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APPLICATON OF FATIGUE WEAR
HYDRODYNAMIC JOURNAL BEARING
Figure 1 :journal bearing
Surface fatigue wear, also called rolling contact fatigue,
predominantly occurs in
rolling element bearings. Fatigue wear in rolling bearings
generally starts with micropitting -
small areas on the bearings surface where material has been
removed due to repetitive stress.
At its terminal point, surface fatigue causes significant
surface spalling - large craters often
several hundreds of microns across, which are easily visible to
the naked eye. While the
effects of fatigue on bearings are well-documented, wear debris
analysis offers a unique
insight into fatigue failure. Because the particles that are
removed from the bearing surface
are deposited in the oil and become the mirror image of the
surface distress, the onset and
progress of rolling contact fatigue can be detected.
Figure 2 :journal bearing fatigue test rig with hydraulic
loading
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The principle of this type of fatigue test machine from figure 2
is similar to internal
combustion engines, in which pressure generated in the cylinders
is transmitted to the
bearings through the connecting rod.The test rig uses an
eccentric shaft located between two
concentric shaft parts. The eccentricity is small therefore the
inertia forces generated bymoving parts are negligible.The test
bearing coupled with the eccentric shaft is mounted in
the big end of the connecting rod. Rotation of the eccentric
shaft results in reciprocating
motion of the connecting rod.The shaft is supported by two main
bearings operating against
the concentric shaft parts. The main bearings provide the shaft
rotation under oscillating
forces transmitted by the connecting rod.The shaft is driven by
an electric motor. The rotation
speed of the test rig is within the range 3000-5000 RPM.
Hydrodynamic lubrication is provided by oil supplied to the
bearings through the holes
in the housings and the upper parts of the bearings.The small
end of the connecting rod is
connected to the hydraulic cylinder piston, which performs
reciprocating motion together with
the rod.When the connecting rod goes up the cylinder volume
increases and oil fills it throughthe inlet valve. At this phase
the oil pressure determined by the oil pump is relatively low.
The outlet valve adjusted to the opening at the peak test
pressure is closed.The piston reaches
the upper position and starts to move down forcing the oil in
the cylinder to flow through the
outlet valve. The oil pressure at this phase is high. The inlet
valve is closed since the pressure
of the pump is lower than the cylinder pressure. The load
generated in the cylinder is
transmitted by the connecting rod to the lower part of the test
bearing.
The adjustable opening pressure of the outlet valve determines
the load applied to the test
bearing. The precise value of the load is measured by strain
gauges mounted on the
connecting rod.Bearing performance has been enhanced through the
use of multi-layered architectures
and the development of lining materials with favorable
mechanical properties. These multi-
layered systems provide the required compromise between
stiffness and tribological
performance through a stiff backing layer and a conformable and
low-friction lining. An
additional attribute of multi-layered structures is the
shielding effect that reduces the growth
rate as cracks approach stiffer layers ,thus increasing service
life. The opposite effect is
observed as a crack approaches a more compliant layer.
Bifurcation events have also been
observed in bi-layered architectures in compact specimens of
plastically-mismatched
materials especially at high levels of loading.
Moreover, hydrodynamic journal bearing is a bearing operating
with hydrodynamic
lubrication, in which the bearing surface is separated from the
journal surface by the lubricantfilm generated by the journal
rotation. The lubricant (oil) film prevents localized
overloading
providing a distribution of the applied force over a relatively
wide area. If the pressure
transmitted by the oil film to the bearing is higher than the
fatigue strength of the material
fatigue cracks form on the bearing surface spreading towards the
back layer and may result in
peeling off the Engine bearing materials.
Oscillating loads applied to a part may cause Engine bearing
failure as a result of the
material Fatigue. This occurs if the load exceeds the fatigue
strength (load capacity)which is
the maximum value of cycling stress that the bearing can
withstand after an infinite number of
cycles.The fatigue wear strength values of different engine
bearing materials undercontrollable conditions (oil temperature,
rotation speed, shaft roughness, loading character)are evaluated in
fatigue test rigs simulating operation of the bearings in real
engines.
http://www.substech.com/dokuwiki/doku.php?id=fatiguehttp://www.substech.com/dokuwiki/doku.php?id=bearings_in_internal_combustion_engineshttp://www.substech.com/dokuwiki/doku.php?id=bearings_in_internal_combustion_engineshttp://www.substech.com/dokuwiki/doku.php?id=lubrication_regimeshttp://www.substech.com/dokuwiki/doku.php?id=lubrication_regimes#hydrodynamic_lubricationhttp://www.substech.com/dokuwiki/doku.php?id=lubrication_regimes#hydrodynamic_lubricationhttp://www.substech.com/dokuwiki/doku.php?id=engine_bearing_materials#engine_bearing_layers_and_their_functionshttp://www.substech.com/dokuwiki/doku.php?id=engine_bearing_materialshttp://www.substech.com/dokuwiki/doku.php?id=engine_bearing_failurehttp://www.substech.com/dokuwiki/doku.php?id=fatiguehttp://www.substech.com/dokuwiki/doku.php?id=fatiguehttp://www.substech.com/dokuwiki/doku.php?id=engine_bearing_failurehttp://www.substech.com/dokuwiki/doku.php?id=engine_bearing_materialshttp://www.substech.com/dokuwiki/doku.php?id=engine_bearing_materials#engine_bearing_layers_and_their_functionshttp://www.substech.com/dokuwiki/doku.php?id=lubrication_regimes#hydrodynamic_lubricationhttp://www.substech.com/dokuwiki/doku.php?id=lubrication_regimes#hydrodynamic_lubricationhttp://www.substech.com/dokuwiki/doku.php?id=lubrication_regimeshttp://www.substech.com/dokuwiki/doku.php?id=bearings_in_internal_combustion_engineshttp://www.substech.com/dokuwiki/doku.php?id=bearings_in_internal_combustion_engineshttp://www.substech.com/dokuwiki/doku.php?id=fatigue
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REFERENCES
1.
http://www.pall.com/main/Industrial-Manufacturing/Why-is-Filtration-Important-3779.
2. Ertugrul Durak , Fazl Duran, Tribological and fatigue failure
properties of porousP/M bearing,International Journal of
Fatigue,Volume 30, Issue 4, April 2008, Pages745-755.
3. M.C. Mwanza, M.R. Joyce ,Microstructural characterisation of
fatigue crackinitiation in Al-based plain bearing alloys
,International Journal of Fatigue, Volume
25, Issues 9-11, September-November 2003, Pages 1135-1145
4. S.M. Muzakkir, Harish Hirani, G.D. Thakre, M.R. Tyagi ,
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mills, Engineering Failure
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