Abstract:
The present application discloses a hydraulic oil cylinder, of which a piston rod (3) is provided with at least two cushion collars ( 4, 11 ) which are axially slidable along the piston rod ( 3 ). Axial throttle oil channels ( 301   a,    301   b ) are provided between the cushion collars ( 4, 11 ) and a piston ( 6 ). A first cushion collar ( 4 ) is provided with a sealing end face ( 401 ), and an end cover of a rod cavity ( 1 ) is provided with a sealing end face ( 101 ). The sealing end face ( 401 ) of the first cushion collar contacts with the sealing end face ( 101 ) of the end cover of the rod cavity to form a seal. Hydraulic oil within the rod cavity is discharged through one axial throttle oil channel ( 301   a ) to an oil passage B. A second cushion collar ( 11 ) is provided with a sealing end face ( 111 ), and an end cover of a rodless cavity ( 12 ) is provided with a sealing end face ( 121 ). The sealing end face ( 111 ) of the second cushion collar contacts with the sealing end face ( 121 ) of the end cover of the rodless cavity to form a seal. Hydraulic oil within the rodless cavity is discharged through another axial throttle oil channel ( 301   b ) to another oil passage A. The hydraulic oil cylinder can operate reliably and achieve a buffer function in a large load, high frequency operating condition, and thus has a longer operating life. And also, precision requirements for manufacturing the hydraulic oil cylinder are low, thereby facilitating production of the hydraulic oil cylinder. The present application also discloses a hydraulic cushion system, an excavator and a concrete pump truck which use the above hydraulic oil cylinder.

Description:
[0001]    The present application claims the benefit of priority to the Chinese Patent Application No. 201010235138.1, titled “HYDRAULIC OIL CYLINDER AND CORRELATIVE DEVICE THEREOF, HYDRAULIC CUSHION SYSTEM, EXCAVATOR AND CONCRETE PUMP TRUCK”, filed with the Chinese State Intellectual Property Office on Jul. 23, 2010, the entire disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present application relates to the field of hydraulic technology, and particularly to a hydraulic cylinder. The present application also provides a hydraulic buffer system, an excavator and a concrete pump truck including the above hydraulic cylinder. 
       BACKGROUND OF THE INVENTION 
       [0003]    The hydraulic cylinder is a component which is widely used in the construction machinery, and during operations, a piston is required to perform reciprocating movement continuously. When a piston rod extends to a limit position, a piston end face gives a great impact to an end cap, which may cause damages to the hydraulic cylinder. Therefore, a buffer device is required to be provided at that position in order to avoid the damages to the hydraulic cylinder caused by the above impact. 
         [0004]    There are great differences between the existing buffer devices due to different application situations and different sizes of the hydraulic cylinders. For small cylinders, compression springs can be employed as buffer devices directly. However, for hydraulic cylinders having a large cylinder diameter and a long stroke, if a compression spring is employed as the buffer device, it is difficult to obtain a spring with sufficient elasticity, and the spring will soon be damaged due to repeated compression. Therefore, for the hydraulic cylinder having a large cylinder diameter and a long stroke, a hydraulic buffering mechanism shown in  FIG. 1  is used generally. 
         [0005]    Referring to  FIG. 1 , a buffer device including a big buffer ring  06  and a big buffer sleeve  04  is shown, wherein the big buffer ring  06  is mounted in an intermediate annular groove arranged at a buffering position of a piston rod, and a big buffer sleeve  04  is arranged at the buffering position. A buffer inner hole  07  corresponding to the big buffer sleeve  04  is provided at an opening of the end cap  01  of the rod cavity of the cylinder, and has an inner diameter fitted with the outer diameter of the big buffer sleeve  04 . When the piston rod extends out, the big buffer sleeve  04  is firstly inserted into the buffer inner hole  07  to block the oil return passage of the rod cavity in the cylinder barrel  02 , and at the same time, a throttle oil channel is formed by a clearance between the big buffer sleeve  04  and the buffer inner hole  07 . In this way, the piston  05  can continue to perform movement in the extending direction, but its movement is slowed down due to the damping effect of the throttle oil channel. Further, the closer the piston  05  gets to the end position of the extension movement of the piston rod  03 , the longer the throttle oil channel between the big buffer sleeve  04  and the buffer inner hole  07  is, the greater the damping of the throttle oil channel is, the slower the movement of the piston  05  becomes, until the piston rod  03  extends out to reach the end position smoothly. 
         [0006]    Currently, the above buffering mechanism is widely used in hydraulic cylinders with a large cylinder diameter and a long stroke to provide a better buffering protection for these hydraulic cylinders. 
         [0007]    However, there are obvious defects in the above buffering mechanism. Firstly, the above hydraulic cylinder with a large cylinder diameter and long stroke tends to work in the working conditions of heavy load and high frequency, for example, a drive cylinder used to drive a digging arm of an excavator or the like. In this case, it is required for the big buffer sleeve  04  in the above buffering mechanism to be inserted into the buffer inner hole  07  repeatedly at a high speed. However, the fit clearance between the big buffer sleeve  04  and the buffer inner hole  07  is very small actually, and the piston rod  03  is very heavy, so that the piston rod  03  is likely tilted to one side under gravity. Therefore, the hydraulic cylinder used in the above situation is prone to failure since the buffer sleeve  04  fails to be inserted into the buffer inner hole  07 , so that the entire hydraulic cylinder can not operate normally. 
         [0008]    Another key problem in the above buffering mechanism is that, the outer diameter of the big buffer sleeve  04  must be precisely fitted with the inner diameter of the buffer inner hole  07 , and otherwise the buffering effect may not be achieved. As a result, requirements for the manufacturing precision of the buffering mechanism are extremely high and it is difficult for manufacturers with ordinary production level to meet the requirements. Due to the excessive high requirements of the manufacture precision, the hydraulic cylinders with a large cylinder diameter and a long stroke become a bottleneck problem in producing excavators and other construction machinery, which severely restricts the production capacity of the various manufacturers in the downstream procedures of the production. 
       SUMMARY OF THE INVENTION 
       [0009]    The embodiment of the present application provides a hydraulic cylinder having a buffer mechanism capable of achieving a buffering effect reliably in a large load, high frequency operating condition, and thus having a longer service life. In addition, the requirement for the manufacturing precision of the hydraulic cylinder is low, which facilitates production. The hydraulic cylinder is particularly applicable for a large cylinder diameter and a long stroke, is easy to manufacture and process, and has a good smooth buffering effect. 
         [0010]    The embodiment of the present application also provides a device associated with the hydraulic cylinder. Such a device may be a piston rod. 
         [0011]    The embodiment of the present application also provides a hydraulic buffer system, an excavator and a concrete pump truck including the above hydraulic cylinder. 
