The valve gear of a steam engine

Steam engine

A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...

 is the mechanism that operates the inlet and exhaust valve

Valve

A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...

s to admit steam

Steam

Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...

 into the cylinder

Cylinder (engine)

A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...

 and allow exhaust steam to escape, respectively, at the correct points in the cycle. It is sometimes referred to as the "motion".
In the simple case, this can be a relatively simple task as in the internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 in which the valves always open and close at the same points. This is not the ideal arrangement for a steam engine, though, because greatest power is achieved by keeping the inlet valve open throughout the power stroke (thus having full boiler pressure, minus transmission losses, against the piston throughout the stroke) while peak efficiency is achieved by only having the inlet valve open for a short time and then letting the steam expand in the cylinder (expansive working).

The point at which steam stops being admitted to the cylinder is known as the cutoff

Cutoff (steam engine)

In a steam engine, cutoff is the point in the piston stroke at which the inlet valve is closed. On a steam locomotive, the cutoff is controlled by the reverser....

and the optimal position for this varies depending on the work being done and the tradeoff desired between power and efficiency. Steam engines are fitted with regulators (throttle

Throttle

A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

s in US parlance) to vary the restriction on steam flow, but controlling the power via the cutoff setting is generally preferable since it makes for more efficient use of boiler steam.

A further benefit may be obtained by admitting the steam to the cylinder slightly before front or back dead centre. This advanced admission (also known as lead steam) assists in cushioning the inertia of the motion at high speed.

In the internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

, this task is performed by cam

Cam

A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...

s on a camshaft

Camshaft

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...

 driving poppet valve

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

s, but this arrangement is not commonly used with steam engines, partly because achieving variable engine timing using cams is complicated. Instead, a system of eccentrics

Eccentric (mechanism)

In mechanical engineering, an eccentric is a circular disk solidly fixed to a rotating axle with its centre offset from that of the axle ....

, cranks and levers is generally used to derive the motion of a D slide valve

D slide valve

The slide valve is a rectilinear valve used to control the admission of steam into, and emission of exhaust from, the cylinder of a steam engine.-Use:...

 or piston valve from the motion of the main crank/big end. Generally, two simple harmonic motion

Simple harmonic motion

Simple harmonic motion can serve as a mathematical model of a variety of motions, such as the oscillation of a spring. Additionally, other phenomena can be approximated by simple harmonic motion, including the motion of a simple pendulum and molecular vibration....

s with different fixed phase angle

Phase angle

In the context of vectors and phasors, the term phase angle refers to the angular component of the polar coordinate representation. The notation A\ang \!\ \theta,   for a vector with magnitude A and phase angle θ, is called angle notation.In the context of periodic phenomena, such as a wave,...

s are added in varying proportions to provide an output motion that is variable in phase and amplitude. A variety of such mechanisms have been devised over the years, with varying success.

Both slide and piston valves have the limitation that intake and exhaust events are fixed in relation to each other and cannot be independently optimised. Lap is provided on steam edges of the valve, so that although the valve stroke reduces as cutoff is advanced, the valve is always fully opened to exhaust. However, as cutoff is shortened, the exhaust events also advance. The exhaust release point occurs earlier in the power stroke and compression earlier in the exhaust stroke. Early release wastes some energy in the steam, and early closure also wastes energy in compressing an otherwise unnecessarily large quantity of steam. Another effect of early cutoff is that the valve is moving quite slowly at the cutoff point, and this causes 'wire drawing' of the steam, another wasteful thermodynamic effect visible on an indicator diagram.

These inefficiencies drove the widespread experimentation in poppet valve

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide...

 gears for locomotives. Intake and exhaust poppet valves could be moved and controlled independently of each other, allowing for better control of the cycle. In the end, not a great number of locomotives were fitted with poppet valves, but they were common in steam cars and lorries, for example virtually all Sentinel

Sentinel Waggon Works

Sentinel Waggon Works Ltd was a British company based in Shrewsbury, Shropshire that made steam-powered lorries, railway locomotives, and later, diesel engined lorries and locomotives.-Alley & MacLellan, Sentinel Works, Jessie Street Glasgow:...

 lorries, locomotives and railcars used poppet valves. A very late British design, the SR Leader class

SR Leader Class

The Leader was a class of experimental 0-6-6-0T articulated steam locomotive, produced in the United Kingdom to the design of the innovative engineer Oliver Bulleid. The Leader was an attempt to extend the life of steam traction by eliminating many of the operational drawbacks associated with...

