NSK has launched a new gas turbine generator bearing specifically for use in electric vertical take-off and landing (eVTOL) aircraft, including large drones. Among the major advances of the bearing are higher speed revolution performance and a new mechanism that reduces the necessary lubricant supply by around 80%, providing a significant contribution to longer eVTOL flight range.
New developments mean that eVTOLs are making significant progress as a means of transportation, with a predicted market size of €124 billion by 2035. Previously, mainstream eVTOL propulsion systems featured complete motorisation. However, to meet the growing need for longer flight range and improved transportation capabilities, most expect the demand for high-output, compact, lightweight hybrid motors to continue growing.
Gas turbine generators stand out from other hybrid motor designs because they can use fuel sources such as biofuels and hydrogen, making them attractive from a carbon neutrality perspective. Today, gas turbine generators are the strongest contender for practical propulsion mechanisms in eVTOL aircraft.
Extending the flight range of eVTOL aircraft hinges on a variety of factors, including the high-speed revolution performance of its gas turbine generator bearings. To meet this demand, NSK set about developing a bearing capable of delivering over 2.5 million dmn (the product of bearing pitch diameter and rotational speed). In addition, the company succeeded in supporting this high-speed revolution performance with minimal yet optimal lubrication needs.
The problem with jet lubrication, the most common lubrication mechanism for high-speed bearings, is that it requires a large amount of lubricating oil and promotes high power loss. Among the alternatives is under-race lubrication, which uses dedicated flow channels in the shaft and bearing inner race to supply oil. While this method offers minimal power loss and oil usage, it has low versatility and comes at a high cost.
Extending the flight range of eVTOL aircraft hinges on a variety of factors, including the high-speed revolution performance of its gas turbine generator bearings. Photo: Aero Development Japan, Inc.
To overcome these limitations, NSK’s new lubrication mechanism features an improved spacer shape. Following temporary storage of lubricating oil in the spacer, the mechanism supplies the minimal yet optimal amount of oil to the inside of the bearing using centrifugal force.
NSK’s innovative solution reduces the amount of lubricating oil that the bearing requires to just 25% of that needed by conventional jet lubrication, while also cutting power loss by a third. The new design also yields a highly versatile, low-cost lubrication solution in comparison with under-race lubrication. As another advantage for OEMs, oil supply scavenge pumps and the lubricant tank can now be lighter in weight, contributing further to the flight range of eVTOL aircraft.
