PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 1
between two mating parts when certain allowances are designed in them. An
allowance is the total difference between the size of a shaft and the hole
in the part that fits over it. This allowance and the resulting fit can be
a clearance (loose) fit, an interference (tight) fit, or a transitional
(somewhere between loose and tight) fit. These three general types of fits
are further identified by classes of fits, with each class having a
different allowance, based on the intended use or function of the parts
A brief description of each type of fit will be given in the
following paragraphs. Any good handbook for machinists has complete charts
with detailed information on each individual class of fit. Most major types
of equipment repaired in machine shops will have the dimensional sizes and
allowances noted, on blueprints, or in the appropriate manufacturer's
Clearance Fits. Clearance fits or running and sliding fits provide a
varying degree of clearance (looseness) depending on which one of the nine
classes is selected for use. The classes of fit range from class 1 (close
sliding fit) to class 9 (loose running fit).
A class 1 fit permits a
clearance allowance of from +0.0004 to +0.0012 inch on the mating parts with
a 2.500 inch basic diameter. A class 9 fit permits a clearance allowance of
from +0.009 to +0.0205 inch on the same parts.
Even for a small basic
diameter (2.500 inch) clearance allowance from a class 1 minimum to a class
9 maximum differs by +0.0201 inch.
As the basic diameter increases, the
allowance increases. Although the class of fit may not be referenced on a
blueprint, the dimensions given for the mating parts are based on the
service performed by the parts and the specific conditions under which they
operate, as described in each of the class of fits. Some parts that fall
within these classes of fits are a shaper ram (close sliding), a babbitt-
lined bearing, and pump wearing rings (loose removal).
Transitional Fits. Transitional fits are subdivided into three types
known as locational clearance, locational transitional, and locational
Each of these three subdivisions contain different
classes of fits.
These classes provide either a clearance or an
interference allowance, depending on the intended use and class selected.
All of the classes of fits in the transitional category are primarily