Principles of Dimensional Tolerances and Fit Selection

Dimensional tolerances play a decisive role in mechanical design and directly influence the functionality and quality of machines, devices and other mechanical products. In mechanical design, tolerances refer to the permitted deviations from the ideal dimensions of a component or an assembly group. There are different fitting systems, such as the standard bore and standard shaft. For more information on tolerance classes, see our blog article “Tolerance Classes According to ISO 22081 and DIN ISO 2768”.

Why are dimensional tolerances important?

In the manufacturing process, it is almost impossible to produce 100-percent precise components without any deviations. Particularly in mass production, there will always be some minimal deviations. If these deviations would not be considered, the components would no longer be compatible with one another. Dimensional tolerances are used to consider these deviations in advance so that components fit together according to the desired type of fit. They form the basis for the precise determination of the deviations permissible from the nominal dimensions of an object. The following aspects are influenced by dimensional tolerances:

• Functionality and fit precision: Only with correctly defined tolerances can it be ensured that components fit together and do not fit too tight or loose. An incorrect fit can significantly impair the function.
• Assembly and production processes: In the manufacturing process, fluctuations can be caused when large quantities are produced, but these are less significant due to tolerances.
• Costs: The more precisely components are manufactured, the more expensive the production becomes. Tolerances can keep costs low, as unnecessarily high precision is not necessary in most cases.

Various standards define, for example, general tolerances (DIN ISO 2768-1, ISO 22081) or different tolerance principles (DIN ISO 8015).

Suitable Fit Selection

The correct fit selection is crucial to ensure that components and assembly groups are functioning properly. There are three types of fit tolerances:

• Clearance fit: a loose fit tolerance, i.e. movement between the components is desired.
• Interference fit: one component is larger than the other component. A press fit ensures firm connection.
• Transition fit: this is a mixture of clearance fit and interference fit, where a certain amount of clearance remains, but joining the components may also require a minor exertion of force.

The selection must consider the following aspects:

• Functional requirements, e.g. what type of movement, force transmission or torque is required?
• Desired accuracy and precision should also consider the costs associated with precise manufacturing. The goal is a good balance.
• Materials, e.g. the thermal expansion coefficient of the materials should be considered.
• Ambient conditions, e.g. temperature and humidity.
• Assembly and disassembly, e.g. for components that often have to be disassembled. An interference fit that is too tight would not be suitable for this purpose.
• Norms and standards that may be binding for some industries and affect the quality of the manufactured components.

At MISUMI you will find an overview of other basics of fit selection. The following table compares different dimensional tolerances and types of fit and shows application examples:

Standard bore hole and standard shaft

The standard bore hole is a fitting system with ISO tolerances in mechanical engineering. The bore hole that is part of a particular fit tolerance is made uniformly while the corresponding tolerance is shifted to the shaft. On the other hand, with the standard shaft, the tolerance is shifted to the bore hole and the shaft is produced uniformly. Standard bores are more common because it is generally easier and more cost-effective to standardize one bore hole.

Calculation of Fit Tolerances

For the calculation of fit tolerances, the names of standard bore hole and standard shaft provide important information. Standard bore holes and standard shafts are marked according to international norms and standards. The designations ensure uniform and precise communication about the specific dimensions and tolerances of bore holes and shafts in the manufacturing industry.

• [1] H bore hole
• [3] Zero line
• [4] Nominal dimension
• [5] Clearance fit
• [6] Transition fit
• [7] Press fit/interference fit

Standard bore holes are described, for example, in a combination of uppercase letters and numbers as well as a diameter (also nominal size), e.g. diameter 50 H9.

• [2] h shaft
• [3] Zero line
• [4] Nominal dimension
• [5] Clearance fit
• [6] Transition fit
• [7] Press fit/interference fit

Standard shafts with a combination of lowercase letters, number and diameter, e.g. diameter 50 h9. H9 is referred to as the tolerance class; the letter is the basic dimension and the number is the degree of tolerance.

This can also be used to assign a basic tolerance according to ISO 286-1 to a bore hole or shaft. There are the basic tolerances IT1-IT17. In the example provided, H9 and h9 would be assigned to the basic tolerance IT9. The upper and lower limit dimensions can then be taken from the corresponding tables. With these limit dimensions, the maximum and minimum dimensions of the bore hole can then be calculated as follows:

• Maximum dimension G _oB = nominal dimension + upper limit dimension
• Minimum dimension G _ub = nominal dimension + lower limit dimension

The maximum and minimum dimension describe the range within which the actual dimensions of a bore hole or shaft can lie and still be acceptable.

Commonly Used Standard Bore Holes and Standard Shafts

The following tables provide an overview of commonly used standard bore holes and standard shafts as well as their fit tolerances.