machines

Article
SRV Method: Lubricating Oil Screening Test for FZG
Davide Massocchi 1 , Marco Lattuada 2 , Steven Chatterton 1, * and Paolo Pennacchi 1

1 Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 1, 20156 Milan, Italy;
davide.massocchi@polimi.it (D.M.); paolo.pennacchi@polimi.it (P.P.)
2 Downstream Product R&D, Eni SpA, 20097 San Donato Milanese, Italy; marco.maria.flavio.lattuada@eni.com
* Correspondence: steven.chatterton@polimi.it

Abstract: Governments and institutions have the following sustainable development goals: the im-
provement of energy efficiency and the reduction of CO2 emissions, in a “green economy” approach,
have currently become the fundamental drivers that push research and development activity toward
the optimization of rotating machine components in the industrial sector, with a special focus on
lubrication systems too. The activity is directed towards the optimization of tribological testing
methods and equipment to better discriminate the performance of lubricants in operating conditions
as predictive as possible of real applications. In this context, the present paper describes the results of
an experimental campaign based on the use of a well-selected linear oscillation SRV * (Schwingung,
Reibung, Verschleiss) tribometer procedure as a screening of a rig test, the FZG ** (Forschungsstelle für
Zahnräder und Getreibebau (German: Research Centre for Gears and Gear; University of Munich; Munich,
Germany)) test, leading to concrete benefits such as saving time (time duration is 76% less without
mentioning visual inspection and mounting/dismounting phase) and operative costs. Four cases
for the determination of the failure load stage of SRV have been defined as links to seizure and
microseizure phenomena. The procedure was tested for ten oils differing in scope (gas turbine oil,
turbine oil, gear oil and circulating oil). The tests have been repeated three times and a procedure
was defined for repeatability (± 1 stage difference between the minimum and maximum) for nine out
of ten cases a failure stage could be defined. The same oils were also tested using the FZG scuffing
Citation: Massocchi, D.; Lattuada, test, and it can be seen that the results are very comforting as follows: a good correlation with the
M.; Chatterton, S.; Pennacchi, P. SRV FZG rig test has been found for eight out of ten oils.
Method: Lubricating Oil Screening
Test for FZG. Machines 2022, 10, 621. Keywords: tribological test; energy efficiency; rig test; screening; SRV; FZG
https://doi.org/10.3390/
machines10080621

Academic Editor: Davide Astolfi

1. Introduction
Received: 28 June 2022
Accepted: 26 July 2022
The growing attention to CO2 emissions and the energy efficiency improvement in the
Published: 28 July 2022
industrial sector are pushing manufacturers to develop machines with higher performance,
characterized by lower power losses, lower energy consumption and downsizing of com-
Publisher’s Note: MDPI stays neutral
ponents. In this compound, lubricating oils can also provide a significant contribution to
with regard to jurisdictional claims in
energy efficiency [1,2].
published maps and institutional affil-
Manufacturers increasingly tend to consider the oil as an integral part of the machine,
iations.
a key element just in the design phase. To support this evolution, it is important to identify
and use lubricants with well-selected rheological and chemical-physical characteristics
combined with appropriate additive systems; in addition, another important lever that
Copyright: © 2022 by the authors.
gives lubricants a lower environmental impact is the use of components derived from
Licensee MDPI, Basel, Switzerland. renewable sources. Tribological experimentation is an extremely important tool supporting
This article is an open access article the assessment of energy-saving characteristics of products, ensuring at the same time the
distributed under the terms and fulfillment of traditional performance targets.
conditions of the Creative Commons This paper has the following two goals: on one hand, identify and optimize tribological
Attribution (CC BY) license (https:// tests in order to better discriminate lubricants with “energy saving” [3] characteristics
creativecommons.org/licenses/by/ before moving on to field testing; on the other side, acquire know-how on the mechanisms
4.0/). underlying tribological couplings, looking for correlations between mechanics (tribological

Machines 2022, 10, 621. https://doi.org/10.3390/machines10080621 https://www.mdpi.com/journal/machines

