Abstract:
Apparatus for providing a linear movement to an actuating member while preventing axial loadings from being transmitted from the actuating member to an engaging member, such as a screw. A pressure pad or springs are interposed between an actuating member and an engaging member such that axial loadings are prevented from being transmitted from the actuating member to the engaging member due to the resilient effect of the springs or pressure pad.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention is related to an actuator comprising a housing, a motor, an actuating member and a screw mechanism providing a linear movement of the actuating member with respect to the housing in response to a rotational movement of the motor, said screw mechanism comprising a screw and a nut one of which is rotatably supported with respect to the housing. 
     2. Description of Related Art 
     Such actuator is known from WO-A-9603301. The actuator comprises a screw mechanism, consisting of a screw and a nut which engage each other by means of rollers having circumferential grooves. Such actuator provides a well defined axial displacement, and also a certain degree of reduction. Thus, a fairly, small axial displacement is obtained in response to a considerable rotation of the screw. Consequently, the screw may exert a considerable axial force on the actuating member. 
     The proper operation of the screw mechanism can be assured as long as the screw, nut and rollers are subject to a well-defined, axial load. In such case, all components are loaded in accordance with their design requirements; moreover, the load may then reach a considerable level without causing damage to the screw mechanism. 
     A very unfavorable case however occurs when the screw mechanism is subjected to loadings having a transverse or radial component. The screw, nut and rollers are not designed to accommodate the transverse load components, and will be damaged. 
     The same occurs in case the actuator is loaded by a bending couple, which is always associated with such transverse forces. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide an actuator as described before, in which the problems related to transverse or radial loadings are circumvented or at least alleviated. This object is achieved in that the screw mechanism and the actuating member engage each other through a resilient intermediate pressure means. The resilient intermediate pressure means is able to transfer the required actuating force from the screw mechanism onto the actuating member. Thus, its stiffness in axial direction should be rather high. In particular, the stiffness should be maintained at a level where the required force/displacement relationship still provides the possibility to obtain the desired actuating force. 
     On the other hand, said resilient intermediate pressure means is not as stiff as a direct connection between the screw mechanism and the actuating member. This adapted stiffness has the advantage that extreme loadings, which have a certain transverse component or bending moment, are not directly and fully transmitted towards the screw mechanism. The resilient aspect of the force transmission between the screw mechanism and actuating member makes these transverse or bending loadings less severe or even absent. 
     According to a first possible embodiment of the invention, the screw mechanism and the actuating member engage each other through spring elements. The spring elements may provide an asymmetric stiffness distribution with respect to the axis of the screw mechanism. The asymmetric character of the spring elements may be obtained in several ways, e.g. by more or stiffer springs at one side of the axis of the actuating member and the screw mechanism than at the opposite side. 
     The advantage of an asymmetric layout of the spring elements is that it may anticipate an asymmetric loading pattern under full loading. An example of a non-aligned loading, which increases with the load level, is to be attributed to flexing of the claw piece in the application of an actuator in a disc brake. 
     According to a second possibility, the screw mechanism and the actuating member engage each other by means of a resilient pressure pad. 
     Such pressure pad may have a relatively small thickness compared to its lateral dimensions. It can therefore easily be accommodated between the actuating member and the screw mechanism, within narrow space constraints. 
     The pressure pad preferably comprises two generally parallel wall members, said wall members being mutually connected along their circumference and enclosing a closed internal space. 
     The actuating member may be carried out as a piston, said piston being accommodated in a cylinder which forms a bore in the housing of the screw mechanism. Thus, the intermediate resilient pressure member is supported against the inside of the piston head. 
     According to a further development, the resilient intermediate pressure means engages a load measuring device. In particular, the load measuring device senses the pressure of the fluid. 
     The pressure developed in the fluid provides a reliable measure of the force exerted by the actuator. Possible a non-axial or excentric loadings will not hamper the measurement of the overall axial loads to which the actuator is exposed. Thus, a reliable measurement is obtained. 
     If convenient, the internal space of the pressure pad is connected to a measuring channel, the free end of said channel being provided with the load measuring device. Thereby, the actual measurement, e.g., by a piezoelectric sensor, may be carried out a particular location which is for instance shielded from the area where the actuating member is located (heat, moisture). 
     The invention is also related to a brake caliper comprising a housing, a motor, an actuating member and a screw mechanism providing a linear movement of the actuating member with respect to the housing in response to a rotational movement of the motor, said screw mechanism comprising a screw and a nut, one of which is rotatably supported with respect to the housing. The screw mechanism and the actuating member engage each other through a resilient intermediate pressure means for pressing the brake pads onto the brake disc. 
     The resilient intermediate pressure means of the brake caliper may engage a load measuring device as well. The internal space of the pressure pad is connected to a measuring channel, the free end of said channel being provided with the load measuring device. 
     The actual load measuring device (e.g. a piezoelectric sensor) may now be situated at a location where the influence of the heat developed by the friction between the brake pads and the brake disc, is reduced. 
     The measuring channel may extend through the screw of the screw mechanism, the load measuring device being situated at the end of the screw which is opposite the pressure means. Furthermore, the load measuring device comprises a load cell or load sensor, the electric signal line thereof extending through thee internal space of the motor towards a connector or the like on the housing. 
     Preferably the fluid is a temperature resistant thermal oil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be explained further with reference to the embodiments shown in the figures. 
     FIG. 1 shows a first embodiment of a brake caliper comprising an actuator according to the invention. 
     FIG. 1 a  is an enlarge view of the actuating member illustrated in FIG. 1, having springs. 
