The present invention relates to a crane, with at least one braked winch from which a hoisting cable runs over cable pulleys, of which at least one is carried by at least one working cylinder attached to the bedframe and adapted to allow alteration in the distance between the bedframe and the pulley.

Cranes which are used at sea, e.g. on board ships or on different types of platforms, are subjected to dynamic stresses resulting from varying weather conditions.- The dynamic stresses are caused by both crane and loading/unloa¬ ding base moving in relation to each other when handling load at sea, resulting in that the hoisting cable is subject to. jerking. Many different systems are known for attenuating such jerking during load handling.

For a crane of the kind mentioned in the introduction, over¬ loading may be prevented by the use of an overload system where overflow valves prevent the hydraulic pressure in the hydraulic system used exceeding a predetermined value. However, the problem in loading/unloading at sea is that overloads, caused by jerks on the cable also occur when the winch brake is on, and then the overflow valves do not help. This type of jerk can occur, for example, when the hook gets caught in another vessel after terminated load. handling.

To avoid heavy overdi ensioning of the crane it is therefore desirable to make the crane in such a way that stresses resul¬ ting from unintentional jerks in the hoisting cable when the winch brake is on are reduced.

The object of the invention is to provide an improved crane where dynamic stresses as a result of jerks on the hoisting cable with the hoisting winch brake on are automatically limited in a simple and reliable way.

This object is achieved in accordance with the invention by the working cylinder being connected to the brake via a braking force regulator, which is arranged to reduce the braking force, when the brake is on and when a jerk excee- _. ding a predetermined magnitude occurs on the hoisting cable, thereby to reduce jerk stresses on the crane. The working cylinder which is utilized to compensate heaving in normal load handling may thus also be utilized to cater for uninten¬ tional jerks in the hoisting cable and to achieve attenuation of such jerks by easing off the winch brake.

A particularly simple embodiment is obtained when the piston rod side of the working cylinder is connected to the braking force regulator and to a first pressure source, the pressure from this source being selected such that it normally .keeps the working cylinder retracted. To advantage, the braking force regulator may here be a pressure limiting valve dis¬ posed such that when the pressure in the working cylinder exceeds a previously determined value due to a jerk on the hoisting cable it passes pressurized medium to a brake cylinder included in the brake, the cylinder suitably being of the spring accumulator type, for reducing the braking forc from the brake cylinder. In this way, only small alterations need to be carried out on an already existing braking system.

The invention will now be described in detail in the followin with the aid of an embodiment illustrated on the accompanying drawing, where: Fig. 1 is a schematic side view of a crane in accordance with the invention.

Fig. 2 schematically illustrates how a hoisting cable is arranged in a crane according to the invention. Fig. 3 illustrates a section of a hydraulic system for a crane, carried out in accordance with the invention, and Fig. 4 illustrates a variation of the hydraulic system in Fig. 3.

As will be seen from Fig. 1 a crane 1 in accordance with the invention is provided with a bedframe 2 and a jib 3 movable in a vertical plane and articulately connected to the bed¬ frame. A hoisting winch 4 is accommodated in the bedframe 2 and associated with the winch there is a cable barrel 5 and a hoisting cable 6. From the barrel 5 the cable 6 runs over a pulley 7 mounted on _. the bedframe 2, whereafter the cable runs backwards and forwards between pulleys 8 arranged on the jib 3 and at least one pulley 9 carried by a working cylinder 10 attached to the bedframe 2. The distance between the pulley 9 mounted on the cylinder 10 and the bedframe 2 may be varied with the aid of the cylinder.

The detailed arrangement of the hoisting cable 6 will be apparent from Fig. 2. A crane of the type described here has its main features already described in our Swedish patent application No. 8301885-3, for which reason the continued description will be limited to what is novel and essential to the invention.

As will be seen from Fig. 3, the working cylinder 10 is connected via a line 11 to a brake 12 acting on the barrel 5 of the hoisting winch. The brake 12 includes a brake cylinder 13, suitably of the spring accumulator type. A braking force regulator is coupled into the line 11 between the cylinder 10 and brake 12. The working cylinder 10 is also connected to a pressurizing source via a line 15, in this case the source is the hydraulic system 16 driving the hoisting winch. In this case a pump 17 provides the necessary hydraulic pressure to a winch motor 18. The pressure in the hydraulic system 16 is limited with the aid of a pressure limiting valve 19.

Normal operation of the brake 12 takes place via a llhe. 20 connected to an operating system not more closely shown here, and which is suitably supplied with pressure from the hydraulic system 16. The brake 12 is on when the line 20 is without pressure and off when the line 20 is pressurized. There is a non-return valve 21 in the line 11, opening towards the brake cylinder 13, and preventing pressure in

the line 20 from actuating the braking force regulator 14. In a similar way, there is a non-return valve 22 in the line 15 opening towards the working cylinder 10, for preventing pressure increase in the cylinder 10 affecting the hydraulic system 16.

The arrangement illustrated in Fig. 3 functions in the following manner: The chamber 23 on the piston rod side of the working cylinder 10 is normally pressurized with the aid of the hydraulic system 16 so that the piston rod 24 is with¬ drawn well into the cylinder 10. This pressure is not sufficient to open the pressure limiting valve included in the braking force regulator 14. The pressure in the working cylinder 10 therefore does not arrive at the brake cylinder 13. The brake 12 is now on by the line 20 being kept without pressure. Should a sudden jerk occur on the hoisting cable 6, the piston rod 24 is urged out of the cylinder 10 with a resulting increase in pressure in the chamber 23. At a pre¬ determined level of the pressure attained, the braking force regulator 14 releases pressurized medium to the brake cylinder 13 so that the brake eases and the barrel 5 can rotate to a certain extent. There is thus obtained a substan¬ tial reduction of the dynamic stress on the crane. As soon as the jerking stress ceases, the chamber 23 is filled with pressurized medium via the line 15. By means of the inven¬ tion, the dynamic stress as a result of jerks in the hoisting cable can be limited to about 50-60 % of the value it would otherwise have had with a completely clamped brake during the jerking sequence. It is essential here that there is a suffi- cient quantity of pressurized medium in the working cylinder 10 so as to actuate the brake cylinder 13 to a sufficient extent.

The embodiment illustrated in Fig. 4 differs from that in Fig. 3 merely in that the working cylinder 10 is supplied from a separate pressurizing source 25 instead of from the hydraulic system 16, this source being in the form of a pressure accumulator, for example. In this way the hydraulic

system 16 does not need to be pressurized to enable reduction of the stresses from jerks occuring on the hoisting cable.

Instead of as illustrated here, the working cylinder 10 can be attached by the piston rod to the bedframe 2, the pulleys 9 will then be mounted on the cylinder housing instead. It is also possible to use two working cylinders side b side, and to connect them in parallel. Other details included in the crane may, of course, be suited to the requirements in ques- tion within the scope of the invention.