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Tribology International 41 (2008) , Roland 14 3 1. Introduction surface pressures. This is the case in limited slip differ- entials, which normally have a rather low surface sliding speed, and seldom reach the state of lock up. For the drivetrain to work smoothly without any unnecessary noise automatic transmissions that start the engagement at a high difference in rotational speed and then reach a state of lock up. The high speed in these cases implies that the ARTICLE IN PRESS clutch will work in full film lubrication, the largest part of the engagement. Such an engagement process is possible to simulate with good results. There will be a torque 0301-679X/$-see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.triboint.2007.11.014 C3 Corresponding author. E-mail address: par.marklund@ltu.se (P. Marklund). Wet clutches are often used in vehicle drivetrains. The working conditions of different clutches in the transmission greatly vary depending on the application. Wet clutches in automatic transmissions are often used as lock-up clutches between different rotating parts in the gearbox, where the initial sliding velocity of the clutch interface can be quite high. Other parts of the drivetrain can have wet clutches that work with much lower sliding speeds and higher and vibrations, the friction characteristics of the wet clutches have to be thoroughly investigated. Depending on the working conditions some clutches will work in full film, mixed and boundary lubrication, whereas others will work mainly in boundary lubrication regime. To get a better understanding of the frictional behavior of wet clutches, several simulation models have been developed as a complement to traditional measurement methods [1–6]. Most investigations include simulations of clutches in The frictional behavior of wet clutches in vehicle drivetrains is critical for their overall behavior. During the development of new wet clutch systems there is a need to know this friction behavior. The transferred torque is normally investigated in test rigs where the friction in a sliding interface between a friction disc and separator disc is investigated. These test rigs can be designed differently, depending on the working conditions of the investigated clutch. However, it is possible today to simulate the clutch behavior and not limit ourselves to only using measurements from test rigs for the design of the wet clutch. The torque transferred by the clutch during engagement can be roughly divided into full film torque and boundary lubrication torque. The full film regime is possible to simulate quite well, whereas the friction in the boundary regime is much more difficult to simulate due to its strong additive dependency. To obtain a good prediction of the total engagement, friction measurements in the boundary lubrication regime are still needed. These measurements should be easy to perform and fast tests are preferable. Friction coefficients for the whole range of sliding speed, interface temperature and nominal surface pressure should be measured. To use these measurements in simulations and get a better understanding of the friction behavior, it is also preferable to conduct these measurements on a small test sample, for which the temperature and sliding speed can be regarded as constant. Here, the friction of a small sample of a wet clutch friction disc is investigated in a pin on disc test and the temperature is measured in the sample during the tests. Measurements are compared with measurements from a test rig for whole friction discs. A good correspondence between the frictional behaviors of the different measurement methods is achieved. r 2007 Elsevier Ltd. All rights reserved. Keywords: Wet clutch; Friction measurement; Pin on disc Abstract Wet clutch friction characteristic simplified pin Pa¨ r Marklund C3 Division of Machine Elements, Department of Applied Physics and Mechanical Received 2 January 2007; received in revised form Available online 824–830 s obtained from on disc test Larsson Engineering, Lulea? University of Technology, Lulea?, SE-97187, Sweden November 2007; accepted 16 November 2007 January 2008 design, but if only the output torque is measured, the friction coefficient that can be computed for the clutch is simply the mean friction coefficient. Temperature and velocity are not constant in the interface, meaning that the friction is also not constant, see [2,6]. To get a better understanding of how the friction can be described in terms of temperature, velocity and surface pressure, a testing method that measures more local effects has been developed. 