Patent Publication Number: US-2019178336-A1

Title: Composite dual-mass flywheel, system comprising said flywheel, and related method

Description:
TECHNICAL FIELD 
     The present invention relates to improvements to flywheels for use on mechanical transmissions. The invention also relates to systems and methods using these flywheels. 
     Background to the Invention 
     Flywheels are members for smoothing the operation of mechanical systems comprising a power source and a driven member. In automotive, flywheels are arranged on the transmission between the reciprocating IC engine and the driving wheels. In this case, the flywheel function is to make the torque generated by the reciprocating engine more uniform. Dual-mass flywheels are known, in which two coaxial masses are connected to each other through viscous-elastic members, in order to dampen the torsional oscillations. The two masses rotate synchronously except for the angular oscillations due to vibrations. US-A-2012/0234131 discloses a dual-mass flywheel of this type, in which the two masses can be rigidly connected to each other under certain operating conditions, preventing any movement relative to each other. 
     GB-A_2107428 discloses a variable capacity flywheel mechanism, wherein an auxiliary flywheel is connected to a main flywheel by means of an electromagnetic cutch. The clutch is actuated in response to a magnetizing signal in accordance with the rotary speed of an engine crankshaft. 
     In many applications, the load applied to the power source is discontinuous, i.e. the load applies a cyclically discontinuous resistant torque to the mechanical transmission. 
     In this case, the flywheel function is to smooth the operation, which (due to the periodic load) would be subject to continuous speed variations without the flywheel. The greater the inertia of the flywheel, the lower the speed variations generated by the periodic load and the more regular the operation of the driven machine and the driving machine (power source). However, by increasing the flywheel inertia, the starting and stopping phases of the driven machine become more difficult and time consuming. 
     The start is particularly critical when the power source is an IC engine, which tends to turn off or supplies very low starting torque, if the rotational speed is too low. Therefore, in some applications it is necessary to use large IC engines to overcome the starting phase, even if the average power absorbed by the working load does not require all the power deliverable by these motors. 
     In other words, it is necessary to oversize the engine with respect to the power required in steady-state conditions, for the sole purpose of being able to start the driven machine. 
     Problems of this type are frequent in the agricultural sector, where tractors actuate agricultural machines with cyclically variable loads. 
     Nowadays, tractors are equipped with so-called soft start systems, characterized by an initial start-up phase, wherein for a few seconds a clutch inside the tractor transmission limits the torque supplied to the power take-off, in order to avoid that the IC engine switches off. However, the soft start systems are often not enough, as they require too long internal clutch slipping times, with consequent overheating. The operator shall often make some starts in succession with the soft start system, which gradually bring the flywheel and the machine to an initial rotation speed such as to allow the power take-off to start the machine without switching off the IC engine. It is evident that a succession of starts causes overheating that is harmful to the internal friction of the soft start. Moreover, these operations are time-consuming and need a manual intervention that requires particular expertise. 
     Problems of this kind are typically for example in large square balers. These machines are characterized by a compression chamber for pressing the material to be packaged, in which the material is pressed by means of a piston actuated by a rod-crank mechanism. The piston presses the material for about a quarter of the travel generating a periodic load characterized by very high thrust peaks followed by virtually zero thrust strokes. 
     Due to the type of operation, the square balers require large flywheels. Thanks to the continuous search for continuously increasing performance, aimed at increasing the number of bales produced per hour and the density thereof, nowadays these machines are equipped with flywheels of ever larger size with the aim of increasing the inertia thereof and have more energy available to smooth and reduce loads on mechanical transmissions during operation. But this made the start-up phases even more difficult, even for more powerful tractors, for the reasons mentioned above. 
     A need therefore exits to improve the power transmission from a power source to a variable load, in order to alleviate or reduce at least partly the disadvantages described above. 
     SUMMARY OF THE INVENTION 
     According to an aspect, the invention provides a system including a power source, a driven machine and a power transmission therebetween. The transmission includes a composite flywheel comprising a primary flywheel, integral with an input shaft, and a secondary flywheel coaxial with the primary flywheel. The secondary flywheel and the primary flywheel are coupled such that they can rotate freely with respect to each other. Furthermore, a coupling device is provided that, according to the operating conditions of the machine in which the flywheel is inserted, can transmit power from the input shaft to the secondary flywheel, and selectively disconnect the secondary flywheel from the input shaft. 
