TECHNICAL FIELD

The invention relates to an arrangement for a hydraulically operated crane ofthe type which comprises a crane jib, pivotable about a horizontal pivot axle with the aid of a hydraulic lifting cylinder arranged between a first horizontal lifting cylinder attachment axle, whose position is fixed relative to the pivot axle ofthe crane jib, and a second horizontal lifting cylinder attachment axle which is arranged on the crane jib at a distance from the pivot axle ofthe crane jib.

BACKGROUND OF THE INVENTION

Arrangements ofthe type specified above are very commonly used for handling goods, on trucks, ships, forestry machinery, in almost all areas These cranes are hydraulically operated. During the 1960s, the cranes had only so-called open hydraulic circular pumping systems, i.e pumps with fixed displacement were used and, when necessary, the oil flow was choked using a valve in order to achieve the desired pressure in the various functions. These systems were very inefficient and were the cause of considerable power losses and control problems. During the 1970s, the so-called constant pressure system was introduced. This system involves using a pump with variable displacement, the volume flow from the pump being dependent on the consumers. This system too entails considerable energy losses, even if it did represent a great improvement compared to the open circular pumping systems. A more recent introduction was the so-called unloaded constant pressure system, i.e. only when a certain consumer on the machine required oil was a signal given to the pump to increase pressure and flow. This construction also afforded improvements in efficiency.

During the 1980s, the systems were developed further with so-called load sensing, where the pump is controlled in accordance with the actual pressure requirement. This means in fact that the highest pressure is determined by the consumers, i.e. by one of all the hydraulic cylinders, hydraulic motors etc., which are present on the crane, require. As it is not practical to have one pump for each individual consumer, the pressure level is determined by the consumer (function) which requires the highest pressure. This system too afforded important improvements in efficiency, but there is still the phenomenon of hydraulic oil at high pressure

being sent to functions, i.e. hydraulically driven movement members, which do not require this pressure.

In recent years a great deal of work has been done on trying to better balance the hydraulic cylinders and to get all the cylinders operating at maximum system pressure. This in fact means that maximum forces have been determined for the various cylinders and these have then been dimensioned to a common highest pressure level, the result of which is that there is maximum pressure on all functions, despite the fact that these functions do not always require this. To achieve as low an oil consumption as possible, regenerative functions have also been introduced on luffing jib and telescopic jib. This means, for example, as regards the luffing jib, that the downward movement is executed only with the rod cross-section on the luffing cylinder, and that hydraulic oil is led from rod side to piston side by means of a special valve arrangement. However, the regenerative functions mean there is very poor hydraulic/ mechanical efficiency, since the pressure has to be increased substantially when the oil consumption decreases in order to maintain the system functions.

As regards cranes on forest harvesters, i.e. cranes having a harvesting unit at the boom point, these operate with a very high stationary load, but with a very low outer load, hereinafter also referred to as useful load. For example, a commonly used harvester crane can have a reach of 10 m and with a fixed unit weight at the boom end of about 1000 kg, while the useful load which the unit lifts can in many cases be less than 50 kg. A common operating pressure for such a crane is 240 bar and a lowest pressure of 170 bar. This means that just over 70% of the crane's gross lifting capacity is needed to lift the crane jib system without useful load, and that the lowest pressure another function has to operate with is also 170 bar, despite the fact that several functions do not require pressure in excess of 50 bar. Since present-day hydraulic systems do not take account of this, the power requirement of a harvesting crane is often over 100 horse power, which is far in excess ofthe optimal requirement. Moreover, the hydraulic system ofthe harvesting unit has to all intents and purposes never been separated from that of the crane system, which means that the harvesting unit is often provided with pressure reducers for the purpose of reducing the permitted pressure in the unit. Since a pressure reducer functions as a energy dissipator, this means that energy is wasted and the result is poor efficiency.

As regards the telescopic jib, its cylinder is arranged so that the piston side is pressurized upon the outward movement, i.e. the extension, and the rod side is pressurized upon the return movement. The return movement requires high pressures and large amounts of oil for its execution, and the hydraulic systems on the cranes are not normally able to manage this in combination with other functions. This means it has been necessary to introduce limits in the system, to the extent that, for example, the cylinders ofthe luffing jib and telescopic jib cannot be driven simultaneously at full speed. Thus, the crane movement obtained is not continuous.

