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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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