Abstract:
The invention relates to an electromechanically actuatable disk brake with a housing, a friction pad against which an electrically actuatable actuator acts, with the actuator comprising an electric motor and a spindle/nut transmission arrangement, characterised in that the electric motor is configured as an internal rotor motor with a stator and a rotor, the rotor of which is coupled non-rotably with the spindle, the spindle is operatively coupled with the nut in order to transform a rotational movement of the spindle into a translatory movement of the nut, the spindle is supported along its longitudinal extent in at least two places so as to be rotatable relative to the housing, and the nut is guided axially slidable relative to the housing.

Description:
This application is a continuation of PCT/EP99/01081 filed Feb. 19, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to an electromechanically actuatable disk brake with a housing and a friction pad which is slidably supported at the housing, against which an electrically actuatable actuator acts, with the actuator comprising an electric motor and a spindle/nut (transmission) arrangement. 
     Such vehicle brake arrangements, in particular in the form of disk brakes, are known from so-called brake-by-wire systems. 
     These arrangements are based on the problem of transforming the rotational movement of the electric motor into a translatory movement of the friction pads relative to the brake disk, with high feed forces being required on the one hand and high dynamics of the feed motion being required, on the other hand. In addition, there is the requirement for a high reliability with a high mechanical efficiency within a wide temperature range as occurs in vehicle brakes. Moreover, the requirement for a self-releasing capability of the friction pads from the brake disk upon a current failure of the electric motor must be met. In addition, the limited installation space at the vehicle wheel is to be utilised efficiently, and the unsprung masses are to be kept small. Another aspect with the employment of spindle/nut arrangements for the transmission of the rotational movement into a translation movement is that, if possible, no transverse forces or bending moments are introduced into the spindle transmission, because the spindles are generally not designed for this and the spindle efficiency will then considerably deteriorate. 
     The selection of the spindle pitch and the spindle diameter is decisively dependent on the required spindle axial force and the available drive moment at the spindle nut. In addition, the axial force provided by the spindle must be supported by an adequately dimensioned axial bearing. 
     DE 195 11 287 A1 shows a floating caliper brake wherein an electric motor which is formed as an outside rotor drives a nut which is rigidly coupled with the rotor of the electric motor. A screw spindle is displaced in the axial direction via threaded rollers which are in engagement with the nut, when the electric motor sets the nut and thus the threaded rollers in rotation. The screw spindle acts on a friction pad. The screw spindle is supported in the rotor of the electric motor by the threaded rollers. The rotor of the electric motor with its end facing the friction pad is guided via a radial bearing which bears against the housing. 
     From DE 195 43 098 A1 a floating caliper brake is known, wherein an electric motor which is formed as an inside rotor drives a screw spindle. The spindle has a male thread which is in engagement with threaded rollers, which are arranged along the circumference of the spindle in a distributed manner. A female thread of a threaded bushing engages at the periphery of the rollers. The threaded bushing is connected with a friction pad so that the friction pad is displaced in the axial direction upon a rotation of the motor. The rotor of the electric motor is provided with a hollow axle for accommodating the spindle rod. At its rear side, the rotor bears against the housing via a radial bearing and against the spindle rod via an axial bearing. 
     In the two above mentioned arrangements, either transverse forces of the electric motor are acting on the point of contact between the spindle and the rollers, or the friction pads generate reactive forces onto the points of contact between the spindle and the rollers under operating conditions. In both cases the consequence is increased wear and higher friction forces between the points of contact at the spindle or the rollers, respectively. The result of this is that the self-releasing capability and the precision of the actuating movement is affected. 
