Basic Mechanical Fasteners part II

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BasicMechanical Fastenerspart II

• TED126

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Types mechanical fasteners Threaded

• Two very basic types of machine-threaded
  fasteners are
• nuts bolts and screws.
• Their great advantages include the
• ease of assembly, which generally requires no
  special equipment, as well as
• the possibility to de- and re-assemble without
  (much) damage to the joint.
• They may be used for most materials.

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Types mechanical fasteners Threaded

• Screws and bolts are made in a wide range of
  materials, with steel being perhaps the most
  common, in many varieties.
• Where great resistance to weather or corrosion is
  required, stainless steel, titanium, brass or
  bronze may be used, or a coating such as brass,
  zinc or chromium applied.
• Electrolytic action from dissimilar metals can be
  prevented with aluminum screws for double-glazing
  tracks...
• Some types of plastic, such as nylon or teflon,
  can be threaded and used for fastening requiring
  moderate strength and great resistance to
  corrosion or for the purpose of electrical
  insulation.

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Types mechanical fasteners Threaded

• Screws and bolts are usually in tension when
  properly fitted.
• In most applications they are not designed to
  bear large shear forces.
• for example, when two overlapping metal bars
  joined by a bolt are likely to be pulled apart
  longitudinally,
• the bolt must be tight enough so that the
  friction between the two bars can overcome the
  longitudinal force.
• If the bars slip, then the bolt may be sheared in
  half, or friction between the bolt and slipping
  bars may erode and weaken the bolt (called
  fretting).
• For this type of application, high-strength steel
  bolts are used and these should be tightened with
  a torque wrench.

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Types mechanical fasteners Threaded

• High-strength bolts usually have a hexagonal head
  with an ISO strength rating (called property
  class) stamped on the head.
• The property classes most often used are 8.8 and
  10.9.
• The number before the point is the tensile
  ultimate strength in MPa divided by 100.
• The number after the point is 10 times the ratio
  of tensile yield strength to tensile ultimate
  strength.
• For example, a property class 5.8 bolt has a
  nominal (minimum) tensile ultimate strength of
  500 MPa, and a tensile yield strength of 0.8
  times tensile ultimate strength or 0.8(500) 400
  MPa.
• Tensile yield strength is M10, property class 8.8
  bolt can very safely hold a static tensile load
  of about 15 kN

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Types mechanical fasteners Threaded

• A screw is a shaft with a helical groove or
  thread formed on its surface.
• Its main uses are as a threaded fastener used to
  hold objects together, and
• as a simple machine used to translate torque into
  linear force.
• It can also be defined as an inclined plane
  wrapped around a shaft.
• Screws can normally be removed and reinserted
  without reducing their effectiveness.
• They have greater holding power than nails and
  permit disassembly and reuse.

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Types mechanical fasteners

• When screws and bolts cannot be used,
• nailing,
• riveting,
• roll pins,
• pinned shafts,
• welding,
• soldering,
• brazing, and
• gluing are some alternatives.

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Types mechanical fasteners Threaded

• Nuts bolts are generally used with
• pre-drilled holes.
• In the case of a nut and bolt, a simple hole will
  do.
• Access from both sides of the component during
  assembly is generally required,
• although this may not be necessary if captive or
  welded nuts can be used.
• Because of their relatively high cost, nuts
  bolts are mainly used for thicker sections, where
  they are a well-established technique.

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Types mechanical fasteners Threaded

• Nuts bolts
• Typical use is for structural assembly in a wide
  range of engineering applications, particularly
  where high strength is required.
• They can also be found in smaller form in for
  example components where de- and re-assembly may
  be required.

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Types mechanical fasteners Threaded

• Screws
• Self-tapping or thread-forming screws, on the
  other hand, require no nuts or tapped holes.
• Mostly used with pre-drilled holes although
  self-drilling screws are available,
• the screw forms a thread in the materials being
  joined when inserted, avoiding the need for
  tapping of the hole or for access to both sides.
• Flow drilling (causing the material around the
  hole to be extended beyond the normal material
  thickness) usually provides enough material for
  thread engagement, although if required an
  additional nut or clip may be used.

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Types mechanical fasteners Threaded

• Screws
• Self-drilling screws may be used without the need
  for pre-drilled holes.
• In thin materials, a screw with a special tip can
  be used to flow drill the hole in the material,
  providing additional thread engagement.

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Basic Types mechanical fasteners Threaded

• Machine Bolt - a bolt with a square or hexagonal
  head on one end and a threaded shaft on the other
  end tightened with a wrench used to connect
  metal parts.
• Lag bolt / screw - refers to a large wood screw
  with a hexagonal head, driven by a wrench rather
  than a screwdriver.
• Hanger Bolts - have a machine thread on one end,
  wood threads on the other. These bolts are
  frequently found in furniture.
• Stove Bolts - similar to a carriage bolt, but
  usually used in metal. It requires a square hole
  in the metal being bolted to prevent the bolt
  from turning.

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Basic Types mechanical fasteners Threaded

• Carriage Bolts - has a domed or countersunk head,
  and the shaft is topped by a short square section
  under the head. The rib neck carriage bolt has
  several longitudinal ribs instead of the square
  section, to grip into a metal part being fixed.
• Machine screws - has a cylindrical shaft,
  threaded its entire length, and fits into a nut
  or a tapped hole.
• Traditional Wood Screw - has a tapered shaft,
  allowing it to penetrate un-drilled wood.
• Self-tapping Screw - Sheet metal - have sharp
  threads that cut into a material such as sheet
  metal or plastic. They are sometimes notched at
  the tip to aid in chip removal during thread
  cutting.

