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BEARING DAMAGE
This document deals with bearing damage in journal bearings and tilting pad thrust bearings in industrial applications.
It is not an uncommon occurrence, on opening up a piece of machinery, to find that bearings have been damaged in service but the cause of damage may be obscure. The bearings are often blamed and whilst faulty design or manufacture of the bearings may be responsible, the cause most frequently lies in an extraneous source.
The purpose of the document is to direct the operator on the basis of the facts available to the likely causes of bearing damage and to assist in reaching a decision as to the action to be taken. This decision must be based on the nature and severity of the damage and on the significance of the type of damage.
Type of Bearing Damage:
1. Scoring Due to Foreign Matter or "Dirt"
Contamination of the lubricant includes:
A. "Built In" dirt on crankcases, crankshafts, oil galleries, cylinder bores etc. present at the time of machine assembly.
B. Entrained dirt entering through breathers or air filters, and particles derived from combustion of the fuel in the internal-combustion engines.
C. Metallic wear particles resulting from abrasive wear of moving parts
"Dirt" may cause polishing of the surfaces of white metal lined bearings, burnishing of bronze bearings, abrasive wear of overlays or of other bearing linings, and scoring of both bearing and mating surfaces with degrees of severity depending upon the nature and size of the dirt particle, or oil film thickness and type of bearing material.
Journal Bearings
Fig.
1 White metal-lined bearing scored and
pitted by “Dirt”
Fig. 2 White metal-lined bearing showing
“Haloes” caused by dirt particles.
Recommended Action
Bearings in condition shown in Fig. 2 should be scrapped and new bearings
fitted after cleaning journal, oil ways and filter (If Any) and changing oil.
Bearings in condition shown in Fig. 1 should be re-fitted, after cleaning
bearings and journal surfaces, provided clearance increase due to wear can be
tolerated.
Tilting Pad Thrust Bearings
Fig.
3 Concentric scoring of thrust pad due to dirt entering bearing at high speed.
Fig. 4 Scoring by dirt entering bearing at start-up.
Fig. 5 Surface of pad in Fig. 4 at higher magnification showing irregular tracks caused by rolling of shot-blast spherical particles.
Recommended Action
Scrap damaged pads. Clean out lubricating system, change oil, fit new pads.
Fig.1 - Fig. 3
Fig.4 and Fig.5
Type of Bearing Damage:
2. Wiping of Bearing Surfaces
A wiped bearing surface is where surface rubbing, melting and smearing is evident. This is usually due to inadequate running clearance with consequent surface overheating, or may be due to inadequate oil supply, or to both the causes.
A wiped surface may follow disruption of the oil film due to extreme loading, and shaft vibrations due to excessive unbalance or journal instability are other causes.
Journal Bearings
Fig. 6 White metal-lined turbine bearing, wiped in both top and bottom halves due to inadequate clearance.
Fig. 7 Overlay-plated copper lead bearing wiped due to barrelled journal.
Recommended Action
If lightly wiped, re-fit bearing after cleaning surface to remove any loose metal, providing the clearance can be tolerated. If cracked, fit new bearing and increase the clearance when fitting.
If vibration present, (A) Synchronous with shaft rotational frequency: check balance of rotor, alignment of couplings, etc. (B) Vibration frequency at half shaft speed or less: fit anti-whirl bearings.
Tilting Pad Thrust Bearings
Fig. 8 Surface wiping of white metal-lined pad, in successive thin layers, due to excessive steady load at start-up.
Recommended Action
Fit new pads. Improve lubrication at start-up, possibly by using jacking oil system. If possible reduce loading at start-up.
Fig. 6 Fig. 7 Fig.
8
Type of Bearing Damage:
3. Fatigue Cracking
This is caused by the imposition of dynamic loads in excess of the fatigue strength of the bearing material at operating temperature. Fatigue strength especially of low melting point materials such as white metals and lead-base overlays is greatly reduced at high temperatures, hence overheating alone may cause fatigue failure. Other causes are over-loading, cyclic out-of-balance loadings, high cyclic centrifugal loading due to over-speeding, shafts not truly cylindrical due to manufacturing defects such as honing, filing, etc.
Journal Bearings
Fig. 9 Fatigue cracking of white metal lined bearing due to shaft ridging caused by differential wear in way of 180 degree groove.
Fig. 10 Fatigue cracking of 20% Tin Aluminium lining due to misalignment and consequent edge loading.
Fig. 11 Fatigue cracking of white metal-lined bearing due to shaft deflection and edge loading.
Fig. 12 White metal-lined chambered top half turbine bearing, fatigue cracked on lands due to out of balance loading, and excessively wide cut-away.
Recommended Action
Scrap bearings. Investigate and, if possible, rectify causes of misalignment, shaft deflection and overloading. Increase width of lands in top half (Fig. 12). Fit new bearings, possibly of stronger material if under-design is indicated.
Fig. 9 Fig. 10 Fig. 11
Fig.
12
Type of Bearing Damage:
4. Corrosion
Corrosion of the lead in copper-lead and lead-bronze alloys, and of lead base white metals, may be caused by acidic oil oxidation products formed in service, by ingress of water or coolant liquid into the lubricating oil, or by the decomposition of certain oil additives.
