Patent Publication Number: US-8528755-B2

Title: Mobile crane having counterweight

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mobile crane having a counterweight. 
     2. Description of the Background Art 
     Heretofore, there has been known a large mobile crane having a counterweight for increasing a lifting capacity. For example, JP 2008-297112 A discloses a mobile crane which comprises a lower propelling body, a upper slewing body adapted to be slewed on the lower propelling body, a lattice-structured mast attached to the upper slewing body, and a counterweight hung from an upper end of the mast through a guy line, at a position rearward of the upper slewing body. In this type of mobile crane, a crane lifting capacity is adjusted by changing a horizontal distance between a mast foot which is a supported point of a base end of the mast and a center of gravity of the counterweight (the horizontal distance will hereinafter be referred to as “hanging radius of the counterweight”), or changing a mass of the counterweight. For example, when a boom is moved frontwardly while hanging a load therefrom (an operating radius is increased), the counterweight is moved rearwardly with respect to the upper slewing body to increase the hanging radius of the counterweight. 
     The hanging radius of the counterweight is adjusted, for example, by changing an angle of the mast. Specifically, the mast is raised and lowered about the mast foot. The raising and lowering movement of the mast, however, involves a vertical displacement of the mast point at the upper end of the mast, which causes the following problems. When the mast is lowered to increase the hanging radius of the counterweight, the counterweight is displaced downwardly and landed on a ground surface, which hinders the counterweight from functioning as a weight, and further hinders a slewing movement of the upper slewing body and a traveling movement of the mobile crane. On the other hand, when the mast is raised to reduce the hanging radius of the counterweight, the counterweight is lifted up from the ground surface: releasing a load in this state may cause the mobile crane to be inclined rearwardly. 
     To solving the above problems, it is required to preliminarily calculate/measure the mass of the counterweight, the hanging radius of the counterweight, and a length of the guy line for hanging the counterweight (the guy line will hereinafter be referred to as “counterweight guy line”), on a constant basis. The length adjustment of the counterweight guy line, however, requires a lot of time and effort. 
     As a mobile crane designed taking into account this point, there has heretofore been known one type equipped with a cylinder for changing a substantial length of the counterweight guy line. There are shown two examples of this type of mobile crane in  FIGS. 7 and 8 . 
       FIG. 7  shows a mobile crane  101 , which comprises a lower propelling body  10 , a upper slewing body  20 , a boom  25 , a mast  30  having a mast point  30   p  provided at an upper end thereof, a counterweight  50 , and a counterweight guy line  54  for hanging the counterweight  50  from the mast point  30   p , wherein a hydraulic cylinder  160  is interposed in an upper portion of the counterweight guy line  54 . 
     The hydraulic cylinder  160  has a cylinder body  162  surrounding an internal space thereof, a piston  164  provided within the cylinder body  162  to partition the internal space into an upper head-side cylinder chamber  160   h  and a lower rod-side cylinder chamber  160   r , and a rod  166  extending from the piston  164  downwardly beyond an lower end of the cylinder body  162 . The hydraulic cylinder  160  is operated to be extended/retracted depending on an angle of the mast  30 , by means of supply/discharge of a hydraulic pressure to/from the cylinder chambers  160   h  and  160   r , thereby allowing an altitudinal height H of the counterweight  50  from a ground surface G (the altitudinal height will hereinafter be referred to as “counterweight height H”) to be adjusted. 
       FIG. 8  shows a mobile crane  201  which comprises, instead of the cylinder  160  of the above mobile crane  101 , a counterweight support member  241  interconnecting the upper slewing body  20  and the counterweight guy line  54  and a hydraulic cylinder  260  interposed between the counterweight guy line  54  and the counterweight  50 . The counterweight support member  241  is adapted to be extended/retracted in a direction close to a horizontal direction, thereby allow a hanging radius R of the counterweight  50  to be changed without moving the mast  30 . This type of crane, involving no displacement of the mast point  30   p , does not permit a large variation of the counterweight height H; however, if a length of the counterweight guy line  54  was kept constant, the extension/retraction of the counterweight support member  241  could vary the counterweight height H. The hydraulic cylinder  260 , which has a structure similar to that of the cylinder  160 , is adapted to be extended/retracted in an upward-downward direction to allow the counterweight height H to be adjusted. 
