Abstract:
The present invention relates to a fluidic cylinder. This fluidic cylinder is configured in such a manner that a piston unit is received in an axially displaceable manner within a cylinder tube formed in a rectangular cross-sectional shape. The piston unit has: a base body having the front end of a piston rod staked thereto; a wear ring having the base body received therein and having a magnet incorporated therein; and piston packing adjacent to the wear ring. The piston unit is integrally held at one end of the piston rod. The wear ring and the piston packing are formed in a rectangular cross-sectional shape corresponding to the rectangular cross-sectional shape of the cylinder tube and are provided rotatable relative to the piston rod.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a fluid pressure cylinder (fluidic cylinder) for displacing a piston in an axial direction under operation of supplying pressurized fluid. 
       BACKGROUND ART 
       [0002]    Conventionally, as means for transporting a workpiece, etc., a fluid pressure cylinder having a piston displaced under operation of supplying pressurized fluid has been used. 
         [0003]    For example, Japanese Laid-Open Patent Publication No. 06-235405 discloses a fluid pressure cylinder of this type. The fluid pressure cylinder includes a cylindrical cylinder tube, a cylinder cover provided at one end of the cylinder tube, and a piston provided in a displaceable manner inside the cylinder tube. Further, each of the piston and the cylinder tube has a non-circular shape in cross section perpendicular to the axial line. In the structure, in comparison with the case of using a piston having a circular shape in cross section, the pressure receiving surface area is increased, and the outputted thrust force is increased. 
         [0004]    Further, Japanese Laid-Open Patent Publication No. 2011-508127 (PCT) discloses a cylinder apparatus including a piston having a square shape in cross section. The cylinder apparatus includes a cylinder housing also having a square shape in cross section corresponding to the cross sectional shape of the piston. Sealing members are provided through grooves at outer marginal portions of the piston. The sealing members contact inner wall surfaces of the cylinder housing to perform sealing operation. 
       SUMMARY OF INVENTION 
       [0005]    In the fluid pressure cylinders having a non-circular piston as disclosed in Japanese Laid-Open Patent Publication No. 06-235405 and Japanese Laid-Open Patent Publication No. 2011-508127 (PCT), there is a demand to achieve further reduction of the longitudinal dimension in the axial direction. 
         [0006]    A general object of the present invention is to provide a fluid pressure cylinder in which it is possible to increase a thrust force, and achieve reduction in a longitudinal dimension. 
         [0007]    In order to achieve the above object, the present invention provides a fluid pressure cylinder including a cylindrical cylinder tube including an internal cylinder chamber, a pair of cover members attached to both ends of the cylinder tube, a piston provided in a displaceable manner along the cylinder chamber, and a piston rod coupled to the piston. 
         [0008]    Each of the piston and the cylinder tube is formed to have a rectangular shape in cross section, the piston includes a wear ring configured to slide on an inner wall surface of the cylinder tube, and a magnet is provided in the wear ring. 
         [0009]    In the present invention, each of the piston and the cylinder tube of the fluid pressure cylinder has a rectangular shape in cross section. The piston includes the wear ring which slides on the inner wall surface of the cylinder tube, and the magnet is provided in the wear ring. In the structure, in comparison with a fluid pressure cylinder where a wear ring and a magnet are arranged in alignment in an axial direction on an outer circumferential surface of a piston, it is possible to reduce the size in the axial direction in which the piston is displaced. Consequently, by providing the piston having the rectangular shape in cross section to achieve a large pressure receiving surface area, it becomes possible to obtain a larger thrust force, and reduce a longitudinal size of the fluid pressure cylinder including the piston. 
         [0010]    The above object, features, and advantages will be readily understood from the following embodiments which will be described with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is an overall cross sectional view of a fluid pressure cylinder according to a first embodiment of the present invention; 
           [0012]      FIG. 2  is a front view of the fluid pressure cylinder, as viewed from a rod cover of the fluid pressure cylinder in  FIG. 1 ; 
           [0013]      FIG. 3  is an enlarged cross sectional view showing an area around a piston unit of the fluid pressure cylinder in  FIG. 1 ; 
           [0014]      FIG. 4A  is a front view of the fluid pressure cylinder as viewed from the head cover; 
           [0015]      FIG. 4B  is a front view of the fluid pressure cylinder, showing a modified embodiment in which a method of swaging a cylinder tube against the head cover is changed; 
           [0016]      FIG. 5  is a perspective view showing an outer appearance of a piston rod and a piston unit of the fluid pressure cylinder in  FIG. 1 ; 
           [0017]      FIG. 6  is an exploded perspective view of piston unit shown in  FIG. 5 ; 
           [0018]      FIG. 7  is a cross sectional view taken along a line VII-VII in  FIG. 1 ; 
           [0019]      FIG. 8  is a front view of a piston packing; 
           [0020]      FIG. 9  is an enlarged cross sectional view of an area around an outer marginal portion of the piston packing in  FIG. 3 ; 
           [0021]      FIG. 10  is an enlarged cross sectional view of an area around a head cover, showing a modified embodiment in which a swage portion swaged by a head cover is further swaged by a cover portion; 
           [0022]      FIG. 11A  is a front view of a piston packing according to a modified embodiment; 
