- All
- Product Name
- Product Keyword
- Product Model
- Product Summary
- Product Description
- Multi Field Search
Author: Site Editor Publish Time: 2025-01-20 Origin: Site
When designing joints, planning for the minimum expected preload generated by the bolts can effectively avoid the risk of loosening. If you do not use a "safety factor" and instead design based on an average preload, many bolts are likely to loosen. You also need to allow for the loss of preload due to embedment, which usually occurs in the threads and below the contact surface between the bolt head and the nut when the contact surface settles.
In fact, threaded fasteners play an integral role in any engineered product, no matter how complex it is. A key advantage of threaded fasteners over most other connection methods is that they are removable and reusable. This feature is often an important reason why threaded fasteners stand out and are preferred over other connection methods. They often play a vital role in maintaining the structural integrity of the product. However, it should not be overlooked that threaded fasteners are also often a major source of problems in machinery and other components. This is partly because they can inadvertently loosen themselves.
Self-loosening of threaded fasteners has been a phenomenon since the beginning of the Industrial Revolution. Inventors have spent the past 150 years searching for effective solutions to prevent this phenomenon. Many common threaded fastener locking methods were invented more than 100 years ago. However, only recently have the main mechanisms that lead to self-loosening been understood. In fact, there are many different mechanisms that lead to threaded fastener loosening, which can be roughly divided into two categories: rotational loosening 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 called self-loosening, occurs when the fastener rotates under external load. Non-rotational loosening occurs when there is no relative movement between the internal and external threads, but a loss of preload occurs.
Non-rotational loosening can occur due to deformation of the fastener itself or the joint after assembly. This can be the result of partial plastic collapse of these interfaces.
When two surfaces come into contact with each other, the asperities on each surface carry a bearing load. Because the actual contact area can be significantly less than the apparent area, even at moderate loads, the asperities are greater than the yield strength of the material.
This causes the surfaces to partially collapse after the tightening operation is complete. This folding is often referred to as embedding. The amount of clamping force lost due to embedding depends on the stiffness of the bolt and joint, the number of bolt interfaces present in the joint, the surface roughness, and the applied bearing stresses. Under moderate surface stress conditions, the initial collapse typically results in about 1% to 5% of the clamping force being lost within the first few seconds of the joint being tightened. When the joint is subsequently subjected to dynamic loading from applied forces, there is a further reduction due to the pressure changes that occur at the joint interfaces.
Loosening due to embedding loss is problematic in joints that include several thin mating surfaces and a small bolt clamping length. If the surface bearing stress is kept below the compressive yield strength of the joint material, the amount of embedding loss can be calculated, and the joint can be designed to compensate for this loss.
Gerhard Junker published a technical paper in 1969 (“A New Standard for Self-Loosening of Fasteners Under Vibration” SAE Paper 690055, 1969) presenting the results of test work he had done to support his theory on why threaded fasteners self-loosen. His main finding: pre-tensioned fasteners can loosen by rotation as soon as relative motion occurs between the mating threads and between the fastener’s bearing surface and the clamped material. Junker found that transverse dynamic loads produce self-loosening conditions that are much more severe than dynamic axial loads. The reason for this is that radial motion under axial loads is significantly less than radial motion under transverse loads.
Junker showed that pre-tensioned fasteners can self-loosen when relative motion occurs between the mating threads and the fastener’s bearing surface. This relative motion occurs when the transverse forces acting on the joint are greater than the frictional resistance created by the bolt’s preload. For small transverse displacements, relative motion can occur between the thread flanks and the contact surfaces in the bearing area. Once the thread play is overcome, the bolt will be subject to bending forces and, if the lateral sliding continues, the bolt head bearing surface will slip. Once initiated, the threads and bolt head will be temporarily free of friction. The internal closing torque present due to the preload acting on the thread helix angle creates a correlated rotation between the nut and the bolt.
Under repeated lateral movements, this mechanism can completely loosen the fastener. To investigate the causes of loosening, Junker developed a testing machine, the so-called "Junker machine", which will quantify the effectiveness of the anti-loosening properties of the fastener design.
Roller bearings are used to eliminate the effects of friction between the moving plate and the fixed plate. Load cells can continuously monitor the bolt load while lateral movement is applied to the moving plate on which the nut is clamped. This is a major advantage over impact test standards, as the loss of preload can be measured during the test and plotted as a function of preload versus period. The idea behind the Junker machine is that the lateral displacement created by the cam causes a rocking action in the fastener. Overcoming the friction in the fastener creates a self-loosening action.
Tests like the Junker test (test details in specification DIN 65151) allow the performance of various fastener designs to be compared against self-loosening. Over the past two decades, a great deal of work has been done to investigate existing fastener designs in order to compare them against vibration loosening. In order to make a valid comparison, it is essential that the same amplitude is used as this has a significant impact on the results. Typical test results for a helical spring washer are shown here.
Some tests have shown that placing a helical spring washer under the bolt head can accelerate loosening, other tests have shown that using such a washer gives 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 by the continued use of these washers, many organizations appear to be unaware of these findings.
Many locking devices for threaded fasteners are based either on preventing relative thread movement between the bolt and nut threads (such as with nylon insert nuts) or on preventing movement of the nut relative to the joint (such as with various types of 'lock' washers). However, Junker and other later researchers have pointed out the importance of preventing lateral joint movement. Bolted joints are designed so that the clamping force of the bolts is sufficient to prevent lateral movement caused by friction between the joint plates and prevent loosening. This can be achieved at the design stage by selecting fastener size and strength so that the preload can generate sufficient friction to prevent external loads from causing joint movement.
Regarding the phenomenon of self-loosening in threaded fasteners, it is generally believed that joint movement, especially lateral sliding between the bolt threads and the bearing surface, is the main cause, and vibration is not the most critical factor. If sufficient preload can be obtained from the bolt to keep the joint stationary, then there is no need for additional locking devices because the friction is sufficient to hold the parts firmly together. When designing with threaded fasteners, the core issue is to ensure that the preload is sufficient to hold the parts firmly in place even in the face of changing friction conditions. The figure below shows the effect of friction changes on bolt preload. Typically, tightening specifications set a torque range to meet the assembly requirements of the joint while taking into account economy. When this torque range is taken into account, as well as the possible main torques (with maximum and minimum limits), a diagram can be drawn showing the variation in preload due to different assembly specifications. Designing the joint according to the minimum expected preload generated by the bolts effectively eliminates the risk of loosening. On the other hand, if the "safety factor" is not applied and the design is based on the average preload, many bolts are likely to loosen. In addition, a certain allowance must be made for the loss of preload due to embedding in the threaded area and under the bolt head and nut face when the contact surface settles. In order to control the embedding, it is necessary to ensure that the bearing stresses on the nut face, the bolt head and inside the joint are always within the maximum bearing stress range allowed by the clamped materials. In some cases where joint movement cannot be avoided, such as joint movement caused by thermal expansion, reliable locking devices should be used.
