Bearing Repair Services Offer a Cost-Effective Alternative to Expensive Replacement
As the mining industry maneuvers to survive the relentless pressure of plummeting commodity prices, tight credit markets and panicked investors, one of the few prospects for near-term relief is an emerging drop in certain operational costs such as fuel, labor and power which had risen steadily during the prolonged mining boom of recent years.
However, even drastic drops in these costs may not be enough to rescue a project from cancellation or a mine from cutbacks unless they’re coupled with additional cost reductions. Under these conditions, one of the most vulnerable functions in an organization is equipment maintenance and repair, as mine and corporate management look for ways to pare these costs without diminishing worker safety or machine productivity. One low-risk option for saving money while maintaining acceptable performance is through bearing remanufacturing and repair services offered by the major bearing suppliers such as The Timken Co. Regardless of original manufacturer, a wide range of services are available for all bearing types.
As explained by Richard Brooks, manager of reliability services, and Trip Kern, strategic account executive–mining and power generation for Timken, advancements in bearing design, materials, maintenance and repair methods have greatly improved the potential for and popularity of bearing repair as an effective way to extend bearing life. The two offered a wide-ranging description of these services, as offered by Timken, at a presentation during the MINExpo 2008 trade show held earlier this year.
A repaired bearing, depending on the required level of service, often can be returned to like-new specifications quickly and at a savings of up to 60% off the cost of a new bearing. Timken’s experience in this sector has shown that a successfully repaired bearing can offer a life cycle comparable to that of a bearing’s initial service life.
There are several terms describing bearing service options that are used interchangeably within the industry, but do not necessarily reflect the same scope of work to be performed. These include:
Repair, which encompasses a wide-range of services that can be performed to a bearing. In general, the term can be applied any level of work performed to a bearing.
Recertification, which involves certification of a bearing for service. This generally applies to an unused product with an outdated shelf life.
Reconditioning, which includes polishing, honing or tumbling of bearing components to remove very minor surface defects (primarily rust or corrosion) that could lead to more extensive damage if not removed.
Remanufacturing, which is the process of removing aggressive surface damage by using a grinding or hard turning process. This also includes replacement of any unserviceable components.
Benefits of Bearing Repair
Initial bearing design takes into account the use and application of the bearing and establishes a corresponding prediction for service life and fatigue life. Factors such as improper installation, contamination, inadequate lubrication or misalignment often cause bearings to deviate from these expectations. In fact, less than 10% of bearings used in mining applications reach their design L10 life, according to Timken’s data.
A quality repair program can often return a bearing to like-new specifications in about one-third of the time it takes to procure a new bearing, and some bearing repair sources will also offer a warranty on their service.
When compared with the manufacture of a new bearing, bearing repair also could be considered a more environmentally friendly procedure, requiring less energy input and reducing raw material consumption and waste. The majority of energy required to manufacture a new bearing—melting and refining steel, material forging and turning, heat-treatment and grinding—is conserved through bearing repair.
Repaired bearings often reuse materials that have already proved reliable in the application, therefore reducing the risk of bearing failure. Replacement parts are made using materials and tolerances specified by the OEM. Any deviation from OEM specifications will increase the risk of premature failure.
According to Timken, all bearing types are eligible for repair, including tapered, spherical and cylindrical roller bearings—the types most commonly used in mining; along with ball bearings; thrust bearings (including slew rings up to 120 in.); and cross roller bearings
A critical step in any bearing repair program is to recognize potential problems through regular monitoring and inspection. Careful review helps identify the need for repair, such as:
- The bearing is nearing or has exceeded its suggested life expectancy
- Operating temperatures have exceeded 200 degrees F
- Exposure to excessive vibration
- Sudden changes in lubrication and temperatures
- Excessive operating audible sounds
- Loss of bearing seal integrity
The Remanufacturing Process
The type and degree of damage determine whether a bearing can be repaired and the appropriate method of repair. To accomplish this determination, a bearing will go through a thorough cleaning process before being disassembled by repair technicians who measure and record internal clearances and other parameters.
Subsequent inspection of the components looks for major problems or damage, such as fractures, major spalling or bluing due to heat damage. These are indicators that the bearing may not be eligible for repair. Components also are examined to determine the scope of work required to return them to a like-new condition. This ranges from recertifying a yet-unused bearing—cleaning, examining, verifying internal clearances, preserving and packaging—to reclamation, reconditioning or remanufacturing. Recondition generally combines recertification and reclaim services, in which bearing surfaces are polished, then preserved and packaged.
