The term interchangeability is normally employed for the mass
production of identical items within the prescribed limits of sizes. A little
consideration will show that in order to maintain the sizes of the part within
a close degree of accuracy, a lot of time is required. But even then there will
be small variations. If the variations are within certain limits, all parts of
equivalent size will be equally fit for operating in machines and mechanisms.
Therefore, certain variations are recognised and allowed in the sizes of the
mating parts to give the required fitting. This facilitates to select at random
from a
large number of parts for an assembly and results in a
considerable saving in the cost of production. In order to control the size of
finished part, with due allowance for error, for interchangeable parts is called
limit system.
*) Explain Important Terms used in Limit
System
The following terms used in limit system
(or interchangeable system) are important from the subject point of view:
1. Nominal size. It is the
size of a part specified in the drawing as a matter of convenience.
2. Basic size. It is the
size of a part to which all limits of variation (i.e. tolerances)
are applied to arrive at final dimensioning of the mating parts. The nominal or
basic size of a part is often the same.
3. Actual size. It is the
actual measured dimension of the part. The difference between the basic size
and the actual size should not exceed a certain limit, otherwise it will
interfere with the interchangeability of the mating parts.
4. Limits of sizes. There
are two extreme permissible sizes for a dimension of the part. The largest
permissible size for a dimension of the part is called upper or high or
maximum limit, whereas the smallest size of the part is known as lower
or minimum limit.
5. Allowance. It is the
difference between the basic dimensions of the mating parts. The allowance may
be positive or negative. When the shaft size is less than the
hole size, then the allowance is positive and when the shaft size is
greater than the hole size, then the allowance is negative.
6. Tolerance. It is the
difference between the upper limit and lower limit of a dimension. In other words,
it is the maximum permissible variation in a dimension. The tolerance may be unilateral
or bilateral. When all the tolerance is allowed on one side of the
nominal size, e.g.
, then it is said to be unilateral system of
tolerance. The unilateral system is mostly used in industries as it permits
changing the tolerance value while still retaining the same allowance or type
of fit.
Fig. Method of assigning tolerances.
When the tolerance is allowed on both sides of the nominal size, e.g.
, then it is said to be
bilateral system of tolerance. In this case + 0.002 is the upper
limit and – 0.002 is the lower limit. The method of assigning unilateral and
bilateral tolerance is shown in above Fig. (a) and (b) respectively.
7. Tolerance zone. It is the
zone between the maximum and minimum limit size, as shown in below Fig.
Fig. Tolerance zone.
8. Zero line. It is a
straight line corresponding to the basic size. The deviations are measured from
this line. The positive and negative deviations are shown above and below the
zero line respectively.
9. Upper deviation. It is the
algebraic difference between the maximum size and the basic size. The upper
deviation of a hole is represented by a symbol ES (Ecart Superior) and
of a shaft, it is represented by es.
10. Lower deviation. It is the
algebraic difference between the minimum size and the basic size. The lower
deviation of a hole is represented by a symbol EI (Ecart Inferior) and
of a shaft, it is represented by ei.
11. Actual deviation. It is the
algebraic difference between an actual size and the corresponding basic size.
12. Mean deviation. It is the
arithmetical mean between the upper and lower deviations.
13. Fundamental deviation. It is one of
the two deviations which is conventionally chosen to define the position of the
tolerance zone in relation to zero line, as shown in Fig.
Fig. Fundamental
deviation
Fits
The degree of tightness or looseness between the two mating parts
is known as a fit of the parts. The nature of fit is characterised by
the presence and size of clearance and interference. The clearance is
the amount by which the actual size of the shaft is less than the actual size
of the mating hole in an assembly as shown in Fig. 3.5 (a). In other
words, the clearance is the difference between the sizes of the hole and the
shaft before assembly. The difference must be positive.
The interference is the amount by which the actual
size of a shaft is larger than the actual finished size of the mating hole in
an assembly.
Types of Fits
According to Indian standards, the fits are classified into the
following three groups :
1. Clearance fit. In this type
of fit, the size limits for mating parts are so selected that clearance between
them always occur, as shown in Fig. 3.5 It
may be noted that in a clearance fit, the tolerance zone of the hole is
entirely above the tolerance zone of the shaft. In a clearance fit, the
difference between the minimum size of the hole and the maximum size of the
shaft is known as minimum clearance whereas the difference
between the maximum size of the hole and minimum size of the shaft is called maximum
clearance as shown in Fig. (a).
2. Interference fit. In this type
of fit, the size limits for the mating parts are so selected that interference
between them always occur, as shown in Fig.(b). It may be noted that in
an interference fit, the tolerance zone of the hole is entirely below the
tolerance zone of the shaft. In an interference fit, the difference between the
maximum size of the hole and the minimum size of the shaft is known as minimum
interference, whereas the difference between the minimum size of the
hole and the maximum size of the shaft is called maximum interference,
as shown in Fig. (b).
3. Transition fit. In this type
of fit, the size limits for the mating parts are so selected that either a
clearance or interference may occur depending upon the actual size of the
mating parts, as shown in Fig. (c). It may be noted that in a transition
fit, the tolerance zones of hole and shaft overlap.
Basis of
Limit System
The following are two bases of limit system:
1. Hole basis system. When the
hole is kept as a constant member (i.e. when the lower deviation of
the hole is zero) and different fits are obtained by varying the shaft size, as
shown in Fig. (a), then the limit system is said to be on a hole basis.
2. Shaft basis system. When the
shaft is kept as a constant member (i.e. when the upper deviation
of the shaft is zero) and different fits are obtained by varying the hole size,
as shown in Fig. (b), then the limit system is said to be on a shaft
basis.
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