Browse Topic: Gears
The Main Gearbox of a helicopter is a crucial component that delivers the desired performance and ensures the highest possible level of safety of the aircraft; it includes several gears and bearings, which require to be continuously lubricated by a pressurized oil flow. Undesired circumstances may cause the oil to leak from the main circuit, hence reducing its pressure and consequently the oil flow rate targeted towards the rotating components; this modifies their friction coefficient, and subsequently leads to an overheating of the parts with the risk of degenerating in a catastrophic failure. During the design of a helicopter drive system, engineers need to take proper precautions and make sure that the MGB is fully equipped with the proper features to cope with a loss of lubrication event; specifically, the drive system is supposed to be able to run at least 30 minutes after the oil pressure drops to zero. A lot of effort has been put over the years at Leonardo Helicopters to find robust solutions to attain the longest performance of the drive system in no-oil conditions: the most important result is the certification of the AW189 for a 50-minutes “run dry” capability. Nevertheless, the dynamic environment typical of the rotorcraft industry pushes towards continuous innovation, and in the last few years the Transmissions Systems Design department of LH has been asked to investigate suitable ways to further augment the no-oil capabilities of the MGB: the main steps followed and entailed results are presented in this paper. The first part of the manuscript discusses the “state of the art” auxiliary lubrication system, currently flying on the AW189 drive system. The second part tackles the approach adopted to meet the novel requirements, unveiling both the methodology and the final design choice: the latter includes a metering element, able to tune the oil flow rate headed towards the component deemed the most critical in order to satisfy the requirement of longer no-oil performance. Numerical and experimental tools are exploited as complementary tools to properly crystallize the obtained results and corroborate the solution.
The Sikorsky Boeing SB>1 DEFIANT is a technology demonstrator aircraft that was built under the Joint Multi-Role Technology Demonstrator (JMR TD) program to address the next generation performance requirements of the US Army Future Vertical Lift (FVL) initiative. During the development of the SB>1 DEFIANT technology demonstrator aircraft several manufacturing lots of gears were produced with a core hardness that was 10-30% below the minimum engineering requirement. The defect was not detected until a large population of gears was near completion. To prevent significant program cost and schedule impacts, a safe load capacity for the discrepant gears was determined via test. Dynamically loaded ground test articles for SB>1 DEFIANT technology demonstrator aircraft began qualification testing with the low hardness gears. The low hardness issue, root cause, and test method to establish a safe operating load limit are discussed.
This study presents static analyses of transmission error in a single gear pair gearbox for six pairs of hybrid composite-steel spur gear designs developed in a previously presented optimization effort. The results were compared to those of the same gearbox featuring a baseline all-steel gear pair. The gearbox models were developed in the commercial software RomaxDESIGNER R17. A tooth stiffness analysis was also conducted to replace the tooth stiffness values calculated in Romax with values that consider the web effects on tooth stiffness. These stiffnesses are used to calculate the mesh stiffness. This analysis showed a 2-5% difference in average tooth stiffness of the hybrid gears compared to the steel gear. The transmission error analysis with the new tooth stiffnesses showed a 1-3% increase in the transverse error in for hybrid gears compared to the baseline steel.
High speed rotorcraft transmissions are subject to load-independent power losses consisting of drag and pumping loss. Tightly conforming shrouds enclosing the transmission gears are often incorporated to reduce the drag component of the total load independent losses. However, tightly conforming axial shrouding can result in an increase in the pumping loss component. Quantifying the pumping loss of shrouded gear transmissions has been the subject of many studies. This study presents a new approach for estimating pumping loss based on the concept of swept volume borrowed from the positive displacement pump and compressor industry. In this study, pumping loss of shrouded gear transmissions is considered to be related to the swept volume of the gear sets and the downstream flow resistance created by the shroud clearances. The drag loss and pumping loss of a spur gear pair have been determined through testing using the NASA Glenn Research Center Gear Windage Test Facility. The results from this testing have been compared to theoretical results using the formulations presented in this study. Good correlation exist between the test pumping power loss and the predicted pumping power loss for tightly conforming axial shroud configurations.
This study develops an optimization technique for a sinusoidal interlock design of a hybrid spur gear consisting of a metallic outer ring to support high contact stress bonded to a composite inner web for weight reduction. Two objectives (mass and shear traction on the metal-composite interface under static loading conditions) were minimized for four design variables subject to two constraints. Borg MOEA, a multi-objective evolutionary algorithm developed at The Pennsylvania State University, and an in-house finite element solver were used to generate Pareto-optimal solutions to this design problem. Two of the designs were then analyzed in greater detail to determine stress distributions throughout the gear. In the future, this technique will be refined and applied to optimization of more representative rotorcraft gears, with the aim of reducing drive train weight and meeting performance requirements.
