Extending the Life of Bearings

By Jay Alexander 

A bearing showing signs of wear after extended operation in a heavy industrial application is not necessarily destined for the scrap heap. Instead, that bearing can often be repaired – typically multiple times – enabling it to serve additional service life cycles.

Too often, a bearing is discarded without the user realizing there’s an economical option to extend its life. By using bearing repair services, plant managers and technicians can greatly enhance the bearing’s life while saving up to 60% of the cost of a new bearing. Bearing repair services can also return a bearing to like-new specifications in about one-third of the time it would take to procure a new bearing, depending on the required level of service. Furthermore, a successfully repaired bearing can frequently realize two or more subsequent life cycles that are comparable to its first life cycle.

With bearing repair service programs offering significant value in both time and cost compared to replacing bearings, let’s review how to determine whether repair services are right for your operations.

Why repair instead of replace?

A wide variety of factors – load, inadequate lubrication, contamination, temperature, maintenance intervals, installation errors and more – contribute to premature wear that causes less than 10% of bearings used in heavy industrial applications to reach their theoretical L10 life. Theoretical life, or L10 life, is the length of time at which 10% of bearings for a particular application are expected to fail due to fatigue.

Despite this tendency for premature failure, a bearing that has made it through its initial ‘wearing in’ period and reached its first service life cycle is primed for continued reliable operation if a qualified provider repairs the bearing

Advancements in bearing design, steel cleanliness, and maintenance and repair methods have greatly improved the effectiveness of repairs extending bearing life. A common question is whether a repaired bearing will last as long as a new one. Independent studies have shown that a properly repaired bearing will run a second service cycle comparable to the first. It’s often possible to realize up to two additional comparable service cycles before the bearing reaches its theoretical life. Therefore, those users not taking advantage of bearing repair services are likely missing out on years of extended operation, as well as the up-to-60% cost savings that bearing repair services offer.

A high-quality bearing repair program can help determine if and when a bearing can be repaired. It may also offer a warranty on the service. In addition, the program will use replacement parts featuring the same materials and tolerances specified by the OEM, as they have already proven reliable in the application.

Bearing repair eligibility

Bearing repair is an option for heavyduty applications in all industrial markets, including metals, cement, oil and gas, mining, power generation, pulp and paper, aggregates and more. All types of bearings, including tapered, spherical and cylindrical roller bearings, ball bearings, thrust bearings and cross roller bearings, are eligible for repair regardless of the original manufacturer. However, repair is not always the best option for a damaged bearing.

A reliable bearing repair service provider can help organizations make the sometimes difficult decision as to whether repair is an option, as well as which repair option is the most economical and long-term decision. For example, if a bearing is too worn, it’s not a good candidate for repair, as it won’t be able to be restored to like-new condition. In addition, bearings can also become worked into an out-of-round condition based on the roundness of the shaft or housing that supports it. Depending on the amount of this out-of-round condition, the bearing may not be considered for repair.

To help service providers assess bearing repair eligibility, it’s critical for organizations to engage in regular monitoring and inspection activities. Properly trained and experienced personnel involved in routine inspections serve as the first line of defence. Early detection of a problem through routine checks, preventive and predictive maintenance, and vibration analysis can reduce unnecessary downtime and expense and help to maximize the benefits of bearing repair.

Some telltale signs that repair needs are imminent include:

• The bearing nearing or exceeding its suggested life expectancy

• Operating temperatures exceeding 200°F

• Exposure to excessive vibration

• Sudden changes in lubrication and temperatures

• Excessive audible sounds during operation

• Loss of bearing seal integrity.

The repair process

Products returned to a bearing repair service centre undergo a detailed, documented process, from cleaning to final inspection and packaging.

First, the bearing is thoroughly cleaned. Next, it’s disassembled, during which repair technicians record bearing information and internal clearances, tagging each component with unique identifiers.

A detailed inspection of all bearing components follows to determine what’s required to return them to like-new condition. The inspection looks for major problems or damage, such as fractures, major spalling or bluing due to heat damage, as these issues indicate the bearing may not be eligible for repair. In addition, technicians measure the bearing’s bore and outside diameter (O.D.), as well as the roundness of major race components to determine whether repairs are feasible and, if so, the appropriate method of repair.

Bearing repair methods fall into three general service levels based on the extent of damage:

• Type 1 – Recertification: During recertification, bearing assemblies are cleaned, examined, measured for verification of internal clearances, and preserved and packaged for long-term storage and transportation. This process is often performed on unused products with outdated shelf lives so they are again available for use.

