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Today, mobile cranes are indispensable tools for many tasks that involve lifting heavy loads where no tower crane is present. Large models in particular are used for lifting heavy items, for example in wind energy plants. Optimal support should prevent the crane from toppling over while lifting the load. In this context, innovative measurement technology in the supports of the crane can help.
Safety is an important aspect of hoisting technology. However, when mobile cranes are used, there are always uncertainties, which must be clarified before the crane is put to use. Using the precise weight of the load and the necessary jib length, the crane operator can calculate which mobile crane should be used. However, the exact conditions on site – especially the space available for the crane and the stability of the subsoil –also play a significant role. Naturally, the most cost-efficient solution should also be used. Using an excessively large crane for a task would generate unnecessary costs. With regard to most of the parameters, it is never possible to completely eliminate the possibility of accidents while the crane is being operated. This is one of the reasons why standards and regulations in this sector are constantly being improved. The exact safety regulations are set out in the current DIN EN 13000 crane standard.
A mobile crane comes with a support system that generally consists of four hydraulic support cylinders. These are mounted on beams, which can either be swung out or extended. The support ensures that the effective surface covered by the mobile crane can be extended and a high load capacity can therefore be achieved. In addition, mobile cranes also have ballast, located at the back end of the crane structure, which can be adjusted to suit the intended load and allows for further improvement of the stability. The possible working load of the crane depends on the configuration – i.e. ballast, support, jib, etc. Furthermore, modern mobile cranes also feature load torque limitation systems designed to prevent the overloading of a crane. Depending on the configuration, this stops all crane movements that would further increase the load torque. Here, it is standard practice for the crane operator to enter the configuration by hand . If, for example, one of the supports cannot be extended fully due to a limitation of space, the maximum load is reduced, regardless of the direction in which the load is to be hoisted.
Precise knowledge of the configuration parameters, particularly with regard to support, is essential to allow the load torque limitation to be handled in a more flexible manner. Construction machinery manufacturer Liebherr (see text box) has been researching for a long time to find a solution that would allow the forces arising in the supports to be measured during use and thereby optimise the load torque limitation. Measuring the support forces makes it possible to calculate the centre of gravity of the constantly changing crane geometries. The difficulty is that a force transducer in the supports should only determine the vertical force. Movements of the crane also cause lateral forces to be channelled into the supports. The intention is to eliminate these completely using the implemented measuring technology.
In order to realise Force Transducers for Support Cylinders (FTSC) that can measure the forces in mobile cranes reliably and accurately, the team at Liebherr selected sensor specialist tecsis as a system partner. “As well as the expertise tecsis has to offer in the field of force measurement, the company’s development capabilities were also a major factor in this decision”, says Erwin Morath (B. Eng / Dipl.-Ing.), who is responsible for control engineering at Liebherr. As a specialist in the development and production of force transducers with thin film technology, tecsis was a natural choice for this task, which involved the technical development and cost-efficient production of large quantities of force transducers. “The requirements for the measurement technology were very high with regard to this application”, explains Klaus Härle (B. Eng. / Dipl.-Ing (FH)), Product and Key Account Manager of the project at tecsis. This task involved the development, testing and production of force transducers for the various crane types and support cylinders in the measurement range 300 kN to 4,000 kN. The transducers must have a measurement accuracy of better than 1 % and, at the same time, must tolerate a lateral force share of up to 10%.
Since this project involves the production of safety-relevant components, the transducers must achieve Performance Level e (PLe) as set out in the DIN EN ISO 13849-1 standard in order to be able to reach the necessary Performance Level for the machine as a whole. At the same time, it must be possible to guarantee a high level of availability for the force transducers. Furthermore, the environmental requirements for the transducers are also high: the technology used must function within a temperature range of -40 °C to +80 °C and must also demonstrate a very high level of EMC immunity. The experienced development team at tecsis was able to meet all these requirements. In order to make this possible, close coordination and cooperation with the Liebherr management team and with external testing bodies such as TÜV was essential.
“However, the greatest challenge was to construct the transducer in such a way as to allow the effect of the lateral forces to be eliminated almost entirely”, says Härle. Based on the requirements, the tecsis development department came up with two solutions. Both were tested using simulation calculations. During this process, the effect of lateral forces on the measurement accuracy was in focus. Based on these simulations, the team chose a solution with special thin-form cell geometry as a sensor element. A prototype was built, which was put through further tests in the tecsis test centre and later also tested on the crane itself. Here, those tests that focused on the effect of lateral forces were of particular significance. In order to test this aspect, loads were applied on the transducers in the tecsis test laboratory while the lower base plate was placed on a steeply sloped surface. The results showed that the measurement accuracy was better than 1 % even when a load was applied to the transducers on an incline. The next step involved testing the transducers under real conditions in the supports used in Liebherr mobile cranes. These tests proved as well that the measurement accuracy of the prototypes always remained within the required range.
After successful testing, the product had to be transferred to mass production. The production and quality control departments at tecsis were supplied with new machines and systems so that the transducers could be mass produced and tested. One example of the new additions is the new 42 t force testing machine, with its calibration range of 12,000 kN.
The new system, consisting of load torque limitation together with the force transducer for support cylinders was offered as standard for the first time on the LTM1750 mobile crane, which was presented at the Bauma trade fair. The new system makes it possible to base load torque limitation on the current situation, allowing for greater flexibility with regard to the use of the crane. Both, tecsis and Liebherr are very pleased with the result of this development project.
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