Every day we use products which contain numerous threaded fasteners. Screw joints are still the most common way of establishing a demountable, yet secure fixing between two parts. Although they come in various shapes and sizes, all threaded fasteners are used to accomplish the same goal: achieve durable assembly. Hence, it is vital that bolt tension and clamping forces are maintained.

Examples of traditional threadlocking methods

Mechanical devices (e.g. split pins, tab washers)		Friction devices (e.g. nylon ring or deformed nut)		Locking devices (e.g. tooth or ribbed flanged bolts)
Only used to prevent the loss of nuts and bolts.		Increase friction; will not ensure permanent threadlocking under dynamic loads.		Prevent self-loosening, but are expensive; need larger flange-bearing surfaces and damage the surfaces.

 

Challenge

Threaded assemblies generally fail because of a loss in bolt tension.
The main causes for this are relaxation and self-loosening.

Relaxation results in a change in bolt tension which leads to a reduced clamping force. This effect is triggered by: Settling – pressure-induced smoothing of the surface roughness Creeping – time-dependent yielding of substrates when loads exceed the compressive strength of the material e.g. when clamping gaskets. If the elastic capacity of the assembly is inadequate, e.g. if bolts are too stiff or the wrong length to diameter ratio is selected, the loss of bolt tension cannot be compensated.

Self-loosening is caused by any type of dynamic load, such as vibration or changes in temperature. Insufficient clamp load and poorly fitting parts allow relative movements which increase the risk of self-loosening. These load changes lead to short-term frictionless situations where the bolt unwinds from the nut. The sum of these very small movements ultimately results in the loosening of the threaded assembly.

While relaxation can only be reduced by changes in parts design (e.g. length to diameter ratio) or the use of elastic elements, self-loosening can only be avoided by using the appropriate threadlocking devices.

 

Loctite Solution

Loctite threadlockers are single component adhesives which completely fill all voids between the interfacing threads. They cure in the absence of air and in contact with metal to form a tough thermoset plastic, thus creating an interfacial connection that maintains the friction between the threads by adhesion to both flanges and by cohesion within the adhesive layer. This prevents any movement between the threads.

Loctite threadlocker between the interfacing threads

 

What method shows the best performance?

The results of the transverse shock test clearly show the different success levels of the various threadlocking methods. Loctite threadlockers achieve the best clamp-load retention.

				Standard bolt with Loctite threadlocking system
				Bolt with saw-toothed flange
				Nut with polyamide ring
				Bolt with DIN 6797 A tooth lock washer
				Bolt with DIN 127 A spring washer
				Unsecured standard bolt

 

How to use

Generally cleaning is key for optimum performance. Components should be clean and free from contamination such as grease, oil, cutting fluids, protective coatings, etc. Normal practice is to degrease using specialised solvents such as Loctite^® 7063.

Loctite threadlockers can be applied as follows:

For through holes		For blind holes		For post assembly

Note:
Passive materials like stainless steel, aluminium or coated metals may require the use of activators to initiate/speed up cure.

 

Disassembly

1. Remove with standard hand tools.
2. In rare instances where hand tools do not work because of excessive engagement length, apply localised heat to nut or bolt to approximately 250 °C. Disassemble while hot.

 

Check out the new Loctite Health & Safety threadlockers Loctite^® 2400 and Loctite^® 2700 or explore the different technologies in our application stories!