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Author: Site Editor Publish Time: 2024-12-11 Origin: Site
Designing joints based on the minimum expected preload generated by the bolt eliminates any risk of loosening. However, designing based on the average preload without applying a "safety factor" may lead to the loosening of many bolts. It is also necessary to account for preload loss due to embedding, which occurs at the thread interface and beneath the bolt head and nut face during contact surface settling.
Practically every engineering product of complexity uses threaded fasteners. One key advantage of threaded fasteners over most other joining methods is that they can be disassembled and reused. This characteristic often makes threaded fasteners the preferred method of connection, playing a critical role in maintaining structural integrity. However, they are also a major source of mechanical and other component issues. Part of the problem stems from their unintended self-loosening.
Since the Industrial Revolution, self-loosening has been a challenge. Over the past 150 years, inventors have sought ways to prevent it. Many common locking methods for threaded fasteners were invented over 100 years ago, but the primary mechanisms causing self-loosening have only been understood recently. Several mechanisms contribute to threaded fastener loosening, broadly categorized into rotational and non-rotational loosening.
In most applications, threaded fasteners are tightened to apply preload to the joint. Loosening is defined as the subsequent loss of preload after the tightening process is complete. This can occur in one of two ways: rotational loosening, often referred to as self-loosening, occurs when the fastener rotates under external load. Non-rotational loosening occurs when there is no relative motion between the internal and external threads, but preload is still lost.
Fasteners Loosening Due to Non-Rotational Loosening
Post-assembly, deformation of the fastener itself or the joint may lead to non-rotational loosening. This can result from partial plastic collapse at these interfaces.
When two surfaces come into contact, the irregularities on each surface bear the load. Because the actual contact area is often significantly smaller than the apparent area, even moderate loads can exceed the material’s yield strength.
This results in partial collapse of the surface after the tightening operation, a phenomenon commonly referred to as embedding. The magnitude of clamping force loss due to embedding depends on the stiffness of the bolt and joint, the number of interfaces within the joint, surface roughness, and the applied bearing stress. Under moderate surface stress, initial collapse typically leads to a loss of about 1% to 5% of the clamping force within the first few seconds after tightening. When the joint is subsequently dynamically loaded, further reductions occur due to pressure changes at the joint interfaces.
Embedding-related loosening is problematic for joints with multiple thin interfaces and a small bolt clamping length. If the surface bearing stress remains 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.
Junker’s Theory on Fastener Self-Loosening
Gerhard Junker’s 1969 technical paper, “New Criteria for Self-Loosening of Fasteners Under Vibration” (SAE Paper 690055), presented test results supporting his theory of why threaded fasteners self-loosen. His main finding was that preloaded fasteners loosen rotationally once relative motion occurs between the mating threads and the bearing surfaces of the fastener and clamped material. Junker found that self-loosening conditions caused by transverse dynamic loads were far more severe than those caused by dynamic axial loads. This is because radial motion under axial loads is significantly smaller than that under transverse loads.
Junker demonstrated that preloaded fasteners self-loosen when relative motion occurs between the mating threads and the fastener’s bearing surfaces. This relative motion occurs when the transverse force acting on the joint exceeds the frictional resistance generated by the bolt’s preload. For small lateral displacements, relative motion may occur between the thread flanks and the bearing contact surfaces. Once thread clearance is overcome, the bolt undergoes bending forces, and if lateral sliding continues, the bolt head’s bearing surface will slide. Once initiated, there is a temporary absence of friction between the threads and bolt head, leading to relative rotation due to the closing torque exerted by the preload on the thread’s helix angle.
Repeated transverse motion can completely loosen the fastener. To investigate the causes of loosening, Junker developed a testing machine, the so-called Junker Machine, to quantify the effectiveness of fastener designs against self-loosening.
Roller bearings are used to eliminate friction effects between the moving and stationary plates. As transverse motion is applied to the moving plate clamped by the nut, a load cell continuously monitors the bolt load. This provides a significant advantage over impact test standards, as preload loss can be measured during the test and plotted as a function of cycles. The Junker machine simulates a rocking motion within the fastener caused by transverse displacement generated by a cam. Overcoming the fastener’s friction forces induces self-loosening.
Tests like the Junker test (detailed in DIN 65151) allow comparison of various fastener designs’ performance in preventing self-loosening. Over the past two decades, extensive research has been conducted to evaluate the resistance to vibrational loosening of existing fastener designs. Effective comparison requires using the same amplitude, as it significantly impacts results. Typical test results for helical spring washers are shown here.
Some tests suggest that placing a helical spring washer under the bolt head accelerates loosening, while others show that using such washers performs similarly to bolts without any locking devices. Many large OEM manufacturers have acknowledged these findings and no longer specify such washers in their internal standards. However, the continued use of these washers suggests that many organizations remain unaware of these findings.
The most widely accepted cause of self-loosening in threaded fasteners is not vibration per se but joint movement, particularly transverse sliding between the bolt threads and bearing surfaces. If sufficient preload can be obtained from the bolt to prevent joint movement, locking devices are unnecessary, as friction will hold the parts together. The primary challenge in designing with threaded fasteners is ensuring enough preload to securely hold parts together despite changes in frictional conditions. The graph shows the impact of friction changes on bolt preload.
The Key to Preventing Self-Loosening: Adequate Bolt Load
Typically, tightening specifications include a torque range to allow economical joint assembly. Considering this, along with potential primary torque variations (with maximum and minimum limits), a chart can be generated showing preload variations caused by assembly specifications.
Designing joints based on the minimum expected preload generated by the bolt eliminates any risk of loosening. However, designing based on the average preload without applying a "safety factor" may lead to the loosening of many bolts. It is also necessary to account for preload loss due to embedding, which occurs at the thread interface and beneath the bolt head and nut face. To limit embedding, ensure the bearing stress beneath the nut face, bolt head, and within the joint remains within the allowable range for the clamped material.
In cases where joint movement cannot be prevented, such as during thermal expansion, a reliable locking device should be specified.
