Abstract:
The present invention relates to a mechanically steered articulating dolly transport system for transporting large structures such as a drilling mast over the federal highway system. Generally, the invention relates to an articulating dolly system that utilizes a unique mechanical system for steering a first dolly with a path generated by the movement of the load held by the second dolly. More particularly, the invention provides a front dolly steered by a non-rotatable front steering frame, a rear dolly having a pivotally mounted turntable and a pair of steering links connected between the respective sides of the front steering frame and the turntable. The left steering link and the right steering link are located in a first plane. One or both of the turntable and the steering frame being located on a second plane that is not the first plane. Moment forces generated between plane one and plane two elastically deform one or both of the steering frame and the turntable to permit the transport system to make a left-hand or right-hand turn.

Description:
TECHNICAL FIELD OF INVENTION 
     The present invention relates to a mechanically steered articulating dolly transport system for transporting large structures such as a drilling mast over the federal highway system. Generally, the invention relates to an articulating dolly system that utilizes a unique mechanical system for automatically steering a first dolly with a path generated by the movement of the load held by the second dolly. More particularly, the invention provides a mechanical steering system having an isosceles trapezoidal steering system that incorporates deformation of the steering system components. The system mechanically steers itself around relatively tight curves without the assistance of an electrical or hydraulic control system. The system has finite alignment adjustability, a minimized weight, and relies on manual hydraulic steering controls only for loading/unloading or parking maneuvers. Additionally, although two steering links share the invention&#39;s steering loads to form a symmetric assembly, either link alone could maintain the steering control should the other fail, providing high reliability and thus safety. 
     BACKGROUND OF THE INVENTION 
     It is a common requirement in several industries to transport very long and heavy structures across the federal and state highways. As a particular example of such requirement, in the exploration of oil, gas and geothermal energy, drilling operations are used to create boreholes, or wells, in the earth. Drilling rigs used in subterranean exploration must be transported to the locations where drilling activity is to be commenced. These locations are often remotely located and in rough terrain. The transportation of such rigs on state highways requires compliance with highway safety laws and clearance underneath bridges or inside tunnels. Once transported to the desired location, large rig components must each be moved from a transport trailer into engagement with the other components located on the drilling pad. 
     Moving a full-size rig requires disassembly of the mast into two or three sections which must then be transported. The mast sections are then loaded between a tractor trailer and a dolly system designed to follow the tractor trailer. The dolly systems must follow the tractor in a precise manner or the load will become unstable and dangerous. Safety is a paramount issue. Consistent with the safety issue and the high cost of downtime during transportation is reliability of the system. Breakdowns can be both dangerous and costly if the drilling rig is unavailable. It is also critical to transport the mast sections (or other structural loads) without imparting undue stress on their structures such that the structural strength of the mast is not compromised. 
     It is also necessary that the transport be simple, have a relatively small turning radius and not require significant programming or balancing of the loads prior to transport. It is also necessary that the system accurately steer the load, such that undesirable wear on the dolly tires and fuel inefficiency can be avoided. It is also desirable that the system requires minimum maintenance and relies on a minimum of hydraulic and electronic functionality. 
     For example, U.S. Pat. No. 2,167,943 to Fox discloses steering mechanisms for vehicles and more particularly to steering mechanisms between towing and towed vehicles, by which the wheels of the towed vehicle are caused to “track” or follow the paths of the wheels of the towing vehicle in making turns. Fox, however, uses hydraulic components, rather than a simple to implement mechanical linkage steering system, and does not provide for direct control of the dolly with redundant links. As a result, the apparatus of Fox requires more components at the cost of increased weight and complexity. Due to the inclusion of hydraulic assemblies, the apparatus of Fox is less reliable and would require additional maintenance to operate, driving up overall costs for the device. 
     U.S. Pat. No. 3,403,925 to Sauer discloses a self-centering axle for a vehicle with at least one fixed single or multiple axle, and an oscillatable mobile assembly which includes a wheeled axle for supporting the same and is adapted for attachment to highway-drawn load-carrying vehicles including a chassis adapted to support the load. The apparatus of Sauer does not use simple to implement mechanical steering linkages, but instead relies upon a complex assembly of raceways, clips, arms, and shafts to facilitate steering of the axles. As a result, the apparatus of Sauer may be less reliable and more prone to breakdown as opposed to a simple mechanical linkage steering system. Thus, the apparatus of Sauer suffers from increased costs and reliability issues over a mechanical steering system. 
     U.S. Pat. No. 3,542,390 to Fikse discloses a tractor and trailer combination, and more particularly to a trailer including a forward and a rear set of opposed lateral support wheels which swivel to facilitate the trailer making a turn with the rear wheels following the arc of the turn. Fikse further discloses a trailer construction with a steerable forward set and a steerable rear set of opposed lateral support wheels or dollies supported upon swivel means. However, the apparatus of Fikse does not provide controllable steering; rather, it relies on interconnected swivels on the rear dollies to facilitate steering of the trailer. The swivel steering system of Fiske provides a less accurate and less stable method for steering as opposed to a mechanical linkage steering system. 
