Abstract:
A method for manufacturing a driveshaft assembly takes advantage of the asymmetrical nature of the driveshaft tube and end fittings to reduce the initial imbalance of the driveshaft assembly. The method includes the initial steps of providing a driveshaft tube having a heavy side and providing an end fitting having a heavy side. The heavy side of the end fitting is aligned so as to be opposite the heavy side of the driveshaft tube. Finally, the driveshaft tube and the end fitting are secured together. The dimensional characteristics of the driveshaft tube and end fitting improve the balancing capability of the driveshaft assembly by permitting levels of imbalance of the driveshaft assembly to be lowered and better-managed.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to driveshaft assemblies, such as are commonly used in vehicle drive train systems for transmitting rotational force or torque from an engine/transmission to an axle assembly. In particular, this invention relates to an improved method for manufacturing a driveshaft assembly that is balanced for rotation during use. 
     Drive train systems are widely used for generating power from a source and for transferring such power from the source to a driven mechanism. Frequently, the source generates rotational power, and such rotational power is transferred from the source to a rotatably driven mechanism. For example, in most land vehicles in use today, an engine/transmission assembly generates rotational power, and such rotational power is transferred from an output shaft of the engine/transmission assembly through a driveshaft assembly to an input shaft of an axle assembly so as to rotatably drive the wheels of the vehicle. A typical driveshaft assembly includes a hollow cylindrical driveshaft tube having a pair of end fittings, such as a pair of tube yokes, secured to the front and rear ends thereof. The front end fitting forms a portion of a front universal joint that connects the output shaft of the engine/transmission assembly to the front end of the driveshaft tube. Similarly, the rear end fitting forms a portion of a rear universal joint that connects the rear end of the driveshaft tube to the input shaft of the axle assembly. The universal joints provide a rotational driving connection from the output shaft of the engine/transmission assembly through the driveshaft tube to the input shaft of the axle assembly, while accommodating a limited amount of angular misalignment between the rotational axes of these three shafts. 
     Ideally, the driveshaft tube would be formed in the shape of a cylinder that is absolutely round, absolutely straight, and has an absolutely uniform wall thickness. Such a perfectly shaped driveshaft tube would be precisely balanced for rotation and, therefore, would not generate any undesirable noise or vibration during use. Similarly, the end fittings would also be manufactured in such a manner as to be precisely balanced for rotation. Such perfectly shaped end fittings could be secured to the driveshaft tube without affecting the rotational balance characteristics thereof. In actual practice, however, the driveshaft tube and the end fittings usually contain variations in roundness, straightness, wall thickness, and shape that result in minor individual imbalances when rotated at high speeds. As a result, when the end fittings are secured to the driveshaft tube, the combined driveshaft assembly is usually rotationally imbalanced. 
     To prevent such imbalances from generating undesirable noise or vibration when rotated during use, it is commonplace to counteract such imbalances by securing balance weights to selected portions of the driveshaft assembly. The balance weights are sized and positioned to counterbalance the imbalances of the driveshaft assembly such that it is balanced for rotation during use. Traditionally, the balancing process has been performed through the use of a conventional balancing machine. The balancing machine includes a pair of fittings that are adapted to support the ends of the driveshaft assembly thereon. The balancing machine further includes a motor for rotating the driveshaft assembly at a predetermined speed. As the driveshaft assembly is rotated, the balancing machine senses vibrations that are caused by imbalances in the structure of the driveshaft assembly. The balancing machine is responsive to such vibrations for determining the size and location of one or more balance weights that, if secured to the driveshaft assembly, will minimize these imbalances. The rotation of the driveshaft assembly is then stopped to allow such balance weights to be secured to the driveshaft assembly in a conventional manner, such as by welding, adhesives, and the like. The driveshaft assembly is again rotated to confirm whether proper balance has been achieved or to determine if additional balance weights are required. 
     Although this method has been effective, this balancing process has been found to be relatively slow and inefficient. This is because each driveshaft tube must usually be rotated and measured at least two times, a first time to measure the imbalances and determine the size and location of the balance weights, and a second time to confirm that proper balance has been achieved after the balance weights have been secured thereto. This time consuming process is particularly problematic in the context of balancing vehicular driveshaft tubes, which are typically manufactured in relatively large volumes. Thus, it would be desirable to provide an improved apparatus and method for quickly and efficiently balancing an article, such a tube for use in a vehicular driveshaft assembly, for rotation about an axis. 
