How to Install Wheels on Cars —— Talking about the locking principle of tire screws and matters needing attention

A few days ago, at a class reunion, a girl told a story that made her feel "the rest of her life": once she drove back to her hometown in the countryside and had a flat tire on the way, then she found a roadside shop to fix it, and then she went on the road. After driving for a while, I felt something was wrong with the car, the direction was always biased to one side, and the wheels were abnormal. So she stopped by the side of the road and got off to check. Seeing this, she gasped: The newly repaired wheel, the tire screw Quan Song, and the tire almost fell off! If you keep running like this, this tire will have to "sell sesame seeds". Maybe something will happen and the car may be destroyed. Thinking about it really feels like "the rest of my life".

It's no use worrying about it. The urgent task is to repair the car. The tire screw of the original car was broken and could not be fastened, so she removed a screw from each of the other three tires and installed it on the faulty wheel, so that at least the car could continue to drive. Then hurry back, find a bigger repair shop and change all the tire screws. This is meditation. After this incident, every time she travels, the first thing is to see if the tire screws of the car are tight and safe.

The speaker has no intention, but the listener has a heart. I heard about it nearby, and out of professional sensitivity, I immediately thought of tire screws, a small thing that many people ignored. In fact, the tire screw of a car is very important, but because it does not need special maintenance, it is not needed unless the tire is replaced, so many people will ignore it, and even many people have never removed the tire screw. When we watch accident videos or pictures online, sometimes there will be accidents in which the wheels of high-speed cars fly out, mainly because the tire screws are broken or loose. So what's the use of car tire screws? How is it installed? What is its fastening and self-locking principle? What are the precautions in use? Today we will talk about the topic of tire screws.

First of all, let's take a look at what a tire screw is and what its function is. Tire screw refers to the screw installed on the hub and connecting the wheel, brake disc (brake drum) and hub. Its function is to connect the wheel, brake disc (brake drum) and hub reliably. As we all know, the weight of the car is ultimately borne by the wheels, so the connection between the wheels and the car body is realized by these screws. Therefore, these tire screws actually bear the weight of the whole vehicle, and also transmit the torque output by the gearbox to the wheels, and the wheels are simultaneously subjected to tensile force and shear force.

The structure of tire screw is very simple, which consists of screw, nut and washer. According to the different screw structures, it can also be divided into single-headed bolts and double-headed bolts. At present, most cars are single-headed bolts, and small and medium-sized trucks generally use double-headed bolts. There are two ways to install single-headed bolts, one is hub bolt+nut, the bolt is fixedly installed on the hub through interference fit, and then the wheel is fixed with nut. Generally, they are Japanese cars and Korean cars, and most trucks also use this method. The advantages of this method are that the wheels are easier to locate, easier to disassemble and assemble, and more secure. The disadvantage is that it is more troublesome to replace the tire screws, and some wheels need to be disassembled. The other is the wheel fixing bolt, which is directly threaded on the wheel hub, and the tire screw is directly screwed on the wheel hub, which is generally used in small cars in Europe and America. The advantage of this method is that it is easier to remove and replace the tire screws, but the disadvantage is that the safety is slightly worse. If the tire screws are disassembled repeatedly, the threads on the hub will be damaged and the hub must be replaced.

Automobile tire screws are generally made of high-strength steel, and the strength grade of the screws is printed on the tire screw head, which is 8.8, 10.9, 12.9. The higher the value, the higher the strength. Here, 8.8, 10.9 and 12.9 refer to the performance grade labels of bolts, which are composed of two parts, indicating the nominal tensile strength value and yield ratio value of bolt materials respectively, and are generally represented by "X.Y", such as 4.8, 8.8, 10.9 and/kloc. The tensile strength of the bolt with performance grade 8.8 is 800MPa, the yield ratio is 0.8, and the yield strength is 800× 0.8 = 640 MPa. The tensile strength of bolts with performance grade of 10.9 reaches 1000MPa, the yield ratio is 0.9, and the yield strength is 1000×0.9=900MPa. Others and so on. Bolts with general strength of 8.8 or above, made of low-carbon alloy steel or medium-carbon steel and heat-treated are called high-strength bolts. The tire screws of automobiles are all high-strength bolts, and the strength of matching bolts is different for different models and different loads. 10.9 is the most common, 8.8 is generally matched for low-end models, and 12.9 is generally matched for heavy trucks.

If you look closely, you will find that there are four, five, six or even more tire screws installed in the car. Why is there such a difference? Is it safer to have more tire screws?

