Operating conditions
Useful life
The calculated life of a bearing is based on the load it supports and its operating speed, and is inversely proportional to the load of the bearing cubed. The maximum rated load of a bearing is for a life of 1 million rotations, which at 50 Hz (i.e. 3000 RPM) is a life of 5.5 working hours. 90% of such bearings have at least that much life, and 50% of bearings have at least 5 times that life.[8].
The calculation of the industry standard life is based on the work of Lundberg and Palmgren carried out in 1947. The formula assumes that the life is limited by the fatigue of the metal and that its value fits a Weibull distribution. There are many variations of the formula that include factors to take into account material properties, lubrication and loading. Load factoring can be seen as a tacit admission that modern materials demonstrate a different relationship between load and service life than that determined by Lundberg and Palmgren.[8].
Failure modes
If a bearing does not rotate, the maximum load is determined by the force that causes plastic deformation of the elements or the rolling grooves. The cracks caused by the elements can concentrate stresses and generate cracks in the components. The maximum load for bearings that do not rotate or rotate very slowly is called the "static" maximum load.[8].
Additionally, if a bearing is not rotating, the oscillating forces on the bearing can cause impact damage to the bearing grooves or rolling elements, known as punching out. A second minor form called false punching occurs if the bearing only rotates in a short arc and projects lubricant away from the rolling elements.
For a rotating bearing, the dynamic load capacity indicates the load at which the bearing is capable of withstanding 1,000,000 cycles.
If a bearing is rotating, but experiences a heavy load that lasts less than one revolution, the maximum static load must be used in the calculations, since the bearing does not rotate during the maximum load.[8].
If lateral torque is applied to a deep groove radial bearing, the rolling elements apply an unequal ellipse-shaped force on the outer ring, concentrating on two regions on opposite sides of the outer ring. If the outer ring is not strong enough, or if it is not sufficiently reinforced by the supporting structure, it will acquire an oval shape due to lateral torque, until the gap is large enough for the rolling elements to escape. The inner ring then slips out of position and the bearing collapses structurally.
A lateral torque in a radial bearing also applies pressure to the cage that holds the rolling elements at equal distances, because the rolling elements try to slide together at the location of greatest lateral torque. If the cage collapses or breaks, the rolling elements clump, the inner ring loses support and can come off center.
Maximum load
In general, the maximum load on a ball bearing is proportional to the outer diameter of the bearing multiplied by the width of the bearing (where the width is measured in the direction of the axis).[8].
Bearings have static load ratings, which are based on not exceeding a certain amount of plastic deformation in the rolling groove. These ratings may be considerably exceeded for certain applications.
Lubrication
For a bearing to function properly, it must be lubricated. In most cases the lubricant is based on the elastohydrodynamic effect (due to oil or grease) but to work at extreme temperatures, dry lubricated bearings are also available.
For a bearing to reach its rated life at its maximum rated load, it must be lubricated with a lubricant (oil or grease) that has at least the minimum dynamic viscosity (usually indicated by the Greek letter ) recommended for that bearing.[8].
The recommended dynamic viscosity is inversely proportional to the bearing diameter.[8].
The recommended dynamic viscosity decreases with the rotation frequency. As a rough indication: for less than , the recommended viscosity increases by a factor of 6 for a speed decrease of a factor of 10, and for more than , the recommended viscosity decreases by a factor of 3 for a speed increase of a factor of 10.[8].
For a bearing where the average of the bearing outer diameter and the shaft bore diameter is , and which rotates at , the recommended dynamic viscosity is .[8].
It should be taken into account that the dynamic viscosity of the oil varies greatly with temperature: an increase in temperature causes the viscosity to decrease by a factor of 10.[8].
If the lubricant viscosity is higher than recommended, the bearing life increases, approximately proportional to the square root of the viscosity. If the viscosity of the lubricant is lower than recommended, the useful life of the bearing decreases and the amount depends on the type of oil used. For oils with EP ('extreme pressure') additives, the useful life is proportional to the square root of the dynamic viscosity, as is the case with too high a viscosity, while for ordinary oils the useful life is proportional to the square of the viscosity if a lower viscosity than the recommended one is used.[8].
