Abstract:
An independently suspended, driven axle shaft set in which the axle shafts are asymmetric with respect to each other, wherein the asymmetry provides mitigation of powerhop. The asymmetric axle shafts are asymmetrically selected such that the relative torsional stiffness therebetween is different by a ratio substantially between about 1.4 to 1 and about 2.0 to 1. The asymmetry may be provided by any known modality that alters torsional stiffness and is compliant with operational load demands of the axle shafts, as for example the axle shafts having the same length, but differing cross-sectional diameters; or by the axle shafts having the same cross-sectional diameters, but differing lengths; or a combination thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present patent application claims the benefit of provisional patent application Ser. No. 61/014,783, filed on Dec. 19, 2007, which provisional patent application is presently pending. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to driven axle shafts of motor vehicles, and more particularly to a damped axle shaft set in which the axle shafts are asymmetric with respect to each other, whereby the asymmetry provides mitigation of powerhop. 
     BACKGROUND OF THE INVENTION 
     Motor vehicles with driven axle independent suspensions include a pair of axle shafts (also referred to as split axles or half shafts), one for each wheel, as described, merely by way of exemplification, in U.S. Pat. No. 4,699,235 issued on Oct. 13, 1987 to Anderson and assigned to the assignee of the present patent application, the disclosure of which is hereby incorporated herein by reference. 
     Referring now to  FIG. 1 , the split axle drive system of U.S. Pat. No. 4,699,235 will be briefly described for point of reference, it being understood the present invention may apply to two wheel drive or four wheel drive systems. 
     Shown is a schematic plan view of a part-time four-wheel drive vehicle, comprising an internal combustion engine  10 , transmission  12  and transfer case  14  mounted on a vehicle chassis (not shown). The engine  10  and transmission  12  are well-known components as is the transfer case  14  which typically has an input shaft (not shown), a main output shaft  16  and an auxiliary output shaft  18 . The main output shaft  16  is drive connected to the input shaft in the transfer case  14  and is customarily aligned with it. The auxiliary output shaft  18  is drive connectable to the input shaft by a clutch or the like in the transfer case  14  and customarily offset from it. The transfer case clutch is actuated by a suitable selector mechanism (not shown) which is generally remotely controlled by the vehicle driver. 
     The main output shaft  16  is drivingly connected to a rear propeller shaft  20  which in turn is drivingly connected to a rear differential  22 . The rear differential  22  drives the rear wheels  24  through split axle parts in a well-known manner. The auxiliary output shaft  18  is drivingly connected to a front propeller shaft  26  which in turn is drivingly connected to a split axle drive mechanism  28  for selectively driving the front wheels  30  through split axle parts. The split axle drive mechanism  28  is attached to the vehicle chassis by means including a bracket  71  on an extension tube  66 . 
     Suitable split axle parts, commonly referred to as half shafts, are well-known from front wheel drive automobiles. These may be used for connecting the split axle drive mechanism  28  to the front wheels  30 . The drawings schematically illustrate a common type of half shaft for driving connection to independently suspended steerable vehicle wheels comprising an axle shaft  76  having a plunging universal joint  78  at its inboard end adapted for connection to an output such as the flange  72  or  74  and the well-known Rzeppa-type universal joint  80  at its outboard end adapted to be connected to the vehicle wheel  30 . 
     Problematically, axle shafts frequently exhibit “powerhop” when a large amount of torque is applied thereto. Powerhop typically occurs when tire friction with respect to a road surface is periodically exceeded by low frequency (i.e., below about 20 Hz) oscillations in torsional windup of the axle shafts. Powerhop produces oscillatory feedback to suspension and driveline components and can be felt by the vehicle occupants, who may describe the sensation as “bucking,” “banging,” “kicking” or “hopping.” 
     Axle shafts are typically manufactured from steel bar material and, as such, act as very efficient torsional springs. In the interest of reducing unwanted oscillations in the axle shafts, the standard practice has been to adjust the size (i.e., increasing the diameter) of the axle shafts in order to tune the resonating frequencies in such a way to minimize the negative impact of oscillations by increasing the overall torsional stiffness of the axle shafts, thereby reducing powerhop. However, increasing the diameter of the axle shafts results in additional packaging, mass and cost related problems, while not really addressing the core issue of directly damping oscillations that are associated with powerhop, to wit: lack of damping to absorb energy placed into the driveline by the negative damping characteristics of the tires during hard longitudinal acceleration or deceleration. 
