Casting Study: Electrical Auxiliary Switch Cam

A starter for large horse-power motors incorporated a fabricated sheet-metal cam bolted to a bracket, and welded to a shaft. Simplifying manufacturing and reducing cost, the cam design was changed to a single-piece ZA-12, graphite permanent mould casting. Additionally, the motor manufacturer estimates that approximately 20 minutes of tedious set-up time per cam is eliminated by using the ZA-12 cam. ZA-12 alloy also gives the cam the strength to withstand the rigors of field installation. Relatively low-volume requirements point to the use of graphite permanent moulds. Because aluminium’s melting precludes its use in graphite moulds, ZA-12’s lower melting temperature and mechanical properties make it the ideal material for the application. As a design note, plastic was rejected because the cam hub is bolted to the shaft. A plastic cam can deform or even crack during bolting.