Grease replenishment and reliability engineering
Experienced bearing manufacturers agree that single-shielded bearings should be installed with the shield facing the grease cavity. The bearing manufacturer intends that the shield in Figure 1 will serve as a metering orifice. The oil constituent in a grease must bleed through this gap, and the shield is there to prevent overgreasing. Selecting double-shielded radial deep-groove bearings is encouraged because one of the shields will then always be adjacent to the grease reservoir. Oil will “bleed” through the gap by capillary action and not by force. Recall that oil “bleeding” into the raceways is what bearing designers intended.
Nonhydrocarbon grease formulations, although cost-justified in certain fully sealed bearings, are not well-known and are underused. It’s important to understand the applications where nonhydrocarbon PFPE/PTFE (or specific advanced synthesized hydrocarbon, collectively called “synthetics”) is of great importance.
There will never be a substitute for experience; accordingly, not everyone working for a manufacturer will outshine an observant user. And the goals and aspirations of bearing providers are not necessarily aligned with those of equipment users and reliability professionals. So, what should be the course of action for this latter group?
Guidelines have evolved; the following current guidelines reflect the experiences of users and manufacturers:
- Sealed (nonregreasable) bearings are preferred in continuous-duty applications as long as the product of the bearing bore diameter “D” in mm and speed “N” in RPM does not exceed 80,000.
- Shielded (regreasable) bearings are considered the best choice for light to medium loads in the DN range from 108,000 to 300,000, and users can opt for either sealed or shielded configurations in the “gray” DN range from 80,000 to 108,000.
- Once a DN-value of 300,000 is exceeded, liquid oil lubrication is normally preferred over grease.
At one major U.S. manufacturer, there were 156 bearing-related repair incidents per 1,000 electric motors per year, while an affiliated refinery experienced only 18 incidents on very similar motors lubricated with the exact same grease. Three installations in the Middle East averaged 14 replacement events per 1,000 motors per year. Their motor specification insisted on the use of regreasable bearings, and the maintenance technicians at the three facilities always removed the drain plug when replenishing grease. Removing the grease drain plug (item 2 in Figure 1) prevents overgreasing and allows spent grease to be expelled. Adding the pipe extension takes away the human element of having to remember that a drain plug, if used, would have to be removed and reinserted as part of a labor-intensive (and now considered outdated) conventional regreasing routine.
Why the dramatic difference in repair incidents? At the 156 incidents per 1,000 motors per year location, periodic grease replenishment was done with the drain plug left in place. New grease tended to force the spent grease into open, or nonshielded, bearings; in some cases, new grease under pressure deflected or even deformed the shields of shielded bearings. In sharp contrast, the 18 incidents per 1,000 motors per year location saw to it that drain plugs were removed during regreasing. Using these documented findings and assuming 2,000 electric motors at a large refinery or paper mill, proper regreasing could thus avoid 276 bearing replacement incidents. At $4,000 per incident, proper regreasing would save in excess of $1 million per year.
For many plants that adhere to grease lubrication, Figure 2 shows the best solution from technical acceptability and also from “not wanting to argue with my workers” points of view. Some companies opt to emphasize accountability and insist on staffers following instructions. We found out how well this approach worked in the United Arab Emirates, where a large refinery reported replacing seven bearings per 1,000 electric motors per year. When asked what magic grease formulation the refinery was using, a senior manager explained that his workers simply followed instructions and that grease-related bearing failures are infrequent. In other words, the technicians at that refinery know what bearings they have; they remove drain plugs; they regrease with the prescribed amount of grease; and then they move on to do the next electric motor. After allowing two to three hours for grease to settle, a worker returns and reinserts each drain plug.
There’s no substitute for following a proper work execution procedure. Good supervision and managing with integrity prevent failures and generate higher profits. Using an open drain pipe (see Figure 2) instead of a drain plug totally eliminates the possibility of overpressuring as a result of human error. Call it ingenuity at work!