Bearing tolerances explainedKen Thayer | December 21, 2018
Ball bearing precision is an important concept for design engineers to understand. Although most people realize that the ABEC rating is a measure of a bearing's precision, many do not understand what it actually represents. A common perception is that the higher the ABEC rating the better. While it is true that bearings with a higher ABEC rating are manufactured to tighter tolerances, a higher precision bearing is not necessarily better for every application. Many applications are ideally suited for lower precision bearings. As bearing costs typically increase with bearing precision, selecting the incorrect ABEC rating can lead to an unnecessary increase in project cost.
Developed in 1917 as an industry trade group by a committee of the world's leading bearing manufacturers, the American Bearing Manufacturers Association (ABMA) developed tolerance standards for bearing producers to follow when producing radial ball bearings. These quality standards, referred to as the Annular Bearings Engineers Committee (ABEC) tolerance ratings, provide tolerances for a number of ball bearing dimensional criteria, including bore diameter, radial runout, side runout, raceway runout and bearing ring widths. Form tolerances for the bearing inner and outer ring raceways are included in these standards and are important to meet runout specifications. Form tolerances include the roundness, taper, runout, parallelism and race radius of the raceway.
The ABMA defines five main tolerance classes, ABEC 1, ABEC 3, ABEC 5, ABEC 7 and ABEC 9. The higher the class, the higher the precision and the tighter the tolerance specifications. ABEC 1 bearings are precision mechanical components, but bearings rated as ABEC 3, 5, 7 or 9 have extra precision.
As previously stated, ABEC ratings are strictly dimensional and form standards and they do not specify other parameters such as speed, noise, ball precision and material quality. While there may be a relationship between ABEC rating and these attributes, they are not part of the specification. However, there is a direct correlation between ABEC rating and bearing life. A bearing with non-parallel raceways or higher runouts can create an imbalance when rotating at high speeds. This imbalance can result in higher noise and vibration levels as well as a shorter life. Therefore, bearings with higher ABEC ratings will have a longer life at high operating speeds than one of a lower tolerance class, but the ABEC rating itself is not an indication of speed limitations. Due to this ability to run smoother at higher speeds for longer periods of time, ABEC 7 and ABEC 9 bearings, referred to as super precision bearings, are ideally suited for demanding high-speed applications such as machine tool spindles.
ABEC 1 and ABEC 3 bearings are the most common, with more than 98% of the bearings sold falling into this precision range. Mass-production of bearings with low tolerance ratings (ABEC 1 and sometimes ABEC 3) is relatively easy. High-tolerance bearings, such as ABEC 5, ABEC 7 and ABEC 9, were difficult and expensive to manufacture for the better part of the 20th century. As late as the 1980s, these high-precision tolerances were so difficult to routinely obtain that some manufacturers inspected the quality into these bearings — meaning they inspected rings and bearings after manufacturing and then sorted them into the appropriate higher tolerance class. Equipment technology advancements, such as hard turning, have progressed and routinely manufacture high-precision bearings today.
ABEC issues radial ball bearing tolerances for both inner and outer rings, along with gauging measurement methods used during the manufacturing and inspection of the bearings. The tolerance specifications prescribed by these standards are defined by ABMA STD 20.
Some dimensional tolerances, such as bearing bore and inner or outer ring widths, are pretty straightforward to understand while others are less obvious. Below is a brief description of the specifications dictated by the ABEC standards and the recommended methods for gauging them.
Bearing bore is the diameter of the inside diameter of the inner ring. Inside diameter is measured in several places and radial planes using a two-point measuring device. This measuring method can be used on all types of rolling element bearings. If the size and weight of the bearing is such that the bore size is influenced by gravity, the bearing should be placed in a horizontal position.
Inner Ring Width
Inner ring width refers to the individual width of the inner rings, not the total width of the bearing. To measure the width of the inner ring, one side of the inner ring is supported in three places and the outer ring is free. The inner ring width is measured with a calibrated indicator opposite the three support locations.
Inner Ring Width Variation
Inner ring width variation refers to the width difference between the largest and smallest width of the inner ring using the method indicated above.
Radial runout for radial ball bearings (other than angular contact bearings) is measured by mounting the bearing on an arbor that is determined to have a diameter that is straight to less than .0002 in/inch length taper. The outer ring is held stationary while the inner ring (arbor) is rotated one full revolution. The difference between the lowest and highest readings on an indicator placed in the center of the outer ring is the radial runout.
