The connection of threaded fasteners is widely used, and the problem that gives us a headache is the loosening of threaded fasteners during use. In response to this problem, the inventor has been designing a method for fasteners to prevent loosening, and there are many mechanisms that lead to loosening of fasteners. Recently, the Standard Parts Network has learned that the rotational and non-rotational loosening mechanisms of fasteners are as follows. This is the first knowledge we share with you, and I hope it will be helpful to you.
Rotational and non-rotational loosening
In most applications, threaded fasteners are tightened to apply preload in the joint. Loosening can be defined as the loss of preload after tightening is complete. This can happen in either of two ways. Rotational loosening, commonly referred to as self-loosening, refers to the relative rotation of fasteners under external loads. Non-rotational looseness means that there is no relative rotation between the female and male threads, but a loss of preload occurs.
Fastener loosening due to non-rotational loosening
After assembly, deformation of the fastener itself or the joint may cause non-rotational loosening. This may be the result of partial plastic collapse of these interfaces. When the two surfaces are in contact with each other, the asperities on each surface bear the bearing surface pressure load. Since the actual contact area of ​​the bumps may be much smaller than the macroscopic area, even under moderate loads, the stress of the protruding parts due to the surface roughness will be greater than the yield strength of the material, and these protruding parts will bear very high local stress, resulting in plastic deformation.
This can lead to partial collapse of the surface after the tightening operation is complete. This collapse is often referred to as embedding. The magnitude of the loss of clamping force due to embedding depends on the stiffness of the bolt and the connected parts, the number of interfaces present within the joint, the surface roughness and the applied contact stress. Under moderate surface stress conditions, the initial collapse typically results in a loss of clamping force of about 1% to 5%, half of which is lost in the first few seconds after the joint is tightened. When the joint is subsequently dynamically loaded by an applied force, the joint is further reduced due to the pressure changes that can occur at the joint interface.
Loosening due to insertion loss is problematic in joints consisting of several thin joint surfaces and small clamping lengths of the bolts. If the surface bearing stress remains below the compressive yield strength of the joint material, the amount of insertion loss can be calculated and the joint design can compensate for this loss.
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