Abstract:
A torque biasing differential including a planetary case rotatable about an axis, a first output shaft rotatable relative to the planetary case, a second output shaft rotatable relative to the planetary case and the first output shaft, and a planetary assembly coupling the planetary case to the first and second output shafts. The planetary assembly includes first and second intermeshed inboard planet gears. The differential also include torque sinks associated with each of the first and second planetary assemblies to selectively distribute torque between the output shafts and control relative shaft rotation. The various embodiments of the torque biasing differential also describe alternative planetary differential configurations relating to the structure, orientation, and interaction of the sun gears, planet gears, and case.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a torque biasing differential for distributing torque from an input drive to first and second output shafts and, more particularly, to a torque biasing differential having coupled and compound planetary gear sets. 
     Torque biasing differentials are used to bias torque between driven shafts, such as axle half-shafts, in a variety of motor driven vehicles including wheel-driven automobiles and trucks, track-laying vehicles such at tanks, off-road vehicles with paired-wheel steering, and boats with twin propellers. Selectively biasing drive torque between the shafts enhances vehicle performance such as by providing steering augmentation and control of wheel slip in low traction environments. Many conventional torque biasing differentials include bevel gears in combination with planetary gear sets and torque sinks for controlling the distribution of torque. Despite the recognition in the art of the benefits of differential movement of driven shafts and the general benefits of planetary gear sets in such systems, there remains an unfulfilled need for a torque biasing differential that provides the controllable variation of speed and torque between the driven shafts in a concentric configuration that enhances packaging and minimizes the size and weight of the differential. 
     SUMMARY OF THE INVENTION 
     In view of the above, the present invention is directed to a torque biasing differential including a planetary case rotatable about an axis, a first output shaft rotatable relative to the planetary case, a second output shaft rotatable relative to the planetary case and the first output shaft, and a planetary assembly coupling the planetary case to the first and second output shafts. The planetary assembly includes first and second intermeshed inboard planet gears. The differential also include torque sinks associated with each of the first and second planetary assemblies to selectively distribute torque between the output shafts and control relative shaft rotation. The various embodiments of the torque biasing differential also describe alternative planetary differential configurations relating to the structure, orientation, and interaction of the sun gears, planet gears, and case. 
     Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given here below, the appended claims, and the accompanying drawings in which: 
     FIG. 1 is a perspective view of the torque biasing differential of the present invention; 
     FIG. 2 is an axial cross sectional view of the torque biasing differential shown in FIG. 1; and 
     FIG. 3 is an axial cross sectional view of another embodiment of the torque biasing differential of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1 and 2 illustrate a torque biasing differential  10  having a planetary case  14  rotatably driven by an engine driven component  16  (such as the illustrated hypoid gear), first and second coupled and compound planetary gear sets  18  and  118  operably coupling first and second output shafts  22  and  122  to the case  14 . The first and second output shafts  22  and  122  are each rotatable relative to the case  14  about an axis  24  with the first and second compound planetary gear sets  18  and  118  capable of driving the output shafts at different rotational speeds based upon the controlled condition of torque sinks  26  and  126 . 
     As will be described in greater detail below, the torque biasing differential  10  provides the functional benefits of conventional torque biasing arrangements, including biasing torque between the half-shafts in a controllable manner. Moreover, the configuration of the torque biasing differential  10  of the present invention further provides numerous advantages over conventional systems including, but not limited to, concentric gear and shaft packing that reduces the space and weight of the differential and eliminates the need for a traditional bevel differential. 
     The components and operation of the first compound planetary gear set  18  will now be described in detail with reference to FIGS. 1 and 2. It should be appreciated that the second compound planetary gear set  118  has components and an arrangement similar to the first compound planetary gear set  18 . For ease of reference, the corresponding components of the second compound planetary gear set  118  are identified in the drawings by reference numerals increased by one hundred. 
