DMF operation and components
There are different technologies available to filter out the rotational vibrations of the crankshaft. All these technologies can be classified into three main categories:
- active damping: in this case an active component (damper) is used, which can generate an opposite force to the crankshaft vibration force; this way the vibrations are cancelled out resulting in a smooth rotation of the crankshaft; this method gives the best vibration reduction performance but comes with a high cost; also, the active component requires an external energy supply and does not have the required reliability for automotive applications
- semi-active damping: it’s similar with the active damping technology but with less external power requirement
- passive damping: implies the usage of a passive component, which does not require external energy but can dissipate energy; the most common applications usually consist of a spring and damper; this is the most cost-effective solution with a reasonable good vibration reduction characteristic.
One efficient and cost effective solution to reduce the rotational (torsional) vibrations is to use a DMF, which is the abbreviation for Dual Mass Flywheel. The dual mass flywheel (DMF) is a passive damping component and its main function is to isolate the driveline/transmission from the vibration generated by the internal combustion engine. This technique will also improve the overall noise behaviour of the vehicle and reduce fuel consumption.
Compared with a conventional flywheel, which has one mass, a standard DMF consists of a primary mass and a secondary mass. The two masses are decoupled and connected through a system with springs and dampers. Both masses are supported by a deep groove ball bearing or plain bearing so they can rotate against each other. The spring rate and the damping characteristic are crucial in determining
the operating performance of a dual mass flywheel.
The primary mass (2) (see figure below) is tightly bolted to the crankshaft, has the starter ring gear (1) attached and it’s driven by the engine. It encloses, together with the primary cover (6), a cavity which forms the arc spring channel.
The main components of the spring-damper system are the arc springs (3). They sit in guides in the arc spring channels and cost-effectively fulfil the requirements of an “ideal” torsion damper. The guides ensure correct guidance of the springs during operation and the grease around the springs reduces wear between themselves, the guides and the channels.
Between the primary and secondary mass, the torque is transferred via the flange (5). The flange is riveted to the secondary mass (7) with its wings sitting between the arc springs.
The secondary mass helps to increase the mass moment of inertia on the gearbox side. Vents ensure heat created during clutch friction is dissipated efficiently. As the DMF has an integral spring-damper system, a rigid clutch disc without a torsion damper is normally used.
In a vehicle with a conventional flywheel and torsion-damped clutch disc, the torsional vibrations in the idling range are transferred practically unfiltered to the gearbox and cause the gear teeth edges to knock together (gearbox rattle). On the other hand, the spring-damper system of the DMF filters out torsional vibration caused by the engine. This prevents gearbox components knocking against each other – rattling does not occur and the driver’s demands for higher comfort are fully met.
The functioning principle of a dual mass flywheel is simple and efficient. Due to the additional mass on the transmission input shaft, the vibration torque range, which is normally between 1200 rpm and 2400 rpm with original torsion dampers, is moved to a lower resonance speed range. This ensures excellent damping of engine vibration even at idle speed.
The dual mass flywheel (DMF) made it possible to isolate the rotational vibrations of the internal combustion engine from the rest of the drivetrain. The undesired gearbox rattling noises were eliminated and body boom considerably reduced. It also became possible to drive the vehicle at very low engine speeds, by increasing low-end torque, therefore reducing fuel consumption.
There are many other operating points which must also be considered when designing a dual mass flywheel (DMF). Firstly, the engine must be started and later stopped at the end of the journey and perhaps also at traffic lights. The drive itself begins with the vehicle launch. Changes in the accelerator pedal position as well as gear changes cause load changes in the drivetrain, or the vehicle coasts without load. These are only a few of the additional operating points in which there is a high demand for comfort.
In all these operating points, the DMF substantially reduces noise, rotational vibration and overall vehicle comfort.
The primary mass is connected to the crankshaft of the internal combustion engine. The primary mass of the DMF and the crankshaft are combined together to form a whole inertia. Compared to a conventional flywheel, the primary mass of the DMF is significantly more flexible, which helps to relieve the crankshaft load. In addition, the primary mass – together with the primary cover – forms the arc spring channel which is typically divided into two sections, separated by the arc spring stops.
