Best-in-Class Dampers for Every Driveline Concept
Dr. Ad Kooy
Figure 1 Schaeffler production of centrifugal pendulum-type absorbers per year for manual transmissions and double-clutch transmissions (left) as well as automatic transmissions (right)
Figure 2 Applications for the centrifugal pendulum-type absorber in the DMF (left) and other applications (right) on the clutch disk, in the single mass flywheel and as a double CPA in an automatic transmission converter
The CPA in the single-mass flywheel of a commercial vehicle drive is another development. A special feature that must be mentioned here is the flange on which the pendulums are arranged and that represents the geometry of the guide tracks. It is screwed to the flywheel, allowing a modular design.
Design of centrifugal pendulum-type absorbers from idle speed
The design of CPA in the range starting at idle speed has three objectives: First, it is important to achieve good isolation even at low speeds. This requires as much mass as possible and a vibration angle that is as wide as possible. Second, it is important to prevent the pendulum from lifting off from the guide track at low speeds and wide vibration angels to prevent wear Third, the goal is to cause as few damper stops at the end of the guide track as possible at low speeds in order to preserve the rubber stops that absorb part of the impact energy to prevent inherent noise.
Figure 3 Basic simulation for optimization of the guide track curvature of the pendulum and the flange in combination with the roller diameter using the example of an CPA without (top) and with (bottom) spring elements between the pendulums (couple pendulum)
The change in the guide track curvature in relation to the vibration angle in the CPA results in a change of the order. The effect of this change can be seen in the color diagram. If an order different from the second order is used, isolation deteriorates, but the vibration angle is also reduced, preventing the pendulum from stopping. That is why relatively strong imbalances are used at the end of the guide track.
In the simulation of the isolation effect of an CPA it must be remembered that, with large vibration angles, the guide track areas for small vibration angles are also over-swept which affects the resulting integral order. For this reason, the basic simulation is performed initially at full load. At part load with its smaller vibration angles, the curve of the order and thus the isolation effect is different even though the guide track is identical. Since the engine excitations are also lower at part load, however, no compromise is generally required when designing the guide track geometry.
As a result, very good isolation can be achieved with the CPA. It is only in the range just above the idle speed where it is not quite ideal. This is a starting point for improvements. For internal CPA, the use of significantly greater masses is a solution that can be considered if there is sufficient design space. Another option is the use of CPA positioned next to the arc spring and located further outside radially, if there is enough axial design space. The close connection between design space specifications and potential design solutions for damping the torsional vibrations underlines the fact that validation is required at an early stage through valid simulations.
Design in the start-and-stop range below idle speed
Speeds go down quickly when the engine is turned off. Starting at a certain point, gravity dominates centrifugal force, and the pendulums lose contact with the rollers. This may lead to noticeable, undesirable noise, particularly for external CPA whose pendulum masses are positioned far outside radially and that are not encapsulated as they would be in the internal CPA. Compared to this, the engine start is characterized by relatively strong excitations, so that stopping at the end of the guide track dominates here.
For internal CPA integrated into the flange, the noise described when turning off the engine can generally be reduced to a reasonable level. This is more difficult for external pendulums. One solution is the so-called couple pendulum in which the pendulums support each other in a circumferential direction through springs.
Here the spring preload is selected in such a way that the pendulum remains in the guide track even if the engine is at a complete standstill, thus not losing contact between the pendulum, the roller and the flange. The effect of the spring forces overlapping with the centrifugal forces is largely compensated by correcting the order of the track. This type of spring arrangement is particularly helpful for first order pendulums, such as those needed for cylinder deactivation from four to two active cylinders. This is because gravity also generates a first order excitation in a rotating pendulum, which increases the excitation from cylinder deactivation.
Figure 4 (right),
Figure 4 Design of an internal CPA with W stop damper (left) as well as tangential connecting elements on the pendulums (couple pendulum, right)
Figure 5 left.
