Flat Washer vs Spring Washer vs Lock Washer: Which One Do You Need?

What Each Washer Actually Does

Washers look like the simplest component in a bolted joint, but choosing the wrong type — or skipping one where it is needed — is a classic cause of loosened connections and damaged surfaces. There are three families you will meet most often in metric hardware, plus a few specialty types.

Flat Washers (DIN 125 / ISO 7089)

A flat washer does not lock anything. Its job is mechanical: it spreads the clamping load over a larger area, prevents the bolt head or nut from digging into soft materials, and protects painted or machined surfaces from turning damage. Use one under whichever element turns (usually the nut). In structural steelwork, larger square washers (DIN 434) do the same job on channel and I-beam flanges.

Spring Washers (DIN 127)

The spring (split) washer is a helical ring with a split that acts like a small spring: it maintains axial tension in the bolt and its sharp cut edges bite slightly into the contact faces to resist rotation. They work best on hard, flat surfaces with short grip lengths. On long bolts, soft materials, or high-vibration assemblies their effectiveness drops — modern practice often prefers nylon-insert lock nuts or flange nuts instead.

Serrated / Toothed Lock Washers (DIN 6797 / 6798)

These have multiple twisted teeth (external for DIN 6797, internal for DIN 6798) that bite into both the bolt head and the mating surface. They grip far better than a single split washer but scratch the surface — fine on unpainted steel, a problem on finished surfaces and stainless (where dissimilar-metal scratching can initiate corrosion).

Side-by-Side Comparison

Type	Standard	Primary Function	Best For	Avoid When
Flat washer	DIN 125	Load distribution, surface protection	Almost every joint; soft or painted surfaces	Used alone expecting anti-loosening
Square washer	DIN 434	Load distribution on steel profiles	Channel / I-beam flanges, anchors	Confined round holes
Spring (split) washer	DIN 127	Maintains tension, resists rotation	Short, rigid, lightly loaded joints	Long bolts, heavy vibration, soft materials
Serrated lock washer	DIN 6797/6798	High-resistance anti-loosening	Unpainted steel, general machinery	Finished surfaces, stainless joints

How to Choose: Practical Rules

Default to a flat washer under the nut — it costs little and improves both load spread and torque repeatability.
Anti-loosening needed? Prefer a nylon-insert lock nut or a serrated flange nut (DIN 6923) over a spring washer for anything that vibrates.
Soft material (wood, aluminum, composite)? Use a flat washer with a large outside diameter — never a spring washer, which will embed unevenly.
Structural steel: follow the project spec — HSFG assemblies usually require hardened washers (HV) under both head and nut.
Want it pre-assembled? SEMS bolts (pre-assembled screw + washer) eliminate washer handling entirely on assembly lines.

Matching Washer Material to Your Bolt

Pair like with like: a galvanized washer under a galvanized bolt, A2 stainless washers under stainless bolts. Mixing a plain carbon-steel washer into a stainless assembly creates galvanic corrosion; mixing a soft washer under a 12.9 bolt can simply crush it. Match the washer hardness to the bolt grade for anything above class 8.8.

Frequently Asked Questions

Can I stack a spring washer on top of a flat washer?

Yes — this is common practice. Put the flat washer against the surface and the spring washer between it and the nut, so the spring element works on a hard, flat counterface instead of digging into the base material.

Do flange bolts and flange nuts need washers?

No. Their integrated flange already performs the flat washer’s job of distributing load. Adding washers under flange fasteners is redundant unless the base material is very soft.

Are spring washers obsolete?

Not quite. Research (notably by the German fastening body DIN 25201) showed their anti-loosening benefit is smaller than assumed, and they are excluded from some modern rail and structural standards — but they remain a low-cost, adequate solution for general machinery with light vibration.