2.1. Pin on disc ARTICLE IN PRESS International 41 (2008) 824–830 825 contribution from the boundary friction at the end of the engagement. This friction is much more difficult to simulate, since it is very additive dependent. For this part of the engagement there is still a need to do frictional measurements that can be used in simulations. For clutches working mainly in boundary lubrication, during longer periods, the simulations will be very dependent on measurements. Examples of simulation models for this kind of application are temperature simulations used to predict changes in torque transfer during a long engage- ment with high surface pressure and a small limited slip torque as in Marklund et al. [6]. For these kinds of semi- empirical simulation models, we encounter a need for a more local friction measurement than what is possible to measure in a test rig that measures torque transfer from one whole friction disc [3,7], or larger parts of a friction disc, including grooves [8]. A measurement method to do these local measurements should measure on a quite small sample of the friction disc, to get the local effects. It should also be possible to measure the temperature inside this small sample close to the sliding interface, since it is shown in [6,7], that the temperature will affect the friction. If the test sample has no grooves, the measured friction will not be geometry dependent. This is not the case when measuring torque transfer from a whole friction disc, where grooves and other surface patterns also can affect the torque. A measurement method based on a pin on disc test can fulfill all these demands. A special pin is designed with a holder for a small sample of the friction disc where a thermocouple is mounted to monitor the temperature during friction measurements. This method can also give a better understanding about the friction phenomena than what is possible in whole friction disc test rigs. The friction coefficient and its variation with temperature, sliding velocity and surface pressure is measured in this paper in a pin on disc test. The test is relatively fast and the normal range of operational parameters are covered within 2h. Another advantage of the proposed test is the possibility to measure the local friction effect, which is of great interest when using measurements in simulations. 2. Method A measurement of boundary lubrication is needed to be able to simulate torque transfer in wet clutches working in boundary lubrication regime. This friction is very additive dependent, and is therefore a function of the additives adsorption and reaction on the surfaces. Adsorption, desorption and reactions depend critically on the operating conditions temperature, sliding velocity and contact pressure. To obtain a friction coefficient for the whole working range of temperature, velocity and pressure, many measurements are required to describe the friction. The friction in the sliding interface of a wet clutch is often measured as output torque for one whole friction disc in P. Marklund, R. Larsson / Tribology contact with one steel separator disc. This is a good method to measure the final output from an existing wet clutch A special holder is developed for a pin on disc test to enable these local friction measurements for the material combinations used in wet clutch systems. In a pin on disc test, a stationary pin is loaded axially in contact with a rotating disc, as in the schematic sketch shown in Fig. 1. The friction force on the pin can be measured, thus making it easy to compute the friction coefficient. The pin on disc machine used in these tests is a Phoenix Tribology TE67. In these measurements, a special pin, Fig. 2, is made which has a holder for a small specimen made of a friction disc, Fig. 3. A thermocouple that measures the temperature at about 0.3mm from the contact surface is inserted in the specimen. Since the specimen only has a diameter of 3.0mm, a constant velocity and temperature can be assumed over the whole test specimen contact area. This makes the measured friction suitable to use in wet clutch simulations and gives a better understanding of the boundary friction. The friction material on the friction discs used in this investigation is made of sintered bronze. The disc is designed as a holder for a piece of the steel separator disc used in the real wet clutch system. This means that the test specimen will have the same properties as the separator disc used in the clutch. The lubricant used in these experiments is a semi-synthetic oil tailor-made for the Haldex Limited Slip Coupling, which is a limited slip differential manufactured by Haldex Traction AB. This application is further described in [9]. The friction measurements are in this case made to correspond with the working conditions of a wet clutch in a limited slip differential, meaning that the sliding velocities F ax ω Fig. 1. Schematic sketch of a pin on disc apparatus. ARTICLE IN PRESS Fig. 2. Pin with holder P. Marklund, R. Larsson / Tribology826 will be fairly low while there will be quite high tempera- tures and surface pressures. The ranges for temperature, velocity and surface pressure and the resolution of the measurements are shown in Table 1.The sampling rate during the measurements is 10Hz. 2.2. Test procedure The tests