Limits, Fits and Tolerance
BY: MOHIT GARG
Tolerance
The allowable
deviation from a
standard, e.g: the
range of variation
permitted in
maintaining a
specified dimension
in a machined piece.
”
Need Of Tolerance
It is impossible to manufacture a part or
component to an exact size or geometry.
Since variation from the drawing is inevitable,
acceptable degree of variation must be applied.
Large variation may effect the functionality of
the part.
Small variation may effect the economy of the
part.
Some Definitions
Basic Size: The size with reference to which
the limits of size are fixed.
Actual Size: Actual measured dimension of the
part.
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.
Some Definitions
Limits of Size: The two extreme permissible
sizes of a part between which the actual size
should lie.
Maximum Limit of Size: The greater of the
two limits of size. Its also called upper limit.
Minimum Limit of Size: The smaller of the
two limits of size. Its also called the lower limit.
Tolerance: It is the difference between the
upper limit and lower limit of a dimension.
Some Definitions
Shaft: A term used by convention to designate
all external features of a part, including those
which are not cylindrical.
Hole: A term used by convention to designate
all internal features of a part, including those
which are not cylindrical.
Tolerance Zone: It is the zone between the
maximum and minimum limit size.
Some Definitions
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.
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.
Some Definitions
Fundamental Deviation : This is the deviation,
either the upper of the lower deviation which
is nearest to the zero line for either hole or
shaft.
Unilateral Tolerance : In this system, the
dimension of a part is allowed to vary only on
one side of the basic size, i.e. tolerance lies
wholly on one side of the basic size either
above or below it.
Bilateral Tolerance : In this system, the
dimension of the part is allowed to vary on
both the sides of the basic size, i.e. the limits of
Unilateral Tolerance
Contd…
For Unilateral Limits a case may occur when
one of the limits coincides with the basic size,
Bilateral tolerance :
Question
Tolerances are specified :
a) To obtain desired fits
b) Because it is not possible to manufacture a
size exactly
c) To obtain higher accuracy
d) To have proper allowances
Question
Question
Question
Fits
A fit may be defined as the degree of tightness
and looseness between two mating parts.
Clearance Fits
In clearance fit, an air space or clearance exists
between the shaft and hole.
Such fits give loose joint.
There is minimum positive clearance between
high limit of the shaft and low limit of the hole.
Type of Clearance Fits
Loose Fit : It is used between those mating
parts where no precision is required. It
provides minimum allowance and is used on
loose pulleys, agricultural machineries.
Running Fit : For a running fit, the dimension
of shaft should be smaller enough to maintain
a film of oil for lubrication. It is used in bearing
pair.
Slide Fit or Medium Fit : It is used on those
mating parts where great precision is required.
It provides medium allowance and is used in
Question
Question
Interference Fits
In such cases, the diameter of the shaft is always
larger than the hole diameter.
It used for components where motion, power has
to be transmitted.
Types Of Interference Fits
Shrink Fit or Heavy Force Fit : In assembly of
the hole and the shaft, the hole is expanded by
heating and then rapidly cooled in its position.
It is used in fitting of rims.
Medium Force Fit : Considerable pressure is
required to assemble the hole and the shaft. It
is used in car wheels, armature of dynamos etc.
Tight Fit or Force Fit : One part can be
assembled into the other with a hand hammer
or by light pressure. It gives a semi-permanent
fit and is used on a keyed pulley and shaft,
Transition Fits
It may result in either clearance fit or
interference fit depending on the actual value
of the individual tolerances of the mating
components.
They are used for applications where accurate
location is important but either a small amount
of clearance or interference is permissible.
Types Of Transition Fits
Push Fit or Snug Fit : It refers to zero allowance
and a light pressure is required in assembling the
hole and the shaft. The moving parts show least
vibration with this type of fit.
Force Fit or Shrink Fit : A force fit is used when
the two mating parts are to be rigidly fixed so that
one cannot move without the other. It either
requires high pressure to force the shaft into the
hole or the hole to be expanded by heating. It is
used in railway wheels, etc.
Wringing Fit : A slight negative allowance exists
between two mating parts in wringing fit. It
requires pressure to force the shaft into the hole
and gives a light assembly. It is used in fixing keys,
Allowance
It is Minimum clearance or maximum
interference. It is the intentional difference
between the basic dimensions of the mating
parts. The allowance may be positive or
negative
Question
Question
Question
Allowance in limits and fits refers to
(a) Maximum clearance between shaft and hole
(b) Minimum clearance between shaft and hole
(c) Difference between maximum and minimum
size of hole
(d) Difference between maximum and minimum
size of shaft
Different Fundamental Deviations
Some Terms
Basic shaft: It is a shaft whose upper deviation
is zero. i.e. the maximum limit of shaft
coincides with the nominal size.(zero line). Eg:
shaft ‘h’
Basic hole: It is a hole whose lower deviation is
zero. i.e. the minimum limit of hole coincides
with the nominal size.(zero line). Eg: shaft ‘H’
Basis of Fits
Hole Basis system : In this system, the basic
diameter of the hole is constant while the shaft
size is varied according to the type of fit.
