Cov yuam sij semicircular yog ib yam ntawm cov yuam sij, nws sab sauv yog lub dav hlau, sab saum toj yog semicircular arc, ob sab yog tib yam, feem ntau hu ua crescent key.Qhov chaw ua haujlwm ntawm tus yuam sij semi-circular yog ob sab, thiab lub zog torque kis tau los ntawm sab.Nws muaj qhov nruab nrab zoo tib yam li daim ntawv cog lus tiaj tus.Tus yuam sij tuaj yeem ua viav vias nyob ib puag ncig ntawm qhov chaw ntawm arc curvature ntawm qhov chaw hauv qab ntawm qhov zawj hauv qhov zawj, yog li nws tuaj yeem hloov kho mus rau qhov qaij ntawm qhov chaw hauv qab ntawm lub hub keyway.