Mechanical Assembly and Types
- 2. Threaded Fasteners Rivets and Eyelets Assembly Methods Based on Interference Fits Other Mechanical Fastening Methods Design for Assembly 2
- 3. Use of various fastening methods to mechanically attach two or more parts together In most cases, discrete hardware components, called fasteners, are added to the parts during assembly In other cases, fastening involves shaping or reshaping of a component, and no separate fasteners are required 3
- 4. Many consumer products are assembled largely by mechanical fastening methods ◦ Examples: automobiles, large and small appliances, telephones Many capital goods products are assembled using mechanical fastening methods ◦ Examples: commercial airplanes, trucks, railway locomotives and cars, machine tools 4
- 5. 1. Methods that allow for disassembly ◦ Example: threaded fasteners 2. Methods that create a permanent joint ◦ Example: rivets 5
- 6. Ease of assembly – can be accomplished with relative ease by unskilled workers using a minimum of special tooling and in a relatively short time Ease of disassembly – at least for the methods that permit disassembly ◦ Some disassembly is required for most products so maintenance and repair can be performed 6
- 7. Discrete hardware components that have external or internal threads for assembly of parts Most important category of mechanical assembly Threaded fasteners permit disassembly Common threaded fastener types are screws, bolts, and nuts 7
- 8. Screw - externally threaded fastener generally assembled into a blind threaded hole Bolt - externally threaded fastener inserted through holes and "screwed" into a nut on the opposite side Nut - internally threaded fastener having standard threads that match those on bolts of the same diameter, pitch, and thread form 8
- 9. Typical assemblies when screws and bolts are used
- 10. 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 reducing variations.ANSI & ISO standards are used. Differences between threaded fasteners affects tooling Example: different screw head styles and sizes require different screwdriver designs 10
- 11. Greater variety than bolts, since functions vary more Examples: ◦ Machine screws - generic type, generally designed for assembly into tapped holes ◦ Cap screws - same geometry as machine screws but made of higher strength metals and to closer tolerances 11
- 12. Setscrews Hardened and designed for assembly functions such as fastening collars, gears, and pulleys to shafts (a) Assembly of collar to shaft using a setscrew; (b) various setscrew geometries (head types and points)
- 13. Designed to form or cut threads in a pre-existing hole into which it is being turned Also called a tapping screw Self-tapping screws: (a) thread-forming, and (b) thread-cutting 13
- 14. Fasteners are produced by cold forming but thread making is expensive process. Steel is low cost and good strength material. Low carbon or alloy steels are also used. Nickel,Cr,Zn,black oxides and similar materials are used for coating preventation from corrosion. Stainless steel, aluminum alloys, nickel alloys, and plastics for low stress applications. 14
- 15. 15
- 16. Additional Threaded Fasteners and related hardware include studs, screw thread inserts, captive thread fasteners and washers. Stud: An externally threaded fasteners but without usual head possessed by bolt. 16
- 17. Internally threaded plugs or wire coils designed to be inserted into an unthreaded hole and accept an externally threaded fastener Assembled into weaker materials to provide strong threads Upon assembly of screw into insert, insert barrel expands into hole to secure the assembly 17
- 18. Screw thread inserts: (a) before insertion, and (b) after insertion into hole and screw is turned into insert
- 19. Threaded fasteners have been permanently preassembled to one of the parts to be joined. Possible preassembly processes include welding,brazing,press fitting, cold forming. 19
- 20. 20
- 21. Hardware component often used with threaded fasteners to ensure tightness of the mechanical joint Simplest form = flat thin ring of sheet metal Functions: ◦ Distribute stresses ◦ Provide support for large clearance holes ◦ Protect part surfaces and seal the joint ◦ Increase spring tension ◦ Resist inadvertent unfastening 21
- 22. Types of washers: (a) plain (flat) washers; (b) spring washers, used to dampen vibration or compensate for wear; and (c) lockwasher designed to resist loosening of the bolt or screw 22
- 23. Typical stresses action on a bolted or screwed joint include tensile and shear. Once tightened bolt is loaded in tension, parts are in compression, forces acting in opposite direction result in a shear stresses. Shear Stresses throughout the length on threads with nut in a direction parallel to axis of bolt can cause stripping of the threads. Failure can occur on internal threads of nut. 23
- 24. Two measures: Tensile strength - which has the traditional definition Proof strength - roughly equivalent to yield strength ◦ Maximum tensile stress without permanent deformation Typical stresses acting on a bolted joint 24
