The function of tire screws is to firmly connect the automobile wheel hub and the wheel as a whole. Under normal circumstances, the screws only bear axial tension and no radial shear stress. When the automobile wheel hub nut becomes loose, the force on the screws will change from bearing tension to bearing shear stress. If the suddenly increased load exceeds its rated load, the screws will be sheared and broken. Screw breakage is extremely dangerous for automobile driving safety. Combining the knowledge of fastener screws, the following shares the specific causes of screw looseness and breakage (excluding the quality of screws and nuts and processing reasons) as well as effective prevention measures.
1. Causes of screw looseness and breakage excluding the quality of screws and nuts and processing reasons
(1) Failure to tighten according to technical requirements during installation, or large differences in the tightness of each nut. Once individual nuts become loose and lose their bearing capacity, the load on other nuts will increase significantly, and long-term uneven force is likely to lead to breakage.
(2) During tire maintenance and installation operations, many units only use ordinary air wrenches for disassembly and assembly instead of torque-controlled air wrenches for installation, which cannot control the upper limit of torque. Some repairmen deliberately increase the tightening torque for fear that the tire screws are not tightened, increasing the normal tightening torque required for large passenger cars (550~650Nm) to 1000Nm or even higher. Frequent disassembly and assembly of tires will cause the screws to be stretched, which in turn leads to nut loosening.
(3) Wear or deformation of the screw holes of the tire rim. Even if the nut torque meets the standard requirements, the tire rim cannot be pressed tightly, resulting in abnormal force on the screws.
(4) After long-term use, the threads of bolts and nuts are damaged, the disc spring washers fail, or the flat washers are cracked and deformed, which will cause the nuts to fail to play an effective locking role, and then looseness occurs.
(5) The positioning collar of the hub nut is damaged or missing, or the inner hole of the rim is out of round due to extrusion deformation of the screw holes; the operation process is simplified during tire installation, and the hub nuts are not tightened in a diagonal order, but one by one at one time. For bolts without a central positioning sleeve, this operation will cause the bolts to deviate from the central positioning. Although the nuts are tightened during assembly, the asymmetric arrangement of bolts and screw holes will lead to inconsistent tire turning radius, and the nuts will become loose soon after the wheel rotates.
(6) Emphasizing use while neglecting maintenance, drivers and repairmen lack a sense of responsibility and fail to conduct regular inspections in accordance with standard requirements; some inspections are perfunctory and not serious at all, failing to timely find hidden dangers such as loose screws.
2. In view of the causes of tire screw breakage, effective prevention measures should be taken from the following aspects
(1) Drivers must check the technical status of tires and hub bolts before departure, during driving and after work. If looseness is found, they should report for repair immediately, and it is strictly prohibited to drive with faults.
(2) Conduct mandatory maintenance operations in accordance with technical requirements, and do not arbitrarily extend the maintenance interval mileage. Repairmen must strictly follow the operation specifications. During primary maintenance, the hub nuts must be inspected and tightened; during secondary maintenance, it is necessary to clean and inspect the condition of bolts, nuts and threads, clean and inspect the wear of rim screw holes, check the deformation of positioning sleeves, washers, rims and inner rims of rims, as well as the flatness of the rim end face. When replacing a new rim, check the thickness of the rim wall; for dual-wheel vehicles, be sure to re-tighten the inner wheel nuts after disassembling and assembling the outer wheel.
(3) Before pre-tightening the nuts, check whether the center position of the wheel is aligned; the tightening operation must be completed step by step in 2~3 times in a diagonal order. The normal tightening torque for large passenger cars is controlled at 550~650Nm to ensure uniform force on each nut. Excessively tight torque will also lead to premature damage of bolts and nuts, which must be strictly controlled. Timely replace failed spring washers, cracked flat washers, positioning collars, and rims with severely worn screw holes; the inner circle deformation of the rim can be repaired; bolts and nuts with more than 3 damaged threads must be replaced to ensure accurate wheel positioning and firm connection.
(4) For units with conditions, after replacing the tires of the vehicle, return to the maintenance factory to re-tighten the tire screws within 50~100Km of driving to ensure reliable tightening.
(5) Select high-quality hub bolts, and it is strictly prohibited to buy inferior products for cheap. Regularly maintain the air pump, timely discharge water and oil in the air reservoir to reduce the erosion of water and oil on the rim and tire body; regularly check the technical performance of the tire changer to avoid affecting the installation quality due to insufficient torque. If conditions permit, priority should be given to using pneumatic or electric torque-controlled tools to ensure uniform force on the screws.
(6) During vehicle driving, if abnormal noise from the wheels or left-right shaking of the vehicle body is found, stop immediately for inspection to eliminate potential safety hazards. If the hub bolt breaks accidentally, do not sharply turn the steering wheel or apply emergency braking. Stabilize the steering wheel, slowly decelerate to stop, and then take corresponding measures to handle it.
