The present invention concerns a hydraulic boom crane of the kind including a boom pivotable relative to a base about one end in a vertical plane by a hydraulic main lifting cylinder, at least one signal emitter in the form of a pressure transducer being associated with the main lifting cylinder and connected to a control unit such that any loading exceeding the nominal loading causes said control unit to inhibit operating functions of the crane.
The nominal load of cranes is determined with a certain safety margin depending on how great a moment may be absorbed by the turning point of the crane, i.e. the bearing supporting the crane, and which then is transmitted to the support of the bearing, e.g. the chassis of a lorry, and in that connection to possibly extended supporting legs without the crane tipping over.
The known load control systems for use in hydraulically driven cranes are of the kind mounted as accessories on e.g. a lorry or definite mobile cranes. The known load control systems may comprise at least one or more pressure transducers provided on the hydraulic cylinders of the crane. The pressure transducers are connected to a control unit connected with controlable valves in connection with the control valve block of the crane. A such load control system is known from EP 0 708 053 A1, where the control unit is arranged in such a way that it by a registered maximum allowable pressure registered by a sensor placed on the main cylinder of the crane blocks for all further manoeuvring of the crane by its control valve block. However, the blocking may subsequently be deactivated in a short period (3-5 sec.) by a manually controlled liberating element, so that it is possible to get out of an otherwise "impossible lifting situation". An "impossible lifting situation" may be defined in that any movement of the crane boom allowed by the control system with the intention of reducing the load will be impossible in practice because of the physical circumstances in the ongoing lift so that the only possibility of getting out of this lifting situation is a deactivation of the load control system in a short pre-determined period, wherein a forced increase of the load is possible. The said load control system also comprise acoustic and visual signal emitters coming into action at different load levels about the maximum allowable load level for the crane.
FR-A-2592368 discloses a hydraulic boom crane of the kind including a boom pivotable relative to a base about one end in a vertical plane by a hydraulic main lifting cylinder and including at least one relevant signal emitter connected to a control unit which in case of loading the crane in excess of its nominal load produces limitations on the functional capability of the crane whereby the relevant signal emitter consists of a pressure transducer connected to the control unit arranged in such way that in case of a transgression of the nominal loading established for the crane in question will produce a marked or total functional limitation.
DE-A-4219369 discloses a crane with a load control system, which, in case of loading the crane in excess of its nominal load ("erster Schwelle"), produces limitations on the functional capability of the crane in a period of time determined by the control unit (limitations are cancelled by the control unit as soon as a reset-button has been actuated by the operator, after which the crane may be loaded up to an acceptable maximum ("zweiter Schwelle"), the transgression of which will cause the control unit to make the crane inoperable.
Other load control systems may furthermore comprise position indicators on the boom sections to be pivoted in a vertical plane, besides the pressure transducers on the hydraulic cylinders, and which are also connected to the control unit The signals from the position indicators are used in the control unit for indicating whether a movement of the pivotable parts of the boom in a desired direction will cause an increased load on the crane. The control of the load with position indicators works in the way that the control unit at a registered maximum allowable load of the crane (the nominal load) ensures that the operator of the crane cannot perform load increasing operations as the control unit at the activation of the previously mentioned controlable valves is hindering such movements of the boom. However, the control unit allows all movements of the boom that by experience cause an immediate reduction of the moment load in proportion to the maximum allowable moment load of the crane, e.g. a reduction of the distance between the load and the turning point of the crane in an operation wherein the length of the boom is reduced.
Still, experience shows that crane operators almost always and with success try to circumvent the known load control systems. An example of this is that an operation allowed by the load control system that usually cause a load reduction is utilized for increasing the moment load of the crane beyond the allowed level. This will by example be possible by performing a reduction of the boom length in connection with an "unallowable" lift whereby the load is dragged across the ground over at the beginning and then is lifted off the ground. Such a lift will also cause a very disadvantageous and destructive loading of the support bearing of the crane.
The load control systems described above are relatively costly since they often comprise up to several pressure transducers and one or more position indicators that by wired connections are connected to the control unit, which again are connected to the control valves in the control valve block. Looking at service procedures the large number of components and wire connections become expensive.