     FIG. 2 shows a second embodiment. 
     FIG. 2 a  is an enlarged view of an actuating member illustrated in FIG. 2, having a pressure pad. 
     FIG. 3 shows a third embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The brake caliper shown in FIG. 1 comprises a claw piece  1 , having a flange  2  supporting brake pad  3 , and a housing  4  carrying brake pad  5 . Between the brake pads  3  and  5 , a brake disc  6  is accommodated. 
     Brake pad  5  is moveable towards, and away from, the other brake pad  3  by means of an actuator  7  which comprises an electric motor  8 , a reduction gear mechanism  9 , and a screw mechanism  10 . The motor  8  has a stator  11  and a rotor  12 , which rotor  12  is connected to a tubular connecting member  13  carrying a sun gear wheel  14 . The sun gear wheel  14  engages satellite gear wheels  15 , which are accommodated on carrier  16 . The satellite gear wheels  15  also engage the fixed ring gear wheel  17 . The carrier  16 , in particular shaft  18  thereof, engages the screw mechanism  10  by means of tubular intermediate  19 . The tubular intermediate member  19  is supported rotatably within the tubular connecting member  13  by means of bearings  20 ,  21 . The tubular intermediate member  19  is connected to nut  22  of the screw mechanism  10  which by means of rollers  23  engages screw  24 . Screw  24  is held non-rotatably, but can move in an axial direction. The nut  22  forms a unity with the inner ring of the four point contact ball bearing  30 , the outer ring  31  of which is supported in the housing  4 . 
     The screw  24  engages the actuating member  25 , in such a way that by means of the screw mechanism  10  the brake pad  5  can be moved towards, and away from, the other brake pad  3 . 
     During a braking action, the brake pads  3 ,  5 , may exhibit some play transverse with respect to the axis of screw mechanism  10 . Thereby, the screw mechanism  10 , in particular screw  24  thereof, may be loaded in a non-axial or non-aligned way. 
     In order to counteract the effects of such non-axial loadings, the surfaces  26  and  27  of the actuating members respectively the screw  24  engage each other by means of relatively stiff springs  28  and relatively flexible springs  29 , whereby a locally different stiffness is obtained. 
     The stiffness ratio of these springs is selected in such a way that they simulate a flexing which is opposite to the flexing of the brake caliper, which flexing occurs as a result of the clamping forces by means of which the brake pads engage the brake disc. Thus, the caliper flexing is reversed or counteracted in order to obtain an axial load on the screw mechanism. 
     FIG. 1 a  is an enlarged view of the actuating member  25  as illustrated in FIG. 1, connected to the screw  24  by springs  28  and  29 . As discussed previously, the springs  28  and  29  counteract the effects of non-axial loading that would be transmitted to the screw  24  in their absence. 
     The embodiment according to FIG. 2 corresponds to some extent to the embodiment of FIG.  1 . The connecting member  13 , which carries sun gear wheel  14 , is now supported with respect to the housing by means of bearings  33 ,  34 . 
     Furthermore, the carrier  16 , which carries satellite gear wheels  15  which engage both the sun gear wheel  14  and the ring gear wheel  17 , is connected to the nut  22  of the screw mechanism  10 . This nut  22  at the same time constitutes the inner ring of the four point-ball bearing  30 , the outer ring  31  of which is supported in the housing  4 . 
     Nut  22  engages screw  24  by means of rollers  23 . The screw is held against rotation, but is able to move in an axial direction. Thus, upon rotation of the nut  22 , the screw  24  moves backward and forward, thus moving the brake pads  3 ,  5  onto each other and onto the brake discs  6 , or moving them away from each other. 
     The actuating member is carried out as piston  35 , which slidably held within the cylinder space  37  in the housing  4 . By means of a groove nut connection  38 ,  38 ′, the piston  35  is held non-rotatably, but slidably. 
     By means of further groove/nut connection  39 ,  39 ′, screw  24  is also held non-rotatably, but slidably, with respect to piston  35 . 
     The screw engages piston head  40  by means of the interposed pressure pad  36 . This pressure pad  36  has two parallel walls  41 , connected at the circumference  42 , e.g. by means of welding. The internal space enclosed between the parallel walls  41  is filled with a pressure medium  43 , e.g. a hydraulic fluid. 
     In case, as a result of the braking action, the brake pad  5  would tilt somewhat, and thereby causes somewhat misalignment between the axis of the piston  35  and the screw  24 , pressure pad  36  may accommodate this misalignment. In this way, the screw  24  is still mainly loaded in axial direction, thus allowing a proper function of the screw mechanism  10 . 
     FIG. 2 a  is an enlarged view of the actuating member or piston  35  as illustrated in FIG. 2, having a pressure pad  36 . As discussed previously, the pressure pad  36  may accommodate misalignment between the axis of the piston  35  and the screw  24 . 
     In the third embodiment, shown in FIG. 3, a load measuring device  50  has been applied. This may for instance be a piezoelectric sensor. The load measuring device  50  is connected to a measuring channel  51 , which is connected to the internal space  52  of the pressure pad  53 . 
     Through a signal cable  54 , the load measuring device  50  is connected to a control unit for further processing of the data thus obtained. As shown in the figure, the signal table  54  is guided through the internal hollow space of the electric motor  7 . 
     The internal space  52  and the measuring channel  51  may be filled with a thermal oil which is resistant to the high temperatures which may develop as a result of the friction between the brake pad  5  and the brake disc  6 . Furthermore, a ceramic pad  55  may be arranged between the pressure pad  53  and the head of the piston  35 , so as to thermally insulate the thermal oil as a further precaution.