Posted to News on 3rd Nov 2020, 08:58
Beware of the gap when locking threaded fasteners
When designing a new machine its handy to be confident that the threaded fasteners used will remain in place. A secure fastener can mean a secure reputation, so preventing the early failure of a threaded fastener is a good thing to achieve
Threaded bolts that self-loosen during a machine’s operation can cause all manner of problems, and when an East European manufacturer of screen printers began reporting bolt failures on its machines, the company went back to basics. In operation, these large-scale printers generate a lot of two things: vibration and heat. Both were causing the threaded fasteners to self-loosen. The manufacturer was swift to act on field reports and resolved the problem by applying a wicking-grade adhesive thread locker to the threaded bolts on the installed printers. It also adopted adhesive thread locking as a standard assembly method in the factory to eliminate self-loosening at source.
Some engineers are unaware that in a typical threaded fastener assembly, there is as little as 15% thread to thread surface contact. The remaining space allows for movement of the bolt within the nut, ultimately resulting in it loosening after a number of machine cycles: the number itself can vary from small to large dependent upon a variety of individual circumstances, but the end result is usually the same. Changes in temperature can also have a similar effect, causing micro expansion and contraction of the metal components which leads to short-term frictionless situations where the bolt unwinds itself from the nut.
A common antidote to the problem is the application of a mechanical thread locking device such as a spring washer or a double nut. Both help delay the onset of self-loosening but ultimately failure may still be inevitable if the assembly isn’t frequently inspected and re-torqued as necessary. By comparison, a liquid thread locker fills all the voids between the mating threads and then solidifies to form a thermoset plastic which unitises the assembly. It effectively stops the micro-movements that lead to self-loosening and, at the same time, seals the assembly against corrosion and airborne contaminants such as dust.
These adhesives are available in a choice of strengths in easy flowing liquids or semi-solid form to suit different applications but all allow fuss-free disassembly with hand tools for repair and maintenance. The adhesive thread locker which the East European printer manufacturer chose for its bolt assembly is LOCTITE® 243 from Henkel. This is a single component, medium strength thread locker that has recently been upgraded to offer good bond strength on contaminated surfaces, provide higher temperature resistance and perform well on passive metals without an activator.
To provide a comprehensive introduction to the merits of adhesive thread locking, either as a stand-alone solution or to complement a mechanical thread locking device, Henkel has published e-guide. It can be downloaded here.
