Abstract:
The invention relates to a spindle drive for converting between a rotational motion and a translational motion comprising a spindle for translational motion and a spindle nut for rotational motion, wherein the spindle and the spindle nut are coupled to each other by means of threads. The spindle nut comprises a reservoir in axial extension of the thread of the spindle nut, in which reservoir a segment of the spindle extends. A lubricant is accommodated in the reservoir, and a displacement element is attached to the spindle in the region of the reservoir, which displacement element extends further outward radially than the thread of the spindle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a spindle drive. The invention particularly relates to a spindle drive on an actuator for use in a motor vehicle. 
         [0002]    A spindle drive comprises a spindle and a spindle nut which are mounted coaxially to a common axis of rotation. The spindle nut has an internal thread and the spindle an external thread, wherein the two threads mesh with one another. If spindle nut and spindle are rotated in opposite directions about the axis of rotation, a translational movement thus occurs between the spindle and the spindle nut along the axis of rotation. In one embodiment, the spindle nut is configured to be set into rotation about the axis of rotation in order to move the spindle along said axis of rotation. 
         [0003]    The spindle drive can, for example, be used in an electrohydraulic actuator for use on board of a motor vehicle. In so doing, the spindle can act axially on a hydraulic piston which is accommodated in a hydraulic cylinder. An electric motor can be provided to set the spindle nut into rotation. The rotation of the electric motor is converted by the spindle drive into a translational movement and by the piston into a volume flow or a pressure change of a hydraulic fluid in the cylinder. In this way, a braking or coupling device can be actuated by means of the electric motor. 
         [0004]    The German patent publication DE 10 2011 108 962 A1 depicts an electrically driven spindle drive. 
         [0005]    The German patent publication DE 10 2009 005 886 A1 depicts a further spindle drive comprising a lubrication channel in order to apply a lubricant to the threads on the spindle and the spindle nut if the spindle is in a predefined parking position in relation to the spindle nut. 
         [0006]    The spindle drive can be particularly sensitive to a lack of lubricant in the region where the threads engage with one another. If a lubricant film breaks down in the region of the threads so that segments of the thread are dry when meshing with each other, one or both threads can be subject to a great deal of wear. An abrasion of the thread flanks can exert the effect of an abrasive medium on further thread flanks so that these too wear at an accelerated pace. In an extreme case, the spindle can break, axially slip on the spindle nut or spindle and spindle nut can seize to one another so that a further relative movement is not possible. Such damage occurs especially at high mechanical loads, high numbers of cycles or high ambient temperatures as they can occur in the region of actuators in motor vehicles. 
       SUMMARY OF THE INVENTION 
       [0007]    It is the aim of the present invention to specify a spindle drive which has an improved load-bearing capacity and is designed as simply as possible. 
         [0008]    A spindle drive according to the invention for converting between a rotational motion and a translational motion comprises a spindle for translational motion and a spindle nut for rotational motion, wherein the spindle and the spindle nut are coupled to each other by means of threads. The spindle nut comprises a reservoir in axial extension of the thread thereof, in which reservoir a segment of the spindle extends. A lubricant is accommodated in the reservoir, and a displacement element is attached to the spindle in the region of the reservoir, which displacement element extends further outward radially than the thread of the spindle. 
         [0009]    The displacement element moves together with the spindle in the axial direction if the spindle nut is rotated with respect to the spindle. As a result, the lubricant in the reservoir is circulated from the one axial side of the displacement element to the other. As a result of this circulation, lubricant can be applied in an improved manner to the thread of the spindle; thus enabling the lubricant to be distributed along the space between the threads of the spindle nut and the spindle when the spindle repeatedly moves axially. A dry operation of the spindle drive can thereby be prevented. By circulating the lubricant, it can be prevented from clumping together; thus enabling a larger proportion of said lubricant to participate in the lubrication of the two threads. In addition, a separation of different components of the lubricant, for example a base oil and a thickener, can be prevented by the circulation of said lubricant. A spindle drive having a higher load-bearing capacity and improved reliability can be provided by means of the improved lubrication. 
         [0010]    In a preferred manner, the reservoir extends axially at least as far as the spindle can move axially. It is especially preferred if the two axial measurements approximately correspond to one another. In this way, an optimal circulation or mixing of the lubrication in the reservoir can be achieved. In the case of a spindle drive having a known power stroke or maximum stroke, for example in order to actuate a clutch, the axial mobility of the spindle and the axial extension of the reservoir can be exactly adapted to one another. 
