Bolted joints loosen from transverse vibration, not axial pull. When a joint cycles sideways, the friction that holds the threads engaged momentarily drops to zero and the bolt backs off a few degrees per cycle. Split washers, star washers, and even nyloc nuts help less than most engineers assume. Thread-locking adhesives — anaerobic methacrylate resins — solve the problem chemically by filling the thread gaps with a polymer that cures in the absence of air and in contact with metal ions.

Henkel's Loctite line is the reference. The color codes are worth memorizing because they tell you the strength and, more importantly, the removability:

• Purple (222) — low strength. For fasteners smaller than M6, brass, aluminum, or anything you'll disassemble often (calibration screws, adjuster set screws). Removable by hand tools.
• Blue (242/243) — medium strength. The default choice for M6–M20 steel fasteners on machinery, vehicles, and equipment. Removable with hand tools. 243 is oil-tolerant for as-machined parts.
• Red (262/271/272) — high strength. Permanent for practical purposes: you'll need to heat the joint to ~250°C to break the cure before wrenching. Use for studs, bearing retainers, and anything you don't intend to service.
• Green (290) — wicking grade. Low viscosity that penetrates already-assembled fasteners by capillary action.

Real-world example: The set screws that clamp encoder hubs to servo motor shafts. A single M4 cup-point set screw at 2 N·m preload will walk out in weeks on a machine that reverses direction constantly. A drop of blue 242 on the threads holds it indefinitely and still allows removal for reindexing.

Rule of thumb — application: use one drop per 6 mm of thread engagement, applied to the bolt threads at the point of nut engagement, not the tip. Assemble within a few minutes; anaerobics cure fully in 24 hours at room temperature but reach handling strength in 10–20 minutes on active metals (steel, brass). On inactive metals (stainless, aluminum, plated fasteners) cure time doubles or triples — use a primer (7471 or 7649) or bump up to a grade specifically formulated for inactive surfaces.

Two failure modes to avoid: applying threadlocker to oily or greasy threads (cure inhibited — clean with brake cleaner first), and using red where blue was called for (torch-wielding technician six months later will curse your name).