           [0023]      FIG. 11B  is a cross sectional view taken along a line XIB-XIB in  FIG. 11A ; 
           [0024]      FIG. 12  is an overall cross sectional view of a fluid pressure cylinder according to a second embodiment of the present invention; 
           [0025]      FIG. 13  is an enlarged cross sectional view showing an area around a head cover of the fluid pressure cylinder in  FIG. 12 ; 
           [0026]      FIG. 14  is a partially exploded perspective view showing a state in which the head cover shown in  FIG. 13  is detached from a cylinder tube; 
           [0027]      FIG. 15A  is a perspective view showing an outer appearance of a stopper ring according to a first modified embodiment; 
           [0028]      FIG. 15B  is a perspective view showing an outer appearance of a stopper ring according to a second modified embodiment; 
           [0029]      FIG. 15C  is an exploded perspective view of stopper means including a plurality of plates and a tightening bolt; 
           [0030]      FIG. 15D  is an enlarged cross sectional view showing an area around a head cover in a state where the head cover is stopped by the stopper means in  FIG. 15C ; 
           [0031]      FIG. 16  is an overall cross sectional view showing a fluid pressure cylinder according to a third embodiment of the present invention; 
           [0032]      FIG. 17  is an enlarged cross sectional view showing an area around a rod cover of the fluid pressure cylinder in  FIG. 16 ; and 
           [0033]      FIG. 18  is a partially exploded perspective view showing a state in which a rod cover shown in  FIG. 17  is detached from a cylinder tube. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    In  FIG. 1 , a reference numeral  10  denotes a fluid pressure cylinder according to a first embodiment of the present invention. As shown in  FIG. 1 , the fluid pressure cylinder includes a cylinder tube  12  having a rectangular shape in cross section, a head cover (cover member)  14  attached to one end of the cylinder tube  12 , a rod cover (cover member)  16  attached to the other end of the cylinder tube  12 , a piston unit (piston)  18  provided in a displaceable manner inside the cylinder tube  12 , and a piston rod  20  coupled to the piston unit  18 . 
         [0035]    The cylinder tube  12  is a cylindrical body, e.g., made of metal material, and extends with a constant cross sectional area in an axial direction (indicated by arrows A and B). A cylinder chamber  22  is formed in the cylinder tube  12 . The piston unit  18  is placed in the cylinder chamber  22 . 
         [0036]    Further, as shown in  FIG. 2 , a sensor attachment rail  24  is provided outside the cylinder tube  12 . The sensor attachment rail  24  is used for attaching a detection sensor (not shown). This sensor attachment rail  24  has a substantially U-shape opened in a direction away from the cylinder tube  12 . The sensor attachment rail  24  has a predetermined length in the axial direction (indicated by the arrows A and B) of the cylinder tube  12 . The sensor attachment rail  24  is attached to a position adjacent to a corner of the cylinder tube  12  having a rectangular shape in cross section. Further, a detection sensor (not shown) is fixedly attached to the sensor attachment rail  24  for detecting a position of the piston unit  18  in the axial direction. 
         [0037]    As shown in  FIG. 1 , for example, the head cover  14  is made of metal material, and has a substantially rectangular shape in cross section. A connection hole  26  having a predetermined depth is formed at the center of the head cover  14 . The connection hole  26  faces the cylinder tube  12  (in the direction indicated by the arrow A). A first damper  28  is attached to the head cover  14  around the outer circumferential side of the connection hole  26  through a groove formed at an end of the head cover  14 . For example, the first damper  28  is made of elastic material, and has a ring shape. An end of the first damper  28  protrudes slightly from the end of the head cover  14  toward the cylinder tube  12  (in the direction indicated by the arrow A). 
         [0038]    A first fluid port  30  is formed on a side surface of the head cover  14 . Pressurized fluid is supplied/discharged through the first fluid port  30 . The first fluid port  30  is connected to the connection hole  26 . Thus, after pressurized fluid is supplied from a pressurized fluid supply source (not shown) to the first fluid port  30 , the pressurized fluid flows into the connection hole  26 . 
         [0039]    Further, an annular first engagement groove  32  depressed inward is provided along an outer circumferential surface, on a side surface of the head cover  14 , at an end closer to the cylinder tube  12  (in the direction indicated by the arrow A) from the first fluid port  30 . Then, one end of the cylinder tube  12  is pressed inward (toward the head cover  14 ), and deformed to be engaged with the first engagement groove  32  as a caulking or swage portion  12   a . Thus, one end of the cylinder tube  12  and the head cover  14  are coupled together through the swage portion  12   a . Further, a seal member  34   a  provided on a side surface of the head cover  14  contacts an inner surface of the cylinder tube  12 . Thus, leakage of the pressurized fluid through the space between the head cover  14  and the cylinder tube  12  is prevented. 
         [0040]    In this regard, for example, as shown in  FIG. 3 , the swage portion  12   a  of the cylinder tube  12  is bent inward from the axial direction (indicated by the arrows A and B) of the cylinder tube  12  at an inclination angle θ in a range of 45° to 90°. The opening dimension D of the swage portion  12   a  which is perpendicular to the axial line of the cylinder tube  12  is determined to become smaller than the outer dimension D′ of the cylinder tube  12  by 3% to 10%. Stated otherwise, the opening dimension is determined in a manner that the depth of the swage portion  12   a  toward the cylinder tube  12  reaches a position where the opening dimension D becomes smaller than the outer dimension D′ of the cylinder tube  12  by 3% to 10%. 