     Conventional end fittings are typically formed by a forging process. In the forging process, a slug of raw material, usually aluminum, is inserted into the cavity of a die. The cavity defines the general shape of the end fitting. A punch applies a compressive force against the slug to cause the slug to assume the shape of the cavity. As a result of the forging process, an asymmetrical raw part is formed. The raw part is machined to form the end fitting. The end fittings are welded to each end of the driveshaft tube to form the driveshaft assembly. After manufacture, the driveshaft assembly must be precisely balanced for rotation to prevent undesirable noise and vibration. This is typically accomplished by determining the amount and location of imbalance of the driveshaft assembly and securing an appropriate counter weight to the driveshaft assembly to offset such imbalance. By convention, the manufacturing and the balancing of the driveshaft assembly have been performed as two separate and unrelated operations. That is to say, the driveshaft assembly has been completely manufactured and then balanced. This can result in a driveshaft assembly that is greatly imbalanced if the heavy side of one or both of the end fittings is aligned with the heavy side of the driveshaft tube. 
     SUMMARY OF THE INVENTION 
     This invention relates to an improved method for manufacturing a driveshaft assembly that takes advantage of the asymmetrical nature of the driveshaft tube and the end fittings to reduce the imbalance of the driveshaft assembly. The method comprises the initial steps of providing a driveshaft tube having a heavy side and providing an end fitting having a heavy side. The heavy side of the end fitting is aligned so as to be opposite the heavy side of the driveshaft tube. Finally, the driveshaft tube and the end fitting are secured together. The dimensional characteristics of the driveshaft tube and end fitting improve the balancing capability of the driveshaft assembly by permitting levels of imbalance of the driveshaft assembly to be lowered and better-managed. 
     Various objects and advantages 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 
     FIG. 1 is a schematic side elevational view of a vehicle drive train system including a driveshaft assembly in accordance with this invention. 
     FIG. 2 is an enlarged, partially exploded perspective view of the driveshaft assembly illustrated in FIG.  1 . 
     FIG. 3 is a partial perspective view of a rolled tubing that can be used to form the driveshaft tube illustrated in FIGS. 1 and 2. 
     FIG. 4 is a partial perspective view of the end fitting illustrated in FIGS. 1 and 2. 
     FIG. 5 is an enlarged perspective view of the driveshaft assembly illustrated in FIGS.  1  and  2 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, there is illustrated in FIG. 1 a drive train system, indicated generally at  10 , in accordance with this invention. The illustrated drive train system  10 , which is intended to be representative of any drive train system (vehicular or otherwise) for transferring rotational power from a source to a driven device, includes a transmission  12  having an output shaft (not shown) that is connected to an input shaft (not shown) of an axle assembly  14  by a driveshaft assembly  15 . The transmission  12  and the axle assembly  14  are conventional in the art. The driveshaft assembly  15  includes a hollow cylindrical driveshaft tube  16  that extends from a front end adjacent to the transmission  12  to a rear end adjacent to the axle assembly  14 . The driveshaft assembly  15  further includes a pair of universal joints  18  for rotatably connecting the output shaft of the transmission  12  to the front end of the driveshaft assembly  15  and for rotatably connecting the rear end of the driveshaft assembly  15  to the input shaft of the axle assembly  14 . The universal joints  18  are also conventional in the art. An end fitting  20 , commonly referred to as a tube yoke or slip yoke, is provided at the front end of the driveshaft tube  16  for connecting the front end of the driveshaft tube  16  to the front universal joint  18 . An end fitting  20  is also provided at the rear end of the driveshaft tube  16  for connecting the rear end of the driveshaft tube  16  to the rear universal joint  18 . The end fitting  20  is conventional in the art and can be secured to the ends of the driveshaft tube  16  by welding, adhesives, or other relatively permanent securing means. 
     The driveshaft tube  16  may be formed from any suitable material, such as aluminum or steel, and in accordance with any suitable method, such as by a machining method or an extrusion method. However, in a preferred embodiment of the invention, the driveshaft tube  16  is formed from rolled tubing  16   a , as shown in FIG.  2 . The rolled tubing  16   a  is most preferably aluminum. The rolled tubing  16   a  has a seam  16   b  that extends longitudinally from the front end of the driveshaft tube  16  to the rear end of the driveshaft tube  16 . The seam  16   b  is formed or joined using a conventional welding method, such as a high-frequency resistance welding, laser welding, or metal inert gas (MIG) welding, or any other technique that is suited for forming the seam  16   b.    
