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Formation criterion and mechanism of electrical pitting on the lubricated surface under AC electric field
Wear 236 Ž1999. 62–72 www.elsevier.comrlocaterwear
Formation criterion and mechanism of electrical pitting on the lubricated surface under AC electric field Yuang-Cherng Chiou ) , Rong-Tsong Lee, Chung-Ming Lin Department of Mechanical Engineering, National Sun Yat-Sen UniÕersity, Kaohsiung, 80424, Taiwan Received 11 January 1999; received in revised form 6 June 1999; accepted 22 June 1999
Abstract The formation mechanism of electrical pitting on the lubricated surface of steel pair was investigated, and the threshold condition to avoid the occurrence of electrical pitting was also established by using a static electrical pitting tester with high precision under the influence of AC electric field. Experimental results show that when the electrical pitting occurs, the interface voltage, interface impedance, and interface power increases slowly with increasing film thickness at a certain supply current. However, the interface voltage and interface power increases with increasing supply current, and the interface impedance decreases with increasing supply current at a certain film thickness. Furthermore, the pitting area versus the interface power relationship is a cubic function, where the pitting area increases with increasing interface power. It is also found that the supply current is the parameter that influences the pitting area the most. Moreover, three electrical pitting regimes are found under the influences of shaft voltage and oil film thickness, namely, pitting, transition, and no-pitting regimes. The boundary between the transition and no-pitting regimes is called the first threshold voltage, and another boundary between the transition and pitting regimes is called the second threshold voltage. However, the supply current insignificantly influences these two threshold voltages. Correlation formula of threshold voltage and oil film thickness is also derived as the formation criterion of electrical pitting for a wide range of oil film thickness. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Formation criterion; Electrical pitting
1. Introduction The shaft voltage frequently exists in rotating machinery, such as turbines and compressors, due to the asymmetric effect between the rotor and the stator, a potential accidentally applied to the shaft, etc. w1x. This shaft voltage in rotating machinery results in the flow of current through bearings. This bearing current sometimes causes the damage to the bearing surfaces due to arcing across the bearing and journal surfaces. The effect of arcing is the wear of the bearing and journal due to the removal of fused metals in the arc. This wear is known as electrical wear or pitting. As a result, the roughening of the surfaces produced by electrical pitting can considerably accelerate the mechanical wear w2x. Moreover, the deterioration of lubricant is accelerated and the bearing life is reduced due to heat generated and instantaneous temperature rise when the bearing are exposed to electrical current w3–8x.
)
Corresponding author
When the current leaks through the roller bearing in which low resistivity grease had been used, Prashad w9x thought that this leads in the initial stages to the electrochemical decomposition of grease and corrosion, and then gradual formation of flutings and corrugation on the surfaces. On the contrary, when the high resistivity grease is used, the charges accumulate on the bearing surfaces leads to surface pitting by an arc-welding effect and the failure of the bearing. The theoretical analysis has been carried out for evaluating the instantaneous rise of temperature due to the effects of instant charge leakage in roller bearings w6x. Komatsuzaki et al. w10,11x found that the electrical pitting is significantly influenced by the applied current rather than the applied voltage. When the bearing current is larger than 90 mA, the electrical pitting occurs. It has been found that the damage under the influence of shaft voltage is primarily due to arcing. The arcing effect is significantly influenced by the magnitude of the bearing impedance and the voltage across the bearings. Dirt, metallic particles, surface roughness, and oil film thickness reduce the bearing impedance w12x. The effect of operating parameters and lubricant characteristics on the
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 9 . 0 0 2 6 4 - 1
Y.-C. Chiou et al.r Wear 236 (1999) 62–72
threshold voltages to avoid bearing damage has been determined by Prashad w12x. However, since the minimum film thickness is obtained from the Gribin formula, his result is only valid in a narrow range of oil film thickness. As mentioned above, most studies have been often devoted to rotating machinery under various operation conditions and lubricant characteristics for investigating the effects of shaft voltage and bearing current on electrical pitting, where the film thickness is determined by the empirical equation of theoretical analysis. However, it has been known that the contact surfaces would be roughened due to the electrical pitting. As a result, the film thickness obtained from theoretical analysis under the very smooth contacts would be incorrect. Hence, in this study, a static pitting tester is designed to simulate practical conditions, as far as possible, where the oil film thickness can be adjusted and provided accurately controllable running conditions under various shaft voltages and bearing currents. Using this tester, the mechanism of electrical pitting can be clearly investigated on the lubricated surface of steel pair, and a diagram of pitting formation is established for a wide range of supply voltage, supply current, and oil film thickness. 2. Experimental apparatus and procedures 2.1. Experimental apparatus A static electrical pitting tester is designed to evaluate the mechanism of electrical pitting on the lubricated surface of steel pair under different oil film thicknesses, as shown in Fig. 1. In this tester, two micrometer heads of non-rotating spindle type are employed. In each micrometer head, one revolution of the thimble moves the spindle the distance of 0.5 mm. In order to adjust the film thickness in the graduation of 0.025 mm, a 50:1 worm gear reducer is employed to drive the micrometer head. The
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worm is driven by a stepping motor with the resolution of 0.98rstep. The stepping motor can be controlled by a PC program. A pin specimen is attached to the spindle of a first micrometer head. Hence, it can be moved upward and downward through the spindle of a first micrometer head. To eliminate the effect of the backlash of micrometer head on the precision of the micro-movement of the pin specimen, a second micrometer head is used to measure the micro-movement of the pin specimen through the reference plate attached to the spindle of the first micrometer, as shown in Fig. 1. Here a cartridge head probe mounted on a second micrometer head is connected to an electrical comparator or differential type analog mu-checker with a graduation of 0.1 mm. Hence, the micro-movement of the pin specimen through this device can be measured under high precision. The sequence of operation for this device is described in Section 2.3. The block specimen is immersed in the oil tank which is placed on the x–y table. A paraffin base oil Ž26.59 cSt at 408C, 4.76 cSt at 1008C., is used in the oil tank. The oil temperature was maintained at 25 " 38C. In this temperature, the resistivity of the base oil is 1.5 = 10 14 V cm. The block specimen is insulated from the body of the tester by the plastic oil tank, and the pin specimen is held in an insulating holder attached to the spindle of the first micrometer head, so that they can constitute an electric circuit. The AC power supply with the resolution of 0.1 V is employed, so that the current can flow through the test specimens. The supply current between pin and block specimens can be adjusted by a variable resistor. The variations of interface voltage and current can be measured on the data acquisition system and fed to a personal computer for data analysis. Moreover, an digital oscilloscope is also employed to observe the variation of interface voltage. The equivalent circuit for the present experiment is shown in Fig. 2. In this figure, VS is the supply voltage, I
Fig. 1. Schematic diagram of the static electrical pitting tester.
