•Need
for different types of fit
•Limits of size and Tolerance
•Principal Terms Explained
•Definition of different types of Fit
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Machines and Plant such as lathes,
compressors, etc. consist of many different
components assembled together.
Some components need to rotate or slide
whilst others need to be fixed, therefore the
tightness with which they fit together varies.
The difference in size of the components
determines the type of fit.
There are three types of fit sub-divided into
different classes these are Clearance,
Transition and Interference.
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The largest acceptable size
for a component is known
as the Upper Limit of Size
The smallest acceptable
size for a component is
Known as the Lower Limit
of Size
The difference between the
Upper Limit of Size and the
Lower Limit of Size is
Known as the Tolerance or
Tolerance Zone
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Nominal/Basic Size:- The size by which the component the
component feature is known e.g. 20.00mm, it is the exact
functional size by which the limit of size are developed
(sometimes referred to as the line of zero deviation)
Unilateral Tolerance:- One where the tolerance zone is in one
direction and lays to one side of the basic size e.g. 20 +0.01
/20 +002 (20.01 / 20.02)
Bilateral Tolerance:- One where the tolerance zone is in both
directions and crosses the basic size e.g. 20 +0.01 / 20 – 0.01
(20.01 / 19.99)
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With respect to shafts
Maximum Material condition
(MMC) refers to the largest shaft
(upper limit of size) i.e. least
material removed
Least Material Condition (LMC)
refers to the smallest shaft Lower
limit of size) i.e. most material
removed
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With respect to holes
Maximum Material condition
(MMC) refers to the smallest hole
(lower limit of size) i.e. least
material removed
Least Material Condition (LMC)
refers to the largest hole (upper
limit of size) i.e. most material
removed
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Where movement is required
between mating parts a
clearance fit is necessary.
With a clearance fit the upper and
lower limits of size for each
component are set so that a
clearance always exists between
the two components (the upper
limit of the shaft is equal to or
less than the lower limit of the
bearing
With a clearance fit assembly
requires little or no effort
Clearance fits are sub-divided
depending on the amount of
clearance e.g. Free running
clearance, close running
clearance location clearance etc
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An Interference fit is required where
movement between mating
components is not permitted due to
the function of the components e.g.
a gear mounted on a shaft.
With an Interference fit the upper
and lower limits of both component
are set so that Interference always
occurs
The amount of interference varies
and assembly may require a press
fit ,a force fit, or in extreme cases a
shrink fit, This is where the bearing
is heated and the shaft cooled, the
two components are then
assembled and returned to normal
temperature resulting in a
permanent fixture which is near
impossible to dismantle e.g. railway
wagon wheels fitted to axles.
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A Transition fit is required when
two mating components are not
required to rotate or slide but may
need to be dismantled easily for
example a pulley mounted on a
shaft
With a transition fit the upper and
lower limits of size for each
component are set so that either a
small clearance or a small
interference could occur
If the shaft is at its lower limit and
the bearing is at its upper limit a
clearance is present
If the shaft is at its upper limit and
the bearing at its lower limit then
interference is present
Transition fits can be a light press
or push fit