Limits, fits and tolerances

Department of Mechanical Engineering
JSS Academy of Technical Education, Bangalore-560060
MECHANICAL MEASUREMENTS AND METROLOGY
(Course Code:18ME36B)

TEXT BOOKS
• Mechanical Measurements, Beckwith Marangoni and Lienhard, Pearson Education, 6th Ed., 2006.
• Instrumentation, Measurement and Analysis, B C Nakra, K K Chaudhry, 4th Edition, McGraw Hill.
• Engineering Metrology, R.K. Jain, Khanna Publishers, Delhi, 2009
REFERENCE BOOKS:
• Engineering Metrology and Measurements, N.V.Raghavendra and L.Krishnamurthy, Oxford
University Press..
Further Reference:
 National Programme on Technology Enhanced Learning (NPTEL)
http://nptel.ac.in/courses/112104121/1

MECHANICAL MEASUREMENTS AND METROLOGY
CHAPTER 3: Limits, Fits, Tolerance and Gauging

• To equip with knowledge of limits, fits, tolerances and gauging.
Learning Objectives

• Appreciate the significance of different types of limits, fits, and tolerances in
design and manufacturing fields, required for efficient and effective performance
of components.
• Understand the importance of manufacturing components to specified sizes.
• Explain the principle of limit gauging and its importance in inspection in industries.
Outcomes

Module 2
System of Limits, Fits, Tolerance and Gauging:
Definitions, Tolerance, Tolerance analysis (addition & subtraction of tolerances)
Interchangeability & Selective assembly. Class & grade of tolerance, Fits, Types
of fits, Numerical on limits, fit and tolerance. Hole base system & shaft base
system. Taylor’s principle, Types of limit gauges, Numerical on limit gauge
design.

INTRODUCTION
• No two parts can be produced with identical measurements by any
manufacturing process.
• In any production process, regardless of how well it is designed or how carefully
it is maintained, a certain amount of variation (natural) will always exist.

INTRODUCTION
Variations arises from;
• Improperly adjusted machines
• Operator error
• Tool wear
• Defective raw materials etc.
Such variations are referred as ‘assignable causes’ and can be identified
and controlled.

INTRODUCTION
• It is impossible to produce a part to an exact size or basic size, some
variations, known as tolerances, need to be allowed.
• The permissible level of tolerance depends on the functional requirements,
which cannot be compromised.

INTRODUCTION
• No component can be manufactured precisely to a given dimension; it can only
be made to lie between two limits, upper (maximum) and lower (minimum).
• Designer has to suggest these tolerance limits to ensure satisfactory operation.
• The difference between the upper and lower limits is termed permissive
tolerance.

INTRODUCTION
Example
Shaft has to be manufactured to a diameter of 40 ± 0.02 mm.
The shaft has a basic size of 40 mm.
It will be acceptable if its diameter lies between the limits of sizes.
Upper limit of 40+0.02 = 40.02 mm
Lower limit of 40-0.02 = 39.98 mm.
Then, permissive tolerance is equal to 40.02 − 39.98 = 0.04 mm.

• To satisfy the ever-increasing demand for accuracy.
• Parts have to be produced with less dimensional variation.
• It is essential for the manufacturer to have an in-depth knowledge of the
tolerances to manufacture parts economically, adhere to quality and reliability
• To achieve an increased compatibility between mating parts.
Tolerances

• The algebraic difference between the upper and lower acceptable dimensions.
• It is an absolute value.
• The basic purpose of providing tolerances is to permit dimensional variations in
the manufacture of components, adhering to the performance criterion.
Tolerances

Tolerances
Manufacturing Cost and Work Tolerance
Relationship between work tolerance and manufacturing cost
Tolerance is a trade-off between the economical
production and the accuracy required for proper
functioning of the product.

