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Fits and Tolerances in Engineering

Heritage Institute of TechnologyDepartment of Mechanical Engineering

SEMINAR-II MECH-3221

Contents

History

Introduction

Interchangeability

System Terminologies

Tolerances

Fits

Hole and Shaft-basis System

Limit Systems

Designation of holes and shafts

Conclusion

History

Replacement of hand made tools with machine madetools.

This change in production, is now known as theIndustrial Revolution.

Began in England in 18th century.

Instead of utilizing artisans to produce hand-madeitems, machines started to help and eventually takethe place of the artisans.

Inventions aided in speeding up the production ofmanufactured items.

Introduction

In the manufacturing of a machine, quality is aprimary consideration.

Manufacturing precision taken into the productdetermines its quality, its cost and selling price.

Precision is the degree of accuracy necessary toensure the functioning of a part as intended.

There are two types of parts:

o Non-mating parts

o Mating parts

Interchangeability

The concept of mass production originated with theautomobile industry.

MODEL-T of Ford Motors was the first machine tobe mass-produced.

Till the 1940’s, every component was manufacturedin-house.

When one component assembles properly (andwhich satisfies the functionality aspect of theassembly/product) with any mating component,both chosen at random, then it is known asinterchangeability.

Ford Motor Co./AP The 1914 Ford Model T touring car isshown in this Ford Motor Co. handout. The 1914 model wasthe first version built on Henry Ford's moving assembly line.

System Terminologies

Shaft: The term ‘shaft’ used in this standard has a wide meaning and serves forspecification of all outer elements of the part, including those elements, whichdo not have cylindrical shapes.

Hole: The term ‘hole’ can be used for specification of all inner elementsregardless of their shape.

Basic Size: The basic size or normal size is the standard size for the part and isthe same both for the hole and its shaft. This is the size which is obtained bycalculation of strength.

System Terminologies (Contd.)

Actual Size: Actual size is the dimension as measured on a manufacturedpart. As already mentioned, the actual size will never be equal to the basic sizeand it is sufficient if it is within predetermined limits.

Limits of Size: These are the maximum and minimum permissible sizes ofthe part (extreme permissible sizes of the feature of the part).

Maximum Limit: The maximum limit or high limit is the maximum sizepermitted for the part

Minimum Limit: The minimum limit or low limit is the minimum sizepermitted for the part.

Zero Line: In a graphical representation of limits and fits, a zero line is astraight line to which the deviations are referred to. It is a line of zero deviationand represents the basic size. When the zero line is drawn horizontally, positivedeviations are shown above and negative deviations are shown below this line.

Deviation: It is the algebraic difference between a size (actual, limit of a size,etc.) and the corresponding basic size.

System Terminologies (Contd.)

Upper Deviation: It is designated as ES (for hole) and es (for shaft). It is thealgebraic difference between the maximum limit of the size and thecorresponding basic size. When the maximum limit of size is greater than thebasic size, it is a positive quantity and when the maximum limit of size is lessthan the basic size then it is a negative quantity.

Lower Deviation: It is designated as EI (for hole) and ei (for shaft). It is thealgebraic difference between the minimum limits of size and the correspondingbasic size. When the minimum limit of size is greater than the basic size, it is apositive quantity and when the minimum limit of size is less than the basic sizethen it is a negative quantity.

Fundamental Deviations (FD): This is the deviation, either upper or thelower deviation, which is the nearest one to the zero line for either a hole or ashaft. It fixes the position of the tolerance zone in relation to the zero line.

Actual Deviation: This is the algebraic difference between an actual size andthe corresponding basic size.

Mean Deviation: It is the arithmetical mean between the upper limit and thelower limit.

System Terminologies (Contd.)

Tolerance: It is the difference between the upper limit and the lower limit of adimension. It is also the maximum permissible variation in a dimension.

Tolerance Zone: It is a function of basic size. It is defined by its magnitudeand its position in relation to the zero line. It is the zone bounded by the twolimits of size of a part in the graphical presentation of tolerance.

Tolerance Guide: It is the degree of manufacturing. It is designated by theletters IT (stands for International Tolerance). Numbers, i.e., IT0, IT01, IT1,follow these letters up to IT16; the larger the number, the larger the tolerance.

Tolerance Class: The term is used for a combination of fundamentaldeviation and tolerance grade.

Allowance: It is an intentional difference between the maximum materiallimits of mating parts. For a shaft, the maximum material limit will be its highlimit and for a hole, it will be its low limit.

