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4 Industrial Lubrication and Tribology
IntroductionLubricating oils are mixtures of a base
material usually derived from
petroleum and additives. When the
lubricant is required to provide
boundary lubrication, it is these
additives which are critical to the
performance of the oil. Parameters to
consider include its lubricity, static
and dynamic friction coefficients, how
it performs under pressure, whether its
acidity affects mechanical comp-onents, and how its flow
properties are
affected by temperature. Because of
the sophisticated chemistry involved
when designing additives to meet
these criteria, as well as the need for
low production costs, laboratory
testing of lubricating oils is an ongoing
process.
For boundary lubrication, one of the
most important parameters in deter-
mining the lubricants performance is
its friction coefficient, defined by the
equation:
FC = FH C/FNwhere FC is the friction coefficient, F
His the horizontal force, FN is the normal
(downward) force, and C is a constant.
In general, the lower the coefficient of
friction, the better the oil will lubricate,
and the less energy will be lost within a
machine or engine using the oil. Hence
a method for accurately measuring
these parameters is an important step to
assessing the in-service performance of
lubricants.
Test equipmentIn many lubricant laboratories, a
standard friction-coefficient testing
method, of the type shown schemat-
ically in Figure l, is used. In this
system, a weighted puck is pushed
and pulled back and forth (recipro-
cated) across a base plate which
carries a film of the lubricant being
tested. The force required to move the
puck is measured by a transducerwithin the instrument, and the
output
of this transducer is recorded and
analysed by laboratory chemists to
provide the necessary performance
data.
The waveforms produced by this
type of test machine are generallysquare in shape, but there can
be
several types of aberration. For
example, an oil that is formulated
using incorrect additives can exhibit
stick/slip characteristics, where the
force required to drive the sliding puck
changes quickly and often produces a
triangle-wave modulation (superposedsignal) on top of the basic
square
wave.
Another common effect is an over-
shoot which results from the higher
value of friction when the puck is
stationary compared to when it is
moving (sticktion). In many cases,
the waveform is not exactly square, butexhibits bowing along its
horizontal
section. Examples of these effects are
shown in Figure 2.
Although the testing machine should
theoretically produce exactly the same
results both on the forward and reverse
strokes, in practice there is often a
small difference. One way of compen-
sating for this effect is to take the mean
of the heights of the first positive and
negative peaks.
Analysing the resultsNormally, an oscilloscope is used to
monitor the output from the testing
machine, with subsequent analysis
being carried out by hand from plotted
waveforms. This procedure is time-
consuming and can introduce the
possibility of human error. Fortunately,
however, instruments are now avail-
able which can introduce a degree of
automation into the waveform analysis
operation.
Measurement sequencesThe instrument shown in Plate 1
offers,among other benefits, a number ofadvanced measurement
routines plusthe facility for programming in cus-tomized
measurement sequences. Forthe lubricant testing application, it
is
A new approach t o lubricantf rict ion test ingTom Lecklider
Figure 1. Schem atic of a lubricant test ma chine
Drivemechanism Baseplate
Forcetransducer
Amplifierand output
Lubricant
NormalloadSliding puck
Figure 2. Typica l wave forms e nco untered d uring friction
testing
Basic squarewave shape "Stick-slip" modulation
"Bowing" of top and base levels "Improved" lubricant, but
"sticktion"
Vol. 47 No. 6, 1995, pp. 4-6, MCB Univers ity Pres s,
0036-8792
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possible to use this combination offeatures to pick out the
first peak on aheavily modulated square wave andthen carry out the
appropriate analysisfunctions.
The measurement sequence makesuse of the fact that, if one finds
the firstnegative crossing of the differentiatedand filtered
testing-machine wave-form, one is guaranteed to find theupper time
bound of the first peak inthe waveform, irrespective of its
exactshape. The lower time bound is alwaysgiven by the zero
crossing of the basicwaveform itself. Then, by using theability to
bound a measurement by theresults of two previous measurements,one
can determine the peak of the firstovershoot.
Because of the physical nature ofthe test, it is realistic to
assume thatthe phase difference between thebasic square wave and
any stick/slip
modulation will be restricted, asshown by the theoretical
example inFigure 3.
Differentiating the square waveallows all the stick/slip
transitions to beexamined with respect to the zerovoltage level,
rather than with refer-ence to otherwise arbitrary levels.
Thismeans that the number of crossings canbe used to find the nth
positive peak,for example.
In Figure 4, the indicated areas bothof the square wave and its
derivative
have been expanded and overlaid toshow more clearly the time
points usedto bound the max or min function,which then determine
the heights of thefirst positive and negative peaks of thesquare
wave as required.
The sequence shown in Figure 5 isused to acquire the basic
signal, differ-entiate it, filter it, and then apply a
measurement routine to the waveformsto determine the peak
values.
Figure 6 shows plots of an actual
signal used to simulate the output of afriction testing machine.
In Figure 7,
ILT November/December, 1995 5
Figure 3. Differentiation o f a s tick-slip wa veform
Basicwaveform
Differentiatedwaveform
Figure 4. Expanded view of differentiated stick-slip w
aveform
Basic waveformrising crossing
First risingcrossing ofdifferentiatedsignal
Fig u re 5 . B a s i c me a s u r e me n t s e q u e n c e f o
rsignal a cq uisition
Plate 1. The Gould DataS YS 700 is an a utomated m eas uring sys
tem ba sed on a n adva nced digital
storage o scilloscope; the ability to program in customized
measureme nt seque nces ma kes it idea lfor analyses of wa veforms
produced by mec hanical test systems
Figure 6. Output s igna ls
TR1Z: 2V 500s
TR2Z: 2V 500s
Date: 16 Feb 1912Start time: 04:15:35
SEQ UENCE 1 Exit 1
0 S IN GL E SH O T 2
1 TRC3 FUNCTION DIFFERENTIATE ns2 TRC3 SOURCE TRACE1 TRC1 3
3 TRC3 DIFF SCALE 10
4 EXECUTE TRC3
5 TRC2 FUNCTION FILTER
6 TRC2 SOURCE TRACE1 TRC3
7 T R C2 FILT ER FA CTO R F1 R enam e 5
8 EXECUTE TRC2
(APPEND)
