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bearing steel pair under ac electric field
Wear 249 (2001) 133–142
Pitting mechanism on lubricated surface of babbitt alloy/bearing steel pair under ac electric field Chung-Ming Lin, Yuang-Cherng Chiou∗ , Rong-Tsong Lee Department of Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC Received 15 September 2000; received in revised form 3 January 2001; accepted 17 January 2001
Abstract The mechanism of electrical pitting on the lubricated surface of babbitt alloy/steel pair is investigated, and the threshold condition to avoid the occurrence of electrical pitting is also established by using a static electrical pitting tester with high precision under the influence of ac electric field. According to the SEM micro-graph and EDS analysis are, the mechanism of electrical pitting is significantly influenced by the interface power and the oil film thickness. At the smaller oil film thickness, the eroded surface of babbitt alloy exhibits a concave crater with a few micro-porosity in the vicinity of center region with a plateau on its surrounding, especially at high supply current. The polished track can be observed at the plateau. A large amount of tin element transfers to the steel ball surface because the molten tin contacts the ball. At the higher oil film thickness, only a little amount of metal element transfers to each other. The major pitting area of the babbitt alloy is caused at the initial stage of the arc discharge. With increasing arc discharge time, the pitting area increases slowly, and finally reaches a saturated value. When the electrical pitting occurs, correlation formula for the electrical pitting area in terms of interface power and melting point of material has been established. It is found that the higher interface power and the lower melting point of material, the higher electrical pitting area. Two electrical pitting regimes are found, namely, pitting and no-pitting regimes. The boundary between the pitting and no-pitting regimes is called the threshold voltage. Correlation formula for the threshold voltage in terms of oil film thickness and melting point of material is derived. © 2001 Published by Elsevier Science B.V. Keywords: Lubricated surface; Electrical pitting mechanism
1. Introduction The shaft voltage and current in rotating machinery has been recognized as one of the sources of bearing failure [1–4]. The oil film in a hydrodynamic journal bearing acts a capacitor and provides a charging mechanism for shaft voltage build-up. When the voltage is greater than the breakdown voltage, the bearing current flows through the lubricated contacts, and it can damage the bearing. The damage of the bearing by the electrical wear can corrugate the surface and can considerably accelerate the mechanical wear [5]. Its effect on the deterioration of lubricant has also been studied [6,7]. The dielectric strength of oil is the ability to withstand high voltage without breakdown. It was indicated by Busse et al. [8] that the dielectric strength of oil is roughly 15 kV/mm. This dielectric strength is similar to the slope of the threshold voltage to avoid bearing damage versus oil film thickness in the earlier paper [9]. However, it was found by Prashad [10] and Chiou et al. [9] that there exist two thresh∗ Corresponding author. Tel.: +886-7-525-2000; fax: +886-7-525-4299. E-mail address: [email protected] (Y.C. Chiou).
0043-1648/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 5 2 8 - 2
old voltages, and the curve in the plot of threshold voltage versus oil film thickness is not a linear relationship. The damage under the influence of shaft voltage is mainly due to the arcing. It has been known that the bearing, oil film, and journal (shaft) present non-linear impedance including capacitance and resistance. Hence, the arcing effect on the lubricated surface is significantly influenced by the electrical impedance. Dirt, metallic particles, surface roughness, and oil film thickness reduce the interface impedance [10]. However, the major factor to influence the electrical pitting area is found to be the supply current or shaft current [9]. 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 accurately under various supply voltages and supply currents. It is obvious that in hydrodynamic journal bearings, the babbitt has been widely used for the bearing material and the steel for the shaft material. It was indicated by Kaufman and Boyd [2] that the journal surface was much less damaged due to pitting than the bearing surface due to its higher melting point [1]. It was also indicated by Wang et al. [11] for electrical contacts that the erosion volume decreases with the melting temperature and
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Nomenclature a, b, c, d, e, f Ap h I ia Ia M P RL Ra T va Va VL Vs Vt
constants pitting area (103 m2 ) oil film thickness (m) rms value of supply current (A) interface current (A) rms value of interface current (A) melting temperature (K) interface power (W) variable resistance () interface impedance (), Va /Ia test time (s) interface voltage (V) rms value of interface voltage (V) terminal voltage of variable resistance (V) rms value of supply voltage (V) rms value of threshold voltage (V)
the enthalpy of melting, but not with the thermal diffusivity. In the present study, the mechanism of electrical pitting on the lubricated surface of babbitt alloy/steel pair is investigated by using the SEM micro-graph and EDS analysis, and the pitting formation diagram for the babbitt alloy/steel pair is also established. Fig. 1. (a) Schematic diagram of the static electrical pitting tester; (b) the equivalent circuit diagram of the present experiment.
2. Experimental apparatus and procedures The static electrical pitting tester is the same one used in the earlier experiment [9], which consists of two micrometer heads, as shown in Fig. 1(a). The block specimen is insulated from the body of the tester by the plastic oil tank, and a small ball inserted in the pin specimen is held in an insulating holder. They are immersed in the oil tank, so that they can constitute an electric circuit. The equivalent circuit for the present experiment is shown in Fig. 1(b). The film thickness between the ball and the block can be adjusted from 0.2 to 7 m by the micrometer head. Moreover, an analog mu-checker with a graduation of 0.1 m is employed to calibrate this film thickness. The sequence of operation for this device has been described in the earlier work [9]. A paraffin base oil (26.59 cSt at 40◦ C, 4.76 cSt at 100◦ C), is used in the oil tank. The oil temperature was maintained at 25 ± 3◦ C. In this temperature, the resistivity of the base oil is 1.5 × 1014 cm. The specimens of ball and block are made of commercial bearing steel (SUJ2) and babbitt alloy (85.84% Sn, 8.76% Sb, 4.88% Cu, 0.34% Pb), respectively. The size and shape of the test specimens were shown in Fig. 2. The arithmetic average roughness for the ball and the block surfaces are about 0.01 and 0.02 m, respectively. Before the adjustment of the oil film thickness, the rms supply voltage is preset from 1 to 100 V through a 60 Hz sine-wave voltage of ac power supply. A variable resistor is
used to adjust the rms supply current between ball and block specimens from 1 to 8 A. The test time is 30 s. 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, a digital oscilloscope is also employed to observe the variation of interface voltage. 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-to-peak values for the interface voltage and the interface current between pin and block specimens. They are used for determining the rms voltage and current. Hence, in this paper, only the rms values of interface voltage and
Fig. 2. The size and shape of the test specimens: (a) pin specimen; (b) block specimen.
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interface current are presented. The electrical pitting area can be measured by using the optical microscope or scanning electronic microscope (SEM).
