2-lectures lec 38
TRANSCRIPT
-
8/12/2019 2-Lectures LEC 38
1/27
ME 307Machine
Design I
CH-8 LEC 38 Slide 1Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
2/27
ME 307Machine
Design 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 38 Slide 2Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
3/27
ME 307Machine
Design I
8-11 Fatigue Loading of Tension Joints8-12 Shear Joints8-13 Setscrews8-14 Keys and Pins
CH-8 LEC 38 Slide 3Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
4/27
ME 307Machine
Design I Example 5 (Example 8-4 Textbook)
(a) The grip is l = 1.50 in. From Table A-
31, the nut thickness is 35/64 in.
Adding two threads beyond the nut
of 2/11ingives a bolt length of
35 21.50 2.229 in
64 11
L
CH-8 LEC 38 Slide 4Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
5/27
ME 307Machine
Design I
(a) From Table A-17, the next fraction size bolt is 2 1/4 in.
CH-8 LEC 38 Slide 5Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
6/27
ME 307Machine
Design I
2 0.25 in, L 6inT
L d
CH-8 LEC 38 Slide 6Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
7/27
ME 307Machine
Design I
CH-8 LEC 38 Slide 7Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
8/27
ME 307Machine
Design I
CH-8 LEC 38 Slide 8Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
9/27
ME 307Machine
Design I
CH-8 LEC 38 Slide 9Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
10/27
ME 307Machine
Design I
Fatigue Analysisa) External Load
8-7 Tension Joints-The External Load
CH-8 LEC 38 Slide 10Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
-
8/12/2019 2-Lectures LEC 38
11/27
ME 307Machine
Design I8-11 Fatigue Loading of Tension Joints
CH-8 LEC 38 Slide 11Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Fatigue Analysis
In general, bolted joints are subject to 0-Pmax, e.g pressure vessels, flanges, pipes,
ME 307
-
8/12/2019 2-Lectures LEC 38
12/27
ME 307Machine
Design I8-11 Fatigue Loading of Tension Joints
CH-8 LEC 38 Slide 12Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Table 8-16 lists average fatigue stress-concentration factors for the fillet under
the bolt head and also at the beginning of the threads on the bolt shank.
These are already corrected for notch sensitivity and for surface finish.
Useof rolled threads is the predominant method of thread-forming in screwfasteners, where Table 8-16 applies.
ME 307
-
8/12/2019 2-Lectures LEC 38
13/27
ME 307Machine
Design I8-11 Fatigue Loading of Tension Joints
CH-8 LEC 38 Slide 13Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
In thread-rolling the amount of cold-work and strain strengthening is unknown to
the designer; therefore, fully corrected (including Kf) axial endurance strength isreported in Table 8-17.
ME 307
-
8/12/2019 2-Lectures LEC 38
14/27
ME 307Machine
Design I8-11 Fatigue Loading of Tension Joints
Fatigue-loaded bolted joints subjected toFatigue action can be analyzed directly by
the methods of Chapter 7.
Fatigue loading is the one in which the
externally applied load fluctuates between
zeroand some maximum force P.
Fmax= Fb and Fmin= Fi Fa= (Fmax Fmin)/2 = (FbFi)/2
a= Fa/At
CH-8 LEC 38 Slide 14Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307
-
8/12/2019 2-Lectures LEC 38
15/27
ME 307Machine
Design I8-11 Fatigue Loading of Tension Joints
CH-8 LEC 38 Slide 15Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
2
2
b iba
b ibm
F FF
F FF
( )
2 2 2
( )
2 2 2
b i i iba
t t t
b i i i i
bmt t t t
bm ba i
F F CP F F CP
A A A
F F CP F F F CP
A A A A
Fmax= Fb andFmin= Fi Fa= (Fmax Fmin)/2 = (FbFi)/2
Fm= (Fmax + Fmin)/2 = (Fb+ Fi)/2
a= Fa/At
(8-34)
(8-35)
(8-36)
ME 307
On the designers fatigue
-
8/12/2019 2-Lectures LEC 38
16/27
ME 307Machine
Design I
CH-8 LEC 38 Slide 16Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
On the designer s fatigue
diagram, shown in Figure 8-20,
the load line is .
High Preload is especially
important in fatigue. iis aconstant the load line at Fi/Athas
a unit slope, r = 1.0
m a i
Figure 8-20Designers fatigue diagram showing a Goodman
failure locus and how a load line is used to definefailure and safety in preloaded bolted joints infatigue.