Y.-C. Chiou et al.r Wear 236 (1999) 62–72
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Fig. 2. The equivalent circuit diagram of the present experiment.
is the supply current, R L is variable resistance, and VL is terminal voltage of variable resistance. Va and Ia are interface voltage and current between pin and block specimens, respectively. When the switch is connected to the point b, V L s VS , I s VSrR L . When the phenomenon of arcing occurs, Va is also called arcing voltage. 2.2. Test specimens The specimens of pin and block are made of medium carbon steel ŽS50C. and bearing steel ŽSUJ2., respectively. The size and shape of the test specimens were shown in Fig. 3. They were heated to 8508C for about 30 min, and then cooled in the furnace slowly to the room temperature. After heat-treatment, they were polished with grade 400, 600, 1000, 1500, and 2000 emery papers, and diamond paste in that order, so that their surface roughness R a was about 0.01 mm or R max of 0.05 mm. 2.3. Experimental procedure Before the test, the pin and block specimens were cleaned with acetone and an ultrasonic cleaner. The pin specimen is fixed at the jig of the first micrometer head, and the block specimen is fixed in the oil tank. When the pin specimen moves downward with the graduation of 0.025 mm by a stepping motor through a worm gear reducer and the first micrometer head, it comes into contact with the block specimen slightly. However, to eliminate the effect of the backlash of micrometer head on the precision of the micro-movement of the pin specimen, the cartridge head probe mounted on a second micrometer head moves downward and slightly contacts with the reference plate attached to the spindle of the first microme-
ter head. Since a cartridge head probe connected to an analog mu-checker has a graduation of 0.1 mm, the micro-movement of the pin specimen can be observed through this mu-checker. Furthermore, the contact condition between the test specimens is also measured through the digital multimeter. When the pin specimen moves upward, the contact resistance between the test specimens increases from 6 to 200 V gradually, and from 200 V to infinity very quickly. Generally, when the contact resistance is in the range of 6–200 V, the lubrication mode belongs to the mixed lubrication where some asperity contacts between the test specimens have been achieved w13x. Hence, if the contact resistance is 200 V, then the initial gap distance between the pin and the block specimens is set to zero, and the analog device for the differential type mu-checker is zeroed. In this experiment, although the pin specimen can move upward with the graduation of 0.025 mm, an analog mu-checker only has a graduation of 0.1 mm. Consequently, the increment of oil film thickness is set to 0.1 mm. Before the adjustment of the oil film thickness, the supply voltage is preset from 1 to 100 V through a 60 Hz sinewave voltage of AC power supply. The test parameters used are 0.2–7 mm initial gap distance, bi-directional arcs of 8.3 ms duration on the block and the pin specimens, and 1800 arc cycles Žor test time of 30 s.. A variable resistance, which will be referred to as the external resistance in the circuit, is used to adjust the supply current from 1 to 8 A. When the oil film thickness is adjusted to a certain value, turn the switch on to supply a certain value of voltage and current. During the test, the digital oscilloscope and the data acquisition system record the peak-topeak values for the interface voltage between pin and block specimens, and the terminal voltage of variable resistor. They are used for determining the r.m.s. voltage and, subsequently, the ‘‘average’’ bearing impedance using the r.m.s. value of the current. The electrical pitting area can be measured by using the optical microscope or scanning electronic microscope.
3. Experimental results and discussion 3.1. The characteristics of interface Õoltage and impedance Since the oil film between the test specimens possesses high resistivity under static condition in the electrical field,
Fig. 3. The size and shape of the test specimens: Ža. pin specimen, Žb. block specimen.
Y.-C. Chiou et al.r Wear 236 (1999) 62–72
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Fig. 4. The effects of supply current and oil film thickness on the interface voltage and the interface impedance at supply voltage of 15 V.
the circuit shown in Fig. 2 is a complete open loop under low potential difference between the test specimens. As a result, the current does not begin to flow, and the interface voltage is equal to the supply voltage. At higher potential difference than threshold voltage, electrical breakdown occurs in the oil film between the specimens, causing an
arcing effect and allowing the passage of current through the oil film in the contact zone. The flow of the current is significantly influenced by the magnitude of the voltage across the test specimens, the resistivity of the lubricant, the bearing materials and their finishes, and the oil film thickness. To investigate the effect of supply current and
Fig. 5. The effects of supply current and oil film thickness on the interface voltage and the interface impedance at supply voltage of 60 V.
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Y.-C. Chiou et al.r Wear 236 (1999) 62–72
Fig. 6. The effects of supply current and oil film thickness on the interface voltage and the interface impedance at supply voltage of 100 V.
voltage on the characteristics of interface voltage and impedance, a series of static electric pitting tests are conducted under a wide range of supply current, supply voltage, and oil film thickness. The typical experimental results are shown in Figs. 4–6.
