Development of New Ultrasonic Instrumentation to Measure

2014 STLE Annual Meeting & Exhibition
May 18-21, 2014
Disney’s Contemporary Resort
Lake Buena Vista, Florida, USA
Development of New Ultrasonic Instrumentation to Measure
Lubricant Properties in Auto Engines
Track or Category
In Situ Tribology II: Material Tribology and Tribotesting Joint Session
Authors and institutions
Michele Schirru, Rob Dwyer-Joyce
Leonardo Centre for Tribology, Department of Mechanical Engineering, University of Sheffield,
Mappin Street, Shefield, S1 3JD, UK.
Introduction
The viscosity of oil in a thin layers such as an elastohydrodynamic lubricated (EHL) contact is very
different to that in the bulk form. Knowledge of that viscosity would help in the design of both
lubricants and machine elements. This could in turn help to reduce emissions and engine wear and
failure. The inability of traditional viscometers to be employed in the analysis of small volumes of
fluid has forced researchers to look for other suitable rheological techniques to study fluid behaviour
in thin films. The aim of this research is to study physical properties of thin layers of lubricants by
means of ultrasound and to provide techniques and instrumentation with the potential to be used insitu in operating machine parts .
Keywords:
Ultrasound, Viscosity, Viscometer, Lubricant, Lubrication, Thin Layer, Non-Newtonian, Journal
Bearing, Engine, In Situ, Real Time, Maxwell
Theoretical models
When a shear polarised ultrasonic wave strikes a solid – liquid boundary the proportion of the wave
reflected depends on, amongst other things, the viscosity of the liquid. This physical phenomenon has
been employed for the study of fluid properties in the past [1, 2]. Two models in particular have been
developed for the study viscosity of Newtonian fluids: the Newtonian solution of the wave equation
(equation 1) [3] and the “bulk model” (equation 2) [4]. In these models the viscosity of the fluid is
calculated as function of the reflected ultrasound energy at the solid-liquid interface. The amount of
reflected energy is referred to as reflection coefficient R, which has both magnitude and phase . The
two models may be expressed with the following expressions:
| |
| |
| |
(1)
| |
| |
( )
(2)
| |
where
is the viscosity,
is the acoustic impedance of the solid body where the transducers are
mounted, is the fluid density, and is the angular acoustic frequency
In the present work a novel model that better reflects the non-Newtonian nature of lubricants has been
developed and compared with the above methodologies. The response of a solid-liquid interface to an
ultrasonic shear stress excitation is modelled as a mechanical system composed by a damper and a
spring in series [5]. By combining mechanical and acoustical theory it is possible to relate shear
viscosity to the amount of energy reflected from the interface of interest as follows:
|
( (
In equation (3),
| |
))
| |
|
(3)
represents the fluid relaxation time.
Materials and Methods.
Four different Cannon calibrated lubricants were enclosed between two aluminium plates to
reproduce a thin film as shown in Figure 1. A 50 MHz TTI TG5011 waveform function generator
produces a five cycle sine burst pulse excitation at 10 V that makes the transmitting piezoelectric
ultrasonic transducer (PX in Figure 1) vibrate and thus producing an ultrasonic wave at the centre
frequency of 10 MHz. The ultrasonic pulse from the first transducer propagates through the
aluminium wedge until is incident to the solid-liquid interface where part of the wave is transmitted,
and dissipated, in the fluid layer and part is reflected back. The reflected wave from the interface is
received by the second transducer (RX). The signal is recorded on a Lecroy LT342 oscilloscope with
a sampling capacity of 500 MS/s, continuously analysed and stored in real time using an acquisition
interface written in Labview code. This apparatus allows calculating the reflection coefficient
modulus as:
(4)
| |
where
is the amplitude of the ultrasonic wave reflected from the solid-lubricant interface while
is the reference ultrasonic signal acquired from the solid-air interface. The phase angle is calculated as
function of the modulus of the reflection coefficient as follows [6]:
| |
| |
(5)
WF generator
Acquisition and
data processing
Oscilloscope
PX
RX
Solid delay line
Fluid layer
Bottom Layer
Thermocouples
Figure 1: Schematic Diagram of Measurement Apparatus
The reflection coefficient from the sample layer was obtained at different temperatures by cooling the
assembly from 60°C down to 25 °C. Viscosity values were obtained from the reflection coefficient in
this range of temperature by using equations 1, 2 and 3.
