Patent Publication Number: US-10774855-B2

Title: Hydraulic actuator

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/JP2017/039198, filed on Oct. 30, 2017, which claims priority from Japanese Patent Application No. 2016-217526, filed on Nov. 7, 2016, and Japanese Patent Application No. 2017-008960, filed on Jan. 20, 2017. 
     TECHNICAL FIELD 
     The present invention relates to a hydraulic actuator. 
     BACKGROUND ART 
     Conventionally, there has been widely used as an actuator for expanding/contracting a tube a pneumatic actuator having a rubber tube (a tube-shaped body) capable of expanding/contracting by using air as working fluid and a sleeve (a woven reinforcing structure) covering an outer peripheral surface of the tube, i.e. a McKibben type actuator (refer to PTL1, for example). 
     Respective end portions of an actuator main body constituted of a tube and a sleeve as described above are caulked by using a sealing member formed by metal. 
     The sleeve is a cylindrical structure formed by woven high tensile strength fiber cords such as polyamide fibers or metal cords, for regulating expansion movements of the tube within a predetermined range. 
     Such a pneumatic actuator as described above, which is used in various fields, is suitably used as an artificial muscle for a nursing care/healthcare device in particular. 
     CITATION LIST 
     Patent Literature 
     PTL1: JP S61-236905 A 
     SUMMARY 
     Technical Problem 
     However, such a conventional actuator as described above using air as working fluid does not have particularly high strength (pressure resistance), which strength is only around 0.5 MPa at most, for example. 
     In this respect, durability of the conventional actuator is not satisfactory when it is employed as a hydraulic actuator using liquid such as oil, water or the like as working fluid because a hydraulic actuator is generally subjected to high pressure, e.g. 5 MPa. In a case where a sleeve is not adequately designed, in particular, in a hydraulic actuator, a tube of the actuator will have to bear yet larger load, further increasing demand for improved durability of the actuator. 
     In view of this, an object of the present disclosure is to solve the prior art problems described above and provide a hydraulic actuator using liquid as working fluid, which exhibits improved durability. 
     Solution to Problem 
     Primary features of the present disclosure for achieving the aforementioned object are as follows. 
     A hydraulic actuator of the present disclosure has an actuator main body constituted of a cylindrical tube capable of expanding/contracting by hydraulic pressure and a sleeve for covering an outer peripheral surface of the tube, the sleeve having a cylindrical structure formed by cords woven to be disposed in predetermined directions, wherein: 
     the average angle formed by the cords of the sleeve with respect to the axis direction of the actuator with no load and no pressure applied thereon is in a range of 20° or larger and less than 45°; and 
     in a state where the average angle formed by the cords of the sleeve with respect to the axis direction of the actuator is 45° under hydraulic pressure of 5 MPa, a ratio (S 2 /S 1 ) of the total area (S 2 ) of clearances between the cords of the sleeve with respect to an area (S 1 ) of an outer peripheral surface of the actuator main body is 35% or less. 
     The hydraulic actuator of the present disclosure, having the adequately designed sleeve, experiences relatively small load on the tube thereof and thus exhibits improved durability. 
     In a preferable example of the hydraulic actuator of the present disclosure, the cords which form the sleeve is made of at least one fiber material selected from the group consisting of polyamide fiber, polyester fiber, polyurethane fiber, rayon, acrylic fiber, and polyolefin fiber. In this case, durability of the actuator further improves. 
     In another preferable example of the hydraulic actuator of the present disclosure, the sleeve is made of one group of cords disposed in one direction and the other group of cords disposed to intersect the cords of the one group, so that the intersecting points at which the cords or pairs of the cords intersect one cord at the upper/lower side thereof in an alternate manner are shifted, by a single cord, from the intersecting points at which the cords or pairs of the cords intersect another cord (adjacent to the one cord) at the upper/lower side thereof in an alternate manner. In this case, durability of the actuator further improves. 
     In yet another preferable example of the hydraulic actuator of the present disclosure, the sleeve is woven by a twill or plain weave. In this case, durability of the actuator further improves. 
     In yet another preferable example of the hydraulic actuator of the present disclosure, the cords of the sleeve have breaking strength of at least 200 N/one cord. In this case, durability of the actuator further improves. Breaking strength of the cord is measured according to JIS L1017 in the present disclosure. 
     In yet another preferable example of the hydraulic actuator of the present disclosure, the cords of the sleeve each have breaking elongation of at least 2.0%. In this case, durability of the actuator further improves. Breaking elongation of the cord is measured according to JIS L1017 in the present disclosure. 
     In yet another preferable example of the hydraulic actuator of the present disclosure, each of the cords of the sleeve has a diameter in the range of 0.3 mm to 1.5 mm. In this case, durability of the actuator further improves. 
     In yet another preferable example of the hydraulic actuator of the present disclosure, driving density of the cords in the sleeve is in the range of 6.8 cords/cm to 25.5 cords/cm. In this case, durability of the actuator further improves. 
     In yet another preferable example of the hydraulic actuator of the present disclosure, provided that “t” (mm) represents thickness of the tube, “d” (mm) represents a diameter of the cord of the sleeve, “Θ 1 ” represents the average angle formed by the cord of the sleeve with respect to the axis direction of the actuator with no load and no pressure applied thereon, and “Θ 2 ” represents the average angle formed by the cord of the sleeve with respect to the axis direction of the actuator in an actuator contracting state, t, d, Θ 1  and Θ 2  satisfy general formula (1) shown below. 
     
       
         
           
             
               
                 
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     In this case, durability of the actuator further improves. 
     In this respect, the average angle Θ 2  formed by the cord of the sleeve with respect to the axis direction of the actuator in an actuator contracting state is a value measured under the condition of load: 2.5 kN and hydraulic pressure: 5 MPa. 
     Further, provided that “t” (mm) represents thickness of the tube, “d” (mm) represents a diameter of the cord of the sleeve. “Θ 1 ” represents the average angle formed by the cord of the sleeve with respect to the axis direction of the actuator with no load and no pressure applied thereon, and “Θ 2 ” represents the average angle formed by the cord of the sleeve with respect to the axis direction of the actuator in the actuator contracting state, t, d, Θ 1  and Θ 2  more preferably satisfy general formula (2) shown below. 
     
       
         
           
             
               
                 
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     In this case, durability of the actuator even further improves. 
     In yet another preferable example of the hydraulic actuator of the present disclosure, twist coefficient K of the cord of the sleeve, defined by general formula (3) shown below, is in the range of 0.14 to 0.50. 
     
       
         
           
             
               
                 
                   K 
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         [In the formula (3), “T 2 ” represents the second twist number (number/10 cm) of the cord, T 2  should be replaced with the first twist number T 1  (number/10 cm) when the cord is a single twist cord, “D” represents the fineness per one raw yarn (dtex) of the cord, and “ρ” represents the density (g/cm 3 ) of the yarn of the cord.] 
       