         [0041]    Further, the swage portion  12   a  is formed over the entire outer circumference of the head cover  14  by rolling swaging (see  FIG. 4A ). 
         [0042]    It is not essential that the swage portion  12   a  is formed in an annular shape over the entire circumference of the cylinder tube  12 . For example, as in the case of a swage portion  12   a ′ shown in  FIG. 4B , the swage portion  12   a  may have a substantially straight line shape in cross section, and swaged against a first engagement groove  32   a  of the head cover  14  in a manner that the swage portion  12   a  is engaged with only four sides of the cylinder tube  12  having a rectangular shape in cross section. 
         [0043]    As in the case of the head cover  14 , the rod cover  16  is made of metal material, and has a substantially rectangular shape in cross section. A rod hole  36  passes through the center of the rod cover  16  in an axial direction (indicated by the arrows A and B). A rod packing  38  and a bush  40  are provided in an inner circumferential surface of the rod hole  36  through respective annular grooves. When the piston rod  20  is inserted into the rod hole  36 , the rod packing  38  slides on the outer circumferential surface of the piston rod  20 . Thus, leakage of the pressurized fluid through the space between the rod cover  16  and the piston rod  20  is prevented. The bush  40  slides on the outer circumferential surface in a manner that the piston rod  20  is guided in the axial direction (indicated by the arrows A and B). 
         [0044]    Further, as shown in  FIG. 2 , attachment holes  42  each having a predetermined depth in the axial direction are formed near four corners of an end surface of the rod cover  16 . For example, at the time of fixing the fluid pressure cylinder to another device (not shown), etc., fixing bolts inserted into the other device are screwed into the attachment holes  42  of the rod cover  16  to fix the fluid pressure cylinder. 
         [0045]    As shown in  FIG. 1 , a second fluid port  44  is provided on a side surface of the rod cover  16 , for supplying/discharging the pressurized fluid through the second fluid port  44 . The second fluid port  44  is connected to the cylinder chamber  22  through a connection channel  46  extending in the axial direction (indicated by the arrow B) of the rod cover  16 . The pressurized fluid supplied from the second fluid port  44  flows from the connection channel  46  into the cylinder chamber  22 . 
         [0046]    Further, an annular second engagement groove  48  depressed inward is provided along an outer circumferential surface, i.e., on a side surface of the rod cover  16 , at an end closer to the cylinder tube  12  (in the direction indicated by the arrow B) from the second fluid port  44 . Then, the other end of the cylinder tube  12  is pressed inward (toward the rod cover  16 ), and deformed to be engaged with the second engagement groove  48  as a caulking or swage portion  12   b . Thus, the other end of the cylinder tube  12  and the rod cover  16  are coupled together through the swage portion  12   b . Further, a seal member  34   b  provided on the side surface of the rod cover  16  contacts an inner surface of the cylinder tube  12 . Thus, leakage of the pressurized fluid through the space between the rod cover  16  and the cylinder tube  12  is prevented. 
         [0047]    In this regard, as in the case of the swage portion  12   a  at the one end, the swage portion  12   b  of the cylinder tube  12  is bent inward from the axial direction (indicated by the arrows A and B) of the cylinder tube  12  at an inclination angle θ in a range of 45° to 90°. The opening dimension D of the swage portion  12   b  is determined to become smaller than the outer dimension D′ of the cylinder tube  12  by 3% to 10% (0.9 to 0.97 D′). Further, the swage portion  12   b  is formed over the entire outer circumference of the rod cover  16  by rolling swaging. 
         [0048]    That is, the swage portion  12   a  at the one end of the cylinder tube  12  and the swage portion  12   b  at the other end of the cylinder tube  12  have substantially the same shape, and are engaged with the head cover  14  and the rod cover  16 , respectively. 
         [0049]    It should be noted that the cylinder tube  12  may be coupled to the head cover  14  and the rod cover  16  by, e.g., welding, adhesion, etc. instead of swaging. 
         [0050]    As shown in  FIGS. 1, 3, 5, and 6 , the piston unit  18  is provided at one end of the piston rod  20 , and includes a base body (coupling body)  50 , a wear ring  52  provided around the base body  50 , a piston packing  54  adjacent to the wear ring  52 , a plate body  56  adjacent to the piston packing  54 , and a second damper  58  provided adjacent to the plate body  56 , at the position closest to the other end of the piston rod  20  (in the direction indicated by the arrow A). 
         [0051]    For example, the base body  50  is made of metal material, and has a circular disk shape. A swaging hole  60  is formed at the center of the base body  50 . One end of the piston rod  20  is inserted into the swaging hole  60  for caulking or swaging. The diameter of the swaging hole  60  is gradually increased toward one end of the piston unit  18  (in the direction indicated by the arrow B). The diameter at one end of the piston rod  20  is increased in correspondence with the shape of the swaging hole  60  to limit the relative displacement in the axial direction (indicated by the arrows A and B). In this state, the base body  50  and the piston rod  20  are coupled together integrally. 
