The worldwide industries that design equipment incorporating hydraulic and pneumatic technology have changed considerably over the last 30 years — largely in response to the increased expectations of the end user. From the standpoint of sealing, these expectations now call for effectively leak-free systems, regardless of the application.
Whereas two decades ago almost all leading OEM's around the world had their own acceptability curves which aspiring suppliers had either to meet or beat, today their approval procedures simply state that zero leakage is the standard. Much of the credit for this situation lies at the door of the Japanese; not so much for any innovative design but for their attention to detail, and their elevation of the market perception of quality. Part of this, of course, demanded leak-free systems.
Europe in the 1970s responded to the export drives of the large Japanese offhighway equipment manufacturers with tough new quality standards, plus manufacturing, design, and sourcing reviews. One result of these reviews was a move toward higher system pressures to increase machine output. Typical European off-highway equipment now operates between 5000 and 8000 psi. Other sectors followed this trend, and today we see 5000-psi and higher-pressure hydraulic systems in many different industries.
To meet these challenges, leading international seal manufacturers have modified existing materials and developed new ones. These materials enable seals to be made today in profiles and configurations unheard of 30 years ago. Modern hydraulic and pneumatic systems often use the seal materials listed in the chart at right.
TPU and TPE
The greatest strides have been made in the thermoplastic polyurethanes (TPUs). The major limitations of the first-generation TPUs — high lip preload loss (particularly at elevated temperatures, say above 160° F) and poor resistance to water and high humidity — have been overcome. Second-generation TPUs are now available that take the system operating range up to 250° F without suffering serious loss of lip preload, and generally do not require O-ring energization. Hydrolysis (see box, page 33) resistance in some formulations is now so good that TPU seals are used in underground-mining cylinders that operate on highwaterbased, fire-resistant fluids.
Pneumatic cylinder designers also have benefitted from the advances in TPU sealing. Calls for very low friction and ultra-long service life have been accommodated by TPU seals which offer half of nitrile's breakout friction and have lasted for 12 10 6 cycles in 2-in. bore, 10-in. stroke cylinders with non-lubricated air.
Modern thermoplastic elastomers (TPE) have also improved. It is possible to chemically engineer TPEs to produce such desirable properties as outstanding wear and fluid resistance. These characteristics have made them a first choice in many sealing applications — particularly as piston seals where, with suitable energization, extremely efficient performance can be produced. Many of these TPE seals compete with PTFE elements where the elastomeric nature of TPE makes them easier to install and also prevents piston drift. An example is in truck-mounted crane outriggers, where the elastomer can bond into the adjacent surface finish. TPEs with their superior wear resistance and tensile strength are ideal for this use.
In Europe, TPEs have a growing importance in specialty sealing applications, such as the mining and steel industries. TPE's heat and fluid resistance perform well in rolling mills, for instance. For port-passing applications, such as phasing cylinders, by exploiting the wear resistance and hardness of TPE, seals can be designed specifically to overcome problems often associated with this type of cylinder design.
The key to success in today's industry for the seal maker lies in combining the latest material technology with innovative profiles to provide the customer with solutions which work.
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 any gap. At pressures of 5000 to 7000 psi, the strongest elastomeric materials in standard seal configurations resist extrusion without reinforcement. At higher pressures, elastomeric sealing elements 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 exceeding 20,000 psi, the extrusion gap must be closed and the elastomeric seal must be protected by a sequence of progressively harder, highermodulus 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.
Abrasion-resistant and self-lubricating materials should be used at high pressures because friction increases with pressure. Some of these materials are:
Enhanced polyurethane — At the lower end (5000 psi) of the high-pressure continuum, a standard packaged configuration of modified polyurethane energized by a resilient O-ring elastomer 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 to 5000 psi without backups. Molythane comes in a 90-Shore A durometer formulation for packaged 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 5000 psi without reinforcement.
Elastalloy co-polymers — Various elastoplastic or elastalloy copolymers offer high pressure performance capabilities. These ackaged materials, energized by a resilient elastomeric O-ring are suitable for applications to 7000 psi without backups.
PTFE composites — PTFE has the lowest coefficient of friction of all solid materials, but is relatively soft and has poor extrusion resistance. When reinforcing fillers — such as bronze, glass, graphite, or other polymers — are combined with PTFE, however, the composite will have very good extrusion resistance and wear without compromising its low friction characteristics. In addition, PTFE's high and low temperature resistance makes it, in some cases, the only choice for extreme pressure/temperature conditions. PTFE seals are generally used with elastomeric or metal spring energizers of various materials and designs.
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 packaged with a positively actuated Nylatron backup ring inserted to bridge the extrusion gap can be used successful at pressures to 10,000 psi.
For extreme pressures in one direction, a three-part sealing system is recommended. The seal is made of a Type B packaged material, 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 packaged material, 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. Seal 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.
