Abstract:
A hydroformed driveshaft tube formed using a hydroforming process is provided. The hydroformed driveshaft tube comprises a first end portion, a second end portion, and a middle portion. The middle portion is at least partially defined by a circular arc shaped surface of revolution. At least a portion of the middle portion has a diameter greater than a diameter of the first end portion and the second end portion. The middle portion is formed between the first end portion and the second end portion. The middle portion affects a critical speed and a breathing mode frequency of the hydroformed driveshaft tube. The hydroformed driveshaft tube reduces a cost of a driveshaft assembly.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims the benefit of priority to U.S. Provisional Application No. 61/724,154 filed on Nov. 8, 2012, which is incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to driveshafts and more specifically to driveshafts for vehicle formed using a hydroforming process. 
       BACKGROUND OF THE INVENTION 
       [0003]    Rotation of a driveshaft at or near a resonating frequency of the driveshaft may lead to an undesired vibration of the driveshaft. Further, rotation of a driveshaft which is unbalanced may also lead to the undesired vibration of the driveshaft, resulting in customer dissatisfaction. Rotation of the driveshaft with the undesired vibration, regardless of its source, may also lead to excessive wear of a plurality of components of the driveshaft. Center bearings, shaft end components (such as yokes), universal joint crosses, needle bearings, and a tubular portion of the driveshaft may all be excessively worn by the undesired vibration of the driveshaft. 
         [0004]    Typically, as a length of the driveshaft increases, the resonating frequency decreases. In vehicles having long lengths of driveshaft between a vehicle powertrain and a drive axle, such as commercial trucks, the resonating frequency of the driveshaft may approach an operational speed of the driveshaft. To relieve the undesired vibration, the driveshaft may comprise a plurality of sections joined by joints. Unfortunately, adding joints to the driveshaft greatly increases a cost and a weight of the driveshaft, and thus a vehicle the driveshaft is incorporated in. 
         [0005]    Alternately, to relieve the undesired vibration, the diameter of the driveshaft, and thus a diameter of the shaft end components, may be increased. However, increasing the diameter of the driveshaft and the diameter of the shaft end components also greatly increases the cost of the driveshaft, and thus the vehicle the driveshaft is incorporated in. 
         [0006]    Following manufacture of the driveshaft but prior to installation of the driveshaft in the vehicle, the driveshaft is typically balanced. Through the use of a dynamic balancing machine, a mass and a location of a balancing weight on the driveshaft is determined. After application of the balancing weight, the driveshaft is substantially balanced, reducing the undesired vibration of the driveshaft during operation. However, balancing of the driveshaft increases a time of manufacture of the driveshaft and therefore increases the cost of the driveshaft, and thus the vehicle the driveshaft is incorporated in. 
         [0007]    The driveshaft formed from aluminum reduces the weight of the driveshaft. Where formed using a hydroforming process, the driveshaft has an increased resonating frequency and a decreased manufacturing cost. Consequently, the driveshaft formed from aluminum using the hydroforming process is advantageous over the driveshaft formed from a steel using the hydroforming process. However, conventional methods used to hydroform driveshafts as applied to aluminum have been unsuccessful, as a maximum strain limit for forming aluminum is less than a maximum strain limit for forming steel. 
         [0008]    It would be advantageous to develop a driveshaft that may be formed using a hydroforming process, reduces a cost of the driveshaft, and has an increased critical speed. 
       SUMMARY OF THE INVENTION 
       [0009]    Presently provided by the invention, a driveshaft that may be formed using a hydroforming process, reduces a cost of the driveshaft, and has an increased critical speed, has surprisingly been discovered. 
         [0010]    In one embodiment, the present invention is directed to a hydroformed driveshaft tube. The hydroformed driveshaft tube comprises a first end portion, a second end portion, and a middle portion. The middle portion is at least partially defined by a circular arc shaped surface of revolution. At least a portion of the middle portion has a diameter greater than a diameter of the first end portion and the second end portion. The middle portion is formed between the first end portion and the second end portion. The middle portion affects a critical speed and a breathing mode frequency of the hydroformed driveshaft tube. 
