FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
The Patent Rules, 2003
Complete Specification
(See section 10 and rule 13)
Dual Mass Flywheel with Damper Arrangement Containing Torsion Springs
Mahindra and Mahindra Ltd.
An Indian company registered under the Indian Companies Act, 1956.
F: 31, MIDC, Satpur, Nashik - 422 007, Maharashtra State, India
The following specification particularly describes the invention and the manner in
which it is to be performed.

Dual Mass Flywheel with Damper Arrangement Containing Torsion
Springs
Field of Invention
The invention relates to damping mechanisms used in vehicle transmission systems. In particular it relates to mechanisms involving dual mass flywheels.
Background of the invention:
Crankshaft of an IC engine does not rotate at a constant speed. During the power stroke, crankshaft rotates at a faster speed and during the compression stroke it rotates at a slower speed. The change in rotational speed generates torsional vibrations. Fatigue stress because of torsional vibrations decreases lifespan of the drive train components. In addition, torsional vibrations generate rattle noise between various rotating members in the drive train and affects Noise Vibration Harshness (NVH) characteristics of a vehicle.
The advancement in vehicle technology has led to increasing levels of engine torques, which causes higher levels of torsional vibrations. In addition, the installation space has decreased with increasing demand on the real estate because of increasing number of components and compact designs. Also there is a rise in

the comfort expectations from a vehicle. It creates the need for increasingly better NVH solutions.
A damping mechanism in the form of a spring and dashpot arrangement placed inside the clutch driven disc (Figure 2) has been conventionally used to overcome torsional vibrations. It uses a cone spring and a Polyamide friction washer. The constraints of space and of the damper travel that can be achieved limit the damping frequency range and the filtration efficiency of the spring and dashpot type damper. The springs that are typically used in such applications are generally of small diameters which are stiffer than what is required to isolate the NVH and as a result adversely restrict the range of travel of the damper. Hence, its use frequently results in Creep and Drive rattle noises. The advances in vehicle technology have led to higher engine torques which cause higher levels of torsional vibrations. To make the matters worse, due to the increasing number of components under the bonnet, the installation space has also decreased. One way of overcoming the creep and rattle noise would be to increase the damper size in the clutch-driven disc. However, in an era where automobile components are becoming more compact to cope with the increasing space restrictions, this is not a viable alternative.

A classic torsion damper with single mass flywheel damping system has proved inadequate to suppress the torsional vibrations adequately because of relatively low angular travel (18° to 25°). A more effective damper, known as Dual Mass Flywheel (DMF) damper, was built by splitting the flywheel into two (primary flywheel on the engine side and secondary flywheel on the driveline side) and placing the frictional damper and arc springs sandwiched between them.
The conventional design for DMF dampers in the case of low engine speeds is very effective in isolating idle and creep-rattle. However, at higher engine speeds, increasing centrifugal force on the arc springs causes friction between the arc springs and spring guides which house them within a DMF. As the arc spring pushes against the spring guide, the spring coil tightens due to restricted movement in localized areas. The tightening of spring coil increases effective spring stiffness, which in turn causes higher level of hysteresis reducing the damping effectiveness. This is also known as 'parasitic hysteresis' (see Fig. 8). conventional systems use grease to overcome this problem, however, while reducing the parasitic hysteresis grease poses threat of melting and leaking out, coming into contact with clutch friction facing and adversely affects its proper functioning. This reduces the reliability of the conventional DMF system.

