Application of Limits and Fits BS 4500 To achieve different types of fit for mating components it is necessary to set the correct limits of size for each part. To do this it is necessary to consider the following factors. Function of the parts The environment they are in Type of material Type of finish required Cost of production (process/s needed. When all these have been considered then you can decide the type of fit needed and by reference to tables (BS4500) calculate limits of size.

Principal Terms of Limits and Fits

ISO System for Specifying Types of Fit The outer component (Bearing) is described by the Nominal Size followed by Capital Letter and a Number e.g. 20 H6 The inner component (Shaft) is described by the Nominal Size followed by a Lower Case Letter and a Number e.g. 20 h6 The Letter specifies the Fundamental Deviation (tolerance position) in relation to the Nominal/Basic Size. The Number specifies the Tolerance Grade (magnitude of tolerance) sometimes referred to as Fundamental Tolerance

Fundamental Deviation There are 28 designated standard deviations for shafts e.g. a, b, c, etc. and 28 designated standard deviations for holes/Bearings e.g. A, B, C, etc.

Standard Deviation Position producing different Types of fit Clearance Interference

Tolerance Grade There are 18 tolerance grades (IT Grades) that can be applied to the Shaft and the Hole designated 01, 0,1,2…….16 The smaller the number the finer the tolerance

Relationship between IT grade and machining process

Hole and Shaft Basis To obtain a given type of fit between two mating components such as a shaft and a bearing two systems can be employed Hole Basis System:- where the hole/bearing size remains constant and the shaft size changes to achieve the desired fit Shaft Basis System:- Where the shaft size remains constant and the hole/bearing size changes to achieve the desired fit The Hole Basis System is commonly used as it is easier to maintain a standard size of hole with the use of drills and reamers, whilst the shaft size can easily be changed during the turning and grinding process

Selected fits Hole Basis

Selected Fits Shaft basis