Another very common mode of failure of gear teeth is that due to surface damage or pitting, which can eventually result in extensive cratering and loss of the load bearing surface of the tooth. Which of the following best describes the causes, and some of the measures that can be taken to prevent this form of failure. a - Maximum compressive stresses occur at the root of the tooth: - Stress concentrations due to cracks in the tooth flank can increase these stresses: - Failure is due to fatigue loading, and is worse because of shock loads: Asperity to asperity contact, and hence high surface roughness or poor lubrication can make this failure worse: -Surface load capacity can be improved by selecting materials with high surface hardness values, such as carburised steels; - Once pitting has occurred over more than 10 % of the surface, gears are no longer serviceable. b. - Maximum tensile stresses occur at the point of contact between the teeth; - Stress concentrations due to pits or small fillet radii can increase these stresses: - Failure is due to fatigue loading, and is worse because of shock loads. - Surface load capacity can be improved by selecting materials with high UTS values, such as carburised steels: Asperity to asperity contact can also lead to high temperature failures such as scuffing and welding: - Grinding can introduce further tensile stresses, which reduces bending strength. C - Maximum tensile stress occurs at the root of the tooth; - Stress concentrations due to cracks in the flank of the tooth can increase these stresses: - Failure is due to fatigue loading, and is worse because of shock loads: - Surface load capacity can be improved by selecting materials with high UTS values, such as carburised steels; -Asperity to asperity contact can also lead to high temperature failures such as scuffing and welding: Once pitting has occurred over more than 10 % of the surface, gears are no longer serviceable. d. - Maximum compressive stresses occur at the point of contact between the two teeth; - These stresses are due to Hertzian stresses because of elastic deformation arising from the local curvature of the teeth: -Failure is due to fatigue loading, which can cause cracks to open up and propogate under repeated loading: Asperity to asperity contact, and hence high surface roughness or poor lubrication can make this failure. worse: -Carburising can introduce compressive stresses which increases surface compressive strength; Surface load capacity can be improved by selecting materials with high UTS values, such as carburised steels: e. - Maximum compressive stresses occur at the point of contact between the two teeth; - These stresses are due to Hertzian stresses because of elastic deformation arising from the local curvature of the teeth: -Failure is due to fatigue loading, which can cause cracks to open up and propogate under repeated loading: Asperity to asperity contact, and hence high surface roughness or poor lubrication can make this failure worse: Surface load capacity can be improved by selecting materials with high surface hardness values, such as carburised steels; -Once pitting has occurred over more than 10 % of the surface, gears are no longer serviceable.