     U.S. Pat. No. 4,117,905 to Mustered discloses a hydraulic articulated steering system with an equivalent total hydraulic steering system for back-up, emergencies, and for supplementary additional steering power, and without dependence upon additional mechanical linkages. Specifically, Mustered discloses a steering system comprised of steering actuation means and two or more hydraulic telescoping steering means, each of which is positioned proximate to the pivot attachment point of the articulated vehicle with at least one of the two or more hydraulic steering means positioned on a first side of this pivot attachment point and another of the hydraulic telescoping steering means positioned on the first or second side of the pivot attachment. The dual metering control means of Mustered is more complex than a simple mechanical steering linkage, and as a result may provide less reliable steering operation over time. Because Mustered relies upon a hydraulic steering system, it does not provide for direct control of the dolly with redundant mechanical links. Thus, the apparatus of Mustered provides less accurate steering as well as increased weight and complexity over a mechanical linkage steering system. Furthermore, cost and reliability of the steering device is adversely affected. 
     U.S. Pat. No. 4,441,730 to Damm discloses a steering device for a multi-axled goose-neck trailer coupled with a vehicle in front of the trailer, the steering device comprising wheels pivotable about a main axle and connected to stay rods for forcibly steering the rear wheel set through hydraulics in a direction opposite to the steering wheels of the tracking vehicle, so that the trailer more closely follows the track of the tracking vehicle. The apparatus disclosed in Damm would thus put substantial strain and wear on the rear wheels. Further, the use of hydraulic cylinders in Damm for forcibly steering of the rear wheels may lead to faster deterioration of the steering assembly and hydraulics in addition to the increased wear on the wheels. The use of hydraulics further increases the overall weight and costs of the steering system, and also adversely affects reliability of the system. 
     U.S. Pat. No. 5,035,439 to Petrillo discloses a tractor trailer or other vehicle that is equipped with an auxiliary rear steering arrangement which is steered via pneumatic-type suspensions and hydraulic cylinders as well as adjusting the weight of the trailer and load to facilitate steering. The apparatus of Petrillo requires an operator, and is generally directed more towards a more efficient method of weight distribution for the load. In this regard, the apparatus taught by Petrillo provides for a less accurate steering system as opposed to a direct mechanical linkage steering system, and has added drawbacks of increased weight and potential reliability problems due to the use of hydraulics and pneumatics. 
     U.S. Pat. No. 5,234,069 to Krone et al. discloses a steering control system in which one pair of wheels of an articulated steering vehicle are steered in synchronization with the articulation steering and controlled via a pump and valve means for directing fluid to hydraulic cylinders, which are locked in place. The hydraulic steering system of Krone is therefore more complex and prone to failure than a simple mechanical linkage steering system. The hydraulic steering system of Krone further suffers from added weight of the hydraulic assemblies and increased costs as a result. 
     U.S. Pat. No. 5,479,999 to Proia discloses an automatic powered, self-tracking system with powered controls for the rear axles of large vehicles, along with the use of motion detectors to sense the direction and degree of vehicle turning, and are connected to a plurality of either hydraulic or pneumatic axle-pivoting mechanisms which exert a force on preselected components of a vehicle&#39;s suspension system. As a result, the steering apparatus taught in Proia is highly dependent upon use of motion detectors, and the failure of any one of the motion sensors would have a substantially adverse impact on the accuracy of the system. In addition, the use of hydraulics and pneumatics on the apparatus of Proia induces increased weight and costs on the system, drawbacks that are not present in a simple mechanical linkage steering system. 
     U.S. Pat. No. 5,700,023 to Picard discloses an articulated vehicle composed of a series of modules interconnected by a composite articulated connection comprising at least one elastic articulation with controlled movement. Picard further discloses a vehicle articulated in a simple mechanical fashion, making it possible to ensure better relative maintenance in position of the modules of which it is composed near-perfect following of the curves and irregularities in the roadway, and a substantial limitation of extraneous movements of the modules relative to each other, even for road trains composed of two or more modules. While Picard discloses a method of mechanical self-tracking, its steering system is unable to achieve precise articulation for precise steering and navigation of the dolly. 
     U.S. Pat. No. 6,152,475 to Poole discloses a converter dolly that replaces the rear wheels of a truck trailer to provide the trailer with a steering ability about its rear wheels, and is controlled by the driver of the tractor through a control system mounted in the tractor cab. The converter dolly is modified to include a hydraulic actuating system, including a piston, cylinder, and pump assembly, which act upon the tongue of the converter dolly to pivot the converter dolly about a king pin. Thus, Poole suffers several deficiencies from the use of hydraulics, such as increased costs associated with the hydraulics as well as increased weight of the hydraulic assemblies. Additionally, the increased complexity of the steering system disclosed in Poole renders it more prone to reliability issues and failures and further requires that the driver of the tractor manually operate the dolly steering system. 
