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Production Technology of Agril. Machinery 3(2+1)

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 / ► LESSON 13. LIMITS, FITS AND TOLERANCE

LESSON 13. LIMITS, FITS AND TOLERANCE

1.1 Limits Fits and Tolerance

Two extreme permissible sizes of a part between which the
actual size is contained are called limits. The relationship
existing between two parts which are to be assembled with
respect to the difference on their sizes before assembly is called
a fit. Tolerance is defined as the total permissible variation of a
size. It is the difference between maximum limit and minimum
limit of size.
1.2 Fits
When two parts are to be assembled the relation resulting from
the difference between their sizes before assembly is called a fit.
The fit signifies the range of tightness or looseness which may
result from the application of a specific combination of
allowances and tolerances in the design of mating parts.
1.2.1 Types of Fits
The three types of fits are shown in Fig. 1.1 The disposition of
tolerance zones for the three classes of fit are shown in Fig. 1.2.
 Fig. 1.1 Types of fits

Fig. 1.2 Disposition of

tolerance zones for the three classes of fit
There are three general types of fit between the mating parts
1. Clearance fit: A clearance fit is one having limits of size so
prescribed that a clearance always results when mating parts are
assembled.
2. Interference fit: An interference fit is one having limits of
size so prescribed that an interference always results when
mating parts are assembled.
3. Transition fit: A transition fit is one having limits of size so
prescribed that either a clearance or interference may always
result when mating parts are assembled.
1.3 Terminology
The terminology used in fits and tolerances is shown in Fig. 1.3.
The important terms are

1.3 Terminology for fits and tolerances

Basic size: It is the exact theoretical size arrived at by design. It
is also called nominal size.
Actual size: The size of a part as may be found by
measurement.
Maximum limit of size: The greater of the two limits of size.
Minimum limit of size: The smaller of the two limits of size.
Allowance: It is an intentional difference between maximum
material limits of mating parts. It is a minimum clearance or
maximum interference between mating parts.
Deviation: The algebraic difference between a size (actual,
maximum, etc.) and the corresponding basic size.
Actual deviation: The algebraic difference between the actual
size and the corresponding basic size.
Upper deviation: The algebraic difference between the
maximum limit of size and the corresponding basic size.
Upper deviation of hole = ES (& art Superior)
Upper deviation of shaft es
Lower deviation: The algebraic difference between the
minimum limit of size and the corresponding basic size.
Lower deviation of hole = El (Ecart Inferior)
Lower deviation of shaft = ei
Upper deviation Lower deviation + Tolerance
Zero line: It is the line of zero deviation and represents the basic
size.
Tolerance zone: It is the zone bounded by the two limits of size
of the parts and defined by its magnitude, i.e. tolerance and by
its position in relation to the zero line.
Fundamental deviation: That one of the two deviations which
is conveniently chosen to define the position of the tolerance
zone in relation to zero line, as shown in fig. 1.4.
Fig. 1.4 Disposition of fundamental deviation and tolerance
zone with respect to the zero line
Basic shaft: A shaft whose upper deviation is zero.
Basic hole: A hole whose, lower deviation of zero.
Clearance: It is the positive difference between the hole size
and the shaft size.
Maximum clearance: The positive difference between the
maximum size of a hole and the minimum size of a shaft.
Minimum clearance: The positive difference between the
minimum size of a hole and the maximum size of a shaft.
1.4 Standard Tolerances
There are 18 standard grades of tolerances as specified by BIS
with designations ITOI, ITO and IT to IT 16.
The standard tolerances for the various grades are given in Table
1.1 and tolerance grades for various manufacturing processes in
Table 1.2
Table 1.1 Standard tolerances.

Table 1.2 Tolerance grade in various manufacturing

processes.

