Abstract:
An actuating unit for an electromechanically actuated disc brake for automotive vehicles which is composed of a drive unit or an electric motor, an actuating element forming one of two friction linings into engagement with a brake disc, and a reducing gear. A freewheel mechanism that interacts with the electric motor has the function of a parking brake. In order to ensure a high level of reliability in operation of the parking brake and to render it resistant to external influences, in particular, oscillations or vibrations, the present invention suggest that the freewheel mechanism be configured in such a way that its clamping effect is produced by a form-locking torque transmission and that, in its actuated state, it prevents a rotational movement of a bearing in which the rotor of the electric motor is mounted.

Description:
TECHNICAL FIELD 
     The present invention generally relates to brake actuating elements and more particularly relates to an actuating unit for an electromechanically actuated disc brake for automotive vehicles. 
     BACKGROUND OF THE INVENTION 
     EP 0 402 421 B1 discloses an electromechanic actuating unit of this type. The electric motor drives the reducing gear by way of the freewheel mechanism in the prior-art actuating unit. The freewheel mechanism which permits a frictional torque transmission prevents a rotational movement of the electric motor in the direction opposite to its actuating direction or in the release direction of the brake when reaction forces occur and, thus, acts as a parking brake. 
     What is disadvantageous in the prior-art actuating unit is, especially, the functional principle of the freewheel mechanism employed, the clamping effect of which will slowly yield in a high-frequent excitation or under vibration stress due to a micro-sliding action in the loading direction so that the retaining force required cannot be ensured. This means that a freewheel mechanism of this type is inappropriate for use under safety aspects, such as e.g. in the operation of a parking brake, where it is absolutely imperative to maintain the holding effect. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the above, an object of the present invention is to disclose an electromechanic actuating unit of the type initially referred to wherein an inadvertent release of the parking brake is prevented, thereby increasing its reliability in operation. Another objective is to achieve a space-saving, compact construction of the electromechanic actuating unit. 
     According to the present invention, this object is achieved in that the freewheel mechanism is configured so that its clamping effect is ensured by a form-locking torque transmission, and that, in its actuated state, it prevents a rotational movement of a bearing in which the rotor of the electric motor is mounted. Preferably, the bearing may be configured as a ball bearing, a needle bearing, or a roller bearing. 
     In a preferred embodiment, the present invention includes a freewheel mechanism forming a subassembly with the bearing, wherein both the outside and the inside ring of the bearing are extended on one side in such a fashion that they enter into a form-locking engagement with the clamping element of the freewheel mechanism. These measures permit the integration of components in a much more compact manner. 
     In a preferred aspect of the subject matter of the present invention, the inside ring of the bearing has a profile which permits a form-locking accommodation of the clamping element, and the outside ring has at least one radial recess and a subsequent slope or ramp which, along with the profiling, limits at least one clamping slot in which the clamping element is received. 
     In another embodiment of the present invention, the clamping element is biassed in the direction of the recess by means of a spring element. Favorably, the spring element is configured as a circlip or a leaf spring. 
     An actuation of the parking brake of the present invention which is easy to realize and ensures a reliable operation is achieved according to another feature of this invention because the freewheel mechanism is operable by means of an electromagnetic actuating unit. 
     The electromagnetic actuating unit is preferably comprised of an electromagnet and a tappet which is movable into a force-transmitting engagement with the clamping element, and the electromagnet is designed as a bistable electromagnet. 
     The clamping element may be configured as a jamming roller or a ball. 
     In a particularly favorable embodiment of the subject matter of the present invention, the reducing gear is configured as a ball-and-thread drive assembly, the actuating element being the threaded nut of the ball-and-thread drive assembly. 
     To considerably reduce the necessary drive torque to be produced by the electric motor, the force transmission between the electric motor and the reducing gear is effected by means of a second reducing gear. 
