EP1528262A1

EP1528262A1 - A crane and a method for controlling a crane

Info

Publication number: EP1528262A1
Application number: EP03024717A
Authority: EP
Inventors: Peter ASTRÖM
Original assignee: Hiab AB
Current assignee: Hiab AB
Priority date: 2003-10-29
Filing date: 2003-10-29
Publication date: 2005-05-04

Abstract

A crane, comprising at least one hydraulic cylinder/piston unit (2) in which a piston rod (5) is displaceable in a cylinder (3) and in which a piston (4) that is connected to the piston rod separates the cylinder chamber in a cylinder side chamber (6) and a piston rod side chamber (7) that is provided with an opening (8) in the cylinder gable (9) in order to permit a displacement of the piston rod (5) out of and into the cylinder, a first conduit (10) for conducting a pressure fluid between a pressure fluid source (11) and the cylinder side chamber (6), a second conduit (13) for conducting pressure fluid between the piston rod side chamber (7) and a pressure fluid reservoir (14) that has a lower pressure than the pressure fluid source (11), a third conduit (15) for regenerative conduction of a pressure fluid between the piston rod side chamber (7) and the cylinder side chamber (6) when pressure fluid is delivered from the pressure fluid source to the cylinder side chamber (6) for the purpose of ejecting the piston rod (5) out of the cylinder, and means (23) for breaking the regenerative conduction of pressure fluid at a predetermined moment. In at least one of the first and the second conduit (10,13) there is arranged a pressure delimiting member (16) for delimiting the pressure in the cylinder side chamber (6) and the piston rod side chamber (7) respectively.

Description

FIELD OF THE INVENTION

The present invention relates to a crane, comprising
at least one hydraulic cylinder/ piston unit in which a piston rod is displaceable in a cylinder and in which a piston that is connected to the piston rod separates the cylinder chamber in a cylinder side chamber and a piston rod side chamber that is provided with an opening in the cylinder gable in order to permit a displacement of the piston rod out of and into the cylinder, a first conduit for conducting a pressure fluid between a pressure fluid source and the cylinder side chamber, a second conduit for conducting pressure fluid between the piston rod side chamber and a pressure fluid reservoir that has a lower pressure than the pressure fluid source, a third conduit for regenerative conduction of a pressure fluid from the piston rod side chamber to the cylinder side chamber when pressure fluid is delivered from the pressure fluid source to the cylinder side chamber for the purpose of ejecting the piston rod out of the cylinder, and means for breaking the regenerative conduction of pressure fluid at a pre-determined moment.
The present invention also relates to a method of controlling such a crane, wherein, upon a predetermined condition during ejection of the piston rod in connection to delivery of the pressure fluid to the cylinder side chamber, the regenerative flow through the third conduit is reduced and the pressure fluid is permitted to flow through the second conduit towards the reservoir.
The invention is applicable to all types of cranes, but is particularly applicable to cranes for lifting piece goods, presenting a considerable overhang when in an extended position. The invention will therefore be described by way of example with reference to such an application.
It should be understood that the hydraulic unit according to the invention can be utilized for the purpose of imparting swinging movements as well as longitudinal movements of any beam that forms a part of the crane.
Even though not necessary restricted to such an embodiment, the invention is particularly suitable for cranes that comprise a plurality of beams that are arranged in a telescopic way. In order to elongate or extend such a crane, the beams are displaced in relation to each other by means of a plurality of hydraulic cylinder/piston units.