Removal of the overlays by abrasive wear or scoring by dirt exposes the underlying lead in copper-lead or lead-bronze inter layers to attack, while in severe cases the overlays may be corroded.
Journal Bearings
Fig. 13 Severely corroded surface of non plated copper-lead lined bearing caused by attack of lead phase by acidic oil oxidation products.
Fig. 14 Fractured section of bearing shown in Fig. 13 indicating depth of corrosion of lead.
Fig. 15 Corrosion of marine turbine white metal bearing. Water in the oil has caused formation of smooth hard black deposit of tin dioxide on surface.
Fig. 16 "Sulphur Corrosion" of phosphor bronze small end bush, caused by decomposition of lubricating oil additive and gross pitting and attack of bearing surface.
Recommended Action
Scrap bearings. Investigate oil condition to ascertain cause of corrosion. Eliminate water in oil. In case of "Sulphur Corrosion" change bearing material from phosphor bronze to phosphorous-free alloy such as lead-bronze, silicon bronze or gunmetal after consultation with oil and bearing suppliers.
Fig. 13 Fig. 14 Fig. 15
Fig.
16
Type of Bearing Damage:
5. Cavitation Erosion
This is an impact fatigue attack caused by the formation and collapse of vapor bubbles in the oil film under conditions of rapid pressure changes during the crank cycle in internal combustion engines. The harder the bearings material the greater is its resistance to cavitation erosion.
Journal Bearings
Fig. 17 Impact cavitation erosion of non grooved white metal-lined bearing occurring downstream of bearing joint face and groove in the other half.
Fig. 18 Section through main bearing illustrating mechanism of cavitation and modification to groove to limit or reduce damage.
Fig. 19 Discharge cavitation erosion in unloaded half-bearing caused by rapid movement of journal in clearance space during crank cycle.
Fig. 20 Set of diesel engine main bearings showing cavitation erosion of soft overlay while harder tin-aluminium is unaffected.
Recommended Action
Bearings may be put back into service if cavitation attack is not very severe or extensive. Investigate practicability of increasing oil feed pressure, modifying bearing groove, blending edges or contours of grooves to promote streamline flow, reducing running clearance or changing to harder bearing material.
Fig. 17 Fig. 19
Fig.
18
Fig. 20
Type of Bearing Damage:
6. "Black Scab" or "Wire Wool"
A large dirt particle (Probably not less than 0.040 across) carried into the clearance space by the lubricating oil, and becoming embedded in the bearing may form a hard scab of material by contact with the steel journal or thrust collar. This scab will then cause very severe damage to the mating steel surface which is literally machined away with the formation of the so-called "Wire Wool". The action is self propagating once started and susceptibility to scab formation appears to depend upon the nature of the lubricant and the composition of the steel of the rotor shaft or collar. Steels containing chromium or manganese in excess of 1% appear to be particularly susceptible to scab formation, especially in high speed machines with bearing rubbing speeds over 20 meters per second.
Journal Bearings
Fig. 21 White metal lined compressor bearing with formation of "Black Scab".
Fig. 22 13% chromium steel journal running in bearing shown in Fig. 21 showing severe "Machining Damage".
Tilting Pad Thrust Bearings
Fig. 23 "Black Scab" formation on white metal lined thrust pad..
Recommended Action
Scrap bearings. Pay particular attention to cleanliness during assembly, taking care to avoid contamination of bearing surface and oil ways with swarf, etc. Investigate possibility of changing journal or collar surface material by sleeving with mild steel, or hard chrome plating. Changing the bearing alloy is unlikely to be effective.
Fig. 21 Fig. 22
Fig.
23
Type of Bearing Damage:
7. Pitting Due to Electrical Discharge
Electrical discharge through the oil film between journal and bearing in electrical machinery or on the rotors in fans and turbines may occur due to faulty insulation or earthing or to the build-up of static electricity. This can occur at very low voltages and may cause severe pitting of bearing or journal surfaces, or both. In extreme cases damage may occur very rapidly and the cause is sometimes difficult to diagnose as pitting of the bearing surface is followed ultimately by wiping and failure which may obscure the original pitting.
Journal Bearings
Fig. 24 Fine hemispherical pitting and scoring of white metal-lined generator bearing due to electrical discharge.
Fig. 25 Close-up of more severe electrical discharge pitting of white metal.
Recommended Action
Scrap bearings if severely pitted or wiped. Examine, and if necessary, regrind journal to eliminate pitting, Investigate earthing conditions of machine especially around any insulation,with particular attention to fittings such as guards, thermocouple leads, water connections, etc. which may be bridging insulation. Fit new bearings and run for only a short time, depending upon the period run to failure point on previous occasion, and examine.
Fig. 24 Fig. 25
Type of Bearing Damage:
8. Faulty Assembly
Included in this section
(A) Fretting damage due to inadequate interference fit in flimsy housing.
(B) Excessive interference fit causing bearing bore distortion.