     In a preferred aspect of the subject matter of the present invention, the electric motor, the (first) reducing gear, and the second reducing gear are designed as at least two independent assemblies so that the electric motor is arranged outside the flux of force of the clamping force and its function cannot be impaired by disturbing influences. This arrangement has an additional benefit because it uncouples the second reducing gear from the (first) reducing gear so that, especially when the second reducing gear is configured as a planetary gear, an equal position of the rotor of the electric motor in relation to the planet wheels and of the planet wheels in relation to the ring gear can be ensured. Designing the second reducing gear as a planetary gear permits considerably reducing the necessary drive torque that is to be generated by the electric motor, with the planetary gear representing an anti-friction gear which does not require any work of deformation and allows achieving a high efficiency with a small mounting space. 
     It is especially favorable that the electric motor, the first reducing gear, and the second reducing gear is designed as one independent subassembly each. An electromechanic actuating unit of a like design is characterized by high efficiency, extraordinary dynamics of brake actuation, and an extremely compact construction permitting high mass-related brake torques to be transmitted. Besides, subassemblies of a modular design can be constructed and tested separately. 
     In a favorable aspect of the subject matter of the present invention, the second reducing gear is arranged on the side of the electric motor remote from the brake linings. This measure permits a constructive uncoupling of the second reducing gear from the first reducing gear so that deformation of the second reducing gear is effectively prevented and clearances can be maintained constant within the gear unit. 
     A higher gear ratio is achieved in another embodiment of the subject matter of the present invention in that the second reducing gear is configured as a planetary gear, preferably with stepped planet wheels. It is, however, also possible to design the second reducing gear as a two-step differential planetary gear. In the last-mentioned gear type, an optimal overall length is achieved because a large sun wheel can be used. 
     According to another feature of the present invention, uncoupling of the flux of force from the drive unit or the electric motor is ensured because there is provision of a guide member which is supported on a casing that accommodates the ball-and-thread drive assembly and embraces the threaded nut, the threaded spindle being axially supported on the guide member. The axial support of the threaded spindle is carried out by the intermediary of an axial bearing by means of a radial collar. This allows using a bearing with a very small diameter. 
     It is particularly advantageous that force-measuring elements are provided on the guide member, thereby allowing to effect force measurements at the part that is not moved and is subject to a defined deformation. 
     An effective protection of the arrangement against contaminants and the ingress of water is achieved due to an elastic seal or sleeve that is interposed between the threaded nut and the guide member. 
     In another favorable embodiment of the subject matter of the present invention, the sun wheel of the planetary gear is designed on the rotor, while the planet wheels are mounted in a planet cage (which is in a force-transmitting connection with the threaded spindle) and are comprised of a first planet wheel of large diameter that is in engagement with the sun wheel and a second planet wheel of small diameter that is in engagement with a ring gear. 
     The mounting space is optimized in the above-mentioned design because the ring gear of the planetary gear is formed by an internal toothing in a cover which represents a case of the planetary gear and is mounted on the casing of the electric motor. 
     In another embodiment of the subject matter of the present invention, the assembly of the actuating unit of the present invention is considerably simplified because the transmission of force between the planet cage and the threaded spindle occurs by means of a form-locking plug coupling. 
     A low-cost design of the actuating unit of this invention involves that the planet cage is mounted in the cover by means of a radial bearing. A planetary gear of this type is easy to manufacture and allows separate testing. 
     It is suitable that the form-locking plug coupling is coupled to the planet cage in a torsion-proof, radially yielding and flexible fashion. This measure allows an effective isolation of disturbing influences. 
     The threaded spindle may preferably have a multi-part design. 
     In another favorable design of the subject matter of the present invention, the threaded nut at its end remote from the first friction lining includes an axial projection which is movable into abutment with a stop that is provided on the threaded spindle also in an axial direction and acts in a circumferential direction. Jamming or clamping of the first reducing gear is prevented by this provision, in particular in a faulty release action, where the threaded nut is reversed until its stop. 