3. Experimental results and discussion 3.1. Electric behavior of lubricated surface In the circuit shown in Fig. 1(b), when the power supply provides a certain value of voltage Vs , the interface current Ia is still zero, if the oil film of high resistivity is not breakdown. In this case, the interface voltage is equal to the supply voltage. However, when the film thickness decreases, the electrical breakdown occurs because the film impedance decreases with decreasing film thickness. This breakdown causes an arcing effect and allows the passage of current through the oil film in the contact zone. As a result, the electrical pitting can be observed on the contact surface. In this paper, the effects of supply current and voltage on the electrical behavior of lubricated surface are investigated. The typical experimental results for the interface voltage, the interface impedance, and the interface power are shown in Figs. 3–4. Fig. 3 shows the effect of supply current and the oil film thickness on the interface voltage and impedance for a supply voltage of 100 V. In this figure, the interface impedance is calculated from the measured interface voltage and current. It can be expressed as Ra =
Va Ia
(1)
Fig. 3. The effects of supply current and oil film thickness on the interface voltage and the interface impedance at supply voltage of 100 V.
Fig. 4. The effects of supply current and oil film thickness on the interface power at supply voltage of 100 V.
Generally, at larger film thickness, the oil film can achieve an insulating condition where the interface impedance is infinity. With decreasing film thickness, the electric field (E = Vs / h) in the gap increases. When the film thickness is smaller than the critical value, the interface current increases suddenly because of the high enough electrical field across the oil film to cause breakdown, and then the arc initiates. This arc can result in the electrical pitting on the contact surface. In this paper, a variable resistance, RL , is used to adjust the supply current, as shown in Fig. 1. Hence, at certain supply voltage, the higher the supply current, the smaller variable resistance. Consequently, it is seen from Fig. 3 that the interface voltage increases with increasing supply current. The interface current can also be calculated from this figure by using Eq. (1). Generally, the interface current is quite close to the supply current because of the smaller interface impedance during the arcing. Moreover, the interface voltage and impedance slightly increases with increasing oil film thickness at certain supply current. However, when the oil film thickness is larger than a critical value, an insulating condition is achieved, and the arc disappears. In this condition, the interface voltage quickly increases to the magnitude of the supply voltage, and the interface impedance increases abruptly to infinity. It is also seen from Fig. 3 that with increasing supply current, the interface impedance decreases due to the increase amount of electrons and ions across the oil film at a certain value of film thickness. It is seen from the circuit shown in Fig. 1 that before the arcing, since the interface impedance is close to infinity, the interface voltage is equal to the supply voltage. Hence, the major factors to cause breakdown are the supply voltage and the film thickness.
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Table 1 Effect of supply voltage on the electric field to cause breakdown for the different material pairs
implies that the insulating condition has been achieved. On the other hand, at a certain oil film thickness, the interface power increases with increasing supply current.
Supply voltage Material pair
Electrical field (kV/mm)
15 V
60 V
100 V
Steel/steel Aluminum/steel Babbitt/steel
E E E
10 18.75 25
15 17.65 19.35
16.67 18.18 19.61
Consequently, under the action of a certain supply voltage, the breakdown occurs at the same oil film thickness for different supply currents. Hence, under the influence of supply current, the critical electric field (E = Vs / h) to cause breakdown is a constant. For the different supply voltages and material pairs, the critical electrical field is shown in Table 1. It is obvious that the critical electrical field is very dependent upon the material of the contact surface. Generally, the breakdown voltage is very dependent upon the microscopic surface conditions of the cathode and the resistivity of the oil. When the electrical breakdown occurs, an arcing effect and the bearing current can be observed. The mean dissipated power during the test time T is defined as Z 1 T va (t)ia (t) dt (2) P = T 0 Since the interface current and voltage are the sine-wave without time lag, this mean dissipated power is equal to Va2 /Ra . It is seen from Fig. 4 corresponding to Fig. 3 that at a certain supply current, with increasing oil film thickness, the interface power slightly increases, and abruptly decreases to zero at the oil film thickness larger than a critical value. This
3.2. Formation mechanism of electrical pitting As mentioned above, when the supply voltage is enough large to cause the breakdown on the interface, the electrical pitting can be observed on the contact surface due to the arcing effect. In this experiment, the appearance of the electrical pitting can be observed by using the SEM under various supply voltages, supply currents, and oil film thickness. Moreover, the energy dispersion spectrometer (EDS) is also employed to investigate the mass transfer phenomenon between two specimens. Since the bearing steel and babbitt alloy are ferrous alloy and tin alloy, respectively, the EDS can be used to detect the tin element on the ball and the ferrous element on the block to distinguish the mass transfer. The typical SEM micro-graph and the EDS analysis of the electrical pitting are shown in Figs. 5–8. Fig. 5 shows the SEM micro-graph and the EDS analysis of the electrical pitting for supply voltage of 100 V, supply current of 8 A, and oil film thickness of 1 m. Fig. 5(a) shows an obvious concave crater with a few micro-porosity in the vicinity of center region with a plateau on its surrounding like a doughnut for the babbitt alloy block. This plateau is quite flat, where the polished track with a few concave pits can be observed. It is obvious that a crater is caused by the arcing effect. When the arcing strikes across the two specimens, the babbitt alloy first melts and then flows radially outward. When the oil film thickness is smaller, this molten metal with lower melting point is possible to
Fig. 5. SEM micro-graph and EDS analysis of electrical pitting at supply voltage of 100 V, supply current of 8 A, and oil film thickness of 1 m.
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Fig. 6. SEM micro-graph and EDS analysis of electrical pitting at supply voltage of 100 V, supply current of 8 A, and oil film thickness of 5 m.
contact the steel ball with higher melting point. Therefore, a flat plateau around the crater can be observed. On the other hand, a plateau with a few micro-porosity can be observed on the ball surface, as shown in Fig. 5(b). In order to investigate the mass transfer, EDS is used to detect the alloy composition of the pitting surface for Fig. 5(a–b), as shown in Fig. 5(c–d). It is seen from Fig. 5(c) that the little amount of ferrous element is left over the surface of babbitt alloy. This ferrous element comes from
the steel ball. It is seen from Fig. 5(d) that a large amount of tin element is left over the plateau of steel ball surface. This tin element comes from the babbitt alloy block. The formation of this electrical pitting and the mass transfer can be considered as follows. Generally, at the action of electrical field, 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. As a result, this higher temperature causes
Fig. 7. SEM micro-graph and EDS analysis of electrical pitting at supply voltage of 100 V, supply current of 1 A, and oil film thickness of 5 m.
