- Email: [email protected]
Failure analysis of hydro-generator thrust bearing
Wear 225–229 Ž1999. 913–917
Failure analysis of hydro-generator thrust bearing H. Iliev
)
Department of Mechanical Engineering, UniÕersity of Zimbabwe, PO Box MP 167, Harare, Zimbabwe
Abstract The failed bearing is a large thrust bearing with eight spring-supported pads. The bearing supports the vertical shaft of a hydraulic turbine and generator. Oil bath is used for lubrication and running cold water provides cooling. Bearing temperature monitoring gave indication of bearing failure. Pads visual examination has shown that loss of area of lining, cracking and wiping of bearing material have occurred. Sample of used oil has been analysed. Also, metallographic analysis of lining material has been carried out. Probably the bearing was overheated due to excessive friction losses andror inadequate cooling capacity. Unevenly damaged pads suggested misalignment. The possible cause of bearing failure is misalignment of bearing to axis of rotation. This study presents failure investigation findings. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Bearing; Hydro-generator; Misalignment
1. Introduction One of the most heavy duty applications of thrust bearings is in hydroelectric power stations for support of the shaft, carrying hydraulic turbine and electric generator. White metal bearing alloy is the preferred choice as the facing material in those applications because of its outstanding embeddability, conformability, wear and corrosion resistance. On the other hand, white metal falls rapidly with temperature rise and its limitation factor is low fatigue strength. A number of failures of white metal faced thrust bearings prompted the search for alternative materials. Recent work shows the potential of PTFE faced thrust bearings in hydro-generator applications w1x. The failure under discussion occurred in a hydroelectric power station. The failed bearing is a large thrust bearing with eight pads. Each pad is supported by 92 coil springs. The face material is white metal bearing alloy. The bearing supports the vertical shaft of a hydraulic turbine and generator. In addition, the shaft is supported by two guide journal bearings, as shown in Fig. 1. The shaft is able to tilt and self-adjust. The turbine duty cycle is in continuous operation throughout the year. For annual maintenance the turbine is
)
Fax: q263-4-333-407; e-mail: [email protected]
taken out of service for a month. The failed bearing is reported to be in operation for more than 30 years. In this paper bearing failure investigation findings are presented and recommendations for the prevention of such a failure are given. 2. Observations Single pad dimensions are shown in Fig. 2. The shaft angular velocity of rotation is v s 166.7 revrmin. The shaft rotates in clockwise direction. Lubrication is provided by immersing the bearing in an oil-flooded chamber without circulation. Oil used is typical oil ŽMobil DTE, heavy medium. with additives for circulation system of hydraulic turbine. Oil viscosity at 408C is 64 cSt, at 1008C is 8.6 cSt and viscosity index is 104. Cold water, running in copper tubes and installed in the oil bath, is used for cooling. A thermocouple mounted in the body of one of the pads is used for measurement of bearing temperature. It was through bearing temperature monitoring that indication of failure came. The bearing temperature exceeded the warning limit of 708C. The turbine has been taken out of service when the bearing temperature reached 808C. Two pads, namely No. 8 and No. 2 and pieces of bearing material, collected from the oil bath, were brought for inspection. Pads visual examination has shown that in the central part face material Žwhite metal. has become
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 4 1 0 - 4
914
H. IlieÕ r Wear 225–229 (1999) 913–917
Oil film thickness for large thrust bearings is typically 50 mm. Bearing performance is sensitive to film thickness and good alignment of bearing and runner to the axis of rotation is necessary. Poorly aligned bearings are prone to failure by overheating of individual pads w2x. It is important that the contact surface of all pads should lie in the same plane to within close tolerances. Departure by more than 10% of film thickness affects performance—high pads are overloaded, low pads carry little load w2x. Unevenly damaged pad No. 8 suggests misalignment of that particular pad. 3.2. Load factor Fig. 1. Shaft of hydro-generator: Ž1. generator guide journal bearing, Ž2. turbine guide journal bearing, Ž3. thrust bearing.
