Patent Publication Number: US-10316868-B2

Title: Fluid pressure cylinder

Description:
TECHNICAL FIELD 
     The present invention relates to a fluid pressure cylinder which causes a piston to be displaced in an axial direction under the supply of a pressure fluid. 
     BACKGROUND ART 
     As disclosed, for example, in Japanese Laid-Open Utility Model Publication No. 03-044210, the present applicant has proposed a fluid pressure cylinder as a means for transporting a workpiece or the like, the fluid pressure cylinder having pistons that are displaced under the supply of a pressure fluid. 
     The fluid pressure cylinder, for example, includes a cylinder body formed with a wide flat shape, a pair of pistons disposed for displacement in the interior of the cylinder body, piston rods that are connected respectively to the pistons, and a plate that is connected to ends of the piston rods. In addition, by supplying a fluid to cylinder chambers of the cylinder body, the pistons are moved along an axial direction, whereby the plate is moved with respect to the cylinder body in directions to approach toward and separate away from the cylinder body. 
     SUMMARY OF INVENTION 
     With the aforementioned fluid pressure cylinder, there is a demand to further reduce the size and number of components that make up the fluid pressure cylinder. 
     A general object of the present invention is to provide a fluid pressure cylinder in which it is possible to further reduce the size in the longitudinal dimension along the axial direction thereof, as well as to reduce the number of component parts that make up the fluid pressure cylinder. 
     The present invention is characterized by a fluid pressure cylinder that includes a cylinder body including a pair of cylinder chambers to which a pressure fluid is introduced, a pair of pistons disposed displaceably along the cylinder chambers, and an end plate disposed outside of the cylinder body, the end plate being disposed on ends of piston rods that are connected to the pistons. The pistons are moved along the cylinder chambers upon supply of the pressure fluid to the cylinder chambers. 
     In the fluid pressure cylinder, a rod is connected to the end plate substantially in parallel with the direction of movement of the pistons, the rod having a magnet on an outer circumferential surface thereof, and in the interior of the cylinder body, the rod is arranged outside of the cylinder chambers and is moved in the axial direction together with the pistons. 
     According to the present invention, in the fluid pressure cylinder, which includes the cylinder body having the pair of cylinder chambers and the pistons, on the end plate, which is disposed on ends of the piston rods that are connected to the pistons, the rod is disposed substantially in parallel with the direction of movement of the pistons for movement in the axial direction together with the pistons at a location outside of the cylinder chambers. The magnet is provided on the outer circumferential surface of the rod. 
     Consequently, by providing the magnet, which heretofore has been disposed on the pistons in the conventional fluid pressure cylinder, on a rod that is separate from the pistons, in comparison with the conventional fluid pressure cylinder, the pistons can be made smaller in size in the axial direction. Along therewith, while the amount of movement of the pistons in the axial direction is kept the same, the longitudinal dimension in the axial direction of the cylinder body can be suppressed, and thus the fluid pressure cylinder can be made smaller in size. Further, since the position of the pair of pistons can be detected by a single rod on which the magnet is provided, in contrast to the conventional fluid pressure cylinder, in which magnets are provided respectively on the pair of pistons, the number of magnets can be reduced, and thus the number of component parts that make up the fluid pressure cylinder can be reduced. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exterior perspective view of a fluid pressure cylinder according to a first embodiment of the present invention; 
         FIG. 2  is an overall vertical cross-sectional view of the fluid pressure cylinder shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line III-III of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along line IV-IV of  FIG. 2 ; 
         FIG. 5  is a cross-sectional view taken along line V-V of  FIG. 2 ; 
         FIG. 6  is an overall vertical cross-sectional view showing a condition in which an end plate of the fluid pressure cylinder of  FIG. 2  is moved in a direction away from the cylinder body; 
         FIG. 7  is an overall vertical cross-sectional view of a fluid pressure cylinder according to a second embodiment of the present invention; and 
         FIG. 8  is an overall vertical cross-sectional view showing a condition in which an end plate of the fluid pressure cylinder of  FIG. 7  is moved in a direction away from the cylinder body. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As shown in  FIGS. 1 through 4 , a fluid pressure cylinder  10  includes a cylinder body  14  formed with a flattened shape in cross-section and having in the interior thereof a pair of cylinder holes (cylinder chambers)  12   a ,  12   b , a pair of head covers  16  that are mounted in ends of the cylinder holes  12   a ,  12   b , a pair of rod covers  18  mounted in other ends of the cylinder holes  12   a ,  12   b , a pair of pistons  20   a ,  20   b  disposed for displacement along the cylinder holes  12   a ,  12   b , a pair of piston rods  22   a ,  22   b  connected respectively to centers of the pistons  20   a ,  20   b , and an end plate  24  that is connected to ends of the piston rods  22   a ,  22   b.    