DISCLOSURE OF THE INVENTION The main object ofthe invention is to tackle the abovementioned problems. More specifically, the invention has the object of substantially increasing the efficiency ofthe integrated hydraulic system by being able to reduce, at least substantially, the abovementioned dissipation of energy, and thereby also reduce the fuel consumption for driving the hydraulic generator or generators which feed the crane's hydraulic system on the vehicle or operating machinery in question. A further object ofthe invention is to make available a hydraulic system which is able to operate with a mean pressure which is substantially less than the pressures required on present-day systems.

These and other objects can be achieved by virtue ofthe fact that the invention is characterized by the features specified in the attached patent claims. Further characteristics and aspects ofthe invention will become evident from the following description of one embodiment in one conceivable application ofthe invention.

BRIEF DESCRIPTION OF THE FIGURES In the following detailed description of one embodiment in one conceivable application ofthe invention, reference will be made to the figures in the attached drawings, of which: Figure 1 shows a knuckle jib crane for lifting and handling goods, and Figure 2 shows a detail of a luffing jib cylinder for moving a luffing jib forming part ofthe crane according to Figure 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

The figure in the drawing shows a knuckle jib crane 1. The general design structure ofthe crane is conventional. The main parts ofthe crane 1 consist of a column or pillar 2 which can swivel on a crane base 3, a lifting jib 4 (also called the main jib in the industry), a luffing jib 5, a jib 41 (called the telescopic jib in the industry) which can be extended telescopically in the luffing jib 5 via front and rear sliding members 43, a grab device 6, a lifting cylinder 7 for lifting the lifting jib 4 by means of pivoting about the horizontal pivot axle ofthe lifting jib, a luffing cylinder 8 for manoeuvring the luffing jib 5 relative to the lifting jib 4 by means of pivoting on a knuckle hinge 10, and a hydraulic cylinder 11 for the telescopic jib 41, hereinafter referred to as the telescopic cylinder. In the crane 1 shown, the telescopic cylinder 11 is arranged so that the cylinder is mounted on the luffing jib 5 and the piston rod is arranged on the telescopic jib 41.

According to the invention, an auxiliary cylinder 20 is also arranged between the column 2 and the lifting jib 4. In the case which is shown, the piston rod 21 is secured, via a first, lower, horizontal auxiliary cylinder attachment axle 22, in the bearing arrangement 13 on the column, while the auxiliary cylinder 20 is secured, via a second, upper, horizontal auxiliary cylinder attachment axle 23, in a bearing arrangement 24 on the lifting jib. According to the embodiment shown, the attachment axles 22 and 23 ofthe auxiliary cylinder lie at a distance from the two attachment axles 12 and 14 ofthe lifting cylinder 7, more specifically further away from the column 2 and from the pivot axle 9 ofthe lifting jib 4. However, it is also possible, and it may be more advantageous, for practical reasons among others, to place the auxiliary cylinder 20 alongside the lifting cylinder 7 and mounted on the same attachment axles 12 and 14 as the lifting cylinder. It is also possible, and even expedient, to provide two auxiliary cylinders 20, especially when the attachment axles ofthe auxiliary cylinder and ofthe lifting cylinder are common, in which case one auxiliary cylinder can be arranged on either side ofthe lifting cylinder 7.

According to one possible embodiment ofthe invention, the auxiliary cylinder 20 can be fully pneumatic and contain an enclosed gas volume, preferably nitrogen. The auxiliary cylinder 20 in this case consists of a pneumatic spring.

