How thread locking compounds work
What actually holds the thread, and when it is the wrong answer.
Thread locking compound holds a fastener in place by setting hard once it is squeezed into the thin gap between the threads and starved of the air around it, filling the small clearances a thread always has between its mating surfaces with a solid resin that stops the nut and bolt rotating relative to each other, rather than by adding any extra friction or grip in the way a spanner might.
The gap every thread needs, and how vibration uses it
An ordinary thread, even a well-made one, always has small clearances built in so the nut can turn onto the bolt without binding. Those same clearances are what let a tightened joint work loose under vibration, the nut rotating back by tiny increments each time the parts shift slightly within that room.
Thread locking compound is applied to the threads as a liquid before assembly, and it behaves unlike almost any other adhesive most people have handled. It stays liquid indefinitely in open air, exposed to oxygen, and only cures once it is squeezed into a thin film between two metal surfaces and shut away from that air, a trigger that runs in reverse to how most glues are expected to behave. The metal surfaces themselves help the reaction along, which is why the compound cures quickly on steel threads and more slowly on plated or stainless ones. Once cured, the resin fills the thread clearances solid, acting more as a filler than a glue, and locks out the tiny rotational slip that vibration would otherwise use to walk the fastener loose.
Mortar in the tub and mortar between bricks
Mortar mixed and spread on a board in the open stays workable for a useful stretch of time, soft enough to smooth and adjust by hand. The same mortar packed into a narrow joint between two bricks pressed firmly together sets harder and faster once it is squeezed thin and largely shut away from the air around the rest of the batch. Nobody expects the whole tub to set at the rate of the thin bead trapped between two bricks, and thread locking compound relies on the same asymmetry, staying workable indefinitely in its bottle where air can reach it and setting firmly only once it is trapped in the thin, air-starved gap between two tightened threads.
Getting the cure to happen
Since the compound cures by exclusion of air, applying it correctly means making sure it ends up shut away. A thread coated too sparingly, or a joint left loosely fitted while the compound is still liquid, can leave enough of it exposed to the surrounding air that it never cures fully. The joint may feel locked straight after assembly while never developing its full holding strength. Oil left on the threads causes a similar problem by keeping the compound from reaching the bare metal that helps it cure, so threads are usually cleaned before it goes on.
Grades, from hand-tool strength to heat-only
Different grades cure to very different final strengths. The mildest can be broken loose with an ordinary hand tool, a middle grade needs a firm pull on a full-sized spanner, and the strongest usually needs the joint heated well past the temperature of boiling water, typically with a blowtorch, to soften the resin before it will turn at all.
Choosing the wrong grade causes trouble in two opposite directions. A low-strength grade on a fastener that needed permanence gives only a little more resistance to loosening than no compound at all. A high-strength grade on a fastener meant to be serviced turns a five-minute job into one that damages the fastener or the part around it in the effort to break it free.
Joints that need to come apart
That second failure explains where the compound belongs: on fasteners tightened once and then left alone for a long service life under vibration. A fastener that needs regular access is better served by a mechanical solution, a lock nut, a split pin, or a properly designed preload margin, each of which resists vibration loosening without also resisting the routine maintenance the joint will need. Scraping out cured compound and reapplying fresh at every service is far more work than choosing a locking method suited to frequent disassembly in the first place.
Locking the thread without adding clamp
The compound also leaves one thing untouched, which is preload. It locks a thread's clearances against rotation and adds no clamping force of its own, so a joint tightened well below its intended preload still suffers the fatigue consequences of being under-clamped, even with a perfectly cured bead holding its threads from turning further. The compound solves the specific problem of vibration walking a correctly tightened fastener loose over time. A fastener that was never tightened correctly in the first place needs the correct torque, and no amount of resin in its threads will stand in for it.