Abstract:
A hydraulic actuator to which a limit-adjustable mechanical lock device is applied, comprising: a housing having a first hole; side covers coupled at both sides of the housing, and having holder insertion holes formed to be opened toward the first hole side of the housing, and plugs; a first holder of which one side of the outer peripheral surface is inserted into the holder insertion hole at the plug side of the side cover by screw coupling; a second holder fitted and coupled to the inner peripheral surface of the side cover and having one end thereof screw-coupled to the second hole of the first holder; a locking means into which a rod is inserted so as to be movable in an axial direction at a predetermined distance across the second hole of the first holder and the third hole of the second holder.

Description:
BACKGROUND 
       [0001]    The present invention relates to a hydraulic actuator employing a limit adjustable mechanical lock. More particularly, the present invention relates to a hydraulic actuator employing a limit adjustable mechanical lock, capable of preventing an error in the operation of a valve by removing the fine operating clearance of a valve and by stopping the operation of the valve under hydraulic pressure equal to or less than preset pressure. 
         [0002]    In general, various valves have been employed to cut off and control the flow of a fluid. The valves are automatically or manually open or closed. 
         [0003]    Although some of the valves have structures that may be manually manipulated, the valves are open or closed through automatic manipulation in significantly various fields. 
         [0004]    As operating units of the valves are automatically open or closed, electrical actuators based on solenoids and hydraulic actuators based a fluid, such as air or oil, have been extensively employed. 
         [0005]    However, the electrical actuators have been applied to small valves that do not require great operating force while the hydraulic actuators have been applied to medium and large valves. 
         [0006]    Among them, in the case of the hydraulic actuators, significantly various operating structures have been suggested and used. In the case of the hydraulic actuators, there have been suggested a configuration in which a piston formed integrally with a rack gear is moved by a predetermined distance through hydraulic pressure to rotate a pinion gear engaged with the rack gear of the piston and a valve shaft shaft-fixed to the pinion gear, thereby rotating a valve coupled to a valve shaft such that the valve is open or closed. 
         [0007]    However, according to the related art, in the case of the hydraulic actuator, the piston simply operating by the hydraulic pressure is rotated and the valve is open or closed by the rotational force of the piston. Even if hydraulic pressure is applied to the piston in insufficient amount, the piston may move so that errors occur in the operation of the valve. 
         [0008]    In addition, even if a fluid passage is initially closed or open to the maximum extent by the valve, if the rack gear of the piston is engaged with the pinion gear to rotate the valve, the difference may be slightly made between the moment that the rack gear of the piston is engaged with the pinion gear and the moment that the valve is open or closed, and a fluid may leak by the difference. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, the present invention is suggested in order to solve the above problem, and an object of the present invention is to provide a hydraulic actuator employing a limit adjustable mechanical lock, capable of preventing errors in the operation of a valve in advance by preventing the locking state of a piston from being released under preset hydraulic pressure or less. 
         [0010]    In addition, another object of the present invention is to provide a hydraulic actuator employing a limit adjustable mechanical lock, capable of instantly operating a valve through the application of the hydraulic pressure by removing a valve clearance as the coupling degree of the valve is adjusted by inserting a cap housing receiving mechanical locking units into a main housing and screwing the cap housing with the main housing. 
         [0011]    In order to accomplish the above objects, according to the present invention, there is provided a hydraulic actuator employing a limit adjustable mechanical lock, the hydraulic actuator including an assembly of a housing having a first hole to receive a piston, which is formed at one side thereof with a rack gear to be engaged with a pinion gear fixing a valve shaft, such that the piston is movable by a predetermined distance, wherein both opposite portions of the piston are recessed by a predetermined depth, and ball insertion grooves are formed in inner circumferential surfaces of recess parts; side covers coupled to opposite sides of the housing and having holder insertion holes to be open toward a first hole of the housing, wherein opposite end faces about the housing are screwed with plugs to block the holder insertion holes from an outside; a first holder having one side of an outer circumferential surface, which is inserted into the holder insertion hole in the side cover positioned at a side of the plug through screwing with an inner circumferential surface of the side cover, and a second hole hollowed to be open toward the first hole; a second holder having one end screwed with the second hole in the first holder while the second holder is fitted-coupled to the inner circumferential surface of the side cover, having a third hole open toward the first hole, and having an end face which is positioned at a side of the second hole and bored with an inner diameter smaller than a diameter of the second hole; a locking unit having a rod inserted into the second hole and the third hole of the first holder and the second holder, respectively, while passing through a fourth hole of the second holder such that the rod is axially movable by a predetermined distance, wherein the rod allows a ball to be inserted into a ball insertion hole of the piston or to deviate from the ball insertion hole of the piston by hydraulic pressure, which is applied to the first holder and equal to or greater than predetermined pressure, and spring elastic force to lock or unlock the piston and a fluid passage extending from the housing to each of the side covers provided at the opposite sides of the housing such that the hydraulic pressure is selectively applied to both first holes and both second holes formed in the side covers and the housing. 
