What is a Cross head ? An-Overview

The cross head is a mechanism used as part of the slider-crank connecting rod mechanism on long reciprocating compressors and piston engines to relieve side pressure on the pistons. In addition, the cross head allows the connecting rod to move freely out of the cylinder. The stroke ratio of the bore of the cross head engine is so small that if the piston is directly connected to the connecting rod, the connecting rod hits the cylinder wall and prevents the engine from rotating, similar to a mainline engine. Therefore, the vertical dimensions of the traverse must match the stroke of the motor.

The cross head bolt connects the piston rod with the connecting rod. Install the cross head slides on either side of the cross head pin. When the piston and connecting rod are moved back and forth, the slide moves up and down within the cross head guide rail and prevents the upper part of the connecting rod from shifting sideways.


On smaller engines, the connecting rod connects the piston directly to the crankshaft, which causes the crank pin (and the direction in which the force is applied) to move from side to side as the crank is turned. Directional force is transmitted to the piston.  Even a small engine can withstand these lateral forces. The greater power of a larger engine causes unbearable wear and tear on the pistons and cylinders and increases the overall friction of the engine.

The piston rod is connected to the piston and connected to the traverse. A cross head is a large casting that slides on the cross head rail and can only move in the same direction as the piston stroke. The traverse is also equipped with an axle pin (US: wrist pin) that turns the smaller end of the crank. Therefore, the side force only acts on the ring gear and its bearings, not on the piston itself.

Applications of Cross head

The cross head uses in the following major applications:

1) Internal Combustion Engine

For internal combustion engines that use a cross head, the pistons can be easily removed, making it easier to maintain the top of the engine. The piston rod is attached to the underside of the piston and connected to the crossbeam by a single nut on the double-acting motor. Large two-stroke marine diesel engines often use this mode. Crossbeams are essential for double-acting diesel engines (see also H-class battleships: 9-cylinder, 2-stroke, double-acting, 12-person diesel engines). Large diesel engines typically have a piston oil pump connected directly to the cross head to deliver high-pressure oil to the cross head bearings.

2) Steam Engine

With steam engines, if it is a double-acting machine, a crossbeam is essential. Since steam is applied to both sides of the piston, it is necessary to seal the piston rod.

3) Locomotives

The steam locomotive spreaders can be mounted on a single rail mounted on top of the spreader, or one on top and one on the bottom (called crocodile spreader because of their two “jaws”). The former is the first choice for many modern locomotives.

4) Marine Engine

In many ship steam engines of the 19th century, the traverse was a solid metal rod connected to the piston rod and perpendicular to the piston rod. It was sometimes used to remove side forces, like a sharp engine, and sometimes used as a connecting rod: sidebars Engine sidebars or square engine connecting rods.

Advantages of cross head type engine?

  • Cross head motors can provide higher performance than drum motors at lower motor speeds. This is because the space available for the spider bearings is greater than the space available for the axle bearing assemblies within the piston.
  • Crankcase lube oil combustion product contamination is less than that of barrel engines.