Abstract:
The trailer steering mechanism automatically steers the trailer wheels in accordance with the tow vehicle steering and articulation between the tow vehicle and trailer during backing maneuvers to facilitate backing maneuvers and to prevent or greatly reduce the possibility of jackknifing between tow vehicle and trailer. The mechanism includes a pivotally attached steering frame connected between the steerable wheels of the trailer and one or more hydraulic cylinders that, when locked, create a very strong link between the tow vehicle and the trailer and its steering system. An alternative embodiment employs two non-powered hydraulic cylinders in tandem. The cylinders are able to withstand great loads, and more importantly, are able to disengage while under great load without jamming. The cylinders provide links that exert lateral force to the trailer to push or pull the trailer into a turn while backing, in addition to controlling the steering mechanism of the trailer.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 11/783,296 filed on Apr. 6, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to vehicle steering systems, and particularly to a trailer steering mechanism for towing a trailer or other towed implement or machine behind a motor vehicle. 
         [0004]    2. Description of the Related Art 
         [0005]    The towing vehicle and trailer combination is an inherently unstable one during backing maneuvers. Even a simple trailer having a single axle with non-steerable wheels requires some skill by the driver of the towing vehicle during backing maneuvers. This is because of the articulation between the towing vehicle and trailer, which allows the trailer to turn relative to the towing vehicle. Once the trailer begins to turn, even continued straight backing by the towing vehicle will cause the trailer to turn ever more sharply, resulting in the jackknifing of the trailer and towing vehicle in short order. 
         [0006]    While the above problem can be overcome by a skilled driver operating a single axle trailer, other trailer configurations can be impossible to back. For example, truck tractors, or other towing vehicles towing multiple articulated trailers in tandem, can only successfully back the single trailer connected directly to the towing vehicle. There is no way for the driver of the towing vehicle to steer both trailers of such a configuration while backing. Another example is the farm wagon-type trailer, having a front axle with steerable wheels and a rear axle with directionally fixed wheels. This combination cannot be backed due to the instability of the steerable front wheels of the trailer, which causes a castering action when backing. 
         [0007]    A number of different trailer steering mechanisms have been developed over the years in attempts to solve at least some of the above-described problems. 
         [0008]    German Patent No. 3,538,338, published on Apr. 30, 1987, describes (according to the drawings and English abstract) an electrical system incorporating servomotors to drive the trailer steering. The drawings are primarily directed to the electronic circuitry used in the system. 
         [0009]    German Patent No. 4,216,543, published on Dec. 3, 1992, describes (according to the drawings and English abstract) a rigid linkage of two or more struts or tow bars between the towing vehicle and trailer, which angularly lock the trailer relative to the towing vehicle and prevent articulation therebetween. The trailer wheels are not actively steered, but, rather, caster to follow the turns of the towing vehicle during both forward and reverse travel. The lateral hydraulic cylinder disclosed in the drawings is not a steering actuation link or element, but is a shimmy damper, as conventionally applied to castering wheel systems. 
         [0010]    Finally, German Patent No. 10,236,335, published on Feb. 27, 2003, indicates (according to the English title and abstract) that the subject trailer has a steerable front axle. However, no such steering mechanism is apparent in the drawings of the two-axle trailer. It appears that the trailer actually has castering front wheels, with no positive steering system being apparent. 
         [0011]    None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, a trailer steering mechanism solving the aforementioned problems is desired. 
       SUMMARY OF THE INVENTION 
       [0012]    The trailer steering mechanism facilitates the backing of a trailer by steering the trailer wheels in the same direction as the towing vehicle during backing maneuvers. The system automatically locks the trailer wheels straight for forward travel. A pivotally mounted steering frame is provided beneath the trailer frame, with the steering frame acting upon the tie rod(s) of the steerable wheels of the trailer. The steering frame receives its input from one or more hydraulic cylinders extending between the steering frame and a hitch bar, with the hitch bar being rigidly and immovably secured to the towing vehicle. The hydraulic cylinder(s) is/are hydrostatically locked during backing maneuvers, thus causing the steering frame of the trailer to pivot according to articulation between the trailer and towing vehicle when backing. The trailer follows (or more accurately leads, during backing maneuvers) the path of the towing vehicle during such backing maneuvers. 
         [0013]    The locking of the hydraulic cylinder(s) or strut(s) is controlled by one or more valves, which control hydraulic flow between the two ends of the double acting cylinder(s). The valves are, in turn, controlled by an electric motor or other electric drive, which is, in turn, controlled by the conventional electrical back-up signal (backing lights, alarm bell or signal, etc.) provided for most motor vehicles. When no signal or electrical power is provided for backing, the valve(s) is/are open to allow relatively free hydraulic flow through the cylinder(s) or strut(s). This allows the cylinder(s) to extend and retract with articulation between the trailer and towing vehicle, while the steering frame of the trailer remains locked straight ahead to lock the wheels of the trailer. When the tow vehicle is placed in reverse, electrical power is received from the backing circuit of the tow vehicle to close the valve(s), thereby locking the hydraulic cylinder(s) to cause the trailer steering frame to move with the tow vehicle, thereby steering the trailer wheels. 
         [0014]    The trailer steering mechanism may make use of various systems for actuating the hydraulic valve mechanism of the trailer, including multiple valves actuated by a single motor and chain or cable drive, or even a rigid rod linkage between the motor and valves, or a single valve having multiple ports, electrically operated solenoid valves, etc. Alternatively, the valve mechanism may be actuated by a self-contained electrical system on the trailer, or by manual operation if so desired. The hydraulic cylinder control valve(s) or hydraulic circuit may include one or more relief valves to relieve extreme pressures and forces in the system in the event of extreme steering angles by the tow vehicle during backing maneuvers. One or more steering frame and axle centering springs and adjustable steering stops may be provided to assist in centering the trailer steering and limit the steering angle of the trailer wheels. A mechanical latch may be provided to lock the trailer steering frame, and thus the trailer wheels, straight ahead for forward travel, with the latch automatically releasing for backing maneuvers and automatically engaging for forward travel. The mechanism may be applied to trailers having a forward and a rearward axle, i.e., farm wagons, with a link provided to a modified rear axle assembly permitting steering of the rear wheels in concert with the freely steering front wheels of the trailer. 
         [0015]    An additional embodiment employs two hydraulic cylinders in tandem in a non-powered, purely hydraulic system. The only non-hydraulic operation of this alternative system is the actuation (e.g., electric, mechanical, manual, etc.) of the single control valve to open and close the hydraulic flow to and from the cylinders for forward or backing maneuvers. In this system the forwardly disposed steering cylinder extends from the hitch bar to the rearward cylinder and operates similarly to the actuation cylinders described further above, i.e., with the control valve being open during forward operation to allow cylinder extension and retraction, and closing (hydraulically locking) the cylinder during backing maneuvers. The rearward axle lock cylinder is a double-rod steering or hydraulic cylinder, modified to include bias springs coaxially disposed about the rods and valve passages through the piston, the passages being closed only when the trailer axle is straight so that the piston is center in the body of the cylinder. 