THE BACKGROUND OF THE INVENTION AND PRIOR ART

Along with an increasing need of longer overhangs on piece goods cranes, the time and power needed for the extension of the crane to its maximum overhang has become an important issue in connection to the design of such cranes. Generally, there is a need of reducing the time that is required for attaining maximum crane length.
Different ways of reducing the maximum power required for attaining maximum overhang have been proposed. Some prior art uses sequential extension of the beams. However, in sequentially controlled cranes, there is an important flow resistance for the pressure fluid that is used. A sequentially controlled crane will require less power and lower maximum load on the individual cylinder/piston unit than a non-sequentially controlled crane, but will be slower. Moreover, generally, a sequentially controlled crane is more complicated and expensive than a non-sequentially-controlled one.
It has also been proposed to lower the extension resistance of the beams by means of lubrication thereof. However, lubricated beams often result in lubrication oil or the like leaking out to the environment. In some applications, such leakage cannot be accepted and is therefore of disadvantage.
In order to further increase the extension speed of the crane it has been proposed to use regenerative conduction of the pressure fluid, normally oil, from the piston rod side chamber to the cylinder side chamber during the extension operation of the crane. Thereby, less oil has to be conducted from a pressurised oil source all the way to the cylinder side chamber, while, simultaneously the pressure of the oil that exits from the piston rod side chamber is taken advantage of for the purpose of filling the cylinder side chamber instead of only being conducted to a low pressure tank or reservoir.
Accordingly, there is a strive to present a crane the extension of which is not performed sequentially, which does not require any lubrication of vital beam parts in order to attain maximum overhang, and which uses regeneration of oil for maximum speed.
It will be possible to apply the regenerative conduction of oil all the way until maximum overhang has been attained for an individual beam. However, this will require more powerful cylinder/piston units than otherwise needed, or a careful lubrication of the beams, since the hydraulic power needed for extending the beam, when loaded, increases with increasing overhang. Therefore, it has been proposed by prior art to extend an individual beam by use of regenerative conduction up to a pre-determined moment at which the regenerative conduction is substituted by conventional operation in which the oil from the piston rod side chamber is conducted to a low pressure tank. The effect is similar to a change to a lower gear operation. For example, in a crane the operative hydraulic pressure of which is 28 MPa, oil of a pressure of 14 MPa is fed to the cylinder side chambers of the individual hydraulic units, resulting in a pressure increase on the cylinder side as well as on the piston rod side. Regenerative flow is permitted during extension of the crane. At a certain overhang of the crane, when a given pressure has been attained in the cylinder side chamber, for example 10 MPa, the regenerative conduction is stopped, whereby the pressure in the piston rod chamber is reduced to generally the same level as the pressure in the low pressure tank to which it is conducted. Thereby, less over-dimensioning of the individual cylinder/piston units is required, and lubrication of the beam may be omitted.
However, the remaining distance that the piston has to be displaced without regenerative operation in order to attain maximum overhang will be of considerable length, especially if the crane is loaded and performing a lifting movement. Accordingly, on non-sequentially controlled cranes, a considerable part of the total displacement of the piston in relation to the cylinder (note that it might be the cylinder that is the moving element out of the two) will be performed at relatively low speed with this kind of prior art operation.

THE OBJECT OF THE INVENTION

It is an object of the present invention to present a crane and a method as initially defined that, during extension of a hydraulic unit in the crane, takes advantage of regenerative operation in the best possible way and promotes a minimizing of the distance that the moving part of the hydraulic unit or units has to be displaced in conventional manner without use of regenerative operation.
It is a further object of the invention to present a crane and a method as initially defined that is readily applicable to existingcrane constructions and that is advantageous because of its relative constructional simplicity and operation reliability.
It is a further object of the invention to present a crane and a method as initially defined that does not require excessive dimensioning of the individual hydraulic units.

