Why bolts work loose
A tightened bolt holds a joint together by preload: the bolt is stretched like a stiff spring and clamps the parts. Two things destroy that preload:
- Self-loosening (rotation). When the clamped parts slip sideways under vibration, the thread and the nut face briefly lose friction and the nut turns back a little with every cycle. This was shown by the Junker transverse vibration test (DIN 65151, ISO 16130), the standard test for locking devices.
- Settling (no rotation). Surface roughness, paint, soft gaskets and coatings flatten under load, so the bolt shortens its stretch and loses preload without the nut turning at all.
The best protection against both is simple: enough preload, a long, slim bolt where possible, and few, hard, flat clamped parts. A locking device is the second line of defence.
Split spring washers: why DIN withdrew them
The split spring washer (DIN 127, IS 3063) was used for decades. Junker testing showed that once a high-strength bolt is properly tightened, the washer is pressed flat and adds almost nothing. Because of this, DIN 127, DIN 128, DIN 6905, DIN 6907 and similar spring and tooth lock washers were withdrawn without replacement, and they are not recommended for bolts of property class 8.8 and above. They still appear on drawings and in low-strength, low-vibration uses, so we stock them, but they should not be relied on to lock a critical joint.
How wedge lock washers work
A wedge lock washer is a pair of washers with radial serrations on the outside and cams on the inside faces. The cam angle is steeper than the thread lead angle. If the nut tries to turn back, the cams ride up on each other and the joint tries to stretch, which raises the clamp load instead of releasing it. The joint is locked by geometry and preload, not by friction, and the washers can be reused if they are undamaged.
Wedge lock washers are tested by the Junker method and work with high-strength bolts, coated bolts and lubricated threads. As authorised HEICO distributor, we supply HEICO-LOCK® wedge lock washers, wedge lock nuts and the related range.
Locking methods compared
| Method | How it works | Prevents rotation | Good to know |
|---|---|---|---|
| Split spring washer | Spring force and sharp edges | Not reliably on 8.8+ bolts | Withdrawn from DIN; fine for light, non-critical joints |
| Nylon insert nut (nylock) | Prevailing torque from a nylon ring | Slows loosening; holds the nut on | Loss-prevention rather than preload locking; temperature limit of the nylon |
| All-metal prevailing torque nut | Deformed thread grips the bolt | Slows loosening | Works hot; limited re-use |
| Thread adhesive | Anaerobic resin fills the thread | Yes, when cured | Needs clean threads and cure time; removal may need heat |
| Wedge lock washers | Cam geometry raises clamp load | Yes | Reusable; works with lubricated and coated bolts |
Choosing for your joint
- Heavy vibration, dynamic loads (crushers, screens, rail, wind, mobile equipment): wedge lock washers on correctly tightened 8.8 or 10.9 bolts.
- Light vibration, low strength bolts: a prevailing torque nut is usually enough.
- Joints you will rarely open: thread adhesive is economical.
- Soft or painted parts: limit settling first, with hardened washers and fewer layers, then add a locking device.
Figures are typical values for guidance. For design, inspection and purchase, always follow the current edition of the standard on your drawing.