         [0016]    While the term “trailer” has been used herein to describe the towed vehicle, machine, or device, it will be understood that the towed apparatus may comprise any form of towed implement, machine, or the like, e.g., wood chippers and other industrial machines having wheels for towing behind a motor vehicle, farm machinery, military equipment, etc., without limitation, so long as the trailer/towed implement incorporates one of the steering systems of the present invention. Moreover, it will be seen that a motor vehicle towing and backing a trailer using the present steering mechanism is not limited to towing and backing only a single trailer. Two or even more trailers may be towed and backed in tandem, so long as each of the trailers is equipped with a steering mechanism according to the present invention. 
         [0017]    These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is an environmental, perspective view of a towing vehicle with a trailer incorporating a trailer steering mechanism according to the present invention, showing its operation during a backing maneuver. 
           [0019]      FIG. 2  is a bottom plan view of a trailer incorporating the trailer steering mechanism of the present invention, showing the general configuration thereof. 
           [0020]      FIG. 3  is a schematic top plan view showing a towing vehicle and trailer steering mechanism according to the present invention during forward travel, with the trailer wheels being locked straight for forward travel. 
           [0021]      FIG. 4  is a schematic top plan view showing a towing vehicle and trailer steering mechanism according to the present invention during a backing maneuver, with the trailer wheels automatically turning to align the trailer with the path of the towing vehicle. 
           [0022]      FIG. 5  is a schematic top plan view of an alternative embodiment of trailer and trailer steering mechanism according to the present invention, wherein the trailer has forward free-steering wheels and a rearward axle having steerable wheels thereon controlled by a steering mechanism. 
           [0023]      FIG. 6  is a detailed top plan view showing a first embodiment of an actuating mechanism for the hydraulic valves of a trailer steering mechanism according to the present invention. 
           [0024]      FIG. 7  is a detailed top plan view showing another embodiment of an actuating mechanism for the hydraulic valves of a trailer steering mechanism according to the present invention, wherein the system incorporates electrically actuated solenoid valves to control hydraulic flow. 
           [0025]      FIG. 8  is a plan view in section of an exemplary two-way or double-acting pressure relief valve for use in the trailer steering mechanism of the present invention. 
           [0026]      FIG. 9  is a detailed top plan view of an alternative embodiment of a valve mechanism for a trailer steering mechanism according to the present invention, incorporating the entire valve mechanism in a single housing. 
           [0027]      FIG. 10  is a detailed side elevation view in partial section of one of the steering centering and limit devices of a trailer steering mechanism according to the present invention. 
           [0028]      FIG. 11  is a left side elevation view of the locking mechanism for securing the trailer wheels in a straight ahead position for forward travel in a trailer steering mechanism according to the present invention. 
           [0029]      FIG. 12A  is a top plan view of a trailer frame incorporating an alternative embodiment trailer steering mechanism employing an non-powered, completely hydraulic system, with the trailer wheels set in their neutral, straight ahead orientation. 
           [0030]      FIG. 12B  is a top plan view in section of the axle lock cylinder, showing its internal configuration when in its neutral position as would be required in the trailer wheel orientation shown in  FIG. 12A . 
           [0031]      FIG. 13A  is a top plan view of the trailer frame and steering mechanism of  FIG. 12A , showing the trailer wheels oriented for a backing maneuver to the right. 
           [0032]      FIG. 13B  is a top plan view in section of the axle lock cylinder of  FIG. 12B , showing its internal configuration when driven to orient the trailer wheels as shown in  FIG. 13A . 
           [0033]      FIG. 14A  is a top plan view of the trailer frame and steering mechanism of  FIGS. 12A and 13A , showing the trailer wheels oriented for a backing maneuver to the left. 
           [0034]      FIG. 14B  is a top plan view in section of the axle lock cylinder of  FIGS. 12B and 13B , showing its internal configuration when driven to orient the trailer wheels as shown in  FIG. 14A . 
           [0035]      FIG. 15  is a top plan view in partial section of the hydraulic control valve used to control the system of  FIGS. 12A through 14B . 
           [0036]      FIG. 16  is an exploded side elevation view of a steering hub that may be installed upon the conventional wheel spindle of a trailer to provide for trailer wheel steering. 
       
    
    
       [0037]    Similar reference characters denote corresponding features consistently throughout the attached drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    The present invention comprises various embodiments of a steering mechanism for trailers having steerable wheels, and various trailer configurations incorporating such a steering mechanism. The steering mechanism allows the steerable wheels of the trailer to be steered positively according to articulation between the towing vehicle and the trailer during backing maneuvers, while locking the steerable wheels straight ahead for forward travel of the trailer. 
         [0039]      FIG. 1  of the drawings provides an environmental perspective view of a trailer  10  with its steering mechanism, shown hitched behind a towing vehicle V, with  FIG. 2  providing a bottom plan view of the trailer  10  and its steering mechanism. The trailer  10  includes a frame  12  having a forwardly extending rigid tongue  14 , with the tongue  14  being removably and pivotally hitched to a lateral three-ball hitch bar  16  attached to the tow vehicle V. The hitch bar  16  is shown most clearly in the top plan view illustrations of  FIGS. 3 ,  4 , and  5 , and is rigidly and immovably secured to the tow vehicle V by means of a conventional receiver hitch or the like. The hitch bar  16  comprises a three-ball hitch bar, having a central ball for the removable connection of the trailer tongue  14  coupler thereto and opposite left and right balls for the removable connection of the couplers of the left and right hydraulic cylinders (described in detail further below) thereto. 
         [0040]    The steerable trailer  10  includes a steering frame  18  attached beneath the trailer frame  12  at a pivot  20 . The location of the pivot point  20  for the steering frame  18  may vary from that shown, so long as the geometry of the steering linkage (i.e., various arm lengths, arcuate and linear movements, etc.) is adjusted accordingly. In the example of  FIG. 2 , the steering frame  18  includes a rearwardly extending steering arm  22 , which connects to an intermediate link  24  between the distal end of the arm  22  and the tie rod(s)  26  of the steerable trailer wheels  28   a ,  28   b , which extend from each end of the axle  30  of the trailer  10 . The wheels  28   a ,  28   b  are attached to the trailer axle  30  by conventional articulated joints, e.g., king pins, etc., to allow the trailer wheels  28   a ,  28   b  to be steered when the steering frame  18  pivots to move the tie rod(s)  26  by means of the intermediate link  24 . 
         [0041]    The steering frame  18  includes a crossmember  32  which provides for attachment of at least one hydraulic cylinder to the steering frame, with there preferably being one hydraulic strut or cylinder  34   a  installed to the left side of the trailer tongue  14  and a second strut or cylinder  34   b  installed to the right side of the tongue  14 . Alternatively, plural struts or cylinders may be installed to each side of the central tongue  14 , if so desired, depending upon the rating of each hydraulic cylinder or strut and the size of the trailer and maneuvering forces required. Each of the cylinders  34   a ,  34   b  has a hitch coupling end  36   a  and  36   b  and an opposite steering frame attachment end  38   a  and  38   b , with the forward coupling ends  40   a ,  40   b  of the cylinders being pivotally coupled to, and extending from, the corresponding ends of the hitch bar  16 , and the steering frame attachment ends  38   a ,  38   b  attaching to the ends of the steering frame crossmember  32 . The struts or cylinders  34   a ,  34   b  are parallel to the trailer tongue  14  and to one another, with the tongue  14  centered between the two struts or cylinders. 
