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Finishing & Bonded Washers
A comprehensive engineering reference for structural engineers, process plant designers, roofing and cladding engineers and procurement teams — covering flat washers, hardened structural washers, bonded sealing washers (EPDM, Neoprene, Silicone, Nitrile), finishing washers, Nord-Lock wedge washers, Belleville disc springs and specialist washer types. Covering washer function in bolted joints, load distribution mechanics, sealing compound selection, material grades and documentation.
Washer Types, Engineering Functions
& Selection Principles
Belleville · Spring · Structural · Load Distribution
The Engineering Functions of a Washer
A washer is a disc-shaped element placed under the head of a bolt or under a nut to perform one or more of the following engineering functions in a bolted joint: (1) Load distribution — spreading the bolt clamping force over a larger bearing area on the joined material surface, reducing the compressive bearing stress below the nut or bolt head to a level the joined material can sustain without yielding or crushing; (2) Surface protection — providing a smooth, hardened bearing surface under the rotating nut or bolt head to prevent galling and scoring of the joined material during tightening; (3) Sealing — bonded sealing washers provide a weathertight or fluid-tight seal around the fastener penetration through a panel, sheet or roof cladding; (4) Locking — spring washers, Nord-Lock and Belleville washers provide resistance to fastener self-loosening under vibration; and (5) Appearance and finishing — finishing washers (countersunk or ogee) conceal the fastener head and provide a decorative, flush finish in architectural and cladding applications.
Without a washer, the entire bolt clamping load is transmitted through the small annular contact area between the nut (or bolt head) bearing face and the joined material surface. The resulting bearing stress σ_b = F / A_bearing. For soft materials (aluminium, fibre-reinforced composites, timber, plastic, soft gaskets) or oversized clearance holes (e.g. from punching tolerances), this bearing stress may exceed the material's compressive yield strength, causing local crushing that reduces bolt preload and joint stiffness. A washer with a larger OD increases the bearing area A and reduces σ_b proportionally. For structural connections per AISC or Eurocode 3, hardened washers (ASTM F436 for UNC bolts, ISO 7090 for metric) are specified under both bolt head and nut when the joint material is thinner than the bolt head height, or when oversized holes are used.
Washer Types — Engineering Descriptions
Bonded Sealing Washers — Sealing Compound
Selection & Engineering Requirements
Temperature · Chemical Compatibility · Compression Set · UV
Sealing Compound Selection
The elastomeric sealing compound in a bonded washer must be selected based on four criteria: (1) Temperature range — the compound must retain its elastic sealing properties (resilience and compression set resistance) throughout the operating temperature range; (2) UV and weather resistance — for outdoor/roofing/cladding applications, the compound must resist degradation from UV radiation, ozone and weathering over the service life (typically 20–30 years for roofing); (3) Chemical compatibility — the compound must resist the chemicals it will contact (industrial atmosphere acids, oil, hydraulic fluid, potable water, process fluid); and (4) Compression set — the compound must maintain its compressed thickness (and therefore the sealing force) without permanent set over the service life; EPDM and Silicone have the best long-term compression set resistance.
| Compound | Temp Range | UV / Weather | Oil / Fuel | Water | Best Application |
|---|---|---|---|---|---|
| EPDM (Ethylene Propylene) | −40 to +120°C | Excellent | Poor — avoid | Excellent | Roofing, cladding, solar panels, rainwater, general outdoor |
| Neoprene (Polychloroprene) | −30 to +100°C | Very Good | Moderate | Very Good | Marine, offshore, general industrial, mild oil mist |
| Silicone (VMQ) | −60 to +200°C | Excellent | Moderate | Excellent | High-temp, food-grade, pharmaceutical, clean room |
| Nitrile (NBR) | −30 to +100°C | Moderate | Excellent | Good | Hydraulic, fuel, oil, petroleum service; not outdoor UV |
| Viton® (FKM) | −15 to +200°C | Very Good | Excellent | Very Good | High-temp chemical, fuel, aggressive solvents |
| PTFE / Teflon (washer insert) | −200 to +260°C | Excellent | Excellent | Excellent | Chemical plant, pharmaceutical, aggressive media; rigid not elastic |
EPDM is the default choice for building envelope, roofing and cladding applications due to its outstanding UV, ozone and weathering resistance and excellent long-term compression set performance in outdoor service. Neoprene is preferred for offshore and marine applications where some oil mist exposure is possible alongside salt-laden atmosphere. Silicone is mandatory for food-grade, pharmaceutical and high-temperature applications above +120°C. Nitrile (NBR) is the correct choice for petroleum, hydraulic fluid and fuel service but should not be used outdoors due to poor UV resistance. Viton (FKM) for the most demanding high-temperature and chemical environments.
