A HYDRAULIC LOAD LIMITING DEVICE FOR HYDRAULIC CRANES

Abstract: The invention disclosed is a hydraulic load limiting device for hydraulic cranes, in which hydraulic valves producing either an on-off (10, 10′, 10″, 41) and/or a switching action (52, 42) are shifted by a pressure signal generated in one of the boom cylinders, thereby setting up a control signal that triggers the operation of hydraulic valves (7a, 7b, 8a, 8b, 9) installed on the flow lines connecting with the boom cylinders; such a control signal, which may be relayed either directly or via a hydraulic level-sensing device (25), serves to pilot shut-off of pressure flow to the relative cylinder or cylinders whenever further extension or retrac­ tion would cause the boom to assume a configuration such as overloads the crane beyond its maximum rated lift capacity.

Description: The invention described herein relates to a hydraulic load limiting device for hydraulic cranes. A device as disclosed is intended for application to cranes of the general type consisting of a column to which a boom with one or more folding and/or telescopic stage or stages is attached, the single stage being operated by a relative hydraulic cylinder. One of the requirements of safety regulations existing in numerous countries the world over, is that any movement in the boom liable to overload the crane by exceeding its maximum rated lift capacity, must be inhibited. Thus, whenever the crane lifts a load such as sets up a moment of force equal to its maximum rated capacity, any subsequent movement in the boom that would lead to an increase in the load-related moment of force must be disallowed; more exactly, any movement whereby a telescopic boom is further extended, and any rotational movement of the boom stages such as distances the load from the crane's slewing axis, must be prevented from taking place. In cranes of the type in question, an increase in moment of force generated by the load is converted, in substantially linear manner, into a build-up of pressure in the rear end of the hydraulic cylinder which operates the hinged boom stage, or supports the hinged boom assembly; accordingly, control and subsequent limiting of the moment of force generated by the load is effected by monitoring pressure of the fluid that impinges on the rear end of the cylinder in question, and limiting it in relation to a given preset level. The prior art embraces devices in which use is made of an electrical signal, relayed from a pressure switfh triggered by pressure in the rear end of the cylinder, to energize the solenoids of valves that prevent further ingress of hydraulic fluid to the cylinders such as might lead to an increase in the moment of force generated by the load. Prior art devices also offer a level-sensing system by means of which to produce an electrical signal, triggered by the angle of the boom above of below horizontal, such as will operate means whereby movement of the boom is enabled insofar at the load-related moment of force will diminish, and inhibited where bound to increase -i.e., when the load matches the maximum rated lift capacity of the crane. Such devices are simple enough from the design and construction standpoints, but are beset by significant drawbacks as a result of being electric. Cranes are generally utilized and parked outdoors, and faultless operation of the above devices is very often jeopardized by weather conditions. A further drawback of prior art devices is that the wiring required for their operation can deteriorate and fail with the passage of time, and with use of the crane. Another drawback with the circuits of prior art load limiting circuits is that the solenoid valves employed reguire heavy coils, which are cumbersome and consume power. Electrical systems of the type are also notably expensive. Lastly, the inclusion of boom level-sensing systems involves considerable complication of the hydraulic load limiting circuit. The prior art embraces load-limiting devices which are hydraulic but which all the same need boom level-sensing systems to indicate the crane boom angle. One such device is illustrated in DE 3414183. Other devices of this type are illustrated in figs 1-5 of the present application, and are described in detail also in the present application. The devices equipped with boom level-sensing systems as in figs 1-5 are illustrated and described simply as examples of possible embodiments of load-limiting devices with crane boom angle indicators, but they do not form part of the claims for the present invention. Hydraulic laod-limiting devices with boom levelsensing systems do not present problems of an electrical nature, but present however the above-mentioned drawbacks due to the presence of said load-limiting device. One object of the invention described herein is that of overcoming the drawbacks mentioned above, providing a device that can respond to all such safety requirements as are envisaged under law, whilst making no use whatever of electrical circuits, whether for relaying control signals or for actuating the valves which shut off flow of hydraulic fluid to the boom cylinders. A further object of the invention is that of providing a device that will not be sensitive to any accidental surge in the moment of force generated by the load, and which will cut in at the precise overload setting selected, whatever the operating conditions. Another object of the invention is that of providing a device which, at maximum rated lift capacity, will inhibit any movement in the crane liable to produce overload conditions, regardless of the configuration of the crane at that instant, and do so without any need for indication of the angle at which the boom stages happen to be disposed. A clear advantage of the device disclosed is that it is all-hydraulic, and uses the same source of energy for its operation (pressurized fluid) as that used by the crane as a whole; such a feature is especially advantageous in terms of circuit design and cost. The stated objects and advantages, and