     U.S. Pat. No. 7,412,315 to Wildey et al. discloses a steering system for an articulated vehicle that has a microprocessor connected to a proportional solenoid valve which controls the direction, amount and rate of flow of hydraulic fluid to and from hydraulic articulation cylinders, which provide articulation between the frames of the articulated vehicle. Wildey also features a positional feedback sensor to measure the angle of articulation between the frames and communicates the angle of articulation to the processor. In Wildey, the use of solenoids in combination with hydraulics for control and articulation of the steering assembly leaves it more prone to failure over more reliable mechanical linkage steering systems and does not allow for direct control of the dolly. Thus, the steering system of Wildey provides less accurate steering as well as increased weight and complexity over a mechanical linkage steering system. 
     U.S. Pat. No. 7,637,512 to McGhie et al. discloses a boom support vehicle which reduces tire scuffing, reduces structural size, and provides improved turning characteristics. McGhie includes a self-steering castor axle as the front axle of the boom support vehicle front dolly, similar to the castor axle of some three axle semi-trailers, a two member articulated boom support vehicle design which has an articulation point (steering pivot) near the rear of the vehicle. While the dolly steering system disclosed in McGhie is separate from the tractor, it fails to disclose an automatic steering system, but rather relies upon the turning forces on the front axles to facilitate the steering and characteristics upon the back axles. Further, the apparatus of McGhie contains conventional rear axles which are not steerable and merely follow the tracking provided by the front axles. 
     Thus, there remains a need for improvements for the creation of a safe and reliable dolly system for transporting large structural loads on the federal and state highway systems. In particular, it is desirable to transport large loads on a mechanically steered articulating dolly system having a relatively small turning radius. 
     It is further desirable to have a dolly device that is relatively inexpensive to produce and implement, as well as inexpensive to maintain with a high degree of reliability. It is also desirable that such a dolly device would also be relatively lightweight in order to reduce consumption of fuel during transport of the dolly and payload. 
     In summary, the preferred embodiments of the present invention provide a unique solution to the engineering constraints and environmental challenges of providing a durable mechanically actuated steering system. 
     SUMMARY OF THE INVENTION 
     The present invention provides a substantially improved transport system for relocation of a large structure such as a mast section for a drilling rig. In one embodiment, a transport dolly is provided, comprising a front dolly, a rear dolly, and a steering mechanism linking the relative angularity between the front and rear dollies. 
     In one embodiment, the front dolly has at least one axle and the rear dolly has at least two axles. The steering mechanism comprises a steering frame non-rotatably connected to the front dolly, a turntable pivotally mounted to the rear dolly, a left steering link pivotally connected between the steering frame and the turntable, and a right steering link pivotally connected between the steering frame and the turntable. The distance between the connections of the left steering link and the right steering link to the turntable are greater than the distance between connections of the left steering link and the right steering link to the steering frame. 
     In another embodiment, one or both of the steering frame and turntable elastically deforms to permit the transport system to make a left-hand or right-hand turn. In one embodiment, one or both of the left and right steering links elastically deforms to permit the transport system to make a left-hand or right-hand turn. 
     In one embodiment, the left steering link and the right steering link are located in a first plane, and one or both of the turntable and the steering frame are located on a second plane that is not the first plane. Moment forces generated between the first plane and the second plane elastically deform one or both of the steering frame and turntable to permit the transport system to make a left-hand or right-hand turn. 
     In one embodiment, the left steering link is nonparallel in relationship to the right steering link. The left steering link, the right steering link, the turntable, and the steering frame form an isosceles trapezoid when the transport system is traveling in a straight line. In this embodiment, the turntable and the steering frame form the parallel bases of the isosceles trapezoid. 
     In another embodiment, the front dolly has a front platform and two axles connected to the front platform. A front thrust bearing is located on top of the front platform. 
     The rear dolly has a rear platform. A rear thrust bearing is located on top of the rear platform. The rear dolly has a rearward portion located behind the rear platform with three axles connected to the rearward portion. The rear dolly has a forward portion located forward of the rear platform. A cylindrical opening is located on the forward portion. 
     The steering mechanism has a torque tube extending through the cylindrical opening on the forward portion of the rear dolly. The torque tube is mechanically locked to the front dolly, such as by an engaging key and keyway or other mechanism to provide coincident rotation between the torque tube and the front dolly. A steering frame is mounted to the torque tube. A turntable is pivotally mounted to the rear platform. A load cradle is connectable to the top side of the turntable. A left steering link is pivotally connected between the steering frame and the turntable. A right steering link is pivotally connected between the steering frame and the turntable. 
     A structure to be transported is connectable to a tractor. The structure is mounted onto the load cradle of the transport system located behind the tractor. The tractor pulls the structure, the structure pulls the rear dolly, and the rear dolly pushes the front dolly. 
     Changing the path of the tractor rotates the structure to follow the tractor. Rotation of the structure rotates the turntable in relation to the rear dolly. Rotation of the turntable compresses one of the right or left steering links and tensions the other so as to urge the steering frame to rotate the front dolly towards the new direction of the tractor. 
     In another embodiment, the left steering link and right steering link have a compressible and/or expandable component to allow small adjustments to the length of the steering links when sufficient force is imparted due to steered turning of the transport system. 
     In another embodiment, the left and right steering links are pivotally connected between the steering frame and the turntable with eccentric connectors that are rotatable to impart small adjustments to the distance between the steering frame and the rear turntable on each of the left and right sides when sufficient force is imparted due to steered turning of the transport system. 
