First, an overview is needed. Bearings are used to reduce friction in rotating machines by removing as much sliding friction as possible from a system using rolling friction, which has a lower coefficient of friction.
What are the minimum bearing loads?
Although bearings attempt to reduce the total friction in a system, the individual rolling elements within the bearing still require some friction to roll rather than slide. This internal friction is created by applying a load to the bearing. This load can be generated internally with a preload or can be generated by an externally applied load.
Eric Phaneuf, industrial market applications engineer at SKF USA Inc. explains in a white paper that for many radial bearings, it is common to provide some clearance between the rolling elements and raceways to allow thermal expansion and prevent bearing seizure. . “This internal clearance creates what we call a loaded zone and unloaded zone inside the bearing. As the shaft rotates, the rolling elements move in and out of the outer ring load zone. As the rolling elements enter and leave the loaded zone, a change in rolling element speed may occur. Without a minimum load on the rolling elements, accelerations in and out of the load zone can be very damaging.
Why are they important for bearings?
Phaneuf goes on to say that if the minimum loads for a bearing are not met, several conditions can occur that can drastically reduce the life of the bearing. Slippage, which flows between the rolling elements and the raceways, can break the lubricating film and cause damage to the burrs. Burrs not only damage the bearing surfaces, but also cause an increase in temperature.
Loads are placed on the cage(s) inside the bearing. Typically, a cage is designed to prevent rolling elements from touching each other. However, when the minimum loads are not met, that is, the traction forces are not present, the cage must now drive the rotating elements rather than the traction forces of the tracks that do so. This creates unaccounted loads that are applied to the cage and can lead to premature failure of the cage,” he added.
How does temperature affect charge?
“Temperature does not affect the load applied to a bearing,” says Phaneuf. “However, this affects how the bearing internally deals with the applied load. There are some areas that may be affected depending on the magnitude of the temperature. If temperatures are relatively high, the material properties of bearing components may be changed, such as polymer cages, thermal stabilization temperature of steel, seals, etc. ”
“However, bearing internal clearance and how it changes with temperature can have a significant impact on the size of a bearing’s load zone. When there is a large temperature differential in the bearing, i.e. hot shaft and cold housing, the internal clearance in the bearing will be small. This reduction in internal clearance changes the load zone from perhaps 150° to something closer to 360°. The smaller the load zone, the colder the bearing will generally be. However, the larger the load zone, the better the load distribution, but also the higher the operating temperature. If the bearing internal clearance is completely removed and a 360° load zone is created, the bearing internal loads can increase significantly. This will create higher loads on the bearing and greater rolling friction.
What are the main issues to be observed to ensure good bearing life?
The most common cause of premature bearing failure is poor lubrication. This can mean many things, from: incorrect type or amount of lubricant, poor lubrication practices, to unexpected application conditions that cause lubrication to fail. Another common 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 inappropriate techniques are used, as well as bearing contamination issues before it is even placed into service. These conditions can significantly reduce bearing life. ”
How do you select the best bearings for your application?
Selecting the best detection for an application requires in-depth knowledge of the application. Some of the most important factors to consider when choosing a bearing are:
• Loads (direction, axial or radial and magnitude)
• Speed
• Temperature
• Type and method of lubrication
• Environmental considerations, such as process materials or other contaminants that may come into contact with bearings during service
• Required service life of bearings
In conclusion, Phaneuf reminds us that bearings are precision components and not raw materials. They see the most demanding parts of an application (loads and speeds). To ensure a good service life of the bearings, it is necessary to select them correctly according to the application conditions, but they must also be properly maintained.
“This includes proper storage and handling techniques, as well as periodic maintenance requirements during operation. Only in this way can the bearings and the equipment on which they are mounted come closer to the realization of your project. “