Machines 2022, 10, 621 2 of 11

instrumentation), chemistry (lubricating oil and its additives) and material in order to set
up a tribo test as predictive as possible of real industrial applications.
As a concrete example of this framework, the evidence obtained by using a tribometer,
the SRV, for prescreening a rig test, the FZG test, has been reported in this report.
An SRV procedure [4] has been built up for reproducing very similar operating condi-
tions to the FZG test [5].
The FZG [6] is a power re-circulating rig test based on a wheel and pinion coupling
and an increasing step load procedure that lasts till the failure load stage is reached. It is
very well known in the lubricant industry as one of the more severe and significant tests
for the evaluation of industrial and transmission lubricating oils.
FZG failure load stage in the scuffing test is defined as the stage where the sum of the
wear area of all 16 teeth of the pinion exceeds the areas of a single tooth.
A higher failure load stage means higher lubricant performance in terms of load-
carrying capability and antiwear characteristics.
The SRV is a well versatile laboratory machine that aims to evaluate friction and wear
characteristics of lubricants in many different operative conditions by using couplings of
different geometry and materials. In this work, the selected coupling is cylinder on disk,
and the procedure is a load-increment step test till the failure load stage occurs.
For SRV, the failure load stage is the stage where the coefficient of friction (CoF) profile
shows high and sudden peaks that mean seizure occurs. In this regard, the evaluation
criteria defined are deeply discussed in the following chapters.
Using the SRV tribometer to reproduce the same operating conditions as the FZG test
rig and try to obtain a good correlation of the results of both tests, could lead to a faster
screening among many candidate oils.
In addition, other benefits can be achieved, such as less operative cost, less test duration
and higher automation level (SRV is less dependent on the operator).
This paper aims to analyze the “EP lubricants” more used for gear oils and try to
reduce the use of time-consuming and expensive tests such as the FZG test. The next step
of the activity is to analyze the oils with antiwear additives in the additive package (in this
case, it is mandatory to analyze the wear performance even in SRV tests).
In this paper, the results achieved by using only the EP procedure modified will
be shown and the corresponding evaluation criteria will be discussed; in addition, the
relationship between the results of both machines, rig and laboratory tests, for all of the
oils tested, will be investigated.

2. Materials and Methods

A widely used test method to evaluate load-carrying capacity in industrial oils is the
FZG test A/8.3/90 according to DIN ISO 14635-1. An example of FZG [7] test rig is shown
in Figure 1.
A-type gears, wheel and pinion, are loaded stepwise in 12 load step stages (Table 1)
from a Hertzian stress of pC = 146 to 1841 N/mm2 and in some cases also at higher load
if required.
The gear pair type A (Figure 1) shows a considerable profile offset, which causes the
tooth flanks to move at higher speeds relative to each other. This in turn increases the
percentage of sliding movement on the flanks, which makes the teeth more susceptible
to scuffing [8]. They are operated for 15 min at a pitch line velocity of 8.3 m/s and a
starting oil temperature of 90 ± 1 ◦ C in each load stage, under conditions of dip lubrication
without cooling.
Hence during the running time of each load stage, starting from the 4th, the oil
temperature can rise freely. Gear flanks are inspected after each load stage for scuffing [9]
marks. Failure load stage is indicated when the wear area detected on all pinion teeth
exceeds the area of a single tooth.
 case, it is mandatory to analyze the wear performance even in SRV tests).
In this paper, the results achieved by using only the EP procedure modified will be
shown and the corresponding evaluation criteria will be discussed; in addition, the rela-
tionship between the results of both machines, rig and laboratory tests, for all of the oils
tested, will be investigated.
Machines 2022, 10, 621 3 of 11
2. Materials and Methods

Figure 1.
Figure 1. FZG
FZG rig
rig test.
test.

For the experimental campaign it has been used SRV5 tribometer (Optimol Instruments
Prüftechnik, Munich, Germany; load range 0–2500 N ± 1 N) equipped with OCA software
for operation, controlling and test evaluation using the EP procedure modified.

Table 1. FZG load stages.

Load Stage Pinion Torque [Nm] Hertzian Pressure [N/mm2 ]

LS 1 3.3 146
LS 2 13.7 295
LS 3 35.3 474
LS 4 60.8 621
LS 5 94.1 773
LS 6 135.5 929
LS 7 183.4 1080
LS 8 239.3 1223
LS 9 302.0 1386
LS 10 372.6 1539
LS 11 450.1 1691
LS 12 534.5 1841
LS 13 626.9 1996
LS 14 714.2 2130

The SRV tribometer [10,11] (see Figure 2) is a very versatile device for measuring the
coefficient of friction and the wear and EP characteristics of lubricating oils and greases
under various operating conditions.
The basic SRV5 system is equipped with an electromagnetic linear drive, which gen-
erates a periodic sinusoidal translational movement in the frequency range 0.001–500 Hz
with strokes of 0.01–5 mm (oscillation) as the relative movement of the test contact.
The main principle of oscillation motion is the identification of the friction coeffi-
cient of a material coupling with or without an intermediate medium, according to the
following setting:
• Testing variable chosen frequency, stroke, test force, test temperature and test duration;
• Pressure of the opposite body on the main body with a defined normal force;
• Oscillation of the opposite body on the surface of the main body with a sinusoidal motion;
• Measuring the lateral friction force resulting from the movement of the opposite body
on the main body;
• Calculating and recording the friction coefficient during the whole test.
 exceeds the area of a single tooth.
For the experimental campaign it has been used SRV5 tribometer (Optimol Instru-
ments Prüftechnik, Munich, Germany; load range 0–2500 N ± 1 N) equipped with OCA
software for operation, controlling and test evaluation using the EP procedure modified.
The SRV tribometer [10,11] (see Figure 2) is a very versatile device for measuring the
Machines 2022, 10, 621 4 of 11
coefficient of friction and the wear and EP characteristics of lubricating oils and greases
under various operating conditions.
Machines 2022, 10, x FOR PEER REVIEW 4 of 11

