Abstract:
A guidance apparatus is used on a transit vehicle which includes separate steerable drive tires operated by a central guidance platform having four peripherally mounted wheels follow the contour of a guide rail. The guide wheels are spaced above the roadway on a guideway used by the steerable drive tires thereby providing the capability of both on-track and off-track driving of the vehicle. A steering linkage imparts the linkage of a guide plateform to one steerable drive tire and a drag link transmits the rotary motion to the second steerable drive tire.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/123,960, filed Mar. 12, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application is co-pending with an application entitled “Apparatus and Method for Steering a Guideway Vehicle” filed as a provisional application on Mar. 12, 1999 simultaneously with the subject provisional application and having the same inventorship. 
     This invention relates to steering mechanisms for vehicles. More particularly, it relates to an apparatus for steering a vehicle to follow a track or guideway. 
     2. Description of the Related Art 
     Various types of on-track and off-track vehicles are currently used for mass transportation. Frequently, it is desirable for the same vehicle to be available for use on a track following a guideway (on-track) as well as on a roadway without the assistance of a guideway (off-track). This is especially useful for a vehicle that must be removed from a guideway and moved to another location for repair or maintenance. The term “guideway” is used herein to describe a roadway surface with a guide rail for steering a vehicle along a predetermined course. 
     Typically, such a vehicle follows a guide rail embedded below the roadway surface of the guideway and, as a result, certain elements of the vehicle steering mechanism, such as the guide wheels extending from the vehicle undercarriage which track the guide rail, protrude below the roadway surface of the guideway. This arrangement is not acceptable for off-track use because the roadway would interfere with the steering mechanism. 
     Ordinarily, a vehicle adapted for use both on-track and off-track employs a conventional design using pneumatic tires to operate on any substantially flat surface. Accordingly, when operated as an on-track vehicle, the guideway is substantially flat and a guide rail is mounted on the guideway parallel to the roadway surface for guiding the vehicle along a predetermined course. 
     U.S. Pat. No. 3,796,165 describes a guide following steering apparatus in which all of the components of the steering apparatus are above the roadway surface of the guideway. The apparatus uses guide followers protruding from the front and the rear of the vehicle. Each guide follower is associated with two steerable drive tires on each of two axles and the two guide followers are connected together with steering linkages. However, the vehicle is bi-directional and only the leading guide follower, whichever one it may be, controls the steering for wheels on both axles. In certain instances, such a design is undesirable because the protruding guide followers diminish the ride quality of the vehicle. 
     U.S. Pat. No. 3,515,405 describes an axle suspension system for transit vehicles utilizing a central guidance system comprised of four guide wheels attached to a guide frame to follow the direction of a guide rail. In the process, one axle on the transit vehicle orients to follow that guide rail. At least two axles are associated with a separate guidance apparatus and there is no interconnection between the two apparatuses. However, the arrangement disclosed in this patent illustrates a guide rail below the roadway surface of the guideway which makes off-track use of the vehicle impossible. Furthermore, this arrangement pivots the entire axle such that the wheels on both sides of the axle rotate as a unit. This requires the entire axle, axle housing and associate frame to be rotated when it is only necessary to turn the wheels of the axle. 
     It is an object of this invention to provide a guidance apparatus capable of following a guide rail located on a guideway at or above the roadway surface of the guideway using a central guidance platform and steerable axles to alleviate the need to turn the entire axle and axle housing when the vehicle must turn. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a steering apparatus for a vehicle adapted to travel on a guideway having a central guide rail parallel to the desired course of the vehicle. The vehicle has a vehicle body with front and rear wheel assemblies. At least one wheel assembly comprises a support frame attached to the vehicle body and a stationary axle housing mounted to the support frame. The axle housing contains an axle having an axle axis. A first steerable drive tire is mounted at one end of the axle and a second steerable drive tire is mounted at the other end of the axle. A guide frame is pivotally mounted to the axle housing about a vertical guide frame axis. Four guide wheels are pivotally mounted upon the guide frame to pivot the guide frame and turn the steerable drive tire. 
     The guide frame may be pivotally mounted to the axle housing through a ring bearing connecting the two frames. 
     The steering apparatus may include a steering linkage extending from the guide frame to the first steerable drive tire hub whereby when the guide frame rotates relative to the axle housing, the first steerable drive tire hub also rotates relative to the axle housing. A similar geometry may be utilized for the second steerable drive tire hub or, in the alternative, a drag link may connect the second steerable drive tire hub to the first steerable drive tire hub to impart steering to the second drive tire hub. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, along with the object and the advantages thereof, will be more apparent with the consideration of the following detailed description read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic view of a guide rail and a transit vehicle utilizing the steering apparatus of the subject invention; 
     FIG. 2 is a schematic top view of the wheel assembly circled in FIG. 1; and 
     FIG. 3 is an end view of the wheel assembly along arrows “ 3 — 3 ” in FIG.  2 . 