others, are fully realized with a device as described herein and as characterized in the appended claims; a device according to the invention employs hydraulic on-off and/or switching valves which respond to a pressure signal from one of the boom cylinders, and produce a hydraulic control signal, used without the need of employing a level-sensing system to pilot hydraulic valves installed on the lines connecting with the boom cylinders, to the end of shutting off pressure flow to the cylinders whenever further extension or retraction would cause the boom to assume a configuration whereby maximum rated lift capacity is exceeded. The invention will now be described in detail by way of example, with the aid of the accompanying drawings, in which: fig 1 illustrates the hydraulic circuit in a first embodiment of the device using a hydraulic level-sensing system fig 2 illustrates the embodiment of certain of the components making up the device in fig 1; fig 3 shows an alternative embodiment of a part of the device in fig 2, seen on larger scale; fig 4 illustrates the hydraulic circuit in a second embodiment of the device using a hydraulic level-sensing system fig 5 illustrates the device using a hydraulic level-sensing system figs 6, 7, 8 and 9 are schematic diagrams illustrating certain possible configurations of the boom; fig 10 illustrates the hydraulic circuit in fourth embodiment of the device which does not employ a hydraulic level-sensing system. fig 11 illustrates the hydraulic circuit in a fifth embodiment of the device which does not employ a hydraulic level-sensing system. fig 12 illustrates the hydraulic circuit in a sixth embodiment of the device which does not employ a hydraulic level-sensing system. The crane illustrated in the drawings consists of a column 1, a first boom stage 2 which is hinged to the top of the column 1 and rotated about its pivot by a hydraulic cylinder 4, and a second boom stage 3 which is both hinged and telescopic, rotated about its pivot by one hydraulic cylinder 5, and extended. and retracted telescopically by a further hydraulic cylinder 6. The hydraulic cylinders are connected to a directional control valve assembly denoted 15, each single section of which, when operated, provides a relative cylinder with fluid supplied under pressure from a pump 14. 4a, 5a and 6a denote lines connecting each section of the control valve with the rear end of the relative cylinder, whereas 4b, 5b and 6b denote similar lines connecting with the rod end of each cylinder; the ′a′ and ′b′ lines of each circuit thus alternate between pressure and return functions, according to the position of the control valve. 4c, 5c and 6c denote respective holding valves installed one in each of the cylinder circuits to the end of preventing jerky descent of the load. All of the components described thus far are of a conventional type widely used in hydraulic cranage. The device as embodied in figs 1, 4, 10, 11 and 12 comprises two-way two-position hydraulic valves, denoted 7a, 7b, 8a, 8b and 9, which are installed in the circuits operating the two hinged cylinders 4 and 5, and in the line connecting with the rear end of the telescoping cylinder 6, respectively. When in the first position, each such valve blocks pressure flow through the relative line while allowing return flow through the same line; in the second position, flow is permitted in either direction. Each valve is held in the second position by pressure of the fluid through the line upstream of the valve itself, and biased toward the first position by a respective spring 17a, 17b, 18a, 18b and 19, as well as being piloted into the first position by a hydraulic control signal described in detail below. As long as there is no pressure through the relative branch of the circuit, the valve will remain springbiased toward the closed position, whereas pressure will cause the valve to open and allow passage of the fluid through that branch; in the event of there being a load on the hook that matches the crane's maximum rated lift capacity, the valves in question act as a shut-off facility disallowing passage of fluid under pressure to the cylinder. The manner in which shut-off occurs is described in detail below. The device as embodied in figs 1 & 4 comprises load-sensing means incorporating a two-way two-position hydraulic response valve 10, first position open, second position closed. The first port of the valve 10 connects with the pump 14, and the second port with a first send line 11 that carries the hydraulic control signal utilized for direct or indirect operation of the shut-off valves 7a, 7b, 8a, 8b and 9. The response valve 10 is biased into normally closed position by a spring 10a, and piloted to open in direct fashion by a pressure signal generated in the rear end of the first stage cylinder 4 and carried to the valve 10 by a relative line 22; the valve 10 is thus operated directly by pressure impinging on the rear end of the cylinder 4 which, it will be remembered, is proportional to the moment of force generated by the load. 23 denotes a set of restrictions located upstream of the response valve 10 on the load sensing line 22, the purpose of which is to avoid unwarranted operation of the valve 10 caused by accidental pressure surge occurring in the cylinder 4. The device as embodied in figs 1 & 4 also comprises level sensing means in the form of a three-way hydraulic valve 25 with a ball 26 that is free to float within the valve chamber. The first port of such a valve is located in the side wall of the chamber and remains permanently open, whereas the second and the third port are located at either end of the chamber, which also offer relative seats in which the ball 26 may register. The valve 25 is made fast to the third stage 3 of the boom, and thus serves to indicate the position of boom as regards the angle assumed above or below horizontal; more exactly, indication of the position A or B of the boom (see fig 1) is made possible by the fact that the ball 26 will shift