How Do Washers Keep Nuts and Bolts Tight?

September 11, 2017

How Do Washers Keep Nuts and Bolts Tight? Well, the gross addition of any additional surface area does aid the tightening process. The added surface allows more torque to be applied, then the washer compresses slightly while it disperses the mechanical energy, but there are limits to this effect. Still, this seems like a good place to start, so let's open with a look at the effects of potential energy.

Storing Potential Energy

Even a heat-treated bolt, one that's made of a hardened alloy, possesses a finite quantity of material elasticity. If you tighten the nut and bolt, perhaps against two surfaces, the bolt will engage further. It stretches imperceptibly as it passes its fully anchored position. What if a plain bearing is added to the mix? Well, the bolt won't pull through the fastening hole, nor will the tightened head warp the surface it’s holding. Better yet, there's now somewhere for that added torque to go. The energy compresses the washer, distributes the force, and keeps the nut and bolt tight. However, there are limited advantages to be gained here, which means the washer needs a little transformational magic if the locking effect is to be maximized.

Introducing Locking Washers

The obvious solution is a washer design that incorporates added surface contact attributes. A serrated washer, for example, rotates slightly then digs into the underlying surface so that it no longer rotates. Those serrated edges also possess a slight spring. As the bolt is further tightened, the washer exerts an equal and opposite force until the nut no longer turns. Unfortunately, this "digging" effect can severely distress the underlying surface part. Cosmetically, the part is abraded. Functionally, a surface finish is destroyed and the fastener hole gains unsightly ragged edges. It's because of this design drawback that split-ring washers have taken over. Used in conjunction with a regular flat washer, the bolts and nuts tighten without laying waste to the surface or the fastener hole.

A flat washer and alloy fastener work together to balance applied torque. Any remaining torque is then compressed and distributed by that disc-like insert. However, the tightening force delivered by this method is limited. The proper solution is a serrated edge or a split ring design. Just remember, however, if that serrated disc is allowed to rotate, then it will gouge the surface underneath the washer. In other words, purpose-designed anchoring washers are suitable bolt and nut tightening solutions, but they must be fastened properly so that the anchoring process doesn't cause damage to the joined surfaces.

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