Understanding Key Terms in Bearing Sizing and Installation: Basic Size, Dimensional Tolerance, and Limit Dimensions
During the process of bearing selection and installation, we often encounter professional terms such as “dimensional tolerance”, “basic size”, and “limit dimensions”. They sound quite “engineering-specific”, but in fact, understanding these concepts is crucial for improving assembly accuracy and extending bearing service life. Today, let’s clarify these three key terms together!
I. What is Basic Size?
The basic size refers to the theoretical dimension marked on the design drawing of a part, which can also be understood as the dimension in an “ideal state”. For example, if the inner diameter of a bearing is marked as 50mm, this 50mm is its basic size.
- It serves as the starting point of the design;
- However, it is difficult to achieve this exact value 100% in actual manufacturing;
- This is why “tolerance” and “limit dimensions” exist.
II. What is Dimensional Tolerance?
As we all know, mechanical manufacturing cannot achieve “absolute precision”—even the most sophisticated equipment will have a certain degree of error. The allowable range of this error is called dimensional tolerance.
Formula: Dimensional Tolerance = Upper Deviation Value – Lower Deviation Value
It specifies the maximum allowable range for the fluctuation of a part’s size.
For example: If the basic size of a hole is 50mm, and its allowable range is +0.02/-0.01mm, then its dimensional tolerance is 0.03mm.
Tolerances are not arbitrarily determined; they are based on national or international standards (such as ISO, GB, etc.). Different tolerance grades correspond to different precision requirements.
III. What is Limit Dimension?
Limit dimensions are the upper and lower size limits derived from the “basic size” and “dimensional tolerance”.
Let’s continue with the example above:
- Basic size: 50mm
- Tolerance: +0.02/-0.01mm
Then its limit dimensions are:
- Maximum limit dimension = 50 + 0.02 = 50.02mm
- Minimum limit dimension = 50 – 0.01 = 49.99mm
In other words, a part is considered qualified as long as its actual size falls within the range of 49.99mm to 50.02mm.