     In further favorable embodiments of the present invention, the electric motor may be configured as an electronically commutated electric motor (non-brush d-c motor) energized by a permanent magnet, or a switched reluctance motor (SR=Switch Reluctance motor). 
     The types of motors mentioned above are especially suited to produce high torques during standstill. 
     In order to electronically commutate the motor of the actuating unit, it is necessary to provide a position detection system which permits detecting the position of the rotor of the electric motor in relation to the stator and, preferably, includes a Hall sensor or a magnetoresistive element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial cross-sectional view of a design of the electromechanic actuating unit of the present invention. 
     FIG. 2 is a design view of the parking brake device utilized in the actuating unit of FIG.  1 . 
     FIG. 3 is a broken view of the parking brake device of FIG. 2 in its rest position. 
     FIG. 4 is a broken view of the parking brake device of FIG. 2 in its actuated position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The electromechanic actuating unit of the present invention, as shown in the drawings, is used to actuate a floating-caliper disc brake whose brake caliper (only represented) is slidably supported in a stationary holder (not shown). A pair of friction linings  4  and S is arranged in the brake caliper so that they face the left-hand and right-hand lateral surface of a brake disc  6 . 
     In the following, friction lining  4  that is shown on the right in the drawing is referred to as first friction lining, and the other friction lining designated by reference numeral  5  is referred to as second friction lining. While the first friction lining  4  is movable into engagement with the brake disc  6  by the actuating unit directly by means of an actuating element  7 , the second friction lining  5  is urged against the opposite lateral surface of brake disc  6  by the effect of a reaction force generated by the brake caliper, when the assembly is actuated. 
     The actuating unit of the present invention which is fitted to the brake caliper by way of securing means (not shown), has a modular design and is generally comprised of three independent subassemblies or modules, namely a drive unit  1 , a first reducing gear  2  that actuates the first friction lining  4 , and a second reducing gear  3  that is interconnected in terms of effect between drive unit  1  and the first reducing gear  2 . 
     The actuating unit  1  mentioned above is comprised of an electric motor  10  which, in the example shown, is a permanent-magnet-energized, electronically commutated motor whose stator  9  is immovably arranged in a motor casing  8  and whose rotor  11  is provided by an annular carrier  13  that carries a plurality of permanent magnet segments  14 . The first reducing gear  2  is interposed between the electric motor  10  and the above-mentioned actuating element  7  in terms of effect and, in the example shown, is configured as a ball-and-thread drive assembly  16  to  21  accommodated in a gearbox case  15 , which may also be of integral design with the above-mentioned brake caliper. In this arrangement, the ball-and-thread drive assembly comprises a threaded nut  16  and a threaded spindle  17 , with several balls  18  being arranged between the threaded nut  16  and the threaded spindle  17  which circulate upon a rotational movement of the threaded spindle  18  and put the threaded nut  16  into an axial or translatory movement. The threaded nut  16  preferably forms the above-mentioned actuating element  7 . The threaded spindle  17  driven by the electric motor  10  by way of the second reducing gear  3  preferably has a three-part design and is comprised of a tubular first spindle member  19  which is in engagement with the threaded nut  16  by means of the above-mentioned balls  18 , an annular second spindle member  20 , and a third spindle member  21 . 
     The arrangement is preferably chosen so that the rotor  10  of the motor  11  drives the third spindle member  21  by the intermediary of the second reducing gear  3 , and the threaded nut  16  is supported on the first friction lining  4 . 
