limts fit & tolerence
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SYSTEM OF LIMITS, FITS, TOLERANCE & GAUGES
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Why study Limits & Fits?Exact size is impossible to achieve.Establish boundaries within which deviation from perfect form is
allowed but still the design intent is fulfilled.Enable interchangeability of components during assembly
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It is impossible to make anything to an exact size, therefore it isessential to allow a definite tolerance or permissible variation onevery specified dimension
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STUFFING BOX
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Importance of tolerance
Parts from the same machine can be different Speed of operation Temperature
Lubrication Variation of incoming material Other factors
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How to decide tolerance? Functional requirements of mating parts Cost of production Available manufacturing process
Choose as coarse tolerance as possible without
compromising functional requirements Proper balance between cost and quality of parts
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CLEARANCE FIT
Maximum shaft dimension < Minimum hole dimension
The largest permitted shaft diameter is smaller than the diameter of the smallesthole, so that the shaft can rotate or slide through with different degrees offreedom according to the purpose of the mating members
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TRANSITION FIT
The diameter of the largest allowable hole is greater than that of the smallestshaft, but the smallest hole is smaller than the largest shaft, so hat small positive
or negative clearance between the shaft and hole members are employable.
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A fit is indicated by the basic size common to both components, followed bysymbol corresponding to each component, the hole being quoted first.
E.g. 45 H8/g7
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HOLE BASED SYSTEM-Size of hole is kept constant,shaft size is variedto get different fits.
To obtain different types of fits, it is general practice tovary tolerance zone of one of the mating parts
SHAFT BASED SYSTEM-
Size of shaft is kept constant,hole size is variedto get different fits.
Representation ofThe selection of letter freezes
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H : lower deviation ofhole is zero
h : upper deviation ofshaft is zero
E.S. upper deviation
E.I. lower deviation
Representation ofTolerance
1) Letter Symbol
The selection of letter freezesone limit of hole / shaft
(how much away from Basicsize)
45 E8/e 7Basic Size
One can have differentpossible combinations; eg.
45H6g7, 45H8r6, 45E5p7
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Representation of Tolerance2) Number or Grade
IT01, IT0, IT1,.IT16
Tolerance Grade defines range ofdimensions (dimensional variation)
There are manufacturingconstraints on tolerance gradechosen
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RANGE IN A GIVEN TOLERANCE GRADE
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RANGE IN A GIVEN TOLERANCE GRADE
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Representation ofThe selection of
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H : lower deviation ofhole is zero
Representation ofToleranceTolerance grade numberfreezes the other limit of
hole / shaft
Together (Letter & Grade)Representation of Fit
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g ( )on both matingcomponents decide qualityof fit
H7 : Tol Grade 7 mean 21 variation(H means upper deviation zero)
Representation of Fit
0.021
0.0220.013
30.000
30.02130.022
30.035
p6 : Tol Grade 6 means 13 variation(p means upper deviation is 22 )
INTERFERENCEFIT
Tolerance on Components
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p
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Estimate kind of fit
3 0 H 7 s
6
3 0 H 8 e
6
3 0 F 8 r 6
3 0 G 7 r 6
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FITS APPLICATIONS
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FITS AND TOLERANCES
The components of the toleranced dimension shall be indicatedin the following order:
a) the basic size, andb) the tolerance symbol.
If, in addition to the symbols it is necessary to express the values of thedeviations or the limits of size, the additional information shall be shownin brackets.
Permissible deviation
If a dimension needs to be limited in onedirection only, this should be indicated by adding
min or max to the dimension.
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The upper deviation or the upper limit of sizeshall be written in the upper position and the lowerdeviation or the lower limit of size in the lowerposition, irrespective of whether a hole or a shaft is
toleranced.
The tolerance symbol for thehole shall be placed before thatfor the shaft or above it, the
symbols being preceded by thebasic size indicated once only.
Indication of Tolerances on Angular Dimensions
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Max Hole size Basic Size = Upper Deviation
Min Hole size Basic Size = Lower Deviation
HOLE
Max shaft size Basic Size = Upper Deviation
Min shaft size Basic Size = Lower Deviation
SHAFT
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