Patent Publication Number: US-2018031012-A1

Title: Fluid pressure cylinder

Description:
TECHNICAL FIELD 
     The present invention relates to a fluid pressure cylinder in which a piston rod is decelerated by a cushion pressure generated near a stroke end of the piston rod. 
     BACKGROUND ART 
     As conventional fluid pressure cylinders, there is a known fluid pressure cylinder that includes a cushioning mechanism by which a piston rod inserted into a cylinder tube is decelerated by a cushion pressure generated when the piston rod comes to the vicinity of a stroke end. 
     JP2012-193752A discloses a fluid pressure cylinder that includes a piston rod inserted into a cylinder tube, a piston that is provided on a tip end of the piston rod and defines a rod side chamber and a bottom-side chamber in the interior of the cylinder tube, and a cushion bearing that defines a cushion passage through which working fluid is allowed to pass when the piston rod comes to the vicinity of a stroke end. With the fluid pressure cylinder disclosed in JP2012-193752A, the cushion bearing is clamped between the piston and a step portion formed on the piston rod. 
     SUMMARY OF INVENTION 
     With a fluid pressure cylinder, in an event in which an excessive external force is exerted to a piston rod, the piston rod may be plastically deformed so as to be elongated in the axial direction. Among the fluid pressure cylinders having a clamped-type cushion bearing clamped between a step portion of the piston rod and a piston, there is a fluid pressure cylinder that has a function of detecting an abnormal state in which the piston rod is plastically deformed in the axial direction. 
     The fluid pressure cylinder having an abnormality detecting function has an annular gap between an inner circumference of the cushion bearing and an outer circumference of the piston rod. The annular gap inside the cushion bearing communicates with a bottom-side chamber through a connection gap between the piston rod and the piston. With the fluid pressure cylinder having such a configuration, when the piston rod is plastically deformed so as to be elongated, an axial gap is formed between the cushion bearing and the step portion of the piston rod, and a rod side chamber is communicated with the bottom-side chamber through the axial gap, the annular gap, and the connection gap. When the rod side chamber is communicated with the bottom-side chamber, even when a load-holding state is achieved by stopping the supply/ discharge of the working fluid to/ from the fluid pressure cylinder, the fluid pressure cylinder is slightly extended or contracted depending on the direction in which the load is applied. Therefore, with the fluid pressure cylinder having the abnormality detecting function, an operator can detect the abnormal state in which the piston rod is plastically deformed by checking whether the fluid pressure cylinder is extended or contracted in the load-holding state. 
     With the fluid pressure cylinder having such a configuration, in a normal state in which the piston rod is not plastically deformed, the cushion bearing is clamped between the step portion of the piston rod and the piston, and thereby, the gap formed in the axial direction between the cushion bearing and the piston rod is sealed. With such a configuration, in the normal state, because the communication between the rod side chamber and the bottom-side chamber through the annular gap and the connection gap is shut off, it is possible to achieve the load-holding state by stopping supply/discharge of the working fluid to/from the fluid pressure cylinder. 
     As described above, with the fluid pressure cylinder having the abnormality detecting function, it is possible to detect the abnormal state by shutting off the communication between the rod side chamber and the bottom-side chamber in the normal state and by allowing the communication between the rod side chamber and the bottom-side chamber through the annular gap when an abnormality has occurred. 
     However, in the fluid pressure cylinder having such a configuration, when the annular gap is provided inside the cushion bearing in order to detect the abnormal state, even in the normal state, the working fluid supplied to the bottom-side chamber may be guided to the annular gap through the connection gap between the piston rod and the piston. When the working fluid is guided to the annular gap, there is a risk that the cushion bearing is elastically deformed due to the pressure of the working fluid and is expanded in the radial direction. When the cushion bearing is expanded in the radial direction, the cushion passage formed between the cushion bearing and the bearing receiving portion is narrowed, and there is a risk in that stability of cushioning operation is deteriorated. 
     An object of the present invention is to improve stability of cushioning operation of a fluid pressure cylinder having an abnormality detecting function. 