    
     In this case, the hydraulic actuator having the adequately designed sleeve is subjected to relatively small load on the tube thereof and thus exhibits further improved durability. 
     In the hydraulic actuator of the present disclosure, the cord of the sleeve preferably has a ratio (T 1 /D) of the first twist number T 1  (number/10 cm) with respect to the fineness D (dtex) per one raw yarn of the cord in the range of 0.004 to 0.03. In this case, durability of the actuator even further improves. 
     In the hydraulic actuator of the present disclosure, the cord of the sleeve preferably has a ratio (T 1 /T 2 ) of the first twist number T 1  (number/10 cm) with respect to the second twist number T 2  (number/10 cm) in the range of 0.8 to 1.2. In this case, durability of the actuator even further improves. 
     In the hydraulic actuator of the present disclosure, the fineness D per one raw yarn of the cord of the sleeve is preferably in the range of 800 to 5000 dtex. Further, the cord preferably has the first twist number T 1  in the range of 3.2 to 150/10 cm, the second twist number T 2  in the range of 2.6 to 180/10 cm, and the number of the twisted yarns constituting the cord in the range of 2 to 4. In this case, durability of the actuator even further improves. 
     In yet another preferable example of the hydraulic actuator of the present disclosure, thickness of the tube with no load and no pressure applied on the actuator is in the range of 1.0 mm to 6.0 mm. In this case, durability of the actuator even further improves. 
     Advantageous Effect 
     According to the present disclosure, it is possible to provide a hydraulic actuator of which durability has improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, wherein: 
         FIG. 1  is a side view of an embodiment of a hydraulic actuator  10 . 
         FIG. 2  is a partially exploded perspective view of an embodiment of the hydraulic actuator  10 . 
         FIG. 3A  is a partial side view of an embodiment of a sleeve  120  and  FIG. 3B  is a partial side view of another embodiment of the sleeve  120  each in a state of no load and no pressure applied on the actuator. 
         FIG. 4A  is a partial side view of an embodiment of the sleeve  120  and  FIG. 4B  is a partial side view of another embodiment of the sleeve  120 , each in a state where the average angle formed by the cords  121  of the sleeve  120  with respect to the axis direction of the actuator is 45°. 
         FIG. 5  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 , cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 1-1. 
         FIG. 6  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 , cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 1-2. 
         FIG. 7  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 , cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 1-3. 
         FIG. 8  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 A, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 2-1. 
         FIG. 9  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 A, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 2-2. 
         FIG. 10  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 A, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 2-3. 
         FIG. 11  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 B, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 3-1. 
         FIG. 12  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 C, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 3-2. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, the hydraulic actuator of the present disclosure will be demonstratively described in detail based on embodiments thereof and with reference to the drawings. The same functions and structures share the same/similar reference numerals and repetitive or redundant explanations thereof will be omitted. 
     (1) Outline of Entire Structure of Hydraulic Actuator 
       FIG. 1  is a side view of a hydraulic actuator  10  according to an embodiment of the present disclosure. As shown in  FIG. 1 , the hydraulic actuator  10  has an actuator main body  100 , a sealing mechanism  200 , and another sealing mechanism  300 . Respective connection portions  20  are provided at respective ends of the hydraulic actuator  10 . 
     The actuator main body  100  is constituted of a tube  110  and a sleeve  120 . A working fluid flows into the actuator main body  100  via a fitting  400  and a passage hole  410 . The actuator of the present disclosure is hydraulically operated and uses a liquid as the working fluid. Examples of the liquid include oil, water, and the like. The actuator of the present disclosure may employ either oil pressure or water pressure. In a case where the hydraulic actuator employs oil pressure, any suitable hydraulic oil which is conventionally used in a hydraulic driving system employing oil pressure may be used as hydraulic oil. 
     The actuator main body  100 , when the working fluid flows into the tube  110 , contracts in the axis direction D AX  and expands in the radial direction D R  of the actuator main body  100 . On the other hand, the actuator main body  100 , when the working fluid flows out of the tube  110 , expands in the axis direction D AX  and contracts in the radial direction D R  of the actuator main body  100 . The hydraulic actuator  10  functions as an actuator by such changes in configuration of the actuator main body  100  as described above. 
     Further, the hydraulic actuator  10  as described above is what is called a. McKibben type actuator, which is applicable to artificial muscles of course and can also be suitably used for limbs (upper limbs and lower limbs) of a robot, which limbs require higher capacity (contraction force) than artificial muscles. The connection portions  20  are connected to members constituting the limbs, or the like. 
     The sealing mechanism  200  and the sealing mechanism  300  seal end portions of the actuator main body  100  in the axis direction D AX  thereof, respectively. Specifically, the sealing mechanism  200  includes a sealing member  210  and a caulking member  230 . The sealing member  210  seals an end portion in the axis direction D AX  of the actuator main body  100 . The caulking member  230  caulks the actuator main body  100  in collaboration with the sealing member  210 . Indentations  231  as marks made by the caulking jigs are formed at an outer peripheral surface of the caulking member  230 . 
     Differences between the sealing mechanism  200  and the sealing mechanism  300  reside in how the fitting  400  and a fitting  500  (and the passage hole  410  and a passage hole  510 ) function, respectively. 
     The fitting  400  provided in the sealing mechanism  200  protrudes such that the fitting  400  can be mounted to a driving pressure source of the hydraulic actuator  10 , or more specifically a hose (a piping path) connected to a compressor of the working fluid. The working fluid which has flowed into the actuator via the fitting  400  then flows into the inside of the actuator main body  100 , or more specifically the inside of the tube  110 , via the passage hole  410 . 
     On the other hand, the fitting  500  provided in the sealing mechanism  300  protrudes such that it can be used for gas venting when the working fluid is injected into the actuator. When the working fluid is injected into the actuator at the initial operation stage of the actuator, gas present inside the actuator is discharged from the fitting  500  via the passage hole  510 . 
       FIG. 2  is a partially exploded perspective view of the hydraulic actuator  10 . As shown in  FIG. 2 , the hydraulic actuator  10  has the actuator main body  100  and the sealing mechanism  200 . 
     The actuator main body  100  is constituted of the tube  110  and the sleeve  120 , as described above. 
     The tube  110  is a cylindrical, pipe-like member capable of expanding/contracting by hydraulic pressure. The tube  110 , which is to repeat contracting and expanding movements alternately by the working fluid, is made of an elastic material such as rubber. 
     Thickness of the tube  110  with no load and no pressure applied thereon is preferably in the range of 1.0 mm to 6.0 mm and more preferably in the range of 1.4 mm to 5.0 mm. Thickness of the tube  110 ≥1.0 mm improves strength of the tube  110  and suppresses protrusion of the tube  110  from clearances between the cords of the sleeve  120 , thereby further improving durability of the actuator. Thickness of the tube  110 ≤6.0 mm ensures a satisfactorily high contraction rate and thus a satisfactorily large magnitude of contraction/expansion of the tube  110 . 
     Although the tube  110  shown in  FIGS. 1 and 2  has a single-layer structure, it is acceptable in the present disclosure that the tube has a multi-layer structure. Further, the (outer) diameter of the tube  110  may be set appropriately in accordance with the intended application. 
     The sleeve  120  has a cylindrical configuration and covers an outer peripheral surface of the tube  110 . The sleeve  120  has a woven structure formed by weaving cords to be disposed in certain directions, wherein the cords thus disposed intersect each other in a woven manner to provide rhombus configurations in a repetitive and continuous manner. The sleeve  120  having such a configuration as described above can deform like a pantograph and follow contraction/expansion of the tube  110 , while also regulating the contraction/expansion. 
       FIG. 3A  is a partial side view of an embodiment of the sleeve  120  and  FIG. 3B  is a partial side view of another embodiment of the sleeve  120  each in a state of no load and no pressure applied on the actuator. 
     In the present disclosure, the average angle Θ 1  formed by the cords  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator with no load and no pressure applied thereon (i.e. at the initial state thereof) is in a range of 20° or larger and less than 45°, as shown in  FIG. 3A  and  FIG. 3B . Setting the average angle Θ 1  formed by the cords  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator in a state of no load and no pressure applied thereon, to be 20° or larger, enhances durability of the sleeve  120 . If the average angle Θ 1  formed by the cords  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator in a state of no load and no pressure applied thereon exceeds 45°, the actuator fails to exhibit a satisfactorily high contraction when it operates, thereby failing to function in a satisfactory manner as an actuator. 
