Forschung im Ingenieurwesen (2023) 87:541–554

https://doi.org/10.1007/s10010-023-00669-4

ORIGINALARBEITEN/ORIGINALS

Influence of the steel plate on the friction behavior of automotive wet

disk clutches
P. Strobl1 · T. Schneider1 · K. Voelkel1 · K. Stahl1

Received: 31 January 2023 / Accepted: 27 April 2023 / Published online: 30 May 2023
© The Author(s) 2023

Abstract
Friction behavior is the key parameter for the design of automotive wet disk clutches. On the one hand, the Coefficient of
Friction (CoF) level should be high to transmit torque efficiently. On the other hand, the clutch requires a positive slope
of the CoF over sliding speed for good controllability, high comfort, and a low tendency to shudder. Clutches used in
automatic or dual-clutch transmissions mostly use organic friction lining. These friction systems tend to have low CoF at
low sliding speeds due to their high requirements regarding shifting comfort. Nevertheless, they show high values of CoF
at high sliding speeds.
This study investigates the influence of different steel plate finishes on friction behavior in different application-relevant
operation modes such as brake shift, unsteady slip, and micro slip. Each of these operation modes requires an accurate
CoF measurement at different sliding speed ranges. Therefore, we use different test rig setups. We characterize the steel
plates by their areal surface topography measured with an optical system using focus variation. We discuss differences in
the friction behavior of the corresponding tribological systems at different operating conditions.
Results show an influence of the steel plate surface finish on the CoF level. Therefore, the surface finish of the steel plate
influences the functional behavior of wet disk clutches and engineers should consider the surface finish in the early design
phase. We discuss the correlation between commonly used surface parameters and friction parameters.

Einfluss der Stahllamellenendbearbeitung auf das Reibungsverhalten von nasslaufenden

Lamellenkupplungen in Kraftfahrzeugen

Zusammenfassung
Das Reibungsverhalten ist der entscheidende Parameter für die Entwicklung nasslaufender Lamellenkupplungen. Dabei
soll die Reibungszahl zum einen möglichst hoch sein, um Drehmoment zuverlässig zu übertragen, zum anderen benötigt
die Kupplung für eine gute Regelbarkeit, hohen Komfort und eine geringe Reibschwingneigung einen positiven Gradienten
der Reibungszahl über der Gleitgeschwindigkeit. In Automat- und Doppelkupplungsgetrieben werden meist Kupplungen
mit organischen Reibbelägen eingesetzt. Diese Reibsysteme neigen aufgrund ihrer hohen Anforderungen bzgl. Schalt-
komfort zu sehr niedrigen Reibungszahlen bei kleinsten Gleitgeschwindigkeiten Dennoch weisen sie in der Regel hohe
Reibungszahlwert bei hohen Gleitgeschwindigkeiten auf.

 P. Strobl K. Stahl
patrick.strobl@tum.de karsten.stahl@tum.de
T. Schneider 1
thomas.schneider@tum.de School of Engineering and Design, Department of Mechanical
Engineering, Gear Research Center (FZG), Technical
K. Voelkel University of Munich, Boltzmannstraße 15, 85748 Garching
katharina.voelkel@tum.de near Munich, Germany

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Diese Publikation beschäftigt sich mit dem Einfluss von gängigen Stahllamellenendbearbeitungen auf das Reibungsverhal-
ten in unterschiedlichen Betriebsmodi wie Bremsschaltungen, instationären Schlupf und Mikroschlupf. Diese Betriebsmodi
erfordern jeweils eine hochgenaue Messung der Reibungszahl in den unterschiedlichen Gleitgeschwindigkeitsbereichen,
weshalb unterschiedliche Prüfumgebungen genutzt werden. Die Stahllamellen werden anhand ihrer flächenhaften Ober-
flächentopografie charakterisiert. Diese wird mit einem optischen Messsystem mittels des Prinzips der Fokusvariation
bestimmt. Es werden Unterschiede im Reibungsverhalten der entsprechenden Reibsysteme bei diversen Betriebsbedingun-
gen diskutiert.
Die Ergebnisse weisen auf einen Einfluss der Stahllamellenendbearbeitung auf das Reibungszahlniveau hin. Aus diesem
Grund muss die Endbearbeitung der Stahllamellen und dessen Einfluss auf das Funktionsverhalten in der frühen Ent-
wicklungsphase nasslaufender Kupplungssysteme berücksichtigt werden. Es wird der Zusammenhang zwischen üblichen
Oberflächen- und Reibungszahlkennwerten untersucht.

