Transcript Mechanical Methods of Joining - College of Engineering | SIU

Mechanical Methods of
Joining
Chapter 15
IT 208
Chapter 15
1
Competencies



Identify the unique applications for fasteners
Identify the different types of fasteners for shafts
Translate the nomenclature used to identify a
thread
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Chapter 15
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FASTENERS
Needs of the Application
 Two needs of the application that must be
determined are:
• Is the joint temporary or permanent?
• Is relative movement permitted?
IT 208
Chapter 15
3
Threaded Fasteners
Use when the application requires a nonpermanent
fastening.
Threads
•
•
•
•
•
The most common thread is the standard 60° vee-thread
(screws)
Acme threads are used when high strength is required.
(vices)
Buttress thread is used for applications that require a high
force in one direction.
Square threads are applicable for precision positioning.
Knuckle and round threads are often used on glass and
rolled metal applications.
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Threaded Fasteners

Left & Right Handed Threads
• As a threaded part is turned clockwise, it moves
either away from you (a right-hand thread) or
toward you (a left-hand thread).

Thread Pitch
• All screws come in standard diameters and
number of threads per inch (Table 15-1). The
distance between two adjacent peaks or crests on
the thread is the pitch.
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Threaded Fasteners

Two Major Classes of Thread Pitch
• Unified National Coarse (UNC)
• The coarse thread has more resistance to
“stripping” the threads out, but less resistance to
shearing the bolt due to over torquing
• Unified National Fine (UNF)
• The fine thread has more threads per inch; it is
used to make fine adjustments at the expense of
strength.
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Threaded Fasteners
Different Types of Screws
 Machine Screws
•


commercially available with approximately 12 standard
head shapes and with several standard finishes.
Wood screws
•
have a tapered thread so as to wedge their way into the
grain of the wood.
Sheet metal screws
•
are a cross between machine screws and wood screws:
They have a tapered thread at the tip, but a straight thread
for most of their length.
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Specifying a Thread
Example: ¼ - 20 UNC2A – LH – Hex Hd. Cap Screw x 2
The translation of this is:
¼ = ¼-inch nominal diameter
20 = 20 threads per inch
UNC = Unified National Coarse series
2 = class 2 fit
A = external thread
LH = left-hand thread (omit this if right-hand thread)
Hex Hd. = hexagonal head
Cap Screw = type of fastener
2 = 2-inch bolt length
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Grades of Bolts
Bolts are specified and made in several grades depending
on the strength and hardness of the bolt. .
Grades run from 0 to 8 with grade 0 being the softest and
weakest.
•
•
•
A grade 0 bolt has no proof load or specified hardness.
Grade 3 and grade 5 bolts are available at most hardware
stores.
Grade 8 bolts have proof loads of a minimum of 120,000
psi.
Figure 15-4 shows the markings of some of these bolts.
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NONTHREADED FASTENERS
Removable with a small effort:
 Not easily removable:
Nails

• Nails are an excellent choice when the application
•
requires the fastener to withstand considerable
shearing force but very little tension.
Classified as temporary fasteners and work in
only a few materials (wood, mainly)
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NONTHREADED FASTENERS
Staples
• act like two nails side by side, so they solve the
revolution problem mentioned earlier.
Pins
• they all perform the function of resisting shearing
forces.
Rivets
• are pins that have a thick diameter compared to
their length. They are used (in tension) to hold
two parts tightly against each other.
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NONTHREADED FASTENERS
Fasteners for Shafts
What are the needs of the application?
• There is a major need to transfer power from the
•
motor to the pulley.
The pulley should not work its way off the end of
the shaft.
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NONTHREADED FASTENERS
Keys, Keyseats, and Keyways
• A keyseat is a slot that is cut into the motor shaft.
• The keyway is cut into the hole through the
center of the pulley.
Then the slots are lined up and the key is fitted into
both slots simultaneously.
• Splines are toothed external and internal
connectors that allow for longitudinal (back and
forth) motion on a shaft.
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