         [0042]    The trailer steering mechanism is selectively actuated according to forward or reverse travel of the tow vehicle V and trailer  10 , with the steering mechanism being locked in a neutral position with the trailer wheels  28   a ,  28   b  straight ahead regardless of articulation between the towing vehicle V and trailer  10  during forward motion, and with the hydraulic cylinders  34   a ,  34   b  being hydraulically locked to transmit articulation between the hitch bar  16  of the tow vehicle V and the steering frame  18  of the trailer  10  to steer the trailer wheels  28   a ,  28   b  during backing maneuvers. This is controlled by a hydraulic strut or cylinder control mechanism  42 , shown in detail in  FIG. 6  of the drawings. 
         [0043]    In the mechanism  42  embodiment of  FIGS. 1 through 4  and  FIG. 6 , a series of four hydraulic control valves  44  through  50  are controlled by a single valve actuator  52 . The first control valve  44  is positioned between a source of hydraulic fluid or reservoir  54  and the hitch bar attachment end  36   a  of the first or left side hydraulic cylinder or strut  34   a . The second control valve  46  is positioned between the hydraulic reservoir  54  and the steering frame attachment end  38   a  of the first or left side cylinder or strut  34   a . The third control valve  48  is positioned between the hydraulic reservoir  54  and the hitch bar attachment end  36   b  of the second or right side hydraulic cylinder or strut  34   b , with the fourth control valve  50  being positioned between the hydraulic reservoir  54  and the steering frame attachment end  38   b  of the second or right side cylinder or strut  34   b.    
         [0044]    The four valves  44  through  50  are linked to the actuator  52  by a roller chain  56 , which is driven by a drive sprocket  58  extending from the actuator, with the drive sprocket  58 , in turn, driving a valve sprocket  60  extending from each of the valves. When the actuator  52  is rotated, its sprocket  58  drives the chain  56  to rotate each of the driven sprockets  60  of the four valves, thereby turning the valves on or off according to the operation of the actuator. It will be seen that other mechanisms may be used in lieu of the above-described mechanism, e.g., a cable drive, a series of rigid pushrods between the valves, etc., or even manual actuation, as desired. Alternatively, electric solenoid valves may be used in lieu of mechanically or manually actuated valves, if so desired. 
         [0045]    The actuator  52  is driven by an electric motor  62 , which is controlled by a solenoid switch  64 , with the solenoid  64  receiving power from the conventional backup signal  66  (backup lights, bells, etc.) provided on tow vehicles. A separate manually-controllable cutout switch  68  may be included in the circuit, if so desired, in order that the trailer wheels may be locked straight during backing maneuvers, e.g., when maneuvering in tight quarters. Assuming the cutout switch  68  is closed, shifting the tow vehicle into reverse provides electrical power to the solenoid switch  64  to actuate the valve actuator control motor  62 . This rotates the actuator  52 , thereby driving the chain  56  to rotate each of the valve sprockets  60  to close their corresponding valves  44  through  50  to hydraulically lock the lengths of the two cylinders  34   a  and  34   b . This results in articulation between the tow vehicle V and trailer  10  turning the steering frame  18  of the trailer, thereby turning the trailer wheels  28   a  and  28   b , as described further above. This is the preferred means of carrying out the control of the system, i.e. providing electrical power from the tow vehicle. However, electrical valve actuation may be accomplished by electrical power supplied by a source on the trailer, if so desired. 
         [0046]    Shifting of the tow vehicle to a forward gear (or opening the cutout switch  68 ) shuts off the electrical signal to the solenoid  64 , thereby shutting off power to the actuator motor  62 . This allows the actuator  52  to return to its original position, driving the chain  56  and valve sprockets  60  to open the various hydraulic cylinder or strut control valves  44  through  50 . Alternatively, a separate circuit may be provided to drive the motor  62  in the opposite direction to open the valves for forward travel. This allows hydraulic fluid to flow relatively freely through the system, with the two hydraulic cylinders  34   a  and  34   b  extending and retracting with articulation between the tow vehicle V and trailer  10  as the assembly turns during forward motion. The viscosity of the hydraulic fluid flowing through the system during turning maneuvers does provide some beneficial damping of any unwanted sway between the trailer and tow vehicle. Since the hydraulic cylinders are no longer hydraulically locked, they do not impart any significant thrust or force upon the steering frame  18  to steer the wheels of the trailer. However, a positive mechanical lock may be provided for the steering frame  18  to assure that the trailer wheels are locked straight ahead during forward travel for stability of the trailer and tow vehicle assembly. This locking mechanism is illustrated in  FIG. 10 , and discussed further below. 
         [0047]    It will be seen that there is no hydraulic pump installed in this hydraulic system, as none is required. The hydraulic system of the present mechanism merely serves to selectively lock the lengths of the hydraulic cylinders or struts  34   a  and  34   b  to lock the relative orientation of the steering frame  18  relative to the hitch bar  16  of the tow vehicle V when backing, thus causing the locked hydraulic cylinders to steer the steering frame to turn the trailer wheels. Conversely, free flow of hydraulic fluid is permitted back and forth between the cylinders  34   a  and  36   a  when in forward travel, thus allowing articulation between the trailer  10  and tow vehicle V while the steering frame  18  is locked to lock the trailer wheels straight ahead. While no hydraulic pump is required, the closed and sealed system may be pneumatically pressurized, if so desired, by adding some predetermined air (or other gas) pressure to the reservoir  54  to serve as a preload for the system. The provision of a closed, sealed system provides additional benefits as well in preventing contaminants (e.g., road dirt and debris, moisture, etc.) from entering the system, thus adding to the longevity of the hydraulic fluid in the system as well as to the components of the system. 
         [0048]      FIG. 7  provides a top plan view in which the above-described hydraulic system (illustrated in  FIG. 6 ) is controlled by electrohydraulic solenoid valves  44   a  through  50   a . The electrohydraulic solenoid valves  44   a ,  46   a ,  48   a , and  50   a  each include a hydraulic valve selectively permitting or stopping the flow of hydraulic fluid therethrough, with the hydraulic valve being controlled by the mechanical action of an electrically actuated solenoid. Accordingly, the mechanical system shown in  FIG. 6  is not needed for control of the electrically actuated valves  44   a - 50   a  of the embodiment of  FIG. 7 . Electrical power to the electrohydraulic solenoid valves  44   a  through  50   a  may be controlled through an electrical circuit identical to that used for the control and operation of the electric motor  62  driving the valve actuator  52  for the mechanical system of  FIG. 6 , i.e., a solenoid  64  receiving power from the back-up system  66  of the tow vehicle. A separate conventional electrical power source may be provided from the tow vehicle or an electrical source on the trailer (e.g., battery, etc.) if more electrical power is required than can be provided through the back-up system  66 , with the actuation of the separate power source being controlled by another solenoid similar to the solenoid  64  shown in  FIGS. 6 and 7 , or a double pole solenoid or the like. 
