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THERE'S A WAY TO SQUEEZE MORE LIFE out of your bearings, and it can usually be done at less than half the cost of new bearings. For companies in such industries as pulp and paper, steel, railroad and mining, where bearings are often high-cost replacement components, bearing reconditioning can give your used bearings a new, cost-efficient, lease on life. What's more, a new bearing warranty may be part of the deal.
Whether they are immediately returned to service or stored as spares, reworked bearings offer a particular advantage over new bearings: They have already proven themselves in use.
Reworked bearings are also a valuable asset in the storage room, where they lower inventory costs by eliminating the need to store more expensive new bearings as spares. Normally, the charge for reconditioning runs between 25% and 60% of the cost of a new bearing. Given the hefty price tags on some bearings (a good-sized spherical in a steel mill can cost tens of thousands of dollars), reconditioning can be a blockbuster bearing bargain.
What bearings are candidates for reconditioning? Those that would be difficult to replace are potential rework bearings. Size is another criterion, though no longer a clear-cut one. Until recently most reconditioning service companies would only rework bearings with an OD of at least 240 mm. Now, however, bearings as small as 180 mm OD-and sometimes even smaller-qualify for rework. Bearings in continuous slab casters are examples. Here, large numbers of bearings operate in conditions of low RPM, with heavy loads and high temperatures. Depending on their type and volume, bearings in continuous slab casters can be especially appropriate for reconditioning-so much so that special programs exist for their rework. Other steel industry bearings often refurbished include tapered roller bearings in rolling mills and spherical roller bearings in peripheral equipment.
In paper machines, bearings that lend themselves to rework are found in dryer cans, Yankee dryers, calendar stacks, and king and queen rolls. Specialized reconditioning services are now available to address bearings in pulp and paper converting equipment. They include a new rough raceway refinishing process that helps refurbished bearings operate reliably.
Obviously, the condition of a bearing is another key factor in determining whether it can or should be reconditioned. Full-service reconditioning companies have industry specialists that can usually make this determination on-site. Specialists have specific expertise in industries that present the greatest bearing challenges, such as the pulp and paper, metals, mining, railroad, automotive, and petrochemical industries. The specialists probe the periphery of the bearing raceway looking for smearing, spalling, brinelling, static etching, corrosion, and other signs of distress. To an experienced examiner, the bearing's condition is as easy to read as the script on a page.
Selecting a supplier. Once you've identified bearings for reconditioning, you'll find a number of companies both large and small competing for your business. How do you choose the right one? Here are five things to consider before deciding:
1. Facilities. Do the company's facilities measure up to the task of bearing reconditioning? Full-service rework operations will have available fairly extensive technology and equipment. SKF, for example, recently opened a 41,000-square-foot facility dedicated largely to reconditioning. The facility houses a climate-controlled metrology laboratory and is equipped with precision gauges and instrumentation necessary to meet tight bearing rework specifications.
A full range of such resources, along with related services, enables a supplier to add value to the original rework service. Bearing failure analysis, for example, can help you prevent a second bearing failure from the same conditions that caused your first failure. Likewise, training and service engineering can help you achieve greater service life from your bearings-and reduce machine downtime due to bearing malfunctions.
2. Levels of service. A customer-friendly rework program will offer different levels of service to cost-efficiently meet specific needs. The following four levels typify the options that a full-service reconditioning company can offer for large bore bearings:
Level 1-a basic inspection service. A Level 1 service usually takes less than two weeks, and includes disassembly, cleaning, and inspection. Based on the inspection, your supplier should provide a detailed bearing analysis report that describes the bearing's condition and notes findings, such as fretting, edge loading, false brinelling and discoloration.
Level 2- basic inspection service, plus return of the bearing.
Level 3-the basic rework service. After qualified bearings are thoroughly cleaned, all rolling surfaces are polished, and a protective slushing compound is applied. The bearing is repackaged and returned in two to four weeks.
Level 4-extensive rework. A bearing that qualifies for extensive rework undergoes grinding of bearing raceways, cleaning and reworking of the cage if necessary, installation of new rollers and slushing. The bearing is repackaged and returned to the customer in eight to 12 weeks.
3. Scope. A full-service reconditioner should be able to handle a wide variety of bearing types and brands. Companies that manufacture bearings and offer rework services can often recondition bearings they did not manufacture. However, because of their unique design, some bearings can only be reconditioned by the companies that made them. Examples include certain cylindrical traction motor bearings, high-capacity E-type sphericals, and CARBTM bearings that combine the advantages of spherical, cylindrical, and needle roller bearings.
4. Warranty. A valuable extra that buyers can ask for is a warranty. They are rare, but they are available. At SKF, for example, bearings that undergo Level 3 or 4 service will be returned with a new bearing warranty that covers any and all work resulting from faulty reconditioning. Depending on the circumstances, the warranty giver might also provide mounting, dismounting, and application assistance.
5. Keeping the bearing doctor away. Once a reconditioned bearing is back on the job, what's to prevent others like it from succumbing to the same kinds of problems? Your supplier should provide a thorough bearing analysis report as part of any Level 1 basic inspection service, as well as more in-depth services. The report will tell you what can be done to keep similar problems from occurring with bearings in similar applications. n
Tony Clabaugh is manager, Industrial Services Center, SKF USA Inc.
WHILE PROCEDURES VARY SOMEWHAT DEPENDING ON BEARING TYPE, REWORKED BEARINGS WILL TYPICALLY UNDERGO SOME OR ALL OF THE FOLLOWING: While procedures vary somewhat depending on bearing type, reworked bearings will typically undergo some or all of the following:
1. Cleaning, degreasing, and coating. A protective coating is applied to safeguard the bearing during the rework process.
2. Disassembly. If the bearing has a two-part cage, the pins are removed and replaced.
3. Inspection. Outer and inner roller paths are inspected for dents, scratches, spalls, and other defects.
4. Grinding. If grinding is needed, a precision gauge determines the minimum amount required to return the ring to serviceable condition. The grinding operation removes all defects on the roller path and brings the path to required tolerances. After grinding, the roller paths are reinspected and the outer and inner rings are polished.
5. Cage work. Cage pockets are machined to match roller size. Rollers are then placed in cage pockets. For two-part cages, new pins are installed and side plates are secured. The cage is polished and inspected for out-of-round, and roller diameter variation is checked for conformance.
6. Reassembly. The bearing is reassembled and the internal clearance is checked.
7. Washing and coating. Solvent is used to wash out dirt and grime that gets on the bearing during reassembly. Then a protective coating is applied to help prevent oxidation until the bearing is back in use.
8. Packaging. Specially designed packaging safeguards the bearing during shipment and storage.
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