T50推土机主离合器的结构设计【含7张CAD图纸】
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Hebei University of Engineering Graduation Thesis外文资料翻译学生姓名： 专业班级： 工程机械运用与维护 指导教师： Theoretical and experimental studies on the interface phenomena during the engagement of automatic transmission clutchYubo Yang and Robert C. LamBorgWarner Automotive, 11 South Eisenhower Lane, Lombard, IL 60148, USAMathematical models for predicting the temperature and thermal degradation have been developed, which include all important phenomena during the engagement of wet friction clutch. The thermal model consists of heat balance for the separator, friction lining, core plate, and ATF in the clearance. The degradation mechanism and constants for the selected friction material obey the zeroth order reaction. The model predictions show good agreement with experimental measurements.Keywords: wet friction clutch, thermal model, paper-based friction materials, friction material degradation171. IntroductionThe wet disc clutch used in automatic transmissions consists of multiple separators, friction plates, and other components, as illustrated in figure 1. The key element in the clutch pack is the friction plate, made of a low carbon steel core plate with friction materials bonded on each side. The friction plates are splined to the input shaft while the separator plates are lugged to the clutch housing. The function of a clutch pack is to transmit torque from a driving to a driven member when shifting from one gear to another. During engagement (shifting) the separator plates and friction plates are forced together by pressure, and the kinetic energy is converted to heat at the interfaces. The time for the relative rotating speed to reach zero, called the stop time, is typically less than a second.Separators absorb most of the friction heat generated during engagement, and the friction plates (friction lining and core plates) receive much less heat, due to the low conductivity of friction material. This results in a non-uniform temperature distribution in the axial direction in the friction material, i.e., large temperature gradient across the thickness of friction materials. The temperature rise at the interface during engagement is determined by the input energy, and physical and processing parameters. The temperature history near the friction interface is important, since it determines the performance characteristics, and especially, the thermal degradation rate of friction material.Previous studies of wet clutch thermal analysis by other researchers involved both experimental and theoretical work. Considerable work of theoretical analysis in this area has been done by Luo et al. 1, Zagrodzki 2 and Elsherbiny et al. 3. However, some of the important factors affecting the heat transfer and fluid hydrodynamics during the engagement were not considered in the previous studies.This paper presents a comprehensive analysis for the engagement of the wet friction clutch. It includes many phenomena which were not investigated before by others,Figure 1. Automatic transmission clutch pack.such as non-uniform contacting, compressibility of friction lining, ATF (automatic transmission fluid) flow into lining due to lining permeability, thermal degradation of friction materials, etc. The numerical simulation will be compared with the experimental measurements. It should be mentioned that part of the work in this paper was presented elsewhere by the authors 5,6.2. Heat transfer and fluid hydrodynamics analysis2.1. Engagement cycleA typical engagement cycle for a wet clutch on the SAE#2 machine can be divided into four stages. The first stage, Stop Time (Engagement Stage), starts from the pressure being applied and ends at the slipping speed being zero between the friction plate and the separator. From the start of applying lining pressure (t = 0) to the time at which a steady lining pressure is reached (normally around 0.10.2 s), some ATF at the interface permeates into the lining due to its porous structure. Heat is dissipated by convection from the interface into the friction lining due to the ATF flow inside the lining. In addition the ATF flow into the grooves of friction material also cools the interface temperature. The second stage, Soak Period, starts from the end of stop time and ends at the time when the pressure is released. During this stage separators and friction plates are stationary and locked together. The third stage, DwellFigure 2. Repeating unit of traditional clutch. ha: 1/2 thickness ofseparator plate; hb: thickness of friction material; hc: 1/2 thickness of core plate; D z: gap for ATF flow.Period, occurs between the time after releasing the pressure and before turning the flywheel motor on. The last stage, Stabilization Period, is between the motor being turned on and the start of the next engagement cycle. During this stage, ATF flows from the inner radius to the outer radius in the learance between separators and friction plates as well as inside grooves, which cools down the clutch. Meanwhile, viscous heat is generated at the interface due to the ATF