Patent Publication Number: US-10316869-B2

Title: Single-acting hydraulic cylinder

Description:
TECHNICAL FIELD 
     The present invention relates to a single-acting hydraulic cylinder. 
     BACKGROUND ART 
     JP2001-200810A discusses a single-acting hydraulic cylinder including a piston rod inserted into a cylinder tube having a bottomed cylindrical shape, a piston provided in a tip of the piston rod to partition an inside of the cylinder tube into a rod-side chamber and a bottom-side chamber, and a cylinder head provided in an opening of the cylinder tube to slidably support the piston rod. In the single-acting hydraulic cylinder discussed in JP2001-200810A, the rod-side chamber is an air chamber. 
     SUMMARY OF INVENTION 
     In general, in the single-acting hydraulic cylinder in which the air is filled in the rod-side chamber, in order to reduce a sliding friction of the piston rod, a lubricant is filled in an inner circumference of the cylinder head. 
     In the single-acting hydraulic cylinder discussed in JP2001-200810A, the cylinder head has a bushing that slidably supports the piston rod and a lubricant filled in an inner circumferential groove formed on the inner circumference of the cylinder head. In this manner, in the single-acting hydraulic cylinder discussed in JP2001-200810A, lubricity between the piston rod and the bushing is secured by filling the lubricant in the inner side of the cylinder head. 
     However, it is difficult to fabricate such a groove formed on the inner circumference of the cylinder head. For this reason, in the single-acting hydraulic cylinder discussed in JP2001-200810A, producibility may be degraded. 
     An object of the present invention is to improve producibility of the single-acting hydraulic cylinder. 
     According to one aspect of the present invention, a single-acting hydraulic cylinder includes a cylinder tube having a bottomed cylindrical shape provided with an opening in one end; a piston rod inserted into the cylinder tube; a piston connected to a tip of the piston rod to partition an inside of the cylinder tube into a rod-side chamber in which a gas is filled and a bottom-side chamber in which an actuating liquid is supplied or discharged; a cylinder head provided in the opening of the cylinder tube to allow the piston rod to be inserted; a bushing installed on an inner circumference of the cylinder head to slidably support the piston rod; a housing recessed portion formed on the inner circumference of the cylinder head to allow the bushing to be inserted from an inlet portion; and a main seal provided on the inner circumference of the cylinder head, the main seal being sliding contact with an outer circumferential surface of the piston rod to seal the rod-side chamber. A bottom clearance filled with a lubricant material is formed between a bottom face of the housing recessed portion and the bushing. The bottom clearance is provided in the rod-side chamber side relative to the main seal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view partially illustrating a hydraulic cylinder according to a first embodiment of the invention; 
         FIG. 2  is a cross-sectional view illustrating a bushing and a housing recessed portion of the hydraulic cylinder according to the first embodiment of the invention; 
         FIG. 3  is a cross-sectional view illustrating a bushing and a housing recessed portion of a hydraulic cylinder according to a second embodiment of the invention; 
         FIG. 4  is a cross-sectional view illustrating a bushing and a housing recessed portion of a hydraulic cylinder according to a third embodiment of the invention; 
         FIG. 5  is a cross-sectional view illustrating a bushing and a housing recessed portion of a hydraulic cylinder according to a fourth embodiment of the invention; 
         FIG. 6  is a cross-sectional view illustrating a bushing and a housing recessed portion of a hydraulic cylinder according to fifth embodiment of the invention; and 
         FIG. 7  is a cross-sectional view illustrating a bushing and a housing recessed portion of a hydraulic cylinder in a comparison example of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;First Embodiment&gt; 
     A single-acting hydraulic cylinder  100  according to a first embodiment of the invention will now be described with reference to the accompanying drawings. Hereinafter, the single-acting hydraulic cylinder  100  will be simply referred to as a “hydraulic cylinder  100 .” 