Lubrication can be done with grease, which has the advantage that the grease normally stays inside the bearing and releases the lubricating oil as the balls compress it. It provides an environmental protective barrier for the bearing metal, but has the disadvantages that this grease must be replaced periodically and the maximum load of the bearing decreases (because if the bearing becomes too hot, the grease melts and comes out of the bearing). The time between grease replacements decreases greatly with the diameter of the bearing: for a bearing, the grease must be replaced every 5000 working hours, while for a rolling bearing it must be replaced every 500 working hours.[8].
Lubrication can also be done with an oil, which has the advantage of a higher maximum load, but needs some way to keep the oil in the bearing as it usually tends to run out. For oil lubrication, it is recommended that for applications where the oil is not heated above , the oil should be replaced once a year, while for applications where the oil is not heated above , the oil should be replaced 4 times a year. For automobile engines, the oil exceeds , but the engine has an oil filter to maintain the quality of the lubricant; therefore, it is generally changed less frequently than bearing oil.[8].
Load direction
Most bearings are designed to support loads perpendicular to the axis ("radial loads"). Whether they can also support axial loads, and if so how much, depends on the type of bearing. Thrust bearings (commonly found in rotary tables) are specifically designed to support axial loads.[8].
For single row deep groove ball bearings, SKF documentation indicates that the maximum axial load is approximately 50% of the maximum radial load, but also notes that "light" and/or "small" bearings can support axial loads that are 25% of the maximum radial load.[8].
For single row edge contact ball bearings, the axial load can be about 2 times the maximum radial load, and for tapered bearings, the maximum axial load is between 1 and 2 times the maximum radial load.[8].
Conrad type ball bearings often show ellipsoidal contact flattening under axial loads. That means the inner diameter of the outer ring is large enough or the outer diameter of the inner ring is small enough to reduce the contact area between the balls and the rolling groove. When this is the case, stresses in the bearing can significantly increase, often invalidating general rules regarding relationships between radial and axial load capacity. With construction designs other than the Conrad type, the inner diameter of the outer ring can be further reduced and the outer diameter of the inner ring increased to limit this effect.
If axial and radial loads are present, they can be added vector-wise, to result in the total load on the bearing, which in combination with the rated maximum load can be used to predict its service life.[8] However, to correctly predict the rated service life of ball bearings, ISO/TS 16281 should be used with the help of calculation software.
Avoid unwanted axial loads
The part of a bearing that rotates (either the shaft bore or the outer circumference) must be fixed, while for a non-rotating part, this is not necessary (so it can be allowed to slide). If a bearing is loaded axially, both sides must be fixed.[8].
If a shaft has two bearings and the temperature varies, the shaft contracts or expands, and therefore, it is not permissible for both bearings to be fixed on both sides, since the expansion of the shaft would exert axial forces that would damage the bearings. Therefore, at least one of the bearings must be able to slide.[8].
A "free slip fit" is one in which there is at least a 4 µm clearance, presumably because the roughness of a surface machined on a lathe is typically between 1.6 and 3.2 µm.[8].
Adjustment
Bearings can withstand their maximum load only if the mating parts are properly sized. Bearing manufacturers supply tolerances for shaft and housing fitment so this can be achieved. The type of material and its hardness can also be specified.[8].
Non-slip fittings are manufactured with diameters that prevent slipping and, consequently, the mating surfaces cannot be moved into position without applying force. For small bearings this is best done with a press because striking with a hammer damages both the bearing and the shaft, while for large bearings the forces required are so great that there is no alternative to heating a part before assembly, so thermal expansion allows a slip fit to be performed.[8].
Avoid torsional loads
If a shaft is supported by two bearings, and the center lines of rotation of these bearings are not the same, then large forces are exerted on both, which can destroy them. Whether and how much very small misalignment is acceptable depends on the type of bearing. For bearings that are specifically made to be 'self-aligning', acceptable misalignment is between 1.5 and 3 degrees of arc. Bearings that are not designed to be self-aligning can accept a misalignment of only 2 to 10 arcminutes (0.033-0.166 degrees).[8].