Patent Publication Number: US-7908961-B2

Title: Locking device for linear actuators

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a locking device for linear actuators, in particular for fluid-actuated adjusting devices, with a piston rod mounted in an axially displaceable manner relative to a receiving device. The invention is suitable in particular for hydraulic adjusting devices that are used for safety-critical applications. 
     2. Discussion of Background Information 
     From prior art, hydraulic lockings in the form of a flow check valve are known for hydraulic adjusting devices. In particular for safety-critical applications in aviation, flow check valves for locking the hydraulic adjusting device are disadvantageous, since there is the possibility of leakage at each seal situated in the system. A malfunction of the valve can have serious consequences. Furthermore, volume changes of the hydraulic fluid are brought about by the temperature variations present in flight operation, which volume changes in the case of the hydraulic locking could lead to the exit of hydraulic fluid due to positive pressure or the entry of ambient air due to negative pressure. 
     Furthermore, frictional lockings via various clamping principles are known. The disadvantage of these is the high Hertzian pressure necessary to provide the required holding power. Moreover, the constructive expenditure of the clamping connections is extremely high and results in a high structural weight. The disadvantage of clamping connections is their tendency to be irreversible, so that after a locking, a release of the clamping connections is impossible or only possible with great difficulty. 
     From DE 31 33 117 C2 a support strut for retractable aircraft landing gear, the length of which strut can be changed telescopically, is known in which when a landing gear is completely retracted, a locking takes place in the end position by means of form-fitting. Locking in a non-retracted position is not possible. 
     SUMMARY OF THE INVENTION 
     The present invention provides a locking device for linear actuators, in particular for fluid-actuated adjusting devices, with a piston rod mounted in an axially displaceable manner relative to a receiving device, with which piston rod a secure locking is possible over the entire travel path of the linear actuators. 
     This is attained according to the invention by a locking device with the features of claim  1 , in which the piston rod has a hollow space extending in the axial direction, in which form-fitting devices are arranged distributed in the axial direction. In addition, form-fitting elements corresponding to the form-fitting devices are arranged in the piston rod in a radially displaceable manner, which elements are mounted in an axially stationary manner relative to the receiving device. An actuating device that displaces the form-fitting elements radially is assigned to the form-fitting elements. It is thus possible via the actuating device to bring the form-fitting elements into form-fitting engagement with the form-fitting devices inside the piston rod, so that a form-fitting locking is possible inside the piston rod at discrete points over the entire travel path. It is thus possible to enable a secure locking of the linear actuators without a hydraulic locking and with a minimal constructive expenditure. Moreover, the mechanical locking through form-fitting inside the piston rod offers the possibility of a protected accommodation of the locking mechanism. 
     A further development of the invention provides that the form-fitting devices inside the piston rod are embodied as grooves, in particular radially peripheral grooves. In order to avoid a notch effect inside the piston rod and in order not to impair the stability of the piston rod, the form-fitting devices are embodied in the shape of groove rings that are inserted arranged behind one another in the piston rod. In the hollow piston rod, individual rings of different diameter or rings with recessed grooves are introduced and fixed therein. In addition, this has the advantages of a simple manufacture and easy assembly. 
     A further development of the invention provides that the receiving device for the piston rod is a cylinder, in particular a hydraulic or pneumatic cylinder, in which a piston fixed to the piston rod is mounted displaceably. The piston can be acted on by pressure on both sides, so that the piston rod is mounted so that it can be displaced to and fro in the axial direction. 
     Moreover it is provided that the form-fitting elements are embodied radially spring-mounted or mounted radially spring-mounted, in order to be able to actuate a reversibility of the form-fitting engagement simply. It is possible thereby that the form-fitting elements are held under tension disengaged from the form-fitting devices during the operation of the linear actuator and that after the tension has been released, the form-fitting elements are brought by the spring force into engagement with the form-fitting devices. Alternatively it is provided that the form-fitting elements are disengaged in the relaxed position and are brought into the form-fitting engagement with the form-fitting devices only by the actuating device. After removal of the actuating device or after a movement back into the starting position, the form-fitting elements spring back into the relaxed position, so that the free movability of the actuator or the adjusting device is again provided. 
     One embodiment of the invention provides that the form-fitting elements are projections arranged or embodied on a holder. Alternatively, the form-fitting elements are segments mounted displaceably, which segments are displaced radially, or are spring washers embodied radially spring-mounted that either spring into the form-fitting devices or spring out of them. 
     An advantageous variant of the invention provides that the holder is embodied as a spreader sleeve that is provided with slots in the axial direction so that the slotted sections can be displaced radially. The form-fitting elements are embodied or arranged on the slotted sections, preferably at the ends of the sections, so that a locking of the piston rod relative to the holder or to the receiving device is possible over the entire displacement area of the piston rod. The form-fitting elements are thereby mounted on the holder in an axially stationary manner relative to the receiving device. The holder or the spreader sleeve is enclosed by the piston rod at least in the area of the form-fitting elements, so that the form-fitting elements can engage in the form-fitting devices symmetrically, preferably in a circumferential circle inside the piston rod. 
