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Experimental stand for measuring the rheological properties of lubricants, using squeeze film phenomenon.

1. INTRODUCTION

The control of the rheological properties of a fluid during processing is important and can determine the efficiency of the production in addition to the performance of the final product. To date, the control of the properties of process flows has been achieved (to varying levels of success) by the use of capillary viscometers/rheometers (Chiu and Pong, 1998; Covas et al., 2000), cone and plate rheometers and slit rheometers (Magnin and Piau, 1990; Pabedinskas et al., 1991). All these rheometers measure both the viscous and elastic properties of fluids over a wide range of shear rates, giving precisely information about the rheological behavior of the fluids.

If more detailed knowledge of the rheological properties of the fluid is required to control the properties, quality, and reproducibility of the product, test volumes can be quite important and the time consuming procedure is long. In this case, the volume of lost fluid could be large and this could be particularly problematic when processing high value materials (such as pharmaceuticals) or products that cannot be reprocessed or products that incur a disposal cost (for example, an environmental levy).

The experimental stand used in the present investigation is based on the squeeze flow method which is quite different from the controlled-strain instruments commonly employed for quality control. The purpose of this paper is to demonstrate, using a Brookfield cone and plate rheometer, that the modified Weissenberg rheometer based on squeeze film phenomenon produces results for rheological parameters which correlate very well with those from a controlled strain instrument.

2. EXPERIMENTAL STAND

The experimental stand used for the carrying out of the determinations was a modified rheogoniometer Weissenberg, built up from the main structural elements (Figure 1): central working unit, driving system of the superior disc, electric and comand system for the servomotor, pressure transducers, displacement transducer and data acquisition system. The measurement principle of this original system is the squeeze film phenomenom between two circular paralel surfaces. The pressure variation during the squeezing process versus film thickness of the lubricant is measured. For the acquisition and the numerical treatment of the experimental data, the LabVIEW software has been used (Arsenoiu, 1999).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The measurement of the signal provided by the three pressure transducers and the displacement transducer was realized using an acquisition board NI USB-6008.

In order to calibrate this original stand, a Brookfiled rheometer CAP2000+ was used, equipped with a cone and plate geometry (Figure 2). This rheometer offers lower shear rates, making it suitable for many applications where small sample volume and good temperature control are necessary.

An acquisition and data analysis software is provided with the rheometer (CAPCALC 32 software), which is able to determine the rheological parameters of the lubricants, based on four models: power law, Bingham, Casson and Herschel Bulkley (***, 2009).

3. RESULTS

Experimental investigations were carried out at the ambient temperature of 20 [degrees]C, using two types of oil, with different wear degree:

* 10W40 oil, specific for an essence motor vehicle, in fresh state and used with 9560 km way;

* M30 oil, specific for a Diesel motor vehicle, in fresh state and used with 7320 km way.

Both oils have been tested with the two experimental stands (Weissenberg modified rheogoniometer and Brookfield rheometer). Typical results of the measurements are presented in Figure 3 for Weissenberg rheogoniometer and Figure 4 for Brookfield rheometer.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

4. DISCUSSIONS

The experimental results have been numerically interpreted using regression analysis method (Crocker, 1983). In the case of the Weissenberg rheogoniometer, the mean viscosity of all four samples of oils has been determinated, starting from the pressure distribution and taking into account four values of the squeezing velocity: 0.25, 0.50, 0.75 and 1.00 mm/s. For the Brookfield rheometer, the viscosity of the same samples has been obtained, directly from the rheogram of the oils and independent of the velocity. The results of the regression are presented in Table 1.

In order to correlate the measurements of both rheometers and to obtain the calibration curve of the original Weissenberg rheogoniometer, based on squeeze film phenomenon, the data from Table 1 are numerically analized, supposing a linear regression equation, such as:

[[eta].sub.Ws] = a x [[eta].sub.Br] + b (1),

where [[eta].sub.Ws]--viscosity obtained with Weissenberg rheogoniometer; [[eta].sub.Br]--viscosity obtained with Brookfield rheometer; a, b--regression parameters.

The regression parameters for the calibration curves are presented in Table 2 and the corresponding curves are visualized in Figure 5.

[FIGURE 5 OMITTED]

5. CONCLUSIONS

An original experimental stand, derived from a modified Weissenberg rheometer, has been developed. It can be used for the determination of the rheological properties of the lubricants and the principle of the measurement is based on the squeeze film phenomenon between two circular parallel surfaces.

Before this stand can be introduced into an industrial environment, it is necessary to ensure that the results obtained can be correlated with those obtained from the commonly used instruments. Therefore, another rheometer, as Brookfield type, has been used for the measurement of the same properties of the lubricants.

The regression curves which correlate the results obtained from both rheometers are approximately linear, so the modified Weissenberg rheometer can be used as a precisely metrological instrument. In order to complete the metrological validation of the stand, a larger area of fluids must be investigated, with different rheological properties.

6. REFERENCES

Arsenoiu, L., Savu, T. & Szuder, A. (1999). Bazele programarii in LabVIEW (Programming in LabVIEW software), Ed. Printech, Bucuresti (in Romanian)

Chiu, S. H. & Pong, S. H. (1999). Development of an on-line twin capillary rheometer, Polymer Degradation and Stability, Vol. 64, No.2, pp 239-242.

Covas, J., Nobrega, J. & Maia, J. (2000). Rheological measurements along an extruder with an on-line capillary rheometer, Polymer Testing, Vol. 19, No. 2, pp 165-176.

Crocker, D.C. (1983). How to use regression analysis in quality control, American Society for Quality Control, Vol. IX

Magnin, A. & Piau, J.M. (1990). Cone-and-plate rheometry of yield stress fluids. Study of an aqueous gel, Journal of Non Newtonian Fluid Mech., Vol. 36, pp 85-108.

Pabendinskas, A., Cluett, W. & Balke, S. (1991). Development of an in-line rheometer suitable for reactive extrusion rocesses, Polymer Engineering and Science, Vol. 31, No. 5, pp 365-375.

*** (2009) Catalogue CAP 2000+ viscometer, www.brookfieldengineering.com/, Accesed on:2009-05-11
Tab. 1. Experimental results for the viscosity measured with
Weissenberg and Brookfield rheometers

 [[eta].sub.Ws], Pa.s

 [[eta].sub.Br], V = 0.25 V = 0.5 V = 0.75 V = 1.00
Oil Pa.s mm/s mm/s mm/s mm/s

10W40 0.154 0.0540 0.0660 0.0260 0.0217
(fresh)

10W40 0.113 0.0497 0.0713 0.0343 0.0280
(used)

M30 0.383 0.112 0.136 0.0520 0.0390
(fresh)

M30 0.337 0.159 0.129 0.130 0.128
(used)

Tab. 2. Regression parameters for the calibration curves

V, mm/s a b, Pa.s

0.25 0.2532 0.01883
0.50 0.2739 0.03322
0.75 0.0409 0.01935
1.00 0.1047 0.01749
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Author:Radulescu, Alexandru Valentin; Radulescu, Irina; Balan, Corneliu
Publication:Annals of DAAAM & Proceedings
Article Type:Report
Geographic Code:4EUAU
Date:Jan 1, 2009
Words:1189
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