Piston rod

In a piston engine, a piston rod joins a piston to the crosshead and thus to the connecting rod that drives the crankshaft or (for steam locomotives) the driving wheels.

Internal combustion engines, and in particular all current automobile engines, do not generally have piston rods. Instead they use trunk pistons, where the piston and crosshead are combined and so do not need a rod between them. The term piston rod has been used as a synonym for 'connecting rod' in the context of these engines.

Engines with crossheads have piston rods. These include most steam locomotives and some large marine diesel engines.

The first single-acting beam engines, such as Newcomen's, had a single power stroke acting downwards. Rather than a piston rod, they used an iron chain. This could transmit a tensile force, but not a compression force pushing upwards. The piston was sealed in the cylinder around its rim but the top of the cylinder was open. A rudimentary piston rod was used, simply to reduce the cost of a long forged chain.

Watt's development of the steam engine introduced an enclosed upper cylinder. This now required a stuffing box to seal around the piston rod, and for the piston rod to be machined smooth and accurately cylindrical. The engines were still single-acting at this time and the rod was still only acting in tension.

Later developments, also by Watt, produced a double-acting cylinder. The piston rod now had to transmit a force alternately pushing and pulling. 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.

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