Classes of Fits Used in Machine Construction.—In assembling machine parts it is necessary to have some members fit together tightly, whereas other parts such as shafts, etc., must be free to move or revolve with relation to each other. The accuracy required for a fitting varies for different classes of work. A shaft that revolves in its bearing must be slightly smaller than the bearing so that there will be room for a film of lubricant. A crank-pin that must be forced into the crank-disk is made a little larger in diameter than the hole, to secure a tight fit. When a very accurate fitting between two cylindrical parts that must be assembled without pressure is required, the diameter of the inner member is made as close to the diameter of the outer member as is possible. In ordinary machine construction, five classes of fits are used, viz; running fit, push fit, driving fit, forced fit and shrinkage fit. The running fit, as the name implies, is employed when parts must rotate; the push fit is not sufficiently free to rotate; the other classes referred to are used for assembling parts that must be held in fixed positions.

Forced Fits.—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.

Allowances for Different Classes of Fits

(Newall Engineering Co.)

ClassTolerances in Standard Holes[1]
Nominal DiametersUp to 1/2" 9/16" - 1" 11/16" - 2" 21/16" - 3" 31/16" - 4"
AHigh Limit+0.0002+0.0005+0.0007+0.0010+0.0010
Low Limit-0.0002-0.0002-0.0002-0.0005-0.0005
Tolerance 0.0004 0.0007 0.0009 0.0015 0.0015
BHigh Limit+0.0005+0.0007+0.0010+0.0012+0.0015
Low Limit-0.0005-0.0005-0.0005-0.0007-0.0007
Tolerance 0.0010 0.0012 0.0015 0.0019 0.0022
Allowances for Forced Fits
FHigh Limit+0.0010+0.0020+0.0040+0.0060+0.0080
Low Limit+0.0005+0.0015+0.0030+0.0045+0.0060
Tolerance 0.0005 0.0005 0.0010 0.0015 0.0020
Allowances for Driving Fits
DHigh Limit+0.0005+0.0010+0.0015+0.0025+0.0030
Low Limit+0.0002+0.0007+0.0010+0.0015+0.0020
Tolerance 0.0003 0.0003 0.0005 0.0010 0.0010
Allowances for Push Fits
PHigh Limit-0.0002-0.0002-0.0002-0.0005-0.0005
Low Limit-0.0007-0.0007-0.0007-0.0010-0.0010
Tolerance 0.0005 0.0005 0.0005 0.0005 0.0005
Allowances for Running Fits[2]
XHigh Limit-0.0010-0.0012-0.0017-0.0020-0.0025
Low Limit-0.0020-0.0027-0.0035-0.0042-0.0050
Tolerance 0.0010 0.0015 0.0018 0.0022 0.0025
YHigh Limit-0.0007-0.0010-0.0012-0.0015-0.0020
Low Limit-0.0012-0.0020-0.0025-0.0030-0.0035
Tolerance 0.0005 0.0010 0.0013 0.0015 0.0015
ZHigh Limit-0.0005-0.0007-0.0007-0.0010-0.0010
Low Limit-0.0007-0.0012-0.0015-0.0020-0.0022
Tolerance 0.0002 0.0005 0.0008 0.0010 0.0012

[1] Tolerance is provided for holes, which ordinary standard reamers can produce, in two grades, Classes A and B, the selection of which is a question for the user's decision and dependent upon the quality of the work required; some prefer to use Class A as working limits and Class B as inspection limits.

[2] Running fits, which are the most commonly required, are divided into three grades: Class X for engine and other work where easy fits are wanted; Class Y for high speeds and good average machine work; Class Z for fine tool work.