Limits Fits and Tolerances ppt.pdf .

Limits Fits and Tolerances ppt.pdf

1. Metrology & Measurements
(MME 2253)

2. Definitions of Metrology
Metrologyis"thescienceofmeasurement,embracing
bothexperimentalandtheoreticaldeterminationsatany
levelofuncertaintyinanyfieldofscienceand
technology"

3. Thus,inabroadersensemetrologyisnotlimitedtolengthandangle
measurementbutalsoconcernedwithnumerousproblemstheoreticalaswell
aspracticalrelatedwithmeasurementsuchas:
1.Unitsofmeasurementandtheirstandards,whichisconcernedwiththe
establishment,reproduction,conservationandtransferofunitsof
measurementandtheirstandards.
2.Methodsofmeasurementbasedonagreedunitsandstandards.
3.Errorsofmeasurement.
4.Measuringinstrumentsanddevices.
5.Accuracyofmeasuringinstrumentsandtheircare.
6.Industrialinspectionanditsvarioustechniques.
7.Design,manufacturingandtestingofgaugesofallkinds.
Instrumentation(Measurements)isthevarietyofmeasuringinstrumentsto
monitorandcontrolaprocess.Itistheartandscienceofmeasurementand
controlofprocessvariableswithinaproduction,laboratory,ormanufacturing
area.
Instrumentationisbranchofengineeringwhichdealswiththemeasurementsof
physicalentitieslikepressure,tempetc.Italsoincludessensorsandtransducers.
Measurements/Instrumentation

4. Part A -Measurements

5. Part B -Metrology

6. Note:Alltheabovereferencebooks(except6
th
)areimportantfromknowledge
pointofviewandareworthbuying.Forsimplicity2
nd
and3
rd
canbereferred.

7. LIMITS, FITS & TOLERANCES
Chapter 7/11

8. TERMINOLOGY
•NominalSize
•BasicSize
•Zeroline
•ActualSize
•Limits
•Tolerances
•Allowances
•Fits(clearance,interference,transition)
•Deviation(Fundamental,upper,lower)

9. NominalSize
•The nominal size of a dimension or part is the size by which
it is referred to as a matter of convenience (Eg: 50mm shaft or
2 inch shaft)
Basic Size
•Thesizeofapartinrelationtowhichalllimitsof
variationaredetermined(Eg:49±0.5mm)
Zero line
•Linewhichrepresentsthebasicsizesothatthe
deviationfromthebasicsizeiszero(Eg:49mmline)
Actual Size
•This is the measured size (Eg:49.25mm)

10. Unilateral Tolerance

11. Bilateral Tolerance

14. A max = 65.15 –39.85 = 25.3 mm
A min = 64.85 –40.15 = 24.7 mm
B max = 25.5 + 35.8 + 20.5 = 81.8 mm
B min = 24.5 + 34.2 + 19.5 = 78.2 mm

16. Limits
Thesearethetwopermissiblesizesforanydimension(high
andlow)
(Eg:49.5mm,48.5mm)
Tolerances
Itisthedifferencebetweenthehighandlowlimitsofsize.
(Eg:1mm,±0.5mm)

18. Fits
•Therelationresultingfromthedifferencebetweentheirsizesbefore
assemblywhentwopartsaretobeassembled.
Allowance
•Itistheintentionaldifferencebetweentheholedimensionandshaft
dimensionforanytypeofthefit.
•Maximumallowanceisobtainedbysubtractingtheminimumshaftsize
fromthelargestholeandminimumallowanceisthedifferencebetween
thelargestshaftandthesmallestholesize.
Atoleranceisthelimitofacceptableunintendeddeviationfroma
nominalortheoreticaldimension.Therefore,apairoftolerances,upper
andlower,definesarangewithinwhichanactualdimensionmayfall
whilestillbeingacceptable.
Incontrast,anallowanceisaplanneddeviationfromthenominalor
theoreticaldimension.

