Abstract:
A driveshaft assembly includes a male shaft and a female shaft slidably engaged with the male shaft. The shafts absorb energy during axial deformation of the driveshaft while substantially maintaining radial alignment of the male and female shafts.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a driveshaft for a motor vehicle and more particularly to a telescoping driveshaft having an energy absorption feature. 
     DESCRIPTION OF THE PRIOR ART 
     In a rear wheel drive motor vehicle, a driveshaft transmits torque from the transmission through a differential to the rear wheels of the motor vehicle. During a frontal crash, energy is imparted upon the vehicle and deforms the components in a longitudinal manner. Typically, the engine and transmission are driven rearward in a frontal crash, causing the driveshaft to buckle during such an impact. This is likely to cause extensive damage to adjacent underbody components. 
     U.S. Pat. No. 5,580,314 describes an energy-absorbing intermediate shaft for a steering column. With a column as described in the &#39;314 patent, a portion of the intermediate shaft is reduced to more predictably buckle during a crash and thereby absorb energy during a crash. However, the radial excursion of this design while the shaft buckles may cause damage to adjacent components, and therefore may require a large amount of clearance around the shaft to function properly. 
     It would be desirable to provide a telescoping shaft with better energy absorption characteristics and one which provides improved longitudinal deformation during a crash. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to provide a driveshaft which axially collapses during a crash and absorbs energy while remaining substantially axially aligned. 
     A driveshaft assembly according to the present invention includes a male shaft having an outer surface and a female shaft slidably engaged with the male shaft. The shafts absorb energy during axial deformation of the driveshaft while maintaining alignment of the male and female shafts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial sectional side view of a driveshaft according to the present invention. 
     FIG. 2 is the driveshaft of FIG. 1 after compression. 
     FIG. 3 is a rear sectional view through the driveshaft of FIG.  1 . 
     FIGS. 4 and 5 are partial sectional views of the female and male shafts of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, a driveshaft assembly  10  according to the present invention is provided. The driveshaft assembly  10  is illustrated in a position prior to a crash, that is, the driveshaft assembly  10  has not been collapsed. FIG. 2 illustrates the driveshaft assembly  10  after collapse. 
     The driveshaft assembly  10  rotates about an axis to transmit torque in a known manner. A male member  12  is rotatably drivably engaged with a female member  14  through a splined connection  15 . Accordingly, the female member  14  includes an internally splined portion  16  slidably engaged with an externally splined portion  17  of the male member  12 . The internally splined portion  16  extends for a first length from the rear end of the female member  14 . Likewise, the splined portion  17  of the male member  12  extends for a length at the front end thereof. A transition  20 ,  26  is formed between the splined  16 ,  17  and unsplined portions  22 ,  21  of the female and male members  14 ,  12 . 
     While the vehicle is driven, the male member  12  moves axially with respect to the female member  14  through the splined connection  15 . During a crash with a longitudinal component, the female member  14  is urged axially rearwardly over the male member  12 . While the female member  14  thus moves rearwardly, and the internally splined portion  16  is forced beyond the externally splined portion  17 , the internally splined portion  16  engages the transition  26  and the unsplined portion  21  of the male member  12 . 
     As illustrated in FIGS. 3-5, the splined portion  16  of the female member  14  comprises circumferentially spaced outwardly and inwardly projecting splines  31 ,  32  formed as a circumferentially convoluted wall in the female member  14 . The front portion of the male member  12  likewise includes a splined portion  17  with mating outwardly and inwardly projecting splines  33 ,  35 , formed as a circumferentially convoluted wall in the male member  12 . 
     In a preferred embodiment, each of the male and female members  12 ,  14  are formed from cylindrical tubes, the male tube fitting inside of the female tube. The splined portions  16 ,  17  are preferably cold formed in each of the tubes using a Grob process, which is well known to one skilled in the art. The Grob process cold thus forms the convoluted walls. The outwardly projecting splines  31  and  33  form an outer diameter  37 ,  30 , respectively, which is approximately equal to the respective outside diameter of the tubes before the internally projecting splines  32  and  35  are formed in the members  14 ,  12 . The number of splines  31 ,  32 ,  33 ,  35  and depth thereof is application specific to ensure the driveshaft assembly  10  is capable of transmitting the torque for the particular application. In an exemplary preferred embodiment, the depth of the splines is approximately 0.2 inch and 26 externally projecting splines  31 ,  33  are provided in each member  14 ,  12 . The depth is defined as the distance from the top of a spline (indicated, for example, at the minor diameter of the male tube  12  at  34 ) to the outside, or major diameter, (indicated at  30  for the male tube  12 ) of an outwardly projecting spline  33 . 