         [0012]    The hydraulic cylinder according to the embodiment of the present application includes a rod cavity end cap ( 1 ), a cylinder barrel ( 2 ), a piston rod ( 3 ), a piston ( 6 ) and a rodless cavity end cap ( 12 ), the rod cavity end cap ( 1 ) being provided with an oil passage (B), and the rodless cavity end cap ( 12 ) being provided with an oil passage (A), wherein, 
         [0013]    at least one throttle oil channel ( 301   a,    301   b ) is further provided, at least one buffer sleeve is provided on the piston rod ( 3 ), the buffer sleeve includes a first buffer sleeve ( 4 ) located in a rod cavity and/or a second buffer sleeve ( 11 ) located in a rodless cavity, the buffer sleeve ( 4 ,  11 ) is axially slidable along the piston rod ( 3 ); i.e. there are at least a first buffer sleeve ( 4 ) located in the rod cavity and a second buffer sleeve ( 11 ) located in the rodless cavity on the piston ( 3 ), the first buffer sleeve ( 4 ) and the second buffer sleeve ( 11 ) are axially slidable along the piston rod ( 3 ); 
         [0014]    the first buffer sleeve ( 4 ) is provided with a sealing end face ( 401 ), and the rod cavity end cap ( 1 ) is provided with a sealing end face ( 101 ), during an extending movement of the piston, the sealing end face ( 401 ) of the first buffer sleeve is capable of contacting with the sealing end face ( 101 ) of the rod cavity end cap ( 1 ) to form a sealing surface, and hydraulic oil located at a side of the sealing surface close to the piston is discharged into the oil passage (B) via the throttle oil channel ( 301   a ); 
         [0015]    the second buffer sleeve ( 11 ) is provided with a sealing end face ( 111 ), and the rodless cavity end cap ( 12 ) is provided with a sealing end face ( 121 ); during a retracting movement of the piston, the sealing end face ( 121 ) of the second buffer sleeve is capable of contacting with the sealing end face ( 121 ) of the rodless cavity end cap ( 12 ) to form a sealing surface, and hydraulic oil located at a side of the sealing surface close to the piston is discharged into the oil passage (A) via the throttle oil channel ( 301   b ). 
         [0016]    Preferably, the throttle oil channels ( 301   a,    301   b ) are arranged linearly between the piston rod ( 3 ) and the buffer sleeves ( 4 ,  11 ) along axial direction. 
         [0017]    Preferably, when the piston rod ( 3 ) extends to an end of a stroke, the first buffer sleeve ( 4 ) keeps a distance (L 1 ) from an end point of its sliding towards the piston ( 6 ). 
         [0018]    Preferably, when the piston rod ( 3 ) retracts to an end of a stroke, the second buffer sleeve ( 11 ) keeps a distance (L 2 ) from an end point of its sliding towards the piston ( 6 ). 
         [0019]    Preferably, when the sealing end face ( 401 ) of the first buffer sleeve ( 4 ) comes into contact with the sealing end face ( 101 ) of the rod cavity end cap ( 1 ) to form a sealing surface, an area of the first buffer sleeve ( 4 ) subjected to an axial action of hydraulic oil in the rod cavity is larger than an area of the first buffer sleeve ( 4 ) subjected to an axial action of hydraulic oil in the oil passage (B). 
         [0020]    Preferably, when the sealing end face ( 111 ) of the second buffer sleeve ( 11 ) comes into contact with the sealing end face ( 121 ) of the rodless cavity end cap ( 12 ) to form a sealing surface, an area of the second buffer sleeve ( 11 ) subjected to an axial action of hydraulic oil in the rodless cavity is larger than an area of the second buffer sleeve ( 11 ) subjected to an axial action of hydraulic oil in the oil passage (A). 
         [0021]    Preferably, the sealing end face ( 401 ) of the first buffer sleeve ( 4 ) comes into contact with the sealing end face ( 101 ) of the rod cavity end cap ( 1 ) to form a face seal or a line seal. 
         [0022]    Preferably, the sealing end face ( 111 ) of the second buffer sleeve ( 11 ) comes into contact with the sealing end face ( 121 ) of the rodless cavity end cap ( 12 ) to form a face seal or a line seal. 
         [0023]    Preferably, the cross-sectional area of the throttle oil channel ( 301   a,    301   b ) becomes smaller as the buffer sleeve ( 4 ,  11 ) slides on the piston rod ( 3 ) towards the piston ( 6 ). 
         [0024]    Preferably, an elastic element ( 5 ,  7 ) for returning the buffer sleeve ( 4 ,  11 ) is provided inside a cavity of the cylinder barrel ( 2 ). 
         [0025]    Preferably, multiple circumferential balancing oil grooves ( 302   a,    302   b ) are provided on a surface of the piston rod ( 3 ) fitted with the buffer sleeve ( 4 ,  11 ). 
         [0026]    Preferably, the throttle oil channel ( 301   a,    301   b ) is a throttle oil groove linearly arranged on an external surface of the piston rod ( 3 ) along an axial direction, and the cross-sectional area of the throttle oil channel ( 301   a,    301   b ) decreases gradually towards the piston ( 6 ). 
         [0027]    Preferably, the throttle oil channel ( 301   a,    301   b ) is formed by a throttle inclined surface linearly arranged in a sliding region between the buffer sleeve ( 4 ,  11 ) and the piston rod ( 3 ) along an axial direction. 
         [0028]    Preferably, the throttle oil channel ( 301   a,    301   b ) includes: an oil channel ( 3013 ) arranged inside the piston rod ( 3 ) and extending in the axial direction; and a plurality of throttle orifices ( 3014 ) arranged on the external surface of the piston rod ( 3 ) along the axial direction being in communication with the oil channel ( 3013 ). 
         [0029]    Preferably, the aperture diameters of the throttle orifices ( 3014 ) become smaller gradually towards the piston ( 6 ). 
         [0030]    Preferably, the throttle oil channel ( 301   a,    301   b ) includes a first segment of throttle oil channel ( 3012 ) located at an inlet end thereof, and a second segment of throttle oil channel ( 3011 ) located at an outlet end thereof. The first segment of throttle oil channel ( 3012 ) is a throttle oil groove arranged on a surface of the piston rod ( 3 ), and the second segment of throttle oil channel ( 3012 ) is an oil channel arranged inside the piston rod ( 3 ) or the buffer sleeve ( 4 ,  11 ). 
         [0031]    Preferably, the cross-sectional area of the first segment of throttle oil channel ( 3012 ) becomes smaller gradually towards the piston ( 6 ). 
