Regardless of the design or manufacturer, bearings often deviate from expectations due to factors such as improper installation, contamination, inadequate lubrication or misalignment. In fact, less than 10% of bearings used in a heavy industrial application reach their design L10 life. Advancements in technology, materials, condition monitoring and reliability centered maintenance programs combined with economic pressures contribute to an increased potential for successful bearing repair programs.
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. Depending on the scope of work, a repaired bearing can save up to 60% of the cost of buying a new bearing. Some bearing repair sources will also offer a warranty on their service. When compared to the manufacture of a new bearing, bearing repair is considered a more environmentally-friendly procedure, requiring less energy input and reducing raw material consumption and waste.
Bearing repair eligibility
Although it offers many benefits, repair is not always the best option for a damaged bearing. The challenge of properly utilising bearing repair services is determining if and when bearings need to be repaired and deciding which option is the most economical and of value in the long-term. Depending on the repair facility, limitations exist on the minimum and maximum size of bearings and product types that can be repaired. There are many different types of repair suppliers, ranging from small facilities limited in their scope of work and knowledge to large bearing manufacturers with an unlimited range of products and services. The scope of work also limits the size of bearing that can be repaired. All bearing types are eligible for repair regardless of the original manufacturer, including ball bearings, class 3 precision bearings (all types), cross roller bearings, cylindrical roller bearings, spherical roller bearings, tapered roller bearings, thrust bearings (all types) and triple ring bearings.
A critical step in any bearing repair program is to recognise potential problems through regular monitoring and inspection. Careful review of the output will help to identify the need for repair, such as:
The bearing is nearing or has exceeded its suggested life expectancy
Operating temperatures have exceeded 200 degrees Fahrenheit
Exposure to excessive vibration
Sudden changes in lubrication and temperatures
Emits excessive noise
Loss of bearing seal integrity
The re-manufacturing process
Once a product is returned to a repair service centre, all bearings undergo a thorough cleaning process. Next, the bearing is disassembled. During disassembly, trained repair technicians will record the bearing information, record actual internal clearances and complete the disassembly and tag with unique identifiers.
Next, a detailed inspection of all the bearing components is performed and findings are recorded. The initial inspection includes looking 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 are also examined to determine the scope of work required to return them to a like-new condition.
A wide range of repair services/methods are available. Depending on the facility capabilities and the level of damage, some repairs can be performed on-site using existing personnel or a bearing manufacturer’s service personnel. In general, onsite programs are suited for recertification or reconditioning processes, not for the remanufacturing process. Below is a detailed description of the repair service levels.
Recertification (Type 1): Bearing assemblies cleaned, examined, measured for verification of internal clearances, inspected, preserved and packaged. This process is used to recertify a bearing for service –generally applying to an unused product with an outdated shelf-life.
Reconditioning (Type 2): Bearing assemblies cleaned,examined, polished, honed or tumbled to remove minor surface defects (primarily rust or corrosion), measured for verification of internal clearances, inspected, preserved and packaged.
Re-manufacturing (Type 3): Bearing assemblies cleaned, examined, raceways reground, new roller sets and major components manufactured and replaced as required, internal clearances reset, inspected, preserved and packaged.
Once the proper repair choice is made and the process completed, the bearings are reassembled and packaged for storage and transportation. Generally, a final inspection is performed on the bearing to ensure that it meets the assembly criteria specified by the bearing design. Again it must be stated that different suppliers perform different levels of inspection and packaging. Bearing manufacturers that perform bearing repair often follow the same procedures as with a new bearing.
Types of damage
The types of damage discovered during bearing repair vary significantly. Damage may be evident in the form of visible corrosion, scuffs, stains and dents. Or, it may be barely noticeable, such as fine cracks or fractures that indicate the potential for more serious damage to occur.
Typical bearing damage falls into the following categories:
Chemical damage: Etching, stains, corrosion pitting, rust or fretting corrosion
Heat damage: Discoloration or checks
Electrical damage: Burns, fluting or pitting
Mechanical damage: Fatigue flaking, cracks, spalling, fractures, nicks, peeling or smearing, brinelling, indentation, scoring, abrasive wear, installation damage,misalignment or lubrication failure
Most resource manuals describe the above types of damage and offer methods to help eliminate their causes. However, resource manuals often fail to mention that, with the exceptions of torch heat damage, extreme spalling, fractures and heavy etching, most damage conditions are repairable. Therefore, it’s recommended to contact a bearing service technician to provide a damage assessment and determine the feasibility of a repair.
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. Regrinding removes surface material, so it needs to be done carefully. If done correctly, repaired bearings offer like-new performance. However, it is important to recognise 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 since that does not repair all damage. It is 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 is also critical to have any bearing repair performed by properly trained and experienced personnel because unnecessary 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 addition to expertise, proper equipment is required to fix the problem and ensure damage has been reviewed and properly removed. The appropriate measuring equipment, such as laser tracing and profiling equipment, CMM and precise measuring machines, are essential to perform thorough inspections on repaired product.
Choosing bearing repair
When a bearing is damaged, the entire operation will suffer, resulting in additional costs, lengthened maintenance work schedules, unnecessary downtime and extended delivery times to final customers. To reduce these undesirable circumstances, bearing repair offers a fast and economical option to extend the life of a bearing further along its theoretical lifeline. A quality bearing repair program can result in significant time and cost savings compared to discarding and purchasing new bearings. In addition, the lead time for repair is substantially less than that of a new bearing.