Lecture 7

Terminology

Types of Fits

Tolerance grades: There are 20 international tolerance grades

Fits• 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

3

2

3

21

#)()(001.045.0

Di

ikITmimmDDDi

DDD

In numbers and equations

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

mmshaft

mDdeviationupperc

mmmmHole

miITcHfitrunningLoose

mi

mmxD

762.120788.120)252.0212.0(212.0(121

2128.095

25.121121)252.000.0(121

252100*52.2100111111

52.2)96.146(001.096.14645.0

96.1461801203

Design an easy running for a basic size of 130 mm

)063.0042.0(042.0(13055.425.5

063.130130)063.000.0(130

6325*52.225888

52.2)96.146(001.096.14645.0

96.146180120

41.0

3

shaftmDdeviationupperc

mmmmHole

miITfHfitrunningeasy

mi

mmxD

Useful link:

http://www.roymech.co.uk/Useful_Tables/ISO_Tolerances/ISO_LIMITS.htm