OPERATING CONDITIONS OF BEARING

Tolerance

When it comes to size, finish and diameter requirements, all bearings of like type must meet AFBMA (Anti-Friction Bearing Manufacturers Association) standards, regardless of bearing manufacturer or the ultimate use of the bearing. In tapered roller bearings, for example, cups and cones are interchangeable. No matter how sophisticated or refined the production method, there are variances in manufacturing that will affect the bearing’s dimensions.

Tolerance is the amount of deviation from prescribed nominal dimensions permitted by the industry. For example, if the nominal bore dimensions of bearing “x” (bore being the inside diameter for the inner race) is 1.838˝, and the tolerance is +0, -.0003˝, the actual size of the bore must be within the parameters established by tolerance levels. That is, the actual dimension of the bore could be as small – but no smaller – than 1.8377˝. It also can be no larger than 1.838˝, since bearing “x” cannot accommodate a larger diameter

Bearing Bore Diameter

“x” 1.838˝

Tolerance Bore Can Be

+0, -.0003˝ 1.8377-1.838˝

Our tolerance here (+0, -.0003˝) is only one example for a particular bearing “x” and is not representative of all bearings. In addition to the bore, there also are tolerance levels prescribed for the bearing’s outer race diameter, as well as for the width of both the outer and inner races. In addition to the variance allowed for the bore diameter, there is

also a variance permitted for the bearing’s radial runout. That is the running accuracy of the inner and outer races. Radial runout is measured for each race separately.

Internal bearing clearance

Clearance is necessary so that the rollers have room to turn without building up excessive heat and friction during operation. The amount the inner race moves as opposed to the outer race, under a given radial or thrust load, is called bearing clearance. This can be measured by how much space there is between the internal parts during operation Roller bearings need a small amount of space – internal clearance – to

prevent excess heat build-up when the rollers turn at higher operating speeds. For the best performance under radial loads, ball bearings should have minimal clearance. This is because the groove in both races of a ball bearing is designed to provide ample clearance.

Cage clearance

In tapered roller bearings, there should be enough clearance in the housing for the cage as well, because if the cage rubs against the housing it can cause the rollers to drag. If the clearance is not sufficient, the cage may become distorted and worn, resulting in misalignment and slanting of rollers. Premature bearing failure then becomes possible.

Bearing seats

Bearing races are mounted on areas called “seats.” The cup seat is the housing while the cone seat is the shaft. Within these two seats are upward extensions on which the races rest. They are called “shoulders.”

Alignment

The bearing cup and cone seats – the shaft and housing – must be properly aligned. Misalignment will reduce the capacity and life of the bearing proportionately to the amount of misalignment Here’s what happens. When the bearing is misaligned, the rollers will not carry the load along its entire length. They will carry the load, but only on a small portion near or at the ends of the rollers. This causes a concentration of load in a small area on the inner and outer race, which could result in chipping and early bearing failure

Shaft and housing conditions

To assure proper bearing performance, the condition of the area in which the bearing sits – the shaft and housing – is pivotal. Since the seat – the shaft as well as the housing – supports the bearing, there must be good surface-to-surface contact. When either seat has a rough finish or is not round, the bearing does not have the surface contact area necessary for proper performance. There should be no high spots or burrs. Any high spot in the housing will cause a corresponding high spot in the race. The high spot then will cause the load to concentrate in that small area. If the roller continues to hit this area, premature bearing failure will result. A burr on the shoulder or dirt on the shaft also can prevent the race from being seated properly. This may affect bearing adjustment  Correct fit and surface characteristics within both the shaft and housing are as important as surface conditions to bearing performance. If either race fits too loosely, the race will creep or turn during operation. This will wear down the shaft and/or housing, and change the bearing adjustment.

Operating temperature

Type of load, shaft speed, and amount of friction all contribute to one of the most critical conditions for operation – temperature. Each component of the bearing must be constructed of materials that not only handle the load but also accommodate temperature fluctuations. Not all heat is due to environment. The bearings themselves may cause excessive heat, because of:

1. Too heavy of a load, resulting in deformed races and rollers;

2. Friction between the rolling elements, retainer and races;

3. Excessive churning, from too much lubricant;

4. Surface friction, from too little lubricant

Lubrication

Using the right type and amount of lubricant for the job is another factor critical to bearing performance. Whenever bearing use causes excess friction, heat rises accordingly. Regular lubrication helps relieve the heat that results from bearing friction