RIM

The rim is the peripheral annular part of a body of revolution, in particular of a wheel or a Balance; for the balance, the synonym more often used is Felloe (rim base). On a toothed wheel, the rim is the outer Crown carrying the teeth and connected to the hub (center) by the spokes or by a web; its thickness, its flatness, and its concentricity affect the rigidity of the wheel and the quality of the meshing. On a balance, the felloe (rim) carries the Mass determining the Moment of inertia; it is on this that, where applicable, the screws or timing weights for Regulation / Adjustment of theinertia, as well as any compensating devices. The geometry of the Rim is therefore both structural and functional: lightened by cutouts to limit excess Mass while retaining useful Inertia at the periphery. The care given to the Rim (Poising, surface finish, possible Beveling / Angling of the arms) contributes to chronometric performance and to the beauty of the component. The term is common in the description of gear trains.

On antique chronometers, the Rim of the compensation Balance, known as “cut,” was sectioned at two points near the arms to form two free bimetallic segments, each ending in one or more weights; under the effect of Temperature, these segments would curve to compensate for the variation in Elasticity of the Hairspring (balance spring). This solution, described as early as 1766 by Pierre Le Roy and later refined by Arnold and Earnshaw, disappeared with the emergence of athermal Alloys (Invar, elinvar) in the early 20th century: the Rim of modern Balances has once again become solid and one-piece, made of Glucydur or a similar Alloy, with its Mass adjusted by laser ablation around the periphery.