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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


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?


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
  • 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)

  • 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



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.


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