Abstract:
An apparatus for steering a nosewheel on an aircraft, or the like, while the aircraft is on the ground. The apparatus includes a steering shaft that engages a cluster of gears and transfers torque from a steering command to the gears. The steering shaft is rotatable from a centered position in response to steering torque. A centering mechanism imparts a counter-torque on the cluster of gears to restore the steering shaft to the centered position when steering torque is released. The gear cluster comprises two drive gears that accurately guide the steering shaft to the centered position under the influence of the centering mechanism. The gear cluster is configured to minimize and compensate for gear wear, permitting accurate and consistent centering of the main shaft each time a steering torque is released.

Description:
FIELD OF THE INVENTION 
     The present invention relates to steering systems which accurately and consistently restore a steering control to a selected position when the control is released, and more specifically to a steering system for moving aircraft or the like on the ground in which the aircraft proceeds in a straight direction of travel upon return of the steering control to a centered position. 
     BACKGROUND 
     In the present state of the art, aircraft moving on the ground may be steered by turning the orientation of the aircraft&#39;s nose gear. A nose gear system generally consists of a handle mechanism with position transducers that provides handle position information to the control system that adjusts the nose gear orientation. Mechanical components of the handle mechanism are frequently subject to wear after an extended period of use. Mechanical wear may affect performance of steering systems, especially where components are designed with strict tolerances. In particular, mechanical wear, lost motion and gear backlash (i.e. the amount of play between gear teeth) may disrupt the engagement between the steering control and the nosewheel, resulting in inaccurate nosewheel control and drifting from an intended course. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved steering system. The steering system includes a main steering shaft operable from a centered or neutral position and rotatable in response to a steering force or torque applied to a handwheel mounted on the main shaft. The main shaft cooperates with a cluster of gears, such as spur gears, and a centering mechanism. The gears rotate in response to a steering force, or torque, applied to the main shaft, and the centering mechanism imparts an opposing force, or counter-torque, on the gears to return the main shaft to the original centered position when the steering force is released from the handwheel. The counter-torque is supplied by a biasing element that biases the main shaft toward the centered position so that the aircraft automatically returns to a straight course when the handwheel is released. 
     The present invention may be used in conjunction with a rotational variable differential transformer (RVDT) or other position-sensitive transducer. When the handwheel is turned, the RVDT monitors rotational displacement of the main shaft and converts the shaft&#39;s angular position to an electrical signal. The signal is sent to the aircraft&#39;s navigational system which changes the orientation of the nosewheel in accordance with the orientation of the main shaft. After the handwheel is turned and released, the centering mechanism imparts a counter-torque on the spur gears to return the main shaft and handwheel to the centered position. The RVDT monitors the rotational change in the main shaft and sends a corresponding signal to the aircraft&#39;s navigational system to reorient the nosewheel to a centered position so that the aircraft travels in a straight line. 
     The present steering system returns the main shaft to its centered position accurately and consistently each time the handwheel is released. This ensures that the RVDT reads the proper orientation for centering the nosewheel. The steering system is configured to compensate for mechanical limitations, such as gear wear and gear backlash. In particular, the gears are engaged in a unified or integrated cluster, substantially preventing any gear from slipping or moving independently relative to the other gears. The spur gears are maintained in positive engagement with one another by constant loads caused by handwheel rotation and counteracting loads from the centering, mechanism. Meshed gear teeth contact surfaces do not disengage from one another when the steering direction is changed from one direction to the opposite direction. As a result, gear backlash and gear wear are minimized, allowing the centering mechanism to accurately and consistently restore the main shaft to the centered position. 
     The integrated gears are engaged with one another directly or indirectly, such that the gears rotate and change direction simultaneously as loads on the main shaft change. The gears are engaged at multiple interfaces within the gear cluster, minimizing the effects of wear that may occur at one location. Therefore, worn areas on an individual gear do not disrupt or affect steering accuracy or cause gear slippage. The constant engagement between gears, and the arrangement of gears as a unified integrated gear cluster, compensates for any wear and other mechanical limitations that may be present. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The foregoing summary, as well as the following description, will be better understood when read in conjunction with the figures, in which: 
     FIG. 1 is an isometric view of a steering module for use in aircraft or the like, in accordance with the present invention. 
     FIG. 2 is a fragmented exploded isometric view of the steering module in FIG. 1 illustrating component parts of the steering module. 
     FIG. 3 is an enlarged isometric view of a drive gear used in the steering module of FIG.  1 . 
     FIG. 4 is an enlarged exploded isometric view of the gears used in the steering module of FIG. 1, illustrating torque distribution during a right turn. 
     FIG. 5 is an enlarged exploded isometric view of the gears used in the steering module of FIG. 1, illustrating torque distribution during left turn. 
