Page 32 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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In general, a compact design is preferred. After calculation, standard sizes close to the
minimum acceptable dimension are usually specified for standardized elements, like
fasteners, keys and rolling contact bearings. Through the selection of standardized ele-
ments, uniformity of practice and reduced cost are achieved.
1.3.2 Common Failure Modes in Machine Elements
A mechanical failure refers to the incapability of a machine or an element to perform
its intended function [9]. It may arise from poor design detailing, inadequate material
properties, manufacturing deficiencies, hostile service conditions and more often their
interactions. A trivial oversight of any of these aspects may cause large detrimental and
even catastrophic failure, resulting in serious financial, insurance and legal repercus-
sions [12].
Mechanical failure modes commonly observed in industrial practice include defor-
mation, yielding, fracture, fatigue, pitting and spalling, wear, scoring, scuffing, galling
and seizure, corrosion, fretting, creep, buckling and so on.
Deformation includes elastic and plastic deformation, referring to the recoverable
and unrecoverable deformation, respectively. When stresses generated in an element
exceed yield strength, plastic deformation or yielding occurs. Both elastic and plastic
deformation may cause malfunctioning of machine elements. A typical example is shafts
supporting a pair of mating gears. The deformation of a shaft may prevent gears meshing
properly, leading to interference, impact, wear, noise and vibration.
Fracture may occur in both brittle and ductile material due to either static or fluc-
tuating loads. Ductile rupture occurs when plastic deformation reaches the extreme in
ductile materials, leaving a dull, fibrous rupture surface; while brittle fracture happens
when elastic deformation achieves the extreme in brittle materials, leaving a granular,
multifaceted fracture surface [5, 13]. Fatigue fracture is a sudden and catastrophic fail-
ure, taking place due to the initiation and propagation of a microcrack under fluctuating
loads over a period of time. The loads causing fatigue failure are usually far below the
static failure level.
Surface fatigue failures are usually associated with rolling surface in contact. The
repeatedly applied loads produce concentrated cyclic subsurface contact stresses.
Micro-cracks initiate slightly below the contact surfaces and propagate until small bits
of surface material spontaneously dislodge off the surfaces, producing surface pitting
or spalling. Examples of pitting failure are manifested in rolling contact bearings, gear
teeth and metal wheels rolling on rails. Pitted surfaces prevent proper function of
elements, causing vibration and noise.
Wear is gradual removal of discrete particles from sliding contact surfaces, leading
to a cumulative dimensional change on the element profiles. The most common types
of wear are abrasive wear and adhesive wear. With an increase of severity of surface
damage, adhesive wear is classified as scoring, scuffing, galling and seizure. Adhesive
wear is the most common type of wear and the least preventable. The worn surface may
impair element profiles, leading malfunction of machines. For instance, wear on gear
tooth surfaces may cause intolerable noise and damaging vibration.
Corrosion occurs as a consequence of undesired deterioration of material as a result
of chemical or electrochemical interaction with environment. It most happens to the
machine elements exposed to corrosive mediums [5].