In fact, the number of tire screws installed is related to the load of the car and the strength of the tire screws. Generally, small cars are relatively light in weight, and four tire screws are enough; Large and medium-sized cars and SUVs, with heavy vehicles, generally need at least 5 tire screws; In heavier vehicles, more tire screws will be installed, and some trucks even have twelve tire screws, and the bolts have large diameter and high strength, which can adapt to higher cargo load. However, we should not judge heroes by the number of tire screws. The strength and quantity of automobile tire screws are precisely calculated, taking into account the worst working conditions of the automobile, so there is a very large safety margin. Under normal circumstances, they will not break or loosen, which is almost absolutely safe. Therefore, more tire screws can ensure driving safety, and the more the better. For example, some models use four tire screws with a strength of 10.9, while another model uses five tire screws with the same diameter and a strength of 8.8. If the total strength is calculated, the four tire screws are higher.

Let's talk about the fastening and self-locking principle of tire screws first.

Fine-pitch triangular threads are generally used for automobile tire screws. The bolt diameter is between 14~20mm, and the pitch is 1~2mm. Theoretically, this triangular thread can be self-locked: after the tire screw is tightened to a specified torque, the threads of the nut and the bolt are attached to each other, and the huge friction between them can keep them motionless, that is, self-locking. At the same time, the bolts are elastically deformed to tightly fix the wheel and the brake disc (brake drum) on the hub. Using fine pitch can increase the friction area between threads and better prevent loosening. Nowadays, more and more fine thread is used in cars, that is, its anti-loosening effect is better.

However, when the car is running, the wheels are subjected to alternating load, and the tire screws will also be subjected to continuous impact and vibration. In this case, at a certain moment, the friction between the tire bolt and the nut disappears, and the tire screw may be loose; In addition, when the car is accelerating and braking, because the direction of wheel rotation is opposite to the fastening direction of tire screws, a "loosening moment" will be generated, which will lead to the loosening of tire screws. Therefore, tire screws must have reliable self-locking and anti-loosening devices. At present, most automobile tire screws adopt friction self-locking and anti-loosening devices, such as installing elastic washers, machining a matching conical surface or spherical surface between the wheel and the nut, and using spherical spring washers. They can compensate the gap caused by the impact and vibration of the tire screws and prevent the tire screws from loosening.

There are also some models that use "inside-out and outside-out" tire screws, that is, the left wheel tire screws use left-handed threads and the right wheel tire screws use right-handed threads, so that the fastening direction of the threads is consistent with the rotation direction of the wheel, which weakens or even eliminates the "loosening torque" and avoids the automatic loosening of the tire screws. This kind of left-handed tire screw is usually marked with a capital letter "L" at the head of the screw, or a groove is opened in the center of the nut to distinguish the right-handed bolt. This kind of left-handed tire screw was widely used in early Dongfeng, Jiefang and other cars, and now there are some medium and light trucks, light buses, small off-road vehicles and so on.

But why don't heavy trucks and small cars adopt this design now? This is mainly because heavy trucks generally install tires with double tires and single nuts, and the tire nuts use spherical spring washers to prevent the nuts from loosening themselves; On the other hand, in small cars, acceleration and braking are relatively fast, so both acceleration and braking will produce loose torque, so it is of little significance to use left-handed tire screws. Therefore, these two models no longer use left-handed threaded tire screws, which is conducive to reducing the number of parts and facilitating disassembly and maintenance.

So what should I pay attention to when removing tire screws? When disassembling, pay attention to loosen the diagonal evenly, so that the wrench is perpendicular to the tire screw, so as to avoid the tire screw being subjected to bending moment and causing the screw to break. When installing, pay attention to diagonal installation and diagonal fastening, do not tighten them at one time, and do not install the screws on one side of the wheel to make the screws stress evenly and avoid the deformation of the rim. After the jack falls and the wheel touches the ground, tighten the tire screws with a torque wrench to the specified torque, which is generally between 15 ~ 20kg. If the screw is too loose, it will be easy to loosen. If it is too tight, it will lead to tight deformation of the rim and even damage the screw. If there is no torque wrench, the average adult man only needs to hold one end of the tire wrench with both hands and tighten it with all his strength. This force is enough.

In addition, try not to tighten the tire screws with air cannons. Generally, the torque of this kind of air cannon is too large, which will lead to the tire screw stretching, even to the yield point, leading to irreversible plastic deformation of the tire screw. This kind of tire screw can no longer be tightened and must be replaced. In addition, tire screws should not be removed repeatedly. Every disassembly will damage the thread, and the friction between the bolt and the nut will become smaller and smaller. The self-locking function of bolts will weaken or even disappear, and tire screws will not be tightened. Generally, it is recommended to replace the tire screws that have been removed for more than ten times. In addition, don't butter the tire screws. Although it will be easier to disassemble in the future, butter will reduce the friction between threads and the self-locking function of screws, and increase the possibility of loosening screws.

This article comes from car home, the author of the car manufacturer, and does not represent car home's position.