HEAT PROCESSING NEWS
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Vacuum Furnace Maintenance Tips |
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Vacuum Fundamentals by Rich Roberts: O-Ring Maintenance |
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Of the types of seals used on vacuum chambers, the lowly O-ring is one of the most prolific, and arguably the least understood. The most common misconception is the way in which an O-ring creates its seal. The common belief is that the seal is created by compression on the O-ring. The fact is that the seal is created by the O-ring being pushed laterally in the groove by pressure on one side and thus being forced into the gap at the edge of the groove. The only compressive force that is necessary for most vacuum applications is to make sure that the O-ring has full contact with the mating surface. This misunderstanding can result in a seal failure. Often penetrations through the shell, especially for rotating shafts and some power feeds, use O-rings to provide the seal. These O-rings are usually provided with some sort of device to apply an axial force to it to bulge its cross section slightly to seal it against the shaft on its ID and the interior surface of the feed through casing on its OD. The force is usually applied by some sort of threaded collar. The problem arises when someone over-tightens the collar, thinking they are improving the seal. Normally, though, over-tightening only extrudes or tears the O-ring, aggravating the situation or actually causing a leak where there was none. The next step is generally to get a bigger wrench and try again. The process usually ends in total failure of the feed through internals. Leaks of this kind are best corrected by replacing the O-ring, or in a pinch, removing, cleaning and re-lubricating it. On most furnace applications, the seals must be lubricated to allow them to move against the flange surfaces. We recommend a silicone-based high vacuum grease. Large seals that are separated often such as door seals should be wiped down and re-lubricated frequently; each time they are opened, if possible. Remember, the grease is a lubricant. Adding more will not improve sealing. We've seen door seals that have been smeared with huge globs of grease. That will do more harm than good. The idea is to apply enough to just make the surface of the seal shiny. Another problem that occurs with seals such as those on chamber doors is that the flanges are exposed, and can get scratched. A surprisingly small scratch on the mating surface will cause a leak if it crosses the seal area. These scratches can easily be removed with emery cloth or similar abrasive. When troubleshooting a vacuum leak, always inspect the surface in contact with the seal for defects, and correct them. O-rings should also be replaced when the material gets hard or misshapen due to age or overheating. If this is happening too often, you might consider an upgrade to a higher temperature material, or a material that is more resistant to the chemicals in your process. Rich Roberts is a Project Engineer with the USA Vacuum Team. The latest design, materials and equipment specifications should be obtained from the company before any reliance is placed on the enclosed since changes may occur due to product improvement.
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