Abstract:
A differential device ( 11 ) of the type including a clutch pack ( 23 ), engaged by a ball ramp actuator ( 21 ), which is initiated by a viscous coupling ( 19;101 ). The output of the device ( 11 ) is an output shaft ( 13 ) which is hollow and extends through the entire axial extent of the device. The viscous coupling ( 19;101 ) includes, in the embodiment of FIG. 1, an annular input coupling member ( 25 ) and an annular output coupling member ( 27 ), within the input member, to define therebetween a cylindrical viscous shear chamber ( 29 ). The output coupling member ( 27 ) at least partially surrounds the first ramp plate ( 41 ) of the ball ramp actuator ( 21 ), and is in driving engagement therewith, for improved packaging. The clutch pack ( 23 ) and the actuator ( 21 ) are disposed about the output shaft ( 13 ) and axially retained between snap rings ( 55,93 ), so that all axial separation forces generated within the clutch pack and the actuator are taken up within a single member, such as the output shaft ( 13 ), rather than being exerted on the connection of the housing ( 17 ) and the end cap ( 15 ). In the FIG. 5 embodiment, the viscous coupling ( 101 ) is in driven engagement with the inner periphery of the first ramp plate ( 41 ), with the viscous coupling being of the type including interleaved plates ( 107,109 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part (CIP) of co-pending application U.S. Ser. No. 09/109,354, filed Jul. 2, 1998, in the name of Erlen B. Walton for a “VISCOUS ACTUATED BALL RAMP CLUTCH”, which is a CIP of U.S. Ser. No. 08/865,901, filed May 30, 1997, abnd in the name of Erlen B. Walton for a “VISCOUS ACTUATED BALL RAMP CLUTCH”. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE DISCLOSURE 
     The present invention relates to a driveline system for a four wheel drive vehicle, and more particularly, to a differential device for use in such a driveline system. The use of the term “differential” does not imply the presence of conventional differential gearing, but instead, the term is used primarily because the device of the present invention replaces the typical prior art center differential. However, it should be understood that the device illustrated, described and claimed herein has uses other than as a center differential in a four wheel drive vehicle driveline. 
     In many of the vehicles being produced today, the basic vehicle platform is a front wheel drive. However, in many such vehicles, especially in vans and sport utility vehicles, it is considered desirable to be able to provide four wheel drive, at least under certain operating conditions. 
     Typically, the various arrangements for achieving part-time four wheel drive have been complex and expensive, and in some cases, have required some sort of control scheme to achieve the four wheel drive (or rear wheel drive) in response to certain predetermined operating conditions. 
     In certain vehicle applications, it is acceptable to provide only front wheel drive under most operating conditions, and rear wheel drive is required only when the front drive wheels are slipping (e.g., under poor traction conditions). However, the prior art has not provided a suitable device wherein only front wheel drive is provided, with no substantial torque being transmitted to the rear wheels, until the front wheels begin to slip. For example, U.S. Pat. No. 4,562,897 discloses a viscous clutch in the driveline, between the front transaxle and the rear wheels, but requires a selector arrangement in order to choose among transmitting torque through the viscous coupling, or disconnecting the viscous coupling, or bypassing the viscous coupling. 
     It is known from U.S. Pat. No. 5,070,975 and from corresponding European Application EP 0 314 420 to use, as a center differential in a four wheel driveline, a viscous actuated ball ramp type friction clutch to transmit torque to the rear axles when there is a speed difference between the front and rear wheels. It is believed that the torque transmitting capability of the device of the cited patent and application would be somewhat limited by the fact that one of the ball ramp plates comprises the input to the viscous coupling. 
     In certain vehicle applications for four wheel drive drivelines, the differential device must be capable of a “through-shaft” configuration, which the known, prior art devices have not been. It should be understood that, as used herein, the term “through-shaft” is not limited to an arrangement in which there are two concentric shafts extending completely through the differential device, but instead, includes an arragement in which there is an output shaft, or some equivalent member, disposed within, and surrounded by, the differential device, over at least most of the axial length of the differential device. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved differential device which is capable of use in a “through-shaft” configuration. 
     It is a more specific object of the present invention to provide an improved differential device which accomplishes the above-stated object, and which includes additional features which facilitate such a through-shaft configuration. 
