What is in this article?:
- Seals and sealing technology
- The right material is essential
- Low-pressure considerations
- Parting line projections and flash
- Thermoplastic polyurethanes and polyester elastomers
Successful sealing involves containment of fluid within fluid power systems and components while excluding contaminants.
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.
Three primary static sealing methods are 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 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.
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 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 virtually any profile and configuration. Modern hydraulic and pneumatic systems commonly use the seal materials listed in the table at right below.