         [0049]      FIG. 8  of the drawings provides a side elevation view in section of an exemplary double-acting or two-way pressure relief valve  100 , which may be incorporated in the trailer steering mechanism. This two-way valve  100  serves to relieve excessive hydraulic pressure in the system in either direction of flow through the valve. The valve  100  includes a stationary frame  101 , which is secured within a passage through the valve housing or body, e.g., the passage  92  of the rotary valve member  80  of  FIG. 9 , by a retaining ring  103  or the like. A first valve spring  105  extends between the stationary frame  101  and a first or outer poppet valve  107 , with the first valve  107  opening toward the first spring  105  when sufficient pressure is developed to overcome spring pressure. 
         [0050]    The first or outer valve  107  includes a central opening or passage  109  therethrough, which is sealed by a second poppet valve  111 . The second valve  111  is held in position over the first valve opening  109  by a second frame  113 , which extends from the first valve  107  opposite the first or stationary frame  101 , with a second valve spring  115  being captured between the second frame  113  and second valve  111  to urge the second valve  111  against the inner face of the first valve  107 . Thus, a first pressure P 1  of sufficient force to compress the first valve spring  105  will force the second valve  111  against its seat on the first valve  107 , but will push the first valve  107  open from its seat on its stationary frame  101  to relieve the excessive pressure P 1 . However, an opposite second pressure P 2  acts in the same direction as the force developed by the first valve spring  105 , with the first valve  107  accordingly remaining closed. But such a pressure P 2  acts against the face of the second valve  111  and against its spring  115 , pushing the second valve  111  open to relieve excessive pressure in the direction indicated by P 2  in  FIG. 8 . 
         [0051]    It will thus be seen that the pressure relief valve  100  can relieve excessive hydraulic pressure in either direction using only a single valve. Moreover, the two springs  105 ,  115  may be provided with adjustable compressive force by means of threaded retainers between the respective valve frames and valves in order to allow the opening force for each valve  107 ,  111  to be adjusted as desired. It will be seen that such a two-way or double-acting pressure relief valve(s)  100  may be incorporated in the hydraulic circuits of  FIGS. 6  and/or  7 , wherein four separate control valves are used, with each incorporating a single two-way pressure relief valve  100 . Alternatively, single directional relief valves may be incorporated by means of two separate parallel hydraulic paths between each control valve or port and corresponding hydraulic cylinder or strut end. 
         [0052]      FIG. 9  of the drawings illustrates an alternative valve assembly  70 , which may be incorporated in the trailer steering mechanism in lieu of the four separate valves and actuator illustrated in detail in  FIGS. 6 and 7  and described above. The valve assembly  70  of  FIG. 9  incorporates four separate valve ports or outlets, respectively  72  through  78 , which communicate hydraulically with the respective hitch coupling end  36   a  and steering frame arm attachment end  38   a  of the first or left hydraulic cylinder or strut  34   a  and the hitch coupling end  36   b  and steering frame arm attachment end  38   b  of the second or right hydraulic cylinder or strut  34   b . A single rotary member  80  rotates within the valve body  82  to selectively align the four hydraulic passages  84  through  90  of the rotary member  80  with the four outlet ports  72  through  78  of the valve assembly  70  to allow flow between the ends of the hydraulic cylinders  34   a  and  34   b  so that the trailer wheels  28   a ,  28   b  may be locked straight ahead for forward travel of the trailer  10 . The valve assembly  70  is illustrated in this open flow configuration for forward travel in  FIG. 9 . 
         [0053]    When the trailer  10  is to be backed, the rotary center member  80  of the valve assembly  70  is rotated (manually, or by means of an actuator motor and electrical circuit, such as the motor  62  and circuit shown in  FIG. 6  and described further above) through forty-five degrees to align the passages  84  through  90  of the rotary member  80  with the closed interior sidewalls of the valve body  82 , thereby shutting off flow between those passages  84  through  90  and their respective hydraulic cylinders  34   a  and  34   b  to lock the cylinders hydraulically. 
         [0054]    However, it will be noted that the valve assembly  70  includes additional restricted flow passages  92  through  98 , each of which includes a two-way pressure relief valve  100  therein; such a two-way pressure relief valve  100  is shown in detail in  FIG. 8  of the drawings, and has been described in detail above. When the valve  70  has been repositioned to close the primary passages  84  through  90  for backing operations, the pressure relief or restricted flow passages  92  through  98  are aligned with the four ports  72  through  78 . So long as hydraulic pressure remains below a certain predetermined level(s) within the cylinders  34   a ,  34   b  and their hydraulic lines, the relief valves  100  remain closed. 
         [0055]    However, in the event that, e.g., an excessive turning angle is reached that might cause damage to the mechanical steering mechanism, the relief valve(s)  100  will open briefly to relieve hydraulic pressure in the system, and then reset automatically once the pressure has been relieved. This limits the extension and/or retraction of the hydraulic cylinders or struts  34   a ,  34   b , rather than having the locked cylinders exert excessive force upon the mechanical structure of the trailer  10  and its steering mechanism and possibly overstressing and/or damaging the structure. 
         [0056]      FIG. 10  of the drawings provides a detailed side elevation view in partial section of an exemplary steering centering spring and stop limit assembly, which may be incorporated into the trailer steering mechanism. The assembly of  FIG. 10  installs between the trailer frame  12  and the pivotal steering frame  18 , or more precisely one (or both) of the lateral arms or crossmembers  32  of the steering frame. A steering angle limit stop  102  is secured (e.g., welded, etc.) to a bracket  104 , which extends from the trailer frame  12 . A steering stop bumper  108  is adjustably secured to the steering frame arm  32  by a threaded adjuster  110 . The span between the end of the limit stop  102  and the end of the bumper  108  may be adjusted by means of the adjuster  110  to adjust the maximum angle or limit of the steering geometry of the trailer  10 . A steering centering coil spring  112  is installed concentrically over and between the steering limit stop  102  and bumper  108  to urge the wheels to a straight position when the hydraulics are released during forward movement, and to limit the impact forces between the steering limit stop  102  and bumper  108  during extreme turning maneuvers when backing. 
         [0057]    The above-described trailer steering centering mechanism serves to resist angular steering input at and near the predetermined geometric steering limits for the trailer, but the spring force does relatively little to resist steering input from the neutral or straight ahead position of the trailer wheels, particularly when identical centering mechanisms are installed upon each crossmember  32  of the steering frame  18 . The lack of any means for holding the trailer wheels precisely aligned would result in instability and trailer wandering during forward travel. Accordingly, a steering lock mechanism is provided for the trailer steering, as shown in the left side elevation view of  FIG. 11 . 
         [0058]      FIG. 11  illustrates the distal forward end  114  of the forwardly extending arm  116  of the trailer steering frame  18 , disposed generally beneath the hydraulic valve actuator  52  and its motor  62  and other componentry. The valve actuator  52  includes an actuator shaft and arm assembly  118  extending therefrom, with the actuator shaft serving to rotate the hydraulic valve drive sprocket  58  (shown in  FIG. 6 ). A steering frame lock cable  120  (or chain or other flexible member, etc.) extends in tension from the arm of the actuator shaft and arm assembly  118 , ultimately connecting to a steering frame arm lock  122 , which is pivotally attached laterally to the trailer frame  12 , or more precisely to a crossmember  124  extending between the two lateral members of the frame  12 . The cable  120  may pass over or through one or more pulleys  126  or guides, fairleads, etc. in order to achieve the proper routing and direction of operation. A down spring  128  urges the steering frame arm lock  122  down to its engaged position to capture the forward end portion  114  of the forwardly extending steering frame arm  116 . The steering frame arm lock  122  may comprise a laterally disposed length of material having a generally L-shaped cross section, e.g., “angle iron” or the like. The side or face opposite its attachment hinge  130  includes a steering arm engagement slot  132  therein (shown in broken lines in  FIG. 11 ), which fits fairly precisely over the forward arm portion  116  of the steering frame. 