According to another possible and more advantageous embodiment, the auxiliary cylinder 20 is instead a hydraulic cylinder. A hydraulic line 26 is in this case connected to a gas accumulator 27, which can be of any conventional type, preferably ofthe piston type, although a gas accumulator ofthe membrane type can also be used in principle. According to this embodiment, the former type is preferably used. Such a gas accumulator contains a piston, designated symbolically by 28, which divides the gas accumulator cylinder into two compartments, a hydraulic part 29, which communicates with the auxiliary cylinder via the line 26, and a pneumatic part 30 which is filled with nitrogen gas under pressure. A connection for initial supply of nitrogen gas is designated 31. This connection is normally closed so that the gas volume in the compartment 30 is hermetically sealed in. In a branch line 33 connected to the hydraulic line 26 there are two valves connected in parallel, namely, on the one hand, a pressure-limiting valve 34, generally called a surge damping valve in the industry, a term which will be used hereinafter, and, on the other hand, a check valve 35 which functions as a backflow valve, as will be described hereinbelow. The branch line 33 continues, downstream ofthe said valves, to a servo system which is designated symbolically by 36. The servo system 36 holds a certain overpressure, for example 16 bar, which is considerably lower than the operating pressure ofthe crane system.

According to the embodiment, the lifting cylinder consists of a double-acting cylinder with a lower port A and an upper port B. The drawing shows, in simplified form, a hydraulic circuit for the lifting cylinder, comprising an oil trough, a multi-directional valve and hydraulic lines 40, 41 from the valve to the ports on the lifting cylinder, which valve is able to connect the hydrauhc lines to either the pressure source or the oil trough. The hydraulic system comprises a pressure sensor and signal transmitter 43 arranged between the lines 40 and 41.

Nitrogen gas is initially fed into and enclosed in the gas accumulator 27, and this is done upon assembly by the manufacturer. Hydraulic oil has then been fed to the hydraulic system.

The arrangement thus equipped operates in the following way. It is assumed that the lifting jib 4 is situated in its lowermost position, that the hydraulic system is filled with oil, and that the crane system is to be balanced by utilizing the possibilities afforded by the invention. The lifting jib is in this case lifted with the aid ofthe lifting cylinder 7, hydraulic oil being sucked

into the auxiliary cylinder 20 from the servo system 36 via the check valve 35 and the line 26. The ascending movement ofthe lifting jib 4 can continue up to the maximum position. Thereafter, the lifting jib 4 is let down again, the hydraulic oil from the lifting cylinder being diverted to the oil trough 38, and oil from the auxiliary cylinder 20 flowing into the gas accumulator 27 and compressing the gas volume until a balance position is reached. Any excess which has been sucked into the system is then forced out since the torque acting on the auxiliary cylinder is increased, for example by means ofthe luffing jib 5 being folded outwards, so that the surge damping valve 34 opens and lets out oil until its balance position has been reached.

The system is now ready, and when the crane 1 is used for lifting work, the auxiliary cylinder 20 is actively employed for unloading the lifting cylinder 7.

In normal operating circumstances, the crane jib 1 is raised on a command from the driver, whereupon hydraulic oil under pressure is forced into the lifting cylinder 7 through the lower port A from the hydraulic pump 18 via the valve 39 and the line 47 (according to the underlying principle ^' of the invention, the auxiliary cylinder 20 also participates in this movement) at the same time as hydraulic oil is discharged from the chamber ofthe hydraulic cylinder 7 via the upper port B, the line 45, the valve 39 and the drainage line to the oil trough 38. When the crane jib 1 is instead to be lowered, the opposite procedure is followed. The operator gives a command that the crane jib is to be lowered, whereupon the valve 39 is changed to function as an overflow valve which conveys oil from the lower chamber through the port A to the upper chamber through the port B and to the oil trough 38, at the same time as the gas accumulator 27 is charged when the auxiliary cylinder 20 is compressed under the action ofthe crane jib's weight and possible load.

Between the lines 45 and 47 there is a pressure sensor and signal transmitter 43 arranged to emit an impulse to the multi-directional valve 39 if the recorded differential pressure has dropped to a certain set value. This function is used, if so required, to lower the crane jib 1 when the latter has been more or less brought together. If this situation arises, i.e. the luffing jib is driven in so that the total length ofthe crane jib 1 substantially decreases, the downwardly directed force ofthe crane jib on the two cylinders 7 and 20 also decreases. If

the measured differential pressure between the lines 45 and 47 has dropped to the set value, the unit 43 sends an impulse to the multi-directional valve 39, which opens a communication between the hydraulic pump 18 and the line 41 if the operator gives a command that the crane jib 1 is to be lowered. At the same time the valve 39 opens a communication between the line 47 and the oil trough 38, so that the cylinder 7 with hydraulic force forces the crane jib 1 down, at the same time as the gas accumulator 27, according to the main principle ofthe invention, is charged as described above. It should be pointed out that the differential pressure set, at which the unit 43 reacts, is low.