     However, the above technique, using the hydraulic cylinder  160  or hydraulic cylinder  260  to pull up the counterweight  50  by the retraction of the cylinder, lowers driving efficiency. For example, pulling up the counterweight  50  by use of the hydraulic cylinder  160  shown in  FIG. 7  requires a hydraulic fluid to be supplied to the rod-side cylinder chamber  160   r  on the side of the counterweight  50 ; in the rod-side cylinder chamber  160   r , the piston  164  has a pressure receiving area smaller than that in the head-side cylinder chamber  160   h  by a cross-sectional area of the rod  166 . Therefore, in order to produce a sufficiently large pull-up force despite the relatively small pressure receiving area, it is necessary to increase a hydraulic pressure or increase a diameter of the cylinder  160 . Either case requires a costly hydraulic cylinder. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a mobile crane capable of adjusting a counterweight height with no use of a costly hydraulic cylinder. The present invention provides a mobile crane which comprises: a lower propelling body; a upper slewing body slewably mounted on the lower propelling body; a boom attached to the upper slewing body pivotably; a mast disposed at the rear of the boom and adapted to be raised and lowered so as to raise and lower the boom; a counterweight disposed at the rear of the upper slewing body and hung from the mast through a guy line; and a counterweight-lifting cylinder attached to the counterweight and adapted to be extended so as to vertically push up the counterweight while receiving a reaction force from a ground surface. The counterweight-lifting/lowering cylinder has a cylinder body surrounding an internal space thereof, a piston fitted inside the cylinder body to separate the internal space into an upper head-side cylinder chamber and a lower rod-side cylinder chamber, and a rod extending from the piston downwardly beyond an lower end of the cylinder body, the counterweight-lifting cylinder being adapted to be extended by supply of a hydraulic pressure into the head-side cylinder chamber while be retracted by supply of a hydraulic pressure into the rod-side cylinder chamber. 
     According to this mobile crane, the counterweight-lifting cylinder can vertically push up the counterweight while receiving a reaction force from a ground surface against the gravity acting on the counterweight, by supply of a hydraulic pressure into the head-side cylinder chamber of the counterweight-lifting cylinder which has a pressure receiving area greater than that of the rod-side cylinder chamber. For this reason, the driving efficiency of the counterweight-lifting cylinder is higher than that of a cylinder required to be retracted to pull up a counterweight against a gravity acting thereon, i.e., a cylinder in which a hydraulic pressure is supplied into a rod-side cylinder chamber having a relatively small pressure receiving area to pull up the counterweight, such as the cylinders  160  and  260  shown in  FIGS. 7 and 8 . This makes it possible to lift the counterweight with a limited hydraulic pressure for the operation of the hydraulic cylinder and a limited diameter of the hydraulic cylinder, thus enabling the counterweight to be lifted/lowered by use of a low-cost hydraulic cylinder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall view of a mobile crane according to one embodiment of the present invention. 
         FIG. 2  is a top plan view of a propelling body of the mobile crane, when viewing the propelling body in a direction indicated by the arrowed line F 2  in  FIG. 3 . 
         FIG. 3  is a rear view of a propelling body, when viewing the propelling body in a direction indicated by the arrowed line F 3  in  FIG. 2 . 
         FIG. 4A  is an enlarged view of a counterweight in  FIG. 1  and components associated therewith, when viewing the counterweight and the associated components in a direction indicated by the arrowed line F 4 A in  FIGS. 5 and 6 . 
         FIG. 4B  is a schematic diagram showing a counterweight-lifting cylinder adapted to be attached to the counterweight, and components associated thereto, such as hydraulic lines connected thereto. 
         FIG. 5  is a view along the arrowed line F 5 , wherein the counterweight and the associated components are shown when viewed in a direction indicated by the arrowed line F 5  in  FIGS. 4A and 6 . 
         FIG. 6  is a schematic diagram of the counterweight and the associated components, when viewed in a direction indicated by the arrowed line F 6  in  FIGS. 1 ,  4 A and  5 . 
         FIG. 7  is a front view showing a conventional mobile crane. 
         FIG. 8  is a front view showing another conventional mobile crane. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIGS. 1 to 6 , one embodiment of the present invention will now be described. 
       FIG. 1  shows a mobile crane  1 . This mobile crane  1  is a counterbalance type crane, having not only upper slewing body-rear end counterweights  22 , which are one attached to an rear end of the upper slewing body  20 , but also a counterweight  50  (extra counterweight) disposed at the rear of an after-mentioned upper slewing body  20 . Specifically, the mobile crane  1  comprises a lower propelling body  10 , a upper slewing body  20  slewably mounted on the lower propelling body  10 , a boom  25  pivotably attached to the upper slewing body  20 , a mast  30  pivotably attached to the upper slewing body  20  at a position rearward of the boom  25 , and a pair of right and left counterweight support members  41  attached to a rear end of the upper slewing body  20 , the counterweight  50  being hung from the upper end of the mast  30 . Furthermore, as shown in  FIG. 4A , attached to the counterweight  50  are a plurality of hydraulic cylinders  60  each serving as a counterweight-lifting cylinder. 
     The lower propelling body  10  includes a propelling body  11  shown in  FIG. 2  and a pair of crawlers  17  ( FIG. 1 ) attached to right and left sides of the propelling body  11  respectively, provided for traveling of the mobile crane  1 . 
     The propelling body  11 , as shown in  FIG. 2 , has a car body  12 , a pair of right and left crawler frames  13 , and a body lifter  15 . The car body  12  makes up a central portion of the propelling body  11 , and has a rectangular shape when viewed from thereabove. The crawler frames  13  are arranged respective ones of right and left sides of the car body  12  to extend in a frontward-rearward direction. The crawlers  17  are attached to respective ones of the crawler frames  13 . The body lifter  15  is attached to four corners of the car body  12  to lift/lower the lower propelling body  10  during assembling and disassembling of the lower propelling body  10 . 
     The body lifter  15  comprises four arms  16  each having a basal end attached to the car body  12  and adapted to be horizontally rotationally moved about the basal end, and four body lifter cylinders each attached to a distal end of the arms  16  respectively. In this embodiment, the hydraulic cylinders  60  are additionally used as the body lifter cylinders, as described later. 
     The distal end of the arm  16  is formed as a body-lifter-side cylinder-holding portion, namely a second cylinder-holding portion, adapted to detachably hold the hydraulic cylinder  60  used as the body lifter cylinder. More specifically, as shown in  FIG. 3 , the arm  16  has an arm-side bracket  16   b , and the hydraulic cylinder  60 , which can be additionally used as the body lifter cylinder, has a cylinder-side bracket  60   b  adapted to be attached to the mounting bracket  16   b . The brackets  16   b  and  60   b  have respective pin holes, into which respective pins are inserted to fix the hydraulic cylinder  60  to the distal end of the arm  16 . 
     For assembling and disassembling of the lower propelling body  10 , the body lifter  15  is extracted from the car body  12  outwardly in the frontward-rearward direction, as indicated by the solid lines in  FIG. 2 , and then operated to separate the propelling body  11  upward from a ground surface G, as shown in  FIG. 3  (it should be noted that  FIG. 3  expresses the spacing as a change in position of the ground surface G from G 2  to G 1 ). In this state, the crawler frames  13  are attached/detached to/from the car body  12  while the crawlers  17  shown in  FIG. 1  are kept attached to the respective crawler frames  13 . On the other hand, during a period other than the assembling and disassembling of the lower propelling body  10  (e.g., during transportation of the lower propelling body  10 ), the body lifter  15  is retracted along front and rear side surfaces of the car body  12 , as shown in  FIGS. 2 and 3  by the two-dot chain lines. 
     The upper slewing body  20 , as shown in  FIG. 1 , is supported on the lower propelling body  10  through a slewing bearing  19  so as to be able to be slewed relatively to the lower propelling body  10 . The upper slewing body  20  has a rear end portion to which the upper slewing body-rear end counterweight  22  are attached, a front end portion to which the boom  25  is attached, and a portion to which the mast  30  is attached at the rear of the boom  25 . Furthermore, there is attached a crane mast  27  to the upper slewing body  20  at a position below and rearward of the mast  30 . Besides, mounted to the upper slewing body  20  are an operator cabin  21  and non-illustrated components, such as a engine, a hydraulic pump and a hydraulic line. 
     The upper slewing body-rear end counterweight  22  are ones for preventing the mobile crane  1  from being inclined frontwardly due to a load hung by a hook  26  to thereby enlarge a lifting capacity of the mobile crane  1 . The upper slewing body-rear end counterweights  22  in this embodiment are fixed to right and left side surfaces of the rear end of the upper slewing body  20  respectively, as shown in  FIG. 5 . The upper slewing body-rear end counterweights  22  are provided separately from the counterweight  50  disposed rearward of the upper slewing body  20  as described in detail later. 
     The boom  25  is formed in a lattice structure, and attached to the front end of the upper slewing body  20  as shown in  FIG. 1  so as to be derrickable with respect to the upper slewing body  20 . The boom  25  has a distal end formed as a boom top  25   t , from which the hook  26  is suspended to hang a load through a wire rope. 
     The crane mast  27  is a member for raising/lowering the mast  30 , having a hollow rectangular cross-section. The crane mast  27  has a basal end attached to the upper slewing body  20  and a distal end located on an opposite side of the basal end, the distal end connected to a drum of a mast-raising/lowering winch (not shown) mounted on the upper slewing body  20  at a position near the rear end thereof, through a mast raising/lowering rope  28 . 
     The mast  30  is a member for raising/lowering the boom  25 , having a lattice structure in this embodiment. The mast  30  is disposed at the rear of the boom  25  and at the front of and above the crane mast  27 , that is, disposed between the boom  25  and the crane mast  27 . 
     The mast  30  has a basal end serving as a mast foot  30   f  vertically pivotably attached to a portion near the front end of the upper slewing body  20  and a distal end located on an opposite side of the base end and formed as a mast top  30   t , the mast top  30   t  and the boom top  25   t  interconnected through a boom-raising/lowering wire rope  32  and a boom guy line  33 . The boom-raising/lowering wire rope  32  is unreeled from a boom-raising/lowering winch  34  attached to the back surface of the mast  30 , and wound between a plurality of mast-side sheaves  35  aligned in a rightward-leftward direction near the mast top  30   t  and a plurality of guy line-side sheaves  36  connected to a rear end of the guy line  33  and arranged in the rightward-leftward direction. The boom-raising/lowering winch  34  is operable to reel or unreel the boom-raising/lowering wire rope  32  while the mast  30  is fixed, thus raising/lowering the boom  25 . 
     The mast  30  is raised and lowered pivotably in accordance with a vertical swinging movement thereof about the mast foot  30   f . Specifically, the mast top  30   t  and the distal end of the crane mast  27  are interconnected through a mast guy line  31 , and the mast-raising/lowering winch is operable to reel or unreel the mast raising/lowering rope  28  to raise and lower the crane mast  27 , thereby raising and lowering the mast  30 . 
     The counterweight support members  41 , which are so disposed as to extend in the frontward-rearward direction and be aligned in side-by-side relation in the rightward-leftward direction as shown in  FIG. 5 , are adapted to be extended/retracted so as to forcibly change a hanging radius R of the counterweight  50 , i.e., a horizontal distance between the mast foot  30   f  and a center of gravity (gravitational center) of the counterweight  50 . Each of the counterweight support members  41  has a front end attached to the rear end of the upper slewing body  20  and a rear end connected to the counterweight  50 . 
     The counterweight  50  is a weight which is disposed at the rear of the upper slewing body  20  and hung from the mast  30  through a guy line (counterweight guy line  54 ), as shown in  FIG. 1 . The counterweight  50  is provided to suppress the frontward inclination of the mobile crane  1  due to a load hung by the hook  26  to thereby enlarge the lifting capacity of the mobile crane  1 . The counterweight  50 , which is other than the upper slewing body-rear end counterweight  22 , is so-called an “extra counterweight”. This means that, when the upper slewing body  20  is slewed under a condition that the counterweight  50  is spaced from the ground surface G, the counterweight  50  is also slewed about the slewing bearing  19 . In  FIGS. 4A ,  5  and  6 , the “frontward-rearward direction” and the “rightward-leftward direction” are defined on the basis of the upper slewing body  20 . 
     As shown in  FIG. 4A , the counterweight  50  includes a base weight  51  disposed at a lowermost position, a plurality of weight members  53  stacked on the base weight  51 , and a connecting mechanism  52  for connecting the counterweight guy line  54  and the base weight  51 . In this embodiment, there are set a plurality of stacking regions arranged side-by-side in the rightward-leftward direction (in  FIGS. 5 and 6 , three regions consisting of a central region, and right and left regions on both sides of the central region); in each of the stack regions, the weight members  53  are stacked on the base weight  51 , and a lower end of the connecting mechanism  52  is connected to the base weight  51  at two or more positions (in  FIGS. 5 and 6 , two right and left positions between adjacent ones of the stacking areas). 
     The counterweight  50  is hung from the mast  30 , as shown in  FIG. 1 . Specifically, the counterweight guy line  54  is drooped from a mast point  30   p  of the mast top  30   t  in the distal end of the mast  30 , and the connecting mechanism  52  is connected to a lower end of the counterweight guy line  54 , as shown in  FIG. 4A . Through the connecting mechanism  52  and the counterweight guy line  54 , the counterweight  50  is hung from the mast  30 . 
