The present invention relates to a method for increasing the maximum lifting power of a hydraulic crane in the high-position range, i.e. when the first boom sec¬ tion is located above the horizontal plane, the position of the first boom section being detected and the load being sensed. The invention also concerns a hydraulic crane with increased maximum lifting power in the high- position range.

As is well-known, hydraulic cranes have lower maxi¬ mum lifting power in the high-position range than in other working range positions. When the first boom sec- tion is erected from a position in the horizontal plane towards a completely upright position, the leverage is in fact shortened between, on the one hand, the point of articulation between the crane body and the first boom section and, on the other hand, the fixing- point in the first boom section for the piston rod of the piston and cylinder unit operating the first boom section. This reduces the moment of force that the piston and cylinder unit is able to produce at a given pressure in the cylin¬ der, which in turn reduces the maximum lifting power of the crane.

A prior-art device for obviating the inconvenience of a shortened leverage consists of a link mechanism. This mechanism has a first link which is mounted between, on the one hand, the point of articulation between the crane body and .the first boom section and, on the other hand, the end of the piston rod in the lifting cylinder, as well as a second link mounted between the end of the piston rod and the first boom section. When the first boom section is erected, the angle between the first and the second link is maintained, such that the leverage is shortened to a lesser extent than would have been the case without a link

mechanism. Consequently, the maximum lifting power of the crane is also reduced to a lesser extent.

However, the link mechanism suffers from the dis¬ advantage of adding to the weight of the crane, which is contrary to the general aim in the art of providing cranes which are as light as possible in relation to their lift¬ ing power so as to impart maximum net loading capacity to e.g. a crane-equipped vehicle.

Another disadvantage of the link mechanism is that it involves the risk of fall through when the length of the leverage changes rapidly, i.e. a risk that the pressure- limiting valve of the hydraulic system opens and the load falls to the ground.

EP 0,349,359 discloses a safety device for restrict- ing the moment of force acting on the body of a hydraulic crane. The moment is given by the pressure in the lifting cylinder of the crane and the leverage between, on the one hand, the point of articulation between the crane body and the first boom section and, on the other hand, the fixing point in the leverage _^ for the piston rod of the piston and cylinder unit operating the first boom section. This device comprises angle sensors for determining the length of the leverage, as well as a computing unit determining the moment acting on the crane body. If the moment becomes too high, the computing unit emits a signal to the hydraulic system to limit the flow from the pump to the lifting cy¬ linder, so that only very small movements can be executed.

If, in this device, the same maximum moment on the crane body is permitted in the whole working range of the crane, i.e. also in the high-position range, there has to be a higher maximum pressure in the lifting cylinder in the high-position range than in the remainder of the working range of the crane. As a result, the crane has to be dimensioned according to the maximum permissible pressure in the high-position range, which in turn means that the crane will become more expensive as well as heavier than

those cranes where there is a constant maximum pressure in the lifting cylinder for the whole working range.

Normally, cranes are dimensioned to withstand slightl more than the load to which they are subjected in operatio at maximum permissible pressure in the lifting cylinder an maximum speed in the normal working range.

To prevent dynamic overloading, the hydraulic system of the crane normally comprises a relief valve which opens at a predetermined pressure, normally slightly above the permissible pressure in the lifting cylinder. Such setting of the valve reduces the maximum lifting power in the high position range, since the leverage is shortened and the valve limits the maximum pressure in the lifting cylinder. If, on the other hand, the valve is set so as to open at a higher pressure, the maximum lifting power in the high- position range is improved, but the crane then has to be dimensioned to withstand the increased load caused by the higher pressure.

SE 396,208 tried to solve this problem by varying the opening pressure of the relief valve as a function of the angle of the first boom section to the horizontal plane, s that a higher pressure is permitted when the angle to the horizontal plane increases. However, this entails the serious consequence that if the valve opens with the first boom section in a high working position, the closing pressure drops all the time as the first boom section is lowered, while the pressure in the cylinder rises owing to an increased outreach. This goes on in an uncontrolled fashion until the load reaches the ground or the crane tip over.

The object of the present invention is to obviate the above disadvantages and provide a crane with increased maximum lifting power in the high-position range and safe load-carrying performance without any problems of stabilit and without any need to dimension the crane for higher loads than warranted by the maximum permissible load in th working range outside the high-position range.

This object is achieved by a method and a crane hav¬ ing the features recited in the appended claims.

