By Joyce Laird, Contributing Editor
What are minimum loads?
First, an overview is needed. Rolling element bearings are used to reduce friction in rotating machinery by removing as much sliding friction as possible from a system by using rolling friction instead which has a lower coefficient of friction. However, even though rolling element bearings are attempting to reduce the total friction in a system, the individual rolling elements within the bearing still require a certain amount of friction to roll rather than slide. This internal friction is created by applying load to the bearing. This load can either be generated internally with a preload, or it can be generated by an externally applied load.
Eric Phaneuf, Applications Engineer, Industrial Market for SKF USA Inc. explains that for many radial bearings, it is common to provide a certain amount of space between the rolling elements and the raceways to allow for thermal expansion and prevent bearing seizure. “This internal clearance creates what is referred to as a loaded zone and an unloaded zone within the bearing. As the shaft rotates, the rolling elements pass into and out of the outer ring load zone. When the rolling elements pass in and out of the loaded zone, a change in the speed of the rolling element can take place. Without a minimum load on the rolling elements, the accelerations in and out of the load zone can be very detrimental. Minimum load is the force required to generate the traction in the raceway contacts to drive the rolling elements and ensure proper bearing performance,” he said.
Phaneuf goes on to say that if minimum loads are not met for a rolling element bearing, then several conditions could result which can drastically reduce the service life of the bearing. Skidding, which is sliding between the rolling elements and the raceways, can disrupt the lubricant film and lead to smearing damage. Smearing not only damages the rolling surfaces, but it also causes a temperature increase.
Loads are placed on the cage(s) within the bearing. Typically a cage is intended to keep the rolling elements from touching each other. However, when the minimum loads are not met, i.e. the traction forces are not present, the cage now has to drive the rolling elements rather than the traction forces of the raceways doing it. This generates unaccounted for loads that are applied to the cage and can lead to premature cage failure,” he added.
How does temperature affect load?
“Temperature does not affect the applied load to a bearing,” Phaneuf says. “It does however affect how the bearing handles the applied load internally. There are a couple of areas that can be impacted depending upon the magnitude of the temperature. If the temperatures are relatively high, the material properties of the bearing components can be altered such as polymer cages, heat stabilization temperature of the steel, seals, etc. Those are fairly simple to evaluate.”
“However, the internal clearance of the bearing and how it changes with temperature can have a significant impact on the size of the load zone in a bearing. When there is a large temperature differential across the bearing, i.e. hot shaft and cold housing, the internal clearance in the bearing will be reduced. This reduction of internal clearance changes the load zone from perhaps 150° to anywhere approaching 360°. The smaller the load zone, the cooler the bearing will typically operate. However, the larger the load zone, the better the load distribution but also the higher the operating temperature. If the internal bearing clearance is completely removed and a 360° load zone is created, the internal bearing loads can increase dramatically. This will create higher loads within the bearing and higher rolling friction. If a thermal runaway condition develops, bearing seizure is imminent.”
What are the key issues to watch for to ensure good bearing life?
The most common cause for premature bearing failure is poor lubrication. This can mean many things ranging from: incorrect type or quantity of lubricant, poor lubrication practices, to unforeseen application conditions which cause lubrication break down. Another prevalent cause of premature bearing failure is improper storage and handling as well as poor installation practices.
Phaneuf notes that bearings are precision components and can be easily damaged by poor handling and storage practices. “During installation, a bearing can be damaged if improper techniques are used, as well as issues of the bearing becoming contaminated before it is even put into service. These conditions can greatly reduce the service life of the bearing.”
How do you select the best bearings for the application?
Selecting the best bearing for an application requires intimate knowledge of the application. A few of the most important factors to consider when selecting a bearing are:
• Loads (direction, axial or radial, and magnitude)
• Type and method of lubrication
• Environmental considerations such as any process materials or other contaminants that may come in contact with the bearings during service
• Required design life for the bearings
In conclusion, Phaneuf reminds that bearings are precision components, not commodities. They see the most demanding parts of an application (loads and speeds). To ensure good service life of bearings, they need to be properly selected based on the application conditions, but then they also need to be properly maintained.
“This includes proper storage and handling techniques, as well as the periodic maintenance requirements during operation. Only in this way can the bearings and the equipment they are mounted in come close to realizing their design life.”
For more work by Joyce Laid, click here.