Abstract:
A non-hydraulic shimmy damper that can be mounted to existing aircraft landing gear and does not substantially interfere with steering or require significant maintenance. When the shimmy damper requires maintenance it can be readily swapped out as a line-replaceable unit (LRU) thereby reducing aircraft downtime. The shimmy damper is mountable to an aircraft landing gear strut and includes a rotatable coupling member that engages a rotatable steering member of the landing gear strut. A damping assembly is coupled to the rotatable coupling member to provide a damping force. The damping assembly is adjustable so that a desired amount of damping force can be applied and/or maintained.

Description:
PRIORITY CLAIM 
     This application claims priority to provisional patent application No. 61/033,837 filed on Mar. 5, 2008 and is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to landing gear for aircraft. More particularly, the invention relates to a shimmy damper for aircraft landing gear. 
     BACKGROUND OF THE INVENTION 
     Wheel shimmy is a condition in which the landing gear wheel or wheels oscillate from side to side along a straight line parallel to the direction of travel of the aircraft. Wheel shimmy can be caused by a number of conditions such as low torsional stiffness, excessive freeplay in the landing gear, wheel imbalance, or worn parts. Often, however, wheel shimmy occurs even in new aircraft due to resonance between the landing gear and the airframe of the aircraft. This resonance may not be discovered until after a new aircraft is first flown and the design of the landing gear and airframe is well established. Solving a shimmy problem after an aircraft is built presents challenges because it is not usually economical to redesign and replace the landing gear in order to solve the problem. Accordingly, several different concepts have been developed for reducing or eliminating wheel shimmy in existing landing gear. 
     For example, hydraulic shimmy dampers have been used to damp wheel shimmy. Typically, such shimmy dampers consist of a hydraulic shock absorber mounted between components of the landing gear to damp shimmy motions. The hydraulic shock absorber generally consists of a hollow tube filled with oil. A rod and piston move through the fluid to generate velocity-dependent, viscous-damping forces. Such designs require frequent maintenance, and temperature increases can reduce damping efficiency. Further, such shimmy dampers generally do not permit 360 degree rotation of the wheel or wheels and typically are difficult to adjust to provide different levels of damping. 
     Another type of shimmy damper that has been used in the past is a friction-type shimmy damper that mechanically engages a steering collar gear on a landing gear strut. For example, one design includes a belleville spring washer compressed against the steering collar to damp rotation thereof. While such a shimmy damper may be functionally adequate, it is not readily retrofittable to existing landing gear assemblies and installation and/or removal of the belleville spring washers typically requires the landing gear to be substantial disassembled, as many of these types of dampers require the spring washers to be telescoped over the landing gear strut. 
     Hydraulic damping (by addition of a restrictor) is also used as a feature of the many hydraulic steering actuation systems as a method to attenuate shimmy vibration. This method is popular as it adds the least additional weight and is generally effective. In cases where the hydraulic restrictor is not effective due to distance from the vibration source or flexibility of the structural load paths from the actuation system to the vibration, other methods may be required. 
     SUMMARY OF THE INVENTION 
     The present invention provides a non-hydraulic rotary shimmy damper that can be mounted to existing aircraft landing gear and does not substantially interfere with steering or require significant maintenance. When the shimmy damper requires maintenance it can be readily swapped out as a line-replaceable unit (LRU) thereby reducing aircraft downtime. The shimmy damper is adjustable to provide a desired amount of damping for a given application and to permit readjustment of the shimmy damper to account for changes in the aircraft landing gear due to worn parts, wheel imbalance, etc. 
     Accordingly, the present invention provides an aircraft shimmy damper mountable to an aircraft landing gear strut having a steerable member for steering a wheel carried by the landing gear comprising a rotatable coupling member and a damping assembly operatively coupled to the rotatable coupling member for resisting rotation of the rotatable coupling member. The rotatable coupling member is configured to be rotatingly connected to the steerable member such that rotation of the rotatable coupling member rotates the steering member. 
     More particularly, the shimmy damper includes a biasing mechanism for biasing the damping gear into engagement with a steering gear of the landing gear strut. The damping assembly includes a rotation member and at least one friction member, such as a friction pad, biased against the rotation member to resist rotation of the rotation member. The biasing mechanism is adjustable for controlling the bias applied to the rotation member. 
     In accordance with another aspect of the invention, a shimmy damper mountable to an aircraft landing gear having a rotatable steering member for steering a wheel carried by the landing gear comprises a rotation member configured to engage the steering member such that rotation of the rotation member rotates the steering member, and a friction member frictionally engaged with the rotation member to resist rotation of the rotation member. 
     More particularly, an adjustable biasing mechanism for biasing the rotation member against the steering member is provided. In addition, an adjustable biasing member for biasing the friction member against the rotation member is provided and, in an exemplary embodiment, includes a spring washer preloaded against the friction member. 
     In accordance with another aspect of the invention, a method of damping shimmy in an aircraft landing gear is provided comprising biasing a rotatable coupling member against a rotatable steering member of the aircraft landing gear and resisting movement of the rotatable coupling member with a damping assembly. 
     Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational view of an aircraft landing gear including a shimmy damper in accordance with an embodiment of the present invention. 
         FIG. 2  is a partial cross-sectional view taken through the line  2 - 2  of  FIG. 1 . 
         FIG. 3  is a perspective view of a shimmy damper in accordance with an embodiment of the present invention. 
         FIG. 4  is a cross-sectional view taken through line  4 - 4  of  FIG. 3 . 
         FIG. 5  is a partial cross-sectional view of another shimmy damper in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings in detail, and initially to  FIG. 1 and 2 , an exemplary shimmy damper  2  in accordance with the present invention is illustrated installed on an exemplary landing gear  3 . The simplified aircraft landing gear  3  includes a strut  4  having an upper tube  6  that is mounted at an upper end to an aircraft structure (not shown). A lower tube  8  extends from the upper tube  6  and includes a hub  10  mounted thereto to which a wheel assembly (not shown) can be attached. A rotatable steering collar  12 , which can include a collar gear, is mounted to the lower end of the upper tube  6  and is rotatably connected to the hub  10  via torque arm  14  for transmitting steering inputs thereto. A steering motor  16  which typically includes a pinion gear is mounted to the upper tube  6  using a suitable method (not shown and not important to the invention) and a gear cover  18  that encloses the collar and pinion gear providing a lubrication chamber that is sealed from the environment. The aircraft landing gear  3  is shown in simple or outline form, while other structures such as locking mechanisms, retracting mechanisms, and steering mechanisms are not shown in order to avoid obscuring the shimmy damper  2 . Various arrangements of such structures are known in the art and are not critical to the description or understanding of the invention. Further, the shimmy damper  2  can be mounted to an aircraft landing gear in a wide variety of positions and locations other than as shown. 
     In  FIG. 2 , the shimmy damper  2  is mounted for pivotal movement by a pivot connection, which in the illustrated embodiment is a mount bolt  20  and pivot bushing  22  that is received in a counterbore  24  (see  FIG. 3 ) in the gear cover  18 . Accordingly, the shimmy damper  2  is free to slide across the top surface of the gear cover  18 . As will be described in more detail below, a biasing mechanism  26  is secured to the gear cover  18  so as to bias the shimmy damper  2  into engagement with the steering collar  12 . 
     Turning to  FIGS. 3 and 4 , the shimmy damper  2  will be described in detail. The shimmy damper  2  includes a housing  28  having a bottom  30  and a generally cylindrical side wall  32 , which together form an interior cavity. A cover  34  for enclosing the interior cavity is secured with a bolt  36 , and suitable sealing members, such as O-rings  38  and  40 , are provided for sealing the cover  34  to prevent exposure of the interior of the housing  28  to the elements. 
     The housing  28  includes a flat radially extending projection  42  including a mounting hole  44  through which the mount bolt  20  and pivot bushing  22  extend for pivotally securing the shimmy damper  2  to gear cover  18  of the aircraft landing gear  4 . A sealing member, such as O-ring  46 , is provided for sealing the housing  28  to the gear cover  18 . 
     The housing  28  also includes a radially extending projection  48  to which the biasing mechanism  26  is mounted. The biasing mechanism  26  includes a spring  52  and spring retainer and adjustment mechanism  55 . The spring retainer and adjustment mechanism  55  is secured to the gear cover  18  and is configured to compress the spring  52  against the projection  48  for applying a force thereto. Accordingly, when the shimmy damper  2  is mounted to the landing gear strut  4 , the biasing mechanism  26  can be used to bias the shimmy damper  2  into engagement with the gear on the rotatable steering collar  12  of the landing gear strut  4 . 