(C) Effect of joint face stagger or absence of joint face relief in bearing causing overheating and damage in region of bearing split.
(D) Fouling at crankshaft fillets due to incorrect shaft radii.
(E) Misalignment or shaft deflection causing uneven wear of bearing
(F) Entrapment of foreign matter between bearing and housing during assembly causing bearing bore distortion and localised overheating.
(G) Effect of grinding shaft in wrong direction.
Journal Bearings
Fig. 26 Fretting between back of bearing and housing due to inadequate interference fit. Could also be due to flimsy housing design.
Fig. 27 Overheating and fatigue at joint faces due to excessive interference fit causing bearing bore distortion. Could also be caused by joint face stagger on assembly or flimsy housing.
Fig. 28 Fretting at joint faces of connecting rod due to inadequate bolt tension.
Fig. 29 Uneven wear of overlay plated bearing due to misalignment.
Fig. 30 Fouling between shaft radius and end of bearing, causing overheating.
Fig. 31 Severe scoring of white metal-lined compressor bearing, due to wrong direction of shaft grinding.
Fig. 32 Back of Tin Aluminium bearing showing "Flat" on steel shell caused by swarf entrapped between shell and housing.
Fig. 33 Bore of bearing shown in Fig. 32. Entrapped swarf has deformed bearing bore resulting in flexure and fatigue damage.
Fig. 34 Angular misalignment of shaft or housing, or dirt trapped behind carrier ring, causing damage to pads on one side only.
Recommended Action
Investigate and if necessary correct interference conditions and housing design. Correct misalignment. Scrap bearings and fit new bearings or pads.
Fig. 26 Fig. 27
Fig.
28
Fig. 29
Fig. 30 Fig. 31
Fig.
32
Fig. 33 || Fig. 34
Type of Bearing Damage:
9. Inadequate Lubrication
This may be due to inadequate oil pump capacity, inadequate oil gallery or oilway diameters, incorrect grooving design, accidental blockage or cessation of oil supply.
In the case of machines depending only upon shaft driven lubricating oil pumps, it may be due to inadequate oil supply during start-up, perhaps due to failure to operate hand priming pumps in an adequate manner.
Depending upon the duration and severity of the lubrication failure, surface melting and wiping, or complete seizure of the bearing may occur.
Journal Bearings
Fig. 35 Fatigue cracking caused by inadequate lubrication due to axial grooving in loaded area.
Fig. 36 Overheating and damage of Tin Aluminium bearing surface due to reduced oil flow caused by sealed cavity with no drain hole at one end of bearing.
Fig. 37 Seizure of 20% Tin Aluminium bearing due to lubrication failure.
Recommended Action
Investigate and, if necessary, correct groove design. Investigate lubrication conditions. Scrap bearings. Examine, and in cases of seizure, crack detect journals"Dye Penetrant or Magnaflux". Fit new bearings.
Fig. 35
Fig. 36
Fig.
37
Type of Bearing Damage:
10. Fretting Damage Due to Vibration
The damage we are talking about is fretting damage in the bore as distinct from fretting damage elsewhere (See Item "A" under Faulty Assembly). Bearing operating surfaces may suffer fretting damage while the shaft is at rest due to vibrations transmitted to the machine from external sources, etc.
Journal Bearings
Fig. 38 Surface damage caused by external vibration while journal was not rotating.
Recommended Action
Eliminate transmission of vibration from external sources if feasible by spring or rubber-mounting the affected machine. If damage occurs in transit, clamp journal to prevent vibration during shipment.
Fig.
38
Type of Bearing Damage:
11. Overheating
Lining extrusion and cracking may be due to overheating with consequent reduction in strength of linings so that the material yields and cracks due to the effect of normal and shear forces transmitted through very thin oil films.
Wiping does not necessarily occur under such conditions.
Surface deformation can be caused in an anisotropic material by thermal cycling.
Tilting Pad Thrust Bearings
Fig. 39 Cracking of white metal lining of pad due to operation at excessively high temperatures. Note displacement of white metal over edge of pad due to extrusion.
Fig. 40 Cracking and displacement of white metal lining of pad due to overheating under steady load conditions.
Fig. 41 Thermal "Ratchetting" of white metal lining of pad due to thermal cycling through excessive temperature range in service.
Recommended Action
Investigate and if possible reduce maximum operating temperatures. Fit new or reconditioned pads.
Fig. 39
Fig. 40
Fig. 41
Type of Bearing Damage:
12. Pivot Fatigue
Axial vibration can cause damage and fatigue of tilting pad pivots, both pivot and carrier ring suffering damage by indentation or fretting. In some cases tiny cavities of almost hemispherical form may be produced. Damage may occur due to axial vibration imposed upon the journal, or may be caused by thrust collar face running out of true.
Tilting Pad Thrust Bearings
Fig. 42 Pad carrier ring showing damage due to axial shaft vibrations.
Fig. 43 Pad pivot showing hemispherical cavities caused by pivot fretting due to vibration.
Recommended Action
Recondition carrier ring, and fit new or reconditioned pads. Investigate and eliminate cause of axial vibration.
Fig. 42
Fig. 43