     The average angle Θ 1  is preferably 22° or larger and more preferably 23° or larger. The larger average angle Θ 1  results in the smaller load born by the tube  110 , thereby suppressing breakage of the tube  110  at portions thereof not in direct contact with the cords  121  and thus successfully maintaining satisfactory capacity of the actuator over a long period of time. 
     The average angle Θ 1  is preferably equal to 37° or less. The average angle Θ 1 ≤37° ensures a satisfactorily high contraction rate and thus a satisfactorily large magnitude of contraction/expansion of the tube  110 . 
     The average angle Θ 1  formed by the cords  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator in the initial state can be adjusted by, for example, adjusting the direction of the cords  121  when the sleeve  120  is woven and when the sleeve  120  thus woven is formed into a cylindrical shape. 
       FIG. 4A  is a partial side view of an embodiment of the sleeve  120  and  FIG. 4B  is a partial side view of another embodiment of the sleeve  120 , each in a state where the average angle formed by the cords  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator is 45°. In the present disclosure, ±1° is allowed as a margin of error when angles of the cords  121  are measured. 
     In the present disclosure, in a state where the average angle Θ 3  formed by the cords  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator is 45° under hydraulic pressure of 5 MPa, a ratio (S 2 /S 1 ) of the total area (S 2 ) of clearances  122  between the cords  121  of the sleeve  120  with respect to an area (S 1 ) of an outer peripheral surface of the actuator main body  100  is 35% or less, preferably 32% or less, more preferably 30% or less, further more preferably 25% or less, and particularly preferably 20% or less, as shown in  FIG. 4A  and  FIG. 4B . When the ratio (S 2 /S 1 ) of the total area (S 2 ) of clearances  122  between the cords  121  of the sleeve  120  with respect to an area (S 1 ) of an outer peripheral surface of the actuator main body  100  is 35% or less in a state where the average angle Θ 3  formed by the cords  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator is 45°, i.e. in a state where the cords  121  intersect each other at the average intersecting angle of 90°, the tube  110  bears relatively small load and durability of the actuator improves. The lower limit value of the ratio (S 2 /S 1 ) is not particularly restricted but preferably 5% or higher in terms of achieving a satisfactorily large magnitude of contraction/expansion of the actuator. 
     The total area (S 2 ) of clearances  122  between the cords  121  of the sleeve  120  can be adjusted by changing type of weaving the sleeve  120 , and diameter, material, density of the cords  121  provided in the sleeve  120 . 
     In the present disclosure, the total area (S 2 ) of clearances  122  between the cords  121  of the sleeve  120  is measured after the load applied on the actuator has been adjusted such that the average angle Θ 3  formed by the cords  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator is 45° under hydraulic pressure of 5 MPa. In this respect, the total area (S 2 ) is measured or evaluated in a region, of the sleeve  120 , where the diameter of the sleeve  120  contracts by −5% with respect to the maximum diameter thereof when the actuator contracts. The sum of the areas of clearances  122  in the region is then regarded as S 2  and the area of an outer surface of the actuator main body  100  in the region is regarded as S 1 , so that the ratio (S 2 /S 1 ) is calculated. In the present disclosure, the areas of clearances  122  between the cords  121  of the sleeve  120  correspond to the areas where the cord  121  is not present and the tube  110  existing on the inner side of the cords is exposed when the sleeve is viewed from the exterior side. 
     Further, in the present disclosure, the average angles Θ 1 , Θ 2 , Θ 3  formed by the cords  121  with respect to the axis direction D AX  of the actuator represent acute angles of the angles formed by the cords  121  with respect to the axis direction D AX  of the actuator, respectively. 
     It is preferable to use, as the cord  121  of the sleeve  120 , a fiber cord made of at least one fiber material selected from the group consisting of: polyamide fibers such as aramid fiber (aromatic polyamide fiber), polyhexamethylene adipamide (Nylon 6, 6) fiber, polycaprolactam (Nylon 6) fiber and the like; polyester fiber such as polyethylene terephthalate (PET) fiber, polyethylene naphthalate (PEN) fiber and the like; polyurethane fiber; rayon; acrylic fiber; and polyolefin fiber. In this case, durability of the sleeve further improves. It is particularly preferable to use a cord made of aramid fiber in terms of ensuring satisfactory strength of the sleeve  120 . 
     However, the cord  121  is not restricted to such fiber cords as described above. It is acceptable, for example, to use as the cord  121  a cord made of high strength fiber such as PBO (poly para-phenylene benzobisoxazole) fiber or a metal cord made of ultra-fine filaments. 
     Surfaces of the fiber/metal cords described above may be covered with rubber, mixture of a thermosetting resin and latex, or the like. In a case where surfaces of the cords are covered with these materials, it is possible to decrease a friction coefficient of the surfaces of the cords to an adequate level, while improving durability of the cords. 
     A solid content in the mixture of a thermosetting resin and latex is preferably in the range of ≥15 mass % and ≤50 mass % and more preferably in the range of ≥20 mass % and ≤40 mass %. Examples of the thermosetting resin include phenol resin, resorcin resin, urethane resin, and the like. Examples of the latex include vinyl pyridine (VP) latex, styrene-butadiene rubber (SBR) latex, acrylonitrile-butadiene rubber (NBR) latex, and the like. 
     In the present disclosure, it is preferable that the sleeve  120  is, as shown in  FIGS. 3A and 4A , made of one group of cords  121 A disposed in one direction and the other group of cords  121 B disposed to intersect the one group of cords  121 A, so that pairs of the two intersecting points at which pairs of the cords  121  intersect one cord  121  at the upper/lower side thereof in an alternate manner are shifted by a single cord  121 , in terms of the intersecting points, from pairs of the two intersecting points at which pairs of the cords  121  intersect another cord  121  (adjacent to the one cord  121 ) at the upper/lower side thereof in an alternate manner. That is, it is preferable that the sleeve  120  is woven by a twill weave. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     Further, in the present disclosure, it is also preferable that the sleeve  120  is, as shown in  FIGS. 3B and 4B , made of one group of cords  121 A disposed in one direction and the other group of cords  121 B disposed to intersect the one group of cords  121 A, so that the intersecting points at which the cords  121  intersect one cord  121  at the upper/lower side thereof in an alternate manner are shifted, by a single cord  121 , from the intersecting points at which the cords  121  intersect another cord  121  (adjacent to the one cord  121 ) at the upper/lower side thereof in an alternate manner. That is, it is also preferable that the sleeve  120  is woven by a plain weave. The tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability in this case, as well. 
     Yet further, in the present disclosure, it is also preferable that the sleeve  120  is made of the cords  121  woven by a basket weave. The tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability in this case, as well. The number of the cords to be aligned in the basket weave is not particularly limited. In the present disclosure, it is preferable that one pair of two cords is aligned and then another pair of two cords aligned separately is driven into the one pair of the two cords. 
     In the present disclosure, the cords  121  of the sleeve  120  have breaking strength of preferably at least 200 N/one cord, more preferably in the range of ≥250 N/one cord and ≤1000 N/one cord, further more preferably in the range of ≥300 N/one cord and ≤1000 N/one cord, yet further more preferably in the range of ≥500 N/one cord and ≤1000 N/one cord, and most preferably in the range of ≥600 N/one cord and ≤1000 N/one cord. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, the cords  121  of the sleeve  120  each have breaking elongation of preferably at least 2.0%, more preferably in the range of ≥3.0% and ≤6.0%. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, each of the cords  121  of the sleeve  120  has a diameter preferably in the range of 0.3 mm to 1.5 mm, more preferably in the range of 0.4 mm to 1.5 mm, further more preferably in the range of 0.5 mm to 1.5 mm, yet further more preferably in the range of 0.6 mm to 1.3 mm, and most preferably in the range of 0.6 mm to 1.0 mm. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, driving density of the cords  121  in the sleeve  120  is preferably in the range of 6.8 cords/cm to 25.5 cords/cm, more preferably in the range of 10.0 cords/cm to 23.5 cords/cm, and further more preferably in the range of 10.0 cords/cm to 20.0 cords/cm. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, provided that “t” (mm) represents thickness of the tube  110 , “d” (mm) represents a diameter of the cord  121  of the sleeve  120 , “Θ 1 ” represents the average angle formed by the cord  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator with no load and no pressure applied thereon, and “Θ 2 ” represents the average angle formed by the cord  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator in an actuator contracting state, it is preferable that t, d, Θ 1  and η 2  satisfy general formula (1) shown below. 
     