velocity difference in the angular direction between the clearance of separator and friction plate, and this also results in drag loss during the disengagement.2.2. Mathematical modelingThe two-dimensional heat balances (angular symmetric) for the repeating unit as shown in figure 2 can be formulated as follows:Heat balance of separator plate (Part a).,(1)where Ta is the temperature; pa is the density of the separator plate; Ca is the heat capacity of the separator plate; and ka is the conductivity of the separator plate.Heat balance on friction lining (Part b).where Tb is the temperature; pb and pf are the densities of friction lining (saturated with ATF) and ATF, respectively; Cb and Cf are the heat capacities of friction lining and ATF, respectively; kb is the conductivity of friction material; vr and vz are the ATF superficial velocities in the r and z direction inside the lining, respectively, which are equal to the product of porosity and pore velocity. Due to the wide range of constituent molecular weights of ATF, vaporization of ATF starts around 210 C and finishes about 500 C. The percentage of ATF being vaporized is a function of temperature. The latent heat of ATF vaporization is embedded in the ATF heat capacity, Cf, as a function of temperatureHeat balance on core plate (Part c). The heat balance on the core plate is only applicable for the traditional two-sided wet clutch. The single-sided clutch does not have core plates.(3)where Tc is the temperature; pc is the density of the core plate; Cc is the heat capacity of the core plate; and kc is the conductivity of the core plate.Heat balance on ATF at the interface. The heat balance on ATF at the interface applies during the stop and stabilization stages:(4)where is the viscous dissipation term, given byHere is the ATF viscosity, and are the ATF velocities at the interface in the r and directions, respectively. If the temperature field is assumed to be angular symmetric, in equation (4) becomes zero. The above equation assumes that the ATF film occupies the whole clearance, and that is not always true. Both theoretical and experimental studies 7 on the drag loss during the disengagement demonstrate that the ATF film only fills the clearance partially at high rotating speeds, especially for the friction plates with grooves. However, the heat generation due to viscous dissipation is relatively small for a typical clutch operation.Heat balance on ATF in grooves. The one-dimensional heat balance for the ATF inside grooves on the friction plate (during engagement and disengagement) can be written aswhere is the volumetric flow rate of ATF in each groove, Tg is the ATF temperature in the groove as a function of radial position, Ti is the interface temperature, L is the perimeter of groove cross section, hg is the effective heat transfer coefficient from the interface to the groove, and a is the ratio of average groove length to the distance from inner to outer radius.Momentum balance for ATF in friction lining in the z direction.(6)where and are the ATF superficial velocities in the z and r direction, respectively; /x is the ATF viscosity; and dP/dz is the hydrodynamic pressure drop in the z direction. The momentum balance for ATF in friction lining in the r direction can be formulated similarly.Considerations for the traditional clutch. Equations (1) to (6) need to be solved simultaneously during the engagement stage to obtain the temperature. However, after the asperity pressure equals the applied pressure, ATF stops flowing into the lining. As illustrated in figure 2, for the traditional clutch the axial position starts from the symmetric line of the separator plate (z = 0), and ends at the symmetric line of the core plate (z = ha + hb + hc during the engagement and locking stages, and z = ha + hi, + hc + Dz during the cooling and motor-on stages). The boundary condition during the engagement stage at the separator-lining interface (z = ha) is， （7）where Qa and Qb are the friction heat flux transferred to the separator and to the lining, respectively. A complementary condition needs to be satisfied also, i.e.,（8）where Qs is the friction heat flux per unit area.Darcys law 4 determines the ATF velocity into the lining at the interface. For laminar flow of viscous liquid into porous media Darcys law states that（9）where Kz is the permeability of ATF in the z direction in the porous media (friction material), which is a function of porosity, pore size, and structure of lining materialsThe heat flux, Qs (J/m2 s), generated during clutch engagement is a function of time. For a small area dA travelling distance ds, the friction heat generated for a uniform contact isHere Pi is the lining pressure; cus is the slipping speed between the friction lining and the separator plate; /j/ is the friction coefficient; and r is the radius. u is calculated from Newtons lawwhere F is the total force; Tc is the total torque; w0 is the initial slipping speed; If is the moment of inertia of flywheel; Pi(t) is the lining pressure as a function of time; N is the number of friction faces; Ri