A flywheel-driven slider-crank mechanism: crank, connecting rod, crosshead and piston move in true mechanical linkage, venting steam puffs at each stroke end.
Reciprocating steam engines convert the linear thrust of a piston into rotary motion through a connecting rod and crank, exactly as modeled here: the crosshead constrains the rod to slide along the cylinder axis while the crank pin traces a circle on the flywheel. Steam admitted at one end of the cylinder pushes the piston, then is vented (or, in condensing engines, drawn into a separate condenser) as the crank carries past top or bottom dead center, producing the characteristic rhythmic puffs. James Watt's 1765 separate condenser was the single biggest efficiency leap of the 18th century, because it let the cylinder stay hot instead of being cooled and reheated every stroke as in earlier Newcomen engines. The heavy flywheel exists purely to smooth out the uneven torque of a single-cylinder engine, storing rotational energy during the power stroke and releasing it through the rest of the cycle. These engines powered the Industrial Revolution's mills, mines, and railways before being displaced by more efficient steam turbines and internal combustion engines in the early 20th century.