ABSTRACT Rotorcraft gearbox transmissions are required to efficiently transfer power from the turbine engine to the main and tail rotor blades. Losses in transmission efficiency impact mission payload and aircraft range. These systems are expected to deliver high power with high gear pitch line velocities. More recently, shrouding has been employed to reduce windage power losses associated with the high gear rotational speeds. However, recent experimental results from tests conducted by the authors show the negative impact of close clearance shrouds on windage power loss, particularly at the meshed region where flow is ejected, or jetted, from the collapsing tooth spaces. A literature review was conducted to gain further insight into the phenomenon of gear mesh jetting and strategies to mitigate and control the associated losses. An analysis was conducted on windage losses in the mesh region. Test results are given for a modified shroud configuration. Finally, a discussion on observed trends follows with suggestions on future research.
ABSTRACT The focus of this work is to integrate component level design analyses developed for different machine elements of a twin Pericyclic drive into a comprehensive design decisions framework. The integrated system loads, bearing loads, and tooth contact analysis procedure is used for designing a prototype for minimum weight within the constraints posed by assembly, component life, and system efficiency. Simultaneous sizing of the gears, bearings and shafts was performed for given input power, speed, and reduction ratio. The effect of inertial loads due to nutational gear motion is significant on support bearing loads and the gear bodies are designed to minimize these loads. It was demonstrated that a power density close to 1 kW/kg can be achieved for the Pericyclic transmission. The test article is designed to operate at a 50 HP, 5000 RPM input with a speed reduction ratio of 32:1 and system efficiency greater than 93%.
ABSTRACT This study investigates the vibration of the OH-58A and OH-58C planetary gear stages at operating conditions using a finite element/contact mechanics model. These two planetary gears have identical sun, planet, and ring gears. They differ in the number of planets, the planet spacing, and the mesh phasing, which results in substantial differences in their vibration characteristics. The elastic vibrations at fixed points on the ring outer diameter of both planetary gears have spectra with frequency content in clusters near each harmonic of mesh frequency. The OH-58A planetary gear, which has equally-spaced planets and in-phase meshes, has the same frequency components near each harmonic of mesh frequency. They include response at mesh frequency harmonics and upper and lower sidebands at multiples of the planet pass frequency. The OH-58C planetary gear has different frequency content than the OH-58A because it has diametrically-opposed planet spacing and gear meshes that are out-of-phase. The frequency content near odd harmonics of mesh frequency differs from that near even harmonics. There is no response at odd harmonics of mesh frequency, and upper and lower sideband frequencies are prominent. Response occurs at even harmonics of mesh frequency. Upper and lower sideband frequencies occur at multiples of twice the carrier frequency. The amplitudes of the sideband frequencies are sensitive to the system's input torque.
ABSTRACT Steel components, such as gears for rotorcraft transmissions, are quench hardened to improve the hardness, strength, and fatigue performance. During a quench hardening process, components are heated to form austenite, followed by quenching (either gas or liquid) to transform to hard martensite. With High Pressure Gas Quenching (HPGQ), parts are rapidly cooled by using a pressurized gas such as Nitrogen or Helium. Hardening is a highly nonlinear process due to the plastic deformation caused by thermal stresses and phase transformations, both of which lead to distortion. Reducing distortion caused by hardening has always been pursued by heat treaters for the purposes of improving part quality and reducing cost. A new gas quenching method to minimize distortion of Ferrium C64 gear steel was developed by DANTE Solutions as part of an effort with the US Army's Aviation Development Directorate (ADD). This method utilizes a new state-of-the-art gas quench chamber to control the temperature uniformity of parts using a recipe developed through computer modeling.
ABSTRACT The pericyclic transmission provides the opportunity to vastly impact transmission design in rotorcraft due to its ability to provide exceedingly high reduction ratios in a single stage that would normally require multiple gear stages. This could lead to lighter transmissions with fewer components, increased reliability, efficiency, speed and decreased cost to maintain. While many previous studies have focused upon the gearing within the pericyclic transmission, this work focused on what influences pericyclic geometry, and how changes in geometry impact bearing loads. Specifically, the loading of bearings that must deliver power from the input shaft to the nutating and rotating gears of the system were of primary concern. A comprehensive look at dynamic loads generated by nutating bodies was performed. Methods to address these dynamic loads via application of counterbalances, and deviation from conventional pericyclic transmission designs were utilized to negate the dynamic moment of concern. Counterbalances negating the dynamic moment were shown to weigh between 30-50% of the pericyclic motion converter gears in a 40:1 reduction ratio pericyclic design at 12,000 rpm input speed and reduced applied moments by three orders of magnitude. Finally, a static solver was used to determine the bearing loads with updated component geometries and mass moment of inertias that included the required counterbalances.