         [0012]    As described above, according to the hydraulic actuator employing the limit adjustable mechanical lock having the above configuration, when the hydraulic pressure equal to or greater than preset pressure is applied, the valve is normally operated and the operating state of the valve is locked. Accordingly, in the abnormal situation that the hydraulic pressure equal to or less than the preset pressure is applied, the operation of the valve is stopped. Therefore, the operability of the valve can be accurately and stably maintained. 
         [0013]    In addition, according to the present invention, the coupling structure of the first holder and the second holder receiving the locking unit is screwed with the end portion of the side cover. Accordingly, the coupling structure is rotated by using a simple mechanism to restrict the moving clearance of the piston, thereby allowing limit adjustment. Accordingly, the valve can be manipulated with the accurate operating reactivity in the state that the operating clearance of the valve is removed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is an exploded sectional view illustrating a hydraulic actuator employing a limit adjustable mechanical lock according to the present invention. 
           [0015]      FIG. 2  is an assembling sectional view of  FIG. 1 . 
           [0016]      FIG. 3  is an exploded perspective view illustrating the coupling state between a first holder and a second holder in the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
           [0017]      FIG. 4  is an enlarged sectional view illustrating the subject matter of the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
           [0018]      FIG. 5  is an enlarged view illustrating the subject matter of the structure that a locking step is formed on a rod in the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
           [0019]      FIG. 6  is a view illustrating the structure of adjusting the limit using the first holder in the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
           [0020]      FIG. 7  is an operation state view illustrating that the locking state of the piston is released as the hydraulic pressure is applied in the hydraulic actuator employing the limit adjustable mechanical lock. 
           [0021]      FIG. 8  is an operation state view illustrating that the piston is moved as hydraulic pressure is applied to a first hole in the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
           [0022]      FIG. 9  is an operating state view illustrating that an outer ball guide is moved as the piston is moved in the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
           [0023]      FIG. 10  is an operating state view illustrating that the piston is locked by the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    Hereinafter, a hydraulic actuator employing a limit adjustable mechanical lock according to an exemplary embodiment of the present invention will be described with reference to accompanying drawings. 
         [0025]      FIG. 1  is an exploded sectional view illustrating the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention, and  FIG. 2  is an assembling sectional view of  FIG. 1 . 
         [0026]    As illustrated in drawings, the present invention is configured with an assembly mainly including a housing  10 , a side cover  20 , a first holder  30 , a second holder  40 , a locking unit  50 , and a fluid passage  60 . 
         [0027]    The housing  10  according to the present invention is formed in the shape of a tube having an inner portion that is laterally hollowed. 
         [0028]    A piston  12  is movably inserted into a first hole  11  formed in the housing  10 , a rack gear  120  is formed at one side of an outer circumferential surface of the piston  12 , a pinion gear  13  is engaged with the rack gear  120  of the piston  120 , and the pinion gear  13  is coupled to a valve shaft  14  having one end fixed to a valve (not illustrated). 
         [0029]    Accordingly, when the piston  12  is selectively moved leftward or rightward inside the first hole  11  of the housing  10 , the pinion gear  13  engaged with the rack gear  120  of the piston  12  is rotated while the valve shaft  14  coupled to the pinion gear  13  is rotated, so operating units, such as valves, are operated by the valve shaft  14 . 