         [0052]    Further, as shown in  FIG. 3 , the base body  50  has one end having a planar shape perpendicular to the axial line. A first projection  62  protruding toward the adjacent wear ring  52  and a second projection  64  protruding beyond the first projection  62  (in the direction indicated by the arrow A) are formed at the other end of the base body  50 . Each of the first projection  62  and the second projection  64  has a circular shape in cross section. The diameter of the second projection  64  is smaller than the diameter of the first projection  62 . Further, a ring shaped gasket (seal member)  66  is attached to the outer circumferential surface of the first projection  62  through an annular groove. 
         [0053]    For example, the wear ring  52  is made of resin material, and has a substantially rectangular shape in cross section. The outer shape of the wear ring  52  is substantially the same as the cross sectional shape of the cylinder chamber  22 . An attachment hole  68  is formed at the center of the wear ring  52  for attaching the base body  50  to the attachment hole  68 . A pair of magnet holes  72  are formed in one end surface of the wear ring  52 , as one end of the piston unit  18  (in the direction indicated by the arrow B) for attaching magnets  70  to the magnet holes  72 . The attachment hole  68  passes through the wear ring  52  in the thickness direction (indicated by the arrows A and B). 
         [0054]    The diameter of the attachment hole  68  is formed stepwise to have different diameters in the axial direction (indicated by arrows A and B), and the first projection  62  and the second projection  64  of the base body  50  are engaged with the attachment hole  68 . Thus, the base body  50  is placed and held at the center of the attachment hole  68 . In this regard, one end surface of the base body  50  does not protrude from one end surface of the wear ring  52 . That is, these surfaces form the same plane surface (see  FIG. 3 ). 
         [0055]    For example, the magnet holes  72  are formed at pair of corners positioned diagonally with respect to the attachment hole  68  at the center. Each of the magnet holes  72  is opened at one end surface side of the wear ring  52 , and has a circular cross sectional shape to have a predetermined depth. As shown in  FIGS. 2 and 5 , the magnets  70  are inserted into the magnet holes  72 , and for example, fixed using adhesive, etc. 
         [0056]    Since the magnets  70  are thinner than the wear ring  52 , in the state where the magnets  70  are placed in the magnet holes  72 , the magnets  70  are provided in the wear ring  52  without protruding from the end surface of the wear ring  52 . 
         [0057]    Further, as shown in  FIG. 2 , in the state where the wear ring  52  containing the magnets  70  is placed in the cylinder tube  12 , the sensor attachment rail  24  is provided at a position adjacent to a corner of the cylinder tube  12  facing the magnets  70 , i.e., a corner of the cylinder tube  12  that is close to one of the magnets  70 . 
         [0058]    As shown in  FIGS. 3, 8, and 9 , the piston packing  54  is made of elastic material such as rubber, and has a rectangular shape in cross section. Annular lubricant retention grooves  76  are formed adjacent to outer marginal portions at one end and the other end of the piston packing  54 . The lubricant retention grooves  76  are formed at one surface of the piston packing  54  closer to the wear ring  52  (in the direction indicate by the arrow B) and the other end surface of the piston packing  54  closer to the plate body  56  (in the direction indicated by the arrow A). The lubricant retention grooves  76  are depressed to have a predetermined depth in the thickness directions (indicated by the arrows A and B) of the piston packing  54  in parallel at predetermined intervals. The number of the lubricant detection grooves  76  is, e.g., three. 
         [0059]    Further, lubricant such as grease is retained in the lubricant retention grooves  76 , and when the piston unit  18  moves in the axial direction (indicated by arrows A and B) along the cylinder tube  12 , the lubricant is supplied to the inner wall surface of the cylinder tube  12  for lubrication between the piston unit  18  and the cylinder tube  12 . 
         [0060]    A packing hole  78  is opened at the center of the piston packing  54 . The piston packing  54  is inserted into a recess  80  formed on the other end surface of the wear ring  52  through the packing hole  78 . Thus, the piston packing  54  is attached to the wear ring  52  in a manner that the other end surface of the piston packing  54  and the other end surface of the wear ring  52  forms substantially the same plane surface (see  FIG. 3 ). 
         [0061]    The plate body  56  is made of metal material, and is a thin plate having a substantially rectangular shape in cross section. An insertion hole  82  is opened at the center of the plate body  56 . The second projection  64  of the base body  50  is inserted into the insertion hole  82 . 
         [0062]    As show in  FIGS. 1, 5, and 6 , the piston rod  20  comprises a shaft body having a predetermined length in the axial direction (indicated by the arrows A and B). The piston rod  20  includes a body portion  84  formed to have a substantially constant diameter, and a small diameter front end portion  86  formed at one end of the body portion  84 . A border between the front end portion  86  and the body portion  84  are formed stepwise, and the piston unit  18  is supported by the front end portion  86 . 
         [0063]    Further, as shown in  FIG. 1 , the other end of the piston rod  20  is inserted into the rod hole  36  of the rod cover  16 , and the piston rod  20  is supported by the bush  40  provided in the rod hole  36  in a displaceable manner in the axial direction (indicated by the arrows A and B). 
