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Copyright © 2009, Emerald Group Publishing Limited
Running on air
Article Type: Mini features From: Aircraft Engineering and Aerospace Technology: An International Journal, Volume 81, Issue 6
The lubricant for engines that propel aircraft may 1 day become the air itself. Scientists at NRC are testing “oil-free” bearings that have the potential to significantly reduce aircraft fuel consumption and greenhouse gas emissions.
Aircraft turbine motors are now lubricated with oil. But oil loses critical slipperiness at the high-operating temperatures where turbines run most efficiently. And, one rough rule of thumb suggests, up to a quarter of a conventionally oiled turbine's weight consists of lubricating pumps, filters and oil channels.
Oil free bearings – also called “foil” or “compliant” bearings – respond to an industry trend to increase the power density of aircraft turbines by making them smaller, lighter, faster – and hotter. As a result, they use less fuel yet lift larger loads, which could allow bigger aircraft, more passengers or heavier cargo. Cutting fuel use and eliminating the oil changes now needed to fly safely would reduce maintenance costs and carry environmental benefits.
“We can get rid of the lubricating system and reduce the weight of the engine, and at the same time reduce the effort needed to maintain the engine,” says Dr Waldek Dmochowski, Leader of the Tribology/Mechanical Components Group at the NRC Institute for Aerospace Research in Ottawa.
Oil free bearings that lubricate shafts with a microns-thick “air wedge” were first theorized about 50 years ago. They are now in regular, reliable use in auxiliary power units, which are little turbines used to start the main engines of aircraft. But their trial-and error design has been as much art as science, and none are robust enough yet for a full-sized motor. Dr Dmochowski and his colleague, Dr Martin Conlon, believe that new materials and bearing designs should slide oil free bearings into aircraft turbines in the next decade or two.
“In principle, the shaft floats on an air film similar to an air hockey table,” says Dr Conlon. “The only difference is that there is no need to pump air to the bearing itself. The spinning shaft supplies its own air.”
Companies that make oil-free bearings shield their construction as trade secrets, but the NRC researchers say it is relatively straightforward. Bearing surfaces are thin metal foil, backed by a separate metal foil resembling corrugated cardboard, wrapped around a turbine shaft. The bearing's surface has a low-friction coating that “oils” the bearing against the shaft at start-up, until the shaft can rotate fast enough to wedge air between it and the bearing. It lubricates again only when the motor shuts-off.
With increasing speed, the thin bearing surface foil and its backing flex and shift separately to hold an air wedge around the shaft. Such bearings change shape or “conform” as they work to create the optimal gap geometry.
NRC's tribology lab is aiding the eventual adoption of oil-free bearings by investigating new ways to improve load capacities and vibration damping. Drs Dmochowski and Conlon's test bed can measure bearing performance on rotating shafts up to 7.5 cm in diameter, spinning at up to 60,000 rpm. The large shaft sizes and test speeds put the lab's capabilities beyond any other in the world.
According to Dr Dmochowski, NRC tests on the bearings' properties and behaviours contributes to better scientific understanding that will propel their design from black art to solid engineering. “We have a unique, world-class facility,” he says. “We are getting interest from larger engine manufacturers now.”