         [0059]    When the actuator  52  is operated to close the hydraulic control valves  44  through  50  (shown in  FIG. 6 ; a conventional solenoid may be used to reposition a lever or arm, for the solenoid valve system of  FIG. 7 ), or alternatively, the single valve  70  (shown in  FIG. 9 ) to allow the trailer wheels to be steered for backing maneuvers, the actuator shaft and arm  118  rotate to the right, as shown in the side view of  FIG. 11 , pulling the upper portion of the cable  120  to the right and the lower end of the cable upwardly, thereby drawing the steering arm lock  122  upwardly generally to its position as shown in broken lines in  FIG. 11 . This allows the forward end portion  114  of the steering frame arm  116  to swing laterally in accordance with steering input due to the articulation between the towing vehicle V and trailer  10 , to the limits defined by the steering centering and stop mechanism of  FIG. 10  and/or any other mechanical limits. 
         [0060]    However, when the towing vehicle is placed in a neutral or forward gear, i.e., the backup signal is no longer activated, the valve actuator  52  rotates the actuator shaft and arm assembly  118  to the position shown in  FIG. 11 , thereby allowing the down spring  128  to draw the cable  120  and steering arm lock  122  downwardly against the top of the steering frame arm  116 . The lower edge of the steering arm lock  122  will ride against the upper surface of the steering frame arm  116  if the arm is not centered. However, when the trailer  10  is pulled forward slightly, the wheel centering spring(s)  112  cause the steerable wheels  28   a ,  28   b  of the trailer  10  to move to a straight ahead position, thus forcing the arm  116  to center. This allows the engagement slot  132  of the steering arm lock  122  to drop downwardly over the steering frame arm  116 , thus capturing the arm  116  within the slot  132  of the steering arm lock  122  to lock the arm  116  and its steering frame  18  and trailer wheels  28   a ,  28   b  straight for stable towing of the trailer  10  during forward travel. The engagement of the steering arm lock  122  with the steering frame arm  116  is easily detected by the driver of the tow vehicle, as the lock  122  is pulled sharply down over the steering arm  116  when the arm centers. Alternatively, a solenoid actuator may be used to control the steering arm lock  122 . The solenoid actuator would be controlled by the same electrical circuit as used to control a solenoid valve system, e.g., the system of  FIG. 7  of the drawings. 
         [0061]    The above-described trailer towing mechanism and trailer with its steerable wheels is directed to a trailer having a single axle with two steerable wheels, one on each end of the single axle. However, the trailer steering mechanism may also be applied to trailers having multiple axles, such as the farm wagon type trailer  10   a  of  FIG. 5 . The trailer  10   a  of  FIG. 5  has a frame  12   a  (shown as a solid peripheral edge for the trailer  10   a  in FIG.  5 ) carrying a forward axle  30   a  and a rearward axle  30   b , with the forward axle  30   a  having free steering left and right wheels  28   c ,  28   d  at the opposite ends thereof and the rearward axle  30   b  having directionally steerable wheels  28   e ,  28   f  at the opposite ends thereof. The front wheels  28   c ,  28   d  are free steering in the sense that they are directionally articulating, but do not include a steering control mechanism. A tie rod  26   a  extends between the steering mechanism of each steerable rear wheel  28   e ,  28   f , with the tie rod  26   a  being connected to the steering frame  18   a  by an intermediate steering link  24   a.    
         [0062]    The steering mechanism for the trailer  10   a  with its rearward axle  30   b  and its directionally steerable rear wheels  28   e ,  28   f  is generally the same steerable wheel mechanism as described further above for the trailer  10  with its single axle  30  and steerable wheels  28   a ,  28   b , shown in  FIGS. 2 through 4  of the drawings. However, the pivotally mounted steering frame  18   a  includes a rearward steering arm extension  22   a  to which the intermediate steering link  24   a  is attached, with the distal end of the link  24   a  being attached to the rear axle tie rod  26   a . Thus, only the rear wheels  28   e ,  28   f  extending from the rear axle  30   b  of the trailer  10   a  may be steered, with the front wheels  28   c ,  28   d  being free steering, i.e. not connected to or directionally controlled by a steering mechanism. 
         [0063]    The rear axle steering mechanism of the rear wheel steering trailer  10  is controlled by means of the same apparatus as provided with the trailer  10 . The frame  12   a  of the trailer  10   a  includes a forwardly extending rigid tongue  14 , with the tongue  14  being removably and pivotally hitched to a lateral hitch bar  16  rigidly attached to the tow vehicle V. The steering frame  18   a  includes a crossmember  32   a  which provides for attachment of at least one hydraulic cylinder to the steering frame, with there preferably being one hydraulic strut or cylinder  34   a  installed to the left side of the trailer tongue  14  and a second strut or cylinder  34   b  installed to the right side of the tongue  14 . Alternatively, plural struts or cylinders may be installed to each side of the central tongue  14 , if so desired, particularly in the case of a relatively large and heavy multiple axle farm wagon-type trailer  10   a . Each of the cylinders  34   a ,  34   b  has a hitch coupling end  36   a  and  36   b  and an opposite steering frame attachment end  38   a  and  38   b , with the forward ends  40   a ,  40   b  of the cylinder struts being pivotally coupled to, and extending from, the corresponding ends of the hitch bar  16 , and the steering frame attachment ends  38   a ,  38   b  attaching to the ends of the steering frame crossmember  32   a . The struts or cylinders  34   a ,  34   b  are parallel to the trailer tongue  14  and to one another, with the tongue  14  being centered between the two struts or cylinders, just as in the case of the single axle trailer  10  embodiment. 
         [0064]    The above-described steering mechanism is controlled by a hydraulic cylinder or strut control system  42 , as illustrated in  FIGS. 6 and 7  of the drawings and described in detail further above. Alternatively, the multiple valve mechanism of the system  42  may be replaced by the single valve mechanism  70  of  FIG. 9 , if so desired. The system operates in essentially the same manner as described above for the single-axle trailer  10 , but steers only the wheels and tires  28   e  and  28   f  of the rear axle  30   b , with the forward wheels and tires  28   c ,  28   d  of the front axle  30   a  being able to free steer at all times during forward or rearward travel. 
         [0065]    When the tow vehicle V towing the multiple axle trailer  10   a  of  FIG. 5  is towing the trailer forward, the hydraulic system is open to allow hydraulic fluid to flow freely between the two cylinders  36   a  and  36   b , thereby allowing the two cylinders to extend and retract in accordance with articulation between the tow vehicle V and trailer  10   a  without actuating the steering frame  18   a  and associated steering linkage to the rear axle  30   b . Thus, the rear wheels and tires  28   a ,  28   f  track in a straight line relative to the centerline of the trailer  10   a , during forward travel. The two front wheels  28   c ,  28   d  steer freely to allow the trailer  10   a  to negotiate turns while traveling forward. 
