Hydraulic Seals and Pneumatic Seals

Hydraulic seals and pneumatic seals include piston seals, rod seals, U-cups, vee, cup, W, piston, and flange packings. Hydraulic seals and pneumatic seals are designed for the reciprocating motion that is common in hydraulic and pneumatic applications, such as cylinders.  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).  

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 works 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, and radial cross section.  Important service limits parameters to consider include maximum operating speed, maximum operating pressure, vacuum rating, and 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, and natural rubber.  Metallic choices include sintered bronze, cast iron, and stainless steel.  Other materials include felt and 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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