     The above and other objects of the invention are accomplished by the provision of an improved differential device of the type including an input adapted to receive input torque, an output adapted to provide driving torque to an output device, and a clutch pack including at least a first friction disc fixed to rotate with the input, and at least a second friction disc fixed to rotate with said output. The device includes means operable to move said friction discs between a disengaged position and an engaged position, said means comprising a cam ramp actuator including first and second ramp plates, and a plurality of cam members, each operably disposed between said first and second ramp plates, and in engagement with first and second ramp surfaces, respectively. The second ramp plate is disposed axially adjacent the friction discs and is operable to move the friction discs toward the engaged position. The means operable to move said friction discs further comprises a viscous coupling including an input coupling member fixed to rotate with said input, and an output coupling member. 
     The improved differential device is characterized by the output comprising an elongated, generally hollow shaft member extending axially through substantially the entire axial extent of the differential device. The device includes means for interconnecting one of the input coupling member and the output coupling member of the viscous coupling with the first ramp plate for common rotation. 
     In accordance with another aspect of the invention, the improved differential device is characterized by the output comprising a shaft member extending axially through the clutch pack and at least partially through the cam ramp actuator. A first retention means is in engagement with one of the input and the shaft member and operable to limit axial movement of the first ramp plate in a forward direction, relative to one of the input and the shaft member. As a result, substantially all axial separation forces within the clutch pack and the cam ramp actuator are taken up within either the input or the shaft member. 
     In accordance with yet another aspect of the invention, the improved differential device is characterized by one of the input and the output defining a plurality N of elongated cut-out portions on the periphery thereof. One of the first friction disc and the second friction disc defines a plurality N of cut-out portions cooperating with the elongated cut-out portions defined by the one of the input and the output to define a plurality N of openings. There is a plurality N of elongated members, each member being disposed in one of the openings, and operable to transmit torque between the one of the first friction disc and the second friction disc and the one of the input and the output. The openings have an axial length L 1 , and each of the elongated members has an axial length L 2 , less than L 1 , the difference therebetween being selected such that wear within the clutch pack will permit L 1  to decrease to L 2 , but will prevent the cam ramp actuator from moving to an over-center condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial cross-section of the differential device made in accordance with the present invention. 
     FIG. 2 is a transverse plan view of the input ramp plate, viewed from the left in FIG. 1, and on a slightly larger scale than FIG.  1 . 
     FIG. 3 is a transverse cross-section taken on line  3 — 3  of FIG. 1, and on a smaller scale than FIG.  1 . 
     FIG. 4 is an enlarged, fragmentary, axial cross-section illustrating one feature of the present invention. 
     FIG. 5 is an axial cross-section of an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, which are not intended to limit the invention, FIG. 1 is an axial cross-section of a differential device, generally designated  11 , for use in a driveline system for a four wheel drive vehicle, not illustrated herein, but which is illustrated and described briefly in co-pending application U.S. Ser. No. 09/109,354, filed Jun. 2, 1998 in the name of Erlen B. Walton for a “VISCOUS ACTUATED BALL RAMP CLUTCH”. 
     Although the invention is not so limited, the differential device  11  would typically be used as a center differential, and would receive input torque from a front transaxle of the vehicle (disposed to the right in FIG. 1) and would transmit output torque to a rear differential assembly (to the left in FIG.  1 ). 
     In the subject embodiment, input torque is transmitted to a forward end cap  15 , also referred to hereinafter as part of the “input”, while output torque is transmitted from the differential device  11  by means of an output shaft  13  to the rear differential. In many applications, the device  11  could be reversed, i.e., the shaft  13  could be the input and the forward end cap  15  could be the output, and therefore, the use herein of the terms “input” and “output” will be understood as explanatory, and not limiting. As was mentioned previously, it would be understood by those skilled in the art that the term “center differential device” does not mean or imply that the device include a conventional differential gear set, but instead, the term will be understood in its broader sense to mean that the device  11  will permit differentiating action between the input  15  and the output  13 . In the subject embodiment, the front drive wheels of the vehicle are the primary drive wheels, and the rear drive wheels are only secondary drive wheels. However, within the scope of the present invention, such could be reversed, i.e., the rear wheels being the primary drive wheels and the front wheels being the secondary drive wheels. 
     Although the present invention will be described as though the front wheels and the rear wheels normally rotate at the same speed, as the vehicle is travelling straight ahead, those skilled in the art will understand that such is typically not the case. For various reasons, such as the fact that the front wheels normally have a smaller rolling radius, the front drive wheels typically rotate somewhat faster than the rear wheels. Therefore, in a typical drive system of the type utilizing the device  11 , there is almost always at least some torque being transmitted by the device  11 . As a result, the performance requirements for the device  11  are much more stringent than would be the case if the device merely rotated as a “solid” unit for most of its duty cycle. 
