Abstract:
The present disclosure relates to a gearing arrangement for a tandem axle assembly for a vehicle that reduces parasitic losses associated with the bearings of a drive pinion. The gearing arrangement includes a first helical gear in driving engagement with an input shaft and a portion of an interaxle differential; a second helical gear coupled to a pinion shaft with at least two bearings mounted on either side of the second helical gear on the pinion shaft; and a drive pinion coupled to the pinion shaft and meshingly engaged with a ring gear. The ring gear is in driving engagement with a forward differential assembly. The first helical gear and second helical gear are meshingly engaged and have a predetermined gear ratio.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is claiming the benefit, under 35 U.S.C. 119(e), of the provisional application granted Ser. No. 62/233,824 filed on Sep. 28, 2015, the entire disclosure of which is hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to a gearing arrangement for a tandem axle assembly for a vehicle that reduces parasitic losses associated with the bearings of a drive pinion. 
       BACKGROUND 
       [0003]    Increases in fuel efficiency are becoming more important to owner and operators of vehicles, particularly large vehicles such as tandem axle tractor trailers. Every aspect of the vehicle driveline is undergoing scrutiny to determine where parasitic losses can be reduced or eliminated so that fuel efficiency can be improved. 
         [0004]    One structure that has received attention to determine if losses can be reduced or eliminated is the bearings in the vehicle driveline. More particularly, the bearings that support pinion shafts appear to create an inordinate amount of drag as they rotate through lubricant. The parasitic power losses of the bearings is a function of speed due to the amount of parasitic fluid drag resulting from rotating through the lubricant. The slower the axle gear ratio (i.e. the higher numerically) the faster the pinion gear must rotate for a given vehicle speed. Power consumption is a function of the multiplication of torque and rotational speed. Thus, the pinion bearings consume more power the slower the axle gear ratio because the bearings rotate at a faster speed. 
         [0005]    Therefore, it would be advantageous to find a way to reduce the parasitic power losses created by the bearings to increase the vehicle driveline efficiency. 
       SUMMARY 
       [0006]    A gearing arrangement for a tandem axle system of a vehicle including a first helical gear in driving engagement with an input shaft and a portion of an interaxle differential; a second helical gear coupled to a pinion shaft with at least two bearings mounted on either side of the second helical gear on the pinion shaft; and a drive pinion coupled to the pinion shaft and meshingly engaged with a ring gear. The ring gear is in driving engagement with a forward differential assembly. The first helical gear and second helical gear are meshingly engaged and have a predetermined gear ratio. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The above, as well as other advantages of the present embodiments, 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: 
           [0008]      FIG. 1  is a schematic perspective view of a preferred embodiment of a tandem axle assembly in accordance with the present disclosure; 
           [0009]      FIG. 2  is a detailed cutaway side view of one embodiment of a forward axle system of the tandem axle assembly of  FIG. 1 ; 
           [0010]      FIG. 3  is a cutaway schematic side view of one embodiment of a forward axle system of the tandem axle assembly of  FIG. 1 ; 
           [0011]      FIG. 4  is a partial, schematic cutaway top view of the forward axle system depicted in  FIG. 3 ; and 
           [0012]      FIG. 5  is a cutaway schematic side view of one embodiment of a rear axle system of the tandem axle assembly of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    It is to be understood that the embodiments 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 
         [0014]    As depicted in  FIGS. 1-4 , one embodiment of a tandem axle assembly  10  for a vehicle includes a forward axle system  20  and a rear axle system  120 . The forward axle system  20  has a housing  22 . The housing  22  maybe hollow and has integrally formed first arm  24  and second arm  26  extending therefrom. The housing  22  may be of one-piece construction or multi-piece construction. A first wheel hub  28  is rotatably mounted at the end of the first arm  24  and a second wheel hub  30  is rotabably mounted at the end of the second arm  26 . Wheels and tires (neither shown) are mounted on the wheel hubs  28 ,  30 . 
         [0015]    A forward differential assembly  32  is located within the housing  22 . A first axle half shaft  34  is connected to the forward differential assembly  32 . The first axle half shaft  34  extends from the forward differential assembly  32  to the first wheel hub  28  within the first arm  24 . A second axle half shaft  36  is connected to the forward differential assembly  32 . The second axle half shaft  36  extends from the forward differential assembly  32  to the second wheel hub  30  within the second arm  26 . Rotational power from the forward differential assembly  32  is transmitted through the axle half shafts  34 ,  36  to the wheel ends to cause the vehicle to move over the road. 
