VI. Dead centres and the flywheel

A piston pushes in straight lines, but mills want turning. The crank makes the conversion — at a price. Twice every revolution the rod and crank line up, and at those dead centres the steam, push as it may, turns nothing at all. The remedy is tons of iron rim: the flywheel drinks up the surplus of the strong part of the stroke and repays it through the dead points. Thin the wheel down to a ton and watch the needle surge and sag; a parked engine may even refuse to start, which is why crews barred engines round by hand to a strong position.

One chapter of nine. Read the whole Primer, or work the engines themselves.