Submit an R.F.Q. — Flat Washers
Technical sales team responds within 24 hours. All sizes, series, materials and coatings available.
R.F.Q. Submitted Successfully
Our technical team will contact you within 24 hours.
Reference: sales@rrhydraulics.com
Flat Washers
A comprehensive engineering reference for EPC contractors, structural engineers, OEM manufacturers and procurement teams — covering flat washer types, washer series selection, bearing area mechanics, dimensional standards per ISO 7089–7093 and ASME B18.22, material grades, surface treatments and full project documentation requirements.
Flat Washer Types, Bearing Area Mechanics
& Engineering Functions
Normal · Large · Fender · Structural · Sealing · Hardened
Definition and Engineering Functions
A flat washer (also called a plain washer) is a thin, flat disc with a central hole sized to pass the shank of a bolt or screw. Despite its apparent simplicity, the flat washer performs four distinct and critical engineering functions in a bolted joint that directly affect the structural integrity, load distribution and longevity of the connection.
The fundamental purpose of a washer in a structural bolted joint is to keep the bearing stress under the nut below the compressive yield strength of the clamped material. Bearing stress = clamping force F / bearing area A, where bearing area A = π/4 × (OD² − ID²). A standard normal-series washer for an M12 bolt has a bearing area of approximately 530 mm²; a large-series washer for the same bolt has a bearing area of approximately 1,256 mm² — 2.4× larger. For aluminium structural connections, the lower yield strength of aluminium (typically 160–270 MPa vs 250+ MPa for steel) frequently requires large-series or extra-large washers to prevent bearing yield at design bolt preload levels. Specifying the wrong washer series for the substrate material is a common structural error.
Flat Washer Types — Engineering Descriptions
Dimensional Data & Standards
ISO 7089 · ISO 7093 · ASME B18.22 · ASTM F436
ID · OD · Thickness · Bearing Area · M2–M64
ISO 7089 Normal Series — Metric Flat Washer Dimensions
| Bolt Size | ID d1 (mm) | OD d2 (mm) | Thickness h (mm) | Bearing Area (mm²) | ISO 7090 Chamfered? |
|---|---|---|---|---|---|
| M3 | 3.2 | 7.0 | 0.5 | 35 | — |
| M4 | 4.3 | 9.0 | 0.8 | 50 | — |
| M5 | 5.3 | 10.0 | 1.0 | 57 | — |
| M6 | 6.4 | 12.0 | 1.6 | 81 | Yes |
| M8 | 8.4 | 16.0 | 1.6 | 145 | Yes |
| M10 | 10.5 | 20.0 | 2.0 | 227 | Yes |
| M12 | 13.0 | 24.0 | 2.5 | 318 | Yes |
| M16 | 17.0 | 30.0 | 3.0 | 479 | Yes |
| M20 | 21.0 | 37.0 | 3.0 | 729 | Yes |
| M24 | 25.0 | 44.0 | 4.0 | 1028 | Yes |
| M30 | 31.0 | 56.0 | 4.0 | 1705 | Yes |
| M36 | 37.0 | 66.0 | 5.0 | 2365 | Yes |
| M42 | 43.0 | 78.0 | 7.0 | 3324 | — |
| M48 | 50.0 | 92.0 | 8.0 | 4691 | — |
| M64 | 66.0 | 115.0 | 9.0 | 6994 | — |
ISO 7089:2000 / DIN 125A. Bearing area = π/4 × (OD² − ID²). ISO 7090 chamfered washers are dimensionally identical to ISO 7089 but have an inner chamfer on one face and are hardened to 200 HV minimum for use with Grade 8.8, 10.9 and 12.9 high-strength bolts. All dimensions in mm. ID tolerance: H12 (clearance for bolt shank). OD tolerance: h14.
| Bolt Size | ID d1 (mm) | OD d2 (mm) | Thickness h (mm) | Bearing Area (mm²) | vs Normal Series Area |
|---|---|---|---|---|---|
| M5 | 5.3 | 15.0 | 1.2 | 154 | 2.7× |
| M6 | 6.4 | 18.0 | 1.6 | 221 | 2.7× |
| M8 | 8.4 | 24.0 | 2.0 | 396 | 2.7× |
| M10 | 10.5 | 30.0 | 2.5 | 619 | 2.7× |
| M12 | 13.0 | 37.0 | 3.0 | 942 | 3.0× |
| M16 | 17.0 | 50.0 | 4.0 | 1737 | 3.6× |
| M20 | 21.0 | 60.0 | 4.0 | 2483 | 3.4× |
| M24 | 25.0 | 72.0 | 5.0 | 3586 | 3.5× |
| M30 | 31.0 | 90.0 | 6.0 | 5617 | 3.3× |
| M36 | 37.0 | 105.0 | 8.0 | 7289 | 3.1× |
ISO 7093-1:2000 / DIN 9021. Large series washers provide 2.7–3.6× the bearing area of normal series for the same bolt size. Use large series when the substrate material compressive yield strength is less than approximately 60% of standard carbon steel, or when the hole diameter exceeds the standard clearance — e.g. slotted holes, oversize holes for adjustment, or bolt holes in soft materials. All dimensions in mm.
