Why Are Your Bolts Still Loosening? The Truth About Spring and Lock Washers

It comes down to how they fight the "loosening" force. A split spring washer (often called a helical washer) is essentially a one-coil spring. Its job is to provide a constant axial load, keeping the bolt under tension even if the materials expand or contract. A toothed lock washer, on the other hand, is an aggressive mechanical device. It has internal or external teeth that dig into the bolt head and the workpiece. In the fastener world, we say the spring washer "absorbs" the movement, while the lock washer "arrests" it.

What is the technical difference between a split spring washer and a serrated lock washer?

It comes down to how they fight the "loosening" force. A split spring washer (often called a helical washer) is essentially a one-coil spring. Its job is to provide a constant axial load, keeping the bolt under tension even if the materials expand or contract. A toothed lock washer, on the other hand, is an aggressive mechanical device. It has internal or external teeth that dig into the bolt head and the workpiece. In the fastener world, we say the spring washer "absorbs" the movement, while the lock washer "arrests" it.

Why did the split spring washer get such a bad reputation in the NASA fastener safety reports?

If you look at the NASA Fastener Design Manual, they are famously critical of split washers. The reason is simple: when you torque a bolt to its full Property Class 8.8 or 10.9 limit, you often exceed the spring capacity of the washer. Once the washer is flattened, it no longer provides that "spring" tension. Worse, the split ends can act as a lubricant or a ramp, actually making it easier for the bolt to rotate backwards under heavy vibration. This is why Fasto focuses on precise heat treatment—to ensure our spring washers maintain their "rebound" even under high clamp loads.

When should I choose an internal toothed lock washer over an external one?

This is a question of aesthetics versus grip. External toothed lock washers have a wider diameter and provide a more powerful "bite" because the teeth are further from the center of rotation, creating more friction. Internal toothed washers are for when you need a clean finish or a smaller head clearance. I’ve seen external washers strip the paint off a control panel in a week, leading to rust. For export-grade electronics or finished cabinets, Fasto recommends the internal variant to keep the mechanical bite hidden and the surface protection intact.

What is the real cost of buying "standard" washers from a non-integrated broker?

The cost of the washer is pennies; the cost of a loosened bolt in a shipping container is thousands of dollars in insurance claims. In the global supply chain, unbranded washers are often the first place brokers try to cut costs. They use thinner wire or skip the final heat-treat stage. I’ve investigated failures where the "lock" washer was so soft the teeth simply folded over during installation. Partnering with a supplier like Fasto means you aren't just buying a bag of steel; you're buying the technical certainty that your fasteners won't back out when your product hits the high-vibration environment of international transport.

How does Fasto’s manufacturing process prevent the "flattening" problem found in cheap washers?

As a factory-and-trade integrated company, Fasto controls the "spring temper." Most cheap washers are made from low-grade carbon steel that hasn't been properly oil-quenched. They might look right in the box, but they lose their tension the moment the impact wrench hits them. Fasto uses high-quality spring steel (like 65Mn or SUS304) and a strictly monitored tempering cycle. This gives our washers "metallurgical memory." When you loosen a Fasto spring washer, it actually springs back to its original shape, ready to be reused—a sign of true industrial quality.