     Contrary to the composite flywheels used in the automotive sector, in which the two masses are constantly maintained in reciprocal torsional connection through a viscous-elastic coupling, in the composite flywheel described herein the primary flywheel and the secondary flywheel can take a complete torsional disengagement condition, wherein the primary flywheel can rotate around its own axis, while the secondary flywheel remains idle, and no torque is transmitted thereto. 
     In this way, when the coupling device is switched to the condition in which it keeps the secondary flywheel disconnected from the input shaft, the power available from a power source can be integrally used to accelerate the primary flywheel and a driven machine connected to the power source. The secondary flywheel can be coupled, preferably in a gradual manner, to the input shaft after the input shaft and the primary flywheel keyed onto it have achieved a given operating condition, e.g. a given angular speed, i.e. a given number of revolutions. 
     In this way it is possible to start a machine provided with a large mass flywheel, using a relatively limited power source, i.e. a source dimensioned to supply sufficient power under steady-state operating conditions of the driven machine, but which would not have sufficient breakaway power to initiate rotation of the whole inertial mass of the flywheel. By dividing the flywheel into two parts, i.e. a primary flywheel and a secondary flywheel, the angular acceleration of the flywheel mass occurs in two stages. 
     It is also possible to facilitate stopping of the driven machine connected to the composite flywheel, as in this case the secondary flywheel can be released from the primary flywheel braking only the primary flywheel and the machine connected thereto. 
     In principle, it can be provided that the coupling device is adapted to connect the secondary flywheel to the input shaft. In particularly advantageous embodiments, the coupling device is adapted to selectively couple and decouple torsionally the primary flywheel and the secondary flywheel with respect to each other, with a direct connection. In other words, the coupling device can be arranged between the primary flywheel and the secondary flywheel and adapted, so as to selectively couple torsionally the secondary flywheel to, and decouple it from, the primary flywheel directly. In this way, the primary flywheel acts as a driving flywheel and the secondary flywheel as a driven flywheel. 
     In this context, torsional coupling means a connection between two members rotating around an axis, which allows to transmit torque from one to the other of said rotating members. Therefore, torsional connection between the primary flywheel and the secondary flywheel means a mechanical connection, which allows to transfer torque from the primary flywheel to the secondary flywheel. 
     In some embodiments, the primary flywheel is torsionally constrained to the input shaft and to an output shaft. The secondary flywheel can be rotatably supported on the output shaft and can be selectively coupled to and decoupled from the primary flywheel through the coupling device. 
     Advantageously, the coupling device can be adapted to modulate the torque transmitted to the secondary flywheel. In embodiments described herein, the coupling device is adapted to modulate the torque transmitted to the secondary flywheel according to the rotation speed of the secondary flywheel. In other embodiments a time control can be provided, in which the transmitted torque is gradually increased as a function of time. In both cases, the secondary flywheel is accelerated gradually. 
     In advantageous embodiments, the coupling device may comprise a clutch, for example a hydraulically controlled clutch, or in general a hydraulically controlled coupling. In other embodiments, an electrical control may be provided. 
     When the coupling device is hydraulically controlled, the torque transmitted to the secondary flywheel can be controlled through the pressure of a control fluid, for example oil, delivered by a power unit. More generally, when the coupling device is adapted to modulate the torque transmitted to the secondary flywheel, said coupling device can be actuated by means of a modulated signal, which may be the pressure of a control fluid, or an electrical voltage, or another parameter that can be controlled, for example directly or indirectly through a central electronic control unit, comprising one or more processors and associated memory. 
     However, the control may be even manual. For example, a friction coupling device or other device can be provided, having a manual control of the transmitted torque. If the coupling device is of the hydraulic type, a manually operated pressure control valve can be provided. In other embodiments, for example in the case of a mechanically controlled coupling device, a manual actuator can be provided, acting on the coupling device to increase or decrease the transmitted torque. A manual actuator may include, for example, a screw system. 