       
         
           
             
               
                 
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                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 Θ 
                                 2 
                               
                             
                           
                         
                         ) 
                       
                       · 
                       d 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     When t, d, Θ 1  and Θ 2  satisfy general formula (1), the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     Further, provided that “t” (mm) represents thickness of the tube  110 , “d” (mm) represents a diameter of the cord  121  of the sleeve  120 , “Θ 1 ” represents the average angle formed by the cord  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator with no load and no pressure applied thereon, and “Θ 2 ” represents the average angle formed by the cord  121  of the sleeve  120  with respect to the axis direction D AX  of the actuator in the actuator contracting state, it is more preferable that t, d, Θ 1  and Θ 2  satisfy general formula (2) shown below. 
     
       
         
           
             
               
                 
                   t 
                   &gt; 
                   
                     
                       
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               2 
                               ⁢ 
                               
                                 Θ 
                                 2 
                               
                             
                             ) 
                           
                         
                         ⁢ 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               Θ 
                               2 
                             
                             ) 
                           
                         
                       
                       
                         
                           sin 
                           2 
                         
                         ⁡ 
                         
                           ( 
                           
                             2 
                             ⁢ 
                             
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                               1 
                             
                           
                           ) 
                         
                       
                     
                     · 
                     d 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     When t, d, Θ 1  and Θ 2  satisfy general formula (2), the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In present disclosure, twist coefficient K of the cord  121  of the sleeve  120 , defined by general formula (3) shown below, is preferably in the range of 0.14 to 0.50, more preferably in the range of 0.16 to 0.50. 
                   K   =       T   2     ×       0.125   ×     D   ρ         ×     10     -   3                 (   3   )               
[In the formula (3), “T 2 ” represents the second twist number (number/1.0 cm) of the cord, T 2  should be replaced with the first twist number T 1  (number/10 cm) when the cord is a single twist cord, “D” represents the fineness per one raw yarn (dtex) of the cord, and “ρ” represents the density (g/cm 3 ) of the yarn of the cord.]
 