toward one or other end of the valve chamber, according to the angle assumed by the boom. The first port of the level sensing valve 25 is connected to the first send line 11, whereas the second and the third ports are connected to a second and a third send line 12 and 13, respectively. For reasons which will become clear in the course of the description, the hydraulic signal utilized for operation of the valve denoted 9 is taken off the first send line 11. In the embodiment of fig 1, the valves 7a and 8a installed on the lines connecting with the rear ends of the two respective hinged cylinders 4 and 5 are operated by a hydraulic signal supplied through the second send line 12, whereas the valves 7b and 8b on the lines connecting with the rod ends of the same cylinders are operated by a signal supplied through the third send line 13. The mechanical structure of a given crane may be such that no linear relationship can exist between pressure in the rear end of the cylinder 4 and the moment of force generated by the load; the result is that certain movements of the boom may overload the crane beyond rated lift capacity without rear end pressure in the cylinder 4 actually registering at the maximum setting envisaged. This is a drawback which can be overcome by inclusion of a second hydraulic response valve 10′ identical to and connected in parallel with the first such response valve 10; both valves will be integrated into the system in exactly the same fashion, except for the fact that the pressure signal which pilots the second valve 10′ to open is taken off the rear end of the second cylinder 5, or at all events, a cylinder other than the first cylinder 4, and carried by a relative sensing line 22′. Fig 2 illustrates how the second valve 10′ would be incorporated, where appropriate. Pressure through the rod end line 4b of the circuit controlling the first cylinder 4, which pilots the holding valve 4c to open, impinges on the rear end of the cylinder in a measure commensurate with the difference in piston area between the rear and rod ends; such pressure can therefore affect the control signal supplied to the response valve 10 and cause the valve to operate even though rear end pressure in the cylinder 4 may not correspond to the maximum rated lift capacity of the crane. In order to overcome such a drawback, the element of the response valve 10 is piloted to close by a pressure signal taken off the rod end line 4b, and the area of the valve element on which this signal impinges differs from the area invested by the signal from the rear end line 4a to an extent commensurate with the difference in piston area between the rod end and rear end of the cylinder 4. Fig 3 illustrates how the response valve 10 may be embodied so as to remain unaffected by pilot pressure through the rod end line 4b of the cylinder circuit. The embodiment of fig 1 also comprises a manually operated hydraulic unloading valve 20 installed in normally closed configuration between the sensing line 22 and the return line, the purpose of which is described in detail below. Finally, each send line is provided with a respective restriction connecting to tank. In the embodiment of fig 4 one has a third two-way two-position hydraulic response valve 10″, first position open, second position closed, biased into its normally closed position by a spring 10a″, and piloted to open in direct fashion by, pressure signal supplied through the sensing line 22. The first port of the valve 10" connects with the pump 14, and the second port with a fourth send line 51. The first response valve 10 will be piloted to open whenever pressure impinging on the rear end of the cylinder 4 and relayed through the sensing line 22 reflects maximum rated lift capacity of the crane, whereas the third shut-off valve 10" is caused to open by a pressure level marginally in excess of this setting. This second embodiment of the device also incorporates a three-way two-position switching valve 52, a first port of which is connected to the first send line 11, the second and third ports being connected respectively to a fifth and a sixth send line 53 and 54 which supply the control signal for opera(ion of two of the shut-off valves 7b and 7a, respectively; signals for operation of the valves denoted 8a and 8b continue to be supplied by the second and third send lines 12 and 13. The switching valve 52 is biased by a spring 55 into first position, whereby first and second ports are connected, and piloted into the second position, in which the first and third ports connect, by a signal supplied through the fourth send line 51. In the third embodiment of the device illustrated in fig 5, two of the shut-off valves 7a and 8a are dispensed with, whereas those denoted 7b, 8b and 9 remain; the valves denoted 7b and 9 are shifted into closed position operated directly by a control signal supplied via the send line denoted 11, whilst the remaining valve 8b is operated by a signal supplied via the send line denoted 13. In this embodiment, the second port of the level-sensing valve 25 and the second send line 12 are both eliminated. The rear end lines of the circuits operating the two hinged boom cylinders 4 and 5 incorporate respective first and second relief valves 57 and 58 that connect the relative line to tank whenever pressure in the line itself rises above the setting corresponding to maximum rated lift capacity of the crane. In the embodiments of figs 10 and 11, sensing means are provided that comprise a three-way two-position valve 41 which is biased by a spring 45 into its first position, whereby first and third ports are connected; the valve 41 is piloted into its second position, in which second and third ports are connected, by a pressure signal taken off from the rear end of the first cylinder 4 and routed to the valve by way of a branched leg 22a of the sensing line 22. Shift of the valve from first to second position occurs whenever the signal reaches a preset pressure level marginally below that which reflects maximum