Fig. 4 shows the effect of supply current on the interface voltage and impedance at different oil film thicknesses for a supply voltage of 15 V. It is seen from this figure that the interface voltage and impedance slightly increases with increasing oil film thickness at certain
Fig. 7. The effects of supply current and oil film thickness on the interface power at supply voltage of 15 V.
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Fig. 8. The effects of supply current and oil film thickness on the interface power at supply voltage of 60 V.
supply current. However, when the oil film thickness increases to a critical value, the interface voltage increases
quickly to the magnitude of the supply voltage. As mentioned above, this circuit becomes a complete open loop
Fig. 9. The effects of supply current and oil film thickness on the interface power at supply voltage of 100 V.
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Fig. 10. SEM micrograph of electrical pitting for supply voltage of 100 V, supply current of 1 A, and oil film thickness of 3 mm Žoriginal magnification: 1500=..
and the interface impedance also increases abruptly to infinity. When the oil film thickness continues to increase, the arcing effect will no longer occur. Hence, the critical oil film thickness is defined as the maximum film thickness causing an arcing effect and allowing the passage of current through the oil film in the contact zone. It is also seen from Fig. 4 that the interface voltage increases with increasing supply current at certain oil film thickness, but
the interface impedance decreases with increasing supply current. Moreover, the critical oil film thickness is not significantly influenced by the supply current. Figs. 5 and 6 show the variation in interface voltage and impedance with oil film thickness at different supply current for the higher supply voltages of 60 and 100 V, respectively. It is seen from Figs. 5 and 6 that the effects of supply current and oil film thickness on interface voltage and impedance are similar to Fig. 4 at certain supply voltage. However, the critical oil film thickness increases with increasing supply voltage. Furthermore, it was indicated by Erdman et al. w8x that the mineral oil field strength is 10 6 Vrm. Hence, for a typical oil film of 0.2 mm, the bearing breakdown voltage threshold is 0.4 V. They suggest that interface voltage less than 0.3 V is safe, and interface voltage between 0.5 and 1.0 V may develop harmful bearing currents. It is seen from Fig. 4 that for the supply voltage of 15 V and the supply current of 1 A at a oil film of 0.2 mm, the interface voltage is close to 0.41 V where a harmful bearing current is developed. Moreover, the interface voltage increases quickly with increasing supply current. Hence, the present results are in very good agreement with the results obtained by Erdman et al. w8x. Generally, the interface voltage to break down the oil film is significantly influenced by the supply voltage, the supply current, and the oil film thickness, as shown in Figs. 4–6.
Fig. 11. The effects of supply current and oil film thickness on the electrical pitting area at supply voltage of 15 V.
Y.-C. Chiou et al.r Wear 236 (1999) 62–72
Fig. 12. The effects of supply current and oil film thickness on the electrical pitting area at supply voltage of 60 V.
Fig. 13. The effects of supply current and oil film thickness on the electrical pitting area at supply voltage of 100 V.
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3.2. The interface power When the electrical breakdown occurs in the oil film between the test specimens, an arcing effect and the bearing current can be observed. Under this situation, the dissipated power is known as the interface power, and it can be calculated as Va2rR a . By using this definition, the interface power for Figs. 4–6 can be calculated, as shown in Figs. 7–9. It is seen from Fig. 7 that at a certain supply voltage of 15 V, the interface power increases with increasing supply current, but it slightly increases with increasing the oil film thickness. Moreover, the effect of oil film thickness for larger supply current is more obvious than that for smaller supply current. Figs. 8 and 9 show the variation in interface power with oil film thickness at different supply current for supply voltages of 60 and 100 V, respectively. The effects of supply current and oil film thickness on interface power are similar to Fig. 7 at a certain supply voltage. However, the effect of supply voltage on interface power supply is insignificant. As previously mentioned, the interface power is mainly influenced by supply current rather than supply voltage. 3.3. The formation mechanism of electrical pitting When the arcing effect occurs on the interface between the pin and block specimens, the electrical pitting can be observed on the contact surface. In this experiment, the appearance of the electrical pitting can be observed by using the scanning electronic microscope ŽSEM. under
different supply voltage and supply current. The typical SEM micrograph of the electrical pitting is shown in Fig. 10 for a supply voltage of 100 V and oil film thickness of 3 mm. It is clear to see an obvious concave pitting in the vicinity of center region with a convex on its surrounding, like honeycomb. The formation for this electrical pitting can be considered as follows. When the contact surface was attacked by the positive and negative ions, the kinetic energy is transformed into the heat energy, so that higher contact temperature than melting point is found. The relationship between the pitting area and operating parameter is shown in Figs. 11–13. It is seen from those figures that the effects of oil film thickness and supply voltage on the electrical pitting area is not significant, but the electrical pitting area is significantly influenced by the supply current. That is, when the supply current increases, the pitting area obviously increases, since the principal factor to influence the interface power is the supply current, as shown in Figs. 7–9. This interface power is the main energy of electrical pitting. Hence, the relationship between the pitting area ŽFigs. 11–13. and the interface power ŽFigs. 7–9. can be rearranged into Fig. 14. It is seen from Fig. 14 that the relationship between the pitting area, A p Žin unit of 10 3 mm2 ., and the interface power, P Žin W., is a cubic function, or A p s 0.0059P 3 y 0.068 P 2 q 0.46 P .
Ž 1.
After using the data shown in Figs. 11–13 under different operating parameters, the correlation equation derived
Fig. 14. The relationship between the interface power and electrical pitting area.
Y.-C. Chiou et al.r Wear 236 (1999) 62–72
by the least-square method for the electrical pitting area in terms of supply voltage, Vs Žin V., supply current, I Žin A., and oil film thickness, h Žin mm., is given as follows: A p s 0.081h0.116 Vs 0.289 I 1.179 ,
R 2 s 0.928
Ž 2.