     The cylinder body  14  is formed, for example, by extrusion molding from a metal material, and has a pair of main body portions  26   a ,  26   b  that are separated a predetermined distance from each other in a widthwise direction (the direction of the arrow A), and a connecting section  28  that interconnects one of the main body portions  26   a  and another of the main body portions  26   b . More specifically, as shown in  FIGS. 3 and 4 , the cylinder body  14  is formed in a symmetrical shape in which the main body portions  26   a ,  26   b  are formed respectively on both sides in the widthwise direction about the connecting section  28 , which is disposed centrally in the widthwise direction of the cylinder body  14 . 
     The main body portions  26   a ,  26   b  are formed, for example, with substantially rectangular shapes in cross-section, and the cylinder holes  12   a ,  12   b , which are circular in cross-section, penetrate in the axial direction (the direction of arrows B 1 , B 2 ) substantially in the centers of the main body portions  26   a ,  26   b . Further, on side surfaces of the main body portions  26   a ,  26   b , as shown in  FIG. 2 , first side surface ports  30   a ,  30   b  and second side surface ports  32   a ,  32   b  open respectively at positions in the vicinity of one end and the other end of the cylinder body  14 . 
     More specifically, the first side surface port  30   a  and the second side surface port  32   a  are formed as a pair in a side surface on the one main body portion  26   a , and the first side surface port  30   b  and the second side surface port  32   b  are formed as a pair in a side surface on the other main body portion  26   b.    
     As shown in  FIGS. 3 and 4 , the upper surface of the connecting section  28  is formed in a substantially planar shape, and is recessed downwardly at a predetermined depth with respect to upper surfaces of the main body portions  26   a ,  26   b . A pair of sensor attachment grooves  34  is formed substantially in the center in the widthwise direction of the upper surface of the connecting section  28 . The sensor attachment grooves  34  are recessed with respect to the upper surface with substantially semicircular shapes in cross-section, and are formed as straight lines along the axial direction (the direction of arrows B 1 , B 2 ). In addition, detection sensors  36  for detecting positions to which the pistons  20   a ,  20   b  have moved are accommodated respectively in the sensor attachment grooves  34 . 
     Further, first and second upper surface ports  38 ,  40  through which the pressure fluid can be supplied and discharged are formed on the upper surface of the connecting section  28 . As shown in  FIG. 2 , the first upper surface port  38  is disposed on a straight line along a widthwise direction (the direction of the arrow A) connecting the first side surface port  30   a  of one of the main body portions  26   a  and the first side surface port  30   b  of the other of the main body portions  26   b . The second upper surface port  40  is disposed on a straight line along the widthwise direction (the direction of the arrow A) connecting the second side surface port  32   a  of the one main body portion  26   a  and the second side surface port  32   b  of the other main body portion  26   b.    
     More specifically, the pair of first side surface ports  30   a ,  30   b  and the first upper surface port  38  are arranged on a straight line along the widthwise direction of the cylinder body  14 , and the pair of second side surface ports  32   a ,  32   b  and the second upper surface port  40  also are arranged on a straight line along the widthwise direction of the cylinder body  14 . 
     Further, as shown in  FIGS. 3 and 4 , on a lower part of the connecting section  28 , a pair of legs  42  are formed that bulge outwardly in a downward direction (the direction of the arrow C). Lower surfaces of the legs  42  are formed in a flat shape, and are substantially coplanar with the lower surfaces of the main body portions  26   a ,  26   b . In addition, the fluid pressure cylinder  10  is mounted stably by placing the lower surfaces of the main body portions  26   a ,  26   b  and the legs  42  of the connecting section  28  in abutment, for example, against a floor surface or the like. 
