Hi everyone. I'm a college student, and I need help of experts with gears. What I know is that there are two dynamics in gear : rolling and sliding. And I heard that the main cause of failure (wear) of spur gear is rolling. Then... how about the cause of failure in the case of helical gear?? I felt that the sliding works as more important role in helical gear than spur gear because of their geometry. But I can't find any literature that comparing effect of rolling and sliding on the surface of helical gear teeth (especially for failure). And what is a difference between failure by rolling and sliding on the gear teeth?? I hope I could get an answer... Thank you!