Research Papers

Windage Power Losses From Spiral Bevel Gears With Varying Oil Flows and Shroud Configurations

[+] Author and Article Information
Graham Johnson, Budi Chandra, Colin Foord

University Technology Centre in Gas Turbine Transmission Systems, University of Nottingham, University Park, Nottingham NG7 2RD, UK

Kathy Simmons1

University Technology Centre in Gas Turbine Transmission Systems, University of Nottingham, University Park, Nottingham NG7 2RD, UK


Present address: Rolls-Royce plc, P.O. Box 31, Derby DE24 8BJ, UK.

J. Turbomach 131(4), 041019 (Jul 13, 2009) (7 pages) doi:10.1115/1.3072519 History: Received September 03, 2008; Revised October 24, 2008; Published July 13, 2009

In many aero-engines, the power to drive accessories is transmitted through high speed bevel gears in a chamber in the center of the engine. The windage power loss (WPL) associated with these gears makes a significant contribution to the overall heat generation within the chamber. Shrouding the gears provides an effective method of reducing this WPL and managing the flow of lubricating oil. Experimental and computational programs at the University of Nottingham Technology Centre in Gas Turbine Transmission Systems are providing an improved understanding of shroud performance and design. This paper presents the results from a pair of shrouded meshing gears run at representative speeds and oil flow in a rig with speed and torque measurement. A previously published study of a single bevel gear operating in air (Johnson, 2007, “Experimental Investigation Into Windage Power Loss From a Shrouded Spiral Bevel Gear” ASME Paper No. GT2007-27885) found a reduction in torque of up to 70% from shrouding. In this work, the addition of oil and the pinion gear did not lead to high torque due to the buildup of oil under the shrouds, but the reduction in torque due to fitting the shrouds is significantly less than was found for the same gear in air alone. In order to isolate the various parameters, further testing with a single gear was carried out. A fully (360 deg) shrouded gear shows a big improvement over an unshrouded gear when running in air alone, but much of this benefit disappears as soon as a very small amount of oil is introduced under the shroud. This implies that the oil is recirculating under the shroud. Increasing the oil flow beyond this initial level increases the torque by the amount required to accelerate the oil mass flow up to the peripheral speed of the gear. Providing a full width slot in the shroud downstream of the oil jet allows the oil to escape without any recirculation and restores much of the benefit of the shroud. Further insight into the oil behavior is obtained from torque measurements and observations through a transparent shroud and with various slot configurations. Video observation shows evidence of a vortex flow under the shroud that carries some of the oil toward the inner diameter of the gear. The three main windage contributors, air alone, recirculation of oil under the shroud, and acceleration of the feed oil, are quantified and methods for achieving the optimum design are discussed.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

(a) Experimental facility. (b) Experimental facility schematic.

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Figure 3

Shrouded crown gear and pinion. 1. Crown gear; 2. Crown shroud; 3. pinion gear; 4. Pinion – shroud; 5. Into mesh oil jet; 6. Pinion bearing oil jet.

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Figure 4

Effect of pinion bearing oil flow on torque at 10,000 rpm main shaft speed

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Figure 5

Torque distribution with shrouded gear and pinion

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Figure 6

Windage torque for unshrouded gear

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Figure 7

Shroud cross section

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Figure 8

Windage torque at 12,500 rpm

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Figure 9

Shroud with slot B

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Figure 10

Extent and position of slots

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Figure 11

Slotted shrouds at 10,000 rpm

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Figure 12

Oil streak under the shroud at 5000 rpm. 1. Crown gear; 2. Shroud inlet; 3. Into mesh oil jet; 4. Gear outer diameter; 5. Path of oil under the shroud.

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Figure 13

Oil leaving slot B at 5000 rpm. 1. Face of crown gear; 2. Shroud inlet; 3. Shroud exit space; 4. Start of shroud slot; 5. Path of oil exiting the shroud.

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Figure 14

Breakdown of terms that determine windage torque level

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Figure 15

Air alone windage torque and torque coefficient Cm




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