There is a multitude of specialist literature and articles (Lit. 23.1.1-22), which deal with the
identifikation of failures on anti friction bearings
(here named bearings) and its causes. Thereby the macroscopic
and microscopic failure mode/appearance is correlated in a systematic with the most probable cause (Ill.
23.1.3-6). These publications are often in the practical work only limited useful, because the failures
propagated already so widly, that satisfactory conclusions are no more possible. In extensive publications,
especially emphasized, are failure modes/appearances. Expert contributions rather deal with failure mechanisms and
its influence, Both do not satisfy the practician of the aero engine technology.
In this chapter those informations are combined. The typical failure mechanisms of bearings are described and explained, as well as correlated with failure modes/appearances. This shall enable to identify potential dangers in time and be aware to avoid those. Are changes observed at the bearings, the evaluation and assessment correspondent with specifications/instructions and manuals are suppiorted with a deeper understanding.
In aeroengines, two types of bearing are distinguished depending from the sort of application. Main bearings (Lit. 23.1.1-1), which carry the radial and axial (thrust bearing) loads of the rotors and bearings at the connection to accessory devices and inside these (e.g., radialdrive/tower shaft to the accessory gear).
Aeroengine bearings differ basically from those of the common mechanical engineering. What distinguishs them are the demanding operation conditions like high service temperatures and high `n x D' values (high circumferential speeds). To damp vibrationes from unbalances and prevent dynamic overloads on the supporting structures, damped or undamped radial movable (elastic or sliding guided), suspended bearings are used (Fig. "Damped main bearings").
Besides this chapter, bearing problems and failures are also discussed under separate aspects in the further volumes of these publication:
Fig. "Operation influennces of main bearings" (Lit. 23.1.1-38 bis Lit. 23.1.1-40):
'Aviation bearings' are subjected the loading types
of the common machine engineering. However the especially
demanding service conditions with highest safety requirements
are a big challenge. So weight and assembly space are of unusual importance.
This show specific designs. So the outer rings also form an application specific flange (sketch). Gear
shafts are provided with integral bearing inner rings (Fig. "Failures of gear wheels"). Damped
and elastic designs (Fig. "Damped main bearings") guarantee an optimal dynamic behaviour of the rotors. Typical for aeroengines are:
High outer loads: To these belong operation loads and stresses, according to special service conditions. Often shock forces are concerned. In radial direction act e.g., landing bumps and unbalances. Typical are failures on rotor blades or because of rotor bow (volume 2, Ill. 7.1.2-9). Thrust bearings must take unusual, high axial forces from compressor surges (volume 3, Ill. 188.8.131.52-1).
Lubrication must be guaranteed also under high g-loads, also during maneuvers, especially in military missions (volume 2, Ill. 7.1.2-12). Even the short time loss of the oilsupply must be tolerated. Aggravating acts during this, because of the decreased cooling, the oil properties in the limit range, because of the high temperatures. A special problem is the lubrication and cooling of the sliding guided, high speed rotating cages (Fig. "Mechanisms of rolling wear") and rolling elements in the cage pockets.
Extreme rotation speeds have, because of the large main bearing diameter (D x n values) of big aeroengines, a special significance (Fig. "Unbalance of rollers in bearings" and Ill. 23.1.1 -10.3). For aeroengines with little power, the rotation speeds lay above 50 000 U/min, in ranges where small damages act catastrophic within seconds. The centrifugal forces of the rolling elements demand a high part of the bearing loading capacity. Especially high demands are claimed a the dimensional accuracy, respectively the balance condition of the rotating elements (Fig. "Oscillation of the rollers sc skewing weaving"). Effects like cage slippage (skidding 23.1.1-14.1) or roller flutter (roller weaving, Fig. "Oscillation of the rollers sc skewing weaving") urge into the foreground. This is also caused by the high resistance of the oil film. Three shaft aeroengines own so called intershaft bearings. The inner ring and the outer ring of these are operating with different rotation speeds and or counter-rotating. A special problem is the supply and scavenge of the lubrication oil.
High service temperatures demand as well from the bearing materials (Fig. "Improvement of bearing materials"), as also
from the oil a load bearing capacity in the limit range. With this, the ignition of an
oil fire gets more and more probable (volume 2, Ill. 9.2-2). especially demanding is the temperature load on the hot part
(e.g., turbine). The problem are high temperatures and coking of the hot part bearings after shut off of
the engine (heat soaking, Fig. "Problema by heat soaking" and Ill. 23.3.2-6.2).
Environmental influences: Developing of condensation during the shut off periods, as well as sea salt act corrosive. This can demand the application of special materials (Fig. "Improvement of bearing materials"). Here, a corrosion cell formation with the cage material (e.g., bronze, silver plating) acts corrosion promoting .
Assembly conditions like module design can demand different combinations of the bearing inner ring with the outer ring and the cage. To this belongs also the damaging danger of the roller bearing during the pushing together during the assembly of the modules (Fig. "Causes for transport damages" and volume 1, Ill. 4.3-6).
Wear/fretting must be especially considered at oil damped bearings (Fig. "Damped main bearings"), caused by the principal determined oscillating movements. At bearing seats, relative movements caused by the light and elastic design of shafts and casings, can promote fretting. This must be considered with a suitable material selection of the bearing seat. For example the bearing innr ring must not have a direct contact with a titanium alloy. In this case a TC (tungsten carbide) coating at the shaft has proven (volume 2, Ill. 6.2-16 and Ill. 6.2-17).
Fig. "Damped main bearings" (Lit. 23.1.1-2 up to Lit. 23.1.1-4): Anti friction bearings can be
damped as well mechanically (friction) as with an
Today generally oil damped systems prevailed (frame). The oil cushion is located in a
circumferential gap at the outer ring or in a supporting casing ring. The gap is rather sealed at military applications
with sidewise, axially pressed on O rings. If the oil cushion is connected with a feeding and a
drainage (sketch above right), the oilstream can dissipate
heat. Is this not the case, it must taken care that
the O rings get not overheated (chapter 23.4).