1 Introduction Lloyd et al. [22] discuss the static and dynamic friction
behavior. They show differences between different mea-
Wet disk clutches and brakes provide differential speed surement methods (steady-state, break-away, and endpoint
shifting. In automotive drivelines, they are key components friction) for measuring static friction torque and varying the
of dual-clutch and automatic transmissions, as well as of clutch temperature due to different dwell times. Therefore,
limited slip differentials and other applications. Here, the the measurement method itself shows an influence on the
focus is on the friction behavior of wet disk clutches used measured friction behavior. Besides the operation mode,
in dual-clutch and automatic transmissions. In these ap- even a variation of the time of standstill after shifting can
plications, high comfort and fast shifting are the main re- also affect the friction behavior of wet disk clutches [23].
quirements for the machine element. Therefore, the clutch Therefore, methods for good characterization of the fric-
must guarantee a high Coefficient of Friction (CoF) with tion behavior are needed. Meingassner [13], Meingassner
a positive slope of the CoF over sliding speed to prevent et al. [14, 24] and Voelkel et al. [12] develop a method-
self-excited vibrations, also known as shudder. This results ology to investigate the friction coefficient behavior from
in low CoF at low sliding speeds and, therefore, a low very low to comparatively high sliding speeds and shows
static torque capacity. In dual-clutch and automatic trans- a good transferability of the results between each other.
missions, organic friction materials are often used in com- Fluctuations of the frictional behavior due to tolerances
bination with lubricants with appropriate additives to meet are often described in the literature (e.g., [13, 25, 26]).
these conditions. These must be considered in the evaluation methods, es-
Thus, it is only possible to determine the friction behav- pecially at low sliding speeds. Their influence could be
ior of a friction system by experimental investigation due to overlooked due to low sliding paths.
the high number of influencing parameters and their com- The lubricant crucially influences the friction behavior
plex interactions. Therefore, there is ongoing research on of wet disk clutches. Previous investigations show, for ex-
these influences and their interactions. ample, influences of base oil [9], additives [3, 4, 8, 27], or
In this context, mechanical and thermal loads, as well as oil aging [28] on the friction behavior.
the operation mode, can have a substantial impact on the Besides those influences, the used friction material also
frictional behavior. As mentioned above, the CoF should has a strong influence on the friction behavior. Ito et al.
have a positive slope over sliding speed. This results in [1] state that the porosity of the organic friction materials
a strong influence of the sliding speed on friction behavior affects the oil film pressure. However, the Young’s mod-
(e.g., [1–7]). Besides this, the specific surface pressure p ulus of the friction material may also have an impact on
may also affect the friction behavior (e.g., [5, 6]). Many friction behavior, according to Katsukawa [29]. Even the
investigations mention the strong influence of the clutch microstructure of paper-based friction material can affect
temperature on the frictional properties (e.g., [8–14]). To the frictional properties of a friction system [30].
measure this influence, the oil injection temperature is of- In addition to the friction lining and the lubricant, the
ten varied (e.g., [12, 14]) because the clutch temperature influence of the steel friction surface on the friction be-
itself is hard to control directly during the run. Due to its havior of wet disk clutches is investigated in the literature.
importance, there are several ways to determine the temper- Voelkel [31] and Voelkel et al. [32] investigate the influ-
atures of the clutch using simulations (e.g., [15–18]). For ence of different surface finishes on the friction behavior
this reason, there are empirical-based models (e.g., [19–21]) and its change during the run-in. Besides the influence of
to describe the influence of the mechanical and thermal a mechanical surface finish, the nitriding of the steel plates
loads on the friction behavior of wet disk clutches. can also improve the durability and friction behavior of the

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clutch [33]. Furthermore, the manufacturing parameters of