     Accordingly, there is a clearly felt need in the art for axle shafts which are inherently damped very near the source of the oscillation, and thereby provide reduction of powerhop and associated driveline disturbances, such as for example axle shutter. 
     SUMMARY OF THE INVENTION 
     The present invention is an independently suspended, driven axle shaft set in which the axle shafts are asymmetric with respect to each other, wherein the asymmetry provides mitigation of powerhop and associated driveline disturbances, such as for example axle shutter. 
     According to a preferred form of the present invention, the asymmetric axle shafts are asymmetrically tuned such that the relative torsional stiffness therebetween is different by a ratio substantially between about 1.4 to 1 and about 2.0 to 1. The asymmetry may be provided by any known modality that alters torsional stiffness and is compliant with operational load demands of the axle shafts, as for example the axle shafts having the same length, but differing cross-sectional diameters; by the axle shafts having the same cross-sectional diameters, but differing lengths; by the axle shafts having differing solidity (i.e., being solid versus hollow); by the axles shafts having differing material composition; or a combination thereof. 
     The asymmetric axle shafts are operably connected to a limited slip differential in order to provide an axle-to-axle friction torque coupling through which out of phase torque oscillation damping between the asymmetric axle shafts occurs. According to a preferred form of the present invention, the asymmetric axle shafts are suspended in a cradle which is, itself, connected to the vehicle frame or body either directly or via a plurality of resilient cradle mounts having a stiffness which is tuned, per a particular application, to maximize the mitigation of powerhop in conjunction with the asymmetry of the axle shafts. 
     Accordingly, it is an object of the present invention to provide an independently suspended, driven axle shaft set in which the axle shafts are asymmetric with respect to each other, wherein the asymmetry provides mitigation of powerhop and associated driveline disturbances, such as for example axle shutter. 
     This and additional objects, features and benefits of the present invention will become clearer from the following specification of a preferred embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan view of a part-time four-wheel drive vehicle according to the prior art. 
         FIG. 2  is a representation of a rear suspension of a vehicle employing asymmetric axle shafts according to the present invention. 
         FIG. 3  is a side view of an example of a first asymmetric axle shaft according to the present invention. 
         FIG. 3A  is a cross-sectional view, seen along line  3 A- 3 A of  FIG. 3 . 
         FIG. 4  is a first example of a second asymmetric axle shaft which is asymmetric with respect to  FIG. 3 . 
         FIG. 4A  is a cross-sectional view, seen along line  4 A- 4 A of  FIG. 4 . 
         FIG. 5  is a second example of a second asymmetric axle shaft which is asymmetric with respect to  FIG. 3 . 
         FIG. 5A  is a cross-sectional view, seen along line  5 A- 5 A of  FIG. 5 . 
         FIG. 5B  is a cross-sectional view of a third example of a second asymmetric axle shaft which is asymmetric with respect to  FIG. 3 . 
         FIG. 6  is a graph of axle shaft torque versus time of a symmetric axle shaft set according to the prior art. 
         FIG. 7  is a graph of torque versus time of an asymmetric axle shaft set according to the present invention. 
         FIG. 8  is a graph of propeller shaft torque versus time comparing symmetric and asymmetric axle shaft sets. 
         FIG. 9  is a graph of propeller shaft torque versus time comparing a symmetric axle shaft set with highly damped cradle mounts, an asymmetric axle shaft set with minimally damped cradle mounts, and an asymmetric axle shaft set with highly damped cradle mounts. 
         FIG. 10  is a graph of torque versus time of an asymmetric axle shaft set for various values of limited slip differential friction torque. 
         FIG. 11A  is a cross-sectional view of a rear cradle mount. 
         FIG. 11B  is a cross-sectional view of a front cradle mount. 
         FIG. 12  is a schematic view of a front wheel drive system with one of the asymmetric axle shafts including a jackshaft. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the Drawings,  FIGS. 2 through 12  depict various aspects of independently suspended, driven asymmetric axle shafts  100 ,  100 ′ according to the present invention. 
