EP0368811A1

EP0368811A1 - A reset device for hydraulic load control systems

Info

Publication number: EP0368811A1
Application number: EP89830308A
Authority: EP
Inventors: Andrea Storci
Original assignee: Oil Control SpA
Current assignee: Oil Control SpA
Priority date: 1988-11-09
Filing date: 1989-07-04
Publication date: 1990-05-16

Abstract

The device is intended in particular for cranes provided with hydraulic load control systems piloted by a pressure signal proportional to the overturning moment generated by the load, which produce control signals that block any movement of the crane likely to increase the load; use is made of a three-position bleed-off valve (12) which, in a non-stable position assumed momentarily as the spool shifts back and forth between two stable positions, brings about a reduction in the pilot pressure directed into the control system (10) so as to avoid lock-up of the crane that can occur in certain maximum load conditions. A valve (12) of this type also achieves a significant reduction of the hysteresis that traditionally delays the return of such systems (10) to the passive state.

Description

The present invention relates to a reset device for hydraulic load control systems.
For some time now, hydraulic systems have been in use, not least due to their being required by law in many countries of the world, designed to control (i.e. to limit) the load applied to cranes of a general type that consist in a column, and a boom comprising one or more rotatable and/or telescopic arms hinged to the column and operated by hydraulic cylinders.
Such systems are piloted generally by a pressure signal proportional to the overturning moment generated by the suspended load, or attempted lift, taken normally from the main lift cylinder which carries the weight of the entire boom assembly mounted to the column. As soon as the overturning moment exceeds a given threshold, pilot pressure will rise to a level such as inhibits any further movement of the crane likely to increase this overturning moment.
Devices of the type in question are disclosed in detail in Italian patents 1189792 and 1187219, for example, and in application for Italian patent 40135 A/85, filed by the same applicant.
For the sake of brevity, reference is made in the course of the description to 'maximum load', as signifying the maximum overturning moment generated by a load applied to the bearing structure of the crane.
Conventional hydraulic systems of the type in question are beset by the drawback that when the main lift cylinder reaches the end of its stroke, the crane locks up completely. Such a situation is clearly intolerable, and remedied in many instances by the installation of a manually operated check valve which, in the event of the system becoming totally unresponsive in this way, unloads pilot pressure from the control circuit; the effect is that the load control recognizes a resumption of normal operating conditions (albeit false) and the operator can continue to work the crane.
Besides being illegal in many countries, such an expedient presents the obvious drawback of allowing the operator to by-pass the load control system, and thus exceed the maximum permissible load values specified for the crane.
A further drawback affecting load control circuits is that of hysteresis generated in the system, the effect of which is to prevent the crane from ever being used at maximum load capacity. More exactly, as maximum load conditions are reached, certain movements of the crane become inhibited, the only movements allowed being those which have the effect of reducing the load; as the load then decreases, the system will re-enable the formerly inhibited movements, but only when pilot pressure has dropped significantly below the maximum threshold. Thus it happens that the equipment is unsuitable for loads falling between the maximum permissible setting and the pilot pressure at which normal operation is restored.
The object of the present invention is to overcome the drawbacks mentioned above by providing a device that will permit of releasing the hydraulic load control device of a crane in the event of the main lift cylinder accidentally reaching the end of its stroke, without allowing any by-pass whatsoever of the load control system, and which enables a marked reduction of hysteresis in the load control system itself.
A notable advantage of the device disclosed is that it can be integrated into conventional hydraulic load control systems without any modification being required.
The stated object, and others besides, are realized with a device according to the invention and as characterized in the claims appended, intended in particular for hydraulic cranes consisting in a column with rotatable and/or telescoping boom arms raised and lowered and/or extended and retracted by means of hydraulic cylinders, each operated from and connected with a conventional control valve by way of two oil lines carrying pressure and return flow alternately; such a device is applicable to conventional hydraulic systems piloted by a signal proportional to the overturning moment generated by the load and triggering control signals by which selected movements of the crane are inhibited, and characterized in that it comprises a two-way three-position bleed-off valve connected with one port in receipt of the pilot pressure signal and the other port exhausting to tank, of which the first and third positions are stably assumed and designed to block flow between the two ports, and the second position is non-stable and assumed momentarily during the shift from the first position to the third position and viceversa, in which the two ports are connected, and in that the shift between the first and third positions is brought about by a control signal generated from the reactions between two opposing forces.
The invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which:

fig 1 shows a hydraulic circuit diagram of the device incorporated into the load control system of a crane;
fig 2 is a larger scale diagram of the bleed-off valve according to the invention;
fig 3 is the vertical section through a possible embodiment of the bleed-off valve.