     In some embodiments, the flywheel comprises a power output shaft, coaxial with the primary flywheel and the secondary flywheel. The secondary flywheel can be idly supported on the output shaft. When the coupling device is hydraulically controlled, a supply duct can be provided to supply a control fluid to the coupling device, the supply duct being provided in the output shaft. The fluid can enter the duct at an end of the power output shaft facing a driven machine. 
     In some embodiments, the secondary flywheel is torsionally constrained to a shaft supporting the secondary flywheel, which shaft can be suitably hollow so as to house bearings rotatingly supporting the secondary flywheel on a power output shaft. The primary flywheel can be in turn torsionally constrained to a shaft supporting the primary flywheel. The primary flywheel support shaft can be hollow and coaxial with the secondary flywheel support shaft and can be externally supported on it by means of primary flywheel support bearings. 
     In this way, a coaxial configuration of members is achieved, comprising: a power output shaft, on which the secondary flywheel support shaft is mounted by means of bearings; the primary flywheel support shaft, which is hollow and surrounds the secondary flywheel support shaft and is supported on the latter by means of bearings. The coupling device can be adapted to: transmit torque from the primary flywheel support shaft to the secondary flywheel support shaft; and to selectively decouple said two support shafts from each other. 
     According to particularly advantageous embodiments, the primary flywheel and the secondary flywheel have different masses and/or inertias, for example, the moment of inertia of the primary flywheel can be lower than the moment of inertia of the secondary flywheel. 
     In some embodiments, the primary flywheel and the secondary flywheel are adapted so as to reduce the overall axial bulk. To this end, it is for example possible to configure one of the primary flywheel or the secondary flywheel with a mass distributed on an annular volume of larger diameter and the other of the two flywheels with a mass distributed on an annular volume of smaller diameter, so that the lower annular mass is contained inside the greater annular mass. 
     The system disclosed herein comprises a central control unit adapted to supply a controlled torque to the secondary flywheel as a function of at least one operating parameter, for example a parameter correlated to the rotation speed of the primary flywheel or the secondary flywheel. 
     In embodiments disclosed herein, the driven machine is a square baler. In other embodiments and more generally, the driven machine can be a machine with a periodic load, strongly variable between a maximum value and a minimum value. 
     The present invention also concerns a method for operating a driven machine through a power source and a mechanical transmission comprising a composite flywheel as disclosed herein. The method may comprise the steps of:
         starting rotation of the transmission and of the primary flywheel with the coupling device disengaged, gradually increasing the rotation speed of the primary flywheel keeping the secondary flywheel disengaged from the primary flywheel and idle on an output shaft of the composite flywheel;   when a switching operational condition has been achieved, switching the coupling device so as to connect the secondary flywheel to the primary flywheel and to transmit torque from the primary flywheel to the secondary flywheel.       

     According to another aspect, a method is provided for braking a driven machine actuated by means of a power source and a mechanical transmission associated with a composite flywheel as disclosed above. The method comprises the step of disengaging the secondary flywheel from the primary flywheel through the coupling device and braking the driven machine with the secondary flywheel idle with respect to the mechanical transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by following the description and the accompanying drawing, which shows a non-limiting exemplary embodiment of the invention. More in particular, in the drawing: 
         FIG. 1  is a diagram of a power source and a driven machine, connected to each other by means of a mechanical transmission with a flywheel; 
         FIG. 2  is a cross-section of a flywheel in an embodiment; 
         FIG. 3  is an enlargement of  FIG. 2 ; 
         FIG. 4  is a section of a flywheel in a further embodiment. 
     
    
    
     DETAILED DESCRIPTION OF AN EMBODIMENT 
     The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that the particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification is not necessarily referring to the same embodiment(s). Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
       FIG. 1  shows an arrangement  1  comprising a power source  3 , a driven machine  5 , hereinafter also generally referred to as a “load”, and a mechanical transmission  7  connecting the power source  3  to the driven machine  5 . In the example embodiment of  FIG. 1 , the power source is an IC engine  9  of a tractor  10 . Reference number  11  indicates a hydraulic unit, with which the tractor  10  is provided, which supplies pressurized oil for the purposes which will be explained below. In other embodiments, the hydraulic unit  11  can be on board the driven machine  5  instead of the tractor  3 . 
