ME 350 – Lecture 6B

1. ME 350 – Lecture 6B 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?

3. Threaded Fasteners What is the difference between a “screw” and a “bolt”? • Bolt: threaded shaft that goes into a (non-affixed) nut ? • Screw: threaded shaft that goes into a threaded hole ?

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?

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: • of external threads • of internal threads • in cross‑sectional area Most common failure:

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. Rivets used in airplanes. There are about 1.5 million rivets on a Boeing 747!!

11. Eyelets (a) Eyelet, (b) assembly sequence: (1) inserting eyelet through holes and (2) setting operation

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

13. 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)

14. 2. Shrink and Expansion Fits Assembly of two parts (e.g., shaft in collar) that have an interference fit at room temperature • Shrink fitting - part is enlarged by heating; the other part either stays at room temperature • Expansion fitting - part is contracted by cooling and inserted into mating component • Change in diameter:

15. 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

16. 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.

17. 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.

18. 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.

19. Integral Fasteners Components are deformed so they interlock as a mechanically fastened joint Seaming: edges of sheetmetal parts are bent over to form the fastening seam Lanced tabs: to attach wires or shafts to sheetmetal parts

20. DFA Guidelines • Use 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 • Limit the required • 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

21. O.D I.D In class exercise – press fitting • A dowel pin made of steel (elastic modulus = 209 GPa) is to be press fitted into a steel collar. The pin has a nominal diameter of 16.03 mm, and the collar has an O.D. of 27.0 mm and I.D. of 16.00mm. (a) Compute the radial pressure and the maximum effective stress. Dp

22. Next Lecture – Design for Assembly