23.1.3 Prevention and remedies of anti friction bearing failures. Failure investigation and assessment

 Prevention of bearing failures

For anti friction bearings applieds generally: A targeted and successful remedy demands the knowledge of the causative influences (Fig. "Failure modes and appearances for identifications 1"). However this can be, different as to expect from many published systematics, just for anti friction bearings more difficult than for many other mechine elements (Fig. "Bearing failure by Interaction with accessories"). For success, we must direct over sufficient knowledge about the symptoms and specific loads of the particular application. These must be identified and evaluated.

Are the causes known, measures respectively remedies can be developed and introduced. At operational overloads improved bearings and/or load decreasing measures may be used (Fig. "Hints at bearing problems" and volume 2, Ill. 7.2.1-3). Problems with the supply of lubrication oil (Fig. "Oil deficiency shaft failure sequence 1") must be targeted approached in the system. Typical, example is the coking in the bearing area (Fig. "Problema by heat soaking", Fig. "Importance of resting time at idle" and Fig. "Oil coaking endangers main bearings").

 Hints at bearing problems

Fig. "Hints at bearing problems" (Lit. 23.1.3-1 up to Lit. 23.1.3-5): For aeroengines it is already a success, if symptoms are identified, to indicate the problems of a bearing if possible before its failure (Fig. "Identificatgion techniques of bearing failures").
During and outside the operation, direct or indirect observed anomalies at a bearing can give the experienced specialist hints at the problem. Thereby it can be concluded at already existing bearing failures and causative influences. This task is especially demanding, because most of the symptoms can have different reasons. The combination of several anomalies can markedly increase the certainity of the conclusion. (Fig. "Engine health management"). In the most cases, only a first evaluation of the situation is possible, however it can not replace a failure investigation (volume 4, Ill. 17.1-11).

A high bearing temperature is a sign, that a failure is dangerously far progressed and/or the bearing runs under unusual service conditions. Therefore there is little chance to identify the failure in time before a failing of the bearing. At main bearings the oil temperature can give a hint. A conclusion at the concerned bearing is hardly possible without additional observations like chip deposition rate (Fig. "Monitoring particle formation in oil") or vibrations (Fig. "Condition indicator using vibration sensors"). Unusual high temperatures of the bearings from accessory gears can be identified by outer discolourations of the casings/boxes or escaping oil during stand still within maintenence processes, can be caused from a deteriorated seal (Fig. "Problems of radial sealing rings"). In most cases a dangerous bearing temperature is the result of friction heat. This condition can be expected in the endphase of a bearing failure (Fig. "Bearing behaviour by guidance of the cage" up to Fig. "Oscillation of the rollers sc skewing weaving").
High service temperatures as failure cause can develop in different ways:

  • Unsufficient oil supply, e.g., if the oil supply is blocked by coking of pipe lines (Fig. "Oil coaking endangers main bearings") and/or oil jets/nozzles or because a leak (Fig. "Oil deficiency shaft failure sequence 1").
  • Too high oil temperature, e.g. as consequence of an unsufficient oil cooling or the failing of the thermal insulation of a bearing schamber in the hot part region. In the extreme case, oilfire can heat the bearing short-term up to a failure (volume 2, Ill. 9.2-2).
  • Overloading of the bearing by unusual operation conditions like high unbalances. Typical trigger is a blade fracture. Further causes are extreme axial loads, caused by a labyrinth failure (volume 2, Ill. 7.2.1-2).
  • Design caused too high bearing loads.
  • Dimensional problems at the bearing itself, e.g. too small bearing clearance (Fig. "Skidding by unsuitable bearing tolerances") or at the bearing seats (Fig. "Avoidubg skidding with oval race tracks").

Noise: The high noise level of an aeroengine may prevent during operation this identification possibility of a failure. However, during maintenance work, unusual noises during rotating of the rotors, may allow the suggestion at the beginning of a failure. However the assignment may be problematic, for other formation possibilities like rubbing.

Vibrations can be an informative indication of a bearing failure. The casings of aeroengines usually are supplied with several acceleration/vibration sensors, which warn, when an acceleration limit is passed (Fig. "Elements of a vibration monitoring" and Ill. 25.2.1-9). Is in an aeroengine type a bearing identified as weak point with, during the failure occurring vibrations (Fig. "Compressor surge by oil loss", location at the aeroengine, Fig. "Condition indicator using vibration sensors"), this allows more exact conclusions.Are vibration frequencies measured by the sensors, the chance exists, to conclude at the concerned shaft/rotor system.

