Awearandtraction-testingmachineconsistingoftwodisk-shapedspecimensorientedsuchthattheiraxesareparallelandwhosecircumferential,cylindricalsurfacesarecausedtorollorrollandslideagainstoneanother.Therotationratesofeachdiskmaybevariedsoastoproducevaryingdegreesofslidingandrollingmotion.