Deviation
Algebraic difference between a size and a corresponding basic size is called deviation. Limit deviations are upper and lower deviations. Algebraic difference of actual size with basic size is called actual deviation.
Upper Deviation: It is the algebraic difference between the maximum size and the basic size.
The upper deviation of a hole is represented by a symbol ES (Ecart Superior) and of a shaft, it is represented by es.
Lower Deviation: It is the algebraic difference between the minimum size and the basic size.
The lower deviation of a hole is represented by a symbol EI (Ecart Inferior) and of a shaft, it is represented by ei.
Fit
The term used for the relationship that exists between two mating parts is "fit". In engineering, the fit refers to the range of tightness or looseness between two mating parts, such as the fit between a shaft and a bearing or the fit between two interlocking mechanical components. The fit can be categorized as clearance fit, interference fit, or transition fit, depending on the amount of overlap or gap between the parts. Achieving the right fit is critical for the proper functioning of mechanical assemblies and to prevent issues such as excessive wear, vibration, or failure.
