Limits of Size (IQC 5 )

Limits of size are two extreme permissible sizes between which the actual size exists.
 
The maximum limit for a dimension is the largest permissible size, while the minimum limit
for a dimension is the smallest permissible size.

 The maximum or minimum sizes represented by tolerances are called the limits. The basic sizes deviation and tolerances for both shaft and hole are illustrated in Fig

 
                                     Basic size deviation and tolerances for both shaft and hole

Zero Line and Deviation

In graphical representation of limits, straight line to which the deviations are of zero
deviation and the represented of zero deviation are referred.

 The zero line is also known as line of zero deviation is generally drawn as horizontal line and the positive deviations are shown above this line and the negative deviations below it.
 
The algebraic difference between the actual or maximum size and the corresponding
the basic size is called deviation.

 The deviations from the basic dimensions at the
boundaries of the tolerance zone are called upper and lower deviations as depicted in
Fig

 

                                              Upper and lower deviations for both shaft and hole 

Upper Deviation

It is the algebraic difference between the two maximum limit of any size of the part and
the corresponding basic size.

 Lower Deviation

It is the algebraic difference between the minimum limit of any size of the part and the
corresponding basic size.

 Mean Deviation

It is the arithmetical mean between the upper and lower deviations of any size of the
part.