Why Use Washers?
Washers distribute load, protect surfaces, prevent loosening, and span oversized holes. The right washer improves joint strength and longevity—the wrong one (or none) can cause failure.
Flat Washers (Most Common)
Simple discs that distribute load and protect surfaces. Two main types:
- SAE Flat Washers: Smaller OD, designed for machine/automotive use. Standard choice for most applications.
- USS Flat Washers: Larger OD, for structural/construction. Better load distribution on soft materials like wood.
| Bolt Size | SAE OD | USS OD |
|---|---|---|
| 1/4" | 5/8" | 3/4" |
| 3/8" | 13/16" | 1" |
| 1/2" | 1-1/16" | 1-3/8" |
Fender Washers
Extra-large OD with small ID. Named for automotive fender applications.
- Use for: Spanning oversized holes, distributing load on thin/soft materials
- Example: 1/4" bolt with 1-1/4" OD washer
- Common in: Sheet metal, drywall, automotive body panels
Lock Washers
Designed to prevent nuts/bolts from loosening under vibration.
- Split Lock Washers: Most common. The split creates spring tension. Best for medium-duty applications.
- External Tooth Lock Washers: Teeth bite into surface. Better grip than split washers.
- Internal Tooth Lock Washers: Teeth bite under bolt head. For finished/visible surfaces.
- Nord-Lock / Wedge Lock: Premium. Uses wedge action. Best vibration resistance.
Specialty Washers
- Finishing Washers: Countersunk for flat head screws. Provides clean appearance.
- Sealing Washers: Bonded rubber/neoprene. For watertight connections.
- Belleville Washers: Conical spring washers. Maintain tension under thermal expansion.
- Structural Washers: Thick, hardened. Required for structural connections (A325/A490 bolts).
Hardened Washers
Heat-treated for extra strength. Required for high-strength bolts (Grade 8, Class 10.9) to prevent the washer from deforming under load.
- Use with: Grade 8 bolts, structural applications
- Avoid: Standard washers with high-torque applications
Washer Selection Guide
- General purpose: SAE flat washer
- Wood/soft material: USS flat washer
- Vibration concern: Lock washer (or Nyloc nut)
- Oversized hole: Fender washer
- Grade 8 bolt: Hardened flat washer
- Structural steel: F436 structural washer