     The hanging radius R of the counterweight  50 , which is changed according to the extension/retraction of the counterweight support members  41  as shown in  FIGS. 1 and 5 , is also changed according to the raising and lowering of the mast  30 . Meanwhile, because of the constant length of the counterweight guy line  54 , the change in the hanging radius R varies the counterweight height H. For example, when the hanging radius R is changed from R 1  to R 2  as shown in  FIG. 1 , the counterweight height H is changed from H 1  to H 2  as shown in  FIG. 1 . 
     As shown in  FIG. 4A , each of the hydraulic cylinders  60  is also used as a counterweight-lifting cylinder which is attached to the counter weight  50  and is extended so as to vertically push up the counterweight  50  while receiving a reaction force from the ground surface G (in  FIG. 4A , the position of the ground surface G is changed from G 2  to G 1 ). 
     Each of the hydraulic cylinders  60  is an oil-hydraulic cylinder, comprising a cylinder body  62 , a piston  64 , and a rod  66 , as shown in  FIG. 4B . The cylinder body  62  is a hollow cylindrical-shaped container surrounding an internal space thereof. The piston  64  is provided within the cylinder body  62  to partition the internal space into an upper head-side cylinder chamber  63   h  and a lower rod-side cylinder chamber  63   r . The rod  66  extends from the piston  64  downwardly beyond a lower end of the cylinder body  62 , having a lower end formed as a landing portion  68 . 
     The hydraulic cylinder  60  is connected to a hydraulic circuit  70  mounted, for example, to the upper slewing body  20 . The hydraulic circuit  70  has a head-side line  72 H connected to the head-side cylinder chamber  63   h , a rod-side line  72 R connected to the rod-side cylinder chamber  63   r , and a cylinder drive section connected to each of the cylinder chambers  63   h  and  63   r  via a corresponding one of the lines  72 H and  72 R. The cylinder drive section is adapted to supply a hydraulic fluid into the head-side cylinder chamber  63   h  involving a discharge of a hydraulic fluid from the rod-side cylinder chamber  63   r  to thereby extend the hydraulic cylinder  60 , and conversely to supply a hydraulic fluid into the rod-side cylinder chamber  63   r  involving a discharge of a hydraulic fluid from the head-side cylinder chamber  63   h  to thereby retract the hydraulic cylinder  60 . 
     The mobile crane  1  according to this embodiment further comprises a pressure detection sensor  74  operable to detect a hydraulic pressure inside the head-side line  72 H, a controller  76  connected to the pressure detection sensor  74 , and an indicator  78 . The controller  76  is operable to output a command signal to the indicator  78 , when a pressure detected by the pressure detection sensor  74  reaches a predetermined value, to instruct the indicator  78  to present information announcing the landing of the landing portion  68 . The detected pressure in the landed state corresponds to a mass of the counterweight  50 , enabling the controller  74  to present information on a mass of the counterweight, based on the detected pressure, to automatically set a rated load corresponding to the mass, and to present information about the rated load. 
     To be used as the counterweight-lifting cylinders, the hydraulic cylinders  60  are attached to a side surface of the base weight  51 . Specifically, as shown in  FIG. 5 , the hydraulic cylinders  60  are detachably attached to respective four corners of the base weight  51  in top plan view. In other words, the counterweight  50  has a plurality of counterweight-side cylinder-holding portions each adapted to detachably hold each of the hydraulic cylinders  60  as the counterweight-lifting cylinders, the counterweight-side cylinder-holding portions provided to respective holding position, which are four corners in this embodiment, set on the side surface of the base weight  51 . Specifically, as shown in  FIG. 4A , there are provided four weight-side brackets  51   b  each having a pin hole, on respective ones of the four corners of the base weight  51 , so as to protrude beyond the side surface of the base weight  51  in the frontward-rearward direction correspondingly to the respective cylinder-side brackets  60   b  of the hydraulic cylinders  60 . Herein, the hydraulic cylinders  60  are fixed to the base weight  51  by a common pin  60   p  inserted into the pin hole of each of the weight-side brackets  51   b  and the pin hole of the cylinder-side bracket  60   b  corresponding to the weight-side bracket  51   b.    
     The extension/retraction of the hydraulic cylinders  60  allows the counterweight  50  to be brought between the landed state and the hung state without pivot movement of the mast  30 . For example, even when the counterweight height H is changed due to a change in the hanging radius R (see  FIG. 1 ) of the counterweight  50  as shown in  FIG. 1 , the extension/retraction of the hydraulic cylinders  60  allows both of the landed state and the hung state to be selected, in spite of the change in the counterweight height H. Specifically, when no load is hung from the boom  25 , the hydraulic cylinders  60  are driven to be extended so as to let the respective landing portions  68  of the hydraulic cylinders  60  land on the ground surface G (G 1 ), thereby preventing the mobile crane  1  from rearward inclination due to the mass of the counterweight  50 . On contrary, when a load is hung from the boom  25  (see  FIG. 1 ), the hydraulic cylinders  60  are driven to be retracted so as to let the hydraulic cylinders  60  and the base weight  51  to be apart from the ground surface G (G 1 ), thereby enabling the counterweight  50  to serve as a weight to prevent the mobile crane  1  from frontward inclination. 
     In this embodiment, the hydraulic cylinders  60  can also function as the body lifter cylinders shown in  FIGS. 2 and 3 . Specifically, for assembly and disassembly of the lower propelling body  10  shown in  FIG. 1 , the hydraulic cylinders  60  are attached to respective ones of the distal ends of the arms  16  in the body lifter  15  of the propelling body  11  to serve as the body lifter cylinders, as shown in  FIGS. 2 and 3 . On the other hand, in a crane operation, the hydraulic cylinders  60  are detached from the arms  16  in the body lifter  15  of the propelling body  11  and attached to the base weight  51  of the counterweight  50  shown in  FIG. 4A  to serve as the counterweight-lifting cylinders. 
     In the above mobile crane  1 , each of the hydraulic cylinders  60 , attached to the counterweight  50  so as to vertically push up the counterweight  50  while receiving a reaction force from the ground surface by its extension, can be driven in a high efficiency, for example, as compared with the hydraulic cylinders  160  and  260  as shown in  FIGS. 7 and 8 . Specifically, either of the hydraulic cylinders  160  and  260  is required to be retracted to pull up a counterweight  50  against a gravity acting on the counterweight  50 , the retraction requiring a supply of a hydraulic pressure into a rod-side cylinder chamber  160   r  having a pressure receiving area less than that of a head-side cylinder chamber  160   h  (see  FIG. 7 ). In contrast, in the mobile crane  1  according to the above embodiment, each of the hydraulic cylinders  60  is so disposed as to extend to push up the counterweight  50  against a gravity acting on the counterweight  50 , and the extension can be performed by the supply of the hydraulic pressure into the head-side cylinder chamber  63   h  having a relatively large pressure receiving area. This allows the hydraulic cylinder  60  in the above embodiment to lift the counterweight  50  with a low hydraulic pressure or with a small cylinder diameter, as compared with the hydraulic cylinders  160  and  260  used to pull up the counterweight  50 . This permits a low-cost type to be used as the hydraulic cylinder  60 . 
     Furthermore, in the mobile crane  1  according to the above embodiment, the output signal of the pressure detection sensor  74  can provide accurate information about whether the counterweight  50  is landed or not. For example, when a load starts to be gradually lifted up by the boom  25  shown in  FIG. 1  under a condition that the counterweight  50  is landed, the counterweight  50  is gradually brought into separation from the ground surface: the separation lowers a pressure detected by the pressure detection sensor  74 , i.e., an internal pressure of the head-side cylinder chamber  63   h  of the hydraulic cylinder  60 . The detection signal output from the pressure detection sensor  74  is, therefore, a signal useful to determine (i) whether the counterweight  50  is fully landed on the ground surface G, (ii) whether the counterweight  50  is lifted although the landing portion  68  of the hydraulic cylinder  60  is in contact with the ground surface G, or (iii) whether the counterweight  50  and the hydraulic cylinder  60  are fully spaced from the ground surface G. Accordingly, based on the detection signal, the controller  74  can provide accurate information to an operator of the mobile crane  1 . 
     This allows the cost on the mobile crane  1  to be lowered. In a conventional mobile crane, there is required a dedicated limit switch for detecting the landing of the counterweight  50 , as shown, for example, in  FIGS. 4 to 8  of the JP 2008-297112 A. In contract, the mobile crane  1  according to the above embodiment permits the landing to be detected based on a change in the internal pressure of the head-side cylinder chamber  63   h , by no use of dedicated detection means such as a limit switch. 