         [0066]    When the twin-axle trailer  10   a  is to be backed, the two hydraulic cylinders or struts  36   a ,  36   b  are hydraulically locked, as in the case of the single-axle trailer  10 . This results in any articulation between the tow vehicle V and trailer  10   a  rotating the steering frame  18   a , thus steering the rear wheels  28   e ,  28   f . The front wheels  28   c ,  28   d  are free to steer or articulate directionally as required to enable the tow vehicle V and two-axle trailer  10   a  combination to negotiate turns as required. 
         [0067]      FIGS. 12A through 16  illustrate another embodiment of the trailer steering mechanism and various components thereof, wherein a non-powered, purely hydraulic system is used to control the steering of the trailer during backing maneuvers. The hydraulic system of the embodiment of  FIGS. 12A through 16  is completely sealed, i.e., it is not vented to atmosphere and does not utilize any pneumatics, pumps, or other power systems in its operation.  FIGS. 12A ,  13 A, and  14 A provide top plan views of the trailer and its frame and steering mechanism with the wheels straight ( FIG. 12A ) and while turning to the right and left ( FIGS. 13A and 14A ). 
         [0068]    The trailer  210  includes a frame  212  having a forwardly extending rigid tongue  214 , with the tongue  214  being removably and pivotally hitched to a lateral hitch bar  16  attached to the tow vehicle V (as shown in  FIGS. 1 ,  3 ,  4 , and  5 ). The trailer  210  differs from the trailer  10  embodiment of  FIGS. 1 through 5 , in that it is conventional until modified by the addition of the steering system described herein. The hitch bar  16  is rigidly and immovably secured to the tow vehicle V by means of a conventional receiver hitch or the like, as in the trailer and hitch embodiment of  FIGS. 1 through 5 . The hitch bar  16  may be identical to the three-ball hitch bar  16  of  FIGS. 1 and 3  through  5 , but actually requires only one central ball (or other hitch attachment) for the trailer tongue  14  coupler and one laterally offset ball (or other hitch attachment) for the steering connection (described in detail further below). 
         [0069]    A first or steering hydraulic cylinder  216  is installed at the forward portion of the trailer  210 . The steering cylinder  216  has a selectively extendible rod  218  with a tow vehicle (or hitch bar) connection end  220  and a cylinder base  222  opposite the rod  218 . The steering cylinder  216  is a conventional double-acting unit, i.e., retraction of the strut or rod  218  will actuate the internal piston to draw hydraulic fluid into the forward end or portion  224  of the cylinder and expel fluid from the cylinder base end or portion  222 , with extension of the strut or rod  218  expelling fluid from the forward end portion  224  and drawing fluid into the cylinder base end or portion  222 . It should be noted that while the term “steering cylinder” is used for this forward cylinder  216 , it only acts to operate the trailer steering when the trailer is backing. The rod  218  of this steering cylinder  216  is free to extend and retract during forward operation of the trailer  210 , with fluid moving freely to and from the cylinder  216 , as described above. 
         [0070]    A second, rearwardly disposed or axle lock cylinder  226  is immovably affixed to the trailer frame  212 , e.g., by means of conventional clamps and bolts, etc., and connected in tandem with the forward steering cylinder  216 . This axle lock cylinder  226  serves to lock the wheels of the trailer  210  in a straight ahead orientation when the trailer is being pulled forward, in order that the trailer will track straight behind the towing vehicle. The axle lock cylinder  226  is hydraulically open during backing maneuvers to allow the steering linkage to which it is attached to articulate as required. 
         [0071]    The axle lock cylinder  226  is a double-rod steering cylinder modified to function in this manner. The axle lock cylinder  226  has an elongated cylindrical body having end caps at opposite ends of the body. A piston  270  is slidable within the cylindrical body, and defines a forward chamber and a rearward chamber that normally contain hydraulic fluid, the piston  270  having a piston seal around its periphery to prevent fluid flow around the piston  270  between the forward and rearward chambers. Ports are provided on the cylinder body for connecting the forward and rearward chambers to a hydraulic valve  252 , providing hydraulic fluid from a tank or reservoir. 
         [0072]    The axle lock cylinder  226  has a rod assembly  228  that includes a front or forward rod  230  extending from the piston  270  and slidable through a hole in the forward end cap, the hole in the cap having a rod seal or packing to prevent fluid leakage. The front rod  230  is connected directly to the base end  222  of the steering cylinder  216 . The rod assembly also includes a rear of rearward rod  232  extending from the piston  270  opposite the front rod, the rear rod  232  being slidable through a hole in the rear end cap, the hole having a rod seal or packing to prevent fluid leakage. The rear rod  232  is attached to the steering linkage of the trailer  10 , as discussed further below. 
         [0073]    The axle lock cylinder is modified by placing helical compression springs  234  and  236  coaxially around the front rod  230  and rear rod  232 , respectively. The compression springs  234  and  236  are confined between the cylinder&#39;s end caps and annual flanges fixed around the distal ends of the front rod  230  and the rear rod  232 , respectively. The springs  234  and  236  normally bias the piston  270  so that the piston  270  is centered within the cylinder body. 
         [0074]    As shown in  FIG. 12A , the rear rod  232  is pivotally connected to a connector link  238 , which is, in turn, connected to the Pitman arm assembly  240  of the trailer steering linkage. The Pitman arm assembly  240  is immovably affixed (e.g., clamped by conventional bolts, etc.) to the laterally disposed axle  242  of the trailer  210 , the Pitman arm  244  extending therefrom. The Pitman arm  244  is pivotally connected to the tie rod  246 . The opposite ends of the tie rod  246  are pivotally connected to steering arms  248  extending from the steering hubs  250 , which are, in turn, mounted to the axle spindles. The steering hub and axle spindle assembly is shown in detail in  FIG. 16 , and discussed further below. The geometry of the steering linkage illustrated in  FIGS. 12A ,  13 A, and  14 A may be adjusted as required to provide the desired operation, and/or additional idler arms and links, end links, etc. may be incorporated with the above-described trailer steering linkage, as required. The above-described steering system is generally conventional in powered vehicles, but is not found on conventional trailers having directionally fixed wheels, particularly as provided in a kit that may be installed upon an existing trailer having directionally fixed wheels to convert the trailer to a steerable trailer. 
         [0075]    Hydraulic flow to and from the steering and axle lock cylinders  216  and  226  is controlled by a hydraulic control valve  252  that is affixed to the trailer frame  212  or other trailer structure. This control valve  252  is shown in detail in  FIG. 15  of the drawings. The valve  252  is somewhat similar to the hydraulic control valve  70  of  FIG. 9 , but the internal valving is rearranged to provide the required operation of the steering control system of the embodiment of  FIGS. 12A through 16 . A hydraulic tank or reservoir similar to the tank or reservoir  54  provided with the system of  FIGS. 1 through 11 , may be incorporated with the embodiment of  FIGS. 12A through 16  as well. The valve  252  includes a stationary outer body  254  having first through fourth ports  254   a  through  254   d  therein. A rotary inner valve member  256  includes a series of four valve passages  256   a  through  256   d  and four flow blocking ports  256   e  through  256   h  therein, the inner rotary valve passages and blocking ports  256   e  through  256   h  selectively aligning with the four ports  254   a  through  254   d  of the outer body to provide or restrict hydraulic fluid flow, as desired. 
