What is in this article?:
Successful sealing involves containment of fluid within fluid power systems and components while excluding contaminants.
The right material is essential
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 5000 to 7000 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. 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 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. 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.
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.
|Table 1: Average dynamic and static squeeze levels - in.|
| Seal |
|Gland packing space||Squeeze or compression||Percent compression|
Urethane and vegetable oils
The properties of urethane have made it a popular material for a broad range of hydraulic-sealing applications. However, one negative factor is its susceptibility to hydrolysis. As urethanes are produced, water is the byproduct of the chemical reaction. If water is re-introduced to urethanes later at a temperature high enough (generally 140° F) to cause a second chemical reaction, polymer bonds are broken and the urethane begins to deteriorate. The material hardens and then flakes apart. This phenomenon is known as hydrolysis. If a urethane seal is exposed to ambient water - and particularly hot water or steam - for extended periods, the seal may disintegrate completely.Many vegetable oils have an inherent property of water absorption. If such oils are installed in hydraulic systems, their water component introduces a fluid mixture which jeopardizes seal performance. This phenomenon prohibits the use of conventional urethane seals with vegetable oils (as well as water-based or water-mixed fluids) in common hydraulic applications - which typically run at temperatures high enough to precipitate hydrolysis.