       FIG. 2  depicts an example of a motor vehicle rear suspension  102  of a motor vehicle drive system which incorporates the asymmetric axle shafts  100 . The asymmetric axle shafts  100  are in the form of a set of two mutually asymmetric axle shafts: a first axle shaft  100   a  and a second axle shaft  100   b , wherein the asymmetry therebetween is such that each has a different torsional stiffness with respect to the other. The rear suspension  102  includes a cradle  104  which is attached, in this application, by resilient cradle mounts  106  to a frame (not shown) of the motor vehicle. A rear differential module  108  is connected to the cradle  104  via resilient rear differential module mounts  110 , and is further connected, via constant velocity joints  112   a ,  112   b  to the first and second axles shafts  100   a ,  100   b , respectively, of the asymmetric axle shafts  100 . The first and second axle shafts  100   a ,  100   b  are independently suspended via the constant velocity joints  112   a ,  112   b  so they are able to independently articulate along arrows  114   a ,  114   b . A propeller shaft  116  is connected at one end to a transmission (not shown) and at its other end, via a constant velocity (or other type of) joint  118 , to the rear differential module. 
     Referring in addition to  FIGS. 3 through 12 , structural and functional aspects of the asymmetric axles shafts  100 ,  100 ′ will be detailed. 
       FIGS. 3 and 3A  represent a first axle shaft  100   a ,  100   a ′ in which a length L 1  is preselected and a cross-sectional diameter D 1  is also preselected. The selection criteria being that generally standard in the art regarding durability and torque load handling. In this regard, the first axle shaft has a selected torsional stiffness T 1 . By way of example, the first axle shaft  100   a ′ is constructed of solid or hollow steel in a cylindrical configuration, having splines  122   a ,  122   b  at each end for engaging constant velocity joints of the independent suspension. 
     In contradistinction, the second axle shaft  100   b  is asymmetric with respect to the first axle shaft  100   a  such that the physical properties thereof provide a different torsional stiffness T 2 , which may be greater or less than T 1 , wherein the ratio of the torsional stiffness is between about 1.4 to 1 and about 2.0 to 1. By way of example, the second axle shaft  100   b  is constructed of solid or hollow steel in a cylindrical configuration, also having splines  122   a ,  122   b  at each end for engaging constant velocity joints of the independent suspension. 
     Turning attention next to  FIGS. 4 through 5B , depicted are examples of how physical differences between the first and second axle shafts  100   a ,  100   b  may provide the desired difference in torsional stiffness. 
       FIGS. 4 and 4A  show a first example of a second axle shaft  100   b ,  100   b ′ in which the length L 2  is equal to L 1 ; however, the cross-sectional diameter D 2  is different from D 1  (the splines  122   a ,  122   b  are identical to that of  FIG. 3 ). In the example shown, D 2 &gt;D 1 , however, it is also equally possible, of course, to make D 2 &lt;D 1 , all that is required is that D 1  be different from D 2  such as to provide the desired difference in torsional stiffness in which the ratio is between about 1.4 to 1 and 2.0 to 1. 
       FIGS. 5 and 5A  show a second example of a second axle shaft  100   b ,  100   b ″ in which the cross-sectional diameter D 2 ′ is equal to D 1 ; however, length L 2 ′ is different from L 1 , (the splines  122   a ,  122   b  are identical to that of  FIG. 3 ). In the example shown, L 2 ′&lt;L 1 , however, it is also equally possible, of course, to make L 2 ′&gt;L 1 , all that is required is that L 1  be different from L 2  such as to provide the desired difference in torsional stiffness in which the ratio is between about 1.4 to 1 and 2.0 to 1. 
     Of course, it is possible to alter the physical properties in other ways to achieve the torsional stiffness difference as between the first and second axle shafts  100   a ,  100   b , as for example by a selected combination of cross-sectional diameter difference, length difference, solidity difference (i.e., solid vs. hollow construction) or material composition difference (however, since various steels tend to have all about the same torsional stiffness for a given geometry steel materials substitution is unlikely to have, of itself, sufficient difference). An example of torsional stiffness asymmetry due to solidity difference is shown by comparison between  FIGS. 3 and 5B , wherein a third example of a second axle shaft  100   b ,  100   b ′″ is hollow, and may have a larger or smaller cross-sectional diameter than D 1  and a length longer or shorter than L 1 , whereby the torsional stiffness is different therebetween. As mentioned, either or both of the first and second axle shafts  100   a ,  100   b  may be solid or hollow. 