2
where R is the coefficient of determination Ža number between 0 and 1.. The higher the value of R 2 , the more successful is. It is seen from Eq. Ž2. that the major factor to influence the electrical pitting area is the supply current. Furthermore, for a given supply voltage and supply current in a rotating machinery, this equation is useful in predicting the oil film thickness or the lubricating condition by observing the magnitude of the electrical pitting area. 3.4. Pitting formation diagram As previously mentioned, at a certain supply voltage, the arcing effect will no longer take place for the oil film thickness larger than a critical value. Similarly, at a certain supply current and oil film thickness, the arcing effect does not begin until a critical supply voltage or a threshold voltage is achieved. At a certain supply current, the supply voltage versus the oil film thickness can be used to distinct the contact surface into three regimes, namely no-pitting, transition, and pitting regimes, as shown in Fig. 15. In this pitting formation diagram, the boundary between the transition and no-pitting regimes is called the first threshold voltage, and another boundary between the transition and pitting regimes is called the second threshold voltage.
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When the bearing is under different film thickness in the electrical field, the current does not begin to flow until the first threshold voltage is reached. Hence, the electrical pitting cannot be observed on the surfaces of the specimens in no-pitting regime. At a certain oil film thickness and supply current, when the supply voltage achieves the first threshold voltage, arcing takes place in the gap and the momentary flow of current can be measured. However, the current gap distance is larger than the initial gap distance due to the accumulated pitting depth by the arc at the static pitting test. As a result, this momentary flow of current decreases to zero and the arcing effect disappears. Hence, in the transition regime, the supply voltage is still not enough to break down the insulating status of oil film, and a little pitting area can be observed on the surfaces of the specimens in this regime. At a certain oil film thickness and supply current, when the supply voltage achieves or is larger than the second threshold voltage, a very high increase in current can be measured, where the insulating status of oil film has been broken down. Hence, the electrical pitting can be observed on the surfaces of the specimens in the pitting regime. Moreover, this pitting formation diagram also indicates that the first threshold voltage is quite close the second threshold voltage, because the pitting depth is quite small relate to the initial gap distance. When the supply voltage is larger than the second threshold voltage, the insulating status of the oil film is completely destroyed in the pitting regime, so that the
Fig. 15. The diagram of electrical pitting formation at supply current of 1 A.
Y.-C. Chiou et al.r Wear 236 (1999) 62–72
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current continuously passes through the oil film causing the arcing effect. However, the supply current insignificantly influences these two threshold voltages, so the pitting formation diagram for higher supply current does not show in this paper. After using the pitting formation diagram under different supply currents, the correlation equation derived by the least-square method for the first threshold voltage in terms of supply current, I, and oil film thickness, h, is given as follows: Vt s 8.185h1.429 Iy0 .032 ,
R 2 s 0.998
Ž 3.
It is seen from Eq. Ž3. that the pitting formation is significantly influenced by the threshold voltage and oil film thickness, but the effect of the supply current is insignificant.
4. Conclusions The effect of supply voltage, supply current, and oil film thickness on the characteristics of electrical pitting for the lubricated surface of steel pair is investigated by using a static electrical pitting tester with high precision under the influence of shaft voltage of AC electric field. From the experimental results, the following conclusion can be drawn. Ž1. When the electrical pitting occurs, the interface voltage and interface power increase with increasing supply current, but the interface impedance decreases with increasing supply current. However, they are insignificantly influenced by the oil film thickness. Ž2. For the mineral oil, the interface voltage greater than 0.4 V may develop harmful bearing currents at a oil film of 0.2 mm. This result is in very good agreement with the results obtained by Erdman et al. w8x. Ž3. Three electrical pitting regimes are found under the influences of shaft voltage and oil film thickness, namely, pitting, transition, and no-pitting regimes. The boundary between the transition and no-pitting regimes is called the first threshold voltage, and another boundary is called the second threshold voltage. Correlation formula for the first threshold voltage in terms of supply current, I, and oil film thickness, h, is given in Eq. Ž3.. Ž4. When the second threshold voltage exceeds, the higher interface power, the higher pitting area on the contact surfaces. Correlation formula for the electrical pitting area in terms of supply voltage, Vs , supply current, I, and oil film thickness, h, is established in Eq. Ž2..
5. Nomenclature Ap h
Pitting area Ž10 3 mm2 . Oil film thickness Žmm.
I Ia P R2 RL Ra Va VL Vs Vt
Supply current ŽA. Interface current ŽA. Interface power ŽW. Coefficient of determination Variable resistance Ž V . Interface impedance Ž V . Interface voltage ŽV. Terminal voltage of variable resistance ŽV. Supply voltage ŽV. Threshold voltage ŽV.
Acknowledgements The authors would like to express their appreciation to the National Science Council ŽNSC-88-2212-E110-003. in Taiwan, R.O.C. for the financial support.