     According to one aspect of the present invention, a fluid pressure cylinder includes a piston rod having an annular step portion formed on an outer circumferential surface; a cylinder tube into which the piston rod is inserted; a piston connected to a tip end of the piston rod, the piston being configured to slide along an inner circumferential surface of the cylinder tube, the piston defining a rod side chamber and a bottom-side chamber in an interior of the cylinder tube and; a cylindrical cushion bearing clamped between the piston and the step portion of the piston rod, the cushion bearing being provided so as to form an annular gap on the outer circumference of the piston rod; a bearing receiving portion into which the cushion bearing is allowed to enter at vicinity of a stroke end of the piston rod; a cushion passage formed between the cushion bearing and the bearing receiving portion as the cushion bearing enters inside of the bearing receiving portion, the cushion passage being configured to impart resistance to flow of working fluid passing therethrough; and a check seal provided between an inner circumference of the cushion bearing or the piston and the outer circumference of the piston rod. The check seal shuts off the flow of the working fluid from a connection gap between the piston rod and the piston towards the rod side chamber through the annular gap and allows the flow of the working fluid from the annular gap towards the bottom-side chamber through the connection gap. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view showing a part of a fluid pressure cylinder according to an embodiment of the present invention. 
         FIG. 2  is a sectional view showing a cushion bearing and a check seal of the fluid pressure cylinder according to the embodiment of the present invention. 
         FIG. 3  is a view showing a state in which the check seal of the fluid pressure cylinder according to the embodiment of the present invention is accommodated in an accommodating groove and is a sectional view showing a state in which a piston is not assembled. 
         FIG. 4  is a sectional view showing a state in which the check seal and the piston of the fluid pressure cylinder according to the embodiment of the present invention are assembled. 
         FIG. 5  is a view showing the check seal of the fluid pressure cylinder according to the embodiment of the present invention and is a sectional view showing a state in which the fluid pressure cylinder is extended. 
         FIG. 6  is a view showing a part of the fluid pressure cylinder according to the embodiment of the present invention and is a sectional view showing a state in which the fluid pressure cylinder is in an abnormal state. 
         FIG. 7  is a view showing the check seal of the fluid pressure cylinder according to the embodiment of the present invention and is a sectional view showing a state in which the fluid pressure cylinder is in the abnormal state. 
         FIG. 8  is a sectional view showing the check seal of the fluid pressure cylinder according to a comparative example of the embodiment of the present invention. 
         FIG. 9  is a sectional view showing the check seal of the fluid pressure cylinder according to a modification of the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A fluid pressure cylinder according to an embodiment of the present invention will be described below with reference to the drawings. In the following, the case in which the fluid pressure cylinder is a hydraulic cylinder  100  that is driven by using working oil as working fluid will be described. 
     A configuration of the hydraulic cylinder  100  will be described with main reference to  FIG. 1 . 
     The hydraulic cylinder  100  is used as, for example, a bucket cylinder of a hydraulic shovel. A bucket (not shown) of the hydraulic shovel is rotated as the hydraulic cylinder  100  is extended/ contracted. 
     As shown in  FIG. 1 , the hydraulic cylinder  100  includes a piston rod  10  having an annular step portion  13  that is formed on an outer circumferential surface thereof, a cylindrical cylinder tube  20  into which the piston rod  10  is inserted, a piston  30  that is connected to a tip end of the piston rod  10  and slides along an inner circumferential surface of the cylinder tube  20 , and a cylindrical cushion bearing  40  that is provided on an outer circumference of the piston rod  10 . 
     An interior of the cylinder tube  20  is partitioned into a rod side chamber  2  and a bottom-side chamber  3  by the piston  30 . The hydraulic cylinder  100  is extended/contracted by working oil pressure guided from a hydraulic pressure source (working-fluid pressure source) to the rod side chamber  2  or the bottom-side chamber  3 . A gap between an inner circumference of the cylinder tube  20  and an outer circumference of the piston  30  is sealed by a seal member  31 . With such a configuration, communication between the rod side chamber  2  and the bottom-side chamber  3  via the gap between the inner circumference of the cylinder tube  20  and the outer circumference of the piston  30  is shut off. 
     On an open end of the cylinder tube  20 , a cylindrical cylinder head  50  is provided so as to slidably support the piston rod  10 . The cylinder head  50  has a bearing receiving portion  51  that is inserted into the inside of the cylinder tube  20 . The cylinder head  50  is fastened to the cylinder tube  20  with a plurality of bolts (not shown). 
     A bush  55 , an auxiliary seal  56 , a main seal  57 , and a dust seal  58  are interposed on an inner circumference of the cylinder head  50 . 
     The bush  55  is brought into sliding contact with the outer circumferential surface of the piston rod  10 , and thereby, the piston rod  10  is supported so as to be movable in the axial direction of the cylinder tube  20 . 