     The asymmetric axle shafts  100 ,  100 ′ are operably connected to a limited slip differential, which may be electrical or mechanical, (as for example  108  of  FIG. 2  or  306  of  FIG. 12 ) in order to provide an axle-to-axle mechanical coupling through which out of phase torque oscillation damping between the asymmetric axle shafts occurs. The mechanical coupling in a limited slip differential provides a friction torque coupling between the asymmetric axle shafts, wherein, as for example by empirical testing or mathematical modeling, an optimum friction torque is provided that is optimal for a given difference in torsional stiffness between the asymmetric axle shafts, per a particular application. In this regard, if there is no friction torque coupling between the asymmetric axle shafts, then the asymmetry of the axle shafts is unable to provide damping by out of phase torque oscillations axle-to-axle; on the other hand, if an open differential is used instead of a limited slip differential or if the coupling has no slip between the asymmetric axle shafts, then the torque oscillations therebetween will tend to be in phase, and damping will be mitigated, that is, lessened. 
       FIG. 6  is a graph  200  of axle shaft torque versus time for conventional symmetric axle shafts, wherein plots  202 ,  204  are respectively for each axle shaft, and wherein each axle shaft has a torsional stiffness of 525 Nm/deg. (i.e., Newton meters per degree). It will be seen that torque oscillations are in phase, whereby the conditions for powerhop are not mitigated in that the torque oscillations of each axle shaft are constructive with respect to each other. 
       FIG. 7  is a graph  210  of axle shaft torque versus time for asymmetric axle shafts  100  according to the present invention, wherein plot  212  is for the first axle shaft  100   a  which has a torsional stiffness of 270 Nm/deg. and wherein plot  214  is for the second axle shaft  100   b  which has a torsional stiffness of 525 Nm/deg. It will be seen that, unlike  FIG. 6 , torque oscillations are out of phase, whereby the conditions for powerhop are mitigated in that the torque oscillations of each axle shaft are destructive with respect to each other (the out of phase torque oscillations being most pronounced during an initial portion of a powerhop event when powerhop is most likely to be felt by passengers of the vehicle). 
       FIG. 8  is a graph  220  of propeller shaft (see  116  of  FIG. 2 ) torque versus time for conventional symmetric axle shafts at plot  222 , wherein each axle shaft has a torsional stiffness of 525 Nm/deg., wherein the propeller shaft has a torsional stiffness of 138 Nm/deg. and wherein cradle mount (see  106  of  FIG. 2 ) damping is 2 Nsec/mm; and for asymmetric axle shafts  100  according to the present invention at plot  224 , wherein the first axle shaft  100   a  has a torsional stiffness of 270 Nm/deg. and the second axle shaft  100   b  which has a torsional stiffness of 525 Nm/deg., wherein the propeller shaft has a torsional stiffness of 138 Nm/deg. and wherein cradle mount damping is 2 Nsec/mm with an electronic limited slip differential having a friction torque of 400 Nm. It will be seen that the amplitudes of the torque oscillations in initial plot portion  222   a  are high, which is interpreted to mean powerhop is of sufficient amplitude that it may be felt by passengers. On the other hand, initial plot portion  224   a  has lower amplitude torque oscillations than initial plot portion  222   a , which is interpreted to mean powerhop is not of sufficient amplitude that it may be felt by passengers. The fact that following plot portion  224   b  of plot  224  has a residual amplitude larger than that of following plot portion  222   b  of plot  222  is of vanishing consequence, since the amplitudes of these torque oscillations will not be felt by passengers of the vehicle. 
