Designing the joint based on the minimum expected preload of the bolts will eliminate any risk of loosening. If the “safety factor” is not applied and the average preload is used for design, it will lead to the possibility of many bolts loosening. A margin also needs to be left for the preload loss caused by embedding, which occurs in the threads when the contact surface settles and beneath the bolt head and nut face.
Understanding Threaded Fasteners
In fact, threaded fasteners are used in engineering products of any complexity. A key advantage of threaded fasteners over most other connection methods is that they can be disassembled and reused. This feature is often the reason why threaded fasteners are preferred over other connection methods, and they usually play a crucial role in maintaining the structural integrity of the product. However, they are also a significant source of mechanical and other component problems. Part of the reason for this is their unintended self-loosening.
Since the beginning of the Industrial Revolution, self-loosening has been a problem, and inventors have been trying to prevent it from happening for the past 150 years. Many common methods for locking threaded fasteners were invented over 100 years ago, but it was only recently that the main mechanisms causing self-loosening were understood. There are several mechanisms that can cause threaded fasteners to loosen; these can be classified as rotational loosening and non-rotational loosening.
Rotational and non-rotational loosening
In the vast majority of applications, threaded fasteners are tightened to apply preload to the joint. Loosening can be defined as the subsequent loss of preload after the tightening process is completed. This can occur in one of two ways. Rotational loosening, often referred to as self-loosening, occurs when the fastener rotates under the action of external loads. Non-rotational loosening refers to the loss of preload without relative movement between the internal and external threads.
Fasteners become loose due to non-rotational loosening.
Post-assembly deformation of the fastener itself or the joint may cause non-rotational loosening. This could be the result of partial plastic collapse at these interfaces.
When two surfaces come into contact with each other, the unevenness on each surface will bear the axial load. Because the actual contact area may be much smaller than the apparent area, even under moderate loads, it is greater than the yield strength of the material.
Enlarged surface display shows rough contact.
This leads to partial collapse of the surface after the tightening operation is completed. This kind of folding is usually called embedding. The magnitude of the clamping force lost due to embedding depends on the stiffness of the bolt and the joint, the number of bolts in the joint, the interfaces existing in the joint, the surface roughness, and the applied bearing stress.
Under moderate surface stress conditions, the initial collapse usually results in the loss of about 1% to 5% of the clamping force within the first few seconds after the joint is tightened. When the joint is subsequently subjected to dynamic loading by the applied force, the clamping force will further decrease due to the pressure changes occurring at the joint interface.
Loosening caused by embedment loss is a problem at the joint, which includes several thin mating surfaces and a small bolt clamping length. If the surface bearing stress remains below the compressive yield strength of the joint material, the amount of embedment loss can be calculated and the joint can be designed to compensate for this loss.
Junker’s theory of self-loosening of fasteners
In 1969, Gerhard Junker published a technical paper (“New Criteria for Self-Loosening of Fasteners under Vibration” SAE Paper 690055, 1969) presenting the results of his completed test work to support his theory on why threaded fasteners self-loosen. His main finding was that once relative motion occurs between the mating threads and between the support surface of the fastener and the clamped material, the pre-tightened fastener will loosen due to rotation.
Junker found that the self-loosening conditions caused by lateral dynamic loads are much more severe than those caused by dynamic axial loads. The reason is that the radial movement under axial load is significantly smaller than that under lateral load.
Lateral movement of bolted connections
Junker indicated that when there is relative motion between the mating threads and the bearing surface of the fastener, the pre-tightened fastener will loosen by itself. This relative motion occurs when the lateral force acting on the joint exceeds the frictional resistance generated by the bolt’s pre-tightening force. For small lateral displacements, relative motion may occur between the thread flanks and the contact surface of the bearing area.
Once the thread clearance is overcome, the bolt will be subjected to bending force, and if the lateral sliding continues, the bearing surface of the bolt head will slide. Once initiated, the threads and the bolt head will temporarily have no friction. The internal closing torque existing due to the pre-tightening force acting on the thread helix angle generates a corresponding rotation between the nut and the bolt.
Under repetitive lateral movement, the mechanism can completely loosen the fasteners. To investigate the cause of loosening, Junker developed a testing machine, known as the “Junker machine”, which will quantify the effectiveness of the anti-loosening design of the fasteners.
Roller bearings are used to eliminate the frictional influence between the moving plate and the fixed plate. When the nut is clamped on the moving plate and subjected to lateral movement, the load cell can continuously monitor the bolt load. This is a major advantage compared to the impact test standard, as the loss of preload can be measured during the test and a graph of preload versus cycle can be plotted.
The idea behind the Junker machine is that the lateral displacement generated by the cam causes a wobbling motion in the fastener. Overcoming the frictional force of the fastener produces a self-loosening action.
Junker vibration test loosening curve
Tests like the Junker test (test details in DIN 65151) allow for the comparison of the performance of various fastener designs in preventing self-loosening. Over the past two decades, extensive work has been done to investigate existing fastener designs in order to compare their resistance to vibration loosening.
To make effective comparisons, it is crucial to use the same amplitude, as this has a significant impact on the results. Here, typical test results for helical spring washers are shown.
Some tests have shown that placing a helical spring washer under the bolt head can accelerate loosening, while other tests have indicated that using such a washer has similar performance to using a bolt without any locking device. Many large OEM manufacturers are aware of these findings and no longer specify such washers in their internal standards. However, judging from the continued use of these washers, many organizations seem not to be aware of these discoveries.
Many locking devices used for threaded fasteners either prevent the relative thread motion between the bolt and the nut (e.g., by using a nylon insert nut) or prevent the motion of the nut relative to the joint (e.g., by using various types of “locking” washers). However, Junker and other later researchers have pointed out the importance of preventing lateral joint movement.
A bolted joint, when properly designed, can have a clamping force sufficient to prevent lateral movement caused by the frictional force between the joint plates and will not loosen. During the design stage, this can be achieved by selecting the size and strength of the fasteners so that the preload can generate enough frictional force to prevent the joint from moving under external loads.
Research Conclusions
The most widely accepted reason for self-loosening of threaded fasteners is not vibration but joint movement, especially the lateral sliding of the bolt thread and the bearing surface. If sufficient preload can be obtained from the bolt to prevent joint movement, no locking device is needed because friction will hold the parts together.
The main problem in designing with threaded fasteners is to ensure that the preload is sufficient to hold the parts firmly together when the friction conditions change. This figure shows the effect of friction changes on the preload of the bolt.
Typically, tightening specifications include a torque range so that joints can be assembled economically. When this is taken into account along with the possible prevailing torque (with maximum and minimum limits), a chart can be generated showing the variation in preload caused by the assembly specification.
Designing the joint based on the minimum expected preload of the bolt will eliminate any risk of loosening. If the “safety factor” is not applied and the average preload is used for design, it will lead to the possibility of many bolts loosening. A margin should also be left for the preload loss caused by embedding, which occurs in the threads and under the bolt head and nut face when the contact surface settles.
To keep the embedding within limits, it is necessary to ensure that the bearing stress on the nut face, bolt head and within the joint remains within the maximum allowable bearing stress range of the clamped material. In cases where joint movement cannot be prevented, such as in thermal expansion, a reliable locking device should be specified.
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