Bonded Seal: The Reliable Static Sealing Solution for Threaded Connections

Also widely known as Dowty seal, a bonded seal is a composite static sealing washer that combines a rigid metal outer ring and an inner vulcanized elastomer lip permanently bonded together in one single component. It is engineered for face sealing under bolt heads, threaded plugs, hydraulic ports and flat mating joints, delivering stable leak-tight performance for fluid and gas systems.

The basic structure splits two functional parts. The metal washer, commonly carbon steel or stainless steel, acts as a compression limiter. It evenly distributes bolt tightening load, prevents over-compression and elastomer extrusion, and provides mechanical support against vibration and impact. The inner elastomer element is the actual sealing medium. NBR (nitrile rubber) is the standard grade for general oil and hydraulic oil service; FKM (Viton), EPDM and FFKM options are selected for high temperature, chemical corrosive media or aggressive working conditions. When the bolt or fitting is tightened, the rubber lip is axially compressed between two flat machined surfaces, filling tiny surface gaps and blocking leakage paths. Unlike O-rings, bonded seals do not require a precision machined groove for installation, which greatly simplifies part design and assembly workflow.

One of its core advantages is controlled compression. Traditional soft crush washers such as copper or aluminum washers are easy to over-squash and lose sealing stability under fluctuating pressure or vibration. Ordinary O-rings carry high extrusion risk at high pressure without backup rings. The built-in metal stopper of bonded seals solves this problem perfectly, making it capable of handling high pressure up to hundreds of bar with consistent sealing performance. It also tolerates minor surface imperfections on mating faces and shows good resistance to shock and cyclic pressure changes. Self-centering bonded seal variants are available for automated mass assembly, helping operators align the seal on threads quickly and reduce assembly errors.

Bonded seals are foundational components across multiple industries. Hydraulic and pneumatic systems are the largest application field, including manifold blocks, pumps, valves, cylinders and BSP parallel threaded fittings. They are also widely adopted in automotive engine oil circuits, transmission ports, heavy machinery, agricultural equipment, oil & gas equipment and aerospace fluid connections for static threaded sealing points. It is worth noting that bonded seals are designed for flat face sealing on parallel threads; they are not suitable for tapered thread connections that rely on thread interference for sealing.

Correct material selection and installation rules decide service life. Engineers need to match elastomer grade against working medium, temperature range and chemical compatibility. During assembly, avoid scratching the rubber lip, and follow specified torque values. Over-tightening will crack the elastomer; insufficient torque causes leakage.

In sealing engineering, bonded seals fill a unique niche between simple gaskets and groove-mounted elastomer seals. Thanks to compact size, easy installation, high pressure resistance and low maintenance cost, it remains a preferred static sealing choice for threaded ports all over the fluid power industry.