         [0064]    The base body  50  is inserted into the attachment hole  68  from one end surface side of the wear ring  52 , and the plate body  56  is brought into contact with the other end surface of the wear ring  52  to which the piston packing  54  has been attached. In this state, the piston rod  20  is inserted from the plate body  56  into the swaging hole  60  of the base body  50 . In the state where the plate body  56  contacts an end of the body portion  84  of the plate body  56 , the front end portion  86  is crushed by a swaging jig, etc. to increase its diameter. Thus, a coupling portion  88  having the increased diameter is engaged with the swaging hole  60 . 
         [0065]    As a result, as shown in  FIG. 5 , the piston unit  18  is held between the coupling portion  88  (front end portion  86 ) and the body portion  84  of the piston rod  20 . In this regard, in the space between the coupling portion  88  and the body portion  84 , small clearance is formed among the base body  50 , the wear ring  52 , and the plate body  56  in the axial direction (indicated by the arrows A and B). Therefore, the wear ring  52 , the piston packing  54 , and the plate body  56  are rotatably held around the piston rod  20 . 
         [0066]    Further, in the case of limiting rotation of the wear ring  52  and the plate body  56  relative to the piston rod  20 , for example, the plate body  56  and the first projection  62  in the wear ring  52  are designed to have large thickness for allowing the base body  50 , the wear ring  52 , and the plate body  56  to contact together tightly without any clearance among these components. Thus, rotation of the wear ring  52  and the plate body  56  relative to the piston rod  20  is limited, and the piston rod  20  and the piston unit  18  can be fixed together integrally. That is, this structure is suitable in the case where rotation of the piston rod  20  relative to the piston unit  18  is not preferable. 
         [0067]    The fluid pressure cylinder  10  according to the first embodiment of the present invention basically has the above structure. Next, operation and working effects of the fluid pressure cylinder  10  will be described below. In the following description, a state where the piston unit  18  is displaced toward the head cover  14  (in the direction indicated by the arrow B) will be referred to as the initial position ( FIG. 1 ). 
         [0068]    Firstly, the pressurized fluid is supplied into the first fluid port  30  from a pressurized fluid supply source (not shown). In this case, the second fluid port  44  is opened to the atmospheric air by the switching operation of a switching valve (not shown). Thus, the pressurized fluid is supplied from the first fluid port  30  to the connection hole  26 , and the piston unit  18  is pressed toward the rod cover  16  (in the direction indicated by the arrow A) by the pressurized fluid supplied from the connection hole  26  into the cylinder chamber  22 . By the displacement operation of the piston unit  18 , the piston rod  20  is displaced as well. When the second damper  58  contacts the rod cover  16 , the piston unit  18  is stopped at the displacement end position. 
         [0069]    In the case where the piston unit  18  is displaced in the direction (indicated by the arrow B) opposite to the above direction, the pressurized fluid is supplied to the second fluid port  44 , and the first fluid port  30  is opened to the atmospheric air by switching operation of the switching valve (not shown). Then, the pressurized fluid is supplied from the second fluid port  44  to the cylinder chamber  22  through the connection channel  46 . The piston unit  18  is pressed toward the head cover  14  (in the direction indicated by the arrow B) by the pressurized fluid supplied into the cylinder chamber  22 . 
         [0070]    Then, by displacement operation of the piston unit  18 , the piston rod  20  is displaced as well. When the base body  50  of the piston unit  18  contacts the first damper  28  of the head cover  14 , the piston unit  18  returns to the initial position (see  FIG. 1 ). 
         [0071]    As described above, in the first embodiment, the piston unit  18  of the fluid pressure cylinder  10  has a rectangular shape in cross section. The cylinder tube  12  containing the piston unit  18  has a rectangular shape in cross section corresponding to the piston unit  18 . Thus, in comparison with the case of the fluid pressure cylinder equipped with a piston having a circular shape in cross section, when the diameter of the piston having a circular cross section and the length of one side of the piston unit  18  are substantially the same, it is possible to achieve the sufficient pressure receiving surface area. Consequently, it is possible to increase the thrust force in the fluid pressure cylinder  10 , drive the piston unit  18  by supplying the pressurized fluid into the cylinder chamber  22  at low pressure, and save the energy by reducing the quantity of consumed pressurized fluid. 
         [0072]    Further, the piston unit  18  includes the wear ring  52  which slides on the inner wall surface of the cylinder tube  12  for guidance in the axial direction (indicated by the arrows A and B), and the magnets  70  can be provided inside the wear ring  52 . In the structure, in comparison with the case where the wear ring  52  and the magnets  70  are provided in alignment in the axial direction in the outer circumferential surface of the piston, since the size of the piston unit  18  in the axial direction is suppressed, it is possible to achieve size reduction of the fluid pressure cylinder  10 . 
         [0073]    Further, the magnets  70  are provided for the wear ring  52  having a rectangular shape in cross section which does not rotate in the cylinder tube  12 . In the structure, the magnet  70  does not need to have a ring shape for the piston having a circular shape in cross section which might be rotated inside the cylinder tube  12 . As a result, it is possible to reduce the size of the magnets  70 , and reduce the production cost. Stated otherwise, since there is no need to use ring shaped magnets  70 , it is possible to reduce the volume of the magnets  70 . 
         [0074]    Furthermore, since the magnets  70  are provided to face the corners of the cylinder tube  12 , by arranging the sensor attachment rail  24  for attaching the detection sensor at a position adjacent to the corner, it is possible to reliably detect the magnetism of the magnets  70  by the detection sensor. 
