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.
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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.
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.
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 to 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.
Within the framework of low-pressure sealing, several primary design considerations affect sealability:
• seal squeeze,
• compression set,
• sealing force,
• gland surface finish conditions, and
• molding flash.
A seal component is generally installed in a groove machined into one of the surfaces to be sealed. As the two surfaces are brought together to form a gland, they squeeze the diametral cross section of the seal. The mechanical squeezing action deforms the seal cross section; the degree of deformation obviously is a function of the squeezing force. In low-pressure applications, the tendency of the squeezed elastomer to maintain its original shape creates a seal. As the elastomer shape is deformed in its gland, it exerts a counteracting (reaction) force against the mating surfaces equal to the force squeezing it, Figure 7, and hence, provides the available sealing force.
Thus, squeeze is a major low-pressure consideration. The recommended squeeze levels are a function of seal cross section, the application conditions and whether the application is dynamic or static.Dynamic compression typically is lower than static compression, due to seal wear and friction considerations. Table 1 summarizes dynamic squeeze levels as defined by MIL-G-F — a document that serves as a good guide to those parameters. Static data in the table are summarized from common industrial practice.
Compression set reflects the partial loss of memory due to the time effect. In hydraulic systems operating over extreme temperature ranges, it is not uncommon for compression-type seals, such as O-rings, to leak fluid at low pressure because they have deformed permanently or taken a set after used for a period of time. The term compression set refers to the permanent deflection remaining in the seal after complete release of a squeezing load while exposed to a particular temperature level. As related to low-pressure sealing, set-the loss of memory-reduces the compressive sealing force.
Compression set is expressed as a fraction of the initial squeeze. Thus, a 0% compression set value indicates complete recovery from a compressive load, producing the maximum possible compressive sealing force. A 100% set value indicates no recovery or rebound at all. A seal in this condition will no longer provide a sealing force and hence, has no ability to act as a low-pressure seal. The bar graph in Figure 8 depicts the range of typical compression set values for various sealing elastomers. Of course, compression set properties are a major but not the only factor affecting elastomer choice for low-pressure sealing. Compatibility with various hydraulic fluids must be considered as well.
It generally is recommended that a minimum of 0.009-in. squeeze be induced on radial seal cross sections due to compression set considerations. Maximum radial squeeze should be held to 30% because greater squeeze causes assembly difficulties and elastomer deterioration. Compressive sealing load is also directly related to the size of seal's cross-section, Figure 9.
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.
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.
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.
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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 DynamicThe 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.
Most leading OEMs around the world had once their own acceptability curves which aspiring suppliers had either to meet or beat. Today, however, approval procedures simply state that zero leakage is the standard. Much of the credit for this situation lies with the market perception of quality, which, of course, demands leak-free systems.
Europe in the s 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 virtually any profile and configuration. Modern hydraulic and pneumatic systems commonly use the seal materials listed in the table at right below.
The greatest strides in seal materials have been made with thermoplastic polyurethanes (TPUs). Initial limitations of the early TPUs have been overcome. Current TPUs can tolerate system operating temperature 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 TPUmaterials. 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 × 106 cycles in 2-in. bore, 10-in. stroke cylinders with non-lubricated air.
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.
Table 3 Material Applications Positive factors Precautions 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 energizationCast polyurethane gaskets seal tightly under pressure and hold their shape under intense loads. FallLine produces high-performance gaskets and cast urethane seals for demanding industrial environments. We manufacture each part to meet exact specifications for fit, durability, and chemical resistance. Top-of-the-line engineers choose cast urethane seals for applications that demand repeat performance and minimal maintenance.
Both cast polyurethane gaskets and cast urethane seals block out contaminants, absorb impact, and maintain shape under mechanical stress.
These molded components fit tightly within dynamic systems where pressure, friction, and harsh chemicals threaten performance.
FallLine manufactures these high-performance components to exact tolerances for systems that cannot afford failure.
By working directly with engineers and procurement teams, FallLine delivers cast polyurethane gaskets and urethane seals that consistently perform in harsh environments
Some benefits of working with FallLine and our polyurethane gaskets include the following and so much more:
In-house mold design and casting for complete material traceability
Durometer and color customization for application-specific use
Tight tolerances for consistent sealing in high-precision assemblies
Support for prototypes, pre-production, and large production run manufacturing
REQUEST A QUOTE TODAYLet’s quickly take a look at how cast polyurethane gaskets compare to other traditional industrial materials:
PropertyCast Polyurethane GasketsRubber GasketsPlastic GasketsAbrasion ResistanceExcellentGoodFairTear StrengthHighModerateLowChemical ResistanceGoodVariableVariableTemperature Range-60°F to 180°F-40°F to 150°F32°F to 140°FElastic MemoryExcellentGoodPoorLoad-Bearing CapacityHighModerateLowOverall, cast polyurethane gaskets offer a balanced combination of flexibility, durability, and resistance to various environmental factors, making the material the preferred choice for the most demanding applications.
REQUEST A QUOTE TODAYFallLine’s focus on high-impact environments makes cast polyurethane gaskets and urethane seals essential for industries or equipment that operate in challenging operating environments. These parts withstand extreme temperature changes, mechanical stress, and repeated contact while maintaining shape and performance.
Some common applications of these seals and gaskets include:
Storage tank seals for various industries
Public works sector urethane gaskets and seals
Various material handling applications
Regardless of the industry or equipment, FallLine can provide custom-molded parts tailored to your specific needs.
Contact us today to request a quote for your custom order.
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