Common to the known load control system is, however, that they in their own individual way in a rather abrupt way deactivates the functions of the crane completely or partly at the exceeding of the maximum allowable load on the crane, though after a preceding warning from acoustic or visual signal emitters also activated by the control system. However, the known load control systems easily permit the operator to exceed/override the deactivation of the manoeuvrability of the crane in connection with overload situations which inevitably leads to some indifference towards the load control system, since exceeding the maximum allowable load of the crane do not have any consequence as long as operations take place with more or less virtuosity in the alarm range and with an intrepid use of the said deactivation period preceding a total blocking of the control system. The deeper meaning with the alarm is to call the operator's attention to the fact that operation takes place in a potentially dangerous area and that measures for a return to harmless conditions immediately has to be taken, e.g. that the lorry is driven to a position closer to the burden to be lifted. The operator may then decide to try anyway from the consideration that in the said deactivating period he may come in time to discontinue the alarm creating act if the transgression is so great that it downright may give rise to an ultimate blocking signal.
This practice results in appreciable problems that may be read from different accident statistics. The crane manufacturers may decide to place the alarm range in a safer distance from the ultimate border, but this will inevitably raise the cost of the crane in proportion to its capacity. An optimum relationship between price and capacity highly presupposes that the operators respect the alarm range according to its purpose and do not make experiments instead by pressing the crane extremely, causing danger for themselves, the machines and other persons in the close surroundings.
The problem in question is thus not of a purely technical nature, but also involves educational and moral considerations, since previous solutions to the problem have always depended on operator discipline. It has to be acknowledged as a regrettable fact that the time has not yet come for a reasonable expectation of such self-discipline as several other reasons make it attractive for the operators to press the crane to the limit exist, even in already extreme situations and in spite of the hazards belonging to them.
In the present invention it has been recognized that it is possible by technical means to support a more reliable practice of operation, thus avoiding otherwise superfluous overdimensioning or capacity limits on hydraulic boom cranes. This is achieved according by the invention in that the operating functions inhibited by the control unit comprise substantially all operating functions of the crane, whereby the functions remain inhibited for a delay interval, hereinafter designated the "penalty period", the duration of the penalty period imposed at each succeeding occurrence of excess loading being determined by the control unit according to past loading conditions and whereby if the control units detects a loading level exceeding an absolute maximum limit of the crane, all further operation of the crane is disabled.
The invention thus involves a kind of psychological counteraction on the part of the crane against an operator trying to exceed the capacity of the crane into its hazard area. Such a counteraction may be established according to the invention by letting the crane equipment itself produce a capacity reduction of a first or lower safety limit, that is to say by a not very technical but psychological and educationally effective "penalty" to the operator in the form of a more or less total blocking of the crane functions during a certain period of time, e.g. if the operator does not react in an expectedly suitable way after the alarm has been activated the first time, namely by returning the system to a not alarm giving state.
By giving the operator such a time "penalty" with respect to the use of the crane two important aspects will appear, namely that the operator has to reckon with the impossibility of desired capacity gain by using a slightly or heavily overloaded crane, for example because the time was better used in driving the lorry closer to the burden to be lifted, and that the same operator has to face the surroundings' observation that his heroic struggle with the crane to press it to the outermost simply result in stoppage while maybe an alarm signal audible for the whole congregation is sounded. Hereby the operator has to accept a loss of both crane capacity and personal prestige, and both will act in the direction that the operator will keep to the instructions of the crane manufacturer concerning a temporary operative overloading of the crane.
Still, there will exist a few operators who want to test the limits of such a system with the anticipation of accepting a short-term "penalty" at a moderate overload, if it is possible then to continue the work with a greater overload. According to the invention this may be counteracted by producing increasing blocking periods for all crane functions for a correspondingly increasing degree of such overload, whereby the operator soon will experience that any attempt to a further increase of the crane capacity will result in that the same capacity is lessened appreciably because of increasing periods of total blockage of the crane functions. In this way the operator has nothing to gain by his attempts to circumvent the security system, while he at the same time by such ineffective circumventing attempts becomes exposed to scorn from the surroundings eo ipso that the crane is standing still after the sounding of an alarm signal.
Several signal levels may then be introduced by which there is produced increasing "penalty periods", so that the operator will experience a continuously increasing passivity of the crane the more he tries to overload it. At the same time, however, consideration is given to the operator so that the final total blocking of the crane only takes place in quite extreme cases, where the operator in the course of time has had to accept a series of increasing penalty periods and therefore has worked in a quite inefficient way.
