Successful sealing of fluid power cylinders requires a delicate balance of interrelated factors that affect the sealing process in a dynamic system. These factors include lubrication and mating surfaces; design factors, such as shape, material, and process; and environmental factors, such as pressure, temperature, time, dynamics, and assembly.
Understanding the role of lubrication is the first key to successful sealing in dynamic systems. A fully lubricated system will experience some leakage, usually in the form of a fluid film, but friction will decrease, leading to a corresponding decrease in seal wear. Not surprisingly, the opposite is true: In a sparsely lubricated system, the fluid film decreases, but friction—and thus, wear—increase.
Moreover, sealing tends to improve as fluid pressures increase. Because system pressure on the seal surface compresses the seal axially, the seal is forced more tightly into the gland, improving conformability of the seal with the metal surfaces around it. If the seal is correctly designed, as system pressure increases, sealing force and effectiveness also increase.
A seal’s ability to resist extrusion is a third key factor. Seals in dynamic systems must resist shear forces resulting from the differential between the pressurized and unpressurized sides of the seal. Shear forces tend to push the seal into the gap between adjacent metal surfaces (known as the clearance gap), and the seal material and profile must be sufficiently strong and stiff to resist being damaged or destroyed.
Leak-free performance and high service life cannot be ensured by a single sealing element in heavy-duty applications. Because the cylinder’s piston rod is directly exposed to the environment, it’s paramount that the seals keep the fluid in the system. External leakage is easily noticed. Not surprisingly, then, it has become a common criticism of hydraulic power transmission.