     A supply/discharge port  52  that is communicated with the rod side chamber  2  is formed on the cylinder head  50 . The working oil is supplied/discharged to/from the rod side chamber  2  through the supply/discharge port  52 . 
     The piston rod  10  includes a main body portion  11  that is in sliding contact with the inner circumference of the cylinder head  50 , a small-diameter portion  12  that is formed to have a smaller outer diameter than the main body portion  11 , the annular step portion  13  that is formed between the main body portion  11  and the small-diameter portion  12 , and a screw portion  14  that is formed at a tip end of the piston rod  10  and to which the piston  30  is fastened. 
     The cushion bearing  40  is provided on an outer circumference of the small-diameter portion  12  of the piston rod  10 . As shown in  FIGS. 1 and 2 , the cushion bearing  40  is formed to have an inner diameter greater than an outer diameter of the small-diameter portion  12  of the piston rod  10 . In other words, an annular gap  70  is provided between the cushion bearing  40  and the small-diameter portion  12  of the piston rod  10 . In addition, the cushion bearing  40  is formed to have the inner diameter smaller than an outer diameter of the main body portion  11  of the piston rod  10 . Thus, an one end surface  40 A of the cushion bearing  40  is brought into contact with the step portion  13  of the piston rod  10 . 
     As shown in  FIG. 1 , the piston  30  is threaded to the screw portion  14  of the piston rod  10  and is fastened to the piston rod  10  with a predetermined fastening force. Therefore, as shown in  FIGS. 1 and 2 , the cushion bearing  40  is clamped between the piston  30  threaded to the screw portion  14  of the piston rod  10  and the step portion  13  of the piston rod  10 . With such a configuration, an axial gap between the cushion bearing  40  and the step portion  13  of the piston rod  10  and an axial gap between the cushion bearing  40  and the piston  30  are respectively sealed. Thus, communication between the rod side chamber  2  and the annular gap  70  inside the cushion bearing  40  is shut off. 
     As described above, the hydraulic cylinder  100  is a hydraulic cylinder having the clamped-type cushion bearing  40  that is clamped between the piston  30  and the piston rod  10  by fastening the piston  30 . 
     A small gap may be formed between an inner circumference of the piston  30  and the outer circumference of the small-diameter portion  12  of the piston rod  10  and between the inner circumference of the piston  30  and the screw portion  14 . Through such a small gap that is present on the outer circumference of the small-diameter portion  12  of the piston rod  10  and on the threaded portion of the screw, the annular gap  70  inside the cushion bearing  40  is communicated with the bottom-side chamber  3 . In the following, the gap present between the inner circumference of the piston  30  and the outer circumference of the piston rod  10  is referred to as “a connection gap  71 ”. In addition, in  FIGS. 2,4, and 5 , the connection gap  71  is schematically shown as an annular gap. 
     The cushion bearing  40  is formed to have the outer diameter that is smaller than the inner diameter of the bearing receiving portion  51  of the cylinder head  50 , and enters the inside of the bearing receiving portion  51  at the vicinity of a stroke end of the piston rod  10 . As the cushion bearing  40  enters the inside of the bearing receiving portion  51 , a cushion passage  4  is formed between the cushion bearing  40  and the bearing receiving portion  51 . Resistance is imparted to the flow of the working oil passing through the cushion passage  4 . 
     The hydraulic cylinder  100  further includes an annular check seal  60  that is provided between an inner circumference of the cushion bearing  40  and the outer circumference of the piston rod  10 . 
     As shown in  FIG. 2 , the check seal  60  is provided in an accommodating groove  65  that is formed in the axial direction from an opposing surface  40 B of the end surfaces of the cushion bearing  40 . The opposing surface is opposing the piston  30 . The accommodating groove  65  is formed so as to open to the opposing surface  40 B of the cushion bearing  40  opposing the piston  30  and to open to an inner circumferential surface of the cushion bearing  40 . 
     The check seal  60  has a tapered portion  61  at which an outer diameter is gradually increased along the axial direction from the one end surface in the axial direction. The other end surface of the check seal  60  is formed as a flat surface that is perpendicular to the central axis. In addition, a bottom portion  66  of the accommodating groove  65  in the axial direction is formed to have a tapered shape that corresponds to the tapered portion  61  of the check seal  60 . 
