Limits Fits Tolerances

47
Limits, Fits & Tolerances

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fit

Transcript of Limits Fits Tolerances

Page 1: Limits Fits Tolerances

Limits, Fits & Tolerances

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Remember!

“No two things in the nature can be identical, they may be found to be closely similar”

• Every process is a combination of three elements: Man – Machine – Material

• All these three elements are subjected to inherent and characteristic variations

• These variables result in the variation of size of components

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Concept of Limits

Due to inevitable inaccuracy of manufacturing

methods it is impossible to produce a part to

an exact size and it can only be made to lie

between reasonable limits i.e. maximum limit

and minimum limit.

Difference between these two limits is called

the permissive tolerance.

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Tolerance Dimensioning

• Tolerance is the total amount that a specific

dimension is permitted to vary

• It is the difference between the maximum and the

minimum limits for the dimension

• For Example a dimension given as 1.625 ± .002 means

that the manufactured part may be 1.627” or 1.623” ,

or anywhere between these limit dimensions

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Concept of Fits

When parts are assembled together, engineers have to decide how they will fit together and the economics associated with it.

• How they will fit together?

– Clearance fit

• Door Hinges, Sleeve shafts

– Transition fit

• Pulleys, Piston on Piston Rods

– Interference fit

• Bearing bushing in hubs

• Economics?

– Interchangeability

Indian Standard IS 919 – 1993 (Part I & Part II)

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Term

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Relevant Terminology [As per IS: 919 -1993 (Part I)]

• Size: A number expressing, in a particular unit, the numerical

value of a length or of a linear dimension.

• Nominal Size: The nominal size of a dimension or part is the

size by which it is referred to as a matter of convenience. It is

generally expressed in common fractions.

• Basic Size or Normal Size or Basic dimension: The size with

reference to which the limits of size are fixed. The limits of size

are derived by the application of upper and lower deviations.

• Actual Size: The size of a part as may be found by

measurement.

• Fit: When two parts are to be assembled, the relation

resulting from the difference between their size before

assembly is called fit.

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• Shaft: A term used by convention to designate all external

features of a part, including those which are not cylindrical.

• Hole: A term used by convention to designate all internal

features of a part, including those which are not cylindrical.

• Deviation: The algebraic difference between a size (actual,

maximum etc.) and the corresponding basic size. Symbols for

shaft deviations are lower case letters (es, ei) and symbols for

hole deviations are upper case letters (ES, EI).

• Zero line: In a graphical representation of limits and fits, a

straight line to which the deviations are referred. The zero line is

the line of zero deviation and represents the basic size.

• Basic Shaft: A shaft whose upper deviation is zero or where

maximum limit of size is equal to basic size. It is h-shaft in IS:919

• Basic Hole: A hole whose lower deviation is zero or whose

minimum limit of size is equal to basic size. It is H hole in IS:919

Relevant Terminology contd.

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• Upper Deviation: The algebraic difference between the

maximum limit of size and the corresponding basic size. It is a

positive quantity when the maximum limit of size is greater than

the basic size and a negative quantity when the maximum limit

of size is less than the basic size. It is designated by ES for a hole

and es for a shaft.

• Lower Deviation: The algebraic difference between the

minimum limit of size and the corresponding basic size. It is a

positive quantity when the minimum limit of size is greater than

the basic size and a negative quantity when the minimum limit

of size is less than the basic size. It is designated by EI for a hole

and ei for a shaft.

Relevant Terminology contd.

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• Fundamental Deviation: It is that one of the two deviations which is conventionally chosen to define the position of the tolerance zone in relation to the zero line. It is the one nearest the zero line.

• Tolerance: Tolerance is equal to the algebraic difference between the upper and lower deviations and has an absolute value without sign.

• Allowance: The difference between the maximum shaft size and minimum hole is known as allowance.

• Maximum Material Limit: The designation applied to that of the two limits which corresponds to the maximum material size for the feature, i.e. upper limit of size for shaft, or the lower limit of size for a hole.

• Least Material Limit: The designation applied to that of the two limits which corresponds to the minimum material size of the feature, i.e. the lower limit of size for an external feature (shaft), or the upper limit of size for an internal feature (hole).

Relevant Terminology contd.

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De

via

tio

ns

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Fits Between Mating Parts Fit is the general term used to signify the range of tightness or looseness that may result from the application of a specific combination of allowances and tolerances in mating parts.

There are four types of fits between parts

1. Clearance Fit: 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 positive in

a clearance fit

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2. Interference Fit: The internal member is larger than the

external member such that there is always an actual

interference of material. The smallest shaft is 1.2513” and the

largest hole is 1.2506” , so that there is an actual interference

of metal amounting to at least 0.0007” . Under maximum

material conditions the interference would be 0.0019” . This

interference is the allowance, and in an interference fit it is

always negative.

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3. Transition Fit: may result in either a clearance or interference

condition. In the figure below, the smallest shaft 1.2503” will fit

in the largest hole 1.2506” , with 0.003” to spare. But the largest

shaft, 1.2509” will have to be forced into the smallest hole,

1.2500” with an interference of metal of 0.009” .

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Allowance & Clearance

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Basic Hole 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

on and subtracted from the basic

hole size, making the maximum

shaft as 0.498” .

