This study aims at experimentally investigating the sliding friction characteristics of a wet clutch during its lifetime. More precisely, the objective is to understand how the Stribeck and the frictional lag (i.e, sliding hysteresis) parameters evolve as the clutch degradation progresses. For this purpose, a novel test procedure has been proposed and a set of experiments has been carried out on a fully assembled (commercial) clutch using a modified SAE#2 test setup. Furthermore, a systematic methodology for the Stribeck and the frictional lag parameters identification is developed. Regardless of the applied pressure, it appears that the first three identified Stribeck parameters tend to decrease with the progression of the degradation, while the last parameter tends to increase. In regard to the frictional lag parameter, the trend shows pressure dependency.
Adhesive wear and thermal degradation are the main aging sources of clutch friction materials, which are unavoidably present when clutches are in operation. The dominance of these aging sources is determined by many factors, such as the used friction material, oil, and operational condition, Regarding the sliding friction, the characteristics can be classified into two categories, namely,
This study aims at experimentally investigating the typical sliding friction characteristics of a wet friction clutch during its lifetime. More precisely, the objective of the study is to understand how the Stribeck and the frictional lag (i.e., sliding hysteresis) parameters evolve as the degradation progresses. A profound understanding of the evolution of the Stribeck parameters and the sliding hysteresis loop during clutch lifetime may allow to model the evolution of the clutch friction characteristics during the lifetime. This model can then be integrated to a clutch model such that simulations of the dynamic engagement behavior of the clutch with the progression of the friction material degradation is possible. Eventually, the gained knowledge can lead to the derivation of physical features, which are useful for developing a clutch monitoring and prognostic system.
For this purpose, a novel test procedure has been proposed and applied on a fully assembled (commercial) clutch tested on an SAE#2 test setup. The accelerated life test (ALT) concept is utilized to accelerate the progression of the clutch friction material degradation, wherein two additional tests, namely, (i) stationary Stribeck test and (ii) dynamic Stribeck test, are performed between predefined number of duty cycles. This way, important sliding friction characteristics of clutches can be systematically evaluated. The used clutch consists of a predefined number of commercial friction discs and separator discs that are lubricated with a commercial ATF.
The remainder of this paper is organized as follows. The experimental aspects comprising the test setup used in the study and the experimental procedures are presented and discussed in Section
This section begins with some details of the SAE#2 test setup used in the study. An overview concerning the used friction disc, the separator disc, and the ATF is also given. Finally, the experimental procedures carried out in the study are then discussed in the last part of this section.
According to the standard of Society of Automotive Engineer (SAE) (i.e., SAE
Figure
Modified SAE#2 test setup without a flywheel.
As the torque capacity is limited by the design of the test setup, the clutch used in the study only contains one active friction disc, that is, two frictional contacts instead of ten frictional contacts recommended by the design. The lining material of the friction disc is paper-based type and commercially available. Moreover, the friction disc has a waffle groove pattern with the inner diameter
Figure
A flowchart describing the test sequences.
The run-in test designed for this study, in principle, mimics the engagement process of a clutch in automatic transmissions. While the clutch is in open condition (i.e., disengaged phase), the shaft velocity of the motor is controlled at a certain value. When this desired velocity is attained, a certain current signal profile is sent to the proportional valve such that a desired pressure profile, for example, see Figure
A typical pressure profile applied in the run-in test and ALT.
The pressure profile is designed as follows. Initially, there is no electric current signal sent to the proportional valve, such that the clutch pressure is zero (
The control mode of the motor is a velocity control in which the maximum torque of the motor is limited to a certain value. In this study, the velocity is set to 1200 rpm and the maximum torque is set to 20 Nm during the test. As the applied pressure increases, the resulting friction torque increases and the motor delivers more torque to maintain the desired velocity. However, since the maximum torque of the motor is limited, the motor will decelerate when the resulting friction torque exceeds the limit torque and eventually will come to a standstill. The procedure proposed in this paper differs from the standard procedure in an SAE#2 test setup as discussed in [
In principle, the accelerated life test (ALT) is carried out in the same manner as the run-in test discussed previously. However, the energy applied during the ALT is higher than that in the run-in test. For this purpose, the shaft velocity of the motor is set to 2000 rpm.
The main objective of the stationary Stribeck test is to determine the stationary Stribeck curve as a function of the clutch lifetime. It is important to note here that the stationary Stribeck curve is derived from a set of friction torques measured at constant relative velocity and constant applied pressure. In this study, the stationary Stribeck curve is determined for the applied pressure ranging from 4 to 7 bar and for the relative velocity ranging from 20 to 1970 rpm. The pressure step is 0.5 bar and the velocity step is 50 rpm.
For a given pressure level, the velocity is increased step-wise from 20 to 1970 rpm, where the duration for a given velocity level is set to 5 s. After the friction torques for the entire velocity range have been measured, the pressure is increased by 0.5 bar, and in the new pressure level the velocity is restarted from 20 to 1970 rpm. This procedure is repeated until the maximum pressure level, that is, 7 bar, is attained.
