The Walschaerts valve gear is a type of
valve gear
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 g ...
used to regulate the flow of steam to the pistons in
steam locomotives
A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the locomot ...
, invented by
Belgian
Belgian may refer to:
* Something of, or related to, Belgium
* Belgians, people from Belgium or of Belgian descent
* Languages of Belgium, languages spoken in Belgium, such as Dutch, French, and German
*Ancient Belgian language, an extinct languag ...
railway
engineer
Engineers, as practitioners of engineering, are professionals who invent, design, analyze, build and test machines, complex systems, structures, gadgets and materials to fulfill functional objectives and requirements while considering the l ...
Egide Walschaerts
Egide Walschaerts (21 January 1820 – 18 February 1901) was a Belgian mechanical engineer, best known as the inventor of the Walschaerts valve gear used in steam locomotives. He was born in Mechelen, Belgium. In 1838 he was recognised as an ...
in 1844.
The gear is sometimes named without the final "s", since it was incorrectly
patent
A patent is a type of intellectual property that gives its owner the legal right to exclude others from making, using, or selling an invention for a limited period of time in exchange for publishing an enabling disclosure of the invention."A p ...
ed under that name. It was extensively used in steam locomotives from the late 19th century until the end of the steam era.
History
The Walschaerts valve gear was slow to gain popularity. The
Stephenson valve gear
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.
...
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 locomotive
An articulated locomotive is a steam locomotive (rarely, an electric locomotive) with one or more engine units that can move independent of the main frame. Articulation allows the operation of locomotives that would otherwise be too large to neg ...
s.
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
en, Viennese
, iso_code = AT-9
, registration_plate = W
, postal_code_type = Postal code
, postal_code =
, timezone = CET
, utc_offset = +1
, timezone_DST ...
.
In 1874
New Zealand Railways ordered two
NZR B class locomotives. They were
Double Fairlie
A Fairlie is a type of articulated steam locomotive that has the driving wheels on bogies. The locomotive may be double-ended (a double Fairlie) or single ended (a single Fairlie). Fairlies are most famously associated with the Ffestiniog ...
locomotives, supplied by
Avonside
Avonside is an eastern suburb in Christchurch, New Zealand. It is one of the oldest suburbs of the city, with only Heathcote being older.
History
The suburb was named after Holy Trinity Avonside, which was built beside the Avon River in 18 ...
; 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
A cape is a clothing accessory or a sleeveless outer garment which drapes the wearer's back, arms, and chest, and connects at the neck.
History
Capes were common in medieval Europe, especially when combined with a hood in the chaperon. Th ...
.
The
Mason Bogie
Mason Bogie locomotives (also known as Mason Fairlie locomotives) are a type of articulated steam locomotive suited for sharp curves and uneven track, once commonly used on narrow gauge railways in the United States of America. The design is a de ...
, a modified Fairlie locomotive of 1874, was the first to use the Walschaerts gear in
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
.
The first application in Britain was on a
Single Fairlie
A Fairlie is a type of articulated steam locomotive that has the driving wheels on bogies. The locomotive may be double-ended (a double Fairlie) or single ended (a single Fairlie). Fairlies are most famously associated with the Ffestiniog Rail ...
0-4-4T, exhibited in Paris in 1878 and purchased by the
Swindon, Marlborough and Andover Railway
The Midland and South Western Junction Railway (M&SWJR) was an independent railway built to form a north–south link between the Midland Railway and the London and South Western Railway in England, allowing the Midland and other companies' tr ...
in March 1882. According to
Ernest 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 gear was the most commonly used type, especially on larger locomotives. In
Europe
Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
, its use was almost universal, whilst in North America, the Walschaerts gear outnumbered its closest competitor, the derived
Baker valve gear After about 1910, the 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 repla ...
, by a wide margin.
In
Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
and some neighbouring countries, like Poland and Czechoslovakia, the Walschaerts gear is generally named the Heusinger valve gear after
Edmund Heusinger von Waldegg
Edmund Heusinger von Waldegg (12 May 1817 – 2 February 1886) was a German mechanical engineer and railway engineer.
Edmund Heusinger was born in Langenschwalbach (present day Bad Schwalbach) in the state of Hesse in central Germany on 12 May 1 ...
, 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.
Purpose
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:
* The valve opens to admit steam to the cylinder just before the start of a piston stroke. The pressure of this steam provides the driving force.
* Soon before the space on one side of the piston starts to contract, the valve starts to release steam from that space to the atmosphere, so as not to impede the movement of the piston.
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
In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc.
For a he ...
. 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 puffing 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 hissing sound that lasts throughout the exhaust stroke, with the sounds from the two cylinders overlapping to produce a nearly constant sound.
Motion
![Walschaertsgif](https://upload.wikimedia.org/wikipedia/commons/a/a8/Walschaertsgif.gif)
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
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 directi ...
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
A "piston valve" is a device used to control the motion of a fluid along a tube or pipe by means of the linear motion of a piston within a chamber or cylinder.
Examples of piston valves are:
* The valves used in many brass instruments
* The va ...
is twice the total of
lap
A lap is a surface (usually horizontal) created between the knee and hips of a biped when it is in a seated or lying down position. The lap of a parent or loved one is seen as a physically and psychologically comfortable place for a child to ...
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.
Technical details
![Walschaert static](https://upload.wikimedia.org/wikipedia/commons/5/55/Walschaert_static.png)
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
A coupling rod or side rod connects the driving wheels of a locomotive. Steam locomotives in particular usually have them, but some diesel and electric locomotives, especially older ones and shunters, also have them. The coupling rods transfer t ...
or the
connecting rod
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...
. 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).
Inside and outside admission valves
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.
Layout
To lay out Walschaerts gear, the proportions of the combination lever must be chosen. 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.
When the piston is at either dead centre movement of the radius rod should not move the valve rod. Therefore the expansion link die slot should be an arc of a circle of radius equal to the length of the radius rod.
The throw of the return crank must give the required oscillation of the expansion link.
Variants
There have been many variants of Walschaerts valve gear, including:
*
Baker valve gear After about 1910, the 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 repla ...
*
Baguley valve gear
The Baguley valve gear is a type of steam engine valve gear invented by Ernest E. Baguley, the Chief Draughtsman of the W.G. Bagnall company of locomotive manufacturers and patented in 1893. It was used by Bagnall during Baguley's time there, t ...
*
Bagnall–Price valve gear
*
J. T. Marshall valve gear
See also
*
Johnson Bar (locomotive)
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 directi ...
*
Steam locomotive nomenclature
__NOTOC__
This article is a glossary of the main components found on a typical steam locomotive.
The diagram, which is not to scale, is a composite of various designs in the late steam era. Some components shown are not the same, or are not pres ...
Notes
References
Bibliography
*
* See The Encyclopedia of Railroads, O. S. Nock, 1977 The name is Walschaert, changed by the family in 1830 when Belgium became independent of the Netherlands. It was patented under the name Walschaert because that is how the name was spelled in 1844.
*
External links
Valve Gear DemosCharles Dockstader's Valve Gear Demos
ValveGear ConstructorOnline program for designing valve gears semi-automatically
Video: simple explanation of Walschaerts valve gear(with
BR Standard Class 5 73096 as the visual aid)
Danbury Railway Museum - Danbury, CT
{{DEFAULTSORT:Walschaert Valve Gear
Locomotive valve gear