Chapter 5 Bolted Connection-1

8/10/2019 Chapter 5 Bolted Connection-1

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Chapter 5 Bolted Connection

5.1 Screw Pair and Mechanics Characteristics5.2 Materials of Bolt and Screw Fastener

5.3 Clamping and Locking of Bolted Connection

5.4 Strength Design of Bolted Connection

5.5 Measures to Improve Strength of Bolted Connection

5.6 Bolts Group Design

5.7 Power ScrewNut

Washer

Mating

components

Bolt


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Static

connection

Static

DynamicPair (Low pair, High pair)

With

Power

source

Elements Components Mechanism MachineDynamic

(Pair)

Dismountable: Bolts, Keys, Pin, et al

Non-dismountable: Rivets, Welding, Adhesion, et al

Types of Connection

Introduction

Bolted connection(Static connectionComponent)

Power screw(DynamicMechanism)

Screw Pair
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Fasteners

A fasteneris any device used to connect or join two or more

components.

The most common are threaded fasteners referred to by manynames, such as bolts, screws, nuts, studs, lag screws, and set

screws.


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Bolts and Screws

A boltis a threaded fastener designed to pass through holes in

the mating members and to be secured by tightening a nut

from the end opposite to the head of the bolt. Typically, Hexhead bolt.

A screwis a threaded fasteners designed to be insertedthrough a hole in one member to be jointed and into a

threaded hole in the mating member. Typically, Hex head capscrew.

Socket headFlat headRound headFillister head

Bolt Screw


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5.1 Screw Pair and Mechanics Characteristic

Helical thread: A point moves on a cylindered surface, not only

rotating around the axis with constant speed, but also translating

along the axis with constant speed.

d2

Screw

Axial

profile

1. Generation of thread screw


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(1) By the direction of thread

Righthanded screw and Lefthanded screw.

Right handed screw is commonly used. Left handed screwis only used in special situations.

Right Left

Examples

2. Classification of thread screw

Bottom Bracket


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(2) By the axial profile

30

Acme thread

3

30Buttress thread

Square thread

60

Triangular

=60

=30

=30

=15

1=3

2=30

=0

=0

Commonly for static

connection, low efficiency,

self-locking, coarse & fine.

Commonly for power drive,

high efficiency,

difficult to manufacture,

poor concentricity, weak

strength at the tooth root.

Commonly for power drive, high

efficiency, easy to manufacture,

good concentricity, high strength

at the tooth root.

A combination of rectangular

screw and Acme screw,

applicable for unidirectional

drive under great axial force.

-thread angle

- flank angle


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d(Major diameterStandard diameter), such as M8, M10;

d1(Minor diameter) ; d2(Pitch diameter);

p(Pitch); n(No. of threads); s(Lead) ; s=np; (Thread angle);(Flank angle); (Lead angle).

3. Designations of screw

2d

s(np)

tan

np/d2

Internal

Ext.


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Relative movement under axial force

2d

s(np)

Internal

ExternalClamping, Loading,Lifting

Connection

Power drive

Screw pair

F

Fa

Expanding the pitch circle

Slider moves on the inclined plane

4. Mechanic characteristics, efficiency and self-locking


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(1) Rising with constant speed

Fa

F

FnFR

F

FaF

FR+

Angle between FRand FnAngle of friction: tan== F /n

Angle between FRand Fa equals to +

Load Fa(axially, gravity, resistance)

We have the active torque on screw pair

2 2tan( )

2 2

a

d dT F F

F=Fatan( +)

Resultant force FR

Normal supporting force Fn(plane)

Active pushing force F

Friction force: Fn=F

Rectangular screw: (=0)


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A horizontal force Fto resist

the falling process

When F=0

F


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if (equivalent angle of friction)tan=/ cos = (equivalent coefficient of friction)

()

Rising with constant speed: F=Fatan()

Falling with constant speed:F=Fatan(-)

2 2tan( )2 2

a

d dT F F

2 2tan( )2 2

a

d dT F F

Condition of self-locking:

For good self-locking () , (single thread)

Self-locking

Self-locking

Non-rectangular screw


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)(

tg

tg

2d

S

For high

efficiency

max2/45

If too big, difficult to manufacture

25max /\

Equivalent angle of friction, Lead angle

Good self-locking (Single thread) ;

Good efficiency (Multi thread).

, (multi-threads),

Efficiency

When rising by constant speed

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=Effective powerInputting power=FaS/(Fd2)=Fad2 tan/[Fatan(+)d2]


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5.2 Materials of Bolt and Screw Fastener

1. Screw fasteners and materials

There is a variety of screw fasteners. Most of them arestandardized, which are commercially available.

Common screw fasteners include bolts, stud

bolts, cap screw, set screw, and nuts.

(1) BoltsScrew fastener

L

L 0

d

Hex head

L

d

L 0

Small Hex head


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d

L 0L 1

L

L1--Seating lengthL 0--Length for nut

(2) Stud BoltsScrew fastener


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End

type

Captypes

(3) Cap ScrewScrew fastener


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Foundation bolt

lifting eye bolt

T-type bolt

(4) Special ScrewScrew fastener

( )


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Hex nut Flat hex nut Thick hex nut Round nut

(5) NutScrew fastener

There are 16 kinds of nuts in Chinese GB.