References w1x J. Boyd, H.N. Kaufman, The causes and the control of electrical currents in bearings, Lubrication Engineering ŽJanuary 1959. 28–35. w2x H.N. Kaufman, J. Boyd, The conduction of current in bearings, ASLE Conf., Los Angles, CA, October, 1958. w3x S. Komatsuzaki, T. Uematsu, F. Nakano, Bearing damage by electrical wear and its effect on deterioration of lubricating greases, Lubricating Engineering 43 Ž1987. 25–30. w4x H. Prashad, T.S.R. Murthy, Behavior of greases in statically-bounded conditions and when used in non-insulated anti-friction bearings under the influence of electrical fields, Lubrication Engineering 44 Ž1988. 239–246. w5x H. Prashad, Investigation of damaged rolling-element bearings and deterioration of lubricants under the influence of electric fields, Wear 176 Ž1994. 151–161. w6x H. Prashad, Theoretical analysis of the effects of instantaneous charge leakage on roller tracks of roller bearings lubricated with high resistivity lubricants under the influence of electric current, Trans. ASME J. Tribol. 112 Ž1990. 37–43. w7x H. Prashad, Theoretical evaluation of reduction in the life of hydrodynamic journal bearings operating under the influence of different levels of shaft voltages, STLE, Tribol. Trans. 34 Ž1991. 623–627. w8x J.M. Erdman, R.J. Kerkman, D.W. Schlegel, G.L. Skibinski, Effect of PWM inverters on AC motor bearing currents and shaft voltage, IEEE Transactions on Industry Applications 32 Ž2. Ž1996. 250–259. w9x H. Prashad, Diagnosis of failure of rolling-element bearings of alternators — a study, Wear 198 Ž1996. 46–51. w10x S. Komatsuzaki, T. Uematsu, R. Itoh, A study of antielectrowear properties of lubricating greases from the viewpoint of grease composition, J. JSLE 28 Ž1. Ž1983. 54–60. w11x S. Komatsuzaki, T. Uematsu, R. Itoh, A study of antielectrowear properties of lubricating greases from the viewpoint of grease composition, J. JSLE, Int. Ed. 5 Ž1984. 169–174. w12x H. Prashad, Effect of operating parameters on the threshold voltages and impedance response of non-insulated rolling element bearings under the action of electrical currents, Wear 117 Ž1987. 223–240. w13x W.F. Kuo, Y.C. Chiou, R.T. Lee, A study on lubrication mechanism and wear scar in sliding circular contacts, Wear 201 Ž1996. 217–226.
Formation criterion and mechanism of electrical pitting on the lubricated surface under AC electric field Yuang-Cherng Chiou ) , Rong-Tsong Lee, Chung-Ming Lin Department of Mechanical Engineering, National Sun Yat-Sen UniÕersity, Kaohsiung, 80424, Taiwan Received 11 January 1999; received in revised form 6 June 1999; accepted 22 June 1999
Abstract The formation mechanism of electrical pitting on the lubricated surface of steel pair was investigated, and the threshold condition to avoid the occurrence of electrical pitting was also established by using a static electrical pitting tester with high precision under the influence of AC electric field. Experimental results show that when the electrical pitting occurs, the interface voltage, interface impedance, and interface power increases slowly with increasing film thickness at a certain supply current. However, the interface voltage and interface power increases with increasing supply current, and the interface impedance decreases with increasing supply current at a certain film thickness. Furthermore, the pitting area versus the interface power relationship is a cubic function, where the pitting area increases with increasing interface power. It is also found that the supply current is the parameter that influences the pitting area the most. Moreover, three electrical pitting regimes are found under the influences of shaft voltage and oil film thickness, namely, pitting, transition, and no-pitting regimes. The boundary between the transition and no-pitting regimes is called the first threshold voltage, and another boundary between the transition and pitting regimes is called the second threshold voltage. However, the supply current insignificantly influences these two threshold voltages. Correlation formula of threshold voltage and oil film thickness is also derived as the formation criterion of electrical pitting for a wide range of oil film thickness. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Formation criterion; Electrical pitting
1. Introduction The shaft voltage frequently exists in rotating machinery, such as turbines and compressors, due to the asymmetric effect between the rotor and the stator, a potential accidentally applied to the shaft, etc. w1x. This shaft voltage in rotating machinery results in the flow of current through bearings. This bearing current sometimes causes the damage to the bearing surfaces due to arcing across the bearing and journal surfaces. The effect of arcing is the wear of the bearing and journal due to the removal of fused metals in the arc. This wear is known as electrical wear or pitting. As a result, the roughening of the surfaces produced by electrical pitting can considerably accelerate the mechanical wear w2x. Moreover, the deterioration of lubricant is accelerated and the bearing life is reduced due to heat generated and instantaneous temperature rise when the bearing are exposed to electrical current w3–8x.
)
Corresponding author
When the current leaks through the roller bearing in which low resistivity grease had been used, Prashad w9x thought that this leads in the initial stages to the electrochemical decomposition of grease and corrosion, and then gradual formation of flutings and corrugation on the surfaces. On the contrary, when the high resistivity grease is used, the charges accumulate on the bearing surfaces leads to surface pitting by an arc-welding effect and the failure of the bearing. The theoretical analysis has been carried out for evaluating the instantaneous rise of temperature due to the effects of instant charge leakage in roller bearings w6x. Komatsuzaki et al. w10,11x found that the electrical pitting is significantly influenced by the applied current rather than the applied voltage. When the bearing current is larger than 90 mA, the electrical pitting occurs. It has been found that the damage under the influence of shaft voltage is primarily due to arcing. The arcing effect is significantly influenced by the magnitude of the bearing impedance and the voltage across the bearings. Dirt, metallic particles, surface roughness, and oil film thickness reduce the bearing impedance w12x. The effect of operating parameters and lubricant characteristics on the
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 9 . 0 0 2 6 4 - 1
Y.-C. Chiou et al.r Wear 236 (1999) 62–72
