Measuring and testing – Dynamometers – Responsive to force
Reexamination Certificate
2001-04-30
2002-10-15
Noori, Max (Department: 2855)
Measuring and testing
Dynamometers
Responsive to force
Reexamination Certificate
active
06463812
ABSTRACT:
The invention relates to a method for the simulation of loads on lifting appliances, in particular on tower cranes, for the purpose of setting the load limiters of the lifting appliances. This invention is also aimed at a device for carrying out the method. The invention applies to any lifting appliance equipped with a winch-type lifting mechanism, the electric motor of which is controlled by a variable-speed drive unit ensuring regulation of the drive torque.
The conventional procedures for setting the load limiters of a lifting appliance employ actual calibrated loads as a source of the load, the setting being carried out within the following context (with regard to a tower crane).
The function of load limiters is to limit the forces which are attributable to the load suspended on the lifting hook and which are applied to the structure of the crane to acceptable values for which this crane is calculated and designed. A safety margin of 10% is usually provided, in order to take into account the accuracy of the force measurement systems and the dynamic increases attributable to the accelerations during lifting.
The sequence of the operations for setting the load limiters is established so as to protect the personnel carrying out the settings and the equipment against any possible false maneuver. The setting of the torque limiters is therefore carried out before that of the load limiters.
The load limiters are set when the crane is commissioned, at the time of any change in the configuration of the crane and also, in general, at least once a year as a periodic check.
The conventional setting principle, with a device having mechanical sensors, is as follows:
It will be assumed here, as an illustrative example, that the maximum-load limiter of a tower crane is to be set, the permissible maximum load being 10 tonnes. A calibrated load of 10 tonnes is suspended on the hook of the crane, and the device for setting the limiter is brought to the cut-off limit. The lifting of this load at the nominal speed provided is then commanded, and, if lifting does not take place, the setting is repeated in order to make it possible.
The load is then lowered, and an additive load of 10%, that is to say one tonne in the example in question, is added to it. A check is made that the movement of lifting this increased load (equal, here, to 11 tonnes) is impossible, otherwise the setting is repeated in order to bring about the cut-off.
Finally, a check is made that the lifting of the nominal load of 10 tonnes is still possible.
Setting procedures using a device with electronic sensors are also known. By means of such devices, calibration is carried out with any load having a value matched to the scale of loads of the crane, but not made mandatory. The other loads and the torque resulting from each load are calculated automatically by an electronic computer. Despite the use of electronic means, this method still has the disadvantage of requiring at least one actual calibrated load.
In addition to these conventional methods, there are so-called “load simulation” techniques, making it possible to carry out settings without the need for actual calibrated loads.
Thus, in the field of tower cranes, a device for the simulation of loads with the aid of a motor-driven pump or of a jack has already been proposed. The jack is mounted in the region of the mechanical amplifier of the load limiters, in order to exert a pull on this. The load is simulated with the aid of a hydraulic pressure delivered by the motor-driven pump and given by a “pressure/load” cross-reference table. This device does not make it possible to simulate a torque attributable to a load, since it is located in the region of the mechanical sensor of the load limiters.
In the light of this prior art, the object of the invention is to avoid the need for calibrated loads in order to carry out the settings of the load limiters on tower cranes and therefore to limit the operations of handling such loads and also the duration and cost of these settings, whilst also making it possible to set the torque limiters.
To achieve this, the subject of the invention is essentially a method for the simulation of loads on lifting appliances, in particular on tower cranes, the said method involving:
mounting a balance between the lifting hook and a fixed point,
placing the lifting mechanism, with regulation of the drive torque, in a “setting” mode,
displaying desired values for the limitation of the drive torque, commanding the lifting function by means of the control of the said electric motor in the raising direction and reading the tensile force values supplied by the balance.
According to one embodiment, the balance is fastened, at one of its ends, at a fixed point anchored on the ground or is lashed by means of this end to the structure of the crane. In an alternative embodiment, the balance is attached, by one of its ends, to a free mass having a value greater than that of the maximum load to be measured, so that this mass is the equivalent of a fixed point. In any event, the other end of the balance is attached to the lifting hook of the crane.
The method according to the invention may comprise the use of a direct-reading balance, that is to say a balance incorporating a means for displaying the tensile force exerted on this balance. In this case, an operator must be present in the vicinity of the balance, in order to read the displayed values and transmit them to another operator responsible for carrying out the setting, who is usually stationed on the structure of the crane, in particular in the region of the electrical control cabinet of the lifting mechanism.
In an advantageous variant, the method which is the subject of the invention comprises the use of a remote-reading balance, that is to say a balance with means for the remote transmission of the measured tensile force values towards a measured-value display casing which is preferably located within the immediate range of the operator responsible for carrying out the setting. This variant therefore no longer requires the presence of a second operator for reading the measurement given by the balance, and it also avoids any error in communication between operators.
More particularly, the direct-reading or remote-reading balance being in place, and the lifting mechanism having been switched to the “setting” mode, for example by the display or validation of an access code on the control keypad of the variable-speed drive unit which controls the motor actuating the lifting winch, the process of setting the limitation of a load involves, in order to measure the load:
displaying a desired value for the limitation of the drive torque, for example in the form of a percentage of the nominal torque, the said desired value being slightly below that of the load table of the crane (for example: 90%),
commanding the lifting function in the raising direction, until the winch is immobilized, the balance then being subjected to a tensile force corresponding to the previously displayed desired value of the limitation of the torque,
reading the force value supplied by the balance (for example: 5.1 tonnes),
increasing the desired torque-limitation value until the correct value given by the load table (for example: 5.2 to 5.3 tonnes) is obtained at the balance,
if the required value is exceeded, commanding the lifting function in the lowering direction and repeating the preceding phases, whilst at the same time resetting the desired torque-limitation value,
finally, releasing the control, so that the brake acts and immobilizes the winch in position, the force being maintained at the same value at the balance.
The process in question involves, furthermore, with regard more particularly to the setting of a load limiter:
bringing the load limiter to the cut-off limit,
commanding the lifting function in the lowering direction until the lifting cable is slackened,
stopping the crane, until the variable-speed drive unit is completely deactivated (for a time given by the technical specification of the crane, which may be appr
Noori Max
Oliff & Berridg,e PLC
Potain
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