Browse Topic: Tribology
Bench-level tribological experiments were utilized to evaluate material, coating, and lubricant formulation effects on the loss-of-lubricant survivability of tapered roller end and cone rib contacts. Cone rib and roller end contacts were simulated using a single rotating roller and rotating flat disk. The applied load and rotational speeds of the roller and disk were controlled to simulate representative rotorcraft gearbox bearing operating conditions. The contacts were lubricated for an initial period before the lubricant supply was shut off, and the supply tube was then removed. Tests continued to run, without additional oil, until the measured friction force reached a predetermined cutoff value. Weibull-based statistical analysis was used to compare the loss-of-lubrication runtimes.
ABSTRACT The development of a Wedeven Associates Machine (WAMmp) for micro-pitting utilizes an advanced gearbox design and other support components to apply high loads and precision surface velocities while measuring traction under incipient sliding conditions. It is intended to evaluate oil and material pairs for specific performance characteristics related to high cycle fatigue and micropitting. Testing and modeling from WAMmp data creates the opportunity to predict the performance of bearing/gear materials, surface processing, and lubricants during the component design phase. Wedeven Associates, Inc. (WAI) has developed surface finishing processes to axially hone test articles to represent gear tooth finishing. The development of this method provides a meaningful tool for evaluation of new technologies and for predictive modeling for advanced gearbox and drive system designs.
ABSTRACT Loss of the primary lubrication in a helicopter gearbox can result in a very rapid or even immediate failure of the system due to the much-reduced heat removal and the degrading tribological performance of the highly loaded gear contacts. While a limited understanding of this topic may be an acceptable risk for ground vehicles, however, a properly functioning gearbox is flight safety critical for helicopters. Therefore a deeper understanding of the degradation mechanisms is essential to accurately assess the time duration in which the helicopter gearbox can function under oil-out conditions and evaluate designs targeting the desired extension. Current methods for predicting the gearbox life and performance under the loss-of-lubrication situation are indeed largely experimental and experience-based and they provide only limited insights into the underlying physics of the evolving tribology of gears and bearings. One of the major technical barriers that currently limit the physics-based predictive capability is a lack of reliable, quantitative modeling of lubricant retention on the gear tooth surface after the loss of lubrication. This paper first describes the film thickness measurement with the white light interferometry for the lubricant remaining on a glass disc after a certain number of revolutions at a given speed. This is followed by a description of a 3D numerical ANSYS CFX® model which mimics the experimental set-up. The controlling model parameters are the centrifugal and viscous forces, surface tension, temperature, and lubricant-disc contact angle. The predicted effects of rotation speed and temperature are validated by the experimental results. Finally, the modeling methodology is used to simulate the lubricant retention on a gear tooth surface over the range of temperature and speed of a typical helicopter gearbox.
ABSTRACT The current status of an ongoing effort to develop a comprehensive gearbox aero-thermodynamics and tribology simulation tool, named PSULOL, applicable to both well-lubricated and loss-of-lubrication operation is presented. PSULOL employs a multi-scale approach, wherein various physical effects including: meshing tribology, convection heat transfer within the system and to the environment, high-frequency thermodynamic effects induced by the gear meshing frequency, and the long-time response of the overall gearbox temperature to a net imbalance of heat generation and transfer to the environment are simulated separately and coupled with one another iteratively through appropriate boundary and initial conditions. As established in 2014, the first-generation version of PSULOL was the first computational fluid dynamics-based (CFD-based) tool coupled with an all-lubrication regime tribology model capable of simulating transient loss-of-lubrication failure of high-speed gearboxes. This first version was built on an in-house research CFD code, NPHASE-PSU, and relied on a number of physical simplifications, particularly with regard to the geometric configuration of the housing, and to the effects of the coupling between multiphase flow within the system (retained lubricant dynamics) and meshing tribology. Here, present efforts underway to increase the physical fidelity and ease-of-use of PSULOL are outlined, and progress toward simulating physically-realistic gearbox configurations is shown. This includes modeling the disperse multiphase flow of lubricant droplets and film within the system, and transitioning from NPHASE-PSU, an in-house research CFD code, to StarCCM+, a commercial code that is more user-friendly and features additional geometry-handling capabilities.
A newly-developed tribology model is integrated with a reduced-order heat transfer model to construct a simulation platform which is used to conduct system-level simulation of transient multi-physics loss-of-lubrication (LoL) operation of a high-speed gearbox. In the simulations, the processes of increasing gear bulk temperature and of oil depletion are transient. The former is captured by the reduced-order heat transfer model, and the latter is studied parametrically. A nominal problem is studied based on LoL tests of involute spur gears by NASA Glenn. The simulation results qualitatively capture the trend and scale of the "thermal run-away" observed in the experiments. Parametric analyses are carried out to demonstrate the capability of the simulation platform to guide design optimizations in various aspects, including gear configurations, material considerations and secondary emergency lubrication systems.
Polyetheretherketone (PEEK) is a popular material for high performance bearing cages. It is now being considered for aerospace drive train applications due to the 80% weight savings compared to steel, but gaps remain in the understanding of PEEK's tribological performance. In this study, wear testing was performed to investigate the tribological performance of fiber reinforced PEEK under conditions that were representative of a typical aerospace drivetrain application. It was found that the processing conditions used can have a significant effect on the wear resistance. Further, it was found that both of the optimized PEEK materials tested experienced significantly less wear than the silver plated steel used as a baseline. Finally, it was determined that the friction loss of a PEEK bearing cage is expected to be higher than that of a silver plated steel one, but is still quite low with a coefficient of friction of approximately 0.02-0.03.
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