Failure Analysis of Flange Bolts in a Wheel Hub Bearing Unit

ZHANG Hao; LI Xuegang; JI Hongchao; SUN Chaofan; FU Haibing

doi:10.3969/j.issn.1674-6457.2026.05.023

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Journal of Netshape Forming Engineering ›› 2026, Vol. 18 ›› Issue (5) : 248-258. DOI: 10.3969/j.issn.1674-6457.2026.05.023

Advanced Manufacturing Technology and Equipment

Failure Analysis of Flange Bolts in a Wheel Hub Bearing Unit

ZHANG Hao¹, LI Xuegang¹, JI Hongchao^1,2,3,*, SUN Chaofan¹, FU Haibing²

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Abstract

The work aims to analyze and address the failure of all five high-strength bolts in the left rear wheel hub bearing unit of a passenger vehicle, which fracture under Vm operating conditions (a road test for newly developed automobiles) and result in wheel detachment, identify the root cause of the fracture through systematic investigation of the failed bolts, and propose feasible improvement measures to ensure the reliability and safety of automotive wheel hub bolts. In this study, fracture surface morphology observation, metallographic analysis, chemical composition analysis, hardness testing, bearing bolt fastening performance tests, and simulation validation were employed. A comparison with unused samples from the same batch was made to analyze the failure mechanism of the fractured bolts. The fatigue failure was caused by stress concentration initiated by mechanical damage. The fatigue cracks primarily originated from surface damage at the thread root. Specific external factors included insufficient tightening torque (below the recommended value) due to multiple wheel removals and the failure to use the brake disc locating hole, leading to stress concentration in the bolts. Internal factors included the presence of inclusions of approximately 20-30 μm in size in the bolt material, which reduced the material's fatigue strength and accelerated bolt failure. Based on the experimental results, the failure mechanism of bolt fracture is summarized. It is found that the formation of fatigue fracture is closely related to insufficient external tightening torque (the main cause), improper use of brake disc positioning holes, and inclusion particles in the bolt material. The general durability test verifies that bolts would fracture prematurely when the tightening torque is insufficient. Meanwhile, the general durability simulation analysis confirms that insufficient preload leads to an increase in the maximum sliding distance of the clamped parts. It is concluded that insufficient preload will increase fretting wear, causing mechanical damage to the bolts and resulting in fatigue fracture. To prevent similar accidents, the study proposes the following improvement measures: strictly tighten bolts according to the standard torque, and appropriately increase the tightening torque during repeated disassembly and assembly; standardize the use of brake disc positioning holes; optimize the smelting process to reduce inclusions, and strengthen heat treatment quality control to improve material homogeneity and fatigue strength.

Key words

high-strength bolts / SCM435 steel / fracture / tightening torque / preload

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ZHANG Hao, LI Xuegang, JI Hongchao, SUN Chaofan, FU Haibing. Failure Analysis of Flange Bolts in a Wheel Hub Bearing Unit[J]. Journal of Netshape Forming Engineering. 2026, 18(5): 248-258 https://doi.org/10.3969/j.issn.1674-6457.2026.05.023

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Funding

The National Natural Science Foundation of China (52475354); Hebei Provincial Department of Education Industry-University-Research Special Project (CXY2024054)