Limits Fits Engineering

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SUBJECT : QUALITY MANAGEMENT 1 FOR : CP01 SEMESTER 1& 2

LIMITS, FITS &

ENGINEERING TOLERANCES


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4.0 LIMITS AND FITS

1. Type of manufacture.

2. Method of machining. 3. Condition of machine.

4. Skill of machinist.

5. Time taken.

6. Geometry of the product.

7. Condition of measuring tools.

8. Condition of measuring.

It is impossible to produce a component to anabsolute or 100% dimensional accuracy. Thevariations in size of the components arecaused by the following factors.


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Therefore depending upon the functionalrequirements of a work, some variation is

permitted on it. This permissible variation is called tolerance.

Though functional requirement is the primaryconsideration there are other factors like

standardization, methodisation,manufacturing needs etc., which influencethe choice of tolerance.

4.1 LIMIT SYSTEMIt is a system of standard tolerances anddeviations.


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INTERCHANGEABILITY

In earlier times the majority of components in assemblywere matched together, their dimensions being adjusteduntil the required type of fit is obtained.

These methods demanded craftsmanship of high order.Today manufacturing techniques are changed.

In mass production the process is broken in to severalsmaller activities and as a result various components willcome from several shops.

Under such conditions it becomes absolutely essential tohave a strict control over the dimension of parts, whichhave to match with other parts.

Any part selected at random should assemble correctlywith any other matching component that too selected atrandom. When a system of this kind is ensured, it is calledinterchangeable system.


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INTERCHANGEABLE PARTS

Interchangeable parts are those which ensure thepossibility of assembling a unit or machine or

replacing a worn out component without doing

any extra machining or fitting operations.


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ELEMENTS OF LIMIT SYSTEM

NOMINAL SIZE:The nominal size of a dimension is the

size specified in the drawing. It is usually given in the

drawing as rounded of whole millimeters.

BASIC SIZE: The basic size of dimension is the size in

relation to which all limits of variations are determined.

ACTUAL SIZE: the actual size of a dimension is its

measured size.

LIMITS OF SIZE: limits are the two extreme permissible

sizes for that dimension, there being an upper limit and a

lower limit. The upper limit is the largest and lower limitis the smallest permissible sizes.


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TOLERANCETOLERANCE

Tolerance is the difference between upper and lower limit

of size.

BILATERAL TOLERANCE

If the tolerance is allowed on both sides of the basic size it

is called bilateral tolerance.

Eg: 45 0.02 , 30 0.04

UNILATERAL TOLERANCE

If the tolerance is allowed on one side of the basic size it is

called unilateral tolerance.

Eg: 45+0.03 , 30 +0.04


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FITS

1.FIT

The relationship existing between two parts, shaft and

hole, which are to be assembled withrespect to theirdifference in the sizes before assembly is called fit.

1.Hole: The term used by convention to designate all

the internal features of a part

including those, which are not cylindrical.1.SHAFT: The term used by convention to designate

all external features of a part including those, which

are not cylindrical.

4.4.3 TYPES OF FIT:Depending upon the actual limit of the hole or shaft

the fit in Indian Standard shall be divided

into three main classes,


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4.4.3.1: CLEARANCE FIT- The fit, which always provides the

clearance is called clearance fit. Here, the tolerance zone of

the hole is entirely above that of shaft.

4.4.3.2 : INTERFERENCE FIT - The fit, which always provides an

interference is called interference fit.

Here, the tolerance zone of the hole is entirely below that of shaft.

4.4.3.3 : TRANSITION FIT - The fit, which provides either a

clearance or an interference is called transition fit. Here the tolerance

zones of the hole and shaft overlap.


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4.5 Indian standard ISO system of limits and fits

( IS 919 , ISO 286 ).

