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".
The Walschaerts valve gear on a steam locomotive (a PRR E6s).
Soo Line 346 in 1961, showing the Kinkan-Ripken arm on the connecting rod at the right hand edge of the picture
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.
Baker valve gear assembly
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be transformed, by a connecting rod and crank, into rotational force for work. The term "steam engine" is most commonly applied to reciprocating engines as just described, although some authorities have also referred to the steam turbine and devices such as Hero's aeolipile as "steam engines". The essential feature of steam engines is that they are external combustion engines, where the working fluid is separated from the combustion products. The ideal thermodynamic cycle used to analyze this process is called the Rankine cycle. In general usage, the term steam engine can refer to either complete steam plants, such as railway steam locomotives and portable engines, or may refer to the piston or turbine machinery alone, as in the beam engine and stationary steam engine.
A model of a beam engine featuring James Watt's parallel linkage for double action
A mill engine from Stott Park Bobbin Mill, Cumbria, England
A steam locomotive from East Germany. This class of engine was built in 1942–1950 and operated until 1988.
A steam ploughing engine by Kemna