A chain is a rope when pushed and a rod when pulled
Compress a chain and it folds and curves as limply as rope. Pull it tight and it becomes straight, rigid and able to bear enormous loads. That split personality explains why the same basic idea shows up in bicycle drives, ship anchors, medieval armour and old-fashioned toilets.
Chains fall into two broad designs. Lifting, pulling and securing chains, such as those on hoists or bicycle locks, use doughnut-shaped links that let the chain bend in two directions. Power-transmission chains instead have links shaped to mesh with sprocket teeth and bend in only one direction; most are roller chains, though block chains also exist. A bicycle chain is the everyday example, its rigid links hinged on pins so it can wrap around the gears and carry force from pedals to wheel.
Inside a car engine, a timing chain carries the crankshaft's position to the valves and ignition, usually turning at half the crankshaft's speed. Chain pumps lift water by dragging a loop of chain fitted with discs up through a submerged tube. There was even a river vessel called a chain boat, which hauled itself along a steel chain laid on the riverbed.
Some chains work at scale. Navies once stretched enormous chain booms across harbour mouths and rivers to keep out enemy warships. In British nautical usage, a ship's anchor line is called a cable even though the material is chain. Chain mail turned thousands of small linked rings into flexible armour, and chain-link fencing uses zigzag wires hooked into one another.
Chains also carry status and humour. In medieval Europe and in the United Kingdom, a heavy gold collar called a chain of office marks rank or loyalty. Ball and chain originally meant a weight that slowed prisoners down before becoming a jibe about spouses. And although few cisterns still hang overhead with a pull cord, 'pull the chain' survives as a way to say flush the toilet.
Source: Chain
