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Clean Sky's I²BS project keeps watch over UHPE demo bearings

Real-time monitoring of aero-engine bearings means that overhauls and repairs can be safely scheduled when they're needed, as opposed to at fixed intervals determined by time or operation cycles. Clean Sky's Integrated Intelligent Bearing Systems (I²BS) for the Ultra High Propulsive Efficiency (UHPE) Ground Test Demonstrator project is bringing a smarter approach to bearing health monitoring which will make European aero-engines more cost effective to operate while raising safety standards.

The idea of the Integrated Intelligent Bearing Systems (I²BS) for UHPE Ground Test Demo project is to design, develop, evaluate and test interchangeable conventional and smart bearings for the Ultra High Propulsive Efficiency (UHPE) Ground Test Demonstrator. The project kicked off in July 2016 and runs until December 2021.

Smart bearings will be able to deliver information related to their functional characteristics – their health, including temperature, axial and radial load, ball or roller or cage speed, lubrication quality, and radial clearance on each part of the bearing. And all of this information will be in real time, similar to Formula 1 racing cars, to see how they are working within the system.

Dr. Jean-François Brouckaert, Team Leader for Clean Sky's Engine ITDs, references the context of the project by noting that "Safran's UHPE demonstrator has a completely different engine architecture in terms of shaft support, bearing locations and shaft dynamics, because the weight balance along the shaft for an engine like this is very different from a classic direct drive turbofan, so there's limited experience of what the bearings will have to endure in this new architecture. So the concept is to have instrumented bearings to monitor it during the first test and to validate this with respect to the models, and the dynamics of the behaviour of the engine in the longer term. If these smart bearings prove to be reliable, they could be used eventually for in-service health monitoring. But the project's first goal is to understand the dynamics of the shaft on this concept". 

Clearly, anything which is instrumented and fits into the same size as something that isn't instrumented has potential to be used for health monitoring or predictive maintenance.

"In ideal conditions bearings last thousands of hours without problems, but that's not the case inside an aircraft engine. For example, you could have contamination in the oil system, or oil interruption – if the oil pump isn't supplying enough oil for the lubrication you get friction which will lead to degradation of the bearing and eventually bearing failure" says Patrick Mirring, Head of  Product Development Aerospace at Schaeffler Aerospace (formerly FAG Aerospace) and coordinator of the project. "With a smart bearing you will be able to detect any initial damage, so you'll know exactly which component – whether it's the outer ring or the inner ring or if it's a rolling element – has suffered initial damage and consequently you'll also have a prediction of the spall propagation behaviour so that you'll know the bearing's remaining lifetime. And with this knowledge it will be possible to extend the overhaul interval".

Currently the industry uses fixed overhaul inspections based on hours and/or cycles, but with smart bearings, by knowing how many hours of lifetime are left, it becomes possible to extend the inspection intervals and the maintenance intervals, potentially reducing overhaul costs. The project team is also carrying out estimations and calculations regarding the loading and speeding which are applied to the bearings.

"We don't yet know how often overloading situations occur and therefore it could be that some of the bearings are over-engineered" says Mirring. "With instrumentation on the bearings we would get actual data from an engine to know what the maximum load applied is, how often it occurs and so on, and you can take this into consideration to design the bearing according to the real loading and speeding situation and therefore also downsize the bearings – this would be a benefit".

Another benefit would be for the aircraft operator to know, if they land and a bearing fault is detected, for how long it would be safe to operate says Mirring: "If I have a bearing failure and maybe I am somewhere in South Africa and I do not have spare parts available to do the overhaul, the question is can I use the airplane to fly for example to Singapore where they have an overhaul shop, or do I have to ship the part to South Africa. If I have all the remaining lifetime information to confirm it's not going to compromise safety, I can also bring the airplane to my preferred overhaul shop".

One of the project challenges is to provide power to the sensors that monitor the bearings. Ideally, self-powering the sensors by using energy harvesting is preferable, but it means that energy harvesting devices will have to be installed and these must withstand the oil and the heat, and would have to be miniaturised to fit without taking up too much space. The solution being tested is to use a Thermo-Electric Generator (TEG) whereby waste heat from the engine is converted into electricity which is stored in a supercapacitor which powers the sensors. The TEG, at about 5cm³, is small enough to be installed on the outer diameter of the bearing ring.  

"From our perspective it's not necessary to do continuous monitoring" – which would generate terabytes of data says Mirring, "it is sufficient to do it every 30 seconds or every minute because the degradation of the bearing will not be so fast that you have to react within minutes. It is sufficient if you react within half an hour or one hour". 

In terms of testing, subscale testing is underway which means that the size of the bearing is much smaller than conventional main shaft engine bearings, and the goal is firstly to demonstrate that the sensors are able to detect the initial damage. To replicate realistic conditions, white noise is being applied to the test rig to simulate the engine noise. This, in a multi-component and multi-sensor setup where there are lots of similar frequencies being emitted, will help to test whether the algorithm being applied to interpret the data is capable of differentiating between white noise and defective bearing noise to pinpoint the location of any damage. The project is also testing to determine what the smallest defect size that can be detected is. This information will be correlated with the project's existing spall propagation data.

"At the end we will have a test-bed with quite a lot of sensors, though ideally we want to minimise the number of sensors that are necessary to carry out this predictive monitoring because every sensor will require extra space and add cost and weight" says Mirring, who concludes that "Once all of this system with the sensors and TEG has been optimised it will be installed onto a real main shaft engine bearing and tested on a full scale test rig. And after this, we will supply some smart bearings to Safran for ground engine demo tests".