         [0016]    In the depicted embodiment, rotational power is provided to the forward differential assembly  32  from an engine and/or transmission (not shown). The rotational power is provided to the forward differential assembly  32  through an input shaft  38 . A yoke  40  may be connected to the input shaft  38  for connecting with a complementary yoke (not shown). 
         [0017]    The input shaft  38  extends into a hollow interior of the housing  22 . The input shaft  38  is connected to a gearing arrangement  41 . The gearing arrangement  41  includes a first helical gear  42 . The first helical gear  42  is coaxial with the input shaft  38 . The first helical gear  42  is directly meshed with a second helical gear  44 . The second helical gear  44  is located below the first helical gear  42  in the housing  22 . The second helical gear  44  is located on a pinion shaft  46  that is parallel but not coaxial with the input shaft  38 . The pinion shaft  46  is mounted for rotation within the housing  22  on a first bearing  48  and a second bearing  50 . The bearings  48 ,  50  are positioned on either side of the second helical gear  44  on the pinion shaft  46 . 
         [0018]    A drive pinion  52  is mounted on the pinion shaft  46 . The drive pinion  52  is directly connected to a ring gear  54 . In one embodiment, the drive pinion  52  and ring gear  54  have a gear ratio of 2.26. The drive pinion  52  permits the input shaft  38  to be mounted lower in the housing  22  resulting in a vertically compressed forward drive axle system  20 . The ring gear  54  is directly connected to the forward differential assembly  32 . The forward differential assembly  32  includes a differential case  56  that houses at least one pinion gear  58  and at least one side gear  60 . Preferably, the differential case  56  houses two pinion gears  58  mounted on a spider shaft (not depicted) where the spider shaft extends into the differential case  56 . The pinion gears are directly meshed with at least two side gears  60 . The side gears  60  have hollow interiors bounded by splines. The splines mesh with splines on the first and second axle half shafts  34 ,  36 . The forward differential assembly  32  divides rotational drive from the ring gear  54  to the first axle half shaft  34  and the second axle half shaft  36 . 
         [0019]    The first helical gear  42  is drivingly connected to an interaxle differential  62 . The interaxle differential  62  may be comprised of at least one pinon gear  64  and at least one side gear  66 . Preferably, the interaxle differential  62  includes two pinion gears  64  meshed with a first side gear  66   a  and a second side gear  66   b.  The interaxle differential  62  divides rotational drive from the input shaft  38  between the first helical gear  42  and the first side gear  66   a.    
         [0020]    An output shaft  68  is connected to the second side gear  66   b.  The output shaft  68  is co-axial with the input shaft  38  and is mounted for rotation in the housing  22 . The output shaft  68  extends over differential case  56  and the axle half shafts  34 ,  36 . The output shaft  68  extends axially through the rear of the housing  22 . A yoke  70  may be connected to the output shaft  68 . The yoke  70  may be connected to a prop shaft  72 . In one embodiment, the side gear  66   b  is connected to the output shaft  68  to drive prop shaft  72 . 
         [0021]    The first helical gear  42  rotates at the same speed at the input shaft  38  since the two are directly connected to one another without any structure between them to increase or decrease the rotation. If the first and second helical gears  42 ,  44  in a tandem axle assembly have a 1:1 gear ratio, the drive pinion  52  turns at the same speed as the input shaft  38 . However, by intentionally under-driving the drive pinion  52  by adjusting the gear ratio of the helical gears  42 ,  44 , the rotational speed of the drive pinion  52  can be reduced. In one embodiment, the helical gears  42 ,  44  may be designed to have a 1.57 gear ratio. Because the parasitic power loss of the bearings  48 ,  50  is a function of speed, decreasing the speed of the drive pinion  52  increases drive line efficiency. The gear ratios provided in the forward axle system  20  may be selected based on the desired needs and efficiency of the vehicle. Helical gears  42 ,  44  each have tooth inclination, i.e., the teeth are disposed at an angle relative to the axes of the gears  42 ,  44 . The desired gear ratio for the first and second helical gears  42 ,  44  can be achieved by providing helical gears  42 ,  44  with different outer diameters or by varying the number of teeth on each gear. The speed of the helical gears  42 ,  44  are inversely proportional to the ratio of their outer diameters and to the ratio of the number of gear teeth. In one preferred embodiment, the number of teeth on the first helical gear  42  is less than the number of teeth on the second helical gear  44 . Additionally or alternatively, the first helical gear  42  can have an outer diameter smaller than the outer diameter of the second helical gear  44 . 