The work which the lifting cylinder 7 performs consists essentially of useful work, i.e. for handling the load which is in the grab 6. It will be appreciated that the centre of gravity ofthe crane system varies depending on the position ofthe luffing jib 5 and the angular relationship to the lifting jib 4, which means that under unfavourable torque conditions, i.e. when the grab 6 is located far out from the pivot axle 9, the lifting cylinder 7 is to some extent also used for being able to increase the dead weight ofthe integrated crane jib. Nevertheless, the work which the lifting cylinder 7 has to perform, with or without a useful load in the grab 6, is very much less than in conventional systems without the auxiliary cylinder 20 according to the invention.

If the pressure in the hydraulic line 26 were to exceed a certain limit value, preset in the surge damping valve 34, which situation can occur on account of heating ofthe hydraulic oil, the valve 34 opens, with the result that a certain amount of oil flows across to the servo system 36. The hydrauhc system ofthe auxiliary cylinder can then be refilled by sucking hydraulic medium in from the servo system 36 in accordance with the same principles which have been described in conjunction with the above description ofthe balancing ofthe system.

The lifting cylinder 7 and the auxiliary cylinder 20 are expediently dimensioned such that the work which is needed for the ascending movements ofthe lifting jib can be performed substantially with the aid ofthe energy which is stored in the form of compressed gas in the gas accumulator 27, as a result of which the lifting cylinder 7 and the pressure source 18 can be dimensioned primarily to perform the desired useful work, i.e. to lift and handle the load in the grab 6.

According to one possible embodiment ofthe invention, the crane can also be equipped with a pressure accumulator 40 arranged on the telescopic cylinder 11. In the illustrated embodiment ofthe crane, the pressure accumulator 40 is connected via lines 42 to the rod side ofthe telescopic cylinder 11. Thus, a hydraulic circuit 44 which comprises an oil trough 38, pressure source 48 and a valve 50 is provided which, via connections 51, connects only the piston side ofthe cylinder either with the pressure source 48 or the trough 38. The gas accumulator 40 can be of any conventional type, preferably ofthe piston type, although a gas accumulator ofthe membrane type can also be used in principle. According to the embodiment, the former type is preferably used.

Such a gas accumulator contains a piston 52, shown symbolically in the figure, which divides the gas accumulator cylinder into two compartments, a hydraulic part 54, which communicates with the telescopic cylinder 11 via the line 42, and a pneumatic part 56 which is filled with nitrogen gas under pressure. A connection for initial supply of nitrogen gas is designated 58. This connection is normally closed so that the gas volume in the compartment 56 is hermetically sealed in. The gas in the gas accumulator is preferably pre-charged at a pressure which is more than 50% ofthe calculated total pressure which can occur in the accumulator. In a branch line 60 connected to the hydraulic line 42 there are two valves connected in parallel, namely, on the one hand, a pressure-limiting valve 62, generally called a surge damping valve in the industry, a term which will be used hereinafter, and, on the other hand, a check valve 64 which functions as a backflow valve, as will be described hereinbelow. The branch line 60 continues, downstream ofthe said valves, to the servo system.

The normal pattern of movement of a knuckle jib crane is as follows. On the outward movement ofthe crane, normally for picking up a load, the main jib 4 is lowered, the luffing ib 5 is raised, and the telescopic jib 41 is extended, when so required, the latter by means of pressurized oil being introduced via the valve 50 to the piston side ofthe telescopic cylinder. This means that the oil on the rod side flows into the pressure accumulator 40, whereupon the piston 52 in the accumulator is displaced and causes the gas pressure in the pneumatic part 56 to increase.

When the crane is then to be moved back with the load, the normal procedure is that the main jib 4 is raised, the luffing jib 5 is lowered, and the telescopic jib 41 is drawn in, whereupon the valve 50 is set such that the piston side communicates with the oil trough 38. The pressure of the gas then acts on the piston in the accumulator and the hydraulic oil, whereupon the tele- scopic cylinder draws in the telescopic jib 41 with only the pressure built up in the accumulator 40.