At one design the bearing is centered by an elastic spring element („squirrel cage”). This also serves as locking against rotation. Such a spring usually has a shape similar a drum, with axial bars. At smaller bearings we find one piece designs (detail above left), larger bearings are supported by assembled „cages“ (bars as rods with threaded heads).
You also find only oil dampened designs. Here, without an elastic centering, the outer ring is secured against rotation with fixing lugs in radial slots.
Dampened bearings are preferably used near turbine disks. Compared with the stiff rotordrums of a compressor, this turbine shafts are elastic. Operation caused unbalances (thermal distortion, oxidation of the blading) often can only be controlled with damped bearings. With a centering elastic suspension, natural frequencies can be lowered from the opreation range to non-dangerous rotation speeds.
Advantages against an undampened mount:
There are also some disadvantages and potential
Fretting wear: The function caused relative movement of the outer bearing ring against the supporting structure can develop fretting at sealing and contact surfaces. This is especially serious for a direct contact between titanium alloys and the bearing ring from steel (volume 2, Ill. 6.2-16 and Ill. 6.2-17). Is the oil film „penetrated” under high radial forces, fretting wear also forms in the circumferential region.
Cavitation is a sort of material fatigue in connection with the formation of vapour bubbles in the oil (volume 1, Ill. 5.3.1-11.2). The radial amplitudes of the bearing outer ring lead inside the oilgap to heating and locally high pressure fluctuations. In the phase of the pressure drop the formation of vapour bubbles occurs. Those implode in the pressure phase and deteriorate the material surface. Cavitation is promoted by oil contaminations as water or fuel (Ill. 184.108.40.206.1-1). Expecially dampers without elastic centering tend to cavitation.
Changes of the friction conditions: Is the oil film sealed from the side surfaces, arises during the relative movement an additional friction damping. In such cases it is understandable, that the function of this system also is governed from the acting tribological system. So can changes like a wear coating as repair have astonishing negative consequences. Therefore such measures/remedies outside the instructions in the manual require a proving with the OEM.
For civil applications wth its typical long operation periods, radial tight piston rings are used (sketch below right). Its leakage guarantees a cooling oil flow. An additional damping effect has the friction of the piston rings.
Heating of the oil by absorbing damping energy leads to the warming of the oil film. However, this film thould also act cooling at the bearing. With the oil temperature, the risk of cavitation increases (passage before). If the oil temperatures rise very high, an accelerated aging of the oil occurs (chapter 22.3.1 and chapter 22.3.2).
Fractures of the struts from the elastic suspension (squirrel cage) can occur during unnormal high shock loads (forced fractures) or dynamic circumferential forces (fatigue). Skidding (Fig. "Deterioration by cage slipping so called skidding") or unbalances trigger high dynamic friction forces in the bearing, when it comes to the break through of the lubrication film, Extreme unbalances can overload bearings with a shock like circumferential force. Than the struts break in a forced fracture mode.
Fracture of the fixing lugs: Dampings without elastic reset take the circumferential forces of the bearing outer ring with lugs, which slide in flutes. Extreme unbalances can penetrate the oil film on the races. Thereby high shock like circumferential loads occur. With this, the lugs can crack or break out (Fig. "Bearing fractures by shock loads"), up to the fracture of the outer bearing ring.
Worse detectability of failures caused from unsuitable at the casing positioned acceleration pick ups: Vibrations are monitored at modern aeroengines with ascceleration sensors, which are fixed at the casings (Fig. "Condition indicator using vibration sensors"). A damped bearing, from experience can make it difficult (Fig. "Oil dampened roller bearing vibration") to register „lesser“, however for the rotor components (e.g., fatigue of the bearings) dangerous unbalances. This concerns also the partial fracture of a turbine rotor blade.
Abrasion: Damping systems without elastic resetting let the rotor sink about the thickness of the oil film (0,1-0,5 mm), as a result of the declining oil pressure during shut down of the engine. This seems indeed at the first sight little, but can markedly effect the tip gap of the blades (rubbing) and at labyrinth leaks (leakage of the oil during stand still) during start.
Fig. "Stresses between rolling elements and race" (Lit. 23.1.1-5 up to Lit. 23.1.1-9): The
friction in a non friction bearing, influences the
stress distribution inside the races. Increases the friction coefficient
between rolling element and race/ring caused by mixed friction (e.g., during cage slipping =
skidding, Fig. "Deterioration by cage slipping so called skidding" and Fig. "Skidding failure modes at races"), the subsurface
failure effective stress maximum from tension and shear dislocates to the surface (sketch above).
The lower diagram shows the stress distribution perpendicular to the surface. It applies to hydrodynamic lubrication (Fig. "Micropittings and life time") and mixed friction. During the mixed friction the high stress maximum can be identified at a locally, very narrow contact point. With this, also the deteriorating effect of foreigen objects/particles or damages of the race (e.g., indentations with `bell mouth', fusion craters) gets understandable. In its area fatigue pittings form and with this the bearing failure.
Fig. "Fatigue pittings at bearings" (Lit. 23.1.1-9, Lit. 23.1.1-10 and Lit 23.1.1-15): The
„normal” lifetime limiting failure
at anti friction bearings are break outs ( fatigue pits/pittings, detail left) on the races (sketch above).
They are caused of vibration fatigue due to rolling
contact. Its expansion takes place in rolling direction
of the rolling elements. The origin of the
cracking is usually located in the highest stressed zone
below the pitch surface/race (Fig. "Stresses between rolling elements and race"). Under dynamic overload (e.g., unbalances), serve the material
specific weak points (volume 1, Ill. 3-1)as crack starter. Under normal/design according service loads the
early origin of fatigue cracks (Fig. "Reduction of bearing lifetime by particles") is located at
damages of the races/pitch surfaces.