The basic SRV5 system is equipped with an electromagnetic linear drive, which gen-
erates a periodic sinusoidal translational movement in the frequency range 0.001–500 Hz
with strokes of 0.01–5 mm (oscillation) as the relative movement of the test contact.
The main principle of oscillation motion is the identification of the friction coefficient
of a material coupling with or without an intermediate medium, according to the follow-
ing setting:
• Testing variable chosen frequency, stroke, test force, test temperature and test dura-
tion;
• Pressure of the opposite body on the main body with a defined normal force;
• Oscillation of the opposite body on the surface of the main body with a sinusoidal
motion;
• Measuring the lateral friction force resulting from the movement of the opposite
body on the main body;
Figure 2. SRV5 tribometer overview.
• Calculating
Figure and recording
2. SRV5 tribometer overview.the friction coefficient during the whole test.

Thanks to
Thanks to its
its modular
modular design,
design, itit is
is possible
possible toto configure
configure thethe test
test with
with couplings
couplings of
of
different materials and geometry, testing them either in oscillatory-translational
different materials and geometry, testing them either in oscillatory-translational mode or, mode or,
withanother
with anotherset-up,
set-up, inin rotational
rotational mode
mode [12].[12]. The
The software
software isis able,
able, during
during thethe experiment,
experiment,
to measure
to measureandand save
save the
the following
following fundamental
fundamental parameters
parameters of of the
the test:
test: load,
load, temperature,
temperature,
frequency,stroke
frequency, strokeandandcoefficient
coefficientofoffriction
friction(CoF).
(CoF).
The SRV
The SRV [13]
[13] is
is able
abletotoproduce
produceresults
resultsthat comply
that comply with
withthethe
reliability andand
reliability reproduc-
repro-
ibility statements of ASTM D5706, D7421 [14] and related specifications.
ducibility statements of ASTM D5706, D7421 [14] and related specifications.

ExperimentalProcedure
Experimental Procedure
Thealgorithm
The algorithmof ofthe
theprocedure
procedureusedusedconsists
consistsof ofaarunning-in
running-inunder
under50 50N Nfor
for30
30ss then
then
◦ C temperature
there is an increasing step-by-step load up to 17th stage keeping fixed at 90 °C temperature
there is an increasing step-by-step load up to 17th stage keeping fixed at 90
the lower
the lower specimen.
specimen. The test pass to the the following
following step
step ifif no
no seizure
seizure andand microseizure
microseizure
phenomena occur (or go out the threshold cut-off values below described).The
phenomena occur (or go out the threshold cut-off values below described). TheCoF
CoF curve
curve
saved during
saved during thethe test
test is
is then
then evaluated
evaluated by by the
the operator
operator andand after
after an
an accurate
accurate analysis
analysis is is
assigned aa failure
assigned failureload
loadstage.
stage.
The test
The test kinematics
kinematics involves
involves aa periodic
periodic sinusoidal
sinusoidal translational
translational movement
movement at at 50
50 HzHz
with a stroke of 2 mm (oscillation).
with a stroke of 2 mm (oscillation).
Theaim
The aimisistotoreproduce
reproducethe the same
same scuff
scuff marks
marks [15][15] asthe
as in in the
FZGFZGtest,test,
and and to enable
to enable this
and to have the same operating contact conditions (Hertzian pressure
this and to have the same operating contact conditions (Hertzian pressure of the FZG sys- of the FZG system),
the specimens
tem), selected
the specimens to reproduce
selected the pinion/wheel
to reproduce are cylinder
the pinion/wheel and disc.
are cylinder and disc.
The lower specimen is a disk Ø 24 × 7.9 mm, made
The lower specimen is a disk Ø 24 × 7.9 mm, made of hardened rolling of hardened rolling bearing
bearing steel
steel
AISI 52100/100Cr6. The upper◦ specimen [4] is a cylinder Ø 6 × 8 mm. The surfacesurface
AISI 52100/100Cr6. The upper specimen [4] is a cylinder Ø 6 × 8 mm. The is pol-
is polished
ished and aligned
and aligned at 10° atto 10 to the direction
the direction of movement
of movement along itsalong its longitudinal
longitudinal axis (seeaxis
Fig-
(see Figure
ure 3). 3).

Figure3.
Figure 3. Tribological
Tribologicalcontact:
contact:cylinder
cylinderon
ondisk.
disk.

