Sealing Washer

What is a Sealing Washer

A sealing washer is a thin, ring-shaped component placed between a fastener and a mating surface whose job is to create a leak-tight static seal at the fastener interface. It differs fundamentally from a plain flat washer: a flat washer only spreads the clamp load to protect the surface and resist loosening, whereas a sealing washer is designed to deform itself, or to compress a bonded elastomer, so that it fills the microscopic gaps between two metal faces and blocks the path that a pressurized fluid or gas would otherwise take along the threads or under the bolt head.

The need arises wherever a threaded joint must both carry mechanical load and hold pressure. A hydraulic port plug, an engine oil drain plug, a brake-line banjo bolt, a fuel-rail fitting, a gearbox sight plug, and a roofing screw through a steel sheet all face the same problem: the bolt clamps, but the imperfect contact between two machined or pressed faces leaks. A sealing washer turns that imperfect interface into a controlled, repeatable seal without requiring an O-ring groove to be machined into either part.

There are two broad sealing strategies. The first is plastic deformation: a soft, ductile washer of copper, aluminum, or fiber is crushed between the bolt face and the seat, flowing into the surface roughness and staying there. These are single-use crush washers, replaced at every service. The second is elastomer compression: a metal washer carries a rubber ring vulcanized to it, and tightening squeezes the rubber lip while the metal acts as a hard stop. This is the bonded seal, also called a Dowty seal or USIT ring, and it is reusable because the elastomer recovers elastically.

The historical pivot was the bonded seal. Plain copper washers have sealed threaded joints for well over a century, but they cannot be reused and need careful torque control. In the mid-20th century the Dowty Group in the UK developed the bonded seal, a steel washer with a captive elastomer, to give aerospace and hydraulic systems a fast-fitting, reusable, self-controlling seal. That product line later passed to TI, then to Polymer Sealing Solutions, and is today produced by Trelleborg in the UK to the original Dowty specification, while Hutchinson, Freudenberg, Techne, and Parker make equivalent ranges.

In application scale the sealing washer is everywhere and easy to underrate. A single passenger car carries dozens of them on oil plugs, brake banjos, fuel fittings, and air-conditioning ports; a hydraulic excavator carries hundreds on its port connections; a metal roof is held down by thousands of EPDM bonded washers. Because each is cheap and small, the cost of getting one wrong is rarely the part itself but the warranty leak, the lost oil, or the water ingress it allows. Specifying the correct type, material, and size is therefore a disproportionately high-leverage engineering decision.

Sealing Washer Types

Sealing washers split into five practical families by construction and sealing mechanism: metal crush washers, bonded (Dowty) seals, elastomer profile rings, fiber and polymer washers, and EPDM bonded roofing washers. Choosing the wrong family is the most common error, because a part that is correct for an oil drain plug can be entirely wrong for a roofing screw or a high-pressure hydraulic port. The table below summarizes the five families.

Metal crush washers are flat or lipped rings of soft copper or aluminum, dimensioned to standards such as DIN 7603 Form A (flat) and Form B (with a raised inner sealing lip). They seal when the bolt crushes them into the seat, work-hardening as they conform. They are the classic choice for brake banjo bolts, engine oil drain plugs, and instrument fittings. Copper tolerates higher temperature and crushes at lower torque; aluminum is the cheaper substitute. Both are single-use, because once deformed they cannot reflow to reseal a new surface.

Bonded seals, the Dowty or USIT type, are a metal retaining washer with an elastomer ring vulcanized to the inner bore. They are the workhorse of hydraulic and pneumatic threaded joints: the rubber lip does the sealing, the metal ring sets the compression stop and carries the clamp load, and the seal is reusable. They are offered with self-centering variants whose elastomer grips the bolt thread so the seal cannot fall off during assembly, a real advantage on a fast production line.

Elastomer profile rings to DIN 3869, often called ED-rings, are an all-elastomer ring with a trapezoidal cross-section rather than rubber bonded to a flat washer. Seated in the machined port groove, the trapezoidal profile resists twisting and extrusion, so these rings hold higher pressure, up to about 600 bar, and are the standard choice for hydraulic port connections cut to ISO 6149, ISO 9974, or ISO 1179 threads. Fiber and polymer washers, made of vulcanized fiber, nylon, PTFE, or pressed paper, are low-cost conformable gaskets for low-pressure plumbing and fuel fittings; they seal well at modest pressure but are less heat-resistant and not reusable.

EPDM bonded roofing washers are a specialized branch: a steel or stainless washer with an EPDM rubber gasket vulcanized to its underside, fitted to self-drilling and self-tapping roofing and cladding screws. The EPDM is chosen for its outstanding resistance to water, ozone, and UV, and its tendency not to flow under clamp load so the seal stays put through years of thermal cycling. These dominate metal-roof and solar-mount fastening because they keep the penetration watertight for the life of the roof.

Sealing Mechanisms and Construction

Two physical mechanisms underlie every sealing washer, and understanding which one a part uses explains how to install it, whether it can be reused, and how it fails. The first mechanism is plastic deformation of a soft metal or fiber; the second is elastic compression of a captive elastomer against a controlled mechanical stop. The table below contrasts the engineering behavior of the two mechanisms.