     FIG. 6 is a schematic side elevational view of an alternative configuration of the handle and main shaft. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1-5 in general, and to FIG. 1 specifically, a steering module is shown and generally designated as  2 . The steering module  2  and its component parts may be used in a variety of steering applications, including steering applications where a self-centering feature is desired. For example, for purposes of this description, the steering module  2  will be described and illustrated as used in an aircraft&#39;s nosewheel steering system. However, the present invention is also applicable to non-aircraft steering systems, such as other vehicle steering systems, including those that benefit from steer-by-wire. Steer-by-wire systems are beneficial when remote or multiple steering locations are dequired, such as construction or dual-steer refuse vehicle, for example. In addition, the present invention is applicable to any system that requires bi-directional manual input, such as to a lever, handle, or wheel, that also requires automatic rotation to a neutral position when the manual input is removed. For example, game controllers and process controls may also make use of the present invention. 
     Returning to FIG. 1, the steering module  2  comprises a handwheel  4  that is rotatable to control the orientation of a nosewheel on the aircraft while the aircraft is taxiing on the ground. The handwheel  4  is operable from a zero degree (0°) or centered position to steer the aircraft in a straight line. Handwheel  4  is linked in rotational engagement with a centering mechanism  6  which is operable to return the handwheel to the centered position after a steering force is applied and released from the handwheel. 
     Referring to FIGS. 1-2, the handwheel  4  is mounted on a main shaft assembly  20 , which is configured to rotate with the handwheel. The centering mechanism  6  is disposed in rotational engagement with the main shaft assembly  20  and is configured to impart a centering or restoring force on the main shaft assembly to bias the handwheel  4  toward the centered position. The main shaft assembly  20  engages a position sensing transducer  80 , which is configured to monitor the orientation of the main shaft assembly and convert the rotational position to an electrical signal. The signal instructs the aircraft&#39;s electrical system to reposition the nosewheel in accordance with the orientation of the main shaft assembly and handwheel  4 . 
     The centering mechanism  6  is configured to precisely return the main shaft assembly  20  and handwheel  4  to the centered position each time a steering force is released from the handwheel. As such, the centering mechanism is operable to consistently provide a centered position reading on the transducer  80  when the handwheel  4  is released, sending an accurate signal to the aircraft to steer the nosewheel in a straight orientation. 
     Referring now to FIGS. 1-3, the nosewheel steering module  2  will be described in greater detail. The centering mechanism  6  is disposed in a housing assembly  10  which comprises a housing base  12  and housing cover  13 . The housing base  12  comprises a generally centrally located aperture  14  adapted to receive the main shaft assembly  20 . In particular, the main shaft assembly  20  is mounted through a bearing  16  mounted in the aperture  14 . Bearing  16  may be a needle roller bearing or other component configured to allow rotational displacement of the main shaft assembly  20  through the housing base  12 . 
     The handwheel  4  and main shaft  20  are configured for rotational displacement between an extreme counterclockwise position and an extreme clockwise position. Preferably, the handwheel is operable through an angular rotation of up to 150° in either direction from the neutral position. Handle rotation beyond 150° in each direction can be achieved by offsetting the handle  4  from the main shaft  20  and adding gearing to reduce main shaft  20  rotation in relation to handle  4  rotation, as shown in FIG.  6 . The handle  4  is mounted to a handle shaft  7  which is oriented generally parallel to the main shaft  20 . An upper steering gear  3  is mounted on the handle shaft  7  so that the upper gear  3  rotates in unison with rotation of the handle shaft  7 . A lower steering gear  5  is mounted on the main shaft  20  so that the lower gear  5  rotates in unison with rotation of the main shaft  20 . The lower steering gear  5  is located on the main shaft  20  at a position so as to mesh or engage with the upper steering gear  3  to couple rotational motion of the handle  4  to the main shaft  20 . An anti-backlash gear can be incorporated into the upper or lower steering gear  3 ,  5  to eliminate the backlash increase from adding these gears. 
     The angular rotation of the handwheel  4  and main shaft  20  may be limited using a variety of structural arrangements. For instance, the main shaft  20  may have a stop pin  22 , as shown in FIG. 2, that rotates in unison with the main shaft  20 . A pair of set screws  18  are inserted in the housing base  12  and extend into the rotational path of the stop pin to engage the stop pin as it rotates. In this way, the range of angular rotation of the main shaft  20  and hand wheel  4  are limited by the set screws  18 . 
