Hydraulic cylinder seals are used to seal the opening between various components in the hydraulic cylinder. 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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