     On the other hand, as shown in  FIGS. 3 through 5 , a through hole  44  that penetrates in the axial direction (the direction of arrows B 1 , B 2 ) is formed in the interior of the connecting section  28  at a substantially central position in the widthwise direction, and a rod  46 , which is connected to the end plate  24 , is inserted into the through hole  44 . As shown in  FIG. 2 , the through hole  44  is formed substantially in parallel with the cylinder holes  12   a ,  12   b  and the sensor attachment grooves  34 . The through hole  44  is sealed by a ball  48  that is pressed into one end side (in the direction of the arrow B 1 ) thereof. 
     The rod  46  is made up from a shaft, which is formed, for example, with a circular shape in cross-section, and with a predetermined length in the axial direction (the direction of arrows B 1 , B 2 ). The rod  46  is arranged substantially in parallel with the piston rods  22   a ,  22   b . A magnet  50 , which serves as a detecting body, is mounted through an annular groove on an outer circumferential surface on one end of the rod  46 . The magnet  50 , for example, is formed in a cylindrical shape having a predetermined length in the axial direction (the direction of arrows B 1 , B 2 ) of the rod  46 , and is installed so as to cover the outer circumferential side of the one end of the rod  46 . Further, the other end of the rod  46  is connected by threaded engagement with the end plate  24 , as will be described later (see  FIG. 5 ). 
     In addition, when the rod  46  is moved along the axial direction (the direction of arrows B 1 , B 2 ), magnetism from the magnet  50 , which is disposed on the one end thereof, is detected by the detection sensors  36 , which is mounted on the upper surface of the connecting section  28 . As a result, the movement position in the axial direction (the direction of arrows B 1 , B 2 ) of the pistons  20   a ,  20   b , which are connected to the end plate  24  together with the rod  46 , is detected. 
     More specifically, by detecting the position of the rod  46  that moves together with the pistons  20   a ,  20   b , the position of the pistons  20   a ,  20   b  can also be detected. 
     Further, in the interior of the connecting section  28 , as shown in  FIGS. 2 through 4 , a pair of first and second communication passages  52 ,  54  are formed in the widthwise direction (the direction of the arrow A) thereof. The first communication passage  52  and the second communication passage  54  are separated from each other by a predetermined distance in the axial direction (the direction of arrows B 1 , B 2 ) of the cylinder body  14 , and provide communication mutually between one of the cylinder holes  12   a  and the other of the cylinder holes  12   b  in the cylinder body  14 . 
     The first communication passage  52  is disposed in the vicinity of the head covers  16  on one end side (in the direction of the arrow B 1 ) of the cylinder body  14 , and is formed along a straight line with the first side surface ports  30   a ,  30   b . The second communication passage  54  is disposed in the vicinity of the rod covers  18  on the other end side (in the direction of the arrow B 2 ) of the cylinder body  14 , and is formed along a straight line with the second side surface ports  32   a ,  32   b.    
     On the other hand, as shown in  FIG. 2 , on one end of the connecting section  28 , first and second rear surface ports  56 ,  58  are formed through which the pressure fluid can be supplied and discharged. The first rear surface port  56  is connected to a first penetrating passage  60  that penetrates in the axial direction (the direction of arrows B 1 , B 2 ) through the connecting section  28 , and the second rear surface port  58  is connected to a second penetrating passage  62  that penetrates in the axial direction (the direction of arrows B 1 , B 2 ) through the connecting section  28 . The first and second penetrating passages  60 ,  62  are formed substantially in parallel and are separated a predetermined distance from each other. Other ends of the first and second penetrating passages  60 ,  62  are sealed by balls  48 . 
     In addition, the first penetrating passage  60  communicates through the first upper surface port  38  with the first communication passage  52 , and the second penetrating passage  62  communicates through the second upper surface port  40  with the second communication passage  54 . 
