Patent Abstract:
A hydraulic linear drive, particularly a hydraulic transmission actuator, having a piston/cylinder unit, in which an actuating piston longitudinally displaceably arranged in the cylinder housing divided into at least two pressure chambers which can be acted upon by hydraulic oil by way of control conduits. A piston rod is connected with the actuating piston, and a sealing element is arranged on the actuating piston to seal off the two pressure chambers from one another. The actuating piston has a two-piece construction and consists of a first and a second piston part between whose facing faces, a sealing element is arranged.

Full Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a hydraulic linear drive, particularly a hydraulic transmission actuator, in which an actuating piston longitudinally displaceably arranged in the cylinder housing in the cylinder space into at least two pressure chambers which can be acted upon by hydraulic oil by way of control conduits, and having a piston rod connected with the actuating piston, as well as having a sealing element arranged on the actuating piston, by means of which sealing element, the at least two pressure chambers are sealed off from one another. 
     Hydraulic linear drives are used, for example, in the case of automated standard transmissions, for the synchronization of the transmission gears (see, for example, Johannes Loomann, “Zahnradgetriebe”, 2nd Edition, pg, 156, and on). 
     In the case of the hydraulic linear drives of the above-mentioned type, the dual piston bounded by two pressure chambers is in each case pushed toward the left or right as a result of correspondingly being acted upon by pressure. In many of the application cases, the two pressure chambers are sealed off by sealing elements arranged on the outer circumference of the piston. 
     Particularly in the case of hydraulic transmission actuators, high actuating forces are applied during the synchronization of the transmission gears-and require a reliable and durable sealing-off or separation of the two pressure chambers. 
     SUMMARY OF THE INVENTION 
     An object of the present invention to improve the sealing-off of the two pressure chambers in the area of the piston unit. This object has been achieved by the fact that the actuating piston is constructed in two parts and a sealing element is arranged between the two piston parts. The sealing element is clamped between the two piston parts when the piston unit is adjusted and, because of the actuating forces to be applied, for example, during the synchronization of the transmission gear, is pressed radially toward the outside to a certain extent. Thereby the sealing between the actuating piston and the interior cylinder wall is advantageously improved. 
     The sealing element constructed as a sealing ring is received on a sealing device carrier which is axially guided on one of the two piston parts. 
     For a better axial guidance of the sealing device carrier, the latter engages on the face in the first piston part. 
     The sealing device carrier is shaped in one piece out of one of the two piston parts or, as an alternative, is arranged as a separate component between the two piston parts. 
     The sealing device carrier is advantageously longitudinally displaceably disposed on the first piston part, for limiting the contact pressure force exercised upon the sealing ring. The relative movement of the sealing device carrier is limited by two stops constructed on the first piston part. 
     An advantageous embodiment of a hydraulic linear drive which is adapted to the use as a hydraulic transmission actuator is obtained when the two piston parts and the cylinder housing have a stepped construction. As a result of the step piston which provides in this manner, in a first adjusting path, a high adjusting speed can be achieved with a low friction. Because of a large piston diameter, a high actuating force can be generated about the synchronization point and thus a high radial contact pressure force of the sealing ring against the interior wall of the cylinder housing. 
     A longitudinal groove is formed in the surface area of the piston part section having a reduced diameter. The longitudinal groove in each case connects a first hydraulic chamber section with a second hydraulic chamber section of the two step pistons. 
     One control conduit respectively is connected to the two first hydraulic chamber sections of the two step pistons, which control conduit is used for the feeding or removal of hydraulic oil. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of a linear drive with a schematically shown hydraulic control according to a first embodiment of the present invention; and 
         FIG. 2  is a partial cross-sectional view of a linear drive having a hydraulic control according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The hydraulic linear drive illustrated in  FIG. 1  can be used, for example, as a hydraulic transmission actuator for an automated standard transmission. The drive has a two-part cylinder housing  2   a ,  2   b , which parts are both mutually connected, preferably screwed together, on their faces. In the cylinder space formed by the cylinder housing  2   a ,  2   b , two piston parts called step pistons  4 ,  6  are received and, both being equipped with one piston rod  8 ,  10  respectively guided out of the cylinder housing  2 , are longitudinally displaceably guided in the cylinder housing  2 . In this case, the sealing-off of each of the two pistons rods  8 ,  10  takes place by one respective sealing ring  12 ,  14 . The two step pistons  4 ,  6  each have two piston sections  4   a ,  4   b  and  6   a ,  6   b  respectively, in which a sealing device carrier  18  with a sealing ring  16  is arranged between the mutually facing faces of the pistons section  4   b ,  6   b  provided with a larger diameter. 
