PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 1
certain classes of fits, they are referred to as shrink fits.
Using the
same basic diameter as an example, the class 1 fit ranges from an
interference allowance of -0.0006 to -0.0018 inch. In comparison, a class 5
fit ranges from an interference allowance of -0.0032 to -0.0062 inch. The
class 5 fit is normally considered to be a shrink fit class because of the
large interference allowance required.
(1) A shrink fit requires that the part with the external diameter be
chilled; or that the part with the internal diameter be heated.
One can
chill a part by placing it in a freezer, packing it in dry ice, spraying it
with CO2 (do not use a CO2 bottle from a fire station) or by submerging it in
liquid nitrogen.
All of these methods except the freezer are potentially
dangerous, especially the liquid nitrogen.
They should not be used until
all applicable safety precautions have been reviewed and implemented. When
a part is chilled, it actually shrinks in size a certain amount depending on
the type of material, design, chilling medium, and length of time of
exposure to the chilling medium.
A part can be heated by using an
oxyacetylene torch, a heat-treating oven, electrical strip heaters, or by
submerging it in a heated liquid. As with chilling, all applicable safety
precautions must be observed. When a part is heated, it expands in size,
allowing, easier assembly.
All materials expand a different amount per
degree of temperature increased.
This is called the coefficient of
expansion of a metal. It is important to determine the maximum temperature
increase required to expand the part for the amount of shrinkage allowance,
plus enough clearance to allow assembly.
Overheating a part can cause
permanent damage and produce so much expansion that assembly becomes
difficult.
(2) A general rule of thumb for determining the amount of
interference allowance on parts requiring a force or shrink fit is to allow
approximately 0.0015 inch per inch of diameter of the internally bored part.
There are, however, many variables that will prohibit the use of this
general rule.
(a) The amount of interference allowance recommended decreases as
the diameter of the part increases.
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