     The check seal  60  is accommodated in the accommodating groove  65  such that the tapered portion  61  is brought into contact with the bottom portion  66  of the accommodating groove  65 . As described above, because the check seal  60  has the tapered portion  61  and the accommodating groove  65  has the tapered bottom portion  66 , it is possible to prevent misassembly of the check seal  60 . In addition, because the tapered portion  61  of the check seal  60  and the bottom portion  66  of the accommodating groove  65  are in surface contact with each other at the tapered surfaces, sealing performance of the check seal  60  is improved. In order to prevent misassembly of the check seal  60 , it is preferred that the check seal  60  has the tapered portion  61  and the accommodating groove  65  has the tapered bottom portion  66 . However, the bottom portion  66  of the accommodating groove  65  may not be formed to have a tapered shape. For example, the bottom portion  66  of the accommodating groove  65  may be formed to have a flat surface that is perpendicular to the central axis. Even in this case, it is possible to prevent misassembly by providing the tapered portion  61  on one end portion of the check seal  60 . 
     As shown in  FIG. 2 , the check seal  60  has an axial groove  62  that is formed on an outer circumferential surface thereof along the axial direction and a radial groove  63  that is formed on the end surface of the check seal  60  on the piston  30  side along the radial direction and communicates with the axial groove  62 . 
     The check seal  60  is made of a resin material, such as, for example, rubbers, and is an elastic member capable of being deformed by an external force. As shown in  FIG. 3 , in a state accommodated in the accommodating groove  65 , the check seal  60  projects out slightly from the opposing surface  40 B of the cushion bearing  40  opposing the piston  30 . Specifically, the check seal  60  is formed such that a natural length thereof in the axial direction in a state in which an external force is not exerted is longer than the length of the accommodating groove  65  in the axial direction. 
     In the following, a process for assembling the check seal  60  and the cushion bearing  40  will be specifically described with reference to  FIGS. 3 and 4 . 
     As shown in  FIG. 3 , the cushion bearing  40  is first mounted on the outer circumference of the small-diameter portion  12  of the piston rod  10 , and the check seal  60  is accommodated in the accommodating groove  65 . In a state in which the check seal  60  is accommodated in the accommodating groove  65 , the check seal  60  slightly projects out from the opposing surface  40 B of the cushion bearing  40  opposing the piston  30 . 
     Next, the piston  30  is threaded to the screw portion  14  of the piston rod  10 . As the piston  30  is threaded to the screw portion  14  of the piston rod  10 , an opposing surface  30 A of the piston  30  opposing the cushion bearing  40  is brought into contact with the check seal  60 . The piston  30  is further threaded from this state to bring the opposing surface  30 A of the piston  30  and the opposing surface  40 B of the piston rod  10  into contact with each other while compressing the check seal  60  in the axial direction. With such a configuration, as shown in  FIG. 4 , the check seal  60  is accommodated in the accommodating groove  65  by being compressed in the axial direction. The piston  30  is further fastened with a predetermined fastening force, thereby clamping the cushion bearing  40  with the step portion  13  of the piston rod  10 . 
     Next, an operation of the hydraulic cylinder  100  will be described with main reference to  FIGS. 5 to 8 . In  FIGS. 5 to 8 , flow of the working oil is schematically shown with solid line arrows. In  FIG. 8 , the pressure of the working oil acting on the check seal is schematically shown with broken line arrows. 
     When the hydraulic pressure source is communicated with the bottom-side chamber  3  and a tank (not shown) is communicated with the rod side chamber  2 , the working oil is supplied to the bottom-side chamber  3 , and the working oil in the rod side chamber  2  is discharged to the tank. Therefore, the hydraulic cylinder  100  is extended. 
     When the working oil is supplied to the bottom-side chamber  3 , the pressure of the working oil acts on the check seal  60  through the connection gap  71  between the screw portion  14  of the piston rod  10  and the piston  30 . 
     Therefore, as shown in  FIG. 5 , the check seal  60  is pressed against the cushion bearing  40  while being compressed in the axial direction. 
     Here, a hydraulic cylinder according to a comparative example of this embodiment is shown in  FIG. 8 . As shown in  FIG. 8 , with the hydraulic cylinder according to the comparative example, the check seal  60  is accommodated in the accommodating groove  65  so as to form a gap  80  between the bottom portion  66  of the accommodating groove  65  and the piston  30 . In the case in which the check seal  60  is accommodated in the accommodating groove  65  so as to form the gap  80 , when the pressure of the working oil is guided through the connection gap  71 , the pressure of the working oil also acts on an end surface of the check seal  60  on the cushion bearing  40  side through the radial groove  63  and the axial groove  62 . In this case, because the forces acting on both end surfaces of the check seal  60  in the axial direction by the pressure of the working oil are balanced, there may be a case in which the check seal  60  is not pressed against the cushion bearing  40  and the annular gap  70  cannot be sealed. 