     As illustrated in  FIG. 1 , the hydraulic cylinder  100  includes a cylinder tube  10  having a bottomed cylindrical shape and an opening  10 A provided in its one end, a piston rod  20  inserted into the cylinder tube  10 , a piston  30  connected to a tip of the piston rod  20  to partition an inside of the cylinder tube  10  into a rod-side chamber  2  and a bottom-side chamber  3 , and a cylinder head  40  provided in the opening  10 A of the cylinder tube  10  to allow the piston rod  20  to be inserted. 
     The hydraulic cylinder  100  is used as a lift cylinder for lifting or lowering a load of a forklift. The cylinder tube  10  of the hydraulic cylinder  100  is connected to a chassis (not shown) of the forklift, and the piston rod  20  is connected to a fork (not shown) used to place the load. The hydraulic cylinder  100  is mounted on the chassis of the forklift such that its center axis extends in a vertical direction. The fork is lifted or lowered by expanding or contracting the hydraulic cylinder  100 . 
     A gas is filled in the rod-side chamber  2  of the cylinder tube  10 , and a hydraulic fluid as an actuating liquid is supplied to or discharged from the bottom-side chamber  3 . The hydraulic cylinder  100  is expanded by an actuating hydraulic pressure guided from a hydraulic pressure source (actuating liquid pressure source) to the bottom-side chamber  3 . As the actuating hydraulic pressure of the bottom-side chamber  3  decreases, the piston rod  20  and the piston  30  move downward by their self-weights, so that the hydraulic cylinder  100  is contracted. 
     The cylinder head  40  is fixed to the cylinder tube  10 . A main seal  41  and a dust seal  42  are inserted into the inner circumference of the cylinder head  40 . 
     The main seal  41  makes sliding contact with an outer circumferential surface of the piston rod  20  to seal the rod-side chamber  2  of the cylinder tube  10 . The dust seal  42  prevents dust from intruding into the cylinder tube  10 . 
     Here, in order to facilitate understanding of the hydraulic cylinder  100 , the hydraulic cylinder  600  as a comparison example will be described with reference to  FIG. 7 . Like reference numerals denote like elements as in the hydraulic cylinder  100 . 
     As illustrated in  FIG. 7 , the hydraulic cylinder  600  includes an annular bushing  60  provided on the inner circumference of the cylinder head  40  to slidably support the piston rod  20 , a housing recessed portion  550  formed on the inner circumference of the cylinder head  40  to receive the bushing  60  forcibly inserted, and a grease reserving groove  551  formed on the inner circumference of the cylinder head  40  to reserve grease  70  as a lubricant material. As the bushing  60  makes sliding contact with an outer circumferential surface of the piston rod  20 , the piston rod  20  is supported so as to move along an axial direction of the cylinder tube  10 . The bushing  60  is a so-called metal bushing formed of metal. 
     The bushing  60  is inserted from an inlet portion  50 A of the housing recessed portion  550  and is forcibly inserted to the housing recessed portion  550  until it abuts on a bottom portion  553 . 
     The grease reserving groove  551  is formed apart from the housing recessed portion  550  in an axial direction. The grease  70  is filled in the grease reserving groove  551 . As the grease  70  is filled in the grease reserving groove  551  on the inner circumference of the cylinder head  40 , the sliding surface between the piston rod  20  and the bushing  60  is lubricated, so that the piston rod  20  can smoothly slide. 
     However, in the hydraulic cylinder  600 , it is necessary to separately form, on the inner circumference of the cylinder head  40 , the housing recessed portion  550  where the bushing  60  is forcibly inserted and the grease reserving groove  551  where the grease  70  is filled. The grease reserving groove  551  is not opened on an end face of the cylinder head  40 , but is opened only on the inner circumference surface of the cylinder head  40 . For this reason, it is particularly difficult to fabricate the grease reserving groove  551 , fabrication accuracy is easily degraded, and burrs may be easily formed. For this reason, producibility may be degraded in the hydraulic cylinder  600 . 
     In contrast, as illustrated in  FIGS. 1 and 2 , the hydraulic cylinder  100  includes the annular bushing  60  provided on the inner circumference of the cylinder head  40  to slidably support the piston rod  20 , and the annular housing recessed portion  50  formed on the inner circumference of the cylinder head  40  to receive the bushing  60  forcibly inserted. Meanwhile, the hydraulic cylinder  100  has no grease reserving groove  551 . 
     The bushing  60  is forcibly inserted to the housing recessed portion  50  from the inlet portion  50 A opened to the rod-side chamber  2 . As illustrated in  FIG. 2 , the housing recessed portion  50  has a cylindrical forcible insertion portion  52  as an inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted, and a bottom face  53  perpendicular to the center axis of the cylinder head  40 . That is, the housing recessed portion  50  is an annular concave portion having a rectangular cross section. By forming the housing recessed portion  50  in a rectangular cross-sectional shape, it is possible to more easily perform fabrication. 
     As illustrated in  FIG. 2 , the bushing  60  is formed to have a total axial length shorter than a total axial length of the housing recessed portion  50 . The bushing  60  is forcibly inserted by using a forcible insertion tool (not shown) for adjusting a forcible insertion length until an end face opposite to the rod-side chamber  2  substantially matches an end face of the cylinder head  40 . For this reason, a bottom clearance  51  is formed between the bushing  60  and the bottom face  53  of the housing recessed portion  50 . 
     The grease  70  as a lubricant material is filled in the bottom clearance  51  of the bottom portion of the housing recessed portion  50  (refer to  FIG. 1 ). As the grease  70  is filled in the inner side of the cylinder head  40 , a lubricating film is formed on the outer circumference of the piston rod  20 , so that the piston rod  20  smoothly slides. Note that the grease  70  is not illustrated in  FIG. 2 . 
     The lubricant material may be, for example, an impregnant material obtained by impregnating grease or the like. The lubricant material may be formed of any material that can form a lubricating film on the outer circumference of the piston rod  20  to improve lubricity of the piston rod  20 . In addition, a grease nipple for filling the grease  70  in the bottom clearance  51  may be provided in the cylinder head  40 . 
     In this manner, since the grease  70  is filled in the bottom clearance  51  between the bushing  60  and the bottom face  53  of the housing recessed portion  50 , it is not necessary to independently form the grease reserving groove  551  for filling the grease  70  on the inner circumference of the cylinder head  40 . 
     The bottom clearance  51  can be easily formed by increasing a depth of the housing recessed portion  50  having a rectangular cross section or shortening the length of the bushing  60  on the contrary. 
     According to the embodiment described above, the following effects can be obtained. 
     In the hydraulic cylinder  100 , the grease  70  is filled in the bottom clearance  51  between the housing recessed portion  50  and the bushing  60 . For this reason, it is not necessary to independently form the grease reserving groove  551  for filling the grease  70  on the inner circumference of the cylinder head  40 . Therefore, it is possible to improve producibility in the hydraulic cylinder  100 . 
     Next, second to fifth embodiments of the invention will now be described with reference to  FIGS. 3 to 6 . Note that the grease  70  is not illustrated intentionally in  FIGS. 3 to 6 . 
     &lt;Second Embodiment&gt; 
     A hydraulic cylinder  200  according to a second embodiment of the invention will be described with reference to  FIG. 3 . In the following description, the description will be made by focusing on differences from the first embodiment, and like reference numerals denote like elements as in the hydraulic cylinder  100  of the first embodiment and will not be described repeatedly. 
     In the first embodiment described above, the housing recessed portion  50  is an annular rectangular cross-sectional concave portion having a bottom face  53  perpendicular to the central axis of the cylinder head  40 . In contrast, unlike the first embodiment, a housing recessed portion  150  of the hydraulic cylinder  200  has a forcible insertion portion  152  as an inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted, a step portion  154  formed perpendicularly to the center axis of the cylinder head  40  to serve as an abutting portion where the end face of the bushing  60  abuts, and a cylindrical gap partitioning portion  155  formed toward the bottom face  153  from the inner side of the step portion  154  in the radial direction to partition the bottom clearance  151 . 