By the invention it has further been realized that for the surveillance of the moment load of the crane it is hereby in reality only necessary to watch the pressure on the main cylinder of the crane supporting the main boom of the crane, since this pressure is an objective measure for the moment acting on support of the crane when using it. Thereby it is possible to indicate a new and more simply arranged load control system that is cheaper in acquisition as well as in service as compared to the known load control systems, and which in addition gives the operator of the crane more degrees of freedom during special lifting situations and a better feeling of the maximum load of the crane.
By the invention there is thus indicated a new load control system comprising a pressure transducer situated on the main lifting cylinder of the crane and connected to a control unit with the output connected to a by-pass valve, also called "dump valve", controlling the oil supply to the manoeuvring valve block of the crane, which control unit is arranged in such a way that the transgression of the nominal load capability of the crane in question will result in activation of the said "dump valve", so that all the functions of the crane are made inactive in a period of time calculated by the control unit, a so-called penalty period, whereafter the dump valve is deactivated.
Evidently, the load control system according to the invention does not give any immediate possibility for the crane operator to rectify a transgression of the nominal load of the crane since it is not possible to relieve the load as long as the functions of the control valve block of the crane are deactivated during the penalty period.
After the expiration of the penalty period that may be rather short at the first transgression the control unit deactivates the dump valve and the crane becomes manoeuvrable again. If the load on the crane is reduced to a level below the nominal load nothing further will happen, i.e. the work may continue freely.
With the purpose of giving the operator a possibility of remedying any arisen impossible load situations the control unit is arranged in such a way that it allow a further load on the crane to a calculated level (within the safety margin) in excess of the nominal load after the expiration of the penalty period.
If this calculated level for the load is exceeded the dump valve is activated again by the control unit, this time in an perceptibly longer period of time than the first activation. After the expiration of this so-called penalty period the dump valve is deactivated again.
If the load on the crane is reduced the control unit will continuously calculate a new, lower value for the maximum allowable load and penalty period in case of a transgression concurrently with the relieving of load until the nominal load is reached. However, if the operator of the crane tries to increase the load during the relieving the control unit will immediately release a new penalty period.
The load control according to the invention may be programmed with several limits of load levels between the nominal load for the crane and an absolute acceptable maximum within the safety margin. Transgression of the individual load levels/limits will release an increasing penalty period concurrently with the magnitude and number of transgressions.
In order to give the operator the possibility of following how much the crane is loaded in relation to the nominal load the cabinet of the control unit may be provided with a percent graduated scale with light diodes that are switched on and off concurrently with a load increase and decrease, respectively.
In addition, for warning the operator of the crane as well as persons staying within the hazard area of the crane about the nominal load being reached or exceeded, the control unit may also be provided with an acoustic alarm emitting a periodic alarm signal at an existing load between 95% and 100%. During an imposed penalty period at the transgression of the nominal load (100%) a continuous signal is given. Therefore, the operator has to accept that his overloading of the crane is made widely known.
Furthermore, the control unit may be provided with a logging unit that is registering data in a ring buffer memory for all load transgressing actions with the crane, so that data concerning the latest load transgressing events afterwards may be read from the ring buffer memory on e.g. a hand terminal, and that is registering data concerning the largest registered overloads in another data memory, the latter also being readable on a hand terminal.
The load control system described in the above being more simple in arrangement as compared to the known load control systems contains thus some increased degrees of freedom and important educational principles.
The increased degrees of freedom consist in that the operator at the transgression of the nominal load up to an absolute allowable maximum limit within the safety margin and after the expiration of penalty period has the possibility of getting out of an "impossible" lifting situation by utilizing all of the manoeuvring functions of the crane, even with a short-term, smaller load increase allowed after the expiration of a penalty period.
The educational element consist in that the operator of the crane will be inclined to learn a better perception of the load situations implying transgression of the nominal load of the crane, because the penalty period connected with a transgression of the nominal load will slow down the work procedures in connection with the use of the crane and thereby for the operator as well as for the person or persons cooperating in the work within the hazard area of the crane.
The invention is explained in more detail in the following with reference to the drawing, where:
In Fig. 1 there is shown a hydraulic crane 2 intended for mounting on a vehicle, for example a lorry. The crane comprise a basic frame 4 for mounting on the chassis of the lorry. The basic frame has also laterally extendable girders 5 at the ends of which vertically extendable legs 6 are provided. On the basic frame 4 the column 10 of the crane is supported by the bearing 8. In addition, the bearing carries the support for the main cylinder 12 of the crane that is connected to the main arm 14 of the crane. In continuation of the main arm 14 the bending arm 16 of the crane is provided and in this a number oftelescopic arms 18.
