Threaded nuts are commonly used for joining parts temporarily because they offer numerous advantages such as convenience, cost, and time savings. Unfortunately, these products typically begin to loosen under a variety of hostile conditions. To overcome this deficiency the use of anti-loosening nuts has become more popular in a variety of settings. To understand exactly what sets this type of nut apart it is critical to consider what the most common causes of nut loosening are and how they can be overcome.
Unintentional loosening is often referred to as vibrational loosening because constant vibration is considered to be the primary cause of nut failure. Vibration causes the nut or bolt head to move relative to the threads which causes loosening over time. If this does not happen then there is little chance of loosening to occur. In most cases, anti-loosening nut designs focus primarily on preventing this cause of failure.
Fatigue is another common cause of failure. Fatigue failure reduces the clamp force on a joint. As a result, the joint slips and the bolt becomes subjected to bending. The additional bending load subsequently causes joint failure. While the nut itself doesn’t failure, it’s small, incremental slipping causes the additional bending load which leads to failure.
Pre-loaded nuts can also rotate loose when relative motion between male and female threads take place. This motion cancels the friction grip and the off torque rotates the screw loose. There are several common causes of relative motion occurring in threads. They are bending of parts which results in new forces being induced, differential thermal effects, and unintended applied forces on the joint.
Companies have designed a variety of anti-loosening nuts to address these issues. Generally, the design of anti-loosening nuts will address one or more of three issues depending on their anticipated application. The first is a design intended to create sufficient clamp form on the joint interface to reduce or eliminate relative motion between the nut and the joint. The second is to utilize a design in which the joint has the ability to adjust for the effects of embedding and stress relaxation. This is particularly helpful for high-impact applications. The final option is a typical nut and bolt design or anti-loosening nut design intended to work with popular thread locking compounds.
Numerous studies have shown anti-loosening nuts have a much lower loosening tendency than standard fasteners. This is because the design address the most causes of nut loosening including vibrational loosening, fatigue failure, and relative motion between threads. New anti-loosening designs work by approaching this issue in one or more of the following ways: creating sufficient clamp force, using a joint with the ability to adjust for embedding and stress relaxation, and using complimentary thread locking compounds.