Download - Files 2-Lectures LEC 34
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8 LEC 34 Slide 1
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
8-1 Thread Standards and Definitions
8-2 The Mechanics of Power Screws8-3 Strength Constraints8-4 Joints-Fasteners Stiffness8-5 Joints-Member Stiffness8-6 Bolt Strength
8-7 Tension Joints-The External Load8-8 Relating Bolt Torque to Bolt Tension8-9 Statically Loaded Tension Joint with Preload8-10 Gasketed Joints8-11 Fatigue Loading of Tension Joints
8-12 Shear Joints8-13 Setscrews8-14 Keys and Pins8-15 Stochastic Considerations
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Announcements
HW #5 Ch. 18, on WebCT
Due Date for HW #5 is Mon. DEC. 31, 2007
Quiz on Ch. 18, Mon. DEC. 31, 2007 ?????
CH-8 LEC 34 Slide 3
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
8-2 The Mechanics of Power Screws
CH-8 LEC 34 Slide 4
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent JointsCH-8 LEC 34 Slide 5
A power screw is 23 mm in diameter and has a thread pitch of 7 mm.
(a) Find the thread depth, the thread width, the mean and root
diameters, and the lead, provided square threads are used.
(b) Repeat part (a) for Acme threads.
Example-1
Given:
Diameter of the power screw, d= 23 mm
Thread pitch,p = 7 mm
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent JointsCH-8 LEC 34 Slide 6
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8 LEC 34 Slide 7
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
The Mechanics of Power Screws
A power screw is a device used in
machinery to change the angular
motion into linear motion, and
usually, to transmit power.Applications:
Lead screws of lathes
Screws for vises, presses andjacks
Figure 8-4
The Joyce worm-gear screw jack.
CH-8 LEC 34 Slide 8
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
The Mechanics of Power Screws
In Figure 8-5 a square threaded power
screw with single thread having a mean
diameter dm, a pitch anglep, and a lead
angle , and a helix angle is loaded by
the axial compressive force F.
We wish to find an expression for the
torque required to raise this load, and
another expression for the torque
required to lower the load. Figure 8-5 Portion of a power screw(Square)
CH-8 LEC 34 Slide 9
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Imagine that a single thread of the screw is enrolled or developed (Fig. 8-6) for
exactly a single turn. Then on edge of the thread will form the hypotenuse of a
right triangle whose base is the circumference of the mean-thread- circle and
whose height is the lead. The angle is the lead angle of the thread. For raising the
load a force PR acts to the right and to lower the load, PL acts to the left.
CH-8 LEC 34 Slide 10
Figure 8-6 Force Diagrams (a) Lifting the load; (b)lowering the load
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
For raising the loadsin cos 0
cos sin 0
H R
V
F P N f N
F F N f N
(a)
For lowering the loadsin cos 0
cos sin 0
H L
V
F P N f N
F F N f N
(b)
CH-8 LEC 34 Slide 11
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
For raising the load sin coscos sin
R
F fP
f
(c)
For lowering the load (d)
CH-8 LEC 34 Slide 12
EliminatingNfrom the previous equations and solving for P gives
cos sincos sin
R
F fP
f
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
For raising the load
1m
R
m
F l d f P
f l d
(e)
For lowering the load (f)
CH-8 LEC 34 Slide 13
Next, divide the numerator and the denominator of these
equations by cos and use the relation tan ml d
1m
R
m
F f l d P
f l d
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
2
m mR
m
Fd l f d T
d f l
(8-1)
(8-2)
CH-8 LEC 34 Slide 14
The torque is the product of the force P and the mean radius 2m
d
Torque required for raising the load
to overcome thread friction and toraise the loadR
T
2
m mL
m
Fd f d lT
d f l
Torque required for lowering the loadto overcome part of the threadfriction in lowering the loadL
T
ME 307 Self Locking Condition
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 34 Slide 15
Self Locking Condition
If the lead is large or the friction is low, the load will lower itself
by causing the screw to spin without any external effort. In such
cases the torque from Eq. (8-2) will be negative or zero.
When a positive torque is obtained from this equation, the screw
is said to be self locking
LT
Condition for Self Locking: mfd l
Dividing both sides of the above inequality by and recognizing
that , we get
md
tanml d
tanf (8-3)
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ME 307MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 34 Slide 16
The critical coefficient of friction for the lead concerned,
If f = fcr the nut is on the point of moving down the thread without
any torque applied.
Iff > fcr
then the thread is self-locking in that the nut cannot undo
by itself, it needs to be unscrewed by a definite negative torque;
Clearly self-locking behavior is essential for threaded fasteners.