     As illustrated in  FIG. 3 , in the hydraulic cylinder  200 , the bushing  60  is forcibly inserted to the forcible insertion portion  152  until it abuts on the step portion  154 . In this manner, a forcible insertion length of the bushing  60  toward the housing recessed portion  150  is defined by the step portion  154 . The bottom clearance  151  where the grease  70  is filled is partitioned by the end face of the bushing  60  abutting on the step portion  154 , the bottom face  153  of the housing recessed portion  150 , and the gap partitioning portion  155 . 
     The gap partitioning portion  155  is formed in a cylindrical surface shape having an inner diameter smaller than that of the forcible insertion portion. Note that the gap partitioning portion  155  is not limited to the cylindrical surface shape, and may be formed in other shapes such as a tapered shape or a curved shape. 
     According to the second embodiment described above, it is possible to obtain the following effect as well as the same effects as those of the first embodiment. 
     Since the housing recessed portion  150  of the hydraulic cylinder  200  has the step portion  154 , the forcible insertion length of the bushing  60  toward the housing recessed portion  150  is defined by the step portion  154 . Therefore, it is possible to form the bottom clearance  151  in the bottom portion side of the housing recessed portion  150  without using the forcible insertion tool. Therefore, it is possible to improve assemblability of the hydraulic cylinder  200 . 
     &lt;Third Embodiment&gt; 
     Next, a hydraulic cylinder  300  according to a third embodiment of the invention will be described with reference to  FIG. 4 . In the following description, the description will be made by focusing on differences from the second embodiment, and like reference numerals denote like elements as in the hydraulic cylinder  200  of the second embodiment and will not be described repeatedly. 
     According to the second embodiment, the housing recessed portion  150  has the step portion  154  serving as the abutting portion perpendicular to the center axis. In contrast, unlike the second embodiment, a housing recessed portion  250  of the hydraulic cylinder  300  has a forcible insertion portion  252  having a cylindrical surface shape which is the inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted, and a bottom face  253  formed in a tapered shape in which its inner diameter gradually increases along the axial direction, and a boundary portion  253 A between the forcible insertion portion  252  and the bottom face  253  serves as the abutting portion. 
     As illustrated in  FIG. 4 , the housing recessed portion  250  has a forcible insertion portion  252  having a cylindrical surface shape which is the inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted, and a bottom face  253  having a tapered shape in which its inner diameter gradually increases toward the inlet portion  50 A side along the axial direction. Note that the bottom face  253  is not limited to the tapered shape and may be formed in other shapes such as a curved shape as long as the inner diameter gradually increases toward the inlet portion  50 A side. 
     The bushing  60  is forcibly inserted to the forcible insertion portion  252  until it abuts on a boundary portion  253 A with the forcible insertion portion  252  on the bottom face  253  of the housing recessed portion  250 . In this manner, the boundary portion  253 A adjoining with the forcible insertion portion  252  on the bottom face  253  serves as the abutting portion that defines a forcible insertion length of the bushing  60  to the housing recessed portion  250 . Since the bottom face  253  of the housing recessed portion  250  is formed in a tapered shape, a bottom clearance  251  is formed between the end face of the bushing  60  and the bottom face  253  of the housing recessed portion  250 . Similar to the first embodiment, the grease  70  is filled in the bottom clearance  251 . 
     According to the third embodiment described above, it is possible to obtain the following effects. 
     In the hydraulic cylinder  300 , the bottom face  253  of the housing recessed portion  250  is formed in a tapered shape. Therefore, the bushing  60  abuts on the boundary portion  253 A between the forcible insertion portion  252  of the housing recessed portion  250  and the bottom face  253 . In this manner, the forcible insertion length of the bushing  60  to the housing recessed portion  250  is defined by the boundary portion  253 A. Therefore, it is possible to form the bottom clearance  151  between the housing recessed portion  150  and the bottom face  153  without adjusting the forcible insertion length of the bushing  60  using the forcible insertion tool. Therefore, it is possible to improve assemblability of the hydraulic cylinder  200 . 
