Module 2 - Tolerance and Allowances PDF

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MECHANICAL ENGINEERING PROGRAM

MACHINE DESIGN - I ME063C

LEARNING MODULE 2

( Engineering Tolerance and Allowances )

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1 Semester, AY 2020-2021 Copyright 2020

ME063C – Machine Deign-I Engr. Katarungan B. de Pano MBA, PME Page 1 of 16

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Course Description:

The course deals with various mechanical properties of

engineering materials in lieu of the determination of design factor and design stresses. It includes the analyses of simple, variable and combined stresses applied to different mechanical elements such as shafts, mechanical springs. A. Title

:

Engineering Tolerance and Allowances

B. Over view

:

Introduction of the Learning Module 2

The Learning Module-2, comprises the analysis of determining the specified design required to allocate the tolerance if ever the machine reached its specified designed requirements. The Learning Module-2 , also discusses the significant difference of Allowances to Tolerances and how it differs upon the application of machine design calculations. C. Objectives

: After successfully completing the module, the students are expected to:

1.

Distinguish the difference between the technical term Tolerance to Allowance.

2.

Evaluate the working principles based on the designed parameters of the machine.

3.

Apply the design formulas learned to solve the unknown tolerance and allowance in the given sample design problems.

ME063C – Machine Deign-I Engr. Katarungan B. de Pano MBA, PME Page 2 of 16

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D. Discussion: MACHINE DESIGN - is the application of: mathematics, kinematics, statics, dynamics, mechanics of materials, engineering materials, mechanical technology of metals and engineering drawing. It also involves application of other subjects like thermodynamics, electrical theory, hydraulics, engines, turbines, pumps etc. - is defined as the use of imagination, scientific principles and engineering techniques to create a machine or structure economically, in order to satisfy the requirements of a customer. Machine design is the first step involved in creation of a machine. What is Engineering Tolerance in the perspective of Machine Design?

Engineering tolerance is the permissible limit or limits of variation in: a physical dimension; a measured value or physical property of a material, manufactured object, system, or service; ... in mechanical engineering the space between a bolt and a nut or a hole, etc. An engineering tolerance is the acceptable difference between the maximum and minimum size of a mechanical part as a basis for determining the accuracy of its fit with another part

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For Example: A dimension gives as Ø 28±0.2 means that the manufactured part may be Ø 28.2 or Ø 27.8, or anywhere between these limit dimensions.

Terminology Nominal Dimension - The dimension that the tolerance are applied to.

Upper Limit - The maximum allowable size of the part based on the tolerance given Lower Limit - The minimum allowable size of the part based on the tolerance given. Interval of Tolerance - The upper limit minus lower limit, also known as range of tolerance. Functional dimensions- Have to be machined and fit with other mating components

Clearance - mean size of hole minus mean size of shaft

WHY Tolerance No one cannot manufacture a part to its accurate dimension. If you leave a dimension without a tolerance, no one else will know the importance or the unimportant of that dimension. Not only a lack of tolerance lead to improper fits, it will also add to delay and higher costs. Why tolerances are given to the parts- Because it’s impossible to make perfect settings. ME063C – Machine Deign-I Engr. Katarungan B. de Pano MBA, PME Page 4 of 16

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Which type of tolerance provided in drilling mostly - Unilateral because in drilling as dimension is allowed to vary in one direction only. Hole is always oversize, not undersize. Quality control charts doesn’t depend on which factor- Binomial distribution, In establishing of realistic specifications, control charts are considered as a good guide for the design office. Quality control charts are based on the assumptions of random sampling and normal distribution.

Comparison of tolerance with cost

How Effective tolerance helps  The part functions correctly.  Fabrication cost is minimum. Types of Tolerance ME063C – Machine Deign-I Engr. Katarungan B. de Pano MBA, PME Page 5 of 16

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Special tolerance General workshop tolerance

Special Tolerance  Limit Allowance  Unilateral Tolerance  Bilateral Tolerance

How Tolerance differs to Allowance in the perspective of Machine Design? TOLERANCE

M E

 No parts are identical  Variation in size is inevitable  Interchangeability  Specified by two extreme allowable size called limits.  Difference between the maximum and minimum limit size.

ALLOWANCE

ME

 Provided intentionally  Gives the required fit( to meet out functional requirements)  Is a planned deviation between actual dimension and a nominal or theoretical dimension intended to final dimension. Page 6 of 16

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What are the Tolerance Type? Dimensional Tolerance - control dimensions Geometric Tolerance - control dimension and geometry Bilateral tolerance - total tolerance is in both the directions Fundamental

deviation

Calculations: The

fundamental

deviation

is

an allowance rather than tolerance. The allowance is simply the deviation between the hole minimum diameter to the shaft maximum diameter. Where the tolerance refer to either hole (Max. dia of hole-Min. Dia. of the hole) or shaft (Max. Dia. of the shaft- Min. Dia. of the Shaft). There are 25 fundamental deviations are there to fit the shaft in a hole with these 25 allowable deviations. See the following schematic representation of the Fundamental deviation of Holes and shafts (Fundamental Deviation Chart)

Example: A Shaft designation is given as 40H8/f7 H8 = Hole deviation with the international tolerance grade 8 (IT8) f7 = Shaft deviation with the international tolerance grade 7 (IT7)

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But how we can figure out the t tolerances for the given shaft diamete er? Here we go! International Tolerance Grades G We have 18 International Tolerance(IT) grades. These grades deterrmine the tolerances for the given designated shaft value. IT01, IT0, IT1, IT2, IT3, IT4, IT5, IT6, IT7, IT8, IT9, IT10, IT11, IT12, IT13, IT14, IT15, IT16. These are the standard tole erance grades. These tolerance values will be calculated with the help of the relevant mag gnitude given below.

Where

D = Geometric mean diame eter in mm The following list will demon nstrate where these grades are applied according a to their tolerance range in a particu ular application. IT01 to IT4 – For the production of gauges, plug gauges, measuring g instruments IT5 to IT 7 – For fits in prec cision engineering applications IT8 to IT11 – For General Engineering E IT12 to IT14 – For Sheet metal m working or press working IT15 to IT16 – For processes like casting, general cutting work Fundamental tolerances of grades according to the IS: 919 – 1993 ME063C – Machine Deign-I Engr. Katarungan B. de Pano MBA, PME Page 8 of 16

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Fundamental tolerances of grades according to the IS: 919 – 1993

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E.

EVALUATION

Sample Problems

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F.

REFERENCES

1.

Capote & Mandawe (2016). "Solved Problems in MACHINE DESIGN". Published by JAM Publisher. Distributed by PRIME Review Center Junquera Sts, Cebu City.

2.

Faires, Virgil Moring. " Design of Machine Elements" 4th Ed. Collier-McMillan International Editions. EDCA Publishing and Distributing Corporations.

3.

Boldea, Ion and Nasar, Syed A. " The Induction Machines Design Handbook". 2nd Ed. 2010 by Taylor and Francis Group , LLC. Boca Raton London New York

4.

https://basicstomechanicalengineering.blogspot.com/2018/09/tolerance-inmechanical-design.html

5.

https://www.slideshare.net/umairbukhari3/tolerance-and-allowance-2

4.

https://extrudesign.com/fundamental-deviations-for-shaft-and-holes/

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