Increased friction, torque moments can show during an advanced main bearing failure in a markedly braking/drag of the rotor (Fig. "Compressor surge by oil loss"). Thereby the normal operation speed will not be reached and/or the rotor behaves during acceleration markedly dull. In this connection, surging of the compressor is an additional indication.
During stand still and maintenence work, an unusual heavy rotating/turning of the rotor can get noticeable. However, in most cases this may be rather traced back at a rubbing of blades or labyrinths from other causes. In a very seldom case of a bearing failure this is an indirect feature. It can be only expected, when the degree of the failure no more allows a sufficient centering of the rotor. However, in such a case further indications like chips must be expected. Advanced bearing failures, like fatigue (pittings) of the race tracks, can show during stand stillto the attentive visitor. So a rough running and jerky decelerations (at locally fatigue/pits at the circumference of the bearing) can attract the attention during slow rotating/turning of the rotor.

Trapping/jamming, especially of main shafts, may in the most seldom cases caused by a bearing failure. Anyway, a hint is, if this condition does not disappear even after a totally cooling down of the aeroengine. Then the usual jamming by heat expansion can be ruled out (volume 2, Ill. 7.1.2-9).

Vibration fatigue (Fretting) can be hardly identified at a bearing from the outside. It concentrates primarily at the seat surfaces. Indirectly however, a bearing failure caused by vibrations (e.g., rotor bending) and/or alignment problems can act on neighbouring components. Especially spline toothings on shafts can show markedly wear (Fig. "Damage influences at multispline shafts").

Oil leak and smoke formation are often secondary failures/symptoms of a bearing failing (Ill. 23.1.2-3). Thereby, oil escapes through damaged neighboured seals (e.g., bearing chambers). Unusual traces of oil coking in compressor and hot part can be first hints. In an extreme case, an from outside good visible oil fire occurrs (Fig. "Compressor surge by oil loss"). Alarming escape of oil at seals of accessory gears can be caused by the failing of the bearing from a drive/power shaft to the devices.

Drop of the oil pressure is registered from warning signals/displays. It can be the consequence of a bearing failure. Reasons are blocked/clogged filters and sieves or damaged seals. In every case the failure of the bearing propagated to the total breakdown.

Oil analysis indicate in many cases the development of a failure, before a catastrophic breakdown (Fig. "Specifications for oil contaminations" and Fig. "Material specific particle content in oil"). Chapter 22.3.4 deals in detail with this issue.

The indication of the magnetic plug and chip detektors are very helpful for the early indentification and identifying of bearing failures (Fig. "Monitoring particle formation in oil").

 Identificatgion techniques of bearing failures

Fig. "Identificatgion techniques of bearing failures" (Lit. 23.1.3-6 and Lit. 23.1.3-7): The failure development at anti friction bearings, usually takes place (fatigue of the race, vibration fatigue/fretting) exponential after an incubation period (diagram). Therefore the best chance, to identify a failure is during the incubation period of the failure (Fig. "Hints at bearing problems"). This chance grows with the trend to more monitoring sensors in combination with the use of computers (Fig. "Data Fusion and information fusion" and Fig. "Engine health monitoring with data fusion"). So it gets possible, to evaluate different sensor data combined and to make the conclusion more certain (Fig. "Engine health management").
The „moment of detection “ decides about:

  • Degree of the failure respectively its risk.
  • Immediate measures like shut down of the aeroengine.
  • Interpretability of the failure and with thissufficiens certain determination of the cause.
  • Sustainable effective remedies. To these belongsthe guarantee of a sufficient oil supply or theavoidance of coking by `heat soaking' (Ill.22.3.2-6.1 and Fig. "Importance of resting time at idle").

The main phases of a bearing failure and possible features for detection are summarised in the chart.

Early detection means, that the failure will be sufficient early identified. This means, before the failure triggers secondary failures on other components and/or affects the behaviour of the aeroengine. Typica, features are

A late identification occurres, if there is already the dropout of the bearing function. This mis especially likely for high speed bearings of small aeroengines (e.g., helicopters). Here, the failure development from the first sign to the breakdown needs only seconds. The indications are linked with a dropout of the bearing functions, which can reach up to a catastrophic aeroengine failure. To these belong unusual intense vibrations (acceleration data above the limits). The frequencies are in the range of the rotor speeds.

