WO1998017576A1

WO1998017576A1 - Device relating to a crane

Info

Publication number: WO1998017576A1
Application number: PCT/SE1997/001603
Authority: WO
Inventors: Lars Bruun
Original assignee: Lars Bruun
Priority date: 1996-10-18
Filing date: 1997-09-24
Publication date: 1998-04-30
Also published as: SE9603848L; SE9603848D0; SE507514C2; AU4730197A

Abstract

The present invention relates to a device for a hydraulically operated crane (10) of the type which comprises a crane arm (12), which is rotatable around a horizontal axis of rotation by means of a hydraulic lifting cylinder (24), an arm (20), henceforth termed a telescopic arm, arranged displaceably on the crane arm (12) and at least one hydraulic cylinder (30) fitted between the telescopic arm (20) and the crane arm (12) capable of displacing the telescopic arm mainly in parallel with the crane arm to change the radial range of the crane. The invention is characterised in that the piston in said hydraulic cylinder is disposed to compress an enclosed gas volume when the telescopic arm is pushed out.

Description

DEVICE RELATING TO A CRANE

TECHNICAL FIELD The invention relates to a device for a hydraulically operated crane of the type which comprises a crane arm, which is rotatable around a horizontal axis of rotation by means of a hydraulic lifting cylinder, an arm, henceforth termed a telescopic arm, arranged displaceably on the crane arm and at least one hydraulic cylinder fitted between the telescopic arm and the crane arm capable of displacing the telescopic arm mainly in parallel with the crane arm in order to change the radial range of the crane.

BACKGROUND TO THE INVENTION

Devices of the type stated above are very common in the handling of goods, on lorries, ships, forestry machines, in almost all fields. These cranes are operated hydraulically. From the beginning the hydraulic systems were of a very simple nature in so far as no account was taken of power losses in the system owing to throttling, oil flows and similar. Improvements have been made continuously to the systems, such as pumps with variable displacement, constant pressure systems and the like.

During the 1980s, the systems were developed further with so-called load sensing, i.e. load sensing systems, in which the pump is controlled according to the actual pressure requirement. This means in reality that the maximum pressure is determined by the consumers, i.e. any of all the hydraulic cylinders, hydraulic motors etc which are to be found on the crane. Since it is not realistic to have a pump for each individual consumer, the pressure level is determined by the consumer (function) which requires the highest pressure. This system also signifies considerable efficiency improvements, but the phenomenon still exists that hydraulic oil at high pressure is sent to functions, i.e. hydraulically driven movement means, which do not require this pressure.

During recent years a lot of work has been put into trying to balance the hydraulic cylinders better and get all cylinders to work at maximum system pressure. This means in reality that the maximum force has been determined for the various cylinders and they have been designed for a common maximum pressure level, meaning that maximum pressure is obtained for all functions, in spite of the fact that they do not always require it. To achieve minimum oil consumption, regenerative functions have also been introduced on the luffing arm and telescope. As far as the luffing arm is concerned, this means for example that the lowering movement is executed using only the rod area on the luffing cylinder, and that hydraulic oil is conveyed from the rod side to the piston side by a special valve arrangement. However, the regenerative functions signify a very poor level of hydraulic/mechanical efficiency in that the pressure must be increased substantially as the oil consumption is being reduced to maintain the functions in the system.

As far as the telescopic arm is concerned, its cylinder is fitted so that the piston side is pressurized on the outward movement, i.e. extension, and the rod side is pressurized on the return movement. On the return movement, high pressures and large quantities of oil are required to execute the movement, which hydraulic systems for cranes do not normally manage in conjunction with other functions. This means that it has been necessary to introduce restrictions into the system to the extent that the luffing and telescopic arm cylinders for example cannot be operated simultaneously at full speed. Thus continuous movement is not obtained on the crane.

BACKGROUND TO THE INVENTION

The overall object of the invention is to tackle the above problems. More precisely, the invention aims to increase considerably the efficiency of the integrated hydraulic system by at least being able to reduce substantially the aforementioned energy wastage and thereby also reduce the fuel consumption for driving the hydraulic generator or generators which feed the crane's hydraulic systems on the vehicle or working machine in question. The invention also aims to offer a hydraulic system which can operate with a mean pressure which is considerably below the pressures required with present systems.

These and other objects can be achieved by the fact that the invention is characterised by what is stated in the following claims. Further features and aspects of the invention are evident from the following description of an embodiment in a conceivable application of the invention.