ME 307
8 i di f i i
-
8/12/2019 2-Lectures LEC 38
17/27
M 30Machine
Design I8-11 Fatigue Loading of Tension Joints
CH-8 LEC 38 Slide 17Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Next, find the strength components Saand Smof
the fatigue failure locus. These depend on thefailure Criteria.
Goodman1a m
e ut
S S
S S
Gerber 2
1a m
e ut
S S
S S
ASME-Elliptic 22
1a me p
S S
S S
(8-39)
(8-38)
(8-37)
ME 307
For simultaneous solution between Eq. (8-36), as Sa= Sm+ i and each of Eqs. (8-37) to (8-39)
-
8/12/2019 2-Lectures LEC 38
18/27
Machine
Design I
CH-8 LEC 38 Slide 18Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Goodman
e ut ia
ut e
m a i
S SS
S SS S
Gerber
2 21
4 2
2a ut ut e e i ut i e
e
m a i
S S S S S S S
SS S
ASME-Elliptic
2 2
2 2e
a p p e i i e
p e
m a i
SS S S S S
S SS S
(8-43)
(8-42)
(8-40)
(8-41)
q ( ), a m i q ( ) ( )
gives
ME 307
-
8/12/2019 2-Lectures LEC 38
19/27
Machine
Design I
CH-8 LEC 38 Slide 19Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
When using relations of this section, be sure to use K
ffor both
a
and m
.
Otherwise, the slope of the load line will not remain1 to 1.
The factor of safety guarding against fatigue is given by
Applying this to the Goodman criterion, for example, with Eqs. (8-34) and
(8-40) and gives
af
a
Sn
i i tF A
2e ut t i
f
ut e
S S A F n
CP S S
(8-44)
(8-45)
ME 307
With no preload C=1 F =0 and Eq (8-45) becomes
-
8/12/2019 2-Lectures LEC 38
20/27
Machine
Design I
CH-8 LEC 38 Slide 20Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
With no preload, C=1,Fi=0, and Eq. (8-45) becomes
Preload is beneficial for resisting fatigue when is greater than unity. For
Goodman, Eqs.(8-45) and (8-46) with puts an upper bound on the
preloadFiof
If this cannot be achieved, and nfis unsatisfactory, use the Gerber or the ASME-elliptic criterion to obtain a less conservative assessment.
After solving Eq. (8-44), you should also check the possibility of yielding, using
the proof strength
0f f
n n
(8-46)
(8-47)
02
e ut t
f
ut e
S S An
P S S
0 1
f fn n
1i ut tF C S A
pp
m a
Sn
(8-48)
ME 307M hi Example 6 (Example 8 5 Textbook)
-
8/12/2019 2-Lectures LEC 38
21/27
Machine
Design I
CH-8 LEC 38 Slide 21Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Example 6 (Example 8-5 Textbook)
Figure 8-21Pressure-cone frustum member model for a
cap screw. For this model the significant sizesare
Where l = effective grip. The solutions are for=30oand dw=1.5d.
2 2
2
1
2
2
2
tan 1.5 0.577
1.5
w
w
h t t d l
h D t d
D d l d l
D d d
ME 307M hi
-
8/12/2019 2-Lectures LEC 38
22/27
Machine
Design I
CH-8 LEC 38 Slide 22Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307Machine
-
8/12/2019 2-Lectures LEC 38
23/27
Machine
Design I
CH-8 LEC 38 Slide 23Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307Machine
-
8/12/2019 2-Lectures LEC 38
24/27
Machine
Design I
CH-8 LEC 38 Slide 24Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
Figure 8-22
Designers fatigue diagram forpreloaded bolts, dr showing themodified Goodman locus, theGerber locus and the Langer proofstrength locus, with an explodedview of the area of interest, The
strengths used areSp= 85 kpsi, Se= 18.6 kpsi, andSut=120 kpsi. The coordinates are
A: i=63.72 kpsi
B: a=3.1 kpsi, m=66.82 kpsiC: Sa=7.55 kpsi, Sm=71.29 kpsi
D: Sa=10.64 kpsi, Sm=74.35 kpsiE: Sa=11.32 kpsi, Sm=75.04 kpsi
ME 307Machine
-
8/12/2019 2-Lectures LEC 38
25/27
Machine
Design I
CH-8 LEC 38 Slide 25Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307Machine
-
8/12/2019 2-Lectures LEC 38
26/27
Machine
Design I
CH-8 LEC 38 Slide 26Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints
ME 307Machine
-
8/12/2019 2-Lectures LEC 38
27/27
Design I
CH-8 LEC 38 Slide 27Dr. A. Aziz Bazoune Chapter 8: Screws, Fasteners and the Design of Nonpermanent Joints