WONC UM PING UNIVERSITI MALAYSIA SARAWAK

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RELATIONS I if[' BETWEEN TORQUE AND APPLY D LOAD IN A BOLT FOR

DIFFERENT COEFFICIENT OF FRICTIONS

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RELATIONSHIP BETWEEN TORQUE AND APPLIED LOAD IN A BOLT FOR DIFFERENT COEFFICIENT OF FRICTIONS

WONG LIM PING

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FACULTY OF ENGINEERING

UNIVERSITY MALAYSIA SARAWAK

MAC 2002

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Borang Penyerahan Tesis Universiti Malaysia Sarawak

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BORANG PENYERAHAN TESIS

Judul: Relationship between Torque and Applied Load in a bolt for different Coefficient of Friction.

SESI PENGAJIAN: 1999 - 2002

Saya WONG LIM PING

(HURUF BESAR)

mengaku membenarkan tesis ini disimpan di Pusat Khidmat Makiumat Akademik, Univetsiti Malaysia Sarawak dengan syarat-syarat kegunaan seperti berikut:

1. Hakmilik kertas projek adalah di bawah nama penulis melainkan penulisan sebagai projek basama dan dibiayai oleh UNIMAS, hakmiliknya adalah kepunyaan UNIMAS.

2. Naskhah salinan di dalam bentuk kertas atau mikro hanya boleb dibuat dengan kebenaran bertulis daripada penulis.

3. Pusat Khidmat Maklumat Akademik, UNIMAS dibenarkan membuat salinan untuk pengajian ma+eka.

4. Kertas projek hanya boleh diterbitkan dengan kebenaran penulis. Bayaran royalti adalah mengikut kadar yang dipersetujui kelak.

5. " Saya fmmbomAWtidak membenarkan Papustakaan membuat saiinan katas projek ini sebagai baban pertukaran di antara institusi pengajian tinggi.

6. "" Sila tandakan (q)

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SULZT (Mengandungi maklumat yang berdarjah keselamatan stau ke)entiugan Malaysia seperti yang termaktub di dalam AKTA RAHSIA RASMI 1972).

TERHAD (Mengandungi maklumat TERHAD yang telah diteawkan oleh aganisas baden di mane penyelidikan dijalankan).

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TIDAK TERHAD

(TANDATANGAN PENULIS) Alamat tetap: No 1, Lorong 3,

Disahkan Olcýi

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(TANDATAN )

Jalan Bangunan Kerajaan DR. HA HOW UNG

96100 Sarikei, Sarawak, Mala sýia

Tarikh: 30 HB MAC, 2002

CATATAN ^*

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( NamaýP'enyelia )

Tarikh: 3 [io 11-

Potong yang tidak berkenasn.

Jika Kertas Projek ini SULIT stan TERHAD, ills buopirkan Surat daripada püak

berkuasa/ oreanisasi berkesaan denpaa menyertakaa sekaii tempok kertss pevjek. Id perlu dikelaskan sebagal SULIT atau TERHAD.

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This project report entitled "Relationship between Torque and Applied Load in a bolt for different Coefficient of Friction" was prepared by Wong Lim Ping as a partial fulfillment for the Bachelor of Engineering (Hons. ) Mechanical and Manufacturing System Engineering degree programme is hereby read and approved by :

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Dr. Ha How Ung

(Project Supervisor)

Date : ;I Lflo -I-

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Relationship between Torque and Applied Load in a bolt for different Coefficient of Frictions

by

Wong Lim Ping

This report is submitted in partial fulfillment of the requirement for the degree of

Bachelor of Engineering (Hons. ) Mechanical and Manufacturing System Engineering from the

Faculty of Engineering

University Malaysia Sarawak Mac 2002

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Dedicated to my beloved family and friends that always support me

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ACKNOWLEDGEMENTS

The researcher would like to express his sincere gratitude and appreciation to his Project Supervisor, Dr. Ha How Ung for their inspiring guidance, encouragement and thoughtful tips throughout the duration of the project.

In addition, the researcher would like to specifically thank his family and friends for giving their support, help and encouragement during the drift and difficult encounters while doing his laboratory research and report writing. The research would also like to thank the Laboratory Assistants of Faculty of Engineering for their assistance in carrying out laboratory works and testing during the project duration.

Last but not least, not forgetting everyone who involved in one way or another to make this project a success, I would like to thank them very much for their help.

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ABSTRACT

This thesis introduces and examines the relationship between the torque and the applied load within a bolt and how the different coefficient of friction within a bolt can affect them. The aim of the project was to provide information from the experiment done that may enable engineers to understand and specified a surface finish for a specific bolts thread condition. The reason for this was to enable the bolts to work under the best condition and failure will not occur. The experimentation was done on a standard equipment using standard steel bolts. The coefficient of friction at the thread of the bolts was artificially altered using different lubricants, particulate pastes, degreasing agent and also using different materials of nuts. By altering the applied load at the nut, a change in the torque can be measure and record. The experiment resulted in a set of graph that clearly shown the relationship between the torque and applied load as a result of different coefficient of friction. The relationship obtained from this experiment can then be used as a reference guide for monitoring the performance of bolts as this has been widely overlook in industry.