     Just by way of particularly advantageous but not limiting example, the driven machine  5  is a large square baler. As mentioned above, in these machines the resistant torque varies significantly. The power supplied by the power source  3  is used, in this case, to drive a piston  5 . 1  by means of a crankshaft  5 . 2  and a rod  5 . 3 . The piston  5 . 1  moves reciprocatingly (double arrow f 5 ) in a chamber  5 . 4 . 
     The crank shaft  5 . 2  is housed in a casing or box  5 . 6 , in which a transmission shaft enters, described below, to which a composite flywheel  13  is associated, the structure of which will be described in detail hereinafter with reference to  FIGS. 2 and 3 . The flywheel  13  is a composite flywheel, i.e. it comprises a primary flywheel and a secondary flywheel, which can be selectively coupled to each other to rotate together, or can be torsionally disengaged from each other, so that while one rotates integrally with the mechanical transmission  7 , the other one is idle and therefore does not absorb power from the transmission. The primary flywheel and the secondary flywheel form a dual-mass system, where the masses can both be connected to the power source  3 , or one is connected to the power source  3  and the other remains idle. In this way it is possible to modulate the flywheel inertia as required, as described in detail below. 
     With a flywheel of this type, it is possible to have a very high inertial mass, represented by the sum of the primary flywheel and the secondary flywheel, when the system comprising the driven machine  5  and the power source  3  rotate at working speed. The high inertia of the large rotating mass makes the operation uniform. Vice versa, when it is useful to have a lower inertial mass, especially in the start and stop transients, the inertial mass mechanically connected to the transmission  7  to receive power from the power source  3  is only the mass of the primary flywheel. 
     It should be understood that the concept can be extended to more than two masses. In general, there will be a primary flywheel and one or more secondary flywheels, each of which can be selectively torsionally coupled to, or decoupled from, the primary flywheel. 
     Furthermore, while in  FIGS. 2 and 3  the primary flywheel and the secondary flywheel have approximately the same mass and substantially the same inertia, it must be understood that this is not necessary. In some embodiments, for example, the primary flywheel can have a mass smaller or larger than the secondary flywheel and/or such a mass distribution as to present a lower or greater inertia. For example, the primary flywheel can have a mass equal to half the mass of the secondary flywheel. 
     The composite flywheel described herein may be particularly useful in applications in which the flywheel rotates at speeds in the order of 1000-1200 revolutions per minute. In applications of particular interest, for example for the transmission of motion to large square balers, the flywheel can have a diameter of 1 m or more and a weight equal to, or greater than, 600 kg, for example comprised between 600 and 900 kg. Flywheels of this size are particularly suitable for use in large square balers with pistons having for example 45-50 strokes per minute. 
     With reference to  FIGS. 2 and 3 , in the illustrated embodiment the composite flywheel  13  comprises a primary flywheel  15  and a secondary flywheel  17 , coaxial to each other. The axis of the composite flywheel  13 , which coincides with the axis of a shaft  19  of the driven machine or load  5 , is indicated with A-A. The shaft  19  also represents the power output shaft from the composite flywheel  13  towards the driven machine  5 . The primary flywheel  15  is torsionally coupled to the shaft  19  while the secondary flywheel  17  is supported idle on the shaft  19  in the manner described below. 
     The shaft  19  has a first end  19 A facing the power source  3  and a second end  19 B inserted in the casing  5 . 6  of the driven machine  5 . The second end  19 B is provided with a gear, for example in this case a bevel gear  21 , which transmits rotary motion from the shaft  19  to the crankshaft  5 . 2  through mechanical members, not shown, housed in the casing  5 . 6 . The reference numbers  23  and  25  indicate bearings supporting the shaft  19  in the casing  5 . 6 . 
     The primary flywheel  15  is permanently coupled torsionally to the shaft  19  by means of a connecting element  27 . In the illustrated embodiment, the connecting element  27  comprises a disk  29  coaxial with the shaft  19  and coupled torsionally to the first end  19 A of the shaft  19 , for example by means of a coupling  29  with splined profile. The connecting element  27  also forms a mechanical connection with an input shaft  31  of the composite flywheel  13 . The input shaft  31  comprises a flange  31 . 1  and a splined profile  31 . 2 , through which the input shaft  31  is connected to the other components of the mechanical transmission  7 . 