     In contrast, with the hydraulic cylinder  100 , the check seal  60  is accommodated in the accommodating groove  65  by being compressed in the axial direction. Therefore, except for the case in which the hydraulic cylinder  100  is in an abnormal state, which will be described later, as shown in  FIG. 5 , it is possible to reliably seal the annular gap  70  by always bringing the check seal  60  into contact with the bottom portion  66  of the accommodating groove  65 . 
     The annular gap  70  inside the cushion bearing  40  is sealed by the check seal  60 , and thereby, the communication between the annular gap  70  inside the cushion bearing  40  and the connection gap  71  inside the piston  30  is shut off. Therefore, the pressure of the working oil that has been guided through the connection gap  71  is prevented from being guided to the annular gap  70 . 
     The communication between the annular gap  70  and the rod side chamber  2  is shut off by clamping the cushion bearing  40  between the step portion  13  of the piston rod  10  and the piston  30  (see  FIGS. 1 and 2 ). Thus, the flow of the working oil from the rod side chamber  2  to the annular gap  70  inside the cushion bearing  40  is also shut off. 
     As the piston rod  10  is extended and approaches the stroke end, the cushion bearing  40  enters the inside of the bearing receiving portion  51  of the cylinder head  50  (see  FIGS. 1 and 2 ). With such a configuration, the cushion passage  4  is formed by an outer circumferential surface of the cushion bearing  40  and an inner circumferential surface of the bearing receiving portion  51 . Because resistance is imparted by the cushion passage  4  to the flow of the working oil discharged from the rod side chamber  2  through the supply/discharge port  52 , the pressure drop in the rod side chamber  2  is suppressed, and the piston rod  10  is decelerated. By doing so, the cushioning operation is exhibited at the vicinity of the stroke end when the piston rod  10  is extended. 
     In addition, because the flow of the working oil to the annular gap  70  inside the cushion bearing  40  is shut off by the check seal  60 , the cushion bearing  40  is prevented from being expanded in the radial direction by the pressure in the annular gap  70 . Therefore, the cushion passage  4  is also prevented from being narrowed, and it is possible to exhibit the stable cushioning operation. 
     When the hydraulic pressure source is communicated with the rod side chamber  2  and the tank is communicated with the bottom-side chamber  3 , the working oil is supplied to the rod side chamber  2 , and the working oil in the bottom-side chamber  3  is discharged to the tank. Therefore, the hydraulic cylinder  100  is contracted. 
     In addition, due to the weight of the bucket attached to the piston rod  10 , a force acts on the hydraulic cylinder  100  in the extending direction. With the hydraulic cylinder  100  that drives the bucket, the rod side chamber  2  is a load-side pressure chamber on which the load pressure by the load (the bucket) acts. The mutual communication between the rod side chamber  2  and the bottom-side chamber  3  is shut off by the cushion bearing  40  that is clamped between the step portion  13  of the piston rod  10  and the piston  30 . Therefore, when supply/discharge of the working oil to/from the hydraulic cylinder  100  is stopped, the hydraulic cylinder  100  is in a load-holding state in which the load pressure acting on the rod side chamber  2  is held so as to immobilize the bucket, which is the load. 
     Here, when an excessive external force is exerted to the piston rod  10 , the piston rod  10  may be elongated by being plastically deformed. The hydraulic cylinder  100  has an abnormality detecting function that detects such an abnormal state in which the piston rod  10  is plastically deformed. In the following, the abnormality detecting function of the hydraulic cylinder  100  will be described with reference to  FIGS. 6 and 7 . 
     As shown in  FIG. 6 , when the piston rod  10  is plastically deformed in the direction in which the piston rod  10  is elongated, an axial gap  74  is formed between the cushion bearing  40  and the step portion  13  of the piston rod  10  that are arranged in the axial direction. When such an axial gap  74  is formed so as to be adjacent to the cushion bearing  40 , the rod side chamber  2  is communicated with the annular gap  70  through the axial gap  74 . 
