Load carrying capacity of a bearing
The illustration below shows a shaft mounted fan driven by a belt and powered by a motor. Two bearings support the shaft and are subjected to loads as follows:
Radial loads originate from the:
A.Weight of the shaft
B.Weight of the pulley
C.Tension of the belt
D.Weight of the propeller
Note: Radial loads exerted on the ends of the shaft, outside of the two bearings supporting the load (i.e. the belt tension, pulley weight and propeller weight), are compounded by a lever affect and are referred to as overhung loads.
Axial loads originate from the wind (E) induced by the propeller rotation.
Combination loads are the result of both radial load(s) and axial load(s) being combined and exerted on a single bearing.
To ensure a define load carrying capacity of ball and roller bearings for a given application, it is essential to have a clear idea about the running speed, the size and direction of the loads and the designed expectation of life.
The magnitude of load is always influenced by the running speed. For every bearing number its maximum load carrying capacity is specified, corresponding to various running speeds. This load carrying capacity is considered to be valid only for the bearing having normal dimensional and running accuracy, proper methods of lubrication and a speed which yields a life expectance of approximately 5000 running hours.
The Balls (3)
Balls have been defined previously as one of the essential bearing components.They are subjected to the full brunt of the load carried by the bearing. If ball and roller bearings are subjected to high temperatures in service, their load capacity is reduced as a result of softening of the race rings and rolling elements. Once it occurs, the effect is permanent, as the material does not regain its original hardness even if the temperature drops.
Under normal conditions, cages carry very little load. However, when a bearing is not installed properly, is subjected to loads and speeds higher than recommended by the manufacturer, does not maintain proper lubrication, etc., the cage then may be subjected to loads far beyond what it is able to carry. These conditions can lead to premature cage failure.
The Raceways (5)
Raceways are the large, honed (highly polished), track surfaces on the inside of the outer ring (referred to as the outer raceway) and the outside of the inner ring (the inner raceway), that form a closed circle around the circumference of the ring. As the bearing rotates the rolling elements run on these surfaces.
Considering the other factors, the load should always be determined as accurately as possible, particularly with respect to its magnitude and direction. Because it exerts a decisive influence on the life of the bearing, it is very important to take into account of all the force acting on the bearing.
The Load Zone and Contact Points
When a bearing is supporting a radial load, the load is distributed through only a portion of the bearing—approximately one-third (1/3)—at any given time. This area supporting the load is called the bearing load zone.
Radial ball bearings are probably the most widely used and most recognized ball bearing. These bearings have one row of balls (referred to as a single row), that revolves around the ball path. This feature provides another name for the bearings; they are commonly called deep groove ball bearings. Although designed to primarily carry radial loads, a radial ball bearing’s raceways are deep enough that it can also carry reasonable thrust loads. (However, if thrust loads are excessive, an alternative type of bearing should be considered.)