     The end fitting  20  may be formed from any material, such as aluminum or steel, and in accordance with any suitable method, such as by a machining or casting. However, in the preferred embodiment of the invention, the end fitting  20  is forged. In the forging process, a raw material, commonly referred to as a slug (not shown), is provided in a cavity of a metal die (not shown). The cavity is in the general shape of the outside geometry of the end fitting  20 . The slug is most preferably aluminum. A punch (not shown) applies a compressive force against the slug to cause the slug to come into contact with the cavity and thus assume the shape of the cavity. As a consequence, a raw part is formed. The raw part is in the general shape of the end fitting  20 , including the lugs  20   a  and the tube seat  20   b . Holes  20   c  are drilled and broached so as to permit smooth unencumbered insertion of the universal joints  18  therein, the outer surface of the lugs  20   a  are faced so that the universal joint  18  can be axially centered between the lugs  20   a , and the tube seat  20   b  is turned down so as to fit snugly within the open ends of the driveshaft tube  16 . 
     Regardless of the method of formation, the driveshaft tube  16  and the end fittings  20  are likely circumferentially asymmetric and thus imbalanced. In the preferred embodiment of the invention, the distribution of mass of the driveshaft tube  16  is of a nature that the amount of mass is greater circumferentially opposite the seam  16   b  than the mass at the seam  16   b . That is to say, the heavier side of the rolled tubing  16   a  of the driveshaft tube  16  is opposite the welded seam  16   b . This is because the thickness of the wall of the driveshaft tube  16  is greater opposite the seam  16   b . This is exaggerated for illustrative purposes in FIG.  2 . As shown in the drawings, the thickness of the wall of the rolled tubing  16   a  increases gradually starting at a point P 1  at about 150 degrees from the welded seam  16   b  to the thickest point P 2  at about 180 degrees from the seam  16   b  and then decreases back down to a point P 3  at about 210 degrees from the seam  16   b . As a consequence, the amount of mass of any portion of the driveshaft tube  16 , when formed from the rolled tubing  16   a , can be determined based on the circumferential distance of that portion from the seam  16   b . The distribution of mass of the driveshaft tube  16  when formed of the rolled tubing  16   a  is usually consistent and predictable. 
     The predictability of the distribution of mass may not hold true for the forged end fitting  20 . Each end fitting  20  has a wall thickness variation circumferentially around the perimeter of the tube seat  20   b . Much of this variation can be attributed to pressures used when forging the raw part. These pressures cause the die and punch to move as the raw part is forged. Consequently, the distribution of mass of each end fitting  20  may be inconsistent and unpredictable. As a result, the distribution of mass of each end fitting  20  has to be measured and the end fitting  20  marked accordingly to indicate the distribution of mass. In accordance with a conventional method, the end fitting  20  can be placed on a gauge, such as a balancing device (not shown) or a mechanical measuring device (not shown). The balancing device senses the heavy side of the end fitting  20 . The mechanical measuring device measures the variation in wall thickness of the end fitting  20 . Each device correspondingly marks the end fitting  20  so that the heavy side of the end fitting  20  or the side of the end fitting  20  with the thickest wall, which corresponds to the heavy side, can be identified. Even though the distribution of mass of the end fitting  20  is likely to be unpredictable, it is possible that when the end fittings  20  are produced in a repeatable forging operation, the heavy side of the end fittings will be consistently located at a particular circumferential location on the end fittings  20 . In such a case, this predictability can be used to eliminate the need for measuring the mass distribution of the end fittings prior to assembly of the driveshaft assembly  15 . 
     Referring now to FIGS. 3 through 5, there is illustrated the steps in the method of this invention for forming the driveshaft assembly  15  illustrated in FIGS. 1 and 2. Initially, a driveshaft tube  16  is provided, as shown in FIG.  3 . The distribution of mass of the driveshaft tube  16  must be determined. As stated above, the heavy side of the driveshaft tube  16  when formed from the rolled tubing  16   a  is consistently and predictably opposite the seam  16   b . Hence, no additional step is required for determining the distribution of mass of the driveshaft tube  16  when formed from the rolled tubing  16   a . However, the distribution of mass of other driveshafts, such as extruded driveshafts (not shown), is generally inconsistent and unpredictable. Hence, the distribution of mass for such other driveshaft tubes must be determined. This can be accomplished by measuring the distribution of mass of the driveshaft tube and marking the driveshaft tube so that the heavy side of the driveshaft tube can be identified. In accordance with a conventional method, these other driveshaft tubes can be placed on a gauge, such as a balancing device or a mechanical measuring device (not shown), which senses and marks the heavy side of the driveshaft tubes. 
