4. Limits Tolerances
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Transcript of 4. Limits Tolerances
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LIMITS , FITS & TOLERANCES
PPT AS SUPPORT TO CLASS LECTURES
V ADE AUG 2011
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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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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
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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
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