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Robustness and Durability as Measures of REC Gasket Effectiveness
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Home » Products » Rubber-Edged Composite » Select-a-Seal » Validation of the Technology » Robustness and Durability as Measures of REC Gasket Effectiveness
| We use the term Robustness to describe a seal’s ability to perform acceptably in a wide range of circumstances, and the term Durability as the measure of how long a product seals without leaking under a certain set of expected-use conditions. Gasket designs with a high level of robustness will produce durability that is largely insensitive to widely varying initial joint conditions. These properties are measured in the lab using accelerated test methods such as thermal cycling components in an environmental chamber with fluid pressure in the joint. Durability at each test condition is measured as time-to-failure. Robustness is measured by, and inversely related to, the loss of durability that results from increasing the severity of the joint parameters. Some parameters that were changed to represent variations found in production joint designs are bolt torque, flange stiffness, covers and bases of the same or different metals, and internal fluid pressure. These effects are shown in the case studies that follow. |
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| The REC gasket design parameter, volume or size of the rubber sealing edge relative to the thickness of the compressible carrier, was also varied to evaluate its effect on durability in a given joint configuration. Figure 1 shows a plot of the durability test data for a particular application. An REC gasket essentially has a “working range” between a lower “seal limit” and an upper “crush limit”, in which it exhibits full durability, as defined by a 5000 hour thermal cycling test using –40C to 150C cycles and constant fluid exposure. Below the seal limit, the sealing edge is too small to generate adequate compression stress or an adequate adhesive bond. The reduction in durability performance below |
Figure 1.
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| the seal limit is very rapid. Above the crush limit, the rubber may crack and degrade as a seal. The reduction in performance above the crush limit isn’t as dramatic, as the rubber cracks in its thickest section and leaves roughly half of the edge still attached to the composite carrier and functioning as a seal. When a seal is designed and used within its working range, its performance is insensitive to a fairly wide range of edge sizes. In this particular sample case, on a 0.8mm carrier thickness, the seal limit was found to be at a 1.0mm rubber edge height, and the crush limit was found to be at 1.7mm. This wide working range simplifies the requirements of the gasket manufacturing process. |
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Comparative Case Studies
A series of accelerated benchmarking tests were run using production hardware and various gasket styles, to illustrate the Robustness and Durability aspects of those seals compared to REC gaskets. Seal types that were tested include REM and RCM. Two cases are presented here that clearly illustrate the difference between durability and robustness.
Figures 2 and 3 illustrate a case where eight joint configurations were developed by varying the material type and thickness of the cover flange, and the bolt torque. The benchmark gasket is an REM construction, using an AEM edge seal on a 2.4mm aluminum carrier. This joint is used to seal lube oil at pump pressure (0.35 MPa and up to 150C) between two machined aluminum components. The joint has three M6 bolts that are nominally tensioned with 11Nm torque. The REC test gasket uses an ACM edge seal on a 0.8mm composite carrier. The production application for which the REM gasket was designed, uses 12mm thick cast-aluminum flanges. The flanges were varied to 6mm aluminum, and both 5mm and 10mm steel, to introduce variation in joint stiffness. The steel flanges, because they are bolted to aluminum components, become dynamic in thermal cycling tests due to the mismatch of thermal expansion between steel and aluminum. Bolt torque values of 7Nm and 14Nm were used.
Thermal cycle testing was run in an environmental chamber. A continuous, static (non-circulating) oil pressure of 0.35MPa was maintained while the temperature cycled between –40°C and +150°C in four-hour cycles. In these tests, the REM gasket developed about 4.8MPa average initial stress on its AEM edge bead. The REC gasket developed 0.35MPa (minimum) on the composite carrier and 1.7MPa average stress on the ACM edge. Figure 2 shows that the REM gaskets all developed leaks between 800 and 900 hours. This result means that the REM sealing solution’s robustness is very good, with very little sensitivity to design variations, however the durability is not good compared to typical industry testing requirements of 1000 to 2000 test hours. Figure 3 shows that the REC gaskets demonstrated very good durability, with all 8 REC joints remaining sealed until test termination at 2000 hours. The robustness of these REC gaskets was not fully investigated since the durability target of this test series was met.
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Figure 2.
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Figure 3.
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Figures 4 and 5 illustrate a second case, benchmarking an RCM gasket in a water pump application. The production gasket is embossed RCM with an NBR rubber coating. This is the RCM type mentioned earlier that does not use compression limiters, but supports bolt load on the thin rubber layer. The REC gasket tested beside it is an SBR rubber sealing edge on a 0.8mm fiber carrier.
The water pump application for which the RCM gasket was designed, uses a 6mm machined cast aluminum flange and a rigid machined aluminum base. Using seven M8 bolts tensioned to 25 Nm nominal torque, the joint constantly experiences 0.20MPa of internal antifreeze pressure and a maximum temperature of +125°C. In order to open the design space and test robustness, cover flanges were made of both 6mm and 12mm aluminum, and also 10mm steel. The bolts were tensioned to 8 and 25 Nm. The steel flange cases represent dynamic joints in thermal cycling due to thermal expansion differences. |
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Figure 4.
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Figure 5.
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A continuous, static (non-circulating) pressure of 0.20 MPa was maintained in these joints while the temperature cycled between –40oC and +125oC in four-hour cycles. In these tests, the RCM gasket developed about 20.7MPa of initial sealing stress on the embossed areas. The REC gasket developed about 0.35MPa (minimum) on the carrier and 1.7MPa on the SBR edge. As Figure 4 shows, the RCM gasket has very poor robustness within these design variations. While these robustness tests may be outside the RCM gasket’s range of intended use, the gasket was, nonetheless, very sensitive to the variations. In some cases, leaking began 100 hours into the test. The REC gasket showed some lack of robustness in the low-torque (8 Nm) tests, but most joints stayed sealed until test termination at 3,000 hours. This demonstrates reasonable robustness and good durability.
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