black in colour. In fact, all the eight pads experienced blackening of face material in the central part, but only pad No. 8 was badly damaged. Loss of area of lining, extrusion, cracking in mosaic pattern, and wiping of bearing material were observed. The inner part of the pad experienced melting and flow of material only, while the outer part experienced also cracking and loss of lining Žsee Fig. 3.. Plastic flow of lining material occurred in the direction of rotation resulting in notches on the edge like a saw, illustrated in Fig. 4. Also, the surface became smooth and crack lines were not clearly visible. No tracks of rust or corrosion were observed. The contact surface of the runner is reported to be polished and not damaged. Failed pad No. 8 experienced uneven reduction in lining thickness. Variation of lining thickness in radial direction is shown in Fig. 5 Žnew lining has thickness of 3.175 mm.. While at the inner edge thickness was approximately equal to the initial thickness of 3.175 mm, at the outer edge lining thickness was 1.5 mm on the pad’s left side and 0.7 mm on the right side. Analysis of the oil sample, taken prior to turbine inspection, has shown increased iron Ž110 ppm. and copper Ž75 ppm. content, high water content Ž2%. and high TAN Žtotal acid number. value—0.9 mg KOHrg. Viscosity at 408C dropped to 59 cSt. Based on this report the oil has been drained and replaced with a new oil.
Total axial load on the thrust bearing include hydraulic thrust and weight of turbine runner, shaft, rotor and is P s 6.72 MN. The total contact area of the thrust bearing is Žsee Fig. 2. A s p Ž R 2 y r 2 . y nb Ž R y r . ,
Ž 1.
where n is the number of pads. With n s 8, A s 1.9832 m2 . Thus, the specific load on thrust bearing is PrA s 3.39 MPa. The pad’s mean radius is rm s 787.4 mm and mean sliding velocity is Õm s v rm s 13.75 mrs. The recommended maximum specific load for white metal lining at calculated sliding velocity is 3.5–4.0 MPa w2x. 3.3. Bearing temperature Table 1 shows variation of annual average bearing temperature over the years, while Table 2 presents varia-
3. Analysis 3.1. Alignment The failed bearing is a hydrodynamic thrust bearing with spring-supported pads which are able to assume a small angle relative to the moving runner surface. This enables a full hydrodynamic fluid film to be maintained.
Fig. 2. Single pad dimensions: internal diameter ds 2 r s1117.6 mm, external diameter Ds 2 Rs 2032 mm, key width bs 76.2 mm.
H. IlieÕ r Wear 225–229 (1999) 913–917
915
Fig. 3. Failed bearing pad.
tion of weekly average bearing temperature over the weeks of year 1997. Failed bearing temperature readings through the last 14 years show fluctuations below the warning limit. For the first time temperature exceeded 708C in
1997, week 10. Throughout 1997 temperature increased gradually to 808C in week 38, when the turbine was taken for inspection. The temperature of the two guide journal bearings of the same unit is within safe limits. The recorded temperature increase is confirmed by high TAN value of the oil and darkening of the face material. Increased temperature results from excessive friction losses and probably inadequate capacity of cooling system. The flow rate of cooling water is 680 lrmin and its temperature at the exit is 268C in average. 3.4. Bearing material Metallographic analysis of bearing material has been conducted. The micrograph in Fig. 6 shows lining microstructure, where hard cuboids and hard needles are embedded in the ductile matrix. Microhardness Hv of
Fig. 4. Notches on the edge of failed pad due to plastic flow of lining material.
Fig. 5. Variation of failed pad lining thickness in radial direction.
H. IlieÕ r Wear 225–229 (1999) 913–917
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Table 1 Variation of annual average bearing temperature over the years Ž8C. Y
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
Ž1. Ž2. Ž3.