     The first compound planetary gear set  18  includes an inboard sun gear  30 , an outboard sun gear  32 , planet carrier shafts  34  coupled to rotate about axis  24  with the case  14  (functioning as the planet carrier) and about their shaft axes relative to the case, and inboard and outboard planet gears  36  and  38  each mounted on and rotating with the planet carrier shafts  34 . The inboard sun gear  30  is splined or otherwise fixed to rotate with the first output shaft  22  and meshed with the inboard planet gears  36 . The outboard planet gears  38  are meshed with the outboard sun gear  32 . The first and second inboard planet gears  38  and  138  are also intermeshed, such as in the axial gap  41  between the inboard sun gears  30  and  130 . 
     The torque sink  26  may be of any conventional type and is controllable in a known manner to selectively vary the rotational resistance provided by the outboard sun gear  32 . For example, the torque sink may consist of a wet or dry frictional clutch pack, a hydraulic torque control arrangement (such as hydraulic pumps or motors), or electrical mechanisms. A variety of torque sink actuators, including actively or passively controllable electronic, hydraulic, or mechanical actuators may be used with the present invention. The differential  10  of the present invention may be actively controlled thereby permitting communication of different torque levels to the first and second output shafts without the need for a pre-existing rotational speed differential, e.g., wheel slip. By way of example, a controller  40  may be used to control the operative condition of the torque sink  26 . Commonly used controllers receive input from a variety of sensors (e.g., wheel speed, steering wheel angle, brake pedal position, accelerator position, and yaw) that indicate vehicle performance and use conventional control techniques to generate actuation signals to adjust the torque sink between a fully engaged condition wherein the outboard sun gear  32  is rotationally fixed and a fully disengaged condition wherein the outboard sun gear is freely rotatable. 
     During “normal” operation, i.e., straight line driving with adequate wheel traction, the torque biasing differential  10  operates in the following manner to distribute torque equally between the first and second output shafts  22  and  122 . First, the torque sinks  26  and  126  are in their fully disengaged conditions permitting the outboard sun gears  32  and  132  to rotate freely about shafts  22  and  122 , respectively. Thus, the torque delivered from the engine driven component  16  to the case  14  is transmitted to the inboard planet-gears  36  and  136  by the planet carrier shafts  34  and  134  rotating with the case about axis  24 . The inboard sun gears  30  and  130 , meshed with and driven by the respective inboard planet gears  36  and  136 , are fixed to rotate with their respective output shafts  22  and  122 , and cause output shafts  22  and  122  to spin at the rotational speed of the differential case  14 , each receiving fifty percent (50%) of driveline torque. The outboard planet gears  38  and  138  cause the outboard sun gears  32  and  132  to also rotate at the same speed as the differential case  14  resulting in a delta-speed across the torque sinks  26  and  126  equal to the speed of the differential case  14 . 
     The delta-speed across the torque sinks  26  and  126  permit control over the torque distribution between the output shafts. Those skilled in the art will appreciate that it is desirable to distribute different torque to the first and second output shafts  22  and  122  in a variety of circumstances. For example, torque distribution can provide primary steering control in tracked and propeller driven vehicles as well as steering augmentation in commercially available wheeled vehicles. In a steering assist condition, the inboard sun gears  30  and  130  and inboard planet gears  36  and  136  function in a manner similar to a traditional bevel-gear differential to permit rotation of the first and second output shafts at different speeds while still receiving driveline torque. 
     When the vehicle is traversing a left hand turn it is desirable to rotate the left hand (i.e., first) output shaft  22  slower than the right hand (i.e., second) output shaft  122 . In this instance, the controller  40  engages the first torque sink  26  forcing the first inboard sun gear  30  and first output shaft  22  to slow down and the second output shaft  122  to rotate faster. Specifically, engagement of the first torque sink  26  slows the first outboard sun gear  32  and, given the instantaneously constant rotational speed of the case  14 , causes the first outboard planet gears  38 , rigidly connected to the planet carrier shafts  34  and inboard planet gears  36 , to rotate faster about their respective carrier shaft axes, but orbit slower about the case axis  24 . The inboard planet gears  36 , spinning at the same speed as the outboard planet gears  38  and shafts  34 , also increase in rotational speed. Because of the relative gear sizing, the first inboard sun gear  30  and first output shaft  22  slow down relative to the rotation of the case  14 . The first inboard planet gears  36 , now rotating faster than under “normal” operation, are meshed with and drive the second inboard planet gears  136  to rotate faster, but in the opposite direction. As the second torque sink  126  remains in its disengaged condition, the second outboard sun gear  132  freely rotates and the second inboard planet gears  136 , spinning as fast as the first inboard planet gears  36  but in the opposite direction, drive the second inboard sun gear  130  and second output shaft  122  faster than the case  14 . 