     Biasing the shimmy damper  2  against the rotatable steering collar generally reduces freeplay therebetween, and can function to maintain a proper amount of gear lash so as to facilitate smooth operation of the shimmy damper  2 . It will be appreciated that freeplay between the shimmy damper  2  and the rotatable steering member  12  generally has the effect of allowing the rotatable steering collar  12  to rotate with little or no influence from the shimmy damper  2  throughout the range of freeplay. Accordingly, if excessive freeplay exists, a shimmy can develop in the landing gear  4  despite the presence of the shimmy damper  2 . Therefore, minimizing freeplay between the gears is generally desirable. Although in the illustrated embodiment the biasing mechanism  26  is provided to control the freeplay between the shimmy damper  2  and the rotatable steering collar  12 , it will also be understood that freeplay between the components can be reduced or eliminated in other ways, and the invention is not limited to the use of the biasing mechanism  26  as illustrated. 
     A shaft  60  extends from the housing  28  through a centrally located opening therein. The shaft  60  is supported for rotation in the cavity of the housing  28  by suitable means, such as upper bushing  65  and lower bushing  70 . A shaft seal  72  is provided for sealing the shaft  60  to the opening in the housing  28 . A rotatable coupling member, such as engagement gear  75 , is secured to the shaft  60  for rotation therewith. The engagement gear  75  has a plurality of teeth  80  for engaging mating teeth of a steering gear of an aircraft landing gear strut, as will be described. Within the housing  28  a rotation member, disk member  85 , extends radially from the shaft  60 . 
     Disk member  85  is supported for rotation on shaft  60 . It will be appreciated that disk member  85  can be a separate member secured to the shaft  60  by suitable means, such as a key. Alternatively, the disk member  85  can be formed integrally with the shaft  60 , as shown. A radially outer portion of the disk member  85  defines a generally flat engagement area  90  against which a pair of friction pads  95  and  100  are configured to press on opposite surfaces thereof. The friction pads  95  and  100  are fixed against rotation within the housing  28  via suitable means, such as one or more mounting pins  105  press fit in bores  107  in the housing  28 . An adjustable cap assembly  110  for preloading the friction pads  95  and  100  against the disk member  85  is threaded into the housing  28  via threads  112 . 
     The adjustable cap assembly  110  includes a cap  115 , a compression ring  117 , a stack of annular spring disks  122 , which may be belleville spring washers, and a spring disk guide  125 . A spring disk spindle  130  extending from the cap  112  extends through the center of each spring disk  122 , and retains the upper bushing  65  for supporting the shaft  60 . Each spring disk  122  contacts the spring disk below at an inner edge or outer edge as shown. Although four spring disks  122  are illustrated, any suitable number can be used depending on the application and the magnitude of preload pressure required. The bottom spring disk  122  contacts the spring disk guide  125 . The spring disk guide  125  contacts the upper friction pad  95  and applies a preload pressure thereto. The friction between the upper friction pad  95 , lower friction pad  100 , and the disk member  85  produced by the pressure applied by the adjustable cap assembly  110  opposes rotation of the disk member  85 . 
     To adjust the preload pressure on the disk member  85 , the cap  115  is rotated clockwise or counterclockwise (e.g., screwed into or out of the housing  28 ) to adjust the compression of the spring disks  122 . It will be appreciated that rotation of the cap  115  results in a linear translation of the cap  115  so as to force the compression ring  117  downward thereby compressing the stack of spring disks  122 . As the spring disks  122  are compressed, they in turn act on the spring disk guide  125  thereby pressing the upper friction pad  95  into engagement with the upper surface of the engagement area  90  of disk member  85 . The shaft  60 , being supported in the housing  28  such that it can shift axially, can shift downward in response to the pressure applied by the upper friction pad  95  while the lower friction pad  100  applies a reactionary force to the lower surface of engagement area  90  of the disk member  85 . 
     It will now be appreciated that the frictional forces between the friction members  95  and  100  and the disk member  85  developed by the damping assembly convert rotational energy into primarily thermal energy for dissipation to the environment. Accordingly, rotation of the shaft  60  and engagement gear  75 , and any other member operatively connected to the engagement gear  75 , can be dampened a desired amount by adjusting the preload pressure of the shimmy damper  2 . 
     It will now also be appreciated the shimmy damper  2  applies a generally constant force that opposes rotation of the rotatable steering member  12  and, consequently, a wheel mounted to hub  10 . As previously described, the magnitude of the opposing force can be adjusted with the adjustable cap assembly  110  to control the amount of preload pressure the friction members  95  and  100  apply to the disk member  85 . Further, as components of the aircraft landing gear  4  or the friction pads and/or disk member  85  of the shimmy damper  2  wear and/or fatigue through operation, the shimmy damper  2  can be re-adjusted to provide suitable damping to prevent shimmying of the aircraft landing gear. 