     In order to achieve a radial displacement of the form-fitting elements in a simple and reliable manner, the actuating device is embodied as an axially displaceable gripping cone, by means of whose axial displacement the form-fitting elements are caused to engage with or disengage from the form-fitting devices of the piston rod. To this end it is provided that a mating cone complementary to the gripping cone is embodied on the spreader sleeve, via which mating cone the form-fitting elements are held disengaged from the form-fitting devices. An embodiment of this type is provided when the form-fitting elements are held disengaged under tension. To this end the gripping cone is embodied as a cap-shaped outer cone in which the mating cone engages. Alternatively it is provided that the gripping cone is embodied as an inner cone that is inserted into a funnel-shaped mating cone of the spreader sleeve. Through an axial displacement of the gripping cone the spreader sleeve is widened and the form-fitting elements are brought into engagement with the form-fitting devices. 
     In order to prevent the form-fitting elements from escaping radially inwards when the elements are engaged with the form-fitting devices, a tension and locking sleeve is arranged inside the spreader sleeve as a support sleeve, which support sleeve in the locked state is displaced into the area of the slotted sections and exerts a supporting action. To this end it is provided that the gripping cone is coupled with the tension- and locking sleeve via a spring-loaded tension anchor, so that the tension and locking sleeve is displaced via a spring automatically when the tension and locking sleeve is released. 
    
    
     
       BREIF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention is described in more detail below based on the attached figures. These figures show: 
         FIG. 1  shows a cross-sectional view of a hydraulic adjusting device; 
         FIG. 2  shows detail C of  FIG. 1 ; 
         FIG. 3  shows a detailed view of a spreader sleeve; 
         FIG. 4  shows detail B in  FIG. 1 ; and 
         FIG. 5  shows detail D in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
       FIG. 1  shows a linear actuator  1 , in this case a hydraulic adjusting device, with a piston rod  10  that is guided displaceably in a receiving device  30  embodied as a cylinder. The two ends  2 ,  3  of the adjusting device  1  have fixing devices that can be attached to objects that are to be mounted movably relative to one another. The cylinder  30  serving as receiving device for the piston rod  10  is a hydraulic cylinder in which a piston  31 , which is fixed to the piston rod  10 , can be moved in the direction of the double arrow. On the side of the cylinder  30  facing away from the piston rod  10 , a holder  20  is fixed that runs through the piston  31  and projects into a hollow space  11  of the piston rod  10 . On the end of the holder  20  on the piston rod side, form-fitting elements (not shown) are embodied that if required engage in form-fitting devices (likewise not shown) inside the piston rod  10 . A form-fitting locking of the piston rod  10  relative to the cylinder  30  or the holder  20  takes place, whereby the locking can be undertaken in almost every position of the piston rod  10  relative to the cylinder  30 , since the form-fitting devices are composed of grooves arranged axially behind one another, which grooves are arranged in the exemplary embodiment in the hollow space  11  over the entire length of the piston rod  10 . In the state shown, the form-fitting elements of the holder  20  are disengaged from the form-fitting devices inside the piston rod  10 , so that the piston rod  10  is freely movable axially. The holder  20  is connected with the cylinder  30  or with the cylinder housing and attached to the latter, so that the form-fitting elements are also fixed immovably to the cylinder housing  30  in the axial direction. 
     In  FIG. 2  the detail C of  FIG. 1  is illustrated in an enlarged scale and shows in a cross-sectional view the cylinder housing  30  with the holder  20  fixed therein. The holder  20  has a central bore  25  in which a tension anchor  43  for a tension and locking sleeve (not shown) is arranged in a longitudinally displaceable manner, which sleeve is held stationary via a holding device  99 . A base plate  32  that is fixed to the piston  31  is arranged around the outer wall of the holder  20 . The piston  31  in turn is fixed to the piston rod  10 , which has form-fitting devices  12  in its interior. The form-fitting devices  12  are embodied in the form of groove rings that are layered behind one another inside the hollow space  11  of the hollowed-out piston rod  10 . The groove rings  12  are held in the piston rod  10  by means of the base plate  32 . The groove rings  12  embody peripheral grooves. The piston rod  10  and the groove rings  12  are dimensioned such that the piston  31  and the base plate  32  can slide along on the holder  20 . In combination with the base plate  32 , the piston  31  serves as a guide for the holder  20 , which in this way obtains a better buckling strength. 
     On the upper end of the holder  20  (not shown) a spreader sleeve is embodied, as shown in  FIG. 3 . The spreader sleeve  20  is composed thereby of slotted sections  21  that are formed by cuts  24  running axially. The slotted sections  21  are radially displaceable and have form-fitting elements  22  on their exterior in the form of elevations or catches. The upper termination of the spreader sleeve  20  is formed by a mating cone  23  that tapers upwards. 