         [0011]    In another embodiment, the present invention is directed to a hydroformed driveshaft tube. The hydroformed driveshaft tube comprises a first end portion, a second end portion, and a middle portion. The middle portion is at least partially defined by a circular arc shaped surface of revolution. At least a portion of the middle portion has a diameter greater than a diameter of the first end portion and the second end portion. The middle portion comprises a first distension, a constriction, and a second distension. The middle portion is formed between the first end portion and the second end portion. The constriction is formed between the first distension and the second distension. The middle portion affects a critical speed and a breathing mode frequency of the hydroformed driveshaft tube. 
         [0012]    In another embodiment, the present invention is directed to a hydroformed driveshaft tube. The hydroformed driveshaft tube comprises a first end portion, a second end portion, and a middle portion. The middle portion is at least partially defined by a circular arc shaped surface of revolution. At least a portion of the middle portion has a diameter greater than a diameter of the first end portion and the second end portion. The middle portion comprises a first transition portion, a first constriction portion, a second constriction portion, and a second transition portion. The middle portion is formed between the first constriction portion and the second constriction portion. The middle portion affects a critical speed and a breathing mode frequency of the hydroformed driveshaft tube. 
         [0013]    Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above, as well as other advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description when considered in the light of the accompanying drawings in which: 
           [0015]      FIG. 1A  is a perspective view of a driveshaft tube according to an embodiment of the present invention; 
           [0016]      FIG. 1B  is a side plan view of the driveshaft tube illustrated in  FIG. 1A ; 
           [0017]      FIG. 2A  is a perspective view of a driveshaft tube according to another embodiment of the present invention; 
           [0018]      FIG. 2B  is a side plan view of the driveshaft tube illustrated in  FIG. 2A ; 
           [0019]      FIG. 3A  is a perspective view of a driveshaft tube according to another embodiment of the present invention; 
           [0020]      FIG. 3B  is a side plan view of the driveshaft tube illustrated in  FIG. 3A ; 
           [0021]      FIG. 4  is a table displaying experimental data collected from straight tubing used as a control, the driveshaft tube illustrated in  FIG. 1A , the driveshaft tube illustrated in  FIG. 2A , and the driveshaft tube illustrated in  FIG. 3A ; 
           [0022]      FIG. 5  is a bar style chart illustrating a portion of the experimental data shown in  FIG. 4 , comparing a critical speed by a length and a shape of straight tubing used as a control, the driveshaft tube illustrated in  FIG. 1A , the driveshaft tube illustrated in  FIG. 2A , and the driveshaft tube illustrated in  FIG. 3A ; and 
           [0023]      FIG. 6  is a bar style chart illustrating a portion of the experimental data shown in  FIG. 4 , comparing a breathing mode frequency by a length and a shape of straight tubing used as a control, the driveshaft tube illustrated in  FIG. 1A , the driveshaft tube illustrated in  FIG. 2A , and the driveshaft tube illustrated in  FIG. 3A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless the claims expressly state otherwise. 
         [0025]      FIGS. 1A and 1B  illustrate a first driveshaft tube  100  formed using a hydroforming process. The first driveshaft tube  100  is formed from a 6061 aluminum alloy; however, it is understood that other alloys may be used. A tubular aluminum blank (not illustrated) used to form the first driveshaft tube  100  using the hydroforming process may be formed using an extrusion process or a seam welding process. The tubular aluminum blank is a cylindrical aluminum tube. 
         [0026]    The first driveshaft tube  100  includes a first end portion  102 , a middle portion  104 , and a second end portion  106 . Once fitted with a pair of shaft end components (not shown), the first driveshaft tube  100  forms a portion of a driveshaft assembly (not shown) for use with a vehicle. 
