First of all, let's see what a tire bolt is and what it is for. Tire bolts refer to bolts mounted on the wheel hub connecting the wheel, brake disc (brake drum) and wheel hub. Its function is to securely connect the wheels, brake discs (brake drums) and hubs to each other. As we all know, the weight of the car is ultimately borne by the wheels, so the connection between the wheels and the body is through these bolts. Therefore, these tire screws actually carry the weight of the entire vehicle and also transmit the torque generated by the gearbox to the wheels, which are simultaneously subjected to both a pulling force and a shearing force during operation.
The design of a tire bolt is very simple: it consists of a screw, a nut and a washer. According to different screw designs, they can also be divided into single head bolts and double head bolts. Most of the cars now have single head bolts, and double head bolts are commonly used in small and medium trucks. There are two ways to install single head bolts. One is hub bolts + nuts. The bolts are fixed on the hub with an interference fit, and then the wheels are fixed with nuts. Generally, Japanese and Korean cars are widely used, and most trucks also use it in this way. The advantage of this method is that the wheel is easier to find, easier to disassemble and assemble, and the safety is higher, while the disadvantage is that it is more difficult to replace the tire screws, and some require the wheel to be disassembled. wheel hub; Tire screws thread directly onto the wheel hub and are commonly used in small European and American vehicles. The advantage of this method is that it is easier to disassemble and replace the bolts, but the disadvantage is that the safety is slightly lower. hub must be replaced.
Automotive tire bolts are usually made from high strength steel. The screw grade is marked on the screw head, including 8.8, 10.9 and 12.9. The higher the value, the higher the strength. 8.8, 10.9, and 12.9 here refer to the bolt marking, which consists of two part numbers that respectively represent the nominal tensile strength and the yield strength ratio of the bolt material, usually expressed as "XY", such as 4.8, 8.8 , 10.9, 12.9, etc. The tensile strength of class 8.8 bolts is 800 MPa, the yield factor is 0.8, the yield strength is 800×0.8=640 MPa; the tensile strength of class 10.9 bolts is 1000 MPa, the yield factor is 0.9, the yield strength is 1000 × 0.9 = 900 MPa. Others and so on. Generally, the strength is 8.8 grade and above, the bolt material is low carbon alloy steel or medium carbon steel, and it is heat treated as a high strength bolt. Car tire screws are high-strength bolts. Different models and load capacities have different bolt strengths. 10.9 is the most common, 8.8 usually corresponds to lower models, and 12.9 usually corresponds to heavy trucks. .
If you look closely, you will find that the car has four, five, six or even more tire screws. Why such difference? Is it true that the more screws, the safer?
In fact, the number of screws installed depends on the load of the car and the strength of the screws. As a rule, on small cars, the weight is relatively light, and four tire bolts are enough; on medium to large vehicles and SUVs, the vehicle is heavy and typically requires at least five tire bolts; on heavier models, it will have more tire bolts, some trucks even have a dozen tire bolts, and the bolts are also large diameter, very strong and able to withstand higher loads. However, do not judge the hero by the number of tire bolts, the strength and number of car tire bolts are all carefully calculated, and the worst working conditions of the car will be taken into account, so the margin of safety is very large. In general, there will be no breaks, backlashes, etc., and it is almost absolutely safe. Therefore, no matter how many tire bolts are used, driving safety can be guaranteed. The more the better, for example, some models use four 10.9-strength tire bolts, and another model uses five 8.8-strength tire bolts of the same diameter. if the total strength is calculated, four tire screws are higher.
Let's talk about the principle of tightening and self-locking tire bolts.
Automotive tire screws usually have triangular threads with fine teeth, the bolt diameter is 14 to 20 mm, and the thread pitch is 1 to 2 mm. This type of triangular thread can theoretically lock: after tightening the tire screw to a given torque, the threads of the nut and bolt align, and the enormous frictional force between them can hold them in place. In this case, the bolts are elastically deformed, and the wheels and brake discs (brake drums) are rigidly fixed on the hub. Using a fine thread pitch can increase the friction area between the threads, and the loosening effect is better. At present, fine pitch threads are used more and more in automobiles, because their anti-screw effect is better.
But when the car is moving, the wheels are subjected to alternating loads, and the tire bolts are also subjected to constant shock and vibration. At the same time, at a certain moment, the friction force between the tire bolts and nuts disappears, and the tire screws can loosen; in the opposite direction, a "relaxing moment" is created, which in turn causes the tire bolts to loosen. . Therefore, the tire bolt must have a reliable self-locking and anti-loosening device. Most modern automotive tire bolts use frictional self-locking devices to prevent loosening, such as the addition of elastic washers, machining wheels and nuts into appropriate tapered or spherical surfaces, and the use of spherical spring washers. They can compensate for the gap caused by the tire bolts at the moment of impact and vibration, and prevent the tire bolts from loosening.
There are also some models that use "in-and-out" type tire bolts, i.e. left-hand tire bolts are left-handed and right-hand tire bolts use right-hand threads. threads, so that the direction of tightening the threads corresponds to the rotation of the wheel. If the direction matches, the "loosening moment" is weakened or even eliminated, and automatic loosening of the tire bolts is prevented. This type of bolt for left-hand tires is usually marked with a capital "L" on the head of the bolt, or a groove is cut into the center of the nut to distinguish right-handed bolts. This kind of left-hand thread tire screw is widely used in the early cars of Dongfeng, Jiefang and others, and there are still some medium and light trucks, light buses and small SUVs that use this design.
But why don't large and small cars use this design now? This is mainly because heavy duty vehicles usually use dual tires and a single tire nut. Tire nuts use spherical spring washers to prevent nuts from loosening spontaneously. Both are relatively fast, so whether it is acceleration or braking, there will be a loosening moment, and it makes no sense to use the left screws for the tires. Therefore, these two models no longer use left-handed busbar screws, reducing the number of parts and facilitating disassembly and maintenance.
So, what should I pay attention to when disassembling and installing tire bolts? When disassembling, pay attention to even diagonal loosening and keep the wrench perpendicular to the bar screws to avoid applying bending moments to the bar bolts which could cause the bolts to break. When installing, pay attention to diagonal installation and even diagonal tightening in stages, do not tighten all at once, let alone install all the bolts on one side of the wheel, to make the bolts evenly loaded and avoid rim deformation. Once the jack is down and the wheel touches the ground, use a torque wrench to tighten the tire mounting bolts to the specified torque, typically 15 to 20 kg. the bolt can stretch and break. If there is no torque wrench, then it is enough for the average adult man to take one end of the tire wrench with both hands and tighten it with all his might, this effort is quite enough.
Also, avoid using a jackhammer when tightening tire bolts. Generally, the torque of this jackhammer is too high, which will stretch the tire bolts, even if they are stretched to the yield strength of the tire bolts, which will cause when the tire bolts fail Reverse plastic deformation, such bolts are no longer tightened and should be replaced. Also, do not repeatedly disassemble and reassemble tire bolts. Each disassembly is a thread damage. The friction between the bolt and nut will become smaller and smaller, and the self-locking effect of the bolt will weaken or even disappear. It can no longer be tight. Generally, it is recommended to replace the tire screws that have been disassembled and reassembled more than ten times. Also, do not apply oil to the tire screws, as although they will be easier to disassemble later, the oil will reduce the friction between the threads, reduce the self-locking effect of the screws, and increase the chance of the screws being loose.