Transmission system for use in microturbine-powered applications

Monday 16 September 2013

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Transmission system for use in microturbine-powered applications

Abstract
A transmission system for use with a microturbine engine comprising an input shaft operatively coupled to the output shaft of the microturbine engine. The output shaft and microturbine engine input shaft of the transmission system operating at a rotation speed in a range of 50,000 to 250,000 rpm with an output between 200 and 5 HP. The transmission system includes a gear train having a first gear that interfaces to a second gear, the first and second gear tooth surfaces employing opposing-hand helix angle which apply axial forces to the input shaft during both normal operation and start-up operations, which pushes the input shaft to the output shaft of the microturbine engine. Preferably, the helix angle surfaces on respective sides normal drive teeth produce an axial force during normal operation can not be overcome by the axial force produced by the mass acceleration time of the first gear. The interfaces of the gear train to a starter generator preferably through second gear. The gear train preferably provides a reduction ratio has a value greater than 24.
Description 
GOVERNMENT RIGHTS NOTICET
his invention was made under U.S. Government support under Contract DAAH10 - 02- C -0058 awarded by the U.S. Army . The U.S. Government has certain rights in the invention.
Background of the Invention
1. Field of the Invention

This invention relates generally to transmission systems . More particularly, this invention relates to transmission systems that transmit power from a microturbine engine
 .Two . State of the Art 
Unmanned aerial vehicles low cost small ( UAVs ) have been developed and deployed to carry out a variety of military roles , including reconnaissance and attack missions . At present , the spark ignition engines 100 HP intermittent (or less ) of power to all low-speed aircraft UAV. Most of these motors driving propellers without the need for a gearbox . However, these burn gasoline engines , which is highly flammable and therefore undesirable for field service operations . Piston engines also have undesirable vibration characteristics and are difficult to start in cold weather operations .Locust U.S. , Inc of Miami , Florida , assignee of the present invention has developed a microturbine engine and gearbox for use in unmanned aerial vehicles . The microturbine engine , when used in conjunction with heavy jet fuel ( such as JP- 8) , provides a propulsion system for UAVs highly advantageous because of its lighter weight, less fuel use flammable , higher reliability and reduced vibration. Microturbine engines operate at very high rotational speeds , typically in the range 50,000 to 250,000 RPM RPM with an output power of 200 HP and 5 HP . Unmanned aerial vehicles operating in much slower propeller rotation speeds , typically about 2000 RPM to 7000 RPM . These constraints determine that the gearbox provides a reduction ratio of 25:1 to 36:1 by the RPM range and the power range of the microturbine engine .Locust current designs , two different types of drive spline couplings and two different types of axial retention mechanisms can be used to couple the microturbine engine output to the input of the gearbox . The two types of spline drive coupler includes an outer diameter spline driven coupler (sometimes referred to as a " root diameter flat spline fit coupler ") and a spline coupler polygon. The two types of axial locking mechanism comprises a retaining ring and the fastening screws .Figures . 1A and 1B illustrate an outer diameter spline piloted wherein the output coupler spline microturbine engine 2 6 which includes projections projecting radially inwardly from the inner diameter surface for engaging the driven section 5 of the input shaft 3 gearbox . The outer diameter spline driven coupler is well suited for high speed engine RPM produced by the microturbine . Moreover, the axial displacement of the input shaft 3 relative to the output groove 2 which is permitted by the spline outer diameter driven coupler provides for easy and efficient mounting and dismounting of the coupling between the engine and gearbox.In the configuration of FIG . 1A, a snap ring 8 is arranged within an annular groove in the inner diameter surface of the spline output 2 and projecting radially inwards to engage the input shaft 3 in a way that limits the axial shaft input 3 relative to the groove of output 2. The retaining ring 8 is problematic because it is very hard to assemble due to the fact that the retaining ring is "stuck" in a small annular groove in the outer diameter surface of the input shaft 3 .In the configuration of FIG . 1B, the adjustment screws 9 are screwed through holes that pass from the outer diameter surface of the inner diameter surface of the spline output 2 and projecting radially inwards to engage the input shaft 3 of a way limiting the axial travel of the input shaft relative to the output spline 2 . The nine screws are easier to install than the snap ring , but may have problems with imbalance at very high engine and are prone to wear.In small applications , it may be difficult to manufacture the teeth ( projections ) piloted bore spline coupling. Alternatively, in such applications of small size, with a polygonal spline coupling , as shown in Fig . 1C can be used . This coupler uses a polygonal rifling exemplary three lobes polygonal design . This design can be manufactured by grinding very accurate system to produce polygonal adjustments.Therefore, there remains a need in the art for providing a mechanism for coupling a micro-turbine engine to a gear box in a way that is easy to assemble and disassemble while also providing for improved reliability , and incorporating such a mechanism in a small light propulsion system for use in unmanned aerial vehicles . 
