Patent Publication Number: US-2010126291-A1

Title: Double-clutch gearbox

Description:
The present invention relates to a double-clutch gearbox. 
     BACKGROUND OF THE INVENTION 
     Servo-controlled drives, which are structurally similar to a manual drive of the traditional type, except for the clutch pedal and the gear selection lever traditionally operated by the driver which are replaced by corresponding electric or hydraulic servo-controls, are increasingly widespread. When using a servo-controlled drive, the driver only needs to send the order to shift up or down to a drive control unit and the drive control unit autonomously shifts by operating both the engine and the servo-controls associated with clutch and gearbox. 
     In order to reduce the time required for shifting and to eliminate the “drive torque gap” which occurs when shifting by opening the clutch, servo-controlled drives provided with a double-clutch gearbox have been suggested. In a double-clutch gearbox, the drive shaft transmits motion to two coaxial clutches, each of which transmits in turn the motion to a respective primary gearbox shaft; the two primary gearbox shafts are coaxial, are arranged inside each other and are coupled to a common secondary shaft, which transmits the motion to the drive wheels by means of respective gears, each of which has its own drive ratio defining a gear. In order to allow the correct operation of the double-clutch gearbox, all the odd gears are coupled to the same primary shaft, while all the even gears are coupled to the other primary shaft. Typically, each gear comprises a primary toothed wheel which is integral with the respective primary shaft and a secondary toothed wheel which permanently meshes with the primary toothed wheel, is idly mounted to the secondary shaft and may be made integral with the secondary shaft by means of a synchronizer thereof, which is axially movable along the secondary shaft. 
     Normally, each synchronizer is arranged between two secondary gears and is actuated by a respective fork which is axially displaced along the secondary shaft in the two directions for displacing the synchronizer between two limit engaging positions, in each of which the synchronizer engages a respective secondary gear, and an intermediate idle position, in which the synchronizer does not engage any of the two secondary gears. Furthermore, each fork is actuated by a finger integral with a control shaft of a gear actuator; normally, the gear actuator impresses an axial translational movement on the control shaft, and thus on the finger integral with the control shaft, for selecting the gear range (i.e. for selecting the fork to be actuated), and a rotational movement for engaging/disengaging the gears (i.e. for displacing the fork to be actuated). 
     Two gear actuators are included in the currently marketed double-clutch gearboxes, each of which is associated with a respective primary shaft and therefore actuates all and only the forks coupled to its own primary shaft. However, the presence of two different gear actuators implies an increase in the number of components, and thus increasing cost, increasing weight, increasing volumes, and a greater possibility of malfunctions. 
     DE10108881A1 describes a double-clutch gearbox provided with two primary shafts and two secondary shafts; each secondary shaft is provided with two synchronizers, which are actuated by means of two forks provided with respective sliding rods. Each rod is provided at a free end with a first coupling element, which is adapted to be engaged by a second coupling element integral with a control shaft, which is actuated by a single gear actuator, which rotates the control shaft about a longitudinal axis and axially translates the control shaft along the longitudinal axis. The whole is structured to allow the control shaft to couple with a rod for engaging a subsequent gear without preventively disengaging a previously engaged gear. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a double-clutch gearbox, which is free from the above-described drawbacks and which is specifically easy and cost-effective to be implemented in addition to be light, compact and reliable. 
     According to the present invention, a double-clutch gearbox is provided as claimed in the attached claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described with reference to the accompanying drawings, which show a non-limitative embodiment thereof, in which: 
         FIG. 1  is a diagrammatic view, with parts removed for clarity, of a double-clutch gearbox made in accordance with the present invention; 
         FIG. 2  is a diagrammatic, perspective view, with parts removed for clarity, of a fork of a synchronizer of the double-clutch gearbox in  FIG. 1 ; and 
         FIGS. 3-7  are diagrammatic views of some movements made by a movable finger of the double-clutch gearbox in  FIG. 1  for engaging a first gear I and then a second gear II. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , numeral  1  indicates as a whole a servo-assisted drive for a car (not shown) propelled by an internal combustion engine  2 , which is provided with a drive shaft  3 . 
     The servo-assisted drive  1  comprises a servo-assisted double-clutch gearbox  4 , which is provided with a pair of reciprocally coaxial, independent primary shafts  5 , inserted inside each other. Furthermore, the double-clutch gearbox  4  comprises two coaxial clutches  6  arranged in series, each of which is adapted to connect a respective primary shaft  5  to the drive shaft  3  of the internal combustion engine  2 . The double-clutch gearbox  4  comprises a single secondary shaft  7  connected to a differential (not shown) which transmits the motion to the drive wheels (not shown). 
