1. What is minimum load?
First, an overview is needed. Rolling element bearings are used to reduce friction in rotating machinery by using rolling friction with a low coefficient of friction to eliminate as much sliding friction from the system as possible. However, even though rolling element bearings attempt to reduce the total friction in the 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 a load to the bearing. This load can be generated internally by preloading, or it can be generated by an externally applied load.
For many radial bearings, a space is usually provided between the rolling element and raceway to allow for thermal expansion and to prevent the bearing from seizing. “This internal clearance creates so-called loading and unloading zones within the bearing. As the shaft rotates, the rolling element moves in and out of the outer ring load zone. As the rolling element moves in and out of the loading zone, the speed of the rolling element changes. If there is no minimum load on the rolling element, the acceleration in and out of the load zone can be very harmful.
2. Why are they important to the bearing?
If a rolling bearing does not meet the minimum load, a variety of conditions can occur that can significantly shorten the life of the bearing. Slippage in the sliding between the rolling elements and raceways can damage the lubricant film and cause damage to the smear. Smearing not only damages the rolling surfaces, but also causes temperature increases.
The load is placed on the cage inside the bearing. Normally, the cage is designed to prevent the rolling elements from coming into contact with each other. However, when the minimum load is not met, i.e., when no traction exists, the cage must now drive the rolling elements instead of the raceway traction. This creates unexplained loads on the cage and can lead to premature cage failure.
3. How does temperature affect the load?
Temperature does not affect the load on the bearing. However, it does affect the way the bearing internally handles the applied load. Depending on the magnitude of the temperature, there are several areas that may be affected. If the temperature is relatively high, the material properties of the bearing components may change, such as polymer cages, the heat stabilization temperature of steel, seals, etc. These are easy to assess.
However, the internal clearance of a bearing and its variation with temperature can have a significant effect on the size of the load zone in the bearing. When there is a large temperature difference between the two ends of the bearing (i.e., the hot shaft and the cold housing), the internal clearance of the bearing will decrease. This reduction in internal clearance changes the load zone from approximately 150° to anywhere near 360°. The smaller the load zone, the lower the temperature at which the bearing will normally operate. However, the larger the load area, the better the load distribution, but also the higher the operating temperature. If the internal bearing clearance is completely eliminated and a 360° load zone is created, the internal bearing load will increase dramatically. This will produce higher loads and higher rolling friction within the bearing.
4. What are the key issues to look for to ensure good bearing life?
The most common cause of premature bearing failure is poor lubrication. This can mean many things, including: incorrect lubricant type or quantity, improper lubrication practices, and unforeseen application conditions that lead to lubrication failure. Another common cause of premature bearing failure is improper storage and handling and improper installation.
Bearings are precision components that can easily be damaged by improper handling and storage. Bearings can be damaged during installation if improper techniques are used, and bearings can become contaminated even before they are put into service. These conditions can significantly shorten the life of the bearing.
5. How do you choose the best bearing for the application?
Choosing the best bearing for the application requires an in-depth understanding of the application. Some of the most important factors to consider when selecting a bearing are.
(1) Load (direction, axial or radial and size);
(4) Type and method of lubrication;
(5) Environmental factors, such as any process materials or other contaminants that may come in contact with the bearing during use;
(6) The required design life of the bearing;
Finally, a reminder that bearings are precision parts, not commodities. They see the most demanding parts of the application (load and speed). To ensure a good service life for bearings, they need to be selected correctly for the application conditions, but they also need to be properly maintained.
This includes proper storage and handling techniques, as well as regular maintenance requirements during operation. Only then can the bearings and the equipment in which they are mounted come close to achieving their design life.