     In another embodiment, the transport system has a left side and a right side in relationship to a forward direction of travel. The steering frame has a top and an opposite bottom facing the front platform. A flexible left steering bracket is attached to the bottom-left side of the steering frame. A flexible right steering bracket is attached to the bottom-right side of the steering frame. 
     The turntable also has a top and an opposite bottom facing the rear platform. A flexible left turntable bracket is attached to the bottom-left side of the turntable. A flexible right turntable bracket is attached to the bottom-right side of the turntable. The left steering link is pivotally connected at one end to the left steering bracket, and pivotally connected at its opposite end to the left turntable bracket. The right steering link is pivotally connected at one end to the right steering bracket, and pivotally connected at its opposite end to the right turntable bracket. 
     In another embodiment, a front thrust bearing assembly is located between the front platform and the forward portion of the rear dolly. 
     In another embodiment, the front thrust bearing assembly has a hollow interior. The torque tube is positioned in non-rotatable relation, such as by welding, inside the hollow interior of the front thrust bearing assembly. 
     In another embodiment, the torque tube has a hollow interior, and a base plate is located inside the hollow interior of the torque tube. 
     In another embodiment, a rear thrust bearing assembly is located between the rear platform and the turntable. 
     In another embodiment, an override is provided for aligning and parking the system. In this embodiment, a front dolly cylinder bracket is located on the front dolly, forward of the thrust bearing. A rear dolly cylinder bracket is located on the forward portion of the rear dolly, forward of the torque tube. An actuator is pivotally connected between the front dolly cylinder bracket of the front dolly and the rear dolly cylinder bracket of the rear dolly. 
     In another embodiment, the actuator is a hydraulic cylinder. A tank is provided that has sufficient capacity to retain the majority of the hydraulic cylinder fluid when the cylinder is unpowered during normal driving conditions. 
     As will be understood by one of ordinary skill in the art, the system disclosed may be modified somewhat and the same advantageous result obtained. For example, the number of axles may be varied without departing from the teachings and spirit of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the invention will become more readily understood from the following detailed description and appended claims when read in conjunction with the accompanying drawings in which like numerals represent like elements. 
       The drawings constitute a part of this specification and include exemplary embodiments to the invention, which may be embodied in various forms. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention. 
         FIG. 1  is an isometric view of the mechanically steered transport system having features of the present invention, illustrated in a 5-axle embodiment, carrying a mast section of a drilling rig. 
         FIG. 2  is an isometric view of the mechanically steered transport system having features of the present invention. 
         FIG. 3  is an exploded isometric view of the entire steering system, having features of the present invention illustrated from a viewpoint slightly above the mechanism. 
         FIG. 4  is an isometric view of mechanically steered transport system  10 , illustrating steering assembly  300  substantially assembled, and also illustrating in contrast a hydraulic override system that may be used for aligned parking of the system if necessary. 
         FIG. 5  is a partially exploded isometric view of an embodiment of the mechanically steered transport system of  FIG. 3 , and further illustrating the hydraulic steering assembly substantially assembled. 
         FIG. 6  is a top view of a base plate. 
         FIG. 7  is a top view of a retainer. 
         FIG. 8  is a top view of a steering frame connected to a torque tube having a base plate inside. 
         FIG. 9  is a side view of the transport system of the present invention, illustrating the assembly being pulled by a tractor, carrying a mast section of a drilling rig, and going over a raised portion of road. 
         FIG. 10  is a side view of the transport system of  FIG. 3 , illustrating the assembly going over a low portion of road. 
         FIG. 11A  is a top view of the transport system of the present invention, illustrating the transport system being pulled by a tractor, and driving in a straight path. 
         FIG. 11B  illustrates the transport system of  FIG. 11A  being pulled by a tractor and engaging in a relatively tight left-hand turn. 
         FIG. 12A  is a top view of the basic components of the steering system of the transport system in  FIG. 11A , in the orientation found while the transport system is being pulled by a tractor and driving in a straight path. 
         FIG. 12B  is a top view of the basic components of the steering system of the transport system in  FIG. 11B , in the orientation found while the transport system is being pulled by a tractor and engaging in a relatively tight left-hand turn. 
         FIG. 13  is a front end view of the steering frame of  FIG. 8 , illustrating the direction of distortion of the steering frame that results from the forces imparted during a left turn. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
       FIG. 1  is an isometric view of a mechanically steered transport system  10  having features of the present invention, and illustrated in a 5 axle embodiment. As illustrated in  FIG. 1 , transport system  10  is transporting a large structure  12 . In this illustration, structure  12  is a mast of a drilling rig. In the example shown, structure  12  is comprised of a framework  20  that defines a maximum transport width  22 . 
     Transport system  10  is comprised generally of a front dolly  100 , a rear dolly  200 , and a steering system  300 . Structure  12  is supported on one end by a load cradle  42  of a tractor  40 . Behind tractor  40 , structure  12  is separately supported by a load cradle  400  mounted on rear dolly  200 . Transport system  10  is designed to be pulled by structure  12 , through its connection to tractor  40  (see  FIGS. 9-10 ). 