and R0 are the inner and outer radius of lining, respectively. Rm is the averaged radius given byFor a nonuniform contact as a function of radius, equation (10) can be written aswhere f(r) is a function of contact and C is a constant depending on f(r).The boundary condition at z = ha + bb (or z = ha + hb + Dz) for the traditional clutch isThe boundary condition at the symmetric line of the separator (z = 0) and at the symmetric line of the core plate (z = ha + hb + hc, or z = ha + hb + hc + Dz depending on the stage of engagement cycle) isThe boundary conditions of temperature at the inner radius and outer radius is in type ofwhere h is the heat transfer coefficient and Too is the housing temperature. This allows the heat to be transferred to the shaft and housing from the inner and outer radius, respectively.2.3. Results and discussion for thermal analysisFor all the simulation present here, the physical properties of ATF as a function of temperature were taken from Kemp and Lindens paper 8. The physical properties of steel as a function of temperature were taken from a standard handbook, and the physical properties of friction lining were measured experimentally. The other parameters for the test procedures mentioned in the following text are summarized in table 1.Table 1Summary of test procedure parametersTest procedure5003A5004A5006AEnergy levelLevel BLevel BLevel ACycles500050005000Speed (RPM)360062003540Inertia (kg m2)0.3180.1730.285Lining pressure (kPA)6501039669Kinetic energy (J)225973639019588Stop time (s)0.90.780.5Soak period (s)2.12.226.0Dwell period (s)770Stabilization period (s)20205Inlet ATF temp. (C)103 2105 2105 5Oil flow rate (lpm)0.7570.7575.6No. of friction faces668Lining ID (mm)105.6105.688.9Lining OD (mm)126.2126.2121.3Separator thickness (mm)1.81.81.8Lining thickness (mm)0.40.40.51Core thickness (mm)1.01.00.71Total clearance (mm)1.21.21.6Figure 3. Model prediction of interface temperature, Procedure 5003A (Energy Level B).Figure 3 shows the temperatures at the middle point radially (between the inner and outer radius of lining) for a complete engagement cycle of the Test Procedure 5003A, Energy Level B (very severe testing condition). The duration time for the stop time, soak period, dwell period, and stabilization is 0.9, 2.1, 7, and 20 s, respectively. The clutch has six friction facings with an inner diameter of 105.6 mm and an outer diameter of 126.2 mm. The thickness for the separator, lining, and core plate is 1.8 mm, 0.4 mm, and 1.0 mm, respectively. The total input energy to the clutch is 43,930 J with the initial flywheel rotating rate of 3600 RPM. The initial temperature is at 106.2 C. The legends Ts, Tl, and Tc in the figure represent the separa-Figure 4. Temperature distribution in thickness direction during engagement stage, Procedure 5003A (Energy Level B)tor temperature at the middle point of interface, the lining temperature at the middle point of interface, and the core plate temperature at the middle point of its symmetric line, respectively. The engagement pressure was applied for 3 s (engagement stage and soak period). The interface temperature increases rapidly during the engagement stage, reaching the maximum temperature near the end of engagement. The interface temperature then decreases due to the heat conduction from the interface to the other parts of clutch, and due to the heat transfer at the outer and inner radius to the testing machine housing. The temperature at the symmetric line of the core plate increases slowly during the engagement stage because of the low conductivity of friction lining. After the pressure is released (t = 3 s), the separator and the lining are separated by a clearance of 0.2 mm. At that time a large temperature gradient exists across the lining thickness (basically the difference between Ti and Tc). Before releasing the pressure, heat is conducted from the separator (large heat mass) to the lining since the separator has a high average temperature. After releasing the pressure, heat from the separator cannot be transferred into the lining effectively since the gap between the lining and the separator is filled with air (the clutch is not immersed in ATF before stabilization period), and meanwhile the heat at the lining interface is conducted into the lining. As a result, the lining interface temperature drops sharply. After the motor is turned on to generate the kinetic energy of the flywheel for the next engagement cycle, ATF flows from the inner to outer radius through the clearance between the separator and lining, which quickly cools down the interface temperature. At the end of the stabilization stage, Ts, Ti, and Tc almost reach their initial values.:Figure 4 illustrates the temperature distribution across the thickness direction at the middle point of radial direction during the engagement stage at the same conditions as those in figure 4. t in the figure is the dimensionless engagement time (t = t/te, where te is the engagement time, t = 1 is at the end of the engagement stage). TheFigure 5. Separator interface temperature at different