61 36 66
60 38 67
64 36 64
65 37 66
60 40 68
61 40 68
62 42 64
64 46 70
67 41 69
68 48 70
65 52 69
70 43 62
67 44 67
67 49 67
Y, year. Ž1. Generator guide journal bearing; Ž2. turbine guide journal bearing; Ž3. thrust bearing. Table 2 Variation of weekly average bearing temperature over the weeks of year 1997 Ž8C. W
14 15 18 20 22 26 28 30 32 34 35 36 37 38
Ž1. 68 69 68 68 69 68 67 67 69 67 67 Ž2. 48 15 48 49 49 47 46 46 47 46 47 Ž3. 68 69 70 71 74 73 73 74 75 74 76 76 77 80 W, week no. of year 1997. Ž1. Generator guide journal bearing; Ž2. turbine guide journal bearing; Ž3. thrust bearing.
individual phases is as follows: matrix 24.7; cuboids 75.2; needles 83.9. An average lining microhardness obtained is of 26.5 Hv . Lining chemical composition is shown in Table 3. Metallographic analysis confirmed that lining chemical composition, microhardness and microstructure correspond to white metal of British Standard BS 3332r2. Specification BS 3332r2 is for tin-base white metal with hardness 27–32 Hv and melting range 239–3408C. Bonding between lining and substrate was examined and was found to be normal. 3.5. Lubricant Because of the effect of oil quality on hydro-generator performance, regular analysis of oil samples is normal
practice. Any changes due to system malfunction can thus be detected at an early stage, allowing corrective action to be taken before a serious condition occurs w3x. In the case under discussion oil monitoring has not been practised. High-temperature bearing operation causes oxidation of oil. TAN is an indication of the amount of oxidation that an oil has undergone in service. Increase in TAN value is accompanied by viscosity increase. Water content increase also contributes to viscosity increase. On the other hand, temperature increase causes decrease in viscosity. The oil sample report indicates that water content and oxidation indicator TAN exceed the warning limit but viscosity dropped, remaining within specified limit. Obviously, temperature increase is the prevailing factor. Wet oil results in corrosion and can also lead to bacterial growth in the system. Failure of white metal bearing is accelerated by the presence of water in oil w3x. 3.6. Failure Failed bearing is designed for hydrodynamic regime of lubrication with full separation of runner and pads. Presumably, the damaged pad has been misaligned or deflected which affected clearance. Inadequate clearance caused disruption of oil film and consequently high friction losses.
Fig. 6. Microstructure of bearing lining Žoriginal magnification= 100..
H. IlieÕ r Wear 225–229 (1999) 913–917 Table 3 Chemical composition of bearing pad lining Element
Sn
Sb
Cu
Pb
Fe
Zn
Content Žwt.%.
86.52
8.55
4.52
0.36
0.04
0.01
Bearing damage is concentrated on the outer part of the pad, suggesting that the outer edge was higher than it should be and was in ‘metal to metal’ contact with runner. Temperature in the contact area reached white metal melting point and the lining material melted. Thereafter sliding motion caused superficial flow of material in the direction of sliding. In fact, the outer part of the pad was overloaded and in combination with overheating experienced cracking and loss of area of lining by propagation of cracks, which is fatigue failure. Wear of bearing surface is caused by sliding. Two facts support the above statement, namely: Ža. relative motion of runner with respect to stationary pads is sliding in the direction from right to left pad side. Fig. 4 shows serrated edge on pad left side and demonstrates flow of material in the direction of sliding. Žb. Fig. 5 illustrates uneven distribution of wear in tangential direction Žon pad outer edge linear wear on left side is 1.67 mm, while on right side is 2.47 mm.. Once lining melted, the runner acted like a ‘broom’ and removed material in the direction of sliding. In consideration of bearing failure we belittle cavitation and corrosion wear mechanism. Wear pattern does not correspond to cavitation wear marks. If corrosion wear is the leading wear mechanism, most of the pads should be affected. In failed bearing only one pad is damaged and the rest are just normal.
