A DEVICE FOR DAMPING UNDESIRABLE OSCILLATIONS IN A CRANE MEMBER

Abstract: The present invention relates to a device for damping undesirable oscillations of a crane member (1) which is pivotally movable by means of at least one hydraulic motor (5, 5'). The hydraulic circuit (8) has a restriction (13) co-operating with a circuit-closing device (14; 18) the purpose of the restriction being to convert energy inherent in the system to heat in the hydraulic oil. The device includes means (14, 15, 16, 16') for controlling the closing device (14; 18) according to instantaneous conditions and/or changes or conditions, such as pressure and/or flow, in the hydraulic circuit (8). The energy-converting action of the restriction (13) is instantaneously activated as soon as a certain condition and/or a certain change of condition occurs. The conduit (12) in which the restriction (13) lies is completely shut off, rendering flow of oil through the restriction impossible, as long as the energy-converting action of the restriction is not called for.

Description: A DEVICE FOR DAMPING UNDESIRABLE OSCILLATIONS IN A CRANE MEMBER Technical field of the invention This invention relates to a device for damping undesirab oscillations in an arm or other member of a crane that is pivo- tally movable by hydraulic means. It is concerned with a system having at least one hydraulic motor in a hydraulic circuit in which there is a restriction. When a circuit-closing device or function is operated to stop the main motion, this restriction converts energy inherent in the system (deformation energy and kinetic energy) to heat in the hydraulic fluid that still flows as a result of oscillation. The heat dissipates that energy to damp or suppress any oscillations of said member. Background of the invention In cranes, e.g. of the outer-boom type, there can be a plurality of movements which are carried out by means of hydrau lic motors, usually linearly operating motors in the form of cy linder actuators. One such movement is the horizontal slewing o the crane arm between various azimuth positions. This is often accomplished by means of two integrated hydraulic cylinders, which by way of a common toothed rack actuate a gear wheel co¬ axially carried by the post supporting the crane arm. Such a crane movement is schematically illustrated in the appended figure 1 where a crane arm seen from above is designated 1. Thi crane arm 1 is mounted on a post 2 having a coaxial gear wheel 3, which engages a toothed rack 4 connected at opposite ends to motors in the form of two linear hydraulic actuators. These hav cylinders 5, 5' in which pistons 6, 6' define two operating chambers 7, 7'. These two hydraulic actuators are part of a hyd raulic circuit, generally designated 8, in which is also inclu¬ ded a directional valve 9. This is operable to direct oil from pump 10 to either one of the cylinders 5, 5' through main con¬ duits 8a, 8b, while exhaust oil flows to a tank 11. By moving a adjustable lever 9' of the directional valve 9 from a neutral position to either one of two operative positions the crane arm 1 can be slewed either clockwise or anti-clockwise about the axis of the wheel 3. A problem, particularly in cranes having a great reach (today there are cranes having reaches of the order of 14 metre or more), arises when the crane arm has finished slewing, i.e. after the adjustment lever of the directional valve 9 has been returned to the neutral position. Both the arm and the load it carries are subject to oscillatory or reciprocating motions, an the duration of these may be long. They are caused by the fact that deformation energy is easily absorbed in the crane arm, an potential energy in a freely hanging load, when the crane arm i slewed. Assume that the crane arm 1 at a certain moment during cycle of oscillation tends to slew anti-clockwise about the centre of the gear wheel 3. Then the toothed rack 4 is moved to the right in figure 1, the oil pressure in the chamber 7' in¬ creasing at the same time as the oil pressure in the chamber 7 decreases. The directional valve 9 is shut off and will not per¬ mit flow. With such conditions, the arm 1 will act as a spring at the same time as the free-hanging load acts as a pendulum. That will accelerate in the opposite direction when it reaches the end of its swing and then the arm will be set in motion clockwise. The oil pressure in chamber 7 is thus increased. In this manner the crane arm will oscillate. In a 14-metres crane for instance the tip of the crane arm swing or deflect between positions of the order of 2 metres apart and it can be 15 to 20 seconds before the tip and accordingly the load are stopped. Such oscillations are especially troublesome where the load is at the end of a lengthy rope hanging down from the tip of the crane arm. In such cases the crane arm tip oscillates to and fr at the same time as the load oscillates relative to the tip through the rope. Such oscillations may also occur during the slewing of the crane arm tip between two desired positions. The may happen, for example, if the hydraulic flows to the respec¬ tive cylinders 5, 5' are disrupted, as a consequence of sharp lever movements at the directional valve 9 perhaps or as a con¬ sequence of the load or the crane arm hitting obstacles. The prior art In order to cope