Another shortcoming of the traditional DMF design is that the stiffness of damper is constant over longer range of angular travel, whereas, ideally, the stiffness should increase with the torque so that the damper can carry more torque in the shorter travel range.
There have been other attempts to reduce parasitic hysteresis and localized stiffening of springs. These include:
1. Use of special anti-friction coating on spring guides.
2. Use of special types of arc springs in with variable pitch in the windings to help increase the stiffness gradually over longer speed range (torque range).
3. Use of plastic shoes on springs to minimize the hysteresis between spring and the spring guide.
However, none of these methods has proved effective in eliminating the parasitic hysteresis.
The use of torsion spring instead of arc spring is proposed in several prior art such as patent nos. EP 0,259,173, EP 0,339,805, US 5,503,595, US 6,602,140, and US 7,195,111, but none of the patents discloses the design in proposed invention in

terms of the use of torsion springs connected through the special linkages with revolute joint.
There is therefore a need to provide a DMF that will help satisfactorily isolate the constantly increasing engine torques (See Figure 7) and consequential torsional vibrations. There's also a need to provide a simple but very effective solution to isolate the high amplitude of engine vibrations from the rest of the drive line. There's a further need to provide a DMF that will overcome the problems of parasitic hysteresis.
Objects and advantages of the invention:
One of the primary objects of the present invention is to provide a DMF that will help satisfactorily isolate the high amplitude of engine vibrations from the rest of the driveline at all engine speeds. The future trend in engines is of constantly increasing engine torques and hence resulting into higher torsional vibrations. A further object of the present invention is to provide a DMF that will overcome the problems of parasitic hysteresis arising from damper mechanisms such as arc springs used in conventional dampers. A still further objective is to minimize the use of grease in DMF to prevent it from adversely affecting the friction facing as conventional DMFs are susceptible to damage at an elevated temperature.

Summary of the invention:
The invention deploys a dual mass flywheel; it is a flywheel which is split into primary (Engine Side) and secondary (Transmission side) flywheels which are connected to each other through a damping mechanism. The damping mechanism of the present invention is in the form of an arrangement of torsion springs connected to special linkages constrained by revolute joints, which is housed within a DMF. Conventional polyamide friction dampers are also provided to further dissipate energy. As an effect of the critically placed linkages, the angular deflection of the torsion springs and that of the secondary flywheel (6A) vary with respect to each other (see Figure 4). This gives rise to a smooth variation damper stiffness (see Figure 5) with respect to the relative rotation of the DMF (defined as the relative rotation of the primary and secondary wheels). This type of smoothly variable' damping stiffness 'softens' the DMF at low torque transmission (like in idle and creep) and stiffens it up to transmit higher torques (vehicle running condition). Equally importantly, the absence of circumferential arrangement of springs and guides, as is common with conventional systems, results in the elimination of parasitic hysteresis. As there's no significant rubbing of springs, the invention does not use grease in the spring areas, thus further increasing effectiveness of the DMF.

Brief description of figures:
Figure 1: Conventional DMF flywheel damper (arc springs and dashpot type)
Figure 2: Conventional DMF flywheel damper perspective view
Figure 3A: DMF damper with torsion springs with Y-fork linkages and a close up view of a Y-fork linkage
Figure 3B: DMF damper with torsion springs and alternative type of linkages - an exploded view
Figures 3C and 3D: Torsion spring angular deflection measurement
Figure 4: Angular deflection of individual torsion spring v relative rotation of the DMF
Figure 5: Applied torque v relative rotation of the DMF
Figure 6: Trend in magnitude of the engine torque
Figure 7: Hysteresis curve for conventional DMF with and without parasitic hysteresis
List of parts:
Compression Spring - 1 Main plate - 4
A pair of retainer plates - 2 & 3 Torsion spring - 5

Primary flywheel - 6 Revolute joint - 13
Secondary flywheel - 6A Stud - 14
Torsion spring eye - 7 Y-end of Y-fork - 15
Spring holder - 8 Spring end of secondary link - 16
Linkage - 9 Main arm of the torsion spring - 17
Y-fork- 10 Nearly radial end of torsion spring -
18 Secondary link - 11
Tail ofthe Y-fork- 12 S nap-fit type receptor ~ . 9
Hole in the main arm - 20 Spring end of straight linkage- 12A
Hole in secondary wheel - 21 Wheel end of the straight linkage -
12B
Detailed description of the invention:
Figure 1 shows a typical DMF that is currently available. It shows a typical primary flywheel (6) and secondary flywheel (6A) arrangement along with the arc type springs (1) positioned in their respective slots. It is evident that the arc springs are not connected to the secondary wheel (6A) in any way.