the surface finish itself can also affect the friction behavior.
Investigations of steel plates finished with a face traverse
grinding process show this influence with different lubri-
cants [2, 7].
The roughness of both, friction lining and steel plate,
influences the friction behavior of the wet disk clutches.
Therefore, a suitable characterization of the surface topog-
raphy is needed. Many researchers [7, 33] characterize the
surface topography of friction surfaces using the profile Fig. 1 Friction plate (a) and steel plate (b)
method according to DIN EN ISO 21920-2 [34]. In recent
years, the application of three-dimensional or areal mea- to Eq. 2. The nominal friction area A does not consider the
surement of friction surfaces has become more common groove pattern of the friction lining. The groove pattern is
(e.g., [31, 32, 35, 36]). The areal characterization of rough group-parallel multisegmented with a waffle imprint. The
surfaces is standardized according to DIN EN ISO 25178-2 nominal clearance between each friction surface is set to be
[37]. Some investigations use areal parameters for the char- 0.2 mm.
acterization of steel and friction lining plates (e.g. [31, 35, The main goal of this study is the investigation of the
36]). influence of application-relevant surface finishes on the fric-
In summary, there is a gap in the research on the influ- tion behavior of wet disk clutches. For this reason, the inves-
ence of steel plate finish in connection with standard areal tigations consider three typical surface finishes. As a basis
surface parameters on the frictional behavior at low sliding variant, we use steel plates with a polished and acid-etched
speeds. In this publication, we therefore present the results finish from the serial application of a dual-clutch trans-
of investigations with wet disk clutches with organic friction mission (variant a). As variant b, a manufacturer of steel
lining in combination with three near-application finishes plates reworked steel plates of variant a with belt ground
of steel plates. We characterize the surface topography of finish. Variant c is additionally brushed compared to vari-
the steel plate with areal surface parameters determined by ant b. Images made with the Alicona Infinite Focus G4
the principle of focus variation. Then, the friction behavior (see Fig. 7) in Fig. 2 show these surface finish variants
of the friction systems with different steel plate finishings with 50 × magnification. According to the manufacturer, the
in three application-relevant operation modes (brake shifts, hardness of the steel plates is between 280 and 325 HV10.
forced unsteady slip, and micro slip) is discussed. After For the experiments, we use an application-relevant lu-
that, we investigate linear relations between CoF and sur- bricant. The lubricant is a polyalphaolefin used in dual-
face topography parameters. The results are discussed, and clutch transmissions (DCT). Table 2 shows the physical
key statements are summarized. properties of the lubricant available due to its safety data
sheet.
Test rig KLP-260 (see Fig. 3) is a component test rig
2 Method enabling the measurement of the dynamic friction behavior
of whole clutches in different operation modes. A detailed
We investigate the friction behavior of a distinguished com- explanation of the test rig, its technical data, and operation
bination of friction lining and lubricant with different steel modes can be found in literature [38].
plate finishes in various operation modes using two com- The outer carrier is connected to a load cell that pro-
ponent test rigs. Additionally, we measure the surface to- vides the measurement of the friction torque Tf. Due to this
pography of the steel plates after the run-in. First, the tested design, the test rig operates in brake mode with a fixed
parts are introduced. Fig. 1 shows an internally toothed fric- outer carrier. The test rig allows several application rel-
tion plate with organic friction lining from a dual-clutch evant operation modes for automotive clutches. In brake
application and an externally toothed steel plate. For the shift operation, the main drive accelerates the inner shaft
investigations, each clutch consists of four steel plates and
three friction plates leading to a total number of six friction
Table 1 Technical details of the plates
surfaces (z = 6). The plates are mounted alternately in the
Parameter Unit Value
corresponding carriers.
Table 1 gives an overview of the relevant technical details Outer friction diameter da mm 168.0
of the tested parts. The mean radius rm and the nominal Inner friction diameter di mm 141.0
friction area A are used to calculate the CoF according to Mean radius rm mm 77.3
Eq. 1 and the utilized friction coefficient (UFC) according Nominal friction area A mm2 6552.6

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Fig. 2 Visual comparison of the Polished, acid-etched finish (a) + belt ground finish (b) + brushed finish
investigated variants of steel
plate surfaces before the run-in

100 µm

Variant (a) Variant (b) Variant (c)

Table 2 Physical properties of the lubricant

expanded relative measurement uncertainty for the CoF is
Parameter Unit Value
between 1.4 and 2.0% (confidence level 95%).
Density @ 20 °C kg/m3 842 The clutch is lubricated half from the top and half
Kin. Viscosity @ 40 °C mm2/s 24.8 from the inside with an overall specific oil flow rate of
Kin. Viscosity @ 100 °C mm2/s – 0.8 mm3  (mm2  s)–1 with respect to the nominal friction
area A.
over a belt drive. When reaching a given speed, the main In the publication, the measured course of the CoF ac-
drive is disconnected from the inner shaft. After that, the cording to Eq. 1 over the sliding speed vg of the penultimate
axial force Fax is applied by means of a hydraulic piston brake shift is evaluated and depicted.
with integrated force and displacement measurement. The
differential speed is reduced to standstill of the clutch. An- Tf Tf
CoF = = (1)
other mode used for the investigations is forced unsteady Fax  rm  z p  A  rm  z
operation. Here, the axial force Fax is applied first. After In contrast to brake shift operation, the investigation of
that, the inner shaft is cyclically accelerated and deacceler- very low sliding speeds needs special treatment in eval-
ated five times (five slip phases) by a creep drive. Beside uating the friction behavior. To minimize local effects at
these modes, the test rig enables also other operation modes very low sliding speeds such as slightly slanted contact and
for the investigation of stationary slip and combinations of thickness deviations of the plates, the course of the CoF
the shown modes in combination with different lubrication over sliding speed is evaluated using its sliding average.
situations. In addition to the investigation of the friction These local effects have higher relevance in low speed slip
behavior it also enables the investigation of the life time, due to the low number or even percentage of revolutions
damage and shudder behavior. The uncertainty of measure- during an evaluated slip phase. Therefore, the CoF in the
ment for the test rig is discussed in detail in previous pub- deaccelerating part of the third and fourth slip phase in
lications [13, 39, 40] and is dependent on the experimental cycles six to ten over slip speed is used to obtain a Char-
dataset (e.g., axial force). For the shown measurements the acteristic Friction Curve (CFC). With this data, the sliding

Fig. 3 Schematic sketch of the

component test rig KLP-260
according to Meingassner [38]

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Fig. 4 µtop for two types of friction characteristics with equivalent val-
ues of µtop according to Strobl et al. [21]