with the long duration of these oscilla¬ tions, attempts have been made to damp them. By SE 9101544-6 a damping unit is known which is installed in a branch or interme- diate conduit bridging the main conduits between the directiona valve and the hydraulic cylinders. The damping unit is in the form of a restriction associated with a co-operating shut-off device. In SE 9101544-6 this shut-off device consists of a pis- ton captive in a cylinder and freely movable between opposite end positions, the piston shutting off the oil flow through the restriction when it reaches either end position. When the oil passes the restriction a turbulent flow in the oil is generated which converts the energy inherent in the system to heat. In this way of the energy remaining in the system is dissipated, and the duration of the oscillations of the crane arm is reduce to a significant extent. Assuming the crane arm is suddenly stopped from the full velocity of slewing, the duration of oscillatory motions (to when the arm is finally still) can be reduced from 15 to 20 seconds to 3 to 5 seconds. The solution of the problem disclosed in SE 9101544-6 is however, unsatisfactory for several reasons. One reason is that the piston of the damping cylinder may assume more than one dis tinct position when the need for damping has ceased. This means that hydraulic oil may flow in limited amounts through the re¬ striction and accordingly to one of the two hydraulic actuators Therefore, the crane arm will not with certainty assume any positively stationary position. Quite the reverse; the crane ar will have a freedom to swing within a limited range. In certain states of operation, e.g. during loading on the brow of a hill or on inclined ground, this is perilous. Another reason for the known solution being unsatisfactory is that the restricting or choking function becomes irregular as a consequence of the shut off piston in the damping cylinder moving freely all the way to the end position. There it abruptly interrupts further flow and so briefly stops further damping. This results in a bad damping and in a great statical play in the slewing function of the crane dependent on what is prioritized when adjusting the dam¬ ping unit. Objects and eatu es of the invention The present invention aims to alleviate the problem stil left by SE 9101544-6 and provide a damping device which, during the damping operation, promotes regular or even movements of th crane arm and at the same time makes sure that the arm will sto in a reliable manner and in a desired position after slewing. Accordingly, a basic object of the invention is to provide a damping device which is capable of being activated whenever an oscillation tends to occur due to an expected or unexpected ex¬ ternal influence on the hydraulic system, such as intentional o unintentional lever movements or collisions against obstacles. Another object is to provide a damping device which, after the arm has stopped moving, will ensure that any flow of hydraulic fluid through the damping restriction will cease and so prevent uncontrolled movements of the crane arm. According to the invention the above-mentioned objects ar achieved by means of the features defined in the characterizing clause of claim 1. Preferred embodiments of the invention are further defined in the dependent claims. Brief description of the appended drawings In the drawings: Fig 1 is a schematic illustration of a hydraulic system of the type previously mentioned, in which a schematically shown damping device according to the invention is included. More particularly figure 1 shows a first, preferred embo¬ diment having control means including a microcomputer or similar control unit. Fig 2 is a schematic illustration of a second embodiment of a damping device according to the invention, in which the control means is of a mechanical and automatically opera¬ ting nature. Detailed description of preferred embodiments of the invention In a manner that is per se known by SE 9101544-6, a restriction, symbolically indicated at 13, is arranged in a branch or intermediate conduit 12 between the main conduits 8a, 8b. In practice this restriction may consist of a fixed restric- tion in the form of an annular insert or shoulder of reduced diameter located in the conduit 12. The conduit may be a tube o a drilled bore in a block. A shut-off valve, generally designa¬ ted 14, and in the form of a magnetic valve of the on-off-type for example, co-operates with this fixed restriction 13. By means of the shut-off valve the conduit 12 may either be comple tely blocked, whereby oil flow through the restriction 13 is rendered impossible, or opened in order to allow oil to flow through the restriction. This opening and closing of the valve is controlled by a control unit 15 which in practice advanta¬ geously, but not necessarily, may consist of a microcomputer. I the embodiment shown, input data to this control unit are obtai ned by means of two sensors 16, 16' in the form of pressure or flow sensors. These may be connected as shown to the interme- diate conduit 12, or they may be connected to the conduits 8a, 8b, on either side of the restriction 13 and the magnetic valve 14. The purpose of these sensors 16, 16' is to detect signifi¬ cant conditions and/or changes of conditions in the hydraulic oil