         [0032]    Preferably, the piston rod ( 3 ) includes a piston rod body and a transition sleeve ( 304 ). The transition sleeve ( 304 ) is mounted on the piston rod body, and the buffer sleeve ( 4 ,  11 ) is arranged on the transition sleeve ( 304 ). The throttle oil channel ( 301   a,    301   b ) is arranged on the transition sleeve ( 304 ). 
         [0033]    Preferably, the piston rod ( 3 ) includes a piston rod body ( 3   a ) and a buffer shaft ( 3   b ). The piston rod body ( 3   a ) and the buffer shaft ( 3   b ) are connected with each other. The second buffer sleeve ( 11 ) is arranged on the buffer shaft ( 3   b ), and the throttle oil channel ( 301   b ) is arranged on the buffer shaft ( 3   b ). 
         [0034]    The device associated with the hydraulic cylinder according to the embodiment of the present application may be a piston rod including a piston rod body segment in the rod cavity and a buffer shaft segment in the rodless cavity after being assembled. Both the piston rod body segment and the buffer shaft segment are provided with throttle oil channels extending linearly in the axial direction. 
         [0035]    Preferably, the cross-sectional area of each of the throttle oil channels increases gradually from a side of the throttle oil channel close to the piston to the other side of throttle oil channel. 
         [0036]    Preferably, a shaft shoulder for limiting the buffer sleeve ( 4 ) is provided on the piston rod body. 
         [0037]    Preferably, a stop shoulder groove used for a stop shoulder for limiting the second buffer sleeve ( 11 ) is provided at a tail end of the buffer shaft segment of the piston rod ( 3 ) located in the rodless cavity. 
         [0038]    The beneficial effects of the hydraulic cylinder according to the embodiment of the present application are as follows. 
         [0039]    Firstly, the buffer sleeve is provided with a sealing end face, and the rodless cavity end cap and/or the rod cavity end cap are/is provided with a sealing end face. The two sealing end faces come into contact with each other to form a seal. The hydraulic oil in the rodless cavity and/or in the rod cavity is discharged into the oil passage via the throttle oil channel arranged on the buffer sleeve or on the piston rod. Therefore, the enclosed hydraulic oil generates an appropriate buffering pressure that acts on the oil discharging side of the piston, to counteract the inertial force of the piston so as to achieve the purpose of decelerating and braking. The throttle buffering of the mechanism is extremely smooth and reliable, so that the buffering mechanism is avoided from the mechanical failures. In the preferred embodiment, the flowing area of the throttle oil channel is variable, which achieves the purpose of throttle-varied buffering. The cooperation between the buffer sleeve, the piston rod and the throttle oil channel achieves the function of a variable throttle valve. 
         [0040]    Secondly, when the piston rod retracts to the end of the stroke, the second buffer sleeve does not reach the end position and can still slide towards the piston by a certain distance. When the piston rod extends out, oil enters the oil passage A, and under the action of the hydraulic oil, the second buffer sleeve is pushed to slide towards the piston so as to compress a return spring, so that the sealing end face of the second buffer sleeve moves away from the sealing end face of the rodless cavity end cap. The oil passage A comes into direct communication with the rodless cavity, and the hydraulic oil enters into the rodless cavity and pushes the piston to move leftwards. The second buffer sleeve cooperates with the rodless cavity end cap to function as a check valve. In this way, the oil can enter the rodless cavity rapidly so as to push the piston to move. If the second buffer sleeve doesn&#39;t have the function of a check valve and the oil can not enter the rodless cavity rapidly, the piston rod is actuated to extend out slowly, even that the piston rod fails to perform the extending movement. 
         [0041]    When the piston rod extends to the end of the stroke, the first buffer sleeve does not reach the end position, and can still slide towards the piston by a certain distance. When the piston rod retracts back, oil enters the oil passage B, and under the action of the hydraulic oil, the first buffer sleeve is pushed to slide towards the piston so as to compress a return spring, so that the sealing end face of the first buffer sleeve moves away from the sealing end face of the rod cavity end cap. The oil passage B comes into direct communication with the rod cavity, and the hydraulic oil enters into the rod cavity and pushes the piston to move. The first buffer sleeve cooperates with the rod cavity end cap to function as a check valve. In this way, the oil can enter the rod cavity rapidly so as to push the piston to move. If the first buffer sleeve doesn&#39;t have the function of a check valve, and the oil can not enter the rod cavity rapidly, the piston rod is actuated to retract slowly, even that the piston rod fails to perform the retracting movement. 
         [0042]    Thirdly, in a hydraulic cylinder with a large cylinder diameter and a long stroke, it is very difficult merely by ways of spring force to form a reliable sealing surface between the buffer sleeve and the rodless cavity end cap, and this method is also not be the most preferred way. In the hydraulic cylinder according to the embodiment of the present application, when the piston rod retracts to a position being at a set distance from the end of the stroke, the rodless cavity end cap comes into contact with the second buffer sleeve, and the hydraulic oil in the rodless cavity is enclosed in the set oil cavity, causing an increased pressure of the hydraulic oil in the rodless cavity. Since the areas of the two sides of the second buffer sleeve subjected to the axial action of the hydraulic oil are different, i.e., the area of the second buffer sleeve subjected to the axial action of the hydraulic oil in the rodless cavity is larger than the area of the second buffer sleeve subjected to the axial action of the hydraulic oil in the oil passage A, pressure difference is generated between both sides of the second buffer sleeve. Under the action of the hydraulic oil, the second buffer sleeve is pushed to press against the rodless cavity end cap so as to form a seal. Thus, a reliable sealing surface is formed between the second buffer sleeve and the rodless cavity end cap. The hydraulic oil in the rodless cavity is discharged into the oil passage A via the throttle oil channel, therefore solving the problem that it is difficult to form a sealing surface. 
         [0043]    When the piston rod  3  extends to a position being at a set distance from an end of the stroke, the rod cavity end cap comes into contact with the first buffer sleeve, and the hydraulic oil in the rod cavity is enclosed in the set oil cavity, resulting in an increased pressure of the hydraulic oil in the rod cavity. Since the areas of the two sides of the first buffer sleeve subjected to the axial action of the hydraulic oil are different, i.e. the area of the first buffer sleeve subjected to the axial action of the hydraulic oil in the rod cavity is larger than the area of the first buffer sleeve subjected to the axial action of the hydraulic oil in the oil passage B, pressure difference is generated between both sides of the first buffer sleeve. Under the action of the hydraulic oil, the first buffer sleeve is pushed to press against the rod cavity end cap so as to form a seal. Thus, a reliable sealing surface is formed between the first buffer sleeve and the rod cavity end cap. The hydraulic oil in the rod cavity is discharged into the oil passage B via the throttle oil channel, therefore solving the problem that it is difficult to form a sealing surface. 
