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The Walschaerts valve gear on a steam locomotive (a PRR E6s).

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".


In the simple case, this can be a relatively simple task as in the internal combustion 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, 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 (throttles 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, this task is performed by cams on a camshaft driving poppet valves, 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, cranks and levers is generally used to control a D slide valve or piston valve from the motion. Generally, two simple harmonic motions with different fixed phase angles 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 creates a constriction point causes the steam to enter the cylinder at less than full boiler pressure (called 'wire drawing' of the steam, named after the process of making metal wire by drawing it through a hole), another wasteful thermodynamic effect visible on an indicator diagram.

These inefficiencies drove the widespread experimentation in poppet valve 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 lorries, locomotives and railcars used poppet valves. A very late British design, the SR Leader class, used sleeve valves adapted from internal combustion engines, but this class was not a success.

In stationary steam engines, traction engines and marine engine practice, the shortcomings of valves and valve gears were among the factors that lead to compound expansion. In stationary engines trip valves 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 used with semi-rotary Corliss valves or drop valves.[1]

Reciprocating valve gears

Early types

Link gears

Constant lead gear (Walschaerts-type gear)

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

Dual eccentric gear (Stephenson-type gears)
Stephenson's Valve gear. Two eccentrics at nearly 180-degree phase difference work cranks from the main drive shaft. Either can be selected to work the valve slide by shifting the slotted expansion link.
Stephenson's Valve gear. Two eccentrics at nearly 180-degree phase difference work cranks from the main drive shaft. Either can be selected to work the valve slide by shifting the slotted expansion link.

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.

Lever and link gear (Baker-type)

Baker valve gear assembly
Baker valve gear assembly

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.

Poppet valve gears

Conjugating gears

View of Henschel & Son conjugated valve gear mechanism used on Victorian Railways H class locomotive, driven from the outside Walschaerts valve gear
View of Henschel & Son conjugated valve gear mechanism used on Victorian Railways H class locomotive, driven from the outside Walschaerts valve gear

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, 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 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 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.

Bulleid chain-driven valve gear

See Bulleid chain-driven valve gear

Corliss valve gear

See Corliss steam engine

Large stationary engines often used an advanced form of valve gear developed by George Henry Corliss, usually called Corliss valve gear. This gear used separate valves for inlet and exhaust so that the inlet cut-off could be controlled precisely. The use of separate valves and port passages for steam admission and exhaust significantly also reduced losses associated with cylinder condensation and re-evaporation. These features resulted in much improved efficiency.

Controls for valve gear

A locomotive's direction of travel and cut-off are set from the cab by using a reversing lever or screw reverser actuating a rod reaching to the valve gear proper. Some larger steam engines employ a power reverse, which is a servo mechanism, usually powered by steam. This makes control of the reversing gear easier for the driver.

See also


  1. ^ Steam Locomotive Valve Gear Archived 5 January 2012 at the Wayback Machine Animations of Stephenson's, Walschaerts', Baker's, Southern and Young's valve gear., Accessed 1 September 2014
  2. ^ "Slip-eccentric valve gear". Archived from the original on 27 April 2012. Retrieved 2 December 2012.
  3. ^ Van Riemsdijk, J.T. (1994). Compound Locomotives: An International Survey. Penryn: Atlantic Transport Publishers. pp. 23–24. ISBN 0-906899-61-3.
  4. ^ Bradley, D.L. (April 1980) [1961]. The Locomotive History of the South Eastern & Chatham Railway (2nd ed.). London: RCTS. p. 93. ISBN 0-901115-49-5.
  5. ^ "Espacenet - Bibliographic data". Retrieved 12 January 2013.
  6. ^ "Brief Biographies of Mechanical Engineers". Archived from the original on 13 February 2004. Retrieved 12 January 2013.
  7. ^ "Espacenet - Bibliographic data". Retrieved 12 January 2013.
  8. ^ "CIPO - Patent - 204805". 12 October 1920. Archived from the original on 8 March 2014. Retrieved 13 January 2013.
  9. ^ "Railway age gazette". [New York, Simmons-Boardman Pub. Co.] – via Internet Archive.
  10. ^ Dorin, Patrick C. (1979). The Soo Line. Burbank, California: Superior Publishing Co. p. 25. ISBN 0-87564-712-X.
  11. ^ "0-6-4st Badger". Archived from the original on 4 March 2016.
  12. ^ "Correspondence 60". Archived from the original on 4 November 2011.