         [0075]    Further, the wear ring  52 , the piston packing  54 , and the plate body  56  of the piston unit  18  are rotatable relative to the piston rod  20 . Thus, for example, at the time of assembling a transportation table, etc. to the other end of the piston rod  20  by screw engagement or the like, the assembling operation can be performed easily by rotating the piston rod  20 . Thus, even in the case where the fluid pressure cylinder  10  is fixed to another apparatus and cannot be rotated, assembling can be performed efficiently. 
         [0076]    Further, the wear ring  52 , the piston packing  54 , and the plate body  56  of the piston unit  18  are rotatable relative to the piston rod  20 . Thus, even in the case where a load is applied to the piston rod  20  in a direction to rotate the piston unit  18 , by only rotating the piston rod  20  relative to the wear ring  52  and the piston packing  54 , it is possible to avoid application of the load to the wear ring  52  and the piston packing  54  in the rotation direction. As a result, increase in the stress by the contact between the corners and the cylinder tube  12  which may be caused when a load in the rotation direction is applied to the wear ring  52  and the piston packing  54  is prevented, and abrasion of the wear ring  52  and the piston packing  54  is suppressed. Consequently, improvement in the durability is achieved. 
         [0077]    Further, in the above described piston unit  18 , the wear ring  52 , the piston packing  54 , and the plate body  56  are provided rotatably with respect to the piston rod  20 . However, the present invention is not limited in this respect. For example, the wear ring  52 , the piston packing  54 , and the plate body  56  may be fixed to contact one another in the axial direction to limit rotation of the piston rod  20  with respect to the wear ring  52 , the piston packing  54 , and the plate body  56 . That is, depending on the application of the fluid pressure cylinder  10 , it is possible to selectively use the fluid pressure cylinder  10  based on whether or not rotation of the piston rod  20  with respect the piston unit  18  is allowable. 
         [0078]    Further, the inclination angle θ of the swage portions  12   a ,  12   b  swaged against the head cover  14  and the rod cover  16  is determined in a range of 45° to 90° (45°≦θ≦90°) toward the inner circumferential side from the axial direction (indicated by the arrows A and B) of the cylinder tube  12 . Thus, it is possible to couple the cylinder tube  12  to the head cover  14  and the rod cover  16  reliably and firmly. 
         [0079]    Further, at the time of swaging the swage portion  12   a  of the cylinder tube  12  to the head cover  14 , for example, as shown in  FIG. 10 , after the swage portion  12   a  is engaged with the first engagement groove  32 , the head cover  14  adjacent to the first engagement groove  32  may be deformed by pressing the head cover  14  from the outer circumferential side by a jig, etc. (not shown) and form a cover portion  90  partially covering the swage portion  12   a  for further swaging. 
         [0080]    In this manner, by pressing the swage portion  12   a  by the cover portion  90 , the swaging strength of the swage portion  12   a  against the head cover  14  is increased. Consequently, it becomes possible to further increase the coupling strength of the cylinder tube  12  and the head cover  14 . 
         [0081]    It is not essential that this cover portion  90  is provided for the head cover  14 . By forming the cover portion  90  on the rod cover  16  side, the swage portion  12   b  of the cylinder tube  12  may be swaged against the rod cover  16  reliably and firmly. 
         [0082]    Further, as in the case of a piston packing  92  shown in  FIG. 11A , a packing hole  94  formed at the center may have a rectangular shape in cross section like the outer shape of the piston packing  92 . In this case, the recess  80  of the wear ring  52  is also formed in a rectangular shape in cross section. In this manner, by forming the packing hole  94  to have a rectangular shape in cross section, the width E of the piston packing  92  from the packing hole  94  to the outer marginal portion can be kept substantially constant in the circumferential direction of the piston packing  92 . Therefore, it is possible to achieve the uniform surface pressure when the piston packing  92  contacts the cylinder tube  12 . 
         [0083]    As a result, uniform seal function is achieved between the piston packing  92  and the cylinder tube  12  in the circumferential direction of the piston packing  92 . Specifically, ideally, the inner radius R of each corner  96  is determined to satisfy the relationship that the ratio S 1 /S 2  is greater than 1.1, and less than 1.25 (1.1&lt;S 1 /S 2 &lt;1.25), where S 1  denotes the circumferential length of the packing hole  94  having a rectangular shape in cross section, and S 2  denotes the length of the circumference of a virtual circle F inscribed in the packing hole  94 . 
         [0084]    Further, as shown in  FIG. 11B , in the piston packing  92 , one end surface and the other surface each having the lubricant retention grooves  76  are tapered with inclination to get closer to each other toward the outer marginal portion. Stated otherwise, the piston packing  92  gets thinner gradually toward the outer marginal portion. As described above, by reducing the thickness of the outer marginal portion of the piston packing  92 , it becomes possible to achieve the uniform contact surface pressure in the contact between the piston packing  92  and the cylinder tube  12 , improve the sealing performance, and reduce the sliding resistance during movement of the piston unit  18 . 
