limts fit & tolerence

8/11/2019 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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limts fit & tolerence

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