What is in this article?:
Successful sealing involves containment of fluid within fluid power systems and components while excluding contaminants.
Gland surface finishes
Two physical characteristics of the seal contact-band areas can affect how well the available sealing force is transmitted. These are:
- parting line projection and flash on the seal, and
- sealing surface finishes in the gland.
The finish on machined surfaces that come into contact with the seal is a significant factor in achieving optimum seal performance. Finishes can be defined by different systems, which are often misunderstood and sometimes incorrectly specified in hydraulic design. The American Standard Association provides a set of terms and symbols to define basic surface characteristics, such as profile, roughness, waviness, flaws, and lays.
Roughness is the most commonly specified characteristic and is usually expressed in units of µin. Roughness provides a measure of the deviation of the surface irregularities from an average plane through the surface. In most cases, geometric average roughness or root mean square (RMS) is the preferred method. RMS measurement is sensitive to occasional peaks and valleys over a given sample length.
As related to low-pressure sealing, the sealing element must penetrate these micro imperfections and irregularities in order to block the passage of the fluid media across the contact band area. It is generally accepted and recommended that dynamic interfaces should not exceed RMS values of 16 µin. or 0.4 µm. Static interfaces should not exceed RMS values of 32 µin. or 0.8 µm. Special fluid media would benefit from smoother finishes as listed in Table 2.
Parting line projections and flash
Just as there are irregularities in the form of roughness on the gland surface, there are irregularities or imperfections on the sealing element known as parting line projections and flash. A parting line projection is that continuous ridge of material along the line where the mold halves come together at the ID and/or OD of molded rubber seals, such as O- and T-rings. It results from worn or otherwise enlarged corner radii on the mold edges.
Flash is a thinner, film-like material that extrudes from the parting line projection. It is caused by mold separation when material is introduced or inadequate trimming or buffing after molding. Because flash lines are inevitable in clam-shell-type, compression molding processes, the degree of flash must be controlled. Control is especially critical in low-pressure applications and applications sealing gas-oil interfaces. Standards such as MIL-STD-413E and those in the Rubber Manufacturers Association (RMA) Handbooks provide guidelines on allowable flash criteria for manufacturers and users.
Sealing performance characteristics can be enhanced by eliminating the flash line completely from dynamic and static sealing interfaces. This practice is especially desirable in accumulator applications and those requiring low-viscosity fluid media, such as silicone oils. Manufacturers may offer an optional flash-free seal design for these stringent applications.
There are three primary static sealing methods in use today. The flat gasket is the oldest of the three. Where reusability is not required and where the possibility of some leakage can be tolerated, the flat gasket may be the best choice. The O-ring represents a marked improvement over the flat gasket for installations where little or no leakage can be tolerated.
The combination gasket/O-ring seal, Figure 10, represents a significant improvement over both the flat gasket and the O-Ring in a groove for near zero-leakage sealing in static applications. Advantages of the combination gasket/O-ring seal are:
- ease of installation,
- sealing element(s) molded precisely in place,
- limited area of seal exposed to fluid attack,
- visibly inspectable after assembly,
- no re-torquing required,
- high reliability, and
- no special machining of mating flange surface required (no grooves).
The combination gasket/O-ring seal consists of a retainer plate with a groove in one or both element(s). This seal may be either chemically bonded to the groove, Figure 7, and/or mechanically locked in place by cross-holes in the groove, Figure 11. The combination gasket/O-ring seals are relatively more expensive than O-rings.
FEA-assisted seal design
Vitally important to any method of sealing is the ability of the seal to achieve the proper balance between developing enough elastomer stress to provide an adequate seal and not developing too much stress, which would prematurely degrade the seal. Depending on the type and requirements of the seal, this seal/stress relationship will be different.
The study of elastomer stress and its relationship to seal effectiveness has been dramatically enhanced with the advent of Finite Element Analysis (FEA). FEA is a numerical modeling technique that has been used quite successfully for seal applications. FEA can predict seal deformed shapes and stress distributions after installation, in operation and under various conditions. This information is very important in evaluating the following: stability, sealability, thermal deformation, swelling, and seal life. FEA is becoming a very powerful tool for seal design optimization.
The procedure for FEA-assisted seal design can be summarized as follows:
- seal shape sketch,
- material selection,
- material characterization testing (such as tensile stress strain curve, bulk modulus, thermal constants, friction constants, etc.),
- material model selection (Mooney-Rivlin, Ogden, etc.),
- mesh modeling, boundary condition definition,
- numerical analysis,
- post-processing (output), and
- to see if the seal shape needs to be modified.
Figure 12 shows an example of an FEA plot. FEA is also used for flow and mold analyses, which are desired for elastomer processing control.
|Table 2: Surface finishes for special media|
|Fluid media|| Dynamic |
| Static |
| Cryogenic/low |
|4-8 in.||6-12 in.|
| Low viscosity |
fluid and gases
|6-12 in.||6-16 in.|
The worldwide industries that design equipment incorporating hydraulic and pneumatic technology have changed considerably over the last 20 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 off-highway 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 5,000 and 8,000 psi. Other sectors followed this trend, and today we see 5,000-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 20 years ago. Modern hydraulic and pneumatic systems commonly use the seal materials listed in the table at right below.
