Product

Bonded Seals (Dowty Washers)

A bonded seal is a metal washer with an elastomer sealing ring bonded into its bore face, used as a single component under a bolt head or banjo fitting to seal a hydraulic or pneumatic connection in one part instead of a separate O-ring plus washer.

Overview

How it works

The metal washer carries the clamp load from the bolt or banjo fitting and controls how much the bonded elastomer ring is compressed — too little and it won't seal, too much and it will extrude or take a permanent set. Because the elastomer is bonded directly to the washer rather than fitted as a separate part, there's no O-ring to misplace or pinch during assembly, and the joint seals reliably the moment the bolt is torqued down.

Materials

Which compound, and why

Material

NBR

The standard bonded-seal compound — mineral-oil and fuel resistance for general hydraulic connections.

Material

FKM (Viton)

Higher-temperature or synthetic-fluid connections beyond NBR's range.

Applications

Where used

Banjo bolts, bolted flanges and other single-bolt hydraulic or pneumatic connections where fitting a separate gasket or O-ring is impractical.

How Bonded Seals Work

A bonded seal (also known as a Dowty seal or sealing washer) consists of a metal washer with a rubber sealing ring bonded to its inner diameter. When the bolt or fitting is tightened, the metal washer limits compression while the rubber element is squeezed between the fitting and the housing surface, creating a reliable high-pressure static seal. The controlled compression prevents over-tightening damage while ensuring adequate seal force.

Bonded seals are designed as a one-time-use component — once compressed, the rubber element takes a permanent set and should not be reused. They provide a simple, reliable sealing solution for BSP, metric and UNF threaded connections without the need for thread sealant tape or liquid sealants that can contaminate hydraulic systems.

Types and Pressure Ratings

FITCO supplies bonded seals in self-centring and standard (flat) types. Self-centring bonded seals have a raised inner lip on the metal washer that centres the seal in the counterbore, ensuring correct positioning even in vertical or overhead installations. Standard bonded seals rely on the bolt shank for centring and are used in standard drilled and tapped ports. Operating pressures up to 800 bar are achievable with correctly designed port geometry. Available in sizes from M6 to M42 (metric) and 1/8" to 2" BSP.