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Fig. 8. SEM micro-graph and EDS analysis of electrical pitting at supply voltage of 100 V, supply current of 1 A, and oil film thickness of 1 m.
the concave pitting on the contact surfaces. Since the steel ball possesses much higher melting point than the babbitt alloy, its surface would be left over a large amount of tin element, which comes from the babbitt alloy. Consequently, a plateau can be observed on the ball surface, as shown in Fig. 5(b). On the other hand, the formation of the plateau on the surrounding, like a doughnut as shown in Fig. 5(a), is the action of Joule heat of the arcing effect on the interface, and the squeeze action or the constraint of the steel ball. Fig. 6 shows the SEM micro-graph and EDS analysis of the electrical pitting for the same supply voltage and supply current except for oil film thickness of 5 m. It is seen from Fig. 4 that higher oil film thickness has higher interfacial power under the same electrical condition. Consequently, Fig. 6(a) shows the larger pitting on the babbitt alloy surface. On its surrounding, the plateau is not flat and the polished track disappears. This results from the action of larger Joule heating with the pressure of oil film. However, it is seen from Fig. 6(b) that a concave crater with a few micro-porosity is also found on the ball surface. Moreover, it is seen from the EDS analysis of Fig. 6(c–d) that just a little amount of metal element transfers to each other. It is apparent that the ball does obviously not contact to block surface, so that the mass transfer does almost not exist at oil film thickness of 5 m. Hence, it implies that a plateau on the ball surface, as shown in Fig. 5(b), is caused by the contact between the block and the ball at oil film thickness of 1 m. Fig. 7 shows the SEM photomicrograph and EDS analysis of the electrical pitting for the same supply voltage and oil film thickness as Fig. 6 except for supply current of 1 A. It is seen from Fig. 4 that the interface power decreases with increasing supply current. Therefore, Fig. 7(a) shows
a smaller pitting like a rose on the babbitt alloy block. It is apparent that the crater diameter decreases with decreasing interfacial power or supply current. No obvious ferrous element is found on the babbitt alloy surface, as shown in the EDS analysis of Fig. 7(c). Fig. 7(b) and (d) show an obvious pitting with tin element on the ball surface. This tin element comes from babbitt alloy. It is obvious that the melting temperature significantly influence the amount of mass transfer. Fig. 8 shows the SEM micro-graph and EDS analysis of the electrical pitting for the same electrical condition as Fig. 7 except for oil film thickness of 1 m. It is seen from Fig. 4 that the larger the oil film thickness, the larger interface power is. Hence, the pitting area is smaller with shallow crater and several micro-porosity on the babbitt alloy surface and steel ball surface. A few amount of ferrous element is left over on the block surface, as shown in the EDS analysis of Fig. 8(c). Fig. 8(d) shows that a large amount of tin element is left over the ball surface. The covered area of mass transfer on both surfaces is almost equal to the electrical pitting area. It is apparent that the ball contacts to block surface, so that the mass transfer exhibits at oil film thickness of 1 m. It is seen from the above-mentioned eroded surfaces of babbitt alloy block and bearing steel ball that the interface power and the film thickness significantly influence the eroded surface feature. When the oil film thickness is smaller, this molten babbitt alloy contacts to the steel ball, so that a flat plateau around the crater can be observed at the eroded surface of babbitt alloy block. Moreover, a large amount of metal element transfers to each other between the block and the ball specimens. On the contrary, at larger oil film thickness, a plateau around the crater is not flat and the mass transfer becomes insignificant because the molten
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Fig. 9. The effects of supply current and oil film thickness on the electrical pitting area of babbitt alloy block at supply voltage of 100 V.
Fig. 10. The relationship between the interface power and electrical pitting area on the block specimen.
babbitt alloy does obviously not contact to the steel ball during the arcing. Generally, the pitting diameter increases with increasing interfacial power. It is seen from Fig. 9 that the effect of oil film thickness on the electrical pitting area of babbitt alloy block is not significant at low supply current, but this effect becomes obvious at high supply current. Generally, the electrical pitting area increases with increasing supply current. As mentioned above, the formation of the electrical pitting is the action of the Joule heat and the ion strike, which directly relates to the interface power. Hence, the relationship between the pitting area and interface power is rearranged into Fig. 10 for a wide range of supply voltage, supply current, and oil film thickness. It is seen from Fig. 10 that the relationship between the pitting area, Ap (in unit of 103 m2 ), and the interface power, P (in unit of W), is a cubic function, or
It was indicated by Wang et al. [11] that the erosion volume increases with decreasing melting temperature in the erosion experiments of the electrical contacts. Hence, these constants a, b, and c would relate to the melting temperature of electrode material and the resistivity of the base oil. In the present study, the base oil is completely the same, and the average melting temperatures, M, are 513 K for babbitt alloy, 823 K for aluminum alloy, and 1473 K for bearing steal and medium carbon steel, respectively. Hence, using the above data, the correlation equation derived by the least-square method for these constants a, b, and c in term of melting temperature (in K) can be expressed as
Ap = aP3 + bP2 + cP
(3)
where a = 0.016, b = −0.25, and c = 1.42. In the previous paper [9], using the medium carbon steel/bearing steel pair, it was found that a = 0.0059, b = −0.068, and c = 0.46 under the same operation condition. For the aluminum/bearing steel pair, a = 0.01303, b = −0.2093, and c = 1.0969. It is seen from this figure that the correlation between the pitting area and the interface power for the babbitt alloy/bearing steel pair is not so well due to the low melting temperature of babbitt alloy. Generally, the lower melting temperature results in the flow of babbitt alloy due to the arcing effect. Consequently, the pitting area becomes unstable. Hence, it is hard to fit very well for the babbitt alloy/bearing steel pair.
a = −1.04 × 10−5 M + 0.021
(4)
b = 1.97 × 10−4 M − 0.36
(5)
c = −0.998 × 10−3 M + 1.93
(6)
By using Eqs. (3)–(6), the solid, dot-dash, and dashed lines of Fig. 10 indicate the predicted values of babbitt alloy, aluminum alloy, and medium carbon steel against bearing steel, respectively. It is seen from these two curves in Fig. 10 that the higher interface power and the lower melting point of material, the higher electrical pitting area. 3.3. Pitting formation diagram As previously mentioned in Figs. 5–7, at a certain supply voltage and supply current, when the oil film thickness is smaller than a critical value, the arc discharge occurs at the interface to cause the electrical pitting. However, the effect of the supply current on the critical oil film thickness is
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By using Eqs. (7)–(10), the solid, dot-dash, and dashed lines of Fig. 11 indicate the predicted threshold voltage for the babbitt/steel pair, aluminum alloy/steel pair, and steel/steel pair in the same lubricant, respectively. It is seen from this figure that the lower melting point of material, the smaller electrical pitting zone. 3.4. Interfacial lubrication status of arc discharge process
Fig. 11. The diagram of electrical pitting formation.
