Bearing installation and removal basics

It is imperative to measure and check the shaft and housing bore to ensure it is within the recommended dimensional tolerances for the application and bearing size. Calibrated measuring tools such as a micrometer or slide caliper should be used. An eight-point measuring method is recommended for both the shaft and housing (see Figure 1). This involves four measurements at various locations to determine that the shaft and housing are not elliptical and the sides are parallel. Bearing installation guides and machinist’s handbooks will list the various types of fits and the exact dimensional tolerance for both the bearing rings, shafts and housings. If they are not within the suggested specifications, a diminished bearing life will occur. An oversized shaft or undersized housing will reduce the required internal clearance in the bearing. This clearance is necessary to make for free rotation of the rolling elements, lubrication film and the metal expansion that will occur because of thermal changes. Fits that are too loose will cause the bearing to walk or creep, pulling metal, which inevitably ends up in the bearing. Remember, measure prior to mounting.

The three general types of fits are: interference or tight, line-to-line, and clearance or loose. The fit is determined by the nature of the application, load, size, type of bearing and which ring is rotating.

The level of force required to properly mount a bearing is determined by the type and degree of press fit. In most applications, whichever ring is rotating will have the press fit. For example, an electric motor’s shaft rotates with the rotor. Therefore, the inner ring of the bearing will require a press fit and the outer ring will be a line-to-line or a clearance fit.

Bearings less than 50 millimeters (mm) in diameter may be mounted by using a bearing driver if the rings are installed with a light interference fit. Driving tools consist of a metallic tube, polymer impact rings and a dead blow hammer (see Image 2). This type of tube driver can be used with bushings and oil/grease seals. It is important to use the proper sleeve and impact ring size and ensure the bearing does not become cocked while being seated.

An arbor press is a safe method for removal and installation. The press should be bolted to a sturdy workbench or the floor. A rack and pinion along with a long handle are used to provide leverage. Pressing down on the handle lowers the rack. Never attach a pipe to the handle. Apply pressure slowly. A slotted plate, commonly referred to as the anvil, is used to support the machine piece and allow for through access of the shaft. Ideally, use only accessories designed for the press. The proper blocking of the bearing rings is essential as is the alignment and squaring of the workpiece for effective even force. When using a press, make sure that the bearing rings are blocked correctly so that the mounting forces are not transmitted through the rolling elements. These forces will cause dents in the raceway paths, known as true brinelling.

A hydraulic press is a useful and efficient machine to remove and mount bearings but its use comes with several caveats. It is imperative that the maintenance technician be given safety and proper use training. The hazards are numerous for the operator and there is a potential to damage the associated machined components. These presses are usually rated in tons of pressure. Keep your hands clear, identify the start/stop button and wear PPE. The same considerations that apply to the use of hand presses are applicable to the use of hydraulic presses. Squaring and alignment of the workpiece, proper blocking of the bearing and shaft, and slow, even pressure are required. Lower the ram slowly and observe where the force is being applied. The same person should insert/set up the work and operate the press. Do not exceed the working load limit of the press.

Thermal removal, installation & methods

Lowering or raising the temperature of the bearing, shaft, and/or housing within prescribed limits is another method to remove and install bearings. All of the cautionary notes mentioned for mechanical removal methods also apply to thermal methods. The shaft and housing should be clean and clear of nicks and burrs. Time must be allotted to measure and confirm that the shaft and housing are within the recommended dimensions. If the shaft or housing is not within the recommended tolerances, it should be replaced. Although the use of bonding agents may temporarily buy some time, bearing mounting compounds do not replace the requirement for a good dimensional fit.

The use of liquid nitrogen or dry ice to lower the temperature of steel components should only be used when there is an extreme interference fit. During winter months in northern climes, it is common for technicians to place large-diameter shafts outside overnight, then heat the bearing in the morning and assemble them together. Remember, a byproduct of this process may be the formation of water due to rapid temperature changes and the relative dew point. Water on steel equals rust.

There are a variety of bearing heaters such as the cone type, heated plates, pizza oven and the all-too-common oil bath. All of these types have two things in common: They are slow and usually dirty. For the mounting of bearings onto a shaft, the best means is to use a modern bearing induction heater (see Image 3). They are safe, fast, efficient and clean. Induction heaters are available in various sizes, with most being portable by hand or placed on a cart. They also can be used to heat other components such as coupling hubs and bushings.

Bearing induction heaters produce a strong, alternating, magnetic field to induce eddy currents in the metal. These currents cause rapid heating of the ring. It can be compared to a transformer using a primary coil with numerous windings, and a secondary coil with a few windings (the bearing). The secondary coil will deliver a low voltage at high amperage. The bearing acts as a short-circuited, single-turn, secondary coil through which passes a low alternating voltage at high amps. The result: The ring closest to the bar is rapidly heated. One of the byproducts of this process is magnetization. Modern heaters have an automated demagnetization cycle at the end of the process. Remember, the temperature probe should be placed on the inner ring for expansion to accommodate a press fit to shaft. The recommended temperature setting is 110°C or 230°F. The operator has complete control of the settings.

Bearing installation reminders

• Wear your PPE.
• Careful handling and cleanliness is important.
• Avoid steel hammers or sparks if at all possible.
• Inspect the shaft and housing. Remove burrs.
• Measure, measure and measure again.
• The fit matters.
• Use identical replacements.
• Avoid overheating.
• Use the right tools.
• Be safe, think safe, do it safe!

Conclusion

Installing a radial ball or roller bearing properly is critical to achieve the full service life of the component. For proper bearing installation to occur, it is imperative that all safety issues have been addressed, enough time has been allotted for the job and the procedure is understood by all before the work begins.

Richard R. Knotek is a Technical Training Specialist with the Motion Institute, a division of Motion Industries. He has worked 43 years with Motion Industries, holding a variety of positions including driver, inside sales, operations manager, salesperson, branch manager and product specialist. A former adjunct instructor with Northern Michigan University’s Industrial Maintenance Program, Knotek is also the published author of Mechanical Systems & Principles (ISBN 0-13-049417-8). For more information, visit motionindustries.com or the video channel MiHow2.com, which features educational videos including bearing installation tips.