start at an ambient temperature, 221C, and the equipment is gradually heated during the measurements. Before the test starts, the surfaces are run in with the test lubricant. The disc rotates at a speed of 100 revolutions per minute for 10min with an applied load, corresponding to a sliding speed of 0.15m/s. During the test, the velocity is increased from 0 to 0.5m/s, followed by a decrease in speed to a standstill. The whole measurement takes about 30s. When the test is finished, a heater is engaged to warm up the test equipment to the next Fig. 3. Test specimens from bronze friction disc. Specimen to the left with drilled hole for thermocouple. Table 1 Working range and resolution Working range Resolution Nominal surface pressure, p (MPa) 4.0–8.0 Temperature, T e C14 CT 22–100 0.2 Rotational speed (rpm) 0–318 1.0 ) Sliding speed, v (m/s) 0–0.5 0.0016 Friction force, F fric (N) 0–49 0.015 ) Friction coefficient, m (–) o5:3C210 C04 temperature level and a new measurement is conducted. There is a temperature difference of 51C between the temperature levels. The total temperature range for which the friction is measured is 22–1001C. The whole test series is therefore performed for 16 different measurements with different start temperatures. 3. Results and discussion During each test the velocity is increased from standstill to 0.5m/s and then decreased back to standstill. This variation in speed is not linear, and a typical velocity plot for the tests is shown in Fig. 4(a). 3.1. Temperature variation during test The temperature in the test specimen increases due to frictional heating during the velocity increase. When the velocity is decreased, the temperature will decrease. This temperature behavior during the test is visualized in Fig. 4(b) for a start temperature of 251C. It is obvious from this figure that there is no significant delay in the temperature measurement, indicating that the measured temperature is a good measure of the mean temperature in the sliding interface. The sliding interface is located about 0.3mm from for test specimen. International 41 (2008) 824–830 the thermocouple. The temperature measurement is also a good indicator that the temperature is very dependent on the surface heat flux, since the temperature will immediately start to decrease when the velocity is decreased, see Fig. 4.The measured temperature in this point, 0.3mm from the surface, will in this paper be referred to as interface temperature. 3.2. Friction measurements To use the value of the friction coefficient in wet clutch simulations, or for a wet clutch control software, the most important is to describe the friction coefficient as a function of sliding speed, v, interface temperature, T, and nominal surface pressure, p. One test cycle in this pin on disc test will give this frictional behavior for one combina- tion of friction material, lubricant and load. Results from the measurements can be visualized in differently. One way ARTICLE IN PRESS 0 v [m/s] 0 20406 0 0.1 0.2 0.3 0.4 0.5 t [s] Fig. 4. Temperature variation and sliding speed during P. Marklund, R. Larsson / Tribology is to plot the friction coefficient as a function of sliding speed and interface temperature, as in Fig. 5. The measurements are statistically very good with little spread between the measurements. There are basically two ways to describe the relationship between friction coefficient, sliding speed and interface temperature. A mathematical expression can be fitted to the measured friction data, or the data could be stored in a large matrix from which friction coefficients could be interpolated from nearby cases. A mix between these two methods can also be used [6]. The advantage with an approximated function is that it will not need a large storage space, and that is vital for control softwares with small memory capacities. Another advantage with this method is that the friction coefficient will be easy to compute. The disadvantage is the limited flexibility of the chosen expression that can only be applied for a specific frictional behavior. For this case, the expression m ? a 1 ta 2 C1tanhevC1a 3 Tta 4 C1T ta 5 C1T 2 ta 6 C1T 3 ta 7 C1vta 8 C1vT ta 9 C1evTT 2 e1T 0.1 0.2 0.3 0.4 50 75 100 0.04 0.06 0.08 0.1 0.12 afii9839 [-] T [°C] v [m/s] Fig. 5. Friction coefficient as function of sliding speed and interface temperature. Nominal pressure ? 8:0MPa. 0 204060 25 26 27 28 T [° C] t [s] one test cycle: (a) sliding speed; (b) temperature. 0.12 International 41 (2008) 824–830 827 gives a good approximation to the measured frictional data, see Fig. 6. In this equation m is the friction coefficient, v is the sliding velocity and T is the interface temperature. Another way to visualize frictional behavior is to use this expression and plot the friction coefficient as function of sliding speed for different interface temperatures, see Fig. 7. 