     A similar control strategy may be used to limit wheel slip in a wheeled vehicle. For example, when a driven wheel of an automobile encounters a low traction surface, such as ice or gravel, the wheel tends to lose traction and slip. If the wheel driven by the first output shaft  22  (e.g., left side wheel) were to encounter such a condition, the wheel slip may be controlled by engaging the first torque sink  26  to slow the output shaft  22  and transfer torque to second output shaft  122 . 
     Those skilled in the art will appreciate that steering assist in right hand turns and countering wheel slip of the second output shaft  122  may be achieved in the torque biasing differential  10  by engaging the second torque sink  126 . Thus, the control system can engage either the first or second torque sinks to variably increase or decrease the speed and torque delivered to each output shaft. In order to ensure smooth operation of the differential  10 , only one of the first and second torque sinks  26  and  126  should be engaged at a time. 
     Further modifications to the embodiment illustrated in FIGS. 1 and 2 will be apparent to those skilled in the art if it is desired to alter the operation of the differential. For example, the embodiment illustrated in FIGS. 1 and 2 shows the outboard planet gears  38 ,  138  having a larger diameter than the outboard sun gears  32 ,  132  and the inboard planet gears  36 ,  136  having a smaller diameter than the inboard sun gears  30 ,  130  in order to achieve the relative rotational speeds described above. The respective sizes of these elements may be altered to achieve different reduction magnitudes. For example, if the size of these respective elements were changed such that the outboard planet gears  38 ,  138  are the same size as the inboard planet gears  36 ,  136  and the outboard sun gears  32 ,  132  are the same size as the inboard sun gears  30 ,  130 , then actuation of the first torque sink  26  would slow the first output shaft  22  to zero speed and increase the speed of the second output shaft  122  to twice case  14  speed. As another example, if the size of these respective elements were changed such that the outboard planet gears  38 ,  138  are smaller than the outboard sun gears  32 ,  132  and the inboard planets  36 ,  136  larger than the inboard suns  30 ,  130 , actuation of the first torque sink  26  would make the first output shaft  22  spin in the opposite direction of the case  14 , and actuation of the second torque sink  126  would likewise make the second output shaft  122  spin in the opposite direction of the case  14 . 
     From the above description and the illustrations of FIGS. 1 and 2, those skilled in the art will appreciate that the torque biasing differential  10  not only achieves the operational benefits of conventional torque biasing systems but also provides numerous advantages over such systems. For example, the torque biasing differential  10  permits the transmission of differing torque to the first and second output shafts to provide steering control and counteract slip conditions without requiring a traditional bevel-gear arrangement. Moreover, the configuration of the compound and coupled planetary gear sets provide concentric gear and shaft packaging. Further, where the torque sinks  26  and  126  are dry torque sinks, such as the illustrated dry clutch packs, the low viscosity of the air between the clutch plates reduces the viscous drag forces within the clutch-pack, thereby providing negligible torque sink drag when the torque sink is fully disengaged. The invention also achieves simplicity and weight savings by eliminating the need for a ring gear meshed with the inboard and outboard planet gears  36  and  136 . 
     A further embodiment of the torque biasing differential of the present invention is illustrated in FIG.  3 . This torque biasing differential  210  is a kinematic equivalent of the differential described above with reference to FIGS. 1 and 2 and will indicate to those skilled in the art that various other modifications may be made to the embodiments described herein without departing from the scope of the invention defined by the appended claims. For the sake of consistency, components of the torque biasing differential  210  that are similar in structure or function to those of the torque biasing differential  10  of FIGS. 1 and 2 are indicated by reference numerals increased by two hundred. 
