The disc in a rotary crusher of glass and metal in a mercury recycling system encounters fracture failure due to unknown reason. Possible causes are identified and investigated both numerically by finite element analysis and experimentally including modal analysis and materials, acoustic signature and destructive load characterisation. Knowledge is developed on the conditions needed for failure. It is concluded that the observed fracture during operation occur due to a large force rather than fatigue, and a hypothesis on how this may be realized is put forward.