SUMMARY OF THE INVENTION

The object of the present invention is achieved by means of a crane as initially defined, characterised in that, in at least one of the first and the second conduit, there is arranged a pressure delimiting member for delimiting or reducing the pressure in the cylinder side chamber and the piston rod side chamber respectively.
Thereby, by suitable control of existing control valves in the crane, a relatively high pressure (higher than otherwise) can be applied to the cylinder side chamber up to the moment when regenerative operation is ended. Thereby the distance over which piston (or cylinder) displacement is performed by the aid of regenerative operation can be prolonged. The individual piston rod is designed to stand a maximum power or force. According to the invention, regenerative operation is performed up to an absolute pressure on the cylinder side that is substantially above said maximum value. This is possible thanks to the maintained, i.e. delimited, pressure on the piston rod side instead.
At the change to non-regenerative operation, the pressure in the cylinder side chamber can remain high if the pressure delimiting member is provided in the second conduit between the piston rod side chamber and the low pressure reservoir. If, on the other hand, the pressure delimiting member is arranged in the first conduit, it should be of a controllable type that is actuated upon the change to non-regenerative operation in order to permit a reduction of the pressure in the cylinder side chamber while the pressure in the piston rod side chamber is reduced, by means of free flow, to generally the same pressure as the one in the reservoir, which is preferably equal to the atmospheric pressure.
Further preferred embodiments of the crane according to the invention are presented in the dependent claims 2-11.
The object of the invention is also achieved by means of the inventive method as initially defined, which is characterised in that, upon the reaching of said condition, the pressure in the piston rod side chamber is delimited to a pressure that is higher than the pressure in the reservoir but lower than the pressure in the cylinder side chamber, or that the pressure in the cylinder side chamber is reduced while the pressure in the piston rod side chamber is reduced generally to the pressure of the reservoir. The advantage of the method has already been discussed with reference to the crane above.
Further preferred embodiments of the method are presented in the dependent claims 13-17.
Further advantages and features of the invention will be presented in the following detailed description thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference to the annexed drawings, on which.
Fig. 1 is a schematic view of a crane on which the invention might be applied,
Fig. 2 is a hydraulic scheme showing essential parts of the invention according to a first embodiment,
Fig. 3 is a hydraulic scheme showing essential parts of the invention according to a second embodiment, and
Fig. 4 is a hydraulic scheme showing essential parts of the invention according to a third embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 shows a crane according to the invention. The crane comprises a plurality of beams 1 that are telescopically arranged in relation to each other in order to permit adjustment of its length from a shortest length, in which the beams are inserted into each other, to a maximum length when they are fully extended. The crane comprises a plurality of hydraulic units 2 for driving the beams 1 in way known per se. As seen in fig. 2, each hydraulic unit 2 comprises a cylinder 3 and a piston 4 with a piston rod 5. the piston divides the cylinder chamber into a cylinder side chamber 6 and a piston rod side chamber 7. The piston area is larger on the cylinder side than on the piston rod side. The piston rod 5 is arranged to perform a reciprocating movement through an opening 8 in a gable 9 of the cylinder 3 for its operation. For the sake of clarity and for facilitating the description thereof, the hydraulic system or circuit can be divided into a cylinder side and a piston rod side.
As further seen in fig. 2, for the actuation of the hydraulic unit the crane comprises a first conduit 10 that connects the cylinder side chambers 6 of the hydraulic units 2 with a pressure fluid source 11 via a direction valve 12 in a way known per se. The cylinder side chambers 6 are connected in parallel with each other in relation to the source 11. The first conduit can, as here, be divided in a plurality of intermediate conduits, each of which leads from the cylinder side chamber of one hydraulic unit to a cylinder side chamber of another hydraulic unit in a bus-type arrangement. Therefore, the cylinder side chambers 6 may be regarded as an integrated part of the first conduit 10. The fluid pressure in the pressure fluid source may, for example, be 28 MPa.
