Bonded Sealing Washers for Hydraulic and Industrial Fluid Systems

Bonded sealing washers—commonly called bonded seals, metal-bonded sealing washers, hydraulic sealing washers, 

and in some markets Dowty-type seals—combine a rigid metal washer with a bonded elastomer sealing element.

They are commonly used around bolts, plugs, hydraulic fittings, threaded ports and other connections where the assembly must provide both mechanical support and fluid sealing.

Unlike a conventional flat washer, a bonded sealing washer is selected as part of the sealing system.

 Its performance depends on the metal carrier, elastomer compound, mating surfaces, fluid compatibility, temperature, pressure, connection geometry and installation conditions.

For engineers and sourcing teams, this distinction is important: selecting a bonded seal by thread size alone may not be sufficient.

How Does a Bonded Sealing Washer Work?

A typical bonded seal consists of two functional elements:

• Metal outer ring — provides structural support and limits excessive deformation of the sealing element.

• Elastomer inner sealing ring — compresses between the fastener or fitting and mating surface to create the seal.

During tightening, the elastomer deforms within the designed sealing area while the metal portion supports the assembly.

The objective is not simply to apply maximum compression. The objective is to achieve controlled elastomer compression within the actual joint geometry.

This makes bonded sealing washers useful for hydraulic and industrial fluid connections where a separate machined O-ring groove may not be practical.

Bonded Seal vs Flat Washer vs O-Ring: What Is the Difference?

These components should not be treated as interchangeable.

Flat Washer

A standard flat washer is primarily intended to:

• distribute load;

• protect the mating surface;

• provide bearing area;

• support the bolted joint.

A conventional metal flat washer should not automatically be considered a pressure seal.

Bonded Sealing Washer

A bonded sealing washer combines:

• a structural metal carrier;

• an elastomer sealing element;

• localized sealing around the fastener or port.

It is often selected where sealing is required directly beneath a bolt head, plug, fitting or similar interface.

O-Ring

An O-ring normally works within a specifically designed groove or gland. Its performance depends heavily on gland geometry, squeeze, stretch, material and operating conditions.

Engineering decision: Do not replace an O-ring with a bonded seal—or a bonded seal with a flat washer—without reviewing the joint and sealing geometry.

Metal Carrier Materials

Metal selection should be based on mechanical requirements, corrosion environment, mating materials and customer specifications.

Common options may include:

Carbon Steel

Suitable for many general industrial and hydraulic applications when combined with an appropriate protective finish.

Selection should consider:

• corrosion exposure;

• coating compatibility;

• storage conditions;

• operating environment.

Stainless Steel

Stainless steel bonded sealing washers may be specified where improved corrosion resistance is required.

The exact stainless grade should be defined by the drawing, environmental requirements or customer specification rather than assumed from the product name alone.

Elastomer Selection: NBR vs FKM and Other Compounds

The elastomer is one of the most important specification variables in a bonded sealing washer.

NBR

NBR is widely considered for applications involving many petroleum-based oils and hydraulic fluids.

However, suitability depends on the actual:

• fluid formulation;

• operating temperature;

• exposure duration;

• pressure cycle;

• elastomer grade.

FKM

FKM may be considered where higher temperature capability or different chemical resistance is required.

It should not automatically be described as the “better” material. The correct choice depends on the media and operating conditions.

Other Elastomer Requirements

Depending on the project, an OEM may specify:

• elastomer family;

• hardness range;

• temperature requirements;

• chemical compatibility;

• color;

• regulatory requirements;

• specific material documentation.

For critical sealing applications, fluid compatibility should be confirmed rather than inferred from a generic material name.

Where Are Bonded Sealing Washers Used?

Hydraulic Equipment and Fluid Power Systems

Bonded seals are commonly associated with:

• hydraulic fittings;

• threaded ports;

• hydraulic plugs;

• valve assemblies;

• manifolds;

• pumps;

• cylinders;

• mobile hydraulic equipment.

