A recent reliability conference dealt with machinery reliability improvement. After one of the presenters invited attendees to ask questions, two reliability engineers in attendance explained how their plant had repeatedly experienced bearing failures on large electric motors. The average bearing life of these European-built 13.2 kV/3,585 rpm induction motors was reported as 12 months which, of course, is unacceptably low. When asked about the grease path and bearing styles — single shielded, double shielded or open — they had no details. Regarding grease application mode, such as cross-flow, same-side reservoir, plug removed, or plug in place, they again could offer no data.
At the same conference, an attendee from an oil refinery explained that at his facility the electrical department was in charge of motor lubrication. He spoke of moves underway to revert from continuous oil mist lubrication at present to grease lubrication in the future. The refinery’s maintenance crews would then, in the future, periodically replenish the grease on hundreds of electric motors.
|Heinz P. Bloch, P.E., is owner of Process Machinery Consulting (www.heinzbloch.com) in Westminster, Colorado, and the author of numerous articles and books, including “Improving Machinery Reliability” and “Pump Wisdom.” Contact him at firstname.lastname@example.org.|
Because two maintenance technicians and a part-time trainer would probably have to be added to this refinery’s staff to do all the re-greasing, reverting to grease lubrication should be seen as a step backward. Backward steps very often center on gaps in knowledge and a lack of learning. We can confidently make that statement because there are, even as we speak, thousands of electric motors which, with properly configured bearings, well laid-out grease paths, and soundly determined lube-replenishing intervals, give many years of continuous and satisfactory service. Even more dependable are upward of 26,000 electric motors that are lubricated by pure oil mist. Some of them were installed in 1978 and have yet to have their bearings replaced. Savings in maintenance with pure oil mist at one forward-looking petrochemical plant were estimated to exceed $200,000 per 1,000 electric motors, even before the prorated cost of an avoided fire was added to the estimate. Good cost justification calculations take into account that a $4 million fire is likely per 1,000 failures. Therefore, each avoided motor failure would be worth $4,000, according to the 4th edition of “Pump User’s Handbook: Life Extension,” which I co-wrote with A.R. Budris.
Figure 1. Rolling element bearings with two seals (upper left), open and with grease cavity adjacent to bearing (upper right), single-seal bearing with seal shown on wrong side (lower left, open bearing with cross-flow grease path (lower right).
Of course, it would be impossible to purchase a motor without specifying its speed, voltage, and power. Because there are many different combinations of bearing types and lube methods, these, too, should be specified by the buyer or disclosed by the vendor (Figure 1). Although some combinations of bearing styles and lube practices may prove troublesome, a large number of plants still purchase from the lowest bidder and leave lubrication to trial-and-error or chance.
To be counted among the reliability leaders, a user company must understand and successfully implement modern lubrication methods. The managers and staffers who advocate going back to maintenance procedures that were discontinued in the early 1960s are entirely on the wrong track. A safe plant is a reliable plant, and vice versa. Tangible steps to achieving both safety and reliability do not come out of nothing; they are rooted in learning. This learning cannot be sporadic or superficial; it must be highly structured and must embrace both theory and practice. True learning culminates in the acquisition of in-depth knowledge which, when applied, is called wisdom. Wisdom comes after years of being mentored, doing much reading, observing, and applying a considerable amount of common sense over the span of one’s career.
Do not let opinion get in the way of facts. Wisely practicing motor lubrication is granted to those who understand the difference between opinion and facts. Only a mature person will consistently act on facts. And only a highly principled manager will consider it a priority to groom, nurture, and reward mature professionals. Because mature professionals explain facts even in the face of unfounded opinion-based opposition, they are not universally loved by all. They may even be viewed as non-players in a herd of team players.
Not being advised by mature professionals is a serious problem for many managers. Only an informed manager can make good decisions, and he must be careful not to surround himself with the untrained or uninformed or with people who really are imposters. Good managers must make decisions on the basis of facts. When it’s all said and done, we generally find that more has been said than done. And while managers don’t need an in-depth knowledge of details, they must be able to depend on maintenance and reliability professionals who know details and will ascertain that due attention is paid to details.
While there are certainly dozens of examples which could illustrate how important lubrication details are being overlooked, we will let Figure 1 illustrate our point. Figure 1 depicts four different bearing configurations; some of these benefit from a shielded, grease-lubricated bearing. Note that the grease cavity is adjacent to one of these shields. Grease is a mixture of about 90% oil and 10% “soap.” Shields have a small annular gap, and it is through this gap that a few drops of oil are expected to migrate or “bleed” into the bearing in a full day’s time. So then the design intent is for a small amount of oil to bleed from the housing’s grease cavity past the annular gap of a grease shield. If, during the re-greasing process, a maintenance technician applies pressure from a grease gun without first removing the drain plug from the housing, the adjacent shield may be pushed into the rolling elements.
We re-focus our attention on the costly bearing failures reported at the recent conference. If the two engineers had all the details and would act on facts, their bearings would probably live much longer.