     The primary flywheel  15  comprises a main mass  35 , which may have an annular shape to concentrate the mass of the primary flywheel  15  in the peripheral region thereof, thus increasing the inertia thereof. The main mass  35  is rigidly connected to a flange  37 , which is in turn rigidly connected to a support shaft  39  of the primary flywheel  15 . The support shaft  39  is substantially a flanged hollow shaft and houses bearings supporting the primary flywheel  15 , as described below. Externally, the support shaft  39  of the primary flywheel  15  features means for connecting to a coupling device between the primary flywheel  15  and the secondary flywheel  17 . The support shaft  39  of the primary flywheel also constitutes a drawing shaft of the secondary flywheel  17 , as explained below. 
     The support shaft  39  of the primary flywheel  15  houses a support shaft  41  of the secondary flywheel  17 . The support shaft  39  of the primary flywheel  15  and the support shaft  41  of the secondary flywheel  17  are coaxial with each other, and between the two support shafts  39 ,  41  bearings  43 ,  45  are provided, through which the primary flywheel  15  is supported on the support shaft  41  of the secondary flywheel  17 . The support shaft  41  of the secondary flywheel  17  is in turn supported by bearings  47 ,  49  on the shaft  19 . The bearings  47 ,  49  are arranged inside the support shaft  41  of the secondary flywheel  17 . 
     The support shaft  41  of the secondary flywheel  17  is integral, through a flange  51 , with an outer annular body  53 , which is in turn rigidly connected to a flange  55  of the secondary flywheel  17 . The flange  55  is rigidly connected to a main mass  57  of the secondary flywheel  17 . In this way, similarly to the primary flywheel  15 , also for the secondary flywheel  17  the main mass  57  is concentrated in the outer peripheral part so as to increase the inertia of the secondary flywheel  17 . 
     In the illustrated embodiment, the two masses  35  and  57  are approximately equal, but this is not necessary. As mentioned above, the primary flywheel  15  and the secondary flywheel  17  may have different masses and therefore different inertias. In some embodiments, the primary flywheel  15  may have a lower inertia than the secondary flywheel  17 . The different inertia can be obtained with different masses and/or with a different mass distribution with respect to the rotation axis A-A. 
     With the arrangement described above, it is possible to connect the primary flywheel  15  to the power source  3  so as to rotate the primary flywheel  15  through the input shaft  31  around the axis A-A. The primary flywheel  15  integrally rotates with the input shaft  19  of the driven machine  5  thanks to the connection provided by the splined profile  29 A. The primary flywheel  15  is supported by the bearings  43 ,  45  and can rotate independently of the secondary flywheel  17 , which is in turn supported on the shaft  19  by the bearings  47 ,  49 . 
     In some operational conditions, the secondary flywheel  17  can be completely disengaged from the primary flywheel  15 , remaining idle on the shaft  19  while the main flywheel  15  and the shaft  19  rotate. In other operational conditions, the secondary flywheel  17  can be torsionally coupled to the primary flywheel  15  so that a torque is applied thereto, which causes the secondary flywheel to rotate. The reciprocal coupling between primary flywheel  15  and secondary flywheel  17  can be made so as to modulate the transmitted torque, so as to gradually coupling the secondary flywheel  17  to the primary flywheel  15 . In this way the primary flywheel  15  and the shaft  19  can be angularly accelerated to a first operational speed, and then the secondary flywheel  17  can be gradually accelerated to the same operational speed as the primary flywheel  15 . It is also possible, once the synchronization between the primary flywheel  15  and the secondary flywheel  17  has been achieved, to accelerate the two flywheels integrally up to a second operational speed. 
     In order to couple the secondary flywheel  17  to the primary flywheel  15 , a coupling device is provided, indicated as a whole with number  61 . In some embodiments, the coupling device  61  comprises a clutch. In the illustrated embodiment, the coupling device  61  comprises a hydraulic clutch controlled by means of a control valve for controlling the pressure of a control fluid, so that, by modulating the fluid pressure, the torque transmitted from the primary flywheel  15  to the secondary flywheel is modulated. 
