The connection of threaded fasteners is widely used, and the problem that makes us headache is the looseness of threaded fasteners in the process of use. To solve this problem, inventors have been designing methods to prevent looseness of fasteners, and there are many mechanisms leading to looseness of fasteners. Recently, standard parts.com has learned about the rotating and non rotating looseness mechanisms of fasteners. The following is the relevant knowledge we share with you, hoping to help you.
Rotating and non rotating looseness
In the vast majority of applications, threaded fasteners are to be tightened in order to exert preload in the joint. Looseness can be defined as the loss of preload after tightening. This can happen in either way. Rotating looseness, commonly known as self loosening, refers to the relative rotation of fasteners under external load. Non rotating looseness means that there is no relative rotation between internal thread and external thread, but preload loss will occur.
Looseness of fasteners due to non rotating looseness
After assembly, the fastener itself or joint deformation may lead to non rotational loosening. This may be the result of plastic collapse of these interfaces. When the two surfaces contact each other, the asperity on each surface bears the pressure load on the support surface. Because the actual contact area of the bumps may be far less than the macro area, even under medium load, the stress of the bumps due to the surface roughness will be greater than the yield strength of the material, and these bumps will bear very high local stress, resulting in plastic deformation.
This can cause a partial collapse of the surface after the tightening operation. This kind of collapse is often called embedding. The amount of clamping force lost due to embedding depends on the stiffness of bolts and connected parts, the number of interfaces in the joint, surface roughness and applied contact stress. At moderate surface stress conditions, the initial collapse usually results in a loss of about 1% to 5% of the clamping force, half of which is lost in the first few seconds after the joint is tightened. When the joint is loaded dynamically by the applied force, the joint will be further reduced due to the pressure change on the joint interface.
Loosening due to insertion loss is problematic at joints consisting of several thin joint surfaces and small bolt clamping lengths. If the surface bearing stress remains below the compressive yield strength of the joint material, the embedded loss can be calculated and compensated by the joint design.
Junker fastener self loosening theory
Gerhard Junker published a technical paper (SAE paper 6900551969, a new standard for self loosening of fasteners under vibration) in 1969. The results of his experimental work are given to support his theory on the causes of self loosening of threaded fasteners. His key discovery is that once there is relative movement between the engagement threads and between the bearing surface of the fastener and the clamping material, the pre fastener will become loose due to rotation. Junker found that the lateral dynamic load produced more serious self loosening conditions than the axial dynamic load. The reason is that the radial motion under axial load is obviously smaller than that under transverse load.
Junker research shows that the self loosening phenomenon occurs when the pre tightened fastener moves between the matching thread and the supporting surface of the fastener. When the transverse force acting on the joint is greater than the friction force produced by bolt pre tightening, the relative motion will occur. For small lateral displacement, relative motion may occur between the thread side and the contact surface of the support area. Once the thread gap is overcome, the bolt will be subjected to bending force. If the lateral sliding continues, the bearing surface of the bolt head will also slide. Once this happens, the thread and bolt head will only have a small friction coefficient, or even temporarily lose friction. Due to the pre tightening force acting on the helix angle of the thread, the rotation torque generated on the thread, therefore, will generate the relevant rotation between the nut and the bolt.
Under repeated lateral movement, the mechanism can completely loosen the fastener. In order to study the causes of loosening, Juncker has developed a testing machine, the so-called "Juncker machine", which will quantify the effectiveness of loosening resistance of fastener design.
Roller bearings are used to eliminate the effect of friction between the moving and stationary plates. The pressure sensor allows continuous monitoring of the bolt load when lateral movement is applied from the moving plate clamped by the nut. This is a major advantage over the impact test standard, as preload losses can be measured during the test and the relationship between preload and cycle can be plotted. The idea behind the Junker machine is that the lateral displacement generated by the cam will cause the joint to swing (slide), which will produce self loosening effect after overcoming the friction of the fastener.
