1、04FTM9Design of a High Ratio, Ultra Safe, HighContact Ratio, Double Helical CompoundPlanetary Transmission for HelicopterApplicationsby: F.W. Brown, M.J. Robuck, G.K. Roddis and T.E. Beck,The Boeing CompanyTECHNICAL PAPERAmerican Gear ManufacturersAssociationDesign of a High Ratio, Ultra Safe, High
2、ContactRatio, Double Helical Compound PlanetaryTransmission for Helicopter ApplicationsFrederick W. Brown, Mark J. Robuck, G. Keith Roddis and Timothy E. BeckThe Boeing CompanyThe statements and opinions contained herein are those of the author and should not be construed as anofficial action or opi
3、nion of the American Gear Manufacturers Association.AbstractAn ultra-safe, high ratio compound planetary transmission, for application as a helicopter main rotor drive,has been designed under the sponsorship of the National Rotorcraft Technology Center- Rotorcraft IndustryTechnology Association (NRT
4、C/RITA). It is anticipated that this new planetary transmission offersimprovements relative to the current state-of-the-art including, reduced weight, reduced transmitted noiseand improved fail-safety. This paper discusses the analysis and design results for the subject planetarytransmission. Fabric
5、ation and testing of the transmission will be conducted in subsequent phases of theproject.Typically, the final stage in helicopter main rotor drives consists of one or two simple planetary stages whichthemselvesarecomposedofasungear,multipleplanetgears(typicallybetween3and6)andaninternalgearwhich i
6、s generally the fixed member of this epicyclic system. The main rotor transmission is the most criticalandusuallytheheaviestassemblyinthedrivesystemforanyrotarywingaircraft,beitasingleortandemrotorhelicopter or a tilt rotor vehicle.Thenewultra-safe,highratioplanetarytransmissiondesignutilizesacompou
7、ndplanetaryconfigurationwitha17.5:1reductionratiowhichwouldreplaceaconventionaltwostagesimpleplanetarytransmission.Thenewdesignusesultra-safeprinciplessuchassplit-torquepathsandhighcombinedcontactratiogearing.Doublehelical gears in the planet/ ring meshes balance axial tooth forces so that axial bea
8、ring reactions are notrequired. The spur gear sun/planet meshes are staggered to achieve a compact spatial arrangement.In order to provide major, simultaneous improvements in weight, noise, power density, and reliability, aparadigm shift must occur in the basic design of the gear system in this comp
9、onent. This program bringstogether a large number of individual gear system innovations which, taken together, are expected to yield alarge improvement in noise and performance combined with improved reliability and fail safety.Copyright 2004American Gear Manufacturers Association500 Montgomery Stre
10、et, Suite 350Alexandria, Virginia, 22314October, 2004ISBN: 1-55589-832-71Design of a High Ratio, Ultra-Safe, High Contact Ratio, Double Helical Compound Planetary Transmission for Helicopter Applications Frederick W. Brown, Mark J. Robuck, G. Keith Roddis and Timothy E. Beck The Boeing Company, Roto
11、rcraft Division, Philadelphia, PA Introduction Currently, the final gear reduction stages in virtually all helicopter main rotor drives consist of one or two simple spur planetary stages. These simple planetaries are composed of a sun gear, multiple planet gears (typically between 3 and 6) and an in
12、ternal gear that is generally the fixed member of this epicyclic system. While this proven concept provides relatively good power transmission efficiency, it has a tendency to generate high noise levels due to the limited contact ratio of the individual gears and the multiple, often simultaneous, to
13、oth contacts of the several planet gears. The simple planetary drives often used in helicopter main transmissions are subject to several technical limitations. Load capacity is limited as a function of the number of planets that can be accommodated within the internal gear that is, itself sized by t
14、he combination of ratio requirements and allowable stress limits. The planet bearings, which are generally mounted in the bore of each planet gear, must be sized to carry the applied loads and to allow a minimum rim thickness under the planet gear teeth. This requires the planet gear and bearing to
15、be optimized as a unit. In addition, more planets improve load capacity but as the reduction ratio is increased, the number of planets that will fit in the annulus between the sun and ring decreases. Practical reduction ratios, which can be obtained from one-stage simple planetary systems, are limit
16、ed by geometry to approximately 5:1. Because of this limitation, two stages (or more) are often required to obtain higher reduction ratios. Because the simple planetary system is densely packed, a gear tooth failure is generally very destructive as the high power density and very limited space among
17、 the gears prove to be a vulnerable configuration. While it might seem, on the surface at least, that the use of multiple planets would provide parallel load paths, which might offer some redundancy, quite the opposite is true. In a conventional design, the planets themselves provide little in the w