4. Conclusions 1. Specific load calculated does not exceed recommended maximum for white metal lining of large thrust bearings. Therefore failure is not due to overloading. 2. Lining material was identified as a proper white metal BS 3332r2. Bonding with substrate was found to be normal. Bearing material is not to blame for the failure. 3. Although adequacy of cooling system capacity is questionable, we do not think that bearing failure is cooling system failure. 4. Despite the fact that oil deteriorated to a certain degree bearing failure is not oil fault.
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5. Failed bearing has been overheated due to excessive friction losses, which resulted from disruption of oil film. Inadequate film thickness, attributed to pad misalignment or deflection, caused oil film disruption. Pad misalignment, confirmed by uneven wear of bearing surface, has a disastrous effect on bearing performance. Pad misalignment could happen either when pads were mounted or during operation. Overheating, causing reduction in fatigue strength, led to wiping and fatigue failure. In conclusion, pad misalignment caused bearing failure. A year ago all the eight pads of the failed thrust bearing have been replaced with new ones, properly aligned. In this period the bearing is reported to perform well.
5. Recommendations 1. Oil circulation system, which will serve the three bearings of one unit. 2. Regular oil sample monitoring. 3. Periodic removal of water from oil by drainage or centrifuging. 4. Proper sealing of bearing in order to keep oil clean. 5. Proper alignment of bearing pads is of utmost importance. 6. Cooling system copper tubes, being subjected to aging for more than 30 years, need special attention.
Acknowledgements The author wishes to acknowledge Mr. R. Mandebvu for provision of bearing temperature readings and Dr. H. Chikwanda for lining chemical composition and hardness measurements.
References w1x J. Simmons, R. Knox, W. Moss, PTFE faced thrust bearings: state of the art review and hydro-generator application in the UK, World Tribology Congress, Abstract of papers, 8–12 Sept. 1997, London, MEP, London, 1997, p. 208. w2x M. Neale ŽEd.., Tribology Handbook, Butterworths, London, 1973. w3x W. Robertson ŽEd.., Lubrication in Practice, MacMillan, London, 1983.
Failure analysis of hydro-generator thrust bearing H. Iliev
)
Department of Mechanical Engineering, UniÕersity of Zimbabwe, PO Box MP 167, Harare, Zimbabwe
Abstract The failed bearing is a large thrust bearing with eight spring-supported pads. The bearing supports the vertical shaft of a hydraulic turbine and generator. Oil bath is used for lubrication and running cold water provides cooling. Bearing temperature monitoring gave indication of bearing failure. Pads visual examination has shown that loss of area of lining, cracking and wiping of bearing material have occurred. Sample of used oil has been analysed. Also, metallographic analysis of lining material has been carried out. Probably the bearing was overheated due to excessive friction losses andror inadequate cooling capacity. Unevenly damaged pads suggested misalignment. The possible cause of bearing failure is misalignment of bearing to axis of rotation. This study presents failure investigation findings. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Bearing; Hydro-generator; Misalignment
1. Introduction One of the most heavy duty applications of thrust bearings is in hydroelectric power stations for support of the shaft, carrying hydraulic turbine and electric generator. White metal bearing alloy is the preferred choice as the facing material in those applications because of its outstanding embeddability, conformability, wear and corrosion resistance. On the other hand, white metal falls rapidly with temperature rise and its limitation factor is low fatigue strength. A number of failures of white metal faced thrust bearings prompted the search for alternative materials. Recent work shows the potential of PTFE faced thrust bearings in hydro-generator applications w1x. The failure under discussion occurred in a hydroelectric power station. The failed bearing is a large thrust bearing with eight pads. Each pad is supported by 92 coil springs. The face material is white metal bearing alloy. The bearing supports the vertical shaft of a hydraulic turbine and generator. In addition, the shaft is supported by two guide journal bearings, as shown in Fig. 1. The shaft is able to tilt and self-adjust. The turbine duty cycle is in continuous operation throughout the year. For annual maintenance the turbine is
