Prevention of fastener loosening, part 2
In last month’s article on this subject, the main topic of discussion was chemical threadlocking methods, but there is a little more to be said on this before looking at mechanical methods. The anaerobic liquid threadlocking compounds, while being easy to use, have their own problems. As suggested in the previous article, they have an inconsistent effect on the torque-angle relationship, so while we might be reassured that the bolt won’t unwind, we can’t be entirely sure about the pre-load we managed to achieve in the first instance. However, there are ‘dry’ chemical threadlocking processes that take away some of the uncertainty.
It is possible to specify fasteners with a dry threadlock compound applied. In such compounds, an adhesive is encapsulated in a resin binder, typically epoxy. The encapsulated adhesive is not released until sufficient pressure is applied, and this typically happens as the pre-load is developed. The prevailing torque during tightening – that is, the extra torque due to friction – for this type of threadlocker is due simply to the increased drag from the epoxy. Unlike wet threadlockers, there is no torque developed from an unknown and increasing amount of threadlocking compound beginning to cure and set.
These micro-encapsulated adhesive threadlocking systems are available in a variety of specifications, depending on the application and the service temperature, and are applied over a specified number of threads. The recommendation is that they are not re-used, as the holding power on the second and subsequent uses is a function only of the remaining adhesives after the first use. Some compounds are designed to provide a seal in addition to locking the thread.
Mechanical thread locking can take a number of forms, but a convenient starting point is the type which is most similar to the dry threadlocking compounds. It is also possible to specify fasteners that have a number of threads coated with a thin layer or patch of polymer, which acts to put the fastener into slight interference in its hole. There are also options as to the amount of coverage, which will dictate the prevailing torque measured on installation. Nylon is typically used as the locking material.
Continuing with polymeric locking elements, some specialised fasteners are supplied with a polymer bung inserted radially into the thread. As with the previous option of a nylon patch, any prevailing torque is constant after initial compression of the polymer element.
Interference-fit threads, where the male and female components are in radial interference, are not commonly used owing to the unpredictability of the prevailing torque and the possibility of thread damage. Small differences in the amount of interference can make large differences in prevailing torque; however, a degree of axial interference achieved through the use of deliberately mismatched thread pitch is sometimes used on very critical high-strength fasteners. This provides a degree of protection against loosening, and has been shown to improve the distribution of load between the individual threads and to markedly reduce the degree of stress concentration at the first engaged thread.
Written by Wayne Ward