Fig. 6 Measured data, curve fit and SFC—exemplarily with measure-

average over slip speed is generated, which allows a sta-
ments with steel plate variant (a) [ϑoil = 110 °C; p = 1.0 N  mm–2]
ble comparison of different courses of the CoF over sliding
speed. Evaluations only show small deviations between the
individual curves. These are considered in the discussion of rm, the number of friction surfaces z, and the axial force Fax.
the influences in addition to the measurement uncertainty. In this setup, the friction torque can also be calculated by
For parameter-based evaluations, the characteristic value the weight m, the gravitational acceleration g, the length l,
µtop according to Voelkel [31], is used to evaluate the CoF. and the current angle ε of the lever. The axial force can be
Therefore, the CoF at maximum slip speed is determined. written as the specific surface pressure p times the nominal
Fig. 4 shows the evaluation of µtop at different curve types. friction area A. The uncertainty of measurement for the test
Nevertheless, the friction behavior of the systems discussed rig is discussed in detail in previous publications [13, 39]
here is continuously increasing (curve type 1). Here, µtop is and is dependent on the experimental dataset (e.g., axial
evaluated in the last slip phase of cycles six to ten. force). For the shown measurements the expanded relative
On the other hand, component test rig LK-3 (Fig. 5) en- measurement uncertainty for the UFC is between 1.4 and
ables the measurement of the friction behavior at very low 2.3% (confidence level 95%) [39].
sliding speeds at the transition from static to dynamic fric-
tion. It can be used for the investigation of the creep behav- m  g  l  cos.©/
UFC = (2)
ior of wet disk clutches. As in KLP-260, the clutch is tested p  A  rm  z
in brake mode with a fixed outer carrier. First, the clutch The evaluation of every single measurement leads to set
is loaded with a defined axial force Fax via a screw. Then, of creep rate and corresponding UFC value. The measure-
torque is applied to the inner drive via a lever and weights. ments are repeated until no relevant creep rate can be ob-
With this setup, different values of UFC are set, and the served within 10 to 15 min. At this point, the Static Friction
rotation of the lever is measured using a high precision Coefficient (SFC) is obtained.
 With
 all measurements show-
incremental angle encoder and is recalculated to a sliding ing creep a curve of type f vg = a  xb + c with (0 < b < 1)
speed. is fitted according to Voelkel et al. [12] to illustrate the
UFC is defined according to Voelkel et al. [12]. Here, the creep behavior. Fig. 6 exemplarily shows data of the mea-
friction torque Tf is divided by the mean radius of the clutch surements and its corresponding curve fit as well as the

Fig. 5 Component test rig LK-3

according to Meingassner et al.
[24]

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In addition to the friction behavior, we measure the areal

surface topography with the device Alicona Infinite Focus
G4 shown in Fig. 7. This device works with the principle of
focus variation, according to DIN EN ISO 25178-606 [41].
For the investigations, an objective with 50 × magnification
is used to resolve all necessary topography details. Table 3
provides the measuring settings for the determination of
the areal roughness. The surface roughness is determined
at eight evenly spaced circumferential positions for each
side of one of the middle steel plates leading to a to-
tal of sixteen measurements representing the steel friction
surface for each clutch. The measurement field is cut to
a square of 0.75 × 0.75 mm for each measuring position.
This size guarantees a fast and reliable measurement of the
surface topography with the usage of the objective with
Fig. 7 Alicona Infinite Focus G4 [43]
50 × magnification. To calculate roughness parameters in
the mesurement field, the shape is subtracted by alignment
Table 3 Settings for surface measurements
to a reference plane, and the waviness is subtracted by LC
Parameter Setting filtering with a Gaussian filter. The filter is set to be 250 µm
Vertical resolution 50 nm according to recommendations from profile measurements
Lateral resolution 2 µm for values of Ra = 0.02 – 0.1 in DIN EN ISO 4288 [42]. In
Measuring field 0.75 mm × 0.75 mm pre-investigations, the resulting values Ra of the steel sur-
L-filter 250 µm faces showed slightly higher values than 0.1. Taking into
account the size of the measurement field and the resulting
Table 4 Description of surface parameters according to [37] waviness, we decided to use the 250 µm filter. The mea-
Parameter Description surements presented guarantee a good comparison of the
Sa Arithmetical mean height topography of different steel plate surfaces.
Sq Root mean square height
The roughness is determined in terms of standard areal
Sp Maximum peak height
surface parameters calculated according to DIN EN ISO
Sv Maximum pit depth 25178-2 [37]. Most of these areal parameters are derived
Sz Maximum height from corresponding parameters of profile measurement, like
S10z Ten-point-height arithmetical mean height Sa from Ra or core height Sk from
Ssk Skewness
Rk. In this paper, we investigate linear correlations between
Sku Kurtosis the friction behavior and 15 surface parameters. Table 4
Sdq Root mean square gradient gives an overview over the parameters. Further information
Sdr Developed interfacial area ratio on the definition of each of these parameters can be found
Sk Core height
in DIN EN ISO 25178-2 [37].
Spk Reduced peak height
After explanation of the measurement setup, the follow-
Svk Reduced pit depth ing section describes the test procedure, including the test
Smr1 Material ratio of the hills sequence, defined loads, and test conditions. Fig. 8 gives an
Smr2 Material ratio of the dales overview of the test procedure. Variant (b) and (c) are tested
with two clutches to evaluate the reproducibility of the tests.
Due to a limited number of steel plates, variant (a) is only
determined SFC. Later, the curve fit is shown only at the tested once. Nevertheless, clutches from serial production
transition of static and dynamic friction (see zoom). There tend to show good reproducibility.
is a detailed description of the methodology used to study The clutches follow a run-in in brake shift operation
wet disc clutches in creep behavior in literature [12–14]. to guarantee stable friction behavior in subsequent friction
tests, according to Acuner et al. [44]. The load stages shown