The primary mass is a steel stamped component with a sufficient mass moment of inertia. In certain cases, it could be made from cast iron. For engine starting, the starter ring gear is positioned on the primary mass. Depending on the type of DMF, it is either welded or cold pressed on.
The cover is welded to the primary mass to form a sealed chamber containing the curved springs, spring guides and the lubricant.
The engine torque is transferred from the primary mass to the secondary mass via the arc springs and the drive plate. Thanks to the bearing between the primary and secondary mass, independent radial movement of the masses is possible. As with a rigid (single-mass) flywheel, the power output is through the clutch, which is bolted to the secondary mass. The crucial difference, however, is that the engine torque is now largely free of rotational vibration, i.e. it is modulated. Because of this, a clutch disc with torsion damping can be dispensed with in most cases if a DMF is used.
The secondary mass is a cast iron component. One side is machined to form the friction surface of the disc. The secondary mass transmits the engine torque to the clutch, and further to the gearbox and the wheels.
The bearing in the primary mass serves as a rotating connection with the secondary mass. It not only has to absorb the weight-related radial forces of the secondary flywheel and the clutch, but also the axial forces generated by the release force when disengaging.
A dual mass flywheel (DMF) uses two different types of bearings:
- ball bearing: when development of the DMF started, large ball bearings could be used because of the relatively simple design of the internal components; however, the constantly rising demands on the rotary vibration damping made additional components necessary in the DMF; for this reason, further construction space had to be created; this led to a systematic reduction of the diameter of the ball bearing; small ball bearings allow the space-neutral integration of additional rotary vibration dampers and, in this way, increase the efficiency of the DMF.
- plain bearing: in comparison with ball bearings, plain bearings take up less space and are more simply designed; in spite of their low manufacturing costs, they can be universally used and, if necessary, can be designed to allow axial motion.
The primary mass is fitted with a turned hub on which the large-size ball bearing is fitted. A hub flange with the bearing seat (turned or drawn) is mounted onto the primary mass. The bearing seat can be adjusted to mount a small ball bearing – as shown here – or a plain bearing.
In comparison with ball bearings, plain bearings take up less space and are more simply designed. In spite of their low manufacturing costs, they can be universally used and, if necessary, can be designed to allow axial motion.
The task of the drive plate is to transfer torque from the primary mass via the arc springs to the secondary flywheel; in other words, from the engine to the clutch. The drive plate is tightly riveted to the secondary mass with its wings (arrows) sitting between the arc spring channel of the primary mass. The gap between the arc spring stops in the arc spring channel is big enough to enable the drive plate to rotate.
The rigid drive plate is riveted directly to the secondary mass. This allows the use of drive plate wings with different symmetries, which has a positive effect on the isolation of vibration. The simplest form is the symmetrical drive plate, where pull and push sides are identical. Thus, load is applied on the arc springs via both outer and inner areas of the end coil.
The key function of the DMF is to isolate the transmission from the vibration generated by the engine. In order to compensate for the constantly increasing engine torques while the installation space remains the same, the windup curves of the arc springs must rise more steeply. Consequently, their vibration damping capacity deteriorates. Using friction-free internal dampers helps to improve vibration elimination during acceleration. Both the drive plate and the side panels are designed with spring apertures which house straight pressure springs. The excellent vibration damping characteristics of the DMF with internal damper are guaranteed even in the highest torque ranges.
At high engine speeds, the resulting centrifugal forces press the arc springs to the outside against the guides and the coils are disabled. Consequently, the arc spring stiffens and spring action is partly lost. In order to maintain sufficient spring action, straight pressure springs are mounted in the drive plate. Owing to their lower mass and mounting position on a smaller radius, these springs are subject to a lower centrifugal force. Additionally, the convex shape of the upper edge of the spring windows helps to minimise friction. This ensures that neither friction nor the effective spring rate will increase as engine speeds go up.
When an attempt is made to adjust the engine speed very quickly to the speed of the gearbox input shaft, sudden peak loadings occur, so-called impacts. In this way, for example, an impact may be caused by a sudden engagement, leading to stalling of the engine. Here, the arc springs are briefly fully compressed, leading to a disproportionate increase in the loading on the drive plate. In the case of rigid drive plates and those with internal damping, frequent impacts may lead to material deformation, culminating in breakage of the drive plate wings.