         [0085]    Next, a fluid pressure cylinder  100  according to a second embodiment will be described with reference to  FIGS. 12 to 14 . The constituent elements of the fluid pressure cylinder  100  that are identical to those of the fluid pressure cylinder  10  according to the above described first embodiment are labeled with the same reference numerals, and detailed description thereof is omitted. 
         [0086]    The fluid pressure cylinder  100  according to the second embodiment is different from the fluid pressure cylinder  10  according to the first embodiment in that a head cover  102  is provided detachably at one end of the cylinder tube  12  through a stopper ring  104 . 
         [0087]    For example, in this fluid pressure cylinder  100 , as shown in  FIGS. 12 and 13 , a cylindrical body  106  is connected to one end of the cylinder tube  12 . The diameter of the cylindrical body  106  is larger than the diameter of the cylinder tube  12 . For example, the cylindrical body  106  is made of metal material such as stainless steel, and formed in a rectangular shape in cross section. The cylindrical body  106  has a predetermined width in the axial direction (indicated by the arrows A and B). Then, in the state where the inner circumferential surface at one end of the cylindrical body  106  contacts the outer circumferential surface of the cylinder tube  12 , the cylindrical body  106  and the cylinder tube  12  are joined together by welding, adhesion, or the like. 
         [0088]    That is, the cylindrical body  106  is partially overlapped with one end of the cylinder tube  12  in the axial direction (indicated by the arrows A and B), and the inside of the cylindrical body  106  is formed stepwise. 
         [0089]    Further, an annular ring groove  108  depressed toward the outer circumferential side is formed in the inner circumferential surface of the cylindrical body  106 , and the stopper ring  104  described later is engaged with the ring groove  108 . 
         [0090]    Further, a hole  110  passes through the cylindrical body  106  in the radial direction, between a connector portion connected to the cylinder tube  12  and the ring groove  108 . Then, when the head cover  102  is placed inside the cylindrical body  106 , the first fluid port  30  of the head cover  102  becomes coaxial with, and is connected to the hole  110  of the cylindrical body  106 , and a joint or the like (not shown) is connected to the first fluid port  30  through the hole  110 . 
         [0091]    As shown in  FIG. 14 , for example, the stopper ring  104  is made of metal material, and has a substantially octagonal shape in cross section. The stopper ring  104  is configured to apply an elastic force radially outwardly. Jig holes  112  are formed at expanding portions of the open ends of the stopper ring  104  expanding inward in the radial direction. 
         [0092]    Then, by inserting jigs (not shown) to the pair of jig holes  112  of the stopper ring  104 , and displacing the expanding portions having the jig holes  112  toward each other, the stopper ring  104  can be deformed elastically inward in the radial direction, in opposition to the elastic force. 
         [0093]    The head cover  102  is inserted into the cylinder tube  12  and the cylindrical body  106 , and contacts one end of the cylinder tube  12 , and is positioned in the axial direction (indicated by the arrow A). In this state, the stopper ring  104  is engaged with the ring groove  108 . In this manner, the stopper ring  104  is fixed in the state where the stopper ring  104  contacts the end surface of the head cover  102 . Detachment of the head cover  102  from the opening of the cylindrical body  106  is prevented. 
         [0094]    As described above, in the fluid pressure cylinder  100  according to the second embodiment of the present invention, the cylindrical body  106  is provided at one end of the cylinder tube  12 , and in the state where the head cover  102  is placed inside the cylindrical body  106 , the stopper ring  104  is engaged with, and fixed to the ring groove  108  of the cylindrical body  106 . In the structure, by providing the stopper ring  104  detachably for the cylindrical body  106 , it is possible to attach the head cover  102  to, or detach the head cover  102  from the cylinder tube  12  easily and reliably. As a result, in the fluid pressure cylinder  100 , since the head cover  102  can be disassembled, for example, maintenance operation such as replacement of the piston packing  54  or the rod packing  38  can be performed easily. 
         [0095]    Further, the present invention is not limited to the case where the stopper ring  104  has a substantially octagonal ring shape as described above. For example, as shown in  FIG. 15A , a stopper ring  104   a  having a substantially rectangular ring shape in cross section may be adopted. Alternatively, as shown in  FIG. 15B , a stopper ring  104   b  having a substantially hexagonal ring shape in cross section may be adopted. 
         [0096]    Further, instead of using the stopper ring  104 , stopper means  118  made up of four division plates  114   a  to  114   d  and a tightening bolt  116  shown in  FIG. 15C  may be used to fix the head cover  102  inside the cylindrical body  106 . 
         [0097]    The division plates  114   a  to  114   d  have substantially the same rectangular shape. Cutout portions  120  cut in a circular arc shape are formed at corners of the division plates  114   a  to  114   d , respectively. 
         [0098]    The tightening bolts  116  includes a threaded portion  122  where screw threads are engraved, an increased diameter portion  124  provided at an end of the threaded portion  122 , and a head portion  126 . The diameter of the increased diameter portion  124  is larger than the diameter of the threaded portion  122 , and the diameter of the head portion  126  is larger than the increased diameter portion  124 . The threaded portion  122  is screwed into a screw hole  128  formed at an end surface of the head cover  102  (see  FIG. 15D ). 
