Hydraulic and Pneumatic Seals

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Hydraulic and Pneumatic Seals

Hydraulic Seals are relatively soft, non-metallic rings, placed in a groove or fixed in a combination of rings, forming a seal assembly. This assembly is used to block or separate fluid in reciprocating motion applications. Hydraulic seals 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 (a genereric compound of Teflon™, a trademark of The Chemours Company). The type of material is determined by the specific operating conditions or limits due to fluid type, pressure, fluid chemical compatibility or temperature.

Loaded U-Seals come in the following styles, including:

  • RTUB standard loaded rod seals
  • Standard Loaded U-seals
  • Deep Loaded U-seals
  • B-Type Loaded U-seals
  • DRU-style Unloaded U-seals
  • BS (DRTU-style) Unloaded U-seals
  • DPU-style Unloaded U-seals
  • DSU-style Unloaded U-seals
  • HU-style Unloaded U-seals
  • 8400-style Unloaded Symmetrical U-seals (Pneumatics)

A static seal is located in a groove and sees no movement, only sealing within its confined space.  A type dynamic hydraulic seal is called a rod seal and is exposed to movement on its inner diameter along the shaft or rod of a hydraulic cylinder.  Another type of dynamic hydraulic seal is called a piston seal and is exposed to movement on its outer diameter along the tube or bore of a hydraulic cylinder.

This class of seals is a U-seal or u-cup and come in loaded and unloaded styles. 

U-seals are available in standard and metric sizes.

Design & Application Standards

Pneumatic seals, like hydraulic cylinders, something forces a piston to move in the desired direction.  Pneumatic cylinders use the power of compressed gas to produce a force in a reciprocating linear motion.  Pneumatic seals are symmetrical unloaded U-seals that work for both rod and piston applications. These are unloaded, meaning that there is a gap between the inner and outer lips allowing for greater flexibility. They can withstand pressure up to 1,250 psi, and temperatures from -65 to 300F. They have a symmetrical cross section, meaning the inside lip is the same as the outside lip.

Other items used with hydraulic seal applications include:

Pip Rings are complimentary to B-Type U-seals, transforming the B-Type U-seal from uni-directional to bi-directional seals.
Modular Back-up rings, made from urethane or nylon, are commonly used as spacers for U-seals.
Z-seals are symmetrical loaded U-seals that work for both rod and piston applications. The load is much larger, creating two complementary methods of sealing, a nitrile lip, and a urethane base.


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