not so obvious. It only influences the electrical pitting area. Similarly, at a certain oil film thickness, when the supply voltage is larger than a critical value, the electrical pitting can also be observed at the interface due to the arcing. This critical supply voltage is called the threshold voltage. According to the experiments of the interface arcing and the surface observation, the supply voltage versus the oil film thickness can be divided into two regimes, namely, no pitting and pitting regimes, as shown in Fig. 11. Since the melting point of babbitt alloy is much lower than the bearing steel, the first threshold voltage defined in the previous paper [9] disappears. In the present pitting formation diagram, only the second threshold voltage exists. As a result, the transition regime also disappears. It was indicated by Busse et al. [8] that the dielectric strength of oil to withstand high voltage without breakdown is roughly 15 kV/mm. It is seen from Fig. 11 that the relationship between threshold voltage and oil film thickness is close to linear with a slope about 20 kV/mm for the babbitt/steel pair, but it is not linear for the steel/steel pair. Hence, it is obvious that the properties of bearing material influence the threshold voltage too. The correlation equation for the second threshold voltage derived by the least-square method under three different bearing materials in terms of oil film thickness, h (m), and the melting point of material, M (K), is given as follows: Vt = dh2 + eh + f
In the preceding experiment, the erosion mechanism of electrical pitting has been investigated for a wide range of supply voltage, supply current, and oil film thickness at the test time of 30 s. In order to understand the lubrication characteristics during arc discharge process, the discharge time is controlled at the range between 0–30 s. In each test, the new set of specimens is used in the experimental work, because any pitting in the babbitt alloy can lead to different results. After each experiment, the pitting area is measured. Fig. 12 shows the relationship among the pitting area, the interface impedance, and the test time at the supply voltage of 100 V, the supply current of 8 A, and the oil film thickness of 1 m. It is seen from this figure that the pitting area increases quickly to 4.8 × 103 m2 , and the interface impedance decreases to 0.21 at the first second of arc discharge. With increasing arc discharge time, the pitting area increases slowly, and finally reaches a saturated value at test time of 30 s. Meanwhile, the interface impedance decreases slowly, and finally reaches a saturated value too. This result indicates that the major pitting area of eroded surface of the bearing is caused at the initial stage of the arc discharge. It is
(7)
where d = 9.5 × 10−4 M − 0.45,
(8)
e = −6.9 × 10−3 M + 22.6,
(9)
f = −2.56 × 10−2 M + 2.5
(10)
Fig. 12. The relationship among electrical pitting area, interface impedance, and test time at supply voltage of 100 V, supply current of 8 A, and oil film thickness of 1 m.
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returns to 0.21 or partial-EHL condition. However, when the supply voltage is turned off, the required time to increase the interface impedance to infinity increases gradually with increasing times of arc discharge step. This indicates that the increase rate of interface impedance becomes slower. This results from the increase of residual electrons and ions across the oil film with increasing times of arc discharge step.
4. Conclusions In this study, a static electrical pitting tester, SEM, and EDS are employed to investigate the mechanism of electrical pitting for the lubricated surface of babbitt alloy/bearing steel pair for a wide range of supply voltage, supply current, and oil film thickness under ac electric field. The main results are as follows:
Fig. 13. The variation of interface impedance vs. test time at supply voltage of 100 V, supply current of 8 A, and oil film thickness of 1 m.
found from Figs. 5–13 that the concave crater with a plateau on its surrounding has been formed at the initial stage of arcing occurred in the gap. It is seen from the mass transfer and the plateau that the lubrication mode has transferred from the elastohydrodynamic lubrication (EHL) to partial-EHL. This result can be ascertained from the interface impedance of 0.21 . It was indicated by Kuo et al. [12] that when a few asperity are in contact with each other, the interface impedance should remain at low value, ranged from 0.1 to 100 . Fig. 13 shows the variation of interface impedance versus test time under the same operation conditions as Fig. 12, but the arc discharge time is kept 10 s in each step. Meanwhile, in order to understand the change of lubrication mode after the arc discharge, the interface impedance is still measured through a digital multimeter when the supply voltage is stopped. It is seen from Fig. 13 for the first step during the arc discharge that the interface impedance is kept at a certain value about 0.21 , which belongs to partial-EHL condition. This results from the a flat plateau around the crater for the babbitt alloy contacted to the steel ball at small film thickness, as shown in Fig. 5. When the arc discharge is stopped, the molten metal starts to solidify and contract. Hence, the specimens start to separate, so that the interface impedance increases to infinity or the insulating status with increasing time. In the second step, the supply voltage is applied to these specimens. Since the interface has been recovered the insulating status, the arc discharge occurs under the action of the supply voltage. The specimens are contacted to each other again, so that the interface impedance
1. The feature of electrical pitting for the babbitt alloy surface is significantly influenced by the interface power and the oil film. At the smaller oil film thickness, the eroded surface exhibits a concave crater with a few micro-porosity in the vicinity of center region with a plateau on its surrounding, especially at high supply current. The polished track can be observed at the plateau. At the smaller oil film thickness, a large amount of tin element is left over the ball surface because the molten tin contacts the ball. At the higher oil film thickness, only a little amount of mass transfers each other. 2. The plateau influences the lubrication mode from EHL to partial-EHL where a few asperities are in contact with each other at low the interface impedance. 3. The major pitting area of the eroded bearing surface is caused at the initial stage of the arc discharge. With increasing arc discharge time, the pitting area increases slowly, and finally reaches a saturated value. 4. When the electrical pitting occurs, correlation formula for the electrical pitting area in terms of interface power and melting point of material is established. It is found that the higher interface power and the lower melting point of material, the higher electrical pitting area. 5. Two electrical pitting regimes are found for the babbitt alloy, namely, pitting and no-pitting regimes. The boundary between the pitting and no-pitting regimes is called the threshold voltage. Correlation formula for the threshold voltage in terms of oil film thickness and melting point of material is derived.
Acknowledgements The authors would like to express their appreciation to the National Science Council (NSC-88-2212-E110-003) in Taiwan, ROC for the financial support.
