mechanical engineering Mechanical engineering is the study of physical machines and mechanism (engineering), mechanisms that may involve force and movement. It is an engineering branch that combines engineering physics and engineering mathematics, mathematics principl ...

, a crosshead is a mechanical joint used as part of the slider-crank linkages of long stroke

reciprocating engine A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common features of al ...

s (either

internal combustion An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

steam Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

) and reciprocating compressors to eliminate sideways force on the

piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder (engine), cylinder a ...

. The crosshead also allows the

connecting rod A connecting rod, also called a 'con rod', is the part of a reciprocating engine, piston engine which connects the piston to the crankshaft. Together with the crank (mechanism), crank, the connecting rod converts the reciprocating motion of the p ...

to move freely outside the cylinder. Because of the very small bore-to-

stroke Stroke is a medical condition in which poor cerebral circulation, blood flow to a part of the brain causes cell death. There are two main types of stroke: brain ischemia, ischemic, due to lack of blood flow, and intracranial hemorrhage, hemor ...

ratio on such engines, the connecting rod would hit the cylinder walls and block the engine from rotating if the piston were attached directly to the connecting rod as in a trunk engine. Therefore, the longitudinal dimension of the crosshead must be matched to the stroke of the engine.

Overview

On smaller engines, the

links the

and the

crankshaft A crankshaft is a mechanical component used in a reciprocating engine, piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating Shaft (mechanical engineering), shaft containing one or more crankpins, ...

directly, but this transmits sideways forces to the piston, since the

crankpin A crankpin or crank pin, also known as a rod bearing journal, is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to th ...

(and thus the direction the force is applied) moves from side to side with the rotary motion of the crank. These transverse forces are tolerable in a relatively small engine. A larger engine's much greater forces would cause an intolerable degree of wear on the piston and

cylinder A cylinder () has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infinite ...

, as well as increasing overall friction in the engine. A

piston rod In a piston engine, a piston rod joins a piston to the crosshead and thus to the connecting rod that drives the crankshaft or (for steam locomotives) the driving wheels. Internal combustion engines, and in particular all current automobile engin ...

is attached to the piston and links it to the crosshead, which is a large casting sliding in crosshead guides (UK: ''slidebar''), allowing it only to move in the same direction as the piston travel. The crosshead also houses the

gudgeon pin In internal combustion engines, the gudgeon pin (English, wrist pin or piston pin US English) connects the piston to the connecting rod, and provides a bearing for the connecting rod to pivot upon as the piston moves.Nunney, Malcolm James (200 ...

(US: ''wristpin'') on which the small end of the connecting rod pivots. In this way, the transverse forces are applied only to the crosshead and its bearings, not to the piston itself.

Internal combustion engines

Internal combustion engines using crossheads make for easier maintenance of the top end of the engine, since the pistons can be easily removed. The piston rod is mounted on the underside of the piston and connected to the crosshead by a single nut in double acting engines. The large two-stroke

marine diesel engine Marine propulsion is the mechanism or system used to generate thrust to move a watercraft through water. While paddles and sails are still used on some smaller boats, most modern ships are propelled by mechanical systems consisting of an electr ...

s are usually of this pattern. A crosshead is essential in a double-acting diesel engine (see also: battleships: 12 MAN double-acting 2-stroke 9-cylinder diesels). Large diesels often have a plunger oil pump directly attached to the crosshead to supply oil under high pressure to the crosshead bearing.

Steam engines

In the case of the

steam engine A steam engine is a heat engine that performs Work (physics), 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 (locomotive), cyl ...

, a crosshead is essential if the engine is to be ''double acting'' - steam is applied to both sides of the piston, which requires a seal around the piston rod. An exception is the oscillating cylinder steam engine which can be double acting and yet has no crosshead. Early double-acting steam engines, such as those designed by

James Watt James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which was f ...

, relied on a parallel motion linkage in part due to the difficulty of manufacturing guide rails. The cross head was made possible by the introduction of the planer which was suited for producing guides.

Locomotives

Crossheads in a

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, Fuel oil, oil or, rarely, Wood fuel, wood) to heat ...

can be mounted either to one guide mounted above the crosshead or to two, one above and one below (called an ''alligator crosshead'' since it has two "jaws"). The former was preferred in many more modern locomotives.

Marine engines

In many 19th century marine steam engines, the crosshead was a strong metal bar attached to the piston rod and perpendicular to it, which was sometimes used to eliminate transverse forces, as in a steeple engine, and at other times used as a linkage—to side-rods in a side-lever engine or to connecting rods in a

square In geometry, a square is a regular polygon, regular quadrilateral. It has four straight sides of equal length and four equal angles. Squares are special cases of rectangles, which have four equal angles, and of rhombuses, which have four equal si ...

engine.

References