Abstract:
A vehicle suspension system couples an axle to a vehicle frame so that the axle pivots about a first location with respect to the frame when the axle has a first range of articulation. The axle pivots about a second location with respect to the frame extending laterally out from the first location when the axle has a second range of articulation greater than the first range of articulation. When the axle exceeds the second range of articulation, the suspension system retains the axle in a substantially rigid contact with the vehicle frame.

Description:
BACKGROUND OF THE INVENTION 
       FIG. 1  shows a conventional lift truck  10  whose basic function of lifting and transporting cargo is well-known. Drive wheels  12  are connected to a drive axle (not shown) and steer wheels  14  are connected to a steer axle  20  shown in  FIG. 2 . The drive axle is rigidly connected to the frame of lift truck  10 . The steer axle  20  is used for steering the truck  10  and includes a suspension system shown in more detail in  FIG. 2 . 
       FIG. 2  is a top-view of the steer axle  20  used in the lift truck  10  shown in  FIG. 1 . The rear wheels  14  are attached to opposite ends of the steer axle  20 . Stubs  20 A and  20 B extend from the opposite back and front ends, respectively, of the steer axle  20 . A pair of brackets  22 A and  22 B are bolted to the truck frame  24  and hold the stubs  20 A and  20 B, respectively. 
       FIG. 3  is a rear sectional view of the steer axle  20 . From this view, only bracket  22 A is shown. Bracket  22 B is similar to bracket  22 A. The stub  20 A of the steer axle  20  is centered about a center point  21 . The stub  20 A is held in substantially the same relative position within the bracket  22 A about center point  21  by a rubber bushing  30 . The rubber bushing  30  allows the steer axle  20  to pivot about the center point  21  when the steer axle  20  articulates (laterally inclines) either clockwise or counter clockwise. 
     Two articulation stops  32 A and  32 B are located on the upper surface of the steer axle  20 . The spaces between the articulation stops  32 A and  32 B and the truck frame  24  are referred to as articulation gaps  34 A and  34 B, respectively. When the lift truck  10  is at rest, or traveling in a straight line on level terrain  31 , there is little articulation of the steer axle  20  and the articulation gaps  34 A and  34 B remain relatively constant. 
     As long as the articulation stops  32 A and  32 B do not contact frame  24 , the steer axle  20  is free to pivot about center point  21  independently of the frame  24 . This pivoting of steer axle  20  allows the lift truck  10  to maneuver over uneven terrain and obstacles, or make turns, without effecting the lateral displacement of the frame  24 . 
     The size of the articulation gaps  34 A and  34 B determine how far the steer axle  20  can articulate without laterally displacing the frame  24 . If the steer axle  20  articulates far enough on one side, one of the articulation stops  32 A or  32 B contacts frame  24 . In this articulation stop contact position, any further lateral articulation of the steer axle  20  equally articulates the frame  24 . 
     Larger articulation gaps  34 A and  34 B can increase how much the steer axle  20  can articulate before one of the articulation stops  32 A and  32 B contacts frame  24 . Larger articulation gaps  34 A and  34 B allow more articulation of the steer axle  20  without laterally displacing the frame  24 . 
       FIG. 4  shows stability profiles for the suspension system shown in  FIGS. 2 and 3 . The lift truck  10  has a triangular stability profile RSU when the articulation stops  32 A and  32 B ( FIG. 3 ) are not contacting the truck frame  24 . With the triangular stability profile RSU, the lift truck frame  24  is supported at the R and S locations of drive tires  12  and at a U location along the centerline of the steer axle  20 . 
     The lift truck  10  changes to more of a rectangular shaped stability profile RSTV when either of the articulation stops  32 A or  32 B come in contact with the frame  24 . When the steer axle  20  pivots sufficiently to contact either one of the articulation stops  32 A or  32 B ( FIG. 3 ), the steer axle  20  moves into a rigid non-pivoting relationship with the frame  24 . This moves the lateral support locations for the rear end of frame  24  from centerline location U out to the T and V locations at the rear wheels  14 . 
     SUMMARY OF THE INVENTION 
     A vehicle suspension system couples an axle to a vehicle frame so that the axle pivots about a first location with respect to the frame when the axle has a first range of articulation. The axle pivots about a second location with respect to the frame extending laterally out from the first location when the axle has a second range of articulation greater than the first range of articulation. When the axle exceeds the second range of articulation, the suspension system retains the axle in a substantially rigid contact with the vehicle frame. 
     The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of a lift truck. 
         FIG. 2  is a top view of a steer axle used in the lift truck shown in  FIG. 1 . 
         FIG. 3  is a rear sectional view of the steer axle shown in  FIG. 2 . 
