page 205
36.1.3 Interference Fits
•Interference fits always overlap and are used mainly for press fits where the two parts are pushed together, and require no other fasteners
•The figure below shows an interference fit for a hole shaft pair
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Maximum interference (allowance) |
Hole |
Shaft |
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36.2 C.S.A. B97-1 1963 LIMITS AND FITS(REWORK)
• Five types of fits are allowed, with a variety of classes
CODE |
DESCRIPTION |
CLASSES |
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RC |
RUNNING CLEARANCE |
1 TO 9 |
LC |
LOCATIONAL CLEARANCE |
1 TO 11 |
LT |
LOCATIONAL TRANSITION |
1 TO 6 |
LN |
LOCATIONAL INTERFERENCE |
1 TO 6 |
FN |
FORCE OR SHRINK |
1 TO 5 |
page 206
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A |
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z |
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Holes |
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Nominal Dimension |
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H |
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h |
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• This system is hole based (it uses the “H” hole)
“H” Hole
Nominal Dimension
Tolerance Grade
•There are 22 diameter steps from 0” to 200”
•A fit is classified as a grade of “H” hole and a grade of shaft
•For example,
page 207
Assume we are given an RC2 fit with a nominal diameter of 2.25”. (This may be determined during the design stage using experience, lookup tables, etc.).
The RC2 specifies a Running Clearance - Class 2 fit which uses ISO hole H6, and ISO shaft g5.
The CSA tables (given later) provide,
1.hole tolerance
2.allowance
3.shaft tolerance
From the tables for 2.25” dia. RC2 fit
Hole tolerance = +0.0007”
Allowance = 0.0004”
Shaft Tolerance = -0.0005”
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0.0007” |
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H6 |
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dia. 2.25” |
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g5 0.0005”
0.0004”
This diagram translates to
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+0.0000” |
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+0.0007” |
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2.250” |
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2.2496” -0.0005” |
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-0.0000” |
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• To go on further with a more complicated example, consider the hole shaft pair with a bushing
1.000” dia. RC4 fit
1.375” dia. LN3 fit
page 208
Cast Iron Housing |
Brass Bush |
Hardened and Ground Steel shaft
NOTE:
LN is a force fit
Class 3 is a heavy press fit
RC is a running clearance fit
Class 4 is a running fit
Lubricate with grease or oil
We want to detail and dimension the three parts, so first we draw the tolerance diagrams
page 209
LN3
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0.0006” |
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1.375” Nominal |
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0.001” |
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Diameter |
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0.002” |
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RC4
Hole
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1.00” Nominal |
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0.0012” |
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Diameter |
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0.0008”
0.0008”
***NOTE: the values for the clearance are used differently when doing RC and LN fits