SELECTING A BEARING When designing and defining a type of bearing, the application, load to be withstood, its direction, rotational speed, temperature range, working environment etc. should be taken into account.
Average diameter (diameter between centres of the rolling elements)
Diameter of the balls
Number of balls
Compliance between balls and races
INTERNAL DESIGN The combination of the average diameter, the ball diameter and the number of balls, shall determine to a great extent the load capacity of a bearing.
For the construction characteristics of a bearing, contacts occur between the rolling elements and rings which withstand large loads.
Hertz pressures have as a result maximum compression stresses which can reach up to 4200MPa. These stresses cause the appearance of cracks.
By means of the ISO 281 Standard the duration of a bearing based upon material fatigue can be calculated; although when designing special bearings, the incorporation of other variables can alter these calculations.
A bearing is a precision part, and in the majority of cases it is the key element in the proper functioning of an assembly.
When the bearing is under load, the uniform distribution of said load on its rolling elements and raceways is crucial. To that end when designing the bearing several parameters must be taken into consideration.
PARAMETERS AFFECTING THE USEFUL LIFE OF A BEARING
• Compliance between balls and races An important factor in the design of a bearing, is to define the contact area of the balls with the raceways (compliance of the ball, with the radius of the race).
The contact surface between the ball and race, once the load is applied, is an oval surface. When there is increased contact between the race and the ball, the loading capacity increases but also the friction between them. Similarly, with smaller contact area, the friction is reduced, but the wear and tear in a localised area is greater.
• Formal Defect The raceways must be manufactured with a degree of accuracy consistent with the requirements of the bearing. Roundness, form, cylindricity defects, etc. which result in an uneven load distribution on rolling elements will cause stresses and fatigue points which significantly reduces the useful life of the bearing.
• Radial clearance The clearance or gap with which the bearing is assembled is crucial. To the extent that the clearance is increased, the load capacity of a bearing is reduced given that the number of rolling elements in contact with the raceway when it is working under load, is lower.
Similarly, an over-adjustment (reaching the preload) subjects the rolling elements to an excessive load which is added to the operation load.
Of the two circumstances (excessive clearance and preload) the latter is that which affects to a greater extent the useful life of the bearing.
In this arc or sector of its races which withstands the load is called the “LOADING AREA” and is measured in degrees.
The load which a bearing may be subjected to is as follows: