WO2004078630A2

WO2004078630A2 - A method of operating a hoisting device and an accessory for a hoisting device

Info

Publication number: WO2004078630A2
Application number: PCT/IB2004/000603
Authority: WO
Inventors: Charles Patrick De Klerk
Original assignee: Charles Patrick De Klerk
Priority date: 2003-03-05
Filing date: 2004-03-05
Publication date: 2004-09-16
Also published as: WO2004078630A3

Abstract

The invention relates to a method of operating a hoisting device (10) having a drum (12) mounted to rotate about an axis of rotation, an elongate flexible tensile element (14), part of which is wound onto the drum (12) and at a position along the length of which an article to be hoisted is connectable to the tensile element (14), and drum drive means drivingly connected to the drum (12) for rotatably driving the drum (12) about its rotational axis. The method includes the steps of sensing a tension in the tensile element (14) and, when the sensed tension is greater than a first predetermined maximum value or less than a first predetermined minimum value, inhibiting rotation of the drum (12) about its rotational axis. The invention extends to an accessory (50) for a hoisting device (10) and to a hoisting device (10).

Description

A METHOD OF OPERATING A HOISTING DEVICE AND AN ACCESSORY FOR A HOISTING DEVICE

THIS INVENTION relates to the monitoring of tension in a tensile element under load, such as, a rope on a drum winch. More particularly, the invention relates to a method of operating a hoisting device and to an accessory for a hoisting device. It relates also to a hoisting device.

According to one aspect of the invention, there is provided a method of operating a hoisting device having a drum mounted to rotate about an axis of rotation, an elongate flexible tensile element, part of which is wound onto the drum and at a position along the length of which an article to be hoisted is connectable to the tensile element, and drum drive means drivingly connected to the drum for rotatably driving the drum about its rotational axis thereby selectively to wind the tensile element onto or unwind the tensile element from the drum, the method including the steps of sensing a tension in the tensile element at a position along the length of the tensile element; and when the sensed tension is greater than a first predetermined maximum value or is less than a first predetermined minimum value, inhibiting rotation of the drum about its about its rotational axis.

Sensing a tension in the tensile element may include deflecting the tensile element laterally, and monitoring a degree of lateral deflection of at least part of the tensile element, the degree of lateral deflection serving as an indication of the tension in the tensile element. Hence, if the tensile element is deflected laterally beyond a predetermined distance, corresponding to either of the first predetermined maximum value or the first predetermined minimum value for tension, this will be indicative that the tension in the tensile element has risen above the first predetermined maximum value or dropped below the first predetermined minimum value, respectively.

Inhibiting rotation of the drum about its rotational axis may include preventing operation of the drum drive means, eg. by interrupting a power supply to the drum drive means.

The method may include, when the sensed tension drops below a second predetermined minimum value, which is less than the first predetermined minimum value, braking the tensile element at at least one position along the length of the tensile element in order to prevent an article being hoisted from falling to the ground or other support surface.

The hoisting device may be in the form of a tower crane, a gantry (or overhead) crane, or the like.

The method may include the prior step of determining the first predetermined minimum value by sensing a tension in the tensile element when a zero-load is applied to the tensile element. The method may further include determining the first predetermined maximum value by sensing the tension in the tensile element when a load corresponding to the maximum load rating for the tensile element is applied to the tensile element.

The method may further include monitoring the length of tensile element unwound from the drum and adjusting the sensed tension to compensate for a length of tensile element unwound from the drum. According to another aspect of the invention, there is provided an accessory for a hoisting device, said hoisting device having a drum mounted to rotate about an axis of rotation, an elongate flexible tensile element, part of which is wound onto the drum, and drum drive means drivingly connected to the drum for rotatably driving the drum about its rotational axis thereby selectively to wind the tensile element onto or unwind the tensile element from the drum, the accessory including tension sensing means for sensing a tension in the tensile element of said hoisting device to which the accessory is operatively connected; and drum locking means operatively connected to the tension sensing means and configured to inhibit rotation of the drum about its rotational axis thereby preventing the tensile element from being wound onto or unwound from the drum, in response to a sensed tension rising above a first predetermined maximum value and/or falling below a first predetermined minimum value.

The drum locking means may include switch means configured to interrupt a supply of power to the drum drive means in response to a sensed tension rising above the first predetermined maximum value and/or falling below the first predetermined minimum value.