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 which 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 resistance in some formulations is now so good that TPU seals are used in underground-mining cylinders that operate on high-water-based, 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 50% of nitrile's breakout friction and have lasted for 12 X 106 cycles in 2-in. bore, 10-in. stroke cylinders with non-lubricated air.
Modern thermoplastic polyester 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 more easy 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.
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). Other fluid contenders include polyglycols and synthetic esters, but these also present problems - not the least of which is a cost up to ten times that of mineral oil. 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.
|Nitrile||Fluid power cylinders||Inexpensive; good resistance to set||Not tough enough to withstand very smooth surface finishes (<0.4 µin. CLA)|
|Carboxylated nitrile||Better wear resistance than nitrile||Limited low-temperature flexibility, compared with 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||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|
|Polyester||Rubbing faces of seals; Anti-extrusion elements||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 low-temperature applications, a softer material might be preferred because elastomers tend to harden a temperatures drop.
Coefficient of friction (usually only important in dynamic seals) is compound-specific and different for running and break-out. Usually break-out friction is higher. Break-out 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.
Sealing pipe-thread fittings
A review of the important performance properties of compounds of tetrafluoroethylene (TFE) resin and filler materials shows that the resin performs well in many applications without fillers. In fact, fillers can lessen TFE's outstanding electrical and chemical properties. In mechanical applications, however, compounds of TFE and inorganic fillers offer improved wear-resistance, reduced initial deformation and creep, and increased stiffness and thermal conductivity. Hardness is increased, and the coefficient of thermal expansion is decreased. These compounds can make it possible to gain the advantages of TFE in applications where the unfilled resin cannot be used.
Many different fillers can be blended with TFE, but most application requirements have been met with five filler materials: glass fiber, carbon, graphite, bronze, and molybdenum disulfide. The properties of any compound depend on filler type and concentration, and processing conditions. Compounds - such as plain TFE - are made into finished parts by molding, extrusion, or machining.
One example of the application of TFE resin and fillers is O-rings made of TFE. They are used where resistance to solvents and other chemicals, or extremely high- or low-temperature resistance is required. These are applications where elastomeric materials are not suitable. An additional benefit of TFE O-rings, in certain applications, is the material's low coefficient of friction and anti-stick properties. Typical applications are rotary, piston, and valve seals, and gaskets.
Four methods can seal pipe threads:
Trapped dope. The use of drying or non-drying dopes is the oldest and least costly thread-sealing method. Made from ingredients ranging from crushed walnut shells in shellac to other fillers and oils, usually with some thinning volatiles, they are inherently weak, and will shrink when the volatiles evaporate.
Yielding metal. The sealing interface is limited in area and unlimited in force so that yielding takes place. Metal flow fills misalignment and leak paths. These dryseal joints can be effective, but they usually cannot be disassembled and reused without leaking.
Trapped elastomer. Confined O-rings can seal effectively, but also can suffer from sloppy assembly. Damaged threads or pinched rings also can contribute to leakage. O-rings typically are used in high-pressure fluid power systems where the extra cost is more easily justified and freedom from contamination is especially desirable.
Curing resins. Sometimes called machinery adhesives, these anaerobic acrylic materials develop strength by curing. They are very forgiving of tolerances, tool marks, and slight misalignment. They make tapered fittings as effective as O-rings at a fraction of the cost. They lock free-standing fittings - such as gages. They can also improve the 98% effectiveness of yielding-metal joints to 100%. The correct grade must be selected because of their wide range of strengths so that disassembly will not be hampered.
Curing materials are so effective in sealing threads that they are often used on straight threads which enter or plug pressure vessels. In addition, the curing materials are effective even when tapered threads are lightly torqued. Lightly torqued threads (straight or tapered) do not leave high residual stresses in housings or valves that can distort valve bodies to the point of inoperation or long-term fatigue failure.
Design engineers must choose between these options to assure that equipment will function as planned. Specifications should not be left to assembly workers - as is often done.
There probably is little argument that the most significant event in sealing fittings during the past 25 years was the appearance of anaerobic pipe sealant with TFE materials. Since the first appearance of these materials, many other companies have added anaerobic thread sealants to their lines. The new sealant technology offers a variety of benefits:
Convenient curing. Being anaerobic, it cures in the absence of the air, remaining uncured until the parts are assembled. There is no evaporation, hardening beforehand, or other work-life problems.
Lubricity. Containing TFE filler, the material eliminates galling or other component-assembly problems. These products prevent over-torquing to affect a seal.
Fills threads. Due to high wetting ability, the material fills threads so well that leakage past nicks, scratches, and dents does not occur.
Fitting movement. Systems being assembled with anaerobic sealant can be initially readjusted without breaking the seal in the threads.
Vibration resistance. Anaerobic sealant does not permit a fitting to be loosened by vibration. Reusability. Fittings sealed with acrylic and latex-based materials can be disassembled and reused with sealant in the field without danger of leakage.
Freedom from contamination. Unlike the tape most often replaced by the anaerobic material, sealant does not break up to contaminate lines and valves.