     In  FIG. 4  the detail B of  FIG. 1  is shown, in which the cylinder housing  30  with the piston rod  10  mounted displaceably therein can be seen. The form-fitting devices  12  in the form of groove rings are inserted in the hollow space of the piston rod  10 ; alternatively, the grooves can be recessed in the piston rod  10 . Moreover, the grooves can be embodied as screw threads (acme threads or the like), for example as an internal thread inside the piston rod  10 . The form-fitting elements can be embodied on the spreader sleeve  20  as an external thread. The holder  20 , which is embodied as a tube, is guided inside the piston rod  10  and inside the groove rings  12 . In the holder  20  a tension- and locking sleeve  45  is mounted displaceably. Inside the tension and locking sleeve  45 , a tension anchor  43  is mounted that is loaded in the direction of the cylinder-side end of the holder  20  via a spiral spring  45 . The spreader sleeve shown in  FIG. 3  is embodied at the piston rod-side end of the holder  20 , which spreader sleeve is enclosed by the front end of the tension anchor  43 . The front end of the tension anchor  43  is provided with a gripping cone that displaces the mating cone  23  radially inwards by applying an axial force, so that the outside diameter of the spreader sleeve  20  is decreased. The form-fitting elements  22  are thus disengaged from the form-fitting devices  12 , so that the piston rod  10  is freely movable axially. 
     In  FIG. 5  the detail D of  FIG. 4  is shown, based on which it can be seen that the gripping cone  40  as an actuating device displaces the mating cone  23  of the spreader sleeve  20  radially inwards. In this state the spreader sleeve  20  is tensioned and the linear actuator  1  is freely movable. If the axial holding force on the tension and locking sleeve  45  is eliminated in that a holding device  99  as indicated in  FIG. 2  is released, because of the radial tension of the slotted sections  21  of the spreader sleeve  20  the tension anchor  43  is displaced in the direction of the piston rod  10  due to the action of the mating cone  23  on the gripping cone  40 . The spreader sleeve  20  relaxes and the slotted sections  21  spring radially outwards. This causes the form-fitting elements  22  to engage with the form-fitting devices  12  and to lock the piston rod  10  in this position. The catches or the projections  22  press against the groove rings  12  due to the internal stress brought about by the spring action of the slotted sections  21 , as well as due to the tension and locking sleeve  45  loaded by the coil spring, which tension- and locking sleeve  45  exerts a wedge action on the inner flank of the spreader sleeve  20 , which is still deformed inwardly. In so far as the grooves and the projections or catches lie opposite one another, a locking into position occurs. After the locking into position of the form-fitting elements  22  in the groove rings  12 , a cylindrical geometry is present inside the slotted sections  21  of the spreader sleeve  20 . Now the cylindrical tension and locking sleeve  45 , driven by the tensioned coil spring  44 , is displaced completely into the slotted area of the spreader sleeve  20 . The tension- and locking sleeve  45  is supported thereby via the coil spring  44  on the tension anchor  43 , via the gripping cone  240  and the mating cone  23  axially on the mating cone  23 . A slipping back of the tension- and locking sleeve is thus prevented. By these means the slotted sections  21  of the spreader sleeve  20  are prevented from buckling inwards. To unlock the locking, the tension- and locking sleeve  45  is transported in the opposite direction, i.e. in the direction of the cylinder  30 , the coil spring  44  is prestressed. Then the tension anchor  43  is moved concomitantly in a form-fitting manner through the tension- and locking sleeve  45 , the gripping cone  40  engages with the mating cone  23  and displaces the slotted sections  21  radially inwards. The form-fitting elements  22  are thus disengaged from the form-fitting devices  12  and the adjusting device  1  is freely movable axially. 
     The locking device can be used wherever an axial movement must be blocked reversibly, in a form-fitting manner, and if possible over the entire travel path. An extremely fine gradation of the form-fitting locking is possible through the arrangement of the groove rings  12  behind one another. A very good protection from external influences is provided by the locking device arranged inside the cylinder  30 . The entire system is closed, so that damage caused by external influences is almost excluded. The reversible locking at discrete positions over the entire travel path of the adjusting device  1  is guaranteed, so that its use is possible even for safety-critical applications. The actuation of the tension- and locking sleeve  45  can take place through the central bore  25  of the holder  20 , for example by hydraulic or electromotive means. An emergency actuation through a detonator is likewise possible. 
     Alternatively to a hydraulic adjusting device, all linear actuators can be equipped thus. Instead of the version shown of the form-fitting elements  22  held disengaged, it is provided to embody the gripping cone such that it widens the spreader sleeve  20 . Likewise it is possible for latches or segments mounted displaceably, to be arranged inside the spreader sleeve  20  or the holder, which latches or segments are engaged with the form-fitting devices  12  in a form-fitting manner, in that these spring-mounted segments are pressed outwards by a cone. If spring washers are used, these can be held disengaged in an analogous manner and can be unlocked if required.