     The double-clutch gearbox  4  shown in the accompanying figure has six forward gears indicated by Roman numerals (first gear I, second gear II, third gear III, fourth gear IV, fifth gear V, and sixth gear VI) and one reverse (indicated by the letter R). Each primary shaft  5  and the secondary shaft  7  are mechanically coupled to one another by means of a plurality of gear pairs, each of which defines a respective gear and a comprises a primary gear  8  mounted to the primary shaft  5  and a secondary gear  9  mounted to the secondary shaft  7 . In order to allow the correct operation of the double-clutch gearbox  4 , all the odd gears (first gear I, third gear III, fifth gear V) are coupled to a same primary shaft  5   a , while all the even gears (second gear II, fourth gear IV, and sixth gear VI) are coupled to the other primary shaft  5   b.    
     Each primary gear  8  is keyed onto a respective primary shaft  5  to rotate, again integrally, with the primary shaft  5 , and permanently meshes with the respective secondary gear  9 ; on the other hand, each secondary gear  9  is idly mounted to the secondary shaft  7 . Furthermore, the double-clutch gearbox  4  comprises four synchronizers  10 , each of which is coaxially mounted to the secondary shaft  7 , is arranged between two secondary gears  9  (except for the synchronizer  10  of the fifth gear V, which is arranged by the side of a single secondary gear  9 ), and is adapted to be actuated for alternatively engaging the two respective secondary gears  9  to the secondary shaft  7  (i.e. for alternatively making the two respective secondary gears  9  angularly integral with the secondary shaft  7 ). In other words, each synchronizer  10  comprises a guiding sleeve which may be displaced in a direction in order to engage a secondary gear  9  with the secondary shaft  7 , or may be displaced in the other direction in order to engage the other secondary gear  9  with the secondary shaft  7  (except for the synchronizer  10  of the fifth gear V, which engages a single secondary gear  9  with the secondary shaft  7 ). 
     Each synchronizer  10  is arranged between two secondary gears  9  (except for the synchronizer  10  of the fifth gear V, which is arranged by the side of a single secondary gear  9 ) and is actuated by a respective fork  11 , which is axially displaced along the secondary shaft in the two directions for displacing the guiding sleeve of the synchronizer  10  between the two engaging positions, in each of which the synchronizer  10  engages a respective secondary gear  9 , and an intermediate idle position, in which the synchronizer  10  does not engage any of the two secondary gears  9  (as previously mentioned, the synchronizer  10  of the fifth gear V has a single engagement position). Furthermore, each fork  11  is actuated by a finger  12  which is moved by a control shaft  13  of a single, common gear actuator  14 ; the gear actuator  14  impresses an axial translational movement (i.e. parallel to a longitudinal axis  15  of the control shaft  13 ) and a rotational movement about the longitudinal axis  15  of the control shaft  13 . It is worth noting that the control shaft  13  is provided with a single finger  12  which controls all forks  11 ; this constructional solution allows to simplify both the construction and the control of the double-clutch gearbox  4 . 
     As shown in  FIG. 2 , each fork  11  is carried by a rod  16 , which is axially and slidingly mounted to allow the fork  11  to be axially displaced along the secondary shaft  7  in the two directions. Each rod  16  is integral with a plate  17 , in which a catch  18  is defined, i.e. a rectangular seat in which the finger  12  may be inserted for axially pushing the plate  17 , and thus the rod  16 , in the two directions. In  FIG. 2 , it is apparent that the axial translational movement along the longitudinal axis  15  of the control shaft  13  allows to insert the finger  12  into a given catch  18 , and therefore allows to select the gear range, while the rotational movement about the longitudinal axis  15  of the control shaft  13  allows to displace a corresponding fork  11 , and thus allows to engage or disengage a gear of the currently selected range. 
     Each fork  11  is associated with a retaining device  19  which is, for example, mechanically coupled to the rod  16 , is made by means of the known spring and ball architecture, and is adapted to keep the fork  11  in the current position with a constant, predetermined retaining force. The function of each retaining device  19  is to keep the fork  11  in the current position, thus avoiding random, uncontrolled and undesired movements of the fork  11 ; obviously, according to the modes described below, the control shaft  13  is able to apply a driving force to each fork  11 , sufficiently higher than the retaining force generated by the respective retaining device  19 , for ensuring the desired displacement of the fork  11 . 
     As shown in  FIG. 3 , the four catches  18  are arranged parallelly aligned with the longitudinal axis  15  of the control shaft  13 , are equally spaced to one another, and are reciprocally spaced by a distance D 1  greater than the dimension D 2  of the finger  12 , so as to allow the passage of the finger  12  between two catches arranged side-by-side. In other words, there is a void between the two catches  18  arranged side-by-side which has a dimension D 1  greater than the dimension D 2  of the finger  12  so as to allow the passage of the finger  12  in such a void; such a feature is essential for allowing the finger  12  to be displaced between the catches  18  by following the shortest, and therefore fastest, path. 
     According to a preferred embodiment, the distance D 1  between two catches  18  arranged side-by-side is between a minimum value consisting in the dimension D 2  of the finger  12  (actually, it is always slightly larger to take into account both manufacturing tolerances and positioning errors made by the gear actuator  14 ) and a maximum value indicatively equal to double the dimension D 2  of the finger  12 . By containing the distance D 1  between the two catches  18  arranged side-by-side it is possible to make the assembly of catches  18  more compact, and thus reduce the axial stroke that the finger  12  must cover to be displaced between the catches  18 ; in this manner, gear engagement is faster. 
