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Calculations

The synergy that exists between the various disciplines available within Techno Fysica is used abundantly when we do our calculations. That is why you benefit from the added value which a calculation by Techno Fysica brings, in comparison to those done by other companies. We never present calculations merely as a collection of figures and numbers but we also indicate critical areas, as well as offer solutions for the diagnosed problems or advice for improvement.

A torsional vibration calculation done by the supplier of an engine, for example, will be carried out in order to avoid damage to their engine. Aspects such as propeller excitation behaviour, tooth hammer and thermal loads in elastic couplings or other added-on parts are often left out of the equation, or only get brief attention.

Because a calculation done by Techno Fysica is an independent one, to us every component of the installation is equally important. A calculation by Techno Fysica considers all aspects of the installation and not just behaviour of engine, gearbox or propeller.

Situations which, based on the calculations alone, would be considered ‘all right’, will be tested by us because of our practical experience. This helps us to outline potential problems better and is often followed by stricter advice than based merely on calculations.

The result? Increased reliability, more availability, better installation behaviour and, above all, a longer lifespan of the components.

Within the calculation department these are the main calculations which are carried out:

Finite element calculations

Based on special pre- and post-processing programmes as well as powerful, universal, finite element calculating codes, we perform a broad range of calculations. An objective of these calculations can be to provide an insight into the strain and distortions in a construction, or predicting dynamic behaviour such as vibrations. We make calculation models of relatively simple constructions such as shaft lines all the way to dynamic and static calculations of complete models of ships. By means of these finite element calculations results can be attained which would not have been reached through common calculating rules. Furthermore, it is possible to provide an insight into the dynamic behaviour, such as distortion and movement, by means of three-dimensional graphic presentation.

Torsional vibration calculations

These are calculations which considers the effect of the non-uniform rotation generated by the engine. This type of vibrations (with which the shaft flexes), are responsible for a large number of problems in propulsion installations. Here one can think of the premature failure of elastic couples, but also damage to gear wheels and fracture of a propeller or crank shaft. Torsional vibration calculations are compulsory for most seagoing ships, based on requirements by Lloyds, DNV-GL, Bureau Veritas and others. For these calculations we employ software packages created by ourselves alongside commercial ones.

The fact that damage still occurs frequently, for example on elastic couples, gearing, crank shaft fractures and so on, indicates that having a correct torsional vibration calculation done is of greatest importance.

Alignment calculation

As the name already indicates, this is a calculation in which the alignment of the components of a propulsion installation are checked and optimised. With it, problems such as seizure in a bearing can be avoided, as well as optimising the bearing load of the bearings in, for example, a gear box.

We make use of the so-called ‘sag & gap’ method. With this method the alignment of a shaft line is simulated in order to optimise the distribution of force in a coupled situation, after which a second simulation in opened condition shows how much sag (height difference) and gap (angle) there has to be between the two flanges that have to be connected, during the alignment procedure. The result is a more balanced distribution of bearing load, a better contact pattern and better vibration behaviour. Optimisation through the correct distribution of the bearing forces will be beneficial for the longevity of the components of the installation. Often such a calculation is even made compulsory by gear box suppliers, who demand an equal bearing load as a condition for installation. Unfortunately this information is often in the small print in a contract.

Natural frequency calculations

This is a simplified calculation of a three-dimensional design, aimed at prediction the lowest vibration modes. Here you can think of an installation on rubber mounts, or an installation on a simple (steel) frame. A calculation such as this is carried out to avoid interference with the environment (passing on sound or vibration) as much as possible. It is also possible to calculate the movement of the frame itself. In this way it is possible to monitor whether problems can be expected with the natural frequency of the frame as a consequence of excitation from the engine or generator.

Lateral vibration calculations and whirling

For long shafts we make predictions with regard to so-called natural frequencies or resonance frequencies. This, too, is a Class prerequisite for most propulsion installations, and there seems to be more and more attention for what is pointing at the growing sensibility of installations and the problems that come with that. We perform such vibration and whirling calculations by means of 3D FEM models. In the case of propeller shafts this behaviour is influenced by the revolving of the propeller in its plane. This behaviour is called ‘whirling’.

Even though these calculations are relatively simple in design, the behaviour of this type of installations is strongly influenced by aspects such as water moving with the installation, the rigidity of the bearing supports, the behaviour of the oil film with which the shaft rests in the bearing, elastic couplings and so on. Here, too, applies that the added value of the end result is based on the feedback from the measurements.

In practice it turns out that sometimes there are significant differences between theoretical calculation models and the actual installation. This can be because of margins in the characteristics of the material (for example the rubber of an elastic coupling or the actual weight of a propeller), incorrect specifications of certain physical characteristics and the margins of the manufacture. If desired, Techno Fysica is able to compare the calculation models to the actual situation through measurement. This is a powerful combination, especially when researching problems, in which the model can be corrected in such a way that it will closely resemble reality. With a model like this, the effect of a modification can then very reliably be predicted and assessed leading to the desired result, which will prevent a, usually expensive, phase of trial and error.