         [0076]    The first through fourth flow blocking ports  256   e  through  256   h  include two-way relief valves  100  therein. The relief valves  100  of the control valve  252  of  FIG. 15  are essentially the same as the relief valve  100  illustrated in detail in  FIG. 8  of the drawings and described further above. It will be recognized that these valves  100  may completely block hydraulic fluid flow therethrough for all practical purposes if sufficient spring pressure is provided. These valves  100  are intended only to allow sufficient flow to provide some movement of the steering assembly and cylinders, e.g., in the event of the system reaching a physical limit (steering stop, etc.), in order to avoid physical damage to the system. These valves  100  completely block fluid flow therethrough under normal conditions. Alternatively, the valves  100  may be replaced with solid bodies to completely block fluid flow therethrough at all times, if so desired. 
         [0077]    The first outer valve port  254   a  is connected to the forward end  224  of the steering cylinder  216  by a first hydraulic line  258 , while the second outer valve port  254   b  is connected to the opposite cylinder base end  222  of the steering cylinder  216  by a second hydraulic line  260 . The third outer valve port  254   c  connects to the forward end of the axle lock cylinder  226 , i.e., the end closest to the steering cylinder, via a third hydraulic line  262 , while the fourth outer valve port  254   d  connects to the opposite rearward end of the axle lock cylinder  226 , i.e., the end closest to the steering linkage, via a fourth hydraulic line  264 . 
         [0078]    When the trailer  210  is being pulled forward, the trailer wheels  266  are locked in their straight ahead orientation, as shown in  FIG. 12A . The hitch bar  16  is also shown square or perpendicular to the longitudinal axis of the trailer  210  in  FIG. 12A , as would be the case when the towing vehicle is traveling straight ahead with the trailer  210  in tow. However, the hydraulic system of the trailer steering mechanism of the embodiment of  FIGS. 12A through 16  allows the rod  218  of the forward or steering hydraulic cylinder  216  to extend and retract according to turns by the towing vehicle, while the trailer wheels  266  remain straight. This is due to the control valve  252  being set as shown in  FIG. 15 , with the selector switch  268  being set for forward operation. (It will be seen that this selector switch  268  may be manually operated, or actuated electrically or by some other remote means of control, as desired.) This aligns the first and second outer valve ports or passages  254   a  and  254   b  with the open first and second inner valve passages  256   a  and  256   b , thus allowing hydraulic fluid to flow back and forth through the control valve  252  to the first and second hydraulic lines  258  and  260 , and thus to the opposite ends of the forward steering cylinder  216 . Accordingly, even when the axle lock cylinder  226  is locked to hold the trailer wheels  266  straight, as shown in  FIG. 12A , the free movement of the steering cylinder rod  218  due to the open fluid passages  254   a ,  256   a  and  254   b ,  256   b  allows the towing vehicle and its attached hitch bar  16  to turn as desired. 
         [0079]    Hydraulic fluid flow to the rearward axle lock cylinder  226  is simultaneously shut off when flow is permitted to the forward steering cylinder  216  as described above. It will be seen in  FIG. 15  that the rotary inner valve third and fourth blocking ports  256   g  and  256   h , and their highly restrictive relief valves  100 , are aligned with the respective outer body passages or ports  254   c  and  254   d . This effectively shuts off all hydraulic fluid flow to the rearward axle lock cylinder  226  through its hydraulic lines  262  and  264 , thus effectively locking the axle lock cylinder  226  to render it immobile. As the rear rod  230  of the axle lock cylinder  226  is connected directly to the steering linkage, the steering linkage cannot articulate or move and is held straight ahead so long as the rod assembly  228  of the axle lock cylinder  226  is neutral. 
         [0080]    When the trailer  210  is to be backed, the selector switch  268  is moved (manually or by electrical or other remotely controlled means) to the backing position, thus rotating the inner rotary valve member  256  forty-five degrees in the clockwise direction. This aligns the first inner valve blocking port  256   e  with the first outer valve body passage  254   a , and the second inner valve passage  256   f  with the second outer valve body passage  254   b . As these two inner valve blocking ports  256   e  and  256   f  have highly restricted flow due to the restrictor valves  100  installed therein, it will be seen that hydraulic flow is essentially cut off to the forward steering cylinder  216  for backing maneuvers, thus hydraulically locking the steering cylinder  216  to prevent extension or retraction of its rod  218 . 
         [0081]    Simultaneously with the above operation, the third inner valve passage  256   c  is rotated to align with the third outer body valve port  254   c , and the fourth inner valve passage  256   d  is aligned with the fourth outer body valve port  254   d . These two inner valve passages  256   c  and  256   d  are completely open, i.e., they have no hydraulic fluid restriction or blockage therein. This allows unimpeded hydraulic fluid flow back and forth through the control valve  252 , and thus through the second or axle lock cylinder  226  to permit the free movement thereof. 
         [0082]      FIG. 13A  provides an illustration of the operation of the trailer backing system when the towing vehicle begins to back to the right. In such a scenario, the trailer  210  will initially move straight to the rear. The front of the tow vehicle swings to the left as its forwardly disposed steering wheels are turned to the right during backing. This will misalign the rear of the towing vehicle and its hitch bar  16  relative to the trailer  210 , generally as shown in  FIG. 13A . When this occurs, the rod assembly  218  of the forward or steering cylinder  216  is driven rearwardly due to the articulation between the hitch bar  16  and trailer  210 , thus driving the hydraulically locked steering cylinder  216  rearwardly as well. 
         [0083]    As the rearward axle lock cylinder  226  is immovably affixed to the trailer structure or frame  212 , the front rod  230  is forced rearward, compressing the spring  234 . This extends the rear rod  232  and expands spring  236 , thereby actuating the steering linkage, as shown in  FIG. 13A , to swing the trailer  210  toward the right, thus aligning the trailer with the towing vehicle as the backing maneuver continues.  FIG. 14A  illustrates the same process with the turning and steering directions being opposite those shown in  FIG. 13A , i.e., with the towing vehicle initially backing toward the left. 
         [0084]    Once the backing maneuver has been completed, the selector switch  268  of the control valve  252  is returned to its forward setting, generally as illustrated in  FIG. 15 . This opens the flow between the two ends of the forward steering cylinder  216  and closes the flow to the rearward axle lock cylinder  226  to lock the axle lock cylinder hydraulically, as described further above. It will be seen that if this is done while the trailer wheels  266  are turned, e.g., as shown in  FIG. 13A  or  14 A, the wheels will remain locked angularly relative to the trailer  210 , and the trailer will not track in a straight line behind the towing vehicle unless some means is provided to overcome this problem. 
         [0085]      FIGS. 12B through 14B  provide elevation views in section of the axle lock cylinder  226  corresponding to the respective trailer steering scenarios illustrated respectively in  FIGS. 12A through 14A . 
