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Fastener common problems and analysis
Fasteners, also known as standard parts in the market, are mechanical components that can be mechanically fixed or glued together by two or more components.
1. How to tighten bolts and nuts
Threaded connections with bolts and nuts are the most common in fastener applications, and in order to ensure the strength of the connection, self-locking nuts are generally used, or additional mechanical locking is used to prevent loose. The installation of threaded fasteners is also very simple, as long as there is enough torque to achieve the pre-tightening force at the interface, but there are still many people who are not sure whether to tighten through the nut or bolt head.
(1) Should torque be applied to the bolt head or nut when tightening?
(2) apply torque to the bolt head or nut, whether the two are the same result?
There is no fixed answer, and in some cases it is perfectly acceptable to tighten the bolt head while holding the nut in place. However, in some applications, tightening the nut is the only feasible criterion for determination depending on the application (projection bolt or projection nut), the structure of the product itself, as well as the material and installation conditions.
1, interference coordination
In the holes of the interference fit, torque should be applied to the nut for installation.
2, bolt head and nut have different shapes and diameters
When the bolt head and nut have different shapes (such as hexagon head bolt with square nut) or significantly different diameters, 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 capacity.
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 low coefficient of friction, that is, torque tightening on the side that is likely to produce the least friction.
4, long bolt application
When torque is applied to the head of a very long bolt, the problem of torsional winding is very easy to occur, so applying torque to the nut in this case will help to avoid this problem.
There is such an application that the nut is a nut with a flange, but the bolt head is not, that is, the friction radius on one side of the nut is larger than that on the side of the bolt head.
If the tightening torque is determined when the nut is to be tightened, then an overload problem may occur if the bolt head is tightened using that torque. Typically, nearly half of the torque is needed to overcome the friction under the tightening surface, so a smaller radius of friction will cause more torque to enter the threaded joint and thus cause overtightening problems.
Vice versa, assuming that the torque is determined according to the tightening of the bolt head, if the nut is tightened, that is to say, the torque is applied to the side with a larger friction radius, which will cause insufficient preload at the input thread connection, and may cause loose problems.
Why use washers?
(1) If the base material is very soft, the surface of the base plate material will be damaged during the process of screwing in the bolt or screw. By using washers, this extrusion pressure can be distributed over a larger area, avoiding damage to the mother plate surface.
(2) If the base material is very soft, and the hole around the installation fastener has been damaged, then the problem of pull-through failure may occur during the screwing in of the fastener, because in most cases, the force applied in the tightening process is enough to pull the fastener through the material parent plate, and the presence of the washer helps to produce a small gathering pressure.
(3) When the bolt or screw into the mother plate surface is not smooth, flat, the head of the fastener is likely to be stuck, through the use of washers, you can ensure that the surface of the bolt or screw slide and make the installation easier.
(4) Some washers have a special structural design, so that they have the function of preventing nut loosening. We commonly call this type of washer a lock washer, there are many kinds of structural forms, open mouth, star and wave and so on. This type of lock washer needs to be used in conjunction with the nut.
It can be seen that in addition to the specially designed lock washer, the rest of the washer is mostly to distribute the load under the bolt head and nut face, that is, the washer only plays the role of distributing the load for the connection structure.
The reason why threaded fasteners loose after installation is mostly caused by transverse load, because it will cause the joint to slip, so we always emphasize the importance of preload, enough preload can effectively avoid the problem of relative slide of the joint, and then avoid the occurrence of loose. However, for the installation of threaded fasteners, effectively controlling the preload is a very important issue, even if the thread specifications and installation tools have not changed, different operators may also cause the problem of insufficient preload.
In addition, we can not ignore that when the washer is installed, the washer is not kept fixed, it can be rotated along the axis of the threaded fastener, and this will certainly affect the input torque, and then affect the preload force.
In recent years, the use of flange fasteners has gradually increased, because the flange also has the ability to reasonably distribute the load force, and eliminate the rotation problem mentioned above, so the flange fasteners will gradually replace the application of gaskets.
3. The role of preload
When you choose a bolt and nut connection, you need to apply torsion force to achieve locking through a torque tool or a manual wrench, and the torque will eventually be converted into the preload required by the connection part, a certain strength of the preload can ensure that the joint resistance to vibration loosening and fatigue, in most applications, the higher the preload, the better.
How do you describe the preload? When you tighten a bolt or nut, there is tension between the head of the bolt and the head of the nut. Just like when you stretch a spring, the spring always tries to return to its normal state. The stretched bolt also tries to return to its original length to relieve the tension, so there is compression or compression force, which pulls the bolt and nut towards each other. And then clamp the joint together.
4.Sum up
Through the above introduction, we understand whether the torque should be applied to the bolt head or the nut when tightening and the corresponding situation, the role of the washer and the development of the flange fastener, how to ensure the pre-tightening force required for installation.