         [0027]    The first end portion  102  and the second end portion  106  are substantially cylindrical in shape and comprise about 13% of a length of the first driveshaft tube  100 , but it, is understood that other ratios may also be used. A wall thickness of the first end portion  102  and the second end portion  106  are substantially constant. The first end portion  102  and the second end portion  106  respectively meet the middle portion  104  at a first tangential transition  108  and a second tangential transition  110 . A radius of a substantially circular arc of a surface of revolution forming the first tangential transition  108  and the second tangential transition  110  is about four times greater than a radius of the first end portion  102  and the second end portion  106 . A shape of the middle portion  104  is a surface of revolution formed by rotating a substantially circular arc about an axis of the first end portion  102  and the second end portion  106 . As a non-limiting example, the substantially circular arc of the surface of revolution of the middle portion  104  may be defined by an acute angle of about 4 degrees, but it is understood that other angle may also be used. Further, a radius of the substantially circular arc of the surface of revolution of the middle portion  104  is about 200 times greater than a radius of the first end portion  102  and the second end portion  106 , but it is understood that other ratios may also be used. A wall thickness of the middle portion  104  is not constant due to the hydroforming process used to form the first driveshaft tube  100 . A thickness of the middle portion  104  at a thinnest point, at a midpoint of the first driveshaft tube  100 , is about 90% of a thickness of the first end portion  102  and the second end portion  106 , but it is understood that other ratios may be used. The shape of the middle portion  104  of the first driveshaft tube  100  may be commonly described as a barrel shape. 
         [0028]    The first driveshaft tube  100  increases a critical speed or a first bending mode of the driveshaft having a first length by an average of approximately 26% when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to the diameter of the end portions  102 ,  106 . The first driveshaft tube  100  increases a critical speed or a first bending mode of the driveshaft having a second length by an average of approximately 23% when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to the diameter of the end portions  102 ,  106 . The critical speed of the first driveshaft tube  100  is highly dependent on the average diameter of the tubing, so with adjustments to the shape of the forming and percentage of straight tubing forming the first driveshaft tube  100 , this increase in critical speed can be adjusted. 
         [0029]    It has also been discovered through experimentation that a breathing mode frequency of the first driveshaft tube  100  is significantly increased when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to a greatest diameter of the middle portion  104 . The first driveshaft tube  100  having a first length offers an increase over the straight tubing used as a control of about 67%. The first driveshaft tube  100  having a second length offers an increase over the straight tubing used as a control of about 72%. Breathing modes are natural modes of tubing where the circumference of the tube is bent to a non-perfect circle. As this occurs it acts as an amplifying agent to any other noises in the vehicle, typically a whine of a transmission or an axle gear. 
         [0030]      FIGS. 2A and 2B  illustrate a second driveshaft tube  200  formed using a hydroforming process. The second driveshaft tube  200  is formed from a 6061 aluminum alloy; however, it is understood that other alloys may be used. A tubular aluminum blank (not illustrated) used to form the second driveshaft tube  200  using the hydroforming process may be formed using an extrusion process or a seam welding process. The tubular aluminum blank is a cylindrical aluminum tube. 
         [0031]    The second driveshaft tube  200  includes a first end portion  202 , a first transition portion  204 , a first constriction portion  206 , a middle portion  208 , a second constriction portion  210 , a second transition portion  212 , and a second end portion  214 . Once fitted with a pair of shaft end components (not shown), the second driveshaft tube  200  forms a portion of a driveshaft assembly (not shown) for use with a vehicle. 
         [0032]    The first end portion  202  and the second end portion  214  are substantially cylindrical in shape and each comprise about 11% of a length of the second driveshaft tube  200 , but it is understood that other ratios may be used. A wall thickness of the first end portion  202  and the second end portion  214  are substantially constant. The first end portion  202  and the second end portion  214  respectively meet the first transition portion  204  and the second transition portion  212  in a first tangential transition  216  and a second tangential transition  218 . A radius of a substantially circular arc of a surface of revolution forming the first tangential transition  216  and the second tangential transition  218  is about 4.5 times greater than a radius of the first end portion  202  and the second end portion  214 . 
