Additionally, for a bearing to accumulate charges, the ratio of capacitance should be less than unity.
Metallurgical implications
The current passing through a bearing at the line/point contact between the tracks and rolling elements, and the corresponding impedance at the junction generates heat and increases temperature instantaneously. This increases the contact stresses and enables to determine the number of cycles before the slip bands are initiated on the track surfaces of a bearing lubricated with low resistivity lubricant. The contact duration and temperature rise between track surface of races and rolling elements have been theoretically determined depending on kinematics, number of rolling elements in the loaded zone, material properties and the depth of slip bands. Furthermore, a theoretical approach is developed by continuum theory of Griffith to assess the time span/cycles for the development of flutes/corrugation on the track surfaces after appearance of slip formation of craters of the track surface of a bearing lubricated with high resistivity lubricant. Effect of instantaneous leakage of stored energy on track surface
The effect of leakage of charge on the rise of contact temperature on the track surface of a bearing using high resistivity lubricant has been reported. Based on these, mathematical formulations for the evaluation of contact stresses and minimum cycle for the appearance of craters has been developed. The ratio of contact cycles required for charge accumulation and discharge of the accumulated charges on the bearing surfaces depending on the ratio of bearing to shaft voltage have been analysed. The number of cycles, and the number of starts and stops before initiation of craters on track surfaces as against the ratio of bearing to shaft voltage have been theoretically established to restrict the deterioration of bearing.
Lubricants
The recent investigations indicate that the resistivity of lubricant depends on anti-scuff properties, viscosity, torque characteristics and consistency. Resistivity is a function of applied voltage and change in resistivity depends on the nature of impurities or by-products, and the type of additives present in lubricants besides its density, compressibility and structure. Detailed investigations have shown that the low resistivity lubricant tends to "recoup" its resistivity when the applied voltage is switched-off. The percentage of recoupling from the original value varies and depends on the stretching of the molecules. Also, it has been determined that the low resistivity, lithium-base grease decomposes and lithium metal concentration in the aqueous solution increases relatively. The decomposition of carboxylic acid leads to corrosion of the track surfaces before pitting is initiated. However, the applied voltage does not affect the oil content of the grease, but the carboxylate, anion stretching and carboxylic group, present in the soap residue of the grease, undergo changes. Further investigations using X-ray diffraction techniques indicated that the original structure of the soap residue of the fresh grease, i.e. lithium stearate changed to lithium palmitate, and lithium iron oxide and lithium zinc silicate were formed. On the contrary, such changes were not detected under rolling friction without the effect of electric current. It has been further reported that under the influence of electric current, the formation of lithium hydroxide and lithium carbonate makes the dielectric alkaline and corrodes the bearing surfaces; this finally leads to increased wear and failure of the bearing.
Hydrodynamic journal bearings
In a hydrodynamic journal bearing, the zone of minimum film thickness i.e. load-carrying oil film, varies along the circumference of a bearing throughout its length. This forms a capacitor of varying capacitance between the journal and the bearing depending on permittivity ratio, and the clearance ratio. Besides this, load-carrying oil film forms a resistor-capacitor (RC) depending on operating parameters and resistivity of the lubricant. Thus, the load-carrying oil film forms an RC circuit and offers impedance to a current flow. By analyzing the RC circuit, behaviour of a journal bearing has been predicted.
Theoretically, the mathematical model has been developed to determine capacitance, active resistance, capacitive reactance and impedance under different parameters of operation to analyse bearing performance and safe load-carrying capacity. Also, reduction in bearing life under the influence of shaft voltages before the initiation of craters on the bearing surfaces have been observed, and this has been compared to that of the bearings operating without the influence of shaft voltages. The mechanism of formation of craters on the bearing surfaces has been physically explained. Furthermore, it has been established that for safe and reliable operation and adequate life of a bearing, a shaft voltage of 0.5 V must not be exceeded.
A study reported on the capacitive effect and life estimation of a bearing on repeated starts and stops of a machine operating under the influence of shaft voltage gives an in-depth evaluation of bearing performance. Increase in charge accumulation on the bearing liner with time, is established theoretically when the machine is started, and there is a gradual leakage of the accumulated charge from the liner as the shaft voltage falls when the power supply to the machine is switched off. Under these conditions, the variation of shaft revolutions to accumulate charges and to discharge the accumulated surface charges at various levels of bearing-to-shaft voltages have been analysed and mathematically formulated. The variation of the safe limits of starts and stops as against the ratio of bearing-to-shaft voltage has also been reported. From the analysis, it is established that with the increase in ratio of bearing-to-shaft voltage, the ratio of shaft rotations to accumulate and discharge the accumulated charges increases.
Hydrodynamic thrust bearing
Similar to journal bearings, in a thrust bearing, the variation in oil film between pads and the thrust collar forms a capacitor of varying capacitance from lading to trailing edge. This depends on the permitivity of the lubricant, pad width, angle of tilt, and the ratio of oil film thickness at the leading to trailing edge. Besides this, variable oil film thickness offers variation resistance along the pad profile depending on resistivity of the lubricant. A theoretical approach has been developed to determine capacitance, capacitive reactance, and other electrical parameters of a thrust bearing. Variation of capacitance and other parameters with angle of tilt has been mathematically formulated and analysed.
The safe limit of shaft voltage has been assessed for a reliable operation of the thrust bearing operating under the influence of shaft voltages. For this analysis, charge leakage between high "point" of the thrust collar and a pad liner during momentary contact in the zone of load-carrying oil film is used to establish the heat generated and instantaneous temperature rise in each shaft rotation. The contact stresses by instantaneous temperature rise leading to crater formation have been analyzed. It has been established that at the shaft voltage of 2V, the percentage reduction in bearing life is 15.6 times as much as that at the shaft voltage of 0.25 V. Minimum cycles before the formation of craters due to leakage of charge energy on the liner surface of the tilting pads of a thrust bearing have also been analysed.
A developed theoretical model indicates the capacitive effect and life estimation of the pivoted pad thrust bearings on repeated starts and stops of a machine operating under the influence of shaft voltages. The analysis gives the time required for the charge accumulation and the increase of charge with time on the surface of liners, and gradual leakage of the accumulated charges with time as the shaft voltage falls as soon as the power supply to the machine is switched off. This helps to determine the number of repeated starts and stops before initiation of craters on the liner surface of pads of a thrust bearing.
Need for further research
In short, the research in the area of tribology in the electrical environment is in its early stage of development concerning analysis of bearing, diagnosis and frictional processes. As the need to conserve both energy and raw material are becoming very significant, the understanding of basic tribological electromagnetic phenomena a concerning friction/wear process and bearing behaviour/ performance need to be further explored and accelerated in the industries and research establishments across the country.
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