The primary and secondary flywheels are also referred to as primary and secondary wheels, respectively, in this description.
Figure 2 shows a typical conventional dashpot arrangement inside a clutch disc where compression springs (1), sometimes known as arc springs, are confined to special slots made within the pair of retainer plates (2 & 3) and the main plate (4) connected to the hub of the clutch disc (not shown). As the torque from the engine is transmitted to the friction surface of the clutch disc, and hence plates 2 and 3 (as they are directly connected to each other); it compresses the springs (1) against the slots made in the main plate (4), hence the spring pushes the main plate (4) and the hub which transfers torque to the transmission.
In its simplest form, the present invention comprises torsion springs (5) positioned by spreading them on the inside face of the primary flywheel (6) of a DMF. They should be positioned loosely enough to allow their free movement whilst keeping them secured in place. In one preferred embodiment, the eyes (7) of torsion springs, which are the central opening of the spring, are placed on the primary flywheel (6) by loosely positioning the spring eyes (7) around a spring holder (8). In one arrangement, the spring holder (8) is a projection made on the inside face of the primary flywheel (6). In other arrangements, the spring holders (8) may be separate entities fixed onto the inside face of the primary flywheel (6).

Furthermore, the ends of the torsion springs (5) are secured in a variety of ways depending on the type of linkages (9) used for the purpose.
Figure 3A shows an arrangement where linkages (9) are made in two parts, namely a Y-fork (10) and a secondary link (11). The tail (12) (also known as the wheel end of the linkage) of the Y fork (10) is connected through a revolute joint (13) to a stud (14) provided on the secondary flywheel (6A). The Y-end (15) of the Y-fork (10) receives one end of the secondary link (11) in a revolute joint formation. The spring-end (16) of the secondary link (11) rigidly secures main arm (17) of the torsion spring (5). The joints between the Y-fork (10) and the secondary flywheel (6A) are of also preferably of revolute type constructed using any suitable arrangement, for example rivets. It means the Y-fork (10) and the secondary link (11) are free to rotate at the joint, about the axes passing through its center and parallel to the axis of the flywheel.
The number of such spring linkage (9) arrangements used in a DMF damper of the present invention can be varied depending upon the torque transmission requirement.
In general, the generic governing principle on which the invention is based is the following: The novel damping mechanism according to the present invention need to incorporate linkages (9) and torsion spring (5) arrangement such that:

1. A revolute joint between the linkage (9) and the main arm (17) of the torsion spring (5) or any of its connecting parts.
2. A revolute joint between linkage (9) and the secondary flywheel (6A) or connected parts like hub etc.
The arrangements shown in Figure 3A, which works on the above generic principles, is only exemplary and not intended to limit the scope of the invention.
Figure 3B shows another arrangement exemplifying the present invention wherein the DMF is once again split in two parts, namely the primary flywheel (6) and a secondary flywheel (6A). It also shows a number of torsion springs (5), all of which are coiled around axes that are preferably parallel to the axes of the primary and secondary wheels (6 & 6A). A number of such springs (5) are arranged with their main arm (17) in preferably a circumferential or tangential configuration.
The shape of the springs (5) used in this arrangement (Figure 3B) is somewhat different than the one used in Figure 3A. In the present case (Figure 3B), the ends of the springs (5) are positioned so that one arm is positioned in a more or less tangential configuration (the main arm (17)) and the other in a nearly radial orientation. The nearly radial end (18) of the spring is secured preferably in a