Bonded Washer Compression and Sealing Performance
The sealing performance of a bonded washer depends on achieving and maintaining adequate compression of the elastomeric pad. The pad must be compressed sufficiently to: (a) conform to the surface profile of the cladding sheet or panel (which may have minor surface irregularities, profiled troughs or paint coating steps); and (b) generate sufficient contact pressure to prevent water or fluid ingress under the design hydraulic head or wind-driven rain pressure. Key design requirements:
- Minimum compression 20% of the uncompressed pad thickness — below 20% compression, the sealing contact pressure is insufficient for reliable weather exclusion in moderate wind-driven rain conditions. Standard bonded washer pad thickness is 2.0–3.0 mm; minimum compressed thickness is 1.5–2.0 mm.
- Maximum compression 50% — above 50% compression, the rubber pad is over-compressed; the metal washer contacts the sheet surface and carries the bolt load directly (bypassing the rubber), the rubber can extrude from under the washer edge, and the pad loses its ability to reseal if the fastener loosens slightly.
- Compression set limit — the elastomeric pad will permanently set (lose some of its compressed thickness) over time due to creep and thermal cycling. EPDM has excellent compression set resistance — typically 15–25% set at 70 hr/100°C per ASTM D395 Method B. Silicone has the best compression set resistance (5–15% set). Select a compound with ≤25% compression set for long-term sealing performance.
Material Grades, Coating Options
& Applicable Standards
DIN 125 · ISO 7089 · ASTM F436 · F844 · F959
| Material | Grade / Spec | Coating Options | Hardness | Typical Application |
|---|---|---|---|---|
| Carbon Steel | ASTM F436 / DIN 125 | Zinc electroplate, HDG, Dacromet, Xylan | 38–45 HRC (F436 hardened) | Structural bolted connections, general fastening |
| CS Low Carbon | ASTM F844 / DIN 125A | Zinc plate, HDG | Soft — not hardened | General purpose, load distribution, non-structural |
| SS 304 / A2 | ISO 7089/7090 A2 | Passivated (standard) | ≤30 HRC | General SS, indoor/covered, mild corrosion environments |
| SS 316 / A4 | ISO 7089/7090 A4 | Passivated, Xylan coating | ≤30 HRC | Marine, offshore, chemical, food-grade, outdoor |
| SS 316L | ASTM F593 (low carbon) | Passivated | ≤30 HRC | Welding adjacent service, sensitisation resistance |
| Duplex 2205 | A182 F51 / EN 1.4462 | Passivated, Xylan | ≤28 HRC (NACE) | Offshore sour, seawater, high-chloride environments |
| Inconel 718 | AMS 5596 / B670 | Passivated | ≤44 HRC | High-temp Belleville springs, aerospace, cryogenic |
| Titanium Gr.2 | ASTM B265 Gr.2 | Anodised | ≤22 HRC | Seawater, chlorine, desalination bonded washers |
| Brass / Bronze | CuZn37 / CuSn4 | Bare or lacquered | Soft | Electrical/electronic, marine, decorative, non-magnetic |
| Standard | Scope | Key Dimensions / Requirements |
|---|---|---|
| DIN 125 Part A | Plain flat washers — standard series, metric | ID = bolt dia + 0.3–1.0 mm; OD ≈ 2×bolt dia; t = 1.5–4.0 mm |
| DIN 125 Part B | Plain flat washers — large series, metric | Wider OD than Part A for softer materials; otherwise same construction |
| ISO 7089 | Plain washers (normal series), 140 HV, metric | Chamfered outer edge; product grade A; surface finish 140 HV minimum |
| ISO 7090 | Hardened flat washers, 300 HV, metric | Hardness 300–370 HV (approx. 30–38 HRC); for structural high-strength bolts |
| ISO 7092 | Plain washers (small series), metric | Smaller OD than 7089 for confined spaces; same thickness |
| ASTM F436 | Hardened steel washers (inch series) | HRC 38–45; Type 1 (flat), Type 3 (weathering steel); for A325/A490 structural bolts |
| ASTM F844 | Unhardened flat washers (inch series) | Low carbon, not heat-treated; general purpose, non-structural applications |
| ASTM F959 | Direct tension indicator washers (DTI) | Protrusions flatten at specified clamping load — visual preload indicator |