threshold voltages to avoid bearing damage has been determined by Prashad w12x. However, since the minimum film thickness is obtained from the Gribin formula, his result is only valid in a narrow range of oil film thickness. As mentioned above, most studies have been often devoted to rotating machinery under various operation conditions and lubricant characteristics for investigating the effects of shaft voltage and bearing current on electrical pitting, where the film thickness is determined by the empirical equation of theoretical analysis. However, it has been known that the contact surfaces would be roughened due to the electrical pitting. As a result, the film thickness obtained from theoretical analysis under the very smooth contacts would be incorrect. Hence, in this study, a static pitting tester is designed to simulate practical conditions, as far as possible, where the oil film thickness can be adjusted and provided accurately controllable running conditions under various shaft voltages and bearing currents. Using this tester, the mechanism of electrical pitting can be clearly investigated on the lubricated surface of steel pair, and a diagram of pitting formation is established for a wide range of supply voltage, supply current, and oil film thickness. 2. Experimental apparatus and procedures 2.1. Experimental apparatus A static electrical pitting tester is designed to evaluate the mechanism of electrical pitting on the lubricated surface of steel pair under different oil film thicknesses, as shown in Fig. 1. In this tester, two micrometer heads of non-rotating spindle type are employed. In each micrometer head, one revolution of the thimble moves the spindle the distance of 0.5 mm. In order to adjust the film thickness in the graduation of 0.025 mm, a 50:1 worm gear reducer is employed to drive the micrometer head. The
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worm is driven by a stepping motor with the resolution of 0.98rstep. The stepping motor can be controlled by a PC program. A pin specimen is attached to the spindle of a first micrometer head. Hence, it can be moved upward and downward through the spindle of a first micrometer head. To eliminate the effect of the backlash of micrometer head on the precision of the micro-movement of the pin specimen, a second micrometer head is used to measure the micro-movement of the pin specimen through the reference plate attached to the spindle of the first micrometer, as shown in Fig. 1. Here a cartridge head probe mounted on a second micrometer head is connected to an electrical comparator or differential type analog mu-checker with a graduation of 0.1 mm. Hence, the micro-movement of the pin specimen through this device can be measured under high precision. The sequence of operation for this device is described in Section 2.3. The block specimen is immersed in the oil tank which is placed on the x–y table. A paraffin base oil Ž26.59 cSt at 408C, 4.76 cSt at 1008C., is used in the oil tank. The oil temperature was maintained at 25 " 38C. In this temperature, the resistivity of the base oil is 1.5 = 10 14 V cm. The block specimen is insulated from the body of the tester by the plastic oil tank, and the pin specimen is held in an insulating holder attached to the spindle of the first micrometer head, so that they can constitute an electric circuit. The AC power supply with the resolution of 0.1 V is employed, so that the current can flow through the test specimens. The supply current between pin and block specimens can be adjusted by a variable resistor. The variations of interface voltage and current can be measured on the data acquisition system and fed to a personal computer for data analysis. Moreover, an digital oscilloscope is also employed to observe the variation of interface voltage. The equivalent circuit for the present experiment is shown in Fig. 2. In this figure, VS is the supply voltage, I
Fig. 1. Schematic diagram of the static electrical pitting tester.
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Fig. 2. The equivalent circuit diagram of the present experiment.
is the supply current, R L is variable resistance, and VL is terminal voltage of variable resistance. Va and Ia are interface voltage and current between pin and block specimens, respectively. When the switch is connected to the point b, V L s VS , I s VSrR L . When the phenomenon of arcing occurs, Va is also called arcing voltage. 2.2. Test specimens The specimens of pin and block are made of medium carbon steel ŽS50C. and bearing steel ŽSUJ2., respectively. The size and shape of the test specimens were shown in Fig. 3. They were heated to 8508C for about 30 min, and then cooled in the furnace slowly to the room temperature. After heat-treatment, they were polished with grade 400, 600, 1000, 1500, and 2000 emery papers, and diamond paste in that order, so that their surface roughness R a was about 0.01 mm or R max of 0.05 mm. 2.3. Experimental procedure Before the test, the pin and block specimens were cleaned with acetone and an ultrasonic cleaner. The pin specimen is fixed at the jig of the first micrometer head, and the block specimen is fixed in the oil tank. When the pin specimen moves downward with the graduation of 0.025 mm by a stepping motor through a worm gear reducer and the first micrometer head, it comes into contact with the block specimen slightly. However, to eliminate the effect of the backlash of micrometer head on the precision of the micro-movement of the pin specimen, the cartridge head probe mounted on a second micrometer head moves downward and slightly contacts with the reference plate attached to the spindle of the first microme-
ter head. Since a cartridge head probe connected to an analog mu-checker has a graduation of 0.1 mm, the micro-movement of the pin specimen can be observed through this mu-checker. Furthermore, the contact condition between the test specimens is also measured through the digital multimeter. When the pin specimen moves upward, the contact resistance between the test specimens increases from 6 to 200 V gradually, and from 200 V to infinity very quickly. Generally, when the contact resistance is in the range of 6–200 V, the lubrication mode belongs to the mixed lubrication where some asperity contacts between the test specimens have been achieved w13x. Hence, if the contact resistance is 200 V, then the initial gap distance between the pin and the block specimens is set to zero, and the analog device for the differential type mu-checker is zeroed. In this experiment, although the pin specimen can move upward with the graduation of 0.025 mm, an analog mu-checker only has a graduation of 0.1 mm. Consequently, the increment of oil film thickness is set to 0.1 mm. Before the adjustment of the oil film thickness, the supply voltage is preset from 1 to 100 V through a 60 Hz sinewave voltage of AC power supply. The test parameters used are 0.2–7 mm initial gap distance, bi-directional arcs of 8.3 ms duration on the block and the pin specimens, and 1800 arc cycles Žor test time of 30 s.. A variable resistance, which will be referred to as the external resistance in the circuit, is used to adjust the supply current from 1 to 8 A. When the oil film thickness is adjusted to a certain value, turn the switch on to supply a certain value of voltage and current. During the test, the digital oscilloscope and the data acquisition system record the peak-topeak values for the interface voltage between pin and block specimens, and the terminal voltage of variable resistor. They are used for determining the r.m.s. voltage and, subsequently, the ‘‘average’’ bearing impedance using the r.m.s. value of the current. The electrical pitting area can be measured by using the optical microscope or scanning electronic microscope.
3. Experimental results and discussion 3.1. The characteristics of interface Õoltage and impedance Since the oil film between the test specimens possesses high resistivity under static condition in the electrical field,
Fig. 3. The size and shape of the test specimens: Ža. pin specimen, Žb. block specimen.