On the piston rod side the crane comprises a second conduit 13 that connects the piston rod side chamber 7 of a first one of the hydraulic units 2 with a low pressure reservoir 14 for drainage of pressure fluid from the chamber 7 to the reservoir 14. The fluid pressure in the reservoir 14 may, as here, be equal to atmospheric pressure. The second conduit 13 can, as here, be divided in a plurality of intermediate conduits, each of which leads from the piston rod side chamber 7 of one hydraulic unit 2 to a piston rod side chamber 7 of another hydraulic unit 2 in a bus-type arrangement. Therefore, the piston rod side chambers 6 may be regarded as an integrated part of the second conduit 13.
There is also provided a third conduit 15 that in one end is connected to the first conduit 10 and in another end is connected the second conduit 13. The third conduit 15 is arranged for controlled regenerative pressure fluid flow from the piston rod side chamber 7 to the cylinder side chamber 6 during extension of the crane. The third conduit 15 may be connected directly to the cylinder side chamber 6, since the latter may be regarded as an integral part of the first conduit 10, and directly to the piston rod side chamber 7 since the latter may be regarded as an integral part of the second conduit 13.
According to the first embodiment shown in fig. 2 there is provided a pressure delimiting member 16 in the second conduit 13 for the purpose of reducing the pressure on the piston rod chamber side upon a given pressure condition in the first conduit 10.
There is also provided a pilot-controlled valve, here a pilot-controlled non return valve 17, in the second conduit 13. Upon activation of valve 17, it will permit a flow throughthe second conduit 13 towards the reservoir. During extension of the crane under regenerative operation, the regenerative flow through the third conduit 15 is stopped, or at least substantially reduced, as valve 17 is actuated in order to permit a flow of fluid from the piston rod side chamber 7 to the reservoir 14. Thereby, full extension of the hydraulic unit 2 in question can be attained with a remaining high or even further increased pressure on the cylinder side and a reduced, but still substantial, pressure on the piston rod side thanks to the pressure delimiting member 16.
The pressure delimiting member 16 should comprise a counter pressure valve and may comprise any kind of throttle or constriction. It only opens for passage when the pressure in the piston rod side chamber exceeds a given value, in this example 24 MPa, thereby delimiting the pressure in the piston rod side chamber 7 to that pressure. Here, the pressure delimiting member 16 is arranged so as to delimit the pressure in the piston rod side chamber 7 to approximately 24 MPa while the pressure in the cylinder side chamber reaches approximately 28 MPa during the continued piston rod displacement without aid of regenerative flow. In other words, the maximum pressure drop over the pressure delimiting member 16 is equal to the minimum pressure accepted in the piston rod side chamber 7 upon ending of the regenerative operation.
In fig. 3 there is shown a second embodiment according to which there is provided a pressure delimiting member 18 in the first conduit 10 between the direction valve 12 and the cylinder side chamber 6, instead of having a pressure delimiting member corresponding to member 16 in the second conduit 13. According to this embodiment the pressure delimiting member 18 is actuated at the moment when the pressure on the cylinder side has reached a value by which a pilot controlled valve 17 in the second conduit is opened for passage without pressure drop over it 13 and regenerative operation is ended. Thereby the pressure in the cylinder side chamber 6 is reduced to a given value while the pressure in the piston rod side chamber 7 is reduced to generally the same pressure as in the reservoir 14 through activation of valve 17.
The pressure delimiting member 18 in fig. 3 is arranged in a fourth conduit 19 that in one end is connected to the first conduit 10 and in the other end is connected to low pressure reservoir 14, here via second conduit 13. Pressure delimiting member 18 is arranged in series with a pilot-controlled valve 20, here an electrically controlled non-return valve arranged in the fourth conduit 19. Valve 20 is actuated on for the opening thereof at the moment when a given pressure value has been reached on the cylinder side, regenerative operation is stopped, and when valve 17 in second conduit 13 is opened. There is no pressure drop over pilot controlled valve 20 when in its open state. Thus, pressure fluid is drained via the fourth conduit 19 for the purpose of reducing the pressure in the cylinder side chamber 6. The maximum pressure drop (in this example 14 MPa as indicated in the fig. 3) over the pressure delimiting member is equal to the maximum pressure accepted in the cylinder side chamber 6 upon opening of valve 17. A delimited pressure difference between cylinder side and piston side is thereby guaranteed, and even though regenerative operation has been performed up to a high pressure on the cylinder side, full reduction or elimination of the pressure on piston rod side can be permitted without the risk of having a piston rod breakage.
