Despite its unquestionable importance, lubrication science continues to be a neglected field. Because the nuances and implementation details of that science tend to be undervalued and misunderstood, lube application and selection methods often are antiquated and out of tune with today’s professed reliability thinking. As just one example, purchasing lubrication products from the lowest bidder has led to the use of products with inadequate additive formulations.
Fortunately, remedies are available in the form of intelligent specifications. Regardless of bearing type and lubrication method, avoiding lubrication errors is of great value from both a safety and a reliability perspective. Procedural shortcuts and lubrication mistakes often jeopardize equipment reliability and can prove very costly.
Regreasing shielded bearings
Of the billions of rolling element bearings in use in appliances, automobiles, and electric motors, probably 99% are grease-lubricated. The rest, we can assume, are sliding (also called “sleeve”) bearings. With few exceptions, all of these industrial bearings require lubricants as a separating barrier between moving and stationary parts. Having such a barrier will reduce friction and carry away heat.
Here’s a point that bears repeating: There is no single grease type that is best for every conceivable grease lubrication task. Likewise, no one grease replenishing method or procedure will prove superior to all others. Accordingly, a regreasing procedure must fit a particular bearing style, and an approved procedure should be followed for proper regreasing. To be regreasable, a bearing cannot have seals; regreasable bearings require bearing and housing configurations that allow replenishing of grease. Such bearings can have one or two shields or be designed as “open” bearings without shields.
Unfortunately, the incorrect claim that shielded bearings cannot be relubricated with mineral oil-based greases still persists. Most greases consist of approximately 85% oil, with the remaining portion made up of soap and a small amount of additives. There is a body of literature explaining that the oil portion of the grease “bleeds” or “seeps” through the gap between the shields and the bearing inner ring. Seepage can also be called capillary action, whereby the oil leaves the soap and wets out – which is to say the oil provides a lubricant film or coating on the rolling elements. Shielded bearings can be regreased as long as a volume of grease is located adjacent to the bearing shield and the grease is the proper consistency for the application. This means that the bearing’s operating environment must be considered; the suitability of an operating environment depends on bearing housing geometry, shaft speed, bearing temperature, and more.
It is widely known that bearing and lubrication distress are the two primary maintenance items in electric motors; both will greatly affect uptime. That said, a proactive and reliability-focused facility will add bearing and lubrication details to the usual motor procurement specifications that define voltage, speed, power output, service factor, insulation classification, and other parameters. Moreover, and in view of bearing and lubrication concerns, competent reliability professionals at best-in-class facilities insist that maintenance technicians use checklists. These checklists explain the letter code associated with a motor’s drive end (“DE”) and non-drive end (“NDE”). Unless the DE and NDE bearings are identical, each end may require different regreasing routines, and the checklists should contain relevant specifics.
Get the right grease to save $$$
Virtually any substance known to man is affected by temperature changes. Materials usually expand as they grow hot and contract as they become cold. A scientist can rightly point to a few exceptions, but the rule certainly applies to industrial lubricants. This is where grease specifications take on importance. Although “all-purpose grease” may be suitable for horse-drawn carriages, it should not be used at plants and facilities intending to keep machines reliable.
You may think your facility wouldn’t need these reminders, but industry continues to experience equipment and component failures that could have been avoided by simple root-cause analysis and appropriate specification follow-up. A motor shaft failure at a U.S. power plant will illustrate the point: A chain of events commenced when several electric motors in cold weather experienced severe bearing failures. These bearing distress events were followed in at least one case by a massive shaft failure. A team of investigators attributed these incidents to stiff grease not reaching the bearings. In response, a government regulatory agency sent out an expensive advisory message. It advocated that, henceforth, electric motor bearing replacement should become part of time-based preventive maintenance. I don’t recall any suggestions to perform predictive maintenance (PdM) or ensure use of the right grease for the particular environment.
In particular, there didn’t seem to be any effort to investigate why many thousands of electric motors operate flawlessly in the more-severe outdoor crude-oil gathering and refining environments of Canada and other worldwide locations north of the 49th parallel. Nor, it seems, did anyone bother to ask why many of the world’s most profitable paper mills in Finland, Sweden, and Germany use automated periodic regreasing on thousands of pumps and electric motors. It would have been easy to find the answer: The many successful locations use greases with the proper viscosity. There are nine National Lubricating Grease Institute viscosities; the lowest of these, NLGI Grade 000, exhibits the flowability of cooking oil. The highest of the nine grades, NLGI Grade 6, approaches the consistency of cheddar cheese. Of course, industrial greases must be applied in accordance with proven procedures. Many good procedures date back to the 1960s; these are detailed in and are readily accessible in two books I co-authored, “Practical Lubrication for Industrial Facilities” (with Kenneth Bannister) and “Pump User’s Handbook: Life Extension” (with Alan Budris).
The importance of correct regreasing procedures can’t be overstated. Experiments conducted in a Texas refinery shop, for example, have shown that leaving a grease drain plug in place during greasing can yield pressures as high as 15,000 psi (105 kPa) in electric motor bearing housings. At these pressures, shields will be pushed into the rolling elements and cause almost instant failure. Rapid grease deterioration also can be caused by mixing incompatible greases or using contaminated products. Buying sealed bearings and leaving them alone is indeed superior to buying regreasable bearings and then mistreating or abusing them.
On the other hand, by simply implementing correct practices, best-in-class plants manage to prevent both failure-induced downtime and unjustifiable expenditures related to precautionary time-based bearing changeouts.
It’s thus of immense value to determine in a given use case whether the best bearing and lubricant option has been selected. Next, it’s vital to establish whether the application meets the load and speed criteria for periodic and correct regreasing and whether the correct grease formulation has been chosen. Opinions and guesswork have no place here.
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!