A_washer = π/4 × (OD² − ID²) // Net bearing area [mm²] of washer
// WORKED EXAMPLE: M12 bolt, Grade 8.8, 70% preload = 42.6 kN, aluminium substrate (σy = 200 MPa)
// Normal series (ISO 7089): A = 318 mm² → σ = 42,600/318 = 134 MPa ≤ 200/1.5 = 133 MPa — MARGINAL
// Large series (ISO 7093): A = 942 mm² → σ = 42,600/942 = 45 MPa ≤ 133 MPa — OK (3.0× margin)
// Conclusion: use ISO 7093 large series for M12 Grade 8.8 bolt in aluminium substrate
Material Grades, Mechanical Properties
& Surface Treatments
Zinc · HDG · Passivation · EPDM Bonded · Nylon
| Material | Standard | Hardness | Corrosion | Key Application | Notes |
|---|---|---|---|---|---|
| CS Grade 140HV | ISO 7089 / DIN 125 | 140 HV max | Low | General bolting; Grade 4.6–8.8 bolts | Standard grade for normal series flat washers |
| CS Grade 200HV | ISO 7090 / ASTM F436 | 200–300 HV | Low | High-strength bolt joints; Grade 8.8–12.9 | Chamfered inner bore; prevents embedding |
| CS Grade 300HV | ASTM F436 Type 1 | 26–45 HRC | Low | Structural; ASTM A325/A490 bolt connections | Heavy-section structural washer |
| SS 304 (A2) | ISO 7089 / ISO 3506 | ≤220 HV | High | Food, outdoor, marine-adjacent, architectural | Non-magnetic; standard corrosion resistant |
| SS 316 (A4) | ISO 7089 / ISO 3506 | ≤220 HV | Very High | Coastal, offshore, chemical, marine, pool | Mo-bearing; superior chloride resistance |
| Duplex 2205 | ASTM A182 F51 | ≤310 HB | Very High | Offshore, sour service, high-chloride | Higher strength than SS 316; NACE compliant |
| Brass (CuZn37) | BS 2872 | ~120 HV | Good | Electrical, plumbing, marine decorative | Non-magnetic; non-sparking; soft |
| Copper (Cu-ETP) | BS EN 13601 | ~80 HV | Good | Sealing washers; electrical conductivity | Annealed; deforms to seal threads and faces |
| Aluminium (Al 1050) | EN 573-3 | ~35 HV | Good | Lightweight; non-magnetic; electrical | Very soft — not for high-preload bolt joints |
| Nylon PA66 | ISO 527 | ~80 HRR | Excellent | Electrical isolation; chemical resistance | Low load; compressible; non-conductive |
| PTFE | ASTM D1710 | ~50 Shore D | Extreme | Chemical, pharmaceutical, cryogenic | Very low friction; chemically inert |
| Fibre / Asbestos-free | BS 7531 | — | Good | Gasket-washer hybrid; flange sealing | Compressible; sealing; heat-resistant |
Hardness Matching to Bolt Grade — Critical Rule
The hardness of the washer must be matched to the bolt property class. Using a soft washer under a high-strength bolt (Grade 10.9 or 12.9) will result in washer embedding — the washer indents into the clamped surface under the bolt preload, causing the joint to lose clamp force over time as the washer material flows plastically. The embedded washer also permanently marks the clamped surface. The rule is simple: use ISO 7090 hardened washers (200 HV minimum) for all bolted joints using Grade 8.8, 10.9 or 12.9 bolts; use standard ISO 7089 (140 HV maximum) for Grade 4.6 and 5.8 bolts.