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[7] H. Prashad, Investigation of damaged rolling-element bearings and deterioration of lubricants under the influence of electric fields, Wear 176 (1994) 151–161. [8] D. Busse, J. Erdman, R.J. Kerkman, D. Schlegel, G. Skibinski, System electrical parameters and their effects on bearing currents, IEEE Trans. Ind. Application 33 (2) (1997) 577–584. [9] Y.C. Chiou, R.T. Lee, C.M. Lin, Formation criterion and mechanism of electrical pitting on the lubricated surface under ac electrical field, Wear 236 (1999) 62–72. [10] 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 (1987) 223–240. [11] B.J. Wang, N. Saka, E. Rabinowicz, Static-gap, single-spark erosion of Ag–CdO and pure metal electrodes, Wear 157 (1992) 31–49. [12] W.F. Kuo, Y.C. Chiou, R.T. Lee, A study on lubrication mechanism and wear scar in sliding circular contacts, Wear 201 (1996) 217–226.
Pitting mechanism on lubricated surface of babbitt alloy/bearing steel pair under ac electric field Chung-Ming Lin, Yuang-Cherng Chiou∗ , Rong-Tsong Lee Department of Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC Received 15 September 2000; received in revised form 3 January 2001; accepted 17 January 2001
Abstract The mechanism of electrical pitting on the lubricated surface of babbitt alloy/steel pair is investigated, and the threshold condition to avoid the occurrence of electrical pitting is also established by using a static electrical pitting tester with high precision under the influence of ac electric field. According to the SEM micro-graph and EDS analysis are, the mechanism of electrical pitting is significantly influenced by the interface power and the oil film thickness. At the smaller oil film thickness, the eroded surface of babbitt alloy exhibits a concave crater with a few micro-porosity in the vicinity of center region with a plateau on its surrounding, especially at high supply current. The polished track can be observed at the plateau. A large amount of tin element transfers to the steel ball surface because the molten tin contacts the ball. At the higher oil film thickness, only a little amount of metal element transfers to each other. The major pitting area of the babbitt alloy is caused at the initial stage of the arc discharge. With increasing arc discharge time, the pitting area increases slowly, and finally reaches a saturated value. When the electrical pitting occurs, correlation formula for the electrical pitting area in terms of interface power and melting point of material has been established. It is found that the higher interface power and the lower melting point of material, the higher electrical pitting area. Two electrical pitting regimes are found, namely, pitting and no-pitting regimes. The boundary between the pitting and no-pitting regimes is called the threshold voltage. Correlation formula for the threshold voltage in terms of oil film thickness and melting point of material is derived. © 2001 Published by Elsevier Science B.V. Keywords: Lubricated surface; Electrical pitting mechanism
1. Introduction The shaft voltage and current in rotating machinery has been recognized as one of the sources of bearing failure [1–4]. The oil film in a hydrodynamic journal bearing acts a capacitor and provides a charging mechanism for shaft voltage build-up. When the voltage is greater than the breakdown voltage, the bearing current flows through the lubricated contacts, and it can damage the bearing. The damage of the bearing by the electrical wear can corrugate the surface and can considerably accelerate the mechanical wear [5]. Its effect on the deterioration of lubricant has also been studied [6,7]. The dielectric strength of oil is the ability to withstand high voltage without breakdown. It was indicated by Busse et al. [8] that the dielectric strength of oil is roughly 15 kV/mm. This dielectric strength is similar to the slope of the threshold voltage to avoid bearing damage versus oil film thickness in the earlier paper [9]. However, it was found by Prashad [10] and Chiou et al. [9] that there exist two thresh∗ Corresponding author. Tel.: +886-7-525-2000; fax: +886-7-525-4299. E-mail address: [email protected] (Y.C. Chiou).
0043-1648/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 5 2 8 - 2
old voltages, and the curve in the plot of threshold voltage versus oil film thickness is not a linear relationship. The damage under the influence of shaft voltage is mainly due to the arcing. It has been known that the bearing, oil film, and journal (shaft) present non-linear impedance including capacitance and resistance. Hence, the arcing effect on the lubricated surface is significantly influenced by the electrical impedance. Dirt, metallic particles, surface roughness, and oil film thickness reduce the interface impedance [10]. However, the major factor to influence the electrical pitting area is found to be the supply current or shaft current [9]. 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 accurately under various supply voltages and supply currents. It is obvious that in hydrodynamic journal bearings, the babbitt has been widely used for the bearing material and the steel for the shaft material. It was indicated by Kaufman and Boyd [2] that the journal surface was much less damaged due to pitting than the bearing surface due to its higher melting point [1]. It was also indicated by Wang et al. [11] for electrical contacts that the erosion volume decreases with the melting temperature and
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Nomenclature a, b, c, d, e, f Ap h I ia Ia M P RL Ra T va Va VL Vs Vt
constants pitting area (103 m2 ) oil film thickness (m) rms value of supply current (A) interface current (A) rms value of interface current (A) melting temperature (K) interface power (W) variable resistance () interface impedance (), Va /Ia test time (s) interface voltage (V) rms value of interface voltage (V) terminal voltage of variable resistance (V) rms value of supply voltage (V) rms value of threshold voltage (V)
the enthalpy of melting, but not with the thermal diffusivity. In the present study, the mechanism of electrical pitting on the lubricated surface of babbitt alloy/steel pair is investigated by using the SEM micro-graph and EDS analysis, and the pitting formation diagram for the babbitt alloy/steel pair is also established. Fig. 1. (a) Schematic diagram of the static electrical pitting tester; (b) the equivalent circuit diagram of the present experiment.
2. Experimental apparatus and procedures The static electrical pitting tester is the same one used in the earlier experiment [9], which consists of two micrometer heads, as shown in Fig. 1(a). The block specimen is insulated from the body of the tester by the plastic oil tank, and a small ball inserted in the pin specimen is held in an insulating holder. They are immersed in the oil tank, so that they can constitute an electric circuit. The equivalent circuit for the present experiment is shown in Fig. 1(b). The film thickness between the ball and the block can be adjusted from 0.2 to 7 m by the micrometer head. Moreover, an analog mu-checker with a graduation of 0.1 m is employed to calibrate this film thickness. The sequence of operation for this device has been described in the earlier work [9]. A paraffin base oil (26.59 cSt at 40◦ C, 4.76 cSt at 100◦ C), is used in the oil tank. The oil temperature was maintained at 25 ± 3◦ C. In this temperature, the resistivity of the base oil is 1.5 × 1014 cm. The specimens of ball and block are made of commercial bearing steel (SUJ2) and babbitt alloy (85.84% Sn, 8.76% Sb, 4.88% Cu, 0.34% Pb), respectively. The size and shape of the test specimens were shown in Fig. 2. The arithmetic average roughness for the ball and the block surfaces are about 0.01 and 0.02 m, respectively. Before the adjustment of the oil film thickness, the rms supply voltage is preset from 1 to 100 V through a 60 Hz sine-wave voltage of ac power supply. A variable resistor is
used to adjust the rms supply current between ball and block specimens from 1 to 8 A. The test time is 30 s. 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, a digital oscilloscope is also employed to observe the variation of interface voltage. 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-to-peak values for the interface voltage and the interface current between pin and block specimens. They are used for determining the rms voltage and current. Hence, in this paper, only the rms values of interface voltage and
Fig. 2. The size and shape of the test specimens: (a) pin specimen; (b) block specimen.