         [0099]    In the case of fixing the head cover  102  by the stopper means  118 , as shown in  FIG. 15D , in the state where the head cover  102  is placed inside the cylindrical body  106 , each of the division plates  114   a  to  114   d  are brought into contact with the end surface of the head cover  102  such that the cutout portions  120  of the division plates  114   a  to  114   d  face the screw hole  128 . Further, the division plates  114   a  to  114   d  are moved in directions away from the screw hole  128  along the end surface, to insert the outer marginal portions of the division plates  114   a  to  114   d  into the ring groove  108 . 
         [0100]    That is, by arranging the division plates  114   a  to  114   d , substantially a circular hole is formed at the center by the cutout portions  120  of the division plates  114   a  to  114   d.    
         [0101]    Next, the threaded portion  122  of the tightening bolt  116  is screwed into the screw hole  128  through the cutout portions  120  formed in the circular shape. Consequently, the increased diameter portion  124  is brought into contact with the inner surfaces of the cutout portions  120  to limit movement of the division plates  114   a  to  114   d  toward the screw hole  128 , and end surfaces of the division plates  114   a  to  114   d  are pressed by the head portion  126 , and sandwiched and held between the head portion  126  and the end surface of the head cover  102 . 
         [0102]    Thus, in the state where the division plates  114   a  to  114   d  are engaged with the ring groove  108 , the division plates  114   a  to  114   d  are fixed to the end surface of the head cover  102  by the tightening bolt  116 . Thus, the head cover  102  is fixed inside the cylindrical body  106 . Further, by rotating the tightening bolt  116  for detaching the division plates  114   a  to  114   d , it becomes possible to unlock the fixed head cover  102  easily. 
         [0103]    Further, though the fluid pressure cylinder  100  has been described in connection with the structure where the head cover  102  is provided detachably for the cylinder tube  12 , instead of the head cover  102 , the rod cover  16  may be provided detachably for the cylinder tube  12  using the stopper rings  104 ,  104   a ,  104   b  or the stopper means  118 . 
         [0104]    Next, a fluid pressure cylinder  150  according to a third embodiment will be described with reference to  FIGS. 16 to 18 . The constituent elements of the fluid pressure cylinder  150  that are identical to those of the fluid pressure cylinders  10 ,  100  according to the first and second embodiments are labeled with the same reference numerals, and description thereof is omitted. 
         [0105]    The fluid pressure cylinder  150  according to the third embodiment are different from the fluid pressure cylinders  10 ,  100  according to the first and second embodiments in that a rod cover  152  is detachably provided at the other end of the cylinder tube  12  using a plurality of fixing bolts  154 . 
         [0106]    For example, in the fluid pressure cylinder  150 , as shown in  FIGS. 16 to 18 , a pair of holes  156  are formed in an upper surface, and a pair of holes  156  are formed in a lower surface, at the other end of the cylinder tube  12 , and bolt holes  158  are formed in the rod cover  152  inserted into the cylinder tube  12 . The fixing bolts  154  are screwed into the bolt holes  158 , and the bolt holes  158  face the holes  156 . 
         [0107]    For example, each of the fixing bolts  154  includes a head portion having a hexagonal socket (recess)  160 . In the state where the rod cover  152  is placed inside the cylinder tube  12 , the fixing bolts  154  are inserted, and screwed into the bolt holes  158  through the holes  156 . In the structure, the fixing bolts  154  are fixed in the state in which a head portion  162  is inserted in the hole  156 , and the head portions  162  each are caught in the holes  156  to limit movement of the cylinder tube  12  and the rod cover  152  in the axial direction. Thus, the cylinder tube  12  and the rod cover  152  are fixed. In this case, the fixing bolts  154  are provided in the holes  156  without protruding to the outside of the cylinder tube  12 . 
         [0108]    Further, the cylinder tube  12  may be fixed by sandwiching the cylinder tube  12  between the head portions  162  of the fixing bolts  154  and the rod cover  152 . 
         [0109]    By removing the fixing bolts  154  screwed into the side surfaces of the rod cover  152 , the rod cover  152  can be removed from the cylinder tube  12  easily. 
         [0110]    As described above, in the fluid pressure cylinder  150  according to the third embodiment of the present invention, a plurality of holes  156  are formed at the other end of the cylinder tube  12  for allowing the fixing bolts  154  to be inserted into the holes  156 . The bolt holes  158  are formed in the rod cover  152  provided in the other end of the cylinder tube  12 . The fixing bolts  154  inserted into the bolt holes  158  through the holes  156  are tightened to fix the other end of the cylinder tube  12  and the rod cover  152  together. In the structure, by rotating the fixing bolts  154 , it is possible to attach the rod cover  152  to, and detach the rod cover  152  from the cylinder tube  12  easily and reliably. Consequently, by allowing the rod cover  152  to be disassembled in the fluid pressure cylinder  150 , for example, maintenance operation such as replacement of the piston packing  54  or the rod packing  38  can be performed easily. 
         [0111]    Further, though the above fluid pressure cylinder  150  has been described in connection with the case where the rod cover  152  is provided detachably for the cylinder tube  12 , instead of the rod cover  152 , the head cover  14 ,  102  may be provided detachably for the cylinder tube  12  using the fixing bolt  154 . 
         [0112]    The fluid pressure cylinder according to the present invention is not limited to the above described embodiments. It is a matter of course that various structures can be adopted without deviating from the gist of the present invention.