Cause Analysis of Vehicle Tire Bolt Fracture

Cause Analysis of Vehicle Tire Bolt Fracture

With the increase in the number of cars in our country, the topic of vehicle driving safety continues to affect the attention of the majority of car owners and the media. In view of the continuous occurrence of vehicle tire bolt fracture safety accidents, the author has been engaged in failure analysis for many years, and now the reasons that affect tire bolt fracture in axle technology are classified and summarized. Most cars adopt semi-floating axle structure, and the reasons for the failure of tire bolts can be analyzed from the following aspects.

1. Torque moment of tire bolt

The fixed torque of most passenger cars before leaving the factory is 120-150N • m, which is fastened by torque-fixing equipment. When the vehicle is running, the tire bolts are subjected to the shearing force of the forward rim of the vehicle, the torque pressure force of fixing the tire nut, and the pressure stress of the connection process of various related parts. Long-term operation of related parts will cause fretting, and the torque itself will also exist. The recession phenomenon causes a slow decrease in the tightening torque, which needs to be tightened regularly during vehicle maintenance. When dealing with a vehicle accident caused by the shear fracture of all the tire bolts on one side during the driving of an SUV vehicle, it is found that the tightening torque of the tire bolts on the other side is about 90N • m. If only the faulty side is repaired, in the subsequent In operation, due to the continuous decrease in the torque on the lead side, tire bolt breakage is bound to occur.

2. Rim hole condition

The rim is connected to the axle through the tire nut and the bolt. Since the cone surface is attached to the cone surface, there is a frictional force for attachment. It is necessary to check whether there is torsion, wear and deformation in this part, and whether there is quality and friction between the rim and the brake drum or the mounting surface of the brake plate. This helps to determine whether the nut is loose or the torque is degraded before the tire bolt breaks.

3. Determination of bolt position degree

It is necessary to check the offset between the tire bolt and the half shaft hole at the fracture. Generally, the position of the tire bolt after press-fitting through the half shaft hole is required to be about 0.25mm. If the offset is too large, the tire bolt will be offset from the rim hole (the position of the rim hole is generally 0.15-0.25mm) after the tire bolt is connected with the tire nut. Although the fixing torque meets the requirements, there will be an imaginary gap, at the same time, the tire bolts bear the shear force of the eccentric load.

4. Influence of rim material

Generally, the thickness of aluminum alloy rims is 3mm, and the design thickness of steel rims is 6mm. Due to the material characteristics, the plastic deformation of aluminum alloy rims is much larger than that of steel rims, which will accelerate the decline of tire fixing torque. Therefore, the operation of aluminum alloy rims for 3000-5000Km requires calibration of tire nut torque.

5. Breaking position of tire bolt

Determine the R angle of knurling in the tire bolt danger area and the joint position of thread and rod.

6. Breaking surface of tire bolt

Determine whether the tire bolt is brittle fracture or fatigue fracture, etc., is the mechanism or non-mechanism fracture. These determinations may require extensive hands-on experience, with a final detailed analysis conducted by a specialist body or specialist equipment.

7. Vehicle maintenance records

Do you need to pay attention to whether the vehicle is regularly maintained? Where is the maintenance? Generally, regular 4S shops will repair and repair key parts according to the maintenance guidance documents provided by the manufacturer, while ordinary repair stations will use air cannons to tighten, often with excessive torque, which will exceed the limit torque value of tire bolts (the maximum torque moment of 10.9 grade bolts is about 160N • m, which will change according to the diameter of bolts), which will cause potential risks of use.

8. Key inspection items of relevant parts

① Axle position 0.15mm;② Brake drum position 0.3mm;③ Tire nut cone surface runout 0.15mm and angle; ④ Rim position 0.15-0.25mm, cone surface runout and angle; ⑤ Bolt mechanical properties, material, surface decarburization; ⑥ Bolt each R angle size; ⑦ Wheel structure, brake drum, rim and axle stop positioning, check whether there is interference, etc.

Source: Golden Spider Fastener Web