         [0030]    In this case, both end faces of the piston  12  are recessed by a predetermined depth, thereby forming recess parts  121 . A ball insertion groove  122  is formed in an inner circumferential surface of an end portion of the recess part  12 . 
         [0031]    Side covers  20  are screwed with both end portions of the housing  10 , respectively. 
         [0032]    According to the present invention, the side cover  20  has a holder insertion hole  21  hollowed to open the side cover  20  toward the first hole  11 . An end face of the side cover  20  opposite to an end portion of the side cover  20  positioned at the side of the first hole  11  is laterally hollowed with a diameter less than that of the holder insertion hole  21  and screwed with a plug to block the holder insertion hole  21  from the outside. 
         [0033]    In other words, the plug  22  is screwed with the side cover  20  such that the plug  22  is blocked from the outside. 
         [0034]    First holders  30  and second holders  40 , which are screwed with each other in series, are inserted through the holder insertion holes  21  of the both side covers  20 . 
         [0035]    In this case, the first holder  30 , which has a second hole  31  hollowed to open the first holder  30  toward the first hole  11 , is fixedly screwed with an inner circumferential surface of the side cover  20  at the side of the plug  22  of the holder insertion hole  21 . 
         [0036]    In other words, the first holder  30  is configured such that the first holder  30  is inserted into the holder insertion hole  21  of the side cover  20  and a portion of an outer circumferential surface of the first holder  30  is coupled to an inner circumferential surface of the holder insertion hole  21  positioned at the side of the plug  22 . 
         [0037]    In addition, the second holder  40  is coupled to an open end portion of the first holder  30 . 
         [0038]    In other words, the end portion of the first holder  30  is screwed with an end portion of the second holder  40  as illustrated in  FIG. 3 . An outer circumferential surface of one end portion of the second holder  40  having a smaller inner diameter is screwed with an inner circumferential surface of the open end portion of the first holder  30 . 
         [0039]    In this case, similarly to the first holder  30 , the second holder  40  has a third hole  41  having a diameter larger than that of the second hole  31  of the first holder  30  and formed to be open toward the first hole  11 . An end face of the third hole  41  positioned at the side of the second hole  31  has a fourth hole  42  having a diameter smaller than that of the second hole  31  and passing through the second hole  31  such that the third hole  41  has a diameter reduced toward the first holder  30  and is inserted into the first holder  30 . 
         [0040]    Accordingly, in the state that the first holder  30  is screwed with the second holder  40 , the first holder  30  is first inserted into the holder insertion hole  21  through the open end portion of the side cover  20  and screwed with the inner circumferential surface of the side cover  20  positioned at the side of the coupling part of the holder injection hole  21  with the plug  22 , such that the first holder  30  is fixed to the side cover  20 . 
         [0041]    In this case, an outer circumferential surface of the second holder  40  is fitting-inserted into the holder injection hole  21 . 
         [0042]    A locking unit  50  is provided into the second hole  31  in the first holder  30  and the third hole  41  and the fourth hole  42  in the second holder  40 . 
         [0043]      FIG. 4  is an enlarged sectional view illustrating the subject matter of the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
         [0044]    The locking unit  50  according to the present invention is configured with the assembly of a rod  51 , a first spring  52 , a ball guide  53 , an inner ball guide  54 , a second spring  55 , an outer ball guide  56 , and a third spring  57 . 
         [0045]    In other words, the rod  51  of the locking unit  50  has an outer diameter smaller than an inner diameter of the fourth hole  42  to be locked to the first holder  30  and the second holder  40  through the fourth hole  42  such that the rod  51  is axially movable. 
         [0046]    A locking step  510  is formed on an outer circumferential surface of the rod  51  positioned at the side of the second hole  31 , so an end portion of the rod  51  positioned at the side of the first holder  30  and inserted into the second hole  31  has a length shorter than that of the second hole  31 . 
         [0047]    A predetermined widthwise portion of the locking step  510  of the rod  51  has an outer diameter larger than that of the fourth hole  42 . Preferably, the locking step  510  of the rod  51  is formed in at least two stages to form a space having a predetermined width for introducing hydraulic pressure in association with an end face of the second holder  40  as illustrated in  FIG. 5 . 