     In such an abnormal state, when the load-holding state is achieved by stopping supply/discharge of the working oil to/from the hydraulic cylinder  100 , the load pressure in the rod side chamber  2  is guided to the annular gap  70  through the axial gap  74 . By the load pressure that is guided through the annular gap  70 , the check seal  60  is compressed and the check seal  60  is pressed towards the piston  30  side. At this time, the cushion bearing  40  is also pressed towards the piston  30 . 
     As shown in  FIG. 7 , when the check seal  60  is compressed and pressed towards the piston  30  side, an in-groove gap  72  is formed in the accommodating groove  65  between the tapered portion  61  of the check seal  60  and the bottom portion  66  of the accommodating groove  65 . The in-groove gap  72  is communicated with the axial groove  62  and the radial groove  63  of the check seal  60 . Therefore, the annular gap  70  is communicated with the connection gap  71  via the in-groove gap  72 , the axial groove  62 , and the radial groove  63 . As described above, the axial groove  62  and the radial groove  63  serve as a communicating passage through which the connection gap  71  is communicated with the in-groove gap  72 . 
     Therefore, the load pressure in the rod side chamber  2  is guided to the bottom-side chamber  3  through the annular gap  70 , the in-groove gap  72 , the axial groove  62  and the radial groove  63  serving as the communicating passage, and the connection gap  71 . As described above, at the time of the abnormal state, the check seal  60  forms the in-groove gap  72  by the load pressure guided from the annular gap  70  and allows the flow of the working oil from the rod side chamber  2  towards the bottom-side chamber  3  by the in-groove gap  72 , and the axial groove  62  and the radial groove  63 . 
     Therefore, in the abnormal state, even when supply/ discharge of the working oil to/from the hydraulic cylinder  100  is stopped, a small amount of the working oil is guided from the rod side chamber  2  to the bottom-side chamber  3  through the connection gap  71 , causing the hydraulic cylinder  100  to extend by a small amount. Therefore, an operator can detect the abnormal state in which the piston rod  10  is deformed by checking whether the hydraulic cylinder  100  is extended in the load-holding state. 
     As described above, the check seal  60  has a checking function that shuts off the flow of the working oil from the connection gap  71  towards the rod side chamber  2  through the annular gap  70 , and, when an abnormality has occurred, allows the flow of the working oil from the annular gap  70  towards the bottom-side chamber  3  through the connection gap  71 . With such a configuration, it is possible to prevent the cushion bearing  40  from being expanded in the radial direction in the normal state without deteriorating the abnormality detecting function in which, when the abnormality has occurred, the hydraulic cylinder  100  is allowed to extend slightly in the load-holding state by causing the rod side chamber  2  to communicate with the bottom-side chamber  3 . 
     According to the embodiment mentioned above, the advantages described below are afforded. 
     With the hydraulic cylinder  100 , because the flow of the working oil from the connection gap  71  towards the rod side chamber  2  through the annular gap  70  is shut off by the check seal  60 , the working oil is prevented from being guided to the inside of the cushion bearing  40 . Thus, the cushion bearing  40  is prevented from being expanded outwards in the radial direction, and the cushion passage  4  formed at the vicinity of the stroke end is prevented from being narrowed. In addition, because the check seal  60  allows the flow of the working oil from the annular gap  70  towards the bottom-side chamber  3  through the connection gap  71 , when the abnormality, in which the piston rod  10  is plastically deformed and elongated in the axial direction, has occurred, the working oil is guided from the rod side chamber  2  to the bottom-side chamber  3  through the check seal  60 . Therefore, without deteriorating the abnormality detecting function of the hydraulic cylinder  100  having the clamped-type cushion bearing  40 , the cushion passage  4  is prevented from being narrowed. Therefore, according to the hydraulic cylinder  100 , it is possible to improve the stability of the cushioning operation of the hydraulic cylinder  100  having the clamped-type cushion bearing  40 . 
     In addition, with the hydraulic cylinder  100 , the check seal  60  is provided in the accommodating groove  65  that is formed from the opposing surface  40 B of the cushion bearing  40  opposing the piston  30 . As described above, by providing the check seal  60  on the piston  30  side, the working oil is prevented from being guided to the annular gap  70  over the entirety in the axial direction. Therefore, it is possible to further improve the stability of the cushioning operation. 
     In addition, because the check seal  60  is accommodated in the accommodating groove  65  in a state compressed in the axial direction, except for the case in which the abnormality has occurred, the check seal  60  is always brought into contact with the bottom portion  66  of the accommodating groove  65 . Therefore, it is possible to reliably seal the annular gap  70 . 
