Questions & Answers
Q: Why are shafting torsional vibrations so important?
A: Propulsion shafting is one of the most important onboard systems. Therefore, it deserves special consideration during design, manufacturing, and later, operation.
Virtually all onboard structures and systems are subject to static as well as dynamic loads. However, the importance of these two types of loads is not uniformly distributed. Some building blocks are predominantly statically loaded, whereas others are predominantly dynamically loaded. Besides, building blocks have different sensitivity to static and dynamic loads. Therefore, they need different approaches during the design, the manufacturing, and the operation stages.
Design engineers are not able to take into account all the factors acting on the structural parts or machines. However, they must consider the most important loads and processes.
When propulsion shafting is considered, as it is mostly dynamically loaded, its dynamic behavior is the major factor influencing its overall reliability and efficiency. This means that shafting vibration behavior, especially torsional vibration behavior, is the most important factor influencing shafting design.
It must be clearly realized that classification societies' formulas take into account static loads only, and therefore, these formulas are not sufficient for proper shafting designs. Only thorough torsional vibration analysis may confirm or reject the validity of the proposed shafting design!
Q: What is the importance of shafting axial or lateral vibrations?
A: In general, axial and/or lateral vibrations are less severe than torsional vibrations. However, this does not mean that the effects of axial and/or lateral vibrations may be neglected. Propulsion shafting is too important to be designed without considering axial and/or lateral vibration effects.
In general, classification societies require additional vibration analyses and/or measurements if they expect, based on their previous experiences, increased vibration responses.
Q: Can axial or lateral vibrations cause severe damages?
A: Definitely yes! Although axial or lateral vibrations are generally less severe than torsional vibrations, there exists evidence of severe damages caused by axial and/or lateral vibrations.
Moreover, coupled torsional and/or axial and/or lateral vibration is a case of mutual vibrations, when each type of vibration simultaneously contributes to severe damage.
Q: Does a particular engine model produce the same vibration effect independently from the configuration of the propulsion plant?
A: Definitely not! Propulsion engine is not an isolated vibration system. Actual vibration system consists of an engine, shafting, propeller and other devices (e.g., gears, clutches and couplings). Only the vibration system as a whole may be considered as a system that possesses good or bad vibration behavior.
It means that the same engine model may behave completely differently in different propulsion environments, constructed of different propulsion plant components.
Q: The engine runs smoothly. Is there a possibility of severe shafting torsional vibrations, although there is no visible evidence of any vibrations?
A: Unfortunately, yes! Torsional vibrations behave, to some extent, differently from other types of vibrations. For example, floor vibration may be easily detected by the people onboard. Similarly, table or device vibrations are also easily noticeable. On the other hand, shafting torsional vibration behaves differently. Namely, shafting torsional vibrations are the consequence of shafting variable speed of rotation. Since angular displacements are very small, they are invisible to all observers. Moreover, during the high shafting torsional vibrations, the propulsion engine alone may appear stable and free of any vibrations.
The fact that the shafting torsional vibrations are not visible does not mean that they do not exist!
Q: The engine runs at a low speed, say 30% SMCR. Is there a possibility that the propulsion shafting is subjected to severe torsional vibrations?
A: Yes! High vibration stresses are the consequence of resonance conditions. As described elsewhere (e.g., in the Shafting torsional vibrations pages of this primer), major resonant conditions may be met at various engine speeds, even if they are very low. In these cases, shafting may be subjected to high, even severe vibration stresses, although the engine operates at low speed and/or low output power.
The only way to predict hazardous engine speed is to perform shafting vibration analyses. In addition, it is a good engineering practice, if not required by the classification society in charge, to measure actual vibration responses during the sea-trials.