The dynamic Stribeck test aims at determining the dynamic frictional behavior of the clutch in nonstationary conditions as a function of the clutch lifetime. As mentioned previously, the dynamic frictional behavior is typified by the frictional lag phenomenon. When the relative motion is accelerated the resulting friction torque is higher than the friction torque in stationary condition. In contrast, when the relative motion is decelerated, the resulting friction torque is lower than the stationary friction torque. For a sinusoidal relative motion where acceleration and deceleration are evident, a loop is observable around the stationary Stribeck curve. The loop area is strongly determined by the frequency of the motion. In the GMS friction modeling framework [
The dynamic Stribeck test is carried out at different constant pressures ranging from 4 to 7 bar. For a given pressure level, a sinusoidal velocity profile having frequency of 0.1 Hz, amplitude, and offset of 1000 rpm, respectively, is imposed. The total duration of the dynamic Stribeck test for a given velocity profile is 15 s.
This section discusses the results of the stationary and dynamic Stribeck tests. From the tests, the friction torque as a function of three variables, namely, (i) velocity, (ii) pressure, and (iii) degradation level (i.e., number of duty cycles) are obtained. The experimental results of the stationary Stribeck tests are first discussed. Afterwards, the Stribeck parameters identified at different degradation levels and their evolutions during the clutch lifetime are discussed. After addressing the stationary Stribeck characteristics and the evolution of the Stribeck parameters, the dynamic Stribeck behavior, which is characterized by the attractor parameter, is then discussed. Furthermore, the characteristics of the attractor parameter identified at different pressures, frequency excitations, and degradation levels are addressed.
For comparison purposes, photos, and surface profiles of the active friction disc are taken from the same wafle block (see Figure
Photographs of the friction disc (a) before and (b) after the tests.
Surface profiles and the distributions
Figures
Figure
The stationary Stribeck curve measured at initial condition. (a) The 2D Stribeck curve and (b) the 3D Stribeck curve.
The evolution of the Stribeck curve with the progression of degradation is shown in Figure
The stationary Stribeck curves measured for different degradation levels. (a) the evolution of the 3D Stribeck curve and (b) The evolution of the 2D Stribeck curve at 5 bar. Note that the arrow indicates the progression of degradation.
It is evident in Figure
One can also notice the effect of the clutch degradation on the slope of the Stribeck curve in Figure
In order to characterize the evolution of the stationary Stribeck curve, the parameters governing the curve, that is, the Stribeck parameters, are identified at different degradation levels. In the following analysis, the rotational velocity in rpm
In [
Since the function expressed in (
Thus, the optimization problem can now be formulated as
For the Stribeck parameters identification, the
Comparison between the Stribeck curve measured at initial condition and the model.
Figure
The identified Stribeck parameters at initial condition in function of applied pressure.
Figure
The evolution of the identified parameters of the Stribeck curve measured at applied pressure of 5 bar in function of the duty cycles.
For this particular pressure, one can see that the three parameters, namely
Furthermore, Figure
The overall evolutions of the identified parameters of the Stribeck curves measured at different pressures in function of the duty cycles.
As mentioned previously, the dynamic Stribeck test in this study is carried out by applying an imposed sinusoidal velocity profile with a frequency of 0.1 Hz, where the pressure is kept constant during the velocity excitation. In this way, the effects of acceleration and deceleration on the resulting friction torque can be observed. Due to acceleration and deceleration, the friction deviates with respect to the steady-state behavior, that is, the stationary Stribeck curve, thus resulting in the formation of a hysteresis loop which is located around the stationary Stribeck curve, that is referred to as the frictional lag effect.
Figure
Dynamic Stribeck curve of the fresh clutch measured at 5 bar.
The effect of the applied pressure on the frictional lag phenomenon at initial condition (fresh clutch) is shown in Figure
Dynamic Stribeck of the fresh clutch at different applied pressures.
The loop area resulting from the frictional lag in function of applied pressure calculated based on the friction curves in Figure
Figure
The evolution of the dynamic Stribeck curve at applied pressure of 5 bar.
The evolution the frictional lag loop during the clutch service-life at applied pressure of 5 bar.
It is now interesting to observe how the loop area
The evolution the frictional lag loop during the clutch service-life at different applied pressures.
The stationary Stribeck tests reveal that the stationary friction curve drops globally as the friction material degradation progresses. Four parameters, namely, (i) the static friction torque
Under acceleration or deceleration, the resulting friction will deviate from the stationary friction (stationary Stribeck curve). To investigate this effect, the dynamic Stribeck tests have been carried out, wherein a sinusoidal velocity excitation is applied on the clutch. In this way, both acceleration and deceleration effects are present simultaneously in the measured friction, eventually creating a hysteresis loop around the stationary curve that is called the frictional lag effect. The loop area, which is simple to compute, can be considered as a measure of the frictional lag effect. Furthermore, it has been shown in this study that the applied pressure has also an effect on the loop area of the sliding hysteresis. In the beginning of clutch lifetime, the loop area increases with the applied pressure. However, this characteristic changes with the degradation progression, where the hysteresis loop area tends to decrease with the applied pressure at the end of the clutch lifetime.
All the authors would like to thank Tycho Van Peteghem and Wout Vandelaer for performing the experiments. Valuable comments of Professor Hendrik Van Brussel and Professor Farid Al-Bender on this study are appreciated. The experimental support from Dr. Mark Versteyhe of Dana-Spicer Off Highway Belgium is also acknowledged.