     When the twist coefficient K of the cord  121  of the sleeve  120  is equal to 0.14 or larger, the fibers of the actuator bear relatively small load and thus the actuator exhibits further improved durability. When the twist coefficient K of the cord  121  of the sleeve  120  is equal to 0.50 or less, the tube of the actuator bears relatively small load and thus the actuator exhibits further improved durability. 
     In this respect, the twist coefficient K of the cord  121  can be adjusted by changing density and/or fineness of the yarn to be used, the first twist number when the cord is manufactured, and the like. 
     In the present disclosure, the cord  121  of the sleeve  120  has a ratio (T 1 /D) of the first twist number T 1  (number/10 cm) with respect to the fineness D (dtex) per one raw yarn of the cord  121  preferably in the range of 0.004 to 0.03, more preferably in the range of 0.004 to 0.02. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, the cord  121  of the sleeve  120  has a ratio (T 1 /T 2 ) of the first twist number T 1  (number/10 cm) with respect to the second twist number T 2  (number/10 cm) preferably in the range of 0.8 to 1.2, more preferably in the range of 0.9 to 1.1. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, the fineness D per one raw yarn of the cord  121  of the sleeve  120  is preferably in the range of 800 to 5000 dtex, more preferably in the range of 800 to 4000 dtex, further more preferably in the range of 1000 to 4000 dtex, yet further more preferably in the range of 1500 to 4000 dtex, and most preferably in the range of 2000 to 4000 dtex. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, the cord  121  of the sleeve  120  has the first twist number T 1  preferably in the range of 3.2 to 150/10 cm, more preferably in the range of 10 to 36/10 cm, and further more preferably in the range of 10 to 30/10 cm. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, the cord  121  of the sleeve  120  has the second twist number T 2  preferably in the range of 2.6 to 180/10 cm, more preferably in the range of 10 to 36/10 cm, and further more preferably in the range of 10 to 30/10 cm. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, the number of the twisted yarns constituting the cord  121  of the sleeve  120  is preferably in the range of 2 to 4 and particularly preferably 2. In this case, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits further improved durability. 
     In the present disclosure, the fineness D per one raw yarn of the cord  121  of the sleeve  120  is preferably in the range of 800 to 5000 dtex. Further, the cord  121  has the first twist number T 1  preferably in the range of 3.2 to 150/10 cm, the second twist number T 2  preferably in the range of 2.6 to 180/10 cm, and the number of the twisted yarns constituting the cord preferably in the range of 2 to 4. When the fineness D per one raw yarn, the first twist number T 1 , the second twist number T 2 , and the number of the twisted yarns constituting each cord, of the cord  121  of the sleeve  120 , are unanimously within the aforementioned preferable ranges, the tube  110  of the actuator bears yet smaller load and thus the actuator exhibits significantly improved durability. 
     A method for manufacturing the cord  121  is not particularly restricted. For example, in a case where the cord  121  has what is called a double twist structure in which a plurality of yarns (preferably 2 to 4 yarns) are twisted, the cord can be manufactured, for example, by subjecting each yarn to first twist, aligning a plurality of the yarns thus twisted, and subjecting the yarns thus aligned to second twist in the direction opposite to the first twist, thereby obtaining a twisted yarn cord. 
     Alternatively, in a case where the cord  121  has what is called a single twist structure in which the cord is obtained by single twist of yarn(s), the cord can be manufactured, for example, by aligning yarn(s) and then twisting them in one direction, thereby obtaining a twisted yarn cord. In the present disclosure, in a case Where the cord  121  has a single twist structure, the first twist number T 1  represents the number of the twist (number/10 cm) of yarn(s) when a twisted yarn cord is manufactured. Further, in a case where the cord  121  has a single twist structure, the second twist number T 2  (number/10 cm) in the formula (3) should be replaced with the first twist number T 1  (number/10 cm). That is, in a case where the cord  121  has a single twist structure, T 2  in the formula (3) represents the number of the twist (number/10 cm) of yarn(s) when a twisted yarn cord is manufacture. 
     In  FIG. 2 , the sealing mechanism  200  seals an end portion in the axis direction D AX  of the actuator main body  100 . The sealing mechanism  200  includes the sealing member  210 , a first locking ring  220  and the caulking member  230 . 
     The sealing member  210  has a trunk portion  211  and a flange portion  212 . Metal such as stainless steel can be suitably used for the sealing member  210 . However, the material for the sealing member  210  is not restricted to metal and a hard plastic material or the like can be used instead of metal. 
     The trunk portion  211  has a tube-like shape. A passage hole  215  through which the working fluid flows is formed in the trunk portion  211 . The passage hole  215  communicates with the passage hole  410  (see  FIG. 1 ). The trunk portion  211  is inserted into the tube  110 . 
     The flange portion  212 , which is integral with the trunk portion  211 , is positioned further on the side of the axis direction D AX  end portion of the hydraulic actuator  10  than the trunk portion  211 . The flange portion  212  has a larger outer diameter in the radial direction D R  than the outer diameter of the trunk portion  211 . The flange portion  212  is fixedly engaged with the tube  110  having the trunk portion  211  inserted therein and the first locking ring  220 . 
     Irregular portions  213  are formed at an outer peripheral surface of the trunk portion  211 . The irregular portions  213  contribute to suppressing slippage of the tube  110  relative to the trunk portion  211  inserted therein. The irregular portions  213  preferably include at least three projecting portions. 
     Further, a first small diameter portion  214 , of which outer diameter is smaller than that of the trunk portion  211 , is formed in a portion adjacent to the flange portion  212 , of the trunk portion  211 . The configuration of the first small diameter portion  214  will be further described with reference to  FIGS. 5 to 12 . 
     The first locking ring  220  is fixedly engaged with the sleeve  120 . Specifically, the sleeve  120  is folded on the outer side in the radial direction D R  and backward by way of the first locking ring  220  (not shown in  FIG. 2 . See  FIG. 5 ). 
     The outer diameter of the first locking ring  220  is larger than that of the trunk portion  211 . The first locking ring  220  is fixedly engaged with the sleeve  120  at the position of the first small diameter portion  214  of the trunk portion  211 , That is, the first locking ring  220  is fixedly engaged with the sleeve  120  at a position adjacent to the flange portion  212  and on the radial direction D R  outer of the trunk portion  211 . 
     The first locking ring  220  has a configuration split into two portions in the embodiments, so that the first locking ring  220  can be engaged with the first small diameter portion  214  having an outer diameter smaller than that of the trunk portion  211 . It should be noted that the configuration of the first locking ring  220  is not restricted to the aforementioned two-split one. The first locking ring  220  may be split into three or more portions and some of the split portions may be pivotably linked with each other. 
     Any of metal, a hard plastic material or the like, i.e. those similar to the materials for the sealing member  210 , can be used as a material for the first locking ring  220 . 
     The caulking member  230  caulks the actuator main body  100  in collaboration with the sealing member  210 . Metal such as aluminum alloy, brass, iron or the like can be used as a material for the caulking member  230 . Indentations  231  as shown in  FIG. 1  are formed at an outer surface of the caulking member  230  as a result of the caulking member&#39;s being caulked by the caulking jigs. 
     (2) Structure of Sealing Mechanism 
     Next, embodiments of the sealing mechanism  200  will be described with reference to  FIGS. 5 to 12 . 
     (2.1) Embodiment 1-1 
       FIG. 5  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 , cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 1-1. 
     The sealing member  210  has the first small diameter portion  214 , of which outer diameter is smaller than that of the trunk portion  211 , as described above. 
     The first locking ring  220  is disposed on the outer side in the radial direction D R  of the first small diameter portion  214 . The inner diameter R 1  of the first locking ring  220  is smaller than the outer diameter R 3  of the trunk portion  211 . The outer diameter R 2  of the first locking ring  220  may also be smaller than the outer diameter R 3  of the trunk portion  211 . 
     The trunk portion  211  is inserted into the tube  110  such that the tube  110  is in contact with the flange portion  212 . The sleeve  120 , on the other hand, is folded on the outer side in the radial direction D R  and then backward via the first locking ring  220 . As a result, the sleeve  120  has a first folded-back portion  120   a , which has been folded backward by way of the first locking ring  220  at the end in the axis direction D AX  of the actuator. Specifically, the sleeve  120  includes: a sleeve main body  120   b  covering the outer peripheral surface of the tube  110 ; and the first folded-back portion  120   a  folded backward at the end in the axis direction D AX  of the sleeve main body  120   b  to be disposed on the outer peripheral side of the sleeve main body  120   b.    