BRIEF DESCRIPTION OF DRAWINGS

In the following detailed description of an embodiment relating to a conceivable application of the invention, reference will be made to the enclosed drawings, in which

Fig. 1 shows a knuckle boom for lifting and handling goods, and

Fig. 2 shows a detailed view of a luffing arm cylinder for moving a luffing arm forming part of the crane according to Fig. 1. DESCRLPTION OF A PREFERRED EMBODIMENT

The drawings show a knuckle boom The general design of the crane is conventional The main components of the crane 10 are constituted by an upright or pillar 12, which is swivellable on a crane base 14, a lifting arm 16 (also referred to in the trade as the main arm), a luffing arm 18, a telescopically extensible arm 20 (also referred to in the trade as a telescopic arm) arranged in the luffing arm 18 via front and rear sliding devices 19, a gripping device 22, a lifting cylinder 24 for raising the lifting arm 16 by rotation around the horizontal axis of rotation of the lifting arm, a luffing cylinder 26 for operating the luffing arm 18 relative to the lifting arm 16 by rotation in a folding hinge 28, and a hydraulic cylinder 30 for the telescopic arm 20, henceforth termed the telescopic cylinder In the crane 10 shown, the telescopic cylinder 30 is arranged so that the cylinder is fitted to the luffing arm 18 and the piston rod is assigned to the telescopic arm 20

The lifting cylinder 24 and the luffing cylinder 26 are connected in a conventional manner to a hydraulic system and will not be described in greater detail According to a conceivable embodiment of the invention, a pressure accumulator 40 is assigned to the telescopic cylinder 30 In the embodiment of the crane shown, the pressure accumulator 40 is connected via lines 42 to the rod side of the telescopic cylinder 30 Thus a hydraulic circuit 44 is arranged which comprises an oil trough 46, a pressure source 48 and a valve 50, which via connections 51 connects only the piston side of the cylinder either to the pressure source 48 or the trough 46 The gas accumulator 40 can be of any conventional type, preferably of a piston type, although a gas accumulator of the membrane type can also be used in principle According to the embodiment, the former type is preferably used

A gas accumulator of this kind contains a piston 52, shown symbolically in the drawing, which divides the gas accumulator cylinder into two compartments, a hydraulic section 54 which is connected to the telescopic cylinder 30 via the line 42 and a pneumatic section 56, which is filled with nitrogen gas under pressure A connection for the initial supply of nitrogen gas is designated 58 This is normally closed, so that the gas volume in the compartment 56 is hermetically enclosed The gas in the gas accumulator is preferably pre-charged at a pressure which is more than 50% of the calculated total pressure which can occur in the accumulator Located in a branch conduit 60 connected to the hydraulic line 42 are two valves connected in parallel to one another, namely a pressure limiting valve 62 on the one hand, normally termed a choke valve in the trade, a name which will be used below, and a check valve 64 on the other hand, which functions as a back flow valve, as will be described below The branch conduit 60 continues after said valves to a servo system, which is shown symbolically by 66 The servo system 66 maintains a certain positive pressure, which is considerably lower than the working pressure of the crane system

The function of the device will be described below The normal movement pattern for a knuckle boom is as follows In relation to the crane's outgoing movement, normally to collect a load, the main arm 16 is lowered, the luffing arm 18 is raised and the telescopic arm 20 is pushed out if necessary, the last-named due to the fact that pressurized oil is introduced via the valve 50 on the piston side of the telescopic arm This causes the oil on the rod side to flow into the pressure accumulator 40, at which the piston 52 in the accumulator moves and means that the gas pressure in the pneumatic section 56 increases

When the crane is then to be retracted with the load, the normal procedure is that the main arm 16 is raised, the luffing arm 18 is lowered and the telescopic arm 20 is drawn in, at which the valve 50 is set so that the piston side communicates with the oil trough 46 The pressure in the gas then acts on the piston in the accumulator and the hydraulic oil, at which the telescopic arm using only the pressure built up in the accumulator 40 draws in the telescopic arm 20

