Editing Crosshead (section)

==Steam engines==
In the case of the [[steam engine]], 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]], 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 (metalworking) | planer]] which was suited for producing guides. <ref>
{{cite book
	| last = Rolt
	| first = L.T.C.
	| date = 1965
	| title = A Short History of Machine Tools
	| location = Cambridge, MA, USA
	| publisher = The M.I.T. Press
	| page = 101
}}
</ref>

===Locomotives===
Crossheads in a [[steam locomotive]] 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 engine]]s, 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 [[marine steam engine#Steeple|steeple engine]], and at other times used as a linkage&mdash;to side-rods in a [[side-lever]] engine or to connecting rods in a [[marine steam engine#Crosshead (square)|square]] engine.