With reference to the drawings, fig 1 shows the diagram of a hydraulic crane comprising a column 1, a first boom arm 2 hinged to and supported by the top end of the column, and a second boom arm 3 attached to the first; the first boom is rotatable only, and operated, i.e. raised and lowered, by a first hydraulic cylinder 4, whereas the second is rotatable and also telescopic, raised and lowered by a second hydraulic cylinder 5 and extended and retracted by a third cylinder 6.
The cylinders 4, 5 and 6 are operated by respective directional control valves 7, 8 and 9 which, when opened, connect the relative cylinder with the flow of pressurized oil delivered by a pump 14.
Two pipelines connect each control valve to the relative cylinder, one to the rear end, 4a, 5a and 6a respectively, and the other to the rod end 4b, 5b and 6b, respectively; these will function as pressure and return lines alternately, according to the position of the control valve spool.
The hydraulic load control system, represented schematically by a block denoted 10, is piloted to operate by a pressure signal proportional to the overturning moment generated by the load, and when triggered by a load exceeding a given threshold, produces control signals that inhibit certain movements of the crane. conventionally, the pilot signal is taken off the line 4a to the rear end of the main lift cylinder 4 supporting the weight of of the entire rotatable and/or telescopic boom assembly 2 and 3; pressure in the rear end of this cylinder 4, hence pressure through the line 4a to the relative valve, will in fact be proportional to the load applied to the crane.
On arrival at a given load/pressure threshold, the system 10 will produce control signals selectively inhibiting movements of the crane, generally those which would be liable to increase the load.
The drawing shows how signals from the system 10 trigger the operation of servo-pistons that engage the control valve spools and prevent them from shifting in given directions, though the signals could be utilized differently, for example to pilot valves that prevent pressure flow from reaching the pipelines. It will be observed further that the pipeline 6b connected with the rod end of the third cylinder 6 is subject to no control, inasmuch as oil admitted under pressure along this route will have the effect of retracting the cylinder 6, hence the telescoping boom 3, and therefore of reducing the load.
The components and systems thus far described are embraced by the prior art, and their embodiment is a matter of choice.
The device according to the invention comprises a two-way, three-position bleed-off valve 12. The one port 22a of the valve is connected to a signal line 16 connected in turn to the pilot pressure take-off line from the rear end of the main lift cylinder 4, and the other port 22b connected to tank by way of a line denoted 18; more exactly, the signal line 16 is teed into the pilot pressure take-off line at a point downstream of a restriction 22. The three positions of the valve are denoted 12a, 12b and 12c (first, second and third, respectively) in fig 1; the two outer positions 12a and 12c are stable positions in which flow between the two valve ports remains blocked, whereas the centre position 12b is a non-stable position in which the two ports are connected with one another. The connection obtained with the second position 12b occurs by way of a restriction 30, for reasons that will become clear in due course.
The valve 12 is maintained in its first stable position 12a through the elastic force produced by a spring 15, and becomes subject in operation to a thrust force acting in the opposite direction to the elastic force, in such a way that the movement from first position 12a to third position 12c is occasioned when the spring 15 becomes compressed by the opposing thrust force. The opposing force in question is obtained by way of a pressure signal P taken from the oil line 6b to the rod end of the telescoping cylinder 6, which is directed through a relative signal line 17 and applied to a surface of prescribed area within the valve 12, thereby creating the necessary thrust.
In shifting from the first position 12a to the third position 12c, the valve passes momentarily through the position 12b in which its two ports are connected via the restriction 30.
It should be underlined at this juncture, that the second position 12b is no more than a transitional state assumed fleetingly during the passage between the two stable positions 12a and 12c, and cannot be held steady by the valve.
Fig 3 is the sectional illustration of a possible embodiment of the bleed-off valve.
The two valve ports 22a and 22b are fashioned as annular grooves sunk into the external surface of a fixed element 28 provided internally with a coaxial bore slidably accommodating a spool 26. The grooves 22a and 22b are connected with the bore by way of relative sets of passages 25a and 25b set apart one from the other through a prescribed axial distance. 27 denotes a land of diameter smaller than the remainder of the spool 26, and of length marginally greater than the axial distance separating the two sets of porting passages 25a and 25b.
The spool 26 is biased downwards (as viewed in the drawing) by the spring 15, into a position that is denoted 12 in fig 2. In this position, the valve ports 22a and 22b remain mutually unconnected, as the land 27 admits only one set of passages 25a, whereas the other passages 25b remain blocked. 29 denotes an inlet at the bottom of the valve element 28, to which the aforementioned pressure signal P is connected.
The moment that the force generated by pressure P on the bottom end of the spool 26 becomes greater than the mechanical force of the spring 15, the spool shifts upward and into the second stable position of the valve, in which the land 27 admits the passages denoted 25b only; accordingly, the ports 22a and 22b still remain unconnected.
