From The Practical Handbook of Machinery Lubrication

In general, the important parameters influencing bearing wear are contaminant particle size, concentration, hardness and lubricant film thickness. Increases in all of these parameters except film thickness will increase bearing wear.

Increasing lubricant viscosity will reduce bearing wear for a given contamination level.

Bearings operating in a contaminated lubricant exhibit a higher initial rate of wear than those not running in a contaminated lubricant. But with no further contaminant ingress, this wear rate quickly diminishes as the contamination particles are reduced in size as they pass through the bearing contact area during normal operation.

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Equipment that is run in an extremely dirty environment such as in a forge or foundry needs extra care to avoid contamination. We try to keep our equipment sealed and avoid unnecessary opening of the reservoirs. Install quick disconnects in your tanks and on your fill vessels for filling or topping-up hydraulic tanks. All our equipment has mainstream sampling taps installed for uptime or running sampling. This will result in a true indication of the condition of your oil, avoiding any bottom sampling. (Submitted by James Ott, maintenance coordinator, Patriot Forge Co.)

"We have worked through setting and improving our oil cleanliness, but our mill does not have a cleanliness standard for greases. Should we assume that the greases have the same concentration of solid contaminants as the oils that we receive from our supplier? Is this even relevant to our reliability initiative?"

Grease-lubricated bearings generally do not have the same level of reliability as oil-lubricated bearings. One reason for this is our ability to filter oil before and during use in lubrication systems or sumps.

This of course cannot be done with grease. There are no broadly accepted standards or limits on hard particle contaminants in greases. The user is subject to the quality imposed by the grease manufacturer.

There are a few military standards for grease contaminant limits. MIL-G-81322 is used to rate aircraft grease and MIL-G-81937 is used to rate (ultra-pure requirement) instrumentation grease. The G-81322 rating limit still allows the manufacturer to leave enough solid contaminants (thickener, dirt, manufacturing debris, etc.) that would equate to the amount of solid debris in an oil bath at an ISO cleanliness level of 19/16.

Industrial-grade products do not receive the special filtration treatment that these MIL Spec products receive. It is likely that solid particle contaminations are higher in most industrial-grade products.

This begs the question: are you filling your grease guns with lubricants that, measured by your ISO contaminant control expectations, are certain to cause poor reliability? It's something to think about.

Mike Johnson, Noria Corporation