MILLENNIUM STEEL 2012 71 Robust sensors in crane hooks for safer crane transportation of molten metals in ladles To increase safety during crane transportation of molten metals in ladles, BFI has developed a sensor system to ensure proper execution of the load lifting procedure. This is done by accurately detecting the position of the trunnion in the hook mouth and providing a signal to the crane operator. The sensors are well protected against mechanical and thermal stresses and the system is virtually wear-free. W hen transporting molten metals in ladles by cranes, the crane hooks need to be positioned for lifting the load in such a way that the trunnions of the ladle bails lie in the mouth of the lifting hooks (see Figure 1). The load lifting procedure is steered manually by the crane operator who is supported by operational staff on the factory floor, in case, due to lack of attention or insufficient visibility, the ladle is seized by only one lifting hook or is lifted on the tips of the hooks. Both types of incident have led to ladles tipping over or dropping. To help overcome this problem, BFI has developed a piezoelectric sensor system to detect the proper execution of the load lifting procedure (German Patent Office file number: 10 2006 001 423.5). SYSTEM CONCEPT The system consists of piezoelectric sensors installed in the mouth trays of lifting hooks. These sensors are able indirectly to measure the forces in machine structures and tools through strain measurements. With proper threading of the ladle bail trunnions into the hook mouths, the lifting force of the ladle trunnions is induced into the lifting hook mouths and elastic deformation of the lifting hook mouth trays occurs. The deformation is detected by the sensors and evaluated by signal processing electronics. When the load is put down, the mouth trays are discharged, which is also detected by the sensors and evaluated by signal processing electronics. This additional sensor technology is not ‘Safety-related part of control’ as defined in EN ISO 13849-1 ‘Safety of machines – safety-related parts of controls, part 1: General design principles’. When improper threading occurs, for example, when the load is lifted at the hook tips, this sensor signal is not generated. Authors: Ulrich Müller, Andreas Tacke, Thorsten Seising, Klaus Müller and Ingo Brötzmann VDEh-Betriebsforschungsinstitut GmbH and Kranbau Köthen GmbH r Fig 1 Ladle and crane hook STEELMAKING a
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Robust sensors in crane hooks for safer crane transportation of molten metals in ladlesTo increase safety during crane transportation of molten metals in ladles, BFI has developed a sensor system to ensure proper execution of the load lifting procedure. This is done by accurately detecting the position of the trunnion in the hook mouth and providing a signal to the crane operator. The sensors are well protected against mechanical and thermal stresses and the system is virtually wear-free.
When transporting molten metals in ladles by cranes, the crane hooks need to be positioned for lifting the
load in such a way that the trunnions of the ladle bails lie in the mouth of the lifting hooks (see Figure 1). The load lifting procedure is steered manually by the crane operator who is supported by operational staff on the factory floor, in case, due to lack of attention or insufficient visibility, the ladle is seized by only one lifting hook or is lifted on the tips of the hooks. Both types of incident have led to ladles tipping over or dropping.
To help overcome this problem, BFI has developed a piezoelectric sensor system to detect the proper execution of the load lifting procedure (German Patent Office file number: 10 2006 001 423.5).
SYSTEM CONCEPT The system consists of piezoelectric sensors installed in the mouth trays of lifting hooks. These sensors are able indirectly to measure the forces in machine structures and tools through strain measurements. With proper threading of the ladle bail trunnions into the hook mouths, the lifting force of the ladle trunnions is induced into the lifting hook mouths and elastic deformation of the lifting hook mouth trays occurs.
The deformation is detected by the sensors and evaluated by signal processing electronics. When the load is put down, the mouth trays are discharged, which is also detected by the sensors and evaluated by signal processing electronics.
This additional sensor technology is not ‘Safety-related part of control’ as defined in EN ISO 13849-1 ‘Safety of machines – safety-related parts of controls, part 1: General design principles’. When improper threading occurs, for example, when the load is lifted at the hook tips, this sensor signal is not generated.
