its.=--This is the term used when a pin, shaft or other
cylindrical part is forced into a hole of slightly smaller diameter, by
the use of a hydraulic press or other means. As a rule, forced fits are
restricted to parts of small and medium size, while shrinkage fits have
no such limitations and are especially applicable when a maximum "grip"
is desired, or when (as in the construction of ordnance) accurate
results as to the intensity of stresses produced in the parts united are
required. The proper allowance for a forced fit depends upon the mass of
metal surrounding the hole, the size of the work, the kind and quality
of the material of which the parts are composed and the smoothness and
accuracy of the pin and bore. When a pin or other part is pressed into a
hole a second time, the allowance for a given tonnage should be
diminished somewhat because the surface of the bore is smoother and the
metal more compact. The pressure required in assembling a forced fit
will also vary for cast hubs of the same size, if they are not uniform
in hardness. Then there is the personal factor which is much in evidence
in work of this kind; hence, data and formulas for forced fit allowances
must be general in their application.

=Allowance for Forced Fits.=--The allowance per inch of diameter usually
ranges from 0.001 inch to 0.0025 inch, 0.0015 being a fair average.
Ordinarily, the allowance per inch decreases as the diameter increases;
thus the total allowance for a diameter of 2 inches might be 0.004 inch,
whereas for a diameter of 8 inches the total allowance might not be over
0.009 or 0.010 inch. In some shops the allowance is made practically the
same for all diameters, the increased surface area of the larger sizes
giving sufficient increase in pressure. The parts to be assembled by
forced fits are usually made cylindrical, although sometimes they are
slightly tapered. The advantages of the taper form are that the
possibility of abrasion of the fitted surfaces is reduced; that less
pressure is required in assembling; and that the parts are more readily
separated when renewal is required. On the other hand, the taper fit is
less reliable, because if it loosens, the entire fit is free with but
little axial movement. Some lubricant, such as white lead and lard oil
mixed to the consistency of paint, should be applied to the pin and bore
before assembling, to reduce the tendency of abrasion.

Allowances for Different Classes of Fits