The surfaces of the AISI specimens were ultrasonically cleaned in ethanol for 5 min.
Machines 2022, 10, 621 5 of 11

The surfaces of the AISI specimens were ultrasonically cleaned in ethanol for 5 min.
After the cleaning phase the specimens were mounted and locked with a torque
wrench with a load of 30 N. A very small amount of oil sample is needed, and the operator
should dry both the upper and lower specimens during the setting phase.
The load steps for the SRV EP procedure are presented Table 2.

Table 2. SRV EP Procedure vs FZG test load stages.

FZG load step 1 2 4 6 8 10 12 14 16 18

Hertzian stress [N/mm2 ] 146 295 621 929 1223 1539 1841 2170 2465 2777
SRV normal force [N] 7 28 126 282 489 774 1107 1538 1985 2520

The duration of each step is 217 s, which is a result of prior investigations [16], where
it was compared to shorter runs and reflects the following two conflicting physical phe-
nomena: too high step times will lead to too much wear and thereby change the applied
pressure, as the system is load controlled; too short times will not let the tribo-system
establish itself properly and, therefore, do not allow for distinction between the tested oils.
The conditions for the step test are a temperature of 90 ◦ C (changed from the standard
EP procedure to have the same condition as in FZG test), a stroke of 2 mm and a frequency
of 50 Hz.
Have been defined threshold values of some parameters for test execution, the so-
called cut-off values. In particular, for this experimental campaign the following cut-off
values of CoF and stroke have been set: 0.3 as maximum CoF and a stroke deviation of
maximum 55%.
Have been chosen these values for the following two main reasons: on one hand for
safety operation, as to quickly block test run while high seizure occurs; on the other, the
above limits selected allow us to detect also microseizures and seizures (of limited intensity)
helping in the qualitative analysis of CoF profile (otherwise impossible for the operator if
the test suddenly is aborted).
For the evaluation criteria of oil performances and failure load stage assignment the
following four cases have been defined:
• Case 1: during test run the presence of seizure, i.e., a sudden and high increase in
coefficient of friction profile has been detected; in other words, there is a consistent
peak that lasts for at least for few seconds and leads to test stop, according to the
cut-off criteria. In correspondence to the stage where what described occurs, the failure
load stage is assigned;
• Case 2: the oil is able to reach the end of test without presenting any seizure or relevant
microseizures. CoF profile is linear descending and thin. This oil overcomes the SRV
12th stage.
When seizure does not occur, but the CoF profile presents an anomalous trend, a more
interpretative component of the evaluation has to be taken into account and other two cases
can be distinguished.
• Case 3: alternation of frequent and close micro seizures (i.e., peaks not so high as the
seizure) and thickening of the curve of the friction coefficient, before reaching a very
net seizure or without reaching it as follows: from the interpretation of the curve it is
possible to the failure load stage of the SRV in correspondence of the stage where this
phenomenon has begun and continues (seen in some cases that we will present later);
in order to have a more robust and objective evaluation are needed more statistical
data and, eventually, also wear analysis on the specimen in order to better investigate
the behavior of oil;
• Case 4: halfway through the test there is a drastic change in the profile of the coefficient
of friction, microseizures increase and the curve is disturbed although not presenting
appreciable thickening. Moreover, in this case it appears very difficult to assign
Machines 2022, 10, x FOR PEER REVIEW 6 of 11
Machines 2022, 10, 621 6 of 11

assign the failure load stage and are needed at least the FZG result to have a reference
tothe
compare
failure to.
load stage and are needed at least the FZG result to have a reference to
compare to.
3. Experimental Campaign
3. Experimental Campaign [4] has been used with the only difference of temperature (T
The SRV EP procedure
= 90 °C instead
The SRV of EP98 °C).
procedure [4] has been used with the only difference of temperature
(T =The90 ◦procedure
C instead of ◦ C).
is performed for each oil for a minimum of three repetitions.
98
AThegood repeatability
procedure can be defined
is performed for eachwith a difference
oil for a minimum of ±of1 three
stage.repetitions.
Ten oils have
A good been tested
repeatability can for a minimum
be defined with of three repetitions
a difference each. The oils used for
of ± 1 stage.
testingTenwere
oilsgas turbine
have been oils
testedfromforISO VG 15 toof
a minimum ISO VGrepetitions
three 46, a turbine oil ISO
each. The VGoils 46,
usedgear
for
lubricants with kinematic viscosity 320cSt and 460cSt at 40 °C and four circulating prod-
testing were gas turbine oils from ISO VG 15 to ISO VG 46, a turbine oil ISO VG 46, gear
ucts ISO VGwith
lubricants 68. Lubes
kinematicalsoviscosity
differ by320cSt
a mix and
of base oil at
460cSt 40 ◦ C
(from mineral
and four to more synthetic
circulating oil)
products
and EP (extreme pressure) additives.
ISO VG 68. Lubes also differ by a mix of base oil (from mineral to more synthetic oil) and
As can bepressure)
EP (extreme seen in Figure 4, for all gas turbine oils, gear oils and turbine oils there is a
additives.
difference between
As can be seenthein minimum
Figure 4, for andallmaximum
gas turbine failure stageoils
oils, gear of ±and
1. Only for aoils
turbine circulating
there is a
difference between the minimum and maximum failure stage of ± 1. Only
oil is there a difference of five points, and in this case, it was not possible to assign a failure for a circulating
oil is there
stage. a difference
Presumably, this isofdue
fivetopoints, and in this
the different case, it presence
additives’ was not possible to assignand
in the lubricant, a failure
this
stage. Presumably, this is due to the different additives’ presence
variability is assumed to be caused by chemical phenomena that require more detailed in the lubricant, and this
variability
analysis. is assumed
Finally, to be caused
of the products byonly
tested, chemical
one did phenomena that require
not demonstrate good more detailed
repeatability.
analysis. Finally, of the products tested, only one did not demonstrate good repeatability.

