Abstract:
A linear actuator provided for moving a load includes an actuator housing having a tube and a housing bottom at one end of the tube and a housing head at another end of the tube. The linear actuator further includes a piston guided longitudinally in the actuator housing, and an actuator rod assembled with the piston and configured to protrude out of the actuator housing in a manner which is guided in the housing head. The linear actuator is configured such that reliability of operation is increased and damage of structural parts is largely avoided. The linear actuator achieves these results by virtue of the fact that there is at least one plastically deformable buffer element which can be supported on the housing head and can be deformed in the case of an impact of the piston. The buffer element is configured to convert kinetic energy into deformation work.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2014 224 259.2, filed on Nov. 27, 2014 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    The disclosure relates to a linear actuator which serves to move a load. One preferred field of use is movement systems such as driving simulators and flight simulators, in which a cab is mounted on a platform which can be moved in six degrees of freedom by a total of six identical linear actuators. Linear actuators are known which have an actuator housing with a tube, with a housing bottom at the one end of the tube and with a housing head at the other end of the tube, a piston which is guided longitudinally in the actuator housing, and an actuator rod which is assembled with the piston and protrudes out of the actuator housing in a manner which is guided on the housing head. 
         [0003]    Linear actuators of this type are often electromechanical linear actuators, there being a threaded spindle which can be driven rotatably by an electric motor, is mounted rotatably in the housing bottom of the actuator housing and extends in the interior of the actuator housing from the housing bottom in the direction of the housing head, the piston comprising a spindle nut which is in engagement with the threaded spindle, and the actuator rod being hollow and the threaded spindle dipping into the hollow actuator rod. 
         [0004]    It is also known to equip a linear actuator of this type with means for compensating for the static load, up to 75% of the static load being supported, for example, by way of said means. The remaining proportion of the load is absorbed by the threaded spindle. If a plurality of linear actuators are provided to carry the load, each linear actuator of course accepts only a certain proportion of the load. If desired, up to 100% of the static load can also be compensated for. 
         [0005]    In an emergency or in the case of incorrect control, it can occur that, depending on the movement direction and structural design, the piston impacts against the housing bottom or against the housing head or a flange which is fastened to the outside of the actuator rod impacts against the housing head. Here, damage of structurally essential parts can occur which is such that said parts have to be replaced. On account of the damage, the actuator can also be blocked, with the result that emergency operation is no longer possible. There is also the risk that people are injured who are situated in the cab of the simulator and who are subjected to large negative accelerations in the case of an impact of the piston or the actuator rod. 
       SUMMARY 
       [0006]    The disclosure is based on the object of configuring a linear actuator, in such a way that the reliability of operation is increased and damage of structural parts and injury of people are largely avoided. 
         [0007]    In a linear actuator of the generic type, the object is achieved in a first way by virtue of the fact that there is at least one plastically deformable buffer element which can be supported on the housing head on its inner side which faces the interior of the actuator housing and can be deformed in the case of an impact of the piston, kinetic energy being converted into deformation work and, if the buffer element also rebounds somewhat, possibly also potential energy. 
         [0008]    In a linear actuator of the generic type, the object is achieved in a second way by virtue of the fact that there is at least one plastically deformable buffer element which can be supported on the housing head on its outer side which faces away from the interior of the actuator housing and can be deformed in the case of an impact of a flange which is fastened to the actuator rod, kinetic energy being converted into deformation work and, if the buffer element also rebounds somewhat, possibly also potential energy. 
         [0009]    In the case of a linear actuator according to the disclosure, there is therefore at least one plastically deformable buffer element which can be supported on the housing head and can be deformed in the case of an impact of the piston, kinetic energy being converted into deformation work. 
         [0010]    If, in a linear actuator according to the disclosure, the piston impacts the housing head from the inner side or the flange on the actuator rod impacts the housing head from the outer side, the buffer element or elements is/are deformed. The buffer elements therefore form a small crash zone in a similar manner to modern automobiles, within which crash zone the movement energy of the actuator rod with the piston and with the load is converted into deformation work, and the actuator rod is braked to a standstill in a safe way. After an incident of this type, the linear actuator can still be actuated, since no structurally important parts are damaged. The deformed buffer elements are replaced immediately after an above-described incident. 
