THREAD FASTENERS

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THREAD FASTENERS
Chapter 10

• Two basic kinds of fasteners
• Permanent rivets, and welds
• Removable bolts, screws, studs, nuts, pins,
  rings keys

As to right and left- hand threads, threads are
assumed to be right-hand unless designated
otherwise.
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SCREW THREADS
The pitch of a thread P is the distance from a
point on he thread form to the corresponding
point on the next form, measured parallel to the
axis. The lead L is the distance the threaded
part would move parallel to the axis during one
complete rotation in relation to a fixed mating
part ( the distance a screw would enter a
threaded hole in one turn.
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THREAD REPRESENTATION

• True representation of a screw thread is seldom
  used on working drawings
• Symbolic representation of threads is a standard
  practice
• Three type of convention in general use for
  thread representation

Simplified representation used whenever it will
clearly portray the requirements Detailed
representation used to show the detail of a
screw thread for dimensioning in enlarged
views, layouts and assemblies. Schematic
representation as effective as detailed and
easier to draw basically discarded in most
countries in lieu of the Simplified
representation.
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In Simplified Thread Representation the thread
crest (that represent the diameter of the bolt)
except in hidden view, are represented by a thick
outline and the thread roots (minor diameter) are
shown by a thin broken line.
Thread root minor diameter
Simplified External Threads
Crest Diameter of bolt
Simplified Internal Threads
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INCH THREADS
In the US and Canada a great number f threaded
assemblies are still designed using inch-sized
threads
____________1______________ Number of threads
per inch
PITCH
The number of threads per inch is set for
different diameters, also called a thread series.

• Unified National system series
• UNC Unified coarse-thread
• UNF Unified fine-thread
• UNEF Unified extra-fine-thread

CAD packages have threaded fasteners library
components
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THREAD CLASS
Three classes of external thread (classes 1A, 2A
and 3A) Three classes of internal thread (classes
1B, 2B and 3B)
The classes differ in the amount of allowances
and tolerances provided in each class.
Classes 1A and 1B looses fit or greatest amount
of play (free motion) and are useful for work
where ease of assembly and disassembly is
essential like in stove bolts and other rough
bolts and nuts.
Classes 2A and 2B designed for ordinary good
grade of commercial products like machine screws
and fasteners and most interchangeable parts.
Classes 3A and 3B intended for exceptionally
high-grade commercial products, where a snug fit
is essential and the high cost of precision tools
and machines is warranted.
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THREAD DESIGNATION
Designation for inch threads is expressed in a
specific order

• Diameter (nominal or major diameter in decimal
  form with minimum of 3 maximum of 4 decimal
  places)
• Number of threads per inch,
• Thread form and series
• Class of fit (number and letter)

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METRIC THREADS

• Grouped into diameter-pitch combinations
  distinguished from one another by pitch applied
  to specific diameters
• Pitch distance between corresponding point on
  adjacent teeth
• Coarse and fine-pitch series along with series
  of constant pitches
• Coarse-Thread Series intended for use in
  general engineering work and commercial
  applications
• Fine-Thread Series for general use where a
  finer thread than the coarse-thread series is
  desirable
• Stronger in both tensile strength and torsional
  strength
• Less likely to loosen under vibration

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THREAD GRADES CLASSES
Fit of a screw thread is the amount of clearance
between the internal and external threads when
assembled.
For the two main thread elements pitch diameter
and crest diameter there are a number of
tolerance grades that are established. The number
of the tolerance grades reflects the size of the
tolerance.

• Grade 4 tolerances are smaller than grade 6
  tolerances and grade 8 tolerances are larger
  than grade 6 tolerances.
• Grade 6 tolerances should be used for
  medium-quality length-of- engagement
  applications.
• Tolerances below grade 6 are intended for
  applications involving fine quality and/or short
  lengths of engagement.
• Tolerance grades above grade 6 are intended for
  coarse quality and/or long lengths o engagement.

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Along with a tolerance grade, a positional
tolerance is required. Positional Tolerance

• defines the maximum-material limits of the pitch
  and crest diameters of the external and
  internal threads
• Indicates external and internal threads
  relationship to the basic profile

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ISO Metric Screw Thread Designation
ISO metric screw threads are defined by the
nominal size (basic major diameter) and pitch,
both expressed in millimeters.
M

• specifies an ISO metric screw thread
• precedes nominal size
• an x separates the nominal size from the
  pitch
• ONLY in coarse threadsthe pitch is not shown
  unless the dimension for the length of the
  thread is required.

A complete designation for an ISO metric screw
thread includes the basic designation, an
identification for the tolerance class. The
tolerance class designation is separated from the
basic designation by a dash and includes the
symbol for the pitch diameter tolerance followed
immediately by the symbol for crest diameter
tolerance.
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For external threads, the length of thread may be
given as a dimension on the drawing. The length
given is to be the minimum length of full
thread. For threaded holes that go all the way
through the part, the term THRU is sometimes
added to the note. If no depth is given, the
hole is assumed to go all the way through.
For threaded holes that dont go all the way
through, the depth (along with the depth symbol
or word) is given in the note, for example, M12 x
1.75 x 20 DEEP. The depth given is the minimum
depth of full thread.