     Transport system  10  has axles disposed beneath front dolly  100  and rear dolly  200 . In embodiments having an odd number of axles, a number of significant complications related to mechanical steering are present. In the embodiment illustrated, front dolly  100  has a front platform  120 , and a first axle  122  and a second axle  124 . Rear dolly  200  has a third axle  242 , a fourth axle  244 , and a fifth axle  246  connected to rearward portion  240 . 
       FIG. 2  is an isometric view of transport system  10 , having features of the present invention illustrated from a viewpoint slightly above transport system  10 . As seen in  FIG. 2 , steering system  300  generally comprises a steering frame  320 , a turntable  340 , a left steering link  360 , and a right steering link  362 . Turntable  340  is pivotally attached to rear dolly  200 , while steering frame  320  is connected to front dolly  100  in non-rotating relation. 
     In an optional embodiment also illustrated in  FIG. 2 , a manually operable override comprising a front cylinder bracket  150  is connected to front dolly  100 , and a rear cylinder bracket  250  is connected to rear dolly  200 . An actuator  350  is pivotally connected between front cylinder bracket  150  and rear cylinder bracket  250 . Actuator  350  may be a hydraulic cylinder that is permitted to free float during normal driving conditions. Actuator  350  may be engaged for overriding the mechanical steering capability of transport system  10  when reversing the system, or when operating in small spaces, like parking lots or drilling rig sites. 
       FIG. 3  is an exploded isometric view of transport system  10 , having features of the present invention illustrated from a viewpoint slightly above transport system  10 . Front dolly  100  has a front platform  120 , and a first axle  122  and a second axle  124 . A front thrust bearing assembly  130  is located on front platform  120 . Front thrust bearing assembly  130  may be comprised of an upper front bearing  132  and a lower front bearing  134 . In one embodiment, a rotation lock mechanism, such as a keyway  136 , is provided on front platform  120 . As indicated above, a front cylinder bracket  150  may also be provided for pivotal connection to actuator  350 . The preferred location of front cylinder bracket  150  is forward of front platform  120 . 
     Rear dolly  200  generally has a rear platform  220 , a rearward portion  240  rearward of rear platform  220 , and a forward portion  260  forward of rear platform  220 . A rear thrust bearing assembly  230  is located on rear platform  220 . Rear thrust bearing  230  may be comprised of an upper rear bearing  232  and a lower rear bearing  234 . In this embodiment, upper rear bearing  232  is connected to a bottom side of turntable  340 , and lower rear bearing  234  is connected to rear platform  220  such that turntable  340  is mounted in rotatable relation to rear dolly  200 . Rear dolly  200  has a third axle  242 , a fourth axle  244 , and a fifth axle  246  connected to rearward portion  240 . 
     Forward portion  260  of rear dolly  200  connects to front dolly  100 . Forward portion  260  has a cylindrical opening  262 . In one embodiment, a bearing pad  264  is held in place in cylindrical opening  262  by a bolting ring  266  for engagement with steering system  300 . 
     In another embodiment, upper front thrust bearing  132  is connected to the bottom of frontward portion  260  for engagement with lower front thrust bearing  134 , which is connected to front platform  120 . Engagement of upper front thrust bearing  132  with lower front thrust bearing  134  provides a weight bearing pivotal connection between front dolly  100  and rear dolly  200 . 
     As indicated above, a rear cylinder bracket  250  may also be provided for pivotal connection to actuator  350 . The preferred location of rear cylinder bracket  250  is forward of cylindrical opening  262 . 
       FIG. 4  is an isometric view of transport system  10 , illustrating steering assembly  300  substantially assembled. As seen in  FIG. 4 , steering system  300  generally comprises a steering frame  320 , a turntable  340 , a left steering link  360 , and a right steering link  362 . Referring back to  FIG. 3 , left steering link  360  is pivotally connected at one end to turntable  340 , and pivotally connected at its opposite end to steering frame  320 . Right steering link  362  is pivotally connected at one end to turntable  340 , and pivotally connected at its opposite end to steering frame  320 . 
     By design, left steering link  360  and right steering link  362  have the same length. In one embodiment, the pivotal connections connecting left and right steering links  360  and  362  to turntable  340  and steering frame  320  are secured with eccentric connectors  364 . The rotational position of eccentric connectors  364  is controllable to impart precise corrective adjustments to the length of left steering link  360  and right steering link  362  to ensure their close equality. 
     Steering frame  320  has a top side facing upwards, and a bottom side facing, and proximate to, front dolly  100 . As best seen in  FIG. 3 , steering frame  320  has a left steering bracket  326  located on its left end (having a pivot point  382 ), and a right steering bracket  328  located on its right end (having a pivot point  380 ). In a one embodiment, left steering bracket  326  and right steering bracket  328  are proximate to the bottom side of steering frame  320 , and thus beneath the horizontal center plane of steering frame  320 . In another embodiment, left steering bracket  326  and right steering bracket  328  are relatively flexible. 
     Turntable  340  has a top side facing upwards, and a bottom side facing, and proximate to, rear dolly  200 . As best seen in  FIG. 3 , turntable  340  has a left turntable bracket  346  located on its left end, and a right turntable bracket  348  located on its right end. In one embodiment, left turntable bracket  346  and right turntable bracket  348  are proximate to the bottom side of turntable  340 , and thus beneath the horizontal center plane of turntable  340 . In another embodiment, left turntable bracket  346  and right turntable bracket  348  are relatively flexible. 
     Referring back to  FIG. 4 , a pair of cradle brackets  370  is provided on the top side of turntable  340 . Cradle brackets  370  permit pivotal connection of cradle  400  to the topside of turntable  340 . 
     In the embodiment illustrated, a torque tube  310  is connected to steering frame  320  in non-rotating relation. A key  312  is attached to torque tube  310 . Key  312  is designed for complimentary fit into keyway  136  on front dolly  100 . When assembled, torque tube  310  will pass through the center of cylindrical opening  262  and through thrust bearing assembly  130 . In one embodiment, a base plate  314  (see  FIG. 6 ) is located inside torque tube  310  proximate to the engagement of key  312  with keyway  136  to prevent distortion of torque tube  310 . A retainer  316  (see  FIG. 7 ) can be secured to the bottom of torque tube  310 . Retainer  316  prevents vertical movement of torque tube  310  relative to cylindrical opening  262  and rear dolly  200 . 
       FIG. 5  is a partially exploded isometric view of mechanically steered transport system  10  of  FIG. 3 , illustrating steering assembly  300  substantially assembled above rear dolly  200 , which is above front dolly  100 . As illustrated in  FIG. 5  and in  FIG. 3 , a bearing pad  264  may be located at cylindrical opening  262  for providing smooth rotational engagement between torque tube  310  and cylindrical opening  262 . A bolting ring  266  may be used to secure bearing pad  264  in place. 
       FIG. 6  is a top view of base plate  314 . Base plate  314  can be located inside torque tube  310  to prevent buckling of torque tube  310 . 
       FIG. 7  is a top view of retainer  316 . Retainer  316  can be secured to the bottom of torque tube  310 . Retainer  316  prevents vertical movement or disengagement of torque tube  310  relative to cylindrical opening  262  and rear dolly  200 . 
       FIG. 8  is a top view of a steering frame connected to a torque tube having a base plate inside. 
       FIG. 9  is a side view of transport system  10  being pulled by tractor  40 , carrying structure  12  while going over a raised portion of road. 
       FIG. 10  is a side view of transport system  10 , illustrating transport system  10  going over a low portion of road. As described above (referring to  FIG. 4 ), cradle brackets  370  permit pivotal connection of cradle  400  to the topside of turntable  340 . This allows structure  12  to remain connected to tractor load cradle  42  when tractor  40  is negotiating inclines and declines and uneven conditions in the road. 
     Operation 
     In the most general sense, structure  12  is pivotally connected to a tractor  40 . The remaining weight of structure  12  is pivotally mounted to rear dolly  200 . Rear dolly  200  is pivotally connected to front dolly  100 , such that a portion of the weight of structure  12  is distributed between front and rear dollies  100  and  200 , respectively. 
     Tractor  40  pulls on structure  12 , which pulls rear dolly  200  beneath structure  12 . Rear dolly  200  pushes front dolly  100  at the pivotal connection. When tractor  40  turns, structure  12  also turns. Structure  12  is connected to steering system  300  such that rotation of structure  12  steers front dolly  100  so that transport system  10  can follow tractor  40  around turns. What follows is a more detailed treatment of components and the specific relationships between components. 
     The front end of transported structure  12  is pivotally connected to a tractor  40  at a tractor load cradle  42 . The remaining weight of structure  12  is distributed between front and rear dollies  100  and  200 , respectively. Load cradle  400  is designed to support the remaining weight of structure  12 . Tractor  40  pulls on structure  12 , which pulls rear dolly  200  through the connection of load cradle  400  to turntable  340  and rear thrust bearing assembly  230 . 
     Forward portion  260  of rear dolly  200  is pivotally connected to front platform  120  of front dolly  100  by front thrust bearing assembly  130 . Rear dolly  200  pushes front dolly  100  forward through the connection at front thrust bearing assembly  130 . 
     When tractor  40  turns, carried structure  12  (the transported load) rotates with tractor  40 . Structure  12  is supported by load cradle  400  which is mounted to turntable  340 . Turntable  340  is connected to rear dolly  200  through a rear thrust bearing assembly  230 . This allows structure  12 , load cradle  400 , and turntable  340  to rotate independently of rear dolly  200 . 
     Rotation of turntable  340  causes opposing directional movement of steering links  360  and  362  to rotate steering frame  320 . When tractor  40  turns right, turntable  340  tensions (pulls on) right steering link  362  and compresses (pushes on) left steering link  360 . When tractor  40  turns left, turntable  340  tensions (pulls on) left steering link  360  and compresses (pushes on) right steering link  362 . The movement of left and right steering links  360  and  362  forces rotation of steering frame  320 . 
     Torque tube  310  extends downwards from the bottom side of steering frame  320 . Torque tube  310  has one or more keys  312  connected to it that engage keyway  136  such that torque tube  310  is locked in non-rotational relationship with front dolly  100 . Rotation of steering frame  320  thus results in like rotation of torque tube  310 , keys  312 , and front dolly  100 . 
     Since torque tube  310  passes through the center of cylindrical opening  262  and through front thrust bearing assembly  130 , torque tube  310  is isolated from the pushing forces applied to front dolly  100  by rear dolly  200 . Similarly, front thrust bearing assembly  130  is isolated from the steering forces being applied to front dolly  100  by steering system  300 , other than as to experience and permit relative rotation of front dolly  100  and rear dolly  200 . 
     In one embodiment, illustrated in  FIG. 6  (see also  FIG. 8 ), a base plate  314  is provided for location inside torque tube  310  proximate to the engagement of key  312  with keyway  136  to prevent distortion of torque tube  310 . A retainer  316  can be secured to the bottom of torque tube  310 . Retainer  316  prevents vertical movement of torque tube  310  relative to cylindrical opening  262  and rear dolly  200 . 
     The length of turntable  340  is constrained by transport width  22  and by the limits placed on the rotated turntable  340  so it does not conflict with the chassis of the dolly. Using a wider turntable reduces the forces on the link. 
     As stated above, left steering link  360  and right steering link  362  have the same length. However, principal to the present invention, the width of steering frame  320  is less than the width of turntable  340 . As a result, left steering link  360 , right steering link  362 , turntable  340 , and steering frame  320  form an isosceles trapezoid when transport system  10  is traveling in a straight line. 
     Since the distance between the connections of left steering link  360  and right steering link  362  to turntable  340  is greater than the distance between the connections of left steering link  360  and right steering link  362  to steering frame  320 , left steering link  360  is nonparallel in relationship to the right steering link  362 . Turntable  340  and steering frame  320  form the parallel bases of the isosceles trapezoid. In the preferred embodiment, the ratio of the length of the bases (steering frame  320  to that of turntable  340 ) ranges between 0.77 and 0.85. 
     In one embodiment, left steering link  360  is pivotally connected between steering frame  320  and/or turntable  340  with eccentric connectors  364 . The rotational position of eccentric connectors  364  is controllable to impart precise corrective adjustments to the total length of left steering link  360  and right steering link  362  to ensure their close equality. 
     In this embodiment, right steering link  362  is also pivotally connected between steering frame  320  and/or turntable  340  with an eccentric connector  364 . Each eccentric connector  364  is rotatable to impart small adjustments to the distance between steering frame  320  and turntable  340  on each of the left and right sides. 
     In another embodiment, structure  12  to be transported is connected to a tractor  40 . Structure  12  is mounted onto load cradle  400  of transport system  10  located behind tractor  40 . Tractor  40  pulls structure  12 , structure  12  pulls rear dolly  200 , and rear dolly  200  pushes front dolly  100 . 
     As described, a change in the direction of tractor  40  rotates structure  12 . Rotation of structure  12  rotates turntable  340  in relation to rear dolly  200 . Rotation of turntable  340  compresses one of left or right steering links  360  or  362  and tensions the other, so as to urge steering frame  320  to rotate front dolly  100  towards the new direction of tractor  40 . The resistance to rotation of front dolly  100  causes torsional deflection of turntable  340  and steering frame  320  in an amount sufficient to correct for the lack of a rotatability of the quadrilateral steering system of transport system  10 . 
     Transport system  10  has a left side and a right side in relationship to a forward direction of travel. The steering frame  320  has a top  322  (not shown) and an opposite bottom  324  (not shown) facing front platform  120 . A left steering bracket  326  is attached to the bottom-left side of steering frame  320  (having a pivot point  382 ), and a right steering bracket  328  is attached to the bottom-right side of steering frame  320  (having a pivot point  380 ). Turntable  340  has a top  342  (not shown) and an opposite bottom  344  (not shown) facing rear platform  220 . A left turntable bracket  346  is attached to the bottom-left side of turntable  340  (having a pivot point  386 ), and a right turntable bracket  348  is attached to the bottom-right side of turntable  340  (having a pivot point  384 ). Left steering link  360  is pivotally connected at a first end to left steering bracket  326  and pivotally connected at its opposite end to left turntable bracket  346 . Right steering link  362  is pivotally connected at a first end to right steering bracket  328  and pivotally connected at an opposite end to right turntable bracket  348 . The brackets  326 ,  328  flex under load as a result of their location offset below turntable  340  and the relatively flat cross-section of turntable  340  having relatively low torsional stiffness. This flexibility functions to absorb small variations in the unloaded pin to pivot geometry between the steering link pins and steering frame pivot centers. 
     It is important to be able to accurately locate rear dolly  200  when locating structure  12  at their designated delivery locations. For this purpose, as detailed above, a manual override may be provided. In this embodiment, front dolly cylinder bracket  150  is located on front dolly  100 , preferably forward of thrust bearing. Rear dolly cylinder bracket  250  is located on forward portion  260  of rear dolly  200 , preferably forward of torque tube  310 . With actuator  350  pivotally connected between front dolly cylinder bracket  150  and rear dolly cylinder bracket  250 , actuator  350  may be engaged for overriding the steering system  300  of transport system  10  when reversing the system, or when operating in small spaces, like parking lots or drilling rig sites. 
