7/23/2019 Threaded Fasteners Ch 10
1/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Threaded Fasteners andPower Screws
7/23/2019 Threaded Fasteners Ch 10
2/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.1 (p. 386)Helical threads of pitch p, lead L and lead angle .
Some worm and power screws
have multiple threads
Lead L = pitch p for single thread
7/23/2019 Threaded Fasteners Ch 10
3/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.2 (p. 386)Unified and ISO thread geometry. The basic profile of the external thread isshown.
7/23/2019 Threaded Fasteners Ch 10
4/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Table 10.1b (cont.)
7/23/2019 Threaded Fasteners Ch 10
5/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Table 10.2 (p. 388)Basic Dimensions of ISO Metric Screw Threads
7/23/2019 Threaded Fasteners Ch 10
6/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
3 Classes of Fit for Screw Threads
Class 1
Loosest fit and largest tolerances
Class 3
Tightest fit and smallest tolerances
7/23/2019 Threaded Fasteners Ch 10
7/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.4 (p. 389)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 is
approximately equal to (d+ dr)/2.]
Difficult tomanufacture
High axial loadcarrying capacity
7/23/2019 Threaded Fasteners Ch 10
8/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.5 (p. 391)Weight supported by three screw jacks. In each screw jack, only the shaded
member rotates.
7/23/2019 Threaded Fasteners Ch 10
9/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.6 (p. 391)Screw thread forces.
Kinematics:: tanm
L
d
=
Infinitesimal portion of nut
q = tangential force, w = portion of total axial force
W, n = normal force, f = friction coefficient,fn = friction force, q dm/2 = torque applied to nutsegment
7/23/2019 Threaded Fasteners Ch 10
10/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Equilibrium Yields Torque Eq. In tangential direction
In axial direction
Combine and solve for q
Torque
cos cos sin 0nq fn n =
sin cos cos 0nw fn n + =
cos cos sin
cos cos sin
n
n
fq w
f
+=
cos cos sin
2 2 cos cos sin
m m n
n
d Wd f T Q
f
+= =
7/23/2019 Threaded Fasteners Ch 10
11/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Torque (cont.) Using kinematic relationship
If application has bearing surface or thrust collar
For square thread or ACME thread cosn is nearly 1:
For lower load, reverse signs on q and friction force fn
cos
2 cos 2m m n c c
m n
Wd f d L Wf d T
d fL
+= +
2 2m m c c
m
Wd f d L Wf d Td fL
+= +
cos2 cos
m m n
m n
Wd f d LTd fL
+=
cos
2 cos 2 2 2
m m n c c m m c c
m n m
Wd f d L Wf d Wd f d L Wf d T
d fL d fL
= + = +
+ + S uare
7/23/2019 Threaded Fasteners Ch 10
12/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.7 (p. 394)Comparison of thread angles measured in axial and normal planes ( and n).
7/23/2019 Threaded Fasteners Ch 10
13/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Overhauling and Self-Locking Self-locking screw: sufficient friction so that
positive torque required to lower the load Overhauling screw: low enough friction so that
axial load will cause screw to turn
Neglecting collar friction
Beware of vibration that can reduce f, and selflocking screw no longer locks!
cos= tan (square thread)n
m m
L Lf
d d
=
7/23/2019 Threaded Fasteners Ch 10
14/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Efficiency of Power Screws
Output work by power screw for one turn =
WL
Input work = T = 2
Efficiency = output work/input work
cos cos tan
cos cos cot
m n n
m m n n
d fL f Le
d fd L f
= =
+ +
7/23/2019 Threaded Fasteners Ch 10
15/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.8 (p. 396)
Efficiency of Acme screwthreads when collar friction isnegligible. (Note: Values for
square threads are higher by
less than 1 percent.)
7/23/2019 Threaded Fasteners Ch 10
16/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.9 (p. 397)Ball bearing screw assembly with a portion of the nut cut away to show
construction. (Courtesy Saginaw Steering Gear Division, General Motors
Corporation.)
Very low coefficient of friction due to mostly rolling contact-
usually overhauling screw
7/23/2019 Threaded Fasteners Ch 10
17/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.11 (p. 400)Force flow for a bolt in tension.
