Metal-Bonded Sealing Washers: Engineering Selection Guide for Bonded Seals

Metal-bonded sealing washers, commonly called bonded seals or bonded sealing washers,

 combine a metal carrier with a bonded elastomeric sealing element to create a compact static seal around bolts, plugs, threaded fittings and other connection points.

Metric bonded seals are widely used in hydraulic, fluid-power and industrial equipment where the joint requires both mechanical support and controlled elastomeric sealing.

Their function, however, is often oversimplified.

A bonded seal does not perform simply because metal and rubber have been combined into one washer. 

Effective sealing depends on the interaction between the metal carrier, elastomer, seal geometry, mating surfaces, fluid, temperature, pressure and installation conditions.

Understanding these relationships helps engineers specify the correct bonded sealing washer and helps procurement teams avoid sourcing components that are dimensionally similar but functionally unsuitable.

What Is a Metal-Bonded Sealing Washer?

A metal-bonded sealing washer is a composite static sealing component consisting of a metal washer or carrier and an elastomeric sealing element bonded to it.

When the connection is tightened, the elastomer is compressed against the mating surfaces to create the primary sealing interface.

The metal portion provides structural support and helps control the behavior of the elastomer within the joint.

Bonded seals are commonly installed around:

• threaded plugs

• hydraulic fittings

• fluid ports

• valve connections

• pump connections

• manifold connections

• bolts or screws passing through a sealing interface

The exact geometry varies by design and application.

How Does a Bonded Seal Work?

A bonded sealing washer uses controlled elastomer compression to establish a static seal.

During installation, tightening the fitting, plug or fastener applies an axial load to the washer. The elastomer deforms against the mating surfaces while the metal carrier supports the assembly.

This creates two distinct functions within one component:

Metal carrier → mechanical support and compression control

Elastomer → primary sealing interface

This division of functions is one of the most important concepts when selecting bonded seals.

The metal ring itself should not automatically be considered the primary sealing element, and the elastomer should not be treated as an ordinary loose rubber washer.

The geometry of the complete bonded seal determines how these two materials interact.

Are Bonded Seals the Same as Dowty Seals?

“Bonded seal” is the generic engineering term for this type of metal-and-elastomer sealing washer.

“Dowty seal” is also widely used in industry and sourcing searches for bonded sealing washer designs. When specifying or purchasing a component, however, engineers and buyers should rely on the required dimensions, material, seal geometry and applicable specification rather than assuming that every product described by similar terminology is interchangeable.

For replacement sourcing, the safest identification method is usually the original drawing, specification, part number or approved sample.

What Is the Primary Function of a Bonded Sealing Washer?

The primary function is to provide a static sealing interface around a fastener, fitting or connection.

Depending on the assembly, the seal may be intended to control leakage of media such as:

• hydraulic oil

• lubricating oil

• water

• coolant

• air

• fuel or other application-specific fluids

Compatibility must be confirmed for the actual media and operating conditions.

A bonded seal should not automatically be described as leak-proof, high-pressure rated or suitable for a particular fluid merely because it uses a metal carrier and elastomer.

What Bonded Sealing Washers Are Not Designed to Do

Understanding what a component does not do is important in engineering selection.

They Are Not Primarily Friction-Reduction Washers

Bonded seals are not generally selected to reduce friction between moving surfaces.

Their primary engineering purpose is static sealing.

If an assembly requires a bearing surface, low-friction interface or wear component, a different washer or bearing technology may be required.

They Do Not Automatically Prevent Vibration Loosening

The elastomeric sealing element should not automatically be treated as a locking feature.

Fastener loosening under vibration depends on the complete joint, including preload, joint stiffness, surface conditions and the locking method.

If anti-loosening performance is required, it should be addressed separately in the joint design.

They Do Not Automatically Provide a Defined Pressure Rating

Pressure capability depends on the complete sealing system.

Two bonded seals with similar outside dimensions can perform differently because of differences in:

• elastomer

• seal profile

• metal geometry

• mating surfaces

• compression

• fitting design

• temperature

• fluid

• installation

Pressure requirements should therefore be defined as an application requirement rather than assumed from the product name.

Anatomy of a Metal-Bonded Seal

Although bonded seals appear simple, several design variables influence their behavior.

Metal Carrier

The metal carrier provides structural support and establishes the mechanical interface around the elastomer.

Depending on the design specification, metal materials may include:

• carbon steel

• stainless steel

• other specified metals or alloys

Surface treatment may also be relevant where corrosion resistance or compatibility with the surrounding assembly is required.

Metal material and finish should be selected according to the actual application and drawing requirements.

Elastomeric Sealing Element

The elastomer creates the compliant sealing interface.

Common elastomer families may include:

• NBR

• FKM

• EPDM

• other application-specific compounds

The correct material depends on the fluid, temperature, chemical environment and required service conditions.

