Plain Flat Washers (DIN/ISO/SAE/USS Standard Flat Washers)
A bolted joint rarely fails because the washer was “missing” in the BOM—it fails because the joint lost preload or the bearing surface embedded, especially on softer substrates, slotted holes, or painted/coated parts. A plain flat washer is the simplest control element in that interface: it spreads the compressive load under the bolt head or nut, reduces local surface damage, and stabilises clamp force over time.
In maintenance-heavy equipment and production assemblies, washers also act as a practical tolerance buffer—helping prevent galling on stainless joints, protecting coatings from fretting, and improving repeatability when tightening against irregular surfaces. Whether you are specifying DIN 125 / ISO 7089 normal series, DIN 9021 (large OD / fender) for thin sheet, or SAE/USS flat washers for imperial builds, the key is matching OD/ID/thickness and hardness to the real contact pressure and the base material.
- Distribute bearing pressure reliably
- Protect coating and surfaces
- Reduce embedment preload loss
- Improve joint torque repeatability
- Support slotted-hole assemblies
- Enable DIN/ISO/SAE interchangeability
Technical Specifications
Product Name
Plain Flat Washers / Plain Washers / Standard Flat Washers / Flat Metal Washers
Standards
DIN 125 (common), DIN 9021 (large OD / fender), DIN 433 (small OD), ISO 7089 (normal), ISO 7090 (chamfered), ISO 7093 (large series), ANSI B18.22.1, SAE Flat Washers (narrow), USS Flat Washers (wide)
Material Grades
Carbon steel, alloy steel, Stainless Steel 304 (A2), Stainless Steel 316 (A4), brass (by request)
Grades / Hardness
Standard commercial; Hardened washers available (e.g., HV 140 / HV 200 / HV 300) depending on load and application
Diameter Range
Metric: typical M3–M30 (and larger by request). Imperial: common #6–1″ and above (SAE/USS series)
Surface Finish
Plain, Zinc plated, Black oxide, Hot Dip Galvanized (HDG), passivated for stainless
Certifications
ISO 9001:2015, RoHS/REACH declarations on request, EN 10204 3.1 material certificates available for controlled lots
1: Surface embedment → preload drops after tightening
What happens: On aluminium, timber, cast iron skins, powder-coated parts, or softer sheet, local contact pressure exceeds surface capacity. The joint “settles,” losing clamp load.
Washer solution: A correctly sized washer increases bearing area, lowering contact stress and reducing embedment-driven preload loss. For very soft surfaces, consider large OD (DIN 9021 / ISO 7093).
2: Coating damage and fretting in service
What happens: Vibration or thermal cycling can cause micro-slip under the head/nut, abrading coatings and accelerating corrosion initiation.
Washer solution: Washers act as a sacrificial, replaceable contact surface. For corrosion-critical designs, match finish (e.g., zinc/HDG) to the system and avoid dissimilar-metal couples where galvanic risk is high.
3: “False torque” from poor bearing conditions
What happens: Tightening against a rough/painted surface or slotted hole can create inconsistent friction and scatter in achieved preload.
Washer solution: Washers provide a more uniform bearing face, improving torque-to-tension repeatability. If preload control is critical, specify tightening condition (dry/lubricated) and verify K-factor in trials.
4: Hole geometry and thin sheet pull-through
What happens: Thin sheet around clearance holes can deform or pull through under load, especially with wide slots.
Washer solution: Fender washers (DIN 9021 / “penny washers” UK) spread load over a much larger area to reduce pull-through risk.
Example tables below illustrate common series. Final dimensions should be confirmed against the required standard edition and tolerance class.
A) ISO 7089 / DIN 125 (Normal series) — example
| Nominal d (for bolt) | Washer ID (d1) | Washer OD (d2) | Thickness (s) |
|---|---|---|---|
| M6 | 6.4 | 12 | 1.6 |
| M8 | 8.4 | 16 | 1.6 |
| M10 | 10.5 | 20 | 2.0 |
| M12 | 13.0 | 24 | 2.5 |
| M16 | 17.0 | 30 | 3.0 |
B) DIN 9021 / ISO 7093 (Large OD / Fender series) — example
| Nominal d (for bolt) | Washer ID (d1) | Washer OD (d2) | Thickness (s) |
|---|---|---|---|
| M6 | 6.4 | 18 | 1.6 |
| M8 | 8.4 | 24 | 2.0 |
| M10 | 10.5 | 30 | 2.5 |
| M12 | 13.0 | 37 | 3.0 |
| M16 | 17.0 | 50 | 4.0 |
How to select quickly (engineering checks)
If the base material is soft or slotted: prioritise larger OD.
If clamp load is high (structural/high-strength bolts): specify hardened washers to prevent washer cupping and embedding.
If corrosion class is demanding: align finish with the system (e.g., HDG with HDG bolts/nuts).
Torque, preload, and friction control
A washer changes the bearing interface friction; for controlled preload, define the tightening condition (dry vs lightly oiled) and validate torque in trials.
In stainless assemblies, consider lubrication to reduce galling risk; uncontrolled galling can destroy threads before target preload is reached.
Washer hardness must match bolt grade and contact pressure
For high clamp loads (e.g., high-strength joints), soft washers can dish/cup, creating preload loss. Specify hardened flat washers (HV 200/HV 300) where bearing stress is high.
Hole clearance compatibility (ISO 273)
Ensure the washer ID clears the bolt shank and any coating buildup while staying centred. Oversized clearance increases the risk of washer walking on slotted holes.
Stack-up discipline (washers + locking)
Plain washers distribute load; they do not provide locking by themselves. If vibration loosening is a known risk, combine with an appropriate locking method (e.g., prevailing torque nuts, threadlocker, serrated flange designs) and verify by testing.
Corrosion and galvanic considerations
Mixing stainless washers with carbon steel fasteners (or vice versa) can accelerate corrosion in aggressive environments. For outdoor use, align the material system or add protective coatings and isolation strategy.
Related Products
FAQ
What is the purpose of a plain flat washer?
A plain flat washer distributes load under a bolt head or nut to reduce surface damage and preload loss. It helps prevent embedment on softer materials and improves tightening consistency.
What is the difference between DIN 125 and DIN 9021 flat washers?
DIN 125 is a normal OD washer, while DIN 9021 is a large OD (fender) washer. DIN 9021 is typically chosen for thin sheet, slotted holes, or soft substrates to reduce pull-through risk.
When should I specify hardened flat washers (HV 200 / HV 300)?
Hardened flat washers are specified when clamp loads and bearing stress are high and a soft washer could dish or embed. They are common in heavy-duty joints and high-strength bolting where preload retention matters.
Are SAE and USS flat washers interchangeable?
Not always—SAE washers generally have a narrower OD, while USS washers are wider. You should match the series to your drawing requirement and verify OD/ID/thickness before substitution.
Do flat washers prevent loosening under vibration?
No, a plain flat washer is not a locking device. It improves bearing conditions and load distribution, but vibration resistance usually requires a locking method such as prevailing torque nuts, threadlocker, or serrated flange components.