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Torsional vibration calculations

Torsional vibrations are the source of a large percentage of problems and damage of rotating equipment. A sound, numerical analyses of the system can prevent most of these problems. This is done by means of so-called torsional vibration calculations.

A torsional vibration calculation is an analysis of a complex system, usually a propeller installation or drive mechanism, with regards to the non-uniform rotation present in the installation. Because of resonance this non-uniform rotation can lead to very serious vibrations and in some cases even to damage. This damage can vary from light pitting on gear wheels to very serious gear wheel damage, ruptures or even the melting of elastic couplings and the breaking of shafts.

The cause of the presence of bothersome or harmful torsional vibrations can be found in the assembly of the components of the assembled installation. The problem here is mainly that in theory sound components are no guarantee for a good assembled structure.

When there is a case of mismatching of the combination of mass inertia and rigidity of the separate components, problems can arise. Sometimes they only arise during certain operational situations so that problems are not always quickly recognised. An added issue, then, is that the component that breaks is not always the weakest or worst part of the installation! It is usual in such cases to blame the supplier of the weakest part, while this is not always where the problem lies. In this case a reliable mathematical analysis can help solve the problem.

Examples of damage caused by torsional vibrations can be:

  • Shorter longevity of elastic couples
  • Wear of gear teeth
  • Gear hammer (hammering of gear teeth caused by large torque fluctuations), which can result in bothersome noise but also gear teeth breakage.
  • Breaking of screw shafts
  • Breaking of crank shafts

In contrast to ‘ordinary’ linear vibrations, torsional vibrations are difficult to detect without equipment. Where linear vibrations can be felt (low frequency) or heard (high frequency) and sometimes even seen, a torsional vibration in general has only limited external reaction force (?). Because the problem cannot be recognised outright, high torsional vibration levels can lead to unexpected failure like fracturing of an elastic couple, gear teeth damage or even shaft breakage. Remarkably enough, noise disturbance on yachts through torsional reaction forces while the torsional vibration level meets all current standards is also possible. However, this has more to do with the high sensibility of the lightweight structures and the high suppression of other noise sources, through which levels that are normally not noticed suddenly do get classified as bothersome.

To be sure that an installation is free of torsional vibrations, a torsional vibration calculation can be carried out. This calculation, compulsory for larger seagoing ships in order to meet Class requirements, will lower the risk of resonances in the operating speed range from appearing. Such a calculation can be carried out at a relatively low cost, and will prevent the wrong choice of parts. It can often already be helpful to vary the rubber hardness of the elastic coupling to offer an adequate solution. In such a case, making a calculation beforehand may prevent tens of thousands of euros worth of damage.

In many cases, especially in new builds, a torsional vibration calculation is done by the supplier of the engine. However, these calculations are rather limited when it comes to analyses and conclusion, and are mainly aimed at having the engine operate without problem or damage. One conclusion that is often heard is that crank shaft stress and damper load are admissible, but this does not say anything about flexible couplings, gearboxes and other components. The main aim of these calculations is merely get approval by Class.

In more complex installations, for example a dredging installation, with a large number of complex load changes such as dredging, pumping, shore discharging and so on, these calculations are not always complete, and carrying out a good calculation calls for thorough knowledge of the operating conditions and the expected load conditions. This knowledge is abundantly available with Techno Fysica, through years of experience while measuring such installations.

It can also be necessary to do a recalculation later in the life span of an installation, for instance if something in the installation is changed. Examples of this are the addition of a generator, the change of propeller or perhaps operating conditions, with different shaft speeds.

In such a case, Techno Fysica can carry out this new calculation for you, and the fact that we do this regarding the complete installation and for the entire field of operation, is an added advantage.

Finally it can be mentioned that we are an independent company: our advice is not ‘limited’ by connections or agreements with suppliers.