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. A very common often encountered in diesel-driven installations are torsional vibrations. Torsional vibrations exist in any rotating equipment, and are related to the non-uniformity of the speed which exist in any combustion engine.  An example of one of many problems relating to torsional vibrations is given below.

A fishing vessel suffered from repeated damage to the rubber elements of the elastic coupling between the front end of the engine and the PTO generator that is used to power the winches that operate the fishing nets. Although the coupling had a safety factor of 2 (which is not uncommon in the fishing industry, where operating conditions are harsh), the rubber elements were destroyed several times, and, this usually happened within a very small time span.

Through measurements of the  torsional vibrations, it was found out that during handling of the nets, which is an operating condition that exists approximately 15% of the time, with a disengaged propeller and at very low engine power and low speed, the governed system became unstable, causing resonance of the lowest torsional vibration mode of the system. Due to this resonance, large torsional vibration amplitudes and, as a result, high alternating torques occurred in the rubber elements of the elastic couplings.

The governor reacts every four revolutions of the engine which phenomenon is called “8-stroking”. The resulting instability lead to a continuous vibratory torque of 175% of the allowable level, which can be considered excessive. One of the functions of a rubber coupling in a propulsion installations is to dampen torsional vibrations. As a result of the hysteresis, heat is produced in the coupling. This is normally not a problem, as this is radiated or ventilated away to the surrounding environment. However, heat becomes a problem is there is to much heat production, usually in areas of resonance or high torsional amplitudes.

The heat produced by  vibratory torque resulting from the instability lead to such a temperature increase that the rubber element started to melt.

Because improving the behaviour of the governor at these low loads proved to be very difficult, it was decided to install an oversize coupling that could cope with the vibratory torque level without exceeding the allowable heat production in the rubber element.

Figure 4.1.8: Dynamic behaviour of the propulsion system of a fishing vessel at low load