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ABSTRAK

Tesis ini memperkenalkan dan memeriksa hubungan di antara tork dengan beban yang dikenakan dalam sebuah skru dan bagaimana perbezaan dalam pekali geseran dalam skru boleh mempengaruhinya. Tujuan projek ini adalah untuk memberikan maklumat daripada eksperimen yang dijalankan yang akan membolehkan jurutera- jurutera memahami dan memperincikankan sesuatu jehis permukaan untuk sesuatu

keadaan kekisi skru. Alasan untuk ini adalah untuk membolehkan skru berkenaan bekerja dalam keadaan yang paling baik dan kegagalan tidak akan berlaku.

Eksperimen ini telah dilakukan di atas peralatan yang standard menggunakan skru besi piawai. Pekali geseran pada kekisi skru telah diubah menggunakan pelbagai pelincir, agen pelincir dan juga menggunakan pelbagai jenis nut. Dengan mengubah

beban yang dikenakan pada nut, perubahan dalam tork boleh diukur dan direkodkan.

Eksperimen ini memberi keputusan dalam satu set graf yang dengan jelas menunjukkan hubungan di antara tork dengan beban yang dikenakan hasil daripada kepelbagaian dalam pekali geseran. Hubungan yang didapati daripada eksperimen ini boleh digunakan sebagai rujukan untuk memerhatikan keboleharapan skru kerana ini telah secara jelas diabaikan dalam industri.

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Pusat Khidmat Makrumat Akademi UNIVERSITI MALAYSIA SAR: iWAK

9434() Kota Saniarahan

Nut-Bolt Thread Assembly' by Mr. D. G. M. Savidge 17

CHAPTER 3: METHODOLOGY 3.1 Theory of Friction

3.1.1 Introduction

3.1.2 The Law of Friction 3.2 Theory of Wear

3.2.1 Introduction

3.2.2 Type of Wear

3.2.2.1 Adhesive Wear 3.2.2.2 Abrasive Wear 3.2.2.3 Corrosive Wear

3.2.2.4 Surface Fatigue Wear 3.3 Lubrication

3.3.1 Introduction

3.3.2 Fluid Lubrication

18 18 18 19 23 23 23 24 24 25 26 26 26 26

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3.3.3 Boundary Lubrication 3.3.4 Usefulness

