{"history":[],"article":{"id":"92b98657-b9bc-4f96-991f-2cef957e18df","title":"Walschaerts valve gear","slug":"walschaerts-valve-gear","content":":::summary\n:::\n\n# \nThe **Walschaerts [valve gear](https://bliptext.com/articles/valve-gear)** is a type of [valve gear](https://bliptext.com/articles/valve-gear) used to regulate the flow of steam to the pistons in [steam locomotives](https://bliptext.com/articles/steam-locomotives), invented by [Belgian](https://bliptext.com/articles/belgium) railway [engineer](https://bliptext.com/articles/mechanical-engineering) [Egide Walschaerts](https://bliptext.com/articles/egide-walschaerts) in 1844.\nThe gear is sometimes named without the final \"s\", since it was incorrectly [patented](https://bliptext.com/articles/patent) under that name.\nIt was extensively used in steam locomotives from the late 19th century until the end of the steam era.\n# [A set of Walschaerts valve gear on 60163 Tornado. Note that the radius bar is set to reverse. | upload.wikimedia.org/wikipedia/commons/1/10/60163///Tornado///motion.jpg]\n\n# History\nThe Walschaerts valve gear was slow to gain popularity.\nThe [Stephenson valve gear](https://bliptext.com/articles/stephenson-valve-gear) remained the most commonly used valve gear on 19th-century locomotives.\nHowever, 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](https://bliptext.com/articles/locomotive-frame) clear and allowing easy access for service and adjustment, which resulted in it being adopted in some [articulated locomotives](https://bliptext.com/articles/articulated-locomotive).\nThe first locomotive fitted with the Walschaerts valve gear was built in 1849 at the Belgian Tubize workshops.\nEgide Walschaerts was awarded a gold medal, honorable mentions and a diploma at the 1878 [Universal Exhibition](https://bliptext.com/articles/exposition-universelle-1878) in [Paris](https://bliptext.com/articles/paris), and also in [Antwerp](https://bliptext.com/articles/antwerp) in 1883.\nIn 1874, [New Zealand Railways](https://bliptext.com/articles/new-zealand-railways-department) ordered two [NZR B class](https://bliptext.com/articles/nzr-b-class-1874) locomotives.\nThey were [Double Fairlie](https://bliptext.com/articles/fairlie-locomotive) locomotives, supplied by [Avonside](https://bliptext.com/articles/avonside-engine-company); the first use in New Zealand of Walschaerts valve gear and probably the first time that a British manufacturer had supplied it.\nThey were [Cape gauge](https://bliptext.com/articles/cape-gauge).\nThe [Mason Bogie](https://bliptext.com/articles/mason-bogie), a modified Fairlie locomotive of 1874, was the first to use the Walschaerts gear in [North America](https://bliptext.com/articles/north-america).\nThe first application in Britain was on a [Single Fairlie](https://bliptext.com/articles/single-fairlie) 0-4-4T, exhibited in Paris in 1878 and purchased by the [Swindon, Marlborough and Andover Railway](https://bliptext.com/articles/swindon-marlborough-and-andover-railway) in March 1882.\nAccording to [Ernest Ahrons](https://bliptext.com/articles/ernest-l-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.\nIn the 20th century, the Walschaerts valve gear was the most commonly used type, especially on larger locomotives.\nIn [Europe](https://bliptext.com/articles/europe), its use was almost universal, whilst in North America, the Walschaerts gear outnumbered its closest competitor, the derived [Baker valve gear](https://bliptext.com/articles/baker-valve-gear), by a wide margin.\nIn [Germany](https://bliptext.com/articles/germany) and some neighbouring countries, like Poland and Czechoslovakia, the Walschaerts gear is generally named the **Heusinger valve gear** after [Edmund Heusinger von Waldegg](https://bliptext.com/articles/edmund-heusinger-von-waldegg), who invented the mechanism independently in 1849.\nHeusinger's gear was closer to the form generally adopted, but most authorities accept Walschaerts' invention as sufficiently close to the final form.\n# [Swindon, Marlborough and Andover Railway Single Fairlie 0-4-4T of 1878. This was the first British locomotive to be fitted with Walschaerts valve gear | upload.wikimedia.org/wikipedia/commons/8/8b/SM%26AR///Single///Fairlie///1878.jpg]\n\n# Purpose\nThe 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.\nAt any setting, the valve gear satisfies the following two conditions:\nIn 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.\nSince 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.\nThus, the most energy available from the steam (in the absence of a [condenser](https://bliptext.com/articles/surface-condenser)) is used.