They act both as a piston and cylindrical crosshead. The crosshead became part of the piston, and there was no longer any piston rod. Also, the crosshead enables the connecting rod to freely move outside the cylinder. Crankpin and crosshead bearings carry small cup-shaped reservoirs for oil. In most cases a crosshead crank gear is used for this. The air pump is driven from each crosshead. This required the use of a crosshead. The other component comes from a separate source, usually the crosshead. A three bar crosshead was added to the motion.