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Fig. 4. The effects of supply current and oil film thickness on the interface voltage and the interface impedance at supply voltage of 15 V.
the circuit shown in Fig. 2 is a complete open loop under low potential difference between the test specimens. As a result, the current does not begin to flow, and the interface voltage is equal to the supply voltage. At higher potential difference than threshold voltage, electrical breakdown occurs in the oil film between the specimens, causing an
arcing effect and allowing the passage of current through the oil film in the contact zone. The flow of the current is significantly influenced by the magnitude of the voltage across the test specimens, the resistivity of the lubricant, the bearing materials and their finishes, and the oil film thickness. To investigate the effect of supply current and
Fig. 5. The effects of supply current and oil film thickness on the interface voltage and the interface impedance at supply voltage of 60 V.
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Fig. 6. The effects of supply current and oil film thickness on the interface voltage and the interface impedance at supply voltage of 100 V.
voltage on the characteristics of interface voltage and impedance, a series of static electric pitting tests are conducted under a wide range of supply current, supply voltage, and oil film thickness. The typical experimental results are shown in Figs. 4–6.
Fig. 4 shows the effect of supply current on the interface voltage and impedance at different oil film thicknesses for a supply voltage of 15 V. It is seen from this figure that the interface voltage and impedance slightly increases with increasing oil film thickness at certain
Fig. 7. The effects of supply current and oil film thickness on the interface power at supply voltage of 15 V.
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Fig. 8. The effects of supply current and oil film thickness on the interface power at supply voltage of 60 V.
supply current. However, when the oil film thickness increases to a critical value, the interface voltage increases
quickly to the magnitude of the supply voltage. As mentioned above, this circuit becomes a complete open loop
Fig. 9. The effects of supply current and oil film thickness on the interface power at supply voltage of 100 V.
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Fig. 10. SEM micrograph of electrical pitting for supply voltage of 100 V, supply current of 1 A, and oil film thickness of 3 mm Žoriginal magnification: 1500=..
and the interface impedance also increases abruptly to infinity. When the oil film thickness continues to increase, the arcing effect will no longer occur. Hence, the critical oil film thickness is defined as the maximum film thickness causing an arcing effect and allowing the passage of current through the oil film in the contact zone. It is also seen from Fig. 4 that the interface voltage increases with increasing supply current at certain oil film thickness, but
the interface impedance decreases with increasing supply current. Moreover, the critical oil film thickness is not significantly influenced by the supply current. Figs. 5 and 6 show the variation in interface voltage and impedance with oil film thickness at different supply current for the higher supply voltages of 60 and 100 V, respectively. It is seen from Figs. 5 and 6 that the effects of supply current and oil film thickness on interface voltage and impedance are similar to Fig. 4 at certain supply voltage. However, the critical oil film thickness increases with increasing supply voltage. Furthermore, it was indicated by Erdman et al. w8x that the mineral oil field strength is 10 6 Vrm. Hence, for a typical oil film of 0.2 mm, the bearing breakdown voltage threshold is 0.4 V. They suggest that interface voltage less than 0.3 V is safe, and interface voltage between 0.5 and 1.0 V may develop harmful bearing currents. It is seen from Fig. 4 that for the supply voltage of 15 V and the supply current of 1 A at a oil film of 0.2 mm, the interface voltage is close to 0.41 V where a harmful bearing current is developed. Moreover, the interface voltage increases quickly with increasing supply current. Hence, the present results are in very good agreement with the results obtained by Erdman et al. w8x. Generally, the interface voltage to break down the oil film is significantly influenced by the supply voltage, the supply current, and the oil film thickness, as shown in Figs. 4–6.
Fig. 11. The effects of supply current and oil film thickness on the electrical pitting area at supply voltage of 15 V.
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Fig. 12. The effects of supply current and oil film thickness on the electrical pitting area at supply voltage of 60 V.
Fig. 13. The effects of supply current and oil film thickness on the electrical pitting area at supply voltage of 100 V.
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3.2. The interface power When the electrical breakdown occurs in the oil film between the test specimens, an arcing effect and the bearing current can be observed. Under this situation, the dissipated power is known as the interface power, and it can be calculated as Va2rR a . By using this definition, the interface power for Figs. 4–6 can be calculated, as shown in Figs. 7–9. It is seen from Fig. 7 that at a certain supply voltage of 15 V, the interface power increases with increasing supply current, but it slightly increases with increasing the oil film thickness. Moreover, the effect of oil film thickness for larger supply current is more obvious than that for smaller supply current. Figs. 8 and 9 show the variation in interface power with oil film thickness at different supply current for supply voltages of 60 and 100 V, respectively. The effects of supply current and oil film thickness on interface power are similar to Fig. 7 at a certain supply voltage. However, the effect of supply voltage on interface power supply is insignificant. As previously mentioned, the interface power is mainly influenced by supply current rather than supply voltage. 3.3. The formation mechanism of electrical pitting When the arcing effect occurs on the interface between the pin and block specimens, the electrical pitting can be observed on the contact surface. In this experiment, the appearance of the electrical pitting can be observed by using the scanning electronic microscope ŽSEM. under
different supply voltage and supply current. The typical SEM micrograph of the electrical pitting is shown in Fig. 10 for a supply voltage of 100 V and oil film thickness of 3 mm. It is clear to see an obvious concave pitting in the vicinity of center region with a convex on its surrounding, like honeycomb. The formation for this electrical pitting can be considered as follows. When the contact surface was attacked by the positive and negative ions, the kinetic energy is transformed into the heat energy, so that higher contact temperature than melting point is found. The relationship between the pitting area and operating parameter is shown in Figs. 11–13. It is seen from those figures that the effects of oil film thickness and supply voltage on the electrical pitting area is not significant, but the electrical pitting area is significantly influenced by the supply current. That is, when the supply current increases, the pitting area obviously increases, since the principal factor to influence the interface power is the supply current, as shown in Figs. 7–9. This interface power is the main energy of electrical pitting. Hence, the relationship between the pitting area ŽFigs. 11–13. and the interface power ŽFigs. 7–9. can be rearranged into Fig. 14. It is seen from Fig. 14 that the relationship between the pitting area, A p Žin unit of 10 3 mm2 ., and the interface power, P Žin W., is a cubic function, or A p s 0.0059P 3 y 0.068 P 2 q 0.46 P .
