FIT- is the
dimension relationship between mating parts, such as press, shrink or sliding
fit.
CLASSES OF FITS
1. Loose Fit 6.
Tight Fit
2. Free Fit 7.
Medium Fit
3. Medium Fit 8.
Medium Force Fit
4. Snug Fit 9.
Medium Force Fit
5. Wringing Fit
Loose Fit- it
is intended to be used where accuracy is not essential and where consideration
freedom is permissible, such as agricultural, mining, and general purpose
machinery.
Free Fit- It
is suitable for running fits where the speeds are in excess of 600 rpm and the
pressure in excess of 600 psi. It is also suitable for shaft of generators,
motors, engines and some automotive parts.
Snug Fit- It
is a zero allowance fit and is the closest fit than can be assembled by hand.
Wringing Fit-
A fit with a slight positive allowance or metal interference and light pressure
is required to assemble the parts. It is suitable for semi-permanent assembly
for long fits in heavy sections, and for drives in thin section.
Medium Force Fit-
A fit that requires considerable force to assemble. It is suitable for press
fits on locomotive wheels, car wheel, generator and motor armatures, and crank
shafts.
Heavy Force and
Shrink Fit- A fit used for steel external member that have a high yield
stress.
Tolerance- It
is permissible variations in the dimensions of machine parts.
Unilateral
Tolerance System- A term applied to a system by which one of the tolerances
is zero.
Allowance- it
is intentional difference in the basic dimensions of two parts.
Limits- These
are the maximum and minimum acceptable dimension of a produced parts.
Lay – It is
the predominant direction of the marks visible on the surface.
Flaws- These
are irregularities occurring at one or more places on the surface which are not
characteristic tics of the surface and cannot be designed by surface,
waiveness, or lay
DESCRIPTIONS OF
VARIOUS DEGRESS AND SURFACE ROUGNESS
Degree or
Roughness Description
2000 rms µin. Unusually
rough, low grade surface produced by removing excess stock on heavy work to
provide appearances.
1000 rms µin. A
very rough surface made by removing stock to nominal dimension.
500 rms µin. A
very rough surface made by removing stock of nominal dimension.
250 rms µin. Used
as rough finish for soft alloy and for steel, and for hard alloys where moderate
notch sensitivity exists but where a rough finish is otherwise tolerable. It is
also used on heavy work when good surface contact is essential.
125 rms µin. is
the coarse made by removing stock to nominal dimension.
63 rms µin. is
used for highly stressed parts that are not subject to serve stress reversals.
32 rms µin. is
used for bearing where accuracy is essential where motion is continuous and
where loads are light particular if the lay of the surface is I the direction
of motion.
16 rms µin. is
suitable for close tolerance work, journal bearings, for commercial grade
bearing balls and rollers as well as the mating races, the outer diameter of
shafts inside such as bearing are housed.
8 rms µin. is
used for work to very close tolerances where scratches can not be tolerated. It
is suitable for the interior surfaces of hydraulic struts and similar
application where properly lubricated seals must slide. It is also suitable for
raceways balls and rolls of rolling element bearings when the loads are
nominal.
4 rms µin. is
suitable for use where packing and rings much slide across the direction of the
finish lay such as on buffed chrome-plated piston rods and for hydraulic
components. It is also suitable for ball and roller bearings carrying heavy
loads.
2 rms µin. is
suitable for gages, refrigerator shaft seals. It is expensive to produce.
1 rms µin. is
used for micrometer anvils high grades gages, and similar application.
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