Fastening

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Fastening

• Manufacturing Processes

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Outline

• Brazing
• Filler Metals and Fluxes
• Brazing Methods
• Soldering
• Adhesive Bonding
• Adhesives
• Design Consideration
• Threaded Fasteners
• Standard Screw Threads
• Thread Classes
• Rivets

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Brazing

• Brazing
• A joining process in which filler metal is
  melted between the surfaces to be joined the
  base metal is not melted

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Brazing
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Filler Metals and Fluxes

• Filler metals
• Melted between the surfaces to form the join
• Brazing fluxes
• Used to dissolve, combine with, or inhibit
  formation of oxides or other by-products it is
  displaced by the filler metal and removed after
  the operation

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Filler Metals
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Brazing Methods

• Torch brazing
• A flame is used to heat the joint, then filler
  is added
• Furnace brazing
• The parts and filler are heated in a furnace
  flux may not be needed
• Induction brazing
• Uses heat from electrical resistance to an
  induced current in the work
• Resistance brazing
• A current is supplied directly to the part to
  cause resistance heating

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Brazing Methods

• Dip brazing
• Parts are immersed in a molten salt or metal
  bath to heat them
• Infrared brazing
• Uses heat from a high-intensity infrared lamp
• Braze welding
• Applies filler metal to a conventional weld
  joint the base metal does not melt

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Soldering

• Soldering
• A filler metal is melted and distributed between
  the surfaces to be joined the base metal does
  not melt, but the filler metal combines with it
  to form a metallurgical bond

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Soldering
Soldering a wire soldering iron (heat source),
left, solder rod, right
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Examples of Soldered Parts
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Examples of Soldered Parts
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Soldering Materials
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Soldering Fluxes

• Soldering fluxes should
• be molten at soldering temperatures
• remove oxide films and tarnish
• prevent oxidization
• promote wetting of the surfaces
• be displaced readily by the filler metal
• leave residue that is not corrosive or conductive

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Adhesive Bonding
Courtesy of Air Products and Chemicals, Inc
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Adhesive Bonding

• Types of Adhesives
• 1. Natural adhesives
• starch
• animal products
• 2. Inorganic adhesives
• sodium silicate
• magnesium oxychloride
• 3. Synthetic organic adhesives
• thermoplastics (soften when heated, harden
  when cooled)
• thermosetting (harden permanently when heated)

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Adhesive Bonding
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Primers

• Primers used as metal surface conditioners
  perform these functions
• protect corrosion sensitive surfaces
• modify the properties of the bond
• tacky resins help hold or position bonded parts

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Theories of Adhesion

• Chemical bond theory
• Adhesion between adhesive and adherend is
  related to the forces holding atoms and molecules
  together
• The predicted strength of the attractive force
  is far higher than in reality however, the
  theory is valid because chemically active primers
  improve adhesion

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Theories of Adhesion

• Weak boundary layer theory
• Poor adhesion or adhesive failure is caused by
  an inclusion of low-molecular weight liquid or an
  unpolymerized liquid the liquid forms a weak
  boundary layer that prevents the proper bonding

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Theories of Adhesion

• Surface energy theory
• A liquid should have a lower surface tension
  than the critical surface tension of a solid in
  order to spread on the solid surface
• Surface contamination of the solid could lower
  the critical surface tension below the surface
  tension of the liquid, causing the bond to be
  ineffective

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Theories of Adhesion

• Inherent roughness theory
• Once the adhesive has wet and spread over the
  adherend surface, the mechanical strength of the
  resin coupled to the rough surface is the basic
  reason for the strength of the bond
• Roughness would increase the surface area in
  which electrostatic forces could operate

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Theories of Adhesion

• Polar theory
• Polar adhesives bond to polar adherends and
  nonpolar adhesives bond to nonpolar adherends
• Liquids wet and spread more effectively on solid
  surfaces with similar properties

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Common Adhesives

• 1. Epoxies
• thermosetting
• metal, glass, ceramic, wood
• two-component epoxies
• resin
• curing agent
• application time 8-12 hours
• 2. Cyanoacrylates
• liquid monomers
• application time 2 seconds

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Common Adhesives

• 3. Anaerobics
• thermosetting polyester acrylics
• almost any material
• application time6-24 hours 5 min. with
  proper additives and heat
• 4. Acrylics
• thermoplastic
• plastics, metals, ceramics, composites, even
  with dirty surfaces