Concerned are corrosion pits, foreigen objects, respectively its indentations or other deteriorations like fusion craters (electric arc) and deterioration by brinelling (Ill. 23.1.1-12).
So the fatigue of the race is rather a consequence but not a cause of a bearing failure. Because of this, bearings with corrosion pits must be scrapped during overhaul. Therefore corrosion is the most frequent cause for the scrapping of bearings (Lit. 23.1.1-21). If a metallographic cross section is taken in circumferential direction perpendicular to the race (sketch below right), difficult etching zones (white etching areas = WEAs) at the crack origin can be found. Because of its geometry, they are also called butterflies, though this is not the only mode (Lit. 23.1.1-15). Obviously they are in connection with a change in the material structure, caused by extreme heating (up to softening) in the micro region under the dynamic load. WEAs may apply as signs of dangerous high dynamic loads. Its hardness is above this of the base material. This can be explained with a fast cooling from high temperatures (new hardening). The frequency of the WEA depends from load and time. It is about an irreversible material change/deterioration by fatigue.
Are there already break outs/pittings, the failure will accelerate. Thereby the notches and spalled race particles act as foreign objects (Fig. "Reduction of bearing lifetime by particles"). In spite of this, the failure propagation is in the most cases relatively slow. So it is possible, to identify with sensors (magnet plugs, magnet sondes, chapter 22.3.4) in the oil circuit sufficient early a catastrophic failure of the component. The experience shows. that merely bearings of small, very high speed aeroengines (helicopter engines, APUs), develop failures too fast, so that they can not be intercepted.
Fig. "Causes for fatigue at races", -5.2 (Lit. 23.1.1-10 ): The
fatigue of races from anti friction bearings
depends from very many influences, which can also combine. In spite of the offer of systematic specialist literature
it is not easy to suggest, sufficient certainfrom the failure mode/appearance of a bearing at the
causative influences of a fatigue. This is further aggravated from
secondary failures (e.g., roll overs),
which have changed the informative beginning/primary failure.
The summary shall give an impression of the variety possible failure relevant influences without claiming completeness.
Material (Lit. 23.1.1-21): In the bearings
of aeroengines the fatigue strength/
rolling fatigue strength and/or wear (during
mixed friction like in the case of skidding), is the
life governing role. As main cause for the
exchange of bearings during overhaul, the
expert literature however mentions corrosion
damages of the races. But there is no single optimal material property for all service influences.
The experience shows, that fatigue failures of aeroengine bearings rather are in connection with race deteriorations. Less frequent is fatigue at coarse impurities in the material structure. Therefore high rolling fatigue strength demands especially for bearings of aeroengines, which usually run at EHD conditions, an insensitivity as high as possible against particle indentations (Lit. 23.1.1-23). For this is, contrary to bearings with high hertzian compression of the usual machine engineering (Lit. 23.1.1-6), a little less hardness in favour of a better ductility is more beneficial. So the notch effect can be lowered by plastic deformation. The slightly lower hardness also leads to a slower growth of the fatigue cracks. This increases the chance, to prevent a catastrophic bearing failure (Ill. 23.1-5.2). A disadvantage of lower hardness is a higher sensitivity for `brinelling' (Fig. "True and false brinelling") and wear/abrasion at mixed friction.
Fatigue processes at surfaces/races of of rolling faces (Fig. "Mechanisms of rolling wear") may be the reason for the recommended hardness in the range of 62-64 HRc (Lit. 23.1.1-28).
Also abrasive wear can be minimized with a hardness of the race as high as possible.
In contrast adhesive wear (galling/seizing) is less influenced from the hardness. This, for example, occurs during overload of the bearing with a break through of the oil film or during lack of lubricant.
For the rollig fatigue strength, according to the
design, the cleanliness of the bearing
material is of highest importance (Fig. "Improvement of bearing materials"). Good fatigue properties demand a fine and uniform
material structure. Unfortunately this can be only difficult realized with
corrosion insensitive bearing materials. Here larger Cr
carbides disturb, which are traced back to the addition of corrosion protecting chromium.
This dilemma seems to be solvable with highly nitridated bearing
Gear shafts in auxilary gears exist, at which integral bearing race rings come to application (Ill. 23.2-3). For this the inner ring is joined with the shaft by electron beam welding or friction welding. A sufficient hardness of the race is realized with case hardening (Fig. "Improvement of bearing materials"). Influence of dimension and shape: Good measurable, even if requirements for very tight tolerance and low roughness guarantee service properties, according to the design. Perhaps therefore, there are no reports about failures caused by dimensional production deviances. However, it is absolutely possible, that the combination of shape and measure variations inside the prescribed tolerances trigger failures (Fig. "Unbalance of rollers in bearings" and Fig. "Unbalance of forged balls"). Thereby dynamic instabilities of the roller movement are of importance (flutter/ weaving, Fig. "Oscillation of the rollers sc skewing weaving").
An elastic deformation of the bearing rings is possible by the seat, as a result of stiffness differences at the circumference or distortion during service. But those obviously are controlled by dimensioning and design. To prevent skidding (Fig. "Deterioration by cage slipping so called skidding"), the outer ring is elastic deformed (oval, trisectioned, Fig. "Avoidubg skidding with oval race tracks"). With testing it must be proved, that locally mixed friction can not occur and damage the race.
Important is the control of unbalances of the rolling elements (Fig. "Unbalance of rollers in bearings", Fig. "Unbalance of forged balls" and Fig. "Forces and moments inside a bearing") and of the cage (Fig. "Bearing behaviour by guidance of the cage").
Deteriorations during operation are frequently traced back at aeroengines to foreign objects /particles in the oil (chapter 220.127.116.11). To this belong auxilary materials like blasting grit/shot (Fig. "Ol filter helping for diagnistics").
Further potential causes for the deterioration of races:
Assembly, repair and transport can deteriorate anti friction bearings in different ways:
Plastic deformation of the bearing during the assembly process (Fig. "Critical joining of big engine modules 2").