Ž 1.
After using the data shown in Figs. 11–13 under different operating parameters, the correlation equation derived
Fig. 14. The relationship between the interface power and electrical pitting area.
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by the least-square method for the electrical pitting area in terms of supply voltage, Vs Žin V., supply current, I Žin A., and oil film thickness, h Žin mm., is given as follows: A p s 0.081h0.116 Vs 0.289 I 1.179 ,
R 2 s 0.928
Ž 2.
2
where R is the coefficient of determination Ža number between 0 and 1.. The higher the value of R 2 , the more successful is. It is seen from Eq. Ž2. that the major factor to influence the electrical pitting area is the supply current. Furthermore, for a given supply voltage and supply current in a rotating machinery, this equation is useful in predicting the oil film thickness or the lubricating condition by observing the magnitude of the electrical pitting area. 3.4. Pitting formation diagram As previously mentioned, at a certain supply voltage, the arcing effect will no longer take place for the oil film thickness larger than a critical value. Similarly, at a certain supply current and oil film thickness, the arcing effect does not begin until a critical supply voltage or a threshold voltage is achieved. At a certain supply current, the supply voltage versus the oil film thickness can be used to distinct the contact surface into three regimes, namely no-pitting, transition, and pitting regimes, as shown in Fig. 15. In this pitting formation diagram, the boundary between the transition and no-pitting regimes is called the first threshold voltage, and another boundary between the transition and pitting regimes is called the second threshold voltage.
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When the bearing is under different film thickness in the electrical field, the current does not begin to flow until the first threshold voltage is reached. Hence, the electrical pitting cannot be observed on the surfaces of the specimens in no-pitting regime. At a certain oil film thickness and supply current, when the supply voltage achieves the first threshold voltage, arcing takes place in the gap and the momentary flow of current can be measured. However, the current gap distance is larger than the initial gap distance due to the accumulated pitting depth by the arc at the static pitting test. As a result, this momentary flow of current decreases to zero and the arcing effect disappears. Hence, in the transition regime, the supply voltage is still not enough to break down the insulating status of oil film, and a little pitting area can be observed on the surfaces of the specimens in this regime. At a certain oil film thickness and supply current, when the supply voltage achieves or is larger than the second threshold voltage, a very high increase in current can be measured, where the insulating status of oil film has been broken down. Hence, the electrical pitting can be observed on the surfaces of the specimens in the pitting regime. Moreover, this pitting formation diagram also indicates that the first threshold voltage is quite close the second threshold voltage, because the pitting depth is quite small relate to the initial gap distance. When the supply voltage is larger than the second threshold voltage, the insulating status of the oil film is completely destroyed in the pitting regime, so that the
Fig. 15. The diagram of electrical pitting formation at supply current of 1 A.
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current continuously passes through the oil film causing the arcing effect. However, the supply current insignificantly influences these two threshold voltages, so the pitting formation diagram for higher supply current does not show in this paper. After using the pitting formation diagram under different supply currents, the correlation equation derived by the least-square method for the first threshold voltage in terms of supply current, I, and oil film thickness, h, is given as follows: Vt s 8.185h1.429 Iy0 .032 ,
R 2 s 0.998
Ž 3.
It is seen from Eq. Ž3. that the pitting formation is significantly influenced by the threshold voltage and oil film thickness, but the effect of the supply current is insignificant.
4. Conclusions The effect of supply voltage, supply current, and oil film thickness on the characteristics of electrical pitting for the lubricated surface of steel pair is investigated by using a static electrical pitting tester with high precision under the influence of shaft voltage of AC electric field. From the experimental results, the following conclusion can be drawn. Ž1. When the electrical pitting occurs, the interface voltage and interface power increase with increasing supply current, but the interface impedance decreases with increasing supply current. However, they are insignificantly influenced by the oil film thickness. Ž2. For the mineral oil, the interface voltage greater than 0.4 V may develop harmful bearing currents at a oil film of 0.2 mm. This result is in very good agreement with the results obtained by Erdman et al. w8x. Ž3. Three electrical pitting regimes are found under the influences of shaft voltage and oil film thickness, namely, pitting, transition, and no-pitting regimes. The boundary between the transition and no-pitting regimes is called the first threshold voltage, and another boundary is called the second threshold voltage. Correlation formula for the first threshold voltage in terms of supply current, I, and oil film thickness, h, is given in Eq. Ž3.. Ž4. When the second threshold voltage exceeds, the higher interface power, the higher pitting area on the contact surfaces. Correlation formula for the electrical pitting area in terms of supply voltage, Vs , supply current, I, and oil film thickness, h, is established in Eq. Ž2..
5. Nomenclature Ap h
Pitting area Ž10 3 mm2 . Oil film thickness Žmm.
I Ia P R2 RL Ra Va VL Vs Vt
Supply current ŽA. Interface current ŽA. Interface power ŽW. Coefficient of determination Variable resistance Ž V . Interface impedance Ž V . Interface voltage ŽV. Terminal voltage of variable resistance ŽV. Supply voltage ŽV. Threshold voltage ŽV.
Acknowledgements The authors would like to express their appreciation to the National Science Council ŽNSC-88-2212-E110-003. in Taiwan, R.O.C. for the financial support.
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