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MECHANICAL ASSEMBLY

MECHANICAL ASSEMBLY

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MECHANICAL ASSEMBLY•Threaded Fasteners•Rivets and Eyelets•Assembly Methods Based on Interference Fits•Other Mechanical Fastening Methods•Molding Inserts and Integral Fasteners•Design for Assembly©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Mechanical Assembly DefinedUse of various fastening methods to mechanicallyattach two or more parts together•In most cases, discrete hardware components, calledfasteners, are added to the parts during assembly•In other cases, fastening involves shaping orreshaping of a component, and no separate fastenersare required©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Products of Mechanical Assembly•Many consumer products are assembled largely bymechanical fastening methodsExamples: automobiles, large and smallappliances, telephones•Many capital goods products are assembled usingmechanical fastening methodsExamples: commercial airplanes, trucks, railwaylocomotives and cars, machine tools©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Two Major Classes of Mechanical Assembly1. Methods that allow for disassembly Example: threaded fasteners2. Methods that create a permanent joint Example: rivets©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Reasons Why Mechanical Assembly isOften Preferred Over Other Methods•Ease of assembly –can be accomplished withrelative ease by unskilled workers using a minimumof special tooling and in a relatively short time•Ease of disassembly –at least for the methods thatpermit disassemblySome disassembly is required for most productsso maintenance and repair can be performed©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Threaded FastenersDiscrete hardware components that have external orinternal threads for assembly of parts•Most important category of mechanical assembly•In nearly all cases, threaded fasteners permitdisassembly•Common threaded fastener types are screws, bolts,and nuts©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Screws, Bolts, and NutsScrew - externally threaded fastener generallyassembled into a blind threaded holeBolt - externally threaded fastener inserted throughholes and "screwed" into a nut on the opposite sideNut - internally threaded fastener having standardthreads that match those on bolts of the samediameter, pitch, and thread form©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Figure 33.1 - Typical assemblies when screws and bolts are used©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Some Facts About Screws and Bolts•Screws and bolts come in a variety of sizes, threads,and shapes•There is much standardization in threaded fasteners,which promotes interchangeability•U.S. is converting to metric, further reducingvariations•Differences between threaded fasteners affect toolingExample: different screw head styles and sizesrequire different screwdriver designs©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Figure 33.2 - Various head styles available on screws and bolts©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Types of Screws•Greater variety than bolts, since functions vary more•Examples:Machine screws - generic type, generallydesigned for assembly into tapped holesCapscrews - same geometry as machine screwsbut made of higher strength metals and to closertolerances©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


SetscrewsHardened and designed for assembly functions suchas fastening collars, gears, and pulleys to shaftsFigure 33.3 - (a) Assembly of collar to shaft using a setscrew;(b) various setscrew geometries (head types and points)©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Self-Tapping Screws•Designed to form or cut threads in a pre-existing holeinto which it is being turned•Also called a tapping screwFigure 33.4 -Self-tapping screws:(a) thread-forming,and(b) thread-cutting©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Screw Thread InsertsInternally threaded plugs or wire coils designed to beinserted into an unthreaded hole and accept anexternally threaded fastener•Assembled into weaker materials to provide strongthreads•Upon assembly of screw into insert, insert barrelexpands into hole to secure the assembly©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Figure 33.6 - Screw thread inserts: (a) before insertion, and(b) after insertion into hole and screw is turned into insert©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


WasherHardware component often used with threadedfasteners to ensure tightness of the mechanical joint•Simplest form = flat thin ring of sheet metal•Functions:Distribute stressesProvide support for large clearance holesProtect part surfaces and seal the jointIncrease spring tensionResist inadvertent unfastening©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Figure 33.8 - Types of washers: (a) plain (flat) washers; (b) springwashers, used to dampen vibration or compensate for wear; and(c) lockwasher designed to resist loosening of the bolt or screw©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Bolt StrengthTwo measures:•Tensile strength, which has the traditional definition•Proof strength - roughly equivalent to yield strengthMaximum tensile stress without permanentdeformation©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Figure 33.9 - Typical stresses acting on a bolted joint©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Overtightening in Bolted Joints• Potential problem in assembly, causing stresses thatexceed strength of fastener or nut• Failure can occur in one of the following ways:1. Stripping of external threads2. Stripping of internal threads3. Bolt fails due to excessive tensile stresses oncross-sectional area• Tensile failure is most common problem©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Tools and Methods for ThreadedFasteners - Basic Functions:•To provide relative rotation between external andinternal threads during fastening process•To apply sufficient torque to secure the assemblyProduct designer often specifies required preloadto secure assemblyAssembly operator must apply the right torque toachieve the specified preload©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Methods to Apply Required Torque forThreaded Fasteners1. Operator feel - not very accurate, but adequate formost assemblies2. Torque wrench –indicates amount of torque duringtightening3. Stall-motor - motorized wrench is set to stall whenrequired torque is reached4. Torque-turn tightening - fastener is initially tightenedto a low torque level and then rotated a specifiedadditional amount©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


