Patent Publication Number: US-2012031215-A1

Title: Transmission unit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/EP2009/009291, filed Dec. 28, 2009. This application claims the benefit and priority of German Patent Application No. 10 2009 014 595.8 filed Mar. 24, 2009. The entire disclosures of each of the above applications are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to a transmission unit for a motor vehicle having a transmission housing for receiving a transmission and having an actuator for actuating the transmission. The actuator includes an electric motor having a stator, a rotor, and a drive shaft which projects into an inner space of the transmission housing. The actuator furthermore includes a control circuit (for example, a circuit board having a plurality of electronic components) for controlling the electric motor. In addition, the actuator includes a heat sink device (for example, a metal plate having cooling ribs), which is thermally coupled to the control circuit. 
     BACKGROUND 
     A transmission unit can include a transfer case for transferring a driving torque in the longitudinal direction of the vehicle (i.e., for transferring a driving torque selectively to the front axle or to the rear axle of the vehicle). Such transfer cases are known in different embodiments (for example, having a lockable differential gear, or having a variable torque transmission device). The transmission unit can in particular include a friction clutch that is actuated by means of the actuator. A transmission unit is, for example, known from WO 2006/128533 A1 in which an electric motor drives two ramp rings to make a rotary movement via a control cam and two scissor linkages actuated thereby in order hereby to actuate a friction clutch, wherein the friction clutch selectively transmits a portion of the driving torque to a secondary axle of the vehicle. 
     High heat development can occur in both the electric motor and the control circuit during operation of the actuator. It is, therefore, known to couple the electric motor thermally conductively to the transmission housing, which is usually formed from metal, so that the transmission housing for the electric motor serves as a heat sink. The control circuit, in contrast, is thermally conductively coupled to a separate heat sink device so that a different temperature level can be adopted in this separate heat sink device than with respect to the transmission housing in order effectively to avoid an overheating of the electronic components of the control circuit. 
     Such arrangements are known from EP 1 640 204 A2, DE 100 10 636 A1, and U.S. Pat. No. 7,215,115 B2. In this respect, the actuator is typically at least partly arranged in an actuator housing whose front side is flanged to the transmission housing. The heat sink device for the control circuit is located at the rear side of the actuator housing in this embodiment. A good thermal insulation of the heat sink device of the control circuit from the transmission housing can be achieved. However, the vibrations that usually occur in the operation of a transmission unit can prove to be problematic. Since the heat sink device of the control circuit is typically formed by a comparatively heavy metal plate and is arranged at a specific spacing from the transmission housing due to the interposition of the actuator housing, the vibrations of the transmission can result in unwanted vibrations of the heat sink device and of the control circuit associated herewith, whereby resonant effects can arise. There is, therefore, the risk of a curtailed service life of the control circuit. 
     SUMMARY 
     The present disclosure provides a transmission unit having an actuator that has good heat insulation and simultaneously good vibration resistance. 
     This is satisfied by a transmission unit where a stator of the electric motor is directly fastened to the transmission housing, and where the heat sink device of the control circuit is fastened to the transmission housing independently of the fastening of the stator. 
     In the transmission unit in accordance with the present disclosure, the stator of the electric motor is directly fastened to the transmission housing. It is hereby ensured that the heat output produced in the electric motor can be effectively transmitted to the transmission housing which, therefore, serves as a heat sink for the electric motor. The heat sink device of the control circuit is not fastened to the transmission housing indirectly via the stator of the electric motor, but rather independently of the fastening of the stator. The heat sink device is hereby particularly well thermally insulated from the electric motor. In addition, the heat sink device of the control circuit can be fastened particularly rigidly to and at a small spacing from the transmission housing. Lever effects are hereby avoided that can result in unwanted resonant amplifications of vibrations of the transmission housing. The transmission unit in accordance with the present disclosure is thus characterized by an improved vibration resistance with respect to the heat sink device and the control circuit associated herewith. 