For example shut down warnings can be triggered from the bearing itself or from secondary failure at other compomnents. To these belong

  • oil fire with high unbalances, fracture of the rotor (shaft, disks, volume 2, Ill. 9.2-13).
  • Distinct rubbing processes with secondary failures at the blading and seals.

Shut downs are based on instrument displays and warning signals. Not least decides the pilot with consideration of instructions and manual. Typical warnings are:

  • Drop of the oilpressure.
  • Unacceptable intense vibrations.
  • Fire warning.
  • Not influenced drop of rotation speed or unusual dull acceleration behaviour of rotors (Fig. "Compressor surge by oil loss"). In this connection compressor surge.
  • Axial rotor displacement during failing of a thrust bearing (volume 1, Ill. 4.5-4). Triggering of the shut down automatic (volume 1, Ill. 4.5-8).

 Conclusions by trend of particle amount

Fig. "Conclusions by trend of particle amount" (Lit. 23.1.3-7): Particles in the oil, in filters and/or on magnetic plugs/magnetic detectors can in manifold aspects add to the determination of the failure cause (Ill.22.3.4-1). The certainty can be if necessary improved with the combination of the single evidences (Fig. "Engine health management" and Fig. "Engine health monitoring with data fusion").

Lapse of time: The trend (diagrams above) of the gain of failure caused particles (fatigue break-outs, chips) points at

  • Type of the failure mechanism: Fatigue, wear.
  • Beginning of the deteriorating influence: e.g., assembly, oil change, transport, special operation loads.

Is the bearing failure not announced with the formation of particles, respectively arises spontaneous, this allows to suggest causes like

  • sudden overload, e.g., during high unbalances because of a blade fracture or intense surge impulses (Fig. "Bearing fractures by shock loads").
  • Shortage of oil as cause for a self-energising heating with failing of the bearing.
  • At very high-speed bearings, like of gasturbines of little power, also a fatigue failure can lead seemingly abrupt to the total destruction of the bearing.

Chemical analysis of the particles permit the identification of the primary concerned component/bearing in the oil system (Fig. "Material specific particle content in oil" and Fig. "Failure modes and appearances for identifications 1"). For this there help can be found in manuals, which assign analysis of materials and components (Fig. "Specifications for oil contaminations"). The chronological appearance of particles from the parts of a bearing (cage, bearing rings, rolling elements) or other components (e.g., gear wheels, seals) can enable the differentiation between primary and secondary failures as well as conclusions at the development of secondary failures.

Shape/form and outer features of the particles caused by the failure are often characteristic for the failure mechanism during the formation (Fig. "Ol filter helping for diagnistics" and Fig. "What deposits of magnet plugs can tell"). For example, fatigue (sketch below right) and fretting (sketches below left) can be distinguished. Particles as failure cause: To these belong hard ceramic contaminations like from blasting/peening or chipped off hard faces of labyrinth tips (Fig. "Dangerous particle sources in bearings", volume 2, Ill. 7.2.2-3.1). Those can be identfied at their appearance and with analysis (Fig. "Ol filter helping for diagnistics").

 Clarification of failure needed remedies

Fig. "Clarification of failure needed remedies": If the bearing of an aeroengine failed, it is frequently so destroyed (Fig. "Bearuing failure by overheating"), that a sufficient conclusion about the failure mechanism, only from the indications of the part, is no more possible. Especially problematic are bearings, which are operated at the limit of their loading capacity (mechanical, thermal). Seemingly little influences, separate or in combination, can complicate a successful cause analysis also for the experienced expert up to unsolvability. This show cases in which only after a multitude of dangerous parallel cases within years, a successful clarification and/or measures was possible (Fig. "Secondary effects of bearing failues" and Fig. "Compressor surge by oil loss"). Therfore all possibilities for the solution, especially the sourcing, must be used. That applies as well for the consequences for other components (e.g., control units) and the aeroengine (e.g., operation behaviour), as also for all possible concerned components of the oil system. So it is of crucial importance, to analyse stored operation data very exactly, also for seemingly marginal features (Fig. "Monitoring parameters" and Fig. "Data Fusion and information fusion"). Its evidence must be systematic analysed and evaluated, with the indication of the components of the oil system. Therefore it is understandable that, before the investigation, no changes at the oil system are carried out. To these belongs the exchange or the removal of oil and filters. Also the disassembly and the cleaning of the parts must happen only in presence and/or with the approval of the responsible for the investigation.

