Flat Washer
Washers · plain washer · USS washer · SAE washer
Definition: A flat washer is a thin, disc-shaped plate with a central hole that distributes the load of a fastener head or nut and protects the bearing surface.
Anatomy
■ Body■ Hole
Governing standards: ASME B18.22.1 · USS · SAE
Common Sizes
| Inch | Metric | Head width | Head height |
|---|---|---|---|
| 1/4 | M6 | — | — |
| 3/8 | M10 | — | — |
| 1/2 | M12 | — | — |
Representative sizes. Full dimensional data lives in the standards listed below.
Grades & Strength
| Grade | Material | Tensile (psi) | Marking |
|---|---|---|---|
| USS (wide) | low carbon steel | — | None |
| SAE (narrow) | low carbon steel | — | None |
Materials
- Zinc-plated steel
- Stainless steel
- Brass
- Nylon
- Neoprene (sealing)
Pros & Cons
Pros
- Cheap insurance against surface damage
- Spreads clamp load over a larger area
- Covers oversized or slotted holes
Cons
- Adds a part to every joint
- A plain flat washer does not lock against vibration
Typical Uses
- Load distribution under bolt heads and nuts
- Protecting finished surfaces from marring
- Shimming and spacing
- Sealing with bonded washers
Alternatives & Related
- Lock Nut — Built-in anti-loosening where a lock washer is not wanted
- Hex Bolt — The fastener a flat washer typically pairs with
Where to Buy
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FAQ
Do I need a washer with a bolt?
A flat washer is recommended whenever the fastener bears on soft material, a slotted hole, or a finished surface. On hardened steel, it is often unnecessary.
What is the difference between USS and SAE washers?
USS washers are wider and thicker for a given size; SAE washers are narrower and thinner. Use USS for soft materials and SAE for tighter fits and hardened surfaces.