Mechanical Assembly and Types Powerpoint

1. Chapter#33
By:Groover
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2.  Threaded Fasteners
 Rivets and Eyelets
 Assembly Methods Based on Interference Fits
 Other Mechanical Fastening Methods
 Design for Assembly
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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
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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
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5. 1. Methods that allow for disassembly
◦ Example: threaded fasteners
2. Methods that create a permanent joint
◦ Example: rivets
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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
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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
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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
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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
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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
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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
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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.
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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.
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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
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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.
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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
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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
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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.
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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
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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36. Various pin geometries are available
 Straight pins
 Dowel pins
 Taper pins
 Grooved pins
 Knurled pins
 Coiled/spiral pins
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37. 37
Grooved pins
Taper Pins

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Knurled Pins
Coiled/Spiral Pins

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Straight Pins
Dowel Pins

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
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41. 41
Expansion Fits
Shrink Fits

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
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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
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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
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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
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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
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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
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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
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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.
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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
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53. Permanent joining methods that involve shaping or
reshaping one of the components by a manufacturing
process such as:
 Casting
 Molding
 Sheet-metal forming
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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
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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
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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
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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
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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.
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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.
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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
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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
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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.
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