\nThe Walschaerts valve gear enables the engine driver to change the cutoff point without changing the points at which intake starts.\nEconomy also requires that the throttle be wide open and that the boiler pressure is at the maximum safe level to maximise [thermal efficiency](https://bliptext.com/articles/thermal-efficiency).\nFor 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.\nWhen 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.\nWith such a setting, when the exhaust opens, the steam in the cylinder is near full boiler pressure.\nThe pressure in the steam at that moment serves no useful purpose; it drives a sudden pulse of pressure into the atmosphere and is wasted.\nThis 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.\nA 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.\n\n# Motion\nThe valve gear operation combines two motions; one is the primary lead motion which is imparted at the bottom of the combination lever (12).\nThe secondary is the directional/amplitude motion which is imparted at the top.\nConsider that the driver has adjusted the [reversing lever](https://bliptext.com/articles/reversing-lever) such that the die block is at mid-gear.\nIn this position the secondary motion is eliminated and the piston valve travel is shortest, giving minimal injection and exhaust of steam.\nThe travel of the [piston valve](https://bliptext.com/articles/piston-valve-steam-engine) is twice the total of [lap](https://bliptext.com/articles/piston-valve-steam-engine) plus lead.\nContrast this to when the die block is at the bottom of the expansion link (7), giving maximum steam injection and exhaust.\nThis is the most powerful forward setting and is used in accelerating forward from rest.\nConversely when the die block is at the top of the expansion link (7), maximal power in reverse is obtained.\n(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. )\nOnce 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.\nThe engine's tractive effort is then less than it was at starting, but its power is greater.\n\n# Technical details\n[[File:Walschaert static.png|frame|center|The key components of Walschaerts Valve Gear: •

Eccentric crank (UK: return crank)
Eccentric rod
Reach rod
Lifting link
Lifting arm
Reverse arm
Expansion link
Radius bar
Crosshead arm (UK Drop link)
Valve stem guide
Union link
Combination lever
Valve stem
Piston valve

\n]]\nThe primary lead motion is provided by the crosshead arm (9) and the union link (11).\nThis pivoting bar gives the in phase component of motion to the bottom of the combination lever (12).\nThe secondary directional/amplitude motion is derived from a mechanical linkage made up of several components.\nThe eccentric crank (UK: return crank) (1) is rigidly attached to the con-rod pin connected to the main drive wheel.\nNote 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](https://bliptext.com/articles/coupling-rod) or the [connecting rod](https://bliptext.com/articles/connecting-rod).\nThe 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.\nThe eccentric rod provides motion to the expansion link (7) which is pivoted in a central location back to the body of the locomotive.\nThe 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.\nThe 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).\nIn 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).\nThe 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).\n\n## Inside and outside admission valves\nThe Walschaerts gear can be linked to inside or outside admission valves.\nThis 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.\nIf the valves have outside admission the radius bar connects to the combination lever below the valve stem rather than above and the orientation of the eccentric crank is changed so that it leads, rather than trails, the main crank pin.\n\n## Layout\nTo lay out Walschaerts gear, the proportions of the combination lever must be chosen.\nA 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.\nThe 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.\nWhen the piston is at either dead centre movement of the radius rod should not move the valve rod.\nTherefore the expansion link die slot should be an arc of a circle of radius equal to the length of the radius rod.\nThe throw of the return crank must give the required oscillation of the expansion link.