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LIMITS, FITS, TOLERANCES & SURFACE ROUGHNESS

LIMITS, FITS, TOLERANCES & SURFACE ROUGHNESS. Made By: Prof. S. B . Gaikwad. IT’s IMPORTANCE!!. It is impossible to maintain dimensions , geometrical properties & surface roughness of components with absolute accuracy in production process.

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LIMITS, FITS, TOLERANCES & SURFACE ROUGHNESS

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  1. LIMITS, FITS, TOLERANCES & SURFACEROUGHNESS MadeBy: Prof. S. B. Gaikwad

  2. IT’sIMPORTANCE!! • Itisimpossibletomaintaindimensions,geometrical properties&surfaceroughnessofcomponentswith absolute accuracy in productionprocess. • Limits ,fits and tolerance are necessary to ensure INTERCHANGEBILITYofmatingparts,coupledwiththe desireddegreeoftightnessorloosenessofassembly. • Lackoftoleranceleadstoimproperfits,itwillalsoaddto • delay & highercost.

  3. INTRODUCTION BasicTerms: 1)Nominal Size 2)Basic Size 3) ActualSize 4)ActualDeviation 5) UpperDeviation 6)LowerDeviation 7) Zeroline

  4. Limits • Themaximumandminimumsize permitted for afeature. • There are twoextreme possible sizes of acomponent. • The largest permissible size fora component is calledupper • limit and smallestsize is • called lowerlimit. s

  5. MAXIMUM LIMIT : The greater value istaken as maximumlimit. • MINIMUM LIMIT : The smaller value istaken as minimumlimit. • Eg. If basicsizeis Ø=30.then 30+0.035=30.035mm.

  6. TOLERANCES Definition: Algebraic difference between upper limit& lowerlimit.

  7. UNILATERALTOLERANCES Variationfromthespecifieddimensionsis permittedin only one direction. Eitherpositiveornegativebutnotboth.

  8. BILATERALTOLERANCES 1.Variationispermittedinbothpositiveand negative directions from the nominal dimension.

  9. GEOMETRICTOLERANCES • Geometrictolerancesaredifferentfromthetolerances allowed for the size of feature, they specify the allowablevariationoftheshapeofafeature. • There are 3 basictypes: • Form 2.Orientation 3.Positiontolerances • Geometrictolerancesarespecifiedusingacontrolframe consistingofatolerancesymbol,atolerancevalue& • optional datumplanes.

  10. GEOMETRIC TOLERANCESYMBOLS

  11. FITS Fitisthedegreeoflooseness or tightness between two matingparts When FITS are combined betweenshaft&holethey can be classified into 3 categories: 1.Clearance Fit 2.InterferenceFit 3.TransitionFit

  12. ClearanceFit • Alwayshasagapbetweentwomatingparts. • Shaftsizeissmall,comparedtoholesize. • Generallyinthistypeoffit,thelowerlimitsizeoftheholeis greateroratleastequaltotheupperlimitsizeoftheshaft. • Use:-pivots,latchesfitofpartsexposedtocorrosive effects

  13. TransitionFit:- • Itisafitwherebothclearanceandinterferencemayoccur in thecoupling. • Heretolerancezonesoftheholeandshaft are partlyor completelyinterface. • Use:- Pulleys and bushing, Flushed boltsetc. B

  14. InterferenceFit:- • Italwaysoverlap&areusedmainlyforpressfitswherethe two parts are pushed together, and require no other fasteners. • Theupperlimitsizeoftheholeissmalleroratleastequalto the lower limitsizeof theshaft. • Use:-In bearing bushings,flangesetc.

  15. SYSTEM OFFITS

  16. SURFACEROUGHNESS The geometrical characteristics of asurface include, • Macro-deviations, Surface waviness,and Micro-irregularities. Thesurfaceroughnessisevaluatedbytheheight,Rtand meanroughnessindexRaofthemicro-irregularities. •

  17. SURFACE ROUGHNESSNUMBER Representstheaveragedepartureofthesurface from perfection over a prescribed sampling length, (usually selected as 0.8mm) • Surface roughness number (Ra) is expressedin microns. Ra =(h1+h2+-----+hn)/n • • The measurements are usually made along a line, runningatrightangletothegeneraldirectionoftool marks on thesurface. •

  18. INDICATION OF SURFACETEXTURE The basic symbol consists of two legs of unequallengthinclinedatapproximately60’ to the line representing the considered surface Thesymbolmustberepresentedbythinline If the removal of material by machining is required,abarisaddedtothebasicsymbol, Iftheremovalofmaterialisnotpermitted, acircleisaddedtothebasicsymbol. When special surface characteristicshave to beindicated,alineisaddedtothelongerarmof any ofthe above symbols,

  19. Generallytoindicatethesurfaceroughness,thesymbolisusedinsteadofvalue.Generallytoindicatethesurfaceroughness,thesymbolisusedinsteadofvalue. Therelationisgiveninfollowingtable.

  20. CASESTUDY Bearings onshaft

  21. SELECTON OF FITSAND TOLERACES • CONDITIONS OFROTATION • Rotating load b)Stationaryload • c)Directionofloadindeterminant • MAGNITUDE OF THELOAD • a)light load b)normal load c)heavy load d)very heavyload • BEARING INTERNALCLEARANCE • TEMPERATUREDIFFERENCES • DESIGN AND MATERIAL OF THE SHAFT AND HOUSING 6.EASE OF MOUNTING ANDDISMOUNTING

  22. For this example , we have selected m6fit. m- interference fit IT- 6grade Shaft size=70mm

  23. Finalevaluation Shaftdiameter=70mm Fit- m6fit Upper deviation(es)=+30 microns Lower deviation(ei)=+11 microns Basic size =70mm

  24. THANKYOU

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