     The sealing device carrier  18  is disposed on an interior ring flange section  20  of the piston section  4   b  and, on its right face, is screwed to the piston section  6   b  of the step piston  6 , while, on its left face, it engages by way of a ring flange  22  in a gearing manner in a ring groove  24  constructed between a central ring flange section  23  and an outer ring flange section  25  of the piston section  4   b . The sealing ring  16  is pushed onto the ring flange  22  and correspondingly seals off the two pressure chambers  26 ,  28  from one another which are separated by the step pistons  4 ,  6 . 
     For limiting the sealing device carrier  18  longitudinally displaceably disposed on the interior ring flange section  20 , a left and a right stop is provided. The left stop is formed by the central ring flange section  23  of the piston section  4   b , and the right step  32  is formed by a limit stop washer  32   a  which is axially secured by a snap ring  32   b  received in a ring groove. Further, a flat coil spring  34 , which is arranged on the interior ring flange section  20 , is accommodated in a ring groove forming between the interior and central ring flange section  20 ,  23  and is therefore clamped in between the sealing device carrier  18  and the piston section  4   b.    
     The surface area of the two piston sections  4   a ,  6   a  respectively has a respective longitudinal groove  36 ,  38  each of which hydraulically connects the respective pressure chamber  26 ,  28  with a respective second pressure chamber  40 ,  42 . The two pressure chambers  40 ,  42 , namely, first pressure chambers, are bounded in each case by the respective face  41 ,  42  of the piston section  4   a ,  6   a  and the face of the respective sealing ring  12 ,  14 . One hydraulic conduit  44 ,  46  respectively is connected to the two first pressure chambers  40 ,  42  so that by way of a 7/2-way control valve  48 , the pressure chambers  40 ,  42  can optionally be supplied with hydraulic oil from a tank  50 . One return flow conduit  49 ,  51  is in each case connected to the respective two pressure chambers  26 ,  28 , namely, second pressure chambers. The return flow conduit  49 ,  51  can optionally be connected by way of the control valve  48  with the tank  50 . 
     In the control position of the 7/2-way valve  48 , the first pressure chamber  42  is acted upon by hydraulic oil by the hydraulic conduit  46  for the displacement of the two step pistons  4 ,  6  toward the left. The actuating force exercised on the face  43  of the piston section  6   a  displaces the piston unit consisting of the two step pistons  4 ,  6  toward the left. In that case, after a first adjusting path, by way of the longitudinal groove  38  connecting the two pressure chambers  42 ,  28 , the second pressure chamber  28  is also filled with hydraulic oil. 
     After moving a further distance, the hydraulic oil arrives in an unthrottled manner from the first pressure chamber  42  in the second pressure chamber  28  and acts exclusively with respect to the piston section  6   b  with the larger diameter. Thus, on the one hand, the adjusting rate of the actuating piston  4 ,  6  is reduced but, on the other hand, the actuating force acting upon the step piston  6  is increased. Simultaneously, the hydraulic oil situated in the first and second pressure chamber  40 ,  26  of the opposite side is returned into the tank  50  by the return conduit  49  and the hydraulic conduit  44 . The fact that the piston unit  4 ,  6  is displaced against a resistance, has the effect that the sealing ring  16  clamped in between the exterior ring flange section  25  of the piston section  4   b  and the sealing device carrier  18  deforms elastically and is thereby pressed radially against the interior wall of the cylinder housing  2 . 
     The hydraulic linear drive can be used, for example, as a hydraulic transmission actuator, in which case a shift fork engaging in a gearshift sleeve unit is axially displaced by the transmission actuator for establishing a non-rotatable connection between the gearshift sleeve and the transmission gear. In such case, a high adjusting speed with a low friction is reached by way of a first adjusting path via the two piston sections  4   a ,  6   a  respectively which have the smaller diameter, while about the synchronization point, a high radial contact pressure force of the sealing ring  16  can be achieved with respect to the interior cylinder wall via the two piston sections  4   b ,  6   b  respectively which have a larger diameter. 
     The second embodiment of the hydraulic linear drive illustrated in  FIG. 2  differs from that of  FIG. 1  only with respect to the hydraulic control. Instead of the 7/2-way control valve  48  used in the first embodiment, the controlling of the feeding and removal of hydraulic oil now takes place by way of a first 4/2 control valve  56  and a second 3/2 control valve  58 . By way of the first 4/2 control valve  56 , the two first pressure chambers  40 ,  42  respectively can optionally be acted upon by hydraulic oil, while the return of the hydraulic oil from the first two pressure chambers  26 ,  28  respectively is controlled by the control valve  58 . The difference with respect to the first embodiment consists of the fact that, by the respective closing of the conduit  49 ,  51 , the hydraulic oil to be returned from the respective second pressure chamber  26 ,  28  into the tank  50  is returned in this second embodiment by way of the longitudinal groove  36 ,  38  respectively, the respective first pressure chamber  40 ,  42  and the respective hydraulic conduit  44 ,  46 . As a result, an additional damping of the adjusting movement can be achieved, particularly when reaching one of the two end positions of the actuating pistons  4 ,  6 .

Technology Classification (CPC): 5