Results
Figure 2 shows a typical result obtained by applying the three methodologies
1.4
Viscometer Data
Maxwell Model
Bulk Model
Newtonian Model
1.2
Viscosity (Pas)
1.0
0.8
0.6
0.4
0.2
25
30
35
40
45
Temperature (°C)
50
55
60
Figure 2: Ultrasonic viscometer output for a Cannon S600 lubricant
By comparing the expected temperature-viscosity curves for different lubricants with the results
obtained by the application of (1), (2) and (3) it is possible to obtain a viscosity-error graph that
highlights the region of applicability for the different models (Figure 3).
In region 1 is difficult to obtain accurate measurements of lubricant viscosity as the reflection
coefficient is very close to the unity value and equations (1),(2) and (3) become unstable. Region 2 is
the optimum region to use the Bulk model. As it can be noticed in this region most of the results
coming from the Bulk model give an error that is less than the 10%. Region 3 is the optimum region
to use the Maxwell model.
Case study: application to a journal bearing
The previously described methodology and setup has been applied to a journal bearing by bonding
two shear pzt transducers (centre frequency 1.8 MHz) on the brass race of a journal bearing (75 mm
of inner diameter) at the 0 degree angle.
Maxwell model fit
Bulk model fit
Newtonian model fit
140
130
120
1
2
3
110
100
Error %
90
80
70
60
50
40
30
20
10
0.10
0.20
0.30
0.40
0.50
0.60
0.70
Viscosity (Pas)
0.80
0.90
1.00
1.10
1.20
1.30
Figure 3: Viscosity-error chart. Region of applicability for each models is highlighted.
Tests have been run by rotating the journal bearing steel shaft up to 500 rpm and by lubricating the
bearing interface using three different lubricants. As a consequence of this increase in rotating speed,
the temperature at solid-lubricant interface has risen thus producing a change in the apparent lubricant
viscosity. Such change in viscosity has been detected using the ultrasound methodology as shown in
figure 4a in terms of reflection coefficient. The acquired reflection coefficients have been converted in
viscosity values by using the Maxwell algorithm. As it can be noticed the results accuracy is in line
with the accuracy obtained in the plate to plate setup as shown in figure 4b.
Figure 4 a, b: a) Reflection coefficient detected as temperature in the Journal bearing increases for different lubricants. (b) Jouranl bearing results accuracy
when comparing the Maxwell methodology to the expected viscometer data.
Conclusion
In this work three ultrasonic methodologies for measuring viscosity in thin lubricant layers have been
tested in a plate to plate setup at static conditions and in a journal bearing. The bulk formula shows to
be the most precise tool at low viscosities (up to 0.15 Pas), if a delay line that allows a good acoustical
match with the lubricant layer is provided. In this case the error scatters between 1 to 8% in the
optimum regions. The novel Maxwell model has been successfully developed for application where
the viscosity is higher than 0.15 Pas. Applying this methodology shear viscosity can be measured with
an accuracy of 0.5% to 12%.
References
[1] W.P. Mason, “Measurement of shear elasticity and viscosity of liquids at ultrasonic frequencies”,
Physical review, Volume 7s, Number 6, March 15 1949
[2] F. Buiochi, J. C. Adamowski, and C. M. Furukawa, “Measurement of viscosity using wave mode
conversion,” in Proc. IEEE Ultrason. Symp., 1998, pp. 1193–1196.
[3] E.E. Franco, J.C. Adamowski “Viscosity Measurement of Newtonian Liquids Using the Complex
Reflection Coefficient”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control,
vol. 55, no. 10, October 2008
“Measurement of Circumferential
Viscosity Profile in Stationary Journal Bearing by Shear Ultrasonic Reflection” J. Tribol.
133(3), 031501 (Jun 17, 2011)
[4] S.Kasolang , M.A. Ahmad, R. S. Dwyer-Joyce,
[5] R.W.Worlow, “Rheological Techniques”, Horwood Ltd, 1980
[6] V.Shah,K. Balasubramaniam, “Measuring Newtonian viscosity from the phase of
reflected ultrasonic shear wave”, Ultrasonics 38 (2000)