Modern rolling contact bearings, when installed and lubricated properly, can outlast the machines in which they function. In practice, though, less than 10% of all rolling-element bearings reach their full design life. As for the others, 30% of premature failures can be attributed to incorrect installation or damage done during (or prior to) installation.
Jason Tranter, founder and managing director of Mobius Institute, has been involved with machine-vibration analysis, condition monitoring, and reliability improvement since 1984. He offers these important reminders for anyone who handles bearings.
Handling and installation damage
Use care when handling and assembling bearings so their rolling elements, race surfaces, and edges aren’t damaged. Gouges in the raceway or battered and distorted rolling elements will raise metal around the damaged area. This condition causes localized spalling as rolling elements pass over these areas.
Damaged bearing cages or retainers
Careless handling and use of incorrect tools during installation may damage cages or retainers (typically constructed of mild steel, brass, or bronze) and shorten the bearing’s life.
High spots and fitting practices
Carelessness or damage when driving outer races out of housings can cause burrs or high spots in the outer-race seats. If a tool damages the housing-seat surface, it can leave raised areas that transfer through the outer-race rolling surface, causing stress and shorter operating life.
Excessive preload or overload
Excessive preload can generate heat and cause damage similar to inadequate lubrication.
This situation results in a very small load zone and excessive looseness between the rollers and races outside the load zone. In turn, the rollers become unseated and skid and skew as they move in and out of the load zone. The problem is common with non-drive-end bearings, especially on vertical machines, and even more so with cylindrical roller bearings.
Misalignment and inaccurate seat, shoulder machining
The seats and shoulders supporting the bearing must be within specified limits set by the bearing manufacturer. When misaligned, the load on the bearing won’t be distributed along the rolling elements and races as intended, but concentrated on only a portion of the rollers or balls and races.
Improper fit in housings or shafts
The bearing race where the rotating load exists should be applied with a press fit. The stationary race would normally be applied with a light or loose fit. Where the shaft rotates, the inner race should normally be applied with a press fit and the outer race may be applied with a split fit or even a loose fit.
Impact and high static-load damage
Improper mounting practices and/or extremely high operational impact or static loads may lead to brinelling. When mounting a bearing on a shaft with a tight inner race fit, pushing the outer race will exert an excessive thrust load and bring the rolling elements into sharp contact with the race, causing brinell.
Electric current arcing
Arcing inside a bearing happens when an electric current passes through the component between the races and rolling elements. Each time the current is broken while passing between the ball or roller and race, a pit is produced on both surfaces. Eventually fluting develops.
According to Tranter, since premature bearing failure can result from issues other than improper handling, it’s important to always consult your bearing OEM and/or supplier for product-specific best practices. MT
—Jane Alexander, Managing Editor
Mobius Institute (Melbourne, Australia, and Bainbridge Island, WA) is a worldwide provider of a variety of reliability-improvement, vibration-analysis, and precision-maintenance training and certification services. For more information on the company and its upcoming International Machine Vibration Analysis Conference (IMVAC), in Orlando, visit mobiusinstitute.com.