     Moreover, the mobile crane  1  can possess a high safety. A conventional mobile crane requires manual operation for input of a mass of the counterweight, in order to set its crane capacity. In contrast, in the mobile crane  1  according to the above embodiment, the controller  76  can figure out a mass of the counterweight  50  based on a pressure detected by the pressure detection sensor  74  and therefore can automatically set a capacity associated with the mass of the counterweight  50  (e.g., a rated load); thus, manual input is not needed. This results in no erroneous input and high crane safety. Specifically, in the mobile crane  1 , a pressure detected by the pressure detection sensor  74  when the landing portion  68  of each of the hydraulic cylinders  60  is landed though the counterweight  50  is spaced from the ground surface G (when the counterweight  50  is not lifted up by the counterweight guy line  54 ) as shown in  FIG. 4A  corresponds to a mass of the counterweight  50 . 
     In addition, the mobile crane  1  according to the above embodiment, where the hydraulic cylinders  60  to be used as both of body lifter cylinders for lifting/lowering the lower propelling body  10  during the assembling/disassembling of the lower propelling body  10  and counterweight-lifting cylinders to be attached to the counterweight  50  (see  FIG. 4A ) during the crane operation, is reduced in cost, as compared with a mobile crane using a dedicated hydraulic cylinder for each of the counterweight-lifting cylinders. 
     Furthermore, the mobile crane  1 , where the hydraulic cylinders  60  are attached to the counterweight  50  located at a relatively low position, permits the hydraulic cylinders  60  to be easily attached, as compared with the cylinders  160  and  260 , either of which is provided between the counterweight  50  and the mast point  30   p  of the mast  30 , as shown in  FIGS. 7 and 8 . This results in easy assembly of the mobile crane  1 . For example, if the hydraulic cylinder  60  was attached in a vicinity of the upper end of the mast  30 , the mast  30  should be lowered. However, in the mobile crane  1  shown in  FIG. 1 , the attachment of the hydraulic cylinders  60  can be easily performed on the ground without the lowering of the mast  30 . 
     While the above embodiments according to the present invention have been graphically disclosed, the present invention is not limited to the specific structure of the embodiment, but various changes and modifications may be made therein without departing from the spirits and scope of the invention, for example, as follows. 
     There may be additionally provided a jib on the side of the distal end of the boom  25  shown in  FIG. 1 . 
     The present invention may be applied to a wheel-type mobile. 
     The present invention permits the counterweight support members  41  shown in  FIG. 1  to be omitted. 
     The present invention is not limited to the specific number and the attaching positions of the counterweight-lifting cylinders (in the above embodiment, the hydraulic cylinders  60 ); for example, the number of the counterweight-lifting cylinders to be attached to the counterweight can be three or less, or may be five or more. 
     The attaching positions of the counterweight-lifting cylinders are not limited to the base weight  51 . For example, the counterweight may include a loading board for allowing the base weight to be placed thereon, wherein the counterweight-lifting cylinders are attached to the loading board. Alternatively, the counterweight and the cylinder body of each of the counterweight-lifting cylinders may be integrally formed. 
     As described above, the present invention provides a mobile crane capable of adjusting a counterweight height without use of a costly hydraulic cylinder. The mobile crane comprises: a lower propelling body; a upper slewing body slewably mounted on the lower propelling body; a boom attached to the upper slewing body pivotably; a mast disposed at the rear of the boom and adapted to be raised and lowered so as to raise and lower the boom; a counterweight disposed at the rear of the upper slewing body and hung from the mast through a guy line; and a counterweight-lifting cylinder attached to the counterweight and adapted to be extended so as to vertically push up the counterweight while receiving a reaction force from a ground surface. The counterweight-lifting cylinder has a cylinder body surrounding an internal space thereof, a piston provided within the cylinder body to separate the internal space into an upper head-side cylinder chamber and a lower rod-side cylinder chamber, and a rod extending from the piston downwardly beyond an lower end of the cylinder body, the counterweight-lifting cylinder being adapted to be extended by supply of a hydraulic pressure into the head-side cylinder chamber and to be retracted by supply of a hydraulic pressure into the rod-side cylinder chamber. 
     In the mobile crane of the present invention, the counterweight-lifting cylinder can vertically push up the counterweight on a ground surface against gravity acting on the counterweight, by supply of a hydraulic pressure into the head-side cylinder chamber of the counterweight-lifting cylinder which has a pressure receiving area greater than that of the rod-side cylinder chamber. This allows the counterweight-lifting cylinder to be driven in high efficiency, as compared with a cylinder required to be retracted to pull up a counterweight against a gravity acting thereon, i.e., a cylinder in which a hydraulic pressure is supplied into a rod-side cylinder chamber having a relatively small pressure receiving area to pull up the counterweight, as in the cylinders  160  and  260  shown in  FIGS. 7 and 8 . This makes it possible to lift the counterweight with a reduced hydraulic pressure or a reduced cylinder diameter, thereby permitting a low-cost hydraulic cylinder to be used to lift/lower the counterweight. 