18

16

14

12
Failure load stage

10

15 32 46 46 32
0
46
0
68
A
68
B
68
C
68
D
GT GT GT TO GO GO CO CO CO CO

Repeatabilityofofthe
Figure4.4.Repeatability
Figure theexperimental
experimentalresults.
results.

4. Experimental Results and Discussion

4. Experimental Results and Discussion
In this section, some experimental results are shown in detail and the relationship
In thisrigsection,
between test and some experimental
laboratory results are
test according shown
to the cases inwe
detail and the
reported in relationship
the previous
between rig test and
section has been discussed.laboratory test according to the cases we reported in the previous
section has been discussed.
The procedure used better reproduces the FZG scuffing test (FZG test method A/8,
3/90The procedure
[17]) used better
because there reproduces
is a gradual thein
increase FZGloadscuffing
as in the test
FZG(FZGandtest
themethod A/8,
temperature
3/90 [17]) because there is a gradual increase in load as in the FZG and
is the same, thus maintaining the same rheological characteristics of the oil in the two the temperature is
the same, thus maintaining the same rheological characteristics of the
tests. There is currently no work available in the literature using the SRV EP procedure, oil in the two tests.
There
as it isisfairly
currently
new,no butwork
otheravailable
proceduresin the literature
with using the
SRV machines SRVbeen
have EP procedure,
used in the as it isto
past
fairly new, but
reproduce the other
FZG testprocedures with SRV
under different machines
contact have conditions
geometry been used [18].
in the past to repro-
duce the
TheFZG test under
procedures different
used in the contact geometry
past [19,20] conditions
consisted [18]. run-in and the setting
of a short
The procedures used in the past [19,20] consisted of
of a fixed load to simulate the 12th stage. The 12th stage is a referencea short run-in and the setting
value of
that must
abe
fixed load to simulate the 12th stage. The 12th stage is a reference
exceeded for special applications (such as gear oil for wind turbines) where higher value that must be
performance is required, but not all applications require this load capacity.
Machines 2022, 10, x FOR PEER REVIEW 7 of 11
Machines 2022, 10, x FOR PEER REVIEW 7 of 11

Machines 2022, 10, 621 exceeded for special applications (such as gear oil for wind turbines) where higher7 per- of 11
exceeded for special applications (such as gear oil for wind turbines) where higher per-
formance is required, but not all applications require this load capacity.
formance is required, but not all applications require this load capacity.
Figure 5 shows the simplest case (case 1) as follows: during the test, the presence of
Figure 5 shows the simplest case (case 1) as follows: during the test, the presence of
a clear CoF 5peak
Figure hasthe
shows been detected;
simplest casein(case
correspondence
1) as follows:toduring
this, the
thetested oil presence
test, the reaches the
of
a clear CoF peak has been detected; in correspondence to this, the tested oil reaches the
afailure load peak
clear CoF stage.has been detected; in correspondence to this, the tested oil reaches the
failure load stage.
failure load stage.
0.3 2000
0.3 2000

0.25
0.25
1500
1500
0.2
0.2

Load [N]
0.15 1000
CoF

Load [N]
0.15 1000
CoF

0.1
0.1
500
500
0.05
0.05

0 0
0 0 5 10 15 20 25 30 0
0 5 10 15 20 25 30
Time [min]
Time [min]

Figure 5.
Figure 5. Case
Case 1:
1: example.
example.
Figure 5. Case 1: example.
The results
The results of
of case
case 22 are
are shown
shown in
in Figure
Figure 6,
6, where
where in
in the
the last
last steps,
steps, we
we see
see an
an increase
increase
The results of case 2 are shown in Figure 6, where in the last steps, we see an increase
in the
in the CoF
CoF profile
profile without
without huge
huge peaks
peaks oror multiple
multiple small
small peaks
peaks or or any
any thickening
thickening of
of the
the
in the CoF profile without huge peaks or multiple small peaks or any thickening of the
curve; for this reason, we can conclude that the oil has passed all steps of the procedure.
curve; for this reason, we can conclude that the oil has passed all steps of the procedure.
curve; for this reason, we can conclude that the oil has passed all steps of the procedure.
0.18 2500
0.18 2500

