Abstract:
A method for shifting tandem axle loads on a vehicle including an air suspension circuit having a three way valve, a first air spring connected between a drive axle of a tandem and a vehicle frame and a second air spring connected between a tag axle of the tandem and the vehicle frame. A diameter of the first air spring is larger than a diameter of the second air spring. The system also has an air supply, a first fluid line connected between a port one of the three way valve and the first air spring and a second fluid line connected between a port three of the three way valve and the second air spring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/782,054 filed on Mar. 14, 2013. This application is a non-provisional application filed off of U.S. patent application Ser. No. 61/782,054 filed on Mar. 14, 2013, which is incorporated by reference in its entirety herein. This non-provisional application is being filed during the pendency of U.S. patent application Ser. No. 61/782,054. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an air suspension system for use with the tandem axles of a commercial vehicle line haul tractor. 
       BACKGROUND OF THE INVENTION 
       [0003]    The typical North American Class 8 Line Haul Truck uses a 6×4 tractor with two drive axles in the tractor rear tandem. Alternately, a 6×2 drive line is used with a single drive axle and a dead tag axle in the rear tandem. The 6×2 system is lighter and has a lower parasitic loss compared to the 6×4 system, but suffers from a deficiency in tractive effort under conditions of reduced tire to ground coefficient since the drive axle and tag axle will have the same Gross Axle Weight Rating (GAWR). Systems have been offered to increase the 6×2 single drive axle tractive effort such as wheel differential locks and service brake based electronic traction control systems. Additionally, 6×2 air suspension systems are available that can automatically shift load from the tag axle to the drive axle under conditions of low traction to improve the tractive effort of the drive axle but these systems are slow acting, costly and cumbersome. 
         [0004]    In view of the foregoing disadvantages of the prior art, it would be advantageous to have a low cost system that is fast acting, cost effective and easy to incorporate that can safely and effectively improve the 6×2 drive axle traction. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is directed toward a method for shifting tandem axle loads on a vehicle including an air suspension circuit having a three way valve, a first air spring connected between a drive axle of a tandem and a vehicle frame and a second air spring connected between a tag axle of the tandem and the vehicle frame. A diameter of the first air spring is larger than a diameter of the second air spring. The system also has an air supply, a first fluid line connected between a first port of the three way valve and the first air spring and a second fluid line connected between a third port of the three way valve and the second air spring. 
         [0006]    In accordance with the present invention, it has been discovered that the ability to rapidly deliver air from the tag axle to the drive axle during a traction event, while maintaining the same ride height of both axles is highly desirable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]    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: 
           [0008]      FIG. 1  is a side view of a commercial vehicle embodying the present invention; 
           [0009]      FIG. 2  is a detailed side view of a commercial vehicle embodying the present invention; 
           [0010]      FIG. 3  is a chart illustrating air pressures and valve positions according to the present invention; 
           [0011]      FIG. 4  is a graph illustrating load vs. deflection at constant pressure of the drive axle according to an embodiment of the present invention; and 
           [0012]      FIG. 5  is a graph illustrating load vs. deflection at constant pressure of the tag axle according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    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. 
         [0014]      FIGS. 1 and 2  depict an air suspension system  10  for a 6×2 vehicle having a single drive axle  12 , a tag axle  14 , air springs  16 ,  18  on each axle, an air pump  20  and an automatic electro-pneumatic control system  22  to control the flow of air between the air springs  16 ,  18 . The automatic electro-pneumatic control system  22  comprises a three-way valve  24 , but it is understood that the automatic electro-pneumatic control system  22  may comprise a different type of valve and/or a different number of valves. 
         [0015]    The three-way valve  24  is used to route air to a particular location via fluid communication lines  32 ,  34 . In this application, port one  26  of the three-way valve  24  is connected to the front, or drive axle air springs  16 . Port three  28  of the three-way valve  24  is connected to the rear, or tag axle air springs  18 . More specifically, ports one and three  26 ,  28  are connected through the three-way valve  24  to allow the transfer of air between the drive axle and the tag axle air springs  16 ,  18 , as described below. Port two  30  of the three-way valve  24  can be used to evacuate pressure from the drive axle air springs  16  and/or the tag axle air springs  18 . One or both of the air springs  16 ,  18  can be selectively evacuated to return them to their original pressure or if an emergency deflation is required. 
         [0016]    The air pump  20  provides a source for drawing air from an air reservoir (not shown), via the fluid communication lines  32 ,  34  to the three-way valve  24  and into the air springs  16 ,  18 . Air moving through port one  26  is in direct communication with the drive axle air spring  16 , and air moving through port three  28  is in direct communication with the tag axle air spring  18 . Port two  30  acts as a connection between ports one and three  26 ,  28 , to facilitate the flow of air from the drive axle air spring  16  to the tag axle air spring  18  and vice versa. The flow of air can also be reversed to draw air from the air springs  16 ,  18  and back in to the air reservoir. 
         [0017]    The drive axle  12  comprises a larger effective diameter air spring  16  and the tag axle  14  comprises a smaller effective diameter air spring  18 , as shown. The smaller diameter air spring  18  on the tag axle  14  has a higher air pressure at the normal GAWR (Gross Axle Weight Rating) of the tandem as compared to the drive axle  12 . Consequently, when a larger ground load is needed on the drive axle  12  due to a traction event, such as during wet and/or other types of slippery, muddy or snowy conditions, the higher pressure from the tag axle air spring  18  can be used to rapidly and efficiently increase the pressure in the drive axle air spring  16 . The ride height is maintained at or near the desired distance since lowering the load and pressure of the tag axle air spring  18  is easily compensated by the increase in pressure and load of the drive axle air spring  16 . 