, used sleeve valve

Sleeve valve

The sleeve valve is a type of valve mechanism for piston engines, distinct from the usual poppet valve. Sleeve-valve engines saw use in a number of pre-World War II luxury cars and in USA in the Willys-Knight car and light truck...

s adapted from internal combustion engines, but this class was not a success.

In stationary steam engine

Stationary steam engine

Stationary steam engines are fixed steam engines used for pumping or driving mills and factories, and for power generation. They are distinct from locomotive engines used on railways, traction engines for heavy steam haulage on roads, steam cars , agricultural engines used for ploughing or...

s, traction engine

Traction engine

A traction engine is a self-propelled steam engine used to move heavy loads on roads, plough ground or to provide power at a chosen location. The name derives from the Latin tractus, meaning 'drawn', since the prime function of any traction engine is to draw a load behind it...

s and marine engine

Marine steam engine

A marine steam engine is a reciprocating steam engine that is used to power a ship or boat. Steam turbines and diesel engines largely replaced reciprocating steam engines in marine applications during the 20th century, so this article describes the more common types of marine steam engine in use...

 practice, the shortcomings of valves and valve gears were among the factors that lead to compound expansion. In stationary engines trip valve

Trip valve

Trip valve mechanisms are a class of steam engine valve gear developed to improve efficiency. The trip mechanism allows the inlet valve to be closed rapidly, giving a short, sharp cut-off. The valve itself can be a drop valve or a Corliss valve....

s were also extensively used.

Valve gear designs

Valve gear was a fertile field of invention, with probably several hundred variations devised over the years. However, only a small number of these saw any widespread use. They can be divided into those that drove the standard reciprocating valves (whether piston valves or slide valves), those used with poppet valves, and stationary engine trip gears

Trip valve

Trip valve mechanisms are a class of steam engine valve gear developed to improve efficiency. The trip mechanism allows the inlet valve to be closed rapidly, giving a short, sharp cut-off. The valve itself can be a drop valve or a Corliss valve....

 used with semi-rotary Corliss valves or drop valves

Double beat valve

The double-beat valve, drop valve or equilibrium valve is a type of poppet valve arranged to allow it to be opened against a high pressure with a minimum of force....

.

Reciprocating valve gears

• Gab or hook gear - used on earliest locomotives. Allowed reversing but no control of cutoff.

Link gears

Two eccentrics joined by a curved or straight link. A simple arrangement which works well at low speed. At high speed, a Walschaerts-type gear is said to give better steam distribution and higher efficiency.

• Stephenson valve gear
  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 all kinds of steam engine. It is named after Robert Stephenson but was actually invented by his employees....
  
   - most common valve gear in 19th century, normally inside the locomotive frame
  Locomotive frame
  A locomotive frame is the structure that forms the backbone of the railway locomotive, giving it strength and supporting the superstructure elements such as a cab, boiler or bodywork. The vast majority of locomotives have had a frame structure of some kind...
  
  .
• William T. James valve gear 1832, used first by Baltimore and Ohio Railroad
  Baltimore and Ohio Railroad
  The Baltimore and Ohio Railroad was one of the oldest railroads in the United States and the first common carrier railroad. It came into being mostly because the city of Baltimore wanted to compete with the newly constructed Erie Canal and another canal being proposed by Pennsylvania, which...
  
  , USA.
• Allan valve gear, a straight-link valve gear. It is fitted to 0-4-0WT "Dolgoch", preserved on the Talyllyn Railway
  Talyllyn Railway
  The Talyllyn Railway is a narrow-gauge preserved railway in Wales running for from Tywyn on the Mid-Wales coast to Nant Gwernol near the village of Abergynolwyn. The line was opened in 1866 to carry slate from the quarries at Bryn Eglwys to Tywyn, and was the first narrow gauge railway in Britain...
  