As environmental issues continue to influence almost all industries, the hydraulics sector will be no exception. In Europe and the U.S., so-called environmentally friendly fluids are being developed. Vegetable oils, such as rape and sunflower seed, have been tried, but they can cause problems for the system (forming resin above 180° F) and for the seals and other components (forming acid in any water present that can attack elastomers).
New materials and blends will be required to combat the effects of these fluids while still providing the sealing integrity users expect. Preliminary work indicates that there is a long road ahead if this issue becomes a reality.
Commonly used seal materials
|Nitrile||Fluid power cylinders||Inexpensive; good resistance to set||Not tough enough to withstand very smooth surface finishes (<0.4 min. CLA)|
|Carboxylated nitrile||Fluid power cylinders||Better wear resistance than nitrile||Limited low-temperature flexibility, compared to standard nitrile|
|EPDM||Exposure to fire-resistant fluids||Resistant to HFD fluids and Skydrol||Not resistant to mineral oils, greases, other hydrocarbons|
|Fluoroelastomer||High temperatures (to 400°F)||Resistant to most hydraulic fluids||Relatively expensive and difficult to process|
|PTFE||General sealing||Low friction, good chemical resistance||Not elastomeric, requires energization|
|Polyurethane||General sealing elements||Good wear resistance and resistance to set—energization not required||First generation subject to hydrolysis effects of water above 120°F|
Rubbing faces of seals
|Elastomeric; good resistance to wear and fluids||Poor resistance to set; requires energization|
Basic properties of elastomeric seal compounds
Although elastomeric compounds used in aerospace seals are derived from relatively few base polymers (such as nitrile, fluoroelastomer, and ethylene propylene), each seal manufacturer usually develops special compounds of these base polymers to enhance or suppress different chemical or physical properties to fit specific requirements of an application.
Proprietary formulations of these compounds are kept secret. Even the analysis of a finished elastomer seal presents an incomplete picture of the original elastomer compound because some ingredients are consumed in processing.
Of all compound properties, the most critical are the changes that occur. Every property of every compound changes with age, temperature, fluid, pressure, squeeze, and other factors. Standardized tests have been developed to provide comparability in changes among compounds. Compounds with the least tendency to change properties are the easiest to work with; they produce a seal that is adaptable to more applications.
The number of properties evaluated for an application depends on the severity of conditions. Factors are highly interdependent, but typically include resilience and memory, abrasion resistance, coefficient of friction, and fluid compatibility. Let's take a closer look at each of these.
Resilience and memory are defined as a compound's ability to return to original shape and dimensions after a deforming force is removed. Resilience implies a rapid return, while memory implies a slow return. In seals, resilience is important because it permits a dynamic seal to follow variations in the sealing surface. Although elastomer resilience is frequently measured on a Bashore resiliometer, field experience is required to relate ratings to seal performance. Additional attention is required for low-temperature applications. When temperature is too low, a compound loses its memory.
Abrasion resistance — resistance to wear when in contact with a moving surface — is the product of other properties, including resilience, hardness, thermal stability, fluid compatibility, and tear/cut resistance. It also is influenced by the compound's ability to hold a film of protective lubricant on its surface.
Harder compounds are usually more resistant to wear, so dynamic seals of 85-durometer compounds are common. However, if the seals encounter high temperatures, it may be good practice to specify an even harder material to compensate for the softening effect of heat. In lowtemperatureapplications, a softer material might be preferred because elastomers tend to harden as temperature decreases.
Coefficient of friction (usually only important in dynamic seals) is compound-specific and different for running ( force to maintain a body in motion sliding across a surface) and break-out (force required to start a body in motion across a surface) friction. Usually break-out friction is higher. Breakout friction increases with time between cycles.
Coefficient of friction is affected by temperature, lubrication, and surface finish. Aging and the influence of service fluids on the compounds may also affect hardness and, in return, both breakout and running friction.
As far as fluid compatibility is concerned, a fluid is considered incompatible with a compound if the fluid causes enough property changes to reduce sealing function and/or shorten the working life of the compound. Dissimilar chemical structure is the key to fluid compatibility. For non-polar liquids — such as hydrocarbon fuels and oils — nitriles, fluorocarbons, or fluorosilicone polymers are normally used. For polar liquids, such as phosphate ester hydraulic fluids, ethylene propylene compounds are most satisfactory.
Urethane and vegetable oils
The properties of urethane have made it a popular material for a broad range of hydraulicsealing 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 waterbased or water-mixed fluids) in common hydraulic applications — which typically run at temperatures high enough to precipitate hydrolysis.
Tim McCulfor, material development specialist at Busak+ Shamban Americas, Fort Wayne, Ind, contributed to this article, much of which was excerpted from the Fluid Power Handbook & Directory. Since 2009, Busak + Shamban has been part of Trelleborg Sealing Soulutions, a leading supplier of hydraulic seals, rotary shaft seals, o-rings, static seals, gaskets, oil seals and pneumatic seals. For more information, visit their website.