The car tire leaks after being punctured, so it needs to replace the spare tire, and go to the repair shop to repair the tire and replace the tire again. Many old irons should have experienced this process. To replace a tire, it is necessary to disassemble and tighten the wheel bolts. Screw owners often see maintenance technicians using a long extension rod or even stepping on a wrench to tighten the wheel bolts. They use their body's strength to press down on the wrench to achieve tightening, and tell customers that the wheel bolts must be tightened enough, otherwise there is a risk of tire loosening when driving on the highway. Is this really reasonable?
1. Wheel fastening connection structure
The fastening methods of wheels can be roughly divided into two types, namely wheel bolt method and wheel nut method: The wheel bolts pass through the brake disc and tire and are tightened onto the pre threaded hub. The head of the wheel bolt is curved and can resist tangential loads when tightened. Compared to wheel bolts, wheel hub bolts pass through the brake disc and tire from the inside, and wheel nuts are tightened on the threaded bolts. The wheel nut method is used in most regions except Europe (China, North America, Japan, etc.). The advantage is that when installing the wheels, the wheels are hung on the bolts for operation, so the operability is good; Compared to tightening bolts, it is advantageous in terms of torque attenuation. The disadvantage is that when the bolt is damaged, it needs to be replaced (pressed into) the wheel hub bolt, resulting in part replacement labor costs, which is disadvantageous in terms of cost compared to tightening the bolt method.
2. The hazards of over tightening?
Some people believe that wheel bolts are used to fasten tires. When the tire is driven at high speeds or under dynamic loads, the external load is relatively large, so it should be pre tightened with a larger torque. Even like other positions on the chassis, bolts should be tightened by yielding. So, what harm can we do if we keep twisting towards death? 1) When tightened too tightly, the wheel bolts will undergo irreversible plastic deformation after entering a yielding state. Bolts that undergo plastic deformation cannot be reused multiple times, which means that every time the wheel is replaced, the wheel bolts also need to be replaced, which is very inconvenient. 2) When tightened too tightly, the axial preload force of the wheel bolts is large, and when subjected to high external loads, the bolts are prone to fracture and failure. 3) When tightened too tightly, the internal threads of the workpiece bear a large load, and after repeated use, it is easy to cause slipping teeth. At this time, only the opponent parts can be replaced, which is inconvenient to operate and costly. 4) When tightened too tightly, the load on the pressure bearing surface of the workpiece is relatively large, especially for aluminum alloy wheels, which are prone to surface crushing problems.
3. How to tighten?
According to the relevant information of a mainstream vehicle manufacturer referenced by Screw Lord, the connection structure of the commonly used 5 wheel bolts is designed. If steel wheels are used, the clamping force should be ≥ 35KN, and if aluminum wheels are used, the clamping force should be ≥ 25KN. Considering that the wheel bolts need to be reused, after 5 repeated tightening cycles, the clamping force of the wheel bolts still needs to meet the requirements of steel wheel hub ≥ 35KN and aluminum wheel hub ≥ 25KN. In terms of torque, the recommended process torque is 120Nm, with a maximum of 140Nm, which is the assembly torque used in the final assembly workshop. The yield torque of wheel bolts is usually above 180Nm-200Nm. From a design perspective, the process torque is much lower than the yield torque. This is also to ensure that the wheel bolts are always used in the linear segment of the tightening curve, and their performance will not decrease even after repeated use. Therefore, when repairing and tightening the position of the wheel connection, it is necessary to use a fixed torque wrench or a digital wrench to tighten it. The usual tightening torque is about 120Nm or to the torque assembled by the original factory of the vehicle.
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