     Thermal insulation of the heat sink device of the control circuit from the transmission housing can be realized by the heat sink device is not being directly fastened to the transmission housing, but rather by means of a thermal insulation device. This thermal insulation device can, for example, be a plastic plate that extends between the heat sink device of the control circuit and the transmission housing. 
     The required electric connection between the control circuit and the stator of the electric engine can, in contrast, take place via electrical contacts (preferably plug-in contacts) that, however, do not satisfy any mechanical support functions and also do not effect any substantial heat transfer. 
     In accordance with a particularly advantageous embodiment, the transmission housing has a recess, with the stator of the electric motor being arranged at the base of the recess and with the heat sink device of the control circuit forming a cover for the recess of the transmission housing. In other words, the transmission housing in this embodiment has (viewed from the outside) a concave section which receives the stator of the electric motor so that no separate housing is required for the electric motor. The heat sink device provided for the control circuit can form a cover so that the recess can be closed after fastening the stator to the transmission housing without any additional components being required. 
     The heat sink device of the control circuit can be substantially made in plate form to form a flat cover for the recess of the transmission housing. In this manner, a particularly rigid mechanical connection is possible between the heat sink device and the transmission housing without projecting masses so that resonant amplifications of vibrations of the transmission housing are effectively avoided. 
     An opening through which the drive shaft of the electric motor projects into the inner space of the transmission housing can be provided at the base of the named recess of the transmission housing. 
     To prevent oil from entering into the region of the electric motor from the inner space of the transmission housing, a seal device such as a radial shaft seal can be arranged at the drive shaft. 
     For sealing contaminations from outside, sealing lips that contact the transmission housing can be provided that are molded onto the thermal insulation device. The region of the electric motor and the adjoining inner space of the transmission housing are, therefore, hereby sealed with respect to the environment. The molded on sealing lips provide a particularly effective protection if the transmission housing has the recess and the thermal insulation device partly projects with the sealing lips into the recess. 
     The rotor of the electric motor can be directly fastened to the drive shaft by, for example, pressing the rotor onto the drive shaft, which projects into the inner space of the transmission housing. Unlike transmission actuators that have a separate actuator housing in a modular construction, a separate motor shaft and a coupling between such a motor shaft and the drive shaft can hereby be omitted. Furthermore, an additional bearing for the drive shaft can be omitted by such an integrated structure. 
     It is furthermore advantageous if at least one permanent magnet is arranged at the drive shaft of the electric motor concentrically to the axis of rotation of the drive shaft with the control circuit having at least one sensor for detecting an angular position of the permanent magnet and thus of the drive shaft, with the sensor also being arranged concentric to the axis of rotation of the drive shaft. The signal of the sensor can be used for at least one of the commutation of the electric motor and for a position regulation of the actuator. A sufficiently high resolution of the angular position can be achieved by the arrangement of the permanent magnet at the drive shaft (i.e., not, for instance, radially spaced apart therefrom) using a single permanent magnet and thus with a particularly simple structure. 
     A particularly simple structure of the actuator results when the drive shaft of the electric motor is made as a worm shaft that cooperates with a worm wheel that forms a ramp ring for actuating a friction clutch of the transmission unit. A particularly short actuator chain can be formed in this manner that, nevertheless, allows a high stepping down of the speed of the electric motor. 
    