SUMMARY OF THE INVENTION
 Therefore, it is an object of the invention to provide an improved mechanism for coupling a microturbine engine to a transmission system that is reliable and easy to assemble and disassemble .Another object of the invention to employ the improved coupling mechanism as part of a propulsion system lightweight and small size .It is still another object of the invention to employ the improved coupling mechanism as part of a propulsion system lightweight and small size for aircraft applications .Another object of the invention to employ the improved coupling mechanism in conjunction with a microturbine whose output shaft is operating in a range of 50,000 to 250,000 RPM RPM with an output power of 200 HP and 5 HP .It is a further object of the invention to employ the improved coupling mechanism in conjunction with a transmission system that provides a reduction ratio of preferably greater than 24, which is suitable for applications UAVs fixed wing . A ratio lower reduction (eg , around 9:01 ) for applications of unmanned aerial vehicles vertical lifting speed that uses a bevel gear reduction .In accordance with these objects , which will be discussed in detail below, an unmanned aerial vehicle (UAV ) is provided which uses a microturbine engine to propel an aircraft through a transmission system. The aircraft can be a fixed-wing aircraft , vertical lift aircraft , or a hybrid tilt rotor aircraft . The spline microturbine engine output is coupled to the input shaft of the transmission system preferably by a coupling which allows axial displacement of the output spline concerning the input shaft but does not permit radial movement ( for example, a diameter piloted outer spline coupler , which is sometimes referred to as a root diameter flat major adjustment spline coupler or coupler polygonal rifling ) . A retaining clip can be used to limit the axial travel when the engine is off microturbine . The spline microturbine engine output and the input shaft of the transmission operate preferably at a speed of rotation over an extended range between 50,000 and 250,000 RPM RPM with an output power of 200 HP and 5 HP . The transmission system includes a gear train which is coupled to the input shaft . The gear train includes a first gear and a second gear that interface to one another. The first gear is mounted on the input shaft and rotates in the same direction as the input shaft . The second gear is mounted on a counter-rotating shaft and therefore rotates in the opposite direction as the input shaft . The output of the transmission system is obtained from the counter-rotating shaft and is operatively coupled to a propeller of the aircraft. The first and second gears also are operatively coupled to the output of a starter generator which is used during startup , which uses the generator as a motor for starting.In the preferred embodiment , the starter generator is performed by a 4-pole brushless permanent magnet AC type architecture with a plurality (eg, four ) magnets mounted around a perimeter of the rotor. A power control unit converts the AC output to a DC output in a first mode , which is used during normal operation to generate the supply current for the electrical components of the aircraft. The power control unit also converts direct current input ( from a battery) to AC input in a second mode , which is used during startup.The first and second gears operate differently in these two modes . In the first mode (normal operation) , the spline output microturbine engine drives the input shaft and the first gear of the transmission system , which in turn drives the second gear and the starter generator coupled thereto. In the second mode (start-up ) , the starter / generator drives the second gear , which in turn drives the first gear , the input shaft of the transmission system and the output spline microturbine engine coupled to same . The first and second gear teeth are used on surfaces - hand helix angle on respective opposite sides of normal driving and normal coast sides of the teeth. These angled surfaces helix- apply an axial force to the input shaft in both the first mode (normal operation) and the second mode ( operation starting) which pushes the input shaft to the engine output spline microturbine . In the preferred embodiment , the helix angle surfaces on respective sides of the normal drive teeth are designed in such a way as to produce an axial force in the first mode (normal operation) which can not be overcome by the axial force produced by mass times acceleration of the first gear.Be appreciated that the axial thrust force applied by the helix angle surfaces of the teeth of the gears controlling the first and second axial displacement of the output spline connection with the input shaft without the use of a set screw or other retention mechanism awkward . Omitting the set screw or other awkward retention mechanism makes installation and removal simple and efficient while providing better reliability .In the preferred embodiment , the transmission system design employs a two-stage intermediate compound which provides a reduction ratio of preferably greater than 24. Moreover, the transmission system may comprise any (or part ) of a number of configurations, including an in-line shaft configuration lay , a star configuration - online star or star configuration stimulus offset . Such configurations provide a reduction ratio preferably having a value preferably greater than 24 applications UAVs fixed wing . A ratio lower reduction (eg , around 9:01 ) for applications of unmanned aerial vehicles vertical lifting speed that uses a bevel gear reduction .According to one embodiment of the invention, the radial loads applied to the first gear, auto - balance so that the input shaft of the transmission system can be supported without bearings.According to other embodiments of the invention, the transmission system and the drive system based on microturbine of the present invention may be readily adapted for use in other applications, such as marine propulsion systems , automotive applications, applications electric power generation , micro - turbine based HVAC applications and hydraulic applications. Moreover, the drive system based on the present invention microturbine may consume a wide variety of fuels, including liquid (such as liquefied natural gas) or gaseous fuel (such as natural gas or propane ) .Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the figures provided . 
