WO1998033989A1

WO1998033989A1 - Device for hydraulically operated working machine

Info

Publication number: WO1998033989A1
Application number: PCT/SE1998/000072
Authority: WO
Inventors: Lars Bruun
Original assignee: Lars Bruun
Priority date: 1997-01-31
Filing date: 1998-01-19
Publication date: 1998-08-06
Also published as: AU5887798A; SE9700297D0

Abstract

The present invention relates to a device for a hydraulic system, which system comprises a number of hydraulic cylinders (20, 11) and a number of accumulators (27, 40), pre-charged at a certain pressure, which communicate via connections (25, 42) with one side of the hydraulic cylinders, in which the force in one movement direction of the cylinder is obtained mainly from the energy stored in the accumulator. The invention is characterized in that a hydraulic line (26, 84) is connected to said connection between accumulator and hydraulic cylinder, capable of supplying hydraulic oil at a pressure which is equal to or somewhat below the selected pre-charging pressure in the accumulator.

Description

DEVICE FOR HYDRAULICALLY OPERATED WORKIN_G M_ACH_INE

TECHNICAL FIELD

The present invention relates to a device for a hydraulic system, which system comprises a number of hydraulic cylinders and a number of accumulators pre-charged at a certain pressure, which communicate via connections with one side of the hydraulic cylinders, in which the force in one movement direction of the cylinder is obtained mainly from the energy stored in the accumulator.

BACKGROUND TO THE INVENTION

Hydraulically operated and manoeuvred working equipment is very common in a number of fields. One example is hydraulic cranes which are used in the handling of goods, on lorries, vessels, forestry machinery, in almost all spheres of load handling. Another example is digging units on excavators, tractors and similar contract machinery.

Many attempts have been made over the years to improve the efficiency of hydraulic systems in this working equipment by means of so-called load-sensing systems, for example, and balancing of the hydraulic cylinders in the system to get all cylinders operating at maximum system pressure.

The systems developed have improved efficiency somewhat, but no system has actually succeeded in optimizing the systems. Systems have therefore been developed including accumulators for storing energy. According to one device for which a patent has been applied for by the present applicant, an accumulator pre-charged to a certain pressure is connected to an auxiliary cylinder, which auxiliary cylinder is disposed parallel to a lifting cylinder intended to raise a lifting arm or main arm on a crane. When the main arm is lifted, the auxiliary cylinder is filled with oil, which oil, when the arm is lowered, will flow into the accumulator and compress the gas volume in this. The auxiliary cylinder now operates with the pressure in the accumulator actively in the lifting work so that to a large extent it is only useful work which the lifting cylinder performs.

According to the same application, the system with accumulators can be enlarged to include the cylinder for a telescopic arm also. The accumulator is then connected to the drawing-in side of the cylinder, i.e. when the telescopic arm is extended, the pressure on the drawing-in side will increase, at which the pressure m the accumulator increases This pressure is used on retraction The use of the accumulators means that the pressure and flow requirement in the system as a whole is reduced, and the maximum pressure level is reduced Due to the fact that the requirement is reduced, several -unctions can be executed at the same time, which was not possible with previous systems

When using the above patent-pending system, however, some problems arose which could not have been fully foreseen at the design stage

Due to the design with accumulators, a closed hydraulic svstem is obtained which means that certain parts of the svstem are not directly connected to oil pressunzed by the h\ drau c pump or are directly connected to the hydraulic oil tank On the one hand this causes problems with venting of certain parts, for example the rod side of the telescopic cvlinder or the side of the auxiliary cylinder which is connected to the accumulator An air pocket there is very difficult to eliminate, due to the fact that the side is only connected to the tank via an overload valve For the same reason an oil change in these parts is difficult In addition, a shortage of oil mav occur in these parts in certain situations due to the oil leakage which always occurs in principle via overload valves and the like

Another problem is that gas accumulators are thermodvnamicallv influenced I e the pressure changes with the temperature Most working equipment is used in wideh varying temperature conditions, which means that the desired pre-charging pressure mav vary considerably The vanation means that the system cannot be fully utilized One vanant would then be to work with accumulators of such a capacity that thev can compensate for the variations These tend to be so large that thev can only be placed on working equipment with difficulty and so expensive that they seriously curtail the savings they are intended to achieve