18、ay of fail- safety; rather the dense package almost assures consequential damage will occur. Another consideration is that when a planet gear looses load-transmitting capability, the internal load balance of the planets is compromised so that radial mesh loads are no longer balanced on both the sun
19、and internal ring gears. Technical Approach Despite the limitations cited above, the planetary gear system, in a general sense, is far from obsolete. Many still serve quite well in applications flying all over the world. One aircraft which utilizes a two-stage simple planetary design is the CH-47D C
20、hinook helicopter. While this transmission system was designed in the late 1970s, it continues to be a reliable and efficient gearbox to this day. However, improvements are necessary for the next generation of rotorcraft transmissions. Rather than scrapping the entire concept, an improved planetary
21、concept has been investigated that retains that which is good about the simple planetary and adds specific features or design enhancements. These features are directed at increasing load capacity, improving system safety and the ability to withstand single tooth faults, while decreasing the generate
22、d noise level and reducing weight. To circumvent the limitations of current planetary designs and to investigate improved gear arrangements for helicopter main rotor drives, a high ratio, Ultra-Safe, 2high contact ratio, staggered and intermeshed planet, double-helical compound epicyclic gear system
23、 concept was developed. This gearbox arrangement is intended to provide simultaneous improvements in load capacity, noise level, and system safety. A technical effort, under the sponsorship of the National Rotorcraft Technology Center- Rotorcraft Industry Technology Association (NRTC/RITA), with the
24、 following objectives is currently being conducted to address this improved planetary speed reducer design: 1. Develop the concept configuration and basic gear system design for an improved planetary gear system with the following technical goals: Reduce planetary subsystem weight by 25% compared to
25、 the CH-47D baseline planetary Reduce transmitted noise at planetary operating frequencies by 10 dB compared to the CH-47D baseline planetary Improve tolerance for defects and damage 2. Produce detailed designs for all components and test configuration hardware 3. Fabricate the improved planetary, i
26、ncluding the sun, planet, and internal gears and the supporting carrier 4. Test the improved planetary transmission in a full-scale (CH47D) test configuration to simulate operational characteristics and verify the concept. 5. Demonstrate, via test, the weight and noise reduction achieved relative to
27、 the baseline planetary subsystem. As of this writing, items 1 and 2 are complete. Item 3, test component fabrication, is currently underway. Testing and evaluation is scheduled for completion in 2005. There are basically three major milestones in this program design, manufacture, and test. Of cours
28、e, each of these major accomplishments is based on the successful completion of a number of interrelated tasks. Design Approach The CH-47D Chinook rotor transmission planetary drive was selected as the baseline application for the design study. Currently, this transmission utilizes a two-stage conve
29、ntional spur gear planetary design with fixed internal ring gears. Torque is supplied to the first stage sun gear that, in turn, meshes with four planet gears. First stage output is via a planet carrier that drives the second stage sun gear. The second stage sun meshes with six planets and the secon
30、d stage carrier drives the helicopter rotor shaft to turn the helicopter main rotor blades. This two-stage planetary provides a reduction ratio of 17.47-to-one. The planetary gears are designed for continuous operation at 4,500 horsepower at 225 RPM output. From initial concept studies, a planetary
31、configuration emerged that can potentially meet the performance goals of an improved planetary design in the CH47D main rotor drive application. A compound planetary configuration, with unconventional features, has the potential to improve a simple planetary drive. Compound planetary designs are not
32、 new. They are used in automotive, off-road, and gas turbine applications and, more recently, in helicopter main rotor transmissions. In their traditional arrangement, compound planetary reducers encounter some of the same limitations as simple planetary drives. However, enhancements to the basic co
33、mpound epicyclic drive arrangement allow an increase in load capacity, reduction in weight and envelope, reduction in noise, and improved damage tolerance through fail-safety. We will examine each of these aspects as they pertain to the CH-47D transmission design. A load capacity limiting factor of