         [0048]    The end portion of the rod  51 , which is positioned at the side of the first holder  30  and inserted into the second hole  31 , is elastically supported by the first spring  52 . 
         [0049]    In addition, an inner circumferential surface of the third hole  41 , which is positioned at the side of the fourth hole  42  of the second holder  40 , is screwed with an end portion of an outer circumferential surface of the ball guide  53 . 
         [0050]    In this case, the center of the ball guide  53  is bored with a diameter larger than an outer diameter of the rod  51  such that the rod  51  is slidable and an inner diameter of a portion of the ball guide  53 , which is positioned at the side of the first hole  11 , is more widely enlarged along a predetermined length, thereby forming a guide groove  530 . 
         [0051]    The second spring  55  is inserted into the guide groove  530  of the ball guide  53  together with the inner ball guide  54 , and the inner ball guide  54  elastically supported by the second spring  55  is prevented from deviating from an end portion of the rod  51 , which is positioned at the side of the first hole  11 , by a stop ring  531 . 
         [0052]    A plurality of ball insertion holes  532  are formed along an outer circumferential surface of an end portion of the ball guide  53 , which is positioned at the side of the first hole  11 , at regular intervals, and balls  533  having smaller diameters are inserted into the ball insertion holes  532 , respectively. 
         [0053]    The outer circumferential surface of the ball guide  53 , which has the guide groove  530 , is formed with a thickness smaller than the diameter of the ball  533 . An end portion of the ball guide  533 , which includes the ball insertion holes  532  and is positioned at the side of the first hole  11 , has an outer diameter more expanded by a predetermined thickness. 
         [0054]    In addition, an outer circumferential surface of the inner ball guide  54  to be inserted into the guide groove  530  is slidably inserted into the guide groove  530  along an inner circumferential surface of the ball guide  53 . 
         [0055]    More preferably, in the inner ball guide  54 , the outer circumferential surface of a predetermined length at the side of the first hole  11  has a smaller diameter, and a boundary surface stepped between outer circumferential surfaces of the inner ball guide  54  is gently inclined. 
         [0056]    In particular, a predetermined widthwise portion of the ball guide  53  has an outer diameter which is smaller than that of an inner diameter of the third hole  41  of the second holder  40 . Accordingly, the outer ball guide  56  and the third spring  57  are interposed between the inner circumferential surface of the second holder  40  and the outer circumferential surface of the ball guide  53 . 
         [0057]    The outer ball guide  56  supported by the third spring  57  has a predetermined length and an inner diameter enlarged corresponding to the length such that an end portion of the outer ball guide  56  at the side of the first hole  11  has an outer diameter larger than that of the ball guide  53 , so the outer ball guide  56  slidably moves along the outer circumferential surface of the end portion of the ball guide  53  at the side of the first hole  11 . 
         [0058]    Accordingly, as a portion of the outer ball guide  56 , which is positioned at the side of the first hole  11  and has a partially enlarged inner diameter, is locked to an end portion of the ball guide  53  having an enlarged outer diameter, the outer ball guide  56  is prevented from deviating from the ball guide  53  toward the first hole  11 . 
         [0059]    In addition, in the state that the balls  533  provided in the ball guide  53  are inserted into the ball insertion holes  532 , the balls  533  internally touches the outer circumferential surface of the end portion of the inner ball guide  54 , which is supported by the second spring  55 , the outer circumferential surface having an enlarged outer diameter. In this state, the balls  533  externally touches an inner circumferential surface of an end portion of the outer ball guide  56  supported by the third spring  57 , the inner circumferential surface having an enlarged inner diameter and being positioned at the side of the first hole  11 . Accordingly, the balls  533  are prevented from deviating from the ball guide  53 . 
         [0060]    However, in the locking unit  50  according to the present invention, the end portion of the outer ball guide  56 , which is positioned at the side of the first hole  11 , is horizontally formed in line with an end portion of the outer ball guide  56  which is positioned at the side of a recess part  121  of the piston  12 . Accordingly, the outer ball guide  56  slides by the piston  12  and the third spring  57 . 
         [0061]    Meanwhile, as illustrated in  FIG. 2 , according to the present invention, a fluid passage  60  is formed in the housing  10 , the side cover  20 , and the first holder  30  such that a fluid is supplied to the first hole  11 , which is positioned at the front end of the second holder  40 , via the housing  10  and the second hole  31  provided in the first holder  30  through each of side covers  20 . 
