Compatability of hydraulic system materials

摘要

Two oil aging procedures have been employed in this work. In the first procedure, the O-ring seals are suspended in the oil in a "free state" and approximate oil aging of rubber required by several military specifications. Because the whole surface area of the seal is exposed to the oil, maximum changes in properties occur. In a second procedure, the O-rings are oil aged simultaneously under stress and compression by means of a steel jig. This procedure provides a reasonably simulated, static rubber-metal-oil environment of the hydraulic system. Because the seals are contained in the metal jig, under stress and compression, less surface area of the seal is exposed to the oil. Consequently, the changes in seal properties (tensile, elongation, hardness, and rubber swell) are less, depending upon the combined effects of the metal and oil, and considered much more realistic than the changes provided by the first procedure. In addition, since the hydraulic materials (rubber-metal-oil) are aged in contact and simultaneously, changes in the metal and oil can be determined in addition to seal properties after aging under stress and compression. With minor modification of the jig, including dimensional changes to allow better oil circulation, the second procedure will continue to be used in future screening of hydraulic materials at higher temperatures and inert atmospheric pressures. The thermal stability of MIL-H-83282 hydraulic fluid precludes its use in hydraulic systems above 450°F. MIL-H-27601, a highly refined, deeply dewaxed mineral oil, is not stable in an oxidative environment above 350°F, but is the only hydraulic oil known to possess thermal stability within viscosity limitations in an inert atmosphere at temperatures up to 600°F. Therefore, at 400°F and above, an inert atmosphere will be required to maintain current material stability in hydraulic systems. The fluorocarbons are the only elastomeric seals known to be compatible with MIL-H-83282 and MIL-H-27601 hydraulic fluids at temperatures above 350°F. After 72 hours at 400°F, the fluorocarbon seals age comparatively with NBR seals at 275°F. However, the known fluorocarbons available do not remain flexible at -65°F and sealing is precluded under dynamic situations. An experimental fluorocarbon, FR-D type, has been evaluated which has a sealing ability at -40°F to 400°F and above. Research is being conducted for modified fluorocarbons which will hopefully provide, in the near future, sealing capability from -65°F to above 4OO°F. Efforts will continue to alleviate problem areas in order to develop a satisfactory hydraulic package for high temperature hydraulic systems.