Machinery's Handbook, 31st Edition
661 ANSI/ASME Standard Preferred Metric Limits and Fits Upper Deviation: The algebraic difference between the maximum limit of size and the corresponding basic size. Lower Deviation: The algebraic difference between the minimum limit of size and the corresponding basic size. Fundamental Deviation: That one of the two deviations closest to the basic size. For example, it is designated by the letter H in 40H7. Tolerance: The difference between the maximum and minimum size limits on a part. Tolerance Zone: A zone representing the tolerance and its position in relation to the basic size. Max Size
Upper Deviation Lower Deviation International Tolerance Grade IT Number)
Main Size
Fundamental Deviation (Letter)
Basic Size
Lower Deviation Upper Deviation
Fundamental Deviation (Letter)
International Tolerance Grade IT Number)
Main Size
Max Size
Fig. 1. Illustration of Definitions International Tolerance Grade: ( IT ): A group of tolerances that vary depending on the basic size, but that provide the same relative level of accuracy within a given grade. For example, International Tolerance Grade is designated by the number 7 in 40H7 or as IT7. Hole Basis: The system of fits where the minimum hole size is basic. The fundamental deviation for a hole basis system is H. Shaft Basis: The system of fits where the maximum shaft size is basic. The fundamental deviation for a shaft basis system is h. Clearance Fit: The relationship between assembled parts when clearance occurs under all tolerance conditions. Interference Fit: The relationship between assembled parts when interference occurs under all tolerance conditions. Transition Fit: The relationship between assembled parts when either a clearance or an interference fit can result, depending on the tolerance conditions of the mating parts. Tolerances Designation.— An “International Tolerance Grade” establishes the magni tude of the tolerance zone or the amount of part size variation allowed for external and inter- nal dimensions alike (see Fig. 1). Tolerances are expressed in grade numbers that are consis- tent with International Tolerance Grades identified by the prefix IT, such as IT6, IT11, etc. A smaller grade number provides a smaller tolerance zone.
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