Tolerances
1. Unilateral tolerance
2. Bilateral tolerance
3. Compound tolerance
4. Geometric tolerance
Classification of Tolerance

Tolerances
1. Unilateral tolerance
• When the tolerance distribution is only on one side of the basic size.
Either positive or negative, but not both.
Tolerances (a) Unilateral (b) Bilateral

1. Unilateral tolerance: Below zero line: Negative

1. Unilateral tolerance: Above zero line: Positive

2. Bilateral tolerance
When the tolerance distribution lies on either side of the basic size.
• It is not necessary that Zero line will divide the tolerance zone equally on both sides.
• It may be equal or unequal

3. Compound tolerance
Tolerance for the dimension R is
determined by the combined effects of
tolerance on 40 mm dimension, on 60o, and
on 20 mm dimension

Geometric dimensioning and tolerancing (GD&T) is a method of defining parts
based on how they function, using standard symbols.

• Diameters of the cylinders need be concentric with each other.
• For proper fit between the two cylinders, both the centres to be in line.
• This information is represented in the feature control frame.
• Feature control frame comprises three boxes.

• First box: On the left indicates the feature to be controlled, represented
symbolically (example: concentricity).
• Centre box: indicates distance between the two cylinders, centres cannot be
apart by more than 0.01 mm (Tolerance).
• Third box: Indicates that the datum is with X.

Symbolic representation of geometric tolerances

Interchangeability
• Manufacturing a large number of parts, it is not economical to produce both the
mating parts by the same operator.
• Parts to be manufactured within min. possible time without compromising on quality.
• To manufacture identical parts; mass production was the idea.
• The components are manufactured in one or more batches by different persons on
different machines at different locations and are assembled at one place.
• Manufacture of parts under such conditions is called interchangeable manufacture.

Interchangeability
Interchangeability: Any one component selected at random should assemble
with any other arbitrarily chosen mating component.
Condition
Identical components, manufactured by different operators, using different
machine tools and under different environmental conditions, can be assembled
and replaced without any further modification during the assembly, without
affecting the functioning of the component when assembled.

Selective Assembly
• In selective assembly the parts produced are classified into groups according to
their size / dimensions by automatic gauging.
• Both the mating parts are segregated according to their sizes, and matched with
the groups of mating parts are assembled.
• This ensures protection and elimination of defective assemblies.
• Assay. costs are reduced, as the parts are produced with wider tolerances.

MAXIMUM AND MINIMUM METAL CONDITIONS
Consider a shaft having a dimension of 40 ± 0.05 mm and Hole having a dimension of 45 ± 0.05 mm.
For Shaft
Maximum metal limit (MML) = 40.05 mm
Least metal limit (LML) = 39.95 mm
For Hole
Maximum metal limit (MML) = 44.95 mm
Least metal limit (LML) = 45.05 mm

FITS
• The degree of tightness and or looseness between the two mating parts.
Three basic types of fits can be identified, depending on the actual limits of the
hole or shaft.
1. Clearance fit
2. Interference fit
3. Transition fit

FITS
1. Clearance fit
The largest permissible dia. of the shaft is smaller than the dia. of the smallest hole.
E.g.: Shaft rotating in a bush
Upper limit of shaft is less than the lower limit of the hole.

FITS
2. Interference fit
• No gap between the faces and intersecting of material will occur.
• Shaft need additional force to fit into the hole.
Upper limit of the hole is less than the lower limit of shaft.

3. Transition fit
Dia. of the largest permissible hole is greater than the dia. of the smallest shaft.
• Neither loose nor tight like clearance fit and interference fit.
• Tolerance zones of the shaft and the hole will be overlapped between the interference and
clearance fits.

FITS
Detailed classification of Fits

• Tolerance grades indicates the degree of accuracy of manufacture.
• IS: 18 grades of fundamental tolerances are available.
• Designated by the letters IT followed by a number.
• The ISO system provides tolerance grades from IT01, IT0, and IT1 to IT16.
• Tolerance values corresponding to grades IT5 – IT16 are determined using the
standard tolerance unit (i, in μm), which is a function of basic size.
Tolerance Grade

Tolerance Grade
• D = diameter of the part in mm.
• 0.001D = Linear factor counteracts the effect of measuring inaccuracies.
• Value of tolerance unit ‘i ’ is obtained for sizes up to 500 mm.
• D is the geometric mean of the lower and upper diameters.
• D=