Plastic-deformation sealing relies on the washer being softer than both clamped faces. As the bolt is torqued, the soft metal yields and flows into the valleys of the surface roughness, achieving intimate metal-to-metal contact that blocks the leak path. The trade-off is that all the conforming happens once: the metal work-hardens as it deforms, so a crushed washer that is removed and refitted no longer matches the new surface and will leak. This is why a fresh copper or aluminum washer is mandatory at each reassembly. Copper can be annealed back to a soft state by heating it to dull red and quenching, restoring ductility, but in production a new washer is usually cheaper than the labor.

Elastomer-compression sealing separates the two functions. The elastomer ring, bonded to the metal washer, is the seal; the metal ring is a structural element that both carries the bolt clamp load and acts as a hard stop. When the fastener is tightened, the trapezoidal rubber lip is squeezed and flows against the seat, while the metal ring bottoms out and prevents further compression. This design tolerance to over-torque is the key advantage: a worker cannot easily crush the rubber to destruction because the metal protects it, which makes the seal forgiving and repeatable on a fast assembly line.

The construction detail that distinguishes a quality bonded seal is the bond between rubber and metal. The elastomer is chemically vulcanized to the metal ring, not glued, so it cannot peel, fall out, or be displaced during handling and assembly. Self-centering variants extend the elastomer slightly inward so it grips the male thread of the bolt or fitting, holding the seal in place while the fastener is started, a feature that eliminates dropped seals on overhead or robotic assembly.

For DIN 3869 profile rings the construction is an all-elastomer ring with a trapezoidal cross-section seated in the machined port groove. Because that profile is confined by the groove walls it has little room to extrude, even at very high pressure, which is why these rings reach about 600 bar against roughly 400 bar for an open-faced bonded seal. The penalty is a more complex, more expensive part and a tighter dimensional fit with the port. In every case the one constant requirement is a flat, clean, undamaged seating face: no sealing washer of any type can seal across a scratch, a burr, or a pit in the seat.

Materials and Governing Standards

Sealing washer selection is really two material choices: the metal of the washer or ring, and, for bonded types, the elastomer of the sealing lip. The metal governs strength, temperature ceiling, and corrosion compatibility; the elastomer governs media compatibility and the operating temperature window. Getting either wrong, EPDM on an oil line or NBR on a hot-water line, causes the seal to swell, harden, or crack within months.

Metal materials. Crush washers are copper (high ductility, reusable after annealing, tolerant to roughly +300 degrees Celsius) or aluminum (cheaper, seals at lower torque, limited to roughly +200 degrees Celsius). Bonded-seal retaining rings are most often zinc-plated mild steel for general hydraulic use, with stainless steel 304 or 316 specified for corrosive or washdown environments, and occasionally brass or aluminum. The metal ring of a bonded seal must be hard enough to act as a compression stop without yielding under bolt load.

Elastomer materials. The rubber lip is chosen for the media. NBR (nitrile) is the default for mineral hydraulic oils, fuels, and greases. FKM (fluorocarbon, Viton) handles aggressive chemicals, fuels, and high temperature. EPDM is for water, steam, glycol coolants, and brake fluids, but is destroyed by mineral oil. HNBR (hydrogenated nitrile) extends oil resistance to higher temperature for engine and compressor duty. VMQ (silicone) covers extreme temperature and food contact. The table below lists verified working temperature ranges and the media each elastomer suits.

Governing standards. Plain copper and aluminum sealing washers follow DIN 7603, which fixes inside diameter, outside diameter, and thickness per thread size in Form A (flat) and Form B (lipped). For example, the DIN 7603 copper washer for an M8 thread is commonly 8 mm bore by 11.5 to 12 mm outside by 1 to 1.5 mm thick, scaling up through M10 (about 13.5 mm OD) and M12 (about 15.5 mm OD). Elastomer profile sealing rings follow DIN 3869, the ED-ring trapezoidal profile, paired with port threads to ISO 6149, ISO 9974, or ISO 1179. Bonded (Dowty) seals do not carry a single universal ISO number; manufacturers publish metric and BSP size tables traceable to the original Dowty range, so the drawing must cite the maker and size code.

The practical discipline is to call out the full triple on the drawing: standard and form, metal grade, and elastomer grade with hardness, for example "DIN 3869 ED-ring, steel, NBR 70 IRHD" or "bonded seal M14, stainless 316 ring, FKM." A washer specified only as "sealing washer M14" leaves the elastomer and metal undefined and invites a substitution that fails in service.

Key Specification Parameters

A sealing washer datasheet looks trivial but hides several parameters that decide whether the seal holds. Six matter most: thread or nominal size, inside and outside diameter, thickness, elastomer grade and hardness, metal ring grade, and pressure rating. Each is explained below, with the values that drive selection.