Fig. 8 Test procedure

Measurement of
Measurement of Measurement of
Run-in surface Reconditioning
friction behavior creep behavior
topography

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Table 5 Load stages in run-in / reconditioning according to Acuner cooling phase of 10 s starts, in which the inner shaft is
et al. [44] accelerated up to the initial speed of the next brake shift.
Name p / N  mm–2 Vg,max / m  s–1 Cycles After the run-in, the clutches are removed from the test
RI1 0.5 6 100 rig to determine the topography of the surface at eight
RI2 0.5 12 100 evenly distributed measuring positions along the mean fric-
RI3 1.0 12 200 tion radius according to the previously described measuring
RI4 1.0 18 200 settings. For this reason, the friction surfaces are cleaned
with a solvent to avoid any possible influence of lubricant
Table 6 Load stages in brake shift operation artifacts on the optical surface measurement. Before resum-
Name p / N  mm–2 Vg,max / m  s–1 Cycles
ing the investigation of the friction behavior, the clutches
are reconditioned by thirty cycles of the run-in load stage
BS1 0.5 10 10
RI4 at an oil injection temperature of ϑoil = 80 °C.
BS2 0.75 10 10
The investigations on the friction behavior are carried
BS3 1.0 10 10
out in three blocks of different oil injection temperatures
ϑoil (80–110–40 °C). Within each block, the friction sys-
Table 7 Load stages in unsteady slip operation tems are first tested in brake shift operation and after that,
Name p / N  mm–2 Vg,max / m  s–1 Cycles in unsteady slip mode. In the first step, ten brake shifts
US1 0.5 0.005 10 are performed in groups of ascending specific surface pres-
US2 0.75 0.005 10 sure p, see Table 6. Here, the cycle time is increased to
US3 1.0 0.005 10 30 s compared to the run-in and conditioning to prevent the
US4 0.5 0.01 10 clutch from influences of the energy input of prior brake
US5 0.75 0.01 10 shifts.
US6 1.0 0.01 10 In unsteady slip operation, ten cycles are performed with
US7 0.5 0.025 10 a total of eighteen combinations of sliding speed vg and
US8 0.75 0.025 10 specific surface pressure p shown in Table 7. The load stages
US9 1.0 0.025 10 are performed with nominally increasing friction work to
US10 0.5 0.05 10 provide an appropriate prior load and are repeated in blocks
US11 0.75 0.05 10 of US1 to US18. The unsteady slip is determined based on
US12 1.0 0.05 10 five slip phases in each sequence, in which the clutch is
US13 0.5 0.1 10 accelerated in the closed state to a maximum differential
US14 0.75 0.1 10 speed within one second and then decelerated within one
US15 1.0 0.1 10 second again.
US16 0.5 0.2 10 Due to restraints in the stable automated evaluation with
US17 0.75 0.2 10 very low sliding speeds, load stages US1–US3 were not
US18 1.0 0.2 10 considered in the following evaluations. The statements
made later are not affected by these results.
After completion of the investigations at the KLP-260,
in Table 5 are successively run through at an oil injection the tested parts are installed in the LK-3 test rig and exam-
temperature of ϑoil = 80 °C and a connected inertia of around ined for their frictional behavior in micro slip and static fric-
1.1 kg  m2. The cycle time describing the time for the brake tion. Before each test, the clutches are subjected to an axial
shift itself and the following cooling phase is set to be 15 s. force of 1000 N and forced to slide several times within an
In this period, the clutch is closed for 5 s before the open angular range of at least +/– 15°. This guarantees a compa-

Fig. 9 Surface topography of Polished, acid etched finish (a) + belt ground finish (b) + brushed finish height
the steel plates after the run-in 2

-2

-4

-6
Variant (a) Variant (b) Variant (c)

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rable preconditioning of the friction surfaces directly before

the test and helps to reduce the influence of the preload. Due
to high test runtimes, micro slip was investigated at two oil
injection temperatures ϑoil (40 °C; 110 °C) and two specific
surface pressures p (0.5 N  mm–2; 1.0 N  mm–2).

