Chapter 5 Bolted Connection-1
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Transcript of 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
2/45
=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