peritrochium

A peritrochium is the larger wheel in a peritrochio, a classical simple machine consisting of two coaxial cylinders or wheels of different diameters. The peritrochium itself is the outer, larger wheel, while the smaller cylinder (the axle proper) is called the axle or arbor. Together they rotate as one unit on a common axis, but their different radii create a mechanical advantage.

The practical work of a peritrochio depends on the size ratio between peritrochium and axle. Load or effort applied to the rim of the peritrochium translates to proportionally greater force at the smaller axle, or vice versa. This is why capstan winches and ship's wheels were built this way: a hand-friendly large wheel allowed operators to lift heavy loads through a small diameter drum or shaft. The mechanical advantage equals the ratio of the peritrochium radius to the axle radius.

Variants and real applications

A peritrochium may be a solid disk, a spoked wheel, or a drum with rope wound around it. In medieval and early industrial contexts, the peritrochium was often wood with an iron rim or hub. Modern examples include the hand wheel on a valve stem (large outer wheel driving a small threaded shaft) and the pulley wheel on a rope hoist or tackle. In each case, the peritrochium is the point of human contact or load attachment, while the axle drives or is driven by mechanism below.

The term itself comes from Greek, combining peri (around) and trokhios (wheel or running), reflecting the classical naming of machines by their parts. It is rarely used in modern English outside academic study of classical mechanics or historical engineering analysis. Modern engineering typically calls the large wheel simply a wheel and describes the advantage ratio directly, making peritrochio nomenclature largely historical rather than working jargon.