         FIG. 4  is a top view of the chassis for the lift truck shown in  FIG. 1 . 
         FIG. 5  is a rear sectional view of a suspension system according to one embodiment of the invention. 
         FIG. 6  is a rear view of the suspension system shown in  FIG. 5  when the lift truck is traversing over uneven terrain. 
         FIG. 7  is a top view of the lift truck chassis showing the stability profiles provided by the suspension system shown in  FIGS. 5 and 6 . 
         FIGS. 8A–8B  are rear-sectional views of a suspension system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 5  shows a suspension system  15  that includes a bracket  22 A rigidly connected to the truck frame  24 . The bracket  22 A contains a rubber bushing  50  having a hole  23  that receives the steer axle stub  20 A previously shown in  FIG. 2 . A similar bushing  50  is also located at the front end of the steer axle  20  contained in the bracket  22 B shown in  FIG. 2 . 
     The shape of bushings  50  promotes vertical compression or vertical movement of the bushing  50  when sufficient downward force is applied by the stub  20 A. In one embodiment, the shape of bushing  50  forms cavities  52 A and  52 B between the bushing  50  and bracket  22 A. In one embodiment, the bushing  50  forms a concave spacing  52 A in the bottom of bracket  22 A and lateral spaces  52 B on the sides of bracket  22 A. However, other bushing shapes can also be used that allow a vertical downward compression. Different types of elastomeric material can be used for the bushings  50 . 
     When sufficient downward force is applied at the stubs  20 A and  20 B, the cavities  52 A and  52 B allow a potion of the bushing  50  to move downward enabling the center point  25  of the king pins  20 A and  20 B to also move in a downward vertical direction.  FIG. 6  shows a rear view of the steer axle  20  while the tire for left steer wheel  14  traverses over an object  55 . As the tire rolls over the object  55 , the steer axle  20  pivots in a clockwise direction about the center point  25  previously shown in  FIG. 5 . The clockwise pivoting of the steer axle  20  moves the articulation stop  32 A into contact with the frame  24 . 
     If the articulation of steer axle  20  is large enough, the left end of rigid steer axle  20  continues to move upward after articulation stop  32 A contacts frame  24 . This causes the stub  20 A to compress bushing  50  downward. The steer axle  20  in this condition pivots about articulation stop  32 A at location W. The downward movement of the stub  20 A moves the bushing  50  downward partially filling in gaps  52 A and  52 B ( FIG. 5 ). The center points of stubs  20 A and  20 B move vertically downward from location  25  to location  27 . The pivot point of the steer axle  20  effectively moves laterally out from center line y to the articulation stop  32 A at location W. 
     The steer axle  20  may continue to articulate until a bottom end  53  and sides  57  of the rubber bushing  50  fill in a certain portion of the gaps  52 A and  52 B ( FIG. 5 ). In this fully compressed state, the steer axle  20  and the frame  24  move into a semi-rigid fixed relationship with each other. Any additional articulation of the steer axle  20  at this point also articulates the frame  24 . 
     In an alternative embodiment, rubber bushings, or some other type of elastomeric material, can be located on the tops of the articulation stops  32 A and  32 B. Alternatively, springs can be located on the articulation stops  32 A and  32 B. The elastic material or springs dampen the forces applied by the articulation stops  32 A and  32 B when contacting frame  24 . 
       FIG. 7  is a top view of the chassis for the lift truck  10  shown in  FIG. 1  that uses the suspension system  15  previously shown in  FIGS. 5 and 6 . The drive wheels  12  are attached to the drive axle  40  and the steer wheels  14  are attached to the steer axle  20 . The drive axle  40  is rigidly affixed to frame  24  and does not pivot independently with respect to frame  24 . 
     The steer axle  20  pivots about the frame centerline AB in a first articulation range prior to one of the articulation stops  32 A or  32 B ( FIG. 5 ) contacting frame  24 . In this articulation stop non-contact condition, the lift truck has the triangular stability profile RSV defined by the points R, S, and V. 
     When the steer axle  20  moves into a second articulation range, one of the articulation stops  32 A or  32 B contacts the frame  24  as previously shown in  FIG. 6 . This may happen, for example, when the tire for one of the steer wheels  14  travels over an object. When one of the articulation stops contact the frame  24 , the lift truck  10  transitions from the triangular stability profile RSV to a trapezoidal stability profile RSUW corresponding to locations R, S, U, and W. 
     The points R and S correspond to locations on the drive wheels  12 . The locations W and U correspond to the locations where the articulation stops  32 A and  32 B, respectively, contact the frame  24 . After the articulation stop  32 A or  32 B contacts the frame  24 , the steer axle stubs  20 A and  20 B start compressing bushing  50  in a downward direction as described above in  FIG. 6 . This allows the steer axle  20  ( FIG. 6 ) to pivot about point W or point U. 