)
Fax: q263-4-333-407; e-mail: [email protected]
taken out of service for a month. The failed bearing is reported to be in operation for more than 30 years. In this paper bearing failure investigation findings are presented and recommendations for the prevention of such a failure are given. 2. Observations Single pad dimensions are shown in Fig. 2. The shaft angular velocity of rotation is v s 166.7 revrmin. The shaft rotates in clockwise direction. Lubrication is provided by immersing the bearing in an oil-flooded chamber without circulation. Oil used is typical oil ŽMobil DTE, heavy medium. with additives for circulation system of hydraulic turbine. Oil viscosity at 408C is 64 cSt, at 1008C is 8.6 cSt and viscosity index is 104. Cold water, running in copper tubes and installed in the oil bath, is used for cooling. A thermocouple mounted in the body of one of the pads is used for measurement of bearing temperature. It was through bearing temperature monitoring that indication of failure came. The bearing temperature exceeded the warning limit of 708C. The turbine has been taken out of service when the bearing temperature reached 808C. Two pads, namely No. 8 and No. 2 and pieces of bearing material, collected from the oil bath, were brought for inspection. Pads visual examination has shown that in the central part face material Žwhite metal. has become
0043-1648r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 Ž 9 8 . 0 0 4 1 0 - 4
914
H. IlieÕ r Wear 225–229 (1999) 913–917
Oil film thickness for large thrust bearings is typically 50 mm. Bearing performance is sensitive to film thickness and good alignment of bearing and runner to the axis of rotation is necessary. Poorly aligned bearings are prone to failure by overheating of individual pads w2x. It is important that the contact surface of all pads should lie in the same plane to within close tolerances. Departure by more than 10% of film thickness affects performance—high pads are overloaded, low pads carry little load w2x. Unevenly damaged pad No. 8 suggests misalignment of that particular pad. 3.2. Load factor Fig. 1. Shaft of hydro-generator: Ž1. generator guide journal bearing, Ž2. turbine guide journal bearing, Ž3. thrust bearing.
black in colour. In fact, all the eight pads experienced blackening of face material in the central part, but only pad No. 8 was badly damaged. Loss of area of lining, extrusion, cracking in mosaic pattern, and wiping of bearing material were observed. The inner part of the pad experienced melting and flow of material only, while the outer part experienced also cracking and loss of lining Žsee Fig. 3.. Plastic flow of lining material occurred in the direction of rotation resulting in notches on the edge like a saw, illustrated in Fig. 4. Also, the surface became smooth and crack lines were not clearly visible. No tracks of rust or corrosion were observed. The contact surface of the runner is reported to be polished and not damaged. Failed pad No. 8 experienced uneven reduction in lining thickness. Variation of lining thickness in radial direction is shown in Fig. 5 Žnew lining has thickness of 3.175 mm.. While at the inner edge thickness was approximately equal to the initial thickness of 3.175 mm, at the outer edge lining thickness was 1.5 mm on the pad’s left side and 0.7 mm on the right side. Analysis of the oil sample, taken prior to turbine inspection, has shown increased iron Ž110 ppm. and copper Ž75 ppm. content, high water content Ž2%. and high TAN Žtotal acid number. value—0.9 mg KOHrg. Viscosity at 408C dropped to 59 cSt. Based on this report the oil has been drained and replaced with a new oil.
Total axial load on the thrust bearing include hydraulic thrust and weight of turbine runner, shaft, rotor and is P s 6.72 MN. The total contact area of the thrust bearing is Žsee Fig. 2. A s p Ž R 2 y r 2 . y nb Ž R y r . ,
Ž 1.
where n is the number of pads. With n s 8, A s 1.9832 m2 . Thus, the specific load on thrust bearing is PrA s 3.39 MPa. The pad’s mean radius is rm s 787.4 mm and mean sliding velocity is Õm s v rm s 13.75 mrs. The recommended maximum specific load for white metal lining at calculated sliding velocity is 3.5–4.0 MPa w2x. 3.3. Bearing temperature Table 1 shows variation of annual average bearing temperature over the years, while Table 2 presents varia-
3. Analysis 3.1. Alignment The failed bearing is a hydrodynamic thrust bearing with spring-supported pads which are able to assume a small angle relative to the moving runner surface. This enables a full hydrodynamic fluid film to be maintained.