         [0011]    The buffer element or the buffer elements is/are configured in such a way that the construction and people are subjected only to a maximum force or deceleration. Here, the deceleration depends on the area extent and on the strength of the buffer material. A greater area is required with a softer or weaker material, in order to achieve the same damping action as with a firmer or stronger material. As an alternative, the path, on which the buffers are deformed, can be extended, in order to absorb the same amount of energy. A longer path with a softer or weaker material influences the deceleration of the actuator. The optimum maximum force and deceleration can be set by way of the selection of the strength, the area and the deformation path for the buffer elements. 
         [0012]    Advantageous refinements of a linear actuator according to the disclosure can be gathered from the subclaims. 
         [0013]    An existing plastically deformable buffer element advantageously has a honeycomb-like structure, the cavities of the structure extending in the movement direction of the actuator rod. An aluminum material is preferably used as material for the buffer elements. Honeycomb-like structures made from an aluminum material are known per se as so-called aluminum honeycombs. 
         [0014]    A buffer element can be configured as a closed ring which surrounds the actuator rod. A buffer element can also, however, have the shape of an annular segment. In the latter case, it is favorable that a plurality of buffer elements are arranged distributed uniformly over 360° around the actuator rod, in order that the actuator rod is not braked on one side and possibly tilts. 
         [0015]    In the context of a compact design of the housing head and therefore of the entire linear actuator, it is advantageous if the buffer elements are situated within the axial extent of a guide bushing for the actuator rod, which guide bushing belongs to the housing head. A certain length is necessary for the guide bushing which usually accommodates guide bands for the actuator rod, one or more sealing rings and a stripper. If the buffer elements are now situated within the axial extent of the guide bushing, the overall length is not influenced by them. In order that this arrangement of the buffer elements is possible, the guide bushing can be displaced axially with respect to the disk in one particularly advantageous development of a linear actuator according to the disclosure. 
         [0016]    For buffering a retracting movement of the actuator rod, an outer annular space is then formed in a simple way, on that side of the disk which faces away from the interior space of the actuator housing, radially between said disk and the guide bushing, in which outer annular space at least one buffer element is arranged, and into which outer annular space a flange on the actuator rod can dip. During dipping, the flange displaces the guide bushing into the actuator housing. 
         [0017]    For buffering of an extending movement of the actuator rod, one buffer element or a plurality of buffer elements is/are arranged in an inner annular space depending on the required force and deceleration, which inner annular space is delimited axially by the disk and the bushing flange which the guide bushing has at an axial spacing from the disk in front of that side of the disk which faces the interior space of the actuator housing. If the interior space of the actuator housing is loaded with pressure in order to compensate for a static load, the bushing flange is pressed by the pressure against the buffer elements which are situated in the inner annular space. In this way, the guide bushing is held in a defined axial position. At the same time, the buffer elements are secured captively. 