     From DE 196 05 988 A1 a floating caliper brake is known, wherein an electric motor with an inside rotor drives a screw spindle. In order to be able to manage with a small drive motor, the rotor is designed as a cup inside of which the screw spindle is arranged. The screw spindle bears with only one end against the bottom of the cup which is supported at the housing by an axial and two radial bearings. Along its longitudinal extension the screw spindle is not supported further so that the other end of the screw spindle is free. Planetary rollers are abutting the screw spindle, by means of which a rotational movement of the screw spindle is transferred into an axial displacement of a nut. The nut bears radially against the inside of the cup-shaped rotor or its radial bearings, respectively. Due to the fact that the second axial end of the screw spindle is not radially supported, this arrangement is disadvantageous in that a shift of the free end of the screw spindle, which is caused by mechanical stress and wear, results in an eccentric rotational movement so that the planetary rollers are stressed unevenly and the actuation movement becomes unprecise. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object to improve a vehicle brake of the initially mentioned type in such a manner that these drawbacks are eliminated. 
     The inventive solution provides for the electric motor to be configured as an internal rotor motor with a stator and a rotor, the rotor of which is coupled secured against rotation with the spindle, the spindle is operatively coupled with the nut in order to transform a rotational movement of the spindle into a translatory movement of the nut, the spindle is radially supported along its longitudinal extent in at least two places so as to be rotatable relative to the housing, and the nut is guided axially slidable relative to the housing. 
     The bearing arrangements provided in the state of the art do not represent a solution of this problem. By means of the axial sliding guidance of the nut, the inventive configuration compensates transverse forces acting on same from the brake shoes, so that no transverse forces are transferred to the rollers. The double bearing of the spindle has the effect that transverse forces from the electric motor can be taken up by the housing, without being able to cause deformations of the spindle. 
     In a preferred embodiment of the invention, the spindle is in a driving connection with the nut via planetary rollers. 
     The spindle is rotatably supported in the area of its two ends. A first bearing is preferably designed as a locating bearing and a second bearing as a floating bearing relative to the housing in the axial direction. This permits a compensation of forces which cause an axial deflection of the spindle. 
     The nut is slidably guided in a sliding bushing or directly in the housing. A direct guidance of the nut in the housing (without a separate sliding bushing) reduces the manufacturing expenditure. 
     In the area of the end of the spindle which faces the friction pad a bearing shield is provided according to the invention, which forms a bearing position for the spindle and/or a bearing position for the nut. The bearing shield can either be formed integrally with the housing, or can be formed as a component which is separately inserted into a corresponding opening in the housing. Thus, the bearing shield can assume a twofold function with a defined correlation of the bearing position for the spindle and the bearing position for the nut being able to be manufactured precisely in a simple manner. 
     In a particularly preferred embodiment, the nut comprises at least one extension which can protrude through a correspondingly formed opening in the bearing shield, in order to transfer the axial movement of the nut through the bearing shield to the outside to the friction pad. 
     The second bearing for the spindle (i.e. the floating bearing) is preferably accommodated in a corresponding recess in the bearing shield and bears against the housing via webs which are arranged between the opening between the extensions of the nut. This particularly preferred embodiment permits a very compact and closed configuration of the arrangement. 
     The extension or each extension of the nut as well as the respective associated opening are preferably designed in such a manner that the nut is secured against rotation relative to the housing. In this manner, the nut is additionally guided both in the axial and in the radial direction towards the area of the openings. 
     In order to avoid the ingress of dust or humidity into the interior of the housing, an intermediate element is preferably arranged between the friction pad and the extension or the extensions of the nut, which serves as an end piece of the extension(s) of the nut or for accommodating a seal (e.g. in the form of a bellows). 
     In a particularly preferred manner the end of the spindle, which is accommodated at the floating bearing, is biased by a spring arrangement in the axial direction against the housing. 
     This can compensate for expansions of the spindle or the housing, but also of other components, which are primarily caused by temperature variations. 
     In a further configuration the spindle is coupled with a spring-loaded cylinder which upon a feed motion takes up work which is directed opposite said feed motion and which causes a resetting movement of the spindle or the nut, respectively, with a currentless electric motor. In this manner, it is ensured that the friction pad clears the brake disk. 
