Flat Washer (Plain Washer)
Direct Answer
The Flat Washer (Plain Washer, standardized under ISO 7089, DIN 125, and ASME B18.21.1) is a flat annular metallic or polymer disc with a central clearance hole. Positioned under screw heads or nuts, it distributes clamping load across a wider surface area, prevents surface gouging during tightening, and bridges oversized holes.
Source: ISO 7089 (Form A) / DIN 125 / ASME B18.21.1
Specs
| Standard Specifications | ISO 7089 (Form A without chamfer) / ISO 7090 (Form B with chamfer) / DIN 125 / ASME B18.21.1 |
| Form Geometry | Flat annular circular disc with precision inner hole and outer diameter |
| Standard Form Tiers | DIN 125 (Standard outer diameter $\approx 2\times D$) / DIN 9021 (Fender washer $\approx 3\times D$) |
| Primary Functions | Load spreading, surface protection, friction reduction during torque, hole bridging |
| Common Materials | Mild steel, Grade 8 hardened steel (ASTM F436), A2/A4 stainless, brass, nylon, PTFE |
| Hardness Specification | 140 HV to 300+ HV (Hardened washers required for high-tensile structural bolts) |
Load Distribution and Surface Protection
When a bolt is tightened to high tension, the small annular contact area beneath the head can exceed the yield strength of soft materials (aluminum, wood, plastic), causing metal galling and surface crushing. A flat washer spreads the clamp force over 2 to 4 times the area, preventing localized material deformation.
Standard vs Large Fender Washers (DIN 9021)
While standard DIN 125 washers have an outer diameter approximately twice the screw diameter (e.g. M6 washer = 6.4 mm ID, 12.0 mm OD), Fender Washers (DIN 9021) feature an extra-wide outer diameter triple the screw size (M6 = 6.4 mm ID, 18.0 mm OD), designed for thin automotive sheet metal and soft wood construction.