     Referring still primarily to FIG. 1, the differential device  11  will be described in some detail. The device  11  includes a housing  17  which may be made in accordance with the teachings of co-pending U.S. Ser. No. 149,991, filed Sep. 9, 1998 in the names of Wayne K. Leichliter and Erlen B. Walton for a “VISCOUS ACTUATED BALL RAMP CLUTCH AND IMPROVED HOUSING THEREFOR”, assigned to the assignee of the present invention and incorporated herein by reference. However, the present invention is not limited to the housing and method of assembly illustrated and described in the cited co-pending application. 
     Disposed within the housing  17 , which may also be considered part of the input, the differential device  11  may be viewed as comprising three separate portions as follows: a viscous coupling  19 ; a ball ramp actuator  21 ; and a clutch pack  23 . It is one important feature of the device of the present invention that these portions comprise separate, somewhat functionally independent devices, except to the extent noted otherwise hereinafter. 
     The viscous coupling  19  includes an input coupling member  25 , which is fixed to rotate with the housing  17 , in a manner to be described subsequently. The viscous coupling  19  also includes an output coupling member  27 , such that the members  25  and  27  cooperate to comprise a totally functional, self-contained viscous coupling, which defines a viscous shear chamber  29 . Both of the coupling members  25  and  27  are generally annular, and therefore, the viscous shear chamber  29  is generally cylindrical. Toward their opposite, axial ends, the input and output coupling members  25 ,  27  cooperate to define forward and rearward annular seal chambers  31  and  33 , and disposed within each of the chambers  31  and  33  is a seal member  35 , illustrated herein by way of example only as a quad ring. Thus, the seal members  35  define the axial extent of the viscous shear chamber  29 . Disposed at the extreme axial ends of the viscous coupling  19  is a pair of bushings  37 , part of the function of which is to support the members  25  and  27  relative to each other, especially during times of relative rotation between the members  25  and  27 . 
     In accordance with one important aspect of the invention, the generally annular configuration of the viscous coupling  19  makes it possible to improve the “packaging” of the differential device  11 . By way of example only, the viscous coupling  19  is disposed in a generally surrounding relationship to at least part of the ball ramp actuator  21 . More specifically, the ball ramp actuator  21  includes a first ramp plate  41 , wherein FIG. 2 is a plan view of the ramp plate  41 , viewed from the left in FIG.  1 . The first ramp plate  41  includes a plurality of cut-out portions or notches  43  disposed about the outer periphery thereof, and as may best be seen in FIG. 1, the output coupling member  27  defines, about its inner periphery, a plurality of projections  45  which mate with the notches  43 , whereby torque is transmitted from the output coupling member  27  to the first ramp plate  41 , while at the same time, the first ramp plate  41  is able to move axially relative to the coupling member  27 . 
     The ball ramp actuator  21  also includes a second ramp plate  47 , and a plurality of balls  49 , although it should be understood that any form of cam member may be used. Thus, the ball ramp actuator  21  is also referred to hereinafter and in the appended claims as a “cam ramp actuator”. 
     Toward the forward end (right end in FIG. 1) of the output shaft  13  are several reduced diameter portions  51  and  53 . The portion  51  is surrounded by the forward end cap  15 , which also serves as the input to the device  11 , in a manner well known to those skilled in the art. Disposed about the reduced diameter portion  53  is a snap ring  55 . Disposed immediately adjacent the snap ring  55 , and seated against it, is an annular retaining plate  57 , and disposed axially between the plate  57  and the first ramp plate  41  is a thrust bearing set  59 . It is significant that the bearing set  59  comprise a true “rolling” bearing, such as a needle-type bearing, although at least theoretically, a device such as a bronze bushing could be used if handling the axial loading were the only concern. The significance of the snap ring  55  and the retaining plate  57  will be described in greater detail subsequently. 
     Referring now to FIG. 3, in conjunction with FIG. 1, the output shaft  13  includes a relatively larger diameter portion  61  which defines, about its outer periphery, a plurality of elongated, half-circular notches  63 , and the second ramp plate  47  defines, about its inner periphery, a plurality of half-circular tabs  65 , such that the second ramp plate  47  and the output shaft  13  rotate at the same speed. It should be understood that the particular shape illustrated for the notches  63  and the tabs  65  is by way of example only. In the subject embodiment, a number of the parts shown comprise powdered metal, wherein elements such as the notches  63  or the tabs  65  are formed in the powdered metal part, and are utilized as pressed, with no subsequent machining. Therefore, by way of example only, the output shaft  13  does not include any external splines. However, and also by way of example only, the output shaft  13  defines two sets of internal splines S, for splined engagement with a shaft (not shown herein) which would typically comprise the input shaft to the rear wheel inter-wheel differential. Typically, the unshown shaft would have a nut on its forward end, with the nut being trapped against the forward surface of the reduced diameter portion  51 . 