Valves 17 and 20 in fig. 3 are both electrically controlled and activated upon receipt of a control signal emitted, upon a given pressure in cylinder side chamber 6, by a pressure gauge or sensor 21 arranged in the first conduit 10. Pressure gauge 21 could deliver the control signal by wire or wireless and, possibly, via a control unit (not shown). However, it should be understood that other solutions, by which valves corresponding to valves 17 and 20 are hydraulically controlled and operated, are also within the scope of the invention.
In fig. 4 there is shown an alternative to the embodiment shown in fig. 3, in which a pressure delimiting member 22 is arranged in the first conduit 10 and comprises a counter pressure valve arranged for reduction of the pressure in the cylinder side chamber 6. Pressure delimiting member 22 is arranged in parallel with an electrically operated non-return valve 28 which opens fully for flow towards the cylinder side chamber 6, without pressure drop over it, during regenerative operation. Valve 28 is closed upon receipt of a control signal from a pressure gauge 21, as described for the embodiment offig. 3, once the pressure in the first conduit 10, or in the cylinder side chamber 6, has reached a predetermined value. Simultaneously, regenerative flow is ended, as valve 17 is or has just been opened for passage in second conduit 13 upon receipt of a control signal from said pressure gauge 21.
In the first embodiment shown in fig. 2, but possibly also in the alternative embodiments of figs. 3 and 4, although not shown, there is provided a pilot-controlled valve 23, here a non-return valve in the third conduit 15 for regenerative flow. Said pilot controlled valve 23, upon actuation thereof, opens for regenerative flow of pressure fluid from piston rod side chamber 7 to cylinder side chamber 6. In series with pilot-controlled valve 23 there is provided a non-return valve 24 that prevents flow in the opposite direction, that is fromcylinder side chamber 6 to piston rod side chamber 7. Valve 23 defines a means for breaking the regenerative operation, but all valves, that are essential for transition from regenerative to non-regenerative operation contribute to breaking the regenerative operation in one way or another. In other words, even without the specific valve 23, the inventive device is still equipped with means for breaking the regenerative operation as this is a crucial feature of the device for the purpose of its functionality. It should be understood that, as an alternative, the third conduit 15 could be directly connected with, for example, the direction valve 12, and that the latter could then be equipped with a slide valve that would take the place and function of the regeneration valve 23 and the non-return valve in series therewith.
In all embodiments there is provided a means for measuring or sensing the pressure in the cylinder side chamber 6 or at least a pressure directly corresponding to that pressure, such as the pressure in the first conduit 10 or in the second conduit 13 or third conduit 15 during regenerative operation.
In the first embodiment of fig. 2 said sensing means includes a counter pressure valve 25 arranged in a forth conduit 26 that extends from the first conduit 10 to the reservoir via the second conduit 13. The valve 25, which is a non-return valve, opens for flow towards the reservoir when the pressure in the first conduit 10 reaches a value (here 27 MPa) slightly lower than the maximum pressure that is still to be reached in the first conduit during regenerative operation (here 28 MPa). A constriction 29, the maximum pressure drop over which (here 1 MPa) is equal to the maximum pressure to be reached in the first conduit 10 minus the maximum pressure drop of valve 25, is arranged downstream valve 25 in said conduit 26. A connection or leakage conduit 27 that connects conduit 26 with pilot controlled valve 17 at a point between valve 25 and constriction, permits pressure fluid to act on valve 17 when the pressure in the first conduit 10 exceeds the maximum pressure drop of valve 25 in conduit 26. Pilot controlled valve 17 is arranged to open fully, with no pressure drop thereover, as soon as a certain pressure (here1 MPa) has been reached in leakage conduit 27. The opening of valve 17 is continuously, i.e. in a delayed mode, effectuated during pressure build up in conduit 27. Thereby, a smooth transition from maximum pressure (here 28 MPa) in the piston rod side chamber 7 to the delimited pressure (here 24 MPa) as determined by the pressure delimiting member 16 is obtained when regenerative operation is ended. Accordingly, valve member 17 also defines a member for delaying thepressure transition from higher to lower pressure in the piston rod side chamber 7 upon termination of regenerative operation, which has proved itself to be an advantageous feature for the operation as a whole.
In the second embodiment of fig. 3, pilot controlled valve 17 in second conduit 13 and pilot controlled valve 18 in the fourth conduit 19 are controlled based on the pressure value measured by the pressure gauge 21.
In the third embodiment of fig. 4, pilot controlled valve 17 in second conduit 13 and pilot-controlled valve 22 in the first conduit 10 are controlled based on the pressure value measured or sensed by the pressure gauge 21.