    
     
       DRAWING 
       The detailed description of the present disclosure will be described in the following only by way of example with reference to the drawing. The FIGURE ( FIG. 1 ) shows a schematic cross-sectional view of a part of a transmission unit for a motor vehicle. 
     
    
    
     DETAILED DESCRIPTION 
     This transmission unit can include a transfer case, not shown in the FIGURE, as is generally known from the initially already named WO 2006/128533 A1. 
     Referring to  FIG. 1 , the transmission unit includes a transmission housing  11 , only shown in part, for receiving the transmission (not shown) and an actuator  13  for actuating the transmission. The actuator  13  includes an electric motor  15  having a stator  17 , a rotor  19  and a drive shaft  21 . The electric motor  15  is made as a brushless DC motor (BLDC motor). The electric motor  15  is arranged within a cylindrical recess  23  of the transmission housing  11 , with the stator  17  being fastened to the base of the recess  23  by means of two screws  25 . The transmission housing  11  can be formed in multiple parts. The transmission housing  11 , however, surrounds the recess  23  peripherally in one part in the region of the electric motor  15 . The rotor  19  is pressed onto the drive shaft  21 . A bipolar permanent magnet  26  can be fastened to the end of the drive shaft  21  disposed in the recess  23  at the end face of the drive shaft  21  and thus concentrically to the axis of rotation of the drive shaft  21 . 
     The drive shaft  21  of the electric motor  15  is made as a worm shaft. The drive shaft  21  projects through an opening  27  at the base of the recess  23  into the inner space  29  of the transmission housing  11 . The drive shaft  21  is in this respect rotatably supported at the transmission housing  11  by means of two bearings  31  that are formed as ball bearings, with the two bearings  31  being axially offset from one another. A radial shaft sealing ring  33  can be arranged between the two bearings  31  and seals the region of the electric motor  15  (recess  23 ) with respect to the lubrication oil present in the inner space  29  of the transmission housing  11 . 
     The actuator  13  furthermore includes a control circuit  35  for controlling the electric motor  15 . The control circuit  35  has a circuit board  37  at which a plurality of electronic components  39  (e.g., a capacitor  41 ) are provided. The actuator  13  further includes a heat sink device  43  which is formed from metal and has a plurality of cooling ribs  45 . The heat sink device  43  is thermally conductively coupled to the control circuit  35 , in particular by direct contact or due to an interposed thermally conductive paste. 
     The heat sink device  43  is fastened to the transmission housing  11  by means of a plurality of screws  48  via a plate-shaped heat insulation device  47  made from plastic, and indeed independently of the fastening of the stator  17  of the electric motor  15  to the transmission housing  11 . Only an electric connection is provided between the stator  17  and the control circuit  35  via a plurality of electric plug-in contacts  49 . 
     The control circuit  35  further includes a Hall sensor  50  which is arranged in an opposite position to the permanent magnet  26  concentrically to the axis of rotation of the drive shaft  21  and the permanent magnet  26 . The Hall sensor  50  contactlessly detects the angular position of the permanent magnet  26  and thus of the drive shaft  21 . The Hall sensor  50  is embedded in the heat insulation device  47  and is electrically connected via a plurality of plug-in contacts  52  to the circuit board  37  or to the component  39  fastened hereto. 
     Since the heat sink device  43  is also made substantially in plate form (with the exception of the cooing ribs  45 ), the heat sink device  43  forms, together with the heat insulation device  47 , a flat cover for the recess  23  of the transmission housing  11 . 
     Molded-on sealing lips  53  which contact the transmission housing  11  and outwardly seal the electric motor  15  are provided at a flange  51  of the heat insulation device  47  which projects into the recess  23  of the transmission housing  11 . 
     A connector plug  55  is shaped at the heat insulation device  47  and includes a plurality of electric connections  57  which are led to the control circuit  35 , to allow a power supply and a communication with a central control unit of the vehicle. 
     The transmission unit shown has the advantage that the stator  17  of the electric motor  15  is directly fastened to the transmission housing  11  within the recess  23  so that no separate motor housing is required. The heat sink device  43  is fastened to the transmission housing  11  via the heat insulation device  47  independently of the electric motor  15 , with the control circuit  35  being arranged between the heat sink device  43  and the heat insulation device  47 . The control circuit  35  is thus particularly well thermally insulated from the electric motor  15 . In addition, a particularly rigid mechanical connection is ensured between the control circuit  35  and the transmission housing  11  so that a large spacing of the comparatively heavy heat sink device  43  from the transmission housing  11  (for instance due to an interposition of the electric motor  15  or of a motor housing) is avoided and vibrations of the transmission housing  11  thus do not result in unwanted resonant effects. 
     A further advantage of this arrangement is that the rotor  19  of the electric motor  15  is directly fastened to the drive shaft  21  which projects into the inner space  29  of the transmission housing  11 . A two-part drive shaft  21  having a corresponding couponing device and additional bearings  31  is thus not necessary, whereby additional components are saved. 
     A particular advantage of the arrangement shown is also that the control circuit  35  is at least partly integrated into the heat insulation device  47 . The components  39  provided at the circuit board  37  are hereby supported by the heat insulation device  47  so that damage to the control circuit  35  due to the total vibrations of the transmission housing  11  is effectively avoided. It is particularly effective if the heat insulation device  47  is made from plastic and at least a part of the control circuit  35  is overmolded by the heat insulation device  47 . This in particular applies with respect to the capacitor  41  which is usually the heaviest component of the control circuit  35  and which is thus particularly prone to damage due to the named vibrations.