Abstract: The impact stresses of balls in ball mills are much critical to material design of balls and liners. The maximum impact velocity and impact energy are determined. Based on the Hertz elastic contact theory, formulas to calculating the impact stress, the contact duration and the radius of the contact area are deduced. The calculations show that the impact stress is as great as 10GPa level. The contact duration is about 0.1ms level. The contact area is much less than the ball size. The experiment data fit the calculation results very well.

Key words: Ball mill, Impact, Stress