This Indian standard which is identical with ISO 286-1:

ISO system of limits and fits: Bases of tolerances, deviationsand fits was adopted by the Bureau of Indian Standards on the

recommendations of the Engineering Standards Sectional

Committee (LM 01) and approval of the Light Mechanical

Engineering Division Council.In this system fundamental deviations are indicated by letter

symbols for both holes and shafts. [Capital letters A to ZC for

holes and a to zc for shafts]. Letter symbols used to indicate

fundamental deviations are A B C CD D E EF F FG G H J

JS K M N P R S T U V X Y Z ZA Z B ZC. And 20

tolerance grades are indicated by number symbols from IT 01,

IT 0, IT 1 IT 18.


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TERMINOLOGY:

4.5.1 ZERO LINE:

In a graphical representation of limits and fits the straight line

to which deviations are referred is called zero line. It represents

the basic size. When the zero line is drawn horizontally positive

deviations are shown above and negative deviations below it.


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4.5.2 DEVIATIONS :It is the algebraic difference between a

size and corresponding basic size.

UPPER DEVIATION : It is the algebraic difference between

the maximum limit of size and the corresponding basic size.

It is designated by ESfor holes and esfor shafts.

LOWER DEVIATION : It is the algebraic difference between

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

is designated by EIfor holes and eifor shafts.

1. TOLERANCE:It is equal to the algebraic differencebetween the upper and lower deviation.


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4.5.4 TOLERANCE ZONE: In a graphical representation of

tolerance, the zone bounded by two limits of size of the part and

defined by its magnitude and by its position in relation to zero line.

Tolrance zone

Fundamental deviation

(lower deviation)

Min. limit size

Max. limit size

Basic size

Tolerance

Zero line


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4.6 SYMBOL FOR TOLERANCES, DEVIATIONS AND FITS

The tolerance is designated by a number symbol called

grade. The position of tolerance zone is indicated by a

letter symbol. [Capital letter for hole and small for shaft.]The tolerance size is just defined by basic value followed by

a letter and numeral. Eg. 50H7, 35g6. A fit is indicated by

the basic size common to both components followed by

symbols corresponding to each component, the hole beingwritten first.


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The following symbols are used to denote upper and lower

deviations.

Upper deviation of hole - ESLower deviation of hole - EI

Upper deviation of shaft- es

Lower deviation of shaft- ei

ES = EI+IT

es = ei+IT

4.6.1 GO AND NOT GO LIMIT :

Go limit refers to upper limit of shaft and lower limit of hole. It

corresponds to maximum material condition.Not Go limit refers to lower limit of shaft and upper limit of hole. It

corresponds to least material condition.


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4.6.2 HOLE BASIS SYSTEM & SHAFT BASIS SYSTEM

HOLE BASIS SYSTEM

A limit system is said to be on a hole basis, when the hole is held a

constant member and different fits are obtained by varying the sizes of the

shafts. In this system a single hole whose lower deviation is zero( H ) is

used.

SHAFT BASIS SYSTEM

A limit system is said to be on a shaft basis, when the shaft is a constantmember and different fits are obtained by varying the sizes of the holes. In

this system a single shaft whose upper deviation is zero (h) is used.

All modern limit system employed the hole basis system. The chief reason

is that it is easier to vary the size of shaft than that of hole. In majority ofdrawings in engineering work are produced with drill, reamer or some similar

tools and vary the size of the hole would necessitate the use of very large

number of tools of varying sizes. However in some instants shaft basis system

goes to more advantages to use than that of an hole basis system.


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4.6.3 GUIDELINE FOR SELECTION OF FIT

In the hole basis system various grades of holes used are,

H5: This grade can be obtained by precision boring, honingand fine internal grinding.

H6: This can be obtained by fine hand reaming, honing and

precision boring.

H7: This grade can be obtained by Internal grinding,broaching, or careful reaming.

H8: This can be obtained by machine reaming or boring.

H9: This can be obtained by boring and reaming. It is mainly

used for non circular dimensions.

H10: This grade is used for milled widths, drill holes andunimportant parts.

H11: This grade being very coarse is never used for fits. Eg.

coarse drilled and punched holes.

