Grades of Precision
NPB® Bearings are manufactured to standard grades of precision with established tolerances for size and geometric accuracy. These standards are known as ABEC classes as set by the Annular Bearing Engineers Committee of the American Bearing Manufacturer’s Association (ABMA). These standards are also accepted by (ANSI) American National Standards Institute and by international agreement for the standards developed by (ISO) International Organization of Standardization.
The ABEC scale is a system of rating the manufacturing tolerance of precision bearings. The system was developed by the Annular Bearing Engineering Council (ABEC), a division of the American Bearing Manufacturers’ Association ABMA). The ABMA was formerly known as the Anti-Friction Bearing Manufacturers’ Association. Bearings rated under the ABEC systems are typically called “precision bearings”, with a rating or class from 1 to 9. ABEC 1 meets a looser tolerance and ABEC 9 meets high precision tight bearing tolerances. ABEC class is not to be confused with radial play, which is discussed under that section of our website. Tolerances control the dimensional accuracy of a bearing. We use ISO bearing tolerances which start at P0 and then move upwards in precision grade to P6, P5 and then P4. ISO tolerances are measured in thousandths of a millimetre (or microns).
ABMA standard 20 defines 5 classes: ABEC 1, ABEC 3, ABEC 5, ABEC 7, and ABEC 9. Ball bearings are defined in standards 12.1 and 12.2 and carry the suffix “P”. Extra thin and thin section ball bearings defined in standards 12.1 and 12.2 carry the suffix “T”. The higher the number of the classification, the tighter the tolerances become resulting in higher precision of the assembled bearing. When the classification system was first set up, machine tool technology was such that manufacturers mass produced only ABEC 1, and sometimes ABEC 3. Bearings for the higher classifications were selected from the production runs of the lower classification. Today, the technology has advanced to the point where manufacturers can produce ABEC 7 and ABEC 9 bearings routinely. ABMA (ABEC) bearing tolerances are also commonly used and these are measured in ten-thousandths of an inch starting at Abec1 then upwards to Abec3, Abec5 and Abec7. Confusingly there are slightly different Abec tolerance grades for instrument bearings (Abec1P, Abec3P, Abec5P, Abec7P) and for thin-section bearings (Abec5T, Abec7T).
Tolerances have no effect on radial play although it is sometimes mistakenly thought that improving the tolerances will produce a bearing with less play. Assuming that the shaft and housing are manufactured to the same tolerances as the bearing, higher bearing tolerances will produce better mating between shaft/housing and bearing, lower noise and vibration due to improved roundness and lower starting and running torque (also subject to radial play and lubricant).
While all bearings are extremely precise mechanisms, a designer must consider the benefits of tighter tolerances in terms of performance and bearing life. For example, there is a direct correlation with precision class and bearing life. Raceway parallelism, a feature controlled by ABEC, can impact bearing torque. A non-parallel raceway will result in torque spikes. In high speed applications, bearing runout can result in an imbalance in the rotating mass. These conditions can lead to premature failure and unpredictable life. Of course, the higher precision levels have the downside of higher cost.
While the bearing tolerance classes primarily control boundary dimensions of the rings, it should also be noted that there are features that are critical to the bearings performance and life that are not controlled by the ABEC (or ISO) specifications. These include: internal clearance, surface finish, ball accuracy, torque, noise, cage type, and lubrication. These items should be specified along with the precision class when selecting bearings to ensure optimum life and performance. In many cases, it is one of these features, not precision level, that when specified properly yields the desired performance and or life.
Many countries throughout the world have their own standards organizations. For example, Germany, Japan, Korea, Russia, and China are major bearing producers and all have industrial standards that cover ball and roller bearings. In almost all cases, these standards are similar to or state equivalency with ISO 492. The table below shows the equivalent tolerance grades for three of the most common standards.
|ANSI Standard 20||ISO 492||DIN 620|
|ABEC 1||Class Normal||P0|
|ABEC 3||Class 6||P6|
|ABEC 5||Class 5||P5|
|ABEC 7||Class 4||P4|
|ABEC 9||Class 2||P2|
For exact tolerance limits, please view our TOLERANCE TABLES.