     More specifically, in the cylinder body  14 , there are included a total of eight ports made up from the first side surface ports  30   a ,  30   b  and the second side surface ports  32   a ,  32   b , which are provided on the side surfaces of the pair of main body portions  26   a ,  26   b , the first and second upper surface ports  38 ,  40 , which are provided on the upper surface of the connecting section  28 , and the first and second rear surface ports  56 ,  58 , which are provided on the one end of the connecting section  28 . 
     In addition, when the pistons  20   a ,  20   b  are moved toward the rod cover  18  (in the direction of the arrow B 2 ), pressure fluid is supplied selectively to any one of the first side surface ports  30   a ,  30   b , the first upper surface port  38 , and the first rear surface port  56 . On the other hand, when the pistons  20   a ,  20   b  are moved toward the head covers  16  (in the direction of the arrow B 1 ), pressure fluid is supplied selectively to any one of the second side surface ports  32   a ,  32   b , the second upper surface port  40 , and the second rear surface port  58 . 
     A pressure fluid supply source is connected, for example, through non-illustrated tubes, to any of the aforementioned pair of first side surface ports  30   a ,  30   b , the pair of second side surface ports  32   a ,  32   b , the first and second upper surface ports  38 ,  40 , or the first and second rear surface ports  56 ,  58 , and the pressure fluid is supplied through the ports to the cylinder holes  12   a ,  12   b . Further, the ports that are not used and to which tubes are not connected (i.e., in the present embodiment, the first side surface ports  30   a ,  30   b  and the second side surface ports  32   a ,  32   b , and the first and second rear surface ports  56 ,  58 ) are closed by installation of sealing plugs  64  therein. 
     More specifically, among the eight ports made up from the first side surface ports  30   a ,  30   b  and the second side surface ports  32   a ,  32   b , the first and second upper surface ports  38 ,  40 , and the first and second rear surface ports  56 ,  58 , any two of the ports are used selectively depending on the installation environment or layout of tubes, etc., which is used for the fluid pressure cylinder  10 , whereas the other six ports, other than the two used ports, are closed by installing the sealing plugs  64  therein. 
     On the other hand, a damper  66 , which, for example, is made of an elastic material, is mounted in facing relation to the end plate  24  on the other end of the connecting section  28 . The damper  66  is formed in a flat plate-like shape projecting a predetermined height with respect to the other end of the connecting section  28 , and the damper  66  is fixed to the cylinder body  14  by a projection  68  formed in a center region thereof being press-fitted into a recess of the cylinder body  14 . In addition, when the end plate  24  is moved toward the cylinder body  14  (in the direction of the arrow B 1 ), by abutment of the end plate  24  against the damper  66 , shocks and impact sounds are reduced. 
     As shown in  FIG. 2 , the head covers  16  are made, for example, from disk-shaped plate bodies, which are inserted into the cylinder holes  12   a ,  12   b  from the one end side (in the direction of the arrow B 1 ) of the cylinder body  14 . In addition, in the cylinder holes  12   a ,  12   b , by the head covers  16  being pressed and expanded in diameter by a non-illustrated tool such as a jig or the like, the outer edges thereof bite into and engage with the inner circumferential surfaces of the cylinder holes  12   a ,  12   b . Further, the outer edges of the head covers  16  are inclined in a direction toward the one end side (in the direction of the arrow B 1 ) of the cylinder body  14 . 
     Each of the rod covers  18 , for example, is formed in a cylindrical shape having a rod hole defined through the center thereof. The rod covers  18  are inserted respectively from the other end sides (in the direction of the arrow B 2 ) of the cylinder holes  12   a ,  12   b , and are fixed in the interiors of the cylinder holes  12   a ,  12   b  by locking rings  72 , which are engaged with the inner circumferential surfaces of the cylinder holes  12   a ,  12   b . Rod packings  74  are disposed through annular grooves on inner circumferential surfaces of the rod holes. 
     The pistons  20   a ,  20   b  are formed, for example, in disk-like shapes having a predetermined thickness. Piston packings  76  are mounted in annular grooves that are formed on outer circumferential surfaces of the pistons  20   a ,  20   b . In addition, the pistons  20   a ,  20   b  are accommodated respectively in the interiors of the cylinder holes  12   a ,  12   b , such that the pistons  20   a ,  20   b  are movable along the axial direction (the direction of arrows B 1 , B 2 ) in a state in which the piston packings  76  abut against inner circumferential surfaces of the cylinder holes  12   a ,  12   b.    