snap-fit type of receptor (19) positioned on the primary wheel (6). The number of coils in each of the springs (5) is preferably one to two for compact design, however it may be more. It should be noted that this alternative arrangement of the present invention conforms to the generic rules stated earlier. The linkage used in this type of damper system is made from a single member by configuring the linkage ends to facilitate connection with the main arm (17) and the secondary wheel (6A). A hole (20) is made in the tangentially positioned arm, also known as the main arm (17), of the torsion spring (5) into which the spring end of the linkage (12A) secured in a similar revolute joint (13). The other end of the linkage (9) (known as the wheel end (12B)) is secured into another hole (21) made in the secondary wheel (6A) in again a similar revolute joint (13), preferably nearer to the flywheel axis. The linkage (9) used in this arrangement is a simple member configured to suit the purpose.
In any possible arrangements of the present invention, the torsion springs (5) are evenly distributed so that the load on the center bearing will be minimzed, which allows to balance the DMF without much difficulty. The springs employed in the DMF are such that their tensile and fatigue strengths can withstand the stresses generated during the DMF operation.

One of the key features of the present invention is the use of a number of torsion springs, the collective effect of which to provide a damping effect which cannot be achieved through the use of a single spring. As the torque is transmitted from the engine down the transmission system, the primary flywheel rotates. This rotation is transmitted to the system of springs, which after absorbing some energy rotate the secondary wheel. However, as the spring absorbs some of the torque, the rotation of the secondary wheel is different than that of the primary wheel. This results in the relative rotation between the two wheels, and referred to as the relative rotation of the DMF.
As one of its key advantages, the 'smoothly variable' damping stiffness of the DMF damper of the present invention 'softens' the DMF at low torque transmission (like in idle and creep) and stiffens it up to transmit higher torques. As an effect of the innovatively positioned linkages (9), the angular deflection of the torsion springs (5) and the relative rotation of the DMF vary smoothly with respect to each other. This is illustrated in Figure 4, which shows a graph between the angular deflection of the secondary flywheel (6A) and that of the torsion spring (5) (denoted as a).
Figures 3C and 3D illustrates the method of measurement of α . It can be seen from these figures that the wheel end of the linkages follows a path that is part of

a circle drawn around the DMF hub. In doing so, while transmitting the torque the main arm if pulled towards the hub. The angle a is measured as the angle formed by the end of the main arm with the centre of the spring eye between its original position (Figure 3C) and the position (Figure 3D) under torque transmission.
It is thus obvious that the present invention gives rise to a smooth variation damper stiffness with respect to the relative rotation of the DMF. This is evident from Figure 5 which shows the relationship between the secondary flywheel rotation angle and the torque transmitted, the latter being directly proportional to the damper stiffness.
Fig. 5 also shows the characteristic torque v. rotation curve of conventional DMF dampers indicated by straight lines. It is a known fact that some of the conventional dampers contain two springs. One of the springs is of low stiffness to cater for low torque transmissions which typically occur up to 40° deg relative rotation of the DMF. In order to transmit the higher torques, which typically correspond to a 40-45° relative rotation of the DMF, the second spring is of a much greater stiffness, and which is utilized only after a certain amount of relative rotation of the DMF (40-45°). When the relative rotation of the DMF reached this point, the second spring kicks in and there's a sudden step-up in the effective damper stiffness. This is represented by a sharp change in slopes of the two

straight lines represented in Figure 5 which represent these two states - one representing a 'low' torque transmission and the second the high torque. However. as an undesirable and unpleasant effect of such arrangement, the passengers in the vehicle feel the difference in the vehicle vibrations till the damper adjusts itself with the torque being transmitted. As seen in figure 4, the same curve for the present invention is smoothly rising and there are no steps, there are no undesirable vibrations at any engine speed due to abrupt step up of stiffness.
The individual torsion springs (5) of the present invention angularly deflect as the relative rotation of the DMF varies. As the energy absorbed by a single torsion spring (5) is directly proportional to its own angular deflection, it can be concluded that the energy absorbed by the damper system comprising at least one such torsion spring (5), in first 10° is least and is increasing steadily throughout the travel range. It is well known that the effective damper stiffness is proportional to energy absorbed per unit deflection of spring and hence it is increasing too. When there is relative rotation between primary and secondary wheels (6 & 6A), the spring absorbs energy as the linkage (9) pulls the spring's main arm (the tangentially positioned arm (17)) towards the center of the flywheel. Referring to Figure 5, the right side of the zero deflection point corresponds to the drive mode representing vehicle movement phases such as accelerating (where the engine drives the transmission system), whereas the left

