motion
A piece of moving mechanism, such as on a steam locomotive.
motion: the working guts of a steam engine
In steam locomotive and stationary engine work, a motion is the assembly of rods, eccentrics, and valve gear that converts the reciprocating motion of the piston into useful mechanical action. On a locomotive, the motion transfers power from the cylinders to the driving wheels; on a stationary engine, it may drive a pump, press, or mill. The term refers to the entire kinematic system, not just one component, though it is sometimes used loosely to mean the valve gear alone.
The motion typically comprises the connecting rod, crosshead, guide bars, and either a Stephenson link motion, Walschaerts gear, or similar valve control system. These parts work together to open and close steam ports at precisely the right moment in the piston stroke. A locomotive motion must withstand enormous forces, repeated millions of times, while running at speeds up to 60 or 70 mph. The connecting rod alone may be cast or forged steel, 15 to 20 feet long on a large engine, and subject to both tension and bending stress.
Common failure modes and wear
Wear in the motion is inevitable and must be monitored closely. The crosshead bushes, which guide the rod's vertical movement, wear away and develop slop; the pins connecting rod to crank or eccentric develop play. Excessive clearance in any joint leads to hammering, lost power, and accelerated wear in adjacent parts. Bearings overheat if oil supply fails. A cracked connecting rod is a catastrophic failure: the engine cannot operate and the fracture may cause damage to the frame or other components.
The name derives from the fact that this assembly is where motion originates and is organized. It is the motor in the mechanical sense: the device that generates and directs movement. Different engine types use different motion designs; the choice depends on duty cycle, speed range, and the need to reverse (crucial on locomotives, less so on stationary pumps). A proper motion, well-maintained with regular lubrication and adjustment, remains the most reliable means of converting steam power into controlled mechanical work.