     FIG. 3 a  is a detached view of the axle details of a steerable drive tire. 
     FIG. 4 is a schematic side view along arrows “ 4 — 4 ” in FIG. 2; 
     FIG. 5 is a schematic plan view of one steering linkage in accordance with the subject invention; and 
     FIG. 6 is a detailed plan view of one embodiment of the subject invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a schematic of a bi-directional transit vehicle  10  which rides upon a guideway  15  in a direction indicated by arrow A, having a guide rail  20  used to direct a front wheel assembly  25  and a rear wheel assembly  27  to steer the transit vehicle  10 . A vehicle body  30  rests upon the front wheel assembly  25  and the rear wheel assembly  27 . 
     The vehicle  10  has the vehicle body  30  with the front wheel assembly  25  and the rear wheel assembly  27 . While both wheel assemblies  25 ,  27  may be identical, attention will be directed to the front wheel assembly  25  for purposes of this discussion. 
     FIG. 2 illustrates further details of the wheel assembly  25  circled in FIG.  1  and will be discussed in conjunction with FIG.  3 . FIG. 3 a  represents one embodiment of the steerable drive tire axle details. 
     To enable the transit vehicle  10  to travel both on-track along the guideway  15  following the guide rail  20  and off-track, it is necessary for both wheel assemblies  25 ,  27  to be above the road surface  12  (FIG. 3) of the guideway  15 . It is necessary to design the wheel assemblies so that their components are above the road surface  12  of the guideway  15 . FIG. 3 illustrates the guideway  15 , having a guide rail  20  mounted thereon with the front wheel assembly  25  having steerable drive tire hub  35   a,    35   b  of the vehicle  10  which rest upon the roadway  12 . The vehicle  10  will travel on the guideway  15  along the central guide rail  20  in a direction of the desired course of the vehicle  10 . 
     For the most part, the design of the front wheel assembly  25  according to the subject invention is symmetric about line “L” running longitudinally through the center of the front wheel assembly  25 . Therefore, identical reference numbers using an “a” or “b” suffix will be used to describe components on the right and left side respectively of the wheel assembly. 
     Returning to FIGS. 2 and 3, the front wheel assembly  25  is made up of a support frame  45  attached to the vehicle body  30 . The support frame  45  may be attached to the vehicle body  30  using pivoting connections  50   a,    50   b  at respective ends  55   a,    55   b  of the support frame  45 . The connections  50   a,    50   b  pivot upon a common axis  60  which is parallel to a steerable drive tire axle  65 . 
     The wheel assembly  25  may be attached to the vehicle body  30  using a wide variety of designs known by those individuals skilled in the art of transit vehicle design. One example of such a design is illustrated in FIGS. 2 and 3 where the support frame  45  is comprised of two structural members  46   a,    46   b  connected by cross members  47 ,  48 . As will be further explained, the structural members  46   a,    46   b  are generally referred to as suspension arms. The subject invention is directed to the steering apparatus and not the suspension systems. Therefore, it should be appreciated that any number of different suspension systems connecting the wheel assembly  25  to the vehicle body are possible. 
     At opposite ends  57   a,    57   b  of the support frame  45  are a pair  70   a,    70   b  of spring damper systems which contact the underside of the vehicle body  30  to act as a suspension system for the front wheel assembly  25  and vehicle body  30 . 
     It should be noted that the axle  65  is actually two axles driven by rotation from a drive shaft extending into a differential  105  within a differential housing  110 . Furthermore, the attachment of each axle to a steerable drive tire  35   a,    35   b  takes place at a universal joint to transmit rotation from each axle to the steerable drive tires  35   a,    35   b.  The axles, which do not pivot with the steerable drive tires, will be referred to as a single steerable drive tire axle  65  having an axle axis  67 . Steerable drive tires  35   a,    35   b  are mounted at each end of the steerable drive tire axle  65 . They are supported by drive tire hubs  36   a, b  which are connected to the axle housing  75  by pivot axes  37   a, b.    
     A stationary axle housing  75  houses the steerable drive tire axle  65 . As illustrated in FIG. 4, which is a side view along arrows “ 4 — 4 ” in FIG. 2 without the guide frame structure and steering linkage, the axle housing  75  is secured to the support frame  45  using clamps  80   a ,  80   b  (only  80   a  shown) which are secured to a plate  81  which is pivotally secured with a post  82  to the support frame  45 . As mentioned, the support frame is attached to the vehicle body  30  at one end by the pivoting connection  50   a  and is supported at the other end with the spring damper system. The support frame  45  in this arrangement is generally referred to as a suspension arm. 
