Abstract:
A two speed transmission having an input shaft and an output shaft, the two speed transmission being capable of changing the rotating speed of the output shaft from a first speed ratio to a second speed ratio, the shift between the first rotating speed ratio and the second rotating speed ratio being smoothly accomplished by the present invention&#39;s combination of two sets of planetary gear clusters and two electric motors, the electric motors being used to smooth the mechanical shift between the first speed ratio and the second speed ratio.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is related to U.S. Provisional Patent Application No. 60/464,312 filed Apr. 21, 2003 from which priority to such application is claimed. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    Current rotorcraft (helicopter) transmission systems are designed for a single reduction ratio with the engine optimized at a single rotational speed. These transmission systems have no ability to vary the rotor speed other than changing the engine speed. In fact, recent studies have indicated that changing rotor speed using a transmission can provide significant benefits. A 15% rotor speed reduction can result in as much as 5 dB noise-reduction. In contrast, changing the rotor speed by varying the engine speed may have adverse consequences. For example, varying the engine speed impairs engine fuel efficiency and could force the engine to operate closer to the critical stall boundary speed.  
           [0004]    Rotorcrafts and/or tilt wing crafts have two modes of operation: a hover mode and a high-speed forward flight mode. In hover mode, a high rotor speed is desirable to improve maneuverability and load factor. In the high-speed forward flight mode, a reduced rotor speed will significantly reduce the rotor noise and keep the velocity of advancing blades subsonic. Thus, in light of the above, a two-speed transmission is needed.  
           [0005]    Various transmission concepts have been considered. These transmissions fall generally into two broad categories: those that are continuously variable transmissions (CVT) and those that are two-speed transmissions. Most CVT&#39;s were based on traction drives or friction drives where the power was transferred though non-positive engagement frictional contacts. These drives are relatively large and heavy, and their efficiency is poor. Additionally, reliability is a major concern for this type of drive.  
           [0006]    Two-speed transmissions that generally using gears to transmit torque and power have a higher power-density and are more efficient than CVT&#39;s. However, shifting from one speed to another speed has proven to be a challenging task—particularly under high power. The shift is usually abrupt and large power changes take place abruptly within the transmission. Either of these characteristics could cause a momentary loss of output power or damage to the transmission or drive train.  
         SUMMARY OF THE INVENTION  
         [0007]    The current invention relates to variable speed transmissions in general and more specifically to a two-speed transmission with a smooth powered shift. Additional features of the present invention will be in part apparent and in part pointed out hereinafter. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0008]    In the drawings, wherein like numerals and letters refer to like parts wherever they occur.  
         [0009]    [0009]FIG. 1 is a vertical section view of one embodiment of the present invention; and  
         [0010]    [0010]FIG. 2 is a general schematic of one embodiment of the present invention; 
     
    
       [0011]    Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.  
         [0012]    While one embodiment of the present invention is illustrated in the above referenced drawings and in the following description, it is understood that the embodiment shown is merely for purpose of illustration and that various changes in construction may be resorted to in the course of manufacture in order that the present invention may be utilized to the best advantage according to circumstances which may arise, without in any way departing from the spirit and intention of the present invention, which is to be limited only in accordance with the claims contained herein.  
       DETAILED DESCRIPTION  
     Description of the Apparatus and Method  
       [0013]    Referring to FIGS. 1 and 2, a two speed transmission A is shown as one embodiment of the present invention. The two speed transmission A is comprised of a compound planetary train  1  having a first planetary unit U 1  and a second planetary unit U 2 , a first electric machine  3 , a second electric machine  4 , a first locking device  5 , and second locking device  6 .  
         [0014]    The first planetary unit U 1  comprises a first sun gear  7  that is operatively connected to an input shaft  8  which is rotated by an engine (not shown), a first set of planetary gears  9 , and a first ring gear  10 . The first ring gear  10  has a first bull gear  11  that can be selectively connected to a first fixed member  27  in the two speed transmission A and that can be held stationary by activating the first locking device  5 .  
         [0015]    The second planetary unit U 2  comprises a second set of planetary gears  12  and a second ring gear  13 . Like the first ring gear  10  in the first planetary unit U 1 , the second ring gear  13  is integrated with a second bull gear  14  that can be selectively connected to a second fixed member  15  in the two speed transmission A and that can be held stationary by activating the second locking device  6 .  
         [0016]    The first set of planetary gears  9  in the first planetary unit U 1  is compounded with the second set of planetary gears  12  in the second planetary unit U 2  to form a planetary cluster  16 . The planetary cluster  16  is supported on a common carrier  17  which is operatively connected to an output shaft  18  that drives a lift rotor or propeller through reduction gear trains (not shown).  
         [0017]    The first electric machine  3  comprises a first rotor  19  and a first stator  20 . The first rotor  19  is connected to a shaft  21  which in turn is connected to a first pinion gear  22  at its end. The first pinion gear  22  meshes with the first bull gear  11 .  
         [0018]    The second electric machine  4  comprises a second rotor  23  and a second stator  24 . The second rotor  23  is connected to a shaft  25  which in turn is connected to a second pinion gear  26  at its end. The second pinion gear  26  engages the second bull gear  14 .  