         [0062]    In other words, the fluid passage  60  is to supply and discharge the fluid through both side covers  20 . The fluid supplied to the second hole  31  provided in the second holder  40  may be supplied to the first hole  11  through the side cover  20  and the housing  10 . 
         [0063]    Hydraulic pressure is selectively applied to both side covers  20 . Accordingly, if hydraulic pressure is applied to one side cover  20 , the hydraulic pressure is discharged from an opposite side cover  20  while the piston  12  is moving inside the first hole  11 . 
         [0064]    Meanwhile, according to the present invention, the first holder  30 , the second holder  40 , and the ball guide  53  have cylindrical shapes. The inner ball guide  54  and the outer ball guide  56  inserted into the guide groove  530  and the second hole  40 , respectively, have ring shapes. 
         [0065]    Reference numeral  61 , which is not described yet, represents a fluid stopper coupled to open an emergency fluid passage for arbitrarily applying normal hydraulic pressure when the operation of the valve is stopped as abnormal hydraulic pressure is applied to the second hole  31 . 
         [0066]    Hereinafter, the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention will be described in terms of operations and effects. 
         [0067]    According to the present invention, for an assembling work, the piston  12  is first inserted into the housing  10 , the pinion gear  13  fixed to the valve shaft  14  is engaged with the rack gear  120  of the piston  12 . 
         [0068]    Then, the third spring  57  and the outer ball guide  56  are sequentially inserted into the second holder  40  along an inner circumferential surface of the third hole  41 , and the rod  51  is coupled to the resultant structure through the fourth hole  42  of the second holder  40 . 
         [0069]    In this case, a portion of the rod  51 , which has a longer length about the locking step  510 , is inserted toward the third hole  41  through the fourth hole  42 , and the ball guide  53  is inserted into the third hole  41  in the second holder  40 . 
         [0070]    However, the ball guide  53  is screwed with the inner circumferential surface of the third hole  41  in the second holder  40  to be securely fixed to the second holder  40 . 
         [0071]    In this case, the end portion of the ball guide  53  having the more expanded outer diameter prevents the outer ball guide  56 , which is slidably fitted around the ball guide  53  along the outer circumferential surface of the ball guide  53 , from deviating from the ball guide  53 . 
         [0072]    After the second spring  55  and the inner ball guide  54  are sequentially inserted into the guide groove  530  while being interposed between the outer circumferential surface of the rod  51  and the ball guide  53 , the stop ring  531  is coupled to the end portion of the rod  51 , thereby preventing the inner ball guide  54  from deviating from the guide groove  530  to the outside. 
         [0073]    The second holder  40  is inserted into the first holder  30  while the first spring  52  is inserted into an end portion of the rod  51  which partially protrudes through the fourth hole  42 . Accordingly, as illustrated in  FIG. 3 , the end portion of the first holder  30  is screwed with the end portion of the second holder  40 . 
         [0074]    The first holder  30  and the second holder  40 , which are integrally screwed with each other, are inserted into the holder insertion hole  21  in the side cover  20 , and the end portion of the outer circumferential surface of the first holder  30  is screwed with an inner circumferential surface of the holder insertion hole  21 . 
         [0075]    The side covers  20  having the assembly of the first holder  30 , the second holder  40 , and the locking unit  50  are screwed with both end portions of the housing  10 , thereby completing the assembling work according to the present invention. 
         [0076]    However, the operating unit, such as a valve, has an opening and closing angle of 90°. Accordingly, in the state that the valve is first closed, one end portion of the piston  12  pushes the outer ball guide  56  by a predetermined length, and the balls  533  are inserted into the ball insertion part  122  formed in the recess part  121  of the piston  12  inside the first hole  11  of the housing  10 . 
         [0077]    If the balls  533  are inserted into the ball insertion part  122  of the piston  12 , the piston  12  becomes in a locking state in which the piston  12  is not moved any more. 
         [0078]    In this case, at the side cover  20  having the locked piston  12 , the first holder  30  is rotated by using an additional tool through a hole having no the plug  22  as illustrated in  FIG. 6 . 