     Returning to FIGS. 2 and 3, a guide frame  85  is pivotally mounted to the axle housing  75  about a vertical guide frame axis  87  which defines a pivot point. Four guide wheels  90   a,    90   b,  and  95   a,    95   b  are pivotally mounted upon the guide frame  85  about guide wheel axes  92   a,    92   b  and  97   a,    97   b.  As will be explained in further detail, the guide wheels  90   a,    90   b,    95   a,    95   b  straddle the guide rail  20  and, as the guide wheels move along the rail, cause the guide frame  85  to pivot according to the contour of the guide rail. The guide wheel axes  92   a,    92   b  and  97   a,    97   b  are preferably oriented in the vertical direction. 
     As illustrated in FIG. 2, the guide wheels  90   a,    90   b  and  95   a,    95   b  are arranged in a rectilinear pattern within the guide frame  85  and are symmetric about the line “L”. Furthermore, as illustrated in FIG. 2, the guide wheels  90   a ,  90   b  and  95   a ,  95   b  may be symmetric about the axle axis  67 . However there may be situations in which asymmetry about the axle axis  67  is preferred to provide certain turning characteristics to the steering assembly and therefore the subject invention should not be limited to symmetric arrangements. 
     The pivotal mounting of the guide frame  85  to the axle housing  75  may be accomplished through the use of a ring bearing  100  (FIG. 3) having an inner race  102  attached to the axle housing  75  and an outer race  104  attached to the guide frame  85 . The ring bearing  100  is used primarily to permit the guide frame  85  to rotate relative to the axle housing  75  but will tolerate lateral forces generated by the guide frame  85  rotations and will also tolerate vertical forces produced by the weight of the guide frame  85  and guide wheels  90   a,    90   b,    95   a,    95   b.  Additionally, the bearing  100  will tolerate thrust forces. The guide rail  20  may have a narrow middle section  20 . The guide wheels  90   a,    90   b,    95   a,    95   b,  as illustrated in FIG. 3, contact the guide rail  20  and are deflected accordingly. 
     Connected to the steerable drive tire axle  65  is the differential  105  with a differential housing  110 . As best illustrated in FIG. 3, the ring bearing  100  may surround the differential housing  110  and a ring bearing plane  115  defined about the periphery of the ring bearing  100  intersects with the differential housing  110 . By nesting the differential housing  110  within a central opening  101  of the ring bearing  100 , the front wheel assembly  25  and differential housing  110  are more efficiently arranged and vertical space needed beneath the vehicle body  30  is reduced. This feature is especially important for designs in which vertical space beneath the vehicle  10  is minimal as is the case when the guide wheels  90   a,    90   b,    95   a,    95   b  may not extend below the level of the guideway  15 . 
     The ring bearing  100  is mounted about a ring bearing vertical axis  117  located midway between the drive tires  35   a,    35   b  on the steerable drive tire axle  65 . In this way the guide frame  85  rotates at a point central between the steerable drive tires  35   a,    35   b  to produce what is known as a central guidance platform. 
     In some steering mechanisms associated with vehicles using guide wheels, in order to follow a guide rail defining a path for a vehicle, the entire wheel assembly, including the axle housing and associated axles, rotates as a unit with the guide wheels in a fashion which maintains the axles perpendicular to the guide rail. In other words, the wheels do not independently pivot relative to the wheel axles. In the subject invention, however, the steerable drive tires  35   a,    35   b  turn independently from the axle  65  and, as a result, it is not necessary to rotate the entire stationary axle housing  75  and support frame  45 . By utilizing such a design, the inertial forces required to turn the vehicle  10  are significantly reduced and subsequently, the forces required on the guide wheels  90   a,    90   b,    95   a,    95   b  to turn the steerable drive tires  35   a,    35   b  are significantly reduced. This permits hardware associated with turning the drive tires to be smaller and lighter. 
     By utilizing the ring bearing  100  to pivotally attach the axle housing  75  and the guide frame  85  produces an additional advantage. The ring bearing  100  must support only the weight of the guide frame  85  and the guide wheels  90   a,    90   b  and  95   a,    95   b.  This is unlike prior art designs in which the entire axle rotates with the guide wheels to turn the vehicle wheels in unison. If a ring bearing were to be used with this prior art design, then the size of the ring bearing would have to be substantially greater because now the ring bearing would be supporting the weight of the vehicle body. For this reason, the present invention permits the use of the ring bearing  100  significantly smaller than a bearing that would be used in the entire stationary axle housing and where wheels are rotated in unison. 
     To turn the steerable drive tires  35   a,    35   b,  the wheel assembly  25  as illustrated by elements in dotted lines in FIG. 2, the schematic of the driving linkage in FIG.  5  and in the plan view of the embodiment of FIG. 6 includes a steering linkage  200  which extends from the guide frame  85  to the first steerable drive tire  35   a  such that when the guide frame  85  rotates relative to the axle housing  75 , the first steerable drive tire  35   a  rotates relative to the axle housing  75 . 
