Page 356 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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of lubrication and resulting coefficient of friction, f , which was initially obtained by the
McKeebrothersinanactualtestoffriction.
At low values of n/p, sliding surfaces rub together and lubricant film is extremely
thin. There is a great possibility of metal-to-metal contact. Boundary lubrication occurs
with high coefficient of friction. In general, boundary lubrication is expected to occurs
in hydrodynamically lubricated bearings during starting, stopping, overloading or lubri-
cant deficiency.
At high values of n/p, hydrodynamic lubrication produces a complete physical
separation between sliding surfaces and greatly reduces friction and wear. To promote
hydrodynamic lubrication, light loads, high relative speeds between moving surfaces
and a copious supply of high viscosity lubricant are required. Hydrodynamic lubrication
implies minimum power losses and maximum life expectancy for bearings.
Between boundary and hydrodynamic lubrication within the range of point A and B
in Figure 12.3 is mixed-film lubrication. Mixed-film lubrication occurs due to scarcity
of lubricant, low viscosity, low bearing speeds, overload, tight clearance and so on. The
mixed-film lubrication zone should be avoided, as a small change in any of three factors
of , n or p produces a sharp change in friction coefficient f , resulting in unpredictable
performance of machine [3].
From starting-up and accelerating to its operating speed, a sliding bearing may
experience through all three modes of lubrication, which will be discussed in detail in
Section 12.2.4.

12.2.2 Wear

Wear is undesirable cumulative profile change due to progressive loss of materials from
the contact surfaces of mating elements as the result of loads and relative motion. It has
long been recognized as a most important detrimental process in machine elements.
Wear is classified as abrasion, adhesion, fatigue, fretting and corrosion wear by the phys-
ical nature of the underlying process. In practical engineering, two or more types of wear
often occur simultaneously.
Abrasive wear arises either due to direct contact of a hard and a soft surface, or due
to rubbing of abrasive particles on a surface. For the former, the asperities of the harder
surface penetrate the softer surface under a normal load, producing plastic deformation;
while for the latter, small, hard or sharp-edged particles, like grains of sand or of metal
or metal oxide, rub off metal surface. Usually, the harder the surface, the more resistant
it is to abrasive wear.
Adhesive wear occurs because of local welding at surface asperity junctions. The
extremely high local pressure and temperature, combined with continuous relative
motion of contact surfaces, cause metal removal from one surface with a lower yield
strength to another by solid-phase welding. Such adhesive wear or surface damage is
called scoring. A mild adhesive wear is often called scuffing, while a severe adhesive
wear is called galling. Seizure is an extension of the galling process when two surfaces
are virtually welded together and relative motion is no longer possible [1–3]. As before,
harder surfaces are more resistant to adhesive wear.
Fatigue wear is caused by propagation of subsurface microcracks under repeated
cyclic loads. Stresses develop on and below the contact surfaces of mating elements.
As cyclic loads are applied, microcracks may initiate and propagate and, eventually,