| DIN 2093 | Belleville / disc spring washers | Series A (t/h ≤0.4), B (t/h = 0.4–1.0), C (t/h ≥1.0); load-deflection curves |
| DIN 127 / ISO 7089 | Spring lock washers | Split (127A) or double-coil (127B); note — limited effectiveness vs other locking methods |
Applications, Installation Notes
& Quality Control and Documentation
Piping Flanges · Belleville Stacks · EN 10204 MTC
Applications by Industry and Use Case
EPDM bonded sealing washers are the standard fastener component for profiled metal roof sheeting and wall cladding systems. Every screw or self-drilling fastener penetrating the roof membrane or cladding sheet is fitted with an EPDM bonded washer that seals the penetration against wind-driven rain, snow melt and condensate. The sealing washer must be oversized relative to the screw head to provide adequate EPDM coverage around the fastener hole — typically 16 mm OD for 5.5 mm screws, 19 mm OD for 6.3 mm screws, and 25–29 mm OD for M8–M10 through-bolts. The EPDM compound must have a minimum 20-year UV resistance rating for roofing applications — confirmed by accelerated UV weathering test per ASTM G154 or ISO 4892.
Photovoltaic (PV) solar panel mounting frames are assembled with M8 or M10 fasteners through extruded aluminium rail profiles. EPDM bonded sealing washers are specified wherever the fastener passes through the aluminium rail into the roof structure — the EPDM seal prevents water ingress at each fastener penetration. SS 316 bonded washers with EPDM compound are used on coastal or marine sites where the salt-laden atmosphere would corrode a zinc-plated steel washer within the 25–30 year panel design life. The washer must be compatible with aluminium (no galvanic corrosion between SS 316 and aluminium in outdoor service is the key design constraint).
ASTM F436 hardened washers (HRC 38–45) are mandatory under both bolt head and nut in high-strength structural bolted connections using A325 or A490 bolts per AISC/RCSC Specification. The hardened washer provides a consistent, non-deforming bearing surface for the rotating nut, reducing torque scatter and improving the reliability of the torque-preload relationship. Direct tension indicator (DTI) washers per ASTM F959 are used where visual confirmation of bolt preload is required — the protrusions on the DTI washer face crush under the specified clamp force, allowing a feeler gauge to confirm that the gap between protrusions has closed to the required limit.
Hardened flat washers (ISO 7090) are specified in ASME B16.5 flange bolt kits where the flange material is soft (e.g. cast iron, FRP, or plastic-lined) or where the nut bearing face OD must be increased to avoid embedding into the flange face outside the raised face gasket area. Belleville spring washers (DIN 2093) are used in high-temperature process piping flange joints where differential thermal expansion between the stud bolt and the flange material (e.g. alloy steel F91 studs in a carbon steel flange) would cause significant preload relaxation during heat-up and cool-down cycles. The Belleville stack maintains a minimum clamping load even as the joint relaxes during thermal transients.
On offshore platforms, all bonded sealing washers in the topsides structure and equipment enclosures are specified in SS 316 body with EPDM or Neoprene compound. The Neoprene compound provides resistance to the oil mist and hydrocarbon vapours present in the offshore topsides atmosphere alongside the seawater spray and UV exposure. For subsea or splash-zone service, duplex 2205 body washers with a EPDM or Neoprene compound resistant to seawater and CP system compatibility requirements are specified.
Installation Requirements
- Bonded sealing washers — correct orientation: always installed with the rubber pad facing the material being sealed (sheet/panel), and the metal washer face against the nut or bolt head. Reversed installation puts the rubber against the nut bearing face — the rubber will tear under the rotating nut and provide no seal.