in the circuit. It is preferred in practice to use sensors which are capable of detecting significant pressures and/or changes of pressure which occur in the two actuators 5, 5' when the crane arm 1 starts to oscillate. When the control unit 15 receives information from the sensors 16, 16' that pressures and/or pressure changes of a certain magnitude have arisen in the system, the unit will provide a signal to the magnetic valv 14 to open, whereby the valve will allow oil to flow through th restriction 13. In other words the restriction is activated, an a turbulent flow is created in the oil passing to and fro through it. The varying deformation energy and the kinetic energy in the system is converted, in a manner known per se, to heat in the oil, and the oscillatory motions of the crane arm are damped. As soon as these oscillatory motions have subsided to a level at which the sensors no longer detect significant conditions and/or changes of conditions (and that may happen within the course of a few seconds) the control unit 15 provide a signal to the magnetic valve 14 to close. Any further flow of oil through the restriction 8 is thus blocked. It should be noted that the directional valve 9 is kept closed all the time during this damping operation. Therefore, when the valve 14 is shut off even a minimum flow of oil between the two cylinders 5 5' is rendered impossible. This ensures that the crane arm 1 is reliably maintained in its given, desired position. Unlike the solution of the problem disclosed in SE 9101544-6, the damping device of figure 1 makes possible a comp letely, reliably and positively controlled closure of the inter mediate conduit 12 when the crane arm is arrested. Thus, a basi advantage of the invention is that it provides a reliable and positive adjustment of the crane arm to the desired position while the duration of the resultant oscillations is substan¬ tially reduced in comparison with such cranes that lack damping devices. Another advantage in comparison with SE 9101544-6 is that the restriction - and accordingly the damping - can be con trolled in a positive and efficacious manner. The control unit 15, when it is a microcomputer, may be programmed in such a man ner that the magnetic valve 14, which is normally closed, will be opened not only when the crane arm has finished slewing (whe the lever of the directional valve 9 is moved to the neutral position) but also at any moment during slewing. A sequence of oscillations, irrespective of whether it occurs after the basic crane arm movement has stopped or during slewing, manifests it¬ self in a harmonic function the condition of which is read by the microcomputer by means of the sensors. On a given condition existing the computer may send a signal to the magnetic valve t either open or close. In contrast, the freely movable or floa¬ ting piston in the damping cylinder according to SE 9101544-6 cannot be acted on at all from outside in any controllable man¬ ner. Reference is now made to figure 2 which illustrates a crane slewing system as described but with a damping device pro¬ vided with mechanical control means. Similarly to the embodimen shown in figure 1 the device includes a fixed restriction 13 in an intermediate conduit 12. In this case, however, pressure limiting valves 17, 17' are installed at either side of the restriction, said valves being of the type which opens only at certain minimum pressure. The two valves 17, 17' are arranged t co-operate with a common piston-cylinder mechanism, generally designated 18, which includes a cylinder 19 and a piston 20 having opposed piston rods 21, 21'. Each piston rod 21, 21' is connected to an associated valve 17, 17' by.way of a mechanical spring 22, 22'. These springs are for increasing the pressure drop across the pressure limiting valves 17 and 17' and accor¬ dingly for increasing the resistance to movement of the piston towards a valve-closing position. They operate alternatively an successively to decrease the flow of oil through the associated valve until the valve completely closes the intermediate condui 12. Assume that the crane arm 1, commencing a sequence of oscillations when the directional valve 9 is closed, tends to swing clockwise as viewed in the figure. This compresses the oi in the cylinder chamber 7. Oil is forced into the chamber 23 an the piston 20 is urged towards the right in figure 2, i.e. towards the second valve 17' , while compressing the mechanical spring 22 ■ . At a certain pressure the valve 17' is opened and allows a freer flow of oil through the intermediate conduit to the chamber 23 of the cylinder 19 through the restriction 13 (while developing heat in the oil and thus causing damping) . Oi from the chamber 23 ' flows through the restricting or pressure- limiting element of the valve 17'. Gradually, as the spring 22' is compressed, the pressure drop in the valve 17' is increased and accordingly the resistance to movement of the piston 20 is increased until an end position when the flow through the devic is halted and reversed. The piston 20 moves in the opposite direction, compressing the spring 22, and the oil flow through the restriction 13 is in a direction from the chamber 23 toward the cylinder chamber 7. The cycle is repeated until the damping is completed. In contrast to the freely movable