The tension sensing means may define a guide path, for accommodating a length or part of the tensile element and in which the tensile element can be deflected laterally, and may include a pair of guide pulleys positioned to contact the tensile element at longitudinally spaced positions and a deflector pulley which is positioned to contact the tensile element intermediate the guide pulleys and to deflect the tensile element laterally, the deflector pulley being urged towards a rest position by resilient bias means and being displaced away from its rest position against the bias means by the tension in the tensile element. In another embodiment, the tension sensing means defines a guide path, for accommodating a length of the tensile element and in which the tensile element can be deflected laterally, and includes a load cell-type arrangement, including a cantilever beam element located adjacent to the guide path and connectable to the tensile element at one end thereof and contactable with the tensile element at its opposite end such that the cantilever beam element urges the tensile element laterally and is deflected on lateral displacement of the tensile element, the load cell-type arrangement further including a transducer for converting a load acting on the beam element on deflection thereof by the tensile element into an output electrical signal.

The accessory may include processing means configured to condition and/or amplify the output electrical signal of the tension sensing means. In one embodiment, the processing means is configured to convert the output electrical signal from analogue to digital format. The processing means may include a comparator for comparing the output electrical signal to predetermined threshold values corresponding to each of the first predetermined maximum value for tension and the first predetermined minimum value for tension, the processing means being configured to transmit a switching signal to the drum drive means when the sensed tension is greater than the first predetermined maximum value and/or less than the first predetermined minimum value.

The switch means may further be operatively coupled to brake means for braking the tensile element at at least one position along the length of the tensile element in response to a sensed tension falling below a second predetermined minimum value, which is less than the first predetermined minimum value.

The accessory may be operatively connected to a hoisting device including a drum mounted to rotate about an axis of rotation, an elongate flexible tensile element, part of which is wound onto the drum and at a position along the length of which an article to be hoisted is connectable to the tensile element, and drum drive means drivingly connected to the drum for rotatably driving the drum about its rotational axis thereby to wind the tensile element onto or unwind the tensile element from the drum, the tension sensing means being mounted along the length of the tensile element and defining a guide path along which the tensile element passes and in which the tensile element can be deflected laterally, and the switch means being connected to the drum drive means to interrupt operation of the drum drive means in response to lateral deflection of the tensile element beyond a predetermined minimum distance which is indicative of an increase in the tension in the tensile element above the first predetermined maximum value or a decrease in the tension in the tensile element below the first predetermined minimum value.

The hoisting device may further include brake means mounted along the length of the tensile element and defining a passage through which the tensile element passes and in which the tensile element can be braked, the switch means being connected to the brake means to brake the tensile element along its length in response to a sensed tension falling below the second predetermined minimum value.

According to still another aspect of the invention, there is provided a hoisting device which includes a drum mounted to rotate about an axis of rotation; an elongate flexible tensile element, part of which is wound onto the drum and at a position along the length of which an article to be hoisted is connectable to the tensile element; drum drive means drivingly connected to the drum for rotatably driving the drum about its rotational axis thereby to wind the tensile element onto or unwind the tensile element from the drum; and tension sensing means for sensing tension in the tensile element mounted along the length of the tensile element, the tension sensing means defining a guide path through which part of the tensile element passes and in which the tensile element is deflected laterally, the extent of lateral deflection of the tensile element serving as an indication of the tension therein.

The device may include switch means operatively connected to the tension sensing means and configured to interrupt operation of the drum drive means in response to a lateral deflection of the tensile element beyond a predetermined distance which is indicative of an increase in the tension in the tensile element above a first predetermined maximum value and/or a decrease in the tension in the tensile element below a first predetermined minimum value.

The device may further include brake means operatively connected to the switch means for braking the tensile element at at least one position along the length of the tensile element in response to a lateral deflection of the tensile element beyond a predetermined distance which is indicative of a decrease in the tension in the tensile element below a second predetermined minimum, which is less than the first predetermined minimum value.

The tension sensing means may include a pair of guide pulleys positioned to contact the tensile element at longitudinally spaced positions and a deflector pulley which is positioned to contact the tensile element intermediate the guide pulleys and to deflect the tensile element laterally, the deflector pulley being urged towards a rest position by resilient bias means and being displaced away from its rest position against the bias means by the tension in the tensile element. In another embodiment, the tension sensing means includes a load cell-type arrangement, including a cantilever beam element located adjacent to the guide path and connectable to the tensile element at one end thereof and contactable with the tensile element at its opposite end such that the cantilever beam element urges the tensile element laterally and is deflected on lateral displacement of the tensile element, the load cell-type arrangement further including a transducer for converting a load acting on the beam element on deflection thereof by the tensile element into an output electrical signal.

The device may include processing means configured to condition and/or amplify the output electrical signal of the tension sensing means. In one embodiment, the processing means is configured to convert the output electrical signal from analogue to digital format. The processing means may include a comparator for comparing the output electrical signal to predetermined threshold values corresponding to each of the first predetermined maximum value for tension and the first predetermined minimum value for tension, the processing means being configured to transmit a switching signal to the drum drive means when the sensed tension is greater than the first predetermined maximum value and/or less than the first predetermined minimum value.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings.