rated lift capacity of the crane. The sensing means in figs 10 and 11 further comprise a four-way two-position valve 42 biased by a spring 46 into its first position, in which the first port connects with the third and the second port connects with the fourth; the valve 42 is piloted into its second position, in which the first port connects with the fourth, and the second port with the third, by the same pressure signal as aforementioned, which is routed to the valve by way of the remaining leg 22b of the branched sensing line 22. Shift of the valve from first to second position occurs whenever the signal reaches a pressure level reflecting the maximum rated lift capacity of the crane. The first port of both valves 41 and 42 is connected to tank; the second and third ports of the valve denoted 41 are connected to the pump 14, and to the second port of the valve denoted 42, respectively. In the embodiment of fig 10, the third port of the four-way valve 42 connects via a send line 47 with the pilot circuits of two of the shut-off valves 7b and 8b; the fourth port of the valve connects via a further send line 48 with the pilot circuits of the shut-off valves denoted 7a and 8a. The pilot circuit of the remaining shut-off valve 9 connects via an additional send line 49 with the third port of the three-way valve 41. In the embodiment of fig 11, the third port of the four-way valve 42 connects by way of the first send line 47 with the pilot circuits of valves 7a and 8a; the fourth port of the valve connects via the second send line 48 with the pilot circuits of valves 7b and 8b. The pilot circuit of the remaining shut-off valve 9 connects via an additional send line 49 with the third port of the three-way valve 41, as in the previous embodiment. The leg denoted 22b which branches from the sensing line 22 incorporates a check valve 44a allowing flow of hydraulic fluid, hence of the pressure signal, toward the four-way valve 42. 44 denotes a pressure balancing valve that connects the one leg 22a of the branched sensing line 22 with a point on the remaining leg 22b between the check valve 44a and the four-way valve 42. Biased into the normally closed position by a spring, the valve 44 is obliged to close by pressure through the one leg 22a, and piloted to open by pressure through the remaining leg 22b. The pressure balancing valve 44 is set in such a way as to connect the two legs 22a and 22b of the sensing line 22 with one another whenever the difference in pressure between the two is marginally greater than the difference between the respective pressure levels that produce shift of the four-way load sensing valve 42 and the three-way load sensing valve 41 from first to second position. The purpose of the valve 44 in question is to restore normal conditions in the limiting device once both sensing valves 41 and 42 have been piloted into second position, and will become clear in due course. Return of the fourway valve 42 to the first position is enabled only after the three-way valve 41 has been returned to first position. 22c denotes restrictions on the sensing line 22 that perform the identical function to those denoted 23 in fig 1. The embodiment of fig 10 comprises a normally closed manual unloading valve 43 which connects the sensing line 22 to tank, and performs the identical function to that denoted 20 aforedescribed. Sensing means in the embodiment of fig 12 are the same as those illustrated in fig 10, with the exception that the manual unloading valve 43 is dispensed with; valves and connections serving the second and third cylinder circuits 5a-5b and 6a-6b likewise remain the same. The line denoted 4b is connected directly to the rod end of the first cylinder 4, and exhausted to tank by a relief valve 60 whenever pressure through the line itself rises beyond the relative setting; the relief valve 60 is set to operate independently of the limiting device. Shut-off valves 7a and 7b are connected to the rear end line 4a in series, and a further relief valve 61 downstream of the second valve 7b will exhaust the rear end to tank whenever pressure through the line 4a rises above the valve setting; in this instance, the valve 61 is set to a pressure marginally in excess of that reflecting maximum rated lift capacity of the crane. In this embodiment, the pressure signal that pilots the valves denoted 7a and 8a is supplied through the second send line 48, whilst that piloting the valves denoted 7b and 7b is supplied through the first send line 47. The signal which pilots the remaining shut-off valve 9 continues to be supplied via the third send line 49. Finally, 63 denotes a second three-way two-position valve maintained normally in the first position, in which first and third ports are connected; the valve is piloted into its second position by pressure flow from the pump 14, thereby connecting the second and third ports, whenever there is a rise beyond the maximum pressure envisaged during retraction of the cylinder rods, hence descent of the boom. Retract pressure is easily quantifiable, corresponding as it does to descent of the boom with no load, and in any event will be considerably lower than the pressure level reflecting maximum rated lift capacity of the crane. The three ports of the valve 63 are connected thus: first port to tank; second port direct to the pump 14; and third port to a pilot line 64. Pressure registering through the pilot line 64 will be transmitted to the four-way sensing valve 42, on which it impinges in the same direction as the force exerted by the spring 46 and thus assists in biasing the valve 42 into first position. Operation of the load limiting device in its various embodiments will now be described. As long as a load on the hook remains within maximum rated lift capacity, all circuits operate in entirely straightforward fashion, inasmuch as by shifting any one of the control valve spools 15, the corresponding rear and rod end circuit shut-off valves 7a, 7b, 8a, 8b and 9 will open and admit flow under normal conditions. In