     In order to achieve a particularly space-saving arrangement of the spring-loaded cylinder this can be designed as a torsion rod formed in a longitudinal hole of the spindle. This arrangement is advantageous in that the spindle/nut arrangement can be designed self-locking so that a parking brake function can be realised. By a controlled charging or discharging, respectively, of the spring-loaded cylinder, the spindle can be brought into rotation by the spring-loaded cylinder against the feed force, even with a non-functional electric motor, so that the brake disk and the friction pad clear each other. 
     The spring-loaded cylinder is preferably arranged relative to the spindle, the motor, and the housing in such a manner that the charging/discharging process of the spring-loaded cylinder can be controlled by means of a coupling which is coupled with the spring-loaded cylinder. 
     It is particularly preferred that the disk brake comprises a floating caliper which carries two friction pads between which the brake disk is arranged. The floating caliper is designed in such a manner that, upon a feed motion, it takes up work which is directed against said feed motion and which causes a resetting movement of the spindle or the nut, respectively, with a currentless electric motor so that the friction pad clears the brake disk. This configuration permits the omission of a special spring-loaded cylinder and forms a particularly simple construction of the disk brake. 
     In a further embodiment, a further transmission stage, preferably in the form of a planetary transmission, is provided in addition to the roller/spindle or nut/spindle (transmission) arrangement. 
     Further characteristics, properties, and modification possibilities will be explained by means of the description of three embodiments with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first embodiment of an inventive electromechanicanically actuatable disk brake as a schematic illustration of a longitudinal section. 
     FIG. 2 shows a sectional view through the electromechanicanically actuatable disk brake according to FIG. 1 along the section lines  2 — 2 . 
     FIG. 3 shows a second embodiment of an inventive electromechanicanically actuatable disk brake as a illustration of a longitudinal section. 
     FIG. 4 shows a third embodiment of an inventive electromechanicanically actuatable disk brake as a illustration of a longitudinal section. 
     FIG. 5 shows a sectional view through the electromechanicanically actuatable disk brake according to FIG. 4 along the section lines  5 — 5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The electromechanically actuatable disk brake which is schematically shown in FIG. 1 is designed as a floating caliper disk brake with a floating caliper  10  which is securely connected with a housing  12 . In the floating caliper  10  two friction pads  14 ,  16  are accommodated between which a brake disk  18  is arranged which is indicated only schematically. 
     In the housing  12  an internal rotor motor  20  is accommodated. A stator  22  which is built from iron laminations with a corresponding stator winding  24  is pressed into the housing. A rotor  26  of the electric motor  20  comprises a step  28  at its end facing away from the friction pad  14 , which carries a collar  30 . The collar  30  is fitted into a ball bearing  32  which is accommodated in a corresponding recess in the housing  12 . A first end of a screw spindle  36  is accommodated in the collar  30  of the rotor  28  secured against rotation by means of a feather key  34  acting as a n anti-rotation means. Thus, this first end of the screw spindle  36  also bears against the housing through the ball bearing  32 . Due to the fact that this first end of the screw spindle  36  is firmly pressed into the step  30  of the rotor  26 , a locating bearing for the screw spindle  36  is formed thereby. The screw spindle  36  comprises a male thread  38  which is in engagement with several (three or more) cylindrical threaded rollers  40 . 
     The threaded rollers  40  meshing with the screw spindle  36  are dimensioned in the radial direction in such a manner that they are also engaging a nut  42  which has the shape of a hollow cylindrical bushing with a female thread. The nut  42  extends coaxially to the screw spindle  36  to a bearing shield  44  which closes the housing. The bearing shield  44  is designed cup-shaped and carries a sliding bushing  46  as a radial guide for the nut  42  at its cylindrical inner wall. The bearing shield  44  has an outwardly directed collar  48  at its open end, which is accommodated in a corresponding indentation in a cover  50  of the housing  12 . The collar  48  thus prevents a deflection of the bearing shield  44  in the axial direction. 
