Introduction to Basic Manufacturing Processes and ... - always yours

470 Introduction to Basic Manufacturing Processes and Workshop Technology
1. Clearance fit
2. Interference fit
3. Transition fit.
Shaft
Basic
size
Interference fit
Hole
High limit
Shaft
Transition fit
Shaft
Low limit
Clearance fit
Fig. 26.5 Graphical representation of types of fits
26.4.2 Clearance Fit
Clearance fit is one in which two assembled parts are always free to move relative to
each other in the assembly. In the clearance fit, the largest permitted shaft diameter is
smaller than the diameter of the smallest hole. The difference between the size of hole and
the size of shaft is defined as clearance. Clearance fits have limits of size prescribed so that
a clearance always results in a positive allowance, or air space is left between mating parts.
The parts can be assembled by hand. Clearance fit is of two kinds namely sliding and running
fits. Examples of clearance fit are door hinges, wheel and axle, shaft and bearing, etc. used
in the assembly of parts.
26.4.3 Interference Fit
In the interference fit, mating parts in sub assembly or main assembly are joined tightly
together and no relative motion is possible. The minimum permitted diameter of the shaft
is larger than the maximum allowable diameter of the hole. In this case, the shaft and the
hole member in any sub assembly or main assembly is to be attached permanently and used
as a solid component but according to the application of this combination, this type of fit can
be varied. Interference fit is used for permanent or semi-permanent assembly of parts, which
require rigidity and alignment and no relative motion, such as dowel pins and bearings in
casting. The difference between the size of shaft and the size of hole in anysub assembly or
main assembly is called interference or negative clearance or negative allowance. Interference
fit has limits of size so prescribed that interference always results when mating parts are
assembled. Transition fits is of two kinds namely driving or press fit and shrink or force fit.
To assemble, parts are usually pressed together using an arbor press.
26.4.4 Transition Fit
In this type of fit, the diameter of the largest allowable hole is greater than that of the
smallest shaft, but the smallest hole is smaller than the largest shaft, so that small positive
or negative clearance between the shaft and hole member is employable. Thus, transition fit
has limits of size of hole and shaft such that either a clearance or an interference fit may
result when two specific parts from the lot are assembled. Here the tolerance zones of the
hole and shaft overlap. Transition fits are a compromise between clearance and interference