     In FIG. 2, the stop pin  22  is shown press fitted into the exterior of main shaft  20 . The aperture  14  in the housing base  12  comprises a semi-circular channel  15  adapted to receive the stop pin  22  and permit rotation of the stop pin within the aperture as the main shaft  20  is rotated in the aperture. The housing base  12  comprises a pair of bores  17  that extend from a top face of the housing base and extend down through the base where they connect with the aperture  14 . Each bore  17  is adapted to receive one of the threaded set screws  18  such that an end of each set screw protrudes into the interior of the aperture  14 . The bores  17  are threaded to engage with the threading on the set screws  18 . As such, the set screws  18  are displaceable within the bores  17  in response to torsional adjustment. The set screws  18  are configured for insertion through the bores  17  and into the interior of aperture  14 , where the screws engage the stop pin  22  to limit rotation of the main shaft  20  and handwheel  4 . The limits of handwheel rotation are adjustable by adjusting the position of the set screws  18  within the bores  17  and the aperture  14 . In particular, the range of handwheel rotation may be decreased by adjusting the screws  18  so that the screws extend farther into the aperture  14 . Similarly, the range of handwheel rotation may be increased by adjusting the screws so that the screws do not extend as far into the aperture  14 . If adjustable stops are not required, then the adjusting screws  18  may be removed and the ends of the aperture will serve as the end stops. 
     The main shaft assembly  20  is configured to connect the handwheel  4  to the centering mechanism  6  and transfer torque between the handwheel and centering mechanism. The main shaft assembly  20  comprises an enlarged diameter section  28  configured for connection with the handwheel. The handwheel  4  is connected to a cylindrical column  36  having a bore  37  adapted to receive the enlarged diameter section  28  of main shaft  20 . The enlarged diameter section  28  of main shaft  20  is secured within the column  36  on handwheel  4  using any of several mounting methods. For instance, in FIG. 2, the enlarged diameter section  28  is shown having a pin hole  29  configured to align with a pin hole  38  on the wheel column  36  when the enlarged diameter section is inserted into the bore  37  in the wheel column. A pin  39  is configured for insertion through aligned holes  29 , 38  to secure the handwheel  4  to the main shaft  20  such that the handwheel and main shaft are integrally connected and rotatable in unison. 
     The main shaft assembly  20  further comprises a reduced diameter section  30  configured to translate torque from the handwheel  4  to the centering mechanism  6 . Centering mechanism  6  comprises an upper drive gear assembly  40  which translates torque applied to the main shaft  20  in a clockwise direction, and a lower drive gear assembly  50  which translates torque applied to the main shaft in a counterclockwise direction. The upper drive gear assembly  40  comprises a drive gear  42  which includes a cylinderical hub  44  and a slot  46  in the hub  44 . Similarly, the lower drive gear assembly  50  comprises a drive gear  52  which includes a cylinderical hub  54  and a slot  56  in the hub  54 . Components of the upper and lower drive gear assemblies each have cylindrical bores configured to align coaxially with one another. Once aligned, the bores are configured to receive the reduced diameter section  30  of the main shaft  20 . 
     Torque applied to the main shaft  20  is translated to the upper and lower drive gear assemblies  40 , 50  by a pair of pin connections. In particular, a first pin hole  32  is machined through the main shaft  20 , and a second pin hole  34  is machined through the main shaft  20  and is aligned longitudinally and radially with the first pin slot. An elongated radial slot  46  is formed through the upper drive gear hub  44  and generally extends through an obtuse angle on one side of the hub  44 , as seen best in FIG.  3 . Similarly, the lower drive gear hub  54  has an elongated radial slot  56  that generally extends through an obtuse angle on one side of the hub  54 . The length of radial slots  46 , 56  control the available rotation of the centering mechanism  6  and may be selected to permit a desired range of gear rotation. Radial slot  56  has a length generally equal to the length of radial slot  46  to permit synchronized rotation and centering of the drive gears  42 , 52 , as will be explained in more detail below. 
     The slots  46 , 56  are configured to align radially with first and second pin holes  32 , 34 , respectively, when the upper and lower drive gear assemblies  40 , 50  are disposed on main shaft  20 . An upper drive pin  47  extends through radial slot  46  in upper drive gear hub  44  and into the first pin hole  32  to fix the longitudinal position of the upper drive gear assembly relative to the main shaft  20 . Similarly, a lower drive pin  57  extends through radial slot  56  in lower drive gear hub  54  and into the second pin hole  34  to fix the longitudinal position of the lower drive gear assembly relative to the main shaft  20 . The upper and lower drive pins  47 , 57  may be connected to the main shaft  20  by press fitting the pins into holes  32 , 34 , respectively. Upper and lower drive pins  47 , 57  are configured to rotate integrally with main shaft  20  in response to torque applied to the handwheel  4  and the main shaft. 