radial positions for non-uniform contact.Figure 6. Effect of liquid permeability on interface temperature, Kperm 0.1 Darcy.dimensionless thickness = 0 = z/(ha + hi, + hc) is at the symmetric line of the separator, and = 1 is at the symmetric line of the core plate. The symbols in the figure are the mesh points in the axial direction. It is clear from this figure that a large temperature gradient exists across the lining between the interface and the bonding plane because of the low conductivity of lining material (kb =0.11 W/m K). The temperature difference between the interface and the symmetric line of separator is small due to the large conductivity of steel Effect of non-uniform contact. Figure 5 illustrates the effect of non-uniform contact, between the separator and lining at the interface, on the separator interface temperature at various positions. It is assumed that the friction heat flux, Qs, has a normal distribution along the radius with the peak flux at rm = 58 mm (middle point on the radial direction) and the standard deviation a of 2 mm, i.e., the distribution function f(r) in equation (12) is given byThe total amount of friction heat flux remains the same as that for the uniform contact. The interface temperatures at three positions are shown in figure 5, the inner radius Ri, the outer radius R0 and the middle point between the inner and outer radius i?center. The peak interface temperature at the middle point increases from about 550 C for a uniform contact to about 680 C for the “point” contac A larg temperature gradient exists from the middle point to the inner or outer radius in case of “point contact” since mos of the friction heat is generated near the middle point.Effect of liquid permeability of friction materials. Figure 6 illustrates the effect of liquid permeability on the interface temperature. The solid curves in the figure represent the results without considering the permeability of lining material, and the dotted curves represent the resultsby considering the permeability of lining material. All the physical and process parameters for figure 6 are the same as those in figure 2, except that the times for the engagement and soak periods are changed to 0.78 and 2.22 s, respectively. The input energy level is 36,930 J with the initial flywheel rotating speed at 6200 RPM, and the initial temperature is 106.1 C. All related parameters are illustrated in table 1 - Procedure 5004A. It is found that a higher permeability material (Kperm = 0.1 Darcy) results in lower interface temperature than lower permeability material (Kperm = 005 Darcy). However, there exists an optimal permeability value, depending on the lining pressure and input energy level.A typical lining has 75% porosity and its thickness is decreased by 3% within the range of the applied lining pressures. It takes 0.1-0.2 s for the lining pressure to rise to the steady pressure during the engagement stage. During this short period of time, some of the heat generated at the interface is transferred by convection due to ATF flow into the porous lining material. The amount of heat transferred to the lining by the ATF flowing inside porous friction material is determined by the applied pressure, permeability of lining material, and the time taken to raise the pressure to the steady state. Comparing the results, obtained with and without considering the permeability in a model simulation, we observed that the peak interface temperature decreases by about 20 C if the lining permeability is 0.1 Darcy.It seems that liquid permeability does not affect the peak interface temperature very much, but it affects the torque response dramatically. Borg-Warner Automotive also developed a torque response model to predict the torque response during the dynamic engagement 10. The effect of liquid permeability on the total torque is illustrated in figure 7. If the liquid permeability is high, the ATF film thickness at the friction interface decreases very quickly at the beginning of the engagement cycle. This causes the asperity contact to act earlier, resulting in higher total torque (hydrodynamic torque plusFigure 7. Effect of liquid permeability on torque responseFigure 8. Comparison of model predictions and experimental measurements, Test Procedure 5006A (Energy Level A).asperity torque) at the beginning of the engagement cycle.To examine the accuracy of the thermal model predictions on the interface temperature, we compared the model predictions on the separator interface temperature with the experimental measurements for Procedure 5006A (Energy Levels A and B). The results of the comparison are shown in figures 8 and 9. The major parameters for the test procedure are listed in table 1. The duration times for the engagement stage, soak period, and stabilization period are 0.5, 6, and 5 s, respectively. The clutch has eight friction faces with an inner diameter of 88.9 mm and an outer diameter of 121.3 mm. The thickness for the separator, lining, and core plate is 1.8 mm, 0.508 mm, and 0.71 mm, respectively. The total input energy to the clutch is 19,588 J