       FIG. 11A  is a top view of transport system  10 , illustrating transport system  10  being pulled by tractor  40  and driving in a straight path.  FIG. 11B  illustrates transport system  10  being pulled by tractor  40  and engaging in a left-hand turn. 
       FIG. 12A  is a top view of the basic components of steering system  300  from  FIG. 11A , illustrating the position of steering system  300  when transport system  10  is proceeding in a straight path.  FIG. 12B  is a top view of the basic components of steering system  300  from  FIG. 11B , illustrating the position of steering system  300  when transport system  10  is engaging in a left-hand turn. 
     As seen in  FIGS. 12A and 12B , steering frame  320  has a width W s , representing the distance between a pivotal connection  380  to right steering link  362  and a pivotal connection  382  to left steering link  360 . Turntable  340  has a width W t , representing the distance between a pivotal connection  384  to right steering link  362  and a pivot connection  386  to left steering link  360 . As illustrated, width W t  of turntable  340  is larger than width W s  of steering frame  320 . Left steering link  360  and right steering link  362  are the same length, within a reasonable tolerance. 
     Referring to  FIG. 12A , a steering centerline  410  is shown passing through pivots  380  and  382 . As illustrated in  FIG. 12A , steering frame centerline  410  is perpendicular to the direction of travel when transport system  10  is traveling in a straight line. A turntable centerline  412  is shown passing through pivots  384  and  386 . As illustrated in  FIG. 12A , turntable centerline  412  is perpendicular to the direction of travel when transport system  10  is traveling in a straight line. Also, when transport system  10  is traveling in a straight path, steering frame centerline  410  and turntable centerline  412  are substantially parallel. 
     As a result of the parallelism of steering frame  320  and turntable  340 , combined with the difference in W s  and W t , combined with the steering links  360  and  362  having substantially the same length, the primary components of steering system  300  form an isosceles trapezoid when steering transport system  10  in a straight direction. 
     Referring to  FIG. 11B , transport system  10  is being pulled by tractor  40  and engaging in a left-hand turn. As seen in  FIG. 11B , tractor  40  turns in a path having a radius R T . Steering system  300  directs front dolly  100  in a path having a radius R 1 . Rear dolly  200  follows tractor  40  in a path having a radius R 2 . As seen in  FIG. 11B , R T  is greater than R 1 , and R 1  is greater than R 2 . The length of path R 1  relative to path R T , and the length of path R 2  relative to R 1 , are determined by the geometry and mechanics of steering system  300 . 
       FIG. 12B  is a top view of steering system  300  from  FIG. 11B , illustrating the position of steering system  300  when transport system  10  is engaging in a left-hand turn. A second steering centerline  420  is shown passing through pivots  380  and  382 . Similarly, a second turntable centerline  422  is shown passing through pivots  384  and  386 . A steering frame angle A S  is formed between centerlines  410  and  420 , representing the angle of rotation of steering frame  320  which results from engaging in the left turn illustrated in  FIG. 11B . A turntable angle A T  is formed between centerlines  412  and  422 , representing the angle of rotation of turntable  340  which results from engaging in the left turn illustrated in  FIG. 11B . Fundamental to the present invention is that A S  is greater than A T . 
     The length of front dolly path R 1  relative to rear dolly path R 2  are critical to safe and efficient movement of transport system  10 . The length of front dolly path R 1  relative to rear dolly path R 2  is determined by the difference between steering frame angle A S  and turntable angle A T . Steering frame angle A S  will be larger than turntable angle A T . In one example, front dolly  100  engages in a 25° turn (steering frame angle A S =25° while rear dolly  200  engages in a 20.13° turn (turntable angle A T =20.13°). Steering frame angle A S  and turntable angle A T  are determined by the precise geometry of the primary components of steering assembly  300 . In particular, in an embodiment in which front dolly  100  has 2 axels, and rear dolly  200  has three axles, the preferred ratio of W s  to W t  is between 0.77 and 0.85. In a more preferred embodiment, the preferred ratio of W s  to W t  is between 0.79 and 0.83. These ratios have been demonstrated to operate very well with transport system  10 . 
       FIG. 13  is a front-end view of steering frame  320 , with the arrows illustrating the direction of distortion generated by the forces acting on steering frame  320  during the left turn of  FIGS. 11B and 12B . Turntable  340  is likewise distorted. 
     During a left turn, steering frame  320  at bracket  326  is placed in compression, and steering frame  320  at bracket  328  is placed in tension. Bracket  326  is twisted in one rotational direction about steering frame  320 , and bracket  328  is twisted in the opposite direction of rotation about steering frame  320 . Similarly, bracket  346  is twisted in one rotational direction about turntable  340 , and bracket  348  is twisted in the opposite direction of rotation about turntable  340 . 
     The designed elasticity of steering frame  320  and turntable  340  to permit distortion under steering loads allows for the functionality of the non-parallelogram steering system  300  of transport system  10 . Specifically, the designed distortion provides a correction to the relative length of the left and right sides of steering assembly  300 .