It will be recalled at this point that the spool land 27 is marginally greater in length than the distance between the two sets of passages 25a and 25b, and accordingly, in passing from the first stable position to the second, the spool connects the two ports momentarily. This brief moment of contact will be repeated when pressure P drops below a level sufficient to maintain the spool 26 in the raised position, and the spool is returned by the spring 15 to the position shown in fig 3.
According to the invention, the spring bias will be notably weak, and the spring 15 itself therefore somewhat soft; consequently, it will be practically impossible to create fluid pressure at the inlet 29 of an order capable of floating the valve in the transitional second position 12b; at all events, to ensure absolutely that this position cannot be held, the option exists of controlling the inlet line 17 with a conventional on/off valve operating to a given pressure threshold, admitting flow only when above the prescribed pressure, and blocking the line when below. Clearly enough, the threshold will reflect a pressure higher than that required to compress the spring 15 and thus shift the valve spool 26.
The difference between the axial length of the spool land 27 and the distance separating the two sets of passages 25a and 25b establishes the degree of restriction 30; needless to say, the smaller the difference, the tighter the restriction obtained.
Operation of the device will now be described. Assuming, by way of example, that maximum load is reached with the crane in the condition of fig 1: the load control system 10 will duly respond by inhibiting any further movements likely to increase the load (in the case of the drawing, a further lowering of the boom 2 and 3 or further extension of the telescopic boom arm 3). At this stage, the operator can execute only such movements with the crane as have the effect of reducing the load, and in practice, the movement almost invariably will be that of retracting the telescopic boom arm 3, as this reduces the load in any given position of the crane.
The telescoping boom 3 is retracted by directing pressure into the rod end line 6b, and this same pressure causes the bleed-off valve 12 to shift from first position 12a to third position 12c, passing momentarily through the non-stable second position 12b.
Thus, in addition to reducing pressure in the rear end of the main lift cylinder 4, hence in the pilot flow to the load control system 10 (the load having been lightened through retraction of the telescopic arm 3), one also has a reduction in pilot pressure to the system 10 brought about through the action of the bleed-off valve 12 during momentary passage through its second position 12b.
If retraction of the boom 3 is stopped at this point by blocking pressure through the relative line 6b, the bleed-off valve will return to the first position 12a, moving again through the second position 12b and unloading momentarily to reduce pilot pressure into the load control system 10. Thus, in returning to normal operating conditions, that is, with all movements of the crane enabled, a load control system 10 incorporating the device disclosed is affected notably less by hysteresis in the hydraulic circuit than prior art load control systems. In short, the system resets with a notably small reduction in load from the maximum specified. Hysteresis can be reduced even more significantly where the crane operator, appropriately instructed, retracts the boom 3 in a succession of short stabs, directing flow intermittently through the relative line 6b; in this way, the valve 12 will be made to assume the transitional second position 12b several times in succession, and a corresponding reduction in pilot pressure to the system 10 thus obtained.
It will be observed that the valve will only begin to bleed off following movements that bring about a reduction in load; accordingly, it is not possible to interfere with the prescribed operation of the system 10, which will reset only in response to a reduction in load. By providing the restriction 22, moreover, it becomes possible to lower the pressure of pilot flow into the system 10 without in any way affecting the pressure level in the rear end of the main lift cylinder 4.
Still referring to the situation shown in fig 1, in the event of the main lift cylinder 4 reaching the limit of its stroke, rear end pressure will rise significantly and the system 10 will duly respond by inhibiting any movement likely to increase the load, i.e. lowering the cylinder; in a condition such as this, the system locks up completely.
By working the telescope control valve 9 in such a way as to direct pressurized flow through the rod end line 6b in bursts (the effect of which, it will be recalled, is to reduce the load on the crane), the bleed-off valve 12 can be made to operate in the manner described above, and pilot pressure in the system 10 reduced to the point where all the directional control valves are re-enabled; in this situation, the lift cylinder 4 can be destroked.
The restriction 30 incorporated into the second position 12b of the valve 12 constitutes a safety feature by preventing any sudden drop in pilot pressure to the load control system 10, and will be calibrated according to the rate of drop in pilot pressure it is wished to obtain with each passage of the valve through the non-stable position 12b.
The valve disclosed will operate perfectly well regardless of the configuration of the load control system 10 and of the means operated by the system for inhibiting movement; accordingly, the device can be used in conjunction with any given hydraulic load control system.
Besides being used for a reset device as described in the foregoing, the valve 12 disclosed can also be utilized for any given application where a need exists, or where it may be advantageous, to bleed off pressurized fluid between two pipelines of different pressure connected to the ports of the valve.