     The configurations, operations, and effects of the embodiment of the present invention will be collectively described below. 
     The hydraulic cylinder  100  includes: the piston rod  10  having the annular step portion  13  formed on the outer circumferential surface of the piston rod  10 ; the cylinder tube  20  through which the piston rod  10  is inserted; the piston  30  that is connected to the tip end of the piston rod  10 , defines the rod side chamber  2  and the bottom-side chamber  3  in the interior of the cylinder tube  20 , and slides along the inner circumferential surface of the cylinder tube  20 ; the cylindrical cushion bearing  40  that is clamped between the piston  30  and the step portion  13  of the piston rod  10  and provided so as to form the annular gap  70  on the outer circumference of the piston rod  10 ; the bearing receiving portion  51  into which the cushion bearing  40  is allowed to enter at the vicinity of the stroke end of the piston rod  10 ; the cushion passage  4  that is formed between the cushion bearing  40  and the bearing receiving portion  51  when the cushion bearing  40  enters the inside of the bearing receiving portion  51  at the vicinity of stroke end and that imparts resistance to the flow of the working oil passing therethrough; and the check seal  60  that is provided between the inner circumference of the cushion bearing  40  and the outer circumference of the piston rod  10 . In the hydraulic cylinder  100 , the flow of the working oil towards the rod side chamber  2  through the annular gap  70  from the connection gap  71 , which is formed between the piston rod  10  and the piston  30 , is shut off by the check seal  60 , and the flow of the working oil from the annular gap  70  towards the bottom-side chamber  3  through the connection gap  71  is allowed. 
     In this configuration, because the flow of the working oil from the connection gap  71  towards the rod side chamber  2  through the annular gap  70  is shut off by the check seal  60 , the working oil is suppressed from being guided to the inside of the cushion bearing  40 . Thus, the cushion bearing  40  is suppressed from being expanded outwards in the radial direction, and the cushion passage  4  formed at the vicinity of stroke end is prevented from being narrowed. In addition, because the check seal  60  allows the flow of the working oil from the annular gap  70  towards the bottom-side chamber  3  through the connection gap  71 , when the abnormality, in which the piston rod  10  is plastically deformed in the axial direction, has occurred, the working oil is guided from the rod side chamber  2  to the bottom-side chamber  3  through the check seal  60 . Therefore, without deteriorating the abnormality detecting function of the hydraulic cylinder  100  having the clamped-type cushion bearing  40 , which is clamped between the piston  30  and the step portion  13  of the piston rod  10 , the cushion passage  4  is prevented from being narrowed. 
     With this configuration, it is possible to improve the stability of the cushioning operation of the hydraulic cylinder  100  having the abnormality detecting function. 
     In addition, in the hydraulic cylinder  100 , the check seal  60  is provided in the accommodating groove  65  that is formed in the opposing surface  40 B of the end surfaces of the cushion bearing  40  opposing the piston  30 . 
     In this configuration, by providing the check seal  60  in the opposing surface  40 B of the cushion bearing  40  opposing the piston  30 , the working oil is prevented from being guided to the annular gap  70  over the entirety in the axial direction, and the cushion passage  4  is prevented from being narrowed. 
     With this configuration, it is possible to further improve the stability of the cushioning operation of the hydraulic cylinder  100  having the clamped-type cushion bearing  40 . 
     In addition, with the hydraulic cylinder  100 , the in-groove gap  72  is formed inside the accommodating groove  65  as the check seal  60  is pressed towards the piston  30  side by the pressure of the working oil guided through the annular gap  70 , and the check seal  60  has the communicating passage (the axial groove  62  and the radial groove  63 ) through which the connection gap  71  is communicated with the in-groove gap  72 . 
     In this configuration, because the in-groove gap  72  is formed by the pressure of the working oil from the annular gap  70  and the check seal  60  has the communicating passage (the axial groove  62  and the radial groove  63 ), the annular gap  70  is communicated with the connection gap  71 . Thus, the check seal  60  allows the flow of the working oil from the annular gap  70  towards the bottom-side chamber  3  through the connection gap  71 . 
     In addition, in the hydraulic cylinder  100 , the communicating passage has the axial groove  62  that is formed on the outer circumferential surface of the check seal  60  along the axial direction and that communicates with the in-groove gap  72  and the radial groove  63  that is formed on the end surface of the check seal  60  on the piston  30  side and through which the axial groove  62  is communicated with the connection gap  71 . 
