Bringing bevel gears back to action cost efficiently with ATA ReVive^TM

Kongsberg Maritime is one of the companies who have benefitted from ATA Gears’ re-engineering service ATA ReVive – for over a year now. The experiences have been good.

We talk about bevel gear pairs, which transmit power to the thruster and thrust the vessel forward or hold it steady, depending on the application. “These are very high-tech products, the bevel gears and their machining – which we have designed together with ATA Gears,” explains Kongsberg’s Contract Team Manager Jussi Miettinen and continues: “The restoring service is a natural continuation of our research project. So far, at least, nothing but positive feedback from our clients.”

Kongsberg’s thrusters are no minor devices: The biggest have a propeller diameter of 4.5-4.7 meters, height 5-6 meters and total weight 120 tons. The diameter of the gear wheel used in thrusters is typically over 1 m.

Joni Keski-Rahkonen, Manager, Technology & Analysis says that at Kongsberg, new bevel gears are practically always designed together with specialist gear suppliers. ”ATA Gears is a key partner and Chief Gear Designer Jesse Rontu, the main man, our principal contact in technical design and service planning. Those are also the typical areas where we co-operate,” Joni Keski-Rahkonen explains.

Regarding new bevel gears, co-operation between Kongsberg and ATA follows a fairly standard procedure, meaning that Jesse and fellow Gear Doctors need certain input from Kongsberg – about the application, loads and conditions. Kongsberg makes certain analyses and defines starting values for design purposes and delivers them to ATA. Soon after this, ATA comes back with a proposal for a bevel gear pair, which Kongsberg’s experts then assess and approve. When the new set of gears has been manufactured, it is installed in the thruster unit for highly specific tests where the actual contact pattern is analysed and compared with the simulated patterns. If necessary, the bevel gear pair is returned for machining and the micro-geometry is fine-tuned. Such gears seldom fail.

But when something unexpected happens in maritime conditions, there is no time to waste.

In ReVive cases there is an existing bevel gear pair, which has sustained some superficial damage to it that can be put right with no need for a total replacement. Savings in both time and money.

In some cases the right treatment is feasible with the vessel in dry dock – it mainly depends on the ship’s location. In all cases, Kongsberg’s and ATA’s experts together evaluate whether a bevel gear can be refurbished or needs to be replaced. A classification society then finally approves a refurbished gear. On all Kongsberg’s projects, those bevel gears, which have undergone the ATA ReVive treatment have without exception returned to their original applications.

ATA ReVive is a result of over a decade of research and process development. ATA Gears’ Sales Director Tami Komssi explains the underlying principles of this lifecycle-conscious service: “The key enabler of ReVive is the accurate heat treatment and the knowledge of true case hardening depth on the tooth. When a gear returns to us for ReVive assessment, we can check from our manufacturing logs all the necessary data, including the remaining case hardness depth compared to the specification. This way we know the remaining machining allowance on the tooth flanks. ATA’s unique knowledge of various failure modes, especially tooth flank fracture (TFF) gives us good confidence in determining whether the gear is worth repairing. Enabling a peace of mind for both ATA and the customers. I believe this is makes a big difference in these challenging days, when significant cost savings are required everywhere.”

There have been several ReVive cases, and the experiences of these have been very positive. Kongsberg intends to continue this co-operation with ATA Gears. Jussi Miettinen reasons: ”For a customer, it is the most cost-efficient way to get a bevel gear back in action again. And at the same time, it is our policy to avoid any unnecessary risks.”

ATA ReVive can be the most cost-efficient solution in a vast range of situations. For example, at the beginning of the design process, the tooth pattern may have been different from the specific use today and this calls for updating and renewal. Or then something may have broken in the bearings with a small particle breaking off and ending up between the gears. Or micro-components in the oil may have damaged the surface of the gear. Yet another cause may be corrosion. In most cases, it is a minor, fixable problem. However, if a loose piece or pieces from a bearing have been working on the gear for months, then the gear will be beyond repair. As in so many things, it’s all down to timing.

Are the thrusters then equipped with analytics or sensors to enable timely fault detection and proaction with ReVive?

A Condition Monitoring System with sensors for gears and bearings has been in use for 10 years already. These provide a good range of data to predict possible problem situations. Joni Keski-Rahkonen describes the next generation thinking:  Monitoring solutions often have a condition-based category and a predictive maintenance category. Since condition-based means directly measuring the current status of the bevel gears, there should be a notable defect on the surface of the bevel gears causing a noticeable change in the vibration pattern. In this case, repair is necessary, not perhaps urgently, but something that should go through re-machining when feasible. The development of such surface defects can be monitored using the CMS system. The next generation of predictive maintenance systems enable forecasting of approaching defects before they even become visible on the surface of components.

It promotes strategic evaluation if a bevel gear should be replaced before embarking on an imminent and perhaps longer contract. It is then possible to estimate the defect and its consequences and decide whether to re-machine or replace the bevel gear to ensure continuous operation.

”Long contract periods are very typical in the offshore industry, for the drilling ships and rigs doing their work on the seven seas of the world. The daily operating costs of these vessels are so high that if they cannot do the work they have been hired to do, the daily losses can rise up extremely high. Compared to that, changing a bevel gear is a marginal cost. And we are using ATA’s gears because we want high quality and reliability,” Jussi Miettinen says confidently.

A thruster sometimes competes with a conventional propulsion system in which a propeller is directly connected to a diesel engine with a shaft.  The efficiency of a thruster is highly competitive compared to the conventional propulsion system because the propeller is situated further away from the hull of the ship providing a cleaner water inflow and more efficient thrust.

Kongsberg has invested heavily in hydrodynamics research and development aiming at lower emissions and carbon load through lower fuel and other operating costs. In some cases, euros can be treated as an indication of carbon footprint, cost saving being also good for the environment.

Delivery cost is a good example of balanced sustainability: Shorter distance and smaller item equals cheaper price and lower footprint. The motto is lower operating costs equals higher environmental benefits. “That’s’ why we like working with ATA’s Gear Doctors,” says Jussi Miettinen and continues: “A case in point, ATA ReVive offers both cost and environmental benefits. Even in real emergencies, when bevel gears are sent out by air, a ReVive process is still an environmentally sustainable way compared to manufacturing a new bevel gear and demolishing the old one.”