Hydraulic seals and guides

Hydraulic cylinder seals are used to seal the opening between various components in the hydraulic cylinder. Fig. 1 shows an example of the seal and guide components of a heavy duty cylinder. There are two main types of hydraulic seals in the system:
  • Dynamic seals
    They seal between components in relative motion. In a hydraulic cylinder the rod sealing system seals dynamic reciprocating motion between the piston rod and head, while the piston sealing system seals dynamic reciprocating motion between the piston and cylinder bore.
  • Static seals
    They seal between components fixed together without relative motion. Hydraulic cylinders use static seals in various locations depending on the design and construction. The most common are static seals between the piston and piston rod and between the head and cylinder bore tube.
Each dynamic seal in a hydraulic cylinder has a special function to contribute to the performance of the system:

Piston seal functions

  • act as pressure barrier and prevent fluid passing the piston – important for controlling the cylinder motion or maintaining the position when at rest

Rod seal functions

  • act as pressure barrier and keep the operating fluid inside the cylinder
  • regulate the fluid film which extends with the surface of the piston rod – important to inhibit rod corrosion and to lubricate the wiper seal and the rod seal itself
  • accept the lubrication film back into the cylinder when the rod retracts

Buffer seal functions

  • protect the rod seal from fluid pressure peaks in excess of system pressure
  • attenuate the fluctuations in system pressure, thereby improving rod seal performance by allowing the rod seal to deal with more constant or gradually changing pressure
  • act as an internal excluder to keep system contaminants, such as metal particles, from damaging the rod seal

Wiper seal functions

  • exclude external contaminants from entering the cylinder assembly and the hydraulic system
  • accept the lubrication film back into the cylinder when the rod retracts

Guide rings (rod and piston) functions

  • prevent metal-to-metal contact between components
  • react the radial load caused by side loads on the cylinder assembly
  • keep the piston rod and piston accurately centred in the cylinder assembly – important for performance of the rod sealing system and piston sealing system
The subsequent sections contain additional information regarding the function of each seal or guide in the system and the selection of them.
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