These applications often require compact sealing around a threaded connection without introducing a separate gasket arrangement.

Automotive and Commercial Vehicle Systems

Potential applications include selected:

• fluid connections;

• lubrication systems;

• hydraulic assemblies;

• transmission-related equipment;

• pumps and valve interfaces.

Automotive use should always be evaluated against the specific OEM drawing, fluid, temperature, pressure and validation requirements.

Industrial Machinery

Bonded sealing washers may be used in:

• machine tools;

• industrial pumps;

• compressors;

• lubrication systems;

• hydraulic power units;

• manufacturing equipment.

Construction and Agricultural Equipment

Mobile equipment frequently combines vibration, hydraulic pressure, outdoor exposure and repeated operating cycles.

This makes material selection and sealing-interface control especially important.

Power and Process Equipment

Depending on the fluid and operating conditions, bonded sealing washers can also be considered for selected connections in:

• power equipment;

• process machinery;

• valves;

• pumps;

• fluid distribution equipment.

The seal compound must be evaluated against the actual media rather than selected solely by industry.

The Most Important Engineering Question: What Is Actually Being Sealed?

One of the most useful questions during bonded-seal selection is not:

“What thread size do you need?”

It is:

“Where is the sealing interface in the assembled joint?”

A thread designation describes the threaded connection, but it does not completely define the sealing interface.

Engineers should review:

1. fastener or fitting geometry;

2. washer inside and outside diameters;

3. available bearing surface;

4. mating-surface condition;

5. elastomer contact area;

6. operating pressure;

7. fluid media;

8. temperature;

9. installation method.

Two connections using the same nominal thread can have different sealing requirements.

Why Surface Condition Matters

A bonded seal cannot compensate for every surface defect.

Potential leakage paths may be created by:

• deep scratches;

• burrs;

• damaged spotfaces;

• excessive surface irregularity;

• contamination;

• corrosion;

• misalignment.

For this reason, sealing performance should be evaluated as a system interaction between the bonded washer and the mating components, not as an isolated property of the washer.

Pressure Alone Is Not Enough to Specify a Bonded Seal

A common sourcing mistake is to request a “high-pressure bonded washer” without defining the rest of the operating conditions.

Pressure capability can depend on:

• seal geometry;

• metal-ring dimensions;

• elastomer compound;

• hardness;

• temperature;

• fluid;

• surface finish;

• joint design;

• installation compression;

• pressure cycling.

JUXIN FASTENERS therefore does not recommend treating a universal pressure rating as applicable to every bonded-seal configuration.

For OEM applications, the required operating conditions should be reviewed together with the drawing or mating-component information.

Installation Considerations

Confirm the Correct Orientation

The washer must seat correctly against the intended sealing surface. Special-profile bonded seals may have orientation requirements.

Inspect the Mating Surface

Before assembly, check for:

• burrs;

• scratches;

• contamination;

• corrosion;

• deformation.

Avoid Damaging the Elastomer

Sharp edges, incorrect handling or unsuitable installation geometry can cut or displace the sealing element.

Control the Fastening Process

Excessive tightening should not be assumed to improve sealing.

The correct tightening condition depends on the complete joint design, including:

• fastener;

• thread;

• mating material;

• bearing surface;

• seal geometry;

• lubrication condition;

• OEM specification.

Inspect Removed Seals Before Reuse

Reuse should never be assumed automatically.

After disassembly, the elastomer may have experienced:

• compression set;

• cutting;

• extrusion;

• chemical degradation;

• thermal aging;

• permanent deformation.

For applications where leakage has significant safety or maintenance consequences, 

replacement requirements should follow the equipment manufacturer's validation and maintenance procedure.

Bonded Sealing Washer Selection Decision Path

For design engineers and sourcing teams, a practical selection sequence is:

Step 1 — Define the Connection

Identify whether the seal is used beneath a:

• bolt head;

• threaded plug;

• hydraulic fitting;

• adapter;

• port connection;

• custom component.

