Many original equipment manufacturers stipulate that if a part is removed, bolts and nuts should be replaced. On the surface, this seems to be a policy aimed at increasing the sales of spare parts.
However, what factors are actually behind the decision of whether a fastener should be reused?
In most applications, there are three main considerations:
1. Is the strength of the fastener affected by the first tightening to the extent that its strength will be weakened if it is tightened again?
2. Have the fasteners been affected by corrosion to the extent that their performance is impaired or they need to be replaced for aesthetic reasons?
3. Has the torque-tension characteristic of the fasteners changed? If they are retightened, will their performance be affected?
The effect of repeated tightening on the torque-tension relationship
By studying the influence of three repeated tightenings on the torque-tension relationship. In many applications, this will be the determining factor in deciding whether fasteners should be reused. The preload or clamping force generated by the fastener is indirectly controlled by specifying the tightening torque.
The maximum uncertainty in the torque-tension relationship is the coefficient of friction between the nut surface and the threads of the fastener. Any change in the friction coefficient will have a significant impact on the bolt tension produced by applying a specific torque value.
The effectiveness of most bolted joints is determined by the clamping force generated by the bolt. In most cases, the clamping force provided by the bolt is controlled by tightening the bolt to a predetermined torque value.
The torque value is determined through testing or by using software with test thread and nut surface friction values. In both cases, the value is typically determined for the first tightening of the fastener.
When fasteners are tightened, their surfaces slide against each other under high pressure. This usually leads to a certain degree of wear on the nut surface, the clamped part and the thread surface. When the fasteners are retightened, the wear increases, which usually results in an increase in the coefficient of friction.
Due to the increase in the coefficient of friction, the clamping force provided by the fasteners decreases at a given torque.
A series of tests were conducted on M12 galvanized bolts and nuts to evaluate the impact of re-tightening on the torque-tension relationship. The bolts and nuts were tested in their as-received condition, and the conditions of the bearing surfaces and thread surfaces after re-tightening were investigated using a scanning electron microscope.
The threaded surface after being retightened 10 times
The above photo (magnified 50 times) shows the pressure surface after the bolt thread was retightened 10 times.
The surface condition of the thread after being retightened five times
The zinc sheet has been worn from the surface. This wear process occurs during the first tightening and gradually worsens. A close-up (magnified 500 times) of the thread after five tightenings shows that the surface has cracked due to the wear process. As shown in the figure, wear particles can also be found on the surface.
The wear particles on the threaded surface magnified 500 times
When tightened for the first time, the friction coefficient of the electro-galvanized finish is relatively high (0.2 to 0.3). It is well known that the wear rate is related to the friction value; the greater the friction, the higher the wear rate. When wear is concentrated on the surface, the surface is damaged, resulting in an increase in the friction value.
It was found that for the received galvanized bolts and nuts, the friction coefficient doubled by the tenth re-tightening. This has a direct impact on the preload generated by the tightening torque. Keeping the same tightening torque usually results in the preload at the tenth tightening being half of that at the first tightening.
Coefficient of friction
By reducing the coefficient of friction, the wear rate will also decrease. It is expected that the reduction in the wear rate will provide a more repeatable torque-tension relationship when re-tightening the fasteners. Lubricants are used to reduce friction and provide a consistent torque-tension relationship.
Even with the use of lubricants, wear will occur, but for a small number of re-tightenings, it will not be noticeable unless the surface is inspected under a microscope.
Under a given torque, the repeated use of fasteners may have a significant impact on the bolt tension. In critical applications, this obviously has serious consequences for the structural integrity of the product.
Therefore, in such applications, only when the test results show that the torque-tension relationship will not be adversely affected, can the fasteners be reused. High-quality professional lubricants, such as molybdenum disulfide, can significantly reduce the increase in friction when retightening.
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