Displays of instruments and warning signals (Fig. "Hints at bearing problems"): Primarily concerned is the monitoring of vibrations, oil pressure, oil temperature and rotor speeds. They give hints at the failure beginning, occurrence and its sequence. So they permit conclusions at the cause.

Also deposits in the oil system, outside the magnetic plugs, filters and sieves can give as well important hints about the failure cause, as also about the failure sequence.
At the cause can point indications for a blocking of the oil supply (e.g., coke formation) and foreign particles (e.g., from seals, overhaul).
Layers of deposits can enable conclusions at the failure sequence. They can be expectend in 'eddy water zones' or where inert forces (centrifugal forces) enable a deposition of layers over the time (volume 1, Ill. 4.5-13). Concerned are zones like

The investigation of such deposits demands an adapted preparation. To this belongs the removal of the oil residues, embedding of the layered sample and the making of a cross section for the microscopic (matellographic) investigations.
Does this succeed, conclusions about causytive foreign particles, failure sequence, primary failing bearing element (e.g., cage or rolling surfaces) and failure mechanisms can be expected.
Surface/appearance and structure of coke deposits (Fig. "Formation conditions shown by coke deposits" and Fig. "Formation mechanisms and oil coke features") can be analysed for the type of origin (oil fire) and development conditions (droplets, vapour, during stand still, high temperatures; Ill. in the chapter about oil). The position of the coke formation gives hints at the associated operation condition (e.g., stand still, Fig. "Oil coaking endangers main bearings") and/or unusual conditions/causes like failing of a thermal bearing chamber insulation.
Deposits in fresh oil (pressure oil) lines and oil jets must be taken especially earnest. Because of the prepositioned filters, there is a high likelihood of a primary effect. At first it must be checked, if they explain a causative dirsurbance of the oil flow.
Further the question arises, if the deposits (mostly oil coke) on site (e.g., in a feed line or oil jet, Fig. "Oil coaking endangers main bearings") here developed or have been washed.

Also an indication of the scavenge oil (return oil) lines os of interest. Scavenge oil lines can get during operation or stand still such high temperatures, that dangerous coke formation occurs. Cause is heat from the outside (radiation, convection) and an unsufficient inner heat dissipation (temporary low flow rate, foam). Coke acts insulating and accelerates the process. A constricted cross section can retain oil in the bearing chamber, up to the overheating of the bearing and ignition of the oil. Unnormal annealing colours or oxidation of the pipe outside point at an oil fire that continues through the scavenge pipe (volume 2, Ill. 9.2-10). In this connection, leakages in scavenge lines must be mentioned. Does here occur a hot gas intrusion, from experience, this can trigger an oil fire with extremely high pipe temperatures.

The investigation of the remains in oil filters and oil sieves will be discussed in detail in Ill. They have the potential for conclusions about failure triggering influences (e.g., nonmagnetic foreign paticles) and failure sequence.

Deposits at magnetic plugs and magnetic chip detectors are the issue of Fig. "What deposits of magnet plugs can tell". Because magnetic particles are concerned, these frequently origin are from the affected bearing. Therefore they are of special interest for the failure mechanism (fatigue, wear processes).

Meaningful investigations of oil samples require, that after the failure occurred, no oil was changed, repoured or drained. Oil samples are suitable to gather at failure specific areas of the oil system. Of special interest are hints at the amount of oil/oil loss, oil type, aging condition (Fig. "Formation of depositions in hot oil systems") and contaminations. Thereby the operation time must be considered.

 What rece tracks of bearings can tell

Fig. "What rece tracks of bearings can tell" (Lit. 23.1.3-1, Lit. 23.1.3-8 up to Lit. 23.1.3-10): Race tracks on rings and rolling elements of anti friction bearings are indeed no failures, but can give important hints about height and direction of the acting loads. In contrast to areas with fatigue pittings (Fig. "Bearing features and terms 1") they are no feature of a material damage. Shape, progression and distribution on the races, that differ from the, according to the design, even alignment are however an indication for a hurtful load distribution.
This is determined by

  • Direction of the force.
  • Changes of the force (e.g. unbalances),
  • Measurement deviations like misalignmentsand inclines in connection with the bearing seats.
  • Deformations like distortion of casings or flexing of shafts.