3 Results

3.1 Surface topography after run-in

Fig. 10 Box-whisker plot of the arithmetic mean roughness Sa for each
clutch
Fig. 9 shows the surface topography of three exemplary
sections of the investigated finishes by means of the height
information after the run-in. The direction of sliding is
horizontal in the image section. In the height information,
smoothing in the direction of sliding is only slightly visible
in variants (a) and (c) but not in variant (b).
Variant (a) shows a directional basic grinding struc-
ture overlaid by clustered elevation and depth ranges.
Variant (b) only shows a directional grinding structure.
Variant (c) shows a directional grinding structure overlaid
by unidirectional patterns resulting from the additionally
Fig. 11 Box-whisker plot of the core roughness Sk for each clutch brushed finish. Due to the uniform choice of height scaling
for all three measurements, it can already be visually rec-
ognized from the color scale that the roughness of the test

Fig. 12 Comparison of the fric-

tion behavior at brake shift
operation of clutches with dif-
ferent steel plates at different
operating conditions

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Fig. 13 Comparison of the fric-

tion behavior at unsteady slip
operation of clutches with dif-
ferent steel plates at different
operating conditions

parts decreases with each machining step. After running- Compared to these influences, the influence of the steel
in, the characteristic surface structures of all variants were plate variant is to be considered minor. Nevertheless, it is
retained. Structures in the sliding direction (horizontal) can noticeable and particularly evident in the comparison of
be seen in a few areas in variant (a) and in many areas in variants (a) and (c). Variant (b) is difficult to classify be-
variant (c) but not in variant (b). tween the two alternative steel plates. Particularly at low
This visual impression can also be quantified by areal sliding speeds < 2 m s–1, variant (c) shows a lower CoF
topography parameters. For example, the mean arithmetic than variant (a).
roughness Sa and the core height Sk are given in Fig. 10
and and Fig. 11 for the tested clutches of different steel 3.3 Friction behavior in unsteady slip
plate variants.
The influence at low sliding speeds can be observed in de-
3.2 Friction behavior in brake shifts tail in the operation mode of unsteady slip. The friction be-
havior is discussed using the Characteristic Friction Curves
First, the friction behavior in brake shifts is discussed. (cfc) at four specific operating conditions with the maxi-
Fig. 12 shows the friction behavior in four parameter combi- mum tested sliding speed (vg,max = 0.2 m  s–1) in Fig. 13.
nations of oil injection temperature ϑoil (top: 40 °C, bottom: All CFC show a positive slope over the sliding speed in
110 °C) and specific surface pressure p (left: 0.5 N  mm–2, unsteady slip operation. A higher specific surface pressure
right: 1.0 N  mm–2) at brake shifts. p and a higher oil injection temperature ϑoil both lead to
All brake shifts show a positive slope over the sliding a reduction in the transmittable torque.
speed. The high increase at the beginning of the brake shift An influence of the steel plate variant on the friction
(high sliding speeds) is caused by the axial force that builds behavior cannot be observed at an oil injection tempera-
up during engagement and is of no further interest. In gen- ture of ϑoil = 40 °C. Thus, at the oil injection temperature of
eral, there is a significant influence of the oil injection tem- ϑoil = 110 °C, a reduction of the level of CoF can be seen
perature and the specific surface pressure on the level and at all specific surface pressures p. Here, variant (a) shows
course of CoF. a higher level of CoF compared to variants (b) and (c).

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Fig. 14 Comparison of the creep

behavior of clutches with dif-
ferent steel plates at different
operating conditions

Overall, variant (c) shows the lowest level of CoF, but the Fig. 14 shows the fitted curves of the creep behavior at slid-
differences between variant (b) and variant (c) under con- ing speeds of under 0.01 mm  s–1. Here, the scaling of the
sideration of all experiments are small. CoF is adjusted compared to the previous figures to show
differences in slip behavior better. In general, the level of
3.4 Creep behavior and static friction the CoF is significantly lower than in unsteady slip and
brake shift operations. Furthermore, the CoF has a steep
In addition to the investigations of the friction behavior gradient at very low sliding speeds.
at brake shifts and unsteady slip, creep behavior is inves- The friction system with the steel plate variant (a) again
tigated at the test rig LK-3 using a different test method. tends to exhibit the highest friction coefficients—especially
at an oil injection temperature of 40 °C—compared with the
additionally reworked variants. However, it is not possible
to clearly differentiate between variants (b) and (c) in terms
of their creep behavior. Overall, both variants show a reduc-
tion in the coefficients of friction with higher oil injection
temperatures. The influence of the specific surface pressure
p on the creep behavior is subordinate.
This is also shown by the SFC determined at the tran-
sition between static and dynamic friction in Fig. 15. At
an oil injection temperature of 40 °C, variant (a) shows the
highest SFC. At an oil injection temperature of 110 °C, this
cannot be observed.