Bearing stress on thread = P/A where
( )2 2 / 4iA d d=
t/p = no. of threads in contact
( )2 24
i
P p
td d
=
This is average value of
Bearing stress for the threads
Thread 1 carries more load. Why?
7/23/2019 Threaded Fasteners Ch 10
18/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
To distribute load more evenly overthreads
Make the nut out of softer material than
the thread Manufacture threads of nut with slightly
greater pitch than bolts
Modify nut design
7/23/2019 Threaded Fasteners Ch 10
19/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.12 (p. 402)A special nut provides more nearly equal distribution of load amount threads incontact.
7/23/2019 Threaded Fasteners Ch 10
20/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Thread stripping stress
Stripping due to shear failure. Shear area for ISOthread:
where t = nut height, d = diameter of shear fracturesurface
For balance between bolt tensile failure and nut
thread shear failure
If made of same material Ssy = 0.58 Sy, so
Nut is usually softer, so t = (7/8) d is standard
( )0.75A d t=
( ) ( )2
0.9 S = d 0.75 S4
t y y syF A S d d
= =
0.47t d=
7/23/2019 Threaded Fasteners Ch 10
21/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.15 (p. 404)Basic threaded fastener types.
7/23/2019 Threaded Fasteners Ch 10
22/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.16 (p. 405)Some common screw (and bolt) head types.
7/23/2019 Threaded Fasteners Ch 10
23/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.17 (p. 405)"Tamper-resistant" screw heads.
7/23/2019 Threaded Fasteners Ch 10
24/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Table 10.4 (p. 407)Specifications for Steel Used in Inch Series Screws and Bolts
7/23/2019 Threaded Fasteners Ch 10
25/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Table 10.5 (p. 408)Specifications for Steel Used in Millimeter Series Screws and Bolts
7/23/2019 Threaded Fasteners Ch 10
26/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.18 (p. 409)Bolt loads and stresses that are due to initial tightening of a nut. M= 0 for the
bolt and nut assembly shown, that is, T1 = T2 + T3 + T4 (where T1 = nut wrench
torque). T2 = nut face friction torque = fFira(where ra is the effective radius of nutface friction forces). T3 = bolt head friction torque fFirh(where rh is the effectiveradius of bolt head friction forces). T4 = wrench torque required to keep bolt headfrom turning. Note that T4 = 0 if fFirh> T1T2.
7/23/2019 Threaded Fasteners Ch 10
27/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.19 (p. 409)Bolt tension versus elongation, resulting from tightening by torquing versus directtensioning, and for black oxide versus galvanized surfaces [5]. Note: Directtension is produced by hydraulic loading; hence, no torsional stresses are
produced.)
7/23/2019 Threaded Fasteners Ch 10
28/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.20 (p. 412)Common types of lock washers.
7/23/2019 Threaded Fasteners Ch 10
29/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.21 (p. 412)(a) Slotted and (b) castle nuts. Each is also shown with a drilled bolt and cotter pin.
7/23/2019 Threaded Fasteners Ch 10
30/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.22 (p. 412)Examples of free-spinning locknuts.
7/23/2019 Threaded Fasteners Ch 10
31/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.23 (p. 413)Examples of prevailing-torquelocknuts. ( Courtesy SPSTechnologies, Inc.)
7/23/2019 Threaded Fasteners Ch 10
32/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.24 (p. 414)Free-body study of bolt tensile loading.
7/23/2019 Threaded Fasteners Ch 10
33/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.25 (p. 414)Fband Fcversus Fe per bolt for soft clamped membersrigid bolt.
7/23/2019 Threaded Fasteners Ch 10
34/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.26 (p. 415)Fband Fcversus Feper bolt for
rigid clamped memberssoft bolt.
7/23/2019 Threaded Fasteners Ch 10
35/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.27 (p. 416)Force relationships for bolted connections.
7/23/2019 Threaded Fasteners Ch 10
36/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.28 (p. 417)One method for estimating the effective area of clamped members (forcalculating kc). Effective area Ac is approximately equal to the average area of
the dark grey section.
7/23/2019 Threaded Fasteners Ch 10
37/37
Fundamentals of Machine Component Design, 4/Eby Robert C. Juvinall and Kurt M. MarshekCopyright 2006 by John Wiley & Sons, Inc. All rights reserved.
Figure 10.29 (p. 418)Examples of nonintended bolt bending.
Top Related