Typical macroscopic feature of a race track is a polished/reflective surface. During microscopic examination a plastic leveling of normal tool marks is normal. Also shallow run in pittigs show the track and must not be a reason for concern (Fig. "Mechanisms of rolling wear"). But do they exceed an, according to the experience, bearing specific tolerable extent, they can count as an early indication for a short dimensioned lifetime. In this case, a non destructive (internal stresses, Fig. "Internal stresses at the racetrac", Lit 23.1.3-9) and/or destructive investigation for signs of fatigue (Fig. "Gathering informations about bearing failures") can be necessary for a risk assessment (Fig. "Gathering informations about bearing failures" and Fig. "Internal stresses at the racetrac").
Remarkable are indications of race damages. To those belong indentations of particles (Fig. "Reduction of bearing lifetime by particles") or first appearances for skidding of the roller elements (Fig. "Skidding failure modes at races").

Fig. "Bearing features and terms 1" (Lit. 23.1.3-6, Lit. 23.1.3-11 and Lit. 23.1.3-12 ): In this survey typical macroscopic manifestations and attributes of antifriction bearing failures are assigned to the causes. This is for an understanding of the technical terms for a better understanding of instructions in specifications and manuals. Above that an assistance for failure investigations should be given.

 Bearing features and terms 1

 Bearing features and terms 2

Identification of the failure cause with help of the failure mode/appearance.

 Failure modes and appearances for identifications 1
Fig. "Failure modes and appearances for identifications 1"
(Lit. 23.1.3-8): Correlation of operation features and failure features with failure mechanismy respectively causes.

 Failure modes and appearances for identifications 2

 Gathering informations about bearing failures

Fig. "Gathering informations about bearing failures" (Lit. 23.1.3-13): For the failure investigation as deterioration proof and evaluation of the mechanical operation load and with this for a risk assessment a metallographical investigation (volume 4, Ill. 17.3.2-6) in the area for the race track (Fig. "What rece tracks of bearings can tell") can be helpful. Internal stresses/residual stresses (volume 4, Ill. on the race track can suppport the conclusions (Ill. 23.1.3-9).
However it shoult be pointed at the difficulties, to make explicite statements. This can be explained with the interaction of shear stresses and normal stresses (combined stress) as well as peak stresses in the region of the race track surface (Fig. "Stresses between rolling elements and race"). Here act factors like lack of lubrication, unfavourable surface condition, slip of the rolling elements and contaminations of the lubrication oil.
Depending from the operation time and the bearing loads respectively stresses, a suitable prepared/etched cross microsection, shows typical evaluable features which can be assessed as signs of fatigue:

Dark etching areas (= DEAs) appear after the etching in the region of the race track (sketch above, axial cut). They get with the operation time darker and larger. Also over the time develop angular to the surface arranged so called white bands (WBs, detail above), up to few tenth of a millimeter deep, which can be identified in a circumferential section. After this form, nearer to the surface additional white bands, with a steeper orientation. Obviously the WBs are a ferrit formation from the hardening structure (martensite breakdown). With this the drop of the hardness in these zones can be explained (diagram). So, if there is sufficient experience with the particular use of the anti friction bearing, this hardness can make statements about the operation load.

„Butterflies” must be basically distinguished from WBs. Obviously it is not annealed martensit caused by a „heat explosion“ from a short locally deformation. So is understandable their higher hardness as the not influenced material. Characteristic also for the name is their form, rememberig a butterfly (sketch below). Its angle of orientation to the surface depends from the thickness of the lubricant film and roughness of the race. During full EHL (Fig. "Micropittings and life time"), i.e. width of the lubrication gap larger than the added roughness of both opposed rolling surfaces, an angle of about 30° to the race track arises. The angle is smaller, if mixed friction exists. Here obviously more pronounced circumferential friction forces become noticeable. These features offer the chance to conclude at bearing loads. Butterflies develop first if a load threshold is exceeded, which triggers slip processes in the microregion of the material (plastic deformations). These concentrate on small material typical structure notches (no failures!) like carbides. Often these can be well identified (sketch below). Size and formation of the butterflies depend from loading and overruns.