BRIEF DESCRIPTION OF THE DRAWINGS

. 1A is a cross section illustrating a prior art coupler piloted spline outer diameter and the retaining ring , which together can be used to couple the output shaft of a microturbine engine to the input shaft of a reduction transmission system speed.. 1B is a cross section illustrating a prior art coupler spline outer diameter and driven screws which together can be used to couple the output shaft of a microturbine engine to the input shaft of a transmission system to reduce speed.. 1C is a cross section illustrating a tri- lobe polygonal coupling spline of the prior art that can be used to couple the output shaft of a microturbine engine to the input shaft of a transmission system to reduce speed.. 2A is a schematic diagram of a propulsion system based on helix including a microturbine engine and transmission system according to the present invention.. 2B is a graphical illustration of an exemplary fixed-wing UAV employing propulsion system based on the propeller of FIG . 2A according to the present invention.. 3A is a schematic diagram depicting an illustrative embodiment of the transmission system of FIGS . 2A and 2B, which is realized by a configuration of two-stage idler displacement compound according to the present invention.. 3B is a cross section showing the main gear of the drive train of the compound idler setting displacement of the figure. 3A.. 3C is a schematic illustration of the first stage gear set . 3A shows the input pinion , the input drive gear , idler gear and driving gear generator generator together with the generator.. 3D is a schematic illustration of the surfaces of double helix angle of the gear teeth of the input pinion of the figures. 3A - 3C according to an exemplary embodiment of the present invention , the drive gears that interface input pinion teeth surfaces each used in double helix configuration having opposite hand with respect to the pinion gear entry .Figures . 4A and 4B are schematic diagrams illustrating the principles of an input stage self - setting equilibrate for compound idler displacement of the figures . 3A - 3C .Figures . 5A and 5B are schematic diagrams illustrating the axial force applied by the pinion teeth and the input drive gear of the input stage of the configuration of the compound idler displacement of the figures . 3A and 3B during normal operation (when the input pinion gear drives the drive ) FIG . 5A illustrates the configurations of the helical teeth to rotate clockwise output shaft of the engine and the transmission input shaft and Fig . 5B illustrates the configurations of the helical teeth for counterclockwise rotation of the output shaft of the engine and the transmission input shaft .Figures . 6A and 6B are schematic diagrams illustrating the axial force applied by the pinion teeth and the input drive gear of the input stage of the configuration of the compound idler displacement of the figures . 3A and 3B during startup operation (in the input pinion gear is driven transmission ), FIG . 6A illustrates the configurations of the helical teeth for clockwise rotation of the output shaft of the engine and the transmission input shaft and Fig . 6B illustrates the configurations of the helical teeth for counterclockwise rotation of the output shaft of the engine and the transmission input shaft .. 7A is a schematic diagram of a propulsion system based on vertical lift rotor , including a microturbine engine and transmission system according to the present invention.. 7B is a pictorial illustration of a vertical elevator which employs UAV exemplary propulsion system of the rotor from vertical elevation of Fig . 7A according to the present invention.Detailed Description of the Preferred EmbodimentsTurning now to FIG . 2A shows a power plant 10 suitable for use in propelling a fixed wing aircraft . The power plant 10 includes a microturbine engine 12 with an output shaft 14. An example of such a microturbine engine is disclosed in detail in U.S. Patent . No. 5,727,378 to Seymour, hereinafter incorporated by reference in its entirety. A coupling mechanism 16 couples the output shaft 14 to the input shaft 18 of a transmission system 20 . The transmission system 20 operates to reduce the speed of the output shaft 14 of the microturbine engine 12 in its own output shaft 22. The output shaft 22 of the transmission system is coupled to a propeller 24 by a coupling mechanism 26. Helix 24 , when driven by the microturbine engine 12 and the transmission system 20 provides a thrust which propels a body of airplane. Note that in the configuration shown in Fig . 2A, the transmission system 20 and the propeller 24 are disposed on the intake side of the microturbine engine 12. This configuration enables the transmission system to be cooled by the engine intake air . Alternatively, the transmission system 20 and the propeller 24 may be disposed on the exhaust side of the microturbine engine 12 as shown best in fixed wing aircraft unmanned Fig . 