     Next, an end fitting  20  is provided, as shown in FIG.  4 . As stated above, the distribution of mass of each end fitting  20  must be determined independently. As stated above, this can be accomplished by placing the end fitting  20  on a gauge that senses the heavy side of the end fitting  20  or that measures the variation in the wall thickness of the tube seat  20   b  of the end fitting  20 . The heavy side of the end fitting  20  corresponds with the side of the tube seat  20   b  having the greatest wall thickness. This is exaggerated for illustrative purposes in FIG.  4 . 
     Next, the tube seat  20   b  of one of the end fittings  20  is inserted into the front end of the driveshaft tube  16  so that the heavy side of the end fitting  20  (indicated by the mark  20   d  on the tube seat  20   b  of the end fitting  20 ) is aligned with the light side of the driveshaft tube  16 , or with the seam  16   b  of the driveshaft tube  16 , or opposite the heavy side of the driveshaft tube  16 , which is opposite the seam  16   b , as shown in FIG.  5 . The tube seat  20   b  of the other end fitting  20  can be inserted into the rear end of the driveshaft tube  16  so that its heavy side (indicated by the mark  20   d  on the tube seat  20   b  of the end fitting  20 ) is also aligned with the light side of the driveshaft tube  16 , or with the seam  16   b , or opposite the heavy side of the driveshaft tube  16 , which is opposite the seam  16   b . In a preferred embodiment of the invention, the outside perimeter of the tube seat  20   b  of each end fitting  20  is turned down during the machining of the end fitting  20  so as to produce an interference press fit between the outer perimeter of the tube seat  20   b  of the end fitting  20  and the inside diameter of the opening of the driveshaft tube  16 . After the tube seats  20   b  are inserted or press fit into the front and rear ends of the driveshaft tube  16 , the end fittings  20  are secured to the front and rear ends of the driveshaft tube  16 . As stated above, this can be accomplished by welding, adhesives, or other relatively permanent securing means. 
     Finally, the driveshaft assembly  15  is balanced to reduce any remaining imbalance in the driveshaft assembly  15 . This can be accomplished in any suitable manner. For example, the driveshaft assembly  15  can be balanced using a conventional dynamic balancer, which rotates the driveshaft assembly  15  at high speeds, measures the imbalance of the driveshaft assembly  15 , and marks the driveshaft assembly  15  or positions the driveshaft assembly  15  so that the heavy side of the driveshaft assembly  15  can be identified. Often, the light side, or the side of the driveshaft assembly  15  opposite the heavy side, is upwardly directed or exposed. A designated amount of weight is secured to the upwardly exposed side of the driveshaft assembly  15 . The dynamic balancer designates the amount of weight to be secured to the driveshaft assembly  15 . The weight can be secured by welding, adhesive, or other relatively permanent securing means. This balancing step may need to be repeated one or more times to ensure that the driveshaft assembly  15  is balanced within an acceptable tolerance. 
     The method according to the preferred embodiment of the invention takes advantage of the consistent and predictable component characteristics of the driveshaft tube  16  to enhance the product quality of the driveshaft assembly  15 . Mating the driveshaft tube  16  and end fittings  20  so that the light side of the driveshaft tube  16  aligns with the heavy side of the end fittings  20  results in a relatively counterbalanced mass distribution of the driveshaft tube  16  and the end fittings  20 , and this counterbalancing partially offsets what would otherwise be a greater imbalance of the driveshaft assembly  15 . This permits levels of the imbalance of the driveshaft assembly  15  to be lowered and better managed. The invention contemplates that the manufacture of the driveshaft assembly  15  be performed in such a manner as to minimize the amount of imbalance that must be addressed in a balancing operation subsequent to the assembly operation. This reduces the amount of deflection observed in the driveshaft assembly  15  at various operating speeds and thus reduces undesirable noise and vibration. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.