STUDY OF A GENERAL USE GREASE RHEOLOGICAL BEHAVIOUR INFLUENCE ON LUBRICATION

The paper presents the rheological behavior of a MP multifunctional grease (additived, for EP, containing Li and Ca soaps– for general use), experimentally obtained on the THERMO-HAAKE Viscotester VT550, as correlations, at different temperatures, between 15-60C. It results a strong non-Newtonian behavior, with threshold, time depending (tixotropy or rheopexy, depending on the operating conditions). With the aid of the rheological parameters, obtained from the experiments, it is analyzed the influence on a ball bearing frictional coefficient in the contact zone ball-inner ring, in the case of the elasto-hydrodynamic-lubrication (EHL).


INTRODUCTION
Adequate lubrication is one of the key elements of the maintenance success; in most of the applications it is an optimum correlation between lubrication and safety.Many technical problems, or even failures, could have origin in lubrication deficiencies so this topic is of a great interest among the maintenance problems.The lubrication can be done: -with dedicated substances: fluids (oils), semi solids (greases) or gases, upon the technical parameters of the application; -with the working fluid (in some special cases when the contamination with lubricants is forbidden -often encountered in the food or chemical industries, for example).Characterization of lubricant rheological behavior can be done by rheological measurements resulting rheograms, viscosity variation with temperature, time dependent behavior, which give accurate results about grease behavior in service.In the present paper it is studied only the grease behavior.
The rheograms of greases usually are linear or nonlinear with threshold, as can be seen in Figure 1 [1].
Greases can show tixotropy or rheopexy, characterized by the fact that the loading and the unloading curves are not identically, the loading curve is above the unloading curve in tixotropy case and vice versa in the case of rheopexy.The viscosity decreases with the time in the first case and the viscosity increases with the time in the second case.

EXPERIMENTAL RESEARCH
The experimental research was done on a cone and plate Thermo-Haake Viscotester VT550 controlled by a computer (provided with a dedicated soft), suitable for greases (high viscosity at high shear rates and at different temperatures).It was tested a MP multifunctional grease, additived, for EP (extreme pressure), contained Li and Ca soaps-for general use.
There were experimentally determined the rheograms (shear stress [Pa] versus shear rate [1/s]) for different temperatures between 15 and 60 o C, presented in Figure 2 (a -f).The viscosity model obtained is Hershel-Bulkley, given by the equation: where 0 is the threshold, Pa; m [ s Pa ], -rheological constants.
The models resulted, from experiments, are presented in Table 1.It can be seen that there are a good concordance between the theoretical model and the experiments (standard deviation approaches 1).
In Table 2 it is presented the measured time dependent rheological behavior -tixotropy/antitixotropy, by means of area between the loading and unloading curves

INFLUENCE ON THE LUBRICATION PERFORMANCE
It is analyzed, with the aid of the rheological parameters, the influence of dynamic viscosity variation of the grease during work, on a ball bearing frictional coefficient in the case of the elasto-hidrodinamic-lubrication (EHL).
It is analyzed the case of a radial bearing with a single row balls, knowing the rotational working speed of 100 RPM; the ball radius R 1 = 7 mm; bearing inner ring radius R 2 = 18,5 mm; roughness: , on the frictional coefficient in the contact zone ball-inner ring, adapting the relation given in [10]: where: r C is a constant depending on the bearing; m, -rheological constants (depending on the grease and the working parameters -rotational speed/shear rates, temperatures); -shear rates; -lubricant density (function of working temperatures); 1 v , 2 v -tangential velocities of the two surfaces in contact (ball-inner ring) [10].
The following values of the mentioned parameters were considered, for the numerical application: The intention is to evaluate, especially, the influence, on the frictional coefficient, of the grease dynamic viscosity (determined in working conditions), described by the following relation: coefficient.It can be seen that the influence coefficient increases ~100 % between 15 and 60 0 C.In the same way would increase the value of the frictional coefficient.
In the literature dedicated to lubrication there are many empirical relationships for the bearings frictional coefficient [6][7][8][9].From our experiments it results that it doesn't matter so much the equation we choose for frictional coefficient in the contact zone ball-inner ring, in comparison with the influence of the working conditions, manifested by temperature and shear rate on grease behavior.

CONCLUSIONS
Conclusions on the grease rheological behavior experimentally determined: a-the grease presents yield point and time dependent behavior; b-the Herschell-Bulckley model was the most appropriates.The tixotropy varied with the temperature; one obtained significant different rheological behavior with the temperature -very important from practical point of view.
During working, when working parameters can change, the grease's behavior changes too.It is important to perform rheological analysis before using the lubricant and periodically (at predetermined period of time) to find out if the flowing properties are still suitable for the practical application or if the lubricant must be replaced.The rheological behavior helps to choose the right lubricant because this allows predicting its behavior during work.
As an application it was presented the influence of the rheological changes of the lubricants, due to working conditions, on the frictional coefficient for a ball bearing, finding it very significant -not to ignore in any case.
; the minimum lubrication film thickness in the contact zone with the inner ball ring (in the case of elasto-hydrodynamics lubrication EHL) the influence of the grease rheological behavior modification, at different temperatures C

-
influence coefficient depending on the dynamic viscosity values at different working temperatures, T. In Figure3it is presented the evolution of the T C

Table 1 .
Model Hershel-Bulkley for dynamic viscosity.It can be seen from the results presented in the Table1that the values of the threshold 0 and of the rheological constant m decrease significantly with the temperature, whereas the exponent values decreases slightly.The thixotropy variation with the temperature suggests a possible slip at 30 o C (Table2).
(obtained at different temperatures), for additivated, multifunctional grease, with Li and Ca soaps.