34、the simple planetary arrangement is the number of 3planets that can be accommodated within the internal ring gear diameter. Note that the sun-planet mesh in a compound planetary is not restricted to fit within the internal ring gear diameter. This means that the sun-planet envelope may be radially l
35、arger than in a simple planetary design. With the compound planetary, “staggering” the planets can circumvent another limitation and substantially increase capacity. In a staggered arrangement, the sun gear is lengthened to allow it to mesh with staggered planetary gears, as seen in Figure 2. This w
36、ill double the number of planet gears which can fit within the compound planetary, resulting in significantly higher capacity and power density for a given gearbox envelope. The use of double helical gears in a compound planetary arrangement is another unique feature. Double helical gears are used e
37、xtensively in heavy industry like mill drives. One benefit of the double helical Figure 1. CH-47 Forward Rotor Transmission Cross Section arrangement is increased total contact ratio due to the action of the helix angles in the double helical gears. The increased contact ratio results in higher mesh
38、 load capacity and reduces mesh-generated noise. Since the thrust loads developed on each helix within the double helical gear are oriented in opposing directions, the net axial load from a double helical mesh is zero. This permits a simpler support bearing arrangement where large axial load capacit
39、y is not required. The double helical design also enhances the fail-safety or Ultra-Safe characteristics of the compound planetary transmission. The premise of Ultra-Safe gear design is to use design guidelines and specific techniques to improve the fail-safe operation of gears in Simple Planetary S
40、econd Stage Simple Planetary First Stage 4critical applications, such as, helicopter rotor transmissions. This differs from designing gears for greater reliability so that the probability of failure is lower. Designing for greater reliability is an important aspect of transmission design, but it is
41、an aspect that is treated in the typical design process. The goal of improved damage tolerance is addressed by implementing Ultra-Safe features in the transmission design. These Ultra-Safe design features are aimed at improving gear system fail-safety relative to standard design practices. Our tradi
42、tional design approach focuses on increasing reliability, or, designing so that the probability of failure (due to typical modes) is reduced. The Ultra-Safe approach addresses the problem of what happens when (not if) a failure occurs. Certain gear design configurations lend themselves to improved f
43、ail-safety. A number of examples are discussed in 1. For double helical gears in a compound planetary transmission, the higher total contact ratio of the double helical gears and having the mesh load distributed between two helices (in parallel), tend to improve the Ultra-Safe characteristics of the
44、 transmission design. This is also true of the “staggered” planetary arrangement that permits a greater number of parallel load paths between the sun and planet gears, contributing to the Ultra-Safe characteristics of the compound planetary transmission.Figure 2. Staggered planet sun meshes in compo
45、und planetary (with early concept 2 piece ring gear) Ring Gear (2 piece shown) Sun Gear Planet 1 Ring Gear Planet 2 Sun Gear Planet 1 Planet 2 5Design and Analysis The preliminary design, detailed design and analysis of the high ratio, Ultra-Safe, high contact ratio, double helical compound planetar
46、y transmission has been completed. Preliminary design efforts evaluated a series of alternative configurations for the basic compound planetary gear system and a down-selection to a finalized configuration for the design, fabrication and testing phases. The primary considerations leading to the down
47、-selection of the final concept were weight, envelope, cost, and noise reduction features. An early version of the concept, which employed helical gears in the high-speed sun-planet mesh, is shown in Figure 3. We found that we could vary the basic configuration in a wide variety of ways to emphasize
48、 one quality over others, depending on the specific needs of the system. The concept shown was selected to demonstrate the configuration and technology for the CH47D baseline application, however, the specific aircraft drive system and subsystem requirements would influence any eventual production e
49、mbodiment of this concept. Figure 3. Compound Planetary (early concept with helical high speed mesh) Detailed design of the transmission proceeded, with this concept as its basis, and encompassed design and drawing preparation, component stress analysis, FE modeling and analysis, assembly simulations, risk evaluations etc. The detailed design phase of this project is focused on producing hardware that can be used to experimentally verify the benefits of the compound planetary concept. Although the CH47D main rotor transmission application was used as th
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