     The first folded-back portion  120   a  is attached to the sleeve main body  120   b  situated on the outer side in the radial direction D R  of the tube  110 . Specifically, an adhesive layer  240  is formed between the sleeve main body  120   b  and the first folded-back portion  120   a , so that the sleeve main body  120   b  and the first folded-back portion  120   a  are fixedly attached to each other by the adhesive layer  240 . An appropriate adhesive can be used for the adhesive layer  240  in accordance with the type of the cords constituting the sleeve  120 . 
     However, the adhesive layer  240  is not essentially needed in the present disclosure and it is acceptable that the first folded-back portion  120   a  is not fixedly attached to the sleeve main body  120   b.    
     The trunk portion  211  of the sealing member  210  is inserted into the caulking member  230  having an inner diameter larger than the outer diameter of the trunk portion  211  and then the caulking member is caulked by the jig members. The caulking member  230  caulks the actuator main body  100  in collaboration with the sealing member  210 . Specifically, the caulking member  230  caulks the tube  110  having the trunk portion  211  inserted therein, the sleeve main body  120   b , and the first folded-back portion  120   a . That is, the caulking member  230  caulks the tube  110 , the sleeve main body  120   b , and the first folded-back portion  120   a  in collaboration with the sealing member  210 . 
     (2.2) Embodiment 1-2 
       FIG. 6  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 , cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 1-2. Hereinafter, Embodiment 1-2 will be described mainly in regard to differences between Embodiment 1-1 and itself. 
     In Embodiment 1-2, a sheet-like elastic member is provided between the first folded-back portion  120   a  of the sleeve  120  and the caulking member  230 . Specifically, a rubber sheet  250  is provided between the first folded-back portion  120   a  and the caulking member  230 . The rubber sheet  250  is provided so as to cover an outer peripheral surface of the cylindrical first folded-back portion  120   a . The type of rubber sheet  250  is not particularly restricted. A rubber material similar to the rubber of the tube  110  may be used for the rubber sheet  250 . The caulking member  230  caulks the actuator main body  100  including the rubber sheet  250  in collaboration with the sealing member  210 . 
     (2.3) Embodiment 1-3 
       FIG. 7  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 , cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 1-3. 
     In Embodiment 1-3, a rubber sheet  260  is used in place of the adhesive layer  240  of Embodiment 1-1. The rubber sheet  260  is a sheet-like elastic member and provided between the sleeve main body  120   b  and the first folded-back portion  120   a . A rubber material similar to the rubber of the rubber sheet  250  may be used for the rubber sheet  260 . 
     (2.4) Embodiment 2-1 
       FIG. 8  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 A, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 2-1. 
     In Embodiment 2-1, a sealing mechanism  200 A is used in place of the sealing mechanism  200  of Embodiments 1-1, 1-2 and 1-3. The sealing mechanism  200 A differs from the sealing mechanism  200  in that the former lacks the first small diameter portion  214  formed in the latter. 
     The sealing mechanism  200 A includes a sealing member  210 A, a first locking ring  220 A, and a caulking member  230 A. 
     A trunk portion  211 A of the sealing member  210 A is inserted into the tube  110 . Since the sealing member  210 A lacks the first small diameter portion  214  provided in the sealing member  210 , the diameter of the first locking ring  220 A is larger than the outer diameter of the entire trunk portion  211 A. Accordingly, the first locking ring  220 A is held by the flange portion  212 A and the caulking member  230 A between the flange portion  212 A and the caulking member  230 A. 
     Since the diameter of the first locking ring  220 A is larger than the outer diameter of the entire trunk portion  211 A, the caulking member  230 A is not in contact with the flange portion  212 A. That is, the first locking ring  220 A is exposed to the exterior at the portion thereof on which the sleeve  120  is folded backward. Further, the first locking ring  220 A need not be split like the first locking ring  220  of the embodiments 1-1, 1-2 and 1-3 because the diameter of the first locking ring  220 A is safely larger than the outer diameter of the entire trunk portion  211 A. 
     An adhesive layer  240  is formed between the sleeve main body  120   b  and the first folded-back portion  120   a  in the present embodiment, as in Embodiment 1-1. 
     (2.5) Embodiment 2-2 
       FIG. 9  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 A, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 2-2. Hereinafter, Embodiment 2-2 will be described mainly in regard to differences between Embodiment 2-1 and itself. 
     In Embodiment 2-2, a sheet-like elastic member is provided between the first folded-back portion  120   a  of the sleeve  120  and the caulking member  230 A. Specifically, a rubber sheet  250 A is provided between the first folded-back portion  120   a  and the caulking member  230 A. The rubber sheet  250 A is provided so as to cover an outer peripheral surface of the cylindrical first folded-back portion  120   a  as the rubber sheet  250  does in Embodiment 1-2. 
     (2.6) Embodiment 2-3 
       FIG. 10  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 A, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 2-3. 
     In Embodiment 2-3, a rubber sheet  260  is used in place of the adhesive layer  240  of Embodiment 2-1. The rubber sheet  260  is a sheet-like elastic member and provided between the sleeve main body  120   b  and the first folded-back portion  120   a , as in Embodiment 1-3. 
     (2.7) Embodiment 3-1 
       FIG. 11  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 B, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 3-1. Embodiment 3-1 and Embodiment 3-2 employ two locking rings. 
     The sealing mechanism  200 B includes a sealing member  210 B, a first locking ring  220 B, a caulking member  230 B, and a second locking ring  270 , as shown in  FIG. 11 . 
     The sealing mechanism  200 B includes the second locking ring  270 , as well as the first locking ring  220 B, as described above. The second locking ring  270  fixedly holds the sleeve  120  at a position on the outer side in the radial direction D R  of a trunk portion  211 B and closer to the center in the axis direction D AX  of the actuator main body  100  than the first locking ring  220 B. 
     Specifically, the sealing member  210 B has a second small diameter portion  216 B, of which outer diameter is smaller than that of the trunk portion  211 B. 
     The second locking ring  270  is provided on the outer side in the radial direction D R  of the second small diameter portion  216 B. The inner diameter of the second locking ring  270  is preferably smaller than the outer diameter of the trunk portion  211 B. The outer diameter of the second locking ring  270  may also be smaller than the outer diameter of the trunk portion  211 B. Due to this structure, the second locking ring  270  is fixedly engaged with the second small diameter portion  216 B. 
     The sleeve  120  has a second folded-back portion  120   c , which has been folded forward by way of the second locking ring  270 . The second folded-back portion  120   c  is continuous with the first folded-back portion  120   a . Specifically, the second folded-back portion  120   c  is folded forward at an end in the axis direction D AX  of the first folded-back portion  120   a  to be disposed on the outer peripheral side of the first folded-back portion  120   a.    
     More specifically, the sleeve  120 , folded toward the center side in the axis direction D AX  of the actuator main body  100  by way of the first locking ring  220 B, forms the first folded-back portion  120   a . The first folded-back portion  120   a  of the sleeve  120  is then folded on the side of the end portion in the axis direction D AX  of the actuator main body  100 , thereby forming the second folded-back portion  120   c.    
     The caulking member  230 B caulks the tube  110  having the trunk portion  211 B inserted therein, the sleeve main body  120   b  situated on the outer side in the radial direction D R  of the tube  110 , the first folded-back portion  120   a , and the second folded-back portion  120   c  in collaboration with the sealing member  210 B. 
     The rubber sheet  260  is provided between the sleeve main body  120   b  and the first folded-back portion  120   a , as in Embodiment 1-3. 
     Further, a sheet-like elastic member is provided between the first folded-back portion  120   a  and the second folded-back portion  120   c , as well. Specifically, a rubber sheet  280  is provided between the first folded-back portion  120   a  and the second folded-back portion  120   c . The rubber sheet  280  is provided so as to cover an outer peripheral surface of the cylindrical first folded-back portion  120   a.    