As indicated in fig. 2, the individual piston rod side chambers 7 are connected via a plurality of fifth conduits 30 that form part of the second conduit 13 and each of which is provided between two consecutive piston rod side chambers in the row of hydraulic units 2. A piston rod side chamber 7 of a first hydraulic unit is connected via the second conduit 13 to the low pressure reservoir 14.
Between two consecutive hydraulic units 2, there is a sixth conduit 31 for regenerative conduction of pressure fluid from the piston rod side chamber 7 of one of the hydraulic units 2 to the cylinder side chamber 6 thereof. Here, a first end of the sixth conduit 31 is connected to the first conduit 10 and a second end thereof is connected to said fifth conduit 30.
The crane comprises means for controlling a flow of pressure fluid from the piston rod side chamber 7 via the fifth conduit 30 to the piston rod side chamber 7 of the adjacent hydraulic unit 2 or via the sixth conduit 31 to the cylinder side chamber 6 of the hydraulic unit 2 in question. Said means comprises a valve member 32, 33 that is arranged to open for a flow through the sixth conduit 31 when a threshold value of the cylinder side chamber pressure is exceeded. Valve member 32 comprises a pilot-controlled valve 32, here a non-return valve, that upon actuation will permit a regenerative flow towards the cylinder side chamber 6 in question. Actuation of valve 32 may be effected by, for example, the previously mentioned means 25, 26 for measuring and sensing the pressure in the first conduit 10. Valve member 32 is arranged to open when the pressure difference between the cylinder side (conduit 10 or 31) and atmospheric pressure exceeds a pre-determined low value, here 1 MPa. Valve member 32 may be a spring loaded valve, the spring of which is loaded by a leak flow of pressure fluid from the first conduit 10. Once the spring force is exceeded by the pressure fluid force, valve member 32 opens for regenerative flow through six conduit 31.
There is also provided a sensing means 34 that senses the flow direction in the fifth conduit 30, and a pilot-controlled valve member 35 that is arranged to open for a regenerative flow in the sixthconduit based on the sensed flow direction in the fifth conduit 30. Means 34 defines a pressure delimiting member and comprises a restriction or throttle that upon sensing a pressure fall in the direction from the adjacent piston rod side chamber ( which is closer to the low pressure reservoir in the row of piston rod side chamber in series) towards the rod side piston chamber 7 in question, actuates valve member 35 for permission of regenerative flow towards cylinder side chamber 6. However, in order to permit regenerative flow, also valve member 32 must permit regenerative flow towards cylinder side chamber 6.
Apart from the elements that have been described above, the crane comprises a number of elements the function of which are important for a reliable and effective operation of the crane. Accordingly there is a non-return valve 36 provided in parallel with the pressure delimiting member 17 in the second conduit, for allowing a flow of pressure fluid from the reservoir 14 towards the piston rod chambers 7 without throttling caused by the pressure delimiting member 17. A non-return valve 37 is arranged in a corresponding way and for a corresponding purpose, in parallel with the sensing means 34.
It should be understood that non-return valves that normally permit the regenerative flow towards the cylinder side chamber 6 but prevent a flow in the opposite direction, are a natural part of any embodiment of the invention, in order to further improve the operation of the crane.
It should be realised that the above presentation of the invention has been made by way of example, and that alternative embodiments will be obvious for a man skilled in the art without going beyond the scope of protection as claimed in the annexed patent claims supported by the description and the annexed drawings.
In particular, it should be understood that the invention comprises embodiments in which there is a pressure delimiting member in both the first and second conduit in order to permit an alternative in which the pressure is reduced in the first conduit as well asin the second conduit simultaneously.
It should be understood that different types of pilot controlled valves can be used and that combinations of electrically controlled valves and hydraulically controlled valves are all within the scope of the invention, and that, therefore, combinations of the embodiments shown are within the scope of the invention.
The pressure in the reservoir 14 has been regarded as a reference pressure with the value zero (0), and all pressures mentioned in the pressure circuits presented above are given in relation to said reference value. In reality the pressure in the reservoir will most probably be equal to atmospheric pressure. Moreover the circuits described have been regarded as ideal in the meaning that no pressure drops along conduits etc. have been regarded. In reality, there will be such pressure drops.