and 39,084 J for energy levels A and B, respectively. The initial rotating rate of the flywheel is 3450 RPM with an ATF flow rate of 5.67 l/min. The applied pressure is 669 and 1300 kPa for energy levels A and B, respectively. The lining (70% porosity) is compressed by about 23% at these low lin-Figure 9. Comparison of model predictions and experimental measurements, Test Procedure 5006A (Energy Level B).ing pressures. The friction lining has the permeability of 0.05 Darcy.The symbols in figures 8 and 9 are the experimental measurements for the steady engagement cycles and the solid curves are the model predictions, which show that the steady cycles are reached after 3 to 4 engagement cycles. Figures 8 and 9 illustrate that the interface temperature, predicted by the theoretical model, is in good agreement with the experimental measurement of interface temperature.3. Friction material degradationThe degradation of friction materials has been the subject of several investigations 1113. However, all the previous works of other researchers to the authors knowledge were experimental in nature, and theoretical work is lacking. In order to predict the life cycle of friction materials for a given application, it is necessary to understand theoretically the mechanism and rate of material degradation. The life of a paper-based friction material is affected by both the thermal degradation (carbonization) and mechanical degradation (wear). The thermal degradation of a friction material is the major factor for determining the life cycle under the normal engagement of a wet clutch due to the high temperature environment at the friction interface, caused by the friction heat generated during clutch engagements. After the friction material is thermally degraded to a certain extent, the friction characteristics will change dramatically and its performance will suffer. Generally, the least heat resistant composition in the friction material will be degraded first at elevated temperatures.In order to predict the thermal degradation of friction materials during the dynamic engagement of wet clutches for any given condition, the mechanism of thermal degradation and its rate constants need to be determined. The temperature history near the friction interface during the dynamic engagement, as computed by the thermal model described above, has to be accounted for when calculatingFigure 10. Weight loss of friction material by TGA analysis at temperatureramp modethe cumulative degradation. A model that predicts the thermal degradation of friction material will be presented, and the comparisons between experimental measurements and model predictions of thermal degradation will be illustrated.3.1. Degradation mechanismBorgWarner Automotive produces many types of friction materials, consisting of different constituents for various applications. For this study a production friction material with low heat resistance was selected for the sake of saving experimental time, since it may take over 10,000 engagement cycles for a friction material with high heat resistance to degrade a measurable amount. A TGA experiment of the raw friction material under a high resolution temperature ramp condition can be utilized to determine the percentages of cellulose fibers and inert components. The inert components are those that start to degrade after all cellulose fibers are carbonized. Figure 10 illustrates the sample weight loss as a function of temperature (the temperature ramp rate was set to 100 C/min for up to 300 C, and 200 C/min for temperatures between 300 C and 950 C). The solid curve is the percentage of sample weight loss, and the dotted curve is the negative gradient of weight loss percentage. The gradient peak at lower temperatures corresponds to the maximum rate of cellulose degradation, and the second peak corresponds to that of inert contents. This figure indicates that cellulose fibers start to carbonize near 200 C and finish the degradation at 420 C under the TGA testing condition. It also reveals that the weight percentage of cellulose fibers for this material is 31.2%.Cellulose fiber is the least heat resistant component in this friction material. As cellulose fibers encounter the high interface temperature during repeated clutch engagements, they carbonize. The cellulose fibers undergo a chemical reaction and the interwoven fibers become brittle pieces of carbon after carbonization. After the carbonization reachesFigure 11. Degradation mechanism of friction materials: zeroth and firstordera certain degree, the friction material loses strength and small chunks of material at the surface may be pulled away.The degradation amount/life cycles depends on the degradation rate and temperature history. The degradation mechanism and degradation rate need to be determined first. The degradation of friction material is a simple chemical reaction. We can first assume that the degradation is a nth order reaction, and then determine the n value.For a nth order degradation, its reaction rate can be expressed
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