Fig 8 - uploaded by Martin Weicker
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Electrical Pitting on the raceway. Grey bearing raceway surface damage with pittings at an average bearing current density of Jb = 0.80 A mm ⁄ after 1024 hours operation, rotor speed n = 1000 rpm, radial bearing type 6205 C3, inverted-fed 1.5 kW-induction motor, radial force Fr = 63 N, axial force Fa = 50 N, bearing temperature ϑb = 60 °C, average EDM bearing current amplitude í µí°¼ ,,. = 0.52 A, lubricant type Arcanol MULTI 3.
Contexts in source publication
Context 1
... currents damage the bearing raceway surface at contact points. In Fig. 8 electrical pittings appeared on the surface of the bearing. The radial bearing, type 6205 C3, with 9 balls, was running for 1024 h under EDM bearing currents (Electric Discharge Machining). Average of the measured peak-to-peak EDM events of the bearing current is í µí°¼ ,,, = 0.52 A. The current is originated due to the high ...
Context 2
... V , which is the bearing voltage bearing just before discharge. When the discharge happens the voltage drops sharply to 0 V. Based on Fig. 7, the voltage values between this bearing voltage and 0 V are enough big to provide the melting and/or softening point of the contact body. Dispersed electrical pittings are observed on the inner raceway, Fig. 8. The pointed pitting has a diameter of 40 μm. In our investigation pittings did not appeared in absence of bearing current under the same mechanical conditions. Therefore, electrical currents caused the damages on the bearing ...
Citations
... The increased surface roughness due the current flow leads to micro-Elasto-Hydrodynamic Lubrication (micro-EHL), where the oil is pressed into the surface valleys [24]. Thus, for a rough surface, asperity peaks may still have electrical contacts (a-spots) [25], even when bearing parts are dominantly separated via a lubrication film [26]. At DC bearing currents, the bearing voltage drop between inner and outer bearing rings remains constant around 1 V, independent of the amplitude of bearing current [27]. ...
... This voltage is sufficient to cause very high local contact temperatures above 1000 °C on moving contact partners in a very short time (below 1 µs). This leads to melting of the micro-size volume of the a-spot with a typical radius of 1 µs [25], allowing the current flow. Similar local temperatures occur at inverter-fed motors at impulse bearing current flow. ...
Electrical bearing currents may disturb the performance of the bearings via electro-corrosion if they surpass a limit of ca. 0.1 to 0.3 A/mm2. A continuous current flow, or, after a longer time span, an alternating current or a repeating impulse-like current, damages the raceway surface, leading in many cases to a fluting pattern on the raceway. Increased bearing vibration, audible noise, and decreased bearing lubrication as a result may demand a replacement of the bearings. Here, an electrically corroded axial ball bearing (type 51208) with fluting patterns is investigated. The bearing was lubricated with grease lubrication and was exposed to 4 A DC current flow. It is shown that the electric current flow causes higher concentrations of iron oxides and iron carbides on the bearing raceway surface together with increased surface roughness, leading to a mixed lubrication also at elevated bearing speeds up to 1500 rpm. The “electrically insulating” iron oxide layer and the “mechanically hard” iron carbide layer on the bearing steel are analysed by WLI, XPS, SEM, and EDS. White Light Interferometry (WLI) is used to provide an accurate measurement of the surface topography and roughness. X-ray Photoelectron Spectroscopy (XPS) measurements are conducted to analyze the chemical surface composition and oxidation states. Scanning Electron Microscopy (SEM) is applied for high-resolution imaging of the surface morphology, while the Focused Ion Beam (FIB) is used to cut a trench into the bearing surface to inspect the surface layers. With the Energy Dispersive X-ray spectrometry (EDS), the presence of composing elements is identified, determining their relative concentrations. The electrically-caused iron oxide and iron carbide may develop periodically along the raceway due to the perpendicular vibrations of the rolling ball on the raceway, leading gradually to the fluting pattern. Still, a simulation of this vibration-induced fluting-generation process from the start with the first surface craters—of the molten local contact spots—to the final fluting pattern is missing.
... When a bearing voltage u b is applied to the bearing at full lubrication, and the resulting electric field strength E b = u b /h in the lubrication film exceeds the breakdown electric field of the lubricant of typically E D ≈ 30 V/µm (at virgin lubricant condition), a lubricant dielectric breakdown occurs via avalanche ionization of the lubricant molecules [23], allowing a bearing current flow i b between the ball and the raceway through the ionized, and therefore now conductive, lubricant [23]. The high local discharge current density melts the bearing surface at the electric contact points in the same way as explained above for the rotor-to-ground current [24]. Through a similar mechanism as noted above, the uneven surface with crater rims reduces the lubrication film thickness for the current path at the next bearing revolutions, or even penetrates the lubrication film and the surface oxide film, connecting ball and raceway directly [13,18,19,22]. ...
The effect of variable DC bearing current amplitude, bearing current polarity, mechanical force, rotation speed, bearing temperature, and number of the balls on the fluting in an axial ball bearing type 51208 is investigated under DC currents. The results are obtained from two different test setups with two different lubricants (mineral-oil-based grease and polyglycol oil). The speed varies between 100 rpm and 2000 rpm, the axial bearing force between 200 N and 2400 N, the DC current amplitude between 0.5 A and 20 A, the bearing temperature between 29 °C and 80 °C, the number of steel balls per bearing between 3 and 15, and the test duration between 6 h and 168 h. The results show that with a higher bearing current density and/or a higher bearing speed, a lower bearing force and/or a lower bearing temperature, a bigger number of roller elements, but also at a negative polarity of a DC electric bearing current, the occurring of fluting is more probable and occurs at an earlier stage of operation.
... However, Xie et al. [13] and Shoji et al. [39] found that weak currents can still cause damage to the bearings. Because the electrical contact area between the bearing roller and raceway is significantly smaller than the Hertz contact area when the bearing current passes through the bearing, the local current density flowing through the roller and raceway may be high [40]. Even if the current is very low, it will cause degradation of lubricating grease, shortening the bearing life. ...
The occurrence of the motor shaft voltage and bearing current caused by the inverter will aggravate bearing damage and lead to the premature failure of bearings. Many types of equipment are being shut down due to bearing currents, such as filters, insulated bearings and grounding brushes. Traditional suppression measures cannot eliminate the bearing current and the bearing damage mechanism under the bearing current is not clear. In this paper, the damage caused by the bearing current to bearings is analyzed in detail. The influences of different working conditions on the bearing current and the damage caused are discussed. The source of bearing currents is introduced and the bearing current model under different working conditions is reviewed. An outlook for future studies is proposed, based on the current research status and challenges.
Bearing currents induced in electrical motors, driven with fast switching power electronic inverters, can lead to bearing surface damages on the raceway. In this paper the electrical contact in the bearing is studied. The influence of surface oxidation in the electric contact is described. The bearing voltage and the bearing current of the axial ball bearing, type 51208, are investigated under influence of injected DC bearing currents. The contact radius of the electric contact spots (
a-
spots) are estimated based on the measured bearing resistance. Measured voltage-current characteristics of the bearing are presented for rotational speeds
n
= 100,000 min
<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup>
, bearing forces
$F_{b}=100\,\rm{N}$
and 1000 N, DC bearing currents between
$I_{b}=10\,\rm{mA}$
and
$9.5\,\rm{A}$
, without and with lubrication and without and with the oxide-cleaning agent, citric acid. This paper is for better understanding of the bearing electrical behavior.
