What are dimensional tolerances, basic dimensions, and limit dimensions?

1、 What is the basic size?

Basic dimensions refer to the theoretical dimensions marked on the design drawings of the parts, which can also be understood as the dimensions in the "ideal state". For example, if the inner diameter of a bearing is marked as 50mm, this 50mm is its basic size.

It is the starting point of design;

However, it is difficult to achieve this value 100% in actual processing;

That's why there are "tolerances" and "limit dimensions".

2、 What is dimensional tolerance?

We all know that mechanical processing cannot achieve "absolute precision", even the most precise equipment will have certain errors. The allowable range of this error is the dimensional tolerance.

Dimensional tolerance=upper deviation value - lower deviation value. It specifies the maximum allowable range within which the dimensions of the part can fluctuate.

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.

Tolerance is not given arbitrarily, it is based on national standards (such as ISO, GB, etc.), and different tolerance levels represent different accuracy requirements.

3、 What is the limit size?

The limit size is the upper and lower limits of the size calculated based on the "basic size" and "size tolerance".

Continuing with the example above:

Basic size: 50mm

L tolerance:+0.02/-0.01mm

So its maximum size is:

Maximum limit size=50+0.02=50.02mm

Minimum limit size=50-0.01=49.99mm

In other words, as long as the actual size falls between 49.99mm and 50.02mm, this part is qualified.

Why do bearings need to pay attention to these dimensions?

As a high-precision component, bearings have very high requirements for installation dimensions. If the size deviation is too large, it will cause the following problems:

Excessive clearance: affects operational accuracy and generates vibration;

Tight fit: increases friction, causing heating or even jamming;

Accelerated wear: reduces bearing life;

Installation difficulties: affecting assembly efficiency and cost.

Therefore, understanding the dimensional tolerances and limit dimensions of bearings is a fundamental skill for every purchaser and engineer!