crosshead

A crosshead is a rigid metal block or beam mounted directly to a piston rod that transfers the linear reciprocating motion of the piston into the angled rotational motion needed by a connecting rod. In large reciprocating engines, particularly slow-speed marine diesel engines and steam engines, the crosshead slides on fixed guide rails while the connecting rod articulates freely from its pin, absorbing the lateral forces that would otherwise bend the piston rod.

The crosshead carries two main bores: one vertical bore receives the piston rod under a press fit or tapered lock, and a second horizontal bore accommodates the connecting rod's large end and its gudgeon pin. High-pressure forces from combustion or steam drive the piston downward, transmitting force through the crosshead to the connecting rod and then to the crankshaft. The guide rails, typically cast iron or bronze, constrain the crosshead to pure vertical motion, preventing the side loads that plague smaller engines without this component.

Crossheads in large slow-speed marine engines are massive ductile iron or steel forgings, often weighing many hundreds of kilograms. The piston rod must clear the cylinder head and pass through the crosshead with a substantial diametral fit. Guide surfaces are hardened and scraped to maintain close tolerances; on some engines, the guides run in oil baths or spray lubrication systems. Connecting rod pins are typically hardened steel and may rotate within white metal or polymer-bronze bushings pressed into the crosshead bore.

The crosshead is rarely seen in passenger car engines, where the smaller forces and higher speeds make simple piston rods acceptable. It remains essential in large stationary engines, two-stroke marine diesels above 5 MW, and some historical steam engines. Wear at the guide surfaces causes excessive rod load and vibration; a loose or scored crosshead guide will eventually damage the piston rod and waste fuel.

The term comes from the Latin cross combined with head, reflecting its position at the top of the connecting rod, the head of the rod connection, and its transverse orientation relative to the main axis of motion.