Authors: Ulrich Müller, Andreas Tacke, Thorsten Seising, Klaus Müller and Ingo BrötzmannVDEh-Betriebsforschungsinstitut GmbH and Kranbau Köthen GmbH
r Fig 1 Ladle and crane hook
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INSTALLATION OF SENSORSThe sensors are installed in the mouth trays of laminated lifting hooks: DIN 15407. The laminated hook (see Figure 2) is equipped with an additional, subdivided (no load carrying) blind lamella, to create a channel inside the hook for cabling, while the back of the hook is machined to create a cable channel.
The sensors are installed in boreholes, which are drilled below the contact area between the trunnions of the ladle bails and the mouth trays, into the mouth trays (see Figure 3). Thus, there are no openings in the bearing areas of mouth trays and so any deterioration in the sensors due to environmental conditions is eliminated. The signal transmission is done via cables passing through the channel in the laminated hook to the traverse.
INTERFACE OF SENSOR SYSTEM AND SIGNAL TRANSMISSIONThe signal processing electronics consists essentially of a charge amplifier and microcontroller with power supply. The charge amplifier is mounted on the traverse and the microcontroller and the power supply in the control room of the crane girder.
On the microcontroller, in addition to the signals from the charge amplifier, the signals ‘Load Lifting’ and ‘Weight’ are required as input. The output signals, for example for an optical and/or acoustic display in the crane cab, are provided on the microcontroller.
LABORATORY TESTING OF SENSORS To test the function of the sensors and to determine their optimal arrangement, a series of experiments were performed using a dummy set-up. Figure 4 shows a press and the dummy, whose shape corresponds to the contour of the mouth tray. The sensors were installed in three boreholes. The compressive force F was varied between 30 and 120t and the temperatures varied between 60 and 120°C. Additionally. the ram was positioned both centrally and eccentrically.
Figure 5 shows an example of the measurement results. For each of the sensors the course of the signal and the applied compression force of the press are depicted.
With respect to installation in a crane, it was concluded that two sensors should be installed at each mouth tray in eccentric boreholes. This will lead to a sufficient redundancy and will avoid low level signals when the trunnion is positioned eccentrically. No problems occurred in the investigated temperature range.
SENSOR BEHAVIOUR IN A 230T CASTING CRANEThe signals produced in a 230t casting crane are shown in Figure 6. The system is activated automatically when
r Fig 3 Installation of sensors in boreholes
r Fig 2 Cross-section of laminated hook with an additional, subdivided blind lamella
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a lifting procedure is started (see green line at the bottom of the graph). Beginning with the contact between load and hook, the load cell of the lifting device measures the increasing weight (see blue line at the bottom of the graph). When the measured weight exceeds an adjustable threshold (see magenta line), the microcontroller starts to evaluate the signals of the sensors (see upper graphs). When the signal from each sensor exceeds a low threshold, a digital signal is provided at the output of the microcontroller (see red lines in the upper graphs), indicating the proper threading of the trunnions of the ladle bails into the mouth of lifting hooks. These signals can be used for an optical display and/or audible signal in the crane cab, for example.
Tests under operating conditions ensured that this sensor signal is not generated by an improper threading of the trunnions of the ladle bails into the lifting hook mouths, for example, when lifting the load at the hook tips.
CONCLUSIONSThe sensor system for detecting proper execution of the load lifting procedure increases safety during crane transportation of liquid metals in ladles. The sensors and cables are extensively protected to prevent deterioration through the harsh environmental conditions around steelwork cranes.
The new technology is currently used in cranes delivered by Kranbau Köthen GmbH. MS
Ulrich Müller, Andreas Tacke and Thorsten Seising are with VDEh-Betriebsforschungsinstitut GmbH, Düsseldorf, Germany. Klaus Müller and Ingo Brötzmann are with Kranbau Köthen GmbH, Köthen, Germany.
r Fig 5 Example of measurement results from laboratory tests
r Fig 6 Signal behaviour of the system in a 230t casting crane
