Often used as bolt seals in hydraulic equipment, today bonded seals (commonly known as Dowty Seals or Dowty Washers) are finding usage in many and varied applications throughout industry. Bonded seals are metal washers with rubber inserts for use in high-pressure applications. The washer resists the bursting forces and limits the deformation of insert, which is self-centralizing and bonded to the washer’s inside diameter (ID).


Most bonded seals consist of a rectangular washer and a trapezoidal rubber insert. The insert is vulcanized to improve material properties such as hardness, strength, and elasticity. As captive assemblies, bonded seal washers can be used in applications with thread undercuts. They are often used in place of copper washers in hydraulic and pneumatic connections that are subject to extreme pressures.




Burst pressure, the maximum pressure which bonded seals can withstand, is an important specification to consider. Permanent degradation or product failure can occur if the burst pressure is exceeded during operation.


Types of Washers


Bonded seals use many different types of metal washers. Material of construction includes aluminum, brass, bronze, copper, mild steel, and stainless steel.


  • Aluminum and aluminum alloys are lightweight, non-ferrous metals with good corrosion resistance, ductility, and strength.
  • Copper, brass, and bronze alloys are also non-ferrous. They provide excellent electrical and thermal conductivity as well as good corrosion resistance, ductility, and strength.
  • Mild steel is a common, low-carbon machine steel that is neither extremely brittle nor ductile.
  • Stainless steels are highly corrosion resistant, ferrous alloys that contain chromium and/or nickel additions.


Surface finishes for bonded seals include cadmium plating, chromic anodizing, clear passivated, zinc plating, and gold passivation. Products with zinc/iron and black chromate, or zinc nickel trivalent passivated are also available.


Types of Inserts


Selecting bonded seal washers requires an analysis of rubber insert materials. Choices include: acrylonitrile butadiene copolymer, ethylene propylene diene monomer (EPDM), fluorocarbon elastomer, nitrile butadiene rubber (NBR), and hydrogenated nitrile butadiene (HNBR).


  • Acrylonitrile butadiene copolymers are resistant to mineral oils, water, hot air, and some chemicals.
  • Fluorocarbon elastomers are resistant to mineral oils, petroleum products, solvents, and chemicals. They are often used in place of NBR and other oil-resistant materials.
  • Hydrogenated nitrile butadiene (HNBR) has a greater tensile strength and wears resistance than NBR. Often, HNBR is used in petroleum exploration applications because of its resistance to heat, hydrogen sulphide, corrosion, and steam.


With years of experience and advanced technology, AOK Valve Seal Ltd. is excellent in producing high quality bonded seals (sealing washers) and obtained ISO 9001 and TS16949 qualifications. AOK focuses on creating value for customers by providing outstanding technical and manufacturing services. For more products details, please don’t hesitate to contact with AOK.



Article Source: http://www.globalspec.com/learnmore/mechanical_components/seals/bonded_seals