         [0018]    In order that the housing pressure acts on the guide bushing on the entire annular area between the actuator rod and the tube of the actuator housing, the inner annular space is connected to atmosphere. A sealing ring between the bushing flange and the tube then prevents oil or gas being lost through the gap between the bushing flange and the tube and the pressure in the housing dropping. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    One exemplary embodiment of a linear actuator according to the disclosure is shown in the drawings. The disclosure will now be explained in greater detail using said drawings, in which: 
           [0020]      FIG. 1  shows the linear actuator partially in a longitudinal section, and in a greatly simplified illustration, 
           [0021]      FIG. 2  shows a longitudinal section through the linear actuator in the region of the piston on an enlarged scale, and 
           [0022]      FIG. 3  shows a longitudinal section through the linear actuator in the region of the housing head of the actuator housing on an enlarged scale. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    As can be seen from  FIG. 1 , the linear actuator has an actuator housing  9  with a tube  10 , at the one end of which a housing bottom  11  is situated and at the other end of which a housing head  12  is situated. A threaded spindle  14  is mounted rotatably in the housing bottom via an anti-friction bearing  13 , which threaded spindle  14  extends with a section  15  which is provided with a thread from the anti-friction bearing into the interior space of the actuator housing  9  in an axially non-displaceable manner. Together with further components, the housing bottom  11  at the same time forms a housing  16  for a toothed belt mechanism  17 , via which the threaded spindle  14  can be driven rotationally by an electric motor  18  which is arranged in parallel next to the actuator housing  9 . The belt mechanism  17  consists of a first toothed pulley wheel  19  which is seated fixedly so as to rotate with it on the shaft  20  of the electric motor  18  which protrudes into the housing  16 , a second toothed pulley wheel  21  which is larger than the first pulley wheel  19  and is seated fixedly so as to rotate with it on a shaft journal  22  of the threaded spindle  14  which protrudes into the housing  16 , and a toothed belt  23  which runs over the two pulley wheels  19  and  21 . The passage of the shaft journal  22  is sealed by way of a shaft sealing ring  24 . 
         [0024]    Furthermore, the linear actuator has an actuator rod  30  which is configured as a tube, is fastened to a piston  31  in the interior of the actuator housing  9  and protrudes through the housing head  12  to the outside. That end of the hollow actuator rod which is situated outside the actuator housing  9  is closed by way of an adapter  32  which is centered with a collar in the actuator rod and is screwed to a flange  33  which is screwed onto the actuator rod. A sealing ring  34  is arranged between the collar of the adapter  32  and the actuator rod  30 , by way of which sealing ring  34  the interior of the actuator rod  30  is sealed to the outside. 
         [0025]    The construction of the piston  31  is apparent in greater detail from  FIG. 2 . According to said figure, the piston  31  has a sleeve-like guide part  35  with an outer circumferential groove, into which a guide band  36  is inserted, by way of which the piston  31  is guided longitudinally in the actuator housing  9 . A plurality of bores  37  which run in the axial direction are situated in the guide part, through which bores  37  that part volume of the interior of the actuator housing  9  which is situated between the wall of the actuator housing and the actuator rod and the part volume on the other side of the piston  31  are open fluidically with respect to one another. From the end side which points in the same direction as the actuator rod  30 , a turned groove  38  is made in the guide part  35  up to a small spacing from the groove which receives the guide band  36 , with the result that there is an annular clearance between the actuator housing  9  and the piston  31 . 
         [0026]    In the center, the sleeve-like guide part  35  of the piston  31  has a projecting collar  39  which is provided with an internal thread and into which the actuator rod  30  which is provided with an external thread over a defined section from its piston-side end is screwed as far as an inwardly projecting shoulder of the guide part  35 . The connection is advantageously secured by way of a threaded pin (not shown in greater detail) which is radially screwed into the collar  39 . 
         [0027]    From the side which faces away from the actuator rod  30 , a spindle nut  40  which is configured as a flange bushing is inserted into the guide part  35  of the piston  31  and is connected fixedly to the guide part  35 . With an internal thread, the spindle nut  40  interacts with the thread of the threaded spindle  14 . During operation, the unit comprising actuator rod  30  and piston  31  including the spindle nut is secured against rotation by way of the fastening of the actuator rod on a load. A rotational movement of the threaded spindle  14  therefore leads to an axial movement of piston and actuator rod. The threaded spindle  14  extends through the spindle nut  40  into the actuator rod to a greater or lesser extent depending on the position of the actuator rod  30 . 
         [0028]    Distributed at an identical angular offset with respect to one another, a plurality of axial bores  45  are made in the collar  39  of the guide part  35 , which axial bores  45  open on the inside into a hollowed-out portion  46  which surrounds the spindle nut over part of its longitudinal extent. The inner end side of the spindle nut is at a spacing from the one end of the hollowed-out portion  46 , with the result that there is an open siphon-like fluidic connection between the interior space of the actuator rod  30  and the interior space of the actuator housing  9  via the axial bores  45  and the hollowed-out portion  46 . 