such conditions, the valves 10 and 10′ in the embodiments of figs 1 and 2 will be normally closed. Supposing that maximum rated capacity is reached in the configuration denoted A in fig 1, then the pressure signal deriving from the rear end of the first cylinder 4 will open the valve denoted 10, connecting the pump 14 to the first send line 11, in which a hydraulic control signal is duly set up. Needless to say, in the event of there being two load sensing valves 10 and 10', the control signal will be set up by whichever valve opens first. The control signal acts directly on the telescoping shut-off valve 9, preventing it from opening even though the relative control valve spool may be moved into the position whereby fluid is directed through the respective line 6a; clearly, any further extension of the telescoping cylinder 6 would increase the load, regardless of the angle A or B. No load limiting components whatever are incorporated into the rod end line 6b of the cylinder 6 in question, since normal retraction reduces the load automatically. The control signal is fed through the first send line 11 and into the level-sensing valve 25, the ball 26 of which blocks the second send line 12 and opens up the third send line 13, given the position of the boom. The third send line 13 being open, the control signal impinges on the shut-off valves 7b and 8b installed on the rod end lines to the first and second cylinders 4 and 5; pressure reaching the rod end of either cylinder at this point would turn the relative boom stage toward the ground and increase the load beyond the permissible limit. The valves 7a and 8a controlling the lines to the rear end of the two cylinders 4 and 5 are in receipt of no such load limiting signal, and shift of the relative control valve spools will direct fluid to the service as normal; ingress of fluid into the rear end will in fact raise the boom, reducing the load. Assuming the boom to be in the B position of fig 1, illustrated in broken line, and lifting to maximum rated capacity, the valve denoted 10 will once again be opened by the signal originating from the rear end of the first cylinder 4; as far as the telescoping cylinder 6 is concerned, the same situation obtains as described above. In the case of the hinged cylinders 4 and 5, the control signal is now transmitted to the valves denoted 7a and 8a by way of the second send line 13, since with the boom positioned at B, the third send line 13 will be blocked by the ball 26 of the level sensing valve 25. Pressure is thus prevented from reaching the rear end of either cylinder 4 and 5, inhibiting upward rotation of the boom that would increase the load; conversely, the ingress of fluid under pressure into the rod ends of the cylinders 4 and 5 is permitted, enabling downward rotation of the boom and reduction of the load. Once the load condition returns below maximum rated lift capacity, the two-way sensing valve 10 closes, and pressure in the send lines is exhausted via the restrictions in readiness for further operation of the entire set of load limiting valves. Should the piston of the first cylinder 4 happen to reach stroke limit, one has an appreciable rise in pressure on the rear end without maximum rated lift capacity being registered, causing the load sensing valve 10 to open and trigger operation of certain of the shut-off valves 7a, 7b, 8a, 8b or 9; the result is that the system could cease to operate. To prevent such a situation from arising, one has incorporation of the manual unloading valve 20 which connects the sensing line 22 to tank for as long as is necessary to reduce pressure through the line and allow the load sensing valve 10 to close. In order to avoid difficulties that could arise from accidental jamming of the unloading valve 20 in open position, a spring-loaded check valve 30 is installed upstream of the manual facility; this will close whenever the difference in pressure between inlet and outlet ports of the valve is of an order greater than the bias spring setting. In the case of the embodiment of figs 4 & 5, arrival at maximum rated lift capacity will once again cause the load sensing valve 10 to open, setting up a control signal through the first send line 11. The effects remain the same as before as far as regards the telescoping cylinder 6 and relative shut-off valve 9. In the embodiment of fig 4, the control signal will be relayed to one or other of the two second stage shut-off valves 8a or 8b, depending on the angle of the second boom stage 3 and the corresponding state of the level-sensing valve 25. In the position shown in the drawing, which corresponds to the diagram of fig 9, the signal will be transmitted through the third send line 13 to the rod end valve 8b, thereby disallowing downward rotation of the boom stage 3 and preventing resultant increase of the load; no such signal will impinge on the rear end valve 8a, since upward rotation of the same stage 3 will automatically reduce the load and is thus permissible. Where the first hinged stage 2 of the boom is concerned, it will be observed that the position shown in fig 4, which corresponds to the diagrams of figs 8 and 9, is such that downward rotation of the stage would produce an increase in the load; accordingly, the movement in question is inhibited by the switching valve 52, the first and second ports of which are connected and thus relay the control signal to the rod end shut-off valve 7b, disallowing descent. Supposing, on the other hand, that the boom is in either of the configurations illustrated in figs 6 and 7, it will be seen that on arrival at maximum load, the control signal set up by the load sensing valve 10 will be relayed via the switching valve 52 to the rod end valve 7b, whereas the rear end valve 7a remains in the enabling condition. With pressure flow thus directed through the rear end line 4a, the first stage 2 lifts, producing an increase in load beyond the permissible limit; in response to such a situation, pressure impinging on the rear end of the