         [0011]    In one embodiment of the invention, the spindle extends into the reservoir on one side only. In a preferred embodiment, the spindle, in contrast, completely passes through the reservoir. In an advantageous manner, the change in the volume of the spindle accommodated in the reservoir over the stroke of the spindle varies less dramatically or not at all. A mechanical resistance can be reduced by the work required for displacement being reduced. 
         [0012]    In one embodiment of the invention, the reservoir is rotationally symmetrical. As a result, lubricant can be prevented from accumulating in a radial pocket, where said lubricant is not circulated. It is particularly preferred that the reservoir has a cylindrical shape. An annular clearance between the radial boundary of the reservoir and the rotational body of the displacement element can provide for a uniform circulation of the lubricant in the reservoir. 
         [0013]    The displacement element can be connected to the spindle in a rotationally fixed manner. In so doing, a rotatory movement can also be impressed upon the lubricant, which can contribute to an improved circulation thereof. 
         [0014]    The displacement element can be rotationally symmetrical. The displacement element can especially comprise a plastic disk which is injection molded around the spindle or is pressed onto said spindle. The radial clearance between the displacement element and the boundary of the reservoir can thereby be especially precisely maintained. A clump of lubricant which is larger than the clearance distance can thereby be reliably prevented from axially crossing the displacement element. The clump can instead be sheared in the clearance. 
         [0015]    In a further embodiment of the invention, the displacement element comprises a disk that is open to one side, wherein the spindle has a radial groove for mounting the disk. Such a disk is known in the form of a retaining ring or lock washer. The assembly of the parts of the spindle drive can be facilitated by the radial mounting thereof. 
         [0016]    The lubricant preferably comprises a grease with a low viscosity. In particular, the lubricant can comprise a high temperature grease. In addition, a dry lubricant such as graphite or molybdenum disulfide can be mixed into the lubricant. 
         [0017]    In still a further embodiment of the invention, a ventilation opening for connecting the reservoir to an ambient environment is provided. The change in the remaining volume in the reservoir, if the spindle is moved axially in relation to the spindle nut, can thus be compensated in an improved manner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention is now explained in greater detail with reference to the attached drawings, in which: 
           [0019]      FIG. 1  shows a spindle drive in a first view: 
           [0020]      FIG. 2  shows the spindle drive from  FIG. 1  in a second view; and 
           [0021]      FIGS. 3A, 3B and 3C  show displacement elements for the spindle drive from one of the  FIG. 1 or 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  shows a spindle drive  100 , in particular for use on board of a motor vehicle. The spindle drive  100  can, for example, be provided to axially actuate a hydraulic piston in a hydraulic cylinder. The spindle drive  100  comprises a spindle  105  having an exterior thread  110  and a spindle nut  115  having an interior thread  120 , wherein the threads  110  and  120  mesh with each other and are disposed coaxially to an axis of rotation  125 . In a purely exemplary manner, the spindle nut  115  depicted is mounted by means of a radial bearing  130  in the radial direction. Another bearing can also be used which is equipped to receive axial or tilting forces. In an exemplary manner, the spindle nut  115  is connected by means of a worm gear  135  to an electric motor (not depicted). The worm gear  135  comprises a worm wheel  140 , which is integrally embodied here with the spindle nut  115  and is configured coaxially to the axis of rotation  125 , and a worm  145  which meshes with the worm wheel  140  and is designed to be fastened to a shaft of the electric motor. Instead of the worm gear  135 , another apparatus for transmitting force to the spindle nut  115  can also be provided. 
         [0023]    A reservoir  150  that is equipped to accommodate a lubricant  155  is configured on the spindle nut  115 . The reservoir  150  is located in the axial extension of the internal thread  120  of the spindle nut  115 . In so doing, the reservoir  150  is preferably designed rotationally symmetrical, in particular circularly cylindrical, in relation to the axis of rotation  125 . 
         [0024]    The spindle  105  is equipped to pass through a maximum axial deflection path along the axis of rotation  125 . A segment of the spindle  105  always lies in the reservoir  150  independently of the position of the spindle  105  on the deflection path thereof. As a result, the spindle  105  can extend with an axial end more or less deeply into the reservoir  150  or, as is shown in the embodiment depicted, can completely pass through the reservoir  150 . A sealing element  160  is preferably provided in order to seal the reservoir  150  at a first axial end with respect to the spindle  105 . The segments of the external thread  110  of the spindle and the internal thread  120  of the spindle nut  115  which mesh with each other are located at the other axial end of the reservoir  150 . 