Claims

1) A reset device for hydraulic load control systems, in particular for hydraulic cranes consisting in a column (1) with rotatable and/or telescoping boom arms (2, 3) raised and lowered and/or extended and retracted by hydraulic cylinders (4, 5, 6) each operated from and connected with a conventional control valve (7, 8, 9) by way of two oil lines carrying pressure and return flow alternately, applicable to conventional hydraulic systems (10) piloted by a signal proportional to the overturning moment generated by the load and triggering control signals by which selected movements of the crane are inhibited,
characterized
-in that it comprises a two-way three-position bleed-off valve connected with one port (22a) in receipt of the pilot pressure signal and the other port (22b) exhausting to tank, of which the first position (12a) and the third position (12c) are stably assumed and designed to block flow between the two ports, and the second position (12b) is a non-stable position, assumed momentarily during the shift from the first to the third position and viceversa, in which the two ports are connected;
-and in that the shift between the first and third positions is brought about by a control signal generated from the reactions between two opposing forces.

2) A device as in claim 1, wherein the connection between the ports (22a, 22b) of the bleed-off valve produced in the second position (12b) occurs by way of a restriction (30).

3) A device as in claim 1, wherein the control signal bringing about operation of the bleed-off valve is generated from the reactions between:
-an elastic force by which the valve is maintained in one of the stable positions;
-a thrust force, applied to the valve in the direction opposite to that of the elastic force, which when stronger than the elastic force causes the shift from one stable position to the other, and when weaker than the elastic force permits a return to the former stable position.

4) A device as in claim 3, wherein the thrust force is generated by hydraulic pressure (P) drawn from the oil line through which pressurized flow is directed in order to retract the outermost boom arm, and applied to a surface of prescribed area.

5) A device as in claim 1, wherein the pilot pressure signal is connected to the relative port (22a) of the bleed-off valve (12) at a point downstream of a restriction (22) incorporated into the line through which pilot pressure is directed to the hydraulic load control system.

6) A device as in claim 1, wherein the bleed-off valve comprises:
-a fixed element (28) provided internally with a coaxial bore, and externally, with two annular grooves coinciding with the ports (22a, 22b) and connected with the bore by way of respective sets of passages (25a, 25b);
-a spool (26), slidably accommodated by the coaxial bore to a fluid-tight fit and affording a land (27) of reduced diameter the axial length of which is marginally greater than the axial distance that separates the sets of passages (25a, 25b) connected with the two ports;
-a spring (15), against which the spool (26) is loaded in such a way as to remain permanently in a first position whereby the land (27) is offered only to the passages (25a) connected with the first port (22a);
-an inlet (29) affording access from externally of the valve to the surface of the spool (26) opposite the surface loaded against the spring (15).