 Internal stresses at the racetrac

Fig. "Internal stresses at the racetrac" Fig. "Internal stresses at the racetrac" (Lit. 23.1.3-1 and Lit. 23.1.3-13): In the race are, already before the operation caused rolling events, internal stresses from the production process. These develop during the heat treating/hardening (sketch left) and the machining of the surface (grinding, honing). The honing of the finishing seems to have an especially strong influence on region near the surface.
Develop slippages in the micro region during rolling over (Fig. "Gathering informations about bearing failures"), the existing internal stresses change. There are two reasons

  • Locally plastic deformations (slippages) with hindrance of the deformation by the surrounding, only elastically deformed material.
  • Changes of the material structure (Fig. "Gathering informations about bearing failures"). Residual austenite in martensite and/or martensite formation from locally „temperature flashes”. Cause are fast locally inner plastic deformations or slippage processes during sliding (skidding) with metallic contact (Fig. "Skidding failure modes at races").

The circumperential and axial internal stresses in the region of the race track depend from the load and are normally not equal. They can differ as well in hight, as also in type (tension or compression). Thereby they also change with the dirstance to the surface.
With the number overuns, e.g., the operation time the trend of the internal stresses changes characteristically over the distance to the race surface. Compression stresses in rolling direction reach a stress maximum below the surface (diagram right). These seem to be a loading creiteria. But they obviously have also a certain protective effect against fatigue.

Fig. "High bearing loadings need fine oil flters" (Lit. 23.1.3-14 and Lit. 23.1.3-15): The bearing life is shortened by particles in the oil (Macpherson curve in Diagram above right). This is the case, if these bridge the lubrication gap (frame above left, Fig. "Micropittings and life time" up to 23.1.1-8). It comes to indentations and burials in the race surfaces (Fig. "Reduction of bearing lifetime by particles"). At these notches, the fatigue of the race begins. The deteriorating particle size depends from:

  • Dimension of the bearing/bearing diameter(diagrams below),
  • Bearing type: Ball bearing (diagram below left) or roller bearing (diagram below right),
  • Bearing load respectively width of thelubrication gap,
  • Bearing capacity of the oil (Fig. "Lubrication oils").

Because the bearing life depends so markedly from the possible maximum partocle size in the oil, the fine-meshing of the fresh (pressure) oil is of relevant meaning. With a trend to increasing temperatures and bearing loads, as well as more thin fluid oil, also the maximum tolerable particles get smaller. I.e. the filters must get always finer.

 High bearing loadings need fine oil flters

 Secondary failures by using plastic cages

Fig. "Secondary failures by using plastic cages": The strength of the cage of high speed bearings can markedly influence the fail safe behaviour in case of a failure. Is the bearing shocklike loaded (Fig. "Bearing fractures by shock loads") during a sudden high unbalance (e.g., blade fracture), extreme stesses in the cage will occur. A too weak cage will fracture. As consequence rolling elements can gather at one side. With this, the shaft comes free. The result are high unbalances with the danger of overloading the shaft (plastic deformation, fracture; sketches below). The experience shows, that such secondary failures must be expected especially at bearings with plastic material cages and/or with a weak riveting. Because for this reason, comparatively cheap so called spindle ball bearings, as they are used for high speedn prodiction machines (e.g., grinding machines), in spite of high „D x n values“ are not suitable for the use in aeroengines. Also for test rigs (e.g., for spin tests or aerodynamic tests) because of the potential danger of extensive secondary failures, there must be an advise against the use.

 Avoidubg skidding with oval race tracks

Fig. "Avoidubg skidding with oval race tracks" (Lit. 23.1.3-16 and Lit. 23.1.3-17): The disposition for skidding (Fig. "Deterioration by cage slipping so called skidding") can be avoided by design.

  • Decrease of the roller number, to increase the load on the individual roller.
  • Smaller rollers to decrease the centrifugal force so they don't take off from the inner ring.
  • Smaller radial clearance, to load more rollers at a stiff outer ring and casing.
  • Centering of the cage at the rotating brearing ring.
  • Most effective is an increase of the radial load. To this belongs the specification of a minimum unbalance.

The radial load can be achieved with a tensioning, acting as preload. This can be realised with special non-circular geometries of the race from the outer ring. Usually it is elliptical or „triple arched” (sketches above). The elliptical configurations with symmetrical opposing preload zones has the disadvantage to tend to instability. This is avoided with the triple arched version.
The ovality/noncircularity of the outer ring can be achieved in different ways (middle sketches). Depending of the bearing ring or the seat in the bearing casing/bearing support has the noncircularity. For this also suitable is an elastic deformation. This needs an adjustment of the stiffness from casing and bearing ring.