     The piston rods  22   a ,  22   b  are constituted from shafts having predetermined lengths in the axial direction (the direction of arrows B 1 , B 2 ). Ends of the piston rods  22   a ,  22   b  are inserted through piston holes, which penetrate through the centers of the pistons  20   a ,  20   b , and are joined by caulking with respect to the pistons  20   a ,  20   b . Consequently, the pistons  20   a ,  20   b  are connected to the ends of the piston rods  22   a ,  22   b.    
     Further, the other ends of the piston rods  22   a ,  22   b  are disposed so as to project outwardly from the cylinder body  14  after having been inserted through the rod holes of the rod cover  18 . At this time, the rod packings  74 , which are mounted on the rod cover  18 , are placed in sliding contact with the outer circumferential surfaces of the piston rods  22   a ,  22   b , whereby leakage of pressure fluid from between the piston rods  22   a ,  22   b  and the rod covers  18  is prevented. 
     The end plate  24 , for example, is formed with a rectangular shape in cross-section having a predetermined width. One end in the widthwise direction (the direction of the arrow A) of the end plate  24  is connected with one of the piston rods  22   a  that is inserted through a hole  78 , and the other end in the widthwise direction (the direction of the arrow A) of the end plate  24  is connected by a bolt  80  with respect to the other of the piston rods  22   b . More specifically, the end plate  24  is connected with respect to the other ends of the pair of piston rods  22   a ,  22   b  perpendicularly to the axial direction of the piston rods  22   a ,  22   b . Further, the height of the end plate  24  is formed to be of substantially the same height or slightly lower in height than the height of the main body portions  26   a ,  26   b  of the cylinder body  14  (see  FIG. 5 ). 
     The fluid pressure cylinder  10  according to the first embodiment of the present invention is constructed basically as described above. Next, operations and advantages of the fluid pressure cylinder  10  will be described. The condition shown in  FIG. 2 , in which the pistons  20   a ,  20   b  are moved to the one end side (in the direction of the arrow B 1 ) of the cylinder body  14 , will be treated as an initial condition. Further, in this state, a case will be described in which pressure fluid is supplied and discharged through the first and second upper surface ports  38 ,  40  of the cylinder body  14 . 
     First, in the initial position shown in  FIG. 2 , by supply of the pressure fluid to the first upper surface port  38  through a tube from the non-illustrated pressure fluid supply source, the pressure fluid passes through the first communication passage  52  and is introduced respectively to the pair of cylinder holes  12   a ,  12   b . In this case, the second upper surface port  40  is in a state of being open to atmosphere. 
     By the pressure fluid that is introduced to the pair of cylinder holes  12   a ,  12   b , the pistons  20   a ,  20   b  are pressed toward the other end side (in the direction of the arrow B 2 ) of the cylinder body  14 , along with the piston rods  22   a ,  22   b  and the end plate  24  being moved together in unison. More specifically, by movement of the pistons  20   a ,  20   b  toward the other end side of the cylinder body  14 , as shown in  FIG. 6 , the end plate  24  is moved in a direction (the direction of the arrow B 2 ) away from the cylinder body  14 . 
     In addition, as shown in  FIG. 6 , the pair of pistons  20   a ,  20   b  come into abutment respectively against the ends of the rod covers  18 , so that a displacement end position is reached. 
     On the other hand, in the case that the end plate  24  is moved to approach again toward the cylinder body  14  (in the direction of the arrow B 1 ), under a switching operation of a non-illustrated switching means, the pressure fluid which had been supplied to the first upper surface port  38  is supplied instead to the second upper surface port  40  from the pressure fluid supply source. In this case, the first upper surface port  38  is placed in a state of being open to atmosphere. 