| Finish | Standard | Thickness (µm) | Salt Spray (hrs) | Application |
|---|---|---|---|---|
| Plain / Bare (self-coloured) | — | — | <24 | Indoor dry; short-term storage; VCI for transport |
| Phosphate + oil | MIL-DTL-16232 | 5–15 | 48–96 | Consistent K-factor; indoor machinery; reduces galling |
| Zinc electroplate | ASTM B633 SC2 | 8–13 | 96–200 | General outdoor sheltered; standard commercial |
| Mechanically galvanised | ASTM B695 Cl.12 | 12 min | 200+ | Outdoor; no hydrogen embrittlement risk |
| Hot-dip galvanised | ASTM A153 / ISO 1461 | 45–86 | 500+ | Structural outdoor; timber connections; ACQ timber |
| Dacromet / Geomet | ISO 10683 | 8–12 | 720+ | High-strength washers (200HV+); no H₂; outdoor |
| SS 304 passivation | ASTM A380 | N/A | 1000+ | All SS 304 washers; outdoor / marine-adjacent |
| SS 316 passivation | ASTM A380 | N/A | 1000+ | Coastal / marine / chemical / pool environments |
| EPDM bonded (rubber face) | Manufacturer spec. | 1–3 mm rubber | N/A | Roofing, cladding, enclosure covers; watertight sealing |
| Neoprene bonded | Manufacturer spec. | 1–3 mm rubber | N/A | General sealing; less UV-stable than EPDM |
| Silicone bonded | Manufacturer spec. | 1–3 mm rubber | N/A | High-temperature sealing (-60°C to +230°C) |
For hardened ISO 7090 washers (200 HV+), avoid standard zinc electroplate without bake-out treatment — the pickling pre-treatment produces hydrogen that can embrittlement hard washers. Specify mechanical zinc (ASTM B695), Dacromet/Geomet or hot-dip galvanise for hardened washers requiring corrosion protection. Plain phosphate + oil is the safest finish for hardened washers where K-factor consistency is required for bolt torque calculation.
Washer Selection Guide, Applications
& Quality Control and Documentation
EPC · Offshore · Pressure Vessel · Food Grade · Marine
Washer Selection Decision Guide
| Bolt Grade | Substrate | Hole Type | Recommended Washer Series | Standard | Reason |
|---|---|---|---|---|---|
| 4.6 / 5.8 | Steel | Standard clearance | Normal series | ISO 7089 | Standard joint; normal bearing area sufficient |
| 8.8 / 10.9 / 12.9 | Steel | Standard clearance | Hardened chamfered | ISO 7090 | Prevents washer embedding under high preload |
| A325 / A490 (AISC) | Structural steel | Standard / oversized | Structural hardened | ASTM F436 | Mandatory per AISC 360 for high-strength structural joints |
| Any grade | Timber / wood | Standard clearance | Large series | ISO 7093 | Low timber bearing strength requires enlarged area |
| Any grade | Aluminium | Standard clearance | Large series | ISO 7093 | Lower compressive yield — verify bearing stress with formula |
| Any grade | FRP / composite | Standard / oversize | Large or extra-large | ISO 7093 / custom | Low bearing strength; stress spreading critical |
| Any grade | Any | Slotted / oversize | Large or fender | ISO 7093 / SAE | Must bridge hole; standard OD may fall through slot |
| Any grade | Roofing / cladding | Through-sheet | EPDM bonded washer | Manufacturer spec. | Weathertight sealing at fastener penetration required |
| Any grade | Electrical / isolate | Any | Nylon or PTFE | ISO 527 / ASTM D1710 | Electrical isolation between bolt and substrate required |
Applications by Industry
Hardened ASTM F436 structural washers are mandatory under both the bolt head and nut in all high-strength structural bolted connections using ASTM A325 or A490 bolts per AISC 360 Chapter J. These heavy-section washers (typically 5–8 mm thick) prevent bolt head and nut embedding into the steel flange face, which would cause progressive loss of preload in slip-critical connections. For connections at sloped beam flanges, bevelled F436 Type 2 washers correct the angular misalignment between the nut bearing face and the sloped steel flange, ensuring uniform clamping force across the full nut face.
Large-series flat washers (ISO 7093 / DIN 9021) in hot-dip galvanised carbon steel are the standard for bolted timber connections in heavy timber frames, glulam (GLT) structures, cross-laminated timber (CLT) buildings and prefabricated wood construction. The low perpendicular-to-grain compressive strength of timber (typically 2–6 MPa for softwood species) means that a normal-series washer would crush into the timber face under even moderate bolt preload, causing joint slackening and structural movement. Large washers distribute the load over enough timber area to stay below the bearing strength. For treated (ACQ/CCA) timber, specify HDG washers per ASTM A153 Class D — standard zinc plate corrodes rapidly in contact with copper-based preservatives.
Flat washers under flange stud bolt nuts on ASME B16.5 and B16.47 flanges distribute the bolt preload over the flange face and protect the soft flange facing material (PTFE gasket seat, raised face serrations) from damage by the rotating nut. SS 316 (A4) hardened washers are standard for SS and duplex stainless flanges; carbon steel hardened washers for A105 and alloy steel flanges. Hardness of the washer must not exceed the hardness of the flange body — a washer harder than the flange face will indent the flange face during bolt-up, causing permanent deformation of the seating surface.