- 25. Potential problem in assembly, causing stresses that exceed strength of fastener or nut. Failure can occur in one of the following ways: Stripping of external threads(bolt & screws) Stripping of internal threads (nut) Bolt fails due to excessive tensile stresses on cross-sectional area Tensile failure is most common problem. 85% Bolt breaks due to combination of tensile and torsion during tightening. 25
- 26. To provide relative rotation between external and internal threads during fastening process To apply sufficient torque to secure the assembly. Simple hand made or powered tools are used. Tools should match the screw/bolt/nut in style & size. Hand made are for single point while powered tools are for interchangeable bits. Hydraulics, pneumatic & electric power is used. 26
- 27. Product designer often specifies required preload to secure assembly. Assembly operator must apply the right torque to achieve the specified preload. T=CtDF T=Torque D=Bolt nominal or screw Dia Ct = torque coefficient 0.15-0.25 F= specified preload tension force 27
- 28. 1. Operator feel - not very accurate, but adequate for most assemblies 2. Torque wrench –indicates amount of torque during Tightening 3. Stall-motor - motorized wrench is set to stall when required torque is reached 4. Torque-turn tightening - fastener is initially tightened to a low torque level and then rotated a specified additional amount. 28
- 29. Widely used fasteners for achieving a permanent mechanically fastened joint High production rate,simplicity,dependibility and low cost. Declined in recent decade due to fatseners,welding& brazing. Used for aircraft & aerospace industries. Unthreaded, headed pin used to join two or more parts by passing pin through holes in parts and forming a second head in the pin on the opposite side. 29
- 30. Five basic rivet types, also shown in assembled configuration: (a) solid, (b) tubular, (c) semi tubular, (d) bifurcated, and (e) compression Deforming is done by hot or cold working, hammering or steady pressing. Rivets are specified by their length,dia,head & type. 30
- 31. Rivets are use for lap joints, Clearance hole into which rivet is inserted must be close to the diameter of the rivet. If hole is too small insertion will be difficult if hole is too large rivet will not fill hole. Design tables are available for optimum hole sizes. Tooling and Methods for Rivets 1. Impact - pneumatic hammer delivers a succession of blows to upset the rivet 2. Steady compression - riveting tool applies a continuous squeezing pressure to upset the rivet 3. Combination of impact and compression Automatic drilling & riveting machines are available for drilling holes and inserting& upsetting the rivets. 31
- 32. These are thin walled tubular fasteners with flange to one end usually made of steel used for permanent lap joints. Substitute of rivets for low stress application to save cost, weight & cost. Forming operation is called setting done by opposing tools hold and curl extended portion of eyelets. Applications include automotive, subassemblies, electrical equipments, toys. 32
- 33. 33
- 34. Assembly methods based on mechanical interference between the two mating parts being joined The interference, either during assembly or after joining, holds the parts together Interference fit methods include: 1. Press fitting 2. Shrink and expansion fits 3. Snap fits 4. Retaining rings 34
- 35. Typical case is where a pin (e.g., a straight cylindrical pin) of a certain diameter is pressed into a hole of a slightly smaller diameter Standard pin sizes are available to accomplish a variety of functions such as 1. Locating & locking components 2. Pivot points to permit rotation 3. Shear pins Other applications include collars, gears, pulleys & similar components on shafts. 35
- 36. Various pin geometries are available Straight pins Dowel pins Taper pins Grooved pins Knurled pins Coiled/spiral pins 36
- 40. Assembly of two parts (e.g., shaft in collar) that have an interference fit at room temperature Shrink fitting - external part is enlarged by heating, and internal part either stays at room temperature or is contracted by cooling Expansion fitting - internal part is contracted by cooling and inserted into mating component - when at room temperature, expansion creates interference Used to fit gears, pulleys, sleeves, and other components onto solid and hollow shafts 40
- 42. Heating equipments are torches,furnances,electrical resistance heaters, induction heaters. Cooling methods include refrigeration, packing dry ice, immersion in cold liquids like nitrogen. Resulting change in dia depend upon thermal coefficient & temperature difference. Consider uniform temperature change in dia is D2-D1=α D1(T2-T1) α=coefficient of liner thermal expansion T2 =parts are cooled or heated T1= ambient Temperature D2= Dia at T2 D1= Dia At T1 42
- 43. Joining of two parts in which mating elements possess a temporary interference during assembly, but once assembled they interlock ◦ During assembly, one or both parts elastically deform to accommodate temporary interference ◦ Usually designed for slight interference after assembly Figure 33.13 - Snap fit assembly, showing cross-sections of two mating parts: (1) before assembly, and (2) parts snapped together 43
- 44. Parts can be designed with self aligning features No special tooling is required Assembly can be accomplished very quickly Originally conceived as a method ideally suited for industrial robots Also easier for human assembly workers 44