The ultimate step in bearing repair is remanufacturing, during which the service shop will clean, examine, grind raceways, manufacture new roller sets and major components as required, reset internal clearances, preserve and package. Further modifications may be applied to existing or new bearing assemblies to enhance performance, retrofit to special applications or upgrade to the most recent product designs.
Degrees of Damage
Specific damage modes encountered during a repair service include:
- Fretting–usually shows up in red or black oxides of iron occurring under close-fit conditions; also called friction oxidation
- Scuffing–smearing, scoring or galling as a result of removed and transferred metal from one bearing component to another due to sliding contact
- Staining–surface discoloration without pitting, such as from oil oxidation
- Wear–contact surface degraded and worn away by mechanical action in use
- Corrosion/etching–chemical action (rust) that attacks bearing component surfaces
- Debris denting–localized surface depressions caused by debris or foreign material
- Brinelling–permanent deformation (displaced metal, not just wear) of bearing surfaces at roller/raceway contact areas caused by excessive load or impact
- Spalling–breaking away of metal on raceway or rolling element in flakes or scale-like particles; also called flaking, fine grain or coarse grain spalling
- Heat checks–surface cracks caused by heat from sliding contact, usually formed in direction of motion
- Crack/fracture–significant visible surface cracks, usually caused by abuse or unusual operating conditions
According to Timken, bearings used in various types of mining equipment most often show damage from debris contamination as well as from shock-loading. Slightly improper concentricity is not as big a problem as it could be in other applications because mining equipment generally operates at relatively low rpm.
Various industries and applications may demand different scopes of repair service, but generally, repair service tends to fall into three types.
Type I service generally describes recertification or cleaning and inspection. Type 2 service involves reconditioning or polishing repair, while Type 3 applies to bearings with extensive damage. The Type 3 level of service is basically remanufacture, involving extensive processes such as regrinding of races, replacement of rollers or cage components and may even include replacement of a bearing race. Often, the regrinding of raceways will require the installation of oversize rollers in order to maintain bearing geometry and clearance in bearings where radial internal clearance is critical. In cases where lateral clearance is held, oversize rollers, new spacers or additional shims would be provided.
These levels of repair have traditionally been suited for bearings with a bore size of 8 in. outside diameter and greater. However, reclamation services can be applied to bearings as small as 3 in. in outside diameter. Smaller bearings that were often thrown away can now be serviced, if received in large quantities. Timken says turnaround time on reconditioning and repairs can be as short as two to four weeks, depending on the need and scope of work required.
The company warns, however, that although bearing repair has proven to be a cost-effective solution, like any service, it is subject to limitations. Bearings can be repaired, often more than once, but not indefinitely. A general rule of thumb is that bearings should not have more than three regrinds.
Timken advises that maximum stock removal on any race should be no more than 0.025 in. of the diameter and roller size should not exceed 0.015 in. in diameter from the OEM standard size. These recommendations help reduce the risk associated with altering the design integrity of the bearing. Timken says its guidelines are not industry standard rules, and some repair centers have no stated limitations on design alterations.
If done correctly, repaired bearings offer like-new performance. However, it is important to recognize and understand how repair options address damage modes. For example, polishing can address a variety of damage modes but is not effective for the removal of debris indentations or wear. Therefore, in such cases, do not expect like-new performance if the bearing is only polished, because that does not repair all damage. It is highly recommended to consult with a bearing manufacturer representative and/or application engineer to help determine the cause, extent and suggested repair of the damage.
It’s also important to have bearing repair performed by properly trained and experienced personnel because improper repairs can lead to additional damage and limited bearing life. Common repair mistakes include:
- Improper polishing techniques that cause changes to geometry and/or profiles that do not correct worn geometry and contact conditions
- Improper grinding techniques and processes that can cause surface cracks and damage or improper geometry and/or profiles
- Mixing of preset components
- Improper profile, internal geometry, finishes and clearance settings that can cause bearing failure
In summary, Timken advises prospective bearing-repair customers to seek out a high-quality bearing supplier with a repair program capable of providing:
- A range of service options
- Detailed explanation of the work scope to be performed
- Significant experience with bearings
- Replacement of any component back to OEM standard
- Product that matches the performance of a new bearing
- A warranty and the service to back it
- Quality assurance documentation
- The ability to service any product type and make.