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7/29/2019 2-Lectures LEC 33
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
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
CH-8 LEC 33 Slide 2
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I 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 33 Slide 3
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
CH-8 LEC 33 Slide 4
8-1 Thread Standards and Definitions
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
Introduction
The fundamental operation in manufacture is the creation of shape
- this includes assembly, where a number of components are
fastened or joined together either permanently by welding (Ch9)
for example or detachably(nonpermanent) by screws,nuts and
bolts and so on. Since there is such a variety of shapes in
engineering to be assembled, it is hardly surprising that there is
more variety in demountable fasteners than in any other machine
element. Fasteners based upon screw threads are the most
common, so it is important that their performance is understood,
and the limitations of the fastened assemblies appreciated.CH-8 LEC 33 Slide 5
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I Introduction
There are two distinct
uses for screw threads andthey usually demand
different behavior from
the threads :
a power screwsuch asa lathe leadscrew or the
screw in a car lifting jack
which transforms rotary
motion into substantial
linear motion (or viceversa in certain
applications), and
CH-8 LEC 33 Slide 6
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
Fasteners
a Threaded Fastenersimilar to a nut and bolt which joins a
number of components together again by transforming rotarymotion into linear motion, though in this case the translation issmall.
CH-8 LEC 33 Slide 7
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
Thread Profiles
Thread profiles.
a) Square (b) ACME; (c) UN, ISO
(c)
CH-8 LEC 33 Slide 8
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I Thread Profile Parameters
Lead=L=n p
Figure 8-1Terminology of
screw threads.Sharp Vee Threads
shown for clarity;the crests and roots
are actually
flattened or
rounded during the
forming operation
CH-8 LEC 33 Slide 9
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
Threads
(a) Single (STANDARD)-, (b) double-, and (c) triple threaded screws.
Text Reference: Figure 15.2, page 667 CH-8 LEC 33 Slide 10
ME 307
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I Thread Systems
A thread 'system' is a set of basic thread proportions which
is scaled to different screw sizes to define the thread
geometry. Whitworth, Sellers, British Standard Pipe (BSP)
are just three of the many systems which proliferated beforethe adoption of the
1. ISO Metric thread system.
2. The American National (Unified) Thread standards isused mainly in the US.
3. Square and ACME
CH-8 LEC 33 Slide 11
ME 307
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I Thread geometry
Thebasic profile of ISO Metric threadsis built upfrom contiguous equiangular triangles of height H disposed
symmetrically about a pitch line which becomes the pitchcylinder of diameter d
2when the profile is rotated about the
axis to form the thread. The distance between adjacent
triangles - the pitch - is p = 2 H /3. The tips of the trianglesare truncated by h/8 to form the major diameter ( size ) d of
the thread, and the bases are truncated by h/4 to form theminor diameter d1 . It follows that d1 = d - 5 h/4 = d - 1.08 p.
This leads to the rule of thumb for suitable tapping size
CH-8 LEC 33 Slide 12
ME 307
Thread Profile For Metric System (M MJ)
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
Thread Profile For Metric System (M, MJ)
pitch
minor
major
H= 0.5(3)1/2 p
US N= # threads/in
a ISO 68= aAmerican National (Unified) thread standard= 60CH-8 LEC 33 Slide 13
ME 307
h d S d d
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I Thread Standards
1) ISO Metric thread system: Table 8-1
Major diameter (mm),
2a= 60 Standard thread is RHSpecifications: e.g.: M12x1.75 orM = Basic Metric, J = round root; 12 = nominal major
diameter (mm); 1.75 = pitch (mm)
Tensile stress area At =p/4(dm+dr)/2 (see footnote T.8.1)
MJ12x1.75
CH-8 LEC 33 Slide 14
ME 307
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I
CH-8 LEC 33 Slide 15
ME 307
Th d St d d
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307MachineDesign I Thread Standards
2) The American National (Unified)
Thread standards is used mainly in the
US: Table-8-2 (Size designation) use dUN=regular thread, UNR=round root (use
root radius)Specifications: 5/8-18 UN, UNC, UNF
UNR, UNRC, UNRF
5/8=d 18 = N (thread size)UN = Unified, F=fine, C=Coarse, R =Round Root
CH-8 LEC 33 Slide 16
ME 307
Thread Standards
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I
Thread Standards
3. Square (a) and The ACME Threads (b)-usedmainly in power screws Table 8.3 gives preferred
pitches for ACME threads
CH-8 LEC 33 Slide 17
ME 307
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I
Power screw thread forms. [Note: All threads shown are external (i.e., on thescrew, not on the nut); dm is the mean diameter of the thread contact and isapproximately equal to (d+ dr)/2.]
CH-8 LEC 33 Slide 18
ME 307
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I
CH-8 LEC 33 Slide 19
ME 307h ACME Th d P fil
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I ACME Thread Profile
Figure 15.5 Details of ACME thread profile. (All
dimensions are in inches.)
Text Reference: Figure 15.5, page 670 CH-8 LEC 33 Slide 20
ME 307M hi ACME Thread Properties
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I ACME Thread Properties
Crest diameter,d
c, in.
Number ofthreads per inch, a
n
Tensile stressarea, A
t, in2
Shear stressarea, A
s, in2
1/4
5/16
3/8
7/16
1/2
5/8
3/4
7/8
1
1 1/8
1 1/4
1 3/8
1 1/2
1 3/4
2
2 1/4
2 1/2
2 3/4
3
3 1/2
4
4 1/2
5
16
14
12
12
10
8
6
6
5
5
5
4
4
4
4
3
3
3
2
2
2
2
2
0.02663
0.04438
0.06589
0.09720
0.1225
0.1955
0.2732
0.4003
0.5175
0.6881
0.8831
1.030
1.266
1.811
2.454
2.982
3.802
4.711
5.181
7.338
9.985
12.972
16.351
0.3355
0.4344
0.5276
0.6396
0.7278
0.9180
1.084
1.313
1.493
1.722
1.952
2.110
2.341
2.803
3.262
3.610
4.075
4.538
4.757
5.700
6.640
7.577
8.511
Crest diameters, threads per inch, and stresses for Acme thread.
N
CH-8 LEC 33 Slide 21
ME 307Machine
Mechanics of Power Screws
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I Mechanics of Power Screws
A power screw is a device thatis common to tools or
machinery that are used to
change angular motion intotranslation. It is also capable of
developing a large amount of
mechanical advantage. Familiar
applications include clamps or
vises, presses, lathes lead
screws, and jacks.
The joyce
warm-gear
screw jack
CH-8 LEC 33 Slide 22
ME 307Machine Mechanics of Power Screws
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I Mechanics of Power Screws
Weight supported by three screw jacks. In each screw jack, only theshaded member rotates.
CH-8 LEC 33 Slide 23
ME 307Machine Mechanics of Power Screws
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I
Example of catalogue:
http://www.roton.com/web/quick.power_screws.jsp
Mechanics of Power Screws
CH-8 LEC 33 Slide 24
ME 307Machine Vehicle Screw Jack
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
MachineDesign I
Vehicle Screw Jack
CH-8 LEC 33 Slide 25
ME 307Machine
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Design I
CH-8 LEC 33 Slide 26
ME 307Machine
Power Screw with collar
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Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Design I
Different types of collars
Power Screw with collar
CH-8 LEC 33 Slide 27
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