Hydraulic Seals and Pneumatic Seals Information

Hydraulic seals and pneumatic seals are designed for the reciprocating motion that is common in hydraulic and pneumatic applications, such as cylinders.  A hydraulic seal is a relatively soft, non-metallic ring, captured in a groove or fixed in a combination of rings, forming a seal assembly, to block or separate fluid in reciprocating motion applications.  Hydraulic seals are vital in machinery. Their use is critical in providing a way for fluid power to be converted to linear motion.
Hydraulic seals can be made from a variety of materials such as polyurethane, rubber or PTFE.  The type of material is determined by the specific operating conditions or limits due to fluid type, pressure, fluid chemical compatibility or temperature.

A static hydraulic seal is located in a groove and sees no movement - only sealing within its confined space, acting like a gasket.  To achieve this, the gasket should be under pressure.  The pressure is applied by tightening of the bolts.

A type of dynamic hydraulic seal called a rod seal is exposed to movement on its inner diameter along the shaft or rod of a hydraulic cylinder.  A type of dynamic hydraulic seal called a piston seal is exposed to movement on its outer diameter along the tube or bore of a hydraulic cylinder.

Hydraulic seals are designed for high-pressure dynamic applications such as hydraulic cylinders.  Pneumatic seals are used in pneumatic cylinders and valves and generally are designed for lower operating pressures than hydraulic seals.  Pneumatic applications also typically demand higher operating speeds and lower friction seals than hydraulic applications.  These seals may be used for rotary and reciprocating motion.
Some hydraulic seals and pneumatic seals are composite.  Composite seals are two-or multi-part seals manufactured as an integral unit.  A typical composite seal consists of an integral PTFE ring and elastomer ring, providing the properties of an elastomeric ring with a rigid, low friction (PTFE) working face.  These seals can have a variety of different cross sections.  Some forms of composite seals are also referred to as polyseals, wedge-action seals, crown seals and tri-seals (3 part seals).

Seal Configurations
  • Common sealing orientation and directions for hydraulic seals and pneumatic seals include internal or rod seal, external or piston seal, symmetric seal, and axial seal.
  • Rod seals are radial seals. The seal is press-fit into a housing bore with the sealing lip contacting the shaft. Also referred to as a shaft seal.
  • Piston seals are radial seals. The seal is fit onto a shaft with the sealing lip contacting the housing bore. V-rings are considered external lip seals.
  • Symmetric seals are symmetrical and work equally well as a rod or a piston seal. 
  • An axial seal seals axially against a housing or machine component.

Sealing direction pertains primarily to hydraulic and pneumatic seals that are used in applications with axial motion, such as cylinders and pistons.  The action can be single or double.  Single acting, or unidirectional seals, offer an effective seal in one axial direction only.  In order to seal in both directions for a reciprocating motion, more than one seal must be used.  Double acting, or bi-directional seals, are effective when sealing in both directions.
Important dimensions to consider when specifying hydraulic seals and pneumatic seals include:
  • Shaft outer diameter or seal inner diameter
  • Housing bore diameter or seal outer diameter
  • Axial cross section or thickness
  • Radial cross section
  • Parameters
Important service limits parameters to consider include:
  • Maximum operating speed
  • Maximum operating pressure
  • Vacuum rating
  • Operating temperature
Rubber sealing element or lip material choices for hydraulic and pneumatic seals include:
  • ethylene acrylic
  • EDPM
  • fluoroelastomer
  • fluorosilicone
  • nitrile
  • nylon or polyamide
  • polychloroprene
  • polyetheretherkeytone
  • polyoxymethylene
  • polytetrafluoroethylene
  • polyurethane or urethane
  • natural rubber
Seal material choices include:
  • Sintered bronze
  • Cast iron
  • Stainless steel
  • Felt
  • Leather
Many seal manufacturers use their own proprietary material.  Consult with manufacturer for proprietary material specifications.
Common features for hydraulic seals and pneumatic seals include spring loaded, integral wiper, and split seal.

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