4 6 4 CLEARANCE FIT


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4.6.4 CLEARANCE FIT

Shafts a, b and c gives large clearance and therefore not widely

used.

Shaft d is used for loose running fit. Shaft e is used for large high-

speed heavily loaded bearing.Shaft f is used for normal grease lubricated or oil lubricated

bearings.

Shaft g is used in precision equipments. Shaft h is used for normal

location and spigot

fits and in the finer grades is used as precision sliding fit.

4.6.5 TRANSITION FIT

Shaft j is used for location fits where a slight interference is

permissible. Also used for spigot fits.

Shaft k is best suited for location fits. Shaft m gives location fits.Eg. Dowell hole, dowel pin.

Shaft n gives clearance only on extreme sides. It is recommended

for generally tightassembly fits.


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4.6.6 INTERFERENCE FIT

Shaft p gives a true interference. It is a standard press fit used

for steel and cast iron. An example of this fit is fixing of bush onto a gear.

Shaft r gives a medium drive fit on ferrous parts, and on non-

ferrous parts a light drive fit which can be easily dismantled

when required.

Shaft s is used for permanent and semi permanent assemblies.Shaft t, u and v give more interference. Shaft x , y , z , za , zb

and zc give a very large interference and therefore these

shafts are not recommended for fits.


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4.6.7 PROBLEMS

1) Calculate the maximum & minimum clearance for the following fits. Take the value of deviations

from tolerance chart .

20 h7/g6

20h7

ES = +21

EI = 0

Maximum hole size = 20.021

Minimum hole size = 20.000

20g6 es = -7

ei = -20Maximum shaft size = 19.993

Minimum shaft size = 19.980

Minimum clearance : Minimum clearance exists when the shaft is made to its maximum size and

hole to its minimum size. i.e when shaft size 19.993 and hole size is 20.00 mm

Minimum clearance =20.000-19.993

= 0.007mm

Maximum clearance : Maximum clearance exists when the shaft is made to its minimum size and

hole its maximum size. i.e when shaft size is19.980mm and hole size is 20.021mm

Maximum clearance = 20.021-19.980

= 0.041mm


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2. Calculate the maximum and minimum interference in the fit 20H7/p6

20H7 ES = +.021

EI = 0

Maximum hole size =20.021mm

Minimum hole size =20.000mm

20p6 - es = +35

ei = +22

Maximum shaft size = 20.035mm

Minimum shaft size =20.022mm

Maximum interference = maximum shaft size - minimum hole size= 20.035 -20.000 = 0.035mm

minimum interference = minimun shaft size - maximum hole size

= 20.022 - 20.021 = 0.001mm


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3) Calculate the maximum and minimum interference and

clearance in the fit 30H7/j6

30H7 -ES =+21EI = 0

Maximum hole size =30.021mm

Minimum hole size = 30.000mm

30j6 es = +9

ei = -4

Maximum shaft size=30.009mm

Minimum shaft size = 29.996mm

Maximum interference = maximum shaft size- minimum hole size

= 30.009-30.000

= 0.009mm

Maximum clearance = maximum hole size - minimum shaft size

= 30.021- 29.996

= 0.025mm


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4.7 SELECTIVE ASSEMBLY

Selective assembly is adopted to reduce the production cost of a job

without sacrificing its quality. In selective assembly the components produced

by a machine are classified into several groups according to size. This is doneboth for hole and shaft and then corresponding groups are matched.

Eg: If some part [shaft and hole] to be assembled are manufactured to

normal tolerance of 0.01mm [and both are within curve of normal distribution]

an automatic gauge can segregate them into ten different groups with in a

0.001mm limit for selective assembly of the individual parts.

Thus part with tolerance of 0.001mm is obtained and both the conditions of

high quality and low cost can be served by selective assembly techniques.

A practical example of selective assembly is found in the production of ballbearings. The balls are sorted into groups according to their size, to facilitatethe assembly of any bearing with balls of uniform size.


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Limits Fits Engineering