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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. Two Types of Mechanical Assembly.

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ME 350 – Lecture 15 – Chapter 32


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me 350 lecture 15 chapter 32
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
two types of mechanical assembly
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”? Shouldn’t 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

threaded fasteners
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

slide4

Setscrews Self-Tapping Screws

Function: to form or cut threads into a hole

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

Self tapping bolt?

YES!

screw thread inserts
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
washer
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

bolt strength
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

tensile stress on a bolt or screw
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 = (π/4)(D – 0.9382p)2 where, D (diameter), p (pitch)

M20 x 2.5 means diameter=20mm, pitch=2.5mm

ANSI: As = (π/4)(D – 0.9743/n)2 where, D (diameter), n (threads / inch)

Preload: torque applied during assembly

T = Ct D F = Ct D σ 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)

rivets
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.

interference fits
Interference Fits

Assembly based on mechanical “interference” between two mating parts

  • Examples:
    • Press fitting
    • Shrink and expansion fits
    • Snap fits
    • Retaining rings
1 press fitting

Dc

Dp

1. Press Fitting
  • Examples: pin-in-hole, or collar-on-shaft, where starting inside dia of hole < 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)
2 shrink and expansion fits
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:
3 snap fits
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
4 retaining ring
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.

slide15

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.

molding inserts
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.

integral fasteners
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

dfa guidelines
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