     It is preferable that the mobile crane of the present invention further comprises a pressure detection sensor operable to output a detection signal indicative on an internal pressure of the head-side cylinder chamber of the counterweight-lifting cylinder. The detection signal of the pressure detection sensor contributes to provision of information about a landing state of the counterweight. Since the upward movement of the counterweight from the ground surface along with lifting of a load by the boom lowers the internal pressure of the head-side cylinder chamber of the counterweight-lifting cylinder, the detection signal can be utilized for determination, for example, on: whether the counterweight is fully landed on the ground surface; whether the counterweight starts to be lifted although the counterweight-lifting cylinder is in contact with the ground surface; or whether the counterweight and the counterweight-lifting cylinder are fully apart from the ground surface. 
     The above mobile crane may further comprise an indicator adapted to indicate information about the detection signal, which enables the information to be announced, for example, to an operator (manipulator) of the mobile crane. 
     While a conventional mobile crane must have a limit switch for detecting whether a counterweight is landed, to provide information of the landing, the above-mentioned pressure detection sensor allows the landed state of the counterweight to be figured out based on the internal pressure of the head-side cylinder chamber of the counterweight-lifting cylinder, thus permitting the dedicated limit switch for detection of the landing to be not used, and permitting a cost of the mobile crane to be reduced. 
     In addition to the pressure detection sensor, the mobile crane preferably comprises a controller operable to set a mobile crane capacity associated with to a mass of the counterweight, based on the detection signal from the pressure detection sensor. While the conventional mobile crane requires a manual input operation on a mass of the counterweight to set a mobile crane capacity, the combination of the above pressure detection sensor and the controller enable a capacity associated with a mass of the counterweight to be automatically set by utilization of a relative relationship between the mass of the counterweight and the internal pressure of the head-side cylinder chamber of the counterweight-lifting cylinder, thus allowing the manual input of the mass of the counterweight to be not required. This results in no erroneous input of the mass of the counterweight and high crane safety. 
     In the mobile crane of the present invention, it is possible to use the counterweight-lifting cylinder also as a body lifter cylinder to be attached to the lower propelling body to lift/lower the lower propelling body during assembling/disassembling of the lower propelling body. Specifically, it is preferable that the counterweight has a first cylinder-holding portion (i.e., counterweight-side cylinder-holding portion) adapted to detachably hold the counterweight-lifting cylinder, and the lower propelling body has a second cylinder-holding portion (i.e., lower propelling body-side cylinder-holding portion) adapted to detachably hold the counterweight-lifting cylinder in such a posture that the extension and the retraction of the counterweight-lifting cylinder lifts or lowers the lower propelling body respectively during assembling and disassembling of the lower propelling body. 
     The additional use of the counterweight-lifting cylinder as the body lifter cylinder permits a cost of the mobile crane to be reduced, as compared with a mobile crane equipped with dedicated cylinders for respective ones of the body lifter cylinder and the counterweight-lifting cylinder. Furthermore, since the counterweight-lifting cylinder is attached to the counterweight originally located at a relatively low position, the attachment of the counterweight-lifting cylinder can be easily performed, as compared with, for example, a cylinder to be disposed between the counterweight and the mast. This allows the mobile crane to be easily assembled. If the counterweight-lifting cylinder was disposed in or in a vicinity of the mast, there would be required a heavy operation for lowering the mast; the counterweight-lifting cylinder in the present invention can be easily attached to the counterweight without the lowering of the mast. 
     More specifically, preferable is that: the counterweight includes a base weight connected to the guy line and a plurality of weight members stacked on the base weight; the base weight having a side surface on which a plurality of holding positions; and the first cylinder-holding portion is provided to each of the holding positions. The counterweight-lifting cylinder held by each of the first cylinder-holding portions can push up the entire counterweight including the base weight from the ground surface. 
     This application is based on Japanese patent application serial no. 2010-026533, filed in Japan Patent Office on Feb. 9, 2010, the contents of which are hereby incorporated by reference. 
     Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention hereinafter defined, they should be construed as being included therein.