cylinder 4 rises beyond the load limit setting and shifts the valve denoted 10″ into second position. The result is that a signal is set up through the fourth send line 51, and the switching valve 52 is piloted into the position in which its first and third ports are connected; the control signal from the first sensing valve 10 is now directed to the rear end shut-off valve 7a, inhibiting further upward movement of the boom while allowing the descent which will automatically mitigate load conditions. In short, whatever the configuration of the column and boom, the load limiting device prevents overload on the crane at the moment that its maximum rated lift capacity is either matched or exceeded. It will be obvious to a person skilled in the art, however, that a load in excess of maximum rated capacity can be avoided altogether simply by setting the one load sensing valve 10" at a pressure to match the maximum load, and setting the other 10 at a level marginally below this same pressure. Once a return within the maximum rated capacity of the crane has been effected using such movements as are enabled by the load limiting device (retraction of the telescopic stage 3, for instance) the sensing valves 10″ and 10 will reassume closed position, and control signal pressure through the send line 11 and the various passages leading to the shut-off valves 7a, 7b, 8a, 8b and 9 will exhaust via their relative restrictions, returning the device to its original configuration in which all movements are enabled. It will be observed that the switching valve 52 returns to first position only when the valve denoted 10 has likewise been returned to closed position, since this is the only situation in which pressure can be exhausted from the send line 11. In short, the switching valve 52 memorizes second position until otherwise instructed by a return to normal operating conditions. In the third embodiment of the device illustrated in fig 5, arrival at maximum rated lift capacity causes the signal set up by the load sensing valve 10 to be transmitted direct to the valves denoted 7b and 9, and neither downward rotation of the first stage 2 nor telescoping of the second stage 3 are allowed, whatever their position; downward rotation of the second stage 3 will be inhibited only where the boom is angled above horizontal as in fig 9. In the configuration of fig 9, in fact, the level-sensed control signal will be relayed via the send line 13 to the valve denoted 8b, which blocks pressure flow to the rod end of the relative cylinder 5. In configurations where the second boom stage 3 is angled below horizontal, as in figs 6, 7 and 8 for instance, the relative valve 8b will be in receipt of no control signal by reason of the fact that the level-sensing valve 25 blocks the send line 13, and downward rotation is therefore permitted. In the case in point, upward rotation of the stage 3 will be disallowed only where the movement is such as to produce an increase in load, reflected by a build-up of rear end pressure in the relative cylinder 5 and consequent operation of the relief valve 58. The same principle applies where upward rotation of the first stage 2 is concerned. Any situation where such movement produces an increase in load (as in figs 6 and 7, for instance) will give rise to an excessive build-up of pressure in the rear end of the relative cylinder 4, hence through the line 4a, with the result that the respective relief valve 57 will operate and prevent further pressurization of the cylinder Likewise in this instance, a return to within the maximum rated lift capacity causes the load sensitive valves 10, 57 and 58 to close, thus allowing a return to normal operating conditions. Referring to the embodiment illustrated in fig 10, when one has a load on the hook slightly less than the maximum permitted, then pressure building up in the rear end of the first cylinder 4 will reach the preset level which causes the three-way load sensing valve 41 to shift from first to second position; it will be remembered that rear end pressure is proportional to the load in substantially linear fashion. The third port of the valve 41 in question, which is connected via the third send line 49 to the pilot circuit of the telescoping shut-off valve 9, and via the second send line 48 to the pilot circuits of the first and second cylinder rear end shut-off valves 7a and 8a, is now connected to the pump 14; pilot pressure is thus relayed to these valves 7a and 8a and 9, which close and disallow passage of fluid under pressure. Extension of the three cylinders 4, 5 and 6 is now inhibited, whereas retraction is enabled. In the event that the configuration assumed by the crane is such that retraction of the cylinders 4, 5 and 6 reduces the load, then the boom will remain enabled exclusively for such movement, and normal operating conditions will be restored; it will be remembered that retraction of the telescoping stage cylinder 6 has the effect of reducing the load in any given configuration of the boom. In the event, on the other hand, of the boom configuration being such that retraction of the two hinged cylinders 4 and 5 produces an increase in the load, then selection of such movements will cause a build-up of pressure in the rear end of the cylinder 4, resulting in the set-up of a signal which is relayed to the four-way load sensing valve 42; with pressure ultimately at the level which matches maximum rated lift capacity, the valve 42 shifts from first to second position. In this situation, the pilot circuit's of the first and second cylinder rear end line shut-off valves 7a and 8a are exhausted to tank, allowing pressure to the service, whereas the rod end line valves 7b and 8b will be piloted to close, and thus positioned to disallow pressure to the service. The end result is that one achieves an inversion of the enabled boom movements, and the cylinders may stroke only in the direction which has the effect of reducing load. With a reduction in load, pressure in the rear end of the cylinder 