ME 350 – Lecture 15 – Chapter 32

1. 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

2. 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

3. 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

4. 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!

5. 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

6. 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

7. 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

8. 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)

9. 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.

10. Interference Fits Assembly based on mechanical “interference” between two mating parts • Examples: • Press fitting • Shrink and expansion fits • Snap fits • Retaining rings

11. 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)

12. 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:

13. 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

14. 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.

15. 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.

16. 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.

17. 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

18. 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