Transcript Introduction - The George W. Woodruff School of Mechanical

ME3180
ME 3180 - Mechanical Engineering Design
Screws and Fasteners
Lecture Notes #1
The George W. Woodruff School of Mechanical Engineering
ME3180
Screws and Fasteners
Success or failure of design can hinge on proper selection and use of its
fasteners
Design and manufacture of fasteners are big businesses and are
significant part of our economy
Thousands to millions of fasteners are used in single complex assembly
such as automobile, ship, train or aircraft
Boeing 747 uses about 2.5 million fasteners, some of which cost several
dollars each
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ME3180
Screws and Fasteners Cont’d
Variety of fasteners are available commercially, such as shown below
FIGURE 14 – 1 (Norton)
A Sample of the Variety of
Commercially Available
Fasteners
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Screws and Fasteners Cont’d
ME3180
Discussions in this class will be limited to design, analysis and selection
of conventional fasteners such as bolts, screws, nuts, etc., used for machine
design applications in which significant loads and stresses are encountered
Screws/bolts are used both to hold things together as fasteners and to
move loads as power screws or lead screws
• Will investigate both of these applications
FIGURE 14-4 (Norton)
An Acme-Thread
Power-Screw Jack
FIGURE 14-5 (Norton)
Servomotor-Driven Lead Screw
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For UseEngineering
as a Positioning Device
ME3180
Screws and Fasteners Cont’d
Screws/bolts as fasteners can be arranged to take tensile loads, shear loads,
or both
Will study application of preloads to bolts/screw and fasteners
Preloads can have significant benefit to their load-carrying
abilities
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Design
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of Screws, Fasteners and Connections - Definitions
Definitions (See Fig. 8-1 or Fig. 14-2)
•
•
•
•
•
•
d = major diameter, largest diameter of screw thread
dr or d1 = minor diameter, smallest diameter of screw thread
At = tensile stressed area.
p = Pitch = 1/N, N = # of threads per inch
l = Lead is distance nut travels parallel to screw axis per revolution
Single threaded l = p, Double threaded l = 2p, Triple threaded l = 3p
(NORTON)
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Design of Screws, Fasteners and Connections – Types of Thread
Standards
ME3180
Types of Thread Standards (See Fig. 8-1 or Fig. 14-2)
1. Unified National Standard (UNS) thread series (inch class)
Two major unified thread series are:
• UN series
• UNR series
Difference between them:
• Root radius must be used in UNR series. Because of reduced thread
stress concentration factors, UNR series thread has improved fatigue
strengths.
• Unified threads are specified by stating nominal major diameter,
number of threads per inch, and thread series.
Example:
5''/8 - 18 UNRF or 0.625''-18 UNRF, d-N UNRF
0.1250 - 40 UNC, 0.1250 - 44 UNF
Unified thread is used in US and Britain, and has 60 thread angle. Thread
Crests may The
be flat
or rounded.
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W. Woodruff School of Mechanical Engineering
ME3180
Design of Screws, Fasteners and Connections – Types of Thread
Standards Cont’d
2. Metric type of threads (See Fig. 8-2 or Fig 14-2)
There are two types:
• M profile
• MJ profile
Differences between them
• M profile is replacement of inch class and is basic ISO (International
Standard Organization) 68 profile with 60 symmetric threads
• ISO 68 is European standard not interchangeable with UNS
• MJ profile has rounded fillet at root of external threads and larger
minor diameter for external and internal threads.
Useful where high fatigue strength is required.
Metric threads are specified by writing the major diameter and pitch in
millimeters M12 x 1.75
The George W. Woodruff School of Mechanical Engineering
Design of Screws, Fasteners and Connections – Types of Thread
Standards Cont’d
ME3180
Square thread, Buttress and ACME threads (See Fig 14-3)
• Used on screws when power is to be transmitted.
FIGURE 14-3 (NORTON) – Square, Acme, and Buttress Threads
Multiple Threads (Fig 15-1)
Multiple threads help nut to latch onto bolt quicker than single threads
would.
It will take single thread one complete turn, double thread one half turn (2The thread
George one
W. Woodruff
of Mechanical
start), triple
third turnSchool
(3-start)
to advanceEngineering
the nut a distance p.
ME3180
Screws and Fasteners
Primary focus here is on fasteners. Since completion of manufacturing
intricate parts often requires assembly of components, engineers are then
confronted with task of fastening various members together. Number of
fastener types can be used for this task including:
• thread fasteners
• riveted joints
• welded joints
• adhesive joints
Interesting information about fastener materials indicate that bolt, rivet, or
welded material should be strong and tough whereas members being
fastened should ideally be soft and ductile.
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ME3180
Screws and Fasteners Cont’d
Common element among screw fasteners is their thread. In general, thread is
helix that causes screw to advance into the work-piece or nut when rotated.