     The components of the centering mechanism  6  will now be described as they would appear when the handwheel  4  is disposed in the centered position. To better illustrate the orientation of each gear, the centering mechanism  6  will be described using the exploded drawing in FIG.  2 . FIG. 2 shows the orientation of each component as it would appear when the handwheel is in the centered position. Radial channels  46 , 56  extend on opposing sides of the main shaft  20 . The drive pins  47 , 57  are disposed at ends in channels  46 , 56  and are configured to transfer torque between the main shaft  20  and drive gear assemblies  40 , 50 , respectively. More specifically, upper drive pin  47  is positioned so as to abut against an end of channel  46  in the clockwise direction such that clockwise rotation of the upper drive pin rotates the upper drive gear  42  and hub  44  in the clockwise direction. Lower drive pin  57  is positioned so as to abut against an end of slot  56  in the counterclockwise direction such that counterclockwise rotation of the lower drive pin rotates the lower drive gear  52  and hub  54  in the counterclockwise direction. 
     The radial slots  46 , 56  allow separate rotation and counter-rotation of the upper and lower drive gears  42 , 52 , respectively. More specifically, radial slot  46  is adapted to allow upper drive pin  47  to rotate counterclockwise within the slot  46  during counterclockwise rotation of the main shaft  20 , such that the upper drive pin  47  does not impart counterclockwise torque on the upper drive gear hub  44  and thus upper drive gear  42 . In other words, pin  47  rides in slot  46  rather than turning the upper gear  42  during counterclockwise rotation of the main shaft  20 . 
     Likewise, radial slot  56  is adapted to allow lower drive pin  57  to rotate clockwise within the slot  56  during clockwise rotation of the main shaft  20 , such that the lower drive pin  57  does not impart clockwise torque on the lower drive gear hub  54  and thus lower drive gear  52 . In other words, pin  57  rides in slot  56  rather than turning the lower drive gear  52  during clockwise rotation of the main shaft  20 . The upper and lower drive gear hubs  44 , 54  slidably engage the reduced diameter section  30  of main shaft  20 . As such, upper drive gear  44  and hub  42  rotate freely relative to main shaft  20  when counterclockwise torque is applied to the main shaft  20 , and lower drive gear  54  and hub gear  52  rotate freely relative to the main shaft  20  when clockwise torque is applied to the main shaft  20 . 
     A biasing gear assembly  60  is disposed on a biasing gear axle  62  in proximity to the main shaft  20 . The biasing gear assembly  60  is configured to restore the main shaft  20  and handwheel  4  to the centered position by imparting a restoring force or counter-torque on the drive gear assemblies  40 , 50 . The biasing gear axle  62  is supported by the housing  10  and is configured to rotate freely within the housing  10 . More specifically, a pair of coaxial apertures  63 , 65  disposed in the housing base  12  and housing cover plate  13 , respectively, are adapted to receive the ends of biasing gear axle  62  and support the axle  62  in a position generally parallel to the main shaft  20 . Biasing gear assembly  60  comprises a biasing gear  66  and a biasing element  64  configured to exert a restoring force through the biasing gear  66 . 
     The biasing gear assembly  60  can be formed using a number of different configurations. In addition, biasing element  64  may be any type of energy storing component, such as a spring or a piston. In FIG. 1, the biasing gear assembly  60  is shown mounted on axle  62  generally parallel to the main shaft  20 . The restoring force is provided by a torsion spring  64  which circumscribes the axle  62 . The torsion spring  64  is preferably comprised of a resilient non-corrosive material, such as a steel alloy. One end of the torsion spring  64  is fixed to the biasing gear  66 , and the opposite end of the torsion spring is fixed to a stationary gear  67  which is maintained in a fixed position. 
     The biasing gear  66  is configured to twist or wind up the torsion spring  64  when the handwheel  4  is turned. More specifically, the biasing gear  66  cooperates directly or indirectly with the upper and lower drive gear assemblies  40 , 50  and rotates in response to rotational displacement of the upper and lower drive gears  42 , 52  when torque is applied to the main shaft  20 . One end  82  of the torsion spring  64  engages the biasing gear  66  and rotates integrally with the biasing gear. The other end  84  of the torsion spring  64  engages the stationary gear  67  so as to remain generally fixed relative to the first end  82 . As such, the torsion spring  64  is configured to wind radially in response to torque transferred to the biasing gear  66  and the first end  82  of the spring from the upper and lower drive gear assemblies  40 ,  50 . The torsion spring  64  is operable to supply a counteracting force or counter-torque capable of reversing the upper and lower drive gears  42 , 52  and restoring the main shaft  20  and handwheel  4  to the centered position. More specifically, the resilient property of the torsion spring  64  is sufficient to reverse the rotation of the biasing gear  66  and apply a counter-torque to the upper and lower drive gear assemblies  40 , 50  to return the main shaft  20  to the centered position. 
