Piston rod

Later, a rudimentary piston rod was used, simply to eliminate the cost of a long forged chain.

The steam engine's general use of an enclosed cylinder, nearly always double-acting, made it dependent on the stuffing box and so the piston rod.

Enclosing the entire tail rod in a sealed casing avoided the need for a stuffing box and any leakage of steam.

However this also gave a risk that condensed water could accumulate in the casing, leading to hydraulic lock and mechanical damage.

They reduced their overall length by placing the crosshead alongside the piston in the form of a hollow tubular trunk.

They thus benefit from reduced reciprocating mass and from a shorter, stiffer linkage between the piston and the connecting rod.

The high-speed steam engine's lightweight trunk piston was thus soon adopted and remains in service today.

Its major changes have been in its sealing rings and the adoption of aluminium alloys to make the pistons lighter.

Some large slow-speed Diesel engines, particularly those for ships, use crossheads and piston rods.

Medium- and high-speed diesel engines operate at faster speeds and so use lighter-weight trunk pistons.