1 Dual Mass Flywheel

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Such a system has two vibrations modes The first mode with a natural. frequency of between 2 and 10 Hz is known as the tip in back out reaction. This is generally excited by a driver induced load change. The second mode where the transmission inertia vibrates against engine. and vehicle has a natural frequency of 40 80 Hz with conventional torsion. dampers This is a typical cause of gear rattle,engine transmission vehicle. vibration model,f1 2 10 Hz,f2 40 80 Hz clutch disc. f2 7 5 15 Hz DFC DMFW,Figure 1 Vehicle drive train with vibration modes. Consequently the tuning of a conventional automotive torsion damper a. clutch disc with its corresponding spring characteristic always involves. compromise The upper graph of Figure 2 shows typical speed fluctuations. in a vehicle with a clutch disc In this case the friction damped resonance. is located at around 1700 rpm Further damping of this resonance leads to. a worsening of the hypercritical isolation of rotational vibrations at speeds. higher than the resonance,conventional system,transmission friction damping low. peak peak speed,amplitude rpm,transmission friction damping high.
1000 2000 3000,peak peak speed,amplitude rpm,transmission. 1000 2000 3000, Figure 2 Torsional vibration isolation with conventional clutch disc and. dual mass flywheel DMFW, The goal of torsion damper development is to keep the torsional vibrations. induced by the engine as far as possible from the rest of the drive train. A conventional system only satisfies this requirement at high engine. speeds because the attainable torsion damper spring rates lead to natural. frequencies which are always within the normal driving range. This unsatisfactory situation led to the development of a new torsion. damper concept the dual mass flywheel DMFW This design shifts part. of the flywheel inertia to the transmission input shaft and drastically lowers. the torsion damper spring rate by introducing new spring designs Figure. 3 thus reducing the resonance speed to very low engine speeds Figure 2. lower graph shows the hypercritical isolation of rotational engine vibrations. starting from idle speed,model design,engine clutch trans vehicle engine transmission. flywheel disc mission,engine clutch,Figure 3 Principle of the dual mass flywheel.
Improvements in driving comfort achieved by the dual mass flywheel. together with low cost designs resulting from goal oriented value analized. development has led to the increased popularity of this system Currently. the LuK dual mass flywheel is used by ten car manufacturers in. approximately 80 different models thus covering a wide range of engines. as shown in Table 1,engine type,number of cylinders gas diesel. 6 14 4 4 5,in development, Table 1 Dual mass flywheel used in production and development. Figure 4 shows a current dual mass flywheel with all its fundamental. components The primary side of the DMFW shown in blue consists of. formed sheet metal parts which make the spring channel and a cast hub. The secondary side of the DMFW shown in red consists of a cast disc. into which the torque is transmitted from the flange The secondary side is. mounted in the primary side over a ball bearing The heart of the system is. the arc spring whose special properties will be described in the following. The arc spring damper characteristic and function, The dual mass flywheel consists of the following main function groups. primary and secondary inertias,the torsion damper spring rate. the damping characteristic, The influence of the moment of inertia has been thoroughly discussed in.
1 2 and so will not be discussed in detail here, The spring rate and the damping characteristic are crucial in determining. the operating performance of a DMFW, What requirements does the ideal torsion damper have to fulfill. Figure 4 Dual mass flywheel,It has to control three basic operating modes. transmission rattle during idle drive and coast, resonance break through during engine start and stop. surging associated with torque changes, Significant characteristics for these operating modes are the frequency and.
the vibration amplitudes,Figure 5 shows how they are interrelated. torque angle,TD requirement, operating mode problem frequency angle spring rate damping. idle drive coast noise high low low low,load cycle surging low high low high. resonance break noise durability low high low high. Figure 5 Torsion damper requirements, Transmission rattle occurs during higher excitation frequencies 20. 400 Hz Vibration angles due to irregular engine torque are very small in. Even for a diesel engine with its characteristically extreme torsional. irregularity the vibration angle is seldom larger than 2 degrees In order to. achieve the best possible hypercritical isolation for this operating mode the. torsion damper should have a low spring rate together with a low damping. characteristic, The second operating point is the resonance break through During engine.