     In this configuration, the in-groove gap  72  is communicated with the connection gap  71  by the axial groove  62  and the radial groove  63  of the communicating passage. Thus, the check seal  60  allows the flow of the working oil from the annular gap  70  towards the bottom-side chamber  3  through the connection gap  71 . 
     In addition, in the hydraulic cylinder  100 , the check seal  60  has the tapered portion  61  at which the outer diameter is gradually increased along the axial direction from the one end portion in the axial direction. 
     With this configuration, it is possible to prevent misassembly of the check seal  60 . 
     In addition, in the hydraulic cylinder  100 , the bottom portion  66  of the accommodating groove  65  in the axial direction is formed to have a tapered shape corresponding to the tapered portion  61  of the check seal  60 . 
     In this configuration, the tapered portion  61  of the check seal  60  and the bottom portion  66  of the accommodating groove  65  are in surface contact with each other at the tapered surfaces. 
     With this configuration, it is possible to prevent misassembly of the check seal  60  and to improve the sealing performance of the annular gap  70  by the check seal  60 . 
     In addition, in the hydraulic cylinder  100 , the check seal  60  is accommodated in the accommodating groove  65  in a state compressed in the axial direction. 
     In this configuration, except for the case in which the hydraulic cylinder  100  is in the abnormal state, the check seal  60  is always brought into contact with the bottom portion  66  of the accommodating groove  65 . 
     With this configuration, it is possible to reliably seal the annular gap  70  by the check seal  60 . 
     Embodiments of this invention were described above, but the above embodiments are merely examples of applications of this invention, and the technical scope of this invention is not limited to the specific constitutions of the above embodiments. 
     In the above-mentioned embodiment, although the working oil is used as the working fluid, instead of this configuration, for example, aqueous alternative fluid etc. may be used. 
     In addition, in the above-mentioned embodiment, the check seal  60  has the tapered portion  61 . Instead of this configuration, the check seal  60  may have, for example, a circular section or other polygonal section. In addition, the accommodating groove  65  is not limited to that having the bottom portion  66 , and the accommodating groove  65  may be formed to have any shape. 
     In addition, the communicating passage is not limited to that having the axial groove  62  and the radial groove  63 , and the communicating passage may be formed to have any shape as long as the connection gap  71  is communicated with the in-groove gap  72 . For example, as shown in  FIG. 9 , the communicating passage may be formed as a single through hole  64  that penetrates through the check seal  60  such that the connection gap  71  is communicated with the in-groove gap  72 . 
     In addition, in the above-mentioned embodiment, the accommodating groove  65  is formed from the opposing surface  40 B of the cushion bearing  40  opposing the piston  30 . In order to prevent the working oil from being guided to the annular gap  70  inside the cushion bearing  40  over the entirety in the axial direction, it is desirable that the accommodating groove  65  is formed at a position at which the cushion bearing  40  and the piston  30  face against with each other. However, the configuration is not limited thereto, and for example, the accommodating groove  65  may be provided in the central portion of the cushion bearing  40  in the axial direction. Also in this case, it is possible to prevent the working oil from being guided to a part of the annular gap  70 , in other words, to the gap between the step portion  13  of the piston rod  10  and the check seal  60 , and thereby, it is possible to suppress the expansion of the cushion bearing  40  in the radial direction by the pressure of the working oil. 
     In addition, in the above-mentioned embodiment, the check seal  60  is provided in the accommodating groove  65  that is formed from the opposing surface  40 B of the cushion bearing  40  opposing the piston  30 . Instead of this configuration, the check seal  60  may be provided between the inner circumference of the piston  30  and the outer circumference of the piston rod  10 . In other words, an accommodating groove may be formed in the inner circumference of the piston  30 . In this case, it is possible to prevent the working oil from being guided over the entirety of the annular gap  70  without providing an accommodating groove so as to open at the opposing surface  30 A of the piston  30  opposing the cushion bearing  40 . As described above, even in a case in which the check seal  60  is provided in the accommodating groove formed in the inner circumference of the piston  30 , the similar effects as those of the above-mentioned embodiment can be afforded. 
     This application claims priority based on Japanese Patent Application No.2015-24358 filed with the Japan Patent Office on Feb. 10, 2015, the entire contents of which are incorporated into this specification.