Tolerance Grade
Standard tolerance units

Tolerances grades for applications

General Terminology
• Basic size: Exact theoretical size arrived at by design. Also called as nominal size.
• Actual size: Size of a part as found by measurement
• Zero Line: Straight line corresponding to the basic size. Deviations are measured
from this line.
• Limits of size: Maximum and minimum permissible sizes for a specific dimension.
• Tolerance: Difference between the maximum and minimum limits of size.
• Allowance: LLH –HLS

General Terminology
• Deviation: Algebraic difference between a size and its corresponding basic size.
It may be positive, negative, or zero.
• Upper deviation: Algebraic difference between the maximum limit of size and its
corresponding basic size.
Designated as ‘ES’ for a hole and as ‘es’ for a shaft.
• Lower deviation: Algebraic difference between the minimum limit of size and its
Designated as ‘EI’ for a hole and as ‘ei’ for a shaft.

General Terminology
• Actual deviation: Algebraic difference between the actual size and its
• Tolerance Zone: Zone between the maximum and minimum limit size.

Hole Basis and Shaft Basis Systems
• To obtain the desired class of fits, either the size of the hole or the size of the
shaft must vary.
Two types of systems are used to represent three basic types of fits, clearance,
interference, and transition fits.
(a) Hole basis system
(b) Shaft basis system.

Hole Basis systems
• The size of the hole is kept constant and the shaft size is varied to give
various types of fits.
• Lower deviation of the hole is zero, i.e. the lower limit of the hole is same as
the basic size.
• Two limits of the shaft and the higher dimension of the hole are varied to
obtain the desired type of fit.

Hole Basis systems
(a) Clearance fit (b) Transition fit (c) Interference fit

Hole Basis systems
This system is widely adopted in industries, easier to manufacture shafts of
varying sizes to the required tolerances.
Standard-size plug gauges are used to check hole sizes accurately.

Shaft Basis systems
• The size of the shaft is kept constant and the hole size is varied to obtain
various types of fits.
• Fundamental deviation or the upper deviation of the shaft is zero.
• System is not preferred in industries, as it requires more number of standard-
size tools, like reamers, broaches, and gauges, increases manufacturing and
inspection costs.

Shaft Basis systems
(a) Clearance fit (b) Transition fit (c) Interference fit

Numerical Examples
1. In a limit system, the following limits are specified for a hole and shaft assembly:

2. The following limits are specified in a limit system, to give a clearance fit between
a hole and a shaft:
Numerical Examples

3. Tolerances for a hole and shaft assembly having a nominal size of 50 mm are as
follows:
Numerical Examples

Solution
(e) Since both maximum and minimum clearances are positive, it can be conclude
that the given pair has a clearance fit.

Tolerance symbols
Example: Consider the designation 40 H7/d9
• Basic size of the shaft and hole = 40 mm.
• Nature of fit for the hole basis system is designated by H
• Fundamental deviation of the hole is zero.
• Tolerance grade: IT7.
• The shaft has a d-type fit, the fundamental deviation has a negative value.
• IT9 tolerance grade.
Used to specify the tolerance and fits for mating components.

Tolerance symbols
• First eight designations from A (a) to H (h) for holes (shafts) are used for
clearance fit
• Designations, JS (js) to ZC (zc) for holes (shafts), are used for interference or
transition fits

Tolerance symbols
• Fundamental Deviation: Deviation either the upper or lower deviation, nearest to the
zero line. (provides the position of the tolerance zone).
It may be positive, negative, or zero.
• Upper deviation: Designated as ‘ES’ for a Hole and as ‘es’ for a shaft.
• Lower deviation: Designated as ‘EI’ for a Hole and as ‘ei’ for a shaft.

Typical representation of different types of fundamental deviations
(a) Holes (internal features) (b) Shafts (external features)
• Upper deviation: Designated as ‘ES’ for a Hole and as ‘es’ for a shaft.
• Lower deviation: Designated as ‘EI’ for a Hole and as ‘ei’ for a shaft.