In the drawings,

Figure 1 shows a side view of a hoisting device in accordance with the invention;

Figure 2 shows a sectional side view of one embodiment of an accessory for a hoisting device, in accordance with the invention;

Figure 3 shows a sectional view taken at Ill-Ill in Figure 2; Figure 4 shows a sectional view taken at IV-IV in Figure 2;

Figure 5 shows a schematic diagram of part of one embodiment of the accessory of Figure 2;

Figure 6 shows a schematic diagram of part of another embodiment of the accessory of Figure 2;

Figure 7 shows a sectional side view of another embodiment of an accessory for a hoisting device in accordance with the invention;

Figures 8A to 8C each show a longitudinal section through part of a hoisting device in accordance with the invention; and

Figure 9 shows a longitudinal section through part of another hoisting device in accordance with the invention.

In Figure 1 of the drawings, reference numeral 10 refers generally to a hoisting device in accordance with the invention, in the form of a tower crane. The device 10 includes a drum 12 mounted to rotate about an axis of rotation, and a tensile element, in the form of a steel cable 14, partly wound onto and extending along the length of the drum 12. The cable 14 has a free or "dead" end 16, between which free end 16 and the drum 12 an article to be hoisted is connectable to the cable 14 via a hook 18. The device 10 further includes drum drive means in the form of a drive motor (not shown) drivingly connected to the drum 12 for rotatably driving the drum 12 about its rotational axis, thereby selectively to wind the cable 14 onto or unwind the cable 14 from the drum 12.

The device 10 includes a framework, generally indicated by reference numeral 11. The framework 11 includes a vertical mast (or tower) 20 to an upper end of which is mounted a housing 26 for accommodating the drum 12 and drive motor. The housing 26 for accommodating the drum 12 may instead be positioned at a foot 27 of the mast 20. The framework 11 further includes an elongate horizontal jib or working arm 22 which projects laterally outwardly cantilever fashion from an upper end of the mast 20 from a position spaced below the housing 26, which jib 22 carries the load or article to be hoisted. A shorter horizontal counterjib 24 extends laterally outwardly at an upper end of the mast 20 from an opposed position to the jib 22. The device 10 further includes an operator's cab 28 mounted to the mast 20 at a position spaced below the jib 22 for accommodating an operator of the hoisting device 10 during operation thereof. A trolley 30 is typically mounted to run along a track (not shown) defined on an undersurface of the jib 22 to displace a load connected to the hook 18 selectively towards and away from the mast 20, the trolley 30 having one or more guide pulleys 32 mounted thereon for guiding the cable 14 proximate the hook 18. The free end 16 of the cable 14 is fastened to an end of the jib 22.

The hoisting device 10 includes an accessory 50 mounted to the framework 11 on the jib 22 at a position along the length of the cable 14 intermediate the free or dead end 16 of the cable 14 and the trolley 30.

Reference is now made to Figures 2 to 4 of the drawings, in which the accessory 50 is shown in more detail and, unless otherwise indicated, the same reference numerals used above are used to designate similar parts. The accessory 50 includes a generally box-shaped housing 40 within which is provided tension sensing means, generally indicated by reference numeral 52, for sensing the tension in the cable 14.

The tension sensing means 52 includes a load cell-type arrangement. A cantilever beam element 54, provided by a steel plate, is connected to the cable 14 at its one end 56 by means of a clamp arrangement, generally indicated by reference numeral 58. The clamp arrangement 58 includes two rectangular block-shaped clamp elements 60 each having a channel-recess 62 (Figure 3) defined in an operatively inner surface 64 thereof which recesses 62 together define a circular cylindrical passage in which the cable 14 can be accommodated to clamp the steel plate 54 at a fixed position on the cable 14 (see Figure 3). The clamp elements 60 are bolted together by bolts 67and are bolted also to the beam element 54 at its end 56 by a bolt 59 so as not to allow for any relative movement of the beam element 54 and clamp arrangement 58 and cable 14, respectively.

The beam element 54 is inclined towards the cable 14 at its opposed end 66 by use of a shim 68 arranged between the clamp arrangement 58 and the beam element 54. A roller arrangement 70 is provided at the end 66 of the beam element 54 by which the beam element 54 indirectly contacts the cable 14 at its end 66. The roller arrangement 70 includes a roller 72 mounted, for rotation about a rotational axis 71 transverse to the cable 14, between two spaced roller mounting brackets 74 which depend from the plate 54 (see Figure 4). The roller 72 is contacted with the cable 14, so as to bias/deflect the cable 14 laterally in the direction of the arrow 75. It will be appreciated that the roller 72 and beam element 54 in turn are displaced, in the direction of the arrow 73, on an increase in the tension of the cable 14. Similarly, the roller 72 and beam element 54 are displaced in the direction of bias 75 when the tension in the cable 14 decreases. The degree of deflection of the beam element 54 provides an indication of the tension in the cable 14. Typically a sleeve 76 is provided around the cable 14 at the position along the length of the cable 14 at which the roller 72 contacts the cable 14. The sleeve 76 is provided by a pair of saddle clamps 77 which are fastened together around the cable 14, the saddle clamps 77 having an internal diameter approximately equal to that of the cable 14. The sleeve 76 permits of smooth rotation of the roller 72 and deflection of the beam element 54 on lateral displacement of the cable 14.

