A Vital Part of the Metric System

In our previous blog post, we covered the tolerance system. By using the ISO tolerance system, any combination is possible to get the needed fit. In order to limit the number of tools as well as the number of machine elements, ISO created the base hole/base shaft system.

In the base hole system, the hole is given a fixed tolerance. Since it is easier to modify the diameter of the shaft, the base hole system should be given preference. Most metric reamers produce a hole tolerance of H7. This results in less required tools.

In the base shaft system, the shaft is given a fixed tolerance in order to achieve the required fit.

Here are some standard combinations of fits:

Snug Fit H7/s6
This is used in high pressure bolted joints. The tolerance of the shaft is larger than the hole tolerance and exceeds the zero line.

Sliding Fit H7/h6
This type of fit is used to fit parts that can be assembled and disassembled freely. The shaft tolerance is slightly under the hole tolerance.

Drive Fit H7/m6
For this type of fit, exact positioning is needed, usually in a shear load. The shaft tolerance is much larger than the hole tolerance and the shaft has to be driven into position by force.

Loose Running Fit H7/d9
In this combination, the shaft tolerance is much smaller than the hole tolerance with ample clearance. Used for bearing in off road machinery, farm equipment, and where there are large temperature variations.

Have more questions about tolerance systems or the different combinations of fits? We have the answers! Contact us at ProvenProductivity@bossard.com to start the conversation.