side corresponds to the coast mode representing vehicle deceleration (downhill motion, for example, where the transmission drives the engine resistance).
The stiffness of an individual torsion spring (5) is nearly constant throughout the range of its angular deflection. However, the novel arrangement of the linkages (9) with the torsion springs (5) together creates a combined system which delivers a non-linear incremental relationship between the angular displacement of the torsion springs (5) with respect to the secondary flywheel rotation. This is evident from Figure 5 which shows the resulting relationship between the applied torque and the relative rotation of the DMF. The stiffness of the damper mechanism (slope of the curve) increases with the increasing engine torque up to approximately 55-60° of angular deflection of the flywheel. The stiffness curve is smooth suggesting that the torque carrying capacity of the flywheel is increasing steadily, both in acceleration and deceleration mode, with the increasing deflection of secondary flywheel, thus having better damping efficiency in a smaller angular displacement as compared to the conventional dampers.
It is clear from the foregoing description and the accompanying figures that the torsion springs (5) are able to expand and contract and generally move freely without adversely coming in contact with any of the flywheel parts, thereby

eliminating friction, the resulting parasitic hysteresis (see Figure 7) and the consequent use of grease.
An integral part of a damper arrangement is the system that dissipates the absorbed energy to absorb shocks or reduce the peaks of vibrations. In the present invention, this is achieved by using conventional polyamide friction washer spring loaded with wavy washers or Belleville washers. They rub against the back side of the secondary flywheel (6A) when it rotates. The energy absorbed by the spring dissipates in the form of heat energy.
To summarize the advantages offered by the present invention, it is evident that the present invention is able to dampen the vibrations more effectively (no sudden stepping up as evident from Figure 5) and efficiently (smoothly rising curve in Fig 5) than the conventional design at all engine speeds. It does not have any sliding parts of spring so it does not use any kind of grease for lubricating them, thus the failure mode of grease meltdown and leakage which happens in conventional dampers is eliminated.

Examples:
Results of tests carried out using the DMF with torsion springs (5) of the present invention have been incorporated in Figures 4 and 5. It is evident from these figures that for first 10° of secondary flywheel rotation, the torsion spring (5) rotates by an angle 'a' of approximately 0.6° and the torque applied is approximately 0.5 Nm; for the next 10° of secondary flywheel rotation, 'α' increases by a further approximately 1.5° and torque by 25 Nm; and for the next 10° of secondary flywheel rotation, 'a' increases still further by approximately 3° and the torque by a further 50 Nm. and so on.
In an embodiment of the present invention, the springs may be made of any material suitable for such application including metals and alloys.
In another embodiment of the present invention, the springs may be made of hollow elements.
It is clear from the foregoing discussion that the present invention has the following items.

1. A dual mass flywheel damper comprising a primary flywheel and a secondary flywheel and a damping mechanism, characterized in that said primary and secondary flywheels are connected to each other through said damping mechanism, said damping mechanism in itself comprising an arrangement of at least one torsion spring connected to the two flywheels through special linkages operating with revolute joints, such that upon relative rotation of said primary and secondary flywheels, the individual torsion springs are strained thereby resisting said relative rotation.
2. A dual mass flywheel damper as described in item 1 wherein the number of said torsion springs is greater than 2.
3. A dual mass flywheel damper as described in any of items I and 2 wherein each of said torsion springs are securely positioned by placing their respective eyes around the respective spring holder, said eyes being either an integral part of said primary wheel or fixedly attached thereto, further with the two ends of each spring secured in a respective linkage, each of said linkage having two ends namely a wheel end and a spring end, wherein a wheel end is connected through a revolute joint to a respective stud placed on the secondary wheel, thereby linking the primary and secondary wheels.

