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Common Problems And Analysis Of Fasteners

Jan. 02. 2020

Fasteners are also called standard parts in the market. They are a kind of mechanical component that can fix or bond two or more components mechanically. fastener manufacturer china to share you Some common problems of fasteners, let us understand together.

How to tighten bolts and nuts

Threaded connections using bolts and nuts are the most common in fastener applications. To ensure the strength of the connection, self-locking nuts are generally used, or additional mechanical locking is used to prevent loosening. The installation of threaded fasteners is also very simple, as long as there is sufficient torque to achieve the pre-tightening force at the interface, but still many people are not sure whether they should be tightened by nuts or bolt heads.

① Should the torque be applied through bolt or nut when tightening? ② Is the same result when applying torque to the bolt head or nut? In fact, there is no fixed answer. In some cases, it is completely acceptable to tighten the bolt head while fixing the nut in place. However, in some applications, the only feasible criterion for tightening a nut depends on the application (projection welding bolt or projection welding nut), the structure of the product itself, and the material and installation conditions.

In some applications you can apply torque to the bolt head or nut and maintain a tight connection; under these application conditions, I am afraid that only one of the components can be torqued:

1. Interference fit In the interference fit hole, torque should be applied to the nut for installation.

2. The bolt head and nut have different shapes and diameters. When the shape of the bolt head and the nut is different (such as a hexagonal head bolt formula nut) or the diameter is significantly different, it is best to apply torque on the smaller side of the bearing surface. For example, if the bolt head is smaller than the nut, torque should be applied to the bolt head. It can be simply understood as a small force and a large force.

3. Different materials When two different materials need to be clamped together, it is best to apply a tightening force on the material with a lower friction coefficient, that is, apply torque to the side where the smallest friction force is likely to be tightened.

4. Application of long bolts When torque is applied to the head of a very long bolt, the problem of twisting and winding is very easy to occur, so in this case, applying torque to the nut will help avoid this problem. In such an application, the nut is a flanged nut, but the bolt head is not. That is to say, the friction radius on the nut side is greater than that on the bolt head side.

If the tightening torque is determined when the nut is to be tightened, an overload problem may occur if the bolt head is tightened with this torque. Generally, nearly half of the torque is needed to overcome the frictional force under the fastening surface, so a smaller friction radius will cause more torque to enter the threaded connection and thus cause over-tightening problems. Vice versa, assuming that the torque is determined according to the tightening of the bolt head, if the nut is tightened, that is, the torque is applied to the side with a larger friction radius, which will lead to insufficient pretension at the input thread connection, and May cause loosening problems.

Why use a washer?

Washers are very common standard parts. We often find them in the tightening process of bolts and nuts. It can even be said that washers are always used with threaded fasteners such as bolts or screws.

Why use a washer?

① If the base material is very soft, the surface of the base mother board material will be damaged during the screwing in of the bolts or screws. By using a washer, this squeezing force can be distributed over a larger area to avoid damage to the surface of the motherboard.

② If the base material is very soft and the periphery of the hole where the fastener is installed has been damaged, then the problem of pull-through failure may occur during the screwing of the fastener. The force during the tightening process is sufficient to pull the fastener through the material mother board, and the presence of the washer helps to generate less build-up pressure.

Common Problems And Analysis Of Fasteners

 Carbon Steel Wood Screw

③ When the surface of the mother board into which the bolt or screw is screwed is not smooth and flat, the head of the fastener is likely to be stuck. By using a washer, the surface of the bolt or carbon steel wood screw can be guaranteed. Slide makes installation easier.

④ Some washers adopt a special structural design to make them prevent the nut from loosening. We commonly refer to this type of washer as a lock washer. There are many types of structures, such as open type, star type and wave type. This type of lock washer needs to be used with a nut.

It can be seen that with the exception of specially designed lock washers, the rest of the washers are mostly used to distribute the load under the bolt head and the nut surface, that is, the washer only plays a role in dispersing the load for the connection structure. The reason for the loosening of the threaded fastener after installation is mostly caused by the lateral load, because it will cause the joint to slip. Therefore, we always emphasize the importance of the pre-tension force. Sufficient pre-load can effectively prevent the joint from occurring. The problem of relative sliding, thus avoiding the occurrence of loosening. However, for the installation of threaded fasteners, it is very important to effectively control the pretension. Even if the thread specifications and installation tools have not changed, different operators may also cause the problem of insufficient pretension. occur. In addition, one thing we cannot ignore is that when using a washer for installation, the washer is not fixed, it can rotate along the axis of the threaded fastener, which will definitely affect the input of torque and then affect Preload. In recent years, the use of flange fasteners has gradually increased, because flanges also have the ability to reasonably disperse load forces, and eliminate the rotation problems mentioned above, so flange fasteners will gradually replace the application of gaskets.