In many cases, fasteners that self-loosen during equipment operation may contribute to wear and fatigue and result in poor operating tolerances, misalignment and sometimes catastrophic failure. Various types of differential stress – such as vibration and shock, thermal expansion and contraction and micro-movement – reduce clamping force on the assembly and ultimately cause machine failure.

Pro-active maintenance of threaded fasteners has helped to increase equipment uptime. Maintenance activities that counteract self-loosening come in two basic categories: mechanical locking devices and chemical machinery adhesives. They can also combine both.

Threadlocking options

Mechanical devices such as spring washers, wire retainers and locking bolts are costly and cannot reliably prevent self-loosening caused by side-sliding motion. They also do not seal or prevent corrosion within the fastener assembly and must be sized appropriately for the specific fastener, resulting in large and costly parts inventories.

Liquid threadlocking adhesives have become one of the most reliable and inexpensive ways to ensure that a threaded assembly will remain locked and leak-proof for its entire service life.  

Applied in drops to fastener threads, liquid anaerobic products fill the voids of threads and cure to a hard thermoset plastic when exposed to active metal ions in the metal fasteners, whilst in the absence of air. Machinery adhesives lock the threaded parts together, ensuring that mating parts ultimately act as one conjoined component that resists failure. 

Most assemblies held together by threaded fasteners will, at some time, be dismantled for repair, maintenance or adjustment. For this purpose, threadlockers are available in low strength formulations for easy removal, medium strength grades that can be removed using common hand tools and high strength or permanent grades suitable for very demanding assemblies with minimal service requirements.

 

Even these highest strength threadlockers, however, can be removed with standard hand tools following direct exposure to temperatures of 230 to 260°C for about five minutes and dismantled at that temperature. 

A range of threadlocking adhesive viscosities is available, although in practise, standard products are used for the majority of applications. High viscosity grades are available for locking lose threads and penetrating grades easily wick into pre-assembled fasteners and then lock them in position.

The strength class of the threadlocker is selected based on the thread length engaged, thread size, the strength class of the bolt and the required easiness of disassembly.  For a low strength threadlocker, the break-loose torque is lower than the tightening-torque.  A medium strength threadlocker has a break-loose torque similar or slightly higher than the tightening-torque.  And a high strength threadlocker has a much higher break-loose torque than the tightening-torque.  

Best practice

Threadlocking adhesives should only be applied to the thread engagement area as only adhesive between the threads will cure. A good tip is to apply the threadlocker to the bolt thread at a distance of 1 x nut height from the part when locking nuts in place, or to the leading threads of the bolts if assembling into a tapped hole.

 

For blind hole assemblies such as studs, threadlockers should be applied to the bolt and to the threads within the blind hole.  If adhesive is applied only to the bolt, air pressure may force the liquid to escape as the bolt is torqued down and may result in an incomplete cure and possible assembly failure.

The operating conditions of the end-use environment dictate the threadlocking formulation selected.  The newest threadlocking technologies offer many advantages over previous generations.  These include surface-insensitive, high temperature and chemically resistant products as well as formulations that withstand extreme vibration.

Recent advances in the stability and reactivity of threadlocking materials have also allowed the introduction of semi-solid ‘stick’ formulations to complement their liquid counterparts.  These work well in overhead or hard-to-see applications where liquids might be too messy or potential migration could be a problem.