     The pressure fluid supplied to the second upper surface port  40  passes through the second communication passage  54 , and is introduced between the rod covers  18  and the pistons  20   a ,  20   b  in the pair of cylinder holes  12   a ,  12   b , whereby the two pistons  20   a ,  20   b  are pressed respectively toward the head covers  16  (in the direction of the arrow B 1 ). As a result, the piston rods  22   a ,  22   b  are moved so as to become accommodated gradually inside the cylinder holes  12   a ,  12   b , along with the end plate  24  being moved to approach toward the other end of the cylinder body  14 . In addition, as shown in  FIG. 2 , the end plate  24  comes into abutment against the damper  66  that is mounted on the cylinder body  14 , so that the initial position is restored. 
     Next, in the aforementioned fluid pressure cylinder  10 , a case will be described in which only one of the pistons  20   a  is pressed under the supply of a pressure fluid, at the time of a returning operation to restore the pistons  20   a ,  20   b  to the one end side (in the direction of the arrow B 1 ) of the cylinder body  14 . 
     In this case, for example, midway in the second communication passage  54 , a communication switching mechanism  82  (shown by the two-dot-and-dashed line in  FIGS. 2 and 6 ) is provided. The communication switching mechanism  82  blocks communication via the second communication passage  54  when the pistons  20   a ,  20   b  are moved to the side of the head covers  16  (in the direction of the arrow B 1 ), and the communication switching mechanism  82  also switches the second communication passage  54  to a communicating state at the time of a pressing operation in which the pistons  20   a ,  20   b  are moved to the side of the rod covers  18  (in the direction of the arrow B 2 ). 
     More specifically, the communication switching mechanism  82  is arranged at a position on the side of the cylinder hole  12   b  relative to the center in the longitudinal direction of the second communication passage  54 . Further, instead of providing the sealing plug  64 , a filter or the like, which is permeable to air, may be disposed in the second side surface port  32   b  on the side of the main body portion  26   b , so as to keep the second side surface port  32   b  open to atmosphere. 
     As the communication switching mechanism  82 , for example, a check valve is used, which is installed in facing relation to the flow path of the second communication passage  54 , and is capable of allowing flow of fluid in one direction only, while blocking flow of the fluid in the opposite direction. More specifically, the check valve operates to block flow of the pressure fluid to the cylinder hole  12   b  from the second upper surface port  40 , yet allows flow of the pressure fluid to the second upper surface port  40  from the cylinder hole  12   b.    
     First, in the case that the pistons  20   a ,  20   b  are moved to the side of the rod covers  18  (in the direction of the arrow B 2 ), under a switching action carried out by the communication switching mechanism  82 , communication is established between one of the cylinder holes  12   a  and the other of the cylinder holes  12   b  through the second communication passage  54 . Therefore, air that is pressed by the pistons  20   a ,  20   b  toward the rod covers  18  is discharged to the exterior from the second communication passage  54  and through the second upper surface port  40 . 
     On the other hand, at the time of a returning operation to move the pistons  20   a ,  20   b  to the side of the head covers  16  (in the direction of the arrow B 1 ), since communication between the one of the cylinder holes  12   a  and the other of the cylinder holes  12   b  through the second communication passage  54  is blocked by the communication switching mechanism  82 , by supplying pressure fluid from the second upper surface port  40 , the pressure fluid that has been introduced to the second communication passage  54  is in turn introduced only to the one cylinder hole  12   a , but is not introduced to the other cylinder hole  12   b.    
     Therefore, only the piston  20   a , which is disposed in one of the cylinder holes  12   a , is pressed toward the head cover  16  (in the direction of the arrow B 1 ), and the piston rod  22   a  and the end plate  24  are moved together therewith. In addition, since the piston  20   b , which is disposed in the other of the cylinder holes  12   b , is not pressed by the pressure fluid, the piston  20   b  is pressed together with the piston rod  22   b  toward the one end side by the end plate  24 . At this time, atmospheric air is introduced to the cylinder hole  12   b  through the second side surface port  32   b , thereby keeping the cylinder hole  12   b  at atmospheric pressure. 
     In the foregoing manner, for example, during the returning operation of the fluid pressure cylinder  10 , in which there is no need for a strong thrust force, by supplying the pressure fluid to only the one cylinder hole  12   a  and pressing the piston  20   a , compared to the case of supplying pressure fluid respectively to the pair of cylinder holes  12   a ,  12   b  to thereby operate both of the pistons  20   a ,  20   b , the thrust force is cut roughly in half and the consumption of the pressure fluid can be reduced by half. 
