I n industry, there has been much discussion on the topic of lubricant incompatibility, particularly with greases. When considering switching grease products, whether it’s for better performance characteristics, implementing a new process or changing suppliers, plant personnel must consider the impact of equipment reliability, production and the bottom line.
NSK Corporation, a manufacturer of rolling element bearings, conducted a compatibility study with 10 greases containing different grease thickeners. For each case, two greases were first tested separately and then blended at three different ratios. The worked penetration test was used on the greases after being blended at room temperature and again after storage at 250°F.
Figure 1. Results of NSK Incompatibility Study
The test results are shown in Figure 1. Every grease was incompatible with at least one other grease. The most incompatible were aluminum complex, calcium complex, clay and polyurea-thickened greases. The most common effect was substantial softening, however lithium grease sometimes hardened. Interestingly, barium grease blends looked like grease on the bottom and oil on the top, which may indicate that the second grease thickener was liquefying upon mixing. It is important to note that even if thickeners are generally compatible, two greases may contain clashing base oil or additive formulations. Also, not all thickeners of the same group are compatible with each other. Polyurea grease is an example of this as two polyurea grease formulations in specific cases may not be compatible with each other.
Where is it absolutely necessary to change the grease type used, there are some precautions that should be taken to minimize the risk of potential incompatibility. First, the following conditions should be met:
Verify that the bearing arrangement allows excess lubricant to be purged from the system. Bearing damage may result in sealed-for-life or shielded bearing arrangements.
Verify that the bearing is operating properly before switching products. Improper fits, clearances, bearing configurations or existing bearing damage cannot be corrected by changes in lubrication.
Verify that the bearing operating condition can accept a full-fill lubrication condition. This procedure should not be applied to bearings designed to operate with limited grease quantities because excessive bearing operating temperature may occur.
Assuming all conditions have been met, the following procedure may be used to change out greases.
Use up as much of the old grease as possible before bringing in the new grease.
While the bearing is running, slowly pump in the new grease until the excess grease being purged from the bearing changes in consistency or color. This waste grease should eventually appear similar to the new product.
Repeat the previous step after one to two hours of operation or after the bearing has returned to normal, steady-state operating condition.
Run the bearing for one week (if the previous relubrication frequency was greater than one week) and relubricate using the normal procedure.
Temporarily increase the regrease volume at least during the first two regrease intervals. The increased grease flow will help move out any remaining old grease and will provide sealing while overly soft grease may still be in the bearings.
Initiate testing (power consumption, amperage draw, relubrication frequency, vibration, etc.).
Prior to reverting to the original regreasing interval, sample the purged grease, test its consistency and check for oil separation.
Some additional tips to keep in mind:
- Always clean grease fittings before relubrication.
- Always pump in grease slowly.
- Always apply new grease to a bearing while in normal operating conditions.
To avoid lost downtime and serious equipment failure from potential grease incompatibility, it is imperative to proceed cautiously when changing lubricants. Follow the recommendations discussed in addition to those provided by the OEM and lubricant supplier.