IMC-2008 23rd International Maintenance Conference - Dec 8-11 - Hyatt Coconut Point Bonita Springs Florida Join or Manage Your Profile Posting Boards Machinery Condition Monitoring and Predictive Maintenance Posts About Lubrication and Oil Analysis How to choose a grease-case?
Join or Manage Your Profile Posting Boards Machinery Condition Monitoring and Predictive Maintenance Posts About Lubrication and Oil Analysis How to choose a grease-case?|
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I have AC motor of 560 kw running DC generator and bearing are 6319 and NU, speed is 1000 rpm and I conducted measurement of temperature of bearing housing. It is between 50-85 degree Celsius. I have small MTBF on those machines. I measure vibration and it in the limits for those machines.
What you recommend for grease for those bearing, re-greasing interval, re-greasing amounts, NLGI grade? What is limit recommended temperature for grease, is it 70 or 60 or... and why is that so? |
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I think SKF recommends to double the greasing interval for every 10-15C above 70C.
Is this a belted application? If not, skidding may be a problem. |
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At that kind of temperature, if the motor is not in an intermittent application, then the NLGI grade isn't going to be critical. What kind of grease do you use now? Does it have a temperature range running up to at least 160C? For that temperature that you're measuring at the case, I'd prefer a grease with a top end temperature of 200C, because I would expect more heat at the inner race than I measure at the bearing case exterior, and it's best to have extra capacity in the grease. I try to use a grease with a top end temperature range of twice what I expect. This helps it last longer. My 'standard grease' is a synthetic that has a top end temperature of 200C. For one application in my plant where the bearings get pretty hot (they might actually get close to 200C) I use a special grease that is good to 300C. Mike the Maintenance Guy, turning wrenches on HDPE extrusion lines. |
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It is not belt driven application. I use generic grease temperature range is -20 +100C NLGI 2. I plan to use SKF LGMT 2 or 3, -30 +120C. So I would make mistake?
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Why not get some input from companies that make grease? SKF is a fine company, and yes, they know a lot about lubrication, but it might be wise to call a local lube guy. Second and third opinions are almost always useful - even if it's just to confirm what someone has already said.
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I agree 100% about getting more advice.
But just to make sure I have properly conveyed what SKF recommends, here is an excerpt:
Note that they're not implying the grease temperature limit is 70C... you can go above 70C by halving the interval up to the grease limit. Now a humorous excerpt from the same publication which perhaps illustrates that apparently bearing manufacturers and bearing users sometimes operate in different worlds:
Sooooo..... how many people actually disassemble the machine to remove all the grease every few years? Not many I'm guessing. Certainly not us. This message has been edited. Last edited by: electricpete, |
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I agree with EPete on the NU bearing. Since there is not any appreciable radial load, I would look at skidding of the rollers as being a culprit for low MTBF. What is the failure mode? Dave |
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Dave,
Temperature is reason of failure, it does overheat so we take it to repair... |
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What is the visual findings after you have removed the bearing?
Are there spalls, or any other signs of failure? Are the races or rollers/balls blue from excessive heat? You know too much grease will cause one to heat. 85 deg. C is hot, but I've seen them hotter on fans and still run OK. We found it was too much grease from an overly agressive greasing schedule. Is the ball or the roller bearing that is too hot, or both? Dave |
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I can not give more detail, but at next failure I will check everything, I agree. I am new to department....so I will watch it more closely... thanks to all...
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Pl. visit SKF website. following are some quick links.
http://webtools.skf.com/lubeselectskf http://www.skf.com/portal/skf/home/aptitudexchange?cont...238021.238024.238759 The second one is subscription based. You will get recommendation for NLGI grade, relubrication interval as well as re-greasing amounts. Regards. Irshad Akhtar |
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