     Turning now to  FIG. 5 , another embodiment in accordance with the present invention will be described. In this embodiment, the shimmy damper  300  includes a generally cylindrical housing  310  into which a cap assembly  315  is screwed via threads  320 . Although not illustrated in  FIG. 5 , the housing  310  can include one or more mount holes for securing the shimmy damper  300  to a gear cover, and may also include a biasing mechanism for biasing the shimmy damper  300  against a rotatable steering member, as previously described. The housing  310  includes a sealing member  322  for sealing the bottom of the housing  310  to a gear cover. 
     The cap assembly  315  in this embodiment includes a cap  325  having a threaded portion  330  for mating with threads  320  on the housing  310 , a compression ring portion  335 , and an upper portion  340  configured to permit the cap assembly  315  to be screwed into the housing  310 . The upper portion  340  also functions as a cover to enclose the interior of the housing  310 . In the illustrated embodiment, the upper portion  340  includes a hex head for engagement with a suitable tool, such as a wrench, for rotating the cap assembly  315 . Other means for turning the cap assembly  315  can be provided. For example, the upper portion  340  could be provided with a slot for receiving a blade of a screw driver. Alternatively, a hollow open end cylinder could be mounted to the upper portion  340  into which a shaft could be inserted to turn the cap assembly  315 . 
     The cap assembly  315  further includes a pair of spring disks  345  and  350 , which may be belleville spring washers, and a spring disk guide  355 . The spring disk guide  355  has a lip  358  upon which the lower spring disk  350  rests. The spring disk guide  355  is fixed relative to the housing  310  by one or more pins  360  and includes an upper friction pad  365  for engaging a disk member  370  on shaft  375 . 
     Shaft  375  extends from the housing  310  through a centrally located opening in the housing  310 . The shaft  375  is supported for rotation in the housing  310  by suitable means, such as upper bushing  380  which is retained in the cap assembly  315 , and lower bushing  385 . A shaft seal  386  is provided for sealing the shaft  375  to the opening in the housing  310 . A rotatable coupling member, such as engagement gear  390 , is secured to the shaft  375  for rotation therewith. The engagement gear  390  has a plurality of teeth  395  for engaging mating teeth of a steering gear  400  of an aircraft landing gear strut. 
     Disk member  370  is supported for rotation with shaft  375 . As mentioned, disk member  370  can be a separate member secured to the shaft  375  by suitable means such as a key, or can be formed as a portion of the shaft  375 , as shown. A radially outer portion of the disk member  370  defines a generally flat engagement area  410  against which friction pad  365  is configured to engage. 
     Adjusting the shimmy damper  300  illustrated in  FIG. 5  is similar to the manner in which the shimmy damper  2  of  FIGS. 1-2  is adjusted. Accordingly, the amount of damping produced by the shimmy damper  300  can be adjusted by rotating the cap assembly  315  clockwise or counterclockwise to achieve a linear translation of the compression ring portion  335  thereof. As will be appreciated, as the cap assembly  315  is screwed into the housing  310 , the compression ring portion  335  compresses the spring disks  345  and  350  thereby applying more pressure on the disk member  370  via the friction pad  365 . Accordingly, a larger damping force is produced by the shimmy damper  300 . 
     The shaft and/or disk members in the described embodiments can be made of any suitable material, such as steel. Depending on the particular application, one or more coatings can be applied to the disk member. For example, a preferred coating is tungsten carbide applied by a high velocity oxygen fueled technique (HVOF) to the upper and lower surfaces of the disk member. A preferred friction pad material for use in conjunction with a tungsten carbide coated disk member is ARLON 1286, a material manufactured by Greene Tweed. However, other materials can be used for the friction pads such as metallic, semi-metallic, or ceramic friction materials. 
     It will be appreciated that other engagement members and/or additional transmission elements can be used in accordance with the present invention. For example, a chain or belt could be provided for coupling the engagement gear with the steering collar gear. Similarly, a rack might be provided for linking the collar gear and the engagement gear. 
     It will now be appreciated that the present invention provides an adjustable rotary shimmy damper mountable to existing aircraft landing gear. The shimmy damper does not substantially interfere with steering or require significant maintenance. The shimmy damper is adjustable to provide a desired amount of damping for a given application and to permit readjustment of the shimmy damper to account for changes in the aircraft landing gear due to worn parts, wheel imbalance, and other causes of wheel shimmy throughout the service life of the aircraft. 
     Principles of the invention can be applied to other types of landing gear and struts, including landing gear utilizing hydraulic steering members. 
     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.