         [0033]    A shape of the first transition portion  204 , the middle portion  208 , and the second transition portion  212  corresponds in shape to a surface of revolution formed by rotating a substantially circular arc about an axis of the first end portion  204  and the second end portion  214 . The first transition portion  204  and the second transition  212  portion each comprise about 11% of a length of the second driveshaft tube  200 , but it is understood that other ratios may be used. The middle portion  208  comprises about 40% of a length of the second driveshaft tube  200 , but it is understood that other ratios may be used. As a non-limiting example, the substantially circular arc of the surface of revolution corresponding in shape to the first transition portion  204 , the middle portion  208 , and the second transition portion  212  may be defined by an acute angle of about 7 degrees, but it is understood that other angles may be used. Further, a radius of the substantially circular arc of the surface of revolution corresponding in shape to the first transition portion  204 , the middle portion  208 , and the second transition portion  212  is about 150 times greater than a radius of the first end portion  202  and the second end portion  214 , but it is understood that other ratios may be used. A wall thickness of the middle portion  208  is not constant due to the hydroforming process used to form the second driveshaft tube  200 . A thickness of the middle portion  208  at a thinnest point, at a midpoint of the middle portion  208 , is about 90% of a thickness of the first end portion  202  and the second end portion  214 , but it is understood that other ratios may be used. The shape of the first transition portion  204 , the middle portion  208 , and the second transition portion  212  is divided by the first constriction portion  206  and the second constriction portion  210 . 
         [0034]    The first constriction portion  206  and the second constriction portion  210  are each a surface of revolution formed by rotating a substantially circular arc about an axis of the first end portion  202  and the second end portion  214 . As a non-limiting example, the substantially circular arc of the surface of revolution of the first constriction portion  206  and the second constriction portion  210  may each be defined by an acute angle of about 20 degrees, but it is understood that other angles may be used. Further, a radius of the substantially circular arc of the surface of revolution of the first constriction portion  206  and the second constriction portion  210  is about 4.5 times greater than a radius of the first end portion  202  and the second end portion  214 , but it is understood that other ratios may be used. A concavity of the first constriction portion  206  and the second constriction portion  210  is opposite a concavity of the first transition portion  204 , the middle portion  208 , and the second transition portion  212 . A wall thickness of the first constriction portion  206  and the second constriction portion  210  are substantially equal to a thickness of the first end portion  202  and the second end portion  214 . A diameter of the first constriction portion  206  and the second constriction portion  210  is about 16% greater than a diameter of the first end portion  202  and the second end portion  214 . The first constriction portion  206  respectively tangentially meets the first transition portion  204  and the middle portion  208  in a third tangential transition  220  and a fourth tangential transition  222 . A radius of a substantially circular arc of a surface of revolution forming the third tangential transition  220  and the fourth tangential transition  222  is about 4.5 times greater than a radius of the first end portion  202  and the second end portion  214 . The second constriction portion  210  respectively tangentially meets the second transition portion  212  and the middle portion  208  in a fifth tangential transition  224  and a sixth tangential transition  226 . A radius of a substantially circular arc of a surface of revolution forming the fifth tangential transition  224  and the sixth tangential transition  226  is about 4.5 times greater than a radius of the first end portion  202  and the second end portion  214 . 
         [0035]    The first constriction portion  206  and the second constriction portion  210  of the second driveshaft tube  200  respectively provide a tertiary datum  226  and a quaternary datum  228  (in addition to the first end portion  202  and the second end portion  214 ) to militate against tube buckling which may occur during the hydroforming process. As a result, the first constriction portion  206  and the second constriction portion  210  of the second driveshaft tube  200  reduce an amount of axial runout that is generated in the second driveshaft tube  200  during the hydroforming process. The first constriction portion  206  and the second constriction portion  210  of the second driveshaft tube  200  are created by a shape of a hydroforming die. The diameter of the second driveshaft tube  200  at the first constriction portion  206  and the second constriction portion  210  is greater than the diameter of the first end portion  202  and the second end portion  214 , which allow the hydroforming die to secure the second driveshaft tube  200  with respect to the first end portion  202  and the second end portion  214  during the hydroforming process. 
         [0036]    The second driveshaft tube  200  increases a critical speed or a first bending mode of the driveshaft having a first length by an average of approximately 29% when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to the diameter of the end portions  202 ,  214 . The critical speed of the second driveshaft tube  200  is highly dependent on the average diameter of the tubing, so with adjustments to the shape of the forming and percentage of straight tubing forming the second driveshaft tube  200 , this increase in critical speed can be adjusted. 
         [0037]    It has also been discovered through experimentation that a breathing mode frequency of the second driveshaft tube  200  is significantly increased when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to a greatest diameter of the middle portion  208 . The second driveshaft tube  200  having a first length offers an increase over the straight tubing used as a control of about 52%. 