       FIG. 9  is a graph  240  of propeller shaft torque versus time for conventional symmetric axle shafts at plot  242 , wherein each axle shaft has a torsional stiffness of 525 Nm/deg., wherein the propeller shaft has a torsional stiffness of 138 Nm/deg. and wherein cradle mount damping is high at about 2 Nsec/mm.; for asymmetric axle shafts  100  according to the present invention at plot  244 , wherein the first axle shaft  100   a  has a torsional stiffness of 270 Nm/deg. and the second axle shaft  100   b  which has a torsional stiffness of 525 Nm/deg., wherein the propeller shaft has a torsional stiffness of 138 Nm/deg. and wherein cradle mount damping is minimal around 0.2 Nsec/mm at around 10 Hz; and for asymmetric axle shafts  100  according to the present invention at plot  246 , wherein the first axle shaft  100   a  has a torsional stiffness of 270 Nm/deg. and the second axle shaft  100   b  which has a torsional stiffness of 525 Nm/deg., wherein the propeller shaft has a torsional stiffness of 138 Nm/deg. and wherein cradle mount damping is high at about 2 Nsec/mm. It will be seen that the amplitudes of the torque oscillations in plot portion  242   a  of plot  242  are high, which is interpreted to mean powerhop is of sufficient amplitude that it may be felt by passengers, while that of initial plot portion  244   a  of plot  244  and initial plot portion  246   a  of plot  246  have amplitudes of the respective torque oscillations are low enough that passengers would not feel powerhop. What is seen further, however, is that while initial plot portion  244   a  has relatively low torque oscillation amplitude, that for following plot portion  244   b , the torque oscillation amplitude increases to a level which may be felt by passengers. On the other hand, plot  246  everywhere has low torque oscillation amplitudes, which is interpreted to mean that powerhop would not be felt by passengers. Accordingly, depending upon the application, it may be desirable to include high damped cradle mounts with the asymmetric axle shafts  100 ; however, it is to be noted that there are applications that will not utilize cradle mounts, yet the asymmetric axle shafts damping will still be provided. 
     An illustration of the effect of limited slip differential friction torque is shown at  FIG. 10 , which is a graph  250  of axle shaft torque versus time for asymmetric axle shafts  100  according to the present invention. In this illustration, the first axle shaft  100   a  has a torsional stiffness of 270 Nm/deg. and the second axle shaft  100   b  which has a torsional stiffness of 525 Nm/deg., wherein the propeller shaft has a torsional stiffness of 138 Nm/deg. and wherein cradle mount damping is 2 Nsec/mm. It will be seen that a friction torque of 100 Nm, per plot  252 , may be too low, a friction torque of 400 Nm may be optimum, per plot  254 , and a friction torque of 2,000 Nm, per plot  256 , may be too high. 
     In the event resilient cradle mounts  106  are used, the stiffness of the cradle mounts is adjusted by the configuration and choice of rubber. By way of exemplification, resilient cradle mounts are depicted at  FIGS. 11A and 11B , wherein  FIG. 11A  depicts a rear cradle mount  106 ′, and  FIG. 11B  depicts a front cradle mount  106 ″. Each cradle mount  106 ′,  106 ″ is composed, respectively, of an upper metal washer  106   a ,  106   a ′, a lower metal washer  106   b ,  106   b ′, a rubber core  106   c ,  106   c ′ and an outer sleeve  106   d ,  106   d′.    
       FIG. 12  is a schematic depiction of a front wheel drive system  300 , including an engine  302 , a transmission  304 , a limited slip differential  306  and asymmetric axle shafts  100 ′. The first axle shaft  100   a ,  100   a ″ is, for example, as depicted at  FIG. 3 . The second axle shaft  100   b ,  100   b ″″ is a combination of a second axle shaft component  100   c  and a jackshaft component  100   d  drivingly connected thereto, by way of example at a cradle mount  106 ′″. It will be understood that the asymmetry as between the first and second axle shafts includes the physical properties (i.e., length, cross-sectional diameter, solidity, composition, etc.) of the first axle shaft  100   a ,  100   a ″ with respect to the second axle shaft  100   b ,  100   b ″″ per each or both of the second axle shaft component  100   c  and the jackshaft component  100   d.    
     By way of exemplification and not limitation, the following example is given merely for referential purposes. 
     EXAMPLE 1 
     Asymmetric axle shafts have the first axle shaft  100   a  with a torsional stiffness of 270 Nm/deg. (right hand axle shaft having a diameter of 35 mm between the splines, a length of 0.6 meters, and composed of solid 300M type steel) and have the second axle shaft  100   b  with a torsional stiffness of 525 Nm/deg. (left hand axle shaft having a diameter of 55 mm between the splines, a length of 0.52 meters composed of hollow 300M type steel with an 8 mm wall thickness); the propeller shaft has a torsional stiffness of 138 Nm/deg.; the friction torque of the limited slip differential is 400 Nm; and the cradle mounts have a vertical damping of 2 Nsec/mm. 
     To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.