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 causes 'wire drawing' of the steam, 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 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.
One component of the motion comes from a crank or eccentric. The other component comes from a separate source, usually the crosshead.
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.
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.
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.
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. This resulted in much improved efficiency.
Some larger steam engines employ a power reverse, which is a servo mechanism, usually powered by steam. This makes control of the valve gear easier for the driver.