         [0086]    The axle lock cylinder  226  has been further modified to include at least one valve shaft  272  therein, with  FIGS. 12B ,  13 B, and  14 B showing two such shafts  272  within the cylinder  226 . The valve shafts  272  extend axially between the opposite end caps of the cylinder  226 , and are immovably affixed relative to the body of the cylinder  226 . The valve shafts  272  pass through corresponding passages  274  through the piston  270 . The valve shafts  272  each have a centrally located seal portion  276  thereon, the seal portion having a diameter essentially equal to that of the passage  274  through the piston  270 . Thus, essentially all hydraulic flow within the cylinder  226  is blocked from the forward chamber to the rearward chamber when the piston  270  is centered within the body of the cylinder  226 , the seal portions  276  being seated within the valve passages  274 , and the control valve  252  is set for forward operation as shown in  FIG. 15 . The seal passages  274  have O-rings or other seal or packing material about their periphery to prevent fluid leakage between the forward and rearward chambers when the seal portions  276  are seated in the passages  274 . 
         [0087]    The valve shafts  272  further have relief portions  278  (e.g., flutes or grooves) extending axially between the seal portion  276  and the ends attached to the end caps of the body of the axle lock cylinder  226 . Thus, when the piston  270  is displaced from the center of the cylinder body, hydraulic fluid may flow along the flutes  278  through the passages  274  in the piston  270  and between the forward and rearward chambers. The flutes or grooved portions  278  of the valve shafts  272  have raised portions of equal diameter to the valve passages  274  to prevent flexure and vibration of the valve shafts within the valve passages  274  when the seal portions  276  are unseated from the valve passages  274 . Alternatively, the valve shafts  272  may have tapered or necked down diameters extending between the seal portions  276  and the end caps, if desired, to allow hydraulic fluid to flow through the valve passages  274  when the seal portions  276  are unseated from the valve passages  274 . 
         [0088]      FIG. 13B  corresponds with the trailer  210  configuration of  FIG. 13A , i.e., with the trailer wheels  266  turned to the right during backing. In this scenario, the control valve  252  of  FIG. 15  would have its selector switch  268  turned to the backing position, thus closing the hydraulic flow through the first and second hydraulic lines to the forward steering cylinder  216  and locking that cylinder and its rod  218  relative to one another. At the same time the third and fourth hydraulic lines  262  and  264  are opened, thereby allowing flow between the control valve and the forward and rearward chambers of the axle lock cylinder  226 . As the hitch bar  16  is turned as shown in  FIG. 13A , the hydraulically locked forward cylinder rod  218  is pushed rearwardly, thus pushing the steering cylinder  216  rearwardly as well. This extends the rear rod  232 , thus actuating the steering linkage to turn the wheels  266  to the right, as shown in  FIG. 13A . Hydraulic fluid flow is also provided through the passages  274 , as the piston  270  is offset from the center of the cylinder body, unseating the seal portions  276  of the valve shafts  272 . 
         [0089]      FIGS. 14A and 14B  correspond with one another as well, but show the trailer wheels  266  as positioned when the trailer  210  is being backed to the left. The control valve  252  is set to the backing configuration, i.e., with the forward steering cylinder  216  hydraulically locked and the rearward axle lock cylinder  226  hydraulically open, as in the case of the right turn shown in  FIGS. 13A and 13B . However, the forward cylinder rod  218  and its hydraulically locked forward or steering cylinder  216  are drawn forward in  FIG. 14A , thus drawing the rear rod  232  forward into the body of the axle lock cylinder  226  and compressing spring  236 . This results in the steering linkage being turned as shown in  FIG. 14A . 
         [0090]    When the backing maneuver has been completed, the selector switch  268  of the control valve  252  is again switched to the forward position, as shown in  FIG. 15 . This opens hydraulic flow to the forward steering cylinder  216 , so the rod  218  may extend and retract freely as the trailer follows the tow vehicle during turning maneuvers while moving forward with the trailer wheels  266  locked in their straight ahead positions as shown in  FIG. 12A . 
         [0091]    This also closes the hydraulic flow to the rearward axle lock cylinder  226 . However, the axle lock cylinder  226  is not hydraulically locked immediately if the wheels of the trailer are not straight and aligned with the towing vehicle, because the piston  270  is not centered and hydraulic fluid is still free to flow through the valve passages  274  in the piston  270 . As the trailer continues to turn and the bias springs  234 ,  236  continue to exert their effect. the axle  242  swings until the trailer wheels are aligned straight behind the tow vehicle, at which time the piston  270  is centered, the seal portions  276  seat in the valve passages  274 , and hydraulic pressure in the forward and rearward chambers lock the trailer axle  242  so that the axle cannot turn again until the trailer is backed again. 
         [0092]    The trailer steering mechanism of the embodiment of  FIGS. 12A through 16  may be provided as a kit for the modification of a conventional trailer with non-steerable wheels to a trailer having steerable wheels, if so desired. The rearward or axle lock cylinder  226  and the control valve  252  are both bolt-on attachments to an existing trailer structure or frame, and the forward or steering cylinder  216  attaches only to the forward or steering cylinder attachment end  230  of the axle lock cylinder rod  228  and to the hitch bar  16 . Only the Pitman assembly  240  of the steering linkage attaches to the axle  242  of the trailer structure, and this attachment may be by means of a bolt-on collar or the like. 
         [0093]    The only other components required to convert a conventional trailer to a steering trailer as shown in  FIGS. 12A ,  13 A, and  14 A, are a pair of steerable hubs  250 , the left one of which is shown in detail in  FIG. 16 .  FIG. 16  shows one end of the axle  242  with the wheel and conventional non-steering hub removed therefrom to expose the spindle  280 . The steerable hub  250  comprises three basic components, i.e., a fixed hub attachment  282  and a steerable wheel carrier  284  with its rotating wheel mounting flange  286 . The wheel carrier  284  is pivotally attached to the hub  282  by conventional kingpins  288  or the like, thus permitting the wheel carrier  284  to swivel or steer relative to the fixed hub attachment  282 . The steering arm  248  shown in plan view in  FIGS. 12A ,  13 A, and  14 A is shown in end view in the elevation view of  FIG. 16 . 
         [0094]    Once the conventional hub has been removed from the spindle  280 , as shown in  FIG. 16 , the steerable hub assembly  250  is placed thereon. The original nut  290  is threaded back onto the threaded end of the spindle to lock the hub  282  thereon, and a cotter pin  292  is installed through the nut  290  and threaded end of the spindle  280 . The tie rod  246  is then attached to the steering arm  248  to complete the task. Conversion of the opposite axle end is accomplished in the same manner, using a mirror image steerable hub assembly to that shown in  FIG. 16 . 
         [0095]    In conclusion, the trailer steering mechanism in its various embodiments provides a means for greatly simplifying the steering or guidance of a trailer during backing maneuvers. The system may automatically switch from a directionally locked mode for forward travel to steering mode for backing maneuvers, or may, alternatively, be manually switched, if so desired. While the trailer steering mechanism is particularly well suited to trailers having a single axle and two wheels, it may be adapted to other types of trailers having multiple axles as well, with little modification. Accordingly, the trailer steering mechanism will prove to be extremely popular with vehicle operators who have need to tow and back a trailer from time to time. 
         [0096]    It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.