- Buyer's Guide
Thanks for your invitation to "weigh in" on your recent article. All your points and counterpoints are quite valid. However, as you know of course, the choice of lube method cannot be made solely on the basis of looking only at the pro vs. con items listed in your article.
That said, let me express the hope that few, if any, true reliability professionals are engaged in the "Enduring Grease vs. Oil Debate." I believe that decades ago they found the answers to the question of grease vs. oil to be equipment-specific and linked to certain application parameters. They probably looked up the answer in a bunch of reference texts sitting on their bookshelves, right in front of their eyes. Alternatively, they have exercised resourcefulness by easily finding it in a hundred different books and articles, if they choose. They know that decades ago, the answers were provided in the engineering catalogs of all major bearing manufacturers, among them SKF, Fafnir, Torrington, FAG, NTN, Timken, and several others I can pull off the bookshelf in front of me. The reliability professionals, or anyone else, can also find the answers in lubrication-related texts by Exxon, Chevron and many others.
As a reliability engineer, I have learned to "bracket" a problem. For example, we might all agree that no reasonable individual will claim grease lubrication to be best for a 500 MW turbo-generator. Likewise, no person with even the most elementary exposure to machinery would say that a standard one-hp induction motor in an average application should be liquid-oil lubricated. As knowledgeable engineers, we have for many decades paid attention to DN-values (bearing bore diameter multiplied by rpm). This parameter, tempered by the bearing environment, allows us to determine if periodic regreasing or life-time grease lubrication is best. We know the experience-based values that favor one or the other, or the values that lie in the "go either way" zone. I have shared them with others in some of my books and articles.
Bracketing, as mentioned above, shows us that the question is relevant only for equipment that measures "somewhere in between" big and small, fast and slow, vertical or horizontal, etc. Again, it can always be answered on the basis of economics, failure risk, criticality, redundancy, spare parts philosophy, cost of labor, and the other parameters mentioned in numerous authoritative texts. The answers are obviously different for motor bearings and bearings in a valve operating gear unit, or in medical equipment, or in automotive applications, and so on. The answers are a function of equipment speed, bearing type, bearing diameter, equipment orientation, ambient environment and a few other considerations. From experience, we also know the answers to "what is preferred and when" are reflected in the procurement standards of best-of-class performers. I can vouch that the answers can also be found in at least one (and probably several) of the books Noria sells in its bookstore.
Here, then, is the true bottom line. All of the answers are accessible to people who read, but the answers will never be found by those who seek to find them by any other means. Fortunately, those who read have chosen to give value to their stakeholders by working on far more important value-adding issues. Those who read and learn have a higher probability of making a decent living, in the long term, than those who refuse to read. Mark Twain had it right when he said that the man who refuses to read is no different from the one who cannot read. It may sound harsh, but it's the truth.
Heinz P. Bloch, Process Machinery ConsultingEditor's Note:
This letter from Heinz Bloch is in response to Jim Fitch's editorial column, "The Enduring Grease vs. Oil Debate."