limits fits tolerances
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Limits, Fits & Tolerances
Remember!
“No two things in the nature can be identical, they may be found to be closely similar”
• Every process is a combination of three elements: Man – Machine – Material
• All these three elements are subjected to inherent and characteristic variations
• These variables result in the variation of size of components
Concept of Limits
Due to inevitable inaccuracy of manufacturing
methods it is impossible to produce a part to
an exact size and it can only be made to lie
between reasonable limits i.e. maximum limit
and minimum limit.
Difference between these two limits is called
the permissive tolerance.
Tolerance Dimensioning
• Tolerance is the total amount that a specific
dimension is permitted to vary
• It is the difference between the maximum and the
minimum limits for the dimension
• For Example a dimension given as 1.625 ± .002 means
that the manufactured part may be 1.627” or 1.623” ,
or anywhere between these limit dimensions
Concept of Fits
When parts are assembled together, engineers have to decide how they will fit together and the economics associated with it.
• How they will fit together?
– Clearance fit
• Door Hinges, Sleeve shafts
– Transition fit
• Pulleys, Piston on Piston Rods
– Interference fit
• Bearing bushing in hubs
• Economics?
– Interchangeability
Indian Standard IS 919 – 1993 (Part I & Part II)
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Relevant Terminology [As per IS: 919 -1993 (Part I)]
• Size: A number expressing, in a particular unit, the numerical
value of a length or of a linear dimension.
• Nominal Size: The nominal size of a dimension or part is the
size by which it is referred to as a matter of convenience. It is
generally expressed in common fractions.
• Basic Size or Normal Size or Basic dimension: The size with
reference to which the limits of size are fixed. The limits of size
are derived by the application of upper and lower deviations.
• Actual Size: The size of a part as may be found by
measurement.
• Fit: When two parts are to be assembled, the relation
resulting from the difference between their size before
assembly is called fit.
• 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.
• Deviation: The algebraic difference between a size (actual,
maximum etc.) and the corresponding basic size. Symbols for
shaft deviations are lower case letters (es, ei) and symbols for
hole deviations are upper case letters (ES, EI).
• Zero line: In a graphical representation of limits and fits, a
straight line to which the deviations are referred. The zero line is
the line of zero deviation and represents the basic size.
• Basic Shaft: A shaft whose upper deviation is zero or where
maximum limit of size is equal to basic size. It is h-shaft in IS:919
• Basic Hole: A hole whose lower deviation is zero or whose
minimum limit of size is equal to basic size. It is H hole in IS:919
Relevant Terminology contd.
• Upper Deviation: The algebraic difference between the
maximum limit of size and the corresponding basic size. It is a
positive quantity when the maximum limit of size is greater than
the basic size and a negative quantity when the maximum limit
of size is less than the basic size. It is designated by ES for a hole
and es for a shaft.
• Lower Deviation: The algebraic difference between the
minimum limit of size and the corresponding basic size. It is a
positive quantity when the minimum limit of size is greater than
the basic size and a negative quantity when the minimum limit
of size is less than the basic size. It is designated by EI for a hole
and ei for a shaft.
Relevant Terminology contd.
• Fundamental Deviation: It is that one of the two deviations which is conventionally chosen to define the position of the tolerance zone in relation to the zero line. It is the one nearest the zero line.
• Tolerance: Tolerance is equal to the algebraic difference between the upper and lower deviations and has an absolute value without sign.
• Allowance: The difference between the maximum shaft size and minimum hole is known as allowance.
• Maximum Material Limit: The designation applied to that of the two limits which corresponds to the maximum material size for the feature, i.e. upper limit of size for shaft, or the lower limit of size for a hole.
• Least Material Limit: The designation applied to that of the two limits which corresponds to the minimum material size of the feature, i.e. the lower limit of size for an external feature (shaft), or the upper limit of size for an internal feature (hole).
Relevant Terminology contd.
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Fits Between Mating Parts Fit is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts.
There are four types of fits between parts
1. Clearance Fit: an internal member fits in an external member (as a shaft in a hole) and always leaves a space or clearance between the parts.
Minimum air space is 0.002” . This is the allowance and is always positive in
a clearance fit
2. Interference Fit: The internal member is larger than the
external member such that there is always an actual
interference of material. The smallest shaft is 1.2513” and the
largest hole is 1.2506” , so that there is an actual interference
of metal amounting to at least 0.0007” . Under maximum
material conditions the interference would be 0.0019” . This
interference is the allowance, and in an interference fit it is
always negative.
3. Transition Fit: may result in either a clearance or interference
condition. In the figure below, the smallest shaft 1.2503” will fit
in the largest hole 1.2506” , with 0.003” to spare. But the largest
shaft, 1.2509” will have to be forced into the smallest hole,
1.2500” with an interference of metal of 0.009” .
Allowance & Clearance
Basic Hole System Minimum hole is taken as the basic size, an allowance is assigned,
and tolerances are applied on both sides of and away from this
allowance.
1. The minimum size of the hole
0.500” is taken as the basic size.
2. An allowance of 0.002” is decided
on and subtracted from the basic
hole size, making the maximum
shaft as 0.498” .
3. Tolerances of 0.002” and 0.003”
respectively are applied to the
hole and shaft to obtain the
maximum hole of 0.502” and the
minimum shaft of 0.495” .