Thread or nominal size is the anchor. For metric ports it follows the bolt thread (M8, M10, M12, M14, M16, up to M60); for BSP and hydraulic fittings it follows the port thread (G1/8, G1/4, G1/2 and so on). The bore of the washer must clear the bolt shank without slop, and the outside diameter must sit fully on the seat without overhanging the bolt-head spotface. Specifying a washer by bolt size alone is not enough, because seat diameters vary between fitting standards.

Inside diameter, outside diameter, and thickness are the seal geometry. The bore controls fit on the shank; an oversize bore lets a bonded elastomer extrude inward, a leading cause of leaks. The outside diameter must match the available seat; too large and it fouls adjacent features, too small and it has insufficient sealing area. Thickness sets the available compression: a DIN 7603 crush washer is typically 1 to 2 mm, a bonded seal 1 to 3 mm, and the metal ring thickness in a bonded seal is what limits how far the elastomer can be squeezed.

Elastomer grade and hardness define media compatibility and sealing behavior. Bonded-seal elastomers are typically supplied at 70 to 80 IRHD / Shore A. Softer rubber conforms to a rougher seat and seals at lower clamp load; harder rubber resists extrusion at higher pressure. The grade (NBR, FKM, EPDM, HNBR, VMQ) must match the media per Chapter 4. A hardness and grade callout, for example "NBR 70 IRHD," should appear on the drawing.

Pressure rating is where families diverge sharply. The list below gives verified static-pressure orders of magnitude; always confirm against the specific maker and size:

• Bonded seal, M8 to M16: up to about 400 bar (5,800 psi), FKM variants slightly higher than NBR.
• Bonded seal, M18 to M30: about 350 bar (5,000 psi) as size grows and bolt stretch increases.
• Bonded seal, M33 to M60: about 280 bar (4,000 psi) for the largest standard sizes.
• DIN 3869 profile (ED) ring: up to about 600 bar, because the trapezoidal elastomer profile is confined by the port groove and resists extrusion.
• Copper / aluminum crush washer: very high static rating limited by the joint and bolt, not the washer, but single-use only.

Metal ring grade closes the spec. Zinc-plated mild steel is the default for general hydraulics; stainless 304 or 316 is specified for corrosion, washdown, marine, or food environments; the plating and grade also set the temperature and corrosion limits of the assembly independent of the elastomer. For copper washers, the relevant metal property is the temper: a soft (annealed) copper washer seals far more reliably than a hardened one, which is why annealing restores a used copper washer to service.

Selection Decision Factors

To convert the preceding chapters into a specific part number, follow the ordered decision sequence below. Most selection mistakes are not a single wrong value but a premature commitment at the wrong level, for example choosing a metal crush washer for a joint that will be opened repeatedly, or specifying NBR on a glycol coolant line. These eight steps form a reusable RFQ template.

1. Seal type by duty: First decide whether the joint is opened once (crush washer acceptable) or serviced repeatedly (bonded seal or profile ring for reusability), and whether it is a structural roof penetration (EPDM bonded washer). This choice drives everything downstream.
2. Media and temperature: Pick the elastomer per Chapter 4. NBR for mineral oil and fuel, EPDM for water, steam, and brake fluid, FKM for chemicals and high temperature, HNBR for hot oil, VMQ for extreme temperature or food contact. Confirm both the continuous and peak temperature.
3. Pressure rating: Match the family to the working pressure with margin. Up to about 400 bar a standard bonded seal suffices in small sizes; for higher pressure or larger threads move to a DIN 3869 profile ring rated to about 600 bar.
4. Thread and port standard: Identify the exact thread and seat (metric M-size, BSP G-size, or an ISO 6149 / 9974 / 1179 port) so the bore, outside diameter, and thickness fit the seat. Never specify by bolt size alone.
5. Metal ring grade: Zinc-plated steel for general use, stainless 304 or 316 for corrosion, washdown, marine, or food contact. For crush washers, specify copper (higher temperature) or aluminum (lower cost) and require the soft temper.
6. Seat condition and installation: Confirm the seating face is flat, clean, and undamaged, and define the fastener torque. No sealing washer seals across a scratch or burr, and crush washers are torque-sensitive while bonded seals are torque-tolerant.
7. Compliance and traceability: Cite the governing standard and form (DIN 7603 Form A/B, DIN 3869 ED), the elastomer grade and hardness, and the maker size code. For aerospace, food, or potable-water duty add the relevant material certification.
8. Total cost in service: A crush washer is cheapest per piece but must be replaced every service; a bonded seal costs more but is reusable and faster to fit. Weigh unit price against assembly labor, rework from dropped seals, and the cost of a field leak, which usually dwarfs the part price.

One last commonly overlooked dimension is supply and serviceability: whether the elastomer grade and ring material are held in stock in the sizes you need, whether self-centering variants are available to cut assembly errors, and whether the maker can certify material grades for regulated duty. Bonded and profile seals from Trelleborg (the original Dowty line), Hutchinson, Freudenberg, Techne, and Parker, and DIN 7603 crush washers from fastener houses such as Wurth, Bossard, JW Winco, Kipp, and McMaster-Carr, cover most industrial needs; for a project, confirm the size table, elastomer grade, and lead time before committing a part number.