Brinelling during transport (Fig. "Causes of brielling on bearings" and volume 4, Ill. 18-10).
Design/dimensioning: Here are unexpected axial loads at thrust bearings to mention Concerned can be too high or too los loads. They are in connection with unusual gaps, respectively lekages of the air seals. These disturb the balance of the design conform axial forces (piston forces, volume 2, Ill. 7.2.1-2).
Was a unsuitable cage guidance/lubrication choosen (Fig. "Bearing behaviour by guidance of the cage" ), this can lead under certain circumstances to an almost spontaneous destruction of the bearing.
Too weak cage material or riveted cages (Fig. "Forces and moments inside a bearing" and Fig. "Oscillation of the rollers sc skewing weaving") are especially sensitive for unexpected load changes (e.g., unbalances, increased forces of the rolling elements).
Oil problems are deeper discussed in chapter 22.3. These are great challenge, especially for bearings of aeroengines with ecxtreme circumferential velocities and temperatures.
Fig. "Micropittings and life time" (Lit. 23.1.1-10 and Lit. 23.1.1-18): For roll off surfaces in an oil three different
lubrication conditions are known.:
At hydrodynamic lubriaction the lubricating film about 2µ thick is sufficient, to fully separate the roll off surfaces. During this pressures of several thousand bar develop in the lubrication film. Thereby no noteworthy elastic deformation of the surfaces will occur. However the high shear forces in the lubrication film produce much heat. It heats the lubrication film and so promotes the aging (chapter 22.3).
Boundary lubrication/mixed friction has a 0,001-0,05µm thickness of the lubrication film. A touch of the roughness tips from the pitch surfaces is not avoided. This condition can be expected at low rotation speeds during start and shut down. In this case the lubrication is determined from chemical and physical properties of the lubricant, as well as from the wetted surfaces. Here the viscosity is not of significance.
Predominantly at bearings of aeroengines is the so called elastohydrodynamic lubrication (=EHL). The thickness of the lubricant film lays here in range of 0,05-2,0µm. The pressures in the lubrication gap are with about 20 000 bar extremely high. This is sufficient for markedly elastic deformations of the pitch surfaces (see marked region in the upper sketch). With this a lifetime relevant fatigue load of the races develops. The viscosity of the oil (no Newtonian fluid) markedly increases by the pressure. This improves the bearing strength of the lubrication film.
In the range of the roughness tips, where material typical hard structure components act (sketch above), it comes to peak stress under EHL-conditions (extreme pressurese, high oil viscosity). Thereby so called running-in pittings (Fig. "Mechanisms of rolling wear") in the size of some 'µm' can form (detail below left).
For the lifetime of the bearing, the thickness of the oil film h0 and the accumulated roughness of both opposing surfaces respectively its ratio S (detail below right) are of importance (Fig. "Influence of lubrication gap at lifetime").
Does it come in case of boundary lubrication (low rotation speeds during start and shut off, overload of the bearing) to the contact of the roughness tips of the pitch surfaces, the deteriorating effect can be compared with the bridging by contamination particles (Fig. "Influence of lubrication gap at lifetime").
Fig. "Mechanisms of rolling wear" (Lit. 23.1.1-10 , 23.1.1-16 and Lit. 23.1.1-17): The high magnifications of today
usual SEM investigations (= Scanning Electron Microscopy, volume 4, Ill. 17.3.2-7) make the
phenomenom assessable. In contrast, photo-optical misinterpretations and a false evaluation of the failure
relevance are rather probable.
Minimum lubrication thickness leads at tiny roughness tips („A“) after many roll-overs, to normal microscopic wear of the race. Such a roughness is typical for grinded and polished races of anti friction bearings. This wear does not influence the design life time. It shows as tiny fatigue break outs in the surface region. If not deeper cracks occur we speak about run in pittings.
During this mechanism, first the roughness tips are plastically deformed to a layer in the µm-range („B”). This work-hardened zone covers the structure of the base material with its typical fine dispersed carbides, necessary for the function.
Rises the load and/or decreases the thickness of the lubricant film (Fig. "Micropittings and life time"), the strain hardened layer separates („C“). Micro-fatigue cracks form, during the direct contact with the carbides, which cause the development of further break-outs in the size of few µm („D”). However these microspalls (micropittings) don't cause in this run-in period larger, dangerous fatigue break-outs (fatigue pittings, Fig. "Fatigue pittings at bearings").
At high speed roller bearings and very low unbalances, the danger exists that the rolling motion passes into a sliding motion (slip, Fig. "Influence of lubrication gap at lifetime"). Thereby the cage is decelerated (cage slip, skidding , Ill. 23.1-14.1). This can lead up to the short time standstill of the cage. Thereby, at the beginning the mentioned surface symtoms arise. Increasing this effect, a markedly lifetime reducing deterioration µm thin plane particles of several tenth millimeter size can develop.
Fig. "Influence of lubrication gap at lifetime" (Lit. 23.1.1-25 ): With
EHL-lubrication (Fig. "Micropittings and life time") the lifetime of an
anti friction bearng is in relation with the roughness of the
race (diagram above left).
We see, that the lifetime of roller bearings reacts especially susceptible on the roughness. Roller bearings with the typicaql larger lubricat on gap surfaces, compared with the ball bearings, reach the maximum lifetime already above2 µm lubrication film thickness. Obviously ball bearings reach this condition only at much thicker lubrication films.
The diagram above right shows the influence of the lubrication gap on the lifetime of a roller bearing. A wideness of the lubrication gap, which correlates the double roughness „h0“ (S =2,0), enables maximum lifetime. This optimal thickness of the lubrication gap is determined by the bearing clearance. So it must be considered during design and production.