     The servo-assisted drive  1  comprises a control unit (diagrammatically shown in  FIG. 1 ), which drives the gear actuator  14  for engaging/disengaging the gears and drives the clutch actuators (not shown) which control the opening and closing of the two clutches  6 . 
     The operation of the double-clutch gearbox  4  is disclosed below with reference to  FIGS. 3-7 ; specifically, the engagement mode of the first gear I from an idle condition ( FIGS. 3-5 ) and the subsequent engagement mode of the second gear II ( FIGS. 6 and 7 ) are described. 
       FIG. 3  shows an idle condition, in which no gear is engaged, the two clutches  6  are open (i.e. the drive shaft  3  is disconnected from both the primary shafts  5 ), and the finger  12  is in the intermediate position between the two coupling catches  18   a  and  18   b . The fork  11  connected to the catch  18   a  is required to be rotated in order to engage the first gear I, accordingly, the gear actuator  14  axially displaces the control shaft  13  along the longitudinal axis  15  so as to make the finger  12  engage the catch  18   a , as shown in  FIG. 4 . Once the finger  12  has been inserted into the catch  18   a , the gear actuator  14  rotates the control shaft  13  (and thus the finger  12  integral with the control shaft  13 ) about the longitudinal axis  15  in order to push the plate  17   a  (and thus the corresponding rod  16  and the corresponding fork  11 ) in the required direction to engage the first gear I, as shown in  FIG. 5 ; if the third gear III had been required to be engaged (or the first gear I had been required to be disengaged), then the gear actuator  14  would have rotated the control shaft  13  (and thus the finger  12  integral with the control shaft  13 ) in the opposite direction. As previously mentioned, when displacing the plate  17   a  (and thus the corresponding rod  16  and the corresponding fork  11 ), the gear actuator  14  must overcome the retaining torque generated by the retaining device  19 . 
     Once the first gear I has been engaged in the above-described manner, the clutch  6   a  may be closed to transmit the motion from the drive shaft  3  to the drive wheels (not shown) at the drive ratio of the first gear I. Furthermore, once the first gear I has been engaged in the above-described manner, the gear actuator  14  also acts to engage the second gear II by displacing the plate  17   c  (and thus the corresponding rod  16  and the corresponding fork  11 ). 
     To engage the second gear II, the gear actuator  14  axially displaces the control shaft  13  without performing any rotation so as to make the finger  12  disengage the catch  18   a ; this operation does not imply the disengagement of the first gear I in virtue of the retaining torque exerted by the retaining device  19  which prevents the plate  17   a  (and thus the corresponding rod  16  and the corresponding fork  11 ) from being displaced, except under the bias of the gear actuator  14 . Once the finger  12  has been disengaged from the catch  18   a , the gear actuator  14  rotates the control shaft  13  to align the finger  12  with the catch  18   c  and thus impresses a new axial displacement on the control shaft  13  so as to make the finger  12  engage the catch  17   c , as shown in  FIG. 6 . Once the finger  12  of the control shaft  13  has been inserted into the catch  18   c , the gear actuator  14  rotates the control shaft  13  about the longitudinal axis  15  for rotating the plate  17   c  (and thus the corresponding rod  16  and the corresponding fork  11 ) in the required direction to engage the second gear II; if the fourth gear IV had been required to be engaged (or the second gear II had been required to be disengaged), then the gear actuator  14  would have rotated the plate  17   c  (and thus the corresponding rod  16  and the corresponding fork  11 ) in the opposite direction. As previously mentioned, when rotating the plate  17   c , the gear actuator  14  must overcome the retaining torque generated by the corresponding retaining device  19 . 
     Once the second gear II has been engaged in the above-described manner, the clutch  6   b  may be closed and at the same time the clutch  6   a  must be opened to transmit the motion from the drive shaft  3  to the drive wheels (not shown) at the drive ratio of the second gear II. Furthermore, once the second gear II has been engaged in the above-described manner, the gear actuator acts to disengage the first gear I, and then to engage the third gear III by rotating the plate  17   a  (and thus the corresponding rod  16  and the corresponding fork  11 ), and so on. 
     According to a possible embodiment, under stand-by conditions, if a gear which transmits the motion is present, the finger  12  is arranged inside the catch  18  corresponding to the gear which transmits the motion to prevent the accidental, involuntary disengagement of the gear. In other words, once the finger  12  has been moved to engage the current gear which transmits the motion and to engage the next gear (determined by means of a prediction algorithm), the finger  12  is arranged inside the catch  18  corresponding to the gear which transmits the motion so as to prevent an accidental, involuntary displacement of the catch  17 , which would determine an accidental, involuntary disengagement of the gear. 
     The above-described double-clutch gearbox  4  has several advantages, because it is simple, cost-effective and compact, and requires the use of only one gear actuator  14 , which is able to effectively and efficiently actuate all the forks  11 .