Mechanical Drawing
(MDP 115)
FirstYear,
Mechanical Engineering Dept.,
Faculty of Engineering,
Fayoum University
Dr. Ahmed Salah Abou Taleb
FITS and TOLERANCES
2
Fits
3
Fits Between Mating Parts
4
ILLUSTRATION OF DEFINITIONS
5
Fits Between Mating Parts
Fit: degree of tightness between two parts.
Fit types:
– Clearance Fit – tolerance of mating parts always
leaves a space
– Interference Fit – tolerance of mating parts
always results in interference
– Transition Fit – sometimes interferes, sometimes
clears
6
Clearance Fit
The mating parts have such upper and lower
limits that a clearance always results when the
mating parts are assembled.
7
Clearance Fit (e.g.: H7/f6)
8
Clearance Fit (pl. H7/f6)
9
Clearance Fit (pl. H7/f6)
10
Interference Fit
The mating parts have such limits that the lowest
shaft diameter is larger than the largest hole
diameter..
11
Interference Fit (e.g. H7/p6)
12
Interference Fit (e.g. H7/p6)
13
Interference Fit (e.g. H7/p6)
14
Transition Fit
Either a clearance or an interference may result
depending on the exact value of the dimensions
of the machined shaft and hole within the
specified tolerance zones.
15
Transition Fit (e.g.: H7/j6)
16
Transition Fit (e.g.: H7/j6)
17
Transition Fit (e.g.: H7/j6)
18
Transition Fit (e.g.: H7/j6)
19
Together (Letter & Grade)
on both mating
components decide quality
of fit
Representation of Fit
0.021
INTERFERENCE
FIT
0.022
0.013
Φ30.035
Φ30.022
Φ30.021
Φ30.000
H7 : Tol Grade 7 mean 21μ variation
(H means upper deviation zero)
p6 : Tol Grade 6 means 13μ variation
20
(p means upper deviation is 22 μ)
21
Φ30G7r6
Φ30F8r6
Φ30H8e6
Φ30H7s6
Estimate kind of fit
22
FITS APPLICATIONS
23
24
Basic Systems for Fit Specification
In order to standardize dimensioning of fits, two basic
systems are used:
1) Basic Hole System: Minimum hole diameter is taken
as the basis. Lower deviation for the hole is equal to
zero. Dmax is prescribed according to the specified
tolerance.
2) Basic Shaft System: Maximum shaft diameter is taken
as the basis. Upper deviation for the Shaft is equal to
zero. dmin is prescribed according to the specified
tolerance.
25
Basic Hole System
• Toleranced dimensions are commonly
determined using the basic hole system in
which the minimum hole size is taken as the
basic size.
26
Fits in Basic Hole System
27
Basic Shaft System
• In this system, the maximum shaft is taken as
the basic size and is used only in specific
circumstances.
28
Fits in Basic Shaft System
29
Indicating Tolerances
30
Two ways of indicating tolerances on technical
drawings
Limits of a dimension or the tolerance values are
specified directly with the dimension.
Questions
?
31
Indicating tolerances
The dimension is given by:
• a shape symbol,
• nominal size,
• a letter indicating the
position of the tolerance
zone in relation to zero line,
• a number indicating the
width of the tolerance zone.
(quality of production?)
32
Specifying Fits in Technical Drawings
33
DIMENSIONING OF TOLERANCES -RULES
• The upper deviation should be
written above the lower
deviation value irrespective of
whether it is a shaft or a hole.
• Both deviations are expressed to
the same number of decimal
places, except in the cases
where the deviation in one
direction is nil.
34
Example
For a nominal diameter of 25 mm and for a fit
specification of H7/j5 determine the following:
• Type of the tolerancing system
• Tolerance on the hole
• Tolerance on the shaft
• Upper and lower limits of the hole (Dmax, Dmin)
• Upper and lower limits of the shaft (dmax, dmin)
• Type of the fit
35
Key
a) H7/j5  Basic Hole System
b) D = 25 mm, from the given table:
H7 
 21 m
0 m
+
-
j5 
 5 m
- 4 m
nominal size
H7
j5
Th  21 m
c) Ts  9 m
36