     As a result, by providing, in the second communication passage  54 , the communication switching mechanism  82  that switches a state of communication between the cylinder holes  12   a ,  12   b , the thrust force is maintained at the time of carrying out the pushing operation for pushing the end plate  24  in a direction to separate away from the cylinder body  14 , while the consumption amount of the pressure fluid is reduced during the returning operation when the end plate  24  is returned to the side of the cylinder body  14 . Therefore, energy conservation in the fluid pressure cylinder  10  can be promoted. 
     In the foregoing manner, according to the first embodiment, in a fluid pressure cylinder  10  having the pair of pistons  20   a ,  20   b  and the pair of piston rods  22   a ,  22   b , the magnet  50  for detecting the movement position of the pistons  20   a ,  20   b  is disposed on the rod  46  which is a separate body apart from the pistons  20   a ,  20   b  and which is movable in the axial direction (the direction of arrows B 1 , B 2 ) of the cylinder body  14 . Stated otherwise, the magnet  50  is disposed outside of the cylinder holes  12   a ,  12   b  in which the pistons  20   a ,  20   b  are accommodated. Therefore, in comparison with the conventional fluid pressure cylinder in which magnets are disposed on outer circumferential surfaces of the pistons, the pistons  20   a ,  20   b  can be reduced in thickness along the axial direction of the pistons  20   a ,  20   b.    
     As a result, while the same amount of movement (stroke length) of the pistons  20   a ,  20   b  is assured, the longitudinal dimension in the axial direction of the cylinder body  14  can be suppressed, so that a reduction in longitudinal size along the axial direction of the fluid pressure cylinder  10  is made possible. 
     Further, since the position of the pair of pistons  20   a ,  20   b  can be detected by the single rod  46  (magnet  50 ), in contrast to the conventional fluid pressure cylinder, in which magnets for position detection are provided respectively on the pair of pistons, the number of magnets  50  can be reduced, and thus the number of component parts and assembly steps that make up the fluid pressure cylinder can be reduced, together with enabling a reduction in manufacturing costs. 
     Furthermore, the ports, which are capable of supplying and discharging the pressure fluid, are disposed on the cylinder body  14  in four directions, i.e., on both sides (the first side surface ports  30   a ,  30   b  and the second side surface ports  32   a ,  32   b ), on the upper surface (the first and second upper surface ports  38 ,  40 ), and on the one end side (the first and second rear surface ports  56 ,  58 ) in the axial direction. Therefore, taking into consideration the installation environment in which the fluid pressure cylinder  10  is used, or the layout of tubes that are connected to the ports, ports that are easiest to use can be selected and used appropriately. As a result, freedom of layout can be enhanced when the fluid pressure cylinder  10  is installed. 
     Further still, since it is unnecessary for the magnet  50  to be of a shape corresponding to the shape (outer diameter) of the pistons  20   a ,  20   b , by using the common rod  46  in fluid pressure cylinders  10  having pistons  20   a ,  20   b  of differing shapes, the magnet  50  can be used in common with various types of fluid pressure cylinders  10 . 
     As a result, in contrast to the conventional fluid pressure cylinder in which different magnets are set respectively for fluid pressure cylinders having differently shaped pistons, by making it possible for a single magnet  50  to be used, the cost required for the magnet  50  can significantly be reduced, together with simplifying component settings. 
     Still further, unlike the conventional fluid pressure cylinder, it is unnecessary to change the thickness of the pistons when changing the length in the axial direction (the direction of arrows B 1 , B 2 ) of the magnet  50  provided on the rod  46 , and the detection range by the detection sensors  36  can easily be changed simply by changing the shape of the rod  46 . More specifically, in the case that the detection range by the detection sensors  36  is to be expanded, for example, by arranging two of the magnets  50  in the axial direction of the rod  46 , the detection range can roughly be doubled. 
     Further, since on the cylinder body  14 , the upper surface of the connecting section  28  is recessed downwardly (in the direction of the arrow C) with respect to the upper surfaces of the pair of main body portions  26   a ,  26   b , for example, when tubes are connected via non-illustrated tube fittings to the first and second upper surface ports  38 ,  40  of the connecting section  28 , the amount by which the tube fittings project in the heightwise direction can be suppressed. Therefore, the height dimension of the fluid pressure cylinder  10  including the tube fittings can suitably be suppressed. 