Occurs in the lubrication gap of a roller bearing skidding (e.g. during cage slip) between the rolling surfaces, withn a breach of the oil film, „S” approximates 0. With this in correlation to the example in the diagram below left, the lifetime drops by the factor 100.
To avoid skidding of the cage, minimum friction between the rolling surfaces is necessary. It must guarantee the impellent of the cage, to overcome the resistance of the oil film against the friction of the rolling elements and the cage (Fig. "Influences at cage slip"). So, to avoid slip, in correlation to the diagram below right a minimum friction coefficient respectively traction coefficient is necessary. Understandably this is for a narrow lubrication gap (S =0,5) much higher and is reached markedly sooner than for a wide gap (S = 2,0). A minimum unbalance can be enough to prevent slip/skidding.
Fig. "Reduction of bearing lifetime by particles" (Lit. 23.1.1-7): In aeroengines,
particles in the oil must be expected. So the typical
high temperatures promote coke formation (chapter 22.3.2). Additionally contaminations of different
origins can appear in the oil (chapter 18.104.22.168). Particles become
failure effective, if they are thicker than the bearing oil film
(Fig. "Micropittings and life time"). The thickness of the lubrication film under EHL conditions is in
the range of 0,05 -2,0 µm (Fig. "Influence of lubrication gap at lifetime"). In contrast a human hair is about 100 times thicker (sketch
Does a dangerous particle (size, hardness) get between the races and rolling elements, in every raceway an indentation forms (sketch above right). Typical is a ring shaped bulging around the indentatation. This can break through the lubrication film. Mostly particles or its fragments are centrifuged or washed away. However, in some cases at least particle fragments got stuck in the raceway. At beginning failures they allow an identification. This is the precondition for targeted, prevention of such contaminations. Later the bulge will be plastically flattenend by rolling.
The bulge can be considered as the main cause for failures of antifriction bearings. The metallic contact disturbs the rolling cinematic and triggers sliding effects as well as stress peaks (Ill. 23.1.1-8). After some time from them a fatigue failure will develop (sketch below right). A shortening of the lifetime, up to a factor 1000 must be expected.
This failure effectivity of the bulge serves as justification for a polishing rework of the races. Tests showed: Are the bulges polished away, in spite of this the indentations will remain, the lifetime can be increased again about the factor 10-100. Naturally such a regeneration effect can be only used, if not already a markedly fatigue deterioration has occurred. To evaluate this is a problem which can not be underestimated. This may be the cause, why repolishing, if at all, can only be recommended for, from experience known sufficient low loaded, used bearings (e.g., in gears). Naturally such a rework seems especially for the expensive main bearings lukcrative. Basically rework must be carried out only according to the manual respectively instructions.
Note: The repolishing of the races, if necessary from used anti friction bearings, must be close to the specifications/instructions, respectively the manual. This appies as well for the process as also for the bearings (location of the application, type of the bearing).
Fig. "Unbalance of rollers in bearings", Fig. "Unbalance of forged balls" (Lit. 23.1.1-8, Lit. 23.1.1-27 and Lit. 23.1.1-31): Demands for higher
performance concentration and higher pressure ratios with as few compressor stages as possible, cause
always higher rotorspeeds (Fig. "Tendency of bearing speeds"). This trend affects the aeroengine bearings. This high-speed
(diagramm right) with according high
dm x n values (dm = diameter of the pitch circle from the center
of the rollers) leads to special effects. Also little, in the usual machine engineering, unnoticed
unbalances of the rollers (100 mg.mm
are considered as an acceptable limit), now get failure effective. The
rolling elements itself reach with this rotation speeds around their own axis in the range of 100 000
rpm. Because of this, for aeroengines the unbalance of the rolling elements is limited/specified and
checked for every single one. Unbalances can be caused by little summarizing
inhomogenties of the material structure
(Fig. "Unbalance of forged balls", sketch above) and geometrical unbalances
respectively dimensional deviations
within the specified tolerances (so no failure!). To this belong especially the
edges of the rollers, which are also changed by wear during service (Fig. "Oscillation of the rollers sc skewing weaving").
Unbalances of cylinder roller bearings: Under a
static unbalance (frame above right), a
symmetric dual mass unbalance at the roller axis in the
resting condition is understood. The masses can
be positoned at the same side or 180° offset. The static unbalance does not directly affect the
operation behaviour of the bearing. However it can influence corrosion during stand still, at the contact
surfaces of the rolling elements.
Of high importance is a dynamic unbalance during operation. It can trigger high loads at the edges of the rollers and flutter movements (skewing, weaving) around a radial axis (Fig. "Oscillation of the rollers sc skewing weaving"). Because of nonuniform load distribution, rollerweaving can cause high stresses at the contact surfaces to the races. Thereby the roller edges rub increasingly at the skirt of the bearing ring and the edges of the cage pockets. This leads to relatively high friction forces with a brake effect at the cage. The increased driving power at the cage, needed for slip free run, must be delivered from the rollers in the circumferential region of the load transfer. This causes high shear stresses at the load transferring rolling surfaces. Is such a penetration of the lubrucation film triggered, it comes to extremely local bearing temperatures and adhesive wear. The consequence is a high material stress with markedly shortened life time. Are radial loads too small for the drive forces of slipping rollers, skidding will occur (Fig. "Deterioration by cage slipping so called skidding" and Fig. "Skidding failure modes at races"). In an extreme case the cage will stop for a short time. In this time period the anti friction bearing functions as a friction bearing.
Dynamic unbalances of the balls (sketch below in Fig. "Unbalance of forged balls") are also dengerous. A ball should find for every revolurion a new race track. The corresponding pattern of the race track is called „ball of wool effekt“. However an unbalance can cause a stabilisation of the axis of rotation. Then the ball rotates aroud its axis of inertia. The most frequent overruns in the new fixed track shorten the fatigue lifetime during a adequate high load. This can be an explanation for the fatigue failure of a single ball in a bearing.