     The pivot point of the steer axle  20  effectively moves from location V to location W or location U. In the second articulation range when the truck  10  has the trapezoidal stability profile RSWU, the steer axle  20  still articulates semi-independently from the frame  24 . This trapezoidal stability profile is caused by the vertical displacement of the stubs  20 A and  20 B inside of bushings  50  ( FIG. 6 ). 
     In a third steer axle articulation range, the steer axle stubs  20 A and  20 B can no longer move downward in bushings  50 . For example, when the gaps  52 A and  52 B ( FIG. 5 ) are substantially filled in by portions of the rubber bushing  50 . In this fully compressed bushing state, the lift truck  10  transitions to a stability profile RSXT approaching a rectangular shape. The stability profile RSXT is larger than the trapezoidal stability profile RSWU and extends out to the wheels  12  and  14 . In the larger stability region RSXT, any further lateral articulation of the steer axle  20  causes substantially the same amount of lateral articulation in frame  24 . 
     In the embodiment of the invention described in  FIGS. 5–7 , the shape of bushing  50  allows vertical displacement of the steer axle center pivot point. One of ordinary skill in the art will recognize that other bushing shapes could also be used that enable a center pivot point of an axle to move vertically up and down. 
     For example,  FIGS. 8A and 8B  show an alternative embodiment that uses compression coil springs  82 ,  83 ,  84 , and  85 . The upper springs  82  and  84  are somewhat larger than the lower springs  83  and  85 . A mounting bracket weldment  80  attaches the steer axle  20  to frame  24 . A steer axle casting  88  is formed as part of the steer axle  20  or is a separate piece rigidly welded or bolted to the steer axle  20 . 
     Two bolts  87  are shown in  FIG. 8B . Bolts  87 , in addition to two other bolts that are not shown, are arranged in a square pattern. Each bolt  87  runs through an opening similar to opening  86  ( FIG. 8B ) in the mounting bracket weldment  80  attaching to the frame  24 . The steer axle casting  88  includes T shaped posts  91  on opposite ends that insert inside of both the upper springs  82  and  84  and inside the lower springs  83  and  85 . A rubber bushing  89  sits between the steer axle casting  88  and weldment  80 . The weldment  80  is mounted to the vehicle frame. Washers  93  retain the rubber bushing  89  and protect the bushing  89  from the springs  82 ,  83 ,  84  and  85 . The frame  24  is supported on the upper springs  82  and  84 . 
     In a first articulation range, the steer axle  20  rotates laterally about center line  90  and the articulation stops  32 A and  32 B do not contact frame  24 . This produces a triangular stability profile similar to triangular stability profile RSV shown in  FIG. 7 . 
     If the steer axle  20  continues to articulate either clockwise or counter clockwise, the articulation stop  32 A or  32 B contacts frame  24  and combinations of the springs  82 ,  83 ,  84 , and  85  continue to compress. This allows the steer axle casting  88  to continue to move in a downward direction in weldment  80 . The steer axle  20  is allowed to pivot about the articulation stop  32 A or  32 B that contacts the frame  24  providing a trapezoidal stability profile similar to trapezoidal profile RSUW shown in  FIG. 7 . 
     In a third articulation range, the steer axle casting  88  fully compresses a combination of the springs  82 ,  83 ,  84 , and  85 . The steer axle  20  in this condition has a substantially rigid non-pivoting contact with frame  24 . The suspension system in this state exhibits a larger stability profile approaching a rectangular shape similar to the stability profile RSXT shown in  FIG. 7 . 
     Other spring designs such as Bellville springs may be used for providing the variable suspension profiles shown above. Alternatively, a single traverse set of variable leaf springs that are stiffer in the upward direction than in the downward direction can be used to bridge the front and rear mounting locations of the steer axle  20 . For example, the leaf springs can bridge the locations where brackets  22 A and  22 B are located in  FIG. 2 . Any other different combination of springs or bushings can also be used to vary the stability profiles of the lift truck as described above. 
     The embodiment of the invention described in  FIGS. 5–7  is a passive system. That is, the bushing  50  merely reacts to laterally applied forces. One of ordinary skill in the art will recognize that an active system may also be used to perform the same function. For example, a hydraulic system may be used to directly raise or lower the center pivot point  25  ( FIG. 5 ) of the steer axle  20  according to sensor inputs. Active systems could also adaptively adjust a spring constant of the movable pivot point according to the weight of the load carried by the lift truck or the degree of lateral inclination of the lift truck. 
     Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.