Fig. 2. Single pad dimensions: internal diameter ds 2 r s1117.6 mm, external diameter Ds 2 Rs 2032 mm, key width bs 76.2 mm.
H. IlieÕ r Wear 225–229 (1999) 913–917
915
Fig. 3. Failed bearing pad.
tion of weekly average bearing temperature over the weeks of year 1997. Failed bearing temperature readings through the last 14 years show fluctuations below the warning limit. For the first time temperature exceeded 708C in
1997, week 10. Throughout 1997 temperature increased gradually to 808C in week 38, when the turbine was taken for inspection. The temperature of the two guide journal bearings of the same unit is within safe limits. The recorded temperature increase is confirmed by high TAN value of the oil and darkening of the face material. Increased temperature results from excessive friction losses and probably inadequate capacity of cooling system. The flow rate of cooling water is 680 lrmin and its temperature at the exit is 268C in average. 3.4. Bearing material Metallographic analysis of bearing material has been conducted. The micrograph in Fig. 6 shows lining microstructure, where hard cuboids and hard needles are embedded in the ductile matrix. Microhardness Hv of
Fig. 4. Notches on the edge of failed pad due to plastic flow of lining material.
Fig. 5. Variation of failed pad lining thickness in radial direction.
H. IlieÕ r Wear 225–229 (1999) 913–917
916
Table 1 Variation of annual average bearing temperature over the years Ž8C. Y
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
Ž1. Ž2. Ž3.
61 36 66
60 38 67
64 36 64
65 37 66
60 40 68
61 40 68
62 42 64
64 46 70
67 41 69
68 48 70
65 52 69
70 43 62
67 44 67
67 49 67
Y, year. Ž1. Generator guide journal bearing; Ž2. turbine guide journal bearing; Ž3. thrust bearing. Table 2 Variation of weekly average bearing temperature over the weeks of year 1997 Ž8C. W
14 15 18 20 22 26 28 30 32 34 35 36 37 38
Ž1. 68 69 68 68 69 68 67 67 69 67 67 Ž2. 48 15 48 49 49 47 46 46 47 46 47 Ž3. 68 69 70 71 74 73 73 74 75 74 76 76 77 80 W, week no. of year 1997. Ž1. Generator guide journal bearing; Ž2. turbine guide journal bearing; Ž3. thrust bearing.
individual phases is as follows: matrix 24.7; cuboids 75.2; needles 83.9. An average lining microhardness obtained is of 26.5 Hv . Lining chemical composition is shown in Table 3. Metallographic analysis confirmed that lining chemical composition, microhardness and microstructure correspond to white metal of British Standard BS 3332r2. Specification BS 3332r2 is for tin-base white metal with hardness 27–32 Hv and melting range 239–3408C. Bonding between lining and substrate was examined and was found to be normal. 3.5. Lubricant Because of the effect of oil quality on hydro-generator performance, regular analysis of oil samples is normal
practice. Any changes due to system malfunction can thus be detected at an early stage, allowing corrective action to be taken before a serious condition occurs w3x. In the case under discussion oil monitoring has not been practised. High-temperature bearing operation causes oxidation of oil. TAN is an indication of the amount of oxidation that an oil has undergone in service. Increase in TAN value is accompanied by viscosity increase. Water content increase also contributes to viscosity increase. On the other hand, temperature increase causes decrease in viscosity. The oil sample report indicates that water content and oxidation indicator TAN exceed the warning limit but viscosity dropped, remaining within specified limit. Obviously, temperature increase is the prevailing factor. Wet oil results in corrosion and can also lead to bacterial growth in the system. Failure of white metal bearing is accelerated by the presence of water in oil w3x. 3.6. Failure Failed bearing is designed for hydrodynamic regime of lubrication with full separation of runner and pads. Presumably, the damaged pad has been misaligned or deflected which affected clearance. Inadequate clearance caused disruption of oil film and consequently high friction losses.