Configuration A“: Cylindrical race, oval seat. The oval shape is achieved with an elastic deformation of the outer ring during force fitting into the casing. Thereby the preload depends from:

  • Radial clearance of the bearing,
  • Distribution of the wall thickness from the outerring.
  • Fit between ring and seat.
  • Differences in stiffness between support/casingand bearing ring.

Configuration B”: The whole outer ring with constant cross section has an elliptic shape. The preload exists already in the disassembled condition. It depends from:

  • Noncircularity of the non tentioned outer ring,
  • Radial clearance of the bearing.
  • Stiffness of both bearing rings.
  • Fit between bearing seat and outer ring.

Because of the nonuniform contact, the bearing seat tends to a crawling movement. This can cause fretting wear. Against this helps a locking device against rotation (e.g., noses, Fig. "Bearing fractures by shock loads"). An advantage of this version is a certain play between outer ring and seat, which adjusts differences in thermal expansions. Especially if the bearing outer ring will expand unexpected strong through heating.

Configuration C“is a combination of „A” and „B“: Cylindrical ring surface and elliptical face surface. With this, the bearing is already tensioned before the assembly. Additionally the pretension depends from:

  • Radial play of the bearing.
  • Stiffnesses of the outer ring and the casing/support.
  • Fit between bearing outer ring and seat surface.

So the bearing can sit suifficient tight in the casing, without markedly influencing the tensioning. This manner saves an, for „B” necessary, locking device.
The sketch below left shows an application of a „triple arched“ bearing configuration. The diagrams below right show the roller load in two different directions, at a respectively same radial load. In diagram above the radial load acts in the region of a tensening zone (210° position), in the diagram below in a recess (150°).

 Gloves to avoid swet by handling bearings

Fig. "Gloves to avoid swet by handling bearings" (Lit. 23.1.3-18): The assembly and disasembly as well as the handling of the anti friction bearings is demanding. For this basics must be considered:

Assembly/mounting: New parts and already run parts should be stored with a corrosion protection oil (conservation oil). Usually mineral oils are applied. Mineral oils don't agree normally synthetic oils (Fig. "Lubrication oils"). So it should, corresponding to the manual instruction, removed as short as possible before the assembly.
Basically the devices and tools, specified in instructions respectively manuals, must be used. Are interim solutions concerned, are they worn or damaged, an application should be ruled out. If the bearings, respectively its components, must be cooled for the mouting (heat expansion) or heated, the specified temperature limits must be kept. The temperature control must be carried out, accordant the indicated method (e.g., oven, hot surface, inductive, oil bath). Time sequences must be kept very exactly (see also Fig. "Influence of shrink fitting with spline toothing 1"). For example, too high temperatures can decrease the bearing hardness unacceptable. For this reason, for bearings of the normal engineering, the temperature must be limited at 100 °C. In no case use torchs for welding or brazing. For supercooling (e.g.,with alcohol or dry ice) of the outer ring, condensate during the assembly must bre expected. Also here the mouting time must be kept, to prevent a unacceptable heating. Because of its corrosive effect this humidity must be removed.
During cleaning, don't use fibrilating cloths. Pay attention during pushing together roller bearings (e.g., joining of modules), that no mechanical damages occur (Fig. "Critical joining of big engine modules 2").

Disassembly: Before all required tools and devices must be on hand. A reuse and/or later inspection/investigation can demand an other approach than the exchange with the scrapping of the bearing.

Handling and storage: For new bearings, because of the danger of corrosion, specified suitable gloves (cotton) must be used. The handling must be carried out with the necessary carefulness to ban damages and contamonations.
Corrosion due to hand perspiration/sweat can be identified in some cases by the distribution of tiny corrosion dots (Ill. below). The failure mode correlates the sweat droplets of a finger print.
After a disassembly, respectively the inspection, the bearings which are destined for reuse, must be cleaned as specified. After this, a conservation must be carried out as fast as possible. Then the bearing must be stored in a specified container/wrapper. Bins must be closed at once after the removal. The storage room must accord the instructions/specifications (e.g., constant temperature, low humidity).

