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Abstract
In the present work, diagnosis of deterioration of lithium greases used in rolling-element bearings by X-ray diffractometry (XRD), and analysis by X-ray fluorescence spectrometry (XRFS) and atomic absorption spectrophotometry (AAS) is reported. These techniques give reproducible reliable data to establish the severity of deterioration of greases recovered from the active zone of the bearings. XRD appears to be suitable to diagnose quickly the chemical changes occurred in the soap residue of the greases, available even in small quantity, compared to the other analytical and performance evaluation techniques.
This paper gives diagnosis of the fresh lithium greases, chemical compositions and formation of new compounds in the greases after being used in the rolling-element bearings operated under the influence of electrical fields, and also under pure rolling friction without the effect of electrical fields on the roller bearing test rig. The deteriorated greases recovered from various motor bearings have also been analyzed and results are compared with that of the greases from the test bearings.
The X-ray diffraction analysis shows that the chemical composition of the soap residue of the fresh lithium grease is lithium stearate (C18 H35 Li O2), which does not change in the bearings operated without the effect of electrical fields under pure rolling friction. However, lithium iron oxide (Li5 Fe O2) peaks have been detected after prolong operation of the bearings. On the contrary, under the influence of electrical fields the chemical composition of the greases is changed to lithium palmitate (C16 H31 Li O2), and peaks of gamma lithium iron oxide (γ-Li Fe O2) and lithium zinc silicate (Li3.6 Zn0.2 Si O2) have been detected. The surface of the bearings is found to be corroded after operation under electrical fields. Atomic absorption spectrophotometry analysis has shown considerable increase of lithium percentage as lithium carbonate and lithium hydrooxide in the used greases in the acquous solution compared to the fresh grease.
The investigations given in this paper, along with the study of damaged/corrugated bearing surfaces, have potential to diagnose the cause of bearing failure under the influence of electrical fields, and also, to establish the severity of deterioration of the lithium greases used in the bearings.