The accessory 50 includes four sensors/transducers, provided by strain gauges 78 which are mounted on a surface of the beam element 54 for sensing a deflection of the beam element 54 and converting a sensed displacement of the beam element 54 into an output electrical signal. A strain, proportional to the deflection of the beam element 54, is introduced into the strain gauges 78 on deflection of the beam element 54 accompanying a lateral displacement of the cable 14. Any change in the tension of the cable 14 will thus introduce strain to the gauges 78. The strain gauges 78 in the embodiment shown are resistive strain gauges and form part of a wheatstone bridge circuit (not shown). Naturally, however, any other suitable type of strain gauge selected, for example, from electromagnetic, variable capacitance or magentostriction strain gauges, may be used.

Reference is now made to Figure 5 of the drawings, in which the same reference numerals used above are used to designate similar parts. The accessory 50 includes processing means, generally indicated by reference numeral 80, to which the output electrical signal of the strain gauges 78/wheatstone bridge circuit is fed. The processing means 80 includes a threshold detection circuit 82.

The threshold detection circuit 82 includes a signal conditioning unit 84, in the form of a low pass filter, to which the output signal from the strain gauges 78 is transmitted. The signal conditioning unit 84 is configured to attenuate high frequency signals and to pass low frequency signals, thereby to filter any spurious signals caused by the switching of ancillary switchgear or by other electrical interference.

The signal conditioning unit 84 is connected to a signal amplifier 86, which in turn is connected to an analogue comparator 88. The comparator 88 compares the conditioned and amplified output electrical signal to predetermined threshold values, with which the comparator 88 has been pre-calibrated and corresponding to each of a predetermined maximum value and a first predetermined minimum value for the sensed tension in the cable 14, and provides an output when these upper and/or lower threshold set points are exceeded.

A resistive revolution counter 90, which is rotationaliy connected to the drum 12 via a reduction gearbox 92, provides an additional input to the analogue comparator 88, equivalent to the number of turns of the drum 12. The revolution counter 90 is configured to provide a compensation signal to the analogue comparator 88, to compensate for the length of cable 14 unwound from the drum 12 in cable tension readings, and hence compensate for the weight and elasticity of the cable 14.

An output port/terminal of the analogue comparator 88 is connected to switchgear 92, which is turn is controllably connected to the drum drive motor (not shown). The switchgear 92 is configured to interrupt the supply of electrical power to the motor when the sensed tension in the cable 14 exceeds the predefined thresholds set in the analogue comparator 88.

In Figure 6 an alternative embodiment of the abovedescribed threshold detection circuit 82, generally indicated by reference numeral 200, is shown schematically and, unless otherwise indicated, the same reference numerals used above designate similar parts. In this embodiment, the output electrical signal from the strain gauges 78 is fed to an analogue-to-digital converter 202, to convert the analogue deflection measure signal to a 12 bit digital signal corresponding to the analogue deflection measure. The digital output from the converter 202 is transmitted to a microprocessor 204, which provides a digital comparator circuit equivalent to the comparator 88 described above with reference to Figure 5. Here, a pulse encoder 206 is mechanically connected to the drum 12 to provide an input to the microprocessor 204 indicating the number of revolutions through which the drum 12 is rotated, to permit of compensation in the tension reading for the length of cable 14 unwound from the drum 12.

The microprocessor 204 is configured to provide an output when the sensed tension exceeds the predetermined maximum and minimum values therefor, more particularly the pre-set upper and/or lower threshold settings for the output electrical signal of the strain gauges 78 corresponding to the maximum and minimum values for tension.

The microprocessor 204 is connected to switchgear 208, which in turn is controllably connected to the drive motor (not shown) and is configured to cut the supply of electrical power to the drum drive motor when the sensed tension exceeds the predefined thresholds set in the microprocessor 204.