Approach in the case of a failure:

  • Photografic documentation of single steps.
  • Description of the bearing condition.
  • Notes about the assembly/ mounting situation.
  • Suitable marks of the mounting positions.
  • Observations of possible causes.
  • Condition of the surrounding of the bearing(e,g., contaminations, coke formation) and of the seals.
  • Oil samples (gather at the right spots!) and ifneeded of contaminations/deposits (according the specification and consulting the investigator).
  • Don't clean a bearing before investigation.Before cleaning, first inform the Investigator.
  • No disassembly of the bearing without consultation.


23.1.3-1 H.J.Böhmer, „Wälzverschleiß und -ermüdung von Bauteilen und Maßnahmen zu ihrer Einschränkung”, Zeitschrift „Materialwissenschaft und Werkstofftechnik“, 29 (1998), page 697-713.

23.1.3-2 Australian Transport Safety Bureau (ATSB), Air Safety Occurrence 200003399, Technical Analysis Report „Examination of PT6A-67R Number-1 Bearing and Components”, Incident vom 13. August 2000, page 1-5.

23.1.3-3 „ALPA warns Pilots on IAE V2500-A5 Bearings“, Air Safety Link, www.alpa.org, Document D=4428, August 2003, page 1 and 2.

23.1.3-4 Federal Aviation Administration (FAA), Docket No. 2003-NE-21-AD, „Airworthiness Directives; International Aero Engines AG (IAE) V25xx-xx Turbofan Engines”, Federal Register Volume 71, Number 10, 17. Januar 2006, page 1-5.

23.1.3-5 M.W.Washo, „A Quick Method of Determining Root Causes and Corrective Actions of Failed Ball Bearings“, Zeitschrift „Lubrication Engineering”, March 1996, page 206-212.

23.1.3-6 P.A. Mucklow, „Engine Reliability Through Bearing Condition Monitoring“, Paper von „South African Airways Symposium on Engine Monitoring”, 19-21 April 1983, page 1-8

23.1.3-7 C.A.Waggoner, „Detection and Diagnosis of Bearing Deterioration in Aircraft Prop ulsionsystems by Wear Debris Analysis“, Zeitschrift „Lubrication Engineering”, March 1996, page 2-1bis 2-13.

23.1.3-8 „Wälzlagerschäden, Schadenserkennung und Begutachtung gelaufener Wälzlager“, FAG Publ. -Nr. WL 82 102 page 1-70.

23.1.3-9 E.Schreiber, „Werkstoffliche Schadensanalyse, ein Instrument zur Ermittlung realer Beanspruchungsverhältnisse”, FAG Publikation, 13 pages. .

23.1.3-10 T.E.Tallian, „Failure Atlas for Hertz Contact Machine Elements“, ASME Press, Chapter 7, page 99-181.

23.1.3-11 D.A.Frith, „Reliable Information From Engine Performance Monitoring”, SAE Technical Paper Series 881444, Paper der „Aerospace Technology Conference and Exposition, Anaheim, California, October 3-6, 1988, page 1-10.

23.1.3-12 H.Brenneke, „Early Failure Detection in Gas Turbine Aero Engines Using Magnetic Plugs“, Zeitschrift „British Journal of NDT”, Vol 35, No2, February 1989, page 87-90. 23.1.3-13 O.Zwirlein, H.Schlicht, „Werkstoffanstrengung bei Wälzlagerbeanspruchung - Einfluss von Reibung und Eigenspannungen“, Zeitschrift „Werkstofftechnik”, 11, 1-4 (1980), page 1-14.

23.1.3-14 F.J.Ebert, W.Trojan, H.W.Zoch, „Hochaufgestickter, martensitischer Wälzlagerstahl ist ermüdungsfest und korrosionsbeständig“, Zeitschrift „Wälzlagertechnik - Industrietechnik” Fa. FAG Kugelfischer, Ausgabe 1992-503DA, page 9-21.

23.1.3-15 „Investigation Team Identifies Causes of CF6-80 Problem“, Zeitschrift „Aviation Week & Space Technology”, February 7, 1983, page 32.

23.1.3-16 D.P.Greby, „What turbine technology is teaching us about High-Speed Roller Bearings“, ca. 1972, 6 pages.

23.1.3-17 B.A.Tassone, „Roller Bearing Slip and Skidding Damage”, Zeitschrift „J. Aircraft“, Vol 12, No. 4, April 1975 , page 281-287.

23.1.3-18 „Wälzlager-Handbuch / Roller bearings handbook”, Fa. KRW, Edition 04/2003, page 78-100.

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