables a three-cylinder locomotive to operate with only the two sets of valve gear for the outside cylinders, and derives the valve motion for the inside cylinder from them by means of levers. The gear is sometimes known as the Gresley-Holcroft gear, acknowledging Holcroft's major contributions to its development.
After about 1910 Baker valve gear was the main competitor to Walschaerts valve gear for steam locomotives in the United States. Strictly speaking it was not a valve gear but a variable expansion mechanism adapted to the Walschaerts layout replacing the expansion link and sliding die block. The Baker arrangement used more pivot bearings or pin joints, but avoided the die slip inherent to the expansion link, with the aim of lessening wear and the need for service; it could also facilitate longer valve travel.
The Stephenson valve gear or Stephenson link or shifting link is a simple design of valve gear that was widely used throughout the world for various kinds of steam engines. It is named after Robert Stephenson but was invented by his employees.
Southern valve gear was briefly popular on steam locomotives in the United States. It combines elements of the Walschaerts and Baker patterns.
The Bulleid chain-driven valve gear is a type of steam locomotive valve gear designed by Oliver Bulleid during the Second World War for use on his Pacific (4-6-2) designs. It was peculiar to the Southern Railway in Britain, and borrowed from motor-vehicle practice in an attempt to create a compact and efficient design with a minimum of service requirements.
Piston valves are one form of valve used to control the flow of steam within a steam engine or locomotive. They control the admission of steam into the cylinders and its subsequent exhausting, enabling a locomotive to move under its own power. The valve consists of two piston heads on a common spindle moving inside a steam chest, which is essentially a mini-cylinder located either above or below the main cylinders of the locomotive.
The Kuhn slide is part of a modified Walschaerts (German:Heusinger) valve gear on steam locomotives and is named after its inventor, Michael Kuhn (1851–1903). The term is also used to refer to this particular type of Walschaerts valve gear system as a whole.
An expansion valve is a device in steam engine valve gear that improves engine efficiency. It operates by closing off the supply of steam early, before the piston has travelled through its full stroke. This cut-off allows the steam to then expand within the cylinder. This expanding steam is still sufficient to drive the piston, even though its pressure decreases as it expands. As less steam is supplied in the shorter time for which the valve is open, use of the expansion valve reduces the steam consumed and thus the fuel required. The engine may deliver two-thirds of the work, for only one-third of the steam.
A Johnson Bar is a control lever on a steam locomotive, used to control the timing of the admission of steam into the locomotive's cylinders. By controlling this timing, the amount of power delivered to the wheels is regulated, as is the direction that the wheels rotate, giving the lever the alternate name of the reversing lever. This is the term employed in British English, while the term 'Johnson Bar' is the norm in the United States.
An oscillating cylinder steam engine is a simple steam-engine design that requires no valve gear. Instead the cylinder rocks, or oscillates, as the crank moves the piston, pivoting in the mounting trunnion so that ports in the cylinder line up with ports in a fixed port face alternately to direct steam into or out of the cylinder.
On a steam locomotive, the reversing gear is used to control the direction of travel of the locomotive. It also adjusts the cutoff of the steam locomotive.
Gab valve gear was an early form of valve gear used on steam engines. Its simplest form allowed an engine to be stopped and started. A double form, mostly used on steam locomotives, allowed easy reversing.
Grasshopper beam engines are beam engines that are pivoted at one end, rather than in the centre.
The Central South African Railways Rack 4-6-4RT of 1905 was a South African steam locomotive from the pre-Union era in Transvaal Colony.
The Hackworth valve gear is a design of valve gear used to regulate the flow of steam to the pistons in steam engines. It is a radial gear, with an actuating lever driven from the crank. The drive may be taken directly from the crank or indirectly via a return crank. The other end of the actuating lever is attached to a die block which slides in a slotted link. When the link is vertical, the engine is in mid-gear. Forward, reverse and cut-off adjustments are made by moving the link away from the vertical. The valve rod is pivoted to a point on the actuating lever.
Valve gear opens and closes valves in the correct order. In rotating engines valve timings can be driven by eccentrics or cranks, but in non-rotative beam engines these options are not available. In the Cornish engine valves are driven either manually or through ‘plug rods’ and tappets driven from the beam. This permits the insertions of delays at various points in the cycle, allowing a Cornish Engine to vary from one stroke in ten minutes, to ten or more strokes in one minute, but also leads to some less familiar components when compared with rotative engines.
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