So the chance of a failure clarification with a microscopic follow-up investigatiion of a not too badly damaged ball surface exists. This chance should be given after test runs or certification runs of bearings and can point at unbalance problems.
The unbalances of a ball are caused by density deviations from enrichments of alloy elements/segregations, a special orientation of the material structure (sketch above in Fig. "Unbalance of forged balls") as result of the forging process for the ball shape, or other inhomogenities of the material structure.
Fig. "Improvement of bearing materials" (Lit. 23.1.1-28): There are
two main groups of bearing materials in aeroengines.
Through hardened, which are especially used in main bearings.
Case haredned materials are used in smaller bearings of gears. These are even in some cases an integral element (Fig. "Failures of gear wheels") of gear shafts.
To reach the material pureness necessary for a sufficient fatigue strength, the
melting must take place in the vacuum (`V…' processes in the diagram). Indeed, such materials show no contaminations.
But under very high dynamic loads (specified, i.e. weak points),
inhomogenities like carbides determine the designed lifetime. Then they are the origin of fatigue failures.
Generally a historical trend to higher fatigue lifetimes can be noticed.
In both cases, the minimum hardness of the rolling surfaces from a bearing is abbout 58 HRc (Fig. "Influence of beariring material hardness").
Through hardened materials have been developed for aeroengine main bearings in the direction to high service temperatures. Concerned are alloys with molybdenum, tungsten, chromium, vanadium, cobalt, aluminium and silicium, which enable sufficient hardness. A representative of this type is `M-50' with 0,8% C, 4,0% Cr, 4,25% Mo for survice temperatures up to 315°C. Indeed the lifetime of the bearing rises at elevated operating temperatures, compared with lower alloyed materials. However, at lower service temperatures the bearing life time drops with the amount of alloy components (Fig. "Fatigue lifetime of bearing materials", Lit. 23.1.1-32).
In the last decades, primarily improvements have been achieved with material structure optimizations like grain orientation and alloying. To this belongs a residual austenite under 3% . For the use in corrosive atmosphere, high chromium contenting materials like`440C' with 1%C and 17%Cr are accessed. But this `corrosion -resistant' material (diagram) can only be used up to about 170°C. It also shows a weakness in the fatigue strength.
Even more corrosion-resistant bearing materials can be expected from highly nitridated Cr alloys (e.g., 15%Cr, 0,35%N, 1%Mo; Lit. 23.1.1-21). These show no weakness of the fatigue lifetime (Lit. 23.1.1-21).
Case hardened materials usually have a carburised hardening zone of > 0,4 mm thickness. Its hardness is in the range between 58 and 63 HRc. The core hardness lies under 48 HRc. They have a service temperature limit of about 170°C. From the high impact strength and fatigue strength is expected. This is especially interresting for parts with loads from outside (shafts). So these materials are predestined for the use in auxilary gears.
Fig. "True and false brinelling" (Lit. 23.1.1-28, Lit. 23.1.1-29 and Lit. 23.1.1-30): During stand still anti friction
bearings can be deteriorated by vibrations. This is called
„brinelling”. Such vibrations primarily occur
during the transport of the whole aeroengine or its modules (Fig. "Causes of brielling on bearings"). Because the, during service
existing separating, damping oil film is displaced, during
stand still the rolling elements under pressure of
the rotor weight or a mass force get in direct
contact with the race rings. So the not damped
impact forces and micro relative movements act wearing (fretting).
We distinguish two variants:
'True brinelling' (upper frame) is a race deterioration at which the force transferring rolling elements produce plastic deformations in the race of the bearing ring. They require a lifting and pitching of the rolling elements. Such overloads during stand still, can be expected from shocks like shunting of wagons, vibration caused by rail joints or road holes. Are assembly devices with modules or aeroengines moved over an uneven hall floor (e.g., contact faces of concrete slabs), the danger of brinelling exists. Obviously also during ultrasonic cleaning, true brinelling was observed (Lit. 23.1.1-16).
`False Brinelling' (frame below) is an unevenness of the race at the contact zone of the rolling elements, caused by wear (fretting). They are the result of micro movements, caused from vibrations. During this process the rolling elements don't lift off. This deterioration does not require vibration forces, corresponding the true brinelling. A `normal' transport or vibrations during the cleaning process (acting of ultrasonic sound, volume 4, Ill. 22.214.171.124-7.1) are sufficient. Here the extreme case of standing contact fatigue (= SCF) should be mentioned. In this case a dynamic load produces through a contacting ball ring shaped fatigue cracks. This load is used in test devices for materials, especially of case hardened surfaces.
Fig. "Causes of brielling on bearings": Vibrations can cause true brinelling (Fig. "True and false brinelling") during the transport of not suitable supported aeroengines or unsufficient tensioned rotors. From experience, such deteriorations trigger, especially on small aeroengines with typical high speed rotors (e.g., APU, helicopter engine) already during start and run up catastrophic failures.
Fig. "Deterioration by cage slipping so called skidding" (Lit. 23.1.1-10, Lit. 23.1.1-20, Lit. 23.1.1-25, Lit. 23.1.1-26 and Lit.
23.1.1-32): Cage slippage
(skidding) primarily occurs at
roller bearings. But there are also single cases, when
this phenomenon concerns ball bearings. During skidding, the rotation speed of the cage is slower than
the roll off of the rolling elements would require. So there are no clean kinematic conditions.
Therefore sliding (slippage) of the roller elements
on the race occurs. In an extreme case, the cage stands
still and the bearing acts as a friction
The conditions for skidding are given, when the driving forces are not sufficient to equalise fhe braking forces which act on the cage (friction on the guiding faces, and the cage pockets). This situation can have several causes:
The slipping rollers are extremely accelerated
in the load transferring zone, during thre break
through of the lubrication film (details below). The metallic contact at the face to the race ring takes
place under high relative velocity. Thereby weldings
(galling, seizing) occur, plastic
deformations (detail above) and
overheatings. At race rings and rollers forms a typical
„dart pattern“ which grows over the circumference into extensive rough deteriorated areas (Fig. "Skidding failure modes at races").