3.4 Theoretical Considerations

3.4.1 Square Threaded Nut and Bolt 3.4.2 Standard Nut and Bolt

3.4.2.1 Theory Behind the Greatest Gradient 3.5 Strain Gauge Calculation

3.6 Specification

27 28 28 28 29 31 34 38

3.6.1 Measuring the Stresses in the Bolt 38

3.6.2 Measuring Friction in a Threaded Form 38

3.7 Testing Equipment 39

3.7.1 Introduction 39

3.7.2 ER 420 Pre-Tensioned Bolt 40

3.7.2.1 Description of ER 420 Pre-Tensioned Bolt 41 3.7.2.2 Operation of the ER 420 Pre-Tensioned Bolt 44

3.7.3 El 616 Strain Gauge Bridge 46

3.7.3.1 Initial Balancing of Measurement Channels 46 3.7.3.2 Strain Measurement on Each Channel 48

CHAPTER 4: RESULTS AND DISCUSSION 51

4.1 Introduction

51

4.2 Testing the ER 420 Pre-Tensioned Bolt 51

4.2.1 Testing for Displacement 52

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4.2.1.1 Result

4.2.2 Testing the Strain of the El 616 Strain Bridge 4.2.2.1 Result

4.3 Experimentation of the Bolt Without Lubricant 4.4 Experimentation of the Bolt With Lubricant

4.5 Discussion

53 59 62 66 70 74

CHAPTER 5: CONCLUSION 81

5.1 Conclusion

5.1.1 Relationship Between the Torque and Stress 5.1.2 The Effect of Coefficient of Friction

5.1.3 The Effect of Unspecific Surface Condition or Method of Lubrications

5.1.4 The Effect of Loading at Torque Other Than the Design Load

5.2 Recommendation

5.2.1 The Effect of Different Material of the Nuts 5.2.2 The Effect of Post Tensioning

5.2.3 The Effect of Bolt Tolerances on the Bolt Tension 5.2.4 The Effect of Thread Form on the Bolt Tension

81 81 81

81

82 82 83 83 84 84

BIBLIOGRAPHY 85

APPENDIX 88

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LIST OF TABLES

TABLES Page

4.1 Result of the Configuration For the Tightening by Hydraulic 53 Tensioner

4.2 Result of Strain Measurement of El 616 Strain Bridge at Pressure

from 10 MPa to 90 MPa 62

4.3 Result of the Sum of All the Strain at Entire Channel at Pressure

from 10 MPa to 90 Mpa 64

4.4 The Voltage Value of the Various Channel at Torque Range from

50 Nm to 100 Nm for Bolt Without Lubricant 67 4.5 The Sum of Entire Channel at Torque Range from 50 Nm to 100

Nm for Bolt Without Lubricant 69

4.6 The Voltage Value of the Various Channel at Torque Range from

50 Nm to 100 Nm for Bolt With Lubricant 71

4.7 The Sum of Entire Channel at Torque Range from 50 Nm to 100

Nm for Bolt With Lubricant 72

4.8 The Coefficient of Friction of Bolts at Different Lubricants 76 4.9 The Torque Against the Stresses of the Bolts Without Lubrication

and With Lubrication 77

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LIST OF FIGURES

FIGURES Page

2.1 Variation of Peak Stresses with the Number of Threads in a

Conventional Design 7

2.2 Example of Preload-Torque Curve 8

2.3 Stresses in Bolt Under Load 10

3.1 Schematic View of a Load on a Horizontal Surface. A Tangential

Force P is Applied 20

3.2 Equilibrium Diagram For an Object on an Inclined Plane. Slippage

Down the Plane is Imminent 22

3.3 Basic Square Threaded Nut and Bolt 29

3.4 Standard Bolt with a 30° Flank Angle 30

3.5 Schematic Point Diagram of the Various Angle of Screw Thread 32

3.6 A Wheatstone Bridge Circuit 34

3.7 Testing Equipment For Stress/Load Measurement 39

3.8 The ER 420 Pre-Tensioned Bolt Equipment 41

3.9 The El 616 Extensometer Bridge 43

3.10 Diagram of the Arrangement For Tightening with a Torque Wrench 44

3.11 Front Panel of El 616 Strain Bridge 46

3.12 Full Bridge and Half-Bridge Set-Up 47

4.1 Configuration of the Bolt and Nut 51

4.2 Configuration For Tightening by Hydraulic Tensioner 52

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4.3 Graph of Configuration For Tightening by Hydraulic Tensioner For

Pressure from 60 MPa to 80 MPa 54

4.4 The Various Force and Compression Force Acting on the Bolts in

the Experiment Setup 56

(a) Theoretical Experimental Setup, (b) Actual Experimental Setup

4.5 Graph of Configuration For Tightening by Hydraulic Tensioner for 57 Pressure from 65 MPa to 90 MPa

4.6 The Full-Bridge Set-Up 60

4.7 Voltage For Channel 1, Channel 2, and Channel 3 at Pressure from

10 MPa to 90 MPa 63

4.8 Voltage of the Sum of All Channel at Pressure from 60 MPa to 80 64 MPa

4.9 Voltage of the Sum of All Channel at Pressure from 65 MPa to 90 65 MPa

4.10 Diagram of the Experimental Arrangement For Tightening with a

Torque Wrench 67

4.11 The Graph of Voltage of Each Channel at Torque Range from 50

Nm to 100 Nm for Bolt Without Lubricant 68

4.12 Voltage of the Sum of All Channel at Torque from 60 Nm to 90

Nm For Bolt Without Lubricant 69

4.13 The Graph of Voltage of Each Channel at Torque Range from 50

Nm to 100 Nm 72

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4.14 Voltage of the Sum of All Channel at Torque from 60 Nm to 90 Nm For Bolt With Lubricant

4.15 Graph of the Sum of Voltage For Bolt Without Lubricant and With Lubricant

4.16 The Torque Against the Stresses of the Bolts Without Lubrication and With Lubrication

4.17 The Torque Against the Stresses of the Bolts Without Lubrication Compare to a Rusty Bolt

73

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CHAPTER 1

INTRODUCTION

1.1 Background

Bolts are very important engineering members because of their principal use in clamping parts together with enough force to seal pressure within a vessel or to prevent motion between parts. In engineering practice, it is common to indicate the torque to which a bolt should be tightens. However, when come to the thread's condition or state of lubricants, peoples always tend to neglect these situations. Little attention has been paid to the mechanics or the practical implication of the thread conditions when tighten a bolt.

It should be clear and apparent that the coefficient of friction induce by lubricants will affect the contacting surface between the nut and the thread and this will have major influent on the bolt tension under particular level of torque. It is a frequent and normal occurrence to applied any handy lubricant such as a smear of grease or a squirt of oil to the thread before assembly, and the effects of these lubricant on the bolt tension have always been neglected.