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interface current are presented. The electrical pitting area can be measured by using the optical microscope or scanning electronic microscope (SEM).
3. Experimental results and discussion 3.1. Electric behavior of lubricated surface In the circuit shown in Fig. 1(b), when the power supply provides a certain value of voltage Vs , the interface current Ia is still zero, if the oil film of high resistivity is not breakdown. In this case, the interface voltage is equal to the supply voltage. However, when the film thickness decreases, the electrical breakdown occurs because the film impedance decreases with decreasing film thickness. This breakdown causes an arcing effect and allows the passage of current through the oil film in the contact zone. As a result, the electrical pitting can be observed on the contact surface. In this paper, the effects of supply current and voltage on the electrical behavior of lubricated surface are investigated. The typical experimental results for the interface voltage, the interface impedance, and the interface power are shown in Figs. 3–4. Fig. 3 shows the effect of supply current and the oil film thickness on the interface voltage and impedance for a supply voltage of 100 V. In this figure, the interface impedance is calculated from the measured interface voltage and current. It can be expressed as Ra =
Va Ia
(1)
Fig. 3. The effects of supply current and oil film thickness on the interface voltage and the interface impedance at supply voltage of 100 V.
Fig. 4. The effects of supply current and oil film thickness on the interface power at supply voltage of 100 V.
Generally, at larger film thickness, the oil film can achieve an insulating condition where the interface impedance is infinity. With decreasing film thickness, the electric field (E = Vs / h) in the gap increases. When the film thickness is smaller than the critical value, the interface current increases suddenly because of the high enough electrical field across the oil film to cause breakdown, and then the arc initiates. This arc can result in the electrical pitting on the contact surface. In this paper, a variable resistance, RL , is used to adjust the supply current, as shown in Fig. 1. Hence, at certain supply voltage, the higher the supply current, the smaller variable resistance. Consequently, it is seen from Fig. 3 that the interface voltage increases with increasing supply current. The interface current can also be calculated from this figure by using Eq. (1). Generally, the interface current is quite close to the supply current because of the smaller interface impedance during the arcing. Moreover, the interface voltage and impedance slightly increases with increasing oil film thickness at certain supply current. However, when the oil film thickness is larger than a critical value, an insulating condition is achieved, and the arc disappears. In this condition, the interface voltage quickly increases to the magnitude of the supply voltage, and the interface impedance increases abruptly to infinity. It is also seen from Fig. 3 that with increasing supply current, the interface impedance decreases due to the increase amount of electrons and ions across the oil film at a certain value of film thickness. It is seen from the circuit shown in Fig. 1 that before the arcing, since the interface impedance is close to infinity, the interface voltage is equal to the supply voltage. Hence, the major factors to cause breakdown are the supply voltage and the film thickness.
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Table 1 Effect of supply voltage on the electric field to cause breakdown for the different material pairs
implies that the insulating condition has been achieved. On the other hand, at a certain oil film thickness, the interface power increases with increasing supply current.
Supply voltage Material pair
Electrical field (kV/mm)
15 V
60 V
100 V
Steel/steel Aluminum/steel Babbitt/steel
E E E
10 18.75 25
15 17.65 19.35
16.67 18.18 19.61
Consequently, under the action of a certain supply voltage, the breakdown occurs at the same oil film thickness for different supply currents. Hence, under the influence of supply current, the critical electric field (E = Vs / h) to cause breakdown is a constant. For the different supply voltages and material pairs, the critical electrical field is shown in Table 1. It is obvious that the critical electrical field is very dependent upon the material of the contact surface. Generally, the breakdown voltage is very dependent upon the microscopic surface conditions of the cathode and the resistivity of the oil. When the electrical breakdown occurs, an arcing effect and the bearing current can be observed. The mean dissipated power during the test time T is defined as Z 1 T va (t)ia (t) dt (2) P = T 0 Since the interface current and voltage are the sine-wave without time lag, this mean dissipated power is equal to Va2 /Ra . It is seen from Fig. 4 corresponding to Fig. 3 that at a certain supply current, with increasing oil film thickness, the interface power slightly increases, and abruptly decreases to zero at the oil film thickness larger than a critical value. This
3.2. Formation mechanism of electrical pitting As mentioned above, when the supply voltage is enough large to cause the breakdown on the interface, the electrical pitting can be observed on the contact surface due to the arcing effect. In this experiment, the appearance of the electrical pitting can be observed by using the SEM under various supply voltages, supply currents, and oil film thickness. Moreover, the energy dispersion spectrometer (EDS) is also employed to investigate the mass transfer phenomenon between two specimens. Since the bearing steel and babbitt alloy are ferrous alloy and tin alloy, respectively, the EDS can be used to detect the tin element on the ball and the ferrous element on the block to distinguish the mass transfer. The typical SEM micro-graph and the EDS analysis of the electrical pitting are shown in Figs. 5–8. Fig. 5 shows the SEM micro-graph and the EDS analysis of the electrical pitting for supply voltage of 100 V, supply current of 8 A, and oil film thickness of 1 m. Fig. 5(a) shows an obvious concave crater with a few micro-porosity in the vicinity of center region with a plateau on its surrounding like a doughnut for the babbitt alloy block. This plateau is quite flat, where the polished track with a few concave pits can be observed. It is obvious that a crater is caused by the arcing effect. When the arcing strikes across the two specimens, the babbitt alloy first melts and then flows radially outward. When the oil film thickness is smaller, this molten metal with lower melting point is possible to
Fig. 5. SEM micro-graph and EDS analysis of electrical pitting at supply voltage of 100 V, supply current of 8 A, and oil film thickness of 1 m.
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Fig. 6. SEM micro-graph and EDS analysis of electrical pitting at supply voltage of 100 V, supply current of 8 A, and oil film thickness of 5 m.
contact the steel ball with higher melting point. Therefore, a flat plateau around the crater can be observed. On the other hand, a plateau with a few micro-porosity can be observed on the ball surface, as shown in Fig. 5(b). In order to investigate the mass transfer, EDS is used to detect the alloy composition of the pitting surface for Fig. 5(a–b), as shown in Fig. 5(c–d). It is seen from Fig. 5(c) that the little amount of ferrous element is left over the surface of babbitt alloy. This ferrous element comes from
the steel ball. It is seen from Fig. 5(d) that a large amount of tin element is left over the plateau of steel ball surface. This tin element comes from the babbitt alloy block. The formation of this electrical pitting and the mass transfer can be considered as follows. Generally, at the action of electrical field, 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. As a result, this higher temperature causes
Fig. 7. SEM micro-graph and EDS analysis of electrical pitting at supply voltage of 100 V, supply current of 1 A, and oil film thickness of 5 m.
