# BS 4500 limits and Fits

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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.
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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
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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.
Clearance
Interference
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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
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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
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