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.
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
What is the difference between
a “screw” and a “bolt”?
technical: threads match “bolt” specifications
technical: anything NOT a specific bolt thread
Function: to form or cut threads into a hole
Function: to fasten collars, gears, and pulleys to shafts
Self tapping bolt?
Internally threaded plugs or wire coils designed to be inserted into an unthreaded hole
(a) plain washers; (b) spring washers, (c) lock washer
Means of bolt failure:
Most common failure: #3
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)
Types: (a) solid, (b) tubular, (c) semitubular, (d) bifurcated, and (e) compression.
Assembly based on mechanical “interference” between two mating parts
Dp1. Press Fitting
where, E – modulus of elasticity,
i – interference (“overlap” between ID & OD)
Dc – outside diameter of collar
Dp – pin or shaft diameter
Assembly of two parts (e.g., shaft in collar) that have an interference fit at room temperature
Mating elements possess a temporary interference during assembly, but once assembled interlock
Fastener that snaps into a circumferential groove on a shaft or tube to form a shoulder
Retaining ring assembled into a groove on a shaft.
Common types: (a) unclinched, (b) standard loop, (c) bypass loop, and (d) flat clinch.
(a) threaded bushing, and (b) threaded stud.
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