| Popis: |
Reliable prediction of the remaining service life for a conveyor chain is still an important requirement of customers which has not been solved satisfactorily up to now. The life time sensor developed through the research project “Intelligent Conveyor Chain” constitutes a new approach for long term data logging of chain loads. Fatigue assessment requires the determination of the curve for standing loads and the actual load range. As part of the accumulated damage assessment the damage arising during each individual load cycle is added up. When the total value reaches 1, this indicates a technical crack which will lead to a failure of the structural element of the chain. Determination of the remaining lifetime is based on the recording time of the associated load range data. The longer the recording time during operation the more representative is the load range data and as a result, the fatigue assessment is more reliable. Whereas the Wöhler curves have been determined experimentally for a wide range of different materials they can alternatively be estimated empirically by taking static strength values. In this case the load range has to be determined based on the original load time function of the structural element, independently from the method of fatigue assessment being used. The automotive and aircraft building industries for example have accumulated a huge amount of reliable load range data based on countless prototype tests. Non-standard manufactured products can not be tested to such a level of complexity. For these types of products the design process is limited to the use of estimated load range data. In the field of Condition Monitoring the customer increasingly demands a fatigue assessment indicating the potential service life of parts. The current market provides many different monitoring systems for measuring the load on parts. Most of these systems have to be connected by wire to the measuring point and as a result they are not appropriate for moving parts. Therefore telemetry systems which use wireless communication between the measuring point and the recording device can be used as an alternative. However the time of recording is mostly restricted by the capacity of the battery due to the high power consumption of these devices. Furthermore the space required for the batteries as well for the antennae is considerable. Such conventional telemetry systems are not suitable for installation directly into a conveyor chain. The newly developed “life time sensor” uses a novel concept which enables, for the first time, the long term data logging of chain loads for a service life of up to 3 years. The extension of service life compared to conventional telemetry has been achieved by miniaturisation of the system and the application of energy efficient components. In addition, by employing a measuring frequency matched to the operating conditions, combined with defined power saving, stand-by modes, the service life of these devices is greatly increased. With an outer diameter of about 25 mm and a length of less than 30 mm, these devices can be fitted directly into a chain conveyor link. Another advantage is the pre-processing of measurement data. Within the “life time sensor” the measured data is classified into matrix form to reduce the memory requirements. The advantage of matrix evaluated data compared to cyclic pitch data based on Rainflow counting or cycle counting Algorithms, is the consideration of the shift in the mean stress. Successful testing of the data logging, measurement data pre-processing and wireless communication has been carried out on various test installations at the company Thiele GmbH. The modified Markov matrix counting and the fatigue assessment of the conveyor chain have been verified on a test stand called “Hydropulser”. When a comparison was made between the chain link damage on the basis of the predetermined load variation and the results based on the measured load variation of the “life time sensor” they were found to match. There is a mismatch between the theoretical and the determined remaining life time due to the type of testing carried out, so that chain damage occurred at a calculated value of 0.46. A reliable adjustment of the fatigue assessment using the relative form is feasible at a first field test in an underground longwall and not at a test stand in a predefined environment. The results of the research and development carried out in this project, have demonstrated that long term data logging combined with measurement data pre processing is possible. Determination of the actual chain load range and thus the remaining chain life is now available. The “life time sensor” offers a reliable basis for condition monitoring of chain conveyors. Further Research and development will demonstrate the possible use of these devices in other fields of application. |