In use, as the cable 14 is displaced laterally, for example, on application or removal of a load to or from the cable 14 causing a lengthening or shortening of the cable 14, respectively, the roller 72, biassed as it is into contact with the cable 14, is displaced with the cable 14 along the sleeve 76. This results in a corresponding deflection of the beam element 54, which deflection is sensed by the strain gauges 78, a strain proportional to the degree of deflection of the beam element 54 arising in the strain gauges 78 and producing an electrical resistance change in the gauges 78 which corresponds to the tension in the cable 14). An output electrical signal of the wheatstone bridge circuit (not shown) in which the gauges 78 are connected is conditioned and amplified and thereafter compared to predetermined threshold values corresponding to a minimum permissible and maximum allowable tension in the cable 14 by the threshold detection circuit 82, 200. Where either of these values is violated, the switchgear 92, 208 is actuated to interrupt operation of the drum drive motor such that the cable 14 ceases to be wound onto or unwound from the drum 12. The sensed tension is adjusted to compensate for the length of cable 14 unwound from the drum 12 by the revolution counter 90 or pulse encoder 206, as the case may be.

The predetermined minimum value to which the tension in the cable 14 may be permitted to fall is typically pre-set by sensing a tension in the cable 14 when a zero-load is applied to the cable 14 and calibrating the threshold detection circuit 82, 200 accordingly. Similarly, the predetermined maximum value may be pre-set by sensing the tension in cable 14 when a load corresponding to the maximum load rating for the cable 14 is applied to the cable 14. The shim 68 mounted between the clamp arrangement 58 and the beam element 54 is typically selected to provide for adjustability of the degree of pre-load of the beam element 54 to cover a range of allowable tension readings extending between the predetermined minimum and maximum values.

In Figure 7 of the drawings, reference numeral 100 refers generally to another accessory for a hoisting device 10 in accordance with the invention and, unless otherwise indicated, the same reference numerals used above are used to designate similar parts. The accessory 100 has a generally box-shaped body 140, within which is provided tension sensing means 152 for sensing the tension in the cable 14.

Side walls 163 and a bottom wall 167 of the body 140 are integrally formed as a unit of inverted top-hat section. A mounting plate 165, secured to the unit at an upwardly open end thereof, defines a top wall 169 of the body 140, and provides means for mounting the body 140 to the framework 11 of a hoisting device 10, more particularly to the counterjib 24 thereof.

The tension sensing means 152 includes two guide sheaves/pulley wheels 153, 154, mounted at transversely spaced positions within the body 140. The sheaves 153, 154 are mounted for rotation about parallel rotational axes 156, 158. Each sheave 153, 154 has a groove (not shown) defined in a rim/periphery 155 thereof, in which groove the cable 14 is receivable. An entry opening 170 and an exit opening 172 are defined in transversely spaced sides 174, 176 of the body 140. A pair of longitudinally spaced right-cylindrical rollers 178, each roller 178 being mounted to rotate about an axis of rotation generally parallel to the axes of rotation 156, 158 of the sheaves 153, 154, is mounted within the body 140 proximate each of the entry and exit openings 170, 172, between which pair of rollers is defined a guide passage 173 through which the cable 14 is passed to be guided onto or from the sheave 153, 154, as the case may be.

The tension sensing means further includes a deflector pulley 160 rotatably mounted at one end of an elongate longitudinally extending biassed arm 162, which arm 162 is mounted in turn to be displaceable laterally relative to the cable 14. The arm 162 is mounted between the sheaves 153, 154, equidistant from each sheave 153, 154. The deflector pulley 160 is thus mounted to contact the cable 14 at a position intermediate the sheaves 153, 154 and to deflect the cable 14 laterally. In the embodiment of the invention shown in the drawings, the arm 162 is resiliently biassed into contact with the cable 14, via the deflector pulley 160, by a tension spring 164, the pulley 160 having a groove (not shown) defined in a rim 166 thereof within which the cable 14 is receivable.

The arm 162 passes with clearance through an opening defined in the mounting plate 165.

The arm 162 is displaced in the direction of arrow 167 against the bias of the spring 164 due to tension in the cable 14. The lateral deflection of the cable 14 provides an indication of the tension in the cable 14. It will be appreciated that the higher the tension in the cable 14, the greater will be the compression of the spring 164 and the lower will be the lateral deflection of the cable 14.

The arm 162 is configured to interact with a limit switch 161 when the arm 162 is displaced under the influence of the spring 164 beyond a first predetermined distance, ie. when the cable 14 is displaced laterally beyond a first predetermined distance which indicates a drop in tension in the cable 14. The limit switch 161 is connected to the drum drive motor to interrupt operation of the drum when the arm 162 is displaced beyond said first predetermined distance.

In a preferred embodiment of the invention, the limit switch 161 is also coupled to locking or braking means in the form of one or more hydraulic clamps having relatively displaceable clamping jaws between which the cable 14 is clamped when the clamp is activated by the limit switch 161. To this end, the limit switch 161 is configured to trigger the locking/braking means to clamp the cable 14 when the arm 162 is displaced, under the influence of the spring 164, beyond a second predetermined distance, which second predetermined distance is beyond the first predetermined distance.

