Abstract:
An in flight automatic lubrication system for periodic lubrication of critical cites in an aircraft landing gear. Pivot joints are lubricated during flight while in an unloaded condition.

Description:
BACKGROUND OF THE INVENTION 
     Modern aircraft landing gear typically comprise a main shock strut having one end pivotally attached to a portion of the airplane for swinging movement of the landing gear between a retracted and extended position. The landing gear also includes a truck beam that is pivotally attached to the other end of the main shock strut, having a plurality of wheels rotatably attached thereon. It is desirable for the truck beam to be pivotally attached to the main shock strut to permit the beam to pivot and absorb the energy associated with traversing a runway discontinuity, as well as permitting the truck beam to be positioned for stowage within the airplane. 
     This pivotal joint along with other landing gear mechanical members are manually lubricated at a plurality of critical sites in accordance with published maintenance schedules. These schedules generally designate the frequency of such lubricative maintenance, the amount of lubricant to be applied, and the noted sites of application. A failure to perform scheduled maintenance can accelerate wear depending on the operation of the aircraft. In situations where an aircraft is operated in non-normal service, such as those aircraft operating on the rough runways characteristic of Eastern Europe, more frequent lubricative maintenance is needed. 
     Aircrafts operating on rough runways experience more aggressive landing gear tress. Premature pivot joint failure can be caused by the heat generated by high frequency truck beam pivoting that is characteristic of rough runway operation. Inspection of failed pivot joints has indicated that the frictional heating adversely affects the metallurgical properties of the associated assemblies causing them to become hard, brittle and susceptible to crack formation leading to ultimate failure. 
     Airplane operators have responded with more frequent lubricative maintenance. However, the additional time required to perform the procedure contributes unfavorably to aircraft productivity. Further, it has been observed that the relubrication of the highly loaded pivot joints typical of an airplane being supported by the extended landing gear is ineffective. In this orientation, the pivot joint bearings are loaded primarily through less than their full bearing circumference. This results in the unloaded portion of the bearing defining a gap 24, through which new lubricants have been found to take the path of least resistance, leaving the loaded portions deficient of lubrication. 
     BRIEF SUMMARY OF THE INVENTION 
     For the foregoing reasons there is a need for an aircraft landing gear autolubrication system that performs periodic lubrication to a plurality of critical sites, without impact to aircraft productivity, and one that can be retrofitted easily to aircraft. The present invention is directed to a method and system that satisfies these needs. According to one aspect of the invention, a method for automatically lubricating an aircraft landing gear is provided. The method includes providing a lubricant supply distribution system for dispensing a lubricant; sensing aircraft landing gear parameters indicative of the landing gear&#39;s extended or retracted positional state; providing a timer to control the duration of the lubrication cycle; measuring intervals of time for the duration of the lubrication cycle, and delivering lubricant from the supply distribution system to lubrication points after determining that the landing gear has experienced a retracted to extended transition, for the duration of a predetermined time interval. 
     According to a second aspect of the present invention, yet another method and apparatus for automatically lubricating an aircraft landing gear is provided. The method includes providing a lubricant supply system for dispensing a lubricant; providing an actuator on the landing gear such that a first end of the actuator that will move relative to and in response to the pivotal movement of the landing gear truck beam; activating the second end of the actuator through movement of the first, and delivering lubricant from the lubricant supply distribution system to one or more selected sites in response to activation of the second end. 
     According to a third embodiment of the present invention, an electromechanical system for automatically lubricating an aircraft landing gear is provided. The system includes a lubricant reservoir; a lubricant pump; a lubricant distribution system that fluidly connects the reservoir to the pump inlet and the pump outlet to at least one desired lubrication point; a means for producing a lubrication signal in response to sensed landing gear position parameters; a lubrication cycle timer for controlling the duration of the lubrication cycle; and a means for activating the pump in response to the lubrication signal for the duration of a predetermined time interval. 
     According to a fourth embodiment of the present invention, yet another electromechanical system for automatically lubricating an aircraft landing gear is provided. The system includes a lubricant supply distribution system for dispensing a lubricant to lubrication critical points; an aircraft landing gear position sensor for indicating the landing gear extended or retracted state; a lubrication cycle timer for controlling the duration of the lubrication cycle; and a control circuit for activating the lubricant supply distribution system in response to determining that the landing gear has experienced a retracted to extended transition, for the duration of a predetermined time interval. 
     According to a fifth embodiment of the present invention, a mechanical system for automatically lubricating an aircraft landing gear is provided. The system includes an actuator associated with the landing gear assembly wherein the actuator that moves relative to and in response to the pivotal movement of the landing gear truck beam, a pump coupled to the actuator, the pump mechanism operable for pumping in response to movement of the actuator relative to a portion of the truck beam; a lubricant supply distribution system coupled to the pump for supplying lubrication to selected locations in response to pump activation. 