RivetsUnthreaded, headed pin used to join two or more parts bypassing pin through holes in parts and forming asecond head in the pin on the opposite side•Widely used fasteners for achieving a permanentmechanically fastened joint•Clearance hole into which rivet is inserted must beclose to the diameter of the rivet©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Figure 33.10 - Five basic rivet types, also shown in assembledconfiguration: (a) solid, (b) tubular, (c) semitubular, (d) bifurcated, and(e) compression©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Rivets –Applications and Advantages• Used primarily for lap joints• Example: a primary fastening method in aircraft andaerospace industries• Advantages: High production rates Simplicity Dependability Low cost©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Tooling and Methods for Rivets1. Impact - pneumatic hammer delivers a succession ofblows to upset the rivet2. Steady compression - riveting tool applies acontinuous squeezing pressure to upset the rivet3. Combination of impact and compression©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Interference FitsAssembly methods based on mechanical interferencebetween the two mating parts being joined•The interference, either during assembly or afterjoining, holds the parts together•Interference fit methods include:Press fittingShrink and expansion fitsSnap fitsRetaining rings©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Press Fitting•Typical case is where a pin (e.g., a straight cylindricalpin) of a certain diameter is pressed into a hole of aslightly smaller diameter•Possible functions:Locating and locking components - to augmentthreaded fasteners by holding parts in fixedalignment with each otherPivot points - to permit rotation of one componentabout the otherShear pins©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Shrink and Expansion FitsAssembly of two parts (e.g., shaft in collar) that have aninterference fit at room temperatureShrink fitting - external part is enlarged by heating,and internal part either stays at room temperature oris contracted by coolingExpansion fitting - internal part is contracted bycooling and inserted into mating component - whenat room temperature, expansion creates interference•Used to fit gears, pulleys, sleeves, and othercomponents onto solid and hollow shafts©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Snap FitsJoining of two parts in which mating elements possessa temporary interference during assembly, but onceassembled they interlockDuring assembly, one or both parts elasticallydeform to accommodate temporary interferenceUsually designed for slight interference afterassembly•Originally conceived as a method ideally suited forindustrial robotsEureka! –it’s easier for humans too©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Figure 33.13 - Snap fit assembly, showing cross-sections of twomating parts: (1) before assembly, and (2) parts snapped together©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Retaining RingFastener that snaps into a circumferential groove on ashaft or tube to form a shoulder•Used to locate or restrict movement of parts on ashaft©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Figure 33.14 - Retaining ring assembled into a groove on a shaft©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


StitchingFastening operation in which U-shaped stitches areformed one-at-a-time from steel wire and immediatelydriven through the two parts to be joined•Applications: sheetmetal assembly, metal hinges,magazine binding, corrugated boxesFigure 33.15 - Common types of wire stitches: (a) unclinched,(b) standard loop, (c) bypass loop, and (d) flat clinch©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


StaplingPreformed U-shaped staples are punched through thetwo parts to be attached•Supplied in convenient strips•Usually applied by portable pneumatic guns•Applications: furniture and upholstery, car seats,various light-gage sheetmetal and plastic assemblyjobs©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Molding Inserts and Integral FastenersPermanent joining methods that involve shaping orreshaping one of the components by a manufacturingprocess such as:CastingMoldingSheet-metal forming©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Placement of a component into a mold prior to plasticmolding or metal casting, so that it becomes apermanent and integral part of the molding or castingFigure 33.17 - Examples of molded-in inserts:(a) threaded bushing, and (b) threaded stud©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Reasons for Molding Inserts andExamples of Applications•Insert has better properties than molded or castmaterial•Insert geometry is too complex or intricate toincorporate into the mold•Examples of applications:Internally threaded bushings and nutsExternally threaded studsBearingsElectrical contacts©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Integral FastenersComponents are deformed so they interlock as amechanically fastened joint•Methods include:Lanced tabsSeamingBeading©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Lanced TabsTo attach wires or shafts to sheetmetal partsFigure 33.18(a) lanced tabs toattach wires orshafts tosheetmetal©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


SeamingEdges of two separate sheetmetal parts or theopposite edges of the same part are bent over toform the fastening seamFigure 33.18 (c) single-lock seaming©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


Design for Assembly (DFA)• Keys to successful DFA:1. Design the product with as few parts as possible2. Design the remaining parts so they are easy toassemble• Assembly cost is determined largely in productdesign, when the number of components in theproduct and how they are assembled is decided Once these decisions are made, little can bedone in manufacturing to reduce assembly costs©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


DFA Guidelines•Use modularity in product designEach subassembly should have a maximum of 12or so partsDesign the subassembly around a base part towhich other components are added•Reduce the need for multiple components to behandled at once©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”


More DFA Guidelines•Limit the required directions of accessAdding all components vertically from above is theideal•Use high quality componentsPoor quality parts jams feeding and assemblymechanisms•Minimize threaded fasteners•Use snap fit assembly©2002 John Wiley & Sons, Inc. M. P. Groover, “Fundamentals of Modern Manufacturing 2/e”

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