Claims

A crane, comprising

at least one hydraulic cylinder/piston unit (2) in which a piston rod (5) is displaceable in a cylinder (3) and in which a piston (4) that is connected to the piston rod separates the cylinder chamber in a cylinder side chamber (6) and a piston rod side chamber (7) that is provided with an opening (8) in the cylinder gable (9) in order to permit a displacement of the piston rod (5) out of and into the cylinder,

a first conduit (10) for conducting a pressure fluid between a pressure fluid source (11) and the cylinder side chamber (6),

a second conduit (13) for conducting pressure fluid between the piston rod side chamber (7) and a pressure fluid reservoir (14) that has a lower pressure than the pressure fluid source (11),

a third conduit (15) for regenerative conduction of a pressure fluid from the piston rod side chamber (7) to the cylinder side chamber (6) when pressure fluid is delivered from the pressure fluid source to the cylinder side chamber (6) for the purpose of ejecting the piston rod (5) out of the cylinder, and

means (23) for breaking the regenerative conduction of pressure fluid at a predetermined moment, characterized in that

in at least one of the first and the second conduit (10,13) there is arranged a pressure delimiting member (16; 18;22) for delimiting the pressure in the cylinder side chamber (6) and the piston rod side chamber (7) respectively.
A crane according to claim 1, characterized in that the pressure delimiting member (16) is arranged in the second conduit (13).
A crane according to claim 1 or 2, characterized in that the pressure delimiting member (16) comprises a counter pressure valve.
A crane according to anyone of claims 1-3, characterized in that it comprises a pilot-controlled valve member (17) arranged for preventing passage of pressure fluid from the piston rod side chamber (7) to the pressure fluid reservoir (14) up to a moment, during ejection of the piston rod (5), when a pre-determined pressure value is reached in the cylinder side chamber (6).
A crane according to claim 4, characterized in that it comprises a means (21,25) for sensing the pressure in the cylinder side chamber (6) and for controlling the pilot-controlled valve member (17) based on the sensed pressure.
A crane according to claim 4 or 5, characterised in that the pressure delimiting member (16) is arranged in series with the pilot controlled valve member (17) in the second conduit (13).
A crane according to anyone of claims 4-6, characterized in that the means (23) for breaking the regenerative flow in the third conduit (15) comprises a pilot-controlled valve (23) arranged to permit a regenerative flow in the third conduit (15) up to the moment when the pressure in the cylinder side chamber (6) reaches the predetermined value.
A crane according to anyone of claims 1-7, characterized in that the pressure delimiting member (18,22) is arranged in the first conduit (10).
A crane according to claim 8, characterized in that the pressure delimiting member (22) comprises a counter pressure valve that is arranged to be actuated when a predetermined pressure is reached in the first conduit (10), for the purpose of decreasing the pressure in the cylinder side chamber (6).
A crane according to claim 8 or 9, characterized in that the pressure delimiting member (21) is arranged in a fourth conduit (19) that leads from the first conduit (10) to a pressure fluid reservoir (14) that has a lower pressure than the pressure fluid source (11).
A crane according to claim 10, characterized in that it comprises a means (18) for sensing the pressure in the cylinder side chamber (6) and for controlling a pilot-controlled valve member (20) arranged in the fourth conduit (19) based on the sensed pressure.
A method for controlling a crane comprising,