     With particular reference to  FIG. 3 , the coupling device  61  comprises a series of internal drawing discs  63 , torsionally coupled to the primary flywheel  15 , for example by means of a splined profile coupling  64 . The internal drive discs  63  rotate integrally with the primary flywheel  15 . Between internal drive disks  63 , external drive disks  65  are intercalated, which are torsionally constrained, by means of a splined profile  67 , to the outer annular body  53  of the secondary flywheel  17  and rotate integrally thereto. Thanks to the splined couplings  64 ,  67  the drive disks  63 ,  65  can be pressed against each other so as to transmit torque from the internal drive disks  63  (and hence from the primary flywheel  15 ) to the external drive disks  65  (and then to the secondary flywheel  17 ). To this end, the drawing discs are provided, as in the usual clutches, with coatings made of a material having high coefficient of friction. 
     In the illustrated embodiment, to press the internal drawing discs  63  and the external drawing discs  65  against each other a toroidal piston  69  is provided, housed in an annular cylinder  71  formed between the support shaft  41  of the secondary flywheel  17  and the flange  51 . The piston  69  and the cylinder  71  form a cylinder-piston actuator of the single action type, controlling the coupling device  61 . Springs  73  push the piston  69  to an idle position, moving it away from the drawing discs  63 ,  65 . Pressure control fluid, for example oil, delivered into the annular chamber formed between the cylinder  71  and the piston  69 , pushes the piston  69  against the action of the springs  73  in order to press the drawing discs  63 ,  65  against each other, thus generating a drawing coupling between the primary flywheel  15  and the secondary flywheel  17 . 
     To feed the pressurized control fluid to the coupling device  61  a duct  77  may be provided in the shaft  19 , preferably coaxial thereto. As can be seen in particular in  FIG. 2 , the duct  77  exits on the end  19 B of the shaft  19 , i.e. on the end facing the driven machine or load  5 . The duct  77  can be connected to a hydraulic circuit  79  through a rotating joint  81 , so that pressurized fluid can be fed into the duct  77  while the shaft  19  rotates. The duct  77  can be connected to one or more radial holes  83 , which open into an annular groove  85 . The annular groove  85  is fluidly coupled to one or more openings  87 , which radially extend in the support shaft  41  of the secondary flywheel  17  and connect the annular groove  85 , and therefore the duct  77 , to the annular chamber formed between the piston  69 , the flange  51  and the support shaft  41  of the secondary flywheel  17 . 
     With this arrangement it is possible to deliver pressurized fluid into the cylinder of the cylinder-piston actuator, of which the toroidal piston  69  is part. The fluid pushes the piston  69  against the drawing discs  63 ,  65 , pressing them against each other to a greater or lesser extent according to the fluid pressure. By modulating the fluid pressure, it is therefore possible to modulate the torque transmitted from the primary flywheel  15  to the secondary flywheel  17 . 
     The composite flywheel  13  can be associated with a control system  89 , controlling the coupling device  61 . In  FIG. 2 , the control system is indicated by the reference number  89 . In the embodiment schematically illustrated in  FIG. 2 , the control system  89  comprises at least a first sensor  91  adapted to detect the rotation speed of the primary flywheel  15 . The control system  89  can comprise a second sensor  93  adapted to detect the rotation speed of the secondary flywheel  17 . The control system  89  can further comprise a central control unit  95 , which comprises one or more processors and associated memory and receives signals from the sensors  91  and  93 . The central control unit  95  can be interfaced with an electro-proportioning pressure-reducing valve  97  connected to the hydraulic circuit  79 . 
     The hydraulic circuit  79  can be connected to the hydraulic unit  11  of the power source  3  ( FIG. 1 ) or to a hydraulic unit (not shown) of the driven machine  5 . Pressurized fluid, for example oil, is fed, controlled by means of the control system  89 , to the coupling device  61 , according to the required operational conditions. 
       FIG. 4  is a cross section, according to a plane containing the rotation axis, of a further embodiment of a flywheel. The same reference numbers indicate the same or equivalent parts to those described with reference to  FIGS. 2 and 3  and they will be not described again. The main difference between the embodiment of  FIGS. 2, 3  and the embodiment of  FIG. 4  is that in  FIG. 4  the sizes and the shapes of the primary flywheel  15  and of the secondary flywheel  17  have been optimized, in order to improve the operation and to optimize the overall dimensions of the dual-mass flywheel  13 . 