     In a manner well known to those skilled in the art, the first ramp plate  41  defines a plurality of first ramp surfaces  67 , while the second ramp plate  47  defines a plurality of second ramp surfaces  69 . In FIG. 1, the ball ramp actuator  21  is shown in its “neutral” or centered position, i.e., each of the balls  49  is disposed in the “valley” of each of the ramp surfaces  67  and  69 , such that the ramp plates  41  and  47  are at their minimum axial distance from each other, or stated another way, the overall axial dimension of the actuator  21  is at a minimum. As is also well known to those skilled in the art, the neutral position of the ball ramp actuator  21  would correspond, typically, to a disengaged condition of the clutch pack  23 . Preferably, the first and second ramp surfaces  67  and  69  include first and second detents (not illustrated herein), which are generally understood in the art, such that a very definite, predetermined speed difference (slip speed) within the viscous coupling  19  must occur to initiate ramping of the actuator  21 . Such ramping will not occur in response merely to the very slight differences which may occur in response to variations in factors such as tire size, etc. 
     Disposed immediately adjacent the second ramp plate  47  is the clutch pack  23 , including a plurality of outer friction discs  71 , and a plurality of inner friction discs  73 , one of which is shown in FIG.  3 . The outer friction discs  71  are in engagement with the housing  17  by means of a plurality of axially extending grooves  75  formed in the inner surface of the housing  17 , and a plurality of mating ears  77  formed about the outer periphery of the outer discs  71 . Preferably, the grooves  75  extend axially over most of the axial extent of the housing  17 , as may best be seen in FIG. 1, and the input coupling member  25  includes axially extending ears or splines  78 , which mate with the grooves  75 , as the member  25  is merely slid into the position shown in FIG.  1 . Thus, input torque is transmitted from the input  15  through the housing  17  to the input coupling member  25 . 
     The inner friction discs  73  are in engagement with the output shaft  13  by means illustrated primarily in FIGS. 3 and 4. Each of the inner friction discs  73  defines a half-circular notch  79 , and upon assembly, the notches  63  and  79  are aligned, rotationally, as shown in FIG. 3, and cooperate to define a cylindrical opening  81  (see FIG.  4 ). Disposed within each opening  81  is a generally cylindrical member  83  which, preferably has one end partially closed, as shown at  85  to provide a seat for a compression spring  87 . The springs  87  bias the members to the position shown in FIG. 4, to keep the members  83  in engagement with the notches  79 , but at the same time, the springs  87  are biasing an end plate  89  to the right, effectively biasing the ball ramp actuator  21  toward its neutral, centered position Thus, the cylindrical members  83  act as drive members to transmit torque from the friction discs  73  to the output shaft  13 , without the need for machined splines, while the springs  87  bias the members  83  to their appropriate position, while also serving as the neutral centering springs, without being disposed within openings in the friction plates as has been done previously. 
     As may best be seen in FIG. 4, each of the openings  81  has an overall axial length L 1 , whereas each of the members  83  has an axial length L 2 , with the length L 2  being somewhat less than the length L 1 . In accordance with one feature of the invention, the difference between the lengths L 1  and L 2  is selected small enough to be sure that the ball ramp actuator  21  can never move to an “over-center” condition, as is well understood by those skilled in the ball-ramp art. As the friction surfaces of the outer and inner friction discs  71  and  73  wear, the length L 1  defined by the openings  81  will gradually decrease. If the members  83  were not present, the ball ramp actuator  21  could eventually have enough axial clearance, because of the wear of the clutch pack  23 , that the ramp plates  41  and  47  could go over-center. In that case, each of the balls  49  would likely “cycle” in and out of the ramp surfaces on one of the plates  41  or  47 , resulting in substantial noise, but not applying proper loading to the clutch pack. The length L 2  of the members  83  is selected such that as the length of the openings L 1  decreases with disc wear, the amount of disc wear is limited, as the length L 1  of the openings can decrease to L 2 , but can never be less than the length L 2  of the members  83 . 
