Tolerance Limits and Fits

Tolerance Limits and Fits2/8/20101ToleranceNo two parts are identical.Variation in size inevitable.Interchangeability.Specified by two extreme allowable size called limits.Difference between the maximum and minimum limit sizes.

Allowance Provided intentionally. Gives the required fit (to meet out functional requirement).

2/8/20102Dimensional ToleranceGeometric ToleranceTolerance TypesWhat is the difference? Dimensional tolerances control ____________ ?

Geometric tolerances control __________ & ___________ ?

DimensionsDimensionsGeometry2/8/20103TerminologyStandard SizeUsed for standard components like bolt, nut, etc..Basic SizeSize calculated by design.Actual SizeSize of manufactured component.Tolerance Difference between maximum and minimum limit sizes.DeviationDifference between the actual size and basic size.LimitsTwo allowable extreme sizes for a component.AllowanceDimensional difference between the maximum limit of two mating components.

2/8/20104Types of deviationUpper deviationDeviation between maximum deviation limit and its basic size.

Lower deviationDeviation between minimum deviation limit and its basic size.

Fundamental deviationIt is the upper or lower deviation closest to the basic size.2/8/20105

2/8/20106Tolerance dependencyManufacturing process.Size of the component.

International Tolerance Grade IT01 to IT7 For measuring tools IT8 to IT15 For Materials IT5 to IT11 For fits IT12 to IT16 Commercial manufacturing2/8/20107

Tolerances Related to Machining Processes2/8/20108

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2/8/201010Specifying Tolerance - Methods1) 1.375 0.004

2) 1.371 & 1.379

3) Fundamental tolerance method

2/8/201011Bilateral and Unilateral TolerancesIf the deviation is on both sides from basic size bilateral

If the deviation is on one side (one of the deviation zero) - unilateral

UnilateralBilateral2/8/201012Fundamental tolerance method Uses letter and number symbol.

Letters A to Z, ZA, ZB and ZC is specified.

Upper case letters for holes and lower case letters for shafts.

Letters I, L, O, Q and W are not used.

Letter symbol represents the degree of closeness of tolerance zone.

Number symbol represents IT grade.2/8/201013

Tolerance Zones for holes2/8/201014

Tolerance Zones for shafts2/8/201015

Fundamental deviations for holes2/8/201016Fundamental deviations for shafts

2/8/201017Fundamental tolerance Example 50E6 Decode this tolerance symbol.

Basic sizeTol. Zone symbolIT grade50E6

E is on the positive side. So add tolerance to basic size.

Lower deviation = 50+0.050 = 50.050Upper deviation = 50+0.05+0.016 = 50.066 2/8/201018Example Contd.. Calculate maximum and minimum sizes of the holes for the given.a) 80J7b) 120ZC8

Letter symbolBasic sizeRangeTol. ZoneToleranceUpper/Lower8065 80 J0.018Lower120100 120 ZC0.690Upper Number symbolBasic sizeRangeIT GradeTolerance8050 80 70.03012080 120 80.054SymbolMinimum sizeMaximum size80J780+0.018 = 80.01880+0.018+0.030 = 80.048120ZC8120 0.690 0.054 = 119.256120 0.690 = 119.3102/8/201019Example Contd.. Calculate maximum and minimum sizes of the shaft for the given.85js10 Letter symbolBasic sizeRangeTol. ZoneToleranceUpper/Lower8580 100 jsIT/2Lower/Upper Number symbolBasic sizeRangeIT GradeTolerance8580 120 100.140SymbolMinimum sizeMaximum size85js1085 (0.140/2) = 84.9385 + (0.140/2) = 85.072/8/201020Example Contd.. A shaft of diameter 30 mm is to be produced in bulk quantities using capstan lathe. Suggest suitable tolerance.

From table, IT9 is obtainable in a capstan lathe.Basic size 30Range 18 30 Tolerance 0.052 (for IT9)Symmetric tolerance 0.026

So tolerance suggested is 300.0262/8/201021FitsIndicates the tightness or looseness of the mating parts.

Part ATolerance of A Part BTolerance of BFit Tolerance: Clearance, Interference, or Transition2/8/201022Types of fit Clearance fit Hole diameter is always bigger than shaft. Transition fit Hole diameter is approximately equal to the shaft diameter. Interference fit Hole diameter is smaller than shaft diameter.2/8/201023Shaft and Hole Clearance and Interference Fits

ClearanceInterference2/8/201024Shaft and Hole Fits Transition Fit

Transition2/8/201025

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2/8/201027System of fits Hole basis Lower deviation for hole is zero. Basic size is taken as the minimum hole [H].

Shaft basis Upper deviation for shaft is zero. Basic shaft size is taken as the maximum shaft diameter [h].2/8/201028Basic Hole SystemClearance/Allowance = Hole Shaft

Cmax = Hmax SminCmin = Hmin Smax

Both Cmax and Cmin >0 Clearance fitBoth Cmax and Cmin 0 Cmin < 0 Transition fit

System Tolerance = Cmax Cmin

Note: Fix hole size.Calculate shaft size from formula.

SMAXSMINHMAXHMIN2/8/201029Basic Shaft SystemClearance/Allowance = Shaft Hole

Cmax = Hmax SminCmin = Hmin Smax

Both Cmax and Cmin >0 Clearance fitBoth Cmax and Cmin 0 Cmin < 0 Transition fit

System Tolerance = Cmax Cmin

Note: Fix shaft size.Calculate hole size from formula2/8/201030Specifying a fitBy two fundamental tolerance grades.

Example: H7 d8 (Hole basis)D8 h7 (Shaft basis)First specification is for hole and the second one for shaft.

How to decide on fundamental tolerances?Based on type of fit suitable for the application.

Knowing application how to select fits?Use standards. (table)

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2/8/201033Example for fitA cylinder casing of basic inside diameter 85 mm is to be shrunk fit with a cylinder liner. Suggest a suitable tolerance for the cylinder casing and liner.

Solution:Hole basis Because the basic size is taken as inside diameter (85)Next select the type of fit. How? (Use table)

2/8/201034SummaryVariation in size during production is inevitable.To control above provide tolerance, while preparing drawings.Tolerance is provided based on the fit required.Required fit is selected from standard tables and based on application.

So a Mechanical engineerNeeds to know aboutTolerance and fits.Must be capable of interpretingTolerance symbols/detailsMust be familiar withStandards related to above and to locate and use standards book.

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