Limits and Fits

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Limits and Fits
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Introduction

• In engineering you are usually concerned with a
  number of parts or components fitting together to
  make an

assembly
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• To assemble components together engineers must
  control the sizes of the components to make sure
  that they fit together correctly.
• If they dont fit together correctly then the
  components will not function properly.

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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

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The Clearance Fit.This is where the shaft slides
through the hole
Called the Clearance
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2. The Transition Fit.This is where the shaft
slides through the hole very tightly.
Called the Transition
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3. The Interference Fit.This is where the shaft
slides through the hole and shears the metal has
it goes in.
Called the Interference
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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
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• 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
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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.

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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?

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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.

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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.

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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.

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The Holes
A
B
H
K
M
The Fundamental Deviation
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The Shafts
Zero Line
a
b
g
m
e
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Any combination of hole and shaft can be use
So be careful when choosing your fits they dont
all work
M/a fit
A/e fit
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British Standards Tolerance zones
BS4500 use numbered tolerance zones (01 to 16)
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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 dont use the zeros
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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 holes.
• For shafts we use
• g6, h7 and f7.

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• Here then is the complete fit
• The Hole The shaft

H9
f7
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BS 4500
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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

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Today's task
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.
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Shaft must be allowed to slide so pulley and bush
can rotate
Pulley
Spacer creates another type of fit
Fit should be Clearance Fit
Clearance fit - H7 hole (width of bush) and g6
for the shaft (length to first shoulder)
Bush pressed in using force. Fit is Interference
Choice is -
H7 Hole in both cases and for the shaft either g6
or h6
H7 Hole P6 Shaft
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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

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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.

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When do Tolerances become important

• Tolerances do not affect the function here

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When do Tolerances become important

• Tolerances are important here !

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When do Tolerances become important

• What about the tolerance levels in this mechanism?

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Straightness
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Geometric tolerance
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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.

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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

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Plug gauge
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Snap go/no go gauge for the OD of a cylindrical
workpie
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Replaceable thread and plug gauges
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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
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Other gauges