One way to compensate for impacts and minimise material damage is a drive plate with a friction clutch. In this case, the drive plate is designed as a diaphragm spring. It is pre-tensioned and positioned by two riveted retaining plates with a thin friction lining. In cross-section, this forms a fork-shaped fixture which allows slipping of the drive plate. In the case of an impact, the drive plate can now rotate in the retaining plates. The surplus energy is dissipated as friction heat. In this way, the load on the drive plate wings is minimised.
During the start-up process, the DMF operates briefly in the resonant frequency range. When this happens, the drive plate wings repeatedly hit the arc springs with unbraked force, producing noise as they do so. An effective countermeasure here is an additional friction device, the friction control plate. This has the effect of delaying the rotation of the drive plate within a defined working range. As a result, the drive plate can be rotated over the secondary mass in the range of clearance angle (α) without noticeable resistance. Only outside the clearance angle, i.e. at greater angles of rotation, does the additional friction come into effect. In this way, the noises produced when starting up or changing the load can be eliminated.
During engine start-up, a high angular deflection occurs between the primary and secondary mass. To limit this deflection and help improve engine start-up, friction washers are added on certain applications. They do not operate in drive mode.
DMF systems help to improve the noise behaviour of the vehicle by using special torsion damper designs. As a direct result, less noise is generated and fuel consumption is reduced. In order to make ideal use of the available space, a coil spring with a large number of coils is fitted in a semicircular position. The arc spring lies in the spring channel of the DMF and is supported by a guide. Under operation, the coils of the arc spring slide along the guide and generate friction and thereby damping. In order to prevent wear on the arc springs, the contact surfaces are smeared with grease. The optimised shape of the spring guides helps to reduce friction significantly. Besides improved vibration damping, arc springs help to reduce wear.
Thanks to the diversity of arc spring designs, a dual mass flywheel system can be manufactured to precisely match the individual load characteristics of each vehicle type. Arc springs of various designs and characteristics are used. The most frequent types are:
- single-stage springs
- two-stage springs: either in a parallel arrangement in one of various different layouts, or in-line arrangement
- damping springs
In practice, the spring types are applied in various different combinations. The spring rate and the damping characteristic are crucial in determining the operating performance of a dual mass flywheel.
Benefits of the arc spring:
- high friction at large rotation angle (start-up process) and low friction at low rotation angle (overrun)
- lower actuation force (spring rate) because of the flexible space utilisation (in comparison with systems with multiple single springs)
- impact damping can be integrated (damping spring)
The basic version of the arc spring is a single spring. This is characterised by its large spring volume and resulting high damping capacity. Because of its simple design, however, it only offers limited possibilities for satisfying rising demands for comfort. For this reason, today’s DMFs are seldom fitted with single springs.
The arc springs in most frequent use today are single-stage parallel springs. It consists of an external and internal spring, of about the same length. The two springs are arranged in parallel. Their individual characteristics add up to the spring set curve.
In two-stage parallel springs, two arc springs are again arranged one inside the other. The internal spring, however, is shorter, thus engaged later. The wind-up curve of the external spring is matched to the requirements of the vehicle when the engine starts. Here, load is applied only on the softer external spring, enabling the system to pass the critical resonance speed range faster. In the higher and maximum torque ranges, load is exerted on the internal spring as well. Both external and internal springs work together in the second stage. The interplay of both springs provides good damping at all engine speeds.
Three-stage arc spring consists of one external spring and two internal springs of different lengths arranged in-line. This design combines the benefits of the parallel and in-line arrangements and therefore allows for optimum torsion damping at each engine torque.
The configuration of the springs in the first-generation dual mass flywheels was identical to conventional torsion dampers, where the pressure springs are mounted in a radial direction close to the centre and can therefore provide only limited spring capacity. This design was sufficient to isolate vibration in 6-cylinder engines, as these produce low resonance speeds.
In contrast, 4-cylinder engines induce higher irregularities and consequently higher resonance speeds. Repositioning of the springs towards the outer edge and the use of high-pressure spring diameters increased the damper capacity 5 times without requiring more space.
The primary side of the dual mass flywheel (shown in blue) consists of formed sheet metal parts which make the spring channel, and a cast hub. The secondary side of the dual mass flywheel (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 system.