Abstract: The relative motion of rice grain with threshing tooth is analyzed. Using the principles of impulse and relative motion, the changes of grain relative velocity and kinetic during impact between threshing tooth and grain are given. Based on the con-servation of energy law and the theories of fracture mechanism, surface energy absorbed by grain is obtained. The quantity of threshing injury to rice grain is calculated. Round and rectan-gular section threshing teeth are compared. Good correlation is found between the calculation and the experimental results. A method for researching the threshing injury to rice grain is given.

Key words: Energy conservation, Relative motion, Rice grain, Threshing injury