Basic O-Ring Elastomers

2-26 means of approximating the low temperature capability of an elastomer compression seal. The low temperature sealing limit is generally about 8°C (15°F) below the TR-10 value. This is the formula that has been used, with a few exceptions, to establish the recommended low temperature limits for Parker Seal Group compounds shown in Figure 2-25 and the Fluid Compatibility Tables in Section VII. This is the lowest temperature normally recommended for static seals. In dynamic use, or in static applications with pulsing pressure, sealing may not be accomplished below the TR-10 temperature, or approximately 8°C (15°F) higher than the low-limit recommendation in the Parker Handbook. These recommendations are based on Parker tests. Some manufacturers use a less conservative method to arrive at low temperature recommendations, but similar compounds with the same TR-10 temperature would be expected to have the same actual low temperature limit regardless of catalog recommendations. A few degrees may sometimes be gained by increasing the squeeze on the O-ring section, while insuffi cient squeeze may cause O-ring leakage before the recommended low temperature limit is reached. The low temperature limit on an O-ring seal may be compromised if the seal is previously exposed to extra high temperature or a fl uid that causes it to take a set, or to a fl uid that causes the seal compound to shrink. Conversely, the limit may be lowered signifi cantly if the fl uid swells the compound. See Figure 2-26. Retraction % TR Test According to ASTM-D 1329/ISO S 2921 for a NBR 70 Shore A Compound 100 90 80 70 60 50 40 30 20 10 °C °F -60 -76 -50 -58 -40 -40 -30 -22 -20 -4 -10 -14 Temperature Test results: TR 10 = -31.5°C (-25°F) TR 50 = -24.0°C (-11°F) TR = -20.0°C (-4°F) 70 0 32 10 50 Figure 2-26: TR Test According to ASTM-D 1329/ISO S2921 for a NBR 70 Shore A Compound < Back Section Contents Parker O-Ring Handbook 20 68 Table of Contents Search With decreasing temperature, elastomers shrink approximately ten times as much as surrounding metal parts. In a rod type assembly, whether static or dynamic, this effect causes the sealing element to hug the rod more fi rmly as the temperature goes down. Therefore, an O-ring may seal below the recommended low temperature limit when used as a rod type seal. When excessive side loads are encountered on maximum tolerance rods or glands, and the pressure is in the low range, leakage may occur at temperatures 5°or 8°C (10°or 15°F) above the TR-10 value. It may be necessary to add as much as 22°C (40°F) to the low temperature shown in the tables for this type of service. See Figure 2-27. 2.13.3 Time The three obvious ?dimensions? in sealing are fl uid, temperature, and pressure. The fourth dimension, equally important, but easily overlooked, is time. Up to this point, temperature limits, both high and low, have been published at conventional short-term test temperatures. These have little bearing on actual long-term service of the seal in either static or dynamic applications. A comparison of the temperature limits of individual compounds in this guide with previous literature will reveal that for comparable materials the upper temperature limit is more conservatively expressed. The narrower temperature range does not imply that the compounds discussed are inferior to others. Rather, those high temperature values based on continuous seal reliability for 1,000 hours are being recommended. As illustrated by the graph (Figure 2-28), short term or intermittent service at higher temperatures can be handled by these materials. For example, an industrial nitrile (Buna-N) compound, N0674- 70, is recommended to only 121°C (250°F), yet it is known to seal satisfactorily for fi ve minutes at 538°C (1,000°F) and at 149°C (300°F) for 300 hours. Therefore, when the application requires a temperature higher than that recommended in the compound and fl uid tables, check the temperature curve to determine if the total accumulated time at high temperature is within the maximum allowable limit. The sealing ability of a compound deteriorates with total accumulated time at temperature. The curves show the safe, cumulative time at a given temperature for specifi c elastomers used as static seals. For dynamic seal applications, temperatures as much as 14°C (25°F) below those indicated may be more realistic. 2.13.4 Pressure The system operating pressure is always a consideration as it effects the choice of seal materials in several ways. First is hardness, as may be required to resist extrusion in dynamic designs or where there is a large gap between sealed members in static applications. Second is at-rest vs operating conditions and requirements for ?leakless? at rest conditions which would suggest due consideration be given to the long-term compression set properties of a given material. Parker Hannifi n Corporation ? O-Ring Division 2360 Palumbo Drive, Lexington, KY 40509 Phone: (859) 269-2351 Fax: (859) 335-5128 Next >

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