Threads may be external (screw/bolt) or internal (nut or threaded hole). After
World War II, threads were standardized in Great Britain, Canada and United
States to what is now called Unified National Standard (UNS) series as shown
in Figure 14.2.
thread angle
2α
dm
(NORTON)
45o chamfer angle
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ME3180
Screws and Fasteners Cont’d
European standard is also defined by International Standard Organization (ISO) and
has same thread cross-sectional shape, but uses metric dimensions, so is not
interchangeable with UNS threads. Both UNS and ISO threads are in general
used in United States. Both use 60 degrees included angle and define thread size
By nominal thread (major) diameter d of external thread. Pitch p shown in Figure
15.1 is distance from one point on one thread to same point on an adjacent thread,
and its unit is meters or inches. Parameter that can be used instead of pitch is
threads per inch, n. Important relationship between pitch and threads per inch is
P = 1/n
dc = crest diameter.
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Figure 15.1 Parameters used in defining terminology of thread profile.
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Screws and Fasteners Cont’d
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Screws and
Fasteners Cont’d
The George W. Woodruff School of Mechanical Engineering
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Screws and Fasteners Cont’d
The George W. Woodruff School of Mechanical Engineering
Screws and Fasteners Cont’d
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Crests and roots are defined as flats to reduce stress concentration from
that of sharp corner. Pitch diameter dp and root diameter dr are defined in
terms of thread pitch p with slightly different ratios used for UNS and ISO
threads.
Lead l of thread is distance that screw would advance relative to the nut in one
revolution. Figure 15.2 shows differences between single-, double-, and
triple- threaded screws. If it is single thread, as shown in Figure 15.2a, l
= p. Screws can also be made with multiple threads, also called multiplestart threads. Double thread (2-start) has two parallel grooves wrapped
around diameter, like pair of helical “railroad tracks,” as shown in Figure
15.2b. In this case l = 2p. Triple thread (3-start) will have l = 3p, etc.
Figure 15.2
(a) Single-,
(b) double-, and
(c) triple-threaded
screws.
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ME3180
Screws and Fasteners Cont’d
Advantages of multiple threads are smaller thread height and increased lead
for fast advance of the nut. Some automotive power-steering screws use
5-start threads. However, most screws are made with only single thread
(1-start). Fishing reels use 2-start threads.
Different number of thread profiles can be used for wide variety of
applications. Figure 15.3 shows two types. ACME thread profile is used
for power screws and machine tool threads. ACME profile has thread
angle of 29 degrees, whereas UNS profile has thread angle of 60
degrees. UNS, is used extensively today. Metric profile (M) is popular and
is quite similar to the UNS profile.
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ME3180
Screws and Fasteners Cont’d
Figure 15.4 shows more details of M and UNS thread profiles that are
shown in Figure 15.3. from this figure
ht = 0.5p/tan 30 = 0.8660p
Once pitch p and largest possible thread height ht are known, various
dimensions of the UNS and M thread profiles can be obtained.
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ME3180
Tape Handling Mechanism
Tape handling mechanism
is used to move 8mm data
tapes between tape storage bins
and tape reader.
Mechanism is from a computer
data backup device
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ME3180
Tensile Stress Area
If threaded rod is subjected to pure tensile loading, one might expect its
strength to be limited by area of its minor (root) diameter, dr. However,
testing of threaded rods in tension shows that their strength is better defined
by the average of minor and pitch diameters.
Tensile-stress area At is defined as:
 d d
At 
4
(
p
r
2
)2
Where for UNS threads: dp = d – 0.649519/N
dr = d – 1.299038/N
and for ISO threads:
dp = d – 0.649519P
dr = d – 1.226869P
d = Outside diameter (major diameter)
N = Number of threads per inch
P = Pitch in mm
Stress in a threaded rod due to pure axial tensile load F is then:
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F
t 
At
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Standard Thread Dimension
Table 14-1 shows the principal directions of UNS threads and Table 14-2
shows the same for ISO threads. UNS threads smaller than 0.25-in diameter
are designated by a gage number. A useful algorithm for determining the
major diameter of numbered threads is to multiply the gage number by 13 and
add 60. The result is its approximate major diameter in thousandth of an inch.
The minor diameter = major diameter – pitch.
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ME3180
The George W. Woodruff School of Mechanical Engineering
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The George W. Woodruff School of Mechanical Engineering
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The George W. Woodruff School of Mechanical Engineering
ME3180
UNS and ISO Standard Thread Series
UNS has three standard series of thread pitch families: coarse-pitch
(UNC), fine-pitch (UNF), and extra-fine-pitch (UNEF).
ISO has two standard thread series: Coarse and fine series of threads
Coarse Thread Series:
Most common
Recommended for ordinary applications, especially where
• Repeated insertions and removals of screws are required
• Where screw is threaded into soft material
Coarse threads are less likely to cross or strip soft material on insertion.
Fine Threads Series:
More resistant to loosening from vibration than coarse threads because of
their smaller helix angle and so are used in automobiles, aircraft, and
other applications that are subject to vibration
Extra Fine Thread Series
Used where wall thickness is limited and their short threads are an
advantage
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ME3180
UNS and ISO Thread Standards Cont’d
Tolerance ranges for internal and external threads used to
control their fits
UNS Series:
There are three fit classes, labeled Classes 1, 2, and 3
Class 1 has the broadest tolerances and is used for “hardware quality” (i.e.,
inexpensive) fasteners intended for casual use around home, etc.