On the main cylinder 12 of the crane there is provided a pressure transducer 20 which by a wire connection 21 transmits an oil pressure signal to a control unit 22. As a consequence of the very quickly varying oil pressures appearing in the main cylinder 12 of the crane and which at the transmission to the control unit will cause "signal confusion", the control unit 22 is provided with a not shown signal receiving block performing filtering, stabilization and compensation of the received oil pressure signal from the pressure transducer 20 before the signal is transmitted to the control unit for further processing.
The control unit is connected to a by-pass valve 24, in the following called dump valve that is placed on the oil supply pipe for the manoeuvring valve block 25 of the crane, through a wire connection 23, so that the dump valve in its active position will cause that the oil in the supply pipe is returned directly to the hydraulic oil tank through a not shown by-pass pipe and around the manoeuvring valve block 25 whereby manoeuvring of the crane is not possible. In the inactive position of the dump valve the hydraulic oil is supplied to the valve of the manoeuvring valve block whereby it is possible to manoeuvre the crane.
The control unit 22 comprise a cabinet the front of which is furnished with a number of bulbs 28 placed in conjunction with a percentage scale, one bulb 30 placed by the word "run", an acoustic signal emitter 32, and a stop switch 34.
The shown scaling indicate the existing load on the crane proportional to the nominal load of the crane in the range between 80 and 100%. At a load below 80% of the nominal load only one bulb 30 opposite the word "run" is lit. At a load on the crane within the range 80-100% the bulbs are lit concurrently with the increased load so that the operator has a possibility of following the development of the existing load of the crane. At a load over 95% of the nominal load the control unit activates the acoustic signal emitter 32, which emits a periodic signal. At a load on the crane corresponding to 100% or more the control unit cause a continuous signal to be sounded from the emitter 32 during the said penalty period.
The mode of operation of the load control system is explained in the following with basis in the functional example for the load control system shown in Fig. 2, which shows a coordinate system with abscissa indicating time and ordinate indicating the load of the crane proportional to the nominal load of the crane.
At a load in the range between 0 and 85% of the nominal load the load control system do not react.
At a load between 85 and 95% the control unit perform a gradual activation of the bulbs 28 opposite the percentage indications on the cabinet of the control unit.
At the transgression of 95% of the nominal load the control unit activate the acoustic signal emitter 32 so that it gives a periodic signal which at 100% nominal load changes to a continuous signal in connection with the running of a penalty period. At the expiration of a penalty period the signal emitter changes to the periodic signal indicating that the crane functions are active again, but the existing load of the crane is more than 95% of the nominal load.
If the crane operator attempts to increase the load above 100% the control unit 22 activates the dump valve 24 in the short period of time calculated by the control unit, a so-called penalty period A, cf. Fig. 2, wherein the functions of the crane are paralyzed. After that the load on the crane should be reduced below nominal load capability. The load on the crane may be increased at the expiration of the penalty period A in order to give the operator of the crane the possibility to get out of an adverse lifting situation which is possible up to 102% of nominal load in the given example. If the load is increased in excess of the level determined by the control unit, the control unit activates the dump valve in at penalty period B that is longer than the first period. During this penalty period the control unit calculates a new stop level, e.g. 102.5% of nominal load, and a new and longer penalty period C coming into use if the crane load is increased more than the new stop level.
In the example shown in Fig. 2 the principle in the development of the length of the penalty period is illustrated as a function of the current load on the crane. The control unit calculates the development in the length of the penalty period in relation to the current load of the crane and the current number of attempts of additional overloading. The control unit 22 further comprises an "override" button 36 which by the operator may be activated in case of an "impossible" lifting situation that has to be rectified immediately. By activating the "override" button it will be possible within the subsequent 5 seconds to perform a smaller load increasing operation with the crane within an interval determined by the load control system, the exceeding of which causes a penalty period of 30 seconds. The "override function" is usable up to the absolutely maximum allowable load level of the crane, whereafter the load control system totally deactivates all of the functions of the crane. In that situation there may possibly be a case of a final adjusting possibility, which however require the breaking of a seal or calling of a security inspector.
It shall be emphasized that by the invention it has been possible to reduce all of the sensor equipment on the crane to the pressure transducer 20 only, but naturally it cannot be precluded that more sensors can be used.