Optionally, the switchgear 92,208 of the embodiment of the accessory 50 of the invention may be coupled to one or more hydraulic clamps providing locking/braking means and having clamping jaws between which the cable 14 is clamped when the switchgear 92, 208 is activated, typically when the sensed tension in the cable 14 drops below a second predetermined minimum value, which is less than the first predetermined minimum value.

In use, when it is desired to monitor the tension in a cable 14 under load, the cable 14 from the drum 12 is passed into the body 140 through the entry opening 170 and between the pair of rollers 178, over the guide sheave 153, the deflector pulley 160 and the guide sheave 154, and via the pair of rollers 178 and exit opening 172 out of the body 140 and on to the hook 18. When bearing a load, the tension in the cable 14, which resists lateral deflection of the cable 14, urges the deflector pulley 160 away from its rest position, towards which it is resiliently biassed by the tension spring 164, towards a displaced position. Should the tension in the cable 14 decrease at any time, ie. should a slack develop in the cable 14, the arm is displaced under the bias action of the spring 164 towards its rest position, maintaining contact with the cable 14 and thereby taking up the slack in the cable 14. A first predetermined distance of lateral displacement of the arm 162 is determined to correspond to a first predetermined minimum tension in the cable 14. That is, when the arm 162 is displaced laterally beyond said first predetermined distance, the tension in the cable 14 has fallen below the first predetermined minimum value to which the tension might drop. Upon the tension in the cable 14 dropping below the first predetermined minimum value, ie. when the arm 162 is displaced beyond the corresponding first predetermined distance laterally, the limit switch is triggered and a power supply to the drum drive motor is thereby interrupted such that the drum 12 ceases to be rotatably driven and the cable 14 ceases to be wound onto or unwound from the drum 12. Where the hoisting device 10 is provided with one or more hydraulic clamps (not shown), when the tension in the cable 14 drops below a second predetermined minimum value, lesser than said first predetermined minimum value, ie. when the arm 162 is displaced beyond a corresponding second predetermined distance laterally, a hydraulic power pack of the or each clamp is deactivated by the limit switch 161 such that jaws of the clamp close on the cable 14 thereby to grip the cable 14 and prevent falling of the article/load being hoisted.

Referring now to Figures 8A to 8C of the drawings, in which reference numeral 280 refers generally to part of a hoisting device 10 and, unless otherwise indicated, the same reference numerals used above are used to designate similar parts. In the drawing, reference numeral 282 refers generally to a casing defined by part of a casting of the hook 18. The casing 282 defines a housing, or so-called block, for holding the one or more guide pulleys 232 proximate the hook 18. In Figure 8A, a single pulley 232 is housed within the casing 282, in Figure 8B a pair of pulleys 232 are located in the casing 282 and in Figure 8C, three pulleys are housed in the casing. A hydraulic clamp 284 is located proximate each pulley 232, either that part of the cable 14 leading the or each pulley 232 or that part of the cable 14 trailing the or each pulley 232 passing through jaws of a hydraulic clamp 284 such that the cable 14 is locked/braked at one or more positions proximate the hook 18, in the event of trigger of the or each hydraulic clamp 284 by the limit switch. This in turn prevents a falling of the block 282 to the ground. A hydraulic power pack (not shown) for powering the or each hydraulic clamp 284 may be mounted to the trolley 30 of the device 10.

It will be appreciated that the tension sensing means 152 may be integrated with the block 282, the block 282 then housing the two guide sheaves 153, 154 and the deflector pulley 170, mounted therebetween, as well as one or more hydraulic clamps 284. Furthermore, the block 282 may form part of a gantry crane. Reference is made to Figure 9 of the drawings in which reference numeral 300 refers generally to part of another hoisting device in the form of a gantry crane and, unless otherwise indicated, the same reference numerals used above are used to indicate similar parts. In Figure 9, three guide pulleys 232 are housed within the block/casing 282. In this embodiment of the invention, the guide pulleys 232 for guiding the cable 14 proximate the hook 18 also provide the guide sheaves 153, 154 and the deflector pulley 170 of the tension sensing means, the laterally spaced guide pulleys 232 providing the guide sheaves 153, 154 and the central guide pulley 232, being mounted to be displaceable laterally relative to the cable 14, providing the deflector pulley 170. A hydraulic clamp 284 is provided proximate one or more of the pulleys 232. The Inventor believes that the accessory 100 for a hoisting device in accordance with the invention provides an arrangement for taking up slack in the cable 14 of a hoisting installation 10, the deflector pulley 160 minimising slack in the cable 14, which slack is detrimental to efficient operation of the hoisting installation 10. The Inventor believes that braking of the cable 14 by use of the hydraulic clamp 184 forming part of the device 10 of the invention will minimise the risk of a load supported by the cable 14 falling and resulting dangers associated therewith. It is an advantage of the present invention that it will improve the safety of hoisting installations, such as tower and gantry cranes, and reduce risks of occupational injury to workers on construction and materials handling sites and the like.

