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Published byAlbert Tiley Modified over 3 years ago

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**Fastening (more complex shapes = better function)**

Non-permanent Bolted Permanent Welded Bonded

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**Outline General Thread Nomenclature & Types Power Screws**

Stresses in Threads Preloading Fasteners/Joints Fasteners in Shear

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**Threads p pitch in./thread d diameter (major) in.**

dp pitch diameter in. dr minor diameter in. L Lead in.

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**Screw Classifications**

Unified National Standard ISO (Metric) Thread Pitch UNC –coarse UNF –fine UNEF –extra fine coarse fine Tolerance Class 1 Class 2 Class 3 several levels d=12mm fine Class 2 d=0.25” metric ¼-20 UNF –2A M12 x 1.75 external threads 20 threads/in. p=1.75 mm/thread **see Tables 14-1 and 14-2 for standard sizes**

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Tensile Stress F F At also in Tables 14-1 and 14-2

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**Outline General Thread Nomenclature & Types Power Screws**

Threads Loads Self-locking Efficiency Stresses in Threads Preloading Fasteners/Joints Fasteners in Shear

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**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)

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**Power Screw Types Square Acme Buttress (contrafuerte) 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

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**Load Analysis What “simple machine” does a power screw utilize? P y x**

dp L N f P F LIFTING y x

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More Completely… LIFTING LOWERING y P x f F N L dp

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For Acme Threads LIFTING LOWERING

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**Friction Coefficients**

oil lubricated= collar w/ bushing=0.15 ± 0.05 collar w/ bearing=0.015 ± 0.005

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**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?

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**Efficiency -for lifting- higher efficiency for lowering**

(also derive with frictionless torque/torque)

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Ball Screw

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**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

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Tensile Stress F F At also in Tables 14-1 and 14-2

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**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

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**Thread Stresses – Bearing**

F p/2 p/2 Abearing=(p/2)(dpnt)

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**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)

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Mohr’s Circle F p/2 p/2 z y x

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Buckling use dr

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**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

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**Preloading & Proof Strength**

Sp stress at which bolt begins to take a permanent set Preloading static loading: preload at roughly 90% of Sp dynamic loading: preload at roughly 75% of Sp

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**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

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**Affected Area of Material**

For material, basic model is as follows (shown for 2 materials being clamped) 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

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**Papplied relieves compression in material &**

Loading & Deflection Pm Pb Fm Fb F b1 m1 P Fi m b MATERIAL BOLT P=Pb+Pm Papplied relieves compression in material & adds tension to bolt Fm=Fi-Pm Fb=Fi+Pb

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**Distribution of Applied Load**

b= m

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**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)

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**Yielding Safety Factor**

Fm = Fi + P(C-1) Fb = Fi + CP Ny=Sy/b

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Separation Separation occurs when Fm=0 Fm = Fi + P(C-1)

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**Strategy Reviewed Given: joint dimensions Find: bolt**

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

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**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

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**Outline General Thread Nomenclature & Types Power Screws**

Stresses in Threads Preloading Fasteners/Joints Fasteners in Shear What is Shear? Straight Direct Shear

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Direct Shear

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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” Norton 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 dowels support shear, but not tensile loads bolts support tensile loads, but not shear

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**Direct Shear Ashear=2x(cross sxn of dowels)**

dowels support shear, but not tensile loads bolts support tensile loads, but not shear

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**Outline Revisited General Thread Nomenclature & Types Power Screws**

Stresses in Threads Preloading Fasteners/Joints Fasteners in Shear

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Chapter 9 Welding, Brazing, Bending, and the Design of Permanent Joints From Shigley & Mischke, Mechanical Engineering Design Part 3 Design of Mechanical Elements

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Welding Symbols

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Butt Welds

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Fillet Welds

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**Welding Issues Requires Can Cause Careful Design Skilled Welder**

Weakened adherends Thermal distortion Removal of heat treatment

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**Welding References AWS (American Welding Society) Lincoln Electric**

ASME Codes & Standards Pressure Vessels & Piping Nuclear Installations Safety Codes Performance Test Codes

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**Bonded Joints (thin members)**

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Bonded Joint Types

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More Types

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Peel Stresses

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Good Practices

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**Bonding Issues Can achieve Beware Lighter joint Less costly joint**

Better sound absorption Beware Peel stresses Environmental effects Thermal mismatch

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Bonding References SAMPE (Society for the Advancement of Material & Process Engineering) ASTM Committee D-14 on Adhesives

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