Shaft Basis system: In this system, the basic
diameter of the shaft is constant while the hole
size is varied according to the type of fit.
Hole Basis Preferred : Holes can be finished
by tools like reamers, drills, broaches, and their
sizes are not adjustable. The shaft sizes can be
obtained by external machining.
Hole And Shaft Basis System
Grades Of Tolerance
Grade is a measure of the magnitude of the
tolerance. Lower the grade the finer the
tolerance.
There are total of 18 grades which are allocated
the numbers IT01, IT0, IT1, IT2..... IT16.
Fine grades are referred to by the first few
numbers. As the numbers get larger, so the
tolerance zone becomes progressively wider.
As the grades get finer, the cost of production
increases at a sharper rate.
Grades Of Tolerance
The tolerance grades may be numerically
determined in terms of the standard tolerance
unit ‘i’ where i in microns is given by
(for basic size upto and including 500 mm)
where
D (in mm) is the geometric mean of the lower
and upper diameters of a particular step in
which the component lies.
Diameter Steps
The various diameter steps specified by ISI are:
1-3 3-6 6-10 10-18
18-30 30-50 50-80 80-120
180-250 250-315 315-400 400- 500
The fundamental deviation of type d, e, f, g
shafts are respectively -16D0.44 , -11D0.41 -5.5D0.41
and -2.5D 0.34
The fundamental deviation of type D, E, F, G
shafts are respectively +16D0.44, +11D0.41 ,
+5.5D0.41 and +2.5D0.34 .
Grade of Tolerance
IT01 to IT04 : For production of gauges,
measuring instruments.
IT5 to IT7 : For fits in precision engg.
Applications
IT8 to IT11 : For general engineering
IT12 to IT14 : For sheet metal or press working
IT15 to IT16 : For casting, general cutting etc.
Designation
Numerical
LIMIT GAUGES
BY: MOHIT GARG
Limit Gauges
A Go-No GO gauge refers to an inspection tool
used to check a work-piece against its allowed
tolerances.
A Go - No Go gauge is a measuring tool that
does not return a size in the conventional sense,
but instead returns a state. The state is either
acceptable (the part is within tolerance and
may be used) or it is unacceptable (and must be
rejected).
They are well suited for use in the production
area of the factory as they require little skill or
interpretation to use effectively and have few,
Limit Gauges
Limit gauges ensure the size of the component
lies within the specified limits.
According to their purpose:
(a) Work shop gauges: Working gauges are those
used at the bench or machine in gauging the
work as it being made.
(b) Inspection gauges: These gauges are used by
the inspection personnel to inspect
manufactured parts when finished.
(c) Reference or Master Gauges: These are used
only for checking the size or condition of other
gauges.
Limit Gauges
According to the form of tested surface:
a) Plug gauges: They check the dimensions of a
hole
b) Snap & Ring gauges: They check the
dimensions of a shaft.
Plug Gauges
Plug gauges are the limit gauges used for
checking holes and consist of two cylindrical
wear resistant plugs.
The plug made to the lower limit of the hole is
known as ‘GO’ end and this will enter any hole
which is not smaller than the lower limit
allowed.
The plug made to the upper limit of the hole is
known as ‘NO GO’ end and this will not enter
any hole which is smaller than the upper limit
allowed.
Ring Gauges
Ring gauges are used
for gauging shafts.
They are used in a
similar manner to
that of GO & NO GO
plug gauges.
A ring gauge consists
of a piece of metal in
which a hole of
required size is
bored.
Taylor’s Principle of Gauge Design:
‘GO’ Limit: ‘No Go’ Limit:
This designation is This designation is
applied to that limit of applied to that limit of
the two limits of size the two limits of size
which corresponds to the which corresponds to the
maximum material limit minimum material
considerations, i.e. upper condition. i.e. the lower
limit of a shaft and lower limit of a shaft and the
limit of a hole. upper limit of a hole.
‘No Go’ gauge should
The GO gauges should be check only one part or
of full form, i.e. they feature of the component
should check shape as at a time, so that specific
well as size. discrepancies in shape
or size can be detected.
Thus a separate ‘No Go’
gauge is required for
each different individual
Gauge Tolerance:
Gauges, like any other jobs require a
manufacturing tolerance due to reasonable
imperfections in the workmanship of the gauge
maker. The gauge tolerance should be kept as
minimum as possible though high costs are
involved to do so. The tolerance on the GO &
NO GO gauges is usually 10% of the work
tolerance.
Wear Allowance:
The GO gauges only are subjected to wear due
to rubbing against the parts during inspection
and hence a provision has to be made for the