ABSTRACT 'Loss of Lube' testing was conducted to experimentally evaluate benefits of Lubricants and Coatings to extend the time to failure for gears and bearings that are experiencing a loss of lubrication event. In this project, the testing of Loss of Lubrication technologies were divided into two categories; Ball-on-Disk screening testing and Gear Specimen Testing. The Ball-on-Disk screening tests were performed at Wedeven Associates using their patented WAM testing apparatus. The screening tests were divided into three 'waves' of testing which consisted of lubricant tests (Wave 1), coating tests (Wave 2) and lubricant-coating refinement testing (Wave 3). Wave 3 testing used the best performing lubricants and coatings from Wave 1 and Wave 2, respectively, to establish consistent results and identify any potential anomalous results from the first two waves of screening testing. The second portion of this study was to perform actual loss of lubrication testing on spur gears at Penn State's Gear Research Institute. Using the data obtained from the Ball-on-Disk screening tests, 3.5" PD spur gears of various base material steels were fabricated, many of them with coated teeth, and subjected to loss of lubrication conditions under load to assess their ability to perform without proper lubrication.
ABSTRACT The wind turbine, aerospace, and helicopter gear industries recognize the importance of surface finish and surface texture for maximizing component and system performance. Optimizing surface finish and surface texture has been shown to reduce failure rates and increase operating safety margins. Isotropic superfinishing in the form of chemically accelerated vibratory finishing has been utilized to increase the performance of new wind turbine, aerospace, and helicopter gears for many years. The wind turbine gearbox industry has also used isotropic superfinishing as a method of repairing damaged gears for over a decade. The aerospace and helicopter gear industries have only minimally employed this technology as a repair technique. As the aerospace and helicopter industries scrap many gears due to only minor surface damage, further consideration of isotropic superfinishing as a repair tool is warranted. This paper will summarize the technical capabilities, recent advancements, and economic benefits of using isotropic superfinishing to repair wind turbine, aerospace, and helicopter gears. With this information, the aerospace and helicopter gear industries will be better positioned to evaluate isotropic superfinishing's potential to recover otherwise scrap gears and thereby reduce sustainment costs.
ABSTRACT Many spiral bevel gear applications are implemented with a two piece gear where the gear and shaft are bolted or inertia welded as an assembly task. Eliminating a bolted, splined, or welded joint between the spiral bevel gear and shaft can reduce complexity and cost in a transmission design. Simulations of the motion of spiral bevel machine tools have been developed to ensure successful integral shaft designs for helicopters. Awareness of the location of the wheel during spiral bevel gear tooth manufacturing enables a design solution to iterate between the gear shaft and spiral bevel gear tooth design. The simulation was revisited to refine a preliminary two dimensional tool as well as create a more detailed three dimensional visualization for use during the design process. The design process was applied to three helicopter bevel gear designs. Physical verification was completed on two designs as a risk reduction in the bevel gear grinding machine before the design was released to manufacturing. The tools used in the design process as well as verification during manufacturing are discussed in this publication for three different integral shaft spiral bevel gears for helicopters.
ABSTRACT Windage power loss in high-speed gearboxes result in efficiency losses and increased heating due to drag on the gear teeth. Meshed spur gear windage power loss test results are presented at ambient oil inlet temperatures both with and without shrouding. The rate of windage power loss is observed to increase above 10,000 ft./min., gear surface speed, similar to results presented in the literature. Shrouding is observed to become more effective above 15,000 ft./min., decreasing power loss by 10% at 25,000 ft./min. The need for gearbox oil drain slots limits the effectiveness of shrouding on reducing windage power loss. Also, windage power loss is observed to decrease with increasing gearbox temperatures and to increase with oil flow. Windage power losses for the unshrouded meshed spur gears are 7x more than losses determined from unshrouded single spur gear tests. A 6x to 12x increase in windage power loss is observed comparing shrouded single spur gear data with shrouded meshed spur gear data. Based on this preliminary study additional research is suggested to determine the effect of oil drain slot configurations, axial and radial shroud clearances, and higher gear surface speeds on windage power loss. Additional work is also suggested to determine the sensitivity of windage power loss to oil temperature and oil flow. Windage power loss of meshed spur gears tested in both the shrouded and unshrouded configurations is shown to be more than double versus the same spur gears run individually in the same shroud configurations. Further study of the physical processes behind these results is needed for optimizing gearbox shrouds for minimum windage power loss.
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.
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