GB2055488A - Overload protection in lifting apparatus - Google Patents

Abstract

The rate of change in the rope lifting force in a lifting apparatus is measured and switching from a higher to a lower lifting speed takes place when the measured rate gets to a value proportional to the difference between a greatest allowable and the measured instantaneous rope lifting force. Preferably, the lower lifting speed is maintained for a certain time, and after this time switching to a higher lifting speed for a preset time takes place. <IMAGE>

Description

SPECIFICATION Overload protection of lifting apparatus The present invention is with respect to a method for overload protection of a lifting system designed for operation with at least two different lifting speeds and in which, more specially, on pulling tight a lifting rope, the load force of this rope is measured and, on its becoming greater than a preset, limiting value, the apparatus is switched over from a higher to a or the lower lifting speed.

It is a noted fact that trouble with the overloading of lifting apparatus is likely at the start of a lifting motion because of oscillation or transients making the load force of the rope very much greater than the force produced by the load on the hook under steady lifting conditions. (In this respect, the word "rope" is used in the present specification and claims for covering all equivalent parts, as for example cords, wire cables and chains.) For putting an end to such undesired effects, caused by oscillation, a suggestion has been made in the German Auslegeschrift specification 2,058,712, having an overload protection system coming into operation when a certain rope load force is got to and shutting down the main lifting speed drive, whereupon lifting then takes place at the inching speed.

In this earlier overload protection system, the rope load force, triggering shutting down of the main speed drive, has a fixed value, which has to be very much lower than the rated loading of the rope, because, as caused by the oscillations or transients noted, the rope load force may become much higher even after shut-down of the main driving speed. The -use of a fixed value as a switching point is responsible in many cases, however, for the lifting apparatus' full capacity not being used and a waste of time.

One purpose of the present invention is that of designing an overload protection system of the sort noted, making possible better use of the capacity of the lifting apparatus by stopping any switching back to a lower speed, generally speaking, which is not completely necessary for reasons of safety.

In this respect, the suggestion of the present invention is for the change, as a function of time, in the rope lifting load force (dF/dt) to be measured and for the decrease in the lifting speed to be triggered at a value pr-oportional to the difference between the maximum allowed and the instantaneous rope lifting force.

Details of the invention will now be seen from the account of a preferred working example of the invention, to be seen in the accompanying drawings.

Figure 1 is a view of a contactor control system of a two-speed lifting apparatus, in connection with an electronic overload protection system of the invention.

Figure 2, in the form of three graphs, 2a, 2b and 2c, gives the changes with time of the lifting speed, the rope lifting force and the differential coefficient of the last-named with respect to time, in the case of a small load.

Figure 3, in the form of three graphs, 3a, 3b and 3c, gives the changes with time of the lifting speed, the rope lifting force and the differential coefficient of the last-named with respect to time, in the case of a heavy load.

Figure 4, in the form of three graphs, 4a, 4b and 4a, gives the changes with time of the lifting speed, the rope lifting force and the differential coefficient of the last-named with respect to time, in the case of an overload.

Turning to figure 1, it will be seen that a contactor control system for the driving motor of the lifting apparatus has four contactors HF, HH, SF and SH, for the functions "inching upwards", "full speed lift", "inching downwards" and "full speed lowering". Operation of the contactors is by placing the winding of the contactor needed between the phase Ph and the neutral conductor Mp.

For control purposes, a many-way switch S1 is used for contactors HF and HH, while for contactors SF and SH use is made of a manycontact switch S2 with, in each case, three positions 1, 2 and 3 and three switching levels a, b and c.

Switching over of switch S1 out of the resting position 1 into the working position 2 does not have any effect in the switching plane a, but, however, in switching plane b, phase Ph switched to contactor HF, this starting "inching upwards".

On further switching over into the position 3 in the plane b, phase Ph is joined up with contactor HH, while the current path for the contactor Hf is changed over into switching plane a, that is to say is kept in being. Putting contactor HH on line is responsible for "full speed lifting" but, at the same time, for opening an idle contact hhl. on the input side of contactor HF so that contactor HF is shut down.

On the same lines, the switching over of switch S2 out of the resting position 1 into the working position 2 in switching plane a is without effect, while, by way of switching plane b, the contactor SF will get the phase Ph, this starting "inching downwards". On switching on further into position 3, contactor SH will now be joined with phase Ph by way of switching plane b, while the current path for contactor SH will be changed over the plane a, that is to say, it is kept in existence.

Operation of contactor SH is responsible for "full speed lowering", while, at the same time, for opening an idle contact sh 1 placed at the input side of contactor SF so that the last-named is shut down.