     &lt;Fourth Embodiment&gt; 
     Next, a hydraulic cylinder  400  according to a fourth embodiment of the invention will be described with reference to  FIG. 5 . In the following description, the description will be made by focusing on differences from the first embodiment, and like reference numerals denote like elements as in the hydraulic cylinder  100  of the first embodiment and will not be described repeatedly. 
     Unlike the first embodiment, a housing recessed portion  350  of the hydraulic cylinder  400  has a forcible insertion portion  352  which is the inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted, and a large diameter portion  354  formed between the forcible insertion portion  352  and the bottom face  353  with an inner diameter larger than that of the forcible insertion portion  352  to partition the bottom clearance  351 . 
     As illustrated in  FIG. 5 , the large diameter portion  354  of the housing recessed portion  350  is formed in a cylindrical surface shape having an inner diameter larger than that of the forcible insertion portion  352 . The bottom clearance  351  for filling the grease  70  is formed by the bottom face  353  of the housing recessed portion  350 , the large diameter portion  354 , and the end face of the bushing  60 . 
     Note that the bottom face  353  may be formed to have an inner diameter gradually increasing toward the inlet portion  50 A side, and may be formed in other shapes such as a tapered shape or a curved shape. In addition, the large diameter portion  354  is not limited to the cylindrical surface shape, and may be formed in other shapes such as a tapered shape or a curved shape. In addition, similar to the second embodiment, the abutting portion where the bushing  60  abuts may be provided between the forcible insertion portion  352  and the large diameter portion  354 . 
     According to the fourth embodiment described above, it is possible to obtain the following effects as well as the same effects as those of the first embodiment. 
     In the hydraulic cylinder  400 , the housing recessed portion  350  has the large diameter portion  354  formed to have an inner diameter larger than that of the forcible insertion portion  352 . Therefore, it is possible to increase a volume of the bottom clearance  351 . Therefore, it is possible to increase the amount of the grease  70  filled in the bottom clearance  351 . 
     &lt;Fifth Embodiment&gt; 
     Next, a hydraulic cylinder  500  according to a fifth embodiment of the invention will be described with reference to  FIG. 6 . In the following description, the description will be made by focusing on differences from the first embodiment, and like reference numerals denote like elements as in the hydraulic cylinder  100  of the first embodiment and will not be described repeatedly. 
     In the hydraulic cylinder  100 , the bushing  60  formed of metal is forcibly inserted to the forcible insertion portion  52  of the housing recessed portion  50  and is fixed to the cylinder head  40 . Instead, in the hydraulic cylinder  500 , the bushing  460  formed of a resin material that can be expanded or contracted by an external force is locked to the positioning portion  454  of the housing recessed portion  450  so as to be installed in the cylinder head  40 . 
     As illustrated in  FIG. 6 , the housing recessed portion  450  of the hydraulic cylinder  500  has an introducing portion  452  formed from the inlet portion  50 A, a positioning portion  454  provided between the introducing portion  452  and the bottom face  453  to lock and position the bushing  460  in the axial direction, and a clearance-forming portion  458  provided between the positioning portion  454  and the bottom face  453  to partition the bottom clearance  451 . 
     The positioning portion  454  has an installation cylindrical surface  455  formed to have an inner diameter larger than the inner diameter of the introducing portion  452  and slightly smaller than the outer diameter of the bushing  460 , a first side surface  456  that connects the installation cylindrical surface  455  and the introducing portion  452  and is perpendicular to the center axis, and a second side surface  457  that connects the installation cylindrical surface  455  and the clearance-forming portion  458  and is perpendicular to the center axis. 