start and stop speed always increases from zero or is reduced to zero. This means that the system always passes through the resonance range of. the drive train When a drive train with a dual mass flywheel is designed. the aim is to achieve hypercritical isolation in the normal operating range. i e engine speeds above 700 rpm This means that the development goal. is to achieve maximum reduction of the resonance speed. The resonance break through is characterized by low frequency vibrations. together with a large vibration angle because the vibration angle of the. engine increases in association with decreasing speed In this case the. torsion damper design requires a low spring rate with a high damping. characteristic in order to avoid resonance magnification while passing. through the resonance range, Load cycling is characterized by low frequency vibrations at large vibration. angles In this case the damper requirements call for the lowest possible. spring rate and a high damping characteristic Sudden excitations of the. drive train result in large wind up angles coupled with high friction damping. in the torsion damper This method dissipates the energy of the free natural. vibrations in order to reduce the vibration amplitudes. Figure 5 represents an idealized damper characteristic designed to meet. these requirements i e produce a low spring rate and a high damping for. large vibration angles It also shows that the damping is very low for small. vibration angles, LuK dual mass flywheels contain an arc spring as the main element in. order to achieve suitable spring rates and damping characteristics Arc. spring principles are illustrated in Figure 6, In order to best use the available space a coil spring with a large number. of coils is inserted into a semicircular channel In the DMFW the coils of. this arc spring are supported by support races mounted in the spring. channel in the DMFW When a load is applied to the spring the movement. of the coils along the support races produce friction creating the damping. The contact surfaces of the arc spring are lubricated with grease. The enlarged area in Figure 6 shows the load equilibrium on one coil i of. the arc spring, As the spring load is transmitted along its curved line of action a normal. reaction Fi is created at the contact surface of each coil In addition to this. there is a speed dependent centrifugal force FZ The sum of these two. loads produces the normal reaction force which in turn produces the. friction load FRi for each coil,wind up direction,Figure 6 Arc spring.
F f c e n t r if u g a l f o r c e d e f le c t io n a n g le to r q u e. F1 F2 F3 F4 Fi,F1 F2 F3 F4 Fi,F1 F2 F3 F4 Fi,Figure 7 Arc spring damper function. Figure 7 illustrates how this system functions It shows the individual. contact points of the arc spring coils that are linked by the spring stiffness. of each coil There is a normal force Fi at each support point and the friction. coefficient acts upon each of these support points. Assuming the system is preloaded to a predetermined operating point and. a low cyclical load is applied around this point an equilibrium will be. reached at the contact points between the external load F the individual. coil spring load FFi and the friction load FRi, This operating condition is typical for normal driving i e small vibration. angles and produces spring rates in excess of the nominal spring rate of. the complete arc spring At the same time however the resulting friction. damping characteristic remains very low, Figure 8 shows just such partial hysteresis loops with a low damping. characteristic in green The spring rate in this case is clearly higher than. the nominal spring rate of the complete arc spring. partial hysteresis loop,during drive with,low damping. high damping during,tip in back out,wind up angle,calculation.
measurement, Figure 8 Characteristic curve of an arc spring damper. When large vibration angles occur in the second operating mode as is. typical for tip in back out or resonance break through all coils of the arc. spring become active This results in a reduced spring rate together with. high damping as shown by the cross hatched area in Figure 8 Figure 8. also shows the close match between the measured curve and the curve. calculated using the method shown above, The dependence of spring rate and friction damping on engine speed and. vibration angle for a special DMFW is shown in Figures 9 and 10. Figure 9 shows that for an increasing speed and a decreasing vibration. angle the spring rate of the arc spring damper increases because of the. deactivation of the coils The diagram also shows the engine performance. curve with the vibration angle as a function of velocity for drive coast in. red and for start stop in yellow The curve represents actual operating. points for a specific 2 5 l Diesel engine, Figure 10 shows the corresponding friction damping pattern It can be seen. that the torsion damper friction increases with increased engine speed But. unlike the spring rate the friction damping characteristic decreases sharply. with reduced vibration angle Again the reason is that some of the arc. spring coils are deactivated, In vehicle performance is determined by a combination of spring rate and. load cycle,50 spring rate,spring rate 0,start stop 10 engine speed.
vibration angle,drive coast, Figure 9 Spring rate of the arc spring damper as a function of engine. speed and vibration angle,load cycle,25 friction,start stop 10. vibration angle engine speed,drive coast, Figure 10 friction damping as a function of engine speed and vibration. start stop,fication 0 5,vibration angle engine speed rpm. drive coast, Figure 11 Magnification factor of the arc spring damper.
The magnification factor of an arc spring damper characterises damper. performance and its effective range Here magnification factor is the ratio. of the speed irregularity from the DMFW output transmission side to the. DMFW input engine depending on operating parameters This calculation. is shown in Figure 11 as a function of engine speed and vibration angle. The line respectively the surface with a magnification of 1 corresponds to. a complete transmission of the engine irregularities into the transmission. input shaft as if a rigid connection existed between the engine and the. transmission, The graph clearly indicates that an excellent isolation effect has been. achieved over wide ranges of engine speed and vibration angle and that a. magnification factor of 1 almost never occurs The engine performance. curve displays the actual established values for the magnification factor at. any given point From low to high engine speeds engine vibration. amplitudes are effectively isolated from the rest of the drive train. preventing transmission and vehicle noises,torsion damper angle. DMFW wind up angle,0 0 25 0 5 0 75,transmission,0 0 25 0 5 0 75. Figure 12 Arc spring damper performance during start up. Figure 12 demonstrates these results using measurements taken during. start up One can see that when resonance break through occurs at. relative large vibration angles the resulting low spring rate and high. damping characteristic do not induce excessive vibrations Even in idle. when friction damping automatically decreases due to decreasing vibration. clutch disc engine transmission engine DMFW clutch disc Figure 3 Principle of the dual mass flywheel Improvements in driving comfort achieved by the dual mass flywheel together with low cost designs resulting from goal oriented value analized development has led to the increased popularity of this system Currently

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