The plane c of switch S1 is used for locking switch S2 and the plane c of switch S2 is used for locking switch S1. For this purpose, in the two switches in the plane c, the position 1 is used as a current path for the other switch in each case, while the positions 2 and 3 are not wired. For this reason, switching on one of the two switches S1 and S2 will have the effect of cutting off current from the other so that, at the time of one of the functions "lifting" and "lowering", the other function will be locked or disenabled. On the other hand, the full speed in the two forms of function has the effect of locking the inching motion because of the idling contacts for hh1 and shl of contactors HH and SH.

A special measure of the contactor circuit is that the incoming wire for phase Ph to the switching planes a and b of switch S1 and to the two contactors HF and HH is looped by way of an idling contact a2 in the circuit of the overload protection system. On the same lines, the connection of contactor HH with the neutral wire Mp is by way of an idling contact a1 in the circuit of the overload control system. Furthermore, the root of the resting contact hh1 of contactor HH is joined up with a point X in the overload protection system so that its electronic system will be able to make out when a lifting operation is taking place.

In the circuit of the overload protection system, there is firstly a sensor 1, measuring the instantaneous value of the rope load force F and sending it, put in the right form, to a processing unit 2. One first function of the processing unit 2 is that of producing the difference between the rope load force or value F and a present value Flax, representative of the maximum allowable rope load force. The difference Fmax - F worked out is furthermore multiplied with a preset factor k, which is a constant dependent on the properties of the lifting apparatus and/or the rope. The instantaneous value of k (Fmax - F) is a measure for the instantaneous allowable change in the rope load force, that is to say (dFldt)zui..

As part of an important teaching of the present invention the value, processed out on these lines, that is to say dF zul. zul. = k (Fmax - F) dt goes to a comparison unit 3, which, furthermore, gets from processing unit 2 the instantaneous value dF/dt produced by differentiating the value coming from the sensor, comparison of the two differential coefficients being undertaken in processing unit 2. If the signal dF/dt processed directly from the output signal of the sensor is greater than or equal to the value produced as the.

difference FmaxF, for (dF/dt)zul, a timer 4 will be triggered by comparison unit 3, timer 4 cutting off contact al for its time constant so that contactor HH no longer gets the magnetizing current. As soon as contactor HH has been opened, its contact hh1 will be shut so that contactor HF will be put into operation again, that is to say there will be a switch over from main speed to inching speed operation.

The dynamic overload protection system noted here only takes effect in the function "full speed lift", in which oscillations or transients of any great amplitude are likely. For this reason, there is in addition a static overload protection system, taking effect on overload in the function "inching upwards" and under the direct control of the measured rope load force F. For this reason, on getting to a rope load force P, fixed as being the highest allowable one, a signal will be sent from processing unit 2 to a circuit unit 5, responsible, thereupon, for opening contact a2 for an unlimited time and disenabling the two lifting speeds. The circuit unit 5 may be reset, no detailed account being given of this.Because the static overload protection system is only of any value for the function "inching upwards", the switching point P may be, in this case, put near the greatest allowable rope load force Fmax SO that there is no loss in the lifting capacity of the lifting apparatus.

For an account of the way of operation, attention is to be given to figure 2 in the form of different graphs of changes with time of the lifting rate, the rope load force and the differential coefficient of the last-named with respect to time in the case of a load of normal weight. In this case, the lifting apparatus is started at t = O by moving the function switch S1 into the working position (position 3), that is to say for full speed operation with 1 00% of the lifting speed. The rope speed so produced on lifting or pulling in is to be seen in figure 2a. As long as the pulled-in rope is still loose or slack, the rope load force in figure 2b will be zero at the point in time t = t1, the slack will have been fully taken up, whereupon an increasing force will be produced in the rope, marked as rope load force F in the graph.This force will become quickly greater and greater till, at the time t = the weight is pulled clear of the floor. From this point onwards, an oscillation or transient will be produced, which slowly gets less. After the transient has fully decayed, the rope load force F will be equal to weight L. In the working example in question here, the rope load force F will, at every point in time, keep under the greatest allowable force FmaX. It will be furthermore lower than the switching point P (set at a somewhat low value) of static overload protection. The weight is lifted without stopping at full speed.

Lastly, figure 2c is a graph of the time coefficient of the rope load force of figure 2b, that is to say the value dF/dt. In this case as well, a broken line is used for marking the curve graphically produced as the difference F max - F from figure 2b, which, furthermore, may be looked upon as the reserve rope force. By putting in a factor k, representative of the properties or data of the lifting apparatus and of the rope we then have

this giving the instantaneously allowable time coefficient of the rope load force. It may be seen from this that the full-line curve has to be under the broken line one at all times if there is to be no rope overload. The comparison unit is responsible for testing at all times to see if the full line curve is in fact under the broken line curve. In the case of figure 2 this condition is kept to.