The driving forces pulsating with the circulation of the rollers, cause high stresses in the cage pockets (Fig. "Forces and moments inside a bearing"). The result is heavy wear/abrasion and fractures of the bars/lands.
As remedy against skidding serve measures, which guarantee every time sufficient radial forces. Usually this is a sufficient minimum unbalance. In especially critical cases, slight elastic deformations (e.g., ovalisation) of the outer bearing ring (Fig. "Avoidubg skidding with oval race tracks") can be used.
Fig. "Influences at cage slip" (Lit. 23.1.1-32): This schematic diagram shows the dependence of the cage slip (skidding) from the radial load of the bearing. These are the driving forces of the cage. The necessity of a minimum load can be seen . The slip rises with the rotation speed. This is explainable on the one hand with increased hydrodynamic friction forces, which act on the cage, on the other hand also the centrifugal forces may become noticeable. These add at high circumferential speeds remarkedly to the decrease of the contact pressure at the inner ring.
Fig. "Skidding failure modes at races" (Lit. 23.1.1-10, Lit. 23.1.1-16, Lit. 23.1.1-20, Lit. 23.1.1-32 and Lit.
23.1.1-33): Failures caused by cage slip (skidding) have in the development phase a typical
appearance. Dartlike dull zones form (sketch left). These are more rough than the unhurt race surface. In
the micro range, with the help of the SEM evidence for a deterioration from the contact of
the metallic surfaces under high relative velocity can be identified. Concerned are flat
micro beak-outs (microspalls, Fig. "Mechanisms of rolling wear"),
microcracking and plastic deformed regions
(smeared grinding structure).
In the advanced stadium, the dull structures at the circumference grow to circumferential ring areas (sketch right), till they cover large areas of the bearing ring race (mostly the innwer ring, Fig. "Deterioration by cage slipping so called skidding").
At the extension of the skidding deterioration small tongues („streamers”) can be identified. They follow the usual fine machining marks (Fig. "Mechanisms of rolling wear"). This appearance can be seen as an affirmation of the described failure mechanism.
For a forced overload of the bearing two effects may get effective:
From experience, damped supported anti friction bearings (Fig. "Damped main bearings") without elastic anti-rotation device, show after such an incident forced failures. To these belongs (sketch above)
In such a case, the lands of the elastic fixing of a dampened bearing are highly endangered by a forced fracture.
Fig. "Bearing behaviour by guidance of the cage" (Lit. 23.1.1-34 up to -36): For the operation behavior of a high speed
anti friction bearing, the type of the radial cage
guidence is of special interrest.
At the circumferential surface (bord), which serves the guidence of the cage, friction heat is procduced in the oil film and/or during mixed friction in direct metallic contact. This leads to heating and thermal expansion of the cage. It heats the cage, especially at a local sliding contact. The expansion, limited at a fraction of the circumference, triggers unbalances.
An inbord guidance hinders by centrifuging the supply of lubrication oil to the sliding surfaces of the cage. The heating expands the cage and so it takes off. This widening of the gap improves with an increasing oil entrance the cooling. At the same time the sliding surface, which must divert the friction heat, gets smaller. This acts stabilising at the cage temperature. Adversely is a worsened guidance, which causes an increasing unbalance. This can locally overload the cage. Outbord guided cages don't get unbalanced, even during heating.
The lubrication is promoted by the centrifugal forces and presses oil from the race into the lubrication gap. However, the centrifugal forces incease the friction forces and with this the heating. This leads to thermal expansion with the increase of the diameter and so the slow down of the cage, up to jamming during stand still. With this it can be expected, that bearings with outbord guided cage are also more susceptible for cage slip (skidding, Fig. "Deterioration by cage slipping so called skidding"). The increase of the friction forces is interactive connected with equivalent heat procuction. It comes to the deformation of the cage and high local loads. With the friction force on the cage, the necessary driving forces, which act from the rolling elements increase. This causes in the cage pockets high forces (Fig. "Wear loaded sliding surfaces in bearings") and friction heat.
Fig. "Forces and moments inside a bearing" (Lit. 23.1.1-35 and Lit. 23.1.1-37): The sketches above show a summary of the operation forces on the cage and the rollers. This gives an impression of their complex interaction. In detail the following forces/loads and its causes are concerned:
The forces act at the cage, rolling elements and race rings, whereat they influence each other. The oscillation of the balls, during passing the loaded circumference area (frame below), pumps dirt and aging products of the oil (chapter 22.3) between the two parts of a riveted cage. This can cause the expansion and loosening of the rivets up to their fracture.
Fig. "Oscillation of the rollers sc skewing weaving" (Lit. 23.1.1-26, Lit. 23.1.1-27 and Lit. 23.1.1-37): Under
roller skewing (gyration, weaving, flutter) a random circular movement is understood around an axis, which is not parallel to the
bearing axis. This leads to a load at the roller
ends/crowns with icreased wear at the roller
faces, the cage pockets and the bords of the race rings. In an extreme case, the cage pocket gets so heavily worn,
that the roller can rotate inside, fully around the radial axis (sketch in the middle).
We distinguish, accordant to the cause, the following types of roller skewing:
Misaligned skewing can already occur at a misalignment of 0,0002 mm/mm. Such misalignments appear at not sufficient careful assembled shafts or during operation durig the flexing of the shaft (unbalances).
Roller gyration skewing is caused by unbalances of the rollers (Fig. "Unbalance of rollers in bearings"; frame below, left sketch). Typical shape determined causes are:
These influences must act diagonal at the opposite roller ends (frame below, left sketch).
