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Lecture 7
Terminology
Types of Fits
Tolerance grades: There are 20 international tolerance grades
Fits
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A fit is a shaft and a hole designation
H7f8
H,h is a zero based designation
The capital letter is for holes and small
letter is for shafts
• In hole based system H is found and in
shaft based systems h is found. The
number after the grade is the tolerance
grade
Types of fits and their designations
In numbers and equations
D  D1 D2
i  0.453 D  0.001D
IT #  k 2i
i
3
D
D(mm) i ( m)
The most commonly used grade numbers are IT6 through IT16, and these are based
on the Renard R5 geometric series of numbers. For these, the basic equation is
where D is the geometric mean of the size range under consideration and is obtained from the formula
The ranges of basic sizes up to 1000 mm for use in this equation are shown in Table
With D determined, tolerance grades IT01 through IT16 are found using Eq.and the following table
Design a loose fit for a basic size of
121mm
D  120 x180  146.96mm
i  0.453 146.96  0.001(146.96)  2.52 m
Loose running fit H 11c11
IT 11  100i  2.52 *100  252 m
Hole 121mm (0.00  0.252)
121  121.25mm
c upper deviation  95  0.8 D  212 m
shaft  121(0.212  (0.212  0.252)
120.788  120.762mm
Design an easy running for a basic
size of 130 mm
D  120 x180  146.96mm
i  0.453 146.96  0.001(146.96)  2.52m
easy running fit H 8 f 8
IT 8  25i  2.52 * 25  63m
Hole 130mm (0.00  0.063)
130  130.063mm
c upper deviation  5.5 D 0.41  42.55m
shaft  130(0.042
 (0.042  0.063)
Useful link:
http://www.roymech.co.uk/Useful_Tables/ISO_Tolerances/ISO_LIMITS.htm