The fatigue characteristics of many materials are very sensitive to material conditions, the service enfiroment and even finish. A good example is aluminum. A standard endurance limit (stress level that the material can withstand for 500,000,000 cycles - also used as infinite life criteria) for smooth 2024-T3 is almost 20,000 psi; for 5061-T6, over 12,000 psi; while for casting alloys the endurance limit is less than 8,500 psi.

Our reduction drive case, for instance, is machined from 6061-T6 billit rather than cast.