Washers per Screw

Washers per through-bolt

A standard through-bolted joint takes at least 1 plain washer between the bolt head and the workpiece, plus one more between the nut and the opposite face. What does the plain washer (DIN 125 / ISO 7089) buy you? It spreads the load across a wider area, keeps the rotating head from chewing up the surface, and cuts down local crushing in soft stuff like wood or aluminum.

If vibration is a worry and you don't want the joint backing off, add a spring lock washer (DIN 127), one more washer that sits between the plain washer and the bolt head (or nut). Under heavy or repeated dynamic loads, go with conical Belleville washers instead. They store elastic energy and hold the clamping force even after the joint settles in. Bolt sizes are metric, and in everyday fabrication you'll mostly run into M6, M8 and M10, each paired with its own washer outer diameter and thickness.

Applications

Bolting metal structures together, anchoring machine bases to concrete, building wooden furniture with through-bolts, holding down automotive parts, mounting flanges and brackets, and pretty much any joint that sees vibration and can't be allowed to come loose.

FAQ

Plain washer on both sides, is it really necessary? Yes, any time the surface is softer than the bolt or you care about keeping the finish intact. You can get away with leaving it out on steel-on-steel joints using hardened bolts, though it's still not the cleanest practice.

Where does the lock washer go? Between the plain washer and the bolt head (or nut). Don't put it between the workpiece and the plain washer, or it bites into the surface and loses its spring action.

When should I use Belleville washers instead? Reach for Belleville when the joint goes through thermal cycling, vibration or gasket relaxation. They hold clamp load far better than a single split washer.