     In the embodiment of the present invention illustrated in the drawings, the necessary engine torque is reduced by an expedient integration of a planetary gear  30 - 34  which forms the above-mentioned second reducing gear  3 . The planetary gear, which is interposed between rotor  11  and threaded spindle  17  in terms of effect, is comprised of a sun wheel  30  which preferably includes an externally toothed area  22  on rotor  11 , a plurality of stepped planet wheels, two of which are shown and have been assigned reference numerals  31  and  32 , and a ring gear  33 . The stepped planet wheels  31 ,  32  accommodated in a planet cage  34  have a first step interacting with the sun wheel  30  and a second step interacting with the ring gear  33 , the first step being formed of toothed wheels  31   a ,  32   a  of large diameter and the second step being formed of toothed wheels  31   b ,  32   b  of small diameter. Preferably, the above-mentioned planet cage  34  is configured so that its area positioned between the points of support of the planet wheels  31 ,  32  and the point where the threaded spindle  17  is coupled allows a small axial clearance and a radial clearance and a small offset angle and, for example, is configured as a lamellar plate or a pleated bellows. Ring gear  33  is formed of an internally toothed area of a cover  23  that represents the case of the planetary gear. 
     The above-mentioned threaded nut  16  of the ball-and-thread drive assembly is guided and mounted in a bowl-shaped guide member  12 . The mounting support of the threaded nut  16  in the guide member  12  is carried out in its area close to the first friction lining  4  by means of a first slide ring  28  arranged in guide member  12  as well as in its end area remote from the friction lining  4  by means of a second slide ring  29  arranged on the threaded nut  16 . 
     Further, it can be taken from FIG. 1 that the second annular spindle member  20  is supported on an axial bearing  26  which is arranged within the guide member  12 , while the third spindle member  21  is connected to the planet cage  34  of the second reducing gear  3  by means of a form-locking plug coupling. For this purpose, the end of the third spindle member  21  is e.g. configured as a Torx connection or a hexagon that is slipped into a matingly shaped opening in the planet cage  34 . It is especially favorable that the form-locking plug coupling is coupled to the planet cage  34  in a torsion-proof, radially yielding and flexible manner. Coupling is effected by means of an outside ring  51  of a radial bearing  50  provided in the cover  23 . An elastic seal or sealing sleeve  27  interposed between the threaded nut  16  and the guide member  12  prevents the ingress of contaminants into the interior of the ball-and-thread drive assembly. 
     Moreover, it is expedient for a proper functioning of the actuating unit of the present invention when the threaded nut  16 , at its end remote from the friction lining  4 , has an axial projection (not shown) which, during its resetting action, interacts with a stop that is provided on the periphery of the second spindle member  20 . Further resetting of the threaded nut  16  is effectively prevented by supporting a lateral surface of the projection on the stop so that the two members  16 ,  20  will not be jammed. 
     There is provision of a position detection system  46  (not shown) to determine the current position of the rotor  11 . The position information is then determined by means of a Hall sensor or a magnetoresistive element. 
     In order to realize the function of a parking brake, the actuating unit of the present invention includes electromechanic means which interact with the rotor  11  of the electric motor  10  and permit it being locked. In the embodiment shown, the electromechanic means is an electromagnetically operable freewheel mechanism which has been assigned reference numeral  35  and interacts with a radial bearing  24  in which the rotor  11  is mounted. The electric actuator means associated with the freewheel mechanism  35  is designed in the way of a mechanic flipflop whose condition changes with each short energization. 
     As can be taken from FIGS. 2 to  4  in particular, the basic parts of freewheel mechanism  35  are integrated in the above-mentioned radial bearing  24 . To this end, both the outside ring  36  and the inside ring  37  of the radial bearing  24  are extended on one side so that they confine an annular chamber in which a clamping element  38  is received. A form-locking engagement between the bearing rings  36 ,  37  and the clamping element  38  is achieved due to the special configuration of the extended area of the bearing rings  36 ,  37 . The outside ring  36 , preferably in its area interacting with the clamping element  38 , includes a radial recess  39  which is confined on one side by a slope or ramp  40 , while the inside ring  37  includes a profiling  41  which corresponds to the contour of the clamping element  38  and, along with the recess, limits a clamping slot. Clamping element  38  which may be designed as a jamming roller or as a ball is biassed towards the above-mentioned recess  39  by means of an annular spring element  42 . 