         [0029]    The housing head  12  of the actuator housing  9  comprises a disk  47  which is screwed onto the tubular part of the actuator housing  9  with a collar  48  which is provided with an internal thread. The disk  47  has a stepped through bore  49  with a section  50  of large diameter adjacently to the outer end side which faces away from the interior of the actuator housing  9  and with a section  51  of smaller diameter adjacently to the inner end side which faces the interior of the actuator housing  9 . The diameter of the section  51  is larger than the external diameter of the actuator rod  30 , however. From the inner end side, a guide bushing  52  which is configured as a flange bushing with a bushing flange  61  is inserted into the disk  47 , the diameter of which in front of the bushing flange is slightly smaller than the diameter of the through bore  49  in the region of the section  51 , and which guide bushing  52  reaches as far as that end side of the disk  47  which faces away from the interior of the actuator housing  9 . On the inside, the guide bushing  52  is provided with annular grooves, into which two guide bands  53  and  54  which interact with the actuator rod, a sealing ring  55  which seals the gap between the guide bushing and the actuator rod in a gastight manner, and a stripper  56  are inserted. The outer diameter of the bushing flange  61  of the guide bushing  52  is slightly smaller than the inner diameter of the tube  10  of the actuator housing  9 . The gap is sealed by way of a sealing ring  57 . On account of the selected dimensions, there is an annular space  62  between the guide bushing  52  and the wall of the section  50  of the through bore  49 , which annular space  62  is covered with a thin plate  60 , in order that it does not become contaminated. 
         [0030]    The bushing flange  61  of the guide bushing  52  is at a spacing from that end side of the disk  47  which faces the interior of the actuator housing  9 , with the result that there is an annular clearance  58  between the disk  47  and the bushing flange  61 . Said clearance  58  is ventilated to the outside via radial bores  59  in the disk  47 . The inner diameter of the bushing flange  61  is somewhat larger than the outer diameter of the collar  39  on the piston  31 , with the result that there is an annular space  63  between the bushing flange  61  and the actuator rod, into which annular space  63  the collar  39  can dip. The annular space  63  and the collar  39  on the piston therefore allow the provision of the necessary thread length for fastening the actuator rod, without the stroke of the actuator rod  30  being shortened by the bushing flange  61 . 
         [0031]    A plurality of (four in the present case, for example, for a defined payload) honeycomb-like annular segments  70  which are spaced apart from one another uniformly and are made from an aluminum material are inserted into the annular space  62 , which annular segments  70  are held in their positions distributed over the circumference of the annular space  62  on their radial outer side by way of an adhesive on the disk  47 . A plurality of (four in the present case, for example) honeycomb-like annular segments  71  which are spaced apart from one another uniformly and are made from an aluminum material are inserted into the annular space  58 , which annular segments  71  are held in their positions distributed over the circumference of the annular space  58  on their radial outer side by way of an adhesive on the tube  10 . The annular segments  70  and  71  serve as deformation elements for the conversion of kinetic energy into deformation energy if the actuator rod moves in an uncontrolled manner in an emergency or in the case of a malfunction of the linear actuator. The annular segments  70  and  71  are therefore the buffer elements which are denoted thus further above in the exemplary embodiment. The annular segments  71  are designed differently from the annular segments  70 , and have, for example, a shorter maximum deformation path than the annular segments  70 . In the case of a greater payload, more than four (for example, eight) annular segments can be situated in the annular spaces  58  and  62 , with an area which is then twice as large as in the case of four annular segments. 
         [0032]    Other cross-sectional shapes than the shape of an annular segment are also conceivable for the buffer elements, for example a circular-cylindrical shape or a parallelepiped shape. 