Class 2 defines closer tolerances for better quality fit between mating
threads and is suitable for general machine design applications.
Class 3 is highest precision and can be specified where closer fits are
needed.
Cost increases with higher class of fit. Letter designator indicates an external
(A) or internal (B) thread.
The George W. Woodruff School of Mechanical Engineering
ME3180
UNS and ISO Thread Standards Cont’d
Example of UNS Thread Specifications
¼ -20 UNC – 2A -> 0.250 in major diameter by 20 threads per inch,
coarse series, Class 2 fit, external thread
Example of ISO (Metric) Thread:
M8 X 1.25 -> 8-mm diameter by 1.25-mm pitch thread in ISO coarse
series
All standard threads are right-hand (RH) by default unless specified as
left-hand by adding LH to specification.
Right-hand thread will advance the nut (or screw) away from you when
either is turned clockwise.
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ME3180
Power Screws
Power screw is device used to change angular (rotational) motion into
linear motion, and it is used in lathes, and screws for vices, presses, and
jacks.
• Usually to transmit power
• Also called lead screws
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ME3180
Power Screws Cont’d
More specifically, power screws are used:
1. To obtain greater mechanical advantage in order to lift weight, as in
screw type of jack for cars
2. To exert large forces, as in home compactor or press
3. To obtain precise positioning of axial movement, as in micrometer
screw or lead screw of lathe
In each of these applications, torque is applied to ends of screws through
set of gears, thus creating load on the device.
Fasteners we have discussed thus far may not be strong enough for all
power screw applications.
Power screws use the following thread profiles: square, ACME and
buttress.
FIGURE
14-3 (NORTON)
Square,
Acme, andEngineering
Buttress Threads
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W. Woodruff–School
of Mechanical
Power Screws Cont’d
ME3180
Square Thread
• Provides greatest strength and efficiency (than other two) and
eliminates any radial component of force between screw and nut.
• More difficult to cut because of its perpendicular face
• Modified square thread (not shown) is made with a 10 degrees included
angle to improve its manufacturability.
ACME Thread
• Has 29 degrees included angle, making it easier to manufacture and
also allowing use of split nut that can be squeezed radially against
screw to take wear
• Common choice for power screws that must take loads in both
directions.
Tensile stress area is:
d d
At 

4
(
p
r
2
)2
The George W. Woodruff School of Mechanical Engineering
ME3180
Power Screws Cont’d
Table 14.3 and Figure 15.5 should be used jointly in evaluating ACME
power screws.
The pitch diameter of ACME power screw thread is:
dp = dc – 0.5p – 0.01
This equation is valid only when inches are used for dc and p.
Buttress Thread
• If axial load on screw is unidirectional, buttress thread can be used to
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obtain greater strength at root than either of other two.
ME3180
The George W. Woodruff School of Mechanical Engineering
ME3180
Power Screw Application
Figure 14-4 shows one possible arrangement of power screw used as jack
to lift load.
Nut is turned by applied torque T and screw translates
up to lift load P or down to lower it.
There needs to be some friction at load surface to
prevent screw from turning with load.
In either case, there will be significant friction between
screw and nut as well as between nut and base,
requiring that thrust bearing be provided as shown
If plain (i.e., non-rolling) thrust bearing is used, it is
possible for bearing interface to generate larger
friction torque than threads.
FIGURE 14-4
An Acme-Thread
The
Power-Screw Jack
Ball-thrust bearings are often used in this application
to reduce these losses.
George W. Woodruff School of Mechanical Engineering
ME3180
Other Applications of Power Screws
In linear actuators, which operate on same principle as shown in Figure 144, either motorize nut rotation to translate screw or motorize screw
rotation to translate nut, shown in Figure 14-5.
FIGURE 14-5
Servomotor-Driven Lead Screw
for Use as a Positioning Device
These devices are used in machine tools to move table and workpiece under
cutting tool, in assembly machines to position parts, and in aircraft to move
control surfaces as well as in many other applications.
If rotation input is provided by servomotor or stepping motor in combination
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with precision lead screw, very accurate positioning can be obtained.
ME3180
Power Screw Force and Torque Analysis
This section is copied from pages 897 through pages 904 of Machine Design - An
Integrated Approach, by Robert L. Norton, published by Prentice Hall, 1996.
Note: Material covered in pages 400-403 in Shigley
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The George W. Woodruff School of Mechanical Engineering
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(for a square thread)
*
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*
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(8-1)
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(8-6)
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*
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14.5o
*
*
The George W. Woodruff School of Mechanical Engineering
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General Expression
The George W. Woodruff School of Mechanical Engineering
ME3180
*
Note:
total torque including
collar
torque
The George W. Woodruff School of Mechanical
Engineering
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Has a larger torque
than 14.5b
ACME
Square
*
*
The George W. Woodruff School of Mechanical Engineering
ME3180
The George W. Woodruff School of Mechanical Engineering
ME3180
The George W. Woodruff School of Mechanical Engineering
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The George W. Woodruff School of Mechanical Engineering
ME3180
The George W. Woodruff School of Mechanical Engineering