Fig. 15 Comparison of the SFC of clutches with different steel plates

at different operating conditions

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Forschung im Ingenieurwesen (2023) 87:541–554 551

3.5 Relation between CoF and surface topography height Sa, the root mean square height Sq, the maximum
in unsteady slip operation peak height Sp, the core height Sk, the reduced peak height
Spk, and the reduced valley depth Svk.
In unsteady slip operation, we analyze the relation between
CoF and surface parameters. Therefore, the median of the
value µtop of the fifth slip phase of cycles six to ten is 4 Discussion
determined for each clutch at every load stage. This rep-
resentative friction coefficient value is compared to the The tests in different operating modes show an influence of
median from the sixteen individual measurements of an the steel plate surface on the friction behavior. The investi-
areal surface topography parameter. Fifteen standard pa- gated steel plate variants show a change in the CoF level but
rameters are used to characterize the surface topography; no change in the friction behavior in the context of general
see Table 4. In Fig. 16 and 17, the relation between CoF gradient characteristics. In brake shift operation the influ-
in unsteady slip operation and surface topography is shown ence of the steel plate surface tends to be observed more
for two standard surface parameters Sa and Sk at load stage at the end of shifting when higher temperatures and lower
US 18 (p = 1.0 N  mm–2, vg,max = 0.2 m  s–1) at three oil injec- sliding speeds are present in the clutch. This underlines the
tion temperatures. Both parameters indicate an influence of results in unsteady slip, where the influence of the steel plate
the surface topography parameter on the friction parameter variant is more likely to be detected at higher oil injection
µtop at ϑoil = 80 °C and ϑoil = 110 °C. Thus, no influence of temperatures and, therefore, higher clutch temperatures. In
these surface parameters on the friction behavior exists at creep behavior, the influence of the steel plate variant can-
ϑoil = 40 °C. The linear curve fit is shown as a black line for not be observed in all systems. Contrary to the unsteady
each oil inlet temperature. slip operation where curves of CoF over sliding speed are
The coefficient of determination R2 is determined for all generated using cfc evaluation, in micro slip, local effects at
investigated operating points in a combination of fifteen very low sliding speeds due to slightly slanted contact and
areal surface parameters. These values of R2 are shown in thickness deviations of the plates are not minimized. Espe-
Fig. 18. Here, values greater than 80% are highlighted dark cially at sliding speeds that low, this might cause problems
green, values in the range of 60 to 79% are highlighted in interpreting variants with small differences in friction
green and values in the range of 50 to 59% are highlighted behavior.
yellow. This provides a visual impression of the goodness Nevertheless, investigation of creep behavior and SFC
of fit for each surface parameter. show a similar behavior of all steel plate variants and sup-
In this table, some surface parameters show high val- port a better understanding of the overall friction behavior
ues of R2 in combination with many operating conditions. of wet disk clutches, where according to measurements of
Still, there are only few parameters showing a high coef- Meingassner [13], Meingassner et al. [14, 24] and Voelkel
ficients of determination at an oil injection temperature of et al. [12] the SFC is significantly lower than the CoF at
110 °C. Some parameters partly show very good correlation higher sliding speeds in unsteady slip or brake shift oper-
with many operating conditions especially at an oil injec- ation. This can be confirmed for the investigated clutches
tion temperature of 80 °C. These are the arithmetical mean with different steel plate variants.

Fig. 16 Relation between the arithmetical mean height Sa and µtop for Fig. 17 Relation between the core height Sk and µtop for different
different steel plate variants analyzed by linear regression (black) at steel plate variants analyzed by linear regression (black) at three
three oil injection temperatures ϑoil in load stage US 18 (p = 1.0 N  mm–2, oil injection temperatures ϑoil in load stage US 18 (p = 1.0 N  mm–2,
vg,max = 0.2 m  s–1) vg,max = 0.2 m  s–1)

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552 Forschung im Ingenieurwesen (2023) 87:541–554

Fig. 18 Coefficient of Determi- R2 of each combination load stage surface parameter / %

