Fig. 1 (a) Seal material must conform to irregularities in metal surfaces to block fluid passage; (b) to adjust to clearance gap size changes, the seal must expand or compress rapidly to follow dimensional variations; (c) to resist being forced into the extrusion gap, the seal must have sufficient modulus and hardness to withstand shear stress produced by system pressure.

Fig. 2. As system fluid pressure increases, (a) to (b), an O-ring seal is progressively forced into the extrusion gap. Finally, (c), the physical limits of the seal material have been exceeded.

Fig. 3. Standard PolyPak in modified urethane such as Molythane can be used at pressures to 5,000 psi. Higher modulus elastalloys such as PolyMyte, in the basic PolyPak configuration, operate successfully to 7,000 psi at moderate temperatures and standard tolerances.

Fig. 4., left, A standard PolyPak with a modular backup ring of elastalloy such as PolyMyte will seal successfully to 12,000 psi.

Fig. 5. A positively activated PolyPak of modified urethane such as Molythane with a backup ring of Nylatron can seal to 10,000 psi.

Fig. 6. At extreme pressures, a metal anti-extrusion device of ductile bronze or brass and a high-strength, high-durometer modular backup ring is required.

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, and

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.

Seal extrusion

The ability of a seal to resist extrusion into the gap depends on the interaction of these five factors:

• 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.

Material is the key

The key to high-pressure sealing is the use of a material or a combination of materials that has sufficient tear strength, hardness, and modulus to prevent extrusion through the gap. At pressures of 5,000 to 7,000 psi, the strongest elastomeric materials in standard seal configurations resist the extrusion without reinforcement. At higher pressures, the elastomeric sealing element must be backed by a higher modulus and harder material. Various more-or-less standard backup configurations have demonstrated their effectiveness over many years.

At pressures in excess of 20,000 psi, the extrusion gap must be closed and the elastomeric seal must be protected by a sequence of progressively harder, higher-modulus materials. Properly designed, this progression of materials prevents extrusion, tearing, cutting, or other destructive deformation of the elastomeric seal and distributes loads more uniformly to the element that bridges the gap.

Designs and materials

In high-pressure applications, material characteristics, such as high modulus, tear strength, self-lubrication, and abrasion resistance, become increasingly important. The following seal configurations and materials are specially suited to high-pressure applications. Although these examples cite proprietary compounds as typical, other manufacturers offer their own proprietary compounds, which generally have similar properties.

Abrasion-resistant and self-lubricating materials should be used at high pressures because friction increases there. Some of these materials are:

Enhanced polyurethane - At the lower end (5,000 psi) of the high-pressure continuum, a standard PolyPak configuration of modified polyure-thane energized by a resilient O-ring elastomer, Figure 3, is sufficient. Polyurethane-based materials - such as Molythane (impregnated with molybdenum disulfide to provide dry lubrication plus good compatibility with lubricating properties of working fluids) - are suitable for application pressures up to 5,000 psi without backups.¹ Molythane comes in a 90-Shore A durometer formulation for PolyPak seals and in a 65-Shore D durometer formulation with a higher modulus for increased extrusion resistance for anti-extrusion devices. Ultrathane K-24, a high-tensile, reduced-friction, enhanced-urethane material also is suitable for applications to 5,000 psi without reinforcement.

Elastalloy co-polymers - Various elastoplastic or elastalloy copolymers, such as PolyMyte - a material with exceptionally high tear strength, abrasion resistance, hardness (Shore D 65), and modulus - offer high pressure performance capabilities. PolyMyte configured as PolyPak and energized by a resilient elastomeric O-ring is suitable for applications up to 7,000 psi without backups.² A high-durometer PolyMyte modular backup, Figure 4, used in conduction with a Molythane PolyPak, can withstand pressures up to 12,000 psi or more.³

Non-elastomeric materials. Non-elastomers include polyamide resins such as nylons and modified nylons and metal backup rings, typically ductile bronze or brass.

One non-elastomer is Nylatron, a glass-filled polyamide resin. A Molythane PolyPak with a positively actuated Nylatron backup ring inserted to bridge the extrusion gap, Figure 5, can be used successful at pressures to 10,000 psi.

For extreme pressures in one direction, a three-part sealing system, Figure 6, is recommended. The seal is made of a Type B PolyPak, backed by a filled-polyamide modular backup beveled at 30°. A wedge-shaped, skive-cut split-ring, machined from ductile bronze or brass, is placed behind the beveled modular backup. The metal backup and seal groove are mated at a 45° angle. Under pressure, the wedge-shaped metal ring expands to close the extrusion gap. This design has operated successfully at pressures to 100,000 psi in a specialized application for making synthetic diamonds.

Compressed by the elastomeric urethane PolyPak, this elastoplastic modular backup expands radially to fill the groove and prevent sealing-element extrusion. Without an additional anti-extrusion device, the elastoplastic modular backup would experience plastic flow into the gap at 100,000 psi. A softer, lower tear-strength urethane back-up element would be nibbled or cut by the metal backup ring especially where the metal ring is split.

These proven designs and materials are typical of those available to increase the pressure capabilities of elastomeric seals in dynamic applications.

Many other materials can be suitable for high-pressure applications. Often, the choice of seal materials is dictated by the fluid medium, system operating temperatures, cost, or system pressure. The potentially higher efficiency of high-pressure systems comes at a slight cost premium. Sealing materials for high pressures are more expensive, and seal designs often are more complicated. Higher sealing pressures increase sealing force and friction. Increased friction causes higher wear rates and may require more frequent seal replacement, but frictional force and wear rates typically increase more slowly than pressure.

Hydraulic system design today often seems to focus on dramatic high-pressure applications. For example, the aerospace industry is presently evaluating 8,000-psi systems for future aircraft in special test beds, such as Lockheed-Georgia's HTTB. Many successful high-pressure systems incorporate innovative seal designs in both static and dynamic modes of operation.

¹ Service recommendations based on test conditions of 100,000 pressure cycles at 60 cycles/min from zero to 5,000 psi at 160°F (71°C) with maximum 0.010-in. diametral and 0.005-in. radial clearances.

² Service recommendations based on test conditions of 100,000 pressure cycles at 60 cycles/min from zero to 7,000 psi at 160°F (71°C) with maximum 0.010-in. diametral and 0.005-in. radial clearances.

³ The standard PolyPak fits a gland width equal to nominal depth (that is, square). The modular backup also is square; it occupies a space identical in size to the PolyPak it backs up.