     In  FIG. 4  the primary flywheel  15  has a smaller diameter than the secondary flywheel  17 . In this way, the mass  35  of the primary flywheel  15  can be contained within the mass  57  of the secondary flywheel  17 . Thus, the composite flywheel  13  has a reduced axial dimension, since the axial bulk of the primary flywheel  15  is contained within the overall bulk of the secondary flywheel  17 . Moreover, the mass of the primary flywheel  15  and the spatial distribution thereof are such that the primary flywheel  15  has a moment of inertia substantially lower than the moment of inertia of the secondary flywheel  17 . For example, the primary flywheel  15  may have a moment of inertia equal to half, or less, the inertia of the secondary flywheel  17 . 
     Below, examples will be illustrated of use of the system described above. 
     When the load  5  is to be started, the IC engine  9 , which represents the power source, is actuated at the appropriate number of revolutions, and the power take-off thereof is mechanically coupled to the input shaft  31  of the flywheel  13 . The coupling device  61  is disengaged, with the piston  69  completely retracted. In this way, no torque is applied to the secondary flywheel  17  and all the mechanical power available from the mechanical power source  3  is used to accelerate the mass of the primary flywheel  15  and, through the shaft  19 , the driven machine  5 . The primary flywheel  15  rotates supported by bearings  43 ,  45 , while the support shaft  41  of the secondary flywheel  17  is idle on the shaft  19  and does not rotate. 
     When the driving shaft of the internal combustion engine  9 , and therefore the primary flywheel  15  and the shaft  19 , have achieved a first given operational speed, which can be detected by the sensor  91 , the central control unit  95  of the control system  89  activates the proportioning valve  97 , through which the pressure of the control fluid is increased in the duct  77  and therefore, through the opening(s)  87 , in the pressure chamber of the coupling device  61 . The gradually increasing pressure of the control fluid pushes the toroidal piston  69  against the force of the springs  73  towards the drawing discs  63 ,  65  causing a gradual increase in the torque transmitted from the support shaft  39  to the secondary flywheel  17  by means of said drawing discs  63 ,  65 . The support shaft  39  of the primary flywheel  15  therefore represents the driving shaft of the secondary flywheel  17 . 
     The secondary flywheel  17  is thus gradually accelerated until the angular speed of the primary flywheel  15  is achieved, when the slipping of the hydraulic clutch represented by the coupling device  61  becomes zero. The sensor  93  can provide the control unit  95  with a speed signal, which allows to verify when the rotation speed of the secondary flywheel  17  reaches the rotation speed of the primary flywheel  15 . At this point, through the transmission  7 , the power source  3  can further accelerate the flywheel  13  and the shaft  19  of the driven machine  5 . 
     It is also possible to perform the acceleration steps described above with a different sequence. For example, the primary flywheel  15  can be brought to the desired final rotation speed and then the secondary flywheel  17  can be gradually accelerated until reaching the same speed as the primary flywheel  15 . 
     In further embodiments, it is possible to bring the primary flywheel  15  to a first operational speed and then to accelerate both the primary flywheel  15  from the first operational speed to a second operational working speed, and the secondary flywheel  17  from zero speed to the second operational working speed. 
     In any case, there is the advantage of starting the operating machine  5  using less power than would be required if the entire mass of the flywheel  13  should be accelerated from zero to the working speed in a single acceleration ramp. 
     The composite flywheel described above can be also used to facilitate the stopping of the driven machine  5 . In fact, when it is necessary to stop the driven machine  5 , the secondary flywheel  17  can be disengaged by opening the coupling device  61  and then slowing down the primary flywheel  15 , the shaft  19  and the driven machine  5  until to stop them. 
     The acceleration, deceleration and stopping steps described above can be controlled by means of the control system  89 , automatically or by means of the sensors  91 ,  93  and the central control unit  95 . In simplified embodiments, the coupling and decoupling of the secondary flywheel  17  can be controlled manually, if necessary with the use of a manually controlled proportioning pressure-reducing valve  97 . 