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Nine Thread Types mechanical fasteners Threaded

• Sharp V thread
• not used much as a thread design.
• American National
• similar as a sharp but stronger.
• Unified Thread Standard UTS (1949)
• current standard.unified with the U.S., United
  Kingdom and Canada. UNF, UNC, UNEF etc.
• Square
• this form is used for power/force transmission
  i.e. linear jacks, clamps. The friction is low
  and there is no radial forces imposed on the
  mating nuts. The square thread is the most
  efficient conventional power screw form. It is
  the most difficult form to machine. It is not
  very compatible for using split nuts-as used on
  certain machine tool system for withdrawing the
  tool carriers. Wood vise.

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Nine Thread Types mechanical fasteners Threaded

• ACME
• Used for power transmission i.e. lathe lead
  screws. Is easier to manufacture compared to a
  square thread. It has superior root strength
  characteristics compared to a square thread. The
  acme screw thread has been developed for machine
  tool drives. They are easy to machine and can be
  used with split nuts.
• Whitworth
• Most, if not all, British cars, motorcycles,
  airplanes and machinery up to the 1970s and
  possibly beyond used Whitworth thread forms.
• Worm
• simply a screw, captured in place and running
  in bearings, that accepts the input motion. Its
  threads engage the teeth of the worm wheel which
  is basically a disk with partial screw threads
  running around its circumference. Worm is widely
  used for speed reduction or increase, torque
  multiplication and resolution, and accuracy
  enhancement for positioning systems.

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Nine Thread Types mechanical fasteners Threaded

• Knuckle
• is usually rolled from sheet metal or cast, and
  it is used for light-bulb bases, bottle caps, and
  glass jars.
• Buttress
• A strong low friction thread. However it is
  designed only to take large loads in on
  direction. For a given size this is the
  strongest of the thread forms. When taking heavy
  loads on the near vertical thread face this
  thread is almost as efficient as a square thread
  form. Car jacks.

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Basic Types of Heads mechanical fasteners

• FLAT AND OVAL HEADS (UNDERCUT) The standard flat
  or oval head 80 to 82 counter sunk screw will
  fit a standard counterbored hole and is
  particularly adaptable to flush assemblies in
  thin stock.
• ROUND HEAD Not recommended for new design (see
  pan head). This head was the most universally
  used design in the past.
• BINDING HEAD (Straight Side) Most generally used
  in electrical and radio work because of its
  identifying undercut beneath the head, which
  binds and eliminates fraying of stranded wire. A
  medium - low head with ordinarily sufficient
  bearing surface. Not recommended as a Phillips
  Recessed head see Pan Head for better
  functional design.

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Basic Types of Heads mechanical fasteners

• HEXAGON HEAD (TRIMMED) This is the standard type
  of wrench-applied hexagon head, characterized by
  clean, sharp corners trimmed to close tolerances.
  Recommended for general commercial applications.
• SQUARE SHOULDER SCREWS An adaptation of the
  standard carriage bolt design. Possesses a truss
  head on a square shank, which resists rotation
  when located or driven into place.

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Basic Types of Drivesmechanical fasteners

• Standard / Straight / Slot head has a single
  slot, and is driven by a flat-bladed screwdriver.
  The slotted screw is common in woodworking
  applications, but is not often seen in
  applications where a power driver would be used,
  due to the tendency of a power driver to slip out
  of the head and potentially damage the
  surrounding material.
• Phillips screw drive has slightly rounded corners
  in the tool recess, and was designed so the
  driver will slip out, or cam out, under high
  torque to prevent over-tightening.

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Basic Types of Drivesmechanical fasteners

• Torx is a star-shaped "hexalobular" drive with
  six rounded points. It was designed to permit
  increased torque transfer from the driver to the
  bit compared to other drive systems.
• TORX is very popular in the automotive and
  electronics industries due to resistance to cam
  out and extended bit life, as well as reduced
  operator fatigue by minimizing the need to bear
  down on the drive tool to prevent cam out.
• Square drive prevents slippage even under maximum
  torque, and the screws have positive holding
  power in a variety of materials.

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Basic Types of Drivesmechanical fasteners

• Allen / Hex Key There are six contact surfaces
  between bolt and driver and torque is constrained
  by the length and thickness of the key.
• Clutch Type A or standard clutch head resembles a
  bow tie. These were common in GM automobiles of
  the 1940s and '50s, particularly for body panels.
• Clutch Type G head resembles a butterfly. This
  type of screw head is commonly used in the
  manufacture of mobile homes and recreational
  vehicles

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mechanical fasteners Installing a wood screw

• Two holes are drilled
• Pilot hole
• First hole drilled through completely through
  first piece and into the second.
• Shank hole
• Larger drill bit - drilled completely through the
  first piece of wood.
• If needed a countersink is used to put the flat
  head screws flush with the materials surface.
• If using a 4 screw, pilot hole for hard wood is
  1/16 and shank hole would be 7/64. Softwood
  pilot would be 3/64 and shank hole is the same
  as the hard wood.

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mechanical fasteners Sizes

• Machine Screws and Bolts
• 1/4 x 20
• First number is the diameter
• Second is the number of threads per inch
• Wood Screws
• 6 x 3
• First number is the diameter
• Second is the length of the screw
• After the size you will have three letters
  indicating the head.
• THE END