Fundamental deviation for
shafts and holes of sizes
from above 500 to 3150 mm

• BIS: 18 grades of fundamental tolerances are available.
• Designated by the letters IT followed by a number.
• ISO/BIS: IT01, IT0, and IT1 to IT16.
• Tolerance values corresponding to grades IT5 – IT16 are determined using the
standard tolerance unit (i, in μm)
Tolerance Grade

Tolerance Grade
• D = diameter of the part in mm.
• 0.001D = Linear factor counteracts the effect of measuring inaccuracies.
• Value of tolerance unit ‘i ’ is obtained for sizes up to 500 mm.
• D is the geometric mean of the lower and upper diameters.
• D=
Tolerance unit,

The various steps specified for the diameter steps are as follows:
• 1–3, 3–6, 6–10, 10–18, 18–30, 30–50, 50–80, 80–120
• 120–180, 180–250, 250–315, 315–400, 400–500
• 500–630, 630–800, and 800–1000 mm.
Tolerance Grade
D=

1. Calculate the limits of tolerance and allowance for a 25 mm shaft and hole pair
designated by H8d9.
Numerical Examples

Solution
25 mm diameter lies in the standard diameter step of 18-30 mm
D= 18 × 30 =23.238
Fundamental tolerance unit = i = 0.45
3
𝐷 + 0.001D
i= 1.307 μ
Fundamental tolerance (tolerance grade table) = 25i = 32.5~33μ =0.033 mm
For ‘H’ Hole, fundamental deviation is 0 (from FD Table)
Hence, hole limits are 25 mm and 25+0.033 = 25.033 mm
Hole Tolerance = 25.033 – 25 = 0.033 mm
For H8 hole

Solution
D= 18 × 30 =23.238
Fundamental tolerance unit = i = 0.45
3
𝐷 + 0.001D
i= 1.307 μ
Fundamental tolerance, (tolerance grade table) = 40i = 40 x 1.3 =52 μ = 0.052 mm
For ‘d’ shaft, fundamental deviation (from FD Table) =−16𝐷0⋅44
= - 0.064 mm
Hence, shaft limits are 25 – 0.064 =24.936 mm and 25 – (0.064+0.052) =24.884 mm
Shaft Tolerance = 24.936 – 24.884 = 0.052 mm
For d9 shaft

2. Determine the tolerance on the hole and the shaft for a precision running fit
designated by 50 H7g6.
Given: 1) 50 mm lies between 30- 50 mm
2) i (microns) = 0.45𝐷1/3 + 0.001D
3) Fundamental deviation for ‘H’ hole = 0
4) Fundamental deviation for ‘g’ shaft = −2.5𝐷0⋅34
5) IT7 = 16i and IT6 = 10i
State the actual maximum and minimum sizes of the hold and shaft and maximum
and minimum clearance.
Numerical Examples

Solution
D= 30 × 50 = 38.7 mm
Fundamental tolerance unit = i (microns) = 0.45𝐷1/3 + 0.001D = 1.5597 μ
Fundamental tolerance (tolerance grade table) = 16i = 24.9μ = 0.025 mm
For ‘H’ Hole, fundamental deviation is 0 (from FD Table)
Hence, hole limits are. +0 ⋅ 025
50−0.000 mm
For H7 hole

Solution
D= 30 × 50 = 38.7 mm
Fundamental tolerance unit = i (microns) = 0.45𝐷1/3 + 0.001D = 1.5597 μ
Fundamental tolerance (tolerance grade table) = 10i = 16μ = 0.016 mm
For ‘g’ shaft, fundamental deviation is −2.5𝐷0⋅34 = 9 μ
Hence, shaft limits are.
−0 ⋅ 009
50−0.025 mm
Hole Tolerance = 25.033 – 25 = 0.033 mm
For g6 shaft

Solution
Maximum clearance = 50.025 - 49.975 = 0.05 mm
Minimum clearance = 50.000 - 49.991 = 0.009

Limits, fits and tolerances

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