     The bearing shield  44  comprises an indentation in alignment with the screw spindle  36 , in which a second ball bearing  52  is accommodated. The ball bearing  52  bears against the bottom of the indentation via a plate spring  54  in such a manner that the plate spring  54  acts on an ring  52   a  of the ball bearing  52 . The ball bearing  52  is accommodated play-free in an axially slidable manner in the opening of the bearing shield  44  so that the ball bearing  52  or the bearing shield  44 , respectively, forms a floating bearing for the screw spindle  36 . 
     The bearing shield  44  comprises semi-circular annular openings  60  (see FIG. 2) further to the outside than the outer circumference of the ball bearing  52 , through which the matingly shaped extensions  62  of the nut  42  extend. Upon an actuation of the electric motor  20 , the nut  42  is brought into a longitudinal movement by the rotational movement of the screw spindle  36  or the threaded rollers  40 , through which the extensions  62  in the openings  60  either protrude or retract depending on the sense of rotation of the electric motor  20 . Between the openings  60  webs  64  are arranged by means of which a rotation of the nut  42  or its extensions  62 , respectively, is prevented. With their free ends the extensions  62  engage in correspondingly formed recesses in a plate-shaped formed intermediate element  70  which serves as an end plate of the extensions  62  of the nut  40 . In addition, the intermediate element  70  serves to accommodate a tubular bellows  72  whose one end is accommodated at the intermediate element  70 , and whose other end is accommodated at the outside of the bearing  44 . By means of the intermediate element  70  which accommodates the extensions  62  in correspondingly designed recesses, it is prevented that the extensions buckle  62  laterally upon an axial stress on the nut  42 . 
     The intermediate element  70  acts flatly on the friction pad  14  so that upon an actuation of the electric motor in the sense of a feed of the disk brake, the friction pad  14  is urged against the brake disk  18 . With the feed motion of the nut  40  that brake disk is thereby also urged against the second friction pad  16 . The floating caliper  10  takes up this work and closes the power train to the housing  12 . The floating caliper  10  can be designed in such a manner that is comprises a certain elasticity and stores work directed against this feed motion in an elastic deformation until the electric motor  20  becomes currentless. Then this work becomes free and urges the spindle or the nut, respectively, into a resetting movement via the brake disk  18  and the friction pad  14 . Thereby, the holding force of a self-locking spindle/nut arrangement can be overcome so that the friction pad  14  clears the brake disk  18 . 
     The bearing  52  and thus the screw spindle  36  as well are resiliently clamped in the housing by the plate spring  54  so that no play worth mentioning can occur between the screw spindle  36  and the threaded rollers  40 , or between the threaded rollers  40  and the nut  42 , respectively. 
     In FIG. 3, identical parts or parts which have the same effect as compared to FIG. 1 or FIG. 2 are given identical reference numerals and are not explained again in detail in the following. The same applies to FIG.  4  and FIG. 5 which will be explained further below. 
     The main difference between the embodiment from FIG.  1  and the embodiment in FIG. 3 is that the screw spindle  36  is coupled with a spring-loaded cylinder in the form of a torsion spring  80 , which upon a feed motion takes up a force directed against said feed motion. With a currentless electric motor ( 20 ), a charged spring-loaded cylinder  80  causes a resetting movement of the spindle or the nut, respectively, directed against the feed motion. Thereby, the friction pad can clear the brake disk, in particular, if a self-locking spindle/nut arrangement is employed. For reasons of space, the torsion spring  80  is arranged in the interior of the screw spindle  36 . For this purpose, the torsion spring  80  is accommodated at its one end (in the area of the floating bearing  52 ) secured against rotation in a hole. The other end of the torsion spring  80  (in the area of the ball bearing  32 ) protrudes from the face end of the screw spindle  36 . The end of the torsion spring  80  which protrudes from the screw spindle  36  is accommodated in a coupling  82  which can be actuated electrically in order to load or release the torsion spring  80  relative to the housing  12  in a controlled manner, with the torque applied by the torsion spring  80  being limited. 
