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
8-2
8-3
8-4
8-5
8-6
8-7
8-8
8-9
8-10
8-11
8-12
8-13
8-14
8-15
Thread Standards and Definitions
The Mechanics of Power Screws
Strength Constraints
Joints-Fasteners Stiffness
Joints-Member Stiffness
Bolt Strength
Tension Joints-The External Load
Relating Bolt Torque to Bolt Tension
Statically Loaded Tension Joint with Preload
Gasketed Joints
Fatigue Loading of Tension Joints
Shear Joints
Setscrews
Keys and Pins
Stochastic Considerations
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 2
ME 307
Machine
Design 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
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 3
ME 307
Machine
Design I
8-1
Thread Standards and Definitions
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 4
ME 307
Machine
Design 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.
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 5
ME 307
Machine
Design I
Introduction
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
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 6
ME 307
Machine
Design I
Fasteners
•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.
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 7
Thread Profiles
ME 307
Machine
Design I
(c)
Thread profiles.
a) Square (b) ACME; (c) UN, ISO
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 8
ME 307
Machine
Design I
Thread Profile Parameters
Figure 8-1
Terminology of
screw threads.
Sharp Vee Threads
shown for clarity;
the crests and roots
are actually
flattened or
rounded during the
forming operation
Dr. A. Aziz Bazoune
Lead=L=n p
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 9
Threads
ME 307
Machine
Design I
(a) Single (STANDARD)-, (b) double-, and (c) triple threaded screws.
Dr. A. Aziz Bazoune
Text Reference:
Figure
page 667
Chapter 8: Screws,
Fasteners
and 15.2,
the Design
of Nonpermanent Joints
CH-8
LEC 33 Slide 10
ME 307
Machine
Design 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 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
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 11
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
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 12
Thread Profile For Metric System (M, MJ)
ME 307
Machine
Design I
major
H= 0.5(3)1/2 p
pitch
US N= # threads/in
minor
a ISO 68= a American National (Unified) thread standard= 60°
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 13
ME 307
Machine
Design I
Thread Standards
1) ISO Metric thread system: Table 8-1
Major diameter (mm),
2a= 60° Standard thread is RH
Specifications: e.g.: M12x1.75 or MJ12x1.75
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)
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 14
ME 307
Machine
Design I
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 15
ME 307
Machine
Design I
Thread Standards
2) The American National (Unified)
Thread standards is used mainly in the
US: Table-8-2 (Size designation) use d
UN=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
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 16
Thread Standards
ME 307
Machine
Design I
3. Square (a) and The ACME Threads (b)-used
mainly in power screws Table 8.3 gives preferred
pitches for ACME threads
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 17
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.]
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 18
ME 307
Machine
Design I
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 19
ME 307
Machine
Design I
ACME Thread Profile
Figure 15.5 Details of ACME thread profile. (All
dimensions are in inches.)
Dr. A. Aziz Bazoune
Text Reference:
Figure
page 670
Chapter 8: Screws,
Fasteners
and 15.5,
the Design
of Nonpermanent Joints
CH-8
LEC 33 Slide 20
ACME Thread Properties
ME 307
Machine
Design I
Cres t diameter,
d c, in.
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
Dr. A. Aziz Bazoune
Crest
N umber of
threads per inch, a
Nn
16
14
12
12
10
8
6
6
5
5
5
4
4
4
4
3
3
3
2
2
2
2
2
Tens ile s tres s
area, A t, in2
S hear s tres s
area, A s, in 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
Chapter 8: Screws,
Fasteners
the Design
of Nonpermanent
Jointsthread.
diameters,
threads
perand
inch,
and stresses
for Acme
CH-8
LEC 33
Slide 21
ME 307
Machine
Design I
Mechanics of Power Screws
 A power
The joyce
warm-gear
screw jack
Dr. A. Aziz Bazoune
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.
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 22
ME 307
Machine
Design I
Mechanics of Power Screws
Weight supported by three screw jacks. In each screw jack, only the
shaded member rotates.
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 23
Mechanics of Power Screws
ME 307
Machine
Design I
Example of catalogue:
http://www.roton.com/web/quick.power_screws.jsp
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 24
ME 307
Machine
Design I
Dr. A. Aziz Bazoune
Vehicle Screw Jack
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 25
ME 307
Machine
Design I
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 26
ME 307
Machine
Design I
Power Screw with collar
Different types of collars
Dr. A. Aziz Bazoune
Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
CH-8
LEC 33 Slide 27