Reasons for the failure of threaded assemblies
One of the reasons why threaded fastener assemblies fail is because clamp force is not maintained or they loosen because of gaps. In order for the assembly to become possible, nuts and bolts must have tolerance that will create the required gap between the threads. These gaps will allow the parts to move with vibrations and side to side movements that can cause expansion or contraction. Failure also happens in certain circumstances where the nut and bolt seize together due to corrosion, rust and environmental conditions like humidity and high temperatures. Threadlockers are the adhesives for perfect locking of metal threaded fasteners inasmuch as they adhere well to metal and once they are cured, they are flexible and chemical resistant. Various strengths of the adhesive can be formulated depending on whether there are future plans for the removal of the threaded fastener.
Making a selection for the threadlocker
Threadlockers are made from different formulas:
- Low strength formulations for easy removal. This threadlocking adhesive is used on screws that are ¼” (6mm) in diameter like adjustment and calibration screws.
- Medium strength formulations are designed for removal using hand tools. It is used for ¼” to ¾” fasteners in machine tools and presses.
- High strength formulations are designed to deliver high strength and can only removed through the application of localized heat and disassembly with hand tools while hot. It is used for ¼” to ¾” fasteners found in permanent assembly applications like heavy equipment.
- Liquid formulas are mainly used for everyday assembly and ideal for fine threads and blind holes
- Semi-solid formulas are ideal for overhead applications
- Tape formulas are cost effective and used for controlled applications. It has to be pre-applied days before the assembly
How threadlocker is applied
Application of the threadlocking adhesive requires that it must wet the whole length of the thread engagement area. Proper wetting is dependent on the size of the thread, viscosity of the threadlocker and parts geometry. If the parts are quite large, both parts would require wetting in order to provide the necessary reliability for adhesion. For the through-hole nut and bolt assemblies, the thread locker is only applied where the bolt and nut meet when the assembly is fully tightened. In blind-hole assemblies, the threadlocking adhesive should only be applied to both the bolt and mating threads. Liquid threadlockers are applied using manual, semi-automatic or automatic dispensers although pre-applied coatings are becoming a better option to the manual or automatic dispensing systems.