- 45. Retaining rings are also called snap rings. Fastener that snaps into a circumferential groove on a shaft or tube to form a shoulder Used to locate or restrict movement of parts on a shaft Retaining rings are available in external or internal applications. Made from sheet metal, wire stock. Heat treated for hardness and stiffness 45
- 46. To assemble rings a special pliers tool is used to elastically deform the ring so that its fit over the shaft and then released on groove. Retaining ring assembled into a groove on a shaft 46
- 47. Fastening operation in which U-shaped stitches are formed one-at-a-time from steel wire and immediately driven through the two parts to be joined Applications: sheet metal assembly, metal hinges, magazine binding, corrugated boxes Common types of wire stitches: (a) unclinched, (b) standard loop, (c) bypass loop, and (d) flat clinch 47
- 48. Preformed U-shaped staples are punched through the two parts to be attached Supplied in convenient strips Usually applied by portable pneumatic guns Applications: furniture and upholstery, car seats,various light-gage sheet metal and plastic assembly jobs 48
- 49. 49
- 50. It is common joining method for soft, flexible parts such as cloth and leather This method involves use of long thread or cord interwoven with parts so as to produce a continuous seam between them. The process is widely used in the needle trades industry for assembly garments. 50
- 51. Fasteners formed half round wire into a single two stem pin vary in dia ranging from 0.8mm to 19mm and in point style. Cotter pins are inserted into holes in mating parts and their legs are split to lock assembly Used to secure parts onto shafts and similar applications 51
- 52. 52
- 53. Permanent joining methods that involve shaping or reshaping one of the components by a manufacturing process such as: Casting Molding Sheet-metal forming 53
- 54. Placement of a component into a mold prior to plastic molding or metal casting, so that it becomes a permanent and integral part of the molding or casting. Inserting a separate component is preferable to molding or casting its shape if strength of insert material are required or geometry achieved through the use of the insert is too complex or intricate to incorporate into mold. Examples of applications: Internally threaded bushings and nuts Externally threaded studs Bearings Electrical contacts 54
- 55. Placing inserts into mold has certain disadvantages in production 1. Design of mold become more complicated 2. Handling and placing the inserts into cavity that reduces production rate 3. Inserts introduce a foreign material into the casting or molding an even in defect the cast metal or plastic cannot be easily reclaimed and recycled. 4. Use of inserts is often the most functional design and least-cost production method. Figure 33.17 - Examples of molded-in inserts: (a) threaded bushing, and (b) threaded stud 55
- 56. Components are deformed so they interlock as a mechanically fastened joint. This assembly method is most common for sheet metal parts. Methods include: Lanced tabs Seaming Beading Dimpling 56
- 57. Lanced Tabs To attach wires or shafts to sheet metal parts Fig.33.18(a)lanced tabs to attach wires or shafts to sheet metal
- 58. Seaming: Edges of two separate sheet metal parts or the opposite edges of the same part are bent over to form the fastening seam. Metal must be ductile single- lock seaming 58
- 59. (d)Beading: in which a tube shaped part is attached to a smaller shaft by deforming outer dia inward to cause an interference around entire circumference. (e)Dimpling: forming of simple round indentations in outer part to retain an inner part. Crimping: edges of one part are deformed over mating a component is another example of integral assembly. Squeezing barrel of electrical terminal onto a wire. 59
- 60. 60
- 61. Keys to successful DFA: 1. Design the product with as few parts as possible 2. Design the remaining parts so they are easy to assemble Assembly cost is determined largely in product design, when the number of components in the product and how they are assembled is decided. Once these decisions are made, there is little can be done in manufacturing to reduce assembly costs. 61
- 62. Following recommendations are compiled Use no. of fewest number of parts possible to reduce amount of assembly required. Reduce no. of threaded fasteners required. Standardize fasteners Reduce parts orientation difficulties Avoids parts that tangle 62
- 63. Following are some recommendations & principles applied to product design. Use modularity in product design Each subassembly should have a maximum of 12 or so parts Design the subassembly around a base part to which other components are added Reduce the need for multiple components to be handled at once Limit the required directions of access Adding all components vertically from above is the ideal 63
- 64. Use high quality components Poor quality parts jams feeding and assembly mechanisms Use snap fit assembly: eliminates the need for threaded fasteners, assembly is simple from above. it requires that the parts be designed with special positive and negative features to facilitate insertion and fastening. 64