Seal Geometry

The elastomer profile determines where and how compression develops during assembly.

Important geometric features can include:

• inside diameter

• outside diameter

• metal washer thickness

• elastomer profile

• sealing lip geometry

• bonded interface

• relationship between the seal and mating component

A dimensional match based only on bolt or thread size is therefore not enough to confirm interchangeability.

How Do You Select the Correct Bonded Seal Size?

Metric bonded seals are often associated with a bolt, fitting or thread size, but nominal thread size alone should not control selection.

Engineers should review the complete sealing interface.

Important dimensional considerations include:

• fastener or fitting diameter

• seal inside diameter

• seal outside diameter

• washer thickness

• available bearing area

• port or fitting geometry

• mating surface width

• elastomer position

• required installed compression

For an existing OEM component, the drawing or approved sample should take priority over assumptions based on nominal thread size.

How Should the Elastomer Be Selected?

Elastomer selection should begin with the actual media and operating environment.

NBR Bonded Seals

NBR is commonly considered for many petroleum-based oil and hydraulic applications.

Its suitability still depends on the actual fluid formulation, temperature and exposure conditions.

FKM Bonded Seals

FKM may be considered where the application requires different chemical or temperature characteristics from standard NBR systems.

The actual fluid and service environment should still be checked before specification.

EPDM Bonded Seals

EPDM may be suitable for selected water-based or weather-exposed applications.

It should not be treated as a universal substitute for elastomers intended for petroleum-based fluids.

Do Not Select Elastomer by Temperature Alone

A more useful selection relationship is:

Elastomer + Fluid + Temperature + Pressure + Exposure + Seal Geometry + Compression = Application Suitability

This is why an RFQ stating only “M12 bonded seal, rubber” may not provide enough information for reliable sourcing.

How Should the Metal Carrier Be Selected?

The metal portion should be considered separately from the elastomer.

Selection factors may include:

• required mechanical support

• mating component material

• corrosion environment

• surface treatment

• temperature

• dimensional requirements

• OEM specification

For example, changing from coated carbon steel to stainless steel changes more than corrosion resistance. 

Material properties, surface condition, mating-material interaction and drawing requirements may also need review.

Material substitution should therefore be approved rather than assumed.

Bonded Seals vs Ordinary Rubber Washers

These products should not be treated as interchangeable.

An ordinary rubber washer consists primarily of an elastomeric washer-shaped component.

A bonded seal combines a defined elastomeric sealing element with a metal carrier.

The metal carrier can provide structural support and influence how the elastomer behaves under compression.

Replacing a bonded seal with a dimensionally similar rubber washer can therefore change:

• compression behavior

• load distribution

• sealing geometry

• extrusion behavior

• installation response

The replacement should be evaluated against the original joint design.

Bonded Seals vs Metal Sealing Washers

Metal sealing washers use a different sealing mechanism.

Copper, aluminum and other metallic sealing washers can rely on controlled metal deformation and surface contact to establish a seal.

Bonded seals rely primarily on elastomeric compression supported by a metal carrier.

Neither technology is universally superior.

Selection depends on the joint design, fluid, temperature, pressure, mating surfaces, installation method and validation requirements.

Bonded Seals vs O-Rings

Both technologies use elastomeric sealing, but their joint architectures are different.

An O-ring normally operates within a designed gland or groove that controls its compression and deformation.

A bonded seal integrates the elastomer with a metal carrier and can be installed around suitable threaded fittings, plugs or fastener interfaces without using the same gland architecture as a conventional O-ring seal.

The choice should therefore be based on the connection design rather than simply comparing elastomer materials.

Where Are Metal-Bonded Sealing Washers Used?

Bonded seals are particularly relevant to equipment containing static threaded fluid connections.

Hydraulic and Fluid-Power Systems

Typical connection points may include:

• hydraulic fittings

• manifolds

• pumps

• valves

• plugs

• fluid ports

• connectors

Hydraulic applications should be evaluated according to the actual fluid, pressure, temperature, fitting geometry and validation requirements.

Industrial Machinery

Bonded seals may be used in:

• lubrication systems

• fluid-control assemblies

• pumps

• machine-tool equipment

• industrial power units

• equipment housings with fluid connections

Construction and Agricultural Equipment

Mobile machinery frequently contains hydraulic and lubrication systems with multiple fittings, plugs and ports.

Component selection should follow the equipment manufacturer's specification and operating environment.

Automotive and Transportation Equipment

Bonded sealing components may be specified in selected vehicle fluid and thermal-management assemblies.

For safety-critical or application-specific automotive systems, suitability must be established through the OEM drawing, material specification and validation requirements.

HVAC and Thermal Management Equipment

Pumps, valves and coolant connections may require compact static sealing components.

Elastomer compatibility with the actual coolant and temperature cycle should be considered during selection.

What Determines Bonded Seal Performance?

