Bearings

1Q. What is the B-10 life of a bearing?

1A. That is a statistical number that says for a given anti-friction bearing type operating at that load and speed, 90% of the bearings are expected to exceed that life.

2Q. What does it mean when the B-10 life of a motor bearing runs out?

2A. B-10 life on motor bearings are usually designed to about 3 years. Statistically the average life is about 7 times the B-10 life. Many times when a bearing fails in 3 years the inexperienced say it's because the bearing exceeded its B-10 life. This of course is false, and is usually because the bearing was not lubricated, it was overloaded or the lubricant was contaminated. Most motor bearings operate in excess of ten years when their B-10 life is three years. The motors usually require cleaning before the bearings reach their B-10 potential.

3Q. Bearings in our large gearbox have undergone several hard hits during start ups. The operating loads are not excessive as per the B-10 life. Is there any concern?

3A. There could be depending on how hard the bearings get "hit". Bearings like many metals also follow the Palmgren-Miner linear damage rule and don't forget harsh treatment. (See Hydrocarbon Processing Magazine June, 2002 Case 9: The Machine Remembered.) We have seen instances where too much axial clearance can actually cause impact damage that later on shows up as spalling of the bearing. Good on-line vibration monitoring can detect this early, and might not save the bearing but could save a catastrophic gearbox wreck.

4Q. We keep failing anti friction type pillow block bearings catastrophically on a fan drive. It never use to happen. Any ideas?

4A. Make sure they are lubricated, and if those permanent type canister greasers are used, make sure they are working. One bearing needs to allow axial play so make sure the design allows for this. Sometimes the wrong bearing is used by mistake on the last repair and the next guy uses the same type wrong bearing. Look up the type in the files. Make sure the belt isn't too tight or a bad coupling, if it's a drive end failure. Make sure there isn't misalignment or a soft foot. These are some of the more common problems on repeat failures. Read the bearing to try to determine the cause. Make sure the bearing fits are OK and the bearing is installed correctly.

5Q. A bearing spun on a shaft and a repair shop said they could have it back in the same day if they knurled it for the fit. Is this OK?

5A. No! Not unless it's on an axle of one of you children's toys. On any serious machinery, the knurling will eventually relax and you will have a loose fit again. Chroming, weld build up, metallizing or a new shaft all have their place if done correctly, and depend on the application.

6Q. We are going to "mothball" pumps for a couple of years in our petrochemical plant. We plan to just fill them with oil and put a nitrogen purge on them. Will this be enough?

6A. I never had much luck with this approach, even if the purge was oil mist. Seems moisture always condenses at the rollers and the race and had to be replaced after start up. The only time this method was acceptable is when we rotated the pump shaft by hand on a monthly schedule.

7Q. We rotate running our primary pump and spare pump on a monthly basis. Is this necessary?

7A. That's one way of insuring it will be available when needed. Some will only start it up on a periodic basis to make sure it is operating OK and so as not to interfere with production. The basic premise is that the time to find out the spare needs repair is not when the operating pump fails. You have to have as much confidence in it as the primary.

8Q. How do you calculate bearing frequencies in a complex gearbox?

8A. For bearings in epicyclical, planetary or multi-shaft gearboxes you first have to determine the shaft speed the bearing is on, which can be pretty complex. Good advice is to call in a specialist who does this type of analysis regularly. You have to know your limitations.