         [0018]    Both the drive axle and tag axle air springs  16 ,  18  can have approximately the same internal air volume at the standard ride height so that the spring rates are nearly the same. At the standard tandem GAWR of 40,000 Lbs. the maximum ground loading will be 34,000 Lbs. due to bridge laws, so that each axle will carry a ground load of 17,000 Lbs. at full payload. Consequently under normal operation mode, the tag axle air spring  18  may require a higher air pressure compared to the drive axle spring  16 ; for example, the tag axle air spring  18  could have an air pressure of 100 psi (6.9 Bar) and the drive axle air spring  16  an air pressure of 70 psi (4.8 Bar) but both axles  12 ,  14  will be at the same ground load. The tire tractive effort distribution is equal in both the drive axle  12  and the tag axle  14 . These numbers are reflective of level ground conditions under normal operation mode. In this condition, all ports  26 ,  28 ,  30  in the three-way valve  24  associated with the automatic electro-pneumatic control system  22  are closed. 
         [0019]    If a traction event is sensed, the automatic electro-pneumatic control system  22  can rapidly deliver air from the tag axle air spring  18  to the drive axle air spring  16  as needed. As illustrated in the chart on  FIG. 3  under tractive mode, air pressure in the drive axle air spring  16  is increased from 80 to 100, and air pressure in the tag axle air spring  18  is decreased from 120 to 100, comparing the columns titled “Normal Mode” and “Tractive Mode”. The transfer of pressure between the air springs  16 ,  18  is achieved by the opening of ports one to three  26 ,  28  in the three-way valve  24  in the automatic electro-pneumatic control system  22 . Ports one to two  26 ,  30  and ports two to three  30 ,  28  remain closed. 
         [0020]    The added pressure in the drive axle air spring  16  combined with the reduced pressure in the tag axle air spring  18  results in a higher drive axle  12  ground load and a lower tag axle  14  ground load at approximately the same ride height. After the added tractive effort is applied and the wheel slip is reduced or eliminated, the automatic electro-pneumatic control system  22  can return the air springs  16 ,  18  to the normal air pressures that are required for equal drive axle  12  and tag axle  14  ground loading. 
         [0021]    During the pressure shift, a connection is made between ports one and three  26 ,  28 . Later, when going back to the original pressures, one air spring must increase in pressure while the other air spring must decrease in pressure. The decrease in air pressure can be achieved via venting air through port two  30 . The increase can happen by either the air pump  20  to port three  28  or via the air pump  20  from ports one to three  26 ,  28  as another embodiment; thus not venting via port two  30 . 
         [0022]    Air lines are most commonly used for connecting the air springs  16 ,  18  and the three-way valve  24 . The front and rear air springs  16 ,  18  and the three-way valve  24  are all in close proximity, so the air transfer from rear tag axle air spring  18  to the front drive axle air spring  16  through the air lines can be very quick. Typically, there is an air compressor (not shown) on all commercial trucks to operate the air brake and air suspension systems. This includes an air tank but the traction drive system could have its own reservoir close to the axles so that air is quickly available and does not need to be robbed from the brakes. While tractive capability is needed for moving forward, it may also be needed for improved braking and stopping. 
         [0023]    Various inputs to the automatic electro-pneumatic control system  22  can be incorporated by using an algorithm to improve the functionality of the load shift algorithm such as temperature, vehicle speed, steer axle turn angle, estimated vehicle total GVW and straight ahead travel wheel speed data. The algorithm may control the shifting of air pressure based on these vehicle parameters. The general concept is to try to predict when added tractive effort is needed and to then allow a weight shift for more traction. For example, if the vehicle is making a turn while at higher speeds, the system would not execute a quick weight shift as this may cause the vehicle to roll. Temperature can be used to determine the likelihood of ice on the road. Temperature also has impacts on pressure. The estimated weight may cause the algorithm to limit or increase the pressure shifts. 
         [0024]      FIG. 4  illustrates one example of a 6×2 dynamic weight shift calculation in both equal loading conditions and in unequal loading conditions. Under the “Normal-Equal Loading” columns is a spread sheet calculation comparing an equally loaded tandem with 17,000 Lb on each axle  12 ,  14  but with different types of air springs  16 ,  18  on the axles  12 ,  14 . The first “Normal” calculation has the drive axle  12  with a large diameter air spring  16  requiring 70 psi to support a 5,067 Lb spring force equal to a 17,000 Lb total axle ground load (reference  FIG. 5  drive axle air spring  16  needs 70 psi at 11.38 in height to support 5,067 Lb load). The tag axle  14  has a small diameter air spring  18  requiring 100 psi to support a 5,067 Lb spring force equal to the 17,000 Lb. required GAWR (reference  FIG. 6  tag axle spring  18  needs 100 psi at 11.38 in to support 5,067 Lb.). 
         [0025]    The spread sheet as depicted on  FIG. 4 , has a “Shifted” calculation that assumes the air springs  16 ,  18  are all at the same average pressure ([100+70]/2=85 psi). The graphs on  FIGS. 5 and 6  for each air spring  16 ,  18  show that the drive axle air spring  16  will support 6,100 Lb. and the tag air spring  18  will support 4,250 lb. at standard ride height which works out to 20,129 Lb on the drive axle  12  and 13,923 Lb. GAWR on the tag axle  14  with some graph interpolation. This means that an 18% increase in drive axle  12  load (20,129/17,000=+18%) can be had for an 18% improvement of tractive effort on the drive axle  12  by equalizing the pressures in the air spring  16 ,  18 . The air springs  16 ,  18  as shown in the graphs ( FIGS. 5 and 6 ) were needed since these air springs  16 ,  18  are not linear and a given pressure increase on one will not be an equal load change gain to the loss on the other spring even though the total tandem always needs to add up to a total 34,000 Lb GAWR.