Chapter 10 Threaded Fasteners

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Engineering Drawing and Design
Sixth Edition
Chapter 10
Threaded Fasteners
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Chapter 10Threaded Fasteners
PhotoDisk
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10-1 Simplified Thread Representation

• How a fastener functions
• Types of threads

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Definitions

• Screw thread
• A ridge of uniform section in the form of a helix
  on the surface of a cylinder.
• Pitch
• The distance from a point on the thread form to
  the corresponding point on the next form.
• Lead
• The distance the threaded part would move
  parallel to the axis during one complete rotation
  in relation to a fixed mating part.

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Single and multiple threads

• A single thread has a single ridge in the form of
  a helix.
• A double thread has two ridges started 180?
  apart.
• A triple thread has three ridges, started 120?
  apart.

The Studio Dog
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Relationship between lead and pitch

• In single threads, the lead is equal to the pitch.

• In double threads, the lead is equal to twice the
  pitch.

• In triple threads, the lead is three times the
  pitch.

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Quick Quiz
?

• What term refers to the distance a threaded part
  would move parallel to the axis in one rotation?

• Lead

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Inch threads

• In the inch thread system, pitch is equal to one
  divided by the number of threads per inch.

pitch
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Inch-size thread series

• Thread series refers to standard number of
  threads per inch set for different diameters.
• They include
• UNC A course-thread series
• UNF A fine-thread series
• UNEF An extra-fine thread series
• and constant pitch threads, with a constant
  number of threads per inch
• 8-thread series
• 12-thread series
• 16-thread series

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Inch thread classes

• Thread Class refers to sets of standard
  tolerances and allowances
• Classes 1A and 1B
• Loose fit for ease of assembly
• Classes 2A and 2B
• Ordinary good grade of commercial products, like
  machine screws and fasteners
• Classes 3A and 3B
• Snug fit, used for high-grade commercial products

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Metric threads

• Grouped into diameter-pitch combinations
• Pitch is expressed as the distance between
  corresponding points on adjacent teeth.

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Metric thread series

• Course-thread series
• For use in general engineering work and
  commercial applications
• Fine-thread series
• Finer and stronger than course-thread, and less
  likely to loosen under vibration

PhotoDisk
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Metric thread grades

• Grade numbers reflect the size of tolerance in
  that grade
• Grade 4 tolerances are smaller than grade 6
  tolerance
• Grade 6 Medium-quality length of engagement
• Grades higher than 6 Course quality and/or long
  length of engagement
• Grades lower than 6 Fine quality and/or short
  length of engagement

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Quick Quiz
?

• Does the term UNF refer to a grade, class or
  series?

• Series

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10-2 Detailed and Schematic Thread Representation

• Detailed representation of V threads, square
  threads, and acme threads
• Threaded assemblies
• Schematic representation

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Drawing detailed representation of V threads
exterior
interior
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Drawing detailed representation of V threads

• Step 1
• 1. Establish the pitch and half-pitch at the
  major diameter.

• 2. Measure intervals the size of the pitch along
  the major diameter from the first two marks.

• 3. Add crest lines.

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Drawing detailed representation of V threads

• Step 2
• 1. Add the V profile for one thread, top and
  bottom, locating the root diameter.

• 2. Add construction lines for the root diameter.

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Drawing detailed representation of V threads

• Step 3
• 1. Add one side of the remaining Vs (thread
  profile).

• 2. Add the other side of the Vs, completing the
  thread profile.

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Drawing detailed representation of V threads

• Step 4
• Add the root lines to complete the detailed
  representation of the threads.

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Schematic thread representation

• Staggered lines perpendicular to the axis of the
  thread represent the thread root and crests.
• Spacing between root and crest lines is any
  convenient size.
• Root lines are any convenient length.

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10-3 Common Threaded Fasteners
Types of fasteners Head and point styles
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Fasteners

• Machine screw
• Used in tapped hole or with a nut

• Cap screw
• Passes through a clearance hole in one part and
  screws into a tapped hole in a second part

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Fasteners

• Captive screw
• Remains attached to parent material even when
  mating part is disengaged

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Fasteners

• Bolt
• Passes through a clearance hole and threads into
  a nut

• Stud
• A shaft threaded at both ends used in assemblies

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Fastener head styles
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Point styles
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Quick Quiz
?

• What is the term for a shaft that is threaded on
  both ends?

• Stud

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Drawing a bolt and nut

• Bolts and nuts are not normally drawn on detail
  drawings
• Some assembly drawings may require showing nuts
  and bolts
• Templates are recommended for manual drafting

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Washer functions

• Span an oversize clearance hole
• Give bearing for nuts or screw faces
• Distribute loads over a greater area

• Act as locking devices for threaded fasteners

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Washer functions

• Maintain spring-resistance pressure
• Protect surfaces from marring
• Provide a seal

• Types of washers are shown on pages 307-308 in
  textbook.

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10-4 Special Fasteners

• Setscrews

Locknuts
Self-retaining nuts
Sealing fasteners
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Setscrews

• Used as semipermanent fasteners
• Hold a collar, sheave, or gear on a shaft against
  rotational or translational forces

J.W. Winco Inc.
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Locknut

• Internally grips a threaded fastener
• Prevents rotation

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Captive or self-retaining nuts

• A permanent, strong, multiple-threaded fastener
  for use on thin materials

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Sealing fasteners

• Seal gases and liquids against leakage

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Quick Quiz
?

• Whats another name for a self-retaining nut?

• Captive nut

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10-5 Fasteners for Light-Gage Metal,
Plastic, and Wood

• Types of tapping screws

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Tapping screw

• Cuts or forms a mating thread when driven

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Special tapping screws include

• Self-captive screws
• Combine coarse-pitched starting thread with finer
  pitch farther along the screw shank
• Double-thread combinations
• Sealing tapping screws