0.16
0.16
2000
0.14 2000
0.14

0.12
0.12
1500
0.1 1500
0.1
Load [N]
CoF

Load [N]
CoF

0.08
0.08 1000
1000
0.06
0.06

0.04
0.04 500
500
0.02
0.02

0 0
0 0 10 20 30 40 50 60 0
0 10 20 30 40 50 60
Figure 6. Case 2: example.
Figure 6. Case 2: example.
FigureIn6.the
Case 2: example. it can be highlighted that the oil has an SRV failure load stage of
experiments,
>12, even
In theif experiments,
the stages of the SRV
it can beprocedure
highlightedarethat
17. the oil has an SRV failure load stage of
In
In the
fact,experiments,
in order to be it comparable
can be highlighted
to FZG,that
it isthe oil has
enough an itSRV
that failure
exceeds theload stageSRV
twelfth of
>12, even if the stages of the SRV procedure are 17.
>12,
stageeven if theifstages
(so that, the SRVof failure
the SRV procedure
load stage is are 17. than the fifteenth or reaches the end, it
greater
In fact, in order to be comparable to FZG, it is enough that it exceeds the twelfth SRV
wouldIn fact,
not bein so
order to be comparable to FZG, it is enough that it exceeds the twelfth SRV
relevant).
stage (so that, if the SRV failure load stage is greater than the fifteenth or reaches the end,
stage The
(so that,
resultsif the SRV 3failure
of case load stage
are shown is greater
in Figure thanathe
7, where lotfifteenth
of peaksorare
reaches the end,
attributable to
it would not be so relevant).
itmicroseizures
would not bephenomena
so relevant).in the final steps; after the first microseizure, the CoF curve
presents other peaks of higher intensity. The failure load stage is assigned at the stage
where a series of microseizures occurs and continues for at least a few seconds.
 The results of case 3 are shown in Figure 7, where a lot of peaks are attribu
microseizures phenomena in the final steps; after the first microseizure, the Co
The results
presents of case
other peaks of3higher
are shown in Figure
intensity. 7, where
The failure a lot
load of peaks
stage are attribut
is assigned at th
microseizures
where a series phenomena in theoccurs
of microseizures final and
steps; after thefor
continues first microseizure,
at least the CoF
a few seconds.
Machines 2022, 10, 621
presents other peaks of higher intensity. The failure load stage is assigned 8 of 11
at th
where0.3
a series of microseizures occurs and continues for at least 2000
a few seconds.

0.3 2000
0.25

1500
0.25
0.2
1500

0.2

Load [N]Load [N]

0.15 1000
CoF

0.15 1000
CoF

0.1

500
0.1
0.05
500

0.05
0 0
0 5 10 15 20 25 30
Time [min]
0 0
0 5 10 15 20 25 30
Figure 7. Case 3: example. Time [min]

Figure 7. Case 3: example.

FigureThe
7. Case 3: example.
results of case 4 are shown in Error! Reference source not found.Figure 8
a special and unique
The results of case 4 case can be
are shown in highlighted.
Figure 8, whereIna the middle
special of thecase
and unique test,canthe CoF
The results
be highlighted. Inofthe
case 4
middleare ofshown
the test,inthe
Error!
CoF Reference source
profile changes, not
becoming
changes, becoming flat and reaching high values as in the first steps. This behav found.Figure
flat and 8,
aoccurred
special and
reaching
quiteunique
high
exactlycase
values as in the can
first
in all thebetest
steps. highlighted.
This
repetitions.In the middle of the test, the CoF
behavior has occurred quite exactly in all the
test repetitions.
changes, becoming flat and reaching high values as in the first steps. This behav
occurred quite exactly in all the test repetitions.
0.18 2500

0.16
0.18 2500

2000
0.14
0.16

2000
0.12
0.14

1500
0.1
0.12
Load [N]Load [N]
CoF

1500
0.08
0.1
1000
CoF

0.06
0.08
1000

0.04
0.06
500

0.02
0.04
500

0.02 0 0
0 5 10 15 20 25 30 35 40 45
Time [min]
0 0
0 5 10 15 20 25 30 35 40 45
Figure 8. Case 4: example.
Figure 8. Case 4: example. Time [min]

The results of the SRV test are listed in Table 3 as an average value between the
Figure 8. Case
The
repetitions 4: example.
results of out
we carried thefor
SRVeachtest
oil. are
The listed in are
FZG data Table
also3listed
as anin average value
order to have between t
a clear
etitions we carried out for each oil. The FZG data are also listed in order to have
comparison among the results of both machines.
The results
comparison of the
among theSRV test of
results areboth
listed in Table 3 as an average value between t
machines.
etitions we carried out for each oil. The FZG data are also listed in order to have
comparison among the
Table 3. Experimental results
results: of bothand
laboratory machines.
rig tests.