2B . In this alternative configuration , the transmission system and the helix must operate in a hot environment, and therefore must be designed to withstand the increased heat load that derives from the operation in the hot environment on the exhaust side engine 12.The output shaft 14 of microturbine 12 and the input shaft of the transmission system 18 operate very high speeds of rotation , preferably in the range 50,000 to 250,000 RPM RPM with an output power of 200 HP and 5 HP . For applications of unmanned aerial vehicles, low speed , the propeller 24 operates at slower rotation speeds , typically about 2000 RPM to 7000 RPM . These restrictions result in a reduction ratio required microturbine engine RPM to the RPM of the propeller in the order of 25:1 to 36:1 . The transmission system 20 provides this required speed reduction in the range of output power ( 200 HP to 5 HP ) microturbine engine 12. In the preferred embodiment of the present invention , the transmission system 20 and the microturbine engine 12 are of small size and low weight , and therefore suitable for use in advanced UAVs .Figures . 3A and 3B illustrate an embodiment of the transmission system 20 of FIGS . 2A and 2B, which is realized as a setting compound idler Offset two stages. The first stage of this configuration idler Offset Compound P51 includes an input gear that is connected to two diametrically opposed gears G51 . Each gear G51 is connected to a corresponding intermediate shaft with a second pinion 31 P52 combining stage to drive a single output gear G52 . G52 output gear of the second stage is offset from the input pinion P51 (Fig. 3A) . This displacement is possible geometrically because the center distance of the second stage is greater than the center distance of the first stage. The intermediate shafts of the two gears 31 and two pinions P52 G51 (shown in cross section in fig. 3B ) and spline coupled to the output shaft 22 are supported by bearings as shown. P51 input pinion is integral with the input shaft 18 and rotates in the same direction as the input shaft 18. The two gears G51 are integral with the intermediate counter-rotating shafts 31 and thus rotate in the opposite direction as the input shaft 18. The rotational output of the transmission system 20 is derived from the counter-rotating shafts 31 through the interface between the sprockets P52 and the output gear G52 .As shown best in FIG . 3C , Starter Generator 32 interfaces to the G51 marches through an idler gear and pinion GEN1 GEN2 . Preferably, interfaces to GEN1 idler gear G51 so that the generator has adequate clearance and mounted roughly on the center line of transmission. This geometry is selected (as compared to a geometry of cancellation of the load) due to the loads of the generator are small compared with the normal loads of the motor . The idler gear GEN1 allows clearance between the body of the starter generator 32 and the transmission case . Preferably, the pinion gear GEN2 has more teeth than the pinion P51 so that the RPM of the starter generator 32 is slower than the speed of the motor. In the preferred embodiment , the starter generator 32 is performed by a 4-pole brushless AC architecture type permanent magnet with a plurality (eg, 4) of magnets mounted around a perimeter of the rotor. A power control unit converts the AC output to a DC output in a first mode , which is used during normal operation to generate the supply current for the electrical components of the aircraft. The power control unit converts the input current ( from a battery) to AC input in a second mode , which is used during startup. P51 pinion gear and gears G51 function differently in these two modes . In the first mode (normal operation) , the output of spline 14 of the microturbine engine 12 drives the input shaft 18 and the pinion gear of the first stage P51 , which in turn drives the gears G51 and the starter - generator 32 coupled thereto through gearing GEN1 and GEN2 . In the second mode ( start ) , the starter / generator 32 drives one of the gears G51 through GEN1 and GEN2 gears , which in turn drives the sprocket P51 , the input shaft 18 and the output 14 of the microturbine engine 12 spline coupled thereto.According to the present invention, the pinion gear teeth employs P51 having surfaces in - hand helix angle on respective opposite sides of normal driving and normal coast sides of the teeth ( for example , one of the angled surfaces -helix having a right hand orientation and the other surface of helix angle that is oriented to the left ) as shown in Fig . 3D . The drive gears G51 also employ teeth having surfaces in - hand helix angle on respective opposite sides of normal driving and normal coast sides of the teeth ( for example , one of the angled surfaces of which have an orientation helix right-hand helix and the other angle surface having a left orientation ) . Pinion gear G51 P51 and the drive gears are, in essence , helical gears , but helix angles are small enough to be considered with corrections helical gears . During normal operation ( with the microturbine engine driving the transmission system ) , as shown in FIGS . 