     The female member  14  is preferably swaged forward of the unsplined portion  22 . The forward end of the female member  14  is thus reduced to a size which fits a standard weld yoke  24  for attachment to a universal joint as is well known to one skilled in the art. The rear end of the male member  12  is preferably likewise swaged to reduce the diameter thereof to fit an identical weld yoke  25 , as is known to one skilled in the art. 
     The splined portion  16  of the female member  14  has an inside, or minor, diameter  36  at the base of the internal splines  32 . The inside diameter  36  is smaller than the outside diameter of the transition  26  and unsplined portion  21  of the male member  12 . As the female member  14  is urged rearwardly, the interference therebetween causes the female member  14  to plastically deform and expand radially outwardly as illustrated in FIG.  2 . This expansion requires a large amount of energy due to the cold working of the female member  14  and thus absorbs a portion of the crash energy. Because the driveshaft assembly  10  is thereby permitted to collapse axially and does not buckle significantly, the vehicle may be in a condition to permit towing or driving of a damaged vehicle which would have otherwise required a trailer to transport the vehicle to a repair facility. More significantly, because the driveshaft assembly  10  does not buckle, adjacent components remain undamaged after a crash and therefore the damage to the vehicle is reduced. 
     The female member  14  includes an unsplined portion  22  forward of the splined portion  16 . The unsplined portion  22  preferably has an inside diameter larger than the outside diameter  30  of the splines of the male member  12 . Thus, during axial compression of the driveline assembly  10  as described above, the male member  12  moves freely within the unsplined portion  22 , as long as the male member  12  is not forced beyond the unsplined portion  22 . Preferably the female member  14  in front of the splined portion  16  has an axial length that is adequate to prevent the male member  12  from contacting the weld yoke  24  of the female member  14  during most crashes. 
     In an alternative embodiment, the unsplined portion  22  of the female member  14  has a diameter slightly less than the outside diameter  30  of the outwardly projecting male splines  33 , thus causing a second interference during a crash, and therefore causing radial expansion of the female member  14  at the unsplined portion  22 , or radial compression of the male splined portion  17 . Such expansion or compression occurs in a manner similar to that described above, to absorb additional crash energy. In such an alternative embodiment, this interference may enable a reduction of the amount of interference between the splined and unsplined portions of male and female members  12  and  14 . In this embodiment, the outwardly projecting splines  31  of the female member  14  have a smaller inside diameter than the outside, or major diameter  30 , of the external splines  33  of the male member  12 . Alternatively, the inwardly projecting splines  32  of the female member  14  have a greater depth (smaller inside diameter  36 ) than the minor diameter  34  of the splines  35  of the male member  12 . 
     In a further alternative embodiment, during the axial compression of the driveshaft assembly  10 , the male member  12  is also radially compressed when the splined portion  17  engages the transition  20  and unsplined portion  22  of the female member  14 , likewise absorbing some of the crash energy as described above. The expansion of the female member  14  and compression of the male member  12  occurs due to interferences as described in the preceding paragraphs, but the members  12 ,  14  are modified to enable such compression of the male member  12 . One skilled in the art appreciates that in such an alternative embodiment, the modifications may include providing a thin wall section in the male member  12  or removing material from the male member  12  in the forward portion thereof, such as by providing axial slots, to facilitate compression thereof. 
     In a preferred embodiment, a boot seal  19  is provided to cover the splined connection  15  between the male and female members  12 ,  14 , in part to keep contaminants out of the splined connection  15 . The boot seal  19  is clamped at each end thereof to the outside diameters of the members  12 ,  14  in a known manner. In a preferred embodiment, the boot seal  19  includes a splined inside diameter  23  corresponding with the splined portion of the female member  14  and mates with the splines thereof. The boot  19  engages a smooth cylindrical surface of the male member  12  in a conventional manner. Although not illustrated, one skilled in the art appreciates the boot seal  19  may likewise engage splines on a male member  12  or a smooth surface of a female member  14 . During a crash, the members  12 ,  14  are axially compressed and the boot seal is forced to slide along the outer surface of the male member  12 , thereby absorbing additional energy. 