The advantages of the arrangement according to the invention are several On the one hand, the maximum pressure in the hydraulic system which drives the cylinders can be reduced, and the quantity of hydraulic oil which is consumed is reduced On the outward movement, the main arm is lowered, in which case its cylinder does not require any pressure and does not consume any oil The luffing arm is raised, a relatively high pressure being required for this movement which in a load-sensing system will be definitive for the maximum pressure of the system The movement of the luffing arm does not require such a large quantity of oil, which means that the available quantity of oil can be used simultaneously to move the telescopic arm out, at the same time as the pressure is high, which is utilised to charge the accumulator to a pressure which clearly exceeds the maximum system pressure owing to the difference in area between the piston side and the rod side, approx 30-40 percent In contrast to conventional systems, the energy which is used for the movement is stored, i e the energy supplied is stored in the accumulator without any notable losses and thus without heating of the hydraulic oil On the movement inward, the main arm is raised, pressurized oil being supplied to the lifting cylinder. The luffing arm is lowered, which does not require any high pressure, but a relatively large quantity of oil to fill the piston side of the luffing arm cylinder. At the same time, the valve is opened to the piston side of the telescopic arm, whereupon oil is emitted from there. For this movement it is the pressure in the lifting cylinder which determines the maximum system pressure, as the telescopic arm is pushed in only by means of the pressure in the accumulator. In contrast to conventional systems, the pressure and flow requirement is now considerably lower and continuous movement of the crane can be executed due to the fact that movement of the telescopic arm is activated only by the pressure of the accumulator and not the system pressure and flow. A further advantage is that the gas accumulator pressure is greatest initially when the force required for retracting the telescopic arm is at its greatest, as the sliding supports 19 of the arm lie very close to one another when the telescopic arm is extended with a "drawer effect", which makes it difficult to retract the arm. Another advantage is that, in contrast to conventional cranes, a gentle end stop of the telescopic arm 20 is obtained due to the fact that the pressure in the accumulator suffices to retract the arm to the end position without any remaining excess pressure. In conventional cranes, the arm is moved in until it comes to a stop, in part due to the fact that the operator often cannot see the position of the arm, with great stresses on the crane as a result. The fact that the telescopic arm can be retracted gently to the end position by means of the pressure in the accumulator can be utilised to simplify operation of the crane. For example, a function can be integrated into the control facility which activates the valve, so that this is held open continuously until the operator deactivates the valve by an order. Thus the operator does not need to hold the valve open actively with the control, but can concentrate on the other movements of the crane.

With the device according to the invention, the hydraulic system and the components forming part of the system can be designed in two ways. Either the maximum system pressure can be lowered on existing cranes equipped with cylinders designed for conventional systems, or the system pressure is retained and the flow reduced by reducing the area of the pistons. Whichever method is selected offers significant efficiency improvements for the system. From the point of view of the working environment, advantages are obtained such as the fact that the operator can operate the crane in a more relaxed manner. Studies have shown that operators sit and grip the control harder and harder and thus tense themselves when the crane moves awkwardly due to the fact that the hydraulic system for conventional cranes is not adequate. In order to improve efficiency further and reduce the quantities of oil in the system, the luffing arm cylinder 26 can be designed with a bidirectional piston rod 70, Fig 2 This means that the piston rod 70 runs through the entire cylinder and through the end walls 72 where one end of the piston is attached to the linkage of the luffing arm in a conventional manner The other end is not fastened but runs freely The main object of this design is to reduce the volume in the cylinder on the side of the piston which is pressurized to fold the luffing arm in, which means that a smaller quantity of hydraulic oil is used when the luffing arm is folded in The piston rod itself leads to a reduction in piston area, but since not so much power is required to fold the luffing arm in this is sufficient

It is to be understood that the invention is not restricted to the embodiment described above and shown in the drawings, but can be modified within the scope of the following claims

Claims

1. Device relating to a hydraulically operated crane (10) of the type which comprises a crane arm (12), which is rotatable around a horizontal axis of rotation by means of a hydraulic lifting cylinder (24), an arm (20), henceforth termed a telescopic arm, arranged displaceably on the crane arm (12) and at least one hydraulic cylinder (30) fitted between the telescopic arm (20) and the crane arm (12) capable of displacing the telescopic arm mainly in parallel with the crane arm to change the radial range of the crane, characterised in that the piston in said hydraulic cylinder is disposed to compress an enclosed gas volume when the telescopic arm is pushed out.

2. Device according to claim 1, characterised in that the gas volume is enclosed in a separate accumulator (40) which is connected to the hydraulic cylinder (30).

3. Device according to any of the previous claims, characterised in that the crane also comprises a luffing arm (18) attached articulatedly to the crane arm around a horizontal axis of rotation (28), on which the telescopic arm (20) is arranged, and a hydraulic cylinder (26), henceforth termed a luffing arm cylinder, capable of rotating said luffing arm (18), the luffing arm cylinder (26) being designed with a bidirectional piston rod (70), which runs through both end walls (72) of the cylinder.