     The freewheel mechanism  35  is actuated by an electromagnetic actuating device which is designated by reference numeral  43  in the example shown. The actuating device  43  is basically composed of a bistable electromagnet  44  and a tappet  45  which interacts with the armature of the electromagnet  44  and adjusts the clamping element  38  radially when the electromagnet  44  is activated. Tappet  45  is guided in a tubular guide  47  which is shaped at an annular accommodating member  48  that receives the outside bearing ring  36  and is arranged in the motor casing  8 . 
     The following sequence of operations is provided: Initially, the electromechanic brake is tensioned to reach the necessary level of tensioning force. Rotor  11  or inside bearing ring  37  is moved during tensioning in opposition to the clamping direction of the freewheel mechanism  35 , i.e., to the right in the drawing. When the clamping element  38  is shifted in the direction of the profiling  41  during actuation of the parking brake due to activation of electromagnet  44 , it will roll on the above-mentioned ramp  40  into the tapering clamping slot. When the current supplied to the electric motor  10  is reduced, the spring force of the tensioned brake will tend to turn the rotor  11  or the inside bearing ring  37  in the clamping direction. This will reliably lock the parking brake. The actuated position of the parking brake is illustrated in FIG.  4 . 
     For release purposes, the electric motor  10  must tension the brake only slightly and the electromagnet  44  must be actuated only one time in order to move the tappet  45  in an upward direction. The clamping element  38  which is thereby relieved from load is urged by the force of the spring element  42  biassing it into the recess  39  of the outside bearing ring  36 , and the rotor  11  is freely rotatable in both directions. 
     Of course, various modifications are feasible in the spirit of the present invention. For example, the electric motor that is used as drive unit  1  may be designed as a switched reluctance motor (SR-motor). Other designs of the planetary gear are also possible, such as a two-stepped differential planetary gear or a gear whose planet wheels with their first step are in engagement with a sun wheel and with their second step, by the intermediary of each one spur wheel, are in engagement with a ring gear. Gear units which achieve high reduction ratios due to a deformable toothed ring and eccentricity are of course also feasible. 
     The inside bearing ring, too, is possible with most different types of profiling. It is only important that the clamping element can adopt a safe position. The annular spring element which biasses the clamping element is not required to embrace the entire periphery. It is possible to arrange a leaf spring which is anchored in the vicinity of the clamping element only. A design of the spring element as a so-called ‘beyond dead-center spring’ (bangling tin frog) is also possible. 
     To prevent transverse forces or deformations which are due to the clamping effect, several clamping elements may be arranged evenly on the periphery. 
     List of Reference Numerals: 
       1  drive unit 
       2  reducing gear 
       3  reducing gear 
       4  friction lining 
       5  friction lining 
       6  brake disc 
       7  actuating element 
       8  motor casing 
       9  stator 
       10  electric motor 
       11  rotor 
       12  guide member 
       13  carrier 
       14  permanent magnet segment 
       15  gearbox case 
       16  threaded nut 
       17  threaded spindle 
       18  ball 
       19  spindle member 
       20  spindle member 
       21  spindle member 
       22  area 
       23  cover 
       24  radial bearing 
     
       25 
     
       26  axial bearing 
       27  seal 
       28  slide ring 
       29  slide ring 
       30  sun wheel 
       31  planet wheel 
       31   a  planet wheel 
       31   b  planet wheel 
       32   a  planet wheel 
       32   b  planet wheel 
       33  ring gear 
       34  planet cage 
       35  freewheel mechanism 
       36  outside ring 
       37  inside ring 
       38  clamping element 
       39  recess 
       40  slope 
       41  profiling 
       42  spring element 
       43  actuating unit 
       44  electromagnet 
       45  tappet 
       46  position detection system 
       47  guide 
       48  accommodating member 
     
       49 
     
       50  radial bearing 
       51  outside ring