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Fig. 8. SEM micro-graph and EDS analysis of electrical pitting at supply voltage of 100 V, supply current of 1 A, and oil film thickness of 1 m.
the concave pitting on the contact surfaces. Since the steel ball possesses much higher melting point than the babbitt alloy, its surface would be left over a large amount of tin element, which comes from the babbitt alloy. Consequently, a plateau can be observed on the ball surface, as shown in Fig. 5(b). On the other hand, the formation of the plateau on the surrounding, like a doughnut as shown in Fig. 5(a), is the action of Joule heat of the arcing effect on the interface, and the squeeze action or the constraint of the steel ball. Fig. 6 shows the SEM micro-graph and EDS analysis of the electrical pitting for the same supply voltage and supply current except for oil film thickness of 5 m. It is seen from Fig. 4 that higher oil film thickness has higher interfacial power under the same electrical condition. Consequently, Fig. 6(a) shows the larger pitting on the babbitt alloy surface. On its surrounding, the plateau is not flat and the polished track disappears. This results from the action of larger Joule heating with the pressure of oil film. However, it is seen from Fig. 6(b) that a concave crater with a few micro-porosity is also found on the ball surface. Moreover, it is seen from the EDS analysis of Fig. 6(c–d) that just a little amount of metal element transfers to each other. It is apparent that the ball does obviously not contact to block surface, so that the mass transfer does almost not exist at oil film thickness of 5 m. Hence, it implies that a plateau on the ball surface, as shown in Fig. 5(b), is caused by the contact between the block and the ball at oil film thickness of 1 m. Fig. 7 shows the SEM photomicrograph and EDS analysis of the electrical pitting for the same supply voltage and oil film thickness as Fig. 6 except for supply current of 1 A. It is seen from Fig. 4 that the interface power decreases with increasing supply current. Therefore, Fig. 7(a) shows
a smaller pitting like a rose on the babbitt alloy block. It is apparent that the crater diameter decreases with decreasing interfacial power or supply current. No obvious ferrous element is found on the babbitt alloy surface, as shown in the EDS analysis of Fig. 7(c). Fig. 7(b) and (d) show an obvious pitting with tin element on the ball surface. This tin element comes from babbitt alloy. It is obvious that the melting temperature significantly influence the amount of mass transfer. Fig. 8 shows the SEM micro-graph and EDS analysis of the electrical pitting for the same electrical condition as Fig. 7 except for oil film thickness of 1 m. It is seen from Fig. 4 that the larger the oil film thickness, the larger interface power is. Hence, the pitting area is smaller with shallow crater and several micro-porosity on the babbitt alloy surface and steel ball surface. A few amount of ferrous element is left over on the block surface, as shown in the EDS analysis of Fig. 8(c). Fig. 8(d) shows that a large amount of tin element is left over the ball surface. The covered area of mass transfer on both surfaces is almost equal to the electrical pitting area. It is apparent that the ball contacts to block surface, so that the mass transfer exhibits at oil film thickness of 1 m. It is seen from the above-mentioned eroded surfaces of babbitt alloy block and bearing steel ball that the interface power and the film thickness significantly influence the eroded surface feature. When the oil film thickness is smaller, this molten babbitt alloy contacts to the steel ball, so that a flat plateau around the crater can be observed at the eroded surface of babbitt alloy block. Moreover, a large amount of metal element transfers to each other between the block and the ball specimens. On the contrary, at larger oil film thickness, a plateau around the crater is not flat and the mass transfer becomes insignificant because the molten
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Fig. 9. The effects of supply current and oil film thickness on the electrical pitting area of babbitt alloy block at supply voltage of 100 V.
Fig. 10. The relationship between the interface power and electrical pitting area on the block specimen.
babbitt alloy does obviously not contact to the steel ball during the arcing. Generally, the pitting diameter increases with increasing interfacial power. It is seen from Fig. 9 that the effect of oil film thickness on the electrical pitting area of babbitt alloy block is not significant at low supply current, but this effect becomes obvious at high supply current. Generally, the electrical pitting area increases with increasing supply current. As mentioned above, the formation of the electrical pitting is the action of the Joule heat and the ion strike, which directly relates to the interface power. Hence, the relationship between the pitting area and interface power is rearranged into Fig. 10 for a wide range of supply voltage, supply current, and oil film thickness. It is seen from Fig. 10 that the relationship between the pitting area, Ap (in unit of 103 m2 ), and the interface power, P (in unit of W), is a cubic function, or
It was indicated by Wang et al. [11] that the erosion volume increases with decreasing melting temperature in the erosion experiments of the electrical contacts. Hence, these constants a, b, and c would relate to the melting temperature of electrode material and the resistivity of the base oil. In the present study, the base oil is completely the same, and the average melting temperatures, M, are 513 K for babbitt alloy, 823 K for aluminum alloy, and 1473 K for bearing steal and medium carbon steel, respectively. Hence, using the above data, the correlation equation derived by the least-square method for these constants a, b, and c in term of melting temperature (in K) can be expressed as
Ap = aP3 + bP2 + cP
(3)
where a = 0.016, b = −0.25, and c = 1.42. In the previous paper [9], using the medium carbon steel/bearing steel pair, it was found that a = 0.0059, b = −0.068, and c = 0.46 under the same operation condition. For the aluminum/bearing steel pair, a = 0.01303, b = −0.2093, and c = 1.0969. It is seen from this figure that the correlation between the pitting area and the interface power for the babbitt alloy/bearing steel pair is not so well due to the low melting temperature of babbitt alloy. Generally, the lower melting temperature results in the flow of babbitt alloy due to the arcing effect. Consequently, the pitting area becomes unstable. Hence, it is hard to fit very well for the babbitt alloy/bearing steel pair.