         [0033]    If the actuator rod  30  retracts in an uncontrolled manner in the case of a malfunction, the flange  33  impacts on the guide bushing  52  and via the plate  60  on the deformation elements which are situated in the annular space  62 , and is decelerated, the guide bushing  52  being pushed inward and the deformation elements being deformed. If the actuator rod  30  extends in an uncontrolled manner in the case of a malfunction, the piston  31  impacts on the bushing flange  61  of the guide bushing  52  and displaces the bushing flange and therefore the guide bushing counter to the force which is necessary for deforming the deformation elements which are situated in the annular space  58 . Here, air which is situated in the annular space can escape through the bores  59 . 
         [0034]    The buffer force depends on what angular region of the annular spaces  58  and  62  is filled by deformation elements and how the deformation elements are configured. If an annular space is to be filled completely, this can be achieved by way of a single annular deformation element. 
         [0035]    The entire interior space of the linear actuator is filled with dry nitrogen and oil, there being no dividing element which would separate the nitrogen and the oil from one another. In the case of a completely retracted actuator rod, approximately 60% of the interior space is filled with oil and approximately 40% is filled with nitrogen. In  FIG. 1 , the oil level in the case of a vertical operating position of the linear actuator is indicated by the dashed line  75 . The nitrogen is filled via a valve  65  and is subjected to a pressure which is selected depending on the magnitude of the load. The relatively small volume of nitrogen means that the inner pressure in the actuator is discernibly higher in the case of a retracted actuator rod than in the case of an extended actuator rod. The acceleration and extension of the actuator rod counter to the weight of the payload are assisted by the high inner pressure in the actuator. During retraction of the actuator rod, the weight acts in the movement direction of the actuator rod. The lower inner pressure in the actuator inhibits the acceleration of the payload to a lesser extent. 
         [0036]    The inner pressure also acts on the guide bushing  52  and secures it in an axial position, in which the bushing flange  61  bears against the deformation elements  71  which are situated in the annular space  58  and, via said deformation elements  71 , against the disk  47 . The guide bushing  52  can therefore run easily in the disk  47  and in the tube  10 . 
         [0037]    The linear actuator is preferably provided for applications, in which only limited angles with respect to the vertical are inclined, the actuator rod protruding upward out of the actuator housing, with the result that the oil is situated above the housing bottom  11 . The gas is situated above the oil. During operation, the piston  31  can be dipped completely into the oil or can be situated completely or partially above the oil level. As a result of the siphon-like design of the connection between the interior space of the actuator housing  9  with the axial bores  45  and as a result of the turned groove  38  on the outside of the guide part  35  of the piston  31 , two storage spaces for oil are provided, in which oil is provided even when the piston  31  is situated above the oil level. It is therefore ensured, even if the piston  31  moves in the gas, that the threads of the threaded spindle  14  and the spindle nut  40  and the guide band  36  are lubricated with oil. 
       LIST OF DESIGNATIONS 
       [0000]    
       
           9  Actuator housing 
           10  Tube of  9   
           11  Housing bottom 
           12  Housing head 
           13  Anti-friction bearing 
           14  Threaded spindle 
           15  Section of  14   
           16  Housing 
           17  Toothed belt mechanism 
           18  Electric motor 
           19  Pulley wheel 
           20  Shaft of  18   
           21  Pulley wheel 
           22  Shaft journal of  14   
           23  Toothed belt 
           24  Shaft sealing ring 
           30  Actuator rod 
           31  Piston 
           32  Adapter 
           33  Flange 
           34  Sealing ring 
           35  Guide part of  31   
           36  Guide band 
           37  Bores 
           38  Turned groove 
           39  Collar of  35   
           40  Spindle nut 
           45  Axial bores in  35   
           46  Hollowed-out portion in  35   
           47  Disk of  12   
           48  Collar of  47   
           49  Through bore in  47   
           50  Section of  49   
           51  Section of  49   
           52  Guide bushing 
           53  Guide band 
           54  Guide band 
           55  Sealing ring 
           56  Stripper 
           57  Sealing ring 
           58  Annular space 
           59  Radial bores 
           60  Plate 
           61  Bushing flange on  52   
           62  Annular space 
           63  Annular space 
           65  Valve 
           70  Buffer elements 
           71  Buffer elements 
           75  Oil level