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"Is there a standard method for determining if and when a bearing should be cleaned and repacked?"
Given the practical challenges of hand-packing a bearing with any hope of maintaining grease and bearing cleanliness, it is often best to avoid opening a bearing to clean and repack.
Simply stated, the benefit practically exceeds the costs and risks. However, if circumstances exist where this must be done, then some thoughtful planning is justified.
We should look at the issue as a two-part problem. The first issue is if/when to repack a bearing. If a bearing is intended to be grease-lubricated, then it would be best to include a Zerk or similar fitting to enable replenishment without opening the bearing cavity.
When the bearing is replenished according to sound engineering principles (right product quality, right product selection, right amount, right frequency, no cross-contamination, etc.), the bearing does not require disassembly, cleaning and repacking to maintain lubricant and bearing health. If the bearing cannot be configured with a Zerk and a relief, and a “clean and repack” event is the only option, then scheduling the activity becomes a judgment call based on a multitude of factors as noted below.
The second part of the question is about whether there is a standardized approach for making this decision. After reviewing a few well-respected lubrication texts and speaking with two well-known bearing manufacturers, we conclude that there is no recognized standard with which to make a decision to disassemble and repack a bearing.
Several factors may influence the selected repack frequency decision, including the quality of the environment near the area where the work is to be done, the quality of the grease removed during previous repack events, the size of the housing, the extent to which spent grease has filled up available housing space, flexibility in configuring the housing for routine replenishment, flexibility in configuring the housing to “vent” spent grease during replenishment, evidence of bearing stress (rise in temperature, rise in high-frequency energy, etc.), ease of grease sampling and availability of grease analysis.
The use of acoustics or other high-frequency metrics can be most helpful in supporting the scheduled frequency decision if a manual repack must occur.