Fits: The relationship existing between two parts, shaft and hole, which are tobe assembled, with respect to the difference in their sizes is called fit.

Tolerances

Tolerance is the total amount that a specific dimension ispermitted to vary;

It is the difference between the maximum and the minimumlimits for the dimension.

For Example a dimension given as 1.625 ± .002 means that themanufactured part may be 1.627” or 1.623”, or anywherebetween these limit dimensions.

How to decide tolerance? Functional requirements of mating parts Cost of production Available manufacturing process

Choose as coarse tolerance as possible without compromisingfunctional requirements

Proper balance between cost and quality of parts

Tolerances

The Tolerance is 0.001” for the Hole as well as for the Shaft

Allowances and Clearances

Interchangeable Fit

Specifications of Tolerances

Limit Dimensioning

The high limit is placed above the low limit.

In single-line note form, the lowlimit precedes the high limitseparated by a dash

Specifications of Tolerances (Contd.)

Plus-or-minus Dimensioning

• Unilateral Tolerance

• Bilateral Tolerance

Fits

Fit is the general term used to signify the range of tightness orlooseness that may result from the application of a specificcombination of allowances and tolerances in mating parts.

A fit is the relationship between two meeting parts, viz., shaft andhole. This relationship is nothing but the algebraic differencesbetween their sizes

There are four types of fits between parts:

Clearance Fit

Interference Fit

Transition Fit

Line Fit

Clearance Fit

When an internal member fits in an external member (as a shaft in a hole) and always leaves a space or clearance between the

parts.

Minimum air space is 0.002”. This is the allowance and is always positivein a clearance fit

Interference Fit

The internal member is larger than the external member suchthat there is always an actual interference of material. Thesmallest shaft is 1.2513” and the largest hole is 1.2506”, so thatthere is an actual interference of metal amounting to at least0.0007”. Under maximum material conditions the interferencewould be 0.0019”. This interference is the allowance, and in aninterference fit it is always negative.

Transition Fit

This type of fit may result in either a clearance or interferencecondition. In the figure below, the smallest shaft 1.2503” will fitin the largest hole 1.2506”, with 0.003” to spare. But the largestshaft, 1.2509” will have to be forced into the smallest hole,1.2500” with an interference of metal of 0.009”.

Line Fit

The limits of size are so specified that a clearance or surfacecontact may result when mating parts are assembled.

Hole-basis System

Minimum hole is taken as the basic size, an allowance is assigned, and tolerances are applied on both sides of and away from this allowance.

1. The minimum size of the hole 0.500”is taken as the basic size.

2. An allowance of 0.002” is decided onand subtracted from the basic holesize, making the maximum shaft as0.498”.

3. Tolerances of 0.002” and 0.003”respectively are applied to the holeand shaft to obtain the maximumhole of 0.502” and the minimumshaft of 0.495”.

Minimum clearance: 0.500”-0.498” = 0.002”

Maximum clearance: 0.502” –0.495” = 0.007”

Shaft-basis System

Maximum shaft is taken as the basic size, an allowance is assigned, and tolerances are applied on both sides of and away from this allowance.

1. The maximum size of the shaft0.500” is taken as the basic size.

2. An allowance of 0.002” is decided onand added to the basic shaft size,making the minimum hole as 0.502”.

3. Tolerances of 0.003” and 0.001”respectively are applied to the holeand shaft to obtain the maximumhole of 0.505” and the minimumshaft of 0.499”.

Minimum clearance: 0.502”-0.500” = 0.002”

Maximum clearance: 0.505” –0.499” = 0.006”

International Tolerance Grade (IT)

They are a set of tolerances that varies according to the basic sizeand provides a uniform level of accuracy within the grade.

Indian Standards Specifications and Application

In India we have IS: 919 recommendation for limits and fits forengineering. This standard is mostly based on British StandardsBS: 1916-1953.

Sizes up to 500 min are covered in IS: 919 and sizes above 500mm, up to 3150 mm, are covered in IS: 2101

However, it is yet to adopt several recommendations of ISO: 286.

Conclusion

In manufacturing process after manufacturing the product weneed to check the dimensions of the product.

No manufacturing process is perfect to get perfect dimensionsfor a required product.

After manufacturing process even if you are getting the errors inthe dimensions of the product, those errors can be acceptable upto certain limit. These limits are decided during the designingprocess.

Bibliography

Kulkarni, V.A. , Bewoor, A.K: Metrology and Measurement

I.S. 1871: Commentary on Indian Standard wrought steels for general engineering purposes.

ISO 286: ISO System of limits and fits (in two parts)