Bearing restoration: Whole-life production-line assurance policy Power Transmission Design Magazine
If you stock replacement rolling-element bearings, you must replace the failed bearings with one of your shelf units, and replenish the stock at a leisurely pace. If you don't have spares, however, you panic momentarily, then recover and call your distributor or supplier. But if it is a special or a large bearing that isn't cheap and no one is likely to stock, you may panic again as the equipment silence roars behind you and smiles turn upside down with mounting downtime. Time to consider bearing restoration.
Your distributor may offer some limited capabilities such as cleaning and polishing, but a failed roller bearing needs more than a spit-shine. You can grab the yellow pages and look up "bearing restoration" or "bearing repair" or some such. But you'll be lucky if you find someone. You may be even luckier if you don't in some areas, because the listing may be populated with those who can fix your bearing, sump pump, or Studebaker sedan with equal lack of skill.
According to Robert A. Phillips of PSI Repair Services Inc., "In the U.S. there are roughly half a dozen companies truly qualified to restore roller bearings. Some are bearing manufacturers, and the rest are independent bearing restoration specialists. " The Livonia, Mich. based firm sees itself as one such independent. It provides restoration for most makes of spherical, tapered, and cylindrical roller bearings, with 1- year in-service warranty on bearings.
What is "restoration"? Mr. Phillips says, " Warranties, turnaround time, and prices differ among independent bearing repair companies, but the industry agrees on one thing: quality. In fact, some independents use equipment that rivals that of bearing OEMs. Our restoration equipment includes CNC lathes, CNC grinders with computer-controlled wheel dressers, 3 axis coordinate-measuring machines, Supermicrometers, and 30-in. optical comparators. Each independent has its own repair system its own designation of repair levels- but most are equipped to handle even the most extensive repairs."
PSI's restoration process begins with a free evaluation of the bearing. Here, the unit is fully disassembled, cleaned, and inspected for wear. Based on the evaluation report, the bearing is assigned a restoration level ranging from 1 to 4:
Level 1: cleaning, polishing, assembly, and testing. Level 2: cleaning, grinding of raceways, replacement of rolling elements, assembly, and testing. Level 3: cleaning, replacement of rolling elements, manufacturing of one or more raceways, assembly, and testing. Level 4: new-bearing manufacture.
What does it cost? "In general," says Mr. Phillips, "restoration costs only about 30 to 50% of the cost of a new bearing, usually saving several thousand dollars." When pressed for a breakpoint on restoring vs. buying a new bearing, Mr. Phillips allowed that "it varies across situations. Most of our customers are paper and steel mills using very large (say, 12 to 60-in.-OD) bearings that can cost up to $60,000 new. We commonly charge about half the cost of new for a restored, guaranteed bearing. The savings here is tremendous. However, when dealing with a bearing that goes for $1,000 new, savings of $500 might seem less significant , though our bearing guarantee is longer than that of most manufacturers. In a nutshell, provided that the bearings are not 'blown apart,' even those in the 10 to 12 in. OD range can be economically restored."
The cost of reinstalling a new or a restored bearing is the same, because both are to the same envelope dimensions. You must weigh any other costs, too, such as the costs of downtime, bearing shipment, and insurance.
Get back more than the bearing When a bearing restorer disassembles and inspects your bearing, he often learns something you should know why the bearing failed. It's like your right to look at your doctor's medical record on you. Bearings can fail for any of many reasons, and some leave telltale signs.
Mr. Phillips notes that, besides normal fatigue failure at the end of a bearing's natural life (which many experts feel causes no more than 10 to 30 % of all bearing failures), these are common causes of bearing failure:
Defective bearing seats on shafts and in housings.
Misalignment.
Poor mounting practice.
Incorrect shaft and housing fits.
Inadequate lubrication.
Ineffective sealing.
Vibration while the bearing is idle.
Electronic current passing through the bearing.
Line corrosion due to extended idle periods.
"How many of these conditions exist in your plant?" asks Mr. Phillips. "Rooting them out and correcting them now can add years of extra life to your bearings."
Mr. Phillips also offers the following symptoms of life-threatening bearing-system ailments. He counsels that knowing them can help spot a problem before it becomes unmanageable and costly. Symptoms are in bold-face type.
Noisy bearing. Often results from inadequate lubrication, contaminants in bearing housing, or misaligned seals.
Overheated bearing. Often results from plugged oil-return holes; insufficient lubricant; bearing loose on shaft; or parallel or angular misalignment of shaft.
Vibrating bearing. Often results from foreign matter acting as an abrasive; a flat on a roller due to skidding; excessive bearing internal clearance; or unbalanced loading.
"This is a list of common problems and symptoms," cautions Mr. Phillips, " and is by no means comprehensive There are other dangers that lurk in a mill or factory, and these are often hard to spot. Let's face it mills are noisy place, making it hard to hear a noise or feel a vibration. Sometimes, just getting at your bearing can be tricky, and even the best maintenance plans have a hard time stopping insidious contamination or making up for grueling production schedules." Like the wise and solemnly pleasant undertaker, he adds: " So chances are, you will some day have to deal with a worn or damaged bearing."