         [0038]      FIGS. 3A and 3B  illustrate a third driveshaft tube  300  formed using a hydroforming process. The third driveshaft tube  300  is formed from a 6061 aluminum alloy; however, it is understood that other alloys may be used. A tubular aluminum blank (not illustrated) used to form the third driveshaft tube  300  using the hydroforming process may be formed using an extrusion process or a seam welding process. The tubular aluminum blank is a cylindrical aluminum tube. 
         [0039]    The third driveshaft tube  300  includes a first end portion  302 , a first distension  304 , a constriction  306 , a second distension  308 , and a second end portion  310 . Once fitted with a pair of shaft end components (not shown), the third driveshaft tube  300  forms a portion of a driveshaft assembly (not shown) for use with a vehicle. 
         [0040]    The first end portion  302  and the second end portion  310  are substantially cylindrical in shape and each comprise about 7% of a length of the third driveshaft tube  300 , but it is understood that other ratios may be used. A wall thickness of the first end portion  302  and the second end portion  310  are substantially constant. The first end portion  302  and the second end portion  310  respectively meets the first distension  304  and the second distension  308  at a first tangential transition  312  and a second tangential transition  314 . A radius of a substantially circular arc of a surface of revolution forming the first tangential transition  312  and the second tangential transition  314  is about four times greater than a radius of the first end portion  302  and the second end portion  310 . 
         [0041]    A shape of the first distension  304  is a surface of revolution formed by rotating a substantially circular arc about an axis of the first end portion  302  and the second end portion  310 . As a non-limiting example, the substantially circular arc of the surface of revolution of the first distension  304  may be defined by an acute angle of about 10 degrees, but it is understood that other angles may be used. Further, a radius of the substantially circular arc of the surface of revolution of the first distension  304  is about 40 times greater than a radius of the first end portion  302  and the second end portion  310 , but it is understood that other ratios may be used. A wall thickness of the first distension  304  is not constant due to the hydroforming process used to form the third driveshaft tube  300 . A thickness of the first distension  304  at a thinnest point, at a midpoint of the first distension  304 , is about 90% of a thickness of the first end portion  302  and the second end portion  310 , but it is understood that other ratios may be used. The shape of the first distension  304  of the third driveshaft tube  300  may be commonly described as a barrel shape. 
         [0042]    The constriction  306  is a surface of revolution formed by rotating a substantially circular arc about an axis of the first end portion  302  and the second end portion  310 . As a non-limiting example, the substantially circular arc of the surface of revolution of the constriction  306  may be defined by an acute angle of about 6 degrees, but it is understood that other angles may be used. Further, a radius of the substantially circular arc of the surface of revolution of the constriction  306  is about four times greater than a radius of the first end portion  302  and the second end portion  310 , but it is understood that other ratios may be used. A concavity of the constriction  306  is opposite a concavity of the first distension  304  and the second distension  308 . A wall thickness and a diameter of the constriction  306  are substantially equal to a thickness and a diameter of the first end portion  302  and the second end portion  310 . The constriction  306  respectively meets the first distension  304  and the second distension  308  at a third tangential transition  316  and a fourth tangential transition  318 . A radius of a substantially circular arc of a surface of revolution forming each of the third tangential transition  316  and the fourth tangential transition  318  is about 4 times greater than a radius of the first end portion  302  and the second end portion  310 . 
         [0043]    A shape of the second distension  308  is a surface of revolution formed by rotating a substantially circular arc about an axis of the first end portion  302  and the second end portion  310 . As a non-limiting example, the substantially circular arc of the surface of revolution of the second distension  308  may be defined by an acute angle of about 10 degrees, but it is understood that other angles may be used. Further, a radius of the substantially circular arc of the surface of revolution of the second distension  308  is about 40 times greater than a radius of the first end portion  302  and the second end portion  310 , but it is understood that other ratios may be used. A wall thickness of the second distension  308  is not constant due to the hydroforming process used to form the third driveshaft tube  300 . A thickness of the second distension  308  at a thinnest point, at a midpoint of the second distension  308 , is about 90% of a thickness of the first end portion  302  and the second end portion  310 , but it is understood that other ratios may be used. The shape of the second distension  308  of the third driveshaft tube  300  may be commonly described as a barrel shape. 