Table 3. Experimental results: laboratory and rig tests.

Machines 2022, 10, 621 9 of 11

Table 3. Experimental results: laboratory and rig tests.

Oil Oil ISO SRV FZG

Name Type VG Failure Stage Failure Stage
GT15 Gas turbine 15 11 12
GT32 Gas turbine 32 11 12
GT46 Gas turbine 46 12 12
TO46 Turbine 46 8 9
GO320 Gear 320 >12 >12
GO460 Gear 460 >12 >12
CO68A Circulating 68 >12 >12
CO68B Circulating 68 N.D. 12
CO68C Circulating 68 >12 N.A.
CO68D Circulating 68 >12 >12

For only one of the tested products, no FZG data are available.
Only for one oil, the SRV failure load stage (it belongs to case 4) is not assumed.
For all of the other lubricants, a difference of only ± 1 stage in the values of the failure
load stage obtained by the two machines has been noticed.
Based on this, a good correlation between SRV and FZG tests has been found, appro-
priately distinguished in the cases previously identified (the oils tested mainly belong to
cases 1 and 2).
For lubricants belonging to case 1, characterized by the presence of a net maximum in
the CoF profile due to seizure, it is possible to assume a good correlation between SRV and
FZG failure load stages, with a tolerance of ± 1 stage.
For oils belonging to case 2, characterized by a linear, thin and descending CoF profile
without any detectable seizure or microseizures, it is possible to assume that lubricants that
overcome the twelfth SRV stage can reach or overcomes also the twelfth FZG stage.
For products classified in case 3, a failure load stage has been assigned for the criteria
previously explained in chapter 2 but must be defined with a more precise tolerance in
order to assume a correlation with the FZG test; in addition, it would be very useful to
collect more data and also add further evidence derived from other analysis (wear detection
on the specimen, for example).
For the lubricating oil classified in case 4 (the 8th oil reported in Table 3) is very difficult
to assign the failure load stage as it presents a unique and anomalous trend. Moreover, in
this case, data gained by other types of analysis could help us in the assessment.
In addition, a great contribution to the global evaluation of these tests can derive
from the knowledge of lubricant formulation in order to predict, estimate or determine the
synergistic/antagonistic effect of oil components.

5. Conclusions
In this paper, the results of an experimental campaign are presented based on the use
of a well-selected SRV procedure as a screening of the FZG rig test, leading to concrete
benefits such as saving time and operative costs.
Four cases for the determination of the failure load stage of SRV have been defined
and, in many cases, a good correlation with the FZG rig test has been found. A very good
repeatability of this method has been proven too.
The tests conducted using SRV compared to the FZG scuffing method are as follows:
• Time saving (test time duration is 76% less without mentioning visual inspection and
preparation for the following step);
As an example of time saving, just consider that each step of the FZG procedure takes
at least 15 min without mentioning the operator’s visual inspection and the preparation for
the following step, while the SRV takes only 217 s per step.
• Cost/Material saving (smaller specimens and less oil sample);
Machines 2022, 10, 621 10 of 11

• Less operator independent (automated machine and no visual inspection needed);

• Create a ranking (With the same failure stage we can rank the wear of the candi-
date’s product).
This method is proposed as a screening tool for the FZG test, thus reducing the use of
the FZG rig, which is very costly (the gears are unusable after each test and several liters of
lubricant are needed) and time-consuming.
The obtained results are useful preliminary indications of the behavior of the lubricant,
thus helping a formulator in the development of lubricating oil. In the case of the develop-
ment of a new lubricant, where the performances of six or more different candidates are
compared, only the two best performers will run the FZG scuffing test (still required to
qualify a new lubricant).
Finally, from the work carried out, the importance of optimizing test methods, such as
tribological ones, really emerges. These can lead to numerous advantages from the point of
view of product development and final machine performance, being an extremely useful
tool for the constant improvement of the energy efficiency of modern industrial systems.

Author Contributions: D.M. performed the theoretical analysis, the computation and the experimen-
tal tests, wrote the draft and revised the manuscript, M.L. wrote the draft and revised the manuscript.
S.C. and P.P. checked the logic described/the validity of the theory in the draft and the final version
of the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The present work was undertaken under the support of the Italian Ministry
for Education, University and Research by means of the project Department of Excellence LIS4.0
(Integrated Laboratory for Lightweight e Smart Structures).
Conflicts of Interest: The authors declare no conflict of interest.