Raceway Axial Runout with Reference Side
The raceway axial runout with reference side is measured by supporting the outer ring in a horizontal orientation such that it is held stationary. An indicator is placed on the top, center of the inner ring and an arbor with a taper of less than .0002 in is placed in the bore. Apply a force to the inner ring via the arbor sufficient to fully seat the balls in the raceway. The raceway axial runout is the difference between the maximum and minimum reading through one revolution of the inner ring.
Reference Side Runout with Bore
Reference side runout with bore uses the same set up as the one used for radial runout, except the indicator is placed on the center and side of the inner ring. The side runout is the difference between the minimum and maximum readings through one revolution of the arbor.
The chart below lists inner ring tolerances for ABEC 1 through 9 bearings. Precision bearings rated ABEC 5 and higher may have additional tolerance requirements such as bore taper.
Tolerance and measurement standards for outer rings are similar to those for inner rings.
The outside diameter of the outer ring is a two-point measuring technique that can be used on all rolling element bearing types. Measure the diameter of the bearing in several angular directions. If the size and weight of the bearing is such that the bore size is influenced by gravity, the bearing should be placed in a horizontal position.
Outer Ring Width
The outer ring is measured in much the same way as the inner ring. One side of the outer ring is supported in three places and the inner ring is free. The outer ring width is measured with a calibrated indicator opposite the three support locations.
Outer Ring Width Variation
As with inner ring width variation, outer ring width variation refers to the width difference between the largest and smallest width of the outer ring using the method indicated above.
Radial runout for the outer ring is performed with the same setup that is used for measuring the inner ring radial runout, except the inner is held stationary and the outer ring is rotated one full revolution. The bearing is mounted on an arbor determined to have a diameter that is straight to less than .0002 in/inch length taper. The difference between the lowest and highest readings on an indicator placed in the center of the outer ring through one revolution of the outer ring is the radial runout.
Axial Runout of Raceway with Reference Side
To perform the axial runout of raceway with reference side measurement, place the inner ring on a stationary arbor with a taper less than .0002 in/inch length. Apply a load to the outer ring sufficient to seat the balls in the raceway in a manner that provides repeatable readings. With an indicator placed on the center, side of the outer ring, rotate the outer ring one revolution. The difference between the minimum and maximum readings is the axial runout of raceway.
Outside Diameter Runout with Reference Side
Outside diameter runout with reference side is obtained by placing the outer ring on a flat reference surface such that the inner ring is free to rotate. The outside diameter of the outer ring is placed against a stop and an indicator is placed against the outer ring directly above the stop. Rotate the inner ring one full revolution and calculate the outside diameter runout by subtracting the minimum indicator reading from the maximum reading.
The chart below is a listing of outer ring tolerances for ABEC 1 through 9 bearings.
Skateboard bearings, designated as skate rated, may claim to be ABEC 8, 10, 11, 12, 13, 15, 18 or 20. Despite using the ABEC terminology, there is no correlation or connection with true ABEC standards set by the ABMA. Any ABEC designation other than 1, 3, 5, 7 or 9 is not a reputable ABMA rating.
The American National Standards Institute (ANSI) has approved the ABEC standards from the ABMA and adopted them as U.S. national standards (ANSI/AMBA 20-2011). Other standards developing organizations (SDOs) produce ABEC equivalent standards, including ISO 492:2014 by the International Organization for Standardization (ISO), JSA JIS B 1514-1 by the Japanese Standards Association (JSA) and DIN 620-1 by the Deutsch Industrie Norm (DIN).
ABMA’s Roller Bearings Engineers Committee (RBEC) develops standards for spherical and cylindrical roller bearings that follows similar designations as ABEC. All of the SDOs also develop standards for roller bearings, including tapered roller bearings, and instrument (miniature) bearings.
The following table displays bearing tolerance classes for various bearing types and SDOs. Tables, similar to the ABEC ratings above, are available for each SDO, bearing type and tolerance class.
ABEC standards, developed by the AMBA, are important bearing precision specifications that indicate the tolerances of a ball bearing. The ABMA issues five ABEC quality standards: 1, 3, 5, 7 and 9 — the higher the ABEC rating, the tighter the tolerances.
Tolerances also change with different bearing bores and outside diameters. The smaller the bore and OD, the tighter the tolerances. Although bearings with higher ABEC ratings may be capable of higher speeds, the ABEC rating does not specify performance criteria such as speed or noise.
As bearings with higher ABEC ratings are typically more expensive than lower-rated bearings, engineers should take care to specify the proper bearing that optimizes performance requirements and costs for a given application.