Reliability tip of the month
This monthly bulletin features short technical articles produced by SKF Reliability specialists.
Focusing on practical and technical matters, the Tip of the Month covers various topics such as bearing maintanance, damage analysis, condition monitoring and lubrication.
Bearing Maintenance and Technology
Am I using the wrong tool with bearings?
How do you know if you’re using the wrong tool to mount or dismount a ball or roller bearing?
Simple check: if you have to reach for a hammer, chisel, punch, screwdriver or torch to install or remove a bearing, you’re generally on the wrong track, and you might actually get hurt.
Here are the details:
Hammer and/or chisel:
applying force directly to bearing parts can cause chipping and cracking of the through hardened steel. Too many people have been injured, some seriously, to take this risk.
Hammer and punch:
only proper for mounting an eccentric locking collar-type bearing. Locking nuts will be damaged and chipped out with this combination of tools.
often used to remove bearing seals or shields. This tactic will not improve bearing lubrication! If your bearing isn’t performing, consult SKF for help.
on installation, you risk seriously overheating the bearing, losing hardness of the steel which will cause premature failure. On removal, a torch can crack rings (see above) or permanently bend the shaft you’re working on.
Alignment and soft foot
Machine feet perform many important functions. They constrain the machine for normal operating loads and in the event of a sudden failure such as a shaft lockup or a bearing seizure. Additionally, the feet are used to position the machine with respect to the other machine components in the train. The feet are also an important part of the machines structural or vibration dynamics.
A change in the bolted condition of the machine feet from the manufacturer’s recommendations can affect any or all of these important functions. The use of improper bolt grade, for example, may alter the strength of a foot joint and may not perform as designed in machine accident. Improper bolt torque techniques, damaged washers, and uneven bolting surfaces prevent precision machinery movements while aligning and create a constantly moving target. Changes in bolting stiffness due to incorrect clamping forces can alter machine/base stiffness and may induce resonances in a machine that used to perform well.
Notice in the photograph the uneven foot surface due to a rough casting and repeated bolt tightening. A soft washer was cut at one time in an attempt to fit the radius of the foot pad and is now so severely cupped that it is impossible to accurately position the machine horizontally.
As part of any machine alignment or installation, always inspect the bolts and washers to ensure they are of the proper type and length and that they are in serviceable condition. Replace any soft washers with hardened washers. If the clearance hole in foot is enlarged, damaged, or rough, the use of a ground plate and/or machining of the foot surface will ensure the best possible clamping condition.
Last, and most important, always use a torque wrench with the proper sockets and adaptors when tightening machine feet. Follow a tightening sequence and if all of the softfoot has been removed in advance, the machine will move predictably through the alignment remain in position when placed in service.
Pocket-sized book saves BIG in the pocket
Tip #6 says: “Pay attention to the bearing’s press fit. Use a press for any bearing under 4 in. O.D. Pressure should be applied only to the bearing ring with the press fit, which is usually the ring that rotates after the bearing is installed.”
There are many “whys” beneath the surface of these recommendations. A press fit of the inner ring of a bearing to a shaft literally stretches the ring like a rubber band, although on a very small scale.
Using proper mechanical tools such as a fitting tool evenly distributes mounting forces across the side face of the bearing when mounting the bearing. This is fine for smaller bearings up to 4 inches (100 mm) Outside Diameter (O.D.). The forces required to mount larger bearings could be great enough to either damage the shaft during mounting (adhesive wear) or to crack a ring since they are generally made of throughhardened steel. There is also a risk of personal injury if a chip cracks off during mounting. Don’t risk it.
For bearings larger than 4 inches (100 mm) O.D., use temperature mounting. Heat the bearing or cool the shaft to achieve a 150°F (~80°C) temperature difference between the bearing ring and the shaft. When mounting bearings with a press fit in the housing the same temperature difference is required. Place the wrapped bearing in the freezer and a shop lamp in the housing. Cover with a flameproof blanket to retain the heat. In 30 minutes or so the temperature difference will be safe for mounting. Alternatively, some housings can be heated with an induction heater if the heater has the proper capacity.
Heating a bearing with an induction heater is the safest, fastest method for shaft mounting larger bearings. Ensure that the heater has temperature controls to control heat and expansion and to prevent over heating. Additionally, only use heaters that provide an automatic demagnetization cycle.