at least one hydraulic cylinder/piston unit (2) in which a piston rod (5) is displaceable in a cylinder and in which a piston that is connected to the piston rod (5) separates the cylinder chamber in a cylinder side chamber (6) and a piston rod side chamber (7) that is provided with an opening (8) in the cylinder gable (9) in order to permit a displacement of the piston rod (5) out of and into the cylinder (3),

a first conduit (10) for conducting a pressure fluid between a pressure fluid source (11) and the cylinder side chamber (6),

a second conduit (13) for conducting pressure fluid between the piston rod side chamber (7) and a pressure fluid reservoir (14) of lower pressure than the pressure fluid source (11),

a third conduit (15) for regenerative conduction of a pressure fluid between the piston rod side chamber (7) and the cylinder side chamber (6) when pressure fluid is delivered from the pressure fluid source to the cylinder side chamber (6) for the purpose of ejecting the piston rod (5) out of the cylinder, and

means (23) for breaking the regenerative conduction of pressure fluid at a predetermined ejection condition,

wherein, upon a predetermined condition during ejection of the piston rod (5) in connection to delivery of the pressure fluid to the cylinder side chamber (6), the regenerative flow through the third conduit (15) is reduced and the pressure fluid is permitted to flow through the second conduit (13) towards the reservoir, characterized in that, upon the reaching of said condition, the pressure in the piston rod side chamber (7) is delimited to a pressure that is higher than the pressure in the reservoir (14) but lower than the pressure in the cylinder side chamber (6),

or, the pressure in the cylinder side chamber (6) is reduced while the pressure in the piston rod side chamber (7) is reduced generally to the pressure of the reservoir (14).
A method according to claim 12, characterized in that the pressure in the cylinder side chamber (6) is maintained or increased when the pressure in the piston rod side chamber (7) is delimited to a pressure higher than the pressure in the reservoir (14).
A method according to claim 12 or 13, characterized in that a triggering of the delimitation of the pressure in thepiston rod side chamber (7) is performed when a pressure that is higher than the delimitation pressure is attained in the piston rod side chamber (7), and in that, upon said triggering, the pressure in the piston rod side chamber (7) is reduced from a first triggering level to a second delimitation level.
A method according to claim 14, characterized in that a pressure difference is measured between the pressure in the cylinder side chamber (6) and the pressure in the reservoir (14), and that the triggering is based on when said difference attains a threshold value.
A method according to anyone of claims 12-15, characterized in that the pressure in the cylinder side chamber (6) is increased to a pressure corresponding to the pressure in the pressure fluid source (11), while the pressure in the piston rod side chamber (7) is delimited.
A method according to anyone of claims 12-16, characterized in that, if the pressure in the cylinder side chamber (6) is reduced while the pressure in the piston rod side chamber (7) is reduced generally to the pressure in the reservoir (14), the reduction of the pressure in the cylinder side chamber (6) is effected through an application of a counter pressure in the first conduit (10) or through a drainage of fluid from the first conduit (10) to a reservoir (14) that has a pressure that is lower than the pressure in the pressure fluid source (11).