     Yet further, a rubber sheet  290  having a configuration similar to that of the rubber sheet  250  of Embodiment 1-3 is provided between the second folded-back portion  120   c  and the caulking member  230 B. The rubber sheet  290  is provided so as to cover an outer peripheral surface of the cylindrical second folded-back portion  120   c.    
     (2.8) Embodiment 3-2 
       FIG. 12  is a partial sectional view of the hydraulic actuator  10  including a sealing mechanism  200 C, cut along the axis direction D AX  of the hydraulic actuator, according to Embodiment 3-2. Hereinafter, Embodiment 3-2 will be described mainly in regard to differences between Embodiment 3-1 and itself. 
     Embodiment 3-2 employs a sealing member  210 C in which neither the first small diameter portion  214 B nor the second small diameter portion  216 B is formed. 
     The sealing member  210 C has a trunk portion  211 C. Since neither the first small diameter portion  214 B nor the second small diameter portion  216 B of the sealing member  210 B is formed in the sealing member  210 C, the inner diameter of the first locking ring  220 C and the inner diameter of the second locking ring  270 C are larger than the outer diameter of the trunk portion  211 C, respectively. 
     The caulking member  230 C is positioned between the first locking ring  220 C and the second locking ring  270 C in the axis direction D AX . Accordingly, the first locking ring  220 C and the second locking ring  270 C are exposed to the exterior at the portions thereof on which the sleeve  120  is folded backward/forward. 
     Further, a rubber sheet  281  having a configuration similar to that of the rubber sheet  280  of Embodiment 3-1 is provided between the first folded-back portion  120   a  and the second folded-back portion  120   c . Yet further, a rubber sheet  291  having a configuration similar to that of the rubber sheet  290  of Embodiment 3-1 is provided between the second folded-back portion  120   c  of the sleeve  120  and the caulking member  230 C. 
     EXAMPLES 
     The present disclosure will be described further in detail by Examples hereinafter. The present disclosure is not limited by any means to these Examples. 
     (Preparation of Tube) 
     A rubber composition was prepared by mixing and kneading the following components by a Banbury mixer. 
     High nitrile NBR (acrylonitrile-butadiene rubber, “N220S”, manufactured by JSR Corporation): 45 parts by mass 
     Intermediate-high nitrile NBR (acrylonitrile-butadiene rubber, “N230S”, manufactured by JSR Corporation): 35 parts by mass 
     BR (butadiene rubber, “UBEPOL® BR150”, manufactured by Ube Industries, Ltd.): 20 parts by mass 
     Carbon black (“SEAST 3”, manufactured by Tokai Carbon Co., Ltd.): 50 parts by mass 
     Stearic acid (“STEARIC ACID 50S”, manufactured by New Japan Chemical Co., Ltd.): 1 part by mass 
     Anti-oxidant (“Nocrac 6C”, manufactured by Ouchi Shiko Chemical Industrial Co., Ltd.): 2 parts by mass 
     Resin (“Quintone 100”, manufactured by Zeon Corporation): 10 parts by mass 
     Plasticizer (“SANSO CIZER DOA”, manufactured by New Japan Chemical Co., Ltd.): 8 parts by mass 
     Zinc white (ZnO, “Zinc White No. 3”, manufactured by Hakusui Tech Co., Ltd.): 5 parts by mass 
     Sulfur (“Sulfax Z”, manufactured by Tsurumi Chemical Industry Co., Ltd.): 1 part by mass 
     Vulcanization accelerator CBS (“Nocceler CZ”, manufactured by Ouchi Shiko Chemical Industrial Co., Ltd.): 1 part by mass 
     Vulcanization accelerator TOT (“Nocceler TOT-N”, manufactured by Ouchi Shiko Chemical Industrial Co., Ltd.): 2 parts by mass 
     Test tubes each having a cylindrical configuration (length: 300 mm) were prepared by processing the rubber composition thus obtained, by an extrusion molding machine, respectively. The outer diameter and thickness of each of the test tubes thus prepared are shown in Table 1. 
     (Preparation of Sleeve) 
     Test sleeves each having a cylindrical, woven structure were prepared by weaving 64 cords made of aramid fibers having characteristics shown in Table 1, respectively. Each of the aramid fiber cords was prepared by subjecting the aramid fibers as raw yarns to first twist and then second twist. Accordingly, each test sleeve had a cylindrical, woven structure wherein 64 cords made of the aramid fibers were observed along a circumference of a cross section thereof. 
     Specifically, each test sleeve had a cylindrical, woven structure constituted of one group of 32 aramid fiber cords disposed in parallel to each other at equal intervals therebetween to collectively form a spiral configuration and the other group of 32 aramid fiber cords disposed in parallel to each other at equal intervals therebetween to collectively form another spiral configuration so as to intersect the one group of 32 aramid fiber cords. The one group of 32 aramid fiber cords and the other group of 32 aramid fiber cords were woven to intersect each other alternately. More specifically, the test sleeve was formed so that pairs of the two intersecting points at which pairs of the cords intersect one cord at the upper/lower side thereof in an alternate manner are shifted by a single cord, in terms of the intersecting points, from pairs of the two intersecting points at which pairs of the cords intersect another cord (adjacent to the one cord) at the upper/lower side thereof in an alternate manner, as shown in  FIG. 3A . That is, the test sleeve was woven by a twill weave. 
     The relevant characteristics of each test sleeve, as well as those of the cords constituting the test sleeve, are shown in Table 1. 
     (Preparation of Actuator) 
     Test actuators each having the structures shown in  FIGS. 1 and 2  were prepared by using the test tubes and the test woven sleeves described above, respectively. “UF46” of COSMO SUPER EPOCH was used as hydraulic oil for the tube integrated in the actuator. The angles of the cords constituting of the sleeve of each test actuator thus prepared, as well as durability of the test actuator, were evaluated by the methods described below, respectively. 
     &lt;Method for Evaluating Angle Formed by Cord Constituting Sleeve&gt; 
     The angle formed by the cord constituting the sleeve with respect to the axis direction of the actuator was determined as described below, i.e. by: 
     (1) photographing a relevant portion of the actuator; 
     (2) selecting an image of the middle portion of the actuator (the portion where the image is well focused and the satisfactory image quality for analysis is ensured, the portion corresponding to a region where a decrease in diameter of the sleeve is within 5% with respect to the largest diameter of the sleeve); 
     (3) measuring, in the image of the middle portion thus selected, angles formed by the cords constituting the sleeve with respect to the axis direction centerline of the sealing mechanism; and 
     (4) calculating the average of five values of angles thus measured, and regarding the average as a measurement value. 
     The aforementioned angle was measured for each test actuator in a state of no load and no pressure applied to the actuator and a contracting state with predetermined load and hydraulic pressure (internal pressure) applied thereon, respectively. In Table 1, the angle in the state of no load and no pressure applied to the actuator is indicated as “Initial cord angle Θ 1 ” and the angle in the contracting state with predetermined load and hydraulic pressure applied thereon is indicated as “Contracting-state cord angle Θ 2 ”. 
     &lt;Method for Evaluating the Total Area (S 2 ) of Clearances Between Cords Constituting Sleeve&gt; 
     The total area (S 2 ) of clearances between the cords was determined by a photographic analysis in a manner similar to that of &lt;Method for evaluating angle formed by cord constituting sleeve&gt; described above, while adjusting load applied to the actuator such that the average angle formed by the cords of the sleeve with respect to the axis direction of the actuator under the hydraulic pressure of 5 MPa was set to be 45°. Then, a ratio (S 2 /S 1 ) of the total area (S 2 ) thus determined, with respect to an area (S 1 ) of an outer peripheral surface of the actuator main body, was calculated. The ratio is indicated as “Contracting-state clearance rate (S 2 /S 1 )” in Table 1, ±1° was allowed as a margin of error in the actual measurement of angles of the cords. 
     &lt;Method for Evaluating Durability of Actuator&gt; 
     Durability of the test actuator was determined by: injecting the hydraulic oil into the tube and completely substituting air in the tube with the hydraulic oil; then controlling injection of the hydraulic oil such that the pressure of the hydraulic oil in the tube reciprocally changes between 0 MPa and 5 MPa in an alternate and repetitive manner at every 3 second; counting the number of injections until cracks were generated in the tube and the actuator could no longer function; and expressing the count number as an index value relative to the count number of Example 1 being “100”. The larger index value represents the higher durability. 
     Further, the state of malfunction/dysfunction of the broken actuator was observed and evaluated according to the criteria shown below. 
     A: Malfunction/dysfunction of the actuator due to damage on the tube at a portion thereof in direct contact with the cord 
     B: Malfunction/dysfunction of the actuator due to damage on the tube at a portion thereof not in direct contact with the cord 
     C: Malfunction/dysfunction of the actuator due to breakage of the cord. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                   
                   
