Washers, locking methods and screws that will not stay tight
A screw loosens for one of two reasons: it was never tight enough to begin with, or vibration is slowly rotating it. The fix is different for each, and the split washer most people reach for is the weakest option available.
What each washer actually does
- Plain washer — spreads load and protects the surface. It does not lock anything.
- Spring (split) washer — small amount of tension. Testing has repeatedly shown it does little under real vibration.
- Serrated / external tooth washer — teeth bite both surfaces. Genuinely effective, but it marks the finish.
- Wave washer — takes up slack and stops rattle where you cannot fully tighten.
- Wedge locking pair (Nord-Lock style) — two ramped washers that jam if the screw tries to rotate. The strongest mechanical option.
- Nyloc nut — nylon collar grips the thread. Excellent, but single use in practice and not for high heat.
Thread locker
Blue (medium strength) is the everyday choice: it holds, and you can still undo it with hand tools.
Red is high strength and normally needs heat to remove — wrong for anything in electronics.
Purple is made for small screws, M2 to M6, and will not glue a tiny screw in permanently.
The threads must be clean and dry. A drop on oily threads does nothing.
The usual real cause
Most loose screws were under-tightened, or they are clamping something that compresses — plastic, gasket, paint — so the clamp force bleeds away over a few days.
Re-torque once after the first day of use. That single step fixes more rattles than any washer.
Common questions
Do spring washers really work?
Barely. Under transverse vibration they perform little better than a plain washer. A serrated washer, a nyloc nut or thread locker is far more effective.
Which way round does a washer go?
Plain washers have a slightly rounded edge from stamping — that side faces the head. Serrated washers go teeth-down against the metal you want them to bite.
Can I use thread locker on plastic?
Only a plastic-safe formulation. Ordinary cyanoacrylate-based products can craze ABS and polycarbonate and cause it to crack later.
Keep reading
Tools and screws for this job
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Metric screw assortment kit
One box covering the sizes in this guide
Precision driver and bit set
The right bit is what stops rounded heads
Digital caliper
Measure before you buy anything