Figure 5 Design of an CPA with U pendulum (left) and an iso-radial pendulum (right)
The new so-called iso-radial pendulum solution takes an entirely different approach, Figure 5 right. In this approach, the individual pendulums are connected in one point by a ring not located in the torque flow, which means that the pendulum masses are now synchronized. One of the usual two spherical rollers is eliminated, causing the pendulum to carry out a swiveling motion rather than a purely radial motion. This design eliminates the first order excitation from grvity on the individual pendulum masses. However, this principle does not help against contact loss at low speeds while stopping; here too noise has to be controlled by stop elements. One advantage though is the fact that coupling via springs can be eliminated. This way, improved isolation can be achieved, particularly at low speeds.
Centrifugal pendulum-type absorber on clutch disk
Figure 6 Basic simulation for optimization of the guide track curvature of the pendulum and the flange in combination with the roller diameter using the example of an CPA on the clutch disk without (top) and with (bottom) friction elements
Figure 7 The distance between the rollers with multiple pendulum motions (left) and inserted friction elements for roller stabilization (right)
The best compromise between necessary contact forces and isolation behavior can be found by inserting friction elements on the CPA, the normal force coming either from a waved disk or a diaphragm spring between the pendulum and the flange, Figure 7 right. These do not have a noticeable effect on isolation at higher speeds, but they do help optimize the vibration angle of the CPA at lower speeds. In addition, they ensure the axial stabilization of the pendulum and reduce stopping noises. The friction elements are made from plastic, have very high wear resistance and achieve a sufficient life even for the very high number of vibrations. This design principle can be used for both three-cylinder engines and four-cylinder engines.
Centrifugal pendulum-type absorbers in hybrid vehicles
With plug-in hybrids, the goal is for perceptible vibration-related differences between electric motor-only driving and driving with an internal combustion engine to be as small as possible. This results in complex requirements for isolation that are further complicated by the fact that the electric machine often limits the design space for the damper.
The arrangement of the electric motor also plays a significant role. In P0 and P1 arrangements, the additional torque of the electric motor must be transmitted by the damper; this is not the case in P2 to P4 arrangements. In particular for P2 arrangements, the additional inertia of the electric motor helps with the damping of torsional vibrations. The optimum solution depends greatly on the design space. Various operational principles such as masses, spring dampers and CPA can be combined here. Due to the drag start to re-start after sailing with the internal combustion engine turned off, there is another critical operating point in addition to regular starting. The restart should go unnoticed by the driver if possible. If an CPA is used, a couple pendulum, as described above, can help prevent stopping noises.
Figure 8 Reversed small radial damper (RSRD) with CPA and flywheel on front end of the crankshaft
Analyses at the operating principle level
In Schaeffler’s search for new damping concepts, one approach consists of using simple models to analyze different operating principles at the theoretical level. These models focus on operational parameters and are compared on a physical basis while maintaining the same inertia and spring energies. Limiting factors such as design space, costs and feasibility are not included until the second step. If an operating principle proves to be promising, the function of the new concept is analyzed with regard to expanded operating situations such as an engine start. In a simulated vehicle model, this is carried out with all relevant non-linearities.
Generally, there are three types of vibration damper systems:
• Passive systems: Energy is stored temporarily, alternating between kinetic and potential energy, such as in a classic spring-mass damper such as the DMF or the CPA which temporarily stores potential energy in the centrifugal force field.
Good results can also be achieved with an efficient combination of all of these systems.
Figure 9 Various operating principles for damping vibrations
Figure 10 Implementation of the anti-resonance principle using the example of a damper
Anti-resonance can also be achieved by means of a so-called summation damper or power splitting. Here too, torque amplitudes are added in such a way as to ensure that they completely erase each other for one frequency. These systems have a ratio as an additional parameter. The advantage of this is that no additional resonance is generated because there is no additional degree of freedom from an additional spring.
It is true for all anti-resonance systems that the amplitude of the counter excitation should be exactly inverse. It is possible to select the anti-resonance freely by carefully selecting the parameters. However, if the parameters remain constant, the anti-resonance also remains on a fixed frequency. The excitation frequency though changes in the vehicle in proportion to the engine speed. The problem can be solved by changing one of the relevant parameters in proportion to the excitation frequency. The CPA is one example. It uses the centrifugal force as an energy accumulator and thus, in a certain way, has a spring rate depending on the engine speed.