  .
• Gooch valve gear

Radial gears

Both components of the motion come from a single crank or eccentric. A problem with this arrangement (when applied to locomotives) is that one of the components of the motion is affected by the rise and fall of the locomotive on its springs. This probably explains why radial gears were largely superseded by Walschaerts-type gears in railway practice but continued to be used in traction and marine engines.

• Hackworth valve gear invented by John Wesley Hackworth in 1859.
• Joy valve gear
  Joy Valve Gear
  Joy valve gear is a type of locomotive valve gear, patented in 1870, where the movement is derived from a vertical link connected to the connecting rod. The vertical movement is translated into the horizontal movement required by the valve spindle by a die block moving in a slide which can be...
  
   - a design used extensively on the L&YR
  Lancashire and Yorkshire Railway
  The Lancashire and Yorkshire Railway was a major British railway company before the 1923 Grouping. It was incorporated in 1847 from an amalgamation of several existing railways...
  
   and LNWR
  London and North Western Railway
  The London and North Western Railway was a British railway company between 1846 and 1922. It was created by the merger of three companies – the Grand Junction Railway, the London and Birmingham Railway and the Manchester and Birmingham Railway...
  
   in England, and elsewhere. A preserved example is LNWR G2a Class
  LNWR G Class
  The London and North Western Railway G Class along with modifications that made them into G1, G2 and G2a classes, is a class of 0-8-0 steam locomotives designed for heavy freight work. They are known to railway enthusiasts as "Super Ds". This is because a large number of G1 class engines were...
  
   number 49395.
• Marshall valve gear - a modified Hackworth gear, patented in 1879 by Marshall, Sons & Co.
  Marshall, Sons & Co.
  -External links:* – at the Powerhouse Museum, Sydney, Australia...
  
  .
• Southern valve gear
  Southern valve gear
  Southern valve gear was briefly popular on steam locomotives in the United States. It combines elements of the Walschaerts and Baker patterns.-History:...
  
   - Briefly popular in the United States around 1920. It combined elements of the Walschaerts and Baker patterns.

Walschaerts-type gears

One component of the motion comes from a crank or eccentric. The other component comes from a separate source, usually the crosshead.

• Walschaerts or Heusinger valve gear - most common valve gear on later locomotives, normally externally mounted.
• Baker valve gear
  Baker valve gear
  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...
  
   - fairly common in the United States
  United States
  The United States of America is a federal constitutional republic comprising fifty states and a federal district...
  
  , it had no sliding parts.
• Deeley
  Richard Deeley
  Richard Mountford Deeley was a British engineer, chiefly noted for his five years as Chief Mechanical Engineer of the Midland Railway....
  
   valve gear - fitted to several express locomotives on the Midland Railway
  Midland Railway
  The Midland Railway was a railway company in the United Kingdom from 1844 to 1922, when it became part of the London, Midland and Scottish Railway....
  
  . The combination levers were driven, as normal, from the crossheads. Each expansion link was driven from the crosshead on the opposite side of the engine.
• Young valve gear - used the piston rod motion on one side of the locomotive to drive the valve gear on the other side. Similar to the Deeley gear, but with detail differences.
• Baguley valve gear
  Baguley valve gear
  The Baguley valve gear is a type of steam engine valve gear invented by Mr E. E. Baguley the draftsman of the W.G. Bagnall company of locomotive manufacturers. It was patented in 1893....
  
   - used by W.G. Bagnall
  W.G. Bagnall
  W. G. Bagnall was a locomotive manufacturer from Stafford, England. It was founded in 1875 by William Gordon Bagnall and ceased trading in 1962 when it was taken over by English Electric Co Ltd. The company was located at the Castle Engine Works, in Castle Town, Stafford...
  
  .
• Bagnall-Price valve gear
  Bagnall-Price valve gear
  Bagnall-Price valve gear is a type of steam engine valve gear developed at locomotive manufacturer W.G. Bagnall as an alternative to the more common Walschaerts valve gear. The gear was patented in 1903 by W.G. Bagnall and T. S. Price, the manager of the works....
  
