It is concluded that the fracture of the driving gear is due to the looseness of the fastening nut of the starting motor gear, which makes the meshing transmission between the driving gear and the flywheel ring gear unstable. Under the high-speed operation of the motor, the impact load on the driving gear is too large, and the crack source is generated at the tooth root of the dangerous section where the driving gear bears the greatest bending stress, As a result, cracks of different lengths are produced at the root of each tooth of the gear, which eventually leads to overload fracture of the gear from the root after repeated severe impact.
① The fracture morphology analysis shows that the crack source of the gear fracture is intergranular morphology, no fatigue lines, no fatigue characteristics, indicating that the gear fracture is overload fracture.
② The results of chemical composition analysis, metallographic examination analysis and mechanical property examination of the faulty gear showed no abnormality, which can eliminate the gear fracture caused by unqualified raw materials and heat treatment quality of the gear.
③ After inspection, the manufacturer inspected the size of this batch of drive gears, and the size met the requirements of the drawing. Therefore, the gear fracture caused by unqualified size of drive gears can be ruled out.
④ In the meshing process of the driving gear and the flywheel ring gear, the reaction force of the ring gear is applied at the meshing point. Under normal circumstances, the bending stress of the force on the tooth root of the dangerous section is less than the allowable stress. However, when the driving gear is loose, it will have a serious impact on the gear, so that the bending stress at the tooth root is far greater than the allowable stress, and the crack source at the tooth root will continue to expand. Under the repeated excessive impact load, the gear will be overloaded and broken.
In order to verify this conclusion, the fastening nut of the driving gear of the starting motor is tightened and not locked, and the starting process of the engine is simulated. Through the test, after repeated starting for many times, cracks of different lengths appear at the root of the driving gear, and the crack morphology and location are completely consistent with the fault gear, which indicates that the conclusion is reliable.