\n# [The Walschaerts valve gear on a Pennsylvania Railroad E6s | upload.wikimedia.org/wikipedia/commons/0/0d/Walschaerts///valve///gear.jpg]\n\n# Variants\nThere have been many variants of Walschaerts valve gear, including:","current_revision":null,"created_by":"kACnuQmH5k9Nr5VNVEpVeg7XpvUmMctD","created_at":"2025-01-25T21:17:21.357Z","updated_at":"2025-01-25T21:17:21.357Z","search_vector":"'gear':3 'valv':2 'walschaert':1","search_content":" \n \n\n# \nThe **Walschaerts [valve gear](https://bliptext.com/articles/valve-gear)** is a type of [valve gear](https://bliptext.com/articles/valve-gear) used to regulate the flow of steam to the pistons in [steam locomotives](https://bliptext.com/articles/steam-locomotives), invented by [Belgian](https://bliptext.com/articles/belgium) railway [engineer](https://bliptext.com/articles/mechanical-engineering) [Egide Walschaerts](https://bliptext.com/articles/egide-walschaerts) in 1844.\nThe gear is sometimes named without the final \"s\", since it was incorrectly [patented](https://bliptext.com/articles/patent) under that name.\nIt was extensively used in steam locomotives from the late 19th century until the end of the steam era.\n# [A set of Walschaerts valve gear on 60163 Tornado. Note that the radius bar is set to reverse. | upload.wikimedia.org/wikipedia/commons/1/10/60163///Tornado///motion.jpg]\n\n# History\nThe Walschaerts valve gear was slow to gain popularity.\nThe [Stephenson valve gear](https://bliptext.com/articles/stephenson-valve-gear) remained the most commonly used valve gear on 19th-century locomotives.\nHowever, 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](https://bliptext.com/articles/locomotive-frame) clear and allowing easy access for service and adjustment, which resulted in it being adopted in some [articulated locomotives](https://bliptext.com/articles/articulated-locomotive).\nThe first locomotive fitted with the Walschaerts valve gear was built in 1849 at the Belgian Tubize workshops.\nEgide Walschaerts was awarded a gold medal, honorable mentions and a diploma at the 1878 [Universal Exhibition](https://bliptext.com/articles/exposition-universelle-1878) in [Paris](https://bliptext.com/articles/paris), and also in [Antwerp](https://bliptext.com/articles/antwerp) in 1883.\nIn 1874, [New Zealand Railways](https://bliptext.com/articles/new-zealand-railways-department) ordered two [NZR B class](https://bliptext.com/articles/nzr-b-class-1874) locomotives.\nThey were [Double Fairlie](https://bliptext.com/articles/fairlie-locomotive) locomotives, supplied by [Avonside](https://bliptext.com/articles/avonside-engine-company); the first use in New Zealand of Walschaerts valve gear and probably the first time that a British manufacturer had supplied it.\nThey were [Cape gauge](https://bliptext.com/articles/cape-gauge).\nThe [Mason Bogie](https://bliptext.com/articles/mason-bogie), a modified Fairlie locomotive of 1874, was the first to use the Walschaerts gear in [North America](https://bliptext.com/articles/north-america).\nThe first application in Britain was on a [Single Fairlie](https://bliptext.com/articles/single-fairlie) 0-4-4T, exhibited in Paris in 1878 and purchased by the [Swindon, Marlborough and Andover Railway](https://bliptext.com/articles/swindon-marlborough-and-andover-railway) in March 1882.\nAccording to [Ernest Ahrons](https://bliptext.com/articles/ernest-l-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.\nIn the 20th century, the Walschaerts valve gear was the most commonly used type, especially on larger locomotives.\nIn [Europe](https://bliptext.com/articles/europe), its use was almost universal, whilst in North America, the Walschaerts gear outnumbered its closest competitor, the derived [Baker valve gear](https://bliptext.com/articles/baker-valve-gear), by a wide margin.\nIn [Germany](https://bliptext.com/articles/germany) and some neighbouring countries, like Poland and Czechoslovakia, the Walschaerts gear is generally named the **Heusinger valve gear** after [Edmund Heusinger von Waldegg](https://bliptext.com/articles/edmund-heusinger-von-waldegg), who invented the mechanism independently in 1849.\nHeusinger's gear was closer to the form generally adopted, but most authorities accept Walschaerts' invention as sufficiently close to the final form.\n# [Swindon, Marlborough and Andover Railway Single Fairlie 0-4-4T of 1878. This was the first British locomotive to be fitted with Walschaerts valve gear | upload.wikimedia.org/wikipedia/commons/8/8b/SM%26AR///Single///Fairlie///1878.jpg]\n\n# Purpose\nThe 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.\nAt any setting, the valve gear satisfies the following two conditions:\nIn 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.\nSince 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.\nThus, the most energy available from the steam (in the absence of a [condenser](https://bliptext.com/articles/surface-condenser)) is used.