What Are Bonded Seals? Understanding Their Role, Applications, and Selection Criteria

What is a Bonded Seal?

A bonded seal (also referred to as a Dowty Washer) is a composite washer that combines a metal ring with a bonded rubber seal on its inner diameter. The metal ring provides structural strength and resists deformation, while the seal forms a reliable, pressure-tight seal against fluid or gas leakage.

Bonded seals are suitable for use in both high and low temperature and high and low pressure applications. Designed for use with threaded fasteners and flange connections, they are essential in hydraulic, pneumatic, automotive, and industrial systems.

Bonded Seals have replaced cooper washer in high pressure applications.

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Common Material Choices

Bonded Seals are available in a wide range of combinations to suit all applications, when selecting materials for, it's important to consider the specific application, including environmental conditions such as temperature, chemical exposure, mechanical stress, and corrosion potential.

Metal Washer

Steel - A strong, durable, and cost-effective material often used in structural applications. Suitable for general-purpose use where corrosion resistance is not a primary concern.

Stainless Steel - Offers excellent corrosion resistance, making it ideal for marine, chemical, and outdoor applications. Also provides good strength and longevity.

Aluminium - Available on request these bonded seals are lightweight and resistant to corrosion, aluminium washers are commonly used in aerospace and automotive applications where weight is a concern.

Brass - Available on request they are known for their good corrosion resistance and electrical conductivity, brass is often used in plumbing and electrical applications.

Rubber Seal

Nitrile - A widely used elastomer known for its resistance to oils, fuels, and other petroleum-based products. Common in automotive and industrial sealing.

Viton® (FKM) - A high-performance rubber with excellent resistance to high temperatures, chemicals, and aggressive fuels. Used in aerospace, automotive, and chemical processing industries.

EPDM - Excellent resistance to weathering, ozone, and UV radiation. Suitable for outdoor and water-based applications like roofing and automotive weatherstripping.

Silicone - Offers outstanding temperature resistance (both high and low), flexibility, and biocompatibility. Often used in food-grade, medical, and high-temperature sealing applications.

What are the Advantages of Bonded Seals

Wide range of material combinations available
Suitable for Risk of Misalignment and Low pressure applications
Suitable for Risk of Misalignment and low Temperature

Reduced bolt torque with no loss of tightening loads
Metal washer prevents over compression and distrotion

Main Applications of Bonded Seals:

Bonded Seals are used in a wide range of industries and systems including:

Hydraulic Equipment

Components like pumps, valves, and actuators require precision sealing to prevent fluid leakage and maintain system efficiency. Seals must resist wear, pressure spikes, and oil degradation, while washers must withstand mechanical loads and provide long-term structural support

Pneumatic Systems

These systems demand seals that are flexible and airtight to prevent air leaks, while tolerating repeated cycling and vibrations. Washers in these systems often serve to distribute load and reduce wear in lightweight, corrosion-prone environments.

Aerospace

Aerospace systems require materials that can perform reliably in extreme conditions—high altitudes, rapid pressure changes, and wide temperature fluctuations. Hydraulic components in aircraft must be ultra-reliable, with seals that resist aggressive hydraulic fluids and washers that balance weight savings with high mechanical integrity.

Automotive & Heavy Machinery

In vehicles, components endure high temperatures, constant vibration, and exposure to oils and fuels. Seals must resist degradation from automotive fluids and thermal cycling, while maintaining flexibility and durability. Washers ensure proper torque distribution and vibration damping, especially in dynamic load-bearing assemblies.

Marine & Offshore

Marine environments expose components to constant moisture, salt spray, and corrosive chemicals. Seals must maintain elasticity while resisting water ingress and salt-induced degradation. Washers, often made of corrosion-resistant metals, play a crucial role in bolted connections subject to constant vibration and harsh weather conditions.

Medical Devices

Medical equipment demands ultra-precise, contamination-free fluid control. Seals must be biocompatible, non-reactive, and sterilisation-resistant. Washers are typically used in compact assemblies where corrosion resistance, hygiene, and structural reliability are vital for patient safety and device longevity.

Self-Centralising & Features

In addition to the wide range of metal and seal combinations, bonded seals come in two different seal styles:

Self-Centralising Bonded Seals

These feature a specially designed elastomeric lip that helps centre the seal on the bolt or stud during assembly.

Benefits Include:

Easier installation
Reduced risk of seal misalignment
Faster assembly
Improved seal retention (particularly useful in automated production)

Standard (Non-Self-Centralising) Bonded Seals

These have a flush edge and require more precise manual alignment during installation. While just as effective once installed, they may take longer to position and are more prone to assembly error. Choosing between the two often depends on the application’s operational environment and the importance of installation speed and accuracy. Consideration should be given to cost as Self Centralising Bonded Seals tend to have a higher unit cost price, but a lower installation time.

Feature	Feature	Self-Centering Bonded Seal
Installation Time	Installation Time	20 Seconds
Ease of Alignment	Ease of Alignment	Automatic - Self Centering
Risk of Misalignment	Risk of Misalignment	Low
Suitability for Automation	Suitability for Automation	Risk of Misalignment
Risk of Misalignment	Risk of Misalignment	Low

How to Choose the Right Bonded Seal

To ensure the optimal performance and longevity, consider the following when selecting a bonded seal.

Material Compatibility:

Ensure the seal is chemically compatible with the system's fluids and the environment.

Size and Dimensions:

Bonded seals are available in standard imperial and metric sizes. Custom sizes can also be produced for bespoke applications. To size a bonded seal correctly, you need to match the internal diameter (ID) of the seal to the shaft, bolt, or screw size it's intended to seal.

The outer diameter (OD) and thickness must suit the recess or gland in which the seal will sit. Always consider the thread size (metric or imperial), pressure rating, and surface finish. Standard sizing charts from manufacturers help cross-reference part numbers to dimensions.

Operating Conditions:

Consider both maximum and minimum pressures and temperatures. Make sure the selected seal is rated for the most extreme expected conditions.

Surface Finish:

The mating surfaces should be smooth and clean to ensure a reliable seal.

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