Step 2 — Define the Fluid

Specify the actual operating medium rather than simply stating “oil” or “chemical.”

Step 3 — Define the Operating Environment

Provide:

• normal operating temperature;

• temperature extremes;

• working pressure;

• pressure cycling;

• external corrosion exposure.

Step 4 — Select the Elastomer

Evaluate NBR, FKM or another specified compound against the actual media and temperature conditions.

Step 5 — Select the Metal Carrier

Choose carbon steel, stainless steel or another specified material based on structural and environmental requirements.

Step 6 — Confirm Geometry

Review:

• nominal thread or fastener size;

• inside diameter;

• outside diameter;

• thickness;

• sealing-lip profile;

• available seating area.

Step 7 — Validate the Assembly

For critical OEM applications, validation should be performed in the actual joint configuration and operating environment.

This approach is more reliable than purchasing a bonded seal solely from a nominal metric size.

Standard vs Custom Bonded Sealing Washers

Standard metric bonded seals are suitable for many common hydraulic and industrial connections.

A custom solution may be appropriate when the project requires:

• non-standard dimensions;

• special metal thickness;

• special elastomer compound;

• different hardness requirements;

• unusual operating media;

• stainless steel construction;

• customer-specific geometry;

• drawing-controlled manufacturing.

For OEM programs, a drawing or existing sample can significantly reduce ambiguity during supplier evaluation.

What Procurement Teams Should Include in an RFQ

A technically complete RFQ helps prevent repeated clarification and incorrect quotations.

Where available, provide:

• drawing or dimensional specification;

• nominal thread or fastener size;

• washer ID, OD and thickness;

• metal material;

• metal finish;

• elastomer material;

• elastomer hardness requirement;

• operating fluid;

• operating temperature range;

• working pressure or system requirements;

• annual usage;

• order quantity;

• sample requirements;

• inspection or documentation requirements.

If the existing part is being replaced, provide the current sample, drawing or mating-component information where possible.

Sourcing Bonded Sealing Washers for OEM Applications

For procurement engineers and supplier-development teams, unit price should not be the only sourcing criterion.

Important supplier-evaluation factors include:

• dimensional consistency;

• metal and elastomer material control;

• bonding quality;

• sealing-profile consistency;

• surface-finish control;

• lot traceability when required;

• drawing revision control;

• inspection capability;

• sample validation support;

• repeat production consistency.

A bonded sealing washer is inexpensive compared with many hydraulic assemblies, but a leakage failure can create substantially higher maintenance, warranty and downtime costs.

Related Fastening and Sealing Solutions

Engineers evaluating bonded sealing washers may also need related components such as:

• custom washers and stamped components for non-standard bearing and sealing geometries;

• stainless steel washers for corrosion-resistant assemblies;

• industrial fasteners for machinery and equipment;

• custom fasteners manufactured to customer drawings;

• OEM fastening solutions for application-specific assemblies.

These components should be selected as part of the complete joint rather than as unrelated individual hardware.

Why Source Bonded Sealing Washers from JUXIN FASTENERS?

JUXIN FASTENERS supports global OEM, industrial and equipment-manufacturing customers with standard and custom fastening components.

For bonded sealing washer projects, we can review customer requirements based on:

• drawings;

• dimensions;

• metal material;

• elastomer specification;

• operating environment;

• application requirements;

• sample requirements;

• production quantities.

Our objective is not simply to supply a washer by nominal size. For engineering-driven projects, 

the goal is to help customers define the correct component specification before production and purchasing decisions are finalized.

Request a Quote or Engineering Review

For a faster technical and commercial review, send your drawing, specification, sample information or required dimensions together with the application conditions.

Email: info@juxinfasteners.com
Website: www.juxinfasteners.com

JUXIN FASTENERS provides bonded sealing washers, custom washers and fastening solutions for OEM and industrial applications.