mechanical engineering Mechanical engineering is the study of physical machines that may involve force and movement. It is an engineering branch that combines engineering physics and mathematics principles with materials science, to design, analyze, manufacture, ...

, a crosshead is a

mechanical joint A mechanical joint is a section of a machine which is used to connect one or more mechanical part to another. Mechanical joints may be temporary or permanent, most types are designed to be disassembled. Most mechanical joints are designed to all ...

used as part of the slider-crank linkages of long

reciprocating engine A reciprocating engine, also often known as a piston engine, is typically 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 fe ...

s (either internal combustion or

steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporizatio ...

) 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 and is made gas-t ...

. Also, the crosshead enables the

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

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

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

ratio on such engines, the connecting rod would hit the cylinder walls and block the engine from rotating if the piston was attached directly to the connecting rod like on

trunk engine Trunk may refer to: Biology * Trunk (anatomy), synonym for torso * Trunk (botany), a tree's central superstructure * Trunk of corpus callosum, in neuroanatomy * Elephant trunk, the proboscis of an elephant Computing * Trunk (software), in re ...

s. 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 piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecti ...

directly, but this transmits sideways forces to the piston, since the crankpin (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 smaller engine. A larger engine's much greater forces would cause an intolerable degree of wear on the piston and

cylinder A cylinder (from ) 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 ...

, as well as increasing overall friction in the engine. A piston rod 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 (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 engines 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 oilpump 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 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 ...

, 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 An oscillating cylinder steam engine (also known as a wobbler in the US) is a simple steam-engine design (proposed by William Murdoch at the end of 18th century) that requires no valve gear. Instead the cylinder rocks, or oscillates, as the cra ...

which can be double acting and yet has no crosshead.

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, oil or, rarely, wood) to heat water in the loco ...

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 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 A marine steam engine is a steam engine that is used to power a ship or boat. This article deals mainly with marine steam engines of the reciprocating type, which were in use from the inception of the steamboat in the early 19th century to their ...

, and at other times used as a linkage—to side-rods in a side-lever engine or to connecting rods in a square engine.

References