In figure 3 the curves in question for lifting speed, rope load force and the time coefficient of the last-named will be seen in the case of lifting a heavy load. In this case the lifting apparatus is started up again at full speed. Pulling tight of the rope is again started at t = t, but will then go up to a higher level, because the heavier load will keep on the ground on a longer time. For this reason, there will, lastly, be smaller values for the rope force reserver F max - F still able to be used so that the broken line curve in figure 3c will be lower, the point of cutting or of intersection S of the two curves being at t = t2, this triggering the comparison unit and, by way of the timer, causing switching over, for a limited time, to "inching upwards".Roughly from this point onwards, the further increase in the rope load force will be at a smaller slope. The main lifting speed drive is disenabled by the timer at least till it is seen that the rope load force keepts under the limit P, as is the case in this example. At t = t' the load will be pulled clear of the ground on inching upwards and there will only be a very feeble transient oscillation. Then the rope load force will keep unchanging with F = L till, at t = t4, the contact a1 will be switched on again by the timer and a new attempt will be made at lifting at full speed.

The representative curves in the case of an attempt at lifting an overly heavy load are to be seen in figure 4. In this case as well, starting takes place at full speed and pulling tight of the rope is started again at t = t1. At the same point in time of t = t2 as in the case of figure 3, two curves of figure 4c will be cutting or intersecting, whereupon the comparison unit will be responsible for switching over to "inching upwards". Even before the load is pulled clear of the ground, however, the rope load force F will go up as far as the limit P of the circuit unit, the lastnamed opening contact a2 and so stopping the lifting motion. A feeble transient will now be produced, after whose decay the rope load force F will come to a fixed level somewhat higher than limit P. In this case, only lowering is possible so that the overload will not even be cleared from the ground.

As the accounts given will certainly have made clear to the reader, the overload protection system of the present invention is responsible for fiexible adaptation of the lifting apparatus to the load to be lifted so that its lifting capacity is made better use of. Loads less than about 30% of the rated load may be lifted simply at full speed while overly heavy load will not even be pulled clear of the ground.

Although the account of the invention, for making it simpler, has been based on a two-speed lifting apparatus, an expert will readily be able to see that the general teaching of the invention may be used for overload protection of good effect with lifting apparatus with stepless control or stepless automatic control of the speed of turning of the driving system.

Claims (9)

1. A method for overload protection of a lifting system designed for operation with at least two different lifting speeds and in which, more specially, on pulling tight a lifting rope, the load force of this rope is measured and, on its becoming greater than a preset limiting value, the apparatus is switched over from a higher to a or the lower lifting speed, characterized in that the change as a function of time in the rope lifting load force is measured and the decrease in the lifting speed is triggered at a value proportional to the difference between the maximum allowed and the instantaneous rope lifting force.

2. A method as claimed in claim 1, characterized in that on the rope's load force increasing to a value equal to about 90% of the greatest allowable force, switching off of the lifting motion is triggered.

3. A method as claimed in claim 1, characterized in that after switching back in speed, the lower lifting speed is kept to and after this, only for a preset time, lifting motion with a higher speed is started.

4. A method as claimed in claim 2 and claim 3, characterized in that the preset time is greater than the time necessary for testing to see if the rope lifting force is greater than or less than 90% of the greatest allowable rope lifting force.

5. A system designed for using the method as claimed in anyone of claims 1 to 4, characterized in that a sensor is present for measuring the instantaneous rope lifting force and sending an output signal to a processing unit, designed for forming a first output value and differentiating the input signal with respect to time and, for forming a second output value, working out the value k (Fmax - F), and in that a comparison unit is present for comparison of the two output values of the processing unit and, if they are in agreement, triggering a first switch in the control system of the lifting apparatus, said triggering causing switching back of the lifting speed.

6. A system as claimed in claim 5, characterized by a timer placed between the comparison unit and the first switch, said timer switching back the first switch after a preset time.

7. A system as claimed in claim 5 or claim 6, characterized by a circuit unit getting the sensor signal as amplified by the processing unit, said switching unit causing operation, after a certain limiting value has been got to, of a second switch in the control system of the lifting apparatus, by which the lifting motion is stopped.

8. A method as claimed in claim 1, substantially as described above.

9. An apparatus as claimed in claim 5, substantially as described above with reference to and as illustrated in the accompanying figures.