     Referring again to FIGS. 1 and 2, the configuration of the drive gear assemblies  40 , 50  and biasing gear assembly  60  will be described in more detail. The upper drive gear  42  is longitudinally positioned on the main shaft  20  so as to mesh or engage with the biasing gear  66  on the biasing gear axle  62 . The torsion spring  64  is configured to apply a preload or bias force on the biasing gear  66  and upper drive gear  42  to urge the main shaft  20  into the centered position. The biasing gear  66  is configured to rotate in the counterclockwise direction in response to torque transferred from the upper drive gear assembly  40 . 
     The torsion spring  64  may be a standard close-wound torsion spring configured for winding in one direction. More specifically, the torsion spring  64  may be configured to wind up only in response to counterclockwise rotation of the biasing gear  66 . When the upper drive gear  42  is rotated in the clockwise direction, the direct engagement between the upper drive gear and the biasing gear  66  causes the biasing gear to rotate in the counterclockwise direction to wind up the torsion spring  64 . A direct engagement between the lower drive gear  52  and the biasing gear  66  would cause the biasing gear  66  to rotate in the clockwise direction, not the counterclockwise direction. Therefore, a mechanism is provided to reverse the direction of torque imparted by counterclockwise rotation of the lower drive gear  52 . 
     In FIG. 2, an idler gear assembly  70  is shown generally parallel with the centering mechanism  6 . The idler gear assembly  70  is operable to reverse the direction of torque imparted by the lower drive gear  52  and transfer torque to the biasing gear  66  such that the biasing gear  66  rotates counterclockwise to wind up the torsion spring  64 . The idler gear assembly  70  comprises an axle  71  that is supported by the housing, similar to the biasing gear axle  62 . A pair of coaxial apertures  73 , 75  disposed in the housing base  12  and housing cover plate  13 , respectively, are adapted to receive the ends of idler gear axle  71  and support the axle  71  in a position generally parallel to the main shaft  20 . A first idler gear  72  is mounted on axle  71  and meshes with the lower drive gear  52 . The first idler gear  72  is configured to rotate clockwise in response to counterclockwise rotation of the lower drive gear  52  during counterclockwise rotation of the main shaft  20 . A second idler gear  74  is mounted on the idler axle  71  coaxially with and longitudinally offset from the first idler gear  72 . The idler gears  74 ,  72  are of one piece or mechanically joined as by brazing or similar process so that both rotate together freely about axle  71 . As such, the second idler gear  74  rotates clockwise in response to clockwise rotation of the first idler gear  72 . The second idler gear  74  is further configured to transfer torque to the biasing gear assembly  60  to wind up the torsion spring  64  when the main shaft  20  is rotated counterclockwise. More specifically, the second idler gear  74  rotatably engages the biasing gear  66  such that clockwise rotation of idler gears  72 ,  74  imparts torque on the biasing gear  66  to rotate the biasing gear  66  in the counterclockwise direction. 
     Based on the foregoing, the centering mechanism  6  is configured such that biasing gear  66  rotates counterclockwise in response to either clockwise rotation or counterclockwise rotation of the main shaft  20 . The cooperation between the individual gears is illustrated visually in FIGS. 4 and 5. FIG. 4 illustrates the cooperation of the gears when the handwheel  4  is turned from the centered position to the right; i.e. when the main shaft  20  is rotated clockwise. FIG. 5 illustrates the cooperation of gears when the handwheel  4  is turned from the centered position to the left; i.e. when the main shaft  20  is rotated counterclockwise. The letter “T” in each Figure represents the torque transferred from the main shaft  20  to the drive gear being loaded. The dashed lines and arrow heads represent the path in which torque is transferred throughout the centering mechanism  6 . The curved arrows represent the direction of rotation of the individual gears. 
     Referring to FIG. 4, a clockwise torque “T” applied to upper drive gear  42  acts directly on the biasing gear  66  and causes the biasing gear  66  to rotate counterclockwise to wind up the torsion spring  64 . The biasing gear  66  engages the second idler gear  74  and causes the first and second idler gears  72 ,  74  to rotate clockwise. The first idler gear  72  causes the lower drive gear  52  to rotate counterclockwise. Referring to FIG. 5, a counterclockwise torque “T” applied to the lower drive gear  52  is reversed through the idler gear assembly prior to reaching the biasing gear  66 . More specifically, the counterclockwise torque causes the first and second idler gears  72 , 74  to rotate clockwise. The second idler gear  74  causes the biasing gear  66  to rotate counterclockwise to wind up the torsion spring  64 . It should be apparent from FIGS. 4 and 5 that the drive gears  42 ,  52 , idler gears  72 ,  74 , and biasing gear  66  are configured to rotate in the same direction regardless of the direction of torque applied to the handwheel  4  and main shaft  20 . Therefore, steering force applied to the handwheel  4  from the centered position loads the gears in the same direction, regardless of the direction of steering. 
