High-speed, heavily loaded, and lightweight gearing components are commonplace in rotorcraft systems. These systems are expected to deliver high power from the gas turbine engines to the high-torque, low-speed rotor. The gearing systems in these extreme-duty applications can have thermal behavior problems because of high pitch line velocities that approach 25,000 ft/min.
Helical gear train test components.
Long description of figure 1.
A significant number of tests with varying speeds and loads were conducted on a full-scale, aircraft-quality, high-speed helical gear train. These tests validated the effects of gear shrouding and lubricant jet pressure on the thermal and mechanical performance of this gear system.
Because of the high speed, the thermal behavior of mechanical components can change a design that is successful from a load-capacity (bending and contact stress) viewpoint into a thermally induced failure (because of high operational temperatures, gear tooth scoring, and high drive system losses). In rotorcraft drive systems, such as those of tiltrotors, a helical gear train is used to separate the parallel engine and rotor shafting on the aircraft. This part of the drive system operates at very high rotational speeds and carries the full power of the engine during operation. In this type of arrangement, several idler gears are used to transmit power between the engine and the rotor shaft centerlines. Since these gears have two thermal cycles per revolution and are extremely lightweight (low heat-carrying capacity), the successful operation of the system in all possible normal and emergency conditions can be difficult. The test facility at the NASA Glenn Research Center is full scale and can operate to 15,000 rpm (to simulate the engine input rotational speed) and at power levels to 5000 hp. The test facility is being used to assess performance improvements for normal and emergency lubrication operation.
The current test program investigated the effects of gear shrouding and lubricating jet pressure on the performance of this high-speed helical gear train. The temperature increase in the lubricant and power loss on the gearing system as a function of these variables is shown in the following graphs. Test data are shown for two speeds (12,500 and 15,000 rpm, for forward flight and hover conditions, respectively), whereas the load on the system was maintained at similar conditions (approximately 33 percent of full conditions). Both measured parameters, the temperature increase across the test gearbox and the amount of power to drive the entire facility, were affected by the shrouding and the lubricant jet pressure. At either speed condition, the full shroud case was the best over all the speed and load conditions except when the flow was reduced to approximately 60 psi. Using just the bottom half of the shrouds resulted in a high temperature increase across the test gearbox. The no-shroud condition could not be conducted at 15,000 rpm because of operational instability thought to be due to interactions between the lubricant and the bull gear that affected the proper scavenging of the lubricant. A similar trend is shown for the amount of power supplied to the entire test facility for shroud, bottom-half-shroud, and no-shroud conditions. Full-shrouded gears require the least amount of drive motor power for the tests conducted.
Top: Effect of lubricant jet pressure, input shaft speed, and shrouding on the temperature increase across the test gearbox. Bottom: Effect of lubricant jet pressure, input shaft speed, and shrouding on the power to drive the test facility
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Find out more about Glennís Mechanical Componentís Branch: http://www.grc.nasa.gov/WWW/5900/5950/
U.S. Army Research Laboratory at Glenn contact:
Dr. Robert F. Handschuh, 216-433-3969, Robert.F.Handschuh@nasa.gov
Author: Dr. Robert F. Handschuh
Headquarters program office: Aeronautics Research, VSP
Last updated: October 16, 2006
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