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2. Screw fasteners materials

Bolts are made from middle carbon steel and low carbon

steel, such as A2, A3, 10, 35, 45 steel.

For special situations, Bolts are made from alloy steel and

low carbon steel, such as 15Cr, 40Cr and 30CrMnSi steel.

Nuts are made from middle carbon steel.

3. Bolts strength grade

By Chinese GB, there are 10grades of bolts (GB/T 3098.1-

2000).

3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9, with

increasing numbers indicating greater strength.

The numbers before the decimal point are approximately

0.01 times the tensile strength (b min) of the material in MPa.

The last digit with decimal point is the approximate ratio of the

yield strength (s) of the materials to tensile strength (b) .


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4. Engineering graphics of Bolt connection

Bolts connection

Common bolts connection Fit bolts connection

Stud bolts connection

Screw connection

Set screw connection

Types of bolts connection depends mainly on load types,

mechanical structure, requirements on assembling, and et al.

F t Th i t(1) C b lt ti


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Gap between

bolts and holes

d

FeaturesThere exists a gap

between bolt and hole, and the

mating components are not

needed to cut screw, easy to

assemble and disassemble

Applicationscommon

situations requiring frequently

assembling and disassembling.

(1) Common bolts connection

F t Th i t b t b lt(2) Fit b lt ti


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FeaturesThere is not gap between bolt

and hole, and the mating components

are not needed to cut screw, easy to

assemble and disassemble.

Applications: withstanding transverse

loading.

d

(2) Fit bolts connection

Fit between

bolts and holes

C i b t b lt ti d fit b lt ti


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a) Common bolts connection

Mating components can not be too thick,

with holes without screws.

A bolt are inserted into the hole, matingwith a nut.

After the assembly, there is a gap

between bolt and hole.

It is simple structured, and easy toassemble and disassemble. Common bolts

are most widely used.b) Fit bolts connection

After the assembly, there is no gap

between bolt and hole.

The bolt not only endures transverse

load, but also locateds the two components.

Often we use transition fit for fit bolts

connection (H7/m6,H7/n6) by basic holes stem.

d

l1

d0

a

d

l1

d0

a

d

l1

d0

a

d

l1

d0

a

Comparison between common bolts connection and fit bolts connection

(3) S d b l i


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Features: There exists a gap between bolt and hole. One of

mating components has no screws. It is easy to assemble and

disassemble.

Applications: One of components is thick, and the other needs to

be frequently assembled and disassembled.

(3) Stud bolts connection

a

e

l1

H

H1

H2

d


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Features There exists a gap between bolt and hole. One of

mating components has no screws. It is easy to assemble anddisassemble.

Applications: One of components is thick, and the other dose not

need to be frequently assembled and disassembled.

e

l1

H

H1

H2

(4) Cap screw connection

(5) S i


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(5) Set screw connection

It is used to fix the relative position of two mating components,

which is applicable under small force and torque.

5 3 Cl i d L ki f B lt d C ti


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PurposeWithstanding loadreliability, strength, compactnessMagnitude of T0: Total torque

Resistant torque from screw T1

T2= fcFa rfT0T1+T20.2Fad N.mm

1. Clamping torque T0

T0

5.3 Clamping and Locking of Bolted Connection

BoltsTensile load in the bolt

Mating componentsAxial pressing force

Locking force Fs

T1=Fd2/2=Fa tan (+) d2 /2

Resistant torque from frictionon the contact surface T2

T0=T1+T2


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Often we use torque wrench to control Fsroughly.

Measuring torque wrench Constant torque wrench

2 Locking of bolted connection
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2. Locking of bolted connection

Screw pairConnection(Reliable)

Tightening+ Self-lockingAdditional Locking

Self-locking from passive frictionnot reliable,Locking

Key method: Friction type, Mechanical type, and Non removable type

(1) Friction type

Preventing the relative rotation.Between the nut and mating components there always exists a

significant contact force(Axially, Transversely)

A resistant torque is resulted from the friction.

Locking nutSpring washer Double nut

(2) Mechanical type


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An easily changeable metal component is used to constrain

the screw pair.

(2) Mechanical type

Cotter and groove nut

Locking washer Round nut and wing washer

Welding Riveted Adhesion(3) Non removable type


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Welding, Riveted, Adhesion(3) Non removable type

Welding Punching 2~3 spots

Adhesion

Adhesive

Glued joint

Homework 14


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Try to find the leading angle of coarse screw M10 and M30.

Try to demonstrate the self-locking of these two bolts.

Given that

M10:

Pitch P=1.5mm, pitch diameter d2=9.026mm, flank angle=30;

M30:Pitch P=3.5mm, pitch diameter d2=27.27mm, flank angle=30;

Coefficient of friction force=0.10.

Homework-14

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Chapter 5 Bolted Connection-1