What’s Up Doug: Preserving Assembled Bearings
Recently, I was asked was what to do with bearings after they have been installed into a machine. In context, users will rebuild an asset, such as a pump or a motor, and then store it until needed. The time of storage of such assets could be two or three years.
The Question Is, Do We Preserve the Assembled Bearings?
The question refers mostly to open bearings in applications that are oil lubricated. Most often, these are process pumps that have an oil bath and are stored without oil; however, the discussion can apply to any oil-lubricated bearings put into storage “dry.”
Note: Damage due to vibration or shock loading during storage will not be addressed, as this is another topic. However, it is a critical factor when storing rebuilt assets.
There is very little documentation to provide a reasonable answer to assembled, long-term bearing storage from a bearing manufacturer. As soon as the bearing is removed from the box, it is considered the responsibility of the user. As soon as it is assembled into the asset, the bearing is expected to be lubricated and run. Unfortunately, this is not the case in real life, and it is up to the user to take action to protect the assembled bearing.
Some Basic Principles:
• Prevent moisture corrosion to the bearing;
• Ensure the coating or preservative you put on the bearing is compatible with lubricant that will be used to lubricate the bearings;
• Ensure the preservative will not harden over time, as this will affect the startup of the machine.
Over the years, the preservatives put on bearings have evolved. Years ago, a bearing was coated with a thixotropic fluid, which is thick or viscous under static conditions, then will flow over time when shaken, agitated, shear stressed, or otherwise stressed. Once coated, the bearings (typically larger ones) are wrapped tightly with an oil paper (VCI paper) then again with a polymer tape.
Although designed to be displaced by “working” (i.e., when the bearing starts to rotate), the material—due to storage for an extended period or exposure to the environment—can harden and may need to be washed off.
More recently, the preservative used by bearing manufacturers is a vapour phase inhibitor, which is very thin and is constantly evaporating and re-condensing on the metal surfaces such that the surfaces are constantly being re-coated. The downside to such a preservative is that if the bag in which that bearing is sealed is broken and exposed to the atmosphere, the preservative will evaporate into the atmosphere and deplete itself. As such, bearing manufacturers will not accept returns in which the plastic wrap of a bearing has been opened.
Recently, I had an experience with a customer who was greasing their bearings with a standard lithium/mineral oil grease. Although there was no documentation of the process and product used, and there was no startup or premature failure issues, there was; however, an incident in which, the oil in the sight glass of a newly installed pump was a milky colour. At first, it was thought there was water emulsified in the oil. However, upon inspection of the “assembly” grease ad hoc procedure, the error was discovered.
What is the right choice? A thixotropic fluid coats well, but there is a long-term risk of hardening as the solvent evaporates. A vapour phase inhibitor does not require any flushing or cleaning, but if the machine is not airtight, there may be the risk of the loss of protection.
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