19. Clearance fit
•Inthistypeoffitthelargestpermittedshaftdiameteris
smallerthanthediameterofthesmallesthole,sothatthe
shaftcanrotateorslidethroughwithdifferentdegreesof
freedomaccordingtothepurposeofthematingmembers

21. Interference fit
•Inthistypeoffittheminimumpermitteddiameteronthe
shaftislargerthanthemaximumallowablediameterofthe
hole.Inthiscasetheshaftandholeareintendedtobe
attachedpermanentlyandusedasasolidcomponent

23. Transition fit
•Inthistypeoffitthediameterofthelargestallowableholeis
greaterthanthatofthesmallestshaft,butthesmallestholeis
smallerthanthelargestshaft,sothatsmallpositiveor
negativeclearancebetweentheshaftandholemembersare
employable.

25. Deviation
•Itisthealgebraicdifferencebetweenasizeandthecorresponding
basicsize.
Upper Deviation
•Itisthealgebraicdifferencebetweenthemaximumlimitofsizeand
thecorrespondingbasicsize.
•Itisdesignatedbyletters“ES”forholeand“es”forshaft.
•Itisapositivequantitywhenthemaximumlimitofsizeisgreater
thanthebasicsizeandanegativequantitywhenthemaximum
limitofsizeislessthanthebasicsize.
Lower Deviation
•Itisthealgebraicdifferencebetweentheminimumlimitofsizeand
thecorrespondingbasicsize.
•Itisdesignatedbyletters“EI”forholeand“ei”forshaft.
•Itisapositivequantitywhentheminimumlimitofsizeisgreater
thanthebasicsizeandanegativequantitywhentheminimumlimit
ofsizeislessthanthebasicsize.

27. Fundamental Deviation
•Itisoneofthetwodeviationswhichisconventionallychosentodefinethe
positionofthetolerancezoneinrelationtothezeroline.Thismaybeupperor
lowerdeviationwhichisclosesttozeroline.Itfixesthepositionofzeroline.

29. Term Type of fitDifference Between
Minimum Clearance Clearance Minimum hole Max Shaft
Maximum Clearance Clearance/TransitionMaximum Hole Minimum Shaft
Minimum Interference Interference Maximum Hole Minimum Shaft
Maximum InterferenceInterference Minimum hole Max Shaft

32. HoleBasis:Inthissystem,thebasicdiameteroftheholeisconstantwhiletheshaft
sizeisvariedaccordingtothetypeoffit.
SignificanceofHolebasissystem:ThebureauofIndianStandards(BIS)recommends
bothholebasisandshaftbasissystems,buttheirselectiondependsonthe
productionmethods.Generally,holesareproducedbydrilling,boring,reaming,
broaching,etc.whereasshaftsareeitherturnedorground.
Iftheshaftbasissystemisusedtospecifythelimitdimensionstoobtainvarious
typesoffits,numberofholesofdifferentsizesarerequired,whichinturnrequires
toolsofdifferentsizes.[Sincedrillbitscomesinstandardsizesonly]

33. ShaftBasisSystem:Inthissystem,thebasicdiameteroftheshaftisconstant
whiletheholesizeisvariedaccordingtothetypeoffit.
Itmay,however,benecessarytouseshaftbasissystemwheredifferentfitsare
requiredalongalongshaft.
Forexample,inthecaseofdrivingshaftswhereasingleshaftmayhaveto
accommodatetoavarietyofaccessoriessuchascouplings,bearings,collars,etc.,itis
preferabletomaintainaconstantdiameterforthepermanentmember,whichisthe
shaft,andvarytheboreoftheaccessories.

38. IT stands for International Tolerance

40. For shafts up to 500mm diameter
The various diameter steps specified by ISI are:
1-3, 3-6, 6-10, 10-18, 18-30, 30-50, 50-80, 80-120, 120-180, 180-250, 250-315, 315-400, and
400-500 mm.
The value of ‘D’ is taken as the geometric mean for a particular range of size to
avoid continuous variation of tolerance with size.

45. Calculatethelimitsoftoleranceandallowancefora
25mmshaftandholepairdesignatedbyH8d9.Take
thefundamentaldeviationfor‘d’shaftis-16D
0.44
.

