André Böhme has a mission: fighting TFF.
Subsurface fatique, TIFF or TFF, is causing several unforeseen gear failures and thruster replacements every year. It is the predominant failure mode on bevel gears used in azimuth thrusters onboard state-of-the-art vessels. And fatigue or overload can destroy any gear. For good reason, there is very innovative, comprehensive and collaborative research going on between Kongsberg Marine and ATA Gears to find out what it’s all about and make vessels even safer.
Understanding the phenomenon can lead to big savings
Ultrasonic scanning during service inspection has lead to a transition from gear replacements due to actual tooth flank fractures to gear replacements due to detected subsurface cracks, which means that many gears are discarded because of the scanning but actual tooth breakages happen less often. As gears are expensive parts designed to last a lifetime, 20 years or more, much could be gained in savings and safety if only the phenomenon could be understood and rectified.
This has turned into an important mission for Kongsberg and ATA Gears and a lot of new knowledge and insight has been found about it. The research group is lead by André Böhme, Technical Product Manager of the Azipull thruster family at Kongsberg Maritime, who is also completing his doctoral dissertation on the topic.
“The research so far into gear failure modes like pitting, tooth root breakage, scuffing and micro-pitting has by and large ensured failure-free gear and thruster operation. As we are challenging the power density on azimuthing thrusters, we are now reaching not the surface but subsurface limit of our gear steels,” says André Böhme.
Improvements to the current models and standards
André Böhme believes that the DNV standard is still a viable working tool, but in light of his meticulous research together with ATA’s Gear Doctors, he wants to propose some improvements to the current models to make still more durable gears for even safer vessels and more reliable operations.
He likes to talk about Dynamic Positioning, referring to the fact that the otherwise understandable and applicable DNV standard is based on the assumption that a vessel is run at its maximum rated power, when in fact offshore vessels tend to spend most of their service time in DP i.e. Dynamic Positioning, where power is needed to remain stationery or keep a safe distance from an oil platform. “Of the time they operate only a fraction is in transit, leading to an average load far below the levels used in the classification calculation. This then means that the calculation models should be adjusted to reflect real-life situations, as the gear breakages occur under far more moderate loads than is assumed in the calculation models,” André Böhme points out.
More insights from André´s study
Carried out in a rather unique test bench environment together with the team of ATA’s Gear Doctors, André Böhme’s study presents several highly interesting and valuable insights.
Foe example, a strong point is made that the DNV rules point towards a too shallow subsurface fatigue prediction, resulting in increasing surface hardness requirements rather than hardening layer depth. Furthermore, in his opinion, the current standards do not adequately cater for VHCF (very high cycle fatigue) degradation of fatigue properties.
How about the difference in achievable hardness profile between large and small gears, what does that entail? Further still, are larger gears more prone to reduction in material strength and TFF?
Many gears are discarded because of the scanning but actual tooth breakages happen less often.