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Common Adhesives

• 5. Urethanes
• one-part thermoplastic or two-part
  thermosetting
• application time solidifies in 2 min, cures
  fully in 24 hours
• 6. Silicones
• silicone thermosets
• metal, glass, paper, plastics, rubber
• cures slowly

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Common Adhesives

• 7. High-temperature adhesives
• epoxy phenolics, modified silicones or
  phenolics, polyamides, ceramics
• cures slowly
• 8. Hot melts
• thermoplastic resins
• not true structural adhesives applied as
  heated liquids and bond as they solidify

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Application Methods

• Brushing
• Performed manually coating is often uneven
• Manual rollers
• Similar to paint rollers
• Silk screening
• Brushing through open areas of a screen to coat
  selected areas
• Flowing
• Using manually operated flow guns more
  consistent control than brushing

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Application Methods

• Spraying
• Using a spray gun for fast application over
  large areas
• Automatic applicators
• Automatic dispensers and nozzles for medium and
  high production rates
• Roll coating
• A roller is partially immersed in the adhesive
  and transfers it to the part
• Pull-tab sheets
• The adhesive sheet comes prepared covered by a
  protective layer which is removed before use

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Design Consideration

• Material
• porosity, hardness, surface properties
• thermal expansion
• Assembly
• bond area
• stresses
• Temperature
• during use of parts
• temperature extremes, rate of temperature change

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Design Consideration

• Exposure
• to solvents, water, fuels, light, ultraviolet,
  acid, weathering
• Flexibility/stiffness
• Stability
• required length of time
• load
• Appearance
• Adhesive application
• Cost

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Design Consideration
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Limitations

1. Proper selection is required
2. Most are unstable above 350F (180C)
3. High-strength adhesives are often brittle
4. Long term durability and life expectancy are
   difficult to predict
5. Surface condition, adhesive preparation, and
   curing can be critical

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Limitations

1. Assembly times may be greater than other methods
2. Joint quality is difficult to evaluate
3. Some adhesives contain or produce unfavorable
   chemicals
4. Many adhesives deteriorate under certain
   conditions
5. Joints cannot be easily disassembled

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Threaded Fasteners

• Screw
• An externally threaded fastener, generally
  assembled into a blind threaded hole
• Bolt
• An externally threaded fastener inserted through
  holes in parts and screwed into a nut on the
  opposite side

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Types of Screw Heads
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Types of Screws
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Threaded Fastener Units

• U.S. Standard
• measured in inches and fractions of inches ¼,
  5/8,1-1/8 etc.
• Metric (SI)
• measured in millimeters
• A fastener specified to one measurement system
  may have similar sizes to a fastener of the other
  system, but they will not fit together properly

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Types of Screw Threads

• Coarse-thread series
• general use where not subjected to vibration
• Fine-thread series
• automotive and aircraft
• Extra-fine thread series
• thin-walled material
• Eight-thread series
• pipe flanges, cylinder-head studs
• Twelve-thread series
• not used extensively

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Types of Screw Threads

• Sixteen-thread series
• where fine threads are required
• American Acme thread
• transmitting power and motion
• Buttress thread
• Square thread
• 29 Worm thread
• American standard pipe thread
• pipe joints

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Standard Screw Threads
60
d
r
p

• Where p thread pitch, d thread depth, and r
  fillet radius,
• d .54127p
• r .14434p
• This applies to U.S. Standard and Metric bolts,
  but does not necessarily apply to screws

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Standard Screw Threads
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Standard Screw Threads
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Standard Screw Threads
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Thread Classes

• Unified system
• Class 1 special applications
• Class 2 normal production grade
• Class 3 minimum tolerances, tight fit required
• A external thread
• B internal thread

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Thread Classes

• ISO system
• e large tolerance
• g small tolerance
• H no allowance
• 3-9 tolerance grade
• Lower fine quality higher coarse quality
• Grade 6 is roughly equivalent to U.S. class 2A
  and B

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Rivets
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Rivets

• Rivet
• An unthreaded, headed pin used to join parts by
  passing the pin through holes in the parts and
  then forming a second head on the opposite side

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Rivets
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Summary

• Brazing and soldering melt only filler material
  to form a join
• Adhesive bonding uses a viriety of chemicals that
  are applied as a liquid and solidify to form a
  bond
• Screws, bolts, nuts, and rivets are applied and
  hold material together by physical means

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