Estimation of Rolling-Contact Bearings Operational Properties by Electrical Probe Method
Abstract
The substantiation of an electrical probe method using for diagnostic real bearing units on a boundary state is given. The circuit diagram is developed and the way of estimation tribological properties of rolling-contact bearing races surfaces by means of electrical probe methods is offered. Correlation between parameters of conductance, mechanical strength and frictional metal surface characteristics is installed. It is shown, that application of the given methods allows to control the rolling-contact bearing state.
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Introduction
Reliability and durability of agricultural machinery rolling-contact bearings are defined by many factors: bearing steel quality, form and a roughness of balls, inner and outer races, type of their heat treatment, total product design (for example a grain harvester straw rake) assembly technology and unit adjustment, and also antifriction and strength properties of greases [1]. Durability of the rolling-contact bearing is defined by million cycles of multicyclic frictional loading [2].
The analysis of a boundary state means a metal surface state control without lubricant presence on it and interconnection determination between state and tribotechnological properties, and reinforcement and destruction of a metal surface kinetics [3].
Insufficient lubricant ability BLL in tribological conjunction leads to runout of grease from a contact zone in the beginning, and then under the influence of multicyclic and, as a rule, low amplitude tensions, on real contact points microcracks are formed. Fretting fatigue occurring at very small oscillatory amplitudes when only incipient sliding is present, i.e. the contact area includes both, stick and slip zones [4]. Microcracks increase in size and under the influence of multicyclic fatigue unit and form macrocracks that leads to material separation from a body surface in the form of a petal [5]. Proceeding activity of low amplitude tensions leads to formation of developed dislocation structure on the surface, to its dispersing, that causes petal-scaly wear of material [6]. On a place of the separated material, on a surface pits, tears, hollows are formed, i.e. processes typical for pitting proceed. Energy pumping in the material subsurface layers at a multicyclic frictional loading leads to interacting dislocation accumulations and their exit that promotes pores and strips sliding formation [7]. Progressing ripping of the metal surface layer, related with increasing of fractures nucleus number occurs. At a stage of the maximum dispersing this ripping, covering deeper layers causes sharp increasing exfoliated material mass in the form of the total layer. This process acquires ripple character and leads to selective emissions of destruction products from a frictional contact zone [8]. Selective wearing mechanism of material great volumes leads to scuffing, dripping of mated surfaces, separator destruction and bearing wedging.
Forage- and grain-harvesting techniques work seasonally. Terms of grain harvesting and various root crops, preparations of forages are restricted. In this connection any idle time of agricultural machinery leads to considerable material losses. Therefore minimizing of the idle time related with repairing and working capacity restoration of the failed rolling-contact bearings is the important problem at techniques designing.
The work purpose is an analysis of rolling-contact bearings working capacity and criteria of their work regimes estimation with using of an electrophysical probing method.
Conclusion
The substantiation of electrophysical probing methods using for an estimation of operational properties rolling-contact bearings on a condition of the steel bearing surface is given. Electric circuits and techniques are developed. With using of Hertz theory relationships a calculation of contact pressures in system ball races is executed. Dependence of contraction resistance and oxide film on medium contact pressure is installed. On the basis of model surface boundary, estimation techniques of rolling-contact bearings condition control are developed.
Operating modes of rolling-contact bearings depending on steel surfacing process technology are defined. Following condition stages of rolling-contact bearings are installed: formation monomolecular and polymolecular chemisorbed lubricant layers on races; a dynamic equilibrium condition between formation and wearing of lubricant layers; destruction of layers and predominance of the "dry" not greased contacting mode, intensive oxidation of conjugate surfaces and accumulation of elastic energy mode; surfaces wearing mode, mode of intensive selective surfaces wearing; bearing wedging. Definite form of contact resistance dependence, friction factor and temperature are suitable to each stage or a bearing state. So, contact resistance decreasing to level characteristic for the initial "dry" bearing, in each case depending on a surface treatment method it accepts specific value (or a definite values range), for example, for the bearing type ZVL 6302/16 ≈ 0.2-0.5 Om means fracture initiation of monomolecular BLL, and decreasing to ≈ 1 mOm means the beginning of intensive oxidation, dispersing, formation of developed dislocation structure, leading to time local intensive surface wearing and wedging of the rolling-contact bearing. Determination of these criterion values allows at an early stage, before initiation of bearing destruction critical behavior modes, to carry out its condition control at operation. The given criterion can be used at rolling-contact bearings diagnostic. The recommendation to replace lubricant or machines and mechanisms operating modes restriction can be these diagnostic method results, to prevent their catastrophic destruction.