How can nuts and bolts be kept from undoing during operation, so that the joint remains secure? This question became relevant from the very time the thread was invented. And the higher the speeds, the loads and the cost of mechanisms became, the more serious the problem of threaded joints «unscrewing themselves» became. Many locking methods have been invented: locknuts, split pins, Grover spring washers, drops of paint and so on. Each of these methods has its merits, but it also has drawbacks that sometimes make the use of one or another locking method impossible. Thus some methods require special nuts and bolts, others additional space, and others do not always reliably withstand vibration. The most modern and reliable method today is considered to be the use of liquid threadlockers: they provide reliable locking of the joint and at the same time do not have the drawbacks described above. What is more, using them is cheaper than practically all other methods.
What types of threadlocker are there?
The whole variety of existing liquid threadlockers can ultimately be reduced to two types: low strength, or removable, and high strength, which is permanent. That said, the division into removable and permanent is fairly conventional; a joint made with any threadlocker can be taken apart if you know how to do it.
How can different threadlockers be told apart?
As a rule, removable ones are coloured blue and permanent ones red. But not always: with some manufacturers the colour schemes may differ from the generally accepted ones, so it is best to read the information on the packaging and not to rely on the colour alone.
Which threadlocker should be used where?
It is best to consult the repair and operating manual for the mechanism that is to be repaired. If that is not possible, a simple rule can be followed: in joints that require frequent dismantling a low-strength product is used, and in joints that are expected to be dismantled rarely or never at all, a high-strength product. For example, if the cylinder head is attached to the block with studs, then the studs must be screwed into the block using a high-strength threadlocker, whereas the nuts must be treated with a low-strength one. If the head is attached with bolts, only a low-strength threadlocker should be used. A high-strength threadlocker is used when fitting the flywheel, pulleys, the torque converter in an automatic gearbox and wheel studs (but not bolts!). In most other cases a low-strength threadlocker is used.
How does a threadlocker work?
A threadlocker is an anaerobic one-component adhesive. The term «anaerobic» means that the adhesive polymerises in the absence of oxygen. To ensure reliable locking, the thread must be cleaned of dirt and oil contamination before the threadlocker is applied. It is enough to apply the adhesive to the bolt or stud and then assemble the threaded joint. During tightening the threadlocker is distributed evenly over the thread, and since the gaps in a threaded joint are minimal, oxygen from the air does not get in there and the compound acquires its working characteristics. It should also be taken into account that the locking strength depends on the material the nut and bolt are made of. The greatest strength is achieved on steel, on non-ferrous metals the strength is lower, and on plastic a threadlocker is practically useless. The point is that for polymerisation the compound needs the presence of metals, or rather of their salts, and it is best if these are iron salts.
How can a joint assembled with a threadlocker be taken apart?
It may seem a paradox, but a threadlocker, even a high-strength or, in other words, permanent one, makes it possible to dismantle a threaded joint far more easily. The point is that the strongest threadlocker is rust. Anyone who has tried to undo rusted bolts and nuts knows how difficult this is, and sometimes impossible even with the help of a liquid wrench, a torch and other tricks. Besides its direct purpose, a threadlocker also works as a sealant, protecting the threaded joint from moisture and air getting into it. A thread treated with a threadlocker therefore does not rust. As a result, a joint treated with a low-strength threadlocker comes apart without any problems. With a joint treated with a permanent threadlocker things are more difficult, but not by much. Such a joint cannot be taken apart just like that, «head on»; you can only shear off the bolt head or the stud. But the whole secret is that a threadlocker loses strength when heated. It is enough to warm the threaded joint with a heat gun or a gas torch, and the fastener that was stuck fast can be undone without excessive effort. Naturally, care must be taken when heating fasteners so as not to damage low-melting parts located nearby.