Limiting factors here include the limitation of the vibration angle by the design space and the non-linear curve of the tangential return force that decreases as the pendulum angle increases. This causes the CPA to lose potential with decreasing speed.
Application of operating principles on the centrifugal pendulum-type absorber
Figure 11 Theoretical potential of the CPA in the redistribution of the mass down to a reduction of the total mass of the damper system
Application of operating principles for dampers on intermediate flange
However, design spaces are not always ideal in terms of offering sufficient CPA mass. That is why analyzing the interaction of resonance and anti-resonance is well worth the effort. Operating principles can be helpful here too: If the damper is not arranged right on the secondary mass to be damped, but rather symmetrically between the primary and secondary mass, anti-resonance may result – without the interfering resonance in the higher speed range.
Figure 12 Operating principle of the damper on the intermediate flange: Achievement of an anti-resonance without interfering resonance in a higher speed range
Implementation: Damper on intermediate flange
The symmetrical arrangement can be achieved by placing the damper between the two masses on a so-called intermediate flange. This is a potential option in combination with an CPA if an CPA that is too heavy leads to strength problems, if possible limitation stops cause noise or if the available design space leaves too little clearance for radial deflection.
Figure 13 Placing a damper on an intermediate flange to utilize anti-resonance
This paper has shown some of the ways in which Schaeffler reacts to the growing variety of drive concepts and the resulting requirements for vibration damping. These requirements are very diverse and include changeovers from drives based on electric motors to those using internal combustion engines and the resulting start and stop challenges, cylinder deactivation, reduced design spaces, driving at low engine speed and increased NVH requirements. The refinement of the centrifugal pendulum-type absorber principle and the combination of various damper principles lead to function and cost optimized designs. In addition, the approaches presented in this paper allow another decrease in speed for driving in order to reduce consumption and emissions. The design methods for centrifugal pendulum-type absorbers have also been described.
In the search for new damping concepts, the operating principles they are based on play an important role. The resonance and anti-resonance principle in particular is suitable for developing matching damping concepts for complex design spaces.
As a consequence of the increasing variety of drive concepts, an ever greater range of vibrations are generated in the drive train which must be damped between the engine and the transmission. Schaeffler believes that classic internal combustion engines will continue to play an important role for a long time to come . In the future, numerous concepts are expected to help further reduce fuel consumption. Driving with a long ratio, for instance, reduces internal combustion engine losses due to lower speeds. If, however, the internal combustion engine is operated at just above the idle speed range, it is in a particularly economical range but this also produces strong, low-frequency excitations with high vibration amplitudes. In addition, the torque must be increased to prevent loss of performance. Other vibration-related challenges arise from cylinder deactivation, downsizing and the increasing number of hybrid vehicles with various arrangements of the internal combustion engine, electric motor and transmission. The on-demand switching between electric motor drives and internal combustion engine drives that is typical for hybrids and that is intended to go widely unnoticed by the driver leads to additional tasks for the dampers in the starts and stops of the internal combustion engine.
The trends for the drive train of the future result in high requirements for damping systems between the engine and the transmission that are increased even further by the growing expectations drivers have regarding comfort and driving dynamics. In order manage the ever growing number of variants, transmission manufacturers rely on the increased use of CAE tools with the aim of including the requirements for vibration damping and noise behavior (noise vibration harshness, NVH) as early as possible in the development process and reduce the number of tests with real prototypes This requires high development quality, something that Schaeffler ensures with actions such as the simulation of vehicle influences for tolerance evaluations and damper optimization . Beyond that, it is important to develop effective and customized damping systems for what tend to be smaller and more complex design spaces. This paper aims to show some of the ways that Schaeffler looks for even better operational principles and keeps driving damper technology with innovative combinations.
Under tolerances, a slight offset of the pendulum masses may also occur or the flange may be in a tilted position. Both may cause the pendulum to touch the flange. For this reason, ribs on the roller keep the components apart. In addition, sliding elements are used between the flange and the pendulum to minimize wear and friction .
The section below deals exclusively with passive systems. As shown in Figure 9, basic elements for kinetic energy, potential energy and ratio can be combined arbitrarily and thus describe a nearly unlimited number of operating principles .