     Next, a fluid pressure cylinder  100  according to a second embodiment is shown in  FIGS. 7 and 8 . Constituent elements, which are the same as those of the above-described fluid pressure cylinder  10  according to the first embodiment, are denoted by the same reference characters, and detailed description of such features is omitted. 
     The fluid pressure cylinder  100  according to the second embodiment differs from the fluid pressure cylinder  10  according to the first embodiment, in that wear rings  104  are provided on outer circumferential surfaces of pistons  102   a ,  102   b , and in that the length of rod covers  106  in the axial direction (the direction of arrows B 1 , B 2 ) is shortened. 
     In the fluid pressure cylinder  100 , as shown in  FIGS. 7 and 8 , a pair of annular grooves are formed on the outer circumferential surface of each of the pistons  102   a ,  102   b . A wear ring  104  is installed in one of the annular grooves that is positioned on the side of the head cover  16  (in the direction of the arrow B 1 ), whereas a piston packing  108  is installed in another of the annular grooves that is positioned on the side of the rod cover  106  (in the direction of the arrow B 2 ). The wear ring  104  and the piston packing  108  are separated mutually by a predetermined distance in the axial direction of the pistons  102   a ,  102   b.    
     The wear rings  104  are formed in an annular shape from a resin material, for example, and are disposed in sliding contact with inner circumferential surfaces of the cylinder holes  12   a ,  12   b . The pistons  102   a ,  102   b  are guided displaceably along the cylinder holes  12   a ,  12   b  by the wear rings  104 . More specifically, by providing the wear rings  104 , the pistons  102   a ,  102   b  can be displaced with high precision along the axial direction. 
     Further, by placing the piston packings  108  in sliding contact against the inner circumferential surfaces of the cylinder holes  12   a ,  12   b , leakage of pressure fluid from between the pistons  102   a ,  102   b  and the cylinder holes  12   a ,  12   b  is prevented. 
     The rod covers  106 , for example, are formed with a length which is roughly one-third (⅓) the length of the rod covers  18  of the fluid pressure cylinder  10  according to the aforementioned first embodiment. Along with shortening the length dimension of the rod covers  106 , the length dimension of the cylinder body  110  can also be shortened. 
     More specifically, by positioning the ends of the rod covers  106  that face toward the head covers  16  at the same position as the ends of the rod covers  18  in the aforementioned fluid pressure cylinder  10 , without changing or affecting the stroke length along the axial direction (the direction of arrows B 1 , B 2 ) of the pistons  102   a ,  102   b , the length dimension from the other end side of the cylinder body  110  to the one end side on the side of the head covers  16  (in the direction of the B 1 ) can be made shorter. 
     In the foregoing manner, according to the second embodiment, the lengths of the rod covers  106  that guide the piston rods  22   a ,  22   b  in the axial direction are shortened, and the rod covers  106  are arranged without changing the position of the end surfaces thereof that face toward the pistons  102   a ,  102   b . Thus, the length dimension of the cylinder body  110  can be minimized without changing the stroke length of the pistons  102   a ,  102   b  along the axial direction. 
     Further, the wear rings  104  are disposed on outer circumferential surfaces of the pistons  102   a ,  102   b , and as a result of being constructed to be capable of guiding the pistons  102   a ,  102   b  in the axial direction, even though the lengths of the rod covers  106  in the axial direction are shortened and thus the guiding capability of the piston rods  22   a ,  22   b  is diminished, due to the presence of the wear rings  104 , the ability to guide the pistons  102   a ,  102   b  can be enhanced. Therefore, the ability for the pistons  102   a ,  102   b  and the piston rods  22   a ,  22   b  in the fluid pressure cylinder  100  to advance and retract straight in the axial direction can be maintained with high precision. 
     The fluid pressure cylinder according to the present invention is not limited to the embodiments described above, and various alternative or additional structures may be adopted therein without departing from the scope of the invention as set forth in the appending claims.