4. A dual mass flywheel damper as described in any of items 1 to 3 wherein said linkage is a Y-fork type linkage made of two components, namely a Y-fork and a secondary link, such that the two components are connected to each other through a hinge connection at the two-prong end of the Y-fork so that they are able to rotate about an axis parallel to the central axis of the flywheels and the wheel end of said Y-fork being connected through a revolute joint to said respective stud, and the loose end of said secondary link secures the main arm of a torsion spring, so that upon the relative rotation between the primary and the secondary wheels, each of said Y-fork linkages deflects further whereby the main arms of respective torsion springs are pulled towards the centre of the dual mass flywheel.
5. A dual mass flywheel as described in any of claims 1 to 3 wherein each of said linkage is a single-component linkage, the wheel end of said single-component linkage being attached through a revolute joint to the secondary wheel and the spring-end of respective said single-component linkage securing the main arm of a respective said torsion spring, whereby upon the relative rotation between the primary and the secondary wheels, each of said single-component linkages deflects further whereby the main arms of the respective torsion springs are pulled towards the centre of the dual mass flywheel.

6. A dual mass flywheel as described in any of items 1 to 5 wherein said Y-fork linkages are made from metals or alloys.
While the above description contains much specificity, these should not be construed as limitation in the scope of the invention, but rather as an exemplification of the preferred embodiments thereof. It must be realized that modifications and variations are possible based on the disclosure given above without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their legal equivalents.

We claim:
1. A dual mass flywheel damper system comprising a primary flywheel and a secondary flywheel and a damping mechanism, characterized in that said primary and secondary flywheels are connected to each other through said damping mechanism, said damping mechanism in itself comprising an arrangement of at least one torsion spring connected to the two flywheels through special linkages operating with revolute joints, such that upon relative rotation of said primary and secondary flywheels, the individual torsion springs are strained thereby resisting said relative rotation.
2. A dual mass flywheel damper as claimed in claim 1 wherein the number of said torsion springs is greater than 2.
3. A dual mass flywheel damper as claimed in any of claims 1 and 2 wherein each of said torsion springs are securely positioned by placing their respective eyes around the respective spring holder, said eyes being either an integral part of said primary wheel or fixedly attached thereto, further with the two ends of each spring secured in a respective linkage, each of said linkage having two ends namely a wheel end and a spring end, wherein a wheel end is connected through a revolute joint

to a respective stud placed on the secondary wheel, thereby linking the primary and secondary wheels.
4. A dual mass flywheel damper as claimed in any of claims 1-3, wherein said linkage is a Y-fork type linkage made of two components, namely a Y fork and a secondary Jink, such that the two components are connected to each other through a hinge connection at the two-prong end of the Y fork so that they are able to rotate about an axis parallel to the central axis of the flywheels and the wheel end of said Y fork being connected through a revolute joint to said respective stud, and the loose end of said secondary link secures the main arm of a torsion spring, so that upon the relative rotation between the primary and the secondary wheels, each of said Y-fork linkages deflects further whereby the main arms of respective torsion springs are pulled towards the centre of the dual mass flywheel.
5. A dual mass flywheel as claimed in any of claims 1-3 wherein each of said linkage is a single-component linkage, the wheel end of said single-component linkage being attached through a revolute joint to the secondary wheel and the spring-end of respective said single-component linkage securing the main arm of a respective said torsion

spring, whereby upon the relative rotation between the primary and the secondary wheels, each of said single-component linkages deflects further whereby the main arms of the respective torsion springs are pulled towards the centre of the dual mass flywheel.
6. A dual mass flywheel as claimed in any of claims 1-5 wherein said Y-fork linkages are made from metals or alloys.