     As stated earlier, the centering mechanism  6  is configured to restore the main shaft  20  to the centered position when steering force is released from handwheel  4 . The biasing gear assembly  60  is configured to engage the upper drive gear assembly  40  and idler gear assembly  70  and impart a counter-torque that reverses the rotation of the upper drive gear  42  and lower drive gear  52 . Therefore, the biasing gear assembly  60  is operable to impart rotation of the various gears in directions that are reverse to those shown in FIGS. 4 and 5. 
     The torsion spring  64  is capable of providing torque greater than the minimum torque required to restore the main shaft  20  to the centered position after steering force is released from the handwheel  4 . That is, the design torque exceeds the minimum torque required to overcome mechanical limitations such as friction losses between the drive gear hubs  44 , 54  and the main shaft  20 . The excess design torque retains handwheel  4  in the centered position and resists shimmy or movement, as explained below. A design torque that is fifteen percent (15%) greater than the minimum required torque is sufficient to restore and stabilize the main shaft  20  in the centered position. 
     The radial slots  46 , 56  are configured to engage the drive pins  47 , 57  in an opposing manner to return the main shaft  20  to the centered position. In particular, the biasing gear assembly  60  is configured to rotate the upper drive gear  42  counterclockwise such that an end wall of upper radial slot  46  imparts a counterclockwise torque load on upper drive pin  47 . Similarly, the biasing gear assembly  60  is configured to rotate the lower drive gear  52  clockwise such that an end wall of lower radial slot  56  imparts a clockwise torque load on lower drive pin  57 . The upper and lower drive gear hubs  44 , 54  are positioned relative to drive pins  47 , 57  such that the pins are loaded in opposite directions when the main shaft  20  is restored to the centered position. More specifically, the upper drive pin  47  engages the end of the upper gear slot  46  and the lower drive pin engages the end of the lower gear slot  56  at the point that the main shaft  20  is restored to the centered position. The synchronized loading on the upper and lower drive pins  47 , 57  in opposite directions provides a “hard stop” effect on the main shaft  20  that releasably retains the handwheel  4  in the centered position. As stated above, the adjusted torque of the torsion spring  64  is greater than the minimum torque required to return the main shaft  20  to the centered position. However, the main shaft  20  is prevented from rotating clockwise or counterclockwise past the centered position by the equal and opposite loads on the drive pins  47 , 57 . The equal and opposite loading return the shaft to the centered position without appreciable drift. 
     The centering mechanism  6  is configured to compensate for wear on mechanical components. In this way, wear that occurs on an individual gear does not affect the accuracy of the centering mechanism  6  and the resulting input to the RVDT. The various gears in the centering mechanism  6  are under constant load in one direction by steering forces, and under constant load in the opposite direction by the torsion spring  64 . The constant loading on the gears maintains engagement between cooperating gear teeth. The various gears are configured to connect at various points of engagement and rotate simultaneously. Therefore, the gears do not disengage by virtue of the constant load from steering forces and from the torsion spring  64 , and independent motion of any one gear apart from the other gears is substantially prevented. The multiple points of engagement between gears, and the integrated arrangement that controls motion of all the gears at one time, minimizes the effects that a damaged gear tooth could have on the rest of the centering mechanism. As a result, the centering mechanism minimizes wear of mechanical components that may occur over an extended period of time. 
     The design torque of the torsion spring  64  can be selected to restore the main shaft  20  at various rates of return. The speed at which the torsion spring  64  restores the main shaft  20  and handwheel  4  to the centered position may affect the operational feel of the steering module  2 . Frequently, a dampened or slowed rate of return is desirable to avoid excessive “jerk” in the handwheel  4  when the handwheel  4  is released. Therefore, a dampener may be provided to control the handwheel return rate and improve the operational feel of the steering module  2 . For instance, a viscous dampener may be mounted on a shaft adjacent to the main shaft  20  and cooperatively engage a gear on the main shaft  20  to control the rate at which the main shaft  20  is restored to the centered position. The damper could also be a dynamically controlled unit that would change the return rate and force, to rotate the wheel based on electrical input from the vehicle control system. 