     The introducing portion  452  and the clearance-forming portion  458  are formed as a cylindrical surface having an inner diameter smaller than the outer diameter of the bushing  460  and the inner diameter of the installation cylindrical surface  455  of the positioning portion  454 . 
     In order to install the bushing  460  in the housing recessed portion  450 , first, the bushing  460  is inserted into the introducing portion  452  while it is elastically deformed by an external force to reduce the outer diameter. 
     As the bushing  460  is inserted into the positioning portion  454 , the outer diameter of the bushing  460  that has been elastically deformed is returned to its original shape, so that the outer diameter of the bushing  460  becomes substantially equal to the inner diameter of the installation cylindrical surface  455 . As a result, an axial movement of the bushing  460  is restricted by the first and second side walls  456  and  457  of the positioning portion  454  and is locked to the positioning portion  454 . As a result, the bushing  460  is installed in the housing recessed portion  450  while its release from the housing recessed portion  450  is restricted. In this manner, unlike the first embodiment, the bushing  460  may not be forcibly inserted to and fixed to the housing recessed portion  450 . Instead, it may be installed in the housing recessed portion  450  while its axial movement is restricted as in the fifth embodiment. 
     As the bushing  460  is locked to the positioning portion  454  and is installed in the housing recessed portion  450 , the bottom clearance  451  for filling the grease  70  is formed by the bottom face  453  of the housing recessed portion  450 , the clearance-forming portion  458 , and the end face of the bushing  460 . 
     Note that, although the bushing  460  is formed of a resin material that can be relatively easily elastically deformed in the fifth embodiment, the bushing  460  may be formed of other materials without limiting to the resin material. 
     According to the fifth embodiment described above, it is possible to obtain the following effect in addition to the same effects as those of the first embodiment. 
     In the hydraulic cylinder  500 , the bushing  460  is formed of a resin material that can be relatively easily deformed by an external force and is installed in the cylinder head  40  as it is locked to the positioning portion  454  of the housing recessed portion  450 . For this reason, compared to a case where a metal bushing is forcibly inserted, it is possible to easily install the bushing  460  in the cylinder head  40  and reduce man-hours for the assembly work. 
     Configurations, functions, and effects of the embodiments of the invention will now be described in summary. 
     The single-acting hydraulic cylinder  100 ,  200 ,  300 ,  400 ,  500  includes a cylinder tube  10  having a bottomed cylindrical shape and an opening  10 A provided in one end, a piston rod  20  inserted into the cylinder tube  10 , a piston  30  connected to a tip of the piston rod  20  to partition an inside of the cylinder tube  10  into a rod-side chamber  2  filled with a gas and a bottom-side chamber  3  where a hydraulic fluid is supplied or discharged, a cylinder head  40  provided in the opening  10 A of the cylinder tube to allow the piston rod  20  to be inserted, a bushing  60  provided on an inner circumference of the cylinder head  40  to slidably support the piston rod  20 , and a housing recessed portion  50 ,  150 ,  250 ,  350 ,  450  formed on the inner circumference of the cylinder head  40  to receive the bushing  60   460  forcibly inserted from the inlet portion  50 A, wherein the bottom clearance  51 ,  151 ,  251 ,  351 ,  451  filled with the grease  70  is formed between the bottom face  53 ,  153 ,  253 ,  353 ,  453  of the housing recessed portion  50 ,  150 ,  250 ,  350 ,  450  and the bushing  60 ,  460 . 
     The housing recessed portion  50  of the single-acting hydraulic cylinder  100  is formed in a rectangular cross-sectional shape having a forcible insertion portion  52  and a bottom face  53  perpendicular to a center axis of the cylinder head  40 . The forcible insertion portion  52  is the inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted. 