1.5 Hole Basis and Shaft Basis for Fits

1. Hole basis system: In this system, the different clearances
and interferences are obtained in associating various shafts with
a single hole, whose lower deviation is zero.
2. Shaft basis system: In this system, the different clearances
and interferences are obtained in associating various holes with
a single shaft, whose upper deviation is zero.
1.6 Selection of Fits
Hole basis system is the most commonly used system because
due to the fixed character of hole production tools, it is difficult
to produce holes with odd sizes. Commonly used types of fits
are given in Table 1.3. Shafts ‘a’ to ‘h’ produce clearance fit, ‘j’
to ‘n’ transition fit, and ‘p’ onwards interference fit with hole.
Table 1.3 commonly used fits
1.7 Dimensioning of Tolerances -Rules
1. The upper deviation should be written above the lower
deviation value irrespective of whether it is a shaft or a hole
(Fig. 1.5 (a)).
2. Both deviations are expressed to the same number of decimal
places, except in the cases where the deviation in one direction
is nil (Fig. 1.5 (b)).
3. Tolerances should be applied either to individual dimensions
or by a general note, assigning uniform or graded tolerances
(Fig. 1.5 (c)).
4. The use of general tolerance not greatly simplifies the
drawing and saves much labour in its preparation. On the
drawing, the limits on a dimension can be specified in two ways,
i.e. (i) unilateral, and (n) bilateral. In unilateral tolerance system,
the variation in size is permitted in one direction
1.8 Limit Gauges
Two sets of limit gauges are necessary for checking the size of
various parts. There are two gauges: Go limit gauge, and Not Go
limit gauge.
1. Go Limit: The Go limit applied to that of the two limits of
size corresponds to the maximum material condition, i.e. (1) an
upper limit of a shaft, and (ii) the lower limit of a hole. This is
checked by the Go gauge.
2. Not Go Limit: The Not Go limit applied to that of the two
limits of size corresponds to the minimum material condition,
i.e. (1) lower limit of a shaft, and (ii) the upper limit of a hole.
This is checked by the Not Go gauge.
1.9 Machining Symbols
During the manufacture of a machine, some surfaces of a
component are to be machined, which are required to be
indicated in the drawing. This will enable the pattern maker to
provide machining allowance on that surface. Similarly, the
grade of surface finish is required to be indicated on the surface
to enable the machinist to carry out the job accordingly. Thus,
on production drawings it is necessary to indicate the surfaces to
be machined or finished by certain specific symbols. The
machining symbol is indicated to the left of the system as shown
in Fig. 1.6. The value of allowance is expressed in mm.

Fig. 1.6 Indication of machining allowances

The basic symbol used for indication of surface roughness
consists of two legs of unequal length inclined at 600 to the line
representing the surface under consideration, as shown in fig.
1.7. It may only be used alone when the meaning is expressed by
a note.

Fig. 1.7 Basic symbol for indication of surface roughness

The following guidelines may be used while specifying the
machining symbols:
1. When the surface is produced by any method, it is indicated
as shown in Fig. 1.8 (a).
2. When the removal of material by machining is required, a bar
is added to the basic symbol, as shown in Fig. 1.8 (b).
3. Whenever the removal of material is not permitted In a circle
is added to the basic symbol, as shown n Fig. 1.8 (c).

Fig. 5.8 Symbols used for indication of surface roughness

4. When some special surface characteristics are to be indicated
(say a milled surface), a line added to the longer leg of the basic
symbol, as shown in Fig. 1.8.
1.9.1 Indication of Surface Roughness
The roughness values Ra (urn) are given in Table 1.4
Table 1.4 Surface roughness values, Ra (a m)
The value defining the roughness value Ra in micron and
roughness grade symbols are given on production drawings as
shown in Fig. 1.9.

Fig. 1.9 Indication of surface roughness in micrometers or

roughness grade symbols
When it is necessary to specify the maximum and minimum
limits of the surface roughness, both the values or grades should
be given as shown in Fig. .10.

Fig. 1.10 Invocation of the maximum and minimum limits of

surface roughness.
2. If it is necessary to indicate the sampling length, it is shown
adjacent to the symbol (Fig. 1.11 (a))
3. If it is necessary to control direction of lay or the direction of
the predominant surface patterns, it is indicated by a
corresponding symbol added to the surface roughness symbol
(Fig. 1.11 (b))
 Fig. 1.11
4. Whenever, it necessary to specify the value of machining
allowance, it is indicated in the left of the symbol (Fig. 1.11 (c)).
This value is generally expressed in millimetres.
Thus, combining the above points, we can establish that the
specification of surface
Roughness should be placed relative to the symbol as shown in
Fig. 1.11 (d)
Where, a = Roughness value Ra in micrometers or
Roughness grade symbol NI to N12
b = Production method, treatment or coating to be used
c = Sampling length
d = Direction of lay
e = Manufacturing allowance -
f = Other roughness value in bracket
 Fig. 1.12 Use of notes with surface texture symbol
5. If it is necessary to define surface roughness both before and
after treatment should be explained in a suitable note or in
accordance with Fig. 1.12.
Last modified: Tuesday, 29 April 2014, 9:41 AM
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