     If desired, a mechanism is provided to adjust the amount of pre-load or bending resistance in the torsion spring  64 . In FIG. 2, a torsion spring adjuster  90  is shown mounted in proximity to the biasing gear assembly  60 . The spring adjuster  90  comprises a bracket  91  and a set screw  92  disposed within the bracket. The set screw  92  is configured to adjust the degree of initial angular deflection in the torsion spring  64  by adjusting the orientation of the stationary gear  67  and second end of the torsion spring  64 . More specifically, the set screw  92  has a plurality of threads that are configured to rotatably engage the stationary gear  67 . The set screw  92  engages the stationary gear  67  at an angle perpendicular to the biasing gear axle  62 . The head of the set screw  92  comprises a hex fitting  94  and is operable to rotate the set screw  92  within the bracket  91 . The threads on the set screw  92  cooperatively engage the gear teeth on the stationary gear  67  so as to impart a rotational force on the stationary gear  67  when the set screw  92  is rotated. The stationary gear  67  is configured to rotate through a small angle of rotation in response to rotational adjustment of the hex fitting. As such, rotational adjustment of the stationary gear  67  alters the position of the second end of the spring  64  relative to the first end, which changes the available bending stress and bias force in the spring  64 . The hex fitting  94  and set screw  92  may be rotated clockwise or counterclockwise using an allen wrench or other suitable implement to increase or decrease the bending resistance and bias force in the spring  64 . 
     Operation of the nosewheel steering module  2  will now be described. The handwheel  4  is initially maintained in the centered position by the torsion spring  64 , as stated earlier. That is, the torsion spring  64  imparts torque through the centering mechanism  6  which exerts a load on the upper drive pin  47  and an equal and opposite load on the lower drive pin  57 . The opposing loads on the drive pins  47 , 57  maintain the main shaft  20  in the zero position and produce moderate resistance to rotational displacement out of the zero position. To overcome the resistance produced by the torsion spring  64 , a minimal steering force, or “breakout force”, is applied to the handwheel  4  to steer the handwheel  4  out of the centered position. To turn the nosewheel right, the breakout force is applied to the handwheel  4  in the clockwise direction. To turn the nosewheel left, the breakout force is applied to the handwheel  4  in the counterclockwise direction. 
     For purposes of this description, the described operation will begin with a right turning of the nosewheel. The handwheel  4  is rotated clockwise from the centered position by applying a clockwise breakout force. Torque is produced on the handwheel column  36  and transferred to the enlarged diameter section  28  of the main shaft  20  through the pin connection  39 . Torque is further transferred to the reduced diameter section  30  of the main shaft  20  and the upper drive pin  47 . As the main shaft  20  rotates in the clockwise direction, the RVDT reads the change in angular position of the main shaft  20  and sends an electrical signal to the aircraft&#39;s navigational system to rotate the nosewheel to a corresponding position to the right. 
     As the main shaft  20  rotates clockwise, the upper drive pin  47  contacts the end of the radial channel  46  and drives the cylinder  44  and upper drive gear  42  clockwise. As the upper drive gear  42  rotates clockwise, the engagement between the upper drive gear  42  and biasing gear  66  causes the biasing gear  66  to rotate in the counterclockwise direction, as illustrated in FIG.  4 . The first end  82  of torsion spring  64  rotates counterclockwise with the biasing gear  66  and deflects through a counterclockwise angle of rotation relative to the second end  84  of the spring  64 , which remains stationary. As the first end  82  deflects relative to the second end  84 , the spring  64  is wound up on the biasing gear assembly  60 . 
     As the biasing gear  66  rotates counterclockwise, the engagement between the biasing gear and the second idler gear  74  causes the second idler gear  74  to rotate clockwise. Torque on the second idler gear  74  is transferred to the first idler gear  72 , causing the first idler gear  72  to rotate clockwise. Clockwise rotation of the first idler gear  72 , in turn, rotates the lower drive gear  52 . At the same time, the lower drive pin  57  rotates clockwise within the lower gear radial channel  56 . Clockwise rotation of the handwheel  4  continues in response to a clockwise steering force until the stop pin  22  on main shaft  20  engages one of the set screws  18  in the housing base aperture  14 . At this point, the handwheel is disposed in the extreme clockwise position, and further clockwise rotation of the handwheel is prevented by the engagement between the stop pin  22  and the set screw  18 . The channel  56  is sufficiently long so that the lower drive pin  57  moves freely through the channel during clockwise rotation of the main shaft  20  and does not contact the end of the channel as the handwheel  4  is rotated toward the extreme clockwise position. 
     To steer the nosewheel back towards the centered position, clockwise rotation of the handwheel is ceased, and a counterclockwise torque may be applied to the handwheel  4 . As the direction of applied torque is reversed, the cooperating gears in the centering mechanism  6  remain positively engaged under load from the torsion spring  64 , without lost motion or gear slippage. The directions of rotation of the various gears are reversed at the same time. Counterclockwise torque on the handwheel  4  produces a counterclockwise rotation of the main shaft  20 . The RVDT reads the change in angular position of the main shaft and sends an electrical signal to the aircraft&#39;s navigational system to rotate the nosewheel to a corresponding position to the left. As the handwheel  4  is turned back toward the centered position, the torque applied to the handwheel  4  is aided by the bias from the torsion spring  64 . 