         [0044]    The constriction  306  of the third driveshaft tube  300  provides a tertiary datum  320  (in addition to the first end portion  302  and the second end portion  310 ) to militate against tube buckling which may occur during the hydroforming process. As a result, the constriction  306  of the third driveshaft tube  300  reduces an amount of axial runout that is generated in the third driveshaft tube  300  during the hydroforming process. The constriction  306  of the third driveshaft tube  300  is created by a shape of a hydroforming die. The diameter of the third driveshaft tube  300  at the constriction  306  is the same diameter as the first end portion  302  and the second end portion  310 , which allows the hydroforming die to secure a center of the third driveshaft tube  300  with respect to the first end portion  302  and the second end portion  310  during the hydroforming process. 
         [0045]    The third driveshaft tube  300  increases a critical speed or a first bending mode of the driveshaft having a first length by an average of approximately 22% when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to the diameter of the end portions  302 ,  310 . The third driveshaft tube  300  also increases a critical speed or a first bending mode of the driveshaft having a second length by an average of approximately 20% when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to the diameter of the end portions  302 ,  310 . The critical speed of the third driveshaft tube  300  is highly dependent on the average diameter of the tubing, so with adjustments to the shape of the forming and percentage of straight tubing forming the third driveshaft tube  300 , this increase in critical speed can be adjusted. 
         [0046]    It has also been discovered through experimentation that a breathing mode frequency of the third driveshaft tube  300  is significantly increased when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to a greatest diameter of the distensions  304 ,  308 . The third driveshaft tube  300  having a first length offers an increase over the straight tubing used as a control of about 105%. The third driveshaft tube  300  having a second length offers an increase over the straight tubing used as a control of about 112%. 
         [0047]      FIG. 4  is a table which includes experimental data collected from straight tubing used as a control, the first driveshaft tube  100 , the second driveshaft tube  200 , and the third driveshaft tube  300 . The aforementioned results are shown and based upon the experimental data shown in  FIG. 4 . 
         [0048]      FIG. 5  is a bar style chart comparing the critical speed by a length and a shape of straight tubing used as a control (in three instances), the first driveshaft tube  100 , the second driveshaft tube  200 , and the third driveshaft tube  300 . The bar style chart display the experimental data shown in  FIG. 4 . 
         [0049]      FIG. 6  is a bar style chart comparing the breathing mode by a length and a shape of straight tubing used as a control (in three instances), the first driveshaft tube  100 , the second driveshaft tube  200 , and the third driveshaft tube  300 . The bar style chart display the experimental data shown in  FIG. 4 . 
         [0050]    As can be appreciated from  FIGS. 4-6 , the driveshaft tube  100 ,  200 ,  300  has an increased critical speed when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to the diameter of the end portions  102 ,  106 ,  202 ,  214 ,  302 ,  310 . Such a benefit allows the driveshaft assembly including the driveshaft tube  100 ,  200 ,  300  to have critical speed characteristics of a driveshaft tube having a greater diameter than a driveshaft formed from straight tubing having an outer diameter substantially equal to the diameter of the end portions  102 ,  106 ,  202 ,  214 ,  302 ,  310 . The driveshaft assembly including the driveshaft tube  100 ,  200 ,  300  is compatible with driveshaft end fittings having a reduced diameter, which greatly reduces a cost of the driveshaft assembly including the driveshaft tube  100 ,  200 ,  300 . 
         [0051]    As can be appreciated from  FIGS. 4-6 , the driveshaft tube  100 ,  200 ,  300  has an increased breathing mode frequency when compared to straight tubing used as a control, the straight tubing having an outer diameter substantially equal to a greatest diameter of the middle portion  104 ,  208  or the distensions  304 ,  308 . Such a benefit allows the driveshaft assembly including the driveshaft tube  100 ,  200 ,  300  to have breathing mode frequency characteristics of a driveshaft tube having a reduced diameter, while still obtaining the critical speed benefits of a driveshaft tube having an increased diameter. 
         [0052]    In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiments. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.