BRIEF DESCRff TION OF THE INVENTION

The object of the present invention is to eliminate the aforementioned problems which occur when using accumulators in hydraulic systems According to one aspect of the invention, this is achieved by a device for a hydraulic system which svstem comprises a number of hydraulic cylinders and a number of accumulators, pre-charged at a certain pressure which communicate via connections with one side of the hvdrau c cylinders, in which the force in one movement direction of the cylinder is obtained mainly from the energy stored in the accumulator, characterized in that a hydraulic line is connected to said connection between accumulator and hydraulic cylinder, capable of supplying hydraulic oil at a pressure which is equal to or slightly under the selected pre-charging pressure in the accumulator.

This and other aspects and advantages of the present invention will be evident from the detailed description of a conceivable embodiment and from the following patent claims

BRIEF DESCRIPTION OF DRAWINGS

In the following description of a conceivable embodiment, reference will be made to the enclosed drawings, of which

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

DESCRIPTION OF A PREFERRED EMBODIMENT

The drawings show a knuckle boom 1. The general design of the crane is conventional. The main components of the crane 1 are constituted by a column or pillar 2. which is swivellable on a crane base 3, a lifting arm 4 (also referred to in the industry as the main arm), a luffing arm 5, a telescopically extensible arm 41 (also referred to in the industry as a telescopic arm) arranged in the luffing arm 5 via front and rear sliding devices 43. a gripping device 6, a lifting cylinder 7 for raising the lifting arm 4 by rotation around the horizontal axis of rotation of the lifting arm. a luffing cylinder 8 for operating the luffing arm 5 relative to the lifting arm 4 by rotation in a folding hinge 10, and a hydraulic cylinder 11 for the telescopic arm 41 , henceforth termed telescopic cylinder. In the crane 1 shown, the telescopic cylinder 11 is arranged so that the cylinder is fitted to the luffing arm 5 and the piston rod is assigned to the telescopic arm 41.

According to the invention an auxiliary cylinder 20 is also arranged between the column 2 and the lifting arm 4. In the case shown, the piston rod 21 is fastened via a first, lower, horizontal auxiliary cylinder fixing shaft 22 in the bearing device 13 on the column, while the auxiliary cylinder 20 is fastened via a second, upper horizontal auxiliary cylinder fixing shaft 23 in a bearing device 24 on the lifting arm. According to the embodiment shown, the fixing shafts 22 and 23 of the auxiliary cylinder lie at a distance from the two fastening shafts 12 and 14 of the lifting cylinder 7, more precisely defined further away from the column 2 and from the axis of rotation 9 of the lifting arm 4. However, it is also possible and can be more advantageous inter alia for practical reasons to place the auxiliary cylinder 20 by the side of the lifting cylinder 7 and mounted on the same fastening shafts 12 and 14 as the lifting cylinder. It is also possible and also convenient to set up two auxiliary cylinders 20, especially since the fastening shafts of the auxiliary cylinder and the lifting cylinder are common, in which case an auxiliary cylinder can be disposed on each side of the lifting cylinder 7.

According to a conceivable embodiment of the invention, the auxiliary cylinder 20 can be completely pneumatic and contain a closed gas volume, preferably nitrogen. The auxiliary cylinder 20 consists in this case of a gas spring.

According to another conceivable and more advantageous embodiment, the auxiliary cylinder 20 consists instead of a hydraulic cylinder. A hydraulic line 26 connected to the auxiliary cylinder is arranged with a branch line 25, which in this case is connected to a gas accumulator 27, which can be of any conventional type, preferably of the piston type, although a gas accumulator of the diaphragm 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, shown symbolically by 28, which divides the gas accumulator cylinder into two sections, a hydraulic part 29 which is connected to the auxiliary cylinder via the lines 25, 26 and a pneumatic part 30. which is filled with nitrogen gas under pressure. A connection for the initial supply of nitrogen gas is designated 31. This is normally closed, so that the gas volume in the section 30 is hermetically enclosed. Disposed in a branch line 33 connected to the hydraulic line 25 is a pressure limiting valve 34, normally termed a choke valve in the industry, a term which will be used below, the function of which valve will be described below. The choke valve can also be a sequence valve.

