4. Limits Tolerances

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LIMITS , FITS & TOLERANCES

PPT AS SUPPORT TO CLASS LECTURES

V ADE AUG 2011

1UW / UPES / ADE V SEM 2011


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UW / UPES / ADE V SEM 2011 6


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Tolerancing

Tolerances are used to control the variation insize that exists on all manufactured parts.

The amount that a size is allowed to vary dependson the function of the part & its assembly.

electric drill vs. jet engine The more accuracy required in a part (smaller

tolerance) the greater the cost.

Tolerances allow for interchangeable parts, whichpermits the replacement of individual parts in anassembly instead of replacing the whole system if apart goes bad or fails.



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Tolerancing Terms

Nominal size: the name or general size. Oftenexpressed in decimal format

Actual size: the measured size of the finished part.

Limits: the maximum & minimum sizes shown bythe toleranced dimension.

Tolerance the total allowable variance in adimension Tolerance = Upper Limit Lower Limit

Allowance: the min. clearanceor max. interferencebetween 2 parts.

Fit: the degree of tightness between mating parts



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Fit

Fit: the degree of tightness between mating parts. The three most common types of fit found in industry

Clearance fit- there is always a space betweenthe 2 mating parts. (shaft is smaller than the

hole) Interference fit- the 2 mating parts always

interfere with one another in assembly. (shaft isbigger than the hole)

Transition fit- sometimes a clearance fit &sometimes an interference fit between the matingparts.



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UW / UPES / ADE V SEM 2011 10


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A = Clearance fitB = Transition fitC = Interference fit



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Basic Systems

Basic Hole System Standard reamers, etc., are used to produce

holes and standard gauges used to checksizes. Shafts are machined.

Determine Minimumhole size, applyallowance, then apply tolerances.

Basic Shaft System

Not used often. However, advantageouswhen shafts cannot be easily machined tosize or when several parts fit onto one shaft.

Determine Maximumshaft size, apply

allowance, then apply tolerances. 15UW / UPES / ADE V SEM 2011


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Basic Shaft System

Lower deviation(Maximum clearance)

Upper deviation (Minimumclearance / allowance)

Fundamental deviation forthe hole equals theallowance

Basic size applied to shaft the typeof fit will determine whether thebasic size will be the largest shaft orthe smallest shaft.

Fundamental deviation forthe shaft equals the basicsize



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Basic Hole System

Basic size applied to hole the type of fitwill determine whether the basic size will bethe largest hole or the smallest hole.

Upper deviation (Minimumclearance / allowance)

Fundamental deviation forthe shaft equals theallowance

Lower deviation (Maximumclearance)

Fundamental deviation forthe hole equals the basicsize



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EXAMPLE ONLY FOR REFERENCElimits and fits using hole basis

Description Hole Shaft

Loose Running H11 c11

Free Running H9 d9

Easy Running - Good quality easy to do- H8 f8

Sliding H7 g6

Close Clearance - Spigots and locations H8 f7

Location/Clearance H7 h6

Location- slight interference H7 k6

Location/Transition H7 n6

Location/Interference- Press fit which can beseparated

H7 p6

Medium Drive H7 s6

Force H7 u6



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REFER PSG HAND BOOK FOR

DATA

PROBLEMS

1. Examples solved in class tutorials

2. Assignment No 1


4. Limits Tolerances

Documents

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