4 will drop, and the three-way load sensing valve 41 is returned to first position. The four-way valve 42 will remain in second position by reason of the fact that the check valve 44a prevents pressure exhausting from the relative leg 22b of the branched sensing line 22; regular movement of the boom is in no way impeded, however, since with the three-way valve 41 once more in first position, all ports of the four-way valve 42 can exhaust to tank. A further reduction in load produces a further drop in pressure through the leg of the branched sensing line denoted 22a, and once the difference in pressure between the two branches 22b and 22a reaches a preset level, which at all events will be higher than the difference between the respective pressure levels causing shift of the three-way and four-way sensing valves 41 and 42 from first to second position, then the pressure balancing valve 44 will open so as to exhaust pressure from the leg denoted 22b and enable the four-way sensing valve to return to first position. The expression, adopted throughout the description, that the crane is returned to "normal operating conditions", signifies the condition in which a load on the hook remains within maximum rated lift capacity of the crane, and thus implies the return of the load limiting device to its initial, non-operative configuration. Operation of the embodiment illustrated in fig 11 is identical to that of the embodiment in fig 10, with the sole difference that a shift in position of the three-way load sensing valve 41 inhibits retraction of the hinged cylinders 4 and 5, whereas shift of the four-way valve 42 will inhibit extension. In the case of the telescoping cylinder 6, clearly enough, retraction will be enabled and extension inhibited whenever the load limiting device is operative. In contrast to the device as illustrated in fig 10, there is no requirement in the device of fig 11 for an unloading valve 43 -viz, should it happen that the cylinder 4 reach stroke limit and thus trigger operation of the load sensing valves 41 and 42, retraction of the cylinders remains enabled just the same, and normal operation of the device can be restored. In the sixth embodiment of the device illustrated in fig 12, operation remains identical to the embodiment of fig 10 as far as regards the second and third cylinder circuits 5a-5b, 6a-6b and relative shut-off valves 8a, 8b and 9; operation of the sensing means is also identical, save for the inclusion of the valve denoted 63 which is explained below. Flow is permitted in either direction through the rod end line 4b of the first cylinder, any excess pressure being exhausted via the relief valve 60. Flow to and from the service through the rear end line 4a is controlled by the valves denoted 7a and 7b, respectively; accordingly, shift of the threeway and the four-way load sensing valves 41 and 42 to second position disallows flow to the service and from the service, respectively, any excess pressure through the line 4a being exhausted by the relief valve 61. With such a circuit, it becomes possible to inhibit extension and retraction of the cylinder 4 according to requirements, with pressure remaining available to the service through the rod end line 4b. This is a feature bringing two marked advantages. First, in the instance of pressure being supplied through the rod end line 4b and maximum load being reached, limiting action is achieved by disallowing the rear end line 4a to exhaust, thereby avoiding a sudden pressure drop through the rod end line 4b and preventing any instability occasioned by sudden loss of the rear end pressure required for piloted operation of the load holding valve 4c; this pressure, it will be remembered, is conditioned by dimensional factors deriving from construction of the cylinder. Clearly enough, if pressure is cut off from the rod end line 4b by shifting the relative control valve spool 15, the holding valve 4c closes and the system will lock up until the load returns within maximum rated lift capacity, and the limiting device can thus be released. The second advantage is that the unloading valve 43 of fig 10 becomes unnecessary. With arrival of the cylinder 4 at its stroke limit, excess pressure becoming trapped in the stretch of line 4a connecting the cylinder with the holding valve 4c (pressure which is exploited for operation of the load sensing valves 41 and 42) can be relieved by connecting the free rod end line 4b with pressure; the load holding valve 4c can thus be piloted to open, and the excess pressure relieved. Inclusion of the valve denoted 63 provides a further significant advantage. In the event that circuit pressure should rise beyond the maximum pump rating calculated to occur during retraction (the situation in question can arise only during extension, clearly enough), the valve 63 will be shifted into second position, thereby relaying a signal to the four-way load sensing valve 42 that prevents its moving from first to second position under any circumstance. More exactly, the valve 63 remains inoperative as long as the cylinders are retracting, whereas on the extending stroke, the moment that maximum rated lift capacity is reached, pressure through the two lines denoted 4a and 5a will be disallowed regardless of other conditions. This will prevent a particularly skilled operator from juggling the control valves 15 in such a way as to produce pressure surges capable of shifting the four-way load sensing valve 42, and thus managing to supply additional pressure through either line 4a or 5a, the result of which would be to overload the crane beyond maximum rated capacity. Embodiments of the device as illustrated in figs 10, 11 and 12 provide the considerable advantage of requiring no boom level-sensing system for their correct operation. This represents a particularly welcome feature in crane hydraulics when one considers that the installation of level-sensing facilities, which must of course be applied direct to the boom, involves major complication of the system.

