# Introduction of Dimensional Deviation

*2024-09-23*

The dimensional deviation is the algebraic difference of the dimensions minus their nominal dimensions, which can be divided into actual deviation and limit deviation.

1）Actual deviation. The deviation from the actual size minus its nominal size is called the actual deviation.Actual deviation is indicated by "Ea" and "Ea".

2）Limit deviation. The algebraic difference between the limit dimension and its nominal dimension is called the limit deviation. The limit deviation has upper limit deviation and lower limit deviation. The upper limit deviation is the algebraic deviation of upper limit size minus nominal size. While the lower limit deviation is the algebraic deviation of lower limit size minus nominal size. The deviation value is a numeric value that can be positive, negative, or zero. The upper limit deviation of the hole and shaft is denoted by "ES" and "es" respectively, and the lower limit deviation is denoted by "li".

**Basic deviation**

In the national limit and fit standards, the upper limit deviation or lower limit deviation closest to the zero line is called the basic deviation, which is used to determine the deviation of the tolerance zone from the relative position of the zero line.

**Dimensional tolerance**

Dimensional tolerances are allowed variations in dimensions. The dimensional tolerance is equal to the absolute value of the algebraic difference between the upper limit size and the lower limit size and the absolute value of the algebraic difference between the upper limit and the lower limit deviation. The tolerance is absolute and cannot be negative or zero (tolerance is zero, the part will not be machined). Hole and shaft tolerances are indicated by "Th" and "Ts" respectively.

**Standard tolerance**

The tolerance value specified in the national standard to determine the strip size is the standard tolerance. Tolerance zone In a tolerance zone diagram, the dimensions of an area over the limit and lower limit deviations or the size limit and lower limit are represented by two straight lines. It is determined by the magnitude of the tolerance and its position relative to the zero line as the basic deviation (as shown in Figure 1).

**Tolerance zone**

In a tolerance zone diagram, an area defined by two straight lines representing the upper limit deviation and the lower limit deviation or the upper limit dimension and the lower limit dimension. It is determined by the size of the tolerance and its position relative to the zero line, such as the basic deviation (shown in Figure 2).

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