By the invention, the maximum lifting power of a hydraulic crane can be increased in the high-position range. This is achieved by disconnecting the relief valve when detecting that the crane is within the high-position range and the load on the crane exceeds a predetermined load. This guarantees safe holding of the load. In order to avoid overload of the crane when the relief valve is disconnected, the maximum permissible speed is reduced, so that the dynamic additional forces become insignificant enough to be absorbed by the flexibility of the crane structure. To enable operation of the crane when the relief valve has been disconnected, the outlet flow from the relief valve may, for instance, be separate from the regular flow for operating the crane, so that it makes no difference whether the outlet flow is shut off. Alterna¬ tively, the relief valve may be arranged to open as soon as hydraulic fluid is supplied to the first boom section for operation thereof.

Advantageously, the load-sensing means is an analog pressure sensor adapted to measure the pressure on the lifting side of the piston and cylinder unit. Also other load-sensing means which continuously measure the load on the crane can be used, such as strain gauges.

The position-detecting means may be a sensor determin¬ ing the angle between the crane body and the first boom section, a sensor sensing the position of the piston rod in the piston and cylinder unit, or any other suitable sensor indicating the position of the first boom section. The position-detecting means might also consist of several position switches.

One embodiment of the invention will be described below with reference to the accompanying drawings, in which Fig. 1 shows a crane equipped with a device according to the invention, and

Fig. 2 is a diagram illustrating the maximum per¬ missible pressure in the lifting cylinder as a function of the angle of the first boom section to the horizontal plane. Fig. 1 shows a crane comprising a body 1, a first boom section 2 articulated thereto, an outer boom 3 articulated to the first boom section 2, and an extension boom 4 fixed to the outer boom 3. The first boom section is operated by means of a hydraulic lifting cylinder 5, the outer boom 3 is operated by means of a hydraulic outer boom cylinder 6, and the extension boom 4 is operated by means of a hydrau¬ lic extension boom cylinder 7.

The cylinders 5-7 are operated by means of hydraulic fluid which, in conventional manner, is supplied from a tank 9 by a pump 10 to a directional-control-valve block 12 which controls the size and the direction of the flow of hydraulic fluid to each cylinder as a function of the posi¬ tion of a lever 13 for operating each cylinder.

The directional-control-valve block comprises a shunt valve 14 pumping excessive hydraulic fluid back to the tank 9, an electrically-controlled dump valve 15 which can be caused to return the entire hydraulic flow from the pump directly to the tank 9, as well as a directional-control- valve unit 16 for each cylinder 5-7. For the sake of clarity, only the directional-control-valve unit 16 for the lifting cylinder 5 is shown in Fig. 1.

The directional-control-valve block 12 is of load- sensing and pressure-compensating type, which means that the size of the hydraulic flow supplied to a cylinder is at all times proportional to the position of the slide member in the corresponding directional-control-valve unit, i.e. proportional to the position of the lever 13.

The directional-control-valve unit 16 comprises a pressure-limiting device 17, a pressure-compensating device 18 and the directional-control-valve 19 proper.

Directional-control-valve blocks as well as direc¬ tional-control-valve units of this type are well-known and available on the market. One instance of such a direc¬ tional-control-valve unit is HIAB 91 marketed by HIAB AB, Hudiksvall, Sweden.

A load-holding valve 20 is arranged between the lift¬ ing cylinder 5 and the directional-control-valve unit 16. The load-holding valve shown is of the type LHV 91 avail¬ able from Olsbergs Hydraulic AB, Eks o, Sweden. On the lifting side, this valve comprises a relief valve 21 having a separate drain conduit 20 to the tank. Thus, the normal flow for operating the lifting cylinder 5 and the flow from the relief valve 21 are separate from each other.

Further, the load-holding valve 21 has a pressure- reducing device 23 so arranged that the pressure on the side of the load-holding valve facing the directional- control-valve block 12 is always constant, regardless of the pressure in the lifting cylinder 5. As a result, the lowering speed is directly proportional to the position of the slide member in the directional control valve.

An inventive crane with increased maximum lifting power in the high-position range comprises position-detect¬ ing means in the form of a sensor 8 which is arranged close to the piston rod of the lifting cylinder 5 and which con- tinuously senses the position thereof. Further, the crane has a load-sensing means in the form of a pressure sensor 25 adapted to measure the pressure on the lifting side of the lifting cylinder 5. The position sensor 8 and the pressure sensor 25 are each connected to an input of a computing-control unit 26, suitably a microprocessor. This contains a memory in which are stored, among other things, the maximum permissible pressure on the lifting side of the lifting cylinder 5 as a function of the position of the first boom section 2, as well as the maximum permissible speed for operating the first boom section as a function of the pressure on the lifting side of the lifting cylinder 5.

Advantageously, this memory is a read-write memory, so that the stored values may easily be changed, if desired.

Finally, the crane has an electrically-controlled valve 24 which is arranged in the drain conduit 22 from the relief valve 21 and which serves to shut off the drain con¬ duit, and consequently disconnect the relief valve 21. The function of the valve 24 is controlled by the microproces¬ sor 26.