- Bonded sealing washers — tightening: tighten until the rubber pad is visibly compressed to approximately 2/3 of its original thickness (approximately 1.5 mm from an original 2.0 mm pad). Do not over-tighten — this extrudes the rubber from under the washer edge and breaks the seal. Use a calibrated torque wrench or power screwdriver with torque-limiting clutch to avoid over-compression.
- Hardened washers — orientation: ISO 7090 and ASTM F436 hardened washers have a chamfer on one face (the outer edge). The chamfer face should be placed towards the nut or bolt head (not towards the joint material) — the chamfer accommodates the fillet radius under the nut or bolt head. Reversed installation causes the fillet to bear on a sharp edge, producing stress concentration and potential cracking.
- Belleville washers — stacking orientation: for series stacking (multiple Bellevilles in the same orientation), the spring rate decreases and the total deflection increases. For parallel stacking (Bellevilles alternating orientation, nested), the spring rate increases and the deflection stays the same. Always verify the Belleville load-deflection characteristics against the DIN 2093 design tables for the specific stacking configuration before installation.
- Nord-Lock washers — installation: always installed in pairs, cam faces together. The serrated outer faces must grip both the bolt head/nut and the joined material. Nord-Lock washers are not re-usable — once removed they must be discarded and replaced with new pairs.
Quality Control
QC for washers covers: (1) Dimensional inspection — ID, OD and thickness per the specified standard (DIN 125, ISO 7089, ASTM F436); measured per AQL sampling plan; (2) Hardness verification — Rockwell C hardness per ASTM F436 (HRC 38–45) for hardened structural washers; (3) Material verification / PMI — 100% PMI by XRF on SS, duplex and exotic alloy washers; (4) Coating adhesion — for HDG, zinc plate or Dacromet/Geomet coated washers, adhesion and coating thickness per ISO 2808; (5) Bonded sealing washer — rubber adhesion test — 90° peel test on the rubber-to-metal bond; minimum peel strength per manufacturer's specification; rubber must tear cohesively (not delaminate from the metal face); (6) Compression set test — per ASTM D395 for the elastomeric compound; confirms long-term sealing performance.
| # | Document | Standard | Minimum Requirement |
|---|---|---|---|
| 01 | Material Test Certificate (MTC) | EN 10204 3.1 / 3.2 | 3.1 standard; 3.2 (TPI) for offshore, NACE, duplex washers |
| 02 | Dimensional Inspection Report | DIN 125 / ISO 7089 / ASTM F436 | ID, OD, thickness per standard; per AQL; hardness for F436 |
| 03 | Hardness Test Report | ASTM E18 / ISO 6508 | Mandatory for ASTM F436 hardened washers; HRC 38–45 confirmed |
| 04 | PMI Report (XRF) | Project specification | 100% SS, duplex, exotic alloy washers; alloy grade confirmed |
| 05 | Rubber Bond / Peel Strength Certificate | ASTM D429 / DIN 53531 | Mandatory for bonded sealing washers; cohesive rubber tear confirmed; peel strength ≥ spec. |
| 06 | Compression Set Report | ASTM D395 Method B | For bonded washers where long-term sealing life is specified; ≤25% compression set at 70 hr/100°C |
| 07 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packing for international export |
RR Hydraulics supplies the complete range of industrial washers — flat washers (DIN 125A/B, ISO 7089/7090, ASME B18.22.1), hardened structural washers (ASTM F436, ISO 7090), bonded sealing washers (EPDM, Neoprene, Silicone, Nitrile and Viton compounds), finishing washers (countersunk and ogee), Nord-Lock wedge locking washers, Belleville disc spring washers (DIN 2093) and DTI washers (ASTM F959). Available in carbon steel (zinc-plated, HDG, Dacromet), SS 304/316/316L, Duplex 2205, Super Duplex 2507, Inconel 718 and Titanium Grade 2. Sizes M4–M72. EN 10204 3.1/3.2 MTC, dimensional inspection, hardness, PMI, rubber bond/peel strength and compression set test reports. 48-hour express dispatch on standard in-stock sizes.