A bearing is a device that supports, guides, and reduces the friction of motion between fixed and moving machine parts. And a lubricant is a substance, such as grease or oil that reduces friction when applied as a surface coating to moving parts. Current investigations on the performance of bearings and lubricants under the influence of electric current have given insights into the behaviour of bearings and the deterioration of lubricants. The approach adopted for the assessment of various electrical analogies throws light on a hitherto hidden facet of tribology. Peeking below the surface, the genesis of intermolecular forces is in the electrostatic attraction or repulsion during tribological (Tribos is a Greek word for rubbing and tribology is the study of rubbing surfaces) interactions. Thus, nearly all tribological phenomena, occurring in any interacting system, metal-lubricant-metal or metal-metal are electrical in nature. Determination of the precise nature of the microscopic contacts will lead to innovations in the development of wear-resistant lubricants for industry applications. Besides, in any tribological interaction, there are several components like the thermo-electrical, capacitance, acousto-electric, galvanic and the inductive components of oil film thickness.
Due to the tribological interactions between the metal surface and the lubricating oil, the metal surface acquires electrostatic charges at random spots, which emit low charge particles. The frequency of emission depends on the operating parameters, which govern the tribological behaviour of the interacting surfaces. Reason enough for delving into microscopic investigations to arrive at the final word on tribology! Before dealing with the behaviour and response of the bearings and lubricants under the influence of electrical environment, it would be appropriate to examine the environment first. The bearings on the shaft come under the influence of what are called shaft voltages. These could be caused by one of the following :
· Magnetic flux in the shaft
· Homopolar magnetic flux
· Ring magnetic flux
If the shaft has a belt-driven pulley around it, friction between belt and pulley can set up an electrostatic voltage between the shaft and bearings. Accidental grounding of a part of a rotor winding to the rotor core can lead to stray currents through the shaft and bearings and can result in permanent magnetisation of the shaft. Also, shaft voltage and current could be generated when the machine is started. Furthermore, homopolar flux from an air gap or rotor eccentricity can generate voltage. Another cause of shaft voltage is the linkage of alternating flux with the shaft. The flux flows perpendicular to the axis of the shaft, and pulsates in the stator and rotor cores. It is caused by the asymmetries in the magnetic circuit of the machine like:
· Uneven air gap and rotor eccentricity
· Split stator and rotor core
· Segmental punchings
· Axial holes through the cores for ventilation and clamping purposes
· Keyways for maintaining the core stackings
· Segments of different permeability
All the causes listed above develop a magnetic flux, which closes over a yoke and induces voltage on the shaft as the machine rotates. This results in a localized current at each bearing rather than a potential difference between the shaft ends. A current path, however, along shaft, bearings and frame results in a potential difference between the shaft ends. This happens because of axial shaft flux caused by residual magnetization, rotor eccentricity, and asymmetrical rotor winding.
Environments bearing on lubrication
For the analysis and review of response and behaviour of the bearing under electrical environment, papers are categorized into four main topics, ie Rolling Element Bearing, Lubricants, Hydrodynamic Journal Bearing, and Thrust Bearings. Understanding all the papers listed in the references cannot be summarized separately, furthermore, it would be tedious to read neutral summaries of the published papers, so the collective summary of each topic is given in a broader perspective.
In a rolling element bearing, at each revolution of the shaft, part of the circumference of the inner race passes through a zone of maximum radial force, and Hertzian pressure between the rolling elements and raceways at the contact points leads to a maximum shear stress, and this occurs in the sub-surface at a depth approximately equal to half of the radius of the contact surface. It is generally at this point, that the failure of material, if occurring, will be initiated. The author has highlighted that the process of deformation which leads to the formation of corrugation pattern on track surface is accelerated by the passage of electric current, corrosion and oxidation of surfaces, lubricant characteristics and quality of the bearing. The mathematical formulations have been developed separately for roller bearings and ball bearings, depending on the profile of contacts for the evaluation of pitch of corrugations which depends on bearing kinematics, on the profile of contacts for plane of action of radial loading, bearing quality and lubricant characteristics. Also, formulations for the width of corrugations on the track surfaces have been worked out. The comparison of experimental and theoretical data for the pitch and width of corrugations has been reported. The mathematical formulations show that the width of corrugations on the track surfaces is not affected by the frequency of rotation and depends only on load conditions and bearing kinematics. It has been analyzed that the passage of current causes local surface heating which leads to low temperature tempering, and accelerates formation of corrugations with time for a bearing using low resistivity lubricant (105 ohm-m). After protracted operations, softer tempered surfaces of the races become harder, and the re-hardened particles due to localized high temperature and load are ejected from the craters, which intensify the depth of corrugations.
Flux density distribution
Further, it is reported that under the influence of electric current, a rolling element bearing using low resistivity lubricant develops a magnetic flux on the surfaces. This happens because of the passage of high intensity current through a bearing. On the contrary, a bearing using high resistivity lubricant (109ohm-m) does not develop significant flux density on its surface. Theoretical and experimental investigations have been reported on the distribution of magnetic flux density on the track surfaces of races and rolling elements of bearings. By using the developed model and by experimental determination of residual flux density on the track surfaces, the level of current flow through a bearing can be ascertained without the measurement of shaft voltage and bearing impedance. A typical case study of failure of the bearings of the alternators has been analysed by using the developed flux density distribution technique.
Voltages, currents and charges involved
The phenomenon of threshold voltages and investigations pertaining to first/second threshold voltage for the bearings operating under the influence of electrical currents have been reported. It has been shown that threshold voltages depend on the lubricant resistivity, oil film thickness and operating parameters. The defected threshold voltages are primarily responsible for the momentary flow of current and further increase in current intensity with a slight change in potential drop across the bearings. The threshold voltage decreases as the load on bearing increases at a constant speed. It is established that the increase of current intensity reduces the bearing impedance significantly. It has been emphasised that for the reliable operation of a bearing, the safe limit of the potential drop across the bearing elements should be less than the first threshold voltage. Under the influence of potential drop across a bearing, the minimum film thickness between races and rolling-elements offers a maximum capacitance and minimum capacitive reactance depending on the permittivity of the lubricant. The electrical interaction between the races and the rolling-elements in the presence of oil film is like a resistor-capacitor (RC) circuit that offers impedance to the current flow. The capacitance and resistance for roller bearing and ball bearing have been separately determined analytically, and found to depend on film thickness, width of deformation and are governed by the permittivity and resistivity of the used lubricant. Furthermore, it has been analysed that a bearing lubricated with high resistivity lubricant as opposed to a low resistivity one, with the same permittivity, behaves like a capacitor upto the first threshold voltage.