Car lifting jacks would not be of much use if the load fell as soon as
the operating handle was released.
Self Locking Condition
tanf
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Some applications of power screws require overhauling behavior.
1. The Archimedean drill
2. Pump action screwdrivers(Yankee screw drivers)
These devices incorporate verylarge lead angles
Power Screw-Overhauling
Iff < fcrthen the thread is overhauling in that the nut will unscrew byitself under the action of the load unless prevented by a positivetightening torque.
Increasing lead (angle) overhauling
CH-8 LEC 34 Slide 17
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
decreasing thread friction overhauling
Sensitive linear actuators may incorporate recirculating ball screws
such as that illustrated here to reduce thread friction to levels which
go hand-in-hand with overhauling.
CH-8 LEC 34 Slide 18
Power Screw-Overhauling
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
decreasing thread friction overhauling
Sensitive linear actuators may incorporate recirculating ball screws
such as that illustrated here to reduce thread friction to levels which
go hand-in-hand with overhauling.
CH-8 LEC 34 Slide 19
Power Screw-Overhauling
ME 307h Effi i
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 34 Slide 20
Efficiency
If we let in Eq. (8-1), we obtain0f
which, is the torque required to raise
the load.
(8-4)
0
2
FlT
(g)
The efficiency is therefore
RR T
Fl
T
Teefficiency2
0
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
f
CH-8 LEC 34 Slide 21
Efficiency
ME 307Machine P S ACME Th d
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 34 Slide 22
Power Screw- ACME Thread
Fis parallel to screw axis i.e. makes angle
= 14.5with thread surface ignoring the
small effect ofl, theresultant normal force
NisF/cos. The frictional force =f Nisincreased and thus friction terms in Eq.
(8.1) are modified accordingly:
Torque required to raise load F
sec
sec
m mR
m
d l fdT F
2 d fl
(8-5)
ACME thread is not as efficient as square thread because of additional friction
due to wedging action but it is often preferred because it is easier to machine.
ME 307Machine
P S ith C ll
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
In most of power screw applications (load lifting) a collar is to be designed.The presence of collar increases the friction torque. A thrust collar bearing
must be employed between the rotating and stationary members in order tocarry the axial component
CH-8 LEC 34 Slide 23
Power Screw with Collar
ME 307Machine Po er Scre ith Collar
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints CH-8 LEC 34 Slide 24
Power Screw with Collar
ME 307Machine Power Screw with Collar
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MachineDesign I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
fc= collar friction coefficient
dc = collar mean diameter
CH-8 LEC 34 Slide 25
Power Screw with Collar
sec
sec
m m
R c
m
c c
c
d l fdT F T
2 d fl
Ff dT
2
If is the
coefficient of collar
friction, the torque
required is
c
f
(8-6)
ME 307Machine Power Screws-friction coefficients
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Design I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Power Screws-friction coefficients
Friction wears thread surface for safe applications Maxthreadbearing pressureis given in Table 8-4.
CH-8 LEC 34 Slide 26
ME 307Machine Power Screws-friction coefficients
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Design I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Power Screws-friction coefficients
CH-8 LEC 34 Slide 27
Table 8-5 Coefficients of frictionf for Threaded Pairs
ME 307Machine Power Screws-friction coefficients
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Design I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Power Screws-friction coefficients
CH-8 LEC 34 Slide 28
Table 8-6 Thrust Collar friction coefficient,fc
Coefficients of friction around 0.1 to 0.2 may be expectedfor common materials under conditions ofordinary service
and lubrication.
ME 307Machine Example 2
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Design I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Example-2
Problem # 8.8 (modified)Given:
5/8-6ACME? i.e. d=5/8 and N=6
f=fc= 0.15
dc=7/16 in
P = 6 lb
Larm=2 3/4 in
Required:
F, efficiency, Self-Lock?
PLarm
F
CH-8 LEC 34 Slide 29
ME 307MachineD i I Example-2 (Cont d)
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Design I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Lever torque
Clamping force
R
l=1/N
2
dFfT
Tf ld
fdl2
dFT
ccc
c
m
mm
totalR
sec
sec
p/2 =1/2Nd
CH-8 LEC 34 Slide 30
Example-2 (Cont. d)
ME 307MachineDesign I Example-2 (Cont d)
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Design I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Efficiency161 0.1667
0.262 2 16.5
R
FlEfficiency e
T
Self-lock
which is clear that it is self lock
CH-8 LEC 34 Slide 31
0.15 0.5417 0.255
0.1667
m
m
fd l
fd
l
Example-2 (Cont. d)