Loosening Characteristics of Prevailing Torque Nuts
Nuts having a prevailing torque are probably the most common type of 'locking' fastener. The prevailing torque is the torque needed to run the nut down the thread prior to clamping occurring. With updates for the ISO standards for this type of nut in their final draft form (may 2025), it may be an opportune time to review their characteristics. There are several standards covering prevailing torque nuts and they can broadly be categorised into two types. The first are all-metal nuts in which the prevailing torque feature is achieved by distorting part of the nut thread. Standards such as ISO 7719 and ISO 7042 cover such nuts; the latest proposed update includes a number of changes such as the addition of stainless steel nuts. The second category are for nuts having a prevailing torque feature achieved by use of a non-metallic feature (nylon insert nuts). Standards such as ISO 7040 and ISO 7041 define such nuts. Besides the addition of stainless steel nuts, the latest update includes the addition of some additional sizes such as M18 and M22. These standards refer to ISO 2320 (Fasteners - Prevailing torque steel nuts - Functional properties). This is the performance standard for prevailing torque nuts and covers upper and lower bound values for the prevailing torque. This standard specifies that the prevailing torque for nylon insert type nuts, the maximum prevailing torques for first assembly shall be 50% of the values specified for the all-metal nut.
In research published thirty years ago it was shown that:
1. The resistance to loosening depends upon the magnitude of the prevailing torque. The higher the prevailing torque the higher is the resistance to self-loosening. The disadvantage of too high a prevailing torque is that large torsional stresses are induced into the thread. This can result in limiting the preload that can be achieved due to the torsion causing premature yielding.
2. Those in the know, understand that prevailing torque nuts can self-loosen, as can be seen from the results of a Junker test, but they can still be useful in many applications. Under a standard transverse vibration test, commonly called a Junker test, prevailing torque nuts tend to self-loosen initially but retain a residual amount of preload. That is, such nuts will partially come loose on test, but the loosening will stop when a certain level of preload is reached. As a result, the nut does not become detached from the bolt. The test involves inducing transverse movement into a joint whilst simultaneously measuring the fastener preload. A typical preload decay graph from such a test for a prevailing torque nut is shown. The curves are for an M8 nylon insert nut being subjected to transverse vibration of amplitude +/- 0.65 mm. After an initial stage of self-loosening nut rotation stops leaving a residual preload in the fastener. Under transverse vibration, prevailing torque type nuts are not truly 'lock nuts' in that they do not fully prevent rotation i.e. the nut is not locked to the bolt.
There have been a number of cases of prevailing torque nuts becoming detached from bolts leading to a catastrophic failure of the joint. The cause of such detachments was not understood and one reason for this is that such complete loosening is not able to be reproduced using a standard Junker test. Often, they have been put down to 'vibration' without explaining details of the mechanism involved. Without understanding, progress is at best haphazard.
Study of incidents in which detachments of prevailing torque nuts have occurred indicate that the joint would be subjected to axial as well as transverse loading. Previous published research had indicated that axial loading alone acting on a joint does not result in any significant self-loosening. To investigate the causes of nut detachments, a Junker machine was modified to introduce both axial and transverse loading into a joint. Brief details of the modified machine is shown in the image. Miniature hydraulic jacks were used to apply axial loads to the joint while transverse movement occurred. This setup allows for axial loading alone, transverse displacement, or a combination of both.
Experiments with the modified Junker machine demonstrated that the combination of axial and transverse loads significantly affects the loosening of prevailing torque nuts. The test program revealed the following:
1. When the axial load exceeded the preload retained by the nut in a standard Junker test, the nut continued to rotate until it detached from the bolt. In this scenario, the axial load caused joint separation, creating a gap between the joint plates and resulting in the bolt sustaining all the axial loading. This axial loading generated a loosening torque under transverse movement, continuing the nut's rotation until detachment. The image shown illustrates this process. Initially, the nut is tightened, and an axial load is applied. The bolt sustains a small proportion of the load due to the joint and bolt forming a balanced spring system. The bolt acts as a tension spring and the joint as a compression spring, balancing the tensile and compressive loads. The bolt stretches slightly, sustaining a small proportion of the axial load, while the majority is sustained by reducing the joint's compression. When transverse movement occurs, rapid loosening of the nut is observed. The preload decreases until the joint separates, and the bolt sustains the entire axial load. The nut continues to rotate as long as the axial load is maintained, until the machine is stopped, or the nut detaches.
2. If an intermittent axial load was applied, nut rotation occurred when the load exceeded a threshold value equal to the residual preload retained by the bolt in a standard Junker test. Repeated application of such loading led to complete loosening of the nut, and continued loading resulted in detachment. The image illustrates this, with the dotted line showing the loosening curve without axial load.
The research is also relevant to plain non-locking nuts. In the presence of axial loading, plain nuts can readily detach from bolts. Even a small axial load combined with transverse joint movement can lead to detachment.
The loosening torque generated during transverse movement depends on the bolt preload magnitude. Higher preload results in higher loosening torque. For prevailing torque nuts, loosening under transverse vibration occurs until the loosening torque is resisted by an equal prevailing torque. Once self-loosening reduces the preload below the axial load, the axial loading generates the loosening torque, rotating the nut until detachment.
This study does not imply that prevailing torque nuts are obsolete or ineffective but highlights the need for caution in applications with joints that could potentially experience transverse slip and sustained axial loading.
Further details of this research is available in the Proceedings of Mechanical Engineers, volume 223, part C: Journal of Mechanical Engineering Science.
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