a = −1.04 × 10−5 M + 0.021
(4)
b = 1.97 × 10−4 M − 0.36
(5)
c = −0.998 × 10−3 M + 1.93
(6)
By using Eqs. (3)–(6), the solid, dot-dash, and dashed lines of Fig. 10 indicate the predicted values of babbitt alloy, aluminum alloy, and medium carbon steel against bearing steel, respectively. It is seen from these two curves in Fig. 10 that the higher interface power and the lower melting point of material, the higher electrical pitting area. 3.3. Pitting formation diagram As previously mentioned in Figs. 5–7, at a certain supply voltage and supply current, when the oil film thickness is smaller than a critical value, the arc discharge occurs at the interface to cause the electrical pitting. However, the effect of the supply current on the critical oil film thickness is
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By using Eqs. (7)–(10), the solid, dot-dash, and dashed lines of Fig. 11 indicate the predicted threshold voltage for the babbitt/steel pair, aluminum alloy/steel pair, and steel/steel pair in the same lubricant, respectively. It is seen from this figure that the lower melting point of material, the smaller electrical pitting zone. 3.4. Interfacial lubrication status of arc discharge process
Fig. 11. The diagram of electrical pitting formation.
not so obvious. It only influences the electrical pitting area. Similarly, at a certain oil film thickness, when the supply voltage is larger than a critical value, the electrical pitting can also be observed at the interface due to the arcing. This critical supply voltage is called the threshold voltage. According to the experiments of the interface arcing and the surface observation, the supply voltage versus the oil film thickness can be divided into two regimes, namely, no pitting and pitting regimes, as shown in Fig. 11. Since the melting point of babbitt alloy is much lower than the bearing steel, the first threshold voltage defined in the previous paper [9] disappears. In the present pitting formation diagram, only the second threshold voltage exists. As a result, the transition regime also disappears. It was indicated by Busse et al. [8] that the dielectric strength of oil to withstand high voltage without breakdown is roughly 15 kV/mm. It is seen from Fig. 11 that the relationship between threshold voltage and oil film thickness is close to linear with a slope about 20 kV/mm for the babbitt/steel pair, but it is not linear for the steel/steel pair. Hence, it is obvious that the properties of bearing material influence the threshold voltage too. The correlation equation for the second threshold voltage derived by the least-square method under three different bearing materials in terms of oil film thickness, h (m), and the melting point of material, M (K), is given as follows: Vt = dh2 + eh + f
In the preceding experiment, the erosion mechanism of electrical pitting has been investigated for a wide range of supply voltage, supply current, and oil film thickness at the test time of 30 s. In order to understand the lubrication characteristics during arc discharge process, the discharge time is controlled at the range between 0–30 s. In each test, the new set of specimens is used in the experimental work, because any pitting in the babbitt alloy can lead to different results. After each experiment, the pitting area is measured. Fig. 12 shows the relationship among the pitting area, the interface impedance, and the test time at the supply voltage of 100 V, the supply current of 8 A, and the oil film thickness of 1 m. It is seen from this figure that the pitting area increases quickly to 4.8 × 103 m2 , and the interface impedance decreases to 0.21 at the first second of arc discharge. With increasing arc discharge time, the pitting area increases slowly, and finally reaches a saturated value at test time of 30 s. Meanwhile, the interface impedance decreases slowly, and finally reaches a saturated value too. This result indicates that the major pitting area of eroded surface of the bearing is caused at the initial stage of the arc discharge. It is
(7)
where d = 9.5 × 10−4 M − 0.45,
(8)
e = −6.9 × 10−3 M + 22.6,
(9)
f = −2.56 × 10−2 M + 2.5
(10)
Fig. 12. The relationship among electrical pitting area, interface impedance, and test time at supply voltage of 100 V, supply current of 8 A, and oil film thickness of 1 m.
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returns to 0.21 or partial-EHL condition. However, when the supply voltage is turned off, the required time to increase the interface impedance to infinity increases gradually with increasing times of arc discharge step. This indicates that the increase rate of interface impedance becomes slower. This results from the increase of residual electrons and ions across the oil film with increasing times of arc discharge step.
4. Conclusions In this study, a static electrical pitting tester, SEM, and EDS are employed to investigate the mechanism of electrical pitting for the lubricated surface of babbitt alloy/bearing steel pair for a wide range of supply voltage, supply current, and oil film thickness under ac electric field. The main results are as follows:
Fig. 13. The variation of interface impedance vs. test time at supply voltage of 100 V, supply current of 8 A, and oil film thickness of 1 m.
found from Figs. 5–13 that the concave crater with a plateau on its surrounding has been formed at the initial stage of arcing occurred in the gap. It is seen from the mass transfer and the plateau that the lubrication mode has transferred from the elastohydrodynamic lubrication (EHL) to partial-EHL. This result can be ascertained from the interface impedance of 0.21 . It was indicated by Kuo et al. [12] that when a few asperity are in contact with each other, the interface impedance should remain at low value, ranged from 0.1 to 100 . Fig. 13 shows the variation of interface impedance versus test time under the same operation conditions as Fig. 12, but the arc discharge time is kept 10 s in each step. Meanwhile, in order to understand the change of lubrication mode after the arc discharge, the interface impedance is still measured through a digital multimeter when the supply voltage is stopped. It is seen from Fig. 13 for the first step during the arc discharge that the interface impedance is kept at a certain value about 0.21 , which belongs to partial-EHL condition. This results from the a flat plateau around the crater for the babbitt alloy contacted to the steel ball at small film thickness, as shown in Fig. 5. When the arc discharge is stopped, the molten metal starts to solidify and contract. Hence, the specimens start to separate, so that the interface impedance increases to infinity or the insulating status with increasing time. In the second step, the supply voltage is applied to these specimens. Since the interface has been recovered the insulating status, the arc discharge occurs under the action of the supply voltage. The specimens are contacted to each other again, so that the interface impedance
1. The feature of electrical pitting for the babbitt alloy surface is significantly influenced by the interface power and the oil film. At the smaller oil film thickness, the eroded surface exhibits a concave crater with a few micro-porosity in the vicinity of center region with a plateau on its surrounding, especially at high supply current. The polished track can be observed at the plateau. At the smaller oil film thickness, a large amount of tin element is left over the ball surface because the molten tin contacts the ball. At the higher oil film thickness, only a little amount of mass transfers each other. 2. The plateau influences the lubrication mode from EHL to partial-EHL where a few asperities are in contact with each other at low the interface impedance. 3. The major pitting area of the eroded bearing surface is caused at the initial stage of the arc discharge. With increasing arc discharge time, the pitting area increases slowly, and finally reaches a saturated value. 4. When the electrical pitting occurs, correlation formula for the electrical pitting area in terms of interface power and melting point of material is established. It is found that the higher interface power and the lower melting point of material, the higher electrical pitting area. 5. Two electrical pitting regimes are found for the babbitt alloy, namely, pitting and no-pitting regimes. The boundary between the pitting and no-pitting regimes is called the threshold voltage. Correlation formula for the threshold voltage in terms of oil film thickness and melting point of material is derived.
Acknowledgements The authors would like to express their appreciation to the National Science Council (NSC-88-2212-E110-003) in Taiwan, ROC for the financial support.
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