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A balanced program of computational and experimental research is performed to provide a fundamental understanding of lubrication and its application to a wide variety of complex machinery. Priority is given to developing a high-temperature lubrication system technology consistent with the Integrated High-Performance Turbine Engine Technology (IHPTET) initiatives of DoD. Basically, the research is aimed at achieving good friction, lubrication, and wear properties over a broad range of temperatures and loads. More specifically, the research is to enable bearings and seals to be fully functional from low-temperature startup conditions to full-speed engine conditions where maximum temperatures are encountered.
New and novel concepts such as quasi-hydrodynamic lubrication with triboparticulate matter are also studied. This research combines computational modeling of powder and granular flow characteristics in lubricated contacts with laboratory experiments. Using a kinetic theory model, numerical simulations were performed for granular flow due to shear between two parallel plates. Granular interactions are represented using two-dimensional central force fields with a force cutoff at the granular radius. Angular momentum is exchanged via granular friction. A slider bearing consisting of two parallel counter moving walls are studded with affixed granules has been modeled, and between them is a medium of inter-colliding granules moving about in the intervening gap. The program is run in transient form, computing all particle-particle collisions. Movies were generated using computer animation to illustrate particle flow within the bearing. Macroscopic time-average values for velocity and density distributions, wall stresses, etc., are obtained in the same way as in classical kinetic theory. While the kinetic theory provides insight and is instructive, it is computationally expensive. A continuum approach enables one to make these same calculations without the burden of keeping track of all particle-particle collisions. The experiments led to establishing a constitutive relationship (i.e. stress vs. strain-rate) that assisted in formulating a continuum model. Understanding the rheology of powders as lubricants offers an alternative means (in lieu of liquids) of providing properly designed lubrication systems for high temperature applications. This information has been applied to develop a novel hybrid powder lubricated journal bearing aimed at man rated and expendable engines. Feasibility has been demonstrated and performance characterized with various powder lubricants.
A tilting-pad journal bearing was developed to investigate the overall feasibility, performance, and thermal characteristics of powder lubrication. MoS2 (particle size = 1 to 2 µm) was used as the powder lubricant. The basic feasibility of lubricating journal bearings (1.338 in. dia) to run at speeds to 30,000 rpm and radial loads to 50 lbs. was successfully demonstrated. After 8½ hours run time at various loads and speeds, the measured wear for the shaft and pads was extremely low. Friction coefficients were comparable to a petroleum base liquid lubricant run under the same conditions of load and speed. Bearing pad temperatures were never allowed to exceed 350°C. Thermal stability was achieved at all load and speed combinations. Power loss for the bearing was shown to decrease with an increase in load and speed. These tests were repeated using WS2 as the test lubricant with the same trends exhibited.
Modern surface analysis tools, including Auger electron microscopy with x-ray energy analysis, are used to study the tribological behavior of materials in solid-state contact. Both liquid and solid-lubrication studies include new molecular structures with high-temperature thermal and oxidative stability; also included in these studies is the role of metals in lubrication degradation. In parallel, solid-lubrication research includes conventional and vacuum-deposition techniques for thin films. Polymer and ceramic-metal composite structures also are being examined. A significant laboratory tool is the tribometer, a high- temperature friction and wear-test apparatus that measures the tribological properties of ceramics and metallic alloys from room temperature to 1200°C.
POC: David E. Brewe
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