A crosshead is a mechanism found in larger reciprocating engines that guides a piston rod up and down. This helps to eliminate sideways pressure on the piston, which would otherwise be present on larger engines without a crosshead. Crossheads are an extremely important part of large reciprocating engines since they not only reduce the wear created by sideways pressure, but they also help to regulate and maintain the steadiness of the rotation of the engine. This steadiness is directly effected by the steadiness of the piston's reciprocating movement.
Much like most of the components found in larger steam or diesel reciprocating engines, the crosshead itself is a relatively simple device that is entirely mechanical and consists of very few parts. While the design can vary between engines, they all work in relatively the same way; the device itself consists solely of a pair of crosshead guides, where the crosshead is free to move up and down their length. The crosshead is connected to the piston rod and crosshead bearing of the engine, which are guided up and down the lubricated guides.
Despite the fact that crossheads play a very important role in large engines, smaller engines — especially the internal combustion engines found in automobiles — do not employ them. One of the reasons for this is because the design alone does not require a crosshead; the pistons on gasoline internal combustion engines have no piston rod, as the piston and crankpin are connected directly by the connecting rod. Another reason is because side pressure is far more tolerable on a small engine than on a large engine, as everything is on a far smaller scale. A larger engine without crossheads would get worn down very easily, as the side pressure exerted upon the pistons at higher revolutions per minute (RPMs) would create a great deal of wear.
Besides eliminating side pressure, crossheads have other benefits. One of these is found on internal combustion engines employing crossheads, such as diesel engines. Pistons on reciprocating engines need occasional maintenance or replacement, making it necessary to remove them entirely from the engine. On reciprocating engines without a piston rod, doing this would involve removing the connecting rod from the crankshaft, which can be a difficult process. With a piston rod present, however, removing the piston is as simple as disconnecting the piston rod from the connecting rod. This makes maintaining large diesel engines far easier.