When in doubt…Don’t mix bearing lubricants!
Do not relubricate bearings before checking compatibility.
Never mix greases with different thickeners.
Check the amount of lubricant a bearing requires.
Bearing lubricants are specifically formulated to contribute to bearing service life and to help maximize overall machine reliability. Too often bearing users undermine the effectiveness of these carefully developed lubricants by adding generic factory lubricants to pregreased bearings or by inadvertently mixing greases when relubricating. Maintenance technicians need not be grease experts, but it is important for all personnel charged with bearing responsibility to understand the risks of mixing incompatible lubricants.
Grease formulation and compatibility
Approximately 90% of all rolling bearings are lubricated with grease; the remainder are oil-lubricated. A lubricating grease is produced by suspending a mineral oil or synthetic oil in a thickener, which carries the oil within a network of fibers.
Popular thickeners include polyurea, aluminum complexes, and calcium, sodium, and lithium soaps. During normal operation, oil gradually bleeds from the grease thickener, like water from a sponge, lubricating the bearing’s contacting and sliding surfaces.
As a general rule, greases that have different thickeners, such as lithium or polyurea, should never be mixed. When incompatible greases are mixed, the resulting lubricant is generally softer than either of its components (Fig. 1). The softer mixture tends to slump in the bearing; in extreme cases the base oil will bleed completely out of the grease mass.
In some situations, such as with mixtures involving aluminum complex greases, the opposite effect, grease hardening, can occur. Here, the base oil is bound up too tightly in the grease’s lattice-like network and is unable to bleed properly. Both softening and hardening have negative effects on grease performance and can possibly lead to bearing failure.
Greases that have the same thickener and similar base oils can often be mixed without harming grease effectiveness. But maintenance technicians should be aware that even greases belonging to the same family can differ somewhat in formulation and internal chemistry.
Calcium greases, for example, are available in three varieties: simple calcium soaps, calcium 12 hydroxystearate soaps, and complex calcium soaps. Lithium greases are also available in three basic formulations. Mixing two simple calcium soap greases is usually acceptable, whereas mixing a complex calcium soap with a calcium 12 hydroxystearate soap might lead to compatibility problems.
One way of determining a mixture’s compatibility is through a laboratory procedure called a penetration test. The test involves dropping a cone- shaped object into a grease mixture sample, allowing it to sink for 5 sec, and then measuring the penetration depth. Test samples are rated according to a scale developed by the National Lubricating Grease Institute (NLGI). An NLGI rating of 2 or 3 is generally recommended for rolling bearings (Fig. 2) and relates to operating temperature.
When relubricating with a new grease, make sure the old and new greases are compatible. If there is uncertainty about compatibility, try removing all traces of the old lubricant from the bearing. This removal can be done effectively without resorting to cleaning solvents or other substances that may cause a fire hazard.
While the bearing is in operation, begin adding the new grease and purging the old grease (Fig. 3). Within minutes, the purged mixture will contain traces of both the old and new greases. Continue until only the new grease is purging. This method quickly reduces the proportion of old grease in the bearing to less than 1%, preventing any compatibility problems.
If there are questions regarding the correct grease quantity, check with the bearing manufacturer. The optimal fill range varies, depending on the specific bearing and application.
Compatibility is also an issue with oil-lubricated bearings. Mixing two different mineral oils, or mixing a mineral oil with a synthetic hydrocarbon, is generally acceptable. Mixing a mineral oil with a diester oil falls into a gray area; compatibility must be determined on a case-by-case basis. Mixing a mineral oil with a silicone oil is not recommended; these two oils are not capable of forming a solution.
Whether the lubricants involved are oils or greases, questions of compatibility often come down to a judgment call. In some situations, test results or factory-floor experience may offer adequate assurances to bearing users that two lubricants can be combined without harmful effects. Otherwise, technicians should follow a simple rule: When in doubt… don’t mix lubricants!
— Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135, firstname.lastname@example.org
&HEADLINE>Workbench test for compatibility&/HEADLINE>
Pour equal quantities of the test greases into a container
Stir with a screwdriver
If the mixture turns runny and pours easily rom the container, or hardens into a sludge-like consistency, the two greases are likely to be incompatible.
&HEADLINE>Tips on training&/HEADLINE>
Maintenance supervisors should ensure that plant personnel receive information and training regarding correct bearing lubrication practices. Material Safety Data Sheets (MSDS) enclosed with most lubricants often list lubricant ingredients and contain recommendations on compatibility. Other resources include equipment manuals and training materials provided by lubricant suppliers and bearing manufacturers.
The author is available to answer questions on lubricating bearings. He can be reached at 734-414-6865.
For additional information on bearing lubrication, visit the “Fluid and mechanical power transmission” channel on our web site: www.plantengineering.com.