piston of the damping cylinder according to SE 9101544-6 (which piston moves unresis- ted all the way to the end position, where it abruptly stops an gives rise to a pressure shock in the system) the springs 22, 22' ensure - by increasing the pressure drop across the valve i question - that the piston 20 moves in a smoothly decelerating movement towards the end position without giving rise to abrupt pressure shocks. Moreover, the pressure limiting valves 17, 17' prevent any flow of oil through the restriction 13 as soon as the oscillations have been damped. Unintentional creeping move¬ ments of the crane arm after the damping has stopped are thereb avoided. Conceivable modifications of the invention The invention is not merely limited to the embodiments disclosed above and in the drawings. The embodiment of figure 1 includes a fixed restriction and a magnetic valve separate from the restriction. In practice it is, however, conceivable to use, instead of these separate components, a check valve serving bot as a variable restriction, and for completely shutting-off the hydraulic conduit. Instead of one single fixed restriction (in practice in the form of an insert or a shoulder having a small percolation aperture) it is possible to use two or more restric¬ tions having wider apertures. Instead of pressure sensors it is also possible to use either flow sensors in the hydraulic cir- cuit or position sensors which instantaneously detect the posi¬ tion of the crane member to be damped. Neither is the invention limited to the use of just two sensors, since the necessary information regarding the flow or pressure conditions in the system may also be obtained by means of one single sensor in combination with the characteristics of the restriction. The principle of the invention is in no way limited to the horizon¬ tal slewing of a crane about a vertical axis. Thus, the damping device of the invention may also be used for other members for¬ ming parts of a crane, such as the inner boom and the outer boom, in order to damp oscillations in other planes, such as vertical.

Claims: Claims 1. A device for damping undesirable oscillations in a member (1 of a crane that is pivotally movable by means of at least one hydraulic motor (5, 5') in a hydraulic circuit (8) in which there is a restriction (13) co-operating with a circuit-closing device (14; 18), said restriction serving to convert energy inherent in the system to heat in the hydraulic medium in order to damp or suppress oscillations of said member caused by said energy, c h a r a c t e r i z e d by sensor means (15, 16, 16' 17, 17') responsive to a threshold in conditions relating to th member when oscillating and arranged to operate the circuit clo sing device (14; 18) to permit and block flow through the re¬ striction (13) when the threshold is respectively exceeded and not exceeded. 2. A device according to claim 1, c h a r a c t e r i z e d in that the restriction (13) is in a conduit (12) bridging main conduits (8a, 8b) of the hydraulic circuit to opposed hydraulic motors (5, 5') which operate said member. 3. A device according to claim 1 or 2, c h a r a c t e r i z e in that the sensor means (16, 16') are responsive to pressure and/or flow in the hydraulic circuit. 4. A device according to claim 1 or 2, c h a r a c t e r i z e in that the sensor means (16, 16') are responsive to changes in pressure and/or flow in the hydraulic circuit. 5. A device according to claim 1 or 2, c h a r a c t e r i z e in that the sensor means are responsive to the position of said member. 6. A device according to any preceding claim, c h a r a c t e r i z e d by means (15) for electronically pro cessing information from the sensor means (16, 16') to a form capable of operating said circuit closing device (14) . 7. A device according to any preceding claim, c h a r a c t e r i z e d in that the circuit closing device i a shut-off valve (14) . 8. A device according to claim 7, c h a r a c t e r i z e d in that the shut-off valve is a magnetic valve of the on-off type. 9. A device according to any one of claims 1 to 6, c h a r a c t e r i z e d in that the circuit closing device i a check valve capable of complete and intermediate closure to give variable throttling of the hydraulic medium flowing in the hydraulic circuit. 10. A device according to claim 2, c h a r a c t e r i z e d i that said sensor means include two pressure restricting valves (17, 17') installed on either side of the restriction (13) in the bridging conduit (12) , said valves being of the type which open only at a certain minimum pressure and being coupled through mechanical spring means (22, 22') to opposite sides of piston (20) of a common piston-and-cylinder mechanism (18) , the spring means (22, 22') serving to increase the resistance to movement of the piston (20) towards each end position in which the bridging conduit (12) is shut off, and thereby reducing on each piston stroke the flow of hydraulic medium through the val ves (17, 17') to the point at which one valve completely shuts off the bridging conduit.

Inventors: AASTROEM PETER, SVENSSON GOESTA, AASTROEM, PETER, SVENSSON, GOESTA
Applicants: AASTROEM PETER, HIAB AB, SVENSSON GOESTA, AASTROEM, PETER, SVENSSON, GOESTA
Application year: 1995
Main classification: B66C13
IPC codes: B66C13, B66C23