A vibrating machine sheds bolts. The mechanic adds split spring washers under every nut. They still come loose, and now some washers have flattened out. The spring washer is doing less than people think, and in some cases it makes the joint worse.
Why bolts self-loosen
A threaded fastener holds because of preload: the bolt is stretched, clamping the joint. Vibration and transverse slip let the nut rotate relative to the bolt, which sheds preload. Once the clamp force drops, the joint moves, and the bolt loosens fast. Anything that doesn’t maintain preload doesn’t actually lock the bolt.
A split washer compresses flat as soon as you torque the bolt. Once flat it’s just a flat washer. Under transverse vibration it doesn’t stop the nut from turning. Tests on transverse vibration rigs show plain spring washers perform about as well as no washer at all.
What actually works
The first fix is correct preload. A properly torqued bolt with a hard flat washer under the turned part stays tight longer than any locking device on an under-tightened joint. Make sure the joint is rigid; a soft gasket or a thin plate lets the bolt lose preload as the material settles.
For genuine vibration locking, use a prevailing torque locknut (nylon insert or all-metal distorted thread), a wedge-locking washer pair, or chemical threadlocker. Each holds under conditions where spring washers fail.
Pick the method by the application
Threadlocker is cheap and seals threads, but disassembly needs heat on the high-strength grades. Use medium strength for parts you service. Nylon insert locknuts are reusable a limited number of times and degrade in heat. Wedge washers handle high loads and repeated disassembly but cost more.
Don’t stack devices. A spring washer under a locknut or a double nut doesn’t add locking; it just changes the joint geometry and can reduce preload.
Mark the bolts
After torquing, paint a witness mark across the nut and joint. A quick walk-down shows any rotation without checking every bolt with a wrench. This catches loosening early, which is the point of locking in the first place.
