Limits

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©© DR.AMS2012DR.AMS2012

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CHAPTER -3

LIMITS, FITS & TOLERANCE

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IMPORTANT TERMS

ACCURACY: Capability to get accurate measure of dimension

(Micrometer & Scale) accurate upto one millionth of metre(Micron)

PRECISION : How close together of readings

TOLERANCE: Permissible deviation from the desired value, Basic

size

SURFACE FINISH: Amount of geometric irregularity on the

surface

RELIABILITY: Probability of product functioning in a intended

manner

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TOLERANCE

Basic size

Smallest acceptable

dimension

Largest acceptable dimension

Tolerance

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MEASURING AND INSPECTION INSTRUMENTS

DIRECT MEASURING INSTRUMENTS

Scale, Vernier Caliper, Micrometer

INDIRECT MEASURING INSTRUMENTS

Simple Caliper, comparator

GAUGES:

To check the acceptability of a specified dimension

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THE GUAGE

NO GOGO Dimension of NOGO

opening

Dimension

of GO

opening

Upper limit

(Maximum

acceptable size)

Lower limit

(Minimum

acceptable

size)

Workpiece

toleranceGO

NOGO

Gage

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GO AND NOGO GAUGE

GO NOGOGO NOGOGO

NOGO

Part

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LIMITS AND FITS

A system of limits and fits helps in:

Interchangeability and ease of assembly.

Eliminating the need for minor rectifications (fitting) during

assembly.

Eliminating unnecessary effort being spent in producing

dimensions to a higher degree of accuracy or surface finish than

what is needed for satisfactory operation.

For example, is uneconomical to limit the accuracy of a dimension

to 0.005 mm when 1 mm would prove satisfactory

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TOLERANCE

It is the acceptance of an error in mfg.

Difference between the maximum and minimum limit of size

(Not to scale)

Upper deviation

Tolerance

Dmax

Dn

Dmin

Low

er

devia

tion

0 0

0 0

Upper deviation

Low

er

devia

tion

Dmin

Dn

Dmax

(a) (b)

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TOLERANCE

SELECTION OF TOLERANCE DEPENDS ON

The nominal size.

Manufacturing method used.

Accuracy and quality desired.

Type of fit required

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TOLERANCE

UNILATERAL AND BILATERAL TOLERANCE

(Not to scale)

0.0

30

Nominal size = 24.000

Tolerance = 0.030

24.0

00

Specified as:+0.030- 024.000

or24.03024.000

24.000or24.02523.995

Specified as:

0.0

05

24.0

00

+0.025-0.005

0.0

25

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Specifications of Tolerances

• Unilateral Tolerance

• Bilateral Tolerance

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Allowance & Clearance

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FITS

It is the degree of tightness or looseness between

two mating parts

It is the characteristic of the assembly

TYPES OF FITS

Clearance fits

Interference fits

Transition fits

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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

CLEARANCE FIT

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INTERFERENCE FIT

Ex: dowel pins, bearings in castings

Allowance- Negative Clearance

Maximum

interference

Minimum

interference

Hole

Shaft

(Not to scale)

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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.

INTERFERENCE FIT- CONTD

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TRANSITION FIT

Hole

Maximum

clearance

(Not to scale)

Shaft

Maximum

interference

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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”.

TRANSITION FIT –CONTD---

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GRADES OF TOLERANCE

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EXAMPLES

Find the type of fit obtained in the following case :-

Size of the hole : 29.00

Size of the shaft : 29.00

Maximum clearance = Maximum size of hole – Minimum size of shaft

= 29.013 – 28.987

= 0.026 mm

Minimum clearance = Minimum size of hole – Maximum size of shaft

= 29 – 29

= 0 mm

Since, both clearances are greater than or equal to zero, the resulting

fit is clearance fit.

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EXAMPLES

Solution:

Maximum size of shaft = 50 – 0 = 50 mm

Minimum size of shaft = 50 – 0.011

= 49.989 mm

Maximum size of hole = 50 – 0.026

= 49.974 mm

Minimum size of hole = 50 – 0.0625

= 49.935 mm

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