Minimum clearance: 0.500” -
0.498” = 0.002”
Maximum clearance: 0.502”
– 0.495” = 0.007”
Basic Shaft System Maximum shaft is taken as the basic size, an allowance is assigned,
and tolerances are applied on both sides of and away from this
allowance.
1. The maximum size of the shaft
0.500” is taken as the basic size.
2. An allowance of 0.002” is decided
on and added to the basic shaft
size, making the minimum hole as
0.502” .
3. Tolerances of 0.003” and 0.001”
respectively are applied to the
hole and shaft to obtain the
maximum hole of 0.505” and the
minimum shaft of 0.499” .
Minimum clearance: 0.502” -
0.500” = 0.002”
Maximum clearance: 0.505” –
0.499” = 0.006”
Q. Why hole basis system of fit is generally employed?
Interchangeability
All parts are toleranced to permit them to be assembled
and function without the need for machining or fitting
at assembly. Parts are therefore interchangeable
Complete (functional) Interchangeability is an
Interchangeability, at which all types of parameters are
ensured with the accuracy that allows to perform fitting
less assembling (or re-placement at repair) of any
independently produced parts to obtain finished items.
That is, parts can be manufactured independently in
several shops (factories, towns, countries), and be
assembled into assembly units or items in other factories.
Interchangeability Basic advantages of products produced under
conditions of complete Interchangeability are:
1) Development works for creation of new items are
easier, faster, and cheaper, because basic elements
are standardised (threads, splines, toothed gearing,
etc.);
2) Manufacture of items is easier and cheaper
(accuracy of blanks is specified, improved inspection
methods, easier assembling and others);
3) Exploitation is cheaper (shortening of repair period
and its high quality).
Selective Assembly
• Also termed as group interchangeability or,
incomplete (restricted) interchangeability.
• Parts are mass-produced to specific tolerance
and allowance, then sorted manually or
through a computer-controlled optic system.
• All parts are inspected and sorted into various
size grades according to size.
• Parts are then selectively assembled.
Limit Dimensions: The maximum and minimum values are specified.
Method of giving dimensions
Specifications of Tolerances
Limit Dimensioning
The high limit is placed above
the low limit.
In single-line note form, the low limit
precedes the high limit separated by a
dash
Plus or minus dimensions:
Basic size followed by plus or minus tolerance.
Unilateral: variation in only one direction.
One value should be zero, another may be all +
or -.
Bilateral tolerance
It allows variation in both direction of basic size.
Symmetry in bilateral dimension
Single limit dimension: some time
one limit for example 0.5R MAX
Angular Tolerance: usually bilateral
and in degree minute or in second.
It is best to dimension to each surface so that it
is affected by only one dimension. This can be
done by referring all dimensions to a single
datum surface.
Tolerance related to machining Processes [Decimal Inch system]
Metric system of tolerances and fit
• Specified by International Organization for
Standardization (ISO).
• Basic Size: The size where limits are
assigns.
• Deviation: Different between the basic and
hole or shaft size.
Specifications of Tolerances
2. Plus-or-minus Dimensioning
• Unilateral Tolerance
• Bilateral Tolerance
Cumulative Tolerances
Tolerances Related to Machining Processes
International Tolerance Grade
(IT):
They are a set of tolerances that varies according to the basic
size and provides a uniform level of accuracy within the grade.
Tolerance Zone
It refers to the relationship of the tolerance to basic
size. It is established by a combination of the fundamental deviation indicated by a letter and the IT grade number.
In the dimension 50H8, for the close running fit, the H8 specifies the tolerance zone.
For any basic size, there are 28 different holes available for a combination of fundamental deviations and designated as:
A, B, CD, C, D, E, EF, F, FG, G, H, JS, J, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC.
Tolerance Zone
Each of the 28 holes has a choice of 20 tolerances
which are designated as:
IT01, IT0, IT1, IT2, IT3,…., up to and including IT18
Seven finest grades (IT01 to IT5) cover sizes up to
500 mm and the eleven coarsest grades up to 3150
mm
For Shaft,
There are 25 different shafts designated by small
letters from a to zc.
Also each shaft has 20 grades of tolerance grades
which are designated as for the holes.
Tolerance symbols
They are used to specify the tolerance and fits for mating parts. For the hole-basis system , the 50 indicates the diameter in millimeters; the fundamental deviation for the hole is indicated by the capital letter H, and for the shaft it is indicated by the lowercase letter f. The numbers following the letters indicate this IT grade. Note that the symbols for the hole and shaft are separated by the slash. Tolerance symbols for a 50-mm-diameter hole may be given in several acceptable forms. The values in parentheses for reference only and may be omitted.
Calculation of Standard Tolerances up to
and including 500 mm
Grades IT5 IT6 IT7 IT8 IT9 IT10
Values 7i 10i 16i 25i 40i 64i
Grades IT12 IT13 IT14 IT15 IT16 IT17 IT18
Values 160i 250i 400i 640i 1000i 1600i 2500i
Calculation of Standard Tolerances
Important Formulae for Fundamental
Deviations for Shafts for sizes up to 500
Important Formulae for Fundamental
Deviations for Shafts for sizes up to 500
Fit types and allied applications
Fit types and allied applications
Fit types and allied applications