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Have you ever found yourself staring at a bearing catalog and feeling completely lost? The technical jargon and cryptic codes for bearing tolerances can feel like a foreign language. At Emerson Bearing, we're here to simplify things for you. Our goal is to guide you through the process of selecting the perfect bearing for your needs. To help you understand the language of bearing tolerances, we've put together this blog post.
The basic structure of a bearing includes an inner ring with a specific thickness, an outer ring with its own defined thickness, and rolling elements that sit between them. These components must meet strict standards, such as those outlined by ISO 492 and DIN 620. Bearing tolerances are grouped into three main categories: **dimensional accuracy**, **form accuracy**, and **running accuracy**.
**Dimensional accuracy** pertains to the external dimensions of the bearing. Different letters represent varying dimensions, such as:
- The bore diameter (the size of the inner ring) is denoted as "d."
- The outer diameter is represented by "D."
- The width of the inner ring is labeled as "B."
- The width of the outer ring is marked as "C."
These tolerances measure the difference between the intended size and the actual size of the bearing, often measured across a single plane. The symbols for the average bore and outer diameter tolerances are dmp and Dmp, respectively.
**Form tolerances** focus on how closely a bearing adheres to its ideal shape, including properties like cylindricity and perpendicularity. These are indicated by the letter "V," with Vdmp and VDmp representing the maximum deviations in bore and outer diameter tolerances. Both dimensional and form tolerances play a critical role in ensuring proper fit between the bearing and its housing or shaft.
**Running accuracy**, on the other hand, relates to how well the bearing performs during operation. This includes factors such as radial and axial runout, as well as side runout for both the inner and outer rings (denoted as Kia and Kea). Considering these tolerances early in the design phase helps reduce vibration and misalignment issues in your machinery.
Bearing precision is another key consideration when choosing the right part for your application. Most standard applications work fine with bearings of normal precision (ABEC1/P0). However, for specialized uses like machining tools or precision instruments—where high speed, quiet operation, or exceptional accuracy are required—bearings with tighter tolerances are necessary.
The precision level of a bearing is determined by its tolerance class. In ISO 492 standards, higher numbers indicate greater precision, whereas in DIN 620 standards, lower numbers correspond to better precision (except for normal tolerance, which is P0). Below is a table showing how ISO and DIN standards align in terms of increasing precision.
| **ISO Tolerance Class** | **DIN Tolerance Class** |
|--------------------------|-------------------------|
| ABEC1/P0 | P0 |
| ABEC3/P6 | P6 |
| ABEC5/P5 | P5 |
| ABEC7/P4 | P4 |
| ABEC9/P2 | P2 |
At Emerson Bearing, we're committed to helping you find exactly what you need. Whether you're looking for standard bearings or something more specialized, our team is ready to assist. Reach out to us today or explore our online resources, including our Catalog and the *Art of Precision Bearings* eBook.
For further assistance or to discuss your specific requirements, don’t hesitate to contact us. We’re here to ensure you get the best possible solution for your project.
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June 02, 2025