Nearly all engineering products of varying complexity use threaded fasteners. Compared with most other connection methods, a key advantage of threaded fasteners is that they can be disassembled and reused.
This feature is usually the reason why threaded fasteners are preferred over other connection methods, and they often play a crucial role in maintaining the structural integrity of products.
However, they are also a significant source of problems in machinery and other components. The cause of these problems lies in their self-loosening mechanism. This self-loosening mechanism has long been an issue, and over the past 150 years, designers have been developing methods to prevent this occurrence.
Many common types of locking methods for threaded fasteners were invented more than 100 years ago, yet it is only in recent years that the main mechanisms leading to self-loosening have been understood. There are many mechanisms that can cause threaded fasteners to loosen, which can be divided into rotational loosening and non-rotational loosening.
Rotational and Non-Rotational Loosening
In the vast majority of applications, threaded fasteners are tightened, and preload is applied to the connection. Loosening can be understood as the subsequent loss of preload after the tightening process is completed. This can occur in two ways:
Rotational loosening, commonly referred to as self-loosening, refers to the rotation of the fastener under the action of external loads.
Non-rotational loosening refers to the loss of preload without relative movement between the internal and external threads.
Fastener Loosening Caused by Non-Rotational Loosening
Non-rotational loosening may occur due to deformation of the fastener itself or the connected components after assembly. This is the result of partial plastic collapse of these interfaces.
Magnified View of Rough Surface Contact
When two surfaces are in contact with each other, each surface bears the bearing surface load. Because the actual contact area is much smaller than the surface area, even under moderate loads, very high local stresses are continuously borne, which exceed the yield strength of the material.
This can lead to partial collapse of the surface after the tightening operation is completed; this collapse is usually referred to as embedment.
The amount of clamping force lost due to embedment depends on the stiffness of the bolt and the connected components, the number of contact surfaces present in the connection, surface roughness, and the applied bearing surface stress.
Under moderate surface stress conditions, embedment usually causes a clamping force loss of approximately 1% to 5% within the first few seconds after the joint is tightened. When the joint is subsequently subjected to applied dynamic loads, the clamping force may further decrease due to changes in pressure occurring on the joint contact surface.
If the surface bearing stress is kept below the compressive yield strength of the connected component material, the amount of embedment loss can be calculated and compensated for in the connection design.
Junker's Theory of Fastener Self-Loosening
In 1969, Gerhard Junker used the results of engineering tests to support his theory on why threaded fasteners loosen automatically. His key finding was that once relative movement occurs between the mating threads and between the bearing surface of the fastener and the clamping material, the preloaded fastener will loosen due to rotation.
It was also found that transverse dynamic loads cause more severe loosening than axial dynamic loads. The reason is that the radial movement under axial loads is significantly smaller than that under transverse loads.
Transverse Movement of Bolted Connections
Junker showed that a preloaded fastener will self-loosen when relative movement occurs between the mating threads and the fastener's bearing surface. This happens when the transverse force acting on the joint is greater than the frictional force generated by the bolt preload.
For small transverse displacements, relative movement may occur between the thread flanks and the bearing contact surfaces. Once the thread clearance is overcome, the bolt will be subjected to bending forces, and if transverse sliding persists, sliding at the bearing surface under the bolt head will occur.
Once started, there will be temporarily no friction at the threads and under the bolt head. The self-loosening torque generated by the preload acting on the thread helix angle causes corresponding rotation between the nut and the bolt. Under repeated transverse movements, this mechanism can cause complete loosening of the fastener.
To study the causes of loosening, Junker developed a testing machine, as shown in the figure below, which quantifies the anti-loosening effectiveness of fastener designs.
Junker Fastener Testing Machine
Ball bearings are used to eliminate the frictional effect between the moving and fixed plates. When transverse movement is applied from the moving plate that clamps the nut, the load cell continuously monitors the bolt load.
Compared with ordinary vibration test standards, the loss of preload can be measured during the test, and a graph of preload versus cycle number can be plotted.
The principle of the Junker machine is that the transverse displacement generated by the cam causes the fastener to oscillate, overcoming the frictional force of the fastener to produce loosening.
Screenshot of Junker Testing Machine
Junker Vibration Test Loosening Curve
Through Junker testing, the performance of various fastener anti-loosening designs can be compared. Over the past two decades, a large number of studies on existing fastener anti-loosening designs have been completed to compare their anti-loosening properties.
For effective comparison, it is crucial to use the same vibration amplitude, as this has a significant impact on the results. The figure below shows a typical test result of a spring washer.
The test showed that placing a helical spring washer under the bolt head actually accelerated loosening. Others have also proven that the use of such washers has similar performance to the use of bolts without any locking devices.
Many large OEMs, aware of these findings, no longer specify such washers in their internal standards.
Many locking devices used for threaded fasteners are based on preventing relative movement between the threads (e.g., nylon lock nuts) or relative movement between the bearing surface and the connected components (e.g., various types of "locking" washers).
However, both Junker and other subsequent researchers have pointed out the importance of preventing transverse movement of the joint: a suitable bolted connection design ensures that the clamping force of the bolt is sufficient to prevent transverse movement through the friction of the connecting plates, thus avoiding loosening.
During the design phase, this can be achieved by selecting the appropriate fastener size and strength so that the preload can generate sufficient friction to resist joint movement caused by external loads.
Screw Jun's Conclusion
The fundamental cause of threaded fastener loosening is joint movement, especially transverse sliding of the bolt threads and bearing surfaces. If sufficient preload can be obtained from the bolt to prevent joint movement, no locking device is needed, as friction will hold the parts together.
The main problem in threaded fastener design is ensuring that the preload is sufficient to hold the parts firmly together when changes in frictional conditions are included.
This graph shows the effect of friction changes on bolt preload.
The Key to Preventing Loosening is to Provide Sufficient Bolt Preload
Generally, joints should be designed based on the minimum preload generated at the maximum friction coefficient; designing using the average preload value will lead to loosening of many bolts.
At the same time, it is also necessary to consider the preload loss caused by embedment. To limit the amount of embedment, it is necessary to ensure the maximum stress range that the clamped material can withstand.
In cases where joint movement cannot be prevented, for example, in the presence of thermal expansion, a locking device with proven capability should be specified.