         [0044]    Fourthly, a return spring is provided between the buffer sleeve and the piston, which may, on the one hand, actuate the piston rod rapidly when retracting, and on the other hand, facilitate the buffering and returning between the buffer sleeve and the rod cavity and/or rodless cavity, and also facilitate the sealing. 
         [0045]    Fifthly, multiple circumferential balancing oil grooves are provided on the surfaces of the buffer sleeve and the piston rod fitted with each other so as to improve the service life of the buffer sleeve and the piston rod. 
         [0046]    Sixthly, throttle oil channels are designed as tapered linear throttle oil channels or formed by throttle inclined surfaces, so that the movement of the piston rod and the piston can be slowed down smoothly without too high transient pressure by variable throttling. This kind of structure is manufactured easily, has excellent buffering effect, as well as long service life. 
         [0047]    Seventhly, in order to facilitate incorporating multiple circumferential balancing oil grooves and throttle oil channels with high precision into the piston rod, a transition sleeve is additionally provided on the piston rod, and the multiple circumferential balancing oil grooves and throttle oil channels are manufactured on the transition sleeve; or the piston rod can be divided into two segments to manufacture, the segment located in the rodless cavity can be manufactured separately and connected to the piston rod body by threading and the like. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0048]      FIG. 1  is a structural schematic view of a hydraulic cylinder in the prior art; 
           [0049]      FIG. 2  is a structural schematic view of a hydraulic cylinder according to a first embodiment of the present application; 
           [0050]      FIG. 3  is a structural schematic view of a piston rod part in  FIG. 2 ; 
           [0051]      FIG. 4  is a view taken along line A-A of  FIG. 3 ; 
           [0052]      FIG. 5  is a view taken along line C-C of  FIG. 3 ; 
           [0053]      FIG. 6  is a view taken along line B-B of  FIG. 3 ; 
           [0054]      FIG. 7  is a structural schematic view of a buffer sleeve part in  FIG. 2 ; 
           [0055]      FIG. 8  is a structural schematic view of the hydraulic cylinder in  FIG. 2  with a first buffer sleeve being in a buffering state; 
           [0056]      FIG. 9  is a structural schematic view of the hydraulic cylinder in  FIG. 2  with the first buffer sleeve being in a buffering end state; 
           [0057]      FIG. 10  is a structural schematic view of the hydraulic cylinder in  FIG. 2  with a second buffer sleeve being in a buffering state; 
           [0058]      FIG. 11  is a structural schematic view of the hydraulic cylinder in  FIG. 2  with the second buffer sleeve being in a buffering end state; 
           [0059]      FIG. 12  is a structural schematic view of a hydraulic cylinder according to a second embodiment of the present application; 
           [0060]      FIG. 13  is a structural schematic view of a hydraulic cylinder according to a third embodiment of the present application; 
           [0061]      FIG. 14  is a structural schematic view of a hydraulic cylinder according to a fourth embodiment of the present application; 
           [0062]      FIG. 15  is a structural schematic view of a hydraulic cylinder according to a fifth embodiment of the present application; 
           [0063]      FIG. 16  is a structural schematic view of a hydraulic cylinder according to a sixth embodiment of the present application; 
           [0064]      FIG. 17  is a structural schematic view of a hydraulic cylinder according to a seventh embodiment of the present application; 
           [0065]      FIG. 18  is a structural schematic view of a hydraulic cylinder according to a eighth embodiment of the present application; 
           [0066]      FIG. 19  is a structural schematic view of a hydraulic cylinder according to a ninth embodiment of the present application; 
           [0067]      FIG. 20  is a structural schematic view of a hydraulic cylinder according to a tenth embodiment of the present application; and 
           [0068]      FIG. 21  is a structural schematic view of a hydraulic cylinder according to an eleventh embodiment of the present application. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0069]    In order that the technical solutions of the embodiment of the present application can be better understood by those skilled in the art, the embodiments of the present application will be described in detail in conjunction with the accompanying drawings and the specific embodiments hereinafter. 
         [0070]    Reference is made to the first embodiment of  FIGS. 2 to 11 , which includes a rod cavity end cap  1 , a cylinder barrel  2 , a piston rod  3 , a piston  6  and a rodless cavity end cap  12 . The rod cavity end cap  1  is provided with an oil passage B, and the rodless cavity end cap  12  is provided with an oil passage A. The cavity of the cylinder barrel  2  is divided into a rod cavity and a rodless cavity by the piston rod  3  and the piston  6 . The oil passages A and B are in communication with an oil circuit of the hydraulic system, and both are axial oil passages arranged in the hydraulic cylinder. The oil passage B includes an oil passage hole arranged in the rod cavity end cap  1  and an oil passage formed by a clearance between the piston rod  3  and the rod cavity end cap  1 . The oil passage B extends to a sealing end face  101  of the rod cavity end cap  1 . 
         [0071]    The oil passage B includes the oil passage hole arranged in the rod cavity end cap  1 , and the oil passage formed by a clearance between the piston rod  3  and the rod cavity end cap  1 . The oil passage B extends to a sealing end face  101  of the rod cavity end cap  1 . The oil passage B and the oil passage A can also be connected with each other directly. 
         [0072]    The oil passage A extends to a sealing end face  121  of the rodless cavity end cap  12 . A cavity for accommodating a buffer shaft  3   b  at a tail end of the piston rod  3  is provided in the rodless cavity end cap  12 . The oil passage B and the oil passage A can also be connected with each other directly. 
         [0073]    A first buffer sleeve  4  located in the rod cavity and a second buffer sleeve  11  located in the rodless cavity are provided on the piston rod  3 , and both are axially slidable along the piston rod  3 . An axial throttle oil channel  301   a  is provided between the first buffer sleeve  4  and the piston rod  3 , and an axial throttle oil channel  301   b  is provided between the second buffer sleeve  11  and the piston rod  3 . The throttle oil channels  301   a  and  301   b  can be implemented in various ways, the cross-section of which can be U-shaped, V-shaped, square or in any other shape. 
         [0074]    The first buffer sleeve  4  is provided with a sealing end face  401  for sealing, and the rod cavity end cap  1  is provided with a sealing end face  101  cooperating with the sealing end face  401  to achieve sealing. The sealing end face  401  of the first buffer sleeve  4  can come into contact with the sealing end face  101  of the rod cavity end cap  1  to form a seal, which can break the direct communication between the oil passage B and the rod cavity entirely. The direct communication between the oil passage B and the rod cavity can also be broken partially. 
         [0075]    The second buffer sleeve  11  is provided with a sealing end face  111  for sealing, and the rodless cavity end cap  12  is provided with a sealing end face  121  cooperating with the sealing end face  111  of the second buffer sleeve  11  to achieve sealing. The sealing end face  111  of the buffer sleeve  11  can come into contact with the sealing end face  121  of the rodless cavity end cap  12  to form a seal, which can break the direct communication between the oil passage A and the rodless cavity entirely. The direct communication between the oil passage A and the rodless cavity can also be broken partially. 
