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Fastening (more complex shapes = better function). Non-permanent Bolted Permanent Bolted Welded Bonded. Outline. General Thread Nomenclature & Types Power Screws Stresses in Threads Preloading Fasteners/Joints Fasteners in Shear. Threads. p pitch in./thread d diameter (major) in.
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Fastening (more complex shapes = better function) • Non-permanent • Bolted • Permanent • Bolted • Welded • Bonded
Outline • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Preloading Fasteners/Joints • Fasteners in Shear
Threads p pitch in./thread d diameter (major) in. dp pitch diameter in. dr minor diameter in. L Lead in.
UNC –coarse UNF –fine UNEF –extra fine coarse fine Thread Pitch Class 1 Class 2 Class 3 several levels Tolerance d=12mm metric fine Class 2 d=0.25” p=1.75 mm/thread external threads 20 threads/in. Screw Classifications ISO (Metric) Unified National Standard ¼-20 UNF –2A M12 x 1.75 **see Tables 14-1 and 14-2 for standard sizes**
At also in Tables 14-1 and 14-2 Tensile Stress F F
Outline • General Thread Nomenclature & Types • Power Screws • Threads • Loads • Self-locking • Efficiency • Stresses in Threads • Preloading Fasteners/Joints • Fasteners in Shear
Power Screw Applications Where have you seen power screws? • jacks for cars • C-clamps • vises • Instron material testing machines • machine tools (for positioning of table)
Power Screw Types • Square • strongest • no radial load • hard to manufacture • Acme • 29° included angle • easier to manufacture • common choice for loading in both directions • Buttress (contrafuerte) • great strength • only unidirectional loading
P y x f F L N dp LIFTING Load Analysis What “simple machine” does a power screw utilize?
More Completely… LIFTING LOWERING P y x f F L N dp
LIFTING LOWERING For Acme Threads
Friction Coefficients oil lubricated= collar w/ bushing=0.15 ± 0.05 collar w/ bearing=0.015 ± 0.005
Self-Locking / Back Driving self-locking – screw cannot turn from load P back-driving – screw can be turned from load P for self-locking: would square or Acme of same dimensions lock first?
Efficiency -for lifting- higher efficiency for lowering (also derive with frictionless torque/torque)
Outline • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Body Stresses • Axial • Torsion • Thread Stresses • Bearing • Bending • Buckling • Preloading Fasteners/Joints • Fasteners in Shear
At also in Tables 14-1 and 14-2 Tensile Stress F F
Torsional Stress depends on friction at screw-nut interface • For screw and nut, • if totally locked (rusted together), the screw experiences all of torque • if frictionless, the screw experiences none of the torque • For power screw, • if low collar friction, the screw experiences nearly all of torque • if high collar friction, the nut experiences most of the torque
Thread Stresses – Bearing F p/2 p/2 Abearing=(p/2)(dpnt)
Thread Stresses – Bending F p/2 p/2 transverse shear is also present, but max stress will be at top of tooth For both bearing and bending, F and nt are dependent on how well load is shared among teeth, therefore use Factual=0.38F and nt=1 (derived from experiments)
Mohr’s Circle F p/2 p/2 z y x
Buckling use dr
Outline • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Preloading Fasteners/Joints • Proof Strength • Spring Behavior • Loading & Deflection • Separation of Joints • Fasteners in Shear
Preloading • static loading: preload at roughly 90% of Sp • dynamic loading: preload at roughly 75% of Sp Preloading & Proof Strength • Sp stress at which bolt begins to take a permanent set
Spring Behavior BOTH material being clamped and bolt behave as springs (up to yield/permanent set stresses) for the bolt, threaded vs unthreaded have different spring constants: applied load P
Area is hard to define… from experiments, the following is accurate: When no edges nearby and same materials, even simpler form can be used: A and b are from Table 14-9, pg 916 Affected Area of Material For material, basic model is as follows (shown for 2 materials being clamped)
Pb Fb P Pm Fm m1 b1 P=Pb+Pm Papplied relieves compression in material & adds tension to bolt Fm=Fi-Pm Fb=Fi+Pb Loading & Deflection F Fi m b MATERIAL BOLT
Distribution of Applied Load b= m
Applied Load to Equal Sp How many times more would the loading on the bolt need to be to incur permanent set? (assuming no material separation)
Yielding Safety Factor Fm = Fi + P(C-1) Fb = Fi + CP Ny=Sy/b
Separation Separation occurs when Fm=0 Fm = Fi + P(C-1)
Strategy Reviewed See Example 14-2, p. 906 Given: joint dimensions Find: bolt set preload equal to 90% Sp find lt so that you can find kb find km calculate C, then Pb, Pm, then Fb, Fm find stress in bolt and separation load Such that: factors of safety>1
Dynamic Loading of Fasteners • Bolt only absorbs small % of P • Stresses • Bolt is in tension • Material is in compression • Fatigue is a tensile failure phenomenon • Preloading helps tremendously in fatigue
Outline • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Preloading Fasteners/Joints • Fasteners in Shear • What is Shear? • Straight Direct Shear
Doweled Joints “It is not considered good practice to use bolts or screws in shear to locate and support precision machine parts under shear loads” • Shear can be handled by friction caused by bolts… but, better practice is to use dowels • Bolts need clearances… at best 2 out of a 4 bolt pattern will bear all of load Norton dowels support shear, but not tensile loads bolts support tensile loads, but not shear
Direct Shear Ashear=2x(cross sxn of dowels) dowels support shear, but not tensile loads bolts support tensile loads, but not shear
Outline Revisited • General Thread Nomenclature & Types • Power Screws • Stresses in Threads • Preloading Fasteners/Joints • Fasteners in Shear
Chapter 9 Welding, Brazing, Bending, and the Design of Permanent Joints From Shigley & Mischke, Mechanical Engineering Design Part 3 Design of Mechanical Elements
Welding Issues • Requires • Careful Design • Skilled Welder • Can Cause • Weakened adherends • Thermal distortion • Removal of heat treatment
Welding References • AWS (American Welding Society) • Lincoln Electric • ASME Codes & Standards • Pressure Vessels & Piping • Nuclear Installations • Safety Codes • Performance Test Codes