Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
8-1 8-1 Thread Standards and DefinitionsThread Standards and Definitions8-28-2 The Mechanics of Power ScrewsThe Mechanics of Power Screws8-38-3 Strength Constraints Strength Constraints 8-48-4 Joints-Fasteners StiffnessJoints-Fasteners Stiffness8-58-5 Joints-Member Stiffness Joints-Member Stiffness 8-68-6 Bolt Strength Bolt Strength 8-78-7 Tension Joints-The External LoadTension Joints-The External Load8-88-8 Relating Bolt Torque to Bolt TensionRelating Bolt Torque to Bolt Tension8-98-9 Statically Loaded Tension Joint with PreloadStatically Loaded Tension Joint with Preload8-108-10 Gasketed JointsGasketed Joints8-118-11 Fatigue Loading of Tension JointsFatigue Loading of Tension Joints8-128-12 Shear JointsShear Joints8-138-13 SetscrewsSetscrews8-148-14 Keys and PinsKeys and Pins8-158-15 Stochastic ConsiderationsStochastic Considerations
CH-8 LEC 33 Slide 2
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I AnnouncementsAnnouncements
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
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
CH-8 LEC 33 Slide 4
8-1 8-1 Thread Standards and DefinitionsThread Standards and Definitions
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I IntroductionIntroduction
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
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I IntroductionIntroduction
There are two distinct uses for screw threads and they usually demand different behavior from the threads :
•a power screw such as a lathe leadscrew or the screw in a car lifting jack which transforms rotary motion into substantial linear motion (or vice versa in certain applications), and
CH-8 LEC 33 Slide 6
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I FastenersFasteners
•a Threaded Fastener similar to a nut and bolt which joins a number of components together again by transforming rotary motion into linear motion, though in this case the translation is small.
CH-8 LEC 33 Slide 7
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Thread ProfilesThread Profiles
Thread profiles.
a) Square (b) ACME; (c) UN, ISO
(c)
CH-8 LEC 33 Slide 8
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Thread Profile ParametersThread Profile Parameters
Lead=L=n pFigure 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
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I ThreadsThreads
(a) Single (STANDARD)-, (b) double-, and (c) triple threaded screws.
Text Reference: Figure 15.2, page 667 CH-8 LEC 33 Slide 10
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Thread SystemsThread 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 before the adoption of the
1.ISO Metric thread system. 2.The American National (Unified) Thread standards is used mainly in the US.3.Square and ACME
CH-8 LEC 33 Slide 11
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Thread geometry
The basic profile of ISO Metric threads is built up from contiguous equiangular triangles of height H disposed symmetrically about a pitch line which becomes the pitch cylinder of diameter d2 when 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 triangles are truncated by h/8 to form the major diameter ( size ) d of the thread, and the bases are truncated by h/4 to form the minor 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
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
Thread Profile For Metric System (M, Thread Profile For Metric System (M, MJ)MJ)
pitch
minor
major
H= 0.5(3)1/2 p
US N= # threads/in
a ISO 68= a American National (Unified) thread standard= 60°CH-8 LEC 33 Slide 13
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Thread Standards
1)1) ISO Metric thread systemISO Metric thread system: Table 8-1 Major diameter (mm),
2a= 60° Standard thread is RHSpecifications: e.g.: M12x1.75 or M = 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
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
CH-8 LEC 33 Slide 15
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Thread StandardsThread Standards
2)2) The American National (Unified) The American National (Unified) Thread standards Thread standards is used mainly in the
US: Table-8-2 (Size designation) use d
UN=regular thread, UNR=round root (use root radius)
SpecificationsSpecifications: 5/8”-18 UN, UNC, UNF UNR, UNRC, UNRF
5/8”=d 18 = N (thread size) UN = Unified, F=fine, C=Coarse, R =Round RootCH-8 LEC 33 Slide
16
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Thread StandardsThread Standards
3. Square (a) and The ACME Threads (b)-used mainly in power screws Table 8.3 gives preferred pitches for ACME threads
CH-8 LEC 33 Slide 17
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
Power screw thread forms. [Note: All threads shown are external (i.e., on the screw, not on the nut); dm is the mean diameter of the thread contact and is approximately equal to (d + dr)/2.]
CH-8 LEC 33 Slide 18
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
CH-8 LEC 33 Slide 19
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design 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
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I ACME Thread Properties
Crest diameter,d c, in.
Number ofthreads per inch, a
n
Tensile s tressarea, A t, in2
Shear s tressarea, A s, in2
1/45/163/87/161/25/83/47/81
1 1/81 1/41 3/81 1/21 3/4
22 1/42 1/22 3/4
33 1/2
44 1/2
5
1614121210866555444433322222
0.026630.044380.065890.097200.12250.19550.27320.40030.51750.68810.88311.0301.2661.8112.4542.9823.8024.7115.1817.3389.98512.97216.351
0.33550.43440.52760.63960.72780.91801.0841.3131.4931.7221.9522.1102.3412.8033.2623.6104.0754.5384.7575.7006.6407.5778.511
Crest diameters, threads per inch, and stresses for Acme thread.
N
CH-8 LEC 33 Slide 21
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Mechanics of Power Screws
A power screw is a device that is common to tools or machinery that are used to change angular motion into translation. 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
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Mechanics of Power ScrewsMechanics of Power Screws
Weight supported by three screw jacks. In each screw jack, only the shaded member rotates.
CH-8 LEC 33 Slide 23
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
Example of catalogue:http://www.roton.com/web/quick.power_screws.jsp
Mechanics of Power ScrewsMechanics of Power Screws
CH-8 LEC 33 Slide 24
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I Vehicle Screw Jack
CH-8 LEC 33 Slide 25
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
CH-8 LEC 33 Slide 26
Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307 Machine Design I
Different types of collars
Power Screw with collar
CH-8 LEC 33 Slide 27
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