         [0076]    The sealing formed by the contact between the sealing end face  401  of the first buffer sleeve  4  and the sealing end face  101  of the rod cavity end cap  1  may be face sealing or line sealing. For example, in the first embodiment, the sealing end face  401  contacts the sealing end face  101  to form a plane seal; and in the sixth embodiment, as shown in  FIG. 16 , a line sealing ring is provided on the sealing end face  401 , and configured to contact the sealing end face  101  to form a line seal. In the seventh embodiment, as shown in  FIG. 17 , the sealing end face  101  is a conical surface, and the sealing end face  401  contacts the sealing end face  101  to form a line seal. In the eighth embodiment, as shown in  FIG. 18 , both the sealing end face  401  and the sealing end face  101  are conical surfaces, and the two conical surfaces contact with each other to form a face seal. In addition to the above, other ways are also possible, for example, a curved face seal, or the like. 
         [0077]    Similarly, the sealing formed by the contact between the sealing end face  111  of the second buffer sleeve  11  and the sealing end face  121  of the rodless cavity end cap  12  can be face sealing or line sealing. For example, in the first embodiment, the sealing end face  111  contacts the sealing end face  121  to form a face seal; and in the ninth embodiment, as shown in  FIG. 19 , a line sealing ring is provided on the sealing end face  111 , and configured to contact the sealing end face  121  to form a line seal. In the tenth embodiment, as shown in  FIG. 20 , both the sealing end face  111  and the sealing end face  121  are conical surfaces, and the two conical surfaces contact with each other to form a face seal. In the eleventh embodiment, as shown in  FIG. 21 , the sealing end face  121  is a conical surface, the sealing end face  111  contacts the sealing end face  121  to form a line seal. 
         [0078]    When the piston rod  3  extends to a position being at a set distance from an end of the stroke, the rod cavity end cap  1  comes into contact with the first buffer sleeve  4 , and the hydraulic oil in the rod cavity is enclosed in the set oil cavity, resulting in an increased pressure of the hydraulic oil in the rod cavity. Since the areas of the two sides of the buffer sleeve subjected to the axial action of the hydraulic oil are different, i.e. the area of the first buffer sleeve  4  subjected to the axial action of the hydraulic oil in the rod cavity is larger than the area of the first buffer sleeve  4  subjected to the axial action of the hydraulic oil in the oil passage B, pressure difference is generated between both sides of the first buffer sleeve  4 . 
         [0079]    Under the action of the hydraulic oil, the first buffer sleeve  4  is pushed to press against the rod cavity end cap  1  so as to form a seal. Thus, a reliable sealing surface is formed between the first buffer sleeve  4  and the rod cavity end cap  1 . The hydraulic oil in the rod cavity is discharged into the oil passage B via the throttle oil channel  301   a,  therefore solving the difficulty in forming a sealing surface. 
         [0080]    Similarly, when the piston rod  3  retracts back to a position being at a set distance from the other end of the stroke, the rodless cavity end cap  12  comes into contact with the second buffer sleeve  11 , and the hydraulic oil in the rodless cavity is enclosed in the set oil cavity, resulting in an increased pressure of the hydraulic oil in the rodless cavity. Since the areas of the two sides of the second buffer sleeve  11  subjected to the axial action of the hydraulic oil are different, i.e., the area of the second buffer sleeve  11  subjected to the axial action of the hydraulic oil in the rodless cavity is larger than the area of the second buffer sleeve  11  subjected to the axial action of the hydraulic oil in the oil passage A, pressure difference is generated between both sides of the second buffer sleeve  11 . Under the action of the hydraulic oil, the second buffer sleeve  11  is pushed to press against the rodless cavity end cap  12  so as to form a seal. Thus, a reliable sealing surface is formed between the second buffer sleeve  11  and the rodless cavity end cap  12 . The hydraulic oil in the rodless cavity is discharged into the oil passage A via the throttle oil channel  301   b,  therefore solving the difficulty in forming a sealing surface. 
         [0081]    After the sealing end face  401  of the first buffer sleeve  4  comes into contact with the sealing end face  101  of the rod cavity end cap  1  to form a seal, the direct communication between the oil passage B and the rod cavity is broken entirely. The direct communication between the oil passage B and the rod cavity can also be broken partially. The hydraulic oil in the rod cavity is discharged into the oil passage B via the throttle oil channel  301   a.  Since the oil discharging quantity of the throttle oil channel  301   a  is rather small, the enclosed hydraulic oil generates an appropriate buffering pressure that acts on the oil discharging side of the piston  6  to counteract the inertial force of the piston, so as to achieve the purpose of decelerating or braking The throttle buffering is extremely smooth and reliable, thereby avoiding the buffering mechanism from mechanical failures. 
         [0082]    Similarly, the sealing end face  111  of the second buffer sleeve  11  comes into contact with the sealing end face  121  of the rodless cavity end cap  12  to form a seal, and the direct communication between the oil passage A and the rodless cavity is broken entirely. The direct communication between the oil passage A and the rodless cavity can also be broken partially. The hydraulic oil in the rodless cavity is discharged into the oil passage A via the throttle oil channel  301   b . Since the oil discharging quantity of the throttle oil channel  301   b  is rather small, the enclosed hydraulic oil generates an appropriate buffering pressure that acts on the oil discharging side of the piston  6  to counteract the inertial force of the piston, so as to achieve the purpose of decelerating or braking The throttle buffering is extremely smooth and reliable, thereby avoiding the buffering mechanism from mechanical failures. 
         [0083]    For the structure of the throttle oil channel  301   a  or  301   b,  if the cross-sectional area of the throttle oil channel  301   a  or  301   b  (i.e. the flowing area) is constant during the buffering process of the hydraulic cylinder, the throttle oil channel  301   a  or  301   b  is referred to as a constant throttle oil channel; and if the flowing area is variable automatically during the buffering process of the hydraulic cylinder, the throttle oil channel  301   a  or  301   b  is referred to as a variable throttle oil channel. There are various forms to be selected as set forth below. 
         [0084]    In the first embodiment of the present application, the throttle oil channels  301   a ,  301   b  are arranged in the sliding regions between the piston rod  3  and the first buffer sleeve  4 , the second buffer sleeve  11  (i.e. the throttle oil channel  301   a  is arranged in the sliding region between the piston rod  3  and the first buffer sleeve  4 , and the throttle oil channel  301   b  is arranged in the sliding region between the piston rod  3  and the second buffer sleeve  11 ). The throttle oil channels  301   a,    301   b  are tapered linear throttle oil grooves, with the depth of the throttle oil grooves decreasing gradually towards the piston  6 . Four throttle oil grooves are evenly distributed on the external surface of the piston rod  3  to achieve a throttling-varied smooth buffering effect. 