     One skilled in the art appreciates that the diameters of the male and female member  12  and  14  may vary depending on the vehicle application and materials used. However, in common applications, the outside diameter of a male shaft may range between 2.5 to 4.0 inches where the shafts are preferably formed from a low carbon alloy steel, such as 1015 or 1026 steel. In such a preferred embodiment, the shafts have a wall thickness of approximately 0.065 inches. A preferred clearance between the male and female splines is approximately 0.03 inches on the diameter to facilitate axial movement therebetween. In a preferred embodiment, the interference of the unsplined portion  21  of the male member into the inwardly projecting splines  32  of the female member is 0.4 inches on the diameter. One skilled in the art recognizes this interference is application specific, that is, dependent upon the energy dissipated, the materials selected, the size of the members, etc. One skilled in the art also recognizes the material and thickness are application specific and other materials, such as aluminum, steel alloys, or composite shafts, may be used as may be desirable, based on weight, size, power, and other parameters. In a preferred embodiment, these parameters are selected to create an assembly which expands the female shaft  14  with a force less than 20,000 lbs. 
     In an alternative embodiment (not shown), each the male and female members  12  and  14  have splines formed on the walls of tubes in a more traditional known manner, such as forging or broaching. The radial deformations of the female tube described above occurs due to careful design of the tube cross section thicknessess to enable radial expansion of the female member adjacent the splines. This may require machining of the outside diameter of the female member to reduce the thickness and therefore the radial strength thereof during such a crash. Alternatively, other means may be used to weaken the female member thereat, such as by cutting axial slots in the female member to facilitate the radial expansion. Similar measures may be taken on the male member to facilitate radial compression thereof. 
     In a further alternative embodiment (not shown), the splines extend along the entire length of the male and female members, and therefore the transition from the splines to the unsplined portion as described above and illustrated in the figures is not provided. The female member has a reduced portion which engages the forward end of the splines of the male member during compression of the driveshaft. This reduced portion comprises a circumferentially formed depression rolled into the outwardly projecting splines (the female tube is essentially an annular tube in this region having an inside diameter providing the 0.4 inch interference described above). During a crash, the male member thus engages the depression, and the forward end of the male member causes expansion of the reduced portion, or the male member is compressed within the reduced portion of the female member in a manner similar to that described above. 
     In a further alternative embodiment, the splines likewise extend along the entire length of both the male and female members. The interference described above for expanding the female member is created by outwardly deforming the splines of the male member. In such an embodiment, each of the splines is filled with a material (such as by welding a filler in place between each spline), or integrally forming projections in the tube between splines. In one embodiment, male tube is formed from a flat sheet of metal. The projections are formed by pressing alternating longitudinal depressions in the flat sheet of sheet metal and rolling the sheet into a cylinder and welding the seam. In this embodiment, the longitudinal depressions (the splines) have a break formed therein and therefore a transition is formed therein to cause the interference described above. These projections may alternatively be formed by any other known manner, including swaging (a female tube), extrusion, hydroforming, or any known technique. 
     In a further alternative embodiment (not shown), the transition  26  comprises an circumferential projection (or series of axially spaced projections) formed in the male tube, as opposed to the cylindrical unsplined portion  21  shown in FIGS. 1 and 2. This circumferential projection likewise comprises a larger outside diameter in the male member (between the splines) so the female member deforms outwardly during a crash, thereby dissipating some crash energy. Alternatively, the outward projections are deformed inwardly by the splines of the female member during a crash. 
     In another alternative embodiment, the driveshaft assembly  10  illustrated in FIGS. 1 and 2 includes a male tube  12  having a closed front end (not shown) engaged with the female tube  14 . The female tube  14  has a closed end at the weld yoke  24 . In this embodiment, during the crash, the air within the cavity  27  defined by the closed end of the male tube and the inside of the female tube is compressed, absorbing a further amount of energy. Likewise, the female cavity may be filled with another compressible medium, such as a fluid, gas, or resilient material, such as rubber, or a frangible material, such as a carbon graphite tube, and thereby absorb additional energy. 
     One skilled in the art recognizes the driveshaft may comprise other shapes, such as square, or any other slidably engaged geometry of axially slidably engaged shafts, one of the members having a section which interferes with the other member to absorb energy yet allowing collapse of the shaft during a crash. In such an embodiment, the noncircular shape transmits torque in a known manner and therefore splines are not required. 
     The forms of the invention shown and described herein constitute the preferred embodiments of the invention; they are not intended to illustrate all possible forms thereof. The words used are words of description rather than of limitation, and various changes may be made from that which is described here without departing from the spirit and scope of the invention.