The branch line 33 continues after said valve to a servo system, which is shown symbolically by 36. The servo system maintains a defined over-pressure, e.g. 16 bar, which is considerably lower than the operating pressure of the crane system. The line 26 communicates with the hydraulic pump 18 of the device via a non-return valve 82 and a pressure limiting valve 80. The function of these components will be described below. According to the embodiment, the lifting cylinder consists of a double-acting cylinder with a lower port A and an upper port B. In the drawing, a hydraulic circuit for the lifting cylinder is shown in simplified form, comprising an oil tray 38, a multiple-way valve 39 and hydraulic lines 45, 47 from the valve to the ports on the lifting cylinder, which valve 39 is capable of connecting the hydraulic lines either to the pressure source 18 or the oil tray 38 The hydraulic system comprises a pressure sensor and signal generator 48 disposed between the lines 45 and 47.

Nitrogen gas is initially supplied and enclosed in the gas accumulator 27, this being executed in connection with assembly on the manufacturer's premises The accumulator is thus pre-charged at a certain pressure Following this hydraulic oil is supplied to the hydraulic system

The device thus equipped operates in the following manner. It is assumed that the lifting arm 4 is located in its lowest position, that the hydraulic system is filled with oil and that the crane system is to be balanced by exploiting the options which the invention offers In this connection the lifting arm is raised by means of the lifting cylinder 7 due to oil being routed via the valve 39 into the main piston via Port A The auxiliary cylinder 20 and the accumulator 27 are provided with pressurized oil via the pressure limiting valve 80 and non-return valve 82 The pressure limiting valve 80 is set so that the pressure is equal to or slightly below the selected pre-charging pressure in the accumulator 27 Raising of the lifting arm 4 can continue up to the maximum position The lifting arm 4 is then let back down, the hydraulic oil from the lifting cylinder being drained off to the oil tray 38 The oil pressure in the auxiliary cylinder 20 will then increase, which means that the gas volume in the gas accumulator 27 is compressed until a balancing position is attained If the oil pressure increases above a defined maximum pressure set at the choke valve 34, this will open and let out oil until the balancing position is attained An increase in pressure of this kind can take place for example due to unfolding of the luffing arm 5, or a temperature increase during operation The line 26 with the pressure limiting valve 80 sees to it that the circuit with the auxiliary cylinder/accumulator is always provided with oil pressurized to at least the oil pressure set in the valve, at the same time as the non-return valve 82 ensures that oil is prevented from being pressed out in this direction during charging up of the accumulator The system is now prepared and when the crane 1 is used for lifting work, the auxiliary cylinder 20 is actively exploited to relieve the pressure on the lifting cylinder

7

The work performed by the lifting cylinder 7 basically consists of useful work, i.e. for the purpose of handling the load which is located in the gripper 6. It must be realized that the centre of gravity of the crane system varies depending on the position and angular relationship of the luffing arm 5 with the lifting arm 4, which means that in unfavourable moment conditions, i.e. when the gripper 6 is located far out from the axis of rotation 9, the lifting cylinder 7 is utilized to a certain degree also to be able to raise the integrated crane arm's own weight The work which the lifting cylinder 7 has to perform, with or without an actual load in the gripper 6, is however very considerably less than in conventional systems without the auxiliary cylinder 20 according to the invention

The lifting cylinder 7 and auxiliary cylinder 20 are suitably dimensioned in such a way that the work required for the lifting arm's upward movements can be performed basically by means of the energy which is stored in the form of compressed gas in the gas accumulator 27, due to which the lifting cylinder 7 and pressure source 18 can be dimensioned in the first instance to execute the desired useful work, i.e to lift and handle the load in the gripper 6

According to a conceivable embodiment of the invention, the crane can also be equipped with a pressure accumulator 40 assigned to the telescopic cylinder 11 In the embodiment of the crane shown, the pressure accumulator 40 is connected via lines 42 to the rod side of the telescopic cylinder 11 Assigned to the piston side of the telescopic cylinder in a conventional manner is a hydraulic circuit 44, which comprises an oil tray 38, pressure source 18 and a valve 50, which connects only the piston side of the cylinder with either the pressure source 18 or the tray 38 via connections 51

