Techno Cases

Techno Fysica BV is a Dutch-based engineering company, highly specialised in solving all kinds of problems related to the dynamic loading of machinery and installations. For this, several measuring and analysis techniques are used. In a series of articles, we will give examples of what can go wrong in the engineering and operating of vessels, and what errors lie at the root of the damages observed.

Usually, elastic couplings are fitted between engines and driven equipment to reduce alternating stresses and prevent damage.

However, interaction between the governing system and low frequent torsional vibrations can cause damage to either the elastic coupling itself or to the driven equipment.

The tendency to locate the lowest resonance speed (1^st vibration mode, exited by the 0.5^th order under misfiring conditions) below the minimum operating speed of the installation leads to low natural frequencies in the range between 2 to 4 Hz.

Modern electronic governing systems have no problems to follow speed fluctuations at these frequencies, which can lead to unwanted interaction as can be seen from the following examples.

A Trailing Suction Hopper Dredger (T.S.H.D.) suffered from repeated damage to the gearbox between the two engines and the dredge pump. The damage consisted of cracking of key ways, fretting corrosion of the fit between shafts and gearwheels and damage due to hammering of the teeth of the cog wheels of the clutches.

The torsional vibration calculation showed no dynamic overload under any of the operating conditions. Extensive measurements revealed that the torsional vibration amplitudes were magnified by interaction between torsional behaviour and the governing system which lead to vibratory torque amplitudes in excess of 60% (0-peak) of nominal torque whereas class usually accepts 30% as a maximum.

This dynamic torque occurs during dredging which takes up approximately 40% of the operational profile of this type of ship.

Changing the position of the speed pick-ups and the settings of the parameters of the governors cured the problem; however, systems with such low natural frequencies always remain sensitive for this kind of unwanted interaction.

Mean- and dynamic torque on the input shaft of a pump gearbox of a TSHD