Bonded seal performance cannot be reduced to one material or one pressure number.

A more realistic engineering relationship is:

Seal Performance = Elastomer + Fluid + Temperature + Pressure + Geometry + Mating Surface + Compression + Installation

Changing any one of these variables can change sealing behavior.

Fluid Compatibility

The elastomer must remain suitable for the actual media.

Temperature

Temperature can affect elastomer properties, compression behavior and long-term sealing response.

Pressure

System pressure interacts with seal geometry and the joint interface.

Mating Surface

Surface damage, contamination or unsuitable geometry can create leakage paths even when the sealing washer itself is dimensionally correct.

Compression

Both insufficient and excessive compression can cause sealing problems.

Installation

Alignment, joint geometry and tightening conditions affect how the elastomer contacts the sealing surfaces.

Common Bonded Seal Failure Modes

Understanding failure modes provides more useful engineering information than simply listing product advantages.

Leakage After Installation

Possible causes include:

• insufficient compression

• incorrect seal dimensions

• damaged mating surfaces

• contamination

• incorrect fitting geometry

• installation error

Elastomer Extrusion

Excessive deformation or an unsuitable sealing interface can allow the elastomer to move outside its intended sealing region.

Elastomer Damage

Cutting, pinching or excessive compression during installation can damage the sealing element.

Swelling or Material Degradation

An incompatible fluid or chemical environment can change elastomer properties.

Hardening or Loss of Elastic Response

Operating environment, temperature and aging can affect elastomer behavior over time.

Corrosion of the Metal Carrier

Metal selection and surface protection should reflect the actual environmental conditions.

Incorrect Replacement Part

A washer that looks similar may have a different:

• elastomer

• metal material

• hardness

• seal profile

• thickness

• bonding construction

Visual similarity does not prove functional interchangeability.

Common Procurement Mistakes When Sourcing Bonded Seals

Procurement teams can reduce sourcing risk by avoiding several common shortcuts.

Buying by Thread Size Only

“M10 bonded seal” or “M16 sealing washer” may not fully define the component.

The seal dimensions and construction should also be confirmed.

Specifying Only the Elastomer Name

“NBR” alone does not define the complete sealing component.

The metal carrier, elastomer specification, hardness where controlled, geometry and application conditions may also matter.

Assuming All Bonded Seals Are Interchangeable

Similar-looking parts from different specifications may not have identical geometry or performance.

Substituting Materials Without Engineering Approval

Changing the metal, coating or elastomer can affect application suitability.

Ignoring the Actual Fluid

“Hydraulic application” does not identify the exact fluid chemistry.

Where compatibility is important, provide the actual media or relevant specification.

Standard or Custom Bonded Sealing Washer?

Standard metric bonded seals are useful when an established size and material combination matches the application.

Custom bonded sealing washers may be required when the assembly uses:

• non-standard inside diameter

• non-standard outside diameter

• special washer thickness

• application-specific elastomer profile

• special metal material

• specified coating

• unusual elastomer requirements

• OEM-controlled geometry

• drawing-specific tolerances

For custom OEM components, manufacturing should follow the approved drawing and material requirements rather than trying to force the part into a generic catalog description.

What Should Be Included in a Bonded Seal RFQ?

For accurate quotation and engineering review, provide as much of the following information as applicable:

• customer part number

• 2D drawing

• applicable specification or standard

• nominal thread or fitting size

• inside diameter

• outside diameter

• washer thickness

• seal profile

• metal material

• metal finish or coating

• elastomer material

• elastomer hardness, if specified

• operating fluid or media

• temperature requirements

• pressure requirements

• mating component material

• fitting or port geometry

• inspection requirements

• material documentation requirements

• sample quantity

• production order quantity

• estimated annual usage

For an existing component without a complete specification, a physical sample together with application information can help identify the required construction before quotation.

Why Sample Validation Matters

A bonded sealing washer should be evaluated as part of the assembly rather than only as an isolated component.

Depending on the project, sample validation may include:

• dimensional inspection

• assembly fit

• sealing contact

• installation behavior

• material confirmation

• leakage testing under defined conditions

• customer-specific functional testing

The required validation criteria should come from the application and OEM requirements.

Sourcing Metric and Custom Metal-Bonded Sealing Washers

JUXIN FASTENERS supports industrial OEMs, equipment manufacturers and supply-chain teams sourcing standard and custom sealing and fastening components.

For bonded sealing washer projects, technical review can begin from a:

Drawing → Specification → Existing Part Number → Approved Sample → Application Requirement

For a new or custom project, provide:

Dimensions → Metal Material → Elastomer → Fluid → Temperature → Pressure → Mating Interface → Quantity → Validation Requirements

This allows the component to be evaluated according to its actual sealing function rather than simply matching a nominal metric size.

For OEM, made-to-drawing and production-quantity requirements, send your drawing or specification for engineering and sourcing review.

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