Abstract
Rolling element bearings for aero engine applications have to withstand very challenging operating conditions because of the high thermal impact due to elevated rotational speeds and loads. The high rate of heat generation in the bearing has to be sustained by the materials, and in the absence of lubrication these will fail within seconds. For this reason, aero engine bearings have to be lubricated and cooled by a continuous oil stream. When the oil has reached the outer ring it has already been heated up, thus its capability to remove extra heat from the outer ring is considerably reduced. Increasing the mass flow of oil to the bearing is not a solution since excess oil quantity would cause high parasitic losses (churning) in the bearing chamber and also increase the demands in the oil system for oil storage, scavenging, cooling, hardware weight, etc. A method has been developed for actively cooling the outer ring of the bearing. The idea behind the outer ring cooling concept was adopted from fins that are used for cooling electronic devices. A spiral groove engraved in the outer ring material of the bearing would function as a fin body with oil instead of air as the cooling medium. The method was first evaluated in an all steel ball bearing and the results were a 50% reduction in the lubricating oil flow with an additional reduction in heat generation by more than 25%. It was then applied on a Hybrid ball bearing of the same size and the former results were reconfirmed. Hybrid bearings are a combination of steel made parts, like the outer ring, the inner ring, and the cage and of ceramic rolling elements. This paper describes the work done to-date as a follow up of the work described in, and demonstrates the potential of the outer ring cooling for a bearing. Friction loss coefficient, Nusselt number, and efficiency correlations have been developed on the basis of the test results and have been compared to correlations from other authors. Computational Fluid Dynamics (CFD) analysis with ANSYS CFX has been used to verify test results and also for parametric studies.
Highlights
Ceramics are widely used in the aviation industry because they are lighter, and because they can withstand much higher loads at a much higher endurance when compared to steel [1].Aerospace 2018, 5, 23; doi:10.3390/aerospace5010023 www.mdpi.com/journal/aerospaceAerospace 2018, 5, x FOR PEER REVIEWCeramics are widely used in the aviation industry because they are lighter, and because they can withstand much higher loads at a much higher endurance when compared to steel in the of bearings, the benefit of ceramics is alsoisinalso the in reduction of the friction losses [1]
This is a phenomenon that has been observed in ball bearings and has to do with the pumping action induced by the axial load [4,8]
A 167.5 mm PCD hybrid bearing equipped with a helical cut around its outer ring for active
Summary
Ceramics are widely used in the aviation industry because they are lighter, and because they can withstand much higher loads at a much higher endurance when compared to steel [1]. The bearing races were duplex hardened damper in order to damp vibrations [1]. In its way through the bearing, the oil continuously removes heat from the inner ring,ring, the the rolling elements, bearing cage until it reaches outer ring. Since the the oil oil has has not unlimited capability to remove heat, heat removal from the outer ring is limited. This has not unlimited capability to remove heat, heat removal from the outer ring is limited This problem has initiated the idea of the outerouter ring ring cooling. The hybrid version is equipped squeeze damper, as this on is shown in 3 This enabled damping possible vibrations during testing. 3. Details of the hybrid bearing arrangement with outer ring cooling and squeeze film damping
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