   - a variation of Walschaerts used by W.G. Bagnall
  W.G. Bagnall
  W. G. Bagnall was a locomotive manufacturer from Stafford, England. It was founded in 1875 by William Gordon Bagnall and ceased trading in 1962 when it was taken over by English Electric Co Ltd. The company was located at the Castle Engine Works, in Castle Town, Stafford...
  
  . This gear is fitted to Bagnall 3023 and 3050, both preserved on the Welsh Highland Railway
  Welsh Highland Railway
  The Welsh Highland Railway is a long restored narrow gauge heritage railway in North Wales, operating from Caernarfon to Porthmadog, and passing through a number of popular tourist destinations including Beddgelert and the Aberglaslyn Pass. At Porthmadog it connects with the Ffestiniog Railway...
  
  .
• J. T. Marshall seems to have designed at least two different modifications of Walschaerts gear.
  • One was relatively conventional.
  • The other was very complex and drove separate valves on top of the cylinder (for admission) and underneath the cylinder (for exhaust). After the inventor's death, this gear was fitted experimentally to Southern Railway N Class
    Southern Railway N Class
    The SECR N class was a type of 2-6-0 steam locomotive designed in 1914 by Richard Maunsell for mixed-traffic duties on the South Eastern and Chatham Railway . Built between 1917 and 1934, it was the first non-Great Western Railway type to use and improve upon the basic design principles...
    
     locomotive number 1850, the work taking from 16 October 1933 to 3 February 1934; but it failed on 22 March 1934. Since the inventor was unable to modify the design, the valve gear was replaced by standard Walschaerts gear between 24 March and 11 April 1934.
• Isaacson's patent valve gear - a modified Walschaerts gear. It was fitted to the Garstang and Knot-End Railway's 2-6-0T Blackpool.

Poppet valve gears

• Caprotti valve gear
  Caprotti valve gear
  The Caprotti valve gear is a type of steam engine valve gear invented in the early 1920's by Italian architect and engineer Arturo Caprotti. It uses camshafts and poppet valves rather than the piston valves used in other valve gear...
  
  , British Caprotti valve gear
• Hugo Lentz
  Hugo Lentz
  Hugo Lentz was an Austrian mechanical engineer, born in South Africa. He was the inventor of many award winning improvements to the steam engine....
  
  , oscillating-cam valve gear, rotary-cam valve gear
• Franklin oscillating-cam valve gear
• Franklin rotary-cam valve gear
• Reidinger valve gear

Conjugating gears

These enable a 3-cylinder or 4-cylinder locomotive to be built with only two sets of valve gear. The best known is Gresley conjugated valve gear

Gresley conjugated valve gear

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...

, used on 3-cylinder locomotives. Walschaerts gear is usually used for the two outside cylinders. Two levers connected to the outside cylinder valve rods drive the valve for the inside cylinder. Harold Holcroft

Harold Holcroft

Harold Holcroft was a British railway and mechanical engineer who worked for the Great Western Railway , the South Eastern and Chatham Railway and the Southern Railway...

 devised a different method for conjugating valve gear by linking the middle cylinder to the combination lever assembly of an outside cylinder, creating the Holcroft valve gear

Holcroft valve gear

The Holcroft valve gear was a type of valve gear designed by Harold Holcroft and used on three-cylinder steam locomotives of the South Eastern and Chatham Railway . It bore many similarities to the Gresley conjugated valve gear used on the original LNER A1 design...

derivative. On a 4-cylinder locomotive the arrangement is simpler. The valve gear may be inside or outside and only short rocking-shafts are needed to link the valves on the inside and outside cylinders.

External links

• Valve gear simulation software for MS Windows, showing twenty different valve gear designs.
• Berry accelerator valve gear.
• Diagrams of Walschaerts valve gear and Stephenson valve gear, as supplied on working steam model locomotives.
• http://www.steamlocomotive.com/appliances/valvegear.php Animations of 5 Stephenson, Walschaert, Baker, Southern, and Young valve gears.