What is in this article?:
Successful sealing involves containment of fluid within fluid power systems and components while excluding contaminants.
High-pressure sealing generally refers to confining fluids at pressures above 5,000 psi. Below these pressures, standard energized urethane lip seals and U-cup seals function satisfactorily without special provisions. Above them, some sort of special sealing devices are necessary.
To be effective, seals have to perform three basic functions. They must:
Seal — Sealing elements must conform closely enough to the microscopic irregularities of the mating surfaces (rod to seal groove and/or piston groove to cylinder bore, for example) to prevent pressure fluid penetration or passage, Figure 1.
Adjust to clearance-gap changes — The seal must have sufficient resilience to adjust to changes in the distance between mating surfaces during a cylinder stroke. This clearance gap changes size because of variations in the roundness and diameter of the cylinder parts. The clearance gap also may change size in response to side loads. As the size of the gap changes, the seal must match the size change to maintain compressive sealing force against adjacent mating surfaces.
Resist extrusion — The seal must resist shear forces that result from the pressure differential between the pressurized and unpressurized sides of the seal. These shear forces attempt to push the elastomeric seal into the clearance gap between adjacent metal surfaces, Figure 2. The seal must have sufficient strength and stiffness to resist becoming deformed into the gap and damaged or destroyed.
Higher pressure improves sealing
Elastomeric materials also must seal while accommodating dimensional variations caused by manufacturing tolerances, side loads, and cylinder deformations under pressure. Understand that in general, sealing improves as fluid pressures increase. System pressure on the seal surface attempts to compress the seal axially. This compression forces the seal more tightly into the gland and helps improve conformability of the seal with its contacting metal surfaces.
If the clearance gap increases during the stroke, resilience of the compressed elastomeric seal causes it to expand radially and maintain sealing force against the metal surfaces. System pressure combines with seal resilience to increase compressive sealing forces when the clearance gap increases. It generally is true that, as system pressure increases, sealing force and the resulting sealing effectiveness also increase if the seal is correctly designed.
The seal's internal shear stresses increase as system pressure increases. With increasing pressure, the stresses eventually exceed the physical limits of the seal elastomer, and it extrudes into the gap. Difficulties presented by high pressure are not primarily sealing problems but are problems of keeping the seal in its gland while maintaining its structural integrity as increasing system pressures force the seal into the gap.
Almost all of the design and in-service technology of high-pressure sealing deals with protecting the elastomeric seal from the potentially destructive distortion caused by high system pressures. With proper backup to reduce the size of the gap, relatively fragile elastomers can successfully seal extremely high pressures.
When handling a 90-durometer energized urethane lip seal or U-cup at room temperature, the seal seems to be made of an extraordinarily stiff, tenacious material. It requires well-designed experiments and/or sophisticated computer simulations to visualize the state of such a seal inside a hydraulic cylinder at normal operating temperatures and pressures. At pressures as low as 600 psi for 70-durometer nitrile rubber and 1,500 psi for 90-durometer urethane, the seal cross section is significantly deformed. It changes shape almost instantaneously in response to pressure spikes or changes in the size of the clearance gap. Literally, the seal becomes an annular glob in the seal gland.
The ability of a seal to resist extrusion into the gap depends on the interaction of:
• system operating pressure,
• system operating temperature,
• size and type of clearance gap,
• seal material, and
• seal design.
System operating temperature is especially important in high-pressure applications because most elastomers soften and lose their ability to resist extrusion at higher temperatures. Some design methods that help lower high system temperatures include the use of low-friction materials, an increase in fluid volume, and a decrease in the cycle rate of the system. However, when ambient temperature is high, and operating conditions are extreme, it is possible for system temperatures to exceed design parameters. Under such conditions, it often becomes necessary to upgrade seals, and for anti-extrusion devices to be more temperature-resistant.
The size of the extrusion gap can be controlled throughout the design and manufacture of the cylinder, piston, rod, and end cap. Decreasing manufacturing tolerances increases cylinder cost, however, and also may increase the probability of metal-to-metal interference. In addition, reducing the extrusion gap size is inherently limited by differential thermal expansion of mating metal components.
The actual size of the extrusion gap is a function of:
• the nominal gap designed into the cylinder,
• manufacturing tolerances, including diametrical variation and ovality,
• diametrical expansion of the cylinder caused by system pressure,
• side loads, and
• wear on radial load-bearing surfaces.
Because all these factors vary, and because the variances can be cumulative, seal design and material must resist extrusion through the largest gap likely to be encountered at design pressure and temperature.