Fig. "Intenal currents in bearings" (Lit. 23.1.1-41): It is known, that electrostatic and electromagnetic caused currents, especially in industrial used turbo-machines can trigger failures (Fig. "Electrically caused bearing failure"). Also aeroengines may have a potential for such problems. These effects can develop in two manners:
This occurs without , eye-catching syptoms at the outside. The electric voltages between rotor and casing, respectively the suspension, get only alarming above a certain limit. It is very machine individual and depends on many influences. In such a case, only the OEM if at all, can give an advice.
Experience (without engagement):
With targeted voltage measurements (oscilloscope) at rotors, conclusions can be drawn if there is suspicion. Measurements have to be carried out at every single system which has a coupling (e.g., spline coupling). Thereby it is assumpted, that a current circuit is not always guaranteed. If there is accessibility, a suitable metal brush, like it is used in in mechanical rotating transducers (collector ring), can serve as probe/sensor.
The waveform of the voltage can enable conclusions at the cause. Voltage peaks let evaluate the danger.
Electrostatic shows sharp rectified voltage peaks in constant time periods. The signal depends from service conditons like pressure and temperature.
Electromagnetic produced voltages show a much more regular wave form. The frequencies can be assigned the harmonics/natural frequencies of the shaft rotation speeds. From experience here exist no comparable dependence of the electrostatic effects from the operation conditions. Potential results of circuit continuity are overheating/weld puddles from electric sparks at bearing races, gear teeth flanks and coupling faces. There was for the author no literature available about the endangering of the functions from always more frequent used electric components at aeroengines. However, it can be supposed, that unsufficient protected control units and sensors can react sensitive, respectively are endangered.
Fig. "Bearing failure by lightning strike" (Lit. 23.1.1-11 and Lit. 23.1.1-12): Lightning strike represents a danger, especially for turboprop engines (volume 1, Ill. 5.1.3-4). Concerned are especially antifriction bearings and friction bearings as well as gear tooth flanks in the propeller gear (sketch above). Typical feature is a series of small weld puddles respectively in the advanced condition, fatigue out breaks (pittings). The failure mechanism is shown in the frame below. For the existence of deteriorating electric sparks, respectively arcs, a separating oil film is necessary, which is penetrated by them in short time periods.
Fig. "Bearing behaviour by oil outage" (Lit. 23.1.1-36 and Lit. 23.1.1-37): A good sliding behaviour of the numerous sliding surfaces (Fig. "Forces and moments inside a bearing") from the cage of an antifriction bearing a silver plating provides. This is especially important for military aeroengines. Here, few minutes oil shut-offs are required. These consider hindered oilintake and oil hiding (Fig. "Oil shortage by oil hiding") during especial manoeuvres in flight (e.g., upside-down flight). For such extreme´flight conditions, former cage materials are no more sufficient (bronze with platings of lead and silver). Therefore today cages from steel with a 0,025-0,05 mm thick silver plating are used. Beyond expectations, tests have shown, that a thicker silver plating (0,1-0,2 mm, diagram) acts adverse. Reason is the total moulding of the ball at the cage land. It comes to smearing of the silver and high friction forces. Obviously no sufficient oil film can develop under such conditions. The decelerating of the balls prohibits a kinematic rolling and so triggers `skidding' (Fig. "Deterioration by cage slipping so called skidding"). Further consequences are an extreme cage temperature and the fracture of the cage.
Fig. "Magnetisation of bearings": Magnetised antifriction bearings can get in different ways failure effective.
Magnetic crack detection: If the necessary demagnetisation was imperfect, critical magnetic fields will remain.
Lightning strike: Pulsating cocurrents magnetise the bearing (Fig. "Bearing failure by lightning strike").
Cocurrent flows caused from external currents like from welding. A further possibility is the magnetising by magnetic fields of electric driven transport vehicles. Therefore may an OEM explicit forbid such transports.
Fig. "Electrically caused bearing failure" Lit. 23.1.1-42 op to Lit. 23.1.1-45): Electrical continuity is a big problem of bearings. The currents can be associated with external/parasitic currents from electric machines (motors, generator, starters, Fig. "Bearing failure by electric continuity"). But they can be also produced by rotating magetised components (gears, shafts, bearings, Fig. "Intenal currents in bearings"). The danger may rise with the concept of the so called „more electric engine“. Here the starter-generator sits on a main shaft (at two shaft engines the high pressure shaft) and is driven by it (sketch above). Thereby the generator power of bigger fan aeroengines can be up to some hundred kW. The accessory equipment is no more driven by a radial shaft (tower shaft), but electric. A power line seves as connection. So a conventional accessory gear lacks. This configuration has some advantages:
Is alternating current with variable
frequencies concerned (variable-frequency drive = VFD),
what can be expected for the spectrum of the rotation speeds, an epecially high danger of electric
bearing deterioration exists. This is connected with the
voltage peaks of the electronic frequency control
The increased introduction of plastic components like rotorblades (fan) out of fibre reinforced plastic, can promote the electric charging in the airstream. Does the discharge take place through the bearings these are endangered. Therefore is the knowledge of the typical failure modes for current continuity important (Fig. "Bearing failure by lightning strike").
'Frosting': This term comes from an macroscopic appearance of the failure zone, similar to frost (sketch below left). It is used for different failure causes and failure mechanisms like rolling fatigue (Fig. "Mechanisms of rolling wear"), circuit continuity or cavitation. To the naked eye the dull surface seems like dry blasted. Concerned is the beginning phase of the deterioration. Under the elektron scanning microscope (SEM) a multitude of tiny weld puddels can be seen. It is important not to confuse during macroscopic evaluation this failure with rather harmless running-in pittings (Fig. "Mechanisms of rolling wear").
'Fluting': Applied is the so called „ripple formation”, an advanced failure mode. It indicates from the outside with intense vibrations. During the elektroerosion (electric discharge), longitudinal ribs have formed (flutes, sketch below right), which also are covered with tiny weld puddles.
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23.1.1-13 NTSB Identification: FTW951A064, „Incident Dec-12-94, Aerospitale ATR 72-212”, page 1.
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