SS 316 flat washers with passivation are standard for all bolted connections on offshore topsides, marine jetties, harbour structures and coastal architectural steelwork. Duplex 2205 washers are specified in sour service zones (H≶S zones) and where higher strength is required. For offshore projects, EN 10204 3.1 MTC, PMI and corrosion class certificates are standard documentation requirements for all washers supplied as part of a bolt-nut-washer package.
SS 316 flat washers with electropolish (ASTM A967, Ra ≤ 0.8 µm) are used in food processing equipment, pharmaceutical vessels, cleanroom structures and medical device assemblies. The electropolished surface eliminates crevices around the washer circumference that would harbour bacteria in food-contact and pharmaceutical environments. PTFE washers are specified where complete chemical resistance and non-metallic contact is required — e.g. in direct product contact zones in pharmaceutical dispensing and chemical dosing equipment.
EPDM-bonded metal washers on hex washer head self-drilling screws are the standard for watertight fastening of metal roofing panels (Zincalume, Colorbond, aluminium) to structural steel purlins. The EPDM rubber pad seals around the screw shank and under the washer face, preventing water ingress at the fastener penetration point. EPDM compound must be UV-stable and compatible with the panel coating — verify with the panel manufacturer. Over-driving the screw (causing the EPDM to extrude) or under-driving (insufficient compression to seal) are the two most common installation defects in metal roofing systems.
Quality Control and Documentation
Flat washer QC per ISO 7089–7090 covers: dimensional inspection (ID, OD and thickness per standard tolerances); hardness verification (100 HV–300 HV depending on grade — critical for ISO 7090 chamfered washers where 200 HV minimum ensures prevention of embedding); surface finish and coating inspection (zinc coating thickness by magnetic gauge per ASTM B499; HDG thickness per ASTM A90); and flatness verification (washers must be flat within 0.3–0.5 mm across the diameter to ensure uniform contact with the bolt head and clamped surface). For stainless and duplex washers, PMI (XRF) confirms material grade on every production lot.
Export Packaging and Preservation
- Flat washers packed in heat-sealed polypropylene bags by size, series and material/finish, labelled with standard reference ( ISO 7089/7090/7093), bolt size, OD/ID/thickness, material grade, finish and quantity
- Carbon steel washers (plain, zinc, phosphate) packed with VCI (Volatile Corrosion Inhibitor) desiccant sachet for sea freight; SS washers in clean sealed bags without VCI (SS does not require VCI)
- EPDM-bonded washers must not be stored above 40°C or in direct sunlight; UV and heat degrade the EPDM compound; store in cool, dark conditions
- Bags in double-wall corrugated master cartons; pallets on ISPM-15 heat-treated timber with stretch wrap
- MTC (EN 10204 3.1), dimensional inspection report, hardness test certificate (Grade 200HV+ washers), PMI report (SS/duplex), salt spray certificate (coated washers) and all project documents in waterproof sealed envelope with each pallet
| # | Document | Standard / Reference | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 | 3.1 for EPC structural; 3.2 for offshore / pressure vessel / NACE |
| 02 | Dimensional Inspection Report | ISO 7089–7093 / B18.22 | AQL 1.0 per ISO 2859; ID, OD, thickness and flatness as mandatory chars |
| 03 | Hardness Test Report (HV) | ISO 6507 / ASTM E92 | Mandatory for Grade 200HV (ISO 7090) and structural (ASTM F436); range confirmed |
| 04 | Surface Coating Certificate | ASTM A153 / B633 / A380 | Required for all coated washers; zinc thickness per point confirmed |
| 05 | Salt Spray / Corrosion Test Certificate | ISO 9227 / ASTM B117 | Required for zinc-plated and Dacromet washers; hours-to-rust confirmed |
| 06 | PMI Report (XRF / OES) | Project specification | 100% of SS 304/316, duplex, brass, copper and exotic grade washers |
| 07 | EPDM / Rubber Compound Certificate | Manufacturer / project spec. | Shore A hardness, adhesion, UV resistance — for bonded sealing washers |
| 08 | ISO 9001 Manufacturer Certificate | ISO 9001:2015 | Current; scope must include flat washer manufacture |
| 09 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics manufactures and exports flat washers (plain washers) in all series — normal (ISO 7089/DIN 125A), chamfered hardened (ISO 7090/DIN 125B), large (ISO 7093/DIN 9021), fender, structural (ASTM F436), EPDM-bonded and custom — in carbon steel grades 140HV and 200HV, hot-dip galvanised (ASTM A153), stainless steel SS 304 and SS 316, duplex 2205, brass, copper, aluminium, nylon PA66 and PTFE. M2–M64 metric and #2–4" inch. Finishes: zinc plate, mechanically galvanised, HDG, Dacromet, phosphate + oil, SS passivation, electropolish. EN 10204 3.1 MTC, hardness test certificates, salt spray certificates, PMI reports. 48-hour express dispatch on standard in-stock sizes.