         [0079]    Since the first holder  30  has been already screwed with the side cover  20 , if the first holder  30  is rotated, the first holder  30  is axially slightly moved. 
         [0080]    In addition, since even the second holder  40  is screwed with the first holder  30 , when the first holder  30  is moved, even the second holder  40  is moved while rotating. 
         [0081]    If the second holder  40  is moved and the end portion of the second holder  40  makes close contact with the end portion of the piston  12 , the first holder  30  is not rotated any more. In this state, a zero point of the piston  12  is made. 
         [0082]    If the zero point is made in which the valve is closed as the second holder  40  makes close contact with the piston  12  inside one first hole  11 , the first holder  30  is rotated by using an additional tool through a hole without the plug  22  even at an opposite side cover  20 . 
         [0083]    In this case, regarding the moving length of the first holder  30  at the opposite side cover  20 , since a proper moving degree of the first holder  30  is set when a design for a manufacturing work is made, the first holder  30  at the opposite side may be moved only by the length set when the design is made. 
         [0084]    However, after the moving distance of the piston  12  is accurately restricted by manipulating the rotation of both first holders  30 , both holes into which the tools are inserted are screwed with the plugs  22  to be blocked from the outside. 
         [0085]    Accordingly, if the rotation of the both first holders  30  is manipulated such that the moving clearance of the piston is removed, when the piston  12  is moved by the hydraulic pressure, the operating clearance of the valve is removed, so that the operability of the valve can be accurately implemented. 
         [0086]    After the above assembling work is completed, hydraulic pressure having a predetermined size is selectively applied to the second holes  31  of the first holders  30  through the fluid passages  60  in both side covers  20 . 
         [0087]      FIG. 7  is an operation state view illustrating that the locking state of the piston is released as the hydraulic pressure is applied in the hydraulic actuator employing the limit adjustable mechanical lock. 
         [0088]    As illustrated in drawings, if hydraulic pressure equal to or greater than predetermined pressure is applied to one side of the housing  10 , the hydraulic pressure is applied to the locked second hole  31  of the piston  12  together with the first hole  11 . 
         [0089]    If the rod  51  is moved backward while compressing the first spring  52  by the hydraulic pressure applied to the second hole  31 , as the shaft-fixed inner ball guide  54  prevented from deviating by the stop ring  531  compresses the second spring  55  at the front end of the rod  51 , the balls  533  inserted into the ball insertion holes  532  of the ball guide  53  internally touches the outer circumferential surface of the inner ball guide  54  having a reduced outer diameter. 
         [0090]    Accordingly, if the balls  533  become in a movable state in the ball insertion part  122  of the piston  12  as the rode  51  is moved backward, the piston  12  is pushed by the hydraulic pressure applied to the first hole  11 . In this case, the balls  533  in the ball insertion part  122  of the piston  12  make close contact with the outer circumferential surface of the inner ball guide  54  having the reduced outer diameter, such that the locking state of the piston  12  is released. 
         [0091]      FIG. 8  is an operation state view illustrating that the piston is moved as the hydraulic pressure is applied to the first hole in the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
         [0092]    While the locking state of the piston  12  is released, the piston  12  is moved by the hydraulic pressure, and even the outer ball guide  56  in end-to-end contact with the piston  12  is moved while maintaining the contact state with the piston  12  and provided at a position of the piston  12 , thereby preventing the balls  533  from deviating from the place thereof. 
         [0093]    The piston  12 , which is unlocked from the ball guide  53 , is moved in an opposite direction by the hydraulic pressure applied to the first hole  11 . 
         [0094]    The unlocked piston  12  in the first hole  11  at one side may be moved while the fluid is discharged from the first hole  11  and the second hole  31  at the opposite side. 
         [0095]      FIG. 9  is an operating state view illustrating that the outer ball guide is moved as the piston is moved in the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
         [0096]    As illustrated in drawings, an opposite end portions of the piston  12  moving by the hydraulic pressure makes end-to-end contact with the outer ball guide  56  externally touching the balls  533  of the ball guide  53  while moving the outer ball guide  56 . 
