ME350 Static Machine Components

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ME 350 Static Machine Components
Lecture Note 12 Screws and Fasteners -- II Dr.
Y.B. Guo Mechanical Engineering The University of
Alabama
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Todays Lecture

• Fastener Stiffness
• Bolt Strength
• Connection Mechanics

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Fastener Stiffness

• Bolted Connection
• Clearance
• Bolt in tension
• Parts in compression/preload

(a) bolt
(b) Cap screw
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Fastener Stiffness

• Bolted Connection
• Grip l total thickness of parts
• Threaded length LT
• Length of unthreaded portion
• Length of useful threaded portion

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Fastener Stiffness

• Bolted Connection
• Effective grip l'
• Threaded length LT
• Length of unthreaded portion
• Length of useful threaded portion

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Fastener Stiffness

• Fastener Stiffness
• Two Portions threaded unthreaded
• Two Springs in Series
• Spring Stiffness Force/Deflection

k EA/L
lt ld
Tensile-stress area (Table 8-1, 8-2)
Threaded length Major diameter
Unthreaded length
At Ad
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Fastener Stiffness

• Member Stiffness
• Members (2 or more) in series
• If one member is a soft gasket,
• Other cases, Pressure-Cone method

EG ltlt E1, E2,
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Fastener Stiffness

• Member Stiffness One-Sided Frustum

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Fastener Stiffness

• Member Stiffness Two-Sided Frustum

Hexagonal head bolts cap screws
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Bolt Strength

• Proof Load PL
• The max. load that a bolt withstand without
  acquiring a permanent set (plastic deformation)
• Proof Strength SP
• The quotient of the proof load and the
  tensile-stress area
• Tensile Strength Sut
• The max. load at necking
• SP lt Sut

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Connection Mechanics

• External Load
• Fi preload
• P external load
• Pb portion of P taken by bolt
• Pm portion of P taken by members
• Fb Pb Fi resultant bolt load
• Fm Pm - Fi resultant member load
• C fraction of external load P taken by bolt
• 1-C fraction of external load P taken by member

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Connection Mechanics

• Bolt Torque

Since tanl l/p dm ,
Not l, Error in Eq. (b), page 423
Hexagonal bolts inner diameter d, outer diameter
1.5d
Mean collar diameter,
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Connection Mechanics

• Bolt Torque

Torque Requirement
T for Desired Preload, Fi
K Torque Coefficient
Generally, m mc 0.15 and
K ? 0.20
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Connection Mechanics

• Load Factor against Bolt Plastic Deformation
• Load Factor against Joint Separation
• Preload

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Connection Mechanics

• Example A 0.75-16 UNF2.5 SAE grade 5 bolt is
  subjected to a load P of 6 kip in a tension
  joint. The initial bolt tension is Fi 25 kip.
  The bolt and joint stiffness are kb 6.5 and
  km13.8 Mlb/in, respectively
• (a) Determine the preload and service load
  stresses in the bolt

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Connection Mechanics

• Example A 0.75-16 UNF2.5 SAE grade 5 bolt is
  subjected to a load P of 6 kip in a tension
  joint. The initial bolt tension is Fi 25 kip.
  The bolt and joint stiffness are kb 6.5 and
  km13.8 Mlb/in, respectively
• (b) Find the torque necessary to develop the
  preload