Keep The Wheels Turning

Bearings are a vital component of all major industrial equipment.  They increase efficiency of equipment by reducing friction, and enable it to deliver improved performance. However, the useful life of any bearing often depends on the care and maintenance it receives. This is particularly true in industrial applications, where operating conditions tend to be harsh, loads are heavy, and contamination from dirt and scale common.

To ensure reliable operation and high performance from bearings, it is essential to implement best practices in removing, inspecting and cleaning bearings during maintenance operations and in maintaining and handling bearings during the installation process. If a bearing is contaminated before it is installed, the reliability and performance of the bearing could be compromised. Here are some tips that will help increase the life cycle of bearings.

Removing Bearings
Each type of bearing design has a unique removal process. Regardless of the bearing type, the bearing must be removed with extreme care. If done incorrectly, you can damage the bearings, shafts or housings, requiring repairs. For smaller bearings, there are a variety of pullers available to assist with bearing removal.

For bearings installed with a tight or press fit, or that have become locked in place on a shaft and cannot be removed with a mechanical puller, the inner ring of the bearing can be heated to ease removal. Heat lamps or other heating devices can be used. If a torch is used, it will change the properties of the bearing steel—and could lead to the scrapping of the bearing.

Lifting large bearings
Large bearings can be lifted and moved using a variety of slings, hooks, chains and mechanical devices. Some large bearings are manufactured with tapped holes in the face of inner rings or outer rings. Eyebolts or other points of attachment can be inserted in these lifting holes.
 
Many large bearings have threaded lifting holes in the cage ring that can be used to lift the inner assembly. A clean, heavy-duty nylon sling provides one of the best ways to handle large bearing components because it eliminates the possibility of burring or scratching.
Regardless of what method is used to lift the bearing, exercise extreme care to avoid damaging any of the bearing surfaces. Be especially cautious when lifting or moving bearings that are equipped with a cage. The cage is typically the most deformable component of the bearing, and is more susceptible to damage.

 

Cleaning
After removing a bearing from a piece of equipment, thoroughly clean it to remove all scale, water, lubricant, debris and any other contaminants. Bearings must be cleaned thoroughly to allow for proper bearing inspection.

Smaller bearings can be cleaned in a wash tank that circulates a cleaning solution such a kerosene, mineral sprits or a commercial solvent through the bearing. Use the cleaning solution to remove all lubricant and contamination, making sure that the internal rolling elements are completely clean.

Alkali cleaners, such as trisodium phosphate (TSP) mixed two or three ounces per gallon of hot water, may also used. Hot cleaning solutions are often used as a final cleaning or rinse after the initial cleaning.
For large bearings, or to clean large numbers of bearings, special cleaning equipment such as a large tank containing appropriate cleaning solution is required. Tanks are typically heated with electricity coils, and a pump is used to agitate the cleaning solution. Final cleaning is done by suspending the bearing and using a hose to flush away any contamination.

To reduce bearing contamination from other sources, all parts of the housing, shaft  and gears should also be thoroughly cleaned. After the bearing has been cleared, it can be dried with compressed air, taking care not to let the bearing spin.

After cleaning, the bearing should be carefully inspected for damage and wear. If the bearing is not going to be returned to service immediately, it should be covered with a coating of light oil to protect against rust and corrosion.


Inspection
When a piece of equipment is taken out of service for routine inspection or maintenance, take the opportunity to also inspect and measure the bearings to ensure that they are still within tolerance specifications for the application. In some applications, the expected bearing life may be the limiting factor in the equipment maintenance schedule.

The schedule for equipment tear-downs for bearings inspection will vary depending on operating conditions. Between equipment tear-downs where full bearings inspections are conducted, you should conduct routine inspections to ensure that bearings are operating normally and have proper lubrication. To reduce the need for these inspections, and to more closely monitor bearing and equipment health, condition monitoring systems are available that sense the vibration and temperature in bearings.

The inspection area must be clean and free from dirt and debris to avoid contaminating the bearings. Even a small piece of debris that enters a bearing can create a point of high stress that could lead to spalling, and early fatigue.

In addition to examining the bearing, a full inspection should include the housing and shaft. Check for burrs or metal chips on the inner and outer ring seats and backing surfaces. Burrs or chips can be removed by scraping or filing the damaged surfaces.

Inspect the shaft for proper size, roundness, burrs or other damage. A 12-point check of the shaft with a micrometer is a good idea. If there is evidence of shaft or housing wear, it should be checked against equipment manufacturer’s specifications.

Storing Bearings
While bearings with special anti-corrosion treatment are available, but most bearings are not manufactured from corrosion resistant materials. When handling and storing bearings, care must be taken to ensure that they will not rust or corrode. Even a small amount of moisture or chemical left on an unprotected bearing by a glove or hand can result in a small etched area, which may initiate bearing fatigue.

New and remanufactured Timken bearings are shipped with a protective coating, are typically covered in a protective paper or other wrapping, and are shipped in a carton or crate. When receiving a new or remanufactured bearing, do not remove it from its packaging until ready to install in the application.

Do not store bearings directly on concrete floors, where water can condense and collect on the bearings. Store the bearings on a pallet or shelf, in an area where the bearings will not be subjected to high humidity or sudden and serve temperature changes that may result in condensation forming.

