Pumps & Systems, February 2009
Rolling bearings play vital roles as the hearts of rotating machinery (pumps included) by supporting shafts, locating loads, enabling rotation and reducing friction. When bearings need to be replaced, improper dismounting and mounting techniques can shorten their life expectancy.
For example, when dismounting a bearing to replace it with a new one, potential damage to machinery shafts can occur if the procedure is mishandled. This can jeopardize machine and bearing health. When a new bearing is mounted, poor fitting, brute installation force or incorrect tools can lead directly to premature bearing failure.
These pitfalls can be avoided by following best practices when replacing bearings. Operators should first confirm that a new bearing is identical to the one being replaced (bearing type, bore size, outside diameter, width and any other special requirements). Adopting proper methods and appropriately prescribed tools can contribute to successful outcomes.
Proper Dismounting
The process of dismounting bearings can be a hazardous and demanding task often involving powerful forces, and care must be taken not to damage other machine components, such as the shaft or housing.
No matter how tempting, operators should avoid using welding torches or hammer blows to dismount a bearing. These approaches may be quick, but they can be dangerous, risk damage to machinery shafts and make subsequent bearing failure analysis either difficult or impossible.
Almost universally for most bearing designs, pullers (sometimes hydraulically assisted) have emerged as preferred tools. A wide variety of user-friendly, safe and simple puller designs provide operators with a range of options to meet particular application demands presented by a bearing's size and type.
Choosing the right type of puller that offers the necessary maximum withdrawal force will be critical to completing the job safely and easily.
As examples, two- or three-arm jaw pullers allow for even pulling pressure and are engineered to prevent damage to the bearing or the bearing seating during dismounting; combined internal and external versions can grip both the outside and inside of a component with equal strength; and heavy-duty jaw pullers provide ideal alignment and shaft protection, as well as a superior grip to dismount medium- to large-size bearings. If the bearing is to be reused, a good rule is never to apply pulling force through the rolling elements. Hydraulic or hydraulically assisted versions will also deliver the benefits of effortless force generation.
One type of puller used for dismounting deep groove ball bearings. The claws facilitate a precise fit in the bearing's raceways to provide a good grip and the application of higher dismounting forces.
With the proper tools in hand, these tips can help advance the process:
- Bearings should never be struck directly with any hard object, such as a hammer or chisel. Untold difficulty and damage will follow.
- Puller selection should allow for sufficient pulling force to help prevent puller overload. Overloading a puller can result in puller arm or beam breakage and subsequent operator injury. A safety blanket fitted around the puller and bearing can prove beneficial in case the bearing, the puller's arm or spindle breaks.
- Operators will have to exert considerably higher force when the bearing's interference fit is corroded. Penetrating oil can be applied to dissolve or loosen the rust before the bearing is dismounted.
- Wear protective clothing and goggles as precautionary safety measure.
Mounting Methods From "Cold" to "Hot"
Proper mounting of ball and roller bearings begins with care in their storage and handling.
All rolling bearings should be stored in a cool, clean, low humidity environment free of dust, shocks and vibrations. (For these reasons, avoid storing bearings directly on the floor.) Large rolling bearings should only be stored lying down and, preferably, with support for the side faces of the rings. If kept in a standing position, the weight of the rings and rolling elements may cause permanent distortions, because the rings are relatively thin-walled. Also, false brinelling (marking of the raceways and rollers caused by residual vibration) is much less likely to occur in large bearings that are lying flat.
Users should be cautious when storing sealed or shielded types during long periods of time. The lubricating properties of the grease used to fill these bearings may deteriorate, eventually resulting in associated potential problems.
The importance of cleanliness cannot be over-emphasized. All rolling bearings should be kept clean. Contamination and corrosion will tend to shorten the life of any bearing.
Before a bearing is ready to be mounted, operators should confirm:
- The replacement bearing conforms in all aspects to the type being replaced.
- Housing and shaft are clean and undamaged.
- Housing and shaft are dimensionally accurate.
- Lubricant to be used is clean and correctly specified.
- Necessary tools and equipment are at hand.
The primary methods for proper mounting of a cylindrical bore bearing are commonly referenced as "cold" or "hot," consistent with their enabling technologies. Cold mounting is generally recommended for cylindrical bore bearings with outside diameters up to 4-in, and heat mounting methods are applied for the cylindrical bore larger bearings. Tools have been developed to suit each particular method.
For cold mounting, force can be applied to the bearing by placing a fitting tool, impact ring and sleeve against the bearing ring face with the interference fit and using a press or similar device to advance the bearing evenly to its proper location on the shaft. Before proceeding, the bearing should be positioned exactly at right angles to the shaft, and the shaft should be lightly lubricated with thin oil.
The force required for mounting a bearing will increase consistently with bearing size. Because of this need for greater force, larger bearings cannot easily be pressed on a shaft or in a housing. Hot mounting, where the bearing is pre-heated, provides a practical solution to allow for a bearing's expansion and easier installation, while maintaining specified interference fit when the job is completed.
With this process, the temperature difference between the bearing and seating will depend on the magnitude of the interference fit and the bearing size. Normally, a bearing temperature of 80-deg C to 90-deg C (144-deg F to 162-deg F) above that of the shaft is sufficient. Unless otherwise specified, bearings should never be heated to a temperature greater than 125-deg C (257-deg F); heating limits may apply to sealed/shielded or other prelubricated bearings. Extreme heat can cause the bearing's metallurgy to change, which can alter bearing dimensions or hardness.
Localized overheating of a bearing should always be avoided. In particular, a bearing should never be heated using an open flame, such as a welding torch, which can risk shaft and bearing damage, result in lack of uniform heating and temperature control, and present fire and health hazards.
Proper and safe hot mounting can be accomplished using specialized bearing heaters, which heat the bearing only. Versions include induction heaters, ovens, electric hot plates and new generation products that incorporate three different heaters and specialized design to reduce heating time, power consumption and associated costs.
One caution: Induction heaters produce an electric current, which magnetizes the bearing. Operators should subsequently demagnetize the bearing to prevent the attraction of metal particles and potential contamination leading to premature failure. (Note: Some manufacturers' induction heaters will automatically demagnetize the bearing at the end of the heating cycle.)
Regardless of mounting method, these tips can also prove helpful:
- Do not remove the bearing from its wrapping until the last moment.
- Do not try to wash the bearing. (The bearing's preservative is compatible with mineral based oils and greases and should only be removed from the bore and the outside diameter of the bearing's outer ring. A lint-free cloth dampened with a cleaning agent is suitable for this purpose.)
- Apply mounting forces only to the bearing ring with the interference fit.
- Use a minimum force with maximum control method.
Conclusion
How bearings are replaced will impact their performance, reliability, efficiency and service life and, ultimately, the operation of system machinery. Partnering with an experienced bearing manufacturer and service provider can help optimize results.