Fig. 6. Microstructure of bearing lining Žoriginal magnification= 100..
H. IlieÕ r Wear 225–229 (1999) 913–917 Table 3 Chemical composition of bearing pad lining Element
Sn
Sb
Cu
Pb
Fe
Zn
Content Žwt.%.
86.52
8.55
4.52
0.36
0.04
0.01
Bearing damage is concentrated on the outer part of the pad, suggesting that the outer edge was higher than it should be and was in ‘metal to metal’ contact with runner. Temperature in the contact area reached white metal melting point and the lining material melted. Thereafter sliding motion caused superficial flow of material in the direction of sliding. In fact, the outer part of the pad was overloaded and in combination with overheating experienced cracking and loss of area of lining by propagation of cracks, which is fatigue failure. Wear of bearing surface is caused by sliding. Two facts support the above statement, namely: Ža. relative motion of runner with respect to stationary pads is sliding in the direction from right to left pad side. Fig. 4 shows serrated edge on pad left side and demonstrates flow of material in the direction of sliding. Žb. Fig. 5 illustrates uneven distribution of wear in tangential direction Žon pad outer edge linear wear on left side is 1.67 mm, while on right side is 2.47 mm.. Once lining melted, the runner acted like a ‘broom’ and removed material in the direction of sliding. In consideration of bearing failure we belittle cavitation and corrosion wear mechanism. Wear pattern does not correspond to cavitation wear marks. If corrosion wear is the leading wear mechanism, most of the pads should be affected. In failed bearing only one pad is damaged and the rest are just normal.
4. Conclusions 1. Specific load calculated does not exceed recommended maximum for white metal lining of large thrust bearings. Therefore failure is not due to overloading. 2. Lining material was identified as a proper white metal BS 3332r2. Bonding with substrate was found to be normal. Bearing material is not to blame for the failure. 3. Although adequacy of cooling system capacity is questionable, we do not think that bearing failure is cooling system failure. 4. Despite the fact that oil deteriorated to a certain degree bearing failure is not oil fault.
917
5. Failed bearing has been overheated due to excessive friction losses, which resulted from disruption of oil film. Inadequate film thickness, attributed to pad misalignment or deflection, caused oil film disruption. Pad misalignment, confirmed by uneven wear of bearing surface, has a disastrous effect on bearing performance. Pad misalignment could happen either when pads were mounted or during operation. Overheating, causing reduction in fatigue strength, led to wiping and fatigue failure. In conclusion, pad misalignment caused bearing failure. A year ago all the eight pads of the failed thrust bearing have been replaced with new ones, properly aligned. In this period the bearing is reported to perform well.
5. Recommendations 1. Oil circulation system, which will serve the three bearings of one unit. 2. Regular oil sample monitoring. 3. Periodic removal of water from oil by drainage or centrifuging. 4. Proper sealing of bearing in order to keep oil clean. 5. Proper alignment of bearing pads is of utmost importance. 6. Cooling system copper tubes, being subjected to aging for more than 30 years, need special attention.
Acknowledgements The author wishes to acknowledge Mr. R. Mandebvu for provision of bearing temperature readings and Dr. H. Chikwanda for lining chemical composition and hardness measurements.
References w1x J. Simmons, R. Knox, W. Moss, PTFE faced thrust bearings: state of the art review and hydro-generator application in the UK, World Tribology Congress, Abstract of papers, 8–12 Sept. 1997, London, MEP, London, 1997, p. 208. w2x M. Neale ŽEd.., Tribology Handbook, Butterworths, London, 1973. w3x W. Robertson ŽEd.., Lubrication in Practice, MacMillan, London, 1983.