• Type 2 – Reconditioning: During reconditioning, bearing assemblies are cleaned, examined, polished, measured, preserved and packaged.

• Type 3 – Remanufacturing: During remanufacturing, bearing assemblies are cleaned and examined, raceways and ribs are reground, new rollers and other components are manufactured as required, and internal clearances are reset before the product is preserved and packaged. Often, the regrinding of raceways will require the manufacture of oversized rollers to maintain bearing geometry and clearance in bearings where radial internal clearance is critically held. In cases where lateral clearance is held, oversized rollers, new spacers or additional shims would be provided.

The above levels of repair are traditionally suited for bearings with a 12-in. inside diameter (I.D.) and larger. However, another service option – reclamation – exists for bearings as small as 3 in. in I.D., enabling smaller bearings that would typically be discarded to be returned to service. When handled in large quantities, bearings from 3-8 in. can be reclaimed at a fraction of the cost of purchasing new bearings. During reclamation, components are polished using a proprietary vibratory process that returns the used bearings to like-new condition.

Additionally, end users may use the repair process to modify existing bearing assemblies. Modification services can add special features that enhance performance, facilitate retrofits for special applications and enable upgrades to current product designs.

The goal for a bearing repair service provider is to turn around a quote and initial inspection report as soon as possible, but this is typically a one- to two-week process. Turnaround time on reconditioning can be as short as two to four weeks, depending on the need, complexity and size of the product, as well as the level of damage. The typical turnaround time for reclaiming smaller bearings is four to six weeks depending on the size of the order. The typical process time for remanufacturing is in the eight- to 14-week range based on the scope of work required.

Depending on the facility capabilities and level of damage, some repairs can be performed on-site using existing personnel or a bearing manufacturer’s service personnel. In general, on-site programs are suited for recertification or reconditioning processes, not for remanufacturing.

The equipment, tooling and knowledge to remanufacture bearings require substantial investment and commitment. A qualified bearing repair provider will perform significant repairs and manufacture replacement parts on the same types of equipment that would be used to manufacture new bearings.

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 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.

Repair limitations

Although bearing repair has proven to be a cost-effective solution, like any service, it’s subject to limitations.

Bearings can be repaired, often more than once, but not indefinitely. A rule of thumb is to not regrind bearings more than three times. Regrinding removes surface material, so it’s best to minimize the amount removed to reduce risks associated with altering the design integrity of the bearing. The Timken Company’s standards suggest a maximum stock removal on any race of 0.025 in. of the diameter, as well as not having roller sizes exceed 0.015 in. in diameter from the original equipment manufacturer’s standard size.

When fully restored, repaired bearings offer like-new performance. However, not every repair option addresses every damage type. For example, polishing is not effective for removing debris indentations or wear. If such damage is not repaired, a polished bearing will not deliver like-new performance.

Determining the cause, extent and suggested repair of the damage is best handled by consulting with a bearing manufacturer representative and/or application engineer. It’s also important to have any bearing repair performed by properly trained and experienced personnel, as unnecessary, or inadequate repairs can lead to additional damage and limited bearing life. In addition, proper equipment is required to ensure damage has been reviewed and properly removed.

Maximizing value

To maximize the value of repairs, consider using the bearing repair process as a troubleshooting tool to minimize future bearing damage. Taking corrective actions may improve the likelihood of getting closer to the bearing’s L10 life.

Planning when to remove bearings from service for reconditioning can greatly enhance the total life cycle value of bearings. If removed from service too early, the bearings aren’t being used to their full potential. Leaving bearings in operation for too long increases the risk that they will surpass the point of economical repair. Operating somewhere between these two conditions can optimize cost savings.

A good bearing inspection report is a smart first step in trying to solve performance issues. For example, if the report details black staining and rust on raceways, the likely cause is water ingress. If the raceways show abrasive wear, foreign material has probably entered the bearing. In either case, the bearings’ seals are likely the root cause of the problem. The corrective action is to investigate the seals and improve their maintenance and/or redesign them to minimize bearing contamination.

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 life line. A quality bearing repair program can result in significant time and cost savings compared to discarding worn bearings and purchasing new ones.

 Jay Alexander is general manager for Timken Industrial Bearing Services – North America. For more information, use the link below. http://bit.ly/MRO701Apr15.