     After the handwheel  4  reaches the centered position, additional counterclockwise torque on the handwheel  4  rotates the nosewheel left of the centered position, directing the aircraft in a left turning pattern. At this point, torque applied to the handwheel  4  works against the bias of the torsion spring  64 , as in the scenario when the handwheel  4  is turned to the right from the centered position. Therefore, a counterclockwise breakout force must be applied to the handwheel  4  to turn the nosewheel to the left from the centered position. Counterclockwise torque on the handwheel  4  is transferred to the enlarged diameter section  28  of the main shaft  20  through the pin connection  39 . Torque is further transferred to the reduced diameter section  30  of the main shaft  20  and the lower drive pin  57 . As the main shaft  20  rotates in the counterclockwise direction, the RVDT reads the change in angular position of the main shaft  20  and sends an electrical signal to the aircraft&#39;s navigational system to rotate the nosewheel to a corresponding position to the left. 
     Rotating the main shaft  20  counterclockwise drives the lower drive pin  57  against the end of the radial slot  56 , thereby rotating the hub  54  and lower drive gear  52  counterclockwise. As the lower drive gear  52  rotates counterclockwise, the engagement between the lower drive gear  52  and first idler gear  72  causes the first idler gear  72  to rotate in the clockwise direction, as illustrated in FIG.  5 . The clockwise torque in the first idler gear  72  is transferred to the second idler gear  74 , thereby rotating the biasing gear  66  counterclockwise. As in the right turn scenario, the counterclockwise rotation of the biasing gear  66  winds up the torsion spring  64 . 
     As the biasing gear  66  rotates counterclockwise, the direct engagement between the biasing gear and the upper drive gear  42  causes the upper drive gear  42  to rotate clockwise. At the same time, the upper drive pin  47  rotates counterclockwise within the upper gear radial slot  46 , in response to the counterclockwise torque on the main shaft  20 . Counterclockwise rotation of the handwheel  4  continues until the stop pin  22  on main shaft  20  engages a set screw  18  in the housing base aperture  14 . At this point, the handwheel is disposed in the extreme counterclockwise position, and further counterclockwise rotation is prevented by the engagement between the stop pin  22  and one of the set screws  18 . The slot  46  is sufficiently long so that, as the handwheel  4  is rotated toward the extreme counterclockwise position, the upper drive pin  47  moves freely through the channel during counterclockwise rotation of the main shaft  20  and the upper drive pin  47  does not contact the end of the channel. 
     Release of steering force from the handwheel  4  returns the main shaft  20  to the centered position, at which point the aircraft is directed from a left turning pattern to a straight line. More specifically, when steering force is released from the handwheel  4 , the bias force of the torsion spring  64  is no longer overcome, and the torsion spring  64  is free to unwind and release stored energy to the biasing gear  66 . As the spring  64  unwinds, the stored energy in the spring  64  deflects the first end  82  of the spring through a clockwise angle and causes the biasing gear  66  to rotate clockwise. Clockwise rotation of the biasing gear  66  imparts a counterclockwise rotation on the first drive gear  42  and the second idler gear  74 . Counterclockwise rotation of the second idler gear  74 , in turn, imparts counterclockwise torque and rotation on the first idler gear  72  also. 
     Having facilitated a left turning pattern, the main shaft  20  is disposed in the counterclockwise direction with the lower drive pin  57  engaging an end of the radial slot  56 . Counterclockwise rotation of the first idler gear  72  rotates the lower drive gear  52  and hub  54  clockwise. As a result, the end of radial slot  56  engages the lower drive pin  57  and rotates the lower drive pin  57  clockwise. Clockwise rotation of the lower drive pin  57  rotates the main shaft  20  and upper drive pin  47  in the clockwise direction. At the same time, the upper drive gear  42  rotates counterclockwise in response to rotation of the biasing gear  66 . The upper drive pin  47  meets an end of the upper radial slot  46 , stopping further rotation of the main shaft  20 . At this point, the main shaft  20  is disposed in the centered position and is retained in equilibrium by the opposing forces imposed on the upper and lower drive pins  47 , 57 . The angular orientation of the main shaft  20  is read by the RVDT  80 , and a signal is sent to the aircraft&#39;s navigational system to turn the nosewheel accordingly. More specifically, the centered position of the main shaft  20  provides a centered reading on the RVDT, and the RVDT sends a signal to the navigational system to turn the nosewheel in a straight line orientation. 
     The terms and expressions which have been employed are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized, therefore, that various modifications are possible within the scope and spirit of the invention. For example, the references to clockwise and counterclockwise orientations in the foregoing description and drawings are intended to illustrate one embodiment of the present invention, and are not intended to represent the only configuration that is contemplated for the present invention. The replacement of clockwise references with counterclockwise references in the foregoing description and drawings, and vice versa, may be done without changing the spirit of the invention. Accordingly, the invention incorporates variations that fall within the scope of the following claims.