nation R2 of the linear regression
for every investigated combi-

p / N/mm2

vg / m/s
nation of surface parameters

°C

Smr1

Smr2
S10z
and friction coefficient µtop with

Sdq

Spk
Sku
Ssk

Svk
oil /

Sdr
Sq

Sp
Sa

Sv

Sk
Sz
highlighted high values of R2
0.01 48 48 52 27 26 28 7 4 37 31 54 79 38 3 0
0.025 48 51 65 50 50 52 1 2 63 57 54 93 56 6 0
0.5 0.05 50 49 38 15 14 17 7 11 18 14 49 55 37 0 7
0.1 5 4 0 6 6 4 40 52 2 5 3 11 0 0 8
0.2 9 7 0 10 11 8 31 65 12 16 4 0 1 15 42
0.01 77 79 91 72 71 73 3 4 74 68 85 82 80 0 0
0.025 64 69 76 63 63 66 7 5 67 62 68 81 76 0 1
40 0.75 0.05 63 65 44 29 30 35 7 0 23 20 55 33 65 10 32
0.1 38 38 15 5 5 8 0 8 3 2 29 15 31 12 43
0.2 24 23 3 0 0 0 4 26 1 2 16 4 13 19 54
0.01 76 79 91 75 74 77 6 6 76 71 82 81 84 0 1
0.025 55 61 71 70 72 75 25 19 71 69 58 68 79 0 2
1.0 0.05 63 69 65 62 64 69 29 15 55 53 61 47 84 3 12
0.1 31 36 29 38 42 45 59 28 28 29 26 13 55 7 20
0.2 17 19 6 5 6 9 21 1 2 2 10 2 25 16 46
0.01 66 67 79 54 51 53 1 0 59 52 76 79 58 0 0
0.025 78 79 86 60 57 60 0 0 61 54 87 74 70 0 0
0.5 0.05 87 87 87 60 57 61 0 0 56 50 93 65 76 2 3
0.1 84 84 81 53 50 54 0 0 51 45 88 68 73 2 4
0.2 87 86 74 43 40 45 1 2 39 33 89 54 69 6 10
0.01 67 68 84 62 59 61 0 1 68 61 77 84 64 1 0
0.025 78 80 90 68 66 69 1 2 69 63 86 79 77 0 0
80 0.75 0.05 83 85 89 64 62 66 1 1 63 57 89 74 80 1 2
0.1 86 87 78 54 53 58 2 0 50 44 86 63 82 4 11
0.2 91 92 83 58 56 62 2 0 51 46 91 59 86 6 12
0.01 67 69 86 65 62 64 0 1 71 64 78 84 65 1 1
0.025 74 76 89 67 65 68 1 2 71 65 83 84 74 0 0
1.0 0.05 83 85 92 73 71 75 4 4 71 66 88 76 86 0 2
0.1 78 81 87 72 71 75 7 5 71 66 82 76 86 0 3
0.2 82 86 79 64 64 69 12 5 56 52 81 56 91 5 13
0.01 41 37 30 7 6 7 34 28 9 6 44 36 18 0 1
0.025 42 38 29 6 5 6 34 30 8 5 44 35 18 1 2
0.5 0.05 48 44 32 8 6 8 29 30 9 5 49 35 23 2 5
0.1 52 48 34 9 7 10 25 27 10 6 52 37 27 3 7
0.2 50 46 25 4 3 5 24 37 3 1 46 24 24 9 20
0.01 33 30 26 5 4 5 39 29 8 5 37 37 13 0 0
0.025 44 39 32 8 6 8 32 27 10 6 47 37 20 1 2
110 0.75 0.05 51 47 35 9 7 9 27 27 10 6 52 36 25 3 5
0.1 50 46 34 9 7 9 26 27 10 7 51 39 26 2 5
0.2 57 52 29 6 5 7 20 32 5 2 52 25 29 11 21
0.01 42 38 31 8 6 8 33 27 10 6 45 36 19 0 1
0.025 55 51 44 16 14 16 21 18 18 13 59 44 30 1 2
1.0 0.05 57 53 45 17 14 17 19 17 18 13 60 46 33 1 3
0.1 62 59 45 16 14 17 15 18 17 12 63 45 38 3 8
0.2 78 75 56 25 23 27 5 10 22 17 76 44 54 8 16

In all operation modes, an influence of the oil injec- In further investigations, this could be a first approach to
tion temperature on the friction behavior can be observed. an improved understanding of the interaction between oil
Higher temperatures show a reduction of the CoF. Higher injection temperature, lubricant and friction surface. In ad-
specific surface pressure also leads to a reduction of the CoF dition, the topography of the lining friction surface should
in all operation modes except in creep behavior, where no also be investigated and, if necessary, varied to determine
influence of the specific surface pressure on the friction be- interactions between the steel and friction lining surfaces.
havior can be observed. Thus, in previous investigations, This would support a general understanding of an effect
the specific surface pressure shows an influence on creep mechanism or formulation of a model conception as in the
behavior as well [13]. literature [2, 7], where the influence of different machinery
A relation between oil injection temperature or viscos- settings for a specific surface finish on the friction behavior
ity behavior of the lubricant and the influence of the steel is investigated, and a first model conception is proposed.
plate roughness on the friction behavior cannot be explained Furthermore, several surface topography parameters can
with the investigations since only one lubricant was used. be determined, showing a good relation to the CoF in dif-

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Forschung im Ingenieurwesen (2023) 87:541–554 553

ferent load stages under unsteady slip conditions. These included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included
can be useful for quality control regarding this friction lin-
in the article’s Creative Commons licence and your intended use is not
ing and lubricant combination. Higher values of those sur- permitted by statutory regulation or exceeds the permitted use, you will
face parameters after run-in show higher values of CoF need to obtain permission directly from the copyright holder. To view
which should be the aim for the development of friction a copy of this licence, visit http://creativecommons.org/licenses/by/4.
0/.
systems. Areal surface parameters show good relations to
CoF and low variance over several measurement positions
over the steel plate, which also supports quality control References
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