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The bolts should not be always assumed in tension condition when subjected to load. Indeed, they are intended to do so, and the resulting stresses are tensile, but, the bolt may also subjected to other loads in addition, thus creating a combined and often complex stress situation. Structural bolted joints are frequently loaded in shear, and the shear stresses produced are

added to the tensile stresses due to the axial loading.

It is usually assumed that the stress decreases uniformly through the thickness of the head and nut. The increased stress at the thread run-out (start of the thread) derives from the reduced cross section at the thread root. The height of standard nuts is close to the nominal bolt diameter. The height of nuts, or the engagement length of tap bolts, is determined on the basis of shear strength and shear area (area at the thread root). However, the load is not distributed uniformly. The first thread carries the largest part of the load, and the first three carry the most of it.

Some cases have been record involves the failure of bolts. For examples, in the wheel studs of vehicles, especially to heavy good vehicles. Analysis shown that most of them have failed from fatigue rather than overtightening.

Some of these cases also found to have liberally use of lubricants that would reduce the thread friction and hence lead to bolts failure as the bolt load increases. Therefore, the results of this project, could hopefully be of additional usefulness to the automobile industry.

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1.2 Project Objectives

1.2.1 General

It is generally not a good practice to use unlubricated bolts or nuts as these surfaces tend to gall, especially with repeated use, with the result that it is impossible to predict preload in any meaningful way. Also the galled surfaces tend to become seized together resulting in a joint that cannot be disassembled. Furthermore the damage to the thread surfaces can initiate surface cracks which dramatically decrease the fatigue and fracture life of the assembly and can result in failure under even relatively low loads.

It is, therefore the purpose of this project to investigate the effect of lubricants on the condition and surface finish of the thread in a nut-bolt.

The torque and applied load will also be investigated with relation to these.

1.2.2 Specific

For this project, it is hope that, experimental results are comparable to the theoretical data realized through the basic concept of solid mechanics and mechanical joints design.

Information relating to surface finish and lubricants of the thread form with reference to the torque and induced load within the bolts will be

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illustrated on a comprehensive set of graphs. Information from this experimentation will in return, help the engineers and designers to specify the minimum and maximum applied torque, type and amounts of lubricants that should be use when assembling bolted structure.

Theoretical data stated that the lower the coefficient of friction, the higher the preload values and thus the lower the fatigue life would be. This means that less of the applied torque goes into overcoming friction, and more of it goes into preload. Thus, this experiment will hope to show the effect of lubricants in the practical fatigue environments.

1.3 Definition of Terms

1.3.1 Definition of Torque

Basically, there are two kind of torque namely static torque and dynamic torque. The differences in these two kind of torque is that static torque value tends to be higher than the dynamic torque value because of the static friction higher than the dynamic friction. Static torque are needed to start the tightening process and after the nut or bolt move, dynamic torque will take place.

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1.3.2 Definition of Bolt

A bolt is a fastener which has a thread that usually does not extend all the way to the head. Subcategories of bolts include cap screws and studs. In the majority of cases, the bolts are passes through a hole in the parts being clamped and mates with a nuts.

1.3.3 Definition of Coefficient of Friction

Coefficient of friction is the ratio of the friction force to the normal force.

It has a dimensionless value. The value of coefficient of friction varies for different type of threaded connections, material been used and whether lubricant has been used.

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CHAPTER 2

LITERATURE REVIEW

2.1 Past Research

A particular look at several books, other thesis and a browse through Internet review several research has been undertaken in the area of strength of the fasteners in shear, the effect of bearing stresses on the integrity of a mechanical joint and the threaded fastener.

2.1.1 'Design of Mechanical Joints' by Alexander Blake

In his book entitled `Design of Mechanical Joints", Alexander Blake examined and stated some fundamental relationships for calculating the average shear stress and ultimate strength of a bolt. Basically, at a given point of mechanical system, there are only two kinds of stress; normal and shear. Normal stress act perpendicular to the plane while shear stress acts along the plane. The weakest section of any bolt intension is the threaded portion.

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Blake also acknowledge that the peak stresses in nut or bolt threads fall- off in a nonlinear fashion with the number of threads as indicated in Figure 2.1. The diagram shows a relative stress level which is due to the two main factors. One is the nonlinear distribution of loading between the consecutive threads and the other is concerned with the stress concentration at the bottom of the threads, where the radius of curvature is extremely small.

Figure 2.1 : Variation of peak stresses with the number of threads in a conventional design. (Taken from `Design of

Mechanical Joints', Alexander Blake. )

It appears that the first two or three threads carry the lion share of the bolt load, so that adding more threads and making longer nuts is not going to solve the problem of peak stresses and the nonlinear response.

He also noted that the thread formed also affected the stress factor. Most of the threads are formed by cutting. When the threads are made by rolling, not followed by a heat-treatment, the stress factors can be lowered

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