It is believed that, when the tension in the cable 14 drops below a predetermined minimum level, such as an operational safety standard, or rises above the load capacity for the cable 14, the accessory 50, 100 of the invention will provide an effective means of shutting down operation of the hoisting device 10.

Claims

CLAIMS:

1. A method of operating a hoisting device having a drum mounted to rotate about an axis of rotation, an elongate flexible tensile element, part of which is wound onto the drum and at a position along the length of which an article to be hoisted is connectable to the tensile element, and drum drive means drivingly connected to the drum for rotatably driving the drum about its rotational axis thereby selectively to wind the tensile element onto or unwind the tensile element from the drum, which method includes the steps of sensing a tension in the tensile element at a position along the length of the tensile element; and when the sensed tension is greater than a first predetermined maximum value or is less than a first predetermined minimum value, inhibiting rotation of the drum about its rotational axis.

2. A method as claimed in Claim 1 , in which sensing a tension in the tensile element includes deflecting the tensile element laterally, and monitoring a degree of lateral deflection of at least part of the tensile element, the degree of lateral deflection serving as an indication of the tension in the tensile element.

3. A method as claimed in Claim 1 or Claim 2, in which inhibiting rotation of the drum includes preventing operation of the drum drive means.

4. A method as claimed in any one of the preceding claims, which includes, when the sensed tension drops below a second predetermined minimum value, which is less than the first predetermined minimum value, braking the tensile element at at least one position along the length of the tensile element in order to prevent an article being hoisted from falling.

5. A method as claimed in any one of the preceding claims, which includes the prior step of determining the first predetermined minimum value by sensing a tension in the tensile element when a zero-load is applied to the tensile element.

6. A method as claimed in any one of the preceding claims, which includes the prior step of determining the first predetermined maximum value by sensing the tension in the tensile element when a load corresponding to a maximum load rating for the tensile element is applied to the tensile element.

7. A method as claimed in any one of the preceding claims, which includes adjusting the sensed tension to compensate for a length of tensile element unwound from the drum.

8. An accessory for a hoisting device, said hoisting device having a drum mounted to rotate about an axis of rotation, an elongate flexible tensile element, part of which is wound onto the drum, and drum drive means drivingly connected to the drum for rotatably driving the drum about its rotational axis thereby selectively to wind the tensile element onto or unwind the tensile element from the drum, which accessory includes tension sensing means for sensing a tension in the tensile element of said hoisting device to which the accessory is operatively connected; and drum locking means operatively connected to the tension sensing means and configured to inhibit rotation of the drum about its rotational axis thereby preventing the tensile element from being wound onto or unwound from the drum, in response to a sensed tension rising above a first predetermined maximum value and/or falling below a first predetermined minimum value.

9. An accessory as claimed in Claim 8, in which the drum locking means includes switch means configured to interrupt a supply of power to the drum drive means in response to a sensed tension rising above the first predetermined maximum value and/or falling below the first predetermined minimum value.

10. An accessory as claimed in Claim 9, in which the tension sensing means defines a guide path, for accommodating a length of the tensile element and in which the tensile element can be deflected laterally, and includes a pair of guide pulleys positioned to contact the tensile element at longitudinally spaced positions and a deflector pulley which is positioned to contact the tensile element intermediate the guide pulleys and to deflect the tensile element laterally, the deflector pulley being urged towards a rest position by resilient bias means and being displaced away from its rest position against the bias means by the tension in the tensile element.

11. An accessory as claimed in Claim 9, in which the tension sensing means defines a guide path, for accommodating a length of the tensile element and in which the tensile element can be deflected laterally, and includes a load cell-type arrangement, including a cantilever beam element located adjacent to the guide path and connectable to the tensile element at one end thereof and contactable with the tensile element at its opposite end such that the cantilever beam element urges the tensile element laterally and is deflected on lateral displacement of the tensile element, the load cell-type arrangement further including a transducer for converting a load acting on the beam element on deflection thereof by the tensile element into an output electrical signal.

12. An accessory as claimed in Claim 11 , which includes processing means configured to condition and/or amplify the output electrical signal of the tension sensing means.

13. An accessory as claimed in Claim 12, in which the processing means is configured to convert the output electrical signal from analogue to digital format.