3.3. Comparison with other test rig Traditional friction measurements of wet clutches are performed in test rigs with one whole pair of friction discs [3,10].In[8], larger parts of a friction disc, including grooves, have also been tested in a pin on disc test. With this test method, where a small sample of the friction disc is tested, it is important to investigate the correlation with other performed tests. Fig. 8 shows a comparison between curve fits from the measured friction coefficient in the pin on disc and a wet clutch test rig [10] with nominal pressures of 8.0MPa. 0.1 0.2 0.3 0.4 0.5 25 50 75 100 0.02 0.04 0.06 0.08 0.1 afii9839 [-] T [ ° C] v [m/s] Fig. 6. Friction coefficient versus sliding speed and interface temperature. Measured data and approximative mathematical surface. Nominal pressure ? 8:0MPa. The mesh is the approximation according to Eq. (1). ARTICLE IN PRESS 30°C 50°C 70°C 0.04 0.06 0.08 0.1 0.12 afii9839 [-] P. Marklund, R. Larsson / Tribology828 Friction coefficients from the different measurements show the same trends in variation of friction coefficient throughout the whole range of speed and temperature. However, the friction coefficient is consistently slightly smaller in the wet clutch test rig measurements. The measured friction coefficient should not be exactly the same for the different test rigs, since grooves are not included in the pin on disc test. The nominal pressure on the friction discs used in the wet clutch test rig is calculated for the net surfaced area in contact in the interface, i.e. the groove area subtracted from the total disc area. A smaller difference in this area from the manufacturing process of the discs, could affect the geometry of the friction material and therefore the net 90°C 0 0.1 0.2 0.3 0.4 0.5 0.02 v [m/s] Fig. 7. Friction coefficient as function of sliding speed at different interface temperatures. Based on curve fitted mathematical surface. Nominal pressure ? 8:0MPa. 35°C Pin on Disc 35°C Wet Clutch Test Rig 60°C Pin on Disc 60°C Wet Clutch Test Rig 0 0.04 0.05 0.06 0.08 0.09 0.10 0.11 0.07 afii9839 [-] 0.50.40.30.20.1 v [m/s] Fig. 8. Comparison between friction measurements in pin on disc and wet clutch test rig [10]. Nominal pressure ? 8:0MPa. friction coefficient for 601C is computed. Fig. 10(b) shows the largest absolute deviation from the mean friction coefficient for each sliding velocity. This illustrates that the maximum absolute deviation in friction coefficient for pressure of the three investigated pressures. This makes the difference in friction coefficient to be dependent on the load less plausible. It is possible that the difference in friction coefficient is instead dependent of other vari- ables, such as surface structure and friction material composition, indicating that the friction coefficient could be described just as a function of sliding velocity and temperature for the whole temperature range. Fig. 9 also shows that for sliding velocities about 0.5m/s, the friction coefficient, m, will be about 0.1 for all investigated loads and temperatures. 3.5. Error analysis As described in Section 3.4 the difference in friction coefficient for different surface pressures is not large. The friction coefficient could therefore be described as a function of only interface temperature and sliding velocity without loosing much precision. As a measure of the deviation of the measured data for the maximum and minimum load, the maximum deviation from mean friction coefficient computed for maximum and minimum loads is visualized in Fig. 10. Here, six subsequent measurements at two different loads are investigated. These measurements will contain over 60; 000 measurement points over the measured region described in Table 1. Fig. 10(a) shows plots from the curve fit expression (1) for each measurement at an interface temperature of 601C. From these functions the mean surface area and nominal pressure for a given axial load, which would then influence the friction coefficient’s variation with pressure. 3.4. Load dependence The normal load does not greatly influence on the friction coefficient in these measurements. The fact that the friction coefficient is not very load dependent has also been earlier observed in other experiments, such as Ma¨ ki [11]. Fig. 9 shows the friction coefficient for three different loads in the whole range of sliding velocity. Here, the largest difference in friction coefficient is about 5% at 301C, Fig. 9(a). At 501C, Fig. 9(b), the difference in friction coefficient at different loads is not very large; hence, at these temperatures and higher it is possible to describe friction coefficient as only a function of sliding velocity and temperature without loosing much precision. At higher temperatures, Fig. 9(c) and (d), the difference in friction coefficient for different loads is even smaller. The lowest friction coefficient is achieved for the medium International 41 (2008) 824–830 these measurements in the velocity interval 0.05–0.4m/s is less than 0.004. ARTICLE IN PRESS p=4.0 MPa p=6.0 MPa p=8.0 MPa 0.04 0.06 0.08 0.1 0.12 afii98