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 4 
                 Example 5 
               
               
                   
               
               
                 Tube 
                 Outer diameter of tube 
                 mm 
                 13.0 
                 13.0 
                 13.0 
                 13.0 
                 13.0 
               
               
                   
                 Thickness (t) of tube 
                 mm 
                 2 
                 2.2 
                 2 
                 2.2 
                 2.2 
               
               
                 Sleeve 
                 Initial cord angle Θ 1   
                 degree 
                 25 
                 25 
                 25 
                 25 
                 25 
               
               
                   
                 (under no load and no pressure) 
               
               
                   
                 Contracting-state clearance rate 
                 % 
                 31.9 
                 11.1 
                 26.8 
                 8.7 
                 18.8 
               
               
                   
                 (S2/S1) 
               
               
                   
                 Contracting-state cord angle Θ 2   
                 degree 
                 53.1 
                 52.3 
                 51.3 
                 51.2 
                 51.9 
               
               
                   
                 Diameter (d) of cord 
                 mm 
                 0.51 
                 0.71 
                 0.47 
                 0.71 
                 0.71 
               
               
                   
                 Right side of formula (1) 
                 mm 
                 0.68 
                 0.67 
                 0.66 
                 0.66 
                 0.67 
               
               
                   
                 Right side of formula (2) 
                 mm 
                 1.82 
                 2.01 
                 1.77 
                 2.01 
                 2.01 
               
               
                   
                 Inner diameter of sleeve 
                 mm 
                 14.1 
                 14.1 
                 14.1 
                 14.1 
                 14.1 
               
               
                   
                 Fineness (D) of raw yarn 
                 dtex 
                 2200 
                 2200 
                 1100 
                 2200 
                 2200 
               
               
                   
                 Density (ρ) of raw yarn 
                 g/cm 3   
                 1.44 
                 1.44 
                 1.44 
                 1.44 
                 1.44 
               
               
                   
                 First twist number T 1  of cord 
                 number/10 cm 
                 28 
                 12 
                 15 
                 12 
                 12 
               
               
                   
                 Second twist number T 2  of cord 
                 number/10 cm 
                 28 
                 12 
                 15 
                 12 
                 12 
               
               
                   
                 The number of the twisted yarns 
                 number/cord 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                   
                 Twist coefficient K of cord 
                 — 
                 0.387 
                 0.166 
                 0.147 
                 0.166 
                 0.166 
               
               
                   
                 T 1 /D 
                 — 
                 0.013 
                 0.005 
                 0.014 
                 0.005 
                 0.005 
               
               
                   
                 T 1 /T 2   
                 — 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
               
                   
                 Breaking strength of cord 
                 N/cord 
                 615 
                 633 
                 340 
                 633 
                 633 
               
               
                   
                 Breaking elongation of cord 
                 % 
                 5.2 
                 4.9 
                 4.8 
                 4.9 
                 4.9 
               
               
                   
                 Driving density of cords in sleeve 
                 number/cm 
                 15.6 
                 15.6 
                 23.3 
                 15.6 
                 11.7 
               
               
                   
                 Type of cord weaving 
                 — 
                 Twill weave 
                 Twill weave 
                 Twill weave 
                 Twill weave 
                 Twill weave 
               
               
                 Evaluation 
                 Durability 
                 Index 
                 100 
                 313 
                 215 
                 575 
                 488 
               
               
                   
                 State of malfunction/dysfunction 
                 — 
                 A 
                 A 
                 A 
                 A 
                 A 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Example 6 
                 Comp. Ex. 1 
                 Comp. Ex. 2 
                 Comp. Ex. 3 
               
               
                   
                   
               
               
                   
                 Tube 
                 Outer diameter of tube 
                 mm 
                 13.0 
                 13.0 
                 13.0 
                 13.0 
               
               
                   
                   
                 Thickness (t) of tube 
                 mm 
                 2.2 
                 2 
                 2 
                 2 
               
               
                   
                 Sleeve 
                 Initial cord angle Θ 1   
                 degree 
                 25 
                 25 
                 25 
                 25 
               
               
                   
                   
                 (under no load and no pressure) 
               
               
                   
                   
                 Contracting-state clearance rate 
                 % 
                 16.4 
                 35.2 
                 47.4 
                 42 
               
               
                   
                   
                 (S2/S1) 
               
               
                   
                   
                 Contracting-state cord angle Θ 2   
                 degree 
                 51.0 
                 53.0 
                 52.1 
                 52.9 
               
               
                   
                   
                 Diameter (d) of cord 
                 mm 
                 0.83 
                 0.51 
                 0.33 
                 0.56 
               
               
                   
                   
                 Right side of formula (1) 
                 mm 
                 0.66 
                 0.68 
                 0.67 
                 0.68 
               
               
                   
                   
                 Right side of formula (2) 
                 mm 
                 2.13 
                 1.82 
                 1.63 
                 1.87 
               
               
                   
                   
                 Inner diameter of sleeve 
                 mm 
                 14.1 
                 14.1 
                 14.1 
                 14.1 
               
               
                   
                   
                 Fineness (D) of raw yarn 
                 dtex 
                 3600 
                 2200 
                 1100 
                 1100 
               
               
                   
                   
                 Density (ρ) of raw yarn 
                 g/cm 3   
                 1.44 
                 1.44 
                 1.44 
                 1.44 
               
               
                   
                   
                 First twist number T 1  of cord 
                 number/10 cm 
                 28 
                 28 
                 36 
                 58 
               
               
                   
                   
                 Second twist number T 2  of cord 
                 number/10 cm 
                 28 
                 28 
                 36 
                 52 
               
               
                   
                   
                 The number of the twisted yarns 
                 number/cord 
                 2 
                 2 
                 2 
                 2 
               
               
                   
                   
                 Twist coefficient K of cord 
                 — 
                 0.495 
                 0.387 
                 0.352 
                 0.508 
               
               
                   
                   
                 T 1 /D 
                 — 
                 0.008 
                 0.013 
                 0.033 
                 0.053 
               
               
                   
                   
                 T 1 /T 2   
                 — 
                 1.0 
                 1.0 
                 1.0 
                 1.1 
               
               
                   
                   
                 Breaking strength of cord 
                 N/cord 
                 918 
                 615 
                 312 
                 254 
               
               
                   
                   
                 Breaking elongation of cord 
                 % 
                 4.6 
                 5.2 
                 4.5 
                 6.2 
               
               
                   
                   
                 Driving density of cords in sleeve 
                 number/cm 
                 11.7 
                 11.7 
                 11.7 
                 15.6 
               
               
                   
                   
                 Type of cord weaving 
                 — 
                 Twill weave 
                 Twill weave 
                 Twill weave 
                 Twill weave 
               
               
                   
                 Evaluation 
                 Durability 
                 Index 
                 538 
                 63 
                 25 
                 22 
               
               
                   
                   
                 State of malfunction/dysfunction 
                 — 
                 A 
                 B 
                 C 
                 A 
               
               
                   
                   
               
            
           
         
       
     
     It is understood from Table 1 that the hydraulic actuator according to the present disclosure has high durability. 
     REFERENCE SIGNS LIST 
     
         
         
           
               10 : Hydraulic actuator 
               20 : Connection portion 
               100 : Actuator main body 
               110 : Tube 
               120 : Sleeve 
               120   a : First folded-back portion 
               120   b : Sleeve main body 
               120   c : Second folded-back portion 
               121 : Cord 
               121 A,  121 B: Cord groups 
               122 : Clearance between cords 
               200 ,  200 A,  200 B,  200 C: Sealing mechanism 
               210 ,  210 A,  210 B,  210 C: Sealing member 
               211 ,  211 A,  211 B,  211 C: Trunk portion 
               212 ,  212 A: Flange portion 
               213 : Irregular portions 
               214 ,  214 B: First small diameter portion 
               215 : Passage hole 
               216 B: Second small diameter portion 
               220 ,  220 A,  220 B,  220 C: First locking ring 
               230 ,  230 A,  230 B,  230 C: Caulking member 
               231 : Indentation 
               240 : Adhesive layer 
               250 ,  250 A: Rubber sheet 
               260 : Rubber sheet 
               270 ,  270 C: Second locking ring 
               280 ,  281 : Rubber sheet 
               290 ,  291 : Rubber sheet 
               300 : Sealing mechanism 
               400 ,  500 : Fitting 
               410 ,  510 : Passage hole 
             D AX : Axis direction 
             D R : Radial direction