The advantages ofthe arrangement described above are many. For instance, the maximum pressure in the hydraulic system which drives the cylinders can be reduced, and the amount of hydraulic oil needed is lowered. Upon the outward movement, the main jib is lowered, for which reason its cylinder does not require any pressure and does not use up any oil. The luffing jib is raised, and a relatively high pressure is required for this movement, which in a load-sensing system will determine the maximum pressure ofthe system. The luffing jib movement does not require such a large amount of oil, which means that the available amount of oil can at the same time be used to extend the telescopic jib, at the same time as the pressure is high, which pressure is utilized for charging the accumulator to a pressure which clearly exceeds the maximum pressure ofthe system pressure on account ofthe difference in cross-section between piston side and rod side, approximately 30 - 40 percent. In contrast to conventional systems, the energy which is used for the movement, i.e. supplied energy, is stored in the accumulator without appreciable losses and thus without heating ofthe hydraulic oil.

Upon the inward movement, the main jib is raised, whereupon pressurized oil is supplied to the lifting cylinder. The luffing jib is lowered, which does not require any high pressure, but a relatively large amount of oil in order to fill the piston side ofthe luffing jib cylinder. At the same time, the valve opens to the piston side ofthe telescopic cylinder, whereupon oil is discharged therefrom. For this movement, it is the pressure in the lifting cylinder which defines the maximum pressure ofthe system when the telescopic jib is pushed in only with the aid ofthe pressure in the accumulator. In contrast to conventional systems, the pressure and flow requirement is now considerably less and a continuous movement ofthe crane can be obtained by virtue ofthe fact that the movement ofthe telescopic jib is only activated by the accumulator pressure and not by the system pressure and flow. A further advantage is that the

gas accumulator pressure is greatest at the start when the power requirement for drawing in the telescopic jib is at its greatest, since, with the telescopic jib extended, the sliding supports 43 of the jib lie very close to each other with a "drawer effect" which makes it difficult to draw the jib in Another advantage is that in contrast to conventional cranes, a gentle end stop ofthe telescopic jib 41 is obtained by virtue ofthe fact that the pressure in the accumulator is sufficient to move the jib in to the end position without any remaining excess pressure. In conventional cranes, the jib is driven in until it comes to a stop, partly because the operator often cannot see the position of the jib, with considerable stresses on the crane as a consequence The fact that the telescopic jib can be driven gently to the end position with the aid ofthe pressure in the accumulator can be exploited in order to simplify the operation of the crane For example, it is possible to build into the controls a function which activates the valve so that this is constantly held open until the operator orders deactivation ofthe valve The operator at the controls does not therefore need to actively hold the valve open, and instead can concentrate on the other movements ofthe crane

With the arrangement according to the invention, the hydraulic system and the components included in the system can be designed in two ways Either the maximum system pressure can be lowered on existing cranes provided with cylinders dimensioned for conventional systems, or the system pressure can be retained and the flow reduced by decreasing the cross-section ofthe pistons Whichever option is chosen, significant improvements in the efficiency ofthe system will be obtained As regards the working environment, advantages are that the operator can operate the crane in a more relaxed manner Studies have shown that operators are sitting and gripping the controls tighter and tighter and are therefore straining themselves when the crane moves heavily because the hydraulic system for conventional cranes is inadequate

In order to further improve the efficiency and reduce the amounts of oil in the system, the luffing jib cylinder 26 can be designed with a two-way piston rod 70, Figure 2 This means that the piston rod 70 runs through the whole cylinder and through the end walls 72 where one end ofthe piston is secured to the linkage ofthe luffing jib in a conventional manner. The other end is not secured, and instead runs freely. The primary purpose of this design is to reduce the volume in the cylinder on that side ofthe piston which is pressurized in order to

fold the luffing jib in, which means that the amount of hydraulic oil used is less when the luffing jib is folded in. The piston rod per se leads to a reduction in the piston cross-section, but since such great force is not required to fold the luffing jib in, this is sufficient.

It will be appreciated that the invention is not limited to the embodiment described above and shown in the figures in the drawings, and that it can be modified within the scope ofthe following patent claims.