MAINTENANCE MANUAL - Meritor

Failures and Causes2. Flexing Fatigue:Flexion fatigues at the root are originated byshock and / or repetitive overload, causing cracksfound in the roots of the teeth of the gear.As the mileage increases, the initial cracks increasesand the gear teeth will progressivelyweaken until the break occurs.Flexion fatigue in the root of the drive pinion teethis characterized by the same type of beachmarks left on the axles that has flexion fatiguefailure caused by rotation. When only two orthree teeth breaks, this indicates that the causemay have been an instantaneous shock load.Fatigue-induced by shock loads will also showorigins that are also in line. If all the remainingteeth are broken, the likely cause was a severeshock load with continuous application of voltageor overload of the vehicle.The beach marks causes flexion fatigue cracksat the root, starting at the root of all affectedteeth and progress to the hardened surface ofthe drive pinion and ring gear set teeth. Figure9.2.3. Fatigue Initiated by Shock:Shock loads are often severe enough to breakthe gear’s teeth at the root, parting axles in twoand causing other failures as well. Often, shockloads do not produce instantaneous failure,but leave the component weakened or with acrack. Depending on the severity of the shock,the definite failure may occur only after manykilometers. Figure 9.3.Figure 9.34. Flank CrackingFlank cracking usually causes the peeling of ametal surface very similar to what is a failureby chipping (spalling). However, a gear withcracks in the sides will develop first longitudinalbreaks along the surface of the gear tooth.Once these cracks appear, the failure occursquickly. Many times, just one tooth gives a signof decay while the other teeth are intact. Butwhen breaks appear, soon the metal betweenthem begins to flake off the teeth. Figure 9.4.Figure 9.242 MAINTENANCE MANUAL