14. An accessory as claimed in Claim 12 or Claim 13, in which the processing means includes a comparator for comparing the output electrical signal to predetermined threshold values corresponding to each of the first predetermined maximum value for tension and the first predetermined minimum value for tension, the processing means being configured to transmit a switching signal to the drum drive means when the sensed tension is greater than the first predetermined maximum value and/or less than the first predetermined minimum value.

15. An accessory as claimed in any one of Claims 9 to 14, inclusive, in which the switch means is operatively coupled to brake means for braking the tensile element at at least one position along the length of the tensile element in response to a sensed tension falling below a second predetermined minimum value, which is less than the first predetermined minimum value.

16. An accessory as claimed in Claim 15, which is operatively connected to a hoisting device including a drum mounted to rotate about an axis of rotation, an elongate flexible tensile element, part of which is wound onto the drum and at a position along the length of which an article to be hoisted is connectable to the tensile element, and drum drive means drivingly connected to the drum for rotatably driving the drum about its rotational axis thereby to wind the tensile element onto or unwind the tensile element from the drum, the tension sensing means being mounted along the length of the tensile element and, and the switch means being connected to the drum drive means to interrupt operation of the drum drive means in response to lateral deflection of the tensile element beyond a predetermined distance which is indicative of an increase in the tension in the tensile element above the first predetermined maximum value or a decrease in the tension in the tensile element below the first predetermined minimum value.

17. An accessory as claimed in Claim 16, in which the hoisting device includes brake means mounted along the length of the tensile element and defining a passage through which the tensile element passes and in which the tensile element can be braked, the switch means being connected to the brake means to brake the tensile element along its length in response to a sensed tension falling below the second predetermined minimum value.

18. A hoisting device which includes a drum mounted to rotate about an axis of rotation; an elongate flexible tensile element, part of which is wound onto the drum and at a position along the length of which an article to be hoisted is connectable to the tensile element; drum drive means drivingly connected to the drum for rotatably driving the drum about its rotational axis thereby to wind the tensile element onto or unwind the tensile element from the drum; and tension sensing means for sensing tension in the tensile element mounted along the length of the tensile element, the tension sensing means defining a guide path through which part of the tensile element passes and in which the tensile element is deflected laterally, the extent of lateral deflection of the tensile element serving as an indication of the tension therein.

19. A hoisting device as claimed in Claim 18, which includes switch means operatively connected to the tension sensing means and configured to interrupt operation of the drum drive means in response to a lateral deflection of the tensile element beyond a predetermined distance which is indicative of an increase in the tension in the tensile element above a first predetermined maximum value and/or a decrease in the tension in the tensile element below a first predetermined minimum value.

20. A hoisting device as claimed in Claim 19, which includes brake means operatively connected to the switch means for braking the tensile element at at least one position along the length of the tensile element in response to a lateral deflection of the tensile element beyond a predetermined distance which is indicative of a decrease in the tension in the tensile element below a second predetermined minimum value, which is less than the first predetermined minimum value.

21. A hoisting device as claimed in any one of Claims 18 to 20, inclusive, in which the tension sensing means includes a pair of guide pulleys positioned to contact the tensile element at longitudinally spaced positions and a deflector pulley which is positioned to contact the tensile element intermediate the guide pulleys and to deflect the tensile element laterally, the deflector pulley being urged towards a rest position by resilient bias means and being displaced away from its rest position against the bias means by the tension in the tensile element.

22. A hoisting device as claimed in any one of Claims 18 to 20, inclusive, in which the tension sensing means includes a load cell-type arrangement, including a cantilever beam element located adjacent to the guide path and connectable to the tensile element at one end thereof and contactable under bias with the tensile element at its opposite end such that the cantilever beam element urges the tensile element laterally and is deflected on lateral displacement of the tensile element, the load cell-type arrangement further including a transducer for converting a load acting on the beam element on deflection thereof by the tensile element into an output electrical signal.

23. A hoisting device as claimed in Claim 22, which includes processing means configured to condition and/or amplify the output electrical signal of the tension sensing means.

24. A hoisting device as claimed in Claim 23, in which the processing means is configured to convert the output electrical signal from analogue to digital format.

25. A hoisting device as claimed in Claim 23 or Claim 24, in which the processing means includes a comparator for comparing the output electrical signal to predetermined threshold values corresponding to each of the first predetermined maximum value for tension and the first predetermined minimum value for tension, the processing means being configured to transmit a switching signal to the drum drive means when the sensed tension is greater than the first predetermined maximum value and/or less than the first predetermined minimum value.

26. A method as claimed in Claim 1 , substantially as herein described and illustrated.

27. An accessory as claimed in Claim 8, substantially as herein described and illustrated.

28. A hoisting device as claimed in Claim 18, substantially as herein described and illustrated.

29. A new method of operating a hoisting device, a new accessory for a hoisting device or a new hoisting device, substantially as herein described.