         [0097]    If the end portion of the piston  12  becomes in the close contact with the end portion of the second holder  40  by moving the outer ball guide  56  together with the piston  12 , the piston  12  is not moved any more. In this case, the ball insertion part  122  formed in the recess part  121  of the piston  12  is vertically positioned in line with the balls  533  of the ball guide  53 . 
         [0098]      FIG. 10  is an operating state view illustrating that the piston is locked by the hydraulic actuator employing the limit adjustable mechanical lock according to the present invention. 
         [0099]    If the hydraulic pressure is simultaneously removed from the first hole  11  and the second hole  31 , the rod  51  is moved by the elastic force of the first spring  52 . 
         [0100]    The locking step  510  of the rod  51 , which is formed at the side of the second hole  31 , is moved until the locking step  510  makes close contact with the end face of the second holder  40 . When the rod  51  is moved, the inner ball guide  54  supported on the front end of the rod  51  is simultaneously moved toward the first hole  11  by the elastic force of the second spring  55 . 
         [0101]    As the inner ball guide  54  is moved, the balls  533 , which internally touch the outer circumferential surface of the inner ball guide  54  having the smaller outer diameter, slide the stepped portion of the inner ball guide  54 , which is inclined, and internally touch the outer circumferential surface of the inner ball guide  54  having the larger outer diameter. 
         [0102]    In this manner, the balls  533  are pushed outward from the outer circumferential surface of the inner ball guide  54  having the smaller outer diameter while sliding along the stepped and inclined surface of the inner ball guide  54  such that some of the balls  533  are inserted into the ball insertion part  122  of the piston  12  vertically positioned in line with the balls  533 . 
         [0103]    Accordingly, if the balls  533  are in contact with the outer circumferential surface of the inner ball guide  54  having the larger outer diameter as the rod  51  is moved, the balls  533  are simultaneously inserted into the ball insertion part  122  of the piston  12  through the outer circumferential surface of the inner ball guide  54  having the larger outer diameter and the ball insertion holes  532  of the ball guide  53 . Accordingly, the piston  12  is not moved and is locked. 
         [0104]    If the piston  12  is locked as described above, the valve serving as the operating unit is open. Accordingly, the fluid cut off by the valve can move. 
         [0105]    Meanwhile, in order to close the valve again, the hydraulic pressure is applied to the side cover  20  in which the piston  12  is locked, thereby unlocking the piston  12  while moving the piston  12 . Accordingly, the valve is operated through the rotation of the pinion gear  13  engaged with the rack gear  120  of the piston  12 . 
         [0106]    By the hydraulic actuator employing the limit adjustable mechanical lock of the present invention, if the hydraulic pressure is selectively applied to both side covers  20 , since the piston is locked in the first hole  11  through which the hydraulic pressure is released, the operations of operation units, such as the valve, are exactly performed. 
         [0107]    In particular, in the state that the valve is open or closed, if the piston  12  is locked such that the piston  12  is not moved, the open/closed state of the valve can be stably maintained. 
         [0108]    Accordingly, in the state that the valve is open, if hydraulic pressure, which is equal to or less than preset pressure, is applied to a valve actuator according to present invention due to the failure of a hydraulic circuit, the locking state of the piston  12  is not released and thus the valve is not operated any more. Accordingly, the error in the operation of the valve does not occur. 
         [0109]    In other words, if the hydraulic pressure applied to the valve actuator becomes less than the preset value, the valve actuator and the valve are not operated. 
         [0110]    Accordingly, due to the above-described action, when the circuit of the hydraulic pressure applied to the valve actuator is failed, the operation of the valve is stopped such that the abnormal operation of the valve can be prevented in advance. 
         [0111]    Meanwhile, according to the present invention, when the valve actuator is initially assembled, since the operating limit of the valve can be simply adjusted by rotating the first holder  30 , the moving clearance of the valve is removed, so the operability of the valve can be accurately provided. 
         [0112]    As described above, according to the present invention, the moving clearance of the valve is removed by the simple rotation of the first holder  30  such that valve is accurately operated. In addition, if the hydraulic pressure applied to the side cover  20  becomes less than preset pressure as the hydraulic circuit is failed during the operation of the valve, the operation of the valve is stopped according to the present invention, so that the accident resulting from the error in the operation of the valve can be prevented in advance.