References
1. Chatterton, S.; Pennacchi, P.; Vania, A.; de Luca, A.; Dang, P.V. Tribo-design of lubricants for power loss reduction in the oil-film
bearings of a process industry machine: Modelling and experimental tests. Tribol. Int. 2019, 130, 133–145. [CrossRef]
2. Massocchi, D.; Lattuada, M.; Assanelli, G.; Pennacchi, P.E.L.M.; Chatterton, S. Nuove metodologie di testing a supporto dello
sviluppo di oli lubrificanti “energy-saving” per applicazioni industriali. In Proceedings of the 7◦ Workshop AIT “Tribologia e
Industria”, Pisa, Italy, 4 June 2020.
3. Lattuada, M.; Manni, M. A new methodology for the experimental evaluation of organic antifriction additives. In Proceedings of
the ISFL, International Symposium on Fuel and Lubricants, New Delhi, India, 18–20 April 2016.
4. Baumann, C.; Patzer, G. Screening Engine Oils Using Piston Parts on a Translatory Oscillation Tribometer (SRV® ). Korean
Tribology Society Conference. In Proceedings of the 65th Spring Conference of the Korean Lubrication Society in 2018, April 2018;
pp. 75–76.
5. Grün, T.A.M.; Stoschka, F. Optimization of disc geometry and hardness distribution for better transferability of fatigue life
prediction from disc to FZG tests. Wear 2022, 498, 204329.
6. Concli, F.; Gorla, C. Numerical modeling of the power losses in geared transmissions: Windage, churning and cavitation
simulations with a new integrated approach that drastically reduces the computational effort. Tribol. Int. 2016, 103, 58–68.
[CrossRef]
7. Sagraloff, N.; Dobler, A.; Tobie, T.; Stahl, K.; Ostrowski, J. Development of an oil free water-based lubricant for gear applications.
Lubricants 2019, 7, 33. [CrossRef]
8. Lehtovaara, R.; Bayat, A. Scuffing evaluation of fully formulated environmentally acceptable lubricant using barrel-on-disc
technique. Tribol. Int. 2021, 160, 107002.
9. Hoehn, B.-R.; Michaelis, K.; Oster, P. New Test Methods for the Evaluation of Wear, Scuffing and Pitting Capacity of Gear
Lubricants. AGMA 1998.
10. Zang, L.; Chen, Y.; Wu, Y.; Liu, H.; Ran, L.; Zheng, Y.; Liu, Y. Tribological performance of Mn3 (PO4) 2 coating and PC/MoS2
coating in Rolling–Sliding and pure sliding contacts with gear oil. Tribol. Int. 2021, 153, 106642. [CrossRef]
Machines 2022, 10, 621 11 of 11

11. Patzer, G.; Woydt, M. New Methodologies Indicating Adhesive Wear in Load Step Tests on the Translatory Oscillation Tribometer.
Lubricants 2021, 9, 101. [CrossRef]
12. Balarini, R.; Diniz, G.A.S.; Profito, F.J.; Souza, R.M.D. Comparison of unidirectional and reciprocating tribometers in tests with
MoDTC-containing oils under boundary lubrication. Tribol. Int. 2020, 149, 105686. [CrossRef]
13. Patzer, G.; Shah, R.; Schneider, A.; Iaccarino, P. “New approach to interpreting seizure tests on the translatory oscillation
tribometer (SRV). Lubricants 2019, 7, 93. [CrossRef]
14. ASTMD7421−19; Standard Test Method for Determining Extreme Pressure Properties of Lubricating Oils Using High-Frequency,
Linear-Oscillation (SRV) Test Machine. ASTM: West Conshohocken, PA, USA, 2019.
15. Chen, T.; Zhu, C.; Liu, H.; Wei, P.; Zhu, J.; Xu, Y. Simulation and experiment of carburized gear scuffing under oil jet lubrication.
Eng. Fail. Anal. 2022, 139, 106406. [CrossRef]
16. Patzer, G.; Ebrecht, J. Vorstellung eines Pruefkonzepts als Screeningmethode für Getriebeoele auf dem translatorischen Oszilla-
tionstribometer (SRV® ). Tribol. Schmier. 2016, 63, 58.
17. ISO 14635-1; Gears—FZG Test Procedures—Part 1: FZG Test Method A/8,3/90 for Relative Scuffing Load-Carrying Capacity of
Oils. ISO: Geneva, Switzerland, 2000.
18. Benadé, H.; de Vaal, P. Effect of running-in conditions on repeatability of friction and wear testing results. STLE, Atlanta, USA.
21–25 May 2017.
19. Van de Velde, F.; Willen, P.; de Baets, P.; van Geeteruyen, C. Substitution of Inexpensive Bench Tests for the FZG Scuffing
Test—Part II: Oil Tests©. Tribol. Trans. 1999, 42, 71–75. [CrossRef]
20. Van de Velde, F.; Willen, P.; de Baets, P.; van Geetruyen, C. Substitution of inexpensive bench tests for the FZG scuffing test—Part
I: Calculations. Tribol. Trans. 1999, 42, 63–70. [CrossRef]