Impact of Lubrication Analysis on Improvement of AH-64D Helicopter Component Performance

The purpose of this research study is to provide evidence for the further study and possible inclusion of lubricant data and analysis to common condition based maintenance (CBM) practices as a means of increasing the effectiveness of the AH-64D fleet. Presented in the paper are four different AH-64D aircraft wetted-component case studies, which aim to improve performance through the examining the effect of oil and grease on component performance and fault detection. The first case study was designed to simulate a worst case scenario regarding a leaking output seal on three different tail rotor gearboxes (TGB). The second study presents an oil feasibility experiment on the AH-64D intermediate gearbox (IGB) in which the oil replaced the grease that is traditionally used. The final case study involves two approaches in assessing the theoretical and practical approach in assessing the effectiveness of an oil analysis method to identify a failure by comparing it with the final recommendation provided by AOAP labs. The final case study also examines and compares the efficacy of oil analysis and vibration data at detecting component faults. Though not conclusive, initial correlation observations indicate that more study and research should be conducted to determine and measure the impact that lubricant has on component performance and detecting component faults. Introduction The condition monitoring technologies that determine the health of a machine are crucial for implementing condition based maintenance (CBM) practices. Industrial standards for CBM focus mainly on vibration analysis, with some input from temperature signatures. However, this limited focus may overlook other potentially useful indicators, which, if incorporated into the condition indicators (CIs) that are used to predict component failure, could serve to improve fault detection performance. Lubricant and filter debris analysis is one noticeable area where potentially significant CI and CBM improvements could be made. The two gearbox assemblies that make up the AH-64 tail rotor drive train are termed the intermediate and tail rotor gearboxes (IGB & TGB, respectively). Both gearboxes utilize NS4405-FG grease as their sole lubricant. The primary function of the lubricant is to prevent contact between components in relative motion in a mechanical system by maintaining a thin film between the surfaces. Faults in these two gear boxes are frequent and create significant problems in the field, taking time and money that may not be easily available. The TGB, for example, is prone to leaking grease near the output seal. Additionally, even newly serviced AH64 IGBs have been found to eject large volumes of grease through the gearbox breather port which requires the aircraft to land for immediate maintenance. The Army conducts oil analysis studies in an effort to further understand the impact the lubricant has on the performance of numerous moving components on the aircraft. By understanding the relationship between lubricant properties and AH-64D wetted components, this study hopes to provide motivation for further research and studies into the impact that oil and grease analysis might have for improving tail motor performance and predicting component failures. Industrial lubricants can be generally classified as oil or grease based. Due to the distinct properties of both classes of lubricant, different responses and damage processes are observed in practice. Historically, oil has been a popular lubricant for high-speed machinery as a result of its ability to act as both a coolant and a lubricant [3]. In addition, because oil is a Newtonian fluid, meaning its viscosity is independent of shear rate, its viscosity is easily determined with known temperatures and pressures. Finally, many oilbased lubricants contain additives to resist corrosion, contamination, and extreme temperatures and pressures. The increase in applied stress and the range of environmental conditions under which the lubricant should work are motivating factors driving the selection of grease. Grease is the preferred choice in components prone to leaking because of its high washout resistance. Chemically, grease is composed of a three dimensional thickener network dispersed in base oil. The base oil is trapped in the thickener structure by physical and chemical forces, i.e. capillary effect and van der Waals forces. Lubricating grease exhibits plasticity, viscoelasticity, thixotropy and a complex set of rheological effects [3]. The functional properties of grease are dependent upon shear stability, mechanical stress, volume of contaminants, and variations in temperature and pressure. Grease cannot dissipate heat as well as oil which results in a higher range of peak temperatures present in an operating component. Higher peak temperatures can have noticeable effects on the chemical and physical properties of grease lubricants. For example, the base oil can bleed out of the thickener, leading to the destruction of the thickener network or lubricant starvation on the contact surface. However, in some cases, this chemical change can lead to further lubrication of the contact surface. Grease is a viscoelastic material and falls into the category of nonNewtonian fluids, its viscosity is dependent upon shear rate. Typical greases are shear-thinning fluids (viscosity decreases as shear rate increases across a certain range). Consequently, under zero-shear conditions, these lubricants can act as a sealant but will act as a lubricant at high shear rates. The non-Newtonian effects influence the local film thickness and pressures as well. Even though grease properties cannot be expressed in sample parameters as with other Newtonian fluids, apparent viscosity and consistency are characteristics that can define the effectiveness of grease as a lubricant. Little is known about the impact that oil or grease has on vibration and temperature levels of aircraft components. Currently, Apache helicopter performance is determined by measuring condition indicators regarding temperature and vibration from mounted accelerometers and thermocouples. Therefore, it is necessary to examine the relationship between different lubricating agents and the subsequent response in CIs, in order to be able to recommend adjustments in the CIs to take lubricant characteristics into account. This will ultimately increase the understanding of the gearbox and improve fault diagnostic capabilities. This paper will address three different case studies: a TGB experiment with leaking output seal, an IGB oil feasibility study, and an investigation into the effectiveness of AOAP data on different AH-64D wetted components in detecting faults. The results of these studies demonstrate initial indications that further study into the impact that lubricant analysis might have on CI performance is warranted. Main Body Determining correlations between lubricant analysis and component condition and CIs can either be performed using experiments conducted on actual components or through data analysis. This research utilizes both methods. Component testing of the TGB and IGB was made possible through a unique collaboration between the University of South Carolina (USC) and the South Carolina Army National Guard and the resultant test facility at USC. Data analysis utilized data supplied by the Army Oil Analysis Program (AOAP). CBM testing facility at USC Over the past decade, USC has developed and maintained a strong working relationship with the South Carolina Army National Guard. USC has played a key role in the development of a fully-matured CBM Research Center within the USC Department of Mechanical Engineering which hosts several aircraft component test stands in support of current US Army CBM objectives. The USC test facility possesses a complete AH-64 tail rotor drive train test stand (Figure 1), which is designed to facilitate a scientific understanding of aircraft component conditions and provide empirical understanding of various failure modes and other parameters that can be harbingers of component failure. These observations are necessary for the development of comprehensive and accurate diagnosis algorithms and prognosis models. The test stand emulates the complete tail rotor drive train to the tail rotor swash plate assembly and is composed of true AH-64 aircraft hardware. The test stand was designed and built to accommodate the use of multiple Health and Usage Monitoring Systems (HUMS) and is currently equipped with a Honeywell Modernized Signal Processing Unit (MSPU). Alongside the MSPU, another data acquisition system is employed known as the USC DAQ that operates in parallel to the MSPU. In addition to being used to run the test stand, the USC DAQ collects temperature data from thermocouples and discrete raw vibration time domain data from accelerometers mounted on the components of interest. The availability of this test stand made possible the implementation of the first two experiments, those of the simulated leak in the TGB and the grease/oil comparison in the IGB. It also facilitated the customization of the experiments to simulate the specified conditions and to introduce additional sensors as needed. Figure 1 – USC AH-64 tail rotor drive train test