Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
B.L. Oudman Het opstellen van rekenmodellen voor de bepaling van het dynamisch gedrag van een stacker. Masters thesis, Report 97.3.TT.4958, Transport Engineering and Logistics.
For the ability of a buik terminal to attract lading, it is important for it to have a large storage capacity. Because the investment that a coal stacker requires is large, storage fields are becoming increasingly larger. Large storage fields require a coal stacker with a large reach, which in turn leads to stacker boom lengths up to 60 meters.
The stacker boom is designed to support the conveyor on it. Usually the conveyor with its load form a relatively small part of the total weight of the boom. This means that the design of the boom tends towards a slender structure, which, due to its low stiffness, can give rise to unwanted dynamic behaviour. Within the context of the masters project of the department of Transportation Technology of the Delft University of Technology and by request of Stork-RMO of Amsterdam, research has been done into the cause of unwanted dynamic behaviour of a coal stacker, and the influence various design
parameters have on it. In addition various calculation models have been introduced, with which dynamic behaviour of concept designs of parts of a stacker or a stacker as a whole, can be determined.
Starting point of the search for a design optimum with regards to dynamic behaviour, is increasing the lowest eigen-frequencies. Increasing these will decrease the chance of gusts of wind causing excitation of parts of the stacker. Furthermore the chance of instability of the machine will decrease as the lowest eigen-frequencies increase. This is because the movement of the centre of gravity due to elastic bending of parts of the machine and the chance of buckling of parts will decrease also.
The research into the way stackers are designed has shown that there are seven design parameters or design choices which influence the lowest eigen-frequency of a stacker the most. In order of strongest influence, these parameters or choices are:
1. The torsion stiffness of the boom; determined mostly by the type of boom and height and width of it 2. The bending stiffness of the boom; also determined mostly by the type of boom and height and width of it 3. The mass per meter of the boom; mostly determined by the type of boom
4. The placement of the motors of the conveyor. 5. The placement of the hydraulic cylinders 6. The maximal slewing angle of the tower 7. Method of balancing the boom
For this report research has also been done into the causes of unwanted dynamic behaviour of a stacker. Firstly this research has shown that the chance of gusts of wind causing violent swinging (excitation) of the stacker, depends, among other things, on the dimensions of the main parts of the stacker. The dimensions of a main part determine the frequency which the gusts must have in order for them to load a large piece of this part with a set regularity. If the lowest eigen-frequency of a main part lies above this threshold frequency of that part, than the chance of excitation of that part is small. In addition a literature study has shown that in gusts with frequencies above 0.4 Hz, the energy levels are 50% of the highest possible energy in a gust, for a certain average wind speed. From this it has been concluded that if the lowest eigen-frequency of the main parts, lies above the threshold frequency and 0.4 Hz, than the chance of excitation of these parts is negligible.
Secondly an analysis of a simple singular mass-spring model has shown that, in order to reduce the vibration of the main parts of the stacker,
the times of acceleration, both linear and rotational, have to be adapted to the lowest eigen-frequencies of the parts, in the direction of the acceleration. Thirdly a second analyses of a singular mass-spring model, has been used to show that increasing the lowest eigen-frequencies of a stacker will increase the dynamic load on the parts, when their subjected to an impulse load. As the dimensions of the main parts are mainly calculated on the basis of demands on stiffness, this increase in the dynamic load can easily be absorbed by the structures of the main part without the maximum permissible stress being reached. In relation to buckling, increasing the lowest eigen-frequencies will probably have a positive effect. Even though the pressure loads is the structures of the main parts will increase due to the extra dynamic load, the decrease in the bending of the structure will decrease the chances of buckling occurring. The static and dynamic properties of a specific design of a stacker have been qualitatively and quantitatively discussed in part 1 of book A. For part 2, ANSYS calculations have been done on this specific design. From the results of these calculations, the conclusion was made that the design fulfils all requirements and norms, if some minor adjustments are made. By calculating the lowest 25 eigen-frequencies, it has been verified that the chances of gusts causing excitation of the designed stacker are very small. It has however also shown that the rotational frequency of the conveyor drums coincide with some eigen-frequenties of the boom. In order to avoid excitation of the boom, some adjustments, like adding damping or increasing the stiffness of the boom locally, will have to be made.
Finally a simple calculation model has been introduced in this report, for determining the movement and wheel loads of the stacker, after a lateral collision of the tip of the boom with the storage pile. With the aid of this model the conclusion can be made that such a collision, with the stacker moving at its maximum speed, will not increase the stresses in the stacker beyond the permissible level. Furthermore it has been calculated that with such a collision, the wheel loads will at the most increase with four tonnes. The trucks of the undercarriage will on one side come off the rails if the stacker collides with a speed of 2.9 m/s. This speed is almost six times the maximum speed of the stacker and wil never be reached.
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