The function of the inventive crane will be described below with reference to Figs 1 and 2. In operation, the crane operator controls the first boom section 2 by moving the slide member in the corresponding directional control valve 19 by hand with the aid of a hand lever 13 or by a remote control unit (not shown), thereby generating an electric control signal for positioning the slide member. The pump 10 pumps hydraulic fluid to the directional- control-valve block 12, which in turn feeds the lifting cylinder 5 depending on the position of the slide member in the directional control valve 19. The microprocessor 26 continuously reads the output signals from the pressure sensor 25 and the position sensor 8, and compares the output signal from the pressure sensor 25 with the values of the maximum permissible pressure in the lifting cylinder 5 which are stored in the memory and correspond to the position detected by the position sensor 5.

If the pressure sensed by the pressure sensor 25 is below a first limit value, here set at 25 MPa, i.e. being within the obliquely-hatched area 30 in Fig. 2, operation of all the crane functions at full speed is permitted. This first limit value is constant, regardless of the angle of the first boom section. Dynamic forces arising when a move¬ ment begins or ends are taken up by the relief valve 21, which opens at the predetermined pressure level indicated by the dashed line in Fig. 2, here at about 29 MPa.

If the pressure sensed by the pressure sensor 25 is above the first limit value of 25 MPa and below a second limit value, here 28 MPa, i.e. being within the vertically- hatched area 31 in Fig. 2, the maximum permissible speed for the crane functions is reduced to a predetermined level, e.g. to about 20% of the speed permitted in the area 30. This level may differ for the various crane functions and is stored in the memory. Should the crane operator try to perform a movement at a higher speed than the maximum permissible speed, the microprocessor 26 emits a signal to the dump valve 15, which then dumps all hydraulic fluid to the tank, so that all motions cease. By reducing the maxi¬ mum permissible speed to such an extent, the dynamic forces become small enough to be dampened out by the elastic structure of the crane and the hydraulic system as well as by a very small opening degree of the relief valve 21. As a result, the system still guarantees safe load holding. Also the second limit value of 28 MPa is constant, regardless of the angle of the first boom section to the horizontal plane.

At operating pressures above 28 MPa, the maximum per¬ missible working pressure in the lifting cylinder 5 is a function of the angle of the first boom section, as illus¬ trated in Fig. 2 by the dash-dot line. When the micro- processor 26 senses that the pressure exceeds 28 MPa and the crane is located within the high-position range, it emits a signal to the valve 24, which shuts off the drain conduit 22 of the relief valve 21 to the tank. It should here be pointed out that the relief valve is, of course, only to be closed when the microprocessor has established that the pressure permanently exceeds 28 MPa. This can be done by determining e.g. the mean value or the derivative of the pressure sensed. At the same time as the relief valve is closed, the maximum permissible speed for the crane functions is further reduced, e.g. to 12% of the speed permitted in the area 30, so that the dynamic forces occurring become so small as to be absorbed by the flex-

ibility of the structure. If the pressure sensed by the pressure sensor 25 exceeds the maximum permissible pressure for a given angle of the first boom section, the micro¬ processor 26 emits a signal to the dump valve 25, which dumps the flow to the tank.

When the position-detecting means 8 detects that the first boom section is no longer located in the high-posi¬ tion range and the load has permanently fallen below a predetermined value, the relief valve 21 is again connected by opening the valve 24. At the same time, a higher speed is again permitted. The value at which the relief valve is connected should be lower than the value at which it is disconnected to ensure stable operation. In the example described here, it may be 26.5 MPa. The maximum permissible speed in the areas 31 and 32 is restricted in different ways, depending on whether the crane is operated by the hand lever or by a remote control unit. In the former case, position sensors 40 are con¬ veniently arranged on the slide members in the different directional control valves. If the microprocessor 26 detects that the crane operator performs a slide-member displacement corresponding to an impermissible speed, it emits a signal to the dump valve 15, which dumps the entire hydraulic flow to the tank. In the latter case, the size of the electric signal to the directional control valve is sensed, and is automatically reduced if the microprocessor detects that it corresponds to an imper¬ missible speed.

An alternative to the illustrated load-holding valve with a separate drain conduit from the relief valve to the tank is a load-holding valve controlled by a pilot pressure from the piston side of the cylinder. The load-holding valve then comprises a relief valve which is arranged in the hydraulic conduit between the lifting side of the piston and cylinder unit and the directional control valve and which has no separate drain conduit, as well as a valve serving to disconnect the relief valve. To enable operation

of the lifting crane, the valve is opened as soon as the operator moves the hand lever or the lever on the remote control unit. In actual practice, this can be done by sensing the displacement of the slide member in the directional control valve by means of slide-member-position sensors 40. The signal from these sensors is then used for controlling the opening of the valve.