The gas accumulator can be of any conventional type, preferably of the same type as described above, which comprises a piston 52, a hydraulic part 54 which is connected to the telescopic cylinder 11 via the line 42 and a pneumatic part 56. which is filled with nitrogen gas under pressure, i.e. pre-charged to a certain pressure In a branch line 60 connected to the hydraulic line 42 is a pressure limiting valve 62, normally termed a choke valve in the industry. The branch line 60 continues after said valves to the servo system 36. Connected to the line 42 is a branch line 84, which line provides the rod side and the accumulator with oil pressurized via a pressure limiting valve 86 and a non-return valve 88 to a pressure equal to or just under the pre-charging pressure of the accumulator.

The normal pattern of movement for a knuckle boom is as follows. On the crane's outgoing movement, normally to fetch the load, the main arm 4 is lowered, the luffing arm 5 is raised and the telescopic arm 41 is extended if required, the last-named due to the fact that pressurized oil is introduced via the valve 50 to the piston side of the telescopic arm. This means that the pressure in the oil on the rod side and thus in the pressure accumulator 40 increases, at which the piston 52 in the accumulator is moved and means that the gas pressure in the pneumatic part 56 increases. If the pressure should increase above a maximum pressure set in the choke valve 62, this will trigger and the pressure will be reduced.

When the crane is then to be retracted with the load, the normal procedure is that the main arm 4 is raised, the luffing arm 5 lowered and the telescopic arm 41 retracted, at which the valve 50 is set such that the piston side communicates with the oil tray 46. The pressure in the gas then acts on the piston in the accumulator and the hydraulic oil, whereupon the telescopic arm draws in the telescopic arm 41 using chiefly the pressure built up in the accumulator 40.

The connection of the rod side and the accumulator to a pressure source 18 via the line 84 ensures that a minimum pressure and oil is always present in the circuit. If this connection should not exist, there is a risk that an oil shortage will occur on the rod side if the choke valve releases for different reasons and lets oil through.

With the connections of pressurized oil to the cylinder sides connected to the accumulators, several advantages are gained. Venting can take place of these parts in the system, which were not otherwise in direct communication with the pressure source. Pressure changes in the accumulators due to temperature changes which affect the pre-charging pressure can be compensated for. With the device according to the invention, the system is pressurized at a set minimum pressure. Changing the oil of the aforesaid parts in the system can take place, which could only have been carried out with difficulty without the connections. Compensating for a reduction in the amount of oil in these parts caused by the triggering of choke valves, leakage and the like is likewise possible using the device.

The system described above is expediently also provided with an electric valve 90 connected between the non-return valve and the hydraulic cylinder/accumulator to enable the pressure on the system to be relieved if so required. In the drawing, only a valve 90 for the auxiliary cylinder for the main arm is drawn in. It is to be understood that a valve of this kind is also intended to be connected to the circuit for the telescopic cylinder.

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

Claims

1. A device for a hydraulic system, which system comprises a number of hydraulic cylinders (20, 11) and a number of accumulators (27, 40) pre-charged at a certain pressure, which communicate via connections (25, 42) with one side of the hydraulic cylinders, in which the force in one movement direction of the cylinder is obtained mainly from the energy stored in the accumulator, characterized in that a hydraulic line (26, 84) is connected to said connection between accumulator and hydraulic cylinder, capable of supplying hydraulic oil at a pressure which is equal to or somewhat below the selected pre-charging pressure in the accumulator.

2. A device according to claim 1, characterized in that it includes a maximum pressure limiting valve (34, 62) assigned to said connection (25, 42), which valve communicates with a hydraulic oil tank (38) in that the valve opens when the pressure in the connection between the cylinder and accumulator exceeds a certain set pressure.

3. A device according to claim 1 or 2, characterized in that the hydraulic line to said connection includes a pressure reducing valve (80, 86) for setting the pressure of the hydraulic oil supplied so that this is equal to or somewhat below the selected pre- charging pressure of the accumulator and a non-return valve (82, 88) which prevents the pressure increase which arises in the accumulator during charging up from being lost.