46. 25 H8d9

47. 25 H8d9

48. 25 H8d9

49. •D = (18 x 30) =
•i (in microns)=0.45∛D +0.001D
•i=
•25i=
•40i=
•For H hole FD =0
•UL of Hole=BS+FD+IT
•LL of Hole=BS+FD
•For d shaft FD= -16D
0.44
=
•UL of Shaft=BS+FD=
•LL of Shaft= BS+FD-IT =
D = 23.237mm
i= 1.3 m
25i= 33m
40i=52m
For H hole FD =0
UL of Hole= 25.033mm
LL of Hole= 25.00mm
-16D
0.44
= -64m
UL of Shaft= 24.936mm
LL of Shaft= 24.884mm

50. For H hole FD =0
UL of Hole= 25.033mm
LL of Hole= 25.00mm
FD for Shaft= -64m
UL of Shaft= 24.936mm
LL of Shaft= 24.884mm
HoleTolerance= 33m Shaft Tolerance =52m
Graphical Representation of Limits (Not to Scale)

51. Calculatealltherelevantdimensionsof35H7/f8fit,
dimension35mmfallsinthestepof30-50mm.The
fundamentaldeviationforfshaftis–5.5D
0.41
.
i(inmicrons)=0.45(D)
1/3
+0.001D,IT7=16iandIT8=25i.

52. 35H7/f8

53. Determine the limits on a shaft and a bearing of 20mm nominal diameter, which fit together
with a clearance fit. The tolerance on the shaft is g6 and on the bearing H7
Calculate.
(a) The tolerance on the shaft.(b)The tolerance on the bearing.
(c) The maximum clearance. (d)The minimum clearance.
The fundamental deviation for g shaft = -2.5D
0.34
Ans.
(a)0.013mm
(b)0.021mm
(c)0.041mm
(d)0.007mm
Solve

54. 20H7/g6
(a)The tolerance on the shaft.
(b)The tolerance on the bearing.
(c) The maximum clearance.
(d) The minimum clearance.
Ans.
(a)0.013mm
(b)0.021mm
(c)0.041mm
(d)0.007mm

56. Determine the Limits on a shaft and a pulley of 40mm nominal diameter. The tolerance on the
shaft is h6 and on the pulley is H7
Calculate.
(a) The tolerance on the shaft.
(b) The tolerance on the pulley.
(c) The maximum clearance.
(d) The minimum clearance.
Ans.
(a) 0.016mm (b) 0.025mm
(c) 0.041mm (d) 0.000mm
Solve

57. 40H7/h6
(a)The tolerance on the shaft.
(b)The tolerance on the pulley.
(c) The maximum clearance.
(d) The minimum clearance.
Ans.
(a)0.016mm
(b)0.025mm
(c) 0.041mm
(d) 0.000mm

59. Determine the Limits on a shaft and a pulley of 45mm nominal diameter. The tolerance on
the shaft is g6 and on the pulley is H7
Calculate.
(a) The tolerance on the shaft.
(b) The tolerance on the pulley.
(c) The maximum clearance.
(d) The minimum clearance.
Ans.
(a) 0.016 (b) 0.025
(c) 0.050 (d) 0.009
Solve

60. DeterminethelimitdimensionsforaInterferencefiton
theShaftBasisSystemforabasicsizeof20mm
diameterwithaminimuminterferenceof20μm.The
toleranceontheholeis32μmandontheshaftis
27μm.Representthefitgraphically.
Term Type of fitDifference Between
Minimum Clearance Clearance
Minimum hole Max Shaft
Maximum Clearance
Clearance/Transition
Maximum Hole Minimum Shaft
Minimum Interference Interference Maximum Hole
Minimum Shaft
Maximum Interference
Interference
Minimum hole Max Shaft
Solve

61. Shaft upper limit = 20mm
Shaft lower limit = 20-0.027= 19.973mm
Hole upper limit = 20-0.027-0.020=19.953mm
Hole lower limit= 20-0.027-0.020-0.032=19.921mm
Minimum Interference = 19.973-19.953 = 0.020mm
Maximum Interference = 20-19.921= 0.079mm