     As may best be seen in FIGS. 1 and 4, at the rearward end of the clutch pack  23  is another end plate  91 , which preferably has on its inner periphery, tabs similar to the tabs  65  on the ramp plate  47 , for engagement with the notches  63  on the output shaft  13 . Disposed adjacent the end plate  91  is a snap ring  93 , thus limiting rearward movement of the clutch pack  23  in the same manner as the snap ring  55  limits forward movement of the ball ramp actuator  21 . Therefore, substantially all axial separation forces generated within the clutch pack  23  and within the actuator  21  are taken up within a single element, and in this embodiment, are taken up within the output shaft  13 , which is suitably constructed for that purpose. None of the axial separation forces are transmitted to the housing  17 , and thus, the assembly of the housing  17  and the end cap  15 , and a rearward end cap  95  can be simplified and made substantially less expensive. As is well known to those skilled in the ball ramp clutch art, the separation forces within such devices can be several thousand pounds. The above-described “self-contained” feature, whereby both the clutch pack and the ball-ramp actuator are disposed between retention means on the output shaft, is not limited to use in a through-shaft configuration, but without the self-contained feature, the through-shaft design would be much more difficult to achieve, and the result may not be functionally satisfactory. 
     Referring now primarily to FIG. 5, there is illustrated an alternative embodiment of the invention, wherein the same or similar elements will bear the same reference numerals as in the embodiment of FIGS. 1 through 4, and substantially different elements will bear reference numerals in excess of “100”. Although the device of FIG. 5 operates in substantially the same overall manner as the device of FIG. 1, there are a number of structural differences, which will now be described. It may also be noted by comparing FIGS. 1 and 5 that a number of elements and relationships are reversed in the FIG. 5 embodiment. 
     The most important difference in the device of FIG. 5 relates to the viscous coupling, generally designated  101 , which is disposed radially inward from the ball ramp actuator  21 . The viscous coupling includes an input housing  103 , which is connected to the inner periphery of the first ramp plate  41 , such as by splines or any other suitable means. The coupling  101  also includes an output housing  105 , fixed to rotate with the output shaft  13 . Disposed within the coupling  101  is a plurality of interleaved plates  107  and  109 , the plates  107  being splined to the housing  103 , and the plates  109  being splined to the housing  105 . Within the coupling  101 , viscous shear torque is generated in response to slip speed between the first ramp plate  41  and the output shaft  13 , such that the coupling  101  is , in effect, on the downstream side of the ball ramp actuator  21 . 
     It is an important aspect of the invention that the ball ramp actuator  21  and the viscous coupling  19  or  101  be “nested”, i.e., with the viscous coupling either disposed radially within the actuator  21  or surrounding the actuator  21 , thus foreshortening the overall device. In either case, one element of the viscous coupling, either the input or the output, is in operative engagement with the first ramp plate  41 , at either the radially inner or radially outer periphery of the plate  41 . 
     In the device of FIG. 5, there is a plurality of the generally cylindrical members  83 , but unlike the device of FIG. 1, in the device of FIG. 5, the members  83  are disposed between the housing  17  and the outer friction discs  71 . In the device of FIG. 5, the inner friction discs  73  and preferably splined to the output shaft  13  in a conventional manner, by means of external splines  111  on the outer periphery of the output shaft  13 . Otherwise, the members serve the similar function to those in the FIG. 1 embodiment. In FIG. 5, the members  83  serve as the connection means of the friction discs  71  to the housing  17 , and also prevent the ball ramp actuator  21  from going over-center after the friction discs wear a certain amount. Finally, the springs  87 , biasing the members  83  to the right in FIG. 5, tend to bias the ball ramp actuator toward its centered, neutral position. 
     Another difference in the FIG. 5 embodiment is that the clutch pack  23  and the ball ramp actuator  21  are “self-contained” as in the device of FIG. 1, but in the device of FIG. 5 they are self-contained relative to the housing  17 . Toward the front (right end in FIG. 5) of the housing  17 , there is a snap ring  113  seated within a groove on the inner periphery of the housing  17 , and disposed adjacent the snap ring  113  is an annular retaining plate  115 . The first ramp plate  41  is supported relative to the retaining plate  115  by means of the thrust bearing set  59 , as in FIG.  1 . Thus, those elements in which substantial axial force is generated are retained and contained relative to a single member, in this case the housing  17 . 
     In operation, the device of FIG. 5 receives input at the forward end cap  15 , as does the device of FIG. 1, and as long as there is no speed difference across the device, no viscous shear torque is being generated within the coupling  101 , and the ball ramp actuator  21  remains in neutral. As, by way of example, the front wheels spin out, a viscous shear drag will be generated on the plates  107 , thus tending to retard the rotation of the first ramp plate  41 , relative to the second ramp plate  47 . As a result, the plates  41  and  47  will ramp up, thus compressing the friction discs  71  and  73 , and transmitting torque from the housing  17  to the output shaft  13 , as in the device of FIG.  1 . 
     The invention has been described in great detail in the foregoing specification, and it is believed that various alterations and modifications of the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is intended that all such alterations and modifications are included in the invention, insofar as they come within the scope of the appended claims.