     In this configuration, the grease  70  is filled in the bottom clearance  51 ,  151 ,  251 ,  351 ,  451  between the housing recessed portion  50 ,  150 ,  250 ,  350 ,  450  where the bushing  60 ,  460  is installed and the bushing  60 ,  460 . For this reason, it is not necessary to form a groove for filling the grease  70  on the inner circumference of the cylinder head  40 . Therefore, it is possible to improve producibility of the single-acting hydraulic cylinders  100 ,  200 ,  300 ,  400 ,  500 . 
     The single-acting hydraulic cylinder  200 ,  300  has the abutting portion (the step portion  154  and the boundary portion  253 A) where the bushing  60  abuts on the housing recessed portion  150 ,  250 . 
     The housing recessed portion  150  of the single-acting hydraulic cylinder  200  has the forcible insertion portion  152  which is the inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted, and the abutting portion is the step portion  154  formed perpendicularly to the center axis from the forcible insertion portion  152 . 
     The housing recessed portion  250  of the single-acting hydraulic cylinder  300  has the forcible insertion portion  252  which is the inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted, and the bottom face  253  formed to have an inner diameter gradually increasing toward the inlet portion  50 A. 
     In the single-acting hydraulic cylinder  300 , the boundary portion  253 A between the forcible insertion portion  252  and the bottom face  253  serves as the abutting portion. 
     In this configuration, the forcible insertion length of the bushing  60  toward the housing recessed portion  150 ,  250  is defined by the abutting portion (the step portion  154  and the boundary portion  253 A). Therefore, the bottom clearance  151 ,  251  is formed in the bottom portion of the housing recessed portion  150 ,  250  without adjusting the forcible insertion length of the bushing  60 . Therefore, it is possible to improve assemblability of the hydraulic cylinders  200 ,  300 . 
     The housing recessed portion  350  of the single-acting hydraulic cylinder  400  has the forcible insertion portion  352  which is the inner circumferential surface of the cylinder head  40  where the bushing  60  is forcibly inserted, and a large diameter portion  354  formed between the forcible insertion portion  352  and the bottom face  353  to partition the bottom clearance  351 . The large diameter portion  354  has an inner diameter larger than that of the forcible insertion portion  352 . 
     In this configuration, it is possible to increase a volume of the bottom clearance  351  where the grease  70  is filled. Therefore, it is possible to increase the filling amount of the grease  70 . 
     In the single-acting hydraulic cylinder  500 , the bushing  460  is formed expandably/contractably by an external force in a radial direction, and the housing recessed portion  450  has the introducing portion  452  formed from the inlet portion  50 A and provided with an inner diameter smaller than the outer diameter of the bushing  460 , the positioning portion  454  provided between the introducing portion  452  and the bottom face  453  to lock and position the bushing  460  in the axial direction, and a clearance-forming portion  458  formed between the positioning portion  454  and the bottom face  453  to partition the bottom clearance  451 . 
     In this configuration, the bushing  460  is relatively easily deformed by an external fore and is locked to the positioning portion  454  of the housing recessed portion  450  for installation to the cylinder head  40 . For this reason, compared to a case where a bushing formed of metal is forcibly inserted, it is possible to easily install the bushing  460  in the cylinder head  40  and reduce man-hours for the assembly work. 
     The single-acting hydraulic cylinders  100 ,  200 ,  300 ,  400 ,  500  are lift cylinders for lifting or lowering a fork of a forklift. 
     Embodiments of this invention were described above, but the above embodiments are merely examples of applications of this invention, and the technical scope of this invention is not limited to the specific constitutions of the above embodiments. 
     Although the hydraulic fluid is employed as an actuating liquid in each of the aforementioned embodiments, for example, a water-soluble alternative liquid may also be employed instead. 
     Although the hydraulic cylinder  100 ,  200 ,  300 ,  400 ,  500  is employed as a lift cylinder for lifting a load of the forklift in each of the aforementioned embodiments, they may also be employed in various fields other than the lift cylinder. 
     This application claims priority based on Japanese Patent Application No. 2015-58144 filed with the Japan Patent Office on Mar. 20, 2015, the entire contents of which are incorporated into this specification.