         [0025]    A displacement element  165  is attached to the spindle  105  in the region of the reservoir  150 . The displacement element  165  extends further in the radial direction than the external thread  110  of the spindle  105  and can have various shapes, as is described below in more detail in reference to  FIG. 3 . The displacement element  165  is attached axially to the spindle  105 ; thus enabling said displacement element  165  to move axially through the reservoir  150  if the spindle  105  is moved axially with respect to the spindle nut  115 . Said displacement element  165  circulates the lubricant  155  in the reservoir  150  such that lubricant  155  is better applied to the external thread  110  of the spindle  105 . The rotational movement of the internal thread  120  of the spindle nut  115  can then carry the lubricant  155  further in the axial direction and thus ensure a lubricant film between the internal thread  120  and the external thread  110  along the entire region of engagement. 
         [0026]    The displacement element  165  can be fixedly or loosely secured on the spindle  105  in the circumferential direction. A groove  170  can be introduced into the spindle  105  in order to axially fix the displacement element  165 . In a preferred manner, this relates to a radial annular groove. In one embodiment, a ventilation opening  175  for the pressure equalization of the reservoir with the surrounding environment is provided if, for example, a proportion of the lubricant  155  leaves the reservoir. 
         [0027]      FIG. 2  shows the spindle drive form  FIG. 1  in a second view in a further embodiment. The region of the reservoir  150  is presented here enlarged. The displacement element  165  forms an annular clearance  205  with a radial boundary of the reservoir  150 . The displacement element  165  and the annular clearance  205  divide the reservoir  150  into a first axial section  210 , depicted on top in  FIG. 2 , and a second axial section  215 , depicted on the bottom in  FIG. 2 . If the displacement element  165  is moved together with the spindle  105  in the axial direction, the volume of the one axial section is enlarged and that of the other is reduced. The lubricant  155  fills up the reservoir  150  as far as possible preferably at least immediately after the filling operation. The changing volumes of the sections  210  and  215  force the lubricant to pass through the annular clearance  205 . The lubricant  155  is thereby circulated and mixed in the reservoir  150 . The lubricant  155  can thereby wet the external thread  110  of the spindle  105  in an improved manner and penetrate into the region between the external thread  110  and the internal thread  120  of the spindle nut  115 . The threads  110  and  115  can thus be better provided with lubricant  155 , whereby the service life, the load-bearing capacity or the reliability of the spindle drive  100  can be increased. 
         [0028]    The displacement element  165  can be implemented on the spindle in various ways.  FIGS. 3A, 3B and 3C  show exemplary displacement elements for the spindle drive  100  from  FIGS. 1 and 2 . From top to bottom, a first displacement element  305 , a second displacement element  310  and a third displacement element  315  are depicted. 
         [0029]    The first displacement element  305  shown in  FIG. 3A  is equipped to be axially mounted on the spindle  105 . The first displacement element  305  depicted is implemented in the form of a shaft circlip, which is also known as a lock washer. Another design of an external circlip can also be used. As a result, the first displacement element  305  can rotate in the groove  170  of the spindle  105 . 
         [0030]    The second displacement element  310  shown in  FIG. 3B  is intended to be axially mounted on the spindle  105 . Spring tabs which face radially inwards facilitate a mounting of said second displacement element  310  and hold the second mounting element  310  in the groove  170  in the axial direction. 
         [0031]    The third displacement element  315  shown in  FIG. 3C  has the form of a thrust or flat washer. Said displacement element can, for example, be attached to the spindle  105  in a materially bonded manner, for example by soldering or welding. It is also possible to configure the spindle  105  such that the third displacement element  315  is axially pressed in the groove  170  in order to attach it. In still a further embodiment, the third displacement element  315  is, for example, injection molded from plastic on the spindle  105 . The spindle  105  can likewise be manufactured from plastic or, for example, from steel. 
         [0032]    Further possible variants of the displacement element  165  become apparent to the person skilled in the art immediately upon observing  FIGS. 1 to 3  and the embodiments described above with regard to purpose and properties of the displacement element  165 .