         [0019]    The two electric machines  3  and  4  are electronically connected to each other to supply or receive power to or from each other through a power control and converting unit (not shown). Each electric machine  3  and  4  may also be connected through the power control and converting unit to external power sources (not shown).  
       Detailed Description of the Operation  
       [0020]    In the present embodiment, the two speed transmission A operates primarily in two modes. The first mode results in a rotating speed ratio R 1  between the input shaft  8  and the output shaft  18 . The second mode results in a rotating speed ratio R 2  between the input shaft  8  and the output shaft  18 . It will be appreciated that in either mode of operation in the present embodiment, there is only one mechanical path for power transmission from the input shaft to the output shaft.  
         [0021]    In the first mode, the first ring gear  10  is grounded by engaging the first locking device  5 , fixing it to a first stationary member  27  of the two speed transmission A. The second locking device  6  is disengaged, leaving the second ring gear  13  free-wheeling with the second bull gear  14 . The input power received from the input shaft  8  goes through the first sun gear  7 , to the first set of planetary gears  9  of the planetary cluster  16 , to the common carrier  17 , and then is delivered to the output shaft  18 . The power is transmitted from the input shaft  8  to the output shaft  18  at the first speed ratio R 1 .  
         [0022]    In the second mode, the second locking device  6  is engaged, grounding the second ring gear  13  together with the second bull gear  14  to the second stationary member  15 . The first locking device  5  is disengaged, freeing the first ring gear  10  and the first bull gear  11 . The input power is received from the input shaft  8  and goes through the first sun gear  7 , and through both sets of planetary gears  9  and  12  of the planetary cluster  16 , to the common carrier  17 , and then is delivered to the output shaft  18 . The power is transmitted from the input shaft  8  to the output shaft  18  at the second speed ratio R 2 .  
         [0023]    In the first mode of operation, as the first ring gear  10  is grounded to the first fixed member  27 , it provides a reaction torque to balance the differential torque between the input  8  and the output shaft  18  that are rotating at the first rotating speed ratio R 1 . In the second mode of operation, the second ring gear  13  is grounded and provides a reaction torque to balance the differential torque between the input shaft  8  and the output shaft  18  that rotate at the second rotating speed ratio R 2 . It is understood that in the present embodiment, the reaction torque in the two operation modes is in the same direction.  
         [0024]    To shift from the first mode to the second mode, the first locking device  5  is released. Upon such release, a command is sent to the second electric machine  4  to provide a reaction torque to retard the rotation of the second ring gear  13 . In doing so, the second electric machine  4  converts mechanical power into electrical power.  
         [0025]    The electric power generated during the shift from the first mode to the second mode is fed through the power control and converting unit (not shown) to power the first electric machine  3  to thereby share the reaction torque which otherwise is taken solely by the second electric machine  4 . The first electric machine  3  thus drives and accelerates the first bull gear  11  along with the first ring gear  10 . It is understood that the first ring gear  10  rotates in an opposite direction from the rotation of the second ring gear  13 . As the speed of the first electric machine  3  increases, the speed of the second electric machine  4  decreases, and the share of reaction torque shifts toward the second electric machine  4 . This trend continues until the second electric machine  4  comes to a stop and reaction torque is solely taken by the second electric machine  4 . This completes the shift of the two speed transmission A and the second locking device  15  is then engaged, fixing the second ring gear  13  to ground and lifting the reaction torque off the second electric machine  4 . In general, shifting from the second mode into the first mode is executed by a similar process, with the exception that the above process is generally reversed.  
         [0026]    As one can appreciate, the shift between the first mode and the second mode is substantially continuous and smooth. Additionally, the transition between the two modes is accomplished under full transmission power and with no power interruption. This substantially continuous and smooth power shift occurs because the present embodiment of the invention provides two paths for power transmission from the input shaft  8  to the output shaft  18 . One is a mechanical path, the other is an electrical path and because of an operational overlap between the mechanical path and the electrical path, power variation in the mechanical path during the shifting is fully compensated by the electrical path, thereby resulting in smooth shifting between the modes. It is noted that the power transmitted through the electric path is usually a fraction of the total power.  
         [0027]    The maximum power rating of the first and second electric machines  3  and  4  is determined by a number of factors. The most influential factor is the speed differential between the first rotating speed ratio R 1  and the second rotating speed ratio R 2 . For rotorcraft, the desired differential between R 1  and R 2  is about 1.2. Therefore, the maximum power ratio for the first and second electric machines  3  and  4  is only about 5% of the maximum transmission power rating.  
         [0028]    As can be appreciated, during normal mode of operation, one of the electric machines  3  or  4  is idling. Thus, in this or other embodiments of the present invention, the idling electric machine can be used as generator to generate electric power for onboard electronic accessories.  
         [0029]    While the above description describes various embodiments of the present invention, it will be clear that the present invention may be otherwise easily adapted to fit any configuration where a two speed transmission with a smooth power shift is required. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.