A Quarter of a Century of Damping Vibrations
LuK's Dual Mass Flywheel Celebrates its 25th Birthday
One trigger of torsional vibrations is the result of the periodic stroke of a fourstroke internal combustion engine that, when combined with the cylinder ignition sequence, leads to rotational irregularity of the crankshaft. The drive train, a system with characteristic natural frequencies capable of producing torsional vibrations, is also responsible for converting the rotational irregularities introduced by the engine into torsional vibrations. There is an increased need to deal with the development trend that began in the 1980's that involved optimizing the friction of the drivetrain and increasing its efficiency. However, the modern directinjection diesel engine provided the developers with new challenges more than twenty years ago. While engine torques continued to rise, the optimization of the drive unit also generated an increase in socalled "transmission chatter". Diesel engines with high torques in particular excite additional rotational vibrations that can be heard as a roaring noise from the body of the vehicle. Accordingly, eliminating these unpleasant side effects by finding a solution for eliminating vibrations became an important task for automotive engineers.
Until the volume production readiness in 1985 of the dual mass flywheel (DMF) developed by the clutch and transmission specialist LuK, which is part of the Schaeffler Group, clutch disks with torsion dampers were used for reducing rotational vibrations. The introduction of the dual mass flywheel set new standards in insulating noises and damping vibrations in the drivetrain. Instead of the rigid flywheel located between the engine and the transmission, the new system comprises a flywheel split in two masses. Since the creation of the DMF, the primary inertia masses on the engine side and the secondary inertia masses on the transmission side can be decoupled from each other. They remain connected to each other by means of a spring damping system. Socalled bow springs are at the core of the dual mass flywheel. They are significantly longer than the clutch damper springs previously used and can therefore insulate the vibrations that occur more effectively. The LuK DMF ensured that the critical speeds were reduced to levels under the idling speed for the first time, thereby ensuring secure damping of the vibrations originating in the engine. The significant increase in driving comfort meant that the DMF quickly gained a reputation as a vibration "killer" in the drivetrain.
The focus has now long since turned to another pleasing effect of the dual mass flywheel. Accompanied by high driving comfort, it promotes driving in operating ranges with high fuel efficiency, which also makes an important contribution to reducing CO2 emissions.
LuK heralded a new era in power transmission technology when it began manufacturing the DMF 25 years ago. The enthusiasm of the DMF's infancy remains unbowed, since it still enjoys growing popularity, not least due to the trend for ever increasing engine torques. Due to continuous further development, the DMF is experiencing the same level of innovation as power transmission system. The DMF was originally developed for highperformance engines, but has also long established itself as the standard in the compact vehicle class. Leading vehicle manufacturers all over the world rely on the effective vibration damping provided by the dual mass flywheel. Almost one in three passenger cars in Europe is equipped with this convenient technology. The DMF even makes a significant contribution to the reduction of fuel consumption and CO2 emissions as well as the increase in comfort required today in commercial vehicles.
LuK succeeded in developing an important innovation by integrating a centrifugal pendulumtype absorber in the dual mass flywheel. This milestone in the history of vibration damping opened up a new opportunity in terms of improving comfort, fuel consumption and emissions. Using a centrifugal pendulumtype absorber means the conventional inner damper is no longer required in the DMF. The speedadaptive damper without natural frequency is mounted on the flange of the DMF. The effective rigidity of the pendulum is generated by centrifugal force during operation. The selected order of excitation (for example, the firing frequency of the engine) can therefore be efficiently damped by making the relevant adjustments. Here, a pendulum mass of only one kilogram is all that is required to reduce vibrations in the transmission by more than 60 percent. The DMF with spring mass system continues to provide basic insulation of the vibrations and the remaining vibrations are counteracted by absorption at the engine firing frequency.
The idea of using centrifugal pendulumtype absorbers is not new. It has been used in aircraft construction for several decades. This idea remained in aerospace applications until about five years ago due to the high pendulum mass (around 5 kilos) required and the complexity of the engine application. It was only when the centrifugal pendulumtype absorber was combined with a modern dual mass flywheel did the idea become practicable in the drivetrain.
The DMF with centrifugal pendulumtype absorber made its debut in 2008 with the launch of the BMW 330 d and BMW 320 d Efficient Dynamics Edition, which feature unprecedented smoothness even when driving at low speeds.
Using a DMF in engines with lower speed ranges and in vehicles with lower technical complexity is advisable by all means. Damped clutch disks have dominated this market so far. The ZmartDamp, a costeffective further development of the DMF for small vehicles, means driving comfort will increase significantly in future for this vehicle segment, too.
A quarter of a century of history is a long time for a product in the shortlived and ever dynamic automobile market. However, this does not mean that the DMF's success story is coming to an end. There are several options for further development and adjustments are continuously being made to meet the requirements of the future. The Schaeffler Group manufactures more than six million dual mass flywheels annually in Germany, Hungary and China. Production reached the 50 million mark three years ago.