Nuts that come loose and come off bolts is not a new phenomenon. From the 19th century, there have been improvements to the design of bolts and nuts to prevent them from loosening.
At the moment, most people already know that the lateral movement of the attachment point can completely loosen the non-lock nut. Once relative motion occurs between the threaded surface and the other contact surfaces of the clamped part, the bolt will be almost completely friction-free in the circumferential direction of the connection.
Since the bolt preload acting on the thread is tilted, torque is generated in the circumferential direction, resulting in automatic loosening of the nut. The way to prevent the nut from loosening is to allow the joint to generate enough preload so that joint movement does not occur.
However, this is only a theoretical situation, and it is not always possible to achieve this in practice, because in many applications it is difficult to determine the real forces acting on the connection points.
Therefore, in this case, in order to prevent slippage from occurring, some kind of locking device is usually employed to prevent the nut from loosening.
Fig.1 Transverse joint movement.
Lock nuts are one of the most common ways to provide self-loosening resistance. The patent for such a nut appeared in the 60s of the 19th century. An advantage of this type of nut is that the locking feature can be checked at the time of assembly by measuring the relevant torque.
The key technical requirements are that the screw-in torque in the tightening direction must not exceed the specified maximum value, and the release and release torque must reach the specified minimum value.
Many locknuts have proprietary designs, but in general, they can be divided into two categories, metal lock nuts and non-metallic lock nuts.
Most kinds of non-metallic lock nuts have a polymer ring on the tip of the nut, which creates an anti-loosening torque when tightened onto the bolt. All-metal nuts are usually made with a top thread oval or use spring steel inserts to achieve anti-loosening torque.
Research published thirty years ago shows that:
The resistance to loosening depends on the magnitude of the current torque. The higher the screw-in torque, the higher the self-loosening resistance. The disadvantage of excessive tightening torque is that torsional stresses are transmitted into the thread, resulting in premature yield, limiting the ability to achieve higher preload.
Under the lateral vibration test (DIN 65151), the residual amount of preload is retained even after the locknut is loosened. That is, these nuts will be partially loose during testing, but when a certain level of preload is reached, the loosening will stop so that the nuts will not fall off the bolts.
Figure 2 shows a typical preload attenuation diagram of a locknut under a lateral vibration test. These curves are the amplitude of the M8 nylon insert nut experiencing + -065 mm of lateral vibration curve. After the initial stage of loosening, the nut rotation stops, leaving residual preload in the fastener.
Under lateral vibrations, these torque-type locknuts are not really "lock nuts" because they do not completely prevent rotation.
Fig.2. Typical loosening curve of a popular torque nut.
Studies on accidents that have occurred with loose torque locknuts are:The joints will be subject to axial and lateral loads.
Previously published research has shown that:The axial load acting on the joint alone does not cause any significant loosening.
Foreign scholars have studied how the axial load affects the loosening characteristics of the lock nut in the presence of lateral motion of the joint.
Figure 3 Test solution.
To study the causes of such nut separation, the Junker machine was modified to allow the introduction of axial and lateral loads into the joints, respectively. The details of the improved machine are shown in Figure 3.
Micro hydraulic jacks are used to allow axial loads to be applied to the butt when lateral movement occurs. This arrangement allows individual axial loads, lateral displacements, or a combination of the two to be applied to the joint.
Experiments with improved Junkers machines have shown that the combination of axial and lateral loads has a profound effect on the loosening of popular torque nuts.
The test results showed that:
If the axial load exceeds the preload retained by the nut in the standard Junker test, the nut continues to rotate until it separates from the bolt.
In this case, the axial load causes the joint to separate, i.e. there is a gap between the connectors. The axial load will produce a loose torque generated under lateral motion, which continues to rotate the nut until it is detached from the bolt.
This illustrates this in Figure 4. The nut is tightened first, and then an axial load is applied. At this stage, the bolts are subjected to only a small part of the load due to the balance spring system formed by the joints and bolts.
The bolt acts as a tension spring, the joint acts as a compression spring, and the tension and compression loads balance each other. The bolt is stretched only a small part and therefore only a small part of the axial load.
At this stage, most of the axial load is maintained by reducing the amount of compression experienced by the joint. When the machine is started and the joint undergoes lateral movement, the rapid loosening of the nut can be observed.
The preload is reduced until the joint is detached so that the axial load is fully supported by the bolt. As long as the axial load is maintained, the nut will continue to rotate until the machine stops or the nut separates.
Fig. 4 If an intermittent axial load is applied to the loosening process of the lock nut under constant axial load and joint lateral movement, nut rotation will occur when the load is above the threshold threshold equal to the remaining preload retained by the bolt.
If this load is repeatedly applied to the joint, the nut will come completely loose and detach from the bolt. This is shown in Figure 5. If no axial load is applied, the dashed line indicates a loose curve.
Fig. 5: Loosening the main torque nut under intermittent axial load and transverse joint, which is also applicable to common non-locking nuts. In the presence of axial loads, ordinary nuts can be easily detached from bolts. In the presence of transverse joint movements, even very small axial loads can cause detachment.
The loosening torque generated during lateral movement depends on the amount of bolt preload. The higher the preload, the higher the loosening torque. In the case of lock nuts, loosening occurs under lateral vibration until the loose torque is resisted by an equal amount of locking torque.
Once sufficient self-loosening has occurred so that the axial load is greater than the remaining preload, the axial load creates a loose torque, and it is this load that rotates the nut so that it separates from the bolt.
Based on the completed experiments and the measurements made, the applied axial load causes a self-loosening tendency of the lock nut when the lateral movement of the joint occurs.
Whether or not this type of nut will come completely loose when the joint moves laterally depends on the magnitude of the axial load applied.
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