         [0085]    In the second embodiment of the present application (as shown in  FIG. 11 ), the throttle oil channels  301   a,    301   b  are formed by throttle inclined surfaces arranged on the piston rod  3  respectively. The throttle inclined surface rises gradually towards the piston, i.e. 
         [0086]    the cross-sectional area of the throttle inclined surface decreases gradually towards the piston, so as to achieve a throttling-varied smooth buffering effect. 
         [0087]    In the fifth embodiment of the present application (as shown in  FIG. 14 ), a transition sleeve  304  is provided in the sliding region between the piston rod  3  and the first buffer sleeve  4 . The throttle oil channel  301   a  arranged on the transition sleeve  304  includes a first segment of throttle oil channel  3012  located at an inlet end of the transition sleeve  304 , and a second segment of throttle oil channel  3011  located at an outlet end of the transition sleeve  304 . The first segment of throttle oil channel  3012  is a tapered linear throttle oil groove arranged on the transition sleeve  304 , with the depth of the oil groove decreasing towards the piston  6 ; and the second segment of throttle oil channel  3011  is an oil passage arranged inside the transition sleeve  304 , thereby achieving a throttling-varied smooth buffering effect. 
         [0088]    In the sixth embodiment of the present application (as shown in  FIG. 15 ), a transition sleeve  304  is provided in the sliding region between the piston rod  3  and the first buffer sleeve  4 . The throttle oil channel  301   a  arranged on the transition sleeve  304  includes an oil channel  3013  arranged inside the transition sleeve  304  and extending in the axial direction, and multiple throttle orifices  3014  arranged on the external surface of the transition sleeve  304  along the axial direction of the transition sleeve  304  and being in communication with the oil channel  3013 . When the first buffer sleeve  4  slides towards the piston  6 , the number of the throttle orifices  3014  that are covered by the first buffer sleeve  4  increases gradually, so that the flowing area of the throttle oil channel  301   a  decreases gradually, thereby achieving a throttling-varied smooth buffering effect. The aperture diameter of the throttle orifices  3014  can also decrease gradually towards the piston  6 , so as to achieve the purpose of a constant deceleration. 
         [0089]    In addition to the above illustrative embodiments, the throttle oil channels  301   a ,  301   b  may also be constant throttle oil channel and may be arranged on the first buffer sleeve  4  and the second buffer sleeve  11  respectively. The cross-sectional areas of the throttle oil channels  301   a  and  30  lb gradually decrease in depth and/or in width towards the piston. In the embodiments of the present application, the throttle oil channels  301   a,    301   b  are arranged in the areas where the first buffer sleeve  4 , the second buffer sleeve  11  are slidable with respect to the piston rod  3 , and the throttle oil channels  301   a,    301   b  are tapered linear throttle oil grooves, with the depth of the throttle oil grooves decreasing towards the piston  6 . Compared with the helical throttle oil channel with variable depth, the throttle oil channels  301   a  and  301   b  are processed at a lower cost. Since the processing of the helical throttle oil channel with variable depth is extremely difficult, the processing cost is rather higher, and the processing precision of the helix depth is beyond control, therefore failing to achieve the ideal buffering effect. It is easy to process the tapered linear throttle oil groove and to control the processing precision of the taper, and the ideal buffering effect can be achieved. The first embodiment of the present application is the most preferred embodiment. 
         [0090]    When the piston rod  3  extends out to the end of the stroke, the first buffer sleeve  4  does not reach the end position, and can still slide towards the piston by a certain distance L 1 . When the piston rod  3  retracts, oil enters the oil passage B; under the action of the hydraulic oil, the first buffer sleeve  4  is pushed to slide towards the piston  6  so as to compress a return spring  5 ; thus the sealing end face  401  of the first buffer sleeve  4  moves away from the sealing end face  101  of the rod cavity end cap  1 , so that the oil passage B comes into direct communication with the rod cavity; and the hydraulic oil enters into the rod cavity and pushes the piston  6  to move. During the retracting movement of the piston rod  3 , the first buffer sleeve  4  cooperates with the rod cavity end cap  1  to function as a check valve. The first buffer sleeve  4  keeps a distance L 1  from the end point of its sliding towards the piston  6 . The larger the distance L 1  is, the longer the distance between the sealing end face  401  of the first buffer sleeve  4  and the sealing end face  101  of the rod cavity end cap  1  is, the more the flow of the hydraulic oil entering into the rod cavity is. The smaller the distance L 1  is, the shorter the distance between the sealing end face  401  of the first buffer sleeve  4  and the sealing end face  101  of the end cap  1  of the rod cavity is, the less the flow of the hydraulic oil entering into the rod cavity is. The distance L 1  must allow the oil passage B to be in direct communication with the rod cavity. 
         [0091]    When the piston rod  3  retracts to the end of the stroke, the second buffer sleeve  11  does not reach the end position, and can still slide towards the piston by a certain distance L 2 . When the piston rod  3  extends out, oil enters the oil passage A; under the action of the hydraulic oil, the second buffer sleeve  11  is pushed to slide towards the piston  6  so as to compress a return spring  7 ; thus the sealing end face  111  of the second buffer sleeve  11  moves away from the sealing end face  121  of the rodless cavity end cap  12 , so that the oil passage A comes into direct communication with the rodless cavity; and the hydraulic oil enters into the rodless cavity and pushes the piston  6  to move. During the extending movement of the piston rod  3 , the second buffer sleeve  11  cooperates with the rodless cavity end cap  12  to function as a check valve. The second buffer sleeve  11  keeps a distance L 2  from the end point of its sliding towards the piston  6 . The larger the distance L 2  is, the longer the distance between the sealing end face  111  of the second buffer sleeve  11  and the sealing end face  121  of the rodless cavity end cap  12  is, the more the flow of the hydraulic oil entering into the rodless cavity is. The smaller the distance L 2  is, the shorter the distance between the sealing end face  111  of the second buffer sleeve  11  and the sealing end face  121  of the rodless cavity end cap  12  is, the less the flow of the hydraulic oil entering into the rodless cavity is. The distance L 2  must be sufficient to allow the oil passage A to be in direct communication with the rodless cavity. 