     Having described some embodiments of the composite flywheel, specific features of the present disclosure are set forth in the following clauses: 
     Clause 1. A composite flywheel, comprising a primary flywheel integral with an input shaft and a secondary flywheel coaxial with the primary flywheel; wherein the secondary flywheel and the primary flywheel are coupled so that they can rotate freely with respect to each other; and further comprising a coupling device adapted selectively to: transmit power from the input shaft to the secondary flywheel; and disengage the secondary flywheel from the input shaft. 
     Clause 2. The composite flywheel of clause 1, wherein the coupling device is adapted selectively to couple and decouple torsionally the primary flywheel and the secondary flywheel with respect to each other. 
     Clause 3. The composite flywheel of clause 1 or 2, wherein the primary flywheel is torsionally constrained to the input shaft and to an output shaft and the secondary flywheel is supported rotatable on the output shaft and is adapted to be selectively coupled to and decoupled from the primary flywheel through the coupling device. 
     Clause 4. The composite flywheel of clause 1 or 2 or 3, wherein the coupling device is adapted to modulate the torque transmitted to the secondary flywheel; and wherein the coupling device is preferably adapted to modulate the torque transmitted to the secondary flywheel according to the rotation speed of the secondary flywheel or of the primary flywheel. 
     Clause 5. The composite flywheel of one or more of the previous clauses, wherein the coupling device comprises a clutch. 
     Clause 6. The composite flywheel of one or more of the previous clauses, wherein the coupling device is a hydraulically controlled device. 
     Clause 7. The composite flywheel of clause 6, comprising an output shaft coaxial with the primary flywheel and with the secondary flywheel, provided with a supply duct for supplying a control fluid to the coupling device 
     Clause 8. The composite flywheel of clause 7, wherein the output shaft has a first end for coupling to a power source and a second end for coupling to a driven machine; wherein the supply duct has an opening on the second end of the output shaft; and wherein preferably a rotating joint is associated with the second end of the output shaft. 
     Clause 9. The composite flywheel of one or more of the previous clauses, wherein the secondary flywheel is torsionally constrained to a support shaft of the secondary flywheel, and wherein preferably bearings are arranged inside the support shaft of the secondary flywheel for supporting and rotating the secondary flywheel on an output shaft. 
     Clause 10. The composite flywheel of clause 9, wherein at least one feeding port is arranged inside the support shaft of the secondary flywheel for feeding a control fluid of the coupling device; and wherein preferably said at least one port is fluidly connected with a pressure chamber where a piston of the coupling device is housed. 
     Clause 11. The composite flywheel of clause 9 or 10, wherein the primary flywheel is mounted rotatable on the support shaft of the secondary flywheel. 
     Clause 12. The composite flywheel of one or more of clauses 9 to 11, wherein the primary flywheel is integral with a support shaft of the primary flywheel, coaxial with the support shaft of the secondary flywheel, and wherein preferably bearings are interposed between the support shaft of the primary flywheel and the support shaft of the secondary flywheel, the bearings allowing the rotation of the support shaft of the primary flywheel and the support shaft of the secondary flywheel, one with respect to the other. 
     Clause 13. The composite flywheel of one or more of clauses 9 to 12, wherein the coupling device is arranged outside the support shaft of the secondary flywheel and coaxially thereto. 
     Clause 14. The composite flywheel of one or more of clauses 9 to 13, wherein the support shaft of the secondary flywheel is integral with an external body of the coupling device. 
     Clause 15. The composite flywheel of clause 14, wherein in the external body of the coupling device there are arranged: first drawing discs constrained to the primary flywheel to rotate integrally therewith; second drawing discs constrained to the secondary flywheel to rotate integrally therewith; a piston mounted around the support shaft of the secondary flywheel and adapted to bias the first drawing discs and the second drawing discs against each another; and wherein preferably the support shaft, a flange of the coupling device and the piston form a pressure chamber adapted to receive a control fluid of the coupling device. 
     Clause 16. The composite flywheel of one or more of the previous clauses, wherein the primary flywheel has a different mass and/or a different diameter than that of the secondary flywheel. 
     Clause 17. A system including a power source, a driven machine and a mechanical transmission drivingly coupling the power source to the driven machine, said mechanical transmission including a composite flywheel according to one or more of the preceding clauses, and preferably a control unit adapted to connect the secondary flywheel to the primary flywheel upon achieving a value of an operating parameter of the driven machine, such as for instance a rotary speed.