\nThe Walschaerts valve gear enables the engine driver to change the cutoff point without changing the points at which intake starts.\nEconomy also requires that the throttle be wide open and that the boiler pressure is at the maximum safe level to maximise [thermal efficiency](https://bliptext.com/articles/thermal-efficiency).\nFor 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.\nWhen 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.\nWith such a setting, when the exhaust opens, the steam in the cylinder is near full boiler pressure.\nThe pressure in the steam at that moment serves no useful purpose; it drives a sudden pulse of pressure into the atmosphere and is wasted.\nThis 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.\nA 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.\n\n# Motion\nThe valve gear operation combines two motions; one is the primary lead motion which is imparted at the bottom of the combination lever (12).\nThe secondary is the directional/amplitude motion which is imparted at the top.\nConsider that the driver has adjusted the [reversing lever](https://bliptext.com/articles/reversing-lever) such that the die block is at mid-gear.\nIn this position the secondary motion is eliminated and the piston valve travel is shortest, giving minimal injection and exhaust of steam.\nThe travel of the [piston valve](https://bliptext.com/articles/piston-valve-steam-engine) is twice the total of [lap](https://bliptext.com/articles/piston-valve-steam-engine) plus lead.\nContrast this to when the die block is at the bottom of the expansion link (7), giving maximum steam injection and exhaust.\nThis is the most powerful forward setting and is used in accelerating forward from rest.\nConversely when the die block is at the top of the expansion link (7), maximal power in reverse is obtained.\n(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. )\nOnce 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.\nThe engine's tractive effort is then less than it was at starting, but its power is greater.\n\n# Technical details\n[[File:Walschaert static.png|frame|center|The key components of Walschaerts Valve Gear: • \n• Eccentric crank (UK: return crank) \n• Eccentric rod \n• Reach rod \n• Lifting link \n• Lifting arm \n• Reverse arm \n• Expansion link \n• Radius bar \n• Crosshead arm (UK Drop link) \n• Valve stem guide \n• Union link \n• Combination lever \n• Valve stem \n• Piston valve \n• \n]]\nThe primary lead motion is provided by the crosshead arm (9) and the union link (11).\nThis pivoting bar gives the in phase component of motion to the bottom of the combination lever (12).\nThe secondary directional/amplitude motion is derived from a mechanical linkage made up of several components.\nThe eccentric crank (UK: return crank) (1) is rigidly attached to the con-rod pin connected to the main drive wheel.\nNote 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](https://bliptext.com/articles/coupling-rod) or the [connecting rod](https://bliptext.com/articles/connecting-rod).\nThe 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.\nThe eccentric rod provides motion to the expansion link (7) which is pivoted in a central location back to the body of the locomotive.\nThe 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.\nThe 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).\nIn 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).\nThe 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).\n\n## Inside and outside admission valves\nThe Walschaerts gear can be linked to inside or outside admission valves.\nThis 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.\nIf the valves have outside admission the radius bar connects to the combination lever below the valve stem rather than above and the orientation of the eccentric crank is changed so that it leads, rather than trails, the main crank pin.\n\n## Layout\nTo lay out Walschaerts gear, the proportions of the combination lever must be chosen.\nA 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.\nThe 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.\nWhen the piston is at either dead centre movement of the radius rod should not move the valve rod.\nTherefore the expansion link die slot should be an arc of a circle of radius equal to the length of the radius rod.\nThe throw of the return crank must give the required oscillation of the expansion link.\n# [The Walschaerts valve gear on a Pennsylvania Railroad E6s | upload.wikimedia.org/wikipedia/commons/0/0d/Walschaerts///valve///gear.jpg]\n\n# Variants\nThere have been many variants of Walschaerts valve gear, including:","revisionCount":0},"hasMore":false,"nextCursor":null}