Mechanical Assembly:

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ME 350 Lecture 15 Chapter 32

• 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

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Two Types of Mechanical Assembly

• Methods that allow for disassembly
• Example threaded fasteners
• Methods that create a permanent joint
• Example rivets
• Why are rivets considered assembly if the
  joint is permanent? Shouldnt rivets be an
  example of a joining process like welding?
• Joining is a permanent process because the
  workpart is deformed by the joining process.
• Assembly is when the workpart is not
  permanently deformed

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Threaded Fasteners

• What is the difference between
• a screw and a bolt?
• Bolt threaded shaft that goes into a
  (non-affixed) nut ?
• technical threads match bolt
  specifications
• Screw threaded shaft that goes into a threaded
  hole ?
• technical anything NOT a specific bolt
  thread

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Setscrews Self-Tapping Screws

• Function to fasten collars, gears, and
  pulleys to shafts

Function to form or cut threads into a hole
Self tapping bolt? YES!
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Screw Thread Inserts

• Internally threaded plugs or wire coils designed
  to be inserted into an unthreaded hole
• Usually assembled into weaker materials to
  provide strong threads

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Washer

• Simplest form flat thin ring of sheet metal
• Functions
• Distribute stresses / provide support
• Protect part surfaces and/or seal the joint
• Resist unfastening/ increase spring tension

(a) plain washers (b) spring washers, (c) lock
washer
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Bolt Strength

• Means of bolt failure
• Stripping of external threads
• Stripping of internal threads
• Excessive tensile stress in cross-sectional area
• Most common failure 3

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Tensile Stress on a Bolt (or Screw)

• Bolt proof strength (or tensile stress)
• where, F maximum load, typically proof
  stress or yield strength
• As bolt cross-sectional area
• metric (ISO) As (p/4)(D 0.9382p)2 where,
  D (diameter), p (pitch)
• M20 x 2.5 means diameter20mm, pitch2.5mm
• ANSI As (p/4)(D 0.9743/n)2 where, D
  (diameter), n (threads / inch)
• Preload torque applied during assembly
• T Ct D F Ct D s As
• where, T torque (N-mm)
• Ct torque coefficient (typically between
  0.15-0.25)
• D nominal bolt or screw diameter
• F preload tension force (N)

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Rivets

• Most widely used permanent fastening method
• Typically a pneumatic hammer delivers a
  succession of blows to upset the rivet

Types (a) solid, (b) tubular, (c) semitubular,
(d) bifurcated, and (e) compression.
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Interference Fits

• Assembly based on mechanical interference
  between two mating parts
• Examples
• Press fitting
• Shrink and expansion fits
• Snap fits
• Retaining rings

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1. Press Fitting

• Examples pin-in-hole, or collar-on-shaft, where
  starting inside dia of hole lt outside dia of
  pin
• Radial or interference fit pressure, pf
• where, E modulus of elasticity,
• i interference (overlap between ID OD)
• Dc outside diameter of collar
• Dp pin or shaft diameter
• Maximum Joining Stress (max elastic deformation)

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2. Shrink and Expansion Fits

• 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 the other part either stays at room
  temperature
• Expansion fitting - internal part is contracted
  by cooling and inserted into mating component
• Change in diameter

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3. Snap Fits

• Mating elements possess a temporary interference
  during assembly, but once assembled interlock
• During assembly, one or both parts elastically
  deform to accommodate temporary interference
• Usually designed for slight interference after
  assembly
• Originally conceived to be used by industrial
  robots

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4. Retaining Ring

• 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 ring assembled into a groove on a shaft.

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Stitching or Stapling

• U-shaped steel wire driven through parts
• Applications sheetmetal assembly, metal hinges,
  magazine binding, corrugated boxes

Common types (a) unclinched, (b) standard loop,
(c) bypass loop, and (d) flat clinch.
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Molding Inserts

• Examples
• Internal or external threads
• Bearings
• Electrical contacts
• Advantages
• Insert can be stronger than molded or cast
  material
• Insert can have more intricate geometry

(a) threaded bushing, and (b) threaded stud.
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Integral Fasteners

• Components are deformed so they interlock as a
  mechanically fastened joint

Lanced tabs to attach wires or shafts to
sheetmetal parts
Seaming edges of sheetmetal parts are bent over
to form the fastening seam
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DFA Guidelines

• Use modularity in product design
• subassemblies should have a 12 part maximum
• Design the subassembly around a base part to
  which other components are added
• Reduce the need for multiple components to be
  handled simultaneously or together
• Limit the required directions of access
• Adding all components vertically is the ideal
• Use high quality components
• Poor quality parts jam feeding and assembly
  mechanisms
• Minimize threaded fasteners
• Use snap fit assembly as much as possible