BOLT STRENGTH

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LECTURE 38
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BOLT STRENGTH

• The strength is specified by stating the minimum
  proof strength, or minimum proof load, and the
  minimum tensile strength
• The proof load is the maximum load (force) that a
  bolt can withstand without acquiring a permanent
  set

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• The proof strength is the quotient of the proof
  load and the tensile-stress area
• The proof strength is about 90 percent of the 0.2
  percent offset yield strength
• ASTM threads are shorter (deals mostly with
  structures) and generally loaded in shear

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TENSION CONNECTIONS-THE EXTERNAL LOAD

• A clamping force,(preload) Fi is applied by
  tightening the nut before external force, P is
  applied
• Fi preload
• P external tensile load
• Pb portion of P taken by bolt
• Pm portion of P taken by members

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• Fb Pb Fi resultant bolt load
• Fm Pm Fi resultant load on the members
• The load P is tension, and it causes the
  connection to stretch, or elongate, through some
  distance d
• and

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• Since, P Pb Pm
• The resultant bolt load is
• Fm lt 0

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Fig. 15.9
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Fig. 15.9
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Fig. 15.10
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Fig. 15.9
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Fig. 15.10
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Fig. 15.11
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• Fm lt 0
• These results are valid only as long as some
  clamping load remains in the members

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• In all cases, the members take over 80 percent of
  the external load
• Making the grip longer causes the members to take
  an even greater percentage of the external load

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TORQUE REQUIREMENTS

• A high preload is very desirable in important
  bolted connections
• If the overall length of the bolt can actually be
  measured with a micrometer when it is assembled,
  the bolt elongation due to preload Fi can be
  computed using formula

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• The nut is simply tightened until the bolt
  elongates through the distance d
• This ensures that the desired preload has been
  attained
• The elongation of a screw cannot be measured,
  because the threaded end is often a blind hole

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• The torque wrench has a built-in dial which
  indicates the proper torque
• With impact wrenching, the air pressure is
  adjusted so that the wrench stalls when the
  proper torque is obtained
• Or in some wrenches, the air automatically shuts
  off at the desired torque

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• The snug-tight condition is the tightness
  attained by a few impacts of an impact wrench
• Or the full effort of a person using an ordinary
  wrench
• When the snug-tight condition is attained, all
  additional turning develops useful tension in the
  bolt

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Turn-of-the-Nut Method

• The turn-of-the-nut method is the easiest and
  least expensive method for installing fasteners
  with the proper bolt tension.
• The procedure generally works as follows. An iron
  worker tightens the bolt and nut as tight as
  possible using a spud wrench or a pneumatic
  impact wrench

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• A chalk mark or paint is then made on the bolt
  and nut
• The bolt is tightened further by either
  hammering on the spud wrench or using a pneumatic
  impact wrench until the rotating part has rotated
  the required amount
• The paint or chalk mark shows how far the part
  has rotated and the rotation is always measured
  relative to the rotation of the bolt.

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• The turn-of-the-nut method requires that you
  compute the fractional number of turns necessary
  to develop the required preload from the
  snug-tight condition
• For heavy hexagon structural bolts, the
  turn-of-th-nut specification states that the nut
  should be turned a minimum of 180o from the
  snug-tight condition under optimum conditions

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• A good estimation of the torque required to
  produce a given preload is

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• Th diameter of the washer face of a hexagonal nut
  is the same as the width across flats and equal
  to 1.5 times the nominal size
• dc (11.5d)/2 1.25d

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• Define a torque co-efficient K
• T K Fi d
• The coefficient of friction depends upon the
  surface smoothness, accuracy, and degree of
  lubrication

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• On the average, both f and fc are about 0.15
• K 0.26 for f fc 0.15 no matter what size
  bolts are employed and no matter whether the
  threads are coarse or fine

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BOLT PRELOAD-STATIC LOADING
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• The tensile stress in the bolt can be found by
  dividing both terms of first equation by the
  tensile-stress area, At
• The limiting value of sb is the proof strength,
  Sb

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• With the introduction of a load factor n
• Any value of n gt 1 ensures that the bolt stress
  is less than the proof strength

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JOINT SEPARATION

• For safe joint, external load be smaller than
• that needed to cause the joint to separate
• If separation does not occur, then the entire
• external load will be imposed on the bolt
• Let Po be the value of the external load that
  would cause joint separation

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Fig. 15.15
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• At separation, Fm 0,
• (1 C)Po Fi 0
• Let the factor of saftey against joint separation
  be

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Fig. 8.18
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• Fig. shows stress-strain diagram of a good
  quality bolt material
• No clearly defined yield point
• Fracture at the tensile strength
• No matter how much preload is given to bolt, it
  will retain its load-carrying capacity

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• The pr-tension is the muscle of the joint and its
  magnitude is determined by the strength
• If the full bolt strength is not used, then money
  is wasted
• Good quality bolts can be preloaded into the
  plastic range to develop more strength
• A bolt will either fracture during tightening or
  not at all

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• It is recommended for both static and fatigue
  loading that the following be used for preload
• Where FP is the proof load, obtained from the
  equation FP At SP
• SP is the proof strength obtained from tables 8-4
  to 8-6

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• For other materials, an approximate value is
• SP 0.85 SY

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Fig. 8.17
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Fig. 8.18
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Fig. 8.19