1Fastening (more complex shapes = better function) Non-permanent¾Bolted Permanent¾Bolted¾Welded¾BondedOutline General Thread Nomenclature & Types Power Screws Stresses in Threads Preloading Fasteners/Joints Fasteners in ShearThreadsp pitch in./threadd diameter (major) in.dppitch diameter in.drminor diameter in.L Lead in.Screw ClassificationsUnified National Standard ISO (Metric)Thread PitchUNC –coarseUNF –fineUNEF –extra finecoarsefine¼-20 UNF –2AClass 220 threads/in.d=0.25”external threadsfineM12 x 1.75metricd=12mmp=1.75 mm/threadToleranceClass 1 Class 2Class 3 several levels**see Tables 14-1 and 14-2 for standard sizes**Tensile StressFFttAF=σ2224+=rptddAπAtalso in Tables 14-1 and 14-2Outline General Thread Nomenclature & Types Power Screws¾ Threads¾ Loads¾ Self-locking¾ Efficiency Stresses in Threads Preloading Fasteners/Joints Fasteners in Shear2Power Screw Applications jacks for cars C-clamps vises Instron material testing machines machine tools (for positioning of table)Where have you seen power screws?Power Screw Types Square¾ strongest¾ no radial load¾ hard to manufacture Acme¾ 29° included angle¾ easier to manufacture¾ common choice for loading in both directions Buttress¾ great strength¾ only unidirectional loadingLoad AnalysisWhat “simple machine” does a power screw utilize?πdpLλNfPFLIFTINGyxpdLπλ=tan()()LdLdPdTpppSuµπµπ−+=2Friction Coefficientsµoil lubricated= µcollar w/ bushing=0.15 ± 0.05µcollar w/ bearing=0.015 ± 0.005Ball Screw Outline General Thread Nomenclature & Types Power Screws Stresses in Threads¾ Body Stresses» Axial» Torsion¾ Thread Stresses» Bearing» Bending¾ Buckling Preloading Fasteners/Joints Fasteners in Shear3Outline General Thread Nomenclature & Types Power Screws Stresses in Threads Preloading Fasteners/Joints¾ Proof Strength¾ Spring Behavior¾ Loading & Deflection¾ Separation of Joints Fasteners in ShearPreloading & Proof Strength SpÆ stress at which bolt begins to take a permanent setPreloading• static loading: preload at roughly 90% of Sp• dynamic loading: preload at roughly 75% of SpSpring BehaviorBOTH material being clamped and bolt behave as springs(up to yield/permanent set stresses)lAEk =for the bolt, threaded vs unthreaded have different spring constants:bsbttbEdlEAlk412π+=applied load PStrategy ReviewedGiven: joint dimensionsFind: boltset preload equal to 90% Spfind ltso that you can find kbfind kmcalculate C, then Pb, Pm, then Fb, Fmfind stress in bolt and separation loadSuch that: factors of safety>1See Example 14-2, p. 906Dynamic Loading of Fasteners Bolt only absorbs small % of P Stresses¾Bolt is in tension¾Material is in compression Fatigue is a tensile failure phenomenon ∴ Preloading helps tremendously in fatigueOutline General Thread Nomenclature & Types Power Screws Stresses in Threads Preloading Fasteners/Joints Fasteners in Shear¾What is Shear?¾Straight Direct Shear4Direct Shear Doweled Joints 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“It is not considered good practice to use bolts or screws in shear to locate and support precision machine parts under shear loads”Nortondowels support shear, but not tensile loadsbolts support tensile loads, but not shearDirect Sheardowels support shear, but not tensile loadsbolts support tensile loads, but not shearAshear=2x(cross sxn of dowels)shearAF=τττySSsyN???==ττySSsyN577.0==Outline Revisited General Thread Nomenclature & Types Power Screws Stresses in Threads Preloading Fasteners/Joints Fasteners in ShearChapter9Welding, Brazing, Bending, and the Design of Permanent JointsPart 3Design of Mechanical ElementsFrom Shigley & Mischke, Mechanical Engineering DesignWelding Symbols5Butt Welds Fillet WeldsWelding Issues Requires¾Careful Design¾Skilled Welder Can Cause¾Weakened adherends¾Thermal distortion¾Removal of heat treatmentWelding References AWS (American Welding Society) Lincoln Electric ASME Codes & Standards¾Pressure Vessels & Piping¾Nuclear Installations¾Safety Codes¾Performance Test Codes Bonded Joints (thin members) Bonded Joint Types6More Types Peel StressesGood Practices Bonding Issues Can achieve¾Lighter joint¾Less costly joint¾Better sound absorption Beware¾Peel stresses¾Environmental effects¾Thermal mismatchBonding References SAMPE (Society for the Advancement of Material & Process Engineering) ASTM Committee D-14 on
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