     As is illustrated in FIG. 3, the torque biasing differential  210  includes a planetary differential  220  and first and second planetary gear sets  218  and  318  generally disposed within a planetary case  214 . The planetary differential  220  functions in a manner similar to a traditional differential, such as a bevel gear differential, to permit the first and second output shafts  222  and  322  to rotate relative to one another. The first and second planetary gear sets  218  and  318  communicate drive torque from the engine drive component  216  and case  214  to the first and second output shafts  222  and  322 . The magnitude of the transferred torque is dependent upon the input torque and the engagement state of the torque sinks  226  and  326 . 
     The first and second planetary gear sets  218  and  318  each include an outboard sun gear  232 ,  332  rotatable relate to their respective output shafts  222  and  322 , a planet carrier  228 ,  328  fixed to rotate with the output shaft  222 ,  322 , planet gears  238 ,  338  mounted for rotation on carrier shafts  234 ,  334 , and a ring gear  235  fixed to rotate with the case  214 . The planet gears  238 ,  338  are meshed with the outboard sun gear  232 ,  332  and the ring gear  235 . Just as in the embodiment illustrated in FIGS. 1 and 2, the outboard sun gears  232  and  332  include clutch components, such as the illustrated clutch pack plates, whereby the rotation of the sun gears is controllable by the torque sinks  226 ,  326 . 
     The planetary differential  220  includes a sun gear  230  fixed to rotate with one of the output shafts (e.g., shaft  322  in FIG.  3 ), inner planet gears  236  rotatably mounted on inner carrier shafts  237  and meshed with the sun gear  230 , and outer planet gears  238  rotatably mounted on outer carrier shafts  239  and meshed with the ring gear  235 . The inner planet gears  236  and  238  are intermeshed with one another and their respective inner carrier shafts  237  and  239  rotate with the carrier  228 , which is fixed to the output shaft that is not attached to sun gear  230  (e.g., shaft  222  in FIG.  3 ). 
     In operation, drive torque is transmitted from the driven case  214  and ring gear  235  to the output shafts  222  and  322  via the planet gears  238 ,  338  and carriers  228 ,  328 . During “normal” operation, where the vehicle is moving in a straight path with adequate traction and the torque sinks  226 ,  326  are disengaged, the output shafts  222 ,  322  as well as the outboard sun gears  232 ,  332  rotate at the same speed as the case  214  and the delta-speeds in the torque sinks are equal to the case speed. In order to distribute torque equally between the first and second output shafts  222  and  322 , the distance  243  from the axis  224  to the inner surface of the ring gear  235  is twice the distance  245  from the axis  224  to the outer surface of the inboard sun gear  230 . 
     Engagement of one of the torque sinks  226  or  326  causes the corresponding output shaft  222  or  322  to slow and the other output shaft  322  or  222  to increase in speed. For example, assuming the ring gear  235  rotates at a constant speed, actuation of the first torque sink  226  slows the rotation of the outboard sun gear  232  thereby increasing the rotational speed of planet gears  238  about the carrier shafts  234 , decreasing the rotational speed of the carrier  228  about axis  224 , and decreasing the speed of the output shaft  222  rotating with the carrier  228 . The slower rotating carrier  228  causes the inner planet gears  236  of the planetary differential  220  to rotate faster about their respective shaft axes and, in turn, to rotate the inboard sun gear  230  and second output shaft  322  faster than the case  214 . It should be appreciated that slowing the first shaft  222  and increasing the rotational speed of the second shaft  322  through actuation of the first torque sink  226  is effective for steering assist in left hand turns and to counteract left wheel slip. Actuation of the second torque sink  326  is similarly effective for steering assist in right hand turns and to counteract right wheel slip. 
     The foregoing discussion discloses and describes an exemplary embodiment of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following claims.