MRO Magazine

Bearing Up, Part 1

The industrial world revolves around bearings. We ask a lot of them. Here are some tips for keeping them turning and avoiding unpleasant surprises.


Machinery and Equipment Maintenance

April 1, 2013
By By Carroll McCormick

The industrial world revolves around bearings. We ask a lot of them. Here are some tips for keeping them turning and avoiding unpleasant surprises.

 Tip 1 |

   Press fitting new bearings. “A pump builder once claimed that the bearings they purchased were the wrong size. The new bearings the guys were trying to mount on the shaft were too loose,” recalls Jennifer Moritz, training manager, SKF Canada, Toronto. “We measured the shaft and proved that it was undersized. Then we asked how many times they had put new bearings on the shaft. They said ‘six’.

“Pulling bearings off shafts removes shaft material. You can only do three or four press fits before enough material has been scraped off that the bearing might not fit properly. You need a new shaft or you need to ‘metal it up’, or metal spray it, then turn it down for the right fit,” Moritz advises.

 Tip 2 |

   Proper shaft alignment. Misaligning a motor to the opposing mechanical device can cause excessive stress and catastrophic bearing failure. “We had a large compressor motor come in for repair due to a shaft misalignment issue,” says Brad Stevenson, service manager, Gilbert-McEachern Electric, Brantford, ON. “The bearing had spun on the rotor shaft and damaged the end bell bearing journal [due to misalignment]. The bearing came apart and damaged the windings.

“Welding and machining was required on the rotor shaft and end bell, and the motor had to be rewound. This was a very costly repair due to a preventable mistake.”

 Tip 3 |

   Beware of textbook alignment tolerances regarding seals. Marcus Wickert, engineering division manager, NTN Canada, Mississauga, ON, emphasizes the interdependence of bearings, shafts, housings, seals and lubrication with the exposure of a common misapprehension about alignment. “Most people do not realize that even if bearings can tolerate a specific degree of misalignment, seals do not perform ideally under misaligned conditions.

“Take a common split pillow block. Even though the spherical roller bearing may have a catalogue rating of 1.5 degrees of misalignment, the accompanying seals can rarely accept more than 0.5 degrees of misalignment. Misalignment causes uneven seal lip pressure, with heavy compression on one portion of the contact surface, while leaving the opposite side under minimal contact and open to contamination.

“If you spend time to align, you will ultimately also benefit from improved sealing performance.”

 Tip 4 |   Check bearing shaft and housing fit. This is one of the most underestimated factors affecting bearing performance, according to NTN Canada’s Wickert. “An improper fit can allow the bearing to creep and often results in fretting wear or corrosion between the mating components.

“Fretting corrosion, a dark, rusty looking appearance, or mirror-like fretting wear, are key indicators of relative motion of two contact areas under load. When excessive amounts of either of these are observed, make sure to check that component tolerances are within specifications prior to installing a new bearing onto worn components.”

 Tip 5 |

   Properly size and install journal bearings. Burlington, Ontario-based Thordon Bearings Inc. specializes in water-lubricated, non-metallic journal bearings. To help shops select and correctly install journal bearings, Thordon offers a bearing sizing program. 

“We have a lot of plants that install bearings themselves. There are things that shops sometimes get wrong. If we run into a problem, it may be because the appropriate bearing material was not used, or it was not sized and installed properly,” says Keith Brand, business development manager for pumps and industry at Thordon.

“The sizing program takes a few minutes to fill out. It includes inputs for the application, temperature, pressure, vertical or horizontal pump, and bearing installation type. Once you fill in all this data, you will get a printout of how to size and machine the bearings.”

Don’t be shy about asking for help, Brand says. “We can assist you. This is especially important if a shop is doing this work for the first time.”

 Tip 6 |

   Use seals for high-moisture applications. To protect gearboxes against moisture damage, Gilbert-McEachern Electric’s Stevenson recommends using the proper lubrication, keeping the lubrication level up, changing the lubricant regularly and keeping moisture from getting inside.

Concerning the moisture challenge, SKF Canada’s Moritz advises, “I am against employing unnecessary expenses and fancy solutions when properly sealing the bearing cavity can resolve so many problems. Changing the bearing material doesn’t prevent moisture and/or particle ingress. Only proper and effective sealing – a comparatively cheap solution – can eliminate the root cause. 

“A simple V-ring seal is shockingly effective at contamination prevention, costs only a few dollars and takes little space to implement.”

 Tip 7 |

   Do not under-lubricate. It shouldn’t be allowed to happen, but letting bearings run dry is a common oversight. “A lot of people do not have a good grease schedule,” SKF Canada’s Moritz observes.

Gilbert-McEachern Electric’s Stevenson describes some of the damage he has seen due to under-lubrication. “In certain circumstances the inner race of the bearing has worn the bearing journal on the rotor shaft. This requires weld repairs and machining to get the journal back to the correct bearing tolerance. In some cases the bearings have completely come apart. The balls of some bearing have gone so long without the proper lubrication that they have worn flat.”

Prevention is simple, Stevenson says, “Perform routine lubrication using the proper lubrication specified by the motor manufacturer.”

 Tip 8 |

   Do not over-lubricate. On the flip side, attacking bearings with the old grease gun creates its own problems. Stevenson recalls two choice failures. “One customer lubricated the non-drive end bearing without removing the drain plug in the bottom of the end bell. The grease accumulated inside the bearing cavity, eventually filled the end bell and leaked into the winding/stator of the motor.

“Over-lubrication can also fill the air gap of the rotor stator and lock up the motor. Certain greases can carry the electrical current from the windings to the motor frame, causing it to go to a grounded state.”

One solution is to reduce the frequency of lubrication, says Stevenson. If there is a drain plug, remove it while lubricating the bearing. This lets contaminated grease drain from the bearing cavity and allows the new grease to access the bearing properly. MRO

Carroll McCormick is MRO Magazine’s award-winning senior contributing editor. Part 2 of Bearing Up will appear in our next issue.