Limits and Fits
Introduction
• In engineering you are usually
concerned with a number of parts or
components fitting together to make
an:
assembly
• To assemble components together
engineers must control the sizes of
the components to make sure that
they fit together correctly.
• If they don’t fit together correctly
then the components will not function
properly.
What is a FIT in engineering
terms?
•
There are three types of fit used in
engineering:
1. Clearance Fit
2. Transition Fit
3. Interference Fit
1. The Clearance Fit.
This is where the shaft slides
through the hole
GAP
Called the Clearance
2. The Transition Fit.
This is where the shaft slides
through the hole very tightly.
NO GAP
Called the Transition
3. The Interference Fit.
This is where the shaft slides through
the hole and shears the metal has it
goes in.
SHEARING
Called the Interference
Other Names Used for
the Fits
• Sliding Fit allows one part to slide in
another whilst there is no force
being applied to the shaft
The Clearance Fit
• A Force Fit which needs a “big
Hammer” or preferable a power press
to assemble the shaft and hole.
The Interference Fit
• A Push Fit which means the hole and
the shaft can be push together or
perhaps tapped gentle with a soft
hammer.
The Transition Fit
How do we decide which
fit to use?
• First consider what is required of the fit.
• Then decide how much clearance or
interference you are going to give your fit.
• Then determine what nominal size you are
going to fit together.
What are Nominal
sizes?
• They are basically any size, it maybe any
whole number you like: -
• 16 12 40 60 1200 230,000
• Now you have the size, how much
interference or clearance do we use?
We need a system to help!
Advantages of having a system:
Interchangeable - parts made at different times or
places fit together.
Time saving - reference to a standard system speeds
up decisions on sizes to give particular fits.
Assurance of correct fit - actual fit will be as required
even if design size changes.
Spare Parts – manufacture of these is made easier as
they can be made to the same standard.
British Standards
4500
• Uses the nominal size for both holes and
shafts.
• This size is determined by strength
considerations.
• Calculations of material properties and
strengths are used to obtain the nominal
size.
• Remember: The nominal size is the same
for both hole and shaft.
BS4500
• The British Standard System uses the letters of
the alphabet to describe how much bigger or
smaller a hole or a shaft will deviate from the
nominal size.
• This deviation is called the:-
Fundamental deviation.
• Holes are described by capital letters H, G etc.
• Shafts are described by lower case letters h, g
etc.
The Holes
The Fundamental
Deviation
A
Nominal Size
B
Zero line
H
K
M
The Fundamental
Deviation
The Fundamental Deviation
The Shafts
Fundamental Deviation
Zero Line
a
b
Nominal Size
Nominal Size
g
m
e
Fundamental Deviation
Any combination of hole and shaft can be use
So be careful when
A
choosing
your fits they
don’t all work
a
e
M
M/a fit
A/e fit
British Standards
Tolerance zones
Tolerance Zone
BS4500 use numbered tolerance zones (01 to 16)
British Standards
Tolerance zones
• The most accurate tolerance zones are
numbered 01 02 and 03.
• Whereas for more general engineering
work we use 06, 07 and 08.
• Finally, rough work is done at 14,15 and 16.
In engineering we don’t use the zeros
Round up
• You now have a letter and a number.
• Put them together and you have the
type of fit and the tolerance that will
achieve it. Examples are:
»H7, K8 and M9.
• For shafts we use:
»For holes.
• g6, h7 and f7.
• Here then is the complete fit:
• The Hole
The shaft
H9
f7
BS 4500
Limits of size
• Look on the chart
• Maximum and minimum allowances are
shown in thousandths of a millimetre
• Minimum clearance and minimum
interference is minimum hole size
minus maximum shaft size
• Tolerance is max deviation minus min
deviation
Today's task
Housing
Pulley
Spacer
Shaft
Standard washer
Bush
Decide what fits are required between the shaft
and bush and the bush and pulley. Give reason for
your choices in not less than 600 words.
Pulley
Spacer creates
another type of fit
Shaft must be
allowed to slide
so pulley and
bush can rotate
Fit should be
Clearance Fit
Clearance fit: H7 hole (width of bush)
and g6Choice
for theis:shaft
Hole
in both
(length to H7
first
shoulder)
cases and for
the shaft either
g6 or h6
Bush pressed in
using force. Fit
is Interference
H7 Hole
P6 Shaft
When does Tolerances
become important
• Assemblies: Parts will often not fit
together if their
• dimensions do not fall with in a
certain range of values
When do Tolerances
become important
• Interchangeability
• If a replacement part is used it must
be
• a duplicate of the original part within
certain limits of deviation.
When do Tolerances
become important
• Tolerances do not affect the function here
When do Tolerances
become important
• Tolerances are important here
!
When do Tolerances
become important
•
What about the tolerance levels in this mechanism?
Straightness
Geometric tolerance
Measuring and gauging
• Measuring is a direct reading process
in which the inspection instrument
consists of (or incorporates ) a
continuous series of linear
measurement units, commonly known
as a scale.
Measuring and gauging
• Gauging is checking that a component
fits within tolerance limits
• Tolerances for Go gauges are placed
within working limits
• Tolerances for No Go gauges are
outside working limits
Plug gauge
Snap go/no go gauge for
the OD of a cylindrical
workpie
Replaceable thread and
plug gauges
Plug Gauge operation
Hole Diameter (mm)
Dia of Go End
Dia of No Go end
a, 40 ± 0.02
39.98
40.02
b, 46 + 0.03
46.00
45.03
C, 30 - 0.2
29.80
30.00
Other gauges