         [0092]    In order to enable the smooth slide of the first buffer sleeve  4  and the second buffer sleeve  11  on the piston rod  3  so as to assure the service life and the performance, multiple circumferential balancing oil grooves  302   a,    302   b  are provided between the two buffer sleeves and the piston rod  3 , i.e. multiple circumferential balancing oil grooves  302   a  are provided between the first buffer sleeve  4  and the piston rod  3 , and multiple circumferential balancing oil grooves  302   b  are provided between the second buffer sleeve  11  and the piston rod  3 . The balancing oil grooves  302   a,    302   b  are provided on the external surface of the piston rod  3 . Alternatively, the balancing oil grooves  302   a,    302   b  may be arranged on the internal surfaces of the first buffer sleeve  4  and the second buffer sleeve  11 , i.e. the balancing oil grooves  302   a  are arranged on the internal surface of the first buffer sleeve  4 , and the balancing oil grooves  302   b  are provided on the internal surface of the second buffer sleeve  11 . External surfaces of the piston rod  3  fitted with the first and second buffer sleeves  4 ,  11  can be treated with chromium plating so as to improve the hardness and the surface quality. 
         [0093]    In order to reliably locate the first buffer sleeve  4 , a shaft shoulder  303  for locating the first buffer sleeve  4  is provided on the piston rod  3 . A return spring  5  is provided between the first buffer sleeve  4  and the piston  6  in order to ensure the significant buffering effect of the hydraulic cylinder and a quick return of the piston  6 . One end of the return spring  5  abuts against the piston  6  and the other end abuts against the first buffer sleeve  4 . The return spring  5  is adapted to return and buffer the first buffer sleeve  4 . When the hydraulic cylinder is out of the buffer state, the first buffer sleeve  4  abuts against the shaft shoulder  303  under the applied force of the return spring  5 . The shaft shoulder  303  is provided with an oil discharging groove D which is in communication with the throttle oil channel  301   a.  In order to locate the first buffer sleeve  4  on the piston rod  3 , structures such as a retainer ring may also be arranged on the piston rod  3 . 
         [0094]    In order to reliably limit the second buffer sleeve  11 , a stop shoulder for limiting the second buffer sleeve  11  is provided at the tail end of the piston rod  3 . The stop shoulder includes a key  10 , a key cap  8  and a retainer ring  9 . The key  10  is of two-semicircular ring structure, and is assembled in a corresponding stop shoulder groove at the tail end of the piston rod  3 . The key cap  8  is located between the key  10  and the retainer ring  9  and is adapted to fix the key  10 . The retainer ring  9  is adapted to locate the key cap  8 . The cross section of the key  10  is of an “L” shape, and an oil discharging groove E is arranged on the external surface of the key  10 . The cross section of the key cap is of a square shape. The second buffer sleeve  11  and the hydraulic oil apply a very large force to the key  10 . In order to prevent the applied force from causing damages to the key cap  8  and the retainer ring  9 , the cross section of the key  10  is designed into an “L” shape, and the cross section of the key cap  8  is designed into a square shape, so that an applied force is transmitted onto the piston rod  3  via the key  10  of “L” shape. Therefore, the problem that the second buffer sleeve  11  and the hydraulic oil exert a very large force on the key  10  to cause damages to the key cap  8  and the retainer ring  9  is solved. 
         [0095]    The piston  6  may be connected to the piston rod  3  by means of threading. For example, the piston  6  is fixed on the undercut of the piston rod  3  via a screw  13 , and is sealed against the piston rod  3  via a stationary sealing-ring. The rod cavity end cap  1  and the cylinder barrel  2  are connected by means of bolting, while the rodless cavity end cap  12  and the cylinder barrel  2  are connected by welding. Various ways may be selected to connect the rod cavity end cap  1  and the rodless cavity end cap  12  with the cylinder barrel  2 . For example, both the rod cavity end cap  1  and the rodless cavity end cap  12  can be connected to the cylinder barrel  2  by means of welding or bolting or threading, or they can be produced as an integrated structure as well. 
         [0096]    Seals between the cylinder barrel  2  and the rod cavity end cap  1 , as well as between the cylinder barrel  2  and the rodless cavity end cap  12  can be achieved via a sealing part (K08-D) being of an O-ring adding Glyd-ring form. The rod cavity end cap  1  is provided with a stop shoulder  102  adapted to limit a leftward movement of the piston  6 ; and the rodless cavity end cap  12  is provided with a stop shoulder adapted to limit a rightward movement of the piston  6 . 
         [0097]    The working process of the hydraulic cylinder is described as follows: when the piston rod  3  extends out, the piston  6  moves leftwards; when the piston rod  3  is at an end position of the retraction stroke, the second buffer sleeve  11  and the rodless cavity end cap  12  are in a contact sealed state; in order that the rodless cavity can be fed with oil rapidly, the piston rod  3  is pushed to perform the extending movement. There&#39;s still a distance L 2  between the second buffer sleeve  11  and the end point of its sliding towards the piston  6 ; and under the action of the hydraulic oil, the second buffer sleeve  11  compresses a spring  7  and slides towards the piston  6 . Therefore, the sealing end face  111  of the second buffer sleeve  11  moves away from the sealing end face  121  of the rodless cavity  12 . At this moment, the second buffer sleeve  11  cooperates with the rodless cavity end cap  12  to function as a check valve. 
         [0098]    Hydraulic oil enters into the rodless cavity and pushes the piston  6  to move leftwards. The hydraulic oil in the rod cavity is discharged via the oil passage B; when the piston rod  3  extends to a position away from the end of the stroke by a certain distance, the end face  401  of the first buffer sleeve  4  comes into contact with the end face  101  of the rod cavity to form a seal, breaking the direct communication between the oil passage B and the rod cavity entirely or partially. Hydraulic oil within the rod cavity is discharged through a throttle oil channel  301   a  and an oil discharging groove D to the oil passage B, with the throttle oil channel  301   a  being between the first buffer sleeve  4  and the piston rod  3 . Since the oil discharging quantity of the throttle oil channel  301   a  is rather small, an appropriate buffer pressure being generated in the enclosed hydraulic oil is applied on the oil discharging side of the piston  6 , to counteract with the inertial force of the piston. Thus, the hydraulic cylinder starts to enter into a buffer state in the left side. As the piston rod  3  further extends out, the piston  6  keeps on moving leftwards; the first buffer sleeve  4  slides rightwards with respect to the piston rod  3 , so that the flowing area of the throttle oil channel  301   a  between the first buffer sleeve  4  and the piston rod  3  decreases gradually; the oil discharging quantity decreases as well; the buffer pressure generated in the rod cavity and applied on the oil discharging side of the piston  6  increases gradually; and the movement of the piston  6  is slowed down, thus achieving the object of decelerating and braking and realizing the effect of smooth buffering deceleration. When the left end face of the piston  6  abuts against the stop shoulder  102  of the rod cavity end cap  1 , the piston  6  does not move leftwards any more, and the piston rod  3  extends to the end of the stroke. Thus, the whole buffer process is over. 