     When using a self-locking spindle/nut arrangement together with a non-elastic floating caliper, a parking brake function can be realised as well. For this purpose, the torsion spring  80  is released relative to the housing  12  by the electrically actuatable coupling  82 , so that no resetting movement of the screw spindle  36  or the nut  42 , respectively, directed against the feed motion can take place, so that after actuation for parking, the friction pad ( 14 ,  16 ) remains in frictional engagement with the brake disk  18 . Contrary thereto, for the realisation of the service brake function, the torsion spring  80  is loaded against the housing  12  by the electrically actuatable coupling  82 , so that upon a rotation of the screw spindle  36  the torsion spring is rotated and thus work is applied which is directed against the feed motion. This work will subject the spindle or nut, respectively, to a resetting movement, with the electric motor being currentless, so that the friction pad can clear the brake disk. 
     The embodiment according to FIG. 4 differs from the embodiment according to FIG. 1 insofar, as a second transmission stage in the form of a planetary transmission  90  is provided. For this purpose, a planet carrier  91  which is supported in the housing  12  by means of a ball bearing  32  is provided with several openings which accommodate bearing journals  94  for planetary gears  96 . The planetary gears  96  mesh with a ring gear  100  with internal toothing. In the area of the end of the screw spindle  36  remote of the friction pad  14 , the planet carrier  91  is connected secured against rotation by means of an anti-rotation means  34 . In the area of the end remote from the friction pad  14 , the rotor  26  forms a sun gear  98 . An additional ball bearing  102  forms a floating bearing for the rotor  26 , while the ball bearing  92  forms a locating bearing for the rotor  26 . Through the two-stage design of the transmission, higher feed forces can be achieved. Moreover, the spindle/nut arrangement can be designed self-releasing by a correspondingly selected thread pitch. 
     FIG. 4 is a third embodiment which has the screw spindle  36  coupled with a spring-loaded cylinder in the form of a torsion spring  80  like the embodiment shown in FIG.  3 . However, the face end of the torsion spring  80 , which protrudes from the screw spindle  36 , is accommodated in a torque limiter  110 , which functions purely mechanically. As soon as the torsion spring  80  has reached the torque required for the resetting movement against the feed motion after a certain number of revolutions or a certain proportion of a revolution in the feed direction, this moment is kept almost constant by the torque limiter  110 . 
     The further embodiment is additionally adapted for the parking brake function. As can be seen from the sectional view along the section line  5 — 5  in FIG. 4, a (saw tooth) ring gear  112  is provided for this purpose, which is connected with the screw spindle  36  secured against rotation, which is driven by the electric motor via the axles of the planetary gears  96  of the second transmission stage. The (saw tooth) ring gear  112  which is connected with the screw spindle  36  secured against rotation can be arrested by means of a ratchet  114 , with the ratchet  114  being electrically operable. For parking, the brake is actuated which, among other methods, can be done as it is usual with the service brake via the brake pedal until the necessary clamping force is achieved. Subsequently, the ratchet  114  is operated, which in turn arrests the (saw tooth) ring gear  112  so that a resetting movement is excluded, and consequently the friction pad  14  remains in frictional engagement with the brake disk  18 . Contrary to the embodiment shown in FIG. 3, the spindle/nut arrangement need not necessarily be designed self-locking in this case in order to enable the parking brake function. 
     It is of particular advantage if for the operation of the ratchet  114  a “bistable” (electrotechnical, electromagnetical) changeover switch  116  is employed which requires the supply of current for changing over only, for example, by means of a pulse-shaped current, and which, after changing over, remains currentless in its last assumed position. The same applies for the electrically actuatable coupling according to FIG.  3 . 
     Although the three embodiments have been separately described above, it is understood by those with skill in the art that individual aspects of one embodiment can also be transferred into the respective other two embodiments without departing from the inventive idea.