Washers Selection: Flat, Spring, Belleville, Locking — When to Use Each
Washers are the most mis-used component in fastener assemblies. The spring lock washer that everyone reaches for actually accelerates loosening under vibration. The flat washer that engineers ignore is often the only thing saving the joint from a crushed aluminum housing. Belleville washers that look like trivial conical discs solve thermal-expansion problems that no bolt can handle alone.
The short answer: use flat washers to protect surfaces, Belleville washers for thermal expansion compensation, toothed lock washers for light anti-rotation, and stop using split spring lock washers on anything that vibrates.
Four washer types at a glance
| Washer type | What it does | Best for | When NOT to use |
|---|---|---|---|
| Flat washer (DIN 125) | Spreads load, protects soft surfaces | Aluminum/plastic housings, oversized holes | Never harmful, always safe |
| Split spring lock washer | Points up, claims to lock | Non-critical, low-vibration indoor | Under vibration — accelerates loosening |
| Belleville (conical) | Spring action compensates thermal expansion | High-temp joints, dissimilar materials | Low-temp, room-temp static joints |
| Toothed lock washer (internal/external) | Serrations bite into surface, anti-rotation | Sheet metal, painted surfaces, light assemblies | Hardened steel, vibration above 500 Hz |
Why the spring lock washer is a myth
The split spring lock washer has been sold for 80 years as a vibration-proofing device. Junker transverse-vibration testing shows the opposite: a bolt with a spring washer loses preload faster than a bolt with no washer at all. The reason is mechanical — the split washer introduces a sharp point contact under the bolt head, which reduces the bearing friction that resists unwinding. The bolt effectively sits on a knife edge, and the joint slips sideways sooner.
The spring washer survives in catalogs because it looks like it should work, not because it does. Replace it with a flat washer plus a thread-locking compound for vibrating joints.
When Belleville washers save the joint
When a steel bolt clamps an aluminum housing, the two materials expand at different rates when the temperature rises. Aluminum expands ~2x faster than steel, so the bolt load drops as the joint heats up — and if the joint sees temperature cycles, the preload can vanish entirely. A Belleville washer under the bolt head provides a spring constant that compensates for the differential expansion: as the aluminum grows, the Belleville flattens slightly and maintains the clamp force. This is why engine head bolts and exhaust flange joints always use Belleville or serrated flange bolts.
Toprecision supplies flat washers (DIN 125 / ISO 7089), Belleville washers and serrated flange bolts in standard and custom sizes. Send the joint materials and temperature range — we will recommend the washer type that matches, not the one that looks right on the drawing. Request a washer-selection consult with spring-rate and load-deflection data.
