Limits Fits And Tolerances Nptel.pdf
Limits Fits And Tolerances Nptel.pdf
Limits, fits and tolerances are important concepts in engineering metrology, which deal with the dimensions and relationships of parts in a mechanical system. They are used to ensure the proper functioning, interchangeability and assembly of parts, as well as to control the quality and accuracy of the products. In this article, we will introduce the basic definitions and principles of limits, fits and tolerances, and provide some useful resources for further learning.
What are limits, fits and tolerances?
Limits are the maximum and minimum permissible sizes of a part or a feature, such as diameter, length, angle, etc. They are expressed by two values: the upper limit (UL) and the lower limit (LL). For example, if a shaft has a diameter of 25 mm with a tolerance of 0.05 mm, then its upper limit is 25.05 mm and its lower limit is 24.95 mm.
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Fits are the degree of tightness or looseness between two mating parts, such as a shaft and a hole. They are determined by the difference between the maximum size of the hole and the minimum size of the shaft (called clearance), or the difference between the minimum size of the hole and the maximum size of the shaft (called interference). There are three types of fits: clearance fit, interference fit and transition fit. Clearance fit allows some clearance between the parts, which enables easy assembly and disassembly. Interference fit requires some force or heat to assemble or disassemble the parts, which provides high strength and rigidity. Transition fit may have either clearance or interference depending on the actual sizes of the parts, which allows moderate assembly and disassembly.
Tolerances are the permissible variations in the sizes of parts or features, which account for manufacturing errors, wear, temperature changes, etc. They are expressed by a single value or a symmetrical range around a nominal size. For example, if a hole has a nominal diameter of 20 mm with a tolerance of 0.1 mm, then its actual diameter can vary from 19.9 mm to 20.1 mm. Tolerances can be classified into two types: unilateral tolerance and bilateral tolerance. Unilateral tolerance allows variation only in one direction from the nominal size, such as +0.1 mm or -0.1 mm. Bilateral tolerance allows variation in both directions from the nominal size, such as 0.1 mm.
How to specify limits, fits and tolerances?
There are different methods and standards to specify limits, fits and tolerances for engineering design and manufacturing. One of the most widely used methods is based on the International Organization for Standardization (ISO) system, which defines a set of standard sizes, tolerances and fits for various types of parts and features. The ISO system uses letters and numbers to designate the tolerance grades and fit classes for holes and shafts. For example, H7/g6 means that the hole has a tolerance grade of H7 (which corresponds to a certain range of deviation from the nominal size) and the shaft has a tolerance grade of g6 (which corresponds to another range of deviation from the nominal size). The combination of these two grades results in a certain type of fit (in this case, a transition fit).
The ISO system also provides tables and charts to help select the appropriate tolerance grades and fit classes for different applications and requirements. For example, [this table] shows some examples of standard hole basis fits for different types of assemblies. [This chart] shows some examples of standard shaft basis fits for different types of assemblies.
Where to learn more about limits, fits and tolerances?
If you want to learn more about limits, fits and tolerances in engineering metrology, you can refer to some online courses or books that cover this topic in detail. For example, you can watch [this video lecture series] by Dr Janakarajan Ramkumar from IIT Kanpur, which introduces the basic concepts and definitions of limits, fits and tolerances, as well as their applications in linear measurements, angular measurements, form measurements, surface finish measurements, etc. You can also download [this PDF file], which contains the slides and notes for each lecture.
Alternatively, you can read [this book] by R.K.Jain, which is a comprehensive textbook on engineering metrology, covering the theory and practice of measurement systems, standards, limits, fits, tolerances, errors, uncertainty, calibration, etc. You can also access [this website], which provides some interactive simulations and quizzes on limits, fits and tolerances.
We hope that this article has given you a brief overview of limits, fits and tolerances in engineering metrology, and some useful resources for further learning. If you have any questions or feedback, please feel free to contact us.