         [0022]    If the number of teeth and/or the outer diameter between the helical gears  42 ,  44  differs, it results in the second helical gear  44  rotating at a different speed than the first helical gear  42 . In one preferred embodiment, the second helical gear  44  rotates slower than the first helical gear  42 . The helical gears  42 ,  44  in the forward axle system  20  result in the drive pinion  52  driving the ring gear  54  at a predetermined drive ratio. The result of rotating the second helical gear  44  slower than the first helical gear  42  is that the drive pinion  52  connected to the drive shaft  46  rotates slower than the input shaft  38 . 
         [0023]    In another embodiment, the second helical gear  44  rotates faster than the first helical gear  42 , i.e. the drive pinion  52  is over-driven. The result of rotating the second helical gear  44  faster than the first helical gear  42  is that the drive pinion  52  connected to the drive shaft  46  rotates faster than the input shaft  38 . 
         [0024]    As shown in  FIGS. 1 and 5 , the prop shaft  72  extends from the forward axle system  20  to the rear axle system  120 . The prop shaft is connected to an input shaft  138  of the rear axle system  120 . The rear axle system  120  has a housing  122 . The housing  122  may be of one-piece construction or multi-piece construction. The input shaft  138  is rotatingly mounted within the housing  122 . The housing  122  has integrally formed first  124  and second arm  126  extending therefrom. A first wheel hub  128  is rotatably mounted at the end of the first arm  124  and a second wheel hub  130  is rotabably mounted at the end of the second arm  126 . Wheels and tires (neither shown) are mounted on the wheel hubs  128 ,  130 . 
         [0025]    A drive pinion  152  is located on the end of the input shaft  138 . The drive pinion  152  is co-axial with the input shaft  138 . The drive pinion  152  is engaged with a ring gear  154 . The ring gear  154  is connected to a rear differential assembly  132 . The rear differential assembly  132  is located within the housing  122 . A first axle half shaft  134  is connected to the rear differential assembly  132 . The first axle half shaft  134  extends from the rear differential assembly  132  to the first wheel hub  128  within the hollow first arm  124 . A second axle half shaft  136  is connected to the rear differential assembly  132 . The second axle half shaft  136  extends from the rear differential assembly  132  to the second wheel hub  130  within the hollow second arm  126 . Rotational power from the rear differential assembly  132  is transmitted through the axle half shafts  134 ,  136  to the wheel ends to cause the vehicle to move over the road. The rear differential assembly  132  divides the rotational drive provided by the ring gear  154  between a first rear axle half shaft  134  and a second rear axle half shaft  136 . 
         [0026]    The ring gear  154  is directly connected to a differential  156 . The rear differential assembly  132  includes a differential case (not pictured) that houses at least one pinon gear (not depicted) and at least one side gear (not depicted). Preferably, the differential case houses two pinion gears mounted on a spider shaft (not depicted) where the spider shaft extends into the differential case. The pinion gears are directly meshed with at least two side gears (not depicted). The side gears have hollow interiors bounded by splines. The splines mesh with splines on the first and second axle half shafts  134 ,  136 . 
         [0027]    The first and second forward axle half shafts  34 ,  36  and the first and second rear axle half shafts  134 ,  136  each are located within their respective half shaft housings and extend away from their respective differentials  56 ,  156 . 
         [0028]    The gear ratios provided in the forward and rear tandem axles systems  20 ,  120  may be selected based on the desired needs and efficiency of the vehicle. In one embodiment, the drive pinion  152  driving the ring gear  154  in the rear axle system  120  is not reduced as it is in the front axle system  20 . In one embodiment, the drive ratio for the rear axle system  120  may be, but is not limited to, 3.55. Thus, the drive ratio for the pinion  52  and ring gear  54  for the forward axle system  20  is different, more particularly reduced, compared to the drive ratio for the pinion  152  and ring gear  154  for the rear axle system  120 . 
         [0029]    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 embodiments can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.