5A and 5B for both directions of rotation of the engine different surfaces helix angle of the teeth induce axial loads that push the input shaft 18 to the output shaft 14 of the microturbine engine 12. In the figure. 5A , the pinion teeth used P51 helix- angle surfaces dexterous than your normal share of the unit, the G51 gear teeth used left hand helix angle surfaces than your normal share of the unit, and the input shaft 18 microturbine is urged against the output shaft 14 when the motor is driven clockwise as seen in Fig . In the figure. 5B, the P51 gear teeth surfaces used in left-handed helix angle of your normal unit , the gears teeth using skilled G51 helix- angle surfaces in the normal side drive , and the input shaft 18 is driven into the microturbine output shaft 14 when the motor is driving to the left as seen in Fig . During the startup mode operation ( where the generator 32 drives the transmission system 20 and microturbine engine ) as shown in Figs. 6A and 6B for both motor rotation directions different surfaces helix angle of the teeth induce axial loads that push the input shaft 18 to the output shaft 14 of the microturbine engine 12. In the figure. 6A used P51 pinion teeth surfaces - handed helix in its normal side coast , the gears teeth employ skilled G51 helix- angle surfaces in Normal coast side , and the input shaft 18 is driven against microturbine of the output shaft 14 when the motor is driven clockwise as seen in Fig . In the figure. 6B , the pinion teeth used P51 helix- angle surfaces dexterous than your normal share of the cost , G51 gears teeth used left hand helix angle surfaces in its normal part of the coast, and the input shaft 18 microturbine is driven output shaft 14 when the motor is driven counterclockwise as viewed in Fig . In these configurations, the helical teeth of the input gear G51 P51 and the drive gears employ the construction of FIGS . 5A and 6A to the motor rotating clockwise or Figs building . 5B and 6B for motor rotation anticlockwise. During normal operation , the axial thrust produced by the propeller angled surfaces on the drive side of normal tooth causes the input shaft 18 to operate in contact with the output shaft microturbine spline connection 14 with four transmission of torque and axial retention provided by the axial load produced. Suring launch, the axial thrust produced by the propeller angled surfaces on the side of the normal tooth coast also forces the input shaft 18 to operate in contact with the output shaft microturbine 14 with spline connection 4 of torque transmission and axial retention provided by the axial load produced. Therefore, both during operation and startup, the helix angle surfaces on opposite sides of normal teeth gives a thrust on the input shaft 18 in a direction to force the input shaft 18 toward microturbine output shaft 14. Omitting the set screw, snap ring , or other retaining mechanism awkward makes assembly and dismantling simple and efficient while providing better reliability .In the preferred embodiment , the helix angle surfaces of the teeth are designed in such a way as to produce an external force during normal operation can not be overcome by the axial force produced by the mass and the acceleration of the input pinion P51 . In the illustrative embodiment of FIGS. 3A and 3B, the GEN1 and GEN2 also use helical gears . Helical gear teeth GEN1 is of the opposite hand to helical gear teeth G51 on opposite sides of the tooth. Helical gear teeth GEN2 is of the opposite hand to helical gear teeth GEN1 on opposite sides of the tooth. However, in alternative embodiments, it is contemplated that GEN1 and GEN2 gear can be performed by spur gears which interface to the drive train via a spur gear interface .Another consideration for the design of the transmission system of the present invention is the high speed operation of the input shaft 18. It is very difficult to design bearings suitable for use with radial loads at speeds as high .

Therefore, it is expected that the configuration of the idler wheel is advantageous compound for use in lightweight applications , such as in small unmanned aerial vehicles fixed wing propeller.The rotor 128, when driven by the microturbine engine 112 and transmission system 120, provides a thrust drives an aircraft body , as the body of the vertical lift aircraft 130 of FIG. This configuration enables the transmission system 120 and the bevel gear assembly 124 to be cooled by the engine intake air .In the preferred embodiment of the present invention , the transmission system 120 and the microturbine engine 112 has a small size and low weight. Preferably , the maximum diameter of the transmission system 120 is less than 12 inches.
While we have described particular embodiments of the invention, it is not intended that the invention be limited thereto , as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise . Therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as claimed .