Claims: 1. A hydraulic load limiting device for hydraulic cranes, of the type fitted to cranes having hinged and/or telescopic boom stages (2, 3) rotated and extended by relative hydraulic cylinders (4, 5, 6), each of which actuated by a directional control valve (15) of conventional type that connects with the cylinder by way of a respective rod end line (4b, 5b, 6b) and rear end line (4a, 5a, 6a) which serve alternately as pressure and return lines to and from the service, and comprising sensing means designed to pick up a pressure signal from the rear end of the hydraulic cylinder (4) supporting the entire hinged and telescopic boom assembly and convert it into a control signal for the operation of shut-oft means, installed on the lines connecting each control valve with the relative cylinder, which serve to disallow passage of hydraulic fluid under pressure to the service such as would overload the crane beyond maximum rated lift capacity each of the shut-off means comprises a two-way two-position hydraulic valve (7a, 7b, 8a, 8b, 9) the first position of which disallows pressure flow through the relative line controlled whilst freely allowing return flow, the second position allowing flow in either direction, which is urged into second position by pressure of the fluid in the upstream stretch of the line controlled, biased toward the first position by a respective spring (17a, 17b, 18a, 18b, 19), and piloted into the first position by the control signal; characterized in that the sensing means comprise: a three-way two-position valve (41) biased by a spring (45) into the first position in which first and third ports are connected, and piloted into the second position, in which second and third ports are connected, by a pressure signal of intensity marginally below that which reflects maximum rated lift capacity of the crane; a four-way two-position valve (42) biased by a spring (46) into the first position, in which the first port connects with the third and the second port connects with the fourth, and piloted into the second position, in which the first port connects with the fourth and the second port with the third, by the same pressure signal as pilots the three-way valve (41), whenever the signal reaches a pressure level reflecting maximum rated lift capacity of the crane; and in that the ports of the three-way valve (41) are connected: first, to tank; second, to a pump (14); and third, to the second port of the four-way valve (42), and the ports of the four-way valve (42) are connected: first, to tank; second, to the third port of the three-way valve (41); third and fourth, to respective send lines (47, 48); and the pilot circuit of the third stage shut-off valve (9) connects via a further send line (49) with the third port of the three-way valve (41). 2. A device as in claim 1, wherein the one send line (47) connects with the pilot circuits of the cylinder rod end shut-off valves (7b, 8b) and the other send line (48) connects with the pilot circuits of the cylinder rear end shut-off valves (7a, 8a). 3. A device as in claim 1, wherein the one send line (47) connects with the pilot circuits of the cylinder rear end shut-off valves (7a, 8a) and the other send line (48) connects with the pilot circuits of the cylinder rod end shut-off valves (7b, 8b). 4. A device as in claim 1, the sensing means of which comprise: a sensing line (22) branched into two legs (22a, 22b) that carry pressure- signals to the three-way valve (41) and to the four-way valve (42) respectively; a check valve (44a) installed on one leg (22b) of the branched sensing line, allowing flow of hydraulic fluid toward the four-way valve (42); a pressure balancing valve (44) that connects the one leg (22a) of the branched sensing line with a point on the other leg (22b) between the check valve (44a) and the four-way valve (42), whenever the difference in pressure through the two legs registers marginally above the difference between the pressure level that produces shift of the four-way valve (42) from first to second position and the pressure level that produces shift of the three-way valve (41) from first to second position. 5. A device as in claim 1, comprising a manually operated unloading valve (43) installed in normally closed configuration between sensing line (22) and tank. 6. A device as in claim 1, wherein the sensing line (22) is fitted with at least one restriction (22c). 7. A device as in claim 1, wherein the rod end line (4b) is connected direct to the relative cylinder (4), and shut-off valves (7a, 7b) are connected in series to the rear end line (4a) of the same cylin- der; wherein the one send line (47) is connected to the pilot circuits of two of the shut-off valves (7b, 8b) and the other send line (48) is connected to the pilot circuit; of a further two shut-off valves (7a, 8a); and wherein a relief valve (61) installed downstream of one'of the shut-off valves (7b) exhausts the rear end line (4a) to tank whenever pressure through the line rises marginally above a level reflecting maximum rated lift capacity of the crane. 8. A device as in claim 7, comprising a second three-way two-position valve (63) maintained normally in the first position, in which first and third ports are connected, and piloted into the second position by pressure flow from the pump (14), thereby connecting the second and third ports, whenever there is a rise in pressure beyond the maximum intensity envisaged during retraction of the cylinder rods; wherein the three ports of the valve (63) are connected: first port to tank; second port direct to the pump (14); and third port to a pilot line (64); and wherein pressure registering through the pilot line (64) is transmitted to the four-way valve (42), on which it impinges in such a way as to bias the valve (42) into first position.

Inventors: STORCI, ANDREA
Applicants: OIL CONTROL S.P.A.
Application year: 1986
Main classification: B66C23
IPC codes: B66C23