The Best Methods of Measuring Bearing Tolerances
How well do you know bearing tolerances? The American Bearing Manufacturers Association (ABMA) defines the tolerances and grades them using the ABC grades. Understanding these tolerances is crucial to helping you select, install and maintain them in the perfect condition. This post brings you everything that you need to know about measuring bearing tolerances.
Deviation determines how far the actual measurement is allowed from the nominal dimension. The nominal dimensions are the dimensions that are shown on the manufacturer's catalog. If there were no international tolerance standards outlined for bearings, it implies that every manufacturer would use own dimensions. With deviation tolerance, the bore can be smaller but not bigger than the nominal size.
Mean Bore/OD Deviation
Mean bore (commonly known as single mean bore diameter deviation) is a crucial tolerance when you have closely mate the outer ring and housing or inner ring and shaft. One thing that you need to appreciate is that the bearing is not round. Depending on the point on the inner ring where you take the measurement, you can get readings of about 8 mm and 7.99 mm. Therefore, what should be the bore size? This is where the mean deviation comes in. You need to take several measurements on a single radial plane across the bore.
The above diagram demonstrates an inner bearing ring with the arrows showing the measurements across the bore but on varying directions. Note that additional sets of measurements should also be done in other radial planes to ensure that the bore falls within the right tolerance levels on its length.
This diagram demonstrates how not take the measurements. Note that the measurements are taken at different radial planes (differing points on the bearing ring).
The mean bore size is calculated using the following method. If you say that the mean bore deviation tolerance for a common PO bearing is +0/8 microns, it means that the mean bore can be between 8.000 mm and 7.992. Apply the same principle for the outer ring.
This is the deviation of the single outer or inner width from the factory (nominal) dimensions. Here, the width should be controlled within a specific tolerance. Because the width is less crucial, the tolerances should be wider than for the OD or bearing bore. A width deviation of +0/-120 implies that when measuring the outer or inner ring width at any point, a 4mm bearing, it should not be narrower than 3.88 mm or wider than 4mm.
This tolerance is meant to ensure the roundness of bearings. In the drawing above, of a badly out-of-round 688 inner ring, the longest measurement is 9.000 mm while the shortest is 7.000 mm. Now, get the mean bore (9.000+7.000/2=8.000). While it is true that 8.000mm fall within the bore deviation tolerance, it would be unusable. This means that variations and deviation end up being useless without each other.
Single Bore/OD Variation
Single bore is a Bore/OD Diameter Variation taken on a single radial plane. If you take a look at the diagram above, you will notice that the left section has measurements 7.996mm and 8.000mm. The variation between the smallest and largest is 0,0004mm. This implies that the diameter of the bore in this single radial plane is 4 microns.
Mean Bore/OD Diameter Variation
Because of the single bore/OD deviation and mean bore/OD deviation, the bearing is now close to getting the correct size. Indeed, it is also round enough but one question still remains: “What if the taper on the bore/ OD is too much as indicated on the diagram above?” This is the reason for the development of the mean bore/ OD variation limits.
To get the right mean bore/OD variation, you need to record the OD or mean bore on different radial planes and establish the difference between the smallest and largest. Here is an example. Assume that the measurement on the left image (starting at the top) has a mean bore size of 7.999mm, the middle has 7.997mm, and the bottom has 7.994mm. Now, subtract the smallest from the largest (7.999-7.994mm=0.005mm). The answer is the mean bore.
This is another straightforward variation. Assume, for a specific bearing, the allowed width variation is 10 microns. If you measure the outer or inner ring’s width at different points, the biggest measurement should not surpass 10 microns greater than the smallest measurement.
The radial runout of a fully assembled bearing’s inner or outer ring is another very important component of tolerances. Think of it this way. Suppose the mean deviation of both the inner and outer ring falls within the accepted limits and roundness also within the accepted variance, then, is that all that one would need to think about? The answer is no! Check the above diagram. The width varies. The right width, like everything else, is different at various points on the circumference. However, the radial runout tolerances determine to what this extent this can vary.
Inner ring Runout
The inner ring runout is assessed by measuring all points on a single circle of the inner ring during a complete revolution when the outer ring is stationary. Then, the smallest measurement is subtracted from the largest. The radial runout figures provided in the tolerance tables demonstrate the allowed maximum variation. The difference manifested in ring thickness is exaggerated in order to demonstrate the point more vividly.
Outer ring runout
The outer ring runout is calculated by measuring all points on a single circle of the outer ring on a complete revolution when the inner ring is stationary. Then, the smallest measurement is subtracted from the largest.
This tolerance ensures that the inner surface of your bearing is as close as possible to a right angle with its inner ring face. The tolerance levels for face runout/bore are only provided for bearings of P4 and P5 precision grades. Note that all the points a single circle of the inner ring bore close to the face are calculated in a single revolution when the outer ring remains stationary. Then, the bearing is turned over and the other side checked. Then, the largest measurement is subtracted from the smallest.
Face Runout/OD is the variation of the outside surface generatrix inclination with the face. This tolerance is meant to help ensure the outer ring of the bearing is close enough to a right angle with the face of the outer ring. The figures for the face runout/OD are provided for P4 and P5 grades. Note that all points on a single circle of the outside ring bore adjacent to the face are calculated based on one revolution. However, the inner ring must remain stationary. Then, the bearing is turned over and the outer side of the ring measured. Finally, the largest measurement is taken away from the smallest to get runout/OD bore tolerance.