Bonded Seals (Dowty Washers): Reliable Port Sealing

Introduction to Bonded Seals

Bonded seals, commonly known as Dowty washers or sealing washers, are composite sealing elements consisting of a metal washer with a vulcanized rubber sealing ring bonded to its inner surface. This simple yet highly effective design provides reliable static sealing for threaded port connections in hydraulic systems, pneumatic circuits, and fluid power equipment. The bonded seal has become the industry-standard sealing solution for BSP (British Standard Pipe), metric, and UNF port connections, replacing the older practice of using copper washers or thread sealant tape that often proved unreliable under pressure cycling and vibration.

The genius of the bonded seal design lies in its ability to create a positive seal through controlled compression of the elastomer ring between the port face and the fitting. Unlike thread sealants that rely on filling the helical gap between male and female threads, bonded seals create a face seal that is independent of thread engagement and thread condition. This makes bonded seals more reliable and easier to install correctly than any thread-dependent sealing method.

Construction and Design

Metal Washer Component

The metal washer provides structural support for the elastomer seal and distributes the clamping load from the port plug or fitting. Standard bonded seals use mild steel washers with a zinc or zinc-nickel plating for corrosion protection. For corrosive environments or applications requiring higher material specifications, bonded seals are available with:

• Stainless steel washers (AISI 304 or 316) for corrosive fluid and marine environments
• Aluminum washers for lightweight applications and compatibility with aluminum housings
• Copper washers for high-temperature applications and enhanced corrosion resistance
• Special platings including zinc-nickel, cadmium, and Geomet for specific environmental requirements

Elastomer Sealing Ring

The elastomer sealing ring is molded and vulcanized (bonded) to the inner diameter of the metal washer. The rubber ring protrudes slightly above the metal washer surface, and this protrusion is compressed against the port face when the fitting is tightened, creating the seal. The elastomer material is selected based on the fluid being sealed and the operating temperature:

• NBR (Nitrile): Standard material for hydraulic oil, mineral oil, and fuel applications (-30 to +120 degrees Celsius)
• FKM (Viton): For high-temperature applications, synthetic fluids, and enhanced chemical resistance (-20 to +200 degrees Celsius)
• EPDM: For brake fluid (DOT 3, DOT 4), water-glycol hydraulic fluids, and phosphate ester-based fluids
• Silicone: For food-grade applications and wide temperature range requirements

Thread Standards and Sizes

Bonded seals are manufactured to fit all major thread standards used in hydraulic and pneumatic port connections worldwide:

BSP (British Standard Pipe)

BSP bonded seals are among the most common sizes, fitting BSPP (parallel/straight thread) port connections from 1/8 inch to 2 inches and larger. BSP ports are widely used in European, Asian, and Australian hydraulic systems. The bonded seal sits on the port face and is compressed by the fitting body or port plug to create a face seal independent of the thread engagement.

Metric Threads

Metric bonded seals fit metric port connections commonly found in European hydraulic equipment, particularly systems designed to DIN or ISO standards. Metric sizes range from M8 to M48 and larger, with specific washer outside diameters matched to each metric thread size.

UNF (Unified National Fine)

UNF bonded seals are used primarily in North American hydraulic systems with SAE straight thread O-ring boss (ORB) and other UNF-based port connections. Common sizes range from 7/16-20 UNF to 1-7/8-12 UN.

Self-Centering Bonded Seals

Self-centering bonded seals feature a raised lip or retaining edge on the metal washer that centers the seal on the fitting thread before tightening. This feature prevents the seal from being displaced during installation, which is particularly valuable when installing fittings in awkward or overhead positions where holding the seal in place would otherwise be difficult. The centering lip snaps over the thread root diameter, holding the seal in position while the fitting is threaded into the port.

Self-centering bonded seals are available in most standard BSP, metric, and UNF sizes and are increasingly specified as the default choice for new equipment and maintenance applications due to their installation advantages.

Installation Guidelines

Proper installation of bonded seals is straightforward but must follow correct procedures to ensure reliable sealing. Key installation steps and considerations include:

• Inspect the port face and fitting for damage, corrosion, or debris that could prevent proper sealing
• Ensure the correct size bonded seal is selected for the thread and port face diameter
• Place the bonded seal on the fitting with the rubber side facing the port face (away from the fitting head)
• Thread the fitting into the port by hand until the bonded seal contacts the port face
• Tighten the fitting to the recommended torque value for the thread size and material
• Do not over-tighten, as excessive compression can extrude the rubber and damage the seal
• Replace bonded seals whenever a fitting is removed, as reused seals may not seal reliably

The recommended installation torque varies by thread size and material, with typical values ranging from 10 Nm for 1/8 BSP to 200+ Nm for 1-1/2 BSP connections. Consult the bonded seal manufacturer specifications for the correct torque values for each application.

Applications

Bonded seals are used wherever threaded port connections must be sealed against fluid leakage. Primary applications include hydraulic system test ports, pressure gauge connections, drain plugs, blanking plugs, and port-to-port adapter fittings. They are standard components in mobile hydraulic equipment (excavators, loaders, cranes), industrial hydraulic presses and cylinders, agricultural machinery, and automotive power steering and braking systems.

The reliability and ease of installation of bonded seals have made them the preferred sealing method for hydraulic port connections across all industries, largely replacing copper washers, fiber washers, and thread sealant compounds that were previously used for port sealing.

Tereefek Solutions: Bonded Seal Supplier

Tereefek Solutions stocks a comprehensive range of bonded seals in BSP, metric, and UNF sizes with NBR, FKM, and EPDM elastomer options. Our inventory includes both standard and self-centering designs in steel, stainless steel, and aluminum washer materials. We supply bonded seals individually and in bulk quantities for production and maintenance requirements. Our technical team can assist with size identification, material selection, and torque specification for your specific hydraulic port sealing applications. Contact Tereefek Solutions for competitive pricing and fast delivery on bonded seals and Dowty washers.