     The steering linkage  200  includes a steering tie rod  205  having a first end  207  and a second end  209 . The first end  207  is pivotally connected through a pivot  210  to the guide frame  85  and is pivotally connected at a second end  209  through pivot  215  to the first steerable drive tire hub  36   a.  More specifically, steering tie rod  205  is pivotally connected to the first steerable drive tire  35  through a steering lever  220  having a first end  222  and a second end  224 . The steering lever first end  222  is pivotally connected through pivot  215  to the steering tie rod second end  209  and is fixed at the second end  224  to the steerable drive tire hub  36   a . The second end  224  is pivotally connected at pivot  37 A to the axle housing  75 . The second pivot  37   a  is approximately vertically. 
     A four bar trapezoidal steering geometry from the guide frame  85  to the first steerable drive tire hub  36   a  is thereby defined by 
     a) guide frame virtual link  225  from the guide frame pivot point  87  to the first end  207  of the steering tie rod  205 ; 
     b) the steering tie rod  205  from the guide frame virtual link  225  to the steering lever  220 ; 
     c) the steering lever  220  from the steering tie rod  205  to the first steerable drive tire hub pivot  36   a  at  223 ; and 
     d) an axis virtual link  230  from the first steerable wheel hub pivot  36 a to the guide frame pivot point  87 . 
     By utilizing this four bar trapezoidal steering geometry, the rotation of the guide frame  85  is imparted to the steerable wheel hub  36   a.    
     The first steerable drive tire hub  36   a  pivots about a hub pivot point  37   a.    
     Although not illustrated in the figures, it is entirely possible to duplicate this four bar trapezoidal steering geometry with an independent set of linkages similar to those just described from the guide frame  85  to the second steerable drive tire hub  36   b.    
     Nevertheless, illustrated in the drawings is a more conventional approach in which a drag link  235  having a first end  237  and a second end  239  extends from the first steerable drive tire hub  36   a  to the second steerable drive tire hub  36   b  to impart the rotation from the first steerable drive tire hub  36   a  to the second steerable drive tire hub  36   b.    
     A first drag link lever  240  has a first end  242  fixed to the first steerable drive tire hub  36   a  and is pivotally connected about a pivot  245  to the first end  237  of the drag link  235 . The other pivot axis coincidents with pivot  37   b.  A second drag link lever  250  has a first end  252  and a second  254  wherein the first end  252  is pivotally connected through pivot  255  to the second end  239  of drag link  235 . The second end  254  of the second drag link lever  250  is fixed to the second steerable drive tire hub  36   b.  Through this arrangement of linkages, rotation of the guide frame  85  is imparted to the first steerable drive tire hub  36   a  and then imparted to the second steerable drive tire hub  36   b.  The trapezoid is closed by the axis housing between the pivots of both steering lever, respectively. 
     It should be noted that the goal of the steering linkage is to orient the steerable drive tires  35   a,    35   b  through rotations of the drive tire hubs  36   a, b  around the drive tire hub pivot points  37   a, b  to duplicate an Ackermann steering system such that the radius of curvature of the wheel on the inside of a turn is smaller than the radius of curvature of a wheel on the outside of a turn. 
     It should be noted that the second end  224  of steering lever  220  is fixed not only to the first steerable drive tire hub  36   a  but is also fixed to the first end  242  of the first drag link lever  240 . 
     In addition to illustrating a schematic of a steering mechanism, FIG. 5 also indicates in phantom the manner in which the guide frame  85  rotates and the effect such a rotation has on the steering linkage and the drive tires  35   a,    35   b.  Each of the elements previously identified and shown in solid lines is also shown in dotted lines in its displaced position. Because this phantom motion is clear, duplicate reference numbers are not added to highlight the same part in its phantom position. The angles and lengths associated with each element of the steering linkage  200  will be obvious to one skilled in the art of steering mechanisms and for that reason, these details have not been provided. 
     The detailed plan view of FIG. 6 of one embodiment of the subject invention illustrates the actual hardware for one embodiment of the subject invention and should be used in conjunction with the previously provided schematics to appreciate the orientation of and actual appearance of the elements of the subject invention. Like reference numerals have been added to FIG. 6 to make such an identification convenient. 
     Note however, the guide wheels  90   a,    90   b,    95   a,    95   b  are spaced from longitudinal axis L a greater amount than in FIG.  3 . The width of the actual guide rail used in the FIG. 6 arrangement is wider to engage the guide wheels. 
     Although this invention has been described with respect to preferred embodiments, various modifications, revisions and additions will become evident to persons of ordinary skill in the art. All such modifications, revisions and additions are intended to be encompassed in the scope of this invention which is limited only by the claims appended hereto.