3. Tolerances of 0.002” and 0.003”

respectively are applied to the

hole and shaft to obtain the

maximum hole of 0.502” and the

minimum shaft of 0.495” .

Minimum clearance: 0.500” -

0.498” = 0.002”

Maximum clearance: 0.502”

– 0.495” = 0.007”

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Basic Shaft 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 shaft

0.500” is taken as the basic size.

2. An allowance of 0.002” is decided

on and 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

hole and shaft to obtain the

maximum hole of 0.505” and the

minimum shaft of 0.499” .

Minimum clearance: 0.502” -

0.500” = 0.002”

Maximum clearance: 0.505” –

0.499” = 0.006”

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Q. Why hole basis system of fit is generally employed?

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Interchangeability

All parts are toleranced to permit them to be assembled

and function without the need for machining or fitting

at assembly. Parts are therefore interchangeable

Complete (functional) Interchangeability is an

Interchangeability, at which all types of parameters are

ensured with the accuracy that allows to perform fitting

less assembling (or re-placement at repair) of any

independently produced parts to obtain finished items.

That is, parts can be manufactured independently in

several shops (factories, towns, countries), and be

assembled into assembly units or items in other factories.

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Interchangeability Basic advantages of products produced under

conditions of complete Interchangeability are:

1) Development works for creation of new items are

easier, faster, and cheaper, because basic elements

are standardised (threads, splines, toothed gearing,

etc.);

2) Manufacture of items is easier and cheaper

(accuracy of blanks is specified, improved inspection

methods, easier assembling and others);

3) Exploitation is cheaper (shortening of repair period

and its high quality).

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Selective Assembly

• Also termed as group interchangeability or,

incomplete (restricted) interchangeability.

• Parts are mass-produced to specific tolerance

and allowance, then sorted manually or

through a computer-controlled optic system.

• All parts are inspected and sorted into various

size grades according to size.

• Parts are then selectively assembled.

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Limit Dimensions: The maximum and minimum values are specified.

Method of giving dimensions

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Specifications of Tolerances

Limit Dimensioning

The high limit is placed above

the low limit.

In single-line note form, the low limit

precedes the high limit separated by a

dash

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Plus or minus dimensions:

Basic size followed by plus or minus tolerance.

Unilateral: variation in only one direction.

One value should be zero, another may be all +

or -.

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Bilateral tolerance

It allows variation in both direction of basic size.

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Symmetry in bilateral dimension

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Single limit dimension: some time

one limit for example 0.5R MAX

Angular Tolerance: usually bilateral

and in degree minute or in second.

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It is best to dimension to each surface so that it

is affected by only one dimension. This can be

done by referring all dimensions to a single

datum surface.

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Tolerance related to machining Processes [Decimal Inch system]

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Metric system of tolerances and fit

• Specified by International Organization for

Standardization (ISO).

• Basic Size: The size where limits are

assigns.

• Deviation: Different between the basic and

hole or shaft size.

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Specifications of Tolerances

2. Plus-or-minus Dimensioning

• Unilateral Tolerance

• Bilateral Tolerance

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Cumulative Tolerances

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Tolerances Related to Machining Processes

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International Tolerance Grade

(IT):

They are a set of tolerances that varies according to the basic

size and provides a uniform level of accuracy within the grade.

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Tolerance Zone

It refers to the relationship of the tolerance to basic

size. It is established by a combination of the fundamental deviation indicated by a letter and the IT grade number.

In the dimension 50H8, for the close running fit, the H8 specifies the tolerance zone.

For any basic size, there are 28 different holes available for a combination of fundamental deviations and designated as:

A, B, CD, C, D, E, EF, F, FG, G, H, JS, J, K, M, N, P, R, S, T, U, V, X, Y, Z, ZA, ZB, ZC.

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Tolerance Zone

Each of the 28 holes has a choice of 20 tolerances

which are designated as:

IT01, IT0, IT1, IT2, IT3,…., up to and including IT18

Seven finest grades (IT01 to IT5) cover sizes up to

500 mm and the eleven coarsest grades up to 3150

mm

For Shaft,

There are 25 different shafts designated by small

letters from a to zc.

Also each shaft has 20 grades of tolerance grades

which are designated as for the holes.

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Tolerance symbols

They are used to specify the tolerance and fits for mating parts. For the hole-basis system , the 50 indicates the diameter in millimeters; the fundamental deviation for the hole is indicated by the capital letter H, and for the shaft it is indicated by the lowercase letter f. The numbers following the letters indicate this IT grade. Note that the symbols for the hole and shaft are separated by the slash. Tolerance symbols for a 50-mm-diameter hole may be given in several acceptable forms. The values in parentheses for reference only and may be omitted.

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Calculation of Standard Tolerances up to

and including 500 mm

Grades IT5 IT6 IT7 IT8 IT9 IT10

Values 7i 10i 16i 25i 40i 64i

Grades IT12 IT13 IT14 IT15 IT16 IT17 IT18

Values 160i 250i 400i 640i 1000i 1600i 2500i

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Calculation of Standard Tolerances

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Important Formulae for Fundamental

Deviations for Shafts for sizes up to 500

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Important Formulae for Fundamental

Deviations for Shafts for sizes up to 500

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Fit types and allied applications

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Fit types and allied applications

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Fit types and allied applications