         [0099]    When the piston rod  3  retracts back, the piston  6  moves rightwards. When the piston rod  3  is at an end position of the extending stroke, the first buffer sleeve  4  and the rod cavity end cap  1  are in a contact sealed state; and in order that the rod cavity can be fed with oil rapidly, the piston rod  3  is pushed to perform the retracting movement. There&#39;s still a distance L 1  between the first buffer sleeve  4  and the end point of its sliding towards the piston  6 ; and under the action of the hydraulic oil, the first buffer sleeve  4  compresses a spring  5  and slides towards the piston  6 . Therefore, the sealing end face  401  of the first buffer sleeve  4  moves away from the sealing end face  101  of the rod cavity  1 . At this moment, the first buffer sleeve  4  cooperates with the rod cavity end cap  1  to function as a check valve during the retracting process of the piston rod  3 . 
         [0100]    The hydraulic oil enters into the rod cavity through the oil passage B and pushes the piston  6  to move rightwards, and the piston rod  3  retracts back. The hydraulic oil in the rodless cavity is discharged through the oil passage A; when the piston rod  3  retracts to a position away from the end of the stroke by a certain distance, the end face  111  of the second buffer sleeve  11  comes into contact with the end face  121  of the rodless cavity end cap to form a seal, breaking the direct communication between the oil passage A and the rodless cavity entirely or partially. Hydraulic oil within the rodless cavity is discharged through an throttle oil channel  301   b  and an oil discharging groove E to the oil passage A, with the throttle oil channel  301   b  being between the second buffer sleeve  11  and the piston rod  3 . Since the oil discharging quantity of the throttle oil channel  301   b  is rather small, an appropriate buffer pressure generated in the enclosed hydraulic oil is applied on the oil discharging side of the piston  6 , to counteract with the inertial force of the piston. Thus, the hydraulic cylinder starts to enter into a buffer state. As the piston rod  3  further retracts back, the piston  6  keeps on moving rightwards, the second buffer sleeve  11  slides leftwards with respect to the piston rod  3 , so that the flowing area of the throttle oil channel  301   b  between the second buffer sleeve  11  and the piston rod  3  decreases gradually; the oil discharging quantity decreases as well; the buffer pressure generated in the rodless cavity and applied on the oil discharging side of the piston  6  increases gradually; and the movement of the piston  6  is slowed down, thus achieving the object of decelerating and braking and realizing the effect of smooth buffering deceleration. When the right end face of the piston  6  abuts against the stop shoulder of the rodless cavity end cap  12 , the piston  6  does not move rightwards any more, and the piston rod  3  retracts to the end of the stroke. Thus, the whole buffer process is over. 
         [0101]    Reference is made to the third embodiment of the  FIG. 12 , which is a modification based on the above first embodiment. The third embodiment is different from the first embodiment in that: a transition sleeve  304  is mounted at a position where the piston rod  3  is fitted with the first buffer sleeve  4 , and the transition sleeve  304  is fitted with the first buffer sleeve  4 . Multiple circumferential balancing oil grooves and tapered linear throttle oil grooves are provided on the external surface of the transition sleeve  304 , and the external surface of the transition sleeve  304  fitted with the first buffer sleeve  4  can be treated with chromium plating so as to improve the hardness and the surface quality. 
         [0102]    In the first embodiment, multiple circumferential balancing oil grooves and tapered linear throttle oil grooves are processed on the piston rod  3  directly. Since the piston rod  3  has a large diameter and a long stroke, there are high precision requirements for processing the multiple circumferential balancing oil grooves and tapered linear throttle oil grooves, and the processing is extremely difficult. In the third embodiment, it is relatively easy to process multiple circumferential evenly-distributed balancing oil grooves and tapered linear throttle oil grooves at a high precision on the transition sleeve  304 . 
         [0103]    Reference is made to the fourth embodiment in  FIG. 13 , which is a modification based on the above first embodiment. The fourth embodiment is different from the first embodiment in that: the piston rod  3  includes a piston rod body  3   a  and a buffer shaft  3   b,  and the piston rod body  3   a  and the buffer shaft  3   b  are connected by threading and then fixed via a screw  15 . The buffer shaft  3   b  is fitted with the buffer sleeve  11 , and a shaft shoulder for limiting the buffer sleeve  11  is provided at a tail end of the buffer shaft  3   b.  Since the buffer shaft  3   b  has a short length, it is relatively easy to process multiple circumferential balancing oil grooves and tapered linear throttle oil grooves at a high precision on the buffer shaft  3   b . The piston rod body  3   a  and the buffer shaft  3   b  may be connected together in various ways, for example, by threading, welding, bolting, and the like, as descried herein. 
         [0104]    In the above embodiments, if there is a need for buffering in the rod cavity of the hydraulic cylinder, a buffer sleeve can be arranged only in the rod cavity; if there is a need for buffering in the rodless cavity of the hydraulic cylinder, a buffer sleeve can be arranged only in the rodless cavity; if there is a need for buffering in both the rod cavity and the rodless cavity, buffer sleeves can be arranged in the rod cavity and the rodless cavity respectively. Two or more buffer sleeves may also be arranged in one cavity, depending on actual demands. Multiple circumferential balancing oil grooves and multiple throttle oil channels extending axially may also be arranged on the internal surface of the buffer sleeves, and the cross-sectional area of the throttle oil channel may be constant. 
         [0105]    In the above embodiments, a return spring may be provided between the buffer sleeves and the piston, and may also not to be provided, because the buffer sleeve comes into contact with the rod cavity end cap to form a seal under the action of the hydraulic oil. 
         [0106]    In the hydraulic cylinder according to the embodiment of the present application, in addition to the above embodiments, the throttle oil channel can also be arranged on the rod cavity end cap, the rodless cavity end cap, the buffer sleeve and the piston rod. All such modifications are within the protection scope of the present application. 
         [0107]    When the hydraulic cylinder according to the embodiment of the present application is employed in a hydraulic buffer system to replace the existing oil cylinder, the embodiment of the hydraulic buffer system of the present application can be achieved. 
         [0108]    When the hydraulic cylinder according to the embodiment of the present application is employed in an excavator, the embodiment of the excavator of the present application can be achieved. 
         [0109]    When the hydraulic cylinder according to the embodiment of the present application is employed in a concrete pump truck, the embodiment of the concrete pump truck of the present application can be achieved. The hydraulic cylinder according to the embodiment of the present application may also be employed in construction machinery of other types. 
         [0110]    Although the embodiments of the present application are disclosed above by the preferred embodiments, these preferred embodiments are not intended to limit the application. Any skills in the art can make possible variations and modifications without departing from the spirit and scope of the present application, and the scope of protection of the present application should be defined by the claims of the present application.