Always put oiled paper or, if not available, plastic sheets between rollers and cup races of tapered roller bearings.


Installation of Bearings
Do not remove the bearing from its packing until you are ready to mount it. The packaging provides protection from contamination. When installing a new bearing, do not remove the lubricant or preservative applied by the manufacturer. The preservatives used on almost all bearings are fully compatible with commonly used oils and other lubricants. Leaving it in place will protect the bearings from fingerprints and corrosion.

Bearings should be installed in a clean environment, free from dust, debris, moisture and other contaminants. When installing a bearing in the field, make an effort to ensure clean work area. Use protective screens around the work area, and provide a clean resting surface for the bearing and other components until they can be installed.

Before beginning the installation, plan your work. Be certain that you have the correct replacement bearing and necessary additional adjustment tools. Finally, if the bearing needs to be lubricated as part of the installation process, have the appropriate lubricants and tools available. Planning your work will enable you to perform the installation faster, and shorten the amount of time the bearing is out of the equipment and exposed to contamination and possible handling damage.

Thoroughly clean all machine components near where the bearing will be installed, giving special attention to the mounting surfaces and housings. Housings should be cleaned, including blowing out the oil holes. If the equipment has blind holes where air is ineffective, use a magnetic rod to remove metal chips that might have become lodged there during machining or maintenance.

Shaft surfaces that will support and contact the bearing must be clean and free from nicks or burrs. Shaft shoulders and spacer rings contacting the bearing should be square with the shaft axis. The shaft fillet must be small enough to clear the radius of the bearing.

Do not install bearings in a damaged or worn housing, or on a damaged or worn shaft. Inner and outer ring seat damage should be repaired by using properly fitted sleeves. Shafts can be built up by metal spray and machined to the correct size. If there is not a press fit on the shaft, a weld overlay and re-grind process is recommended to bring the shaft back to specification.

Heating Bearings
In applications that require a tight fit of the inner ring on the shaft, it can be easier to install the bearing if it is heated to expand slightly. For applications that require a tight fit of the outer ring in a housing, it may also be possible to heat the housing to expand it, allowing the bearing to install more easily.

Small bearings can be heated using several methods. They can be heated in a pan or metal container filled with oil. A screen or platform should be used to keep the bearing from resting on the bottom of the pan where heat is applied. A heat lamp can also be used to heat rings, and the temperature regulated by adjusting the distance from the light to the ring.

The fastest method of safely heating bearings is an induction heater. Induction heaters work very quickly. Take care to avoid heating the bearing to temperatures higher than 120 C (250 F).

For larger bearings, you may need to use an oil bath to heat the bearings. Maximum temperature of the oil bath should not exceed 120 C (250 F). The bearing should be positioned in the center of the tank, and allowed to heat long enough to fully expand. Do not allow the bearing to come in direct contact with the heat source. Bearings typically require 20 to 30 minutes of soak time per inch of inner-ring cross section to fully heat in an oil tank. Keep the bearings away from any localized high heat source that may raise the bearing temperature too high, resulting in race hardness reduction.

While the bearing is still warm, remove it from the heater or tank and place it on the shaft. Slide the bearing up the shaft until it squarely contacts the shaft shoulder. Then install the locknut/ washer or clamping plate to prevent the bearing from backing off the shoulder while cooling. As the bearings cools, the locknut or clamping plate should be tightened.

Press-on Bearings
Smaller bearings may be pressed onto the shaft or into a housing with an arbor and mounting tube. Between the press ram and the bearing, use a tube of soft steel with an inside diameter slightly larger than the shaft. The outside diameter of the tube should not exceed the maximum shoulder height for the bearings. The tube should be square at both ends, thoroughly clean inside and out, and long enough to clear the end of the shaft after the bearing is mounted.

Never attempt to make a press fit on a shaft by applying pressure to the outer ring of an assembled bearing. However, because they have a separable outer ring, the outer ring of tapered roller bearings can be pressed into a housing.

Adjusting Bearing Clearance
The space between the rolling elements and the races of an anti-friction bearing is known as the bearing clearance, referred to in tapered roller bearings as the lateral, lateral clearance or end-play. In other types of anti-friction bearings such as spherical, cylindrical, or ball bearings, the radial internal clearance or RIC, is specified. Clearance is desirable in applications where allowance must be provided for thermal growth of components, to accommodate for slight misalignment or other application requirements.

Bearings may also have zero clearance, with the contact surfaces of the rolling elements brought into contact surfaces of the rolling elements brought into contact with races in line-to-line contact.
Finally, bearings may have rolling elements and races brought into contact with a defined initial force, a condition known as preload. This enables precise control over the internal geometry of the mating parts, and is desirable where run-out must be held within critical limits, such as high precision applications.

Bearings with separable races, such as tapered roller bearings, allow the clearances (preload) to be “adjusted” to meet application requirements. Other types of bearings are manufactured with a known clearance or preload, but the clearance can be slightly reduced through an interference fit on the inner or outer race.

Sameer Ticku is Key Account Manager for Reliability Services at Timken India Ltd.