The coefficient of friction of bolts is not a constant value, and the coefficient of friction of different batches of bolts will fluctuate to a certain extent. Usually, the main engine factory will define a qualified friction coefficient range for the first supplier, but Lao Tie, who has done torque management, should be deeply aware that even if the friction coefficient fluctuates within the qualified range, there will often be torque and angle monitoring window alarms.
When the tightening method is the angle method, the increase of the friction coefficient will lead to the rise of the final tension torque, and many old irons may wonder whether the high final tension torque will increase the risk of bolt breakageToday, through the sharing of this article, I hope to have some inspiration for you.
01. Bolt stress analysis
02. Overload fracture analysis of bolts
The following figure shows the ** of a bolt overload tightening and breaking, and the bolt has a necking process before breaking.
03. The influence of friction coefficient on the fracture angle
The angle of fracture of a bolt tightening can be calculated from the moment the bolt yields to the final fracture of the bolt. Screws tested two different friction coefficient states, and the other parameters were the same bolts, and overload was screwed to failure.
The figure below shows the assembly curves of bolts tightened to failure in different friction coefficient states. From the test curve, the torque with a high coefficient of friction is significantly higher than that at the low coefficient of friction, while the angle of fracture with a high coefficient of friction is relatively small. How should this be understood?
Imagine two extreme cases, one is that the bolt only bears tensile load, the bolt will have obvious necking deformation before breaking, and the elongation of the bolt is large;Another situation is that the bolt only bears torsional shear load, the bolt is basically no necking deformation before breaking, and the bolt is basically not elongated.
Therefore, for bolts tightened under compound stress, the smaller the neck shrinkage deformation of the bolt with greater torsional shear stress, the smaller the elongation;The greater the neck shrinkage deformation of a bolt with a large tensile normal stress, the greater the elongation.
According to Equation 5, in order to make full use of the strength of the bolt, a higher axial force can be obtained when the coefficient of friction is reduced.
According to the calculation, when the coefficient of friction is 0At 15, the axial force when the bolt yields is 80% of the tensile yield strength of the bolt;When the coefficient of friction is 03. When the bolt yields, the axial force is 60% of the tensile yield strength of the bolt.
Some people may think that the difference in the fracture angle between the two states in the above figure is not very large. This is mainly related to the stress state of the bolt.
The figure below is another part tested by Screw, the fracture angle of the parts in the high friction coefficient state and the parts in the low friction coefficient state are twice as different.
04. The risk of high bolt tightening torque
Through the above analysis, it should be noted that in the process of bolt tightening, when the torque is high, it is necessary to prevent the shear stress from being too high, which will lead to a small fracture angle and a risk of fracture of the bolt.
Therefore, for high-torque tightening, it is necessary to analyze the bolts and connecting pairs, and the monitoring window should not be easily relaxed.
In addition, the high coefficient of friction is one of the causes of high torque, and the high hardness of fasteners, foreign matter in the threads, and inappropriate size may lead to high tightening torque, which can increase the shear stress of the bolt.
Therefore, when abnormal influencing factors are found, they should be rectified in time to ensure the tightening quality of the connecting pair.
05. Screw Jun's experience and summary
1) The high friction coefficient of the bolt will lead to high tightening torque and high shear stress. The higher the shear stress, the smaller the necking, the smaller the elongation, and the smaller the fracture angle when the bolt breaks.
2) When managing torque in the field, it is necessary to pay attention to the problem of sudden increase in torque. At this point, the bolts and connections should be analyzed to prevent excessive shear stresses, which can increase the risk of bolt breakage.
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