     The present invention provides significant technical advantages including some of the following: A technical advantage of the present invention is that lubrication is automatically performed minimizing unfavorable impacts to aircraft productivity. A second technical advantage of the present invention is that the lubrication is applied to unloaded critical sites, thereby maximizing the lubricant penetration into the normally highly loaded portion of the bearings. A third technical advantage of the present invention is that the invention is easily retrofittable onto aircraft. Yet, a further technical advantage of the present invention is that the lubrication cycle is not continuous. Another feature according to an embodiment of the present invention is that electrical power is not necessarily required. Still a further technical advantage of the present invention is that the reservoir is easily filled and has the ability to self purge air out of and bootstrap the rest of the system. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     For a more complete understanding of the present invention and advantages thereof, reference is now made to the following written description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a bottom view of an airplane showing the general location of the landing gear for which the invention is particularly suited; 
     FIG. 2 is a view illustrating landing gear orientation when extended on the ground and retracted in air; 
     FIG. 3 a  is a view illustrating how the pivot joint bearings are loaded in relation to the landing gear&#39;s “ground” extended position, as illustrated in FIG. 2; 
     FIG. 3 b  is a view illustrating how the pivot joint bearings are loaded in relation to the landing gear&#39;s “air” retracted position, as illustrated in FIG. 2; 
     FIG. 4 is a detailed left side view of the landing gear showing the main shock strut, truck beam, and truck beam pivot joint; 
     FIG. 5 is a cross-sectional view of pivot joint illustrated in FIG. 5, further illustrating internal passages for lubrication; 
     FIG. 6 a  is a schematic diagram of the preferred embodiment of the present invention; 
     FIG. 6 b  is a control circuit diagram for FIG. 6 a;    
     FIG. 7 is a right side view of FIG. 4 with the preferred embodiment of the present invention installed; 
     FIG. 8 is a schematic diagram of an alternative embodiment of the present invention; 
     FIG. 9 a  is a cross-sectional view of the “pump” shown in FIG. 8; 
     FIG. 9 b  is a cross-sectional view of an alternative “pump” shown in FIG. 8; 
     FIG. 10 is a view of FIG. 4 with the alternative embodiment of the present invention installed; and 
     FIG. 11 is a cross-sectional view of the reservoirs ( 56 ,  156 ,  56 ) shown in shown in FIGS. 6 a ,  7  and  8  respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the present invention and its advantages are best understood by referring to FIGS. 1-11 of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     Referring to FIG. 1, an airplane  10  is provided with landing gear assemblies  12  for which the present invention is particularly suited. 
     Referring to FIG. 2, the landing gear assembly  12  may be swingingly attached to the airplane  10  for reciprocating movement between an extended position and a retracted position. 
     Referring to FIGS. 3A &amp; 3B, the relative influence of the landing gear assembly&#39;s  12  extended or retracted orientation respectively on the pivot joint assembly&#39;s  20  bearing loading is shown. The pivot joint bearings are typically loaded primarily through less than their full bearing circumference. This results in the unloaded portion of the bearing to define a gap  24 , through which new lubricants have been found to take the path of least resistance, leaving the loaded portions deficient of lubrication. The orientation of this gap  24  is shown in relation to the landing gear assembly&#39;s  12  extended or retracted position. FIGS. 3A &amp; 3B illustrate that it is therefore advantageous to lubricate the pivot joint assembly when the landing gear is unloaded, preferably when the landing gear is retracted. 
     Referring to FIG. 4, the landing gear assembly  12  includes an elongate shock strut  14 , a truck beam  18 , a plurality of axles  22  and a plurality of wheels  26 . For ease of illustration, only the inboard set of wheels  26  are illustrated. However, it will be apparent that a landing gear assembly normally includes a second set of wheels positioned parallel to the inboard set. Additionally, although the present landing gear assembly is shown for purposes of illustration with a four-wheeled truck, other landing gear assemblies, such as a six-wheeled truck may be utilized in practicing the present invention. 
     The shock strut  14  includes a well known telescoping inner  16 A and outer  14  strut cylinders. The inner shock strut cylinder  16 A is axially slidable within the outer cylinder  14 . A shock absorbing mechanism (not shown) is included inside the telescoping inner  16 A and outer  14  cylinders to dynamically react ground loads encountered during landing and taxiing of the airplane. The lower end of the inner cylinder  16 A includes a bifurcated yoke  16 . Rotation of the inner cylinder with respect to the outer cylinder is prevented by upper and lower torsion links  20 . 
     One end of the upper torsion link is pinned to the outer cylinder  14  by an attachment collar  14 A and a fastener, such as a pin assembly. The other end of the upper torsion link is pinned to one end of the lower torsion link by a second well known fastener, such as a pin assembly. The lower end of the lower torsion link is pinned to the forward end of the yoke  16  by a third well known fastener, such as a pin assembly. As assembled, the upper and lower torsion links and are foldably attached to the inner  16  and outer  14 A; cylinders to resist rotation of the inner cylinder  16  relative to the outer cylinder  14 A. 
     The bifurcated portion of yoke  16  is sized to receive the truck beam  18  therein. The truck beam  18  is pivotably attached to the yoke  16  by a pivot joint assembly  20  to maintain the truck beam  18  parallel to the direction of travel of the airplane. The pivot joint assembly  20  permits the truck beam  18  to pivot about the pivot joint assembly  20  in response to a variety of conditions, such as ground loads encountered during taxiing of the airplane or positioning the truck beam  18  for stowage within the airplane. The wheels  26  are rotatably attached to the truck beam  18  by the axles  22 . Although a total of two sets of landing wheels  22  are illustrated in the preferred embodiment, a landing gear assembly having more or fewer sets of wheels, such as four sets or a single set of wheels may be utilized in practicing the present invention. 
     Referring now to FIG. 6A, a first embodiment of the present invention is shown. The embodiment includes a lubricant supply distribution system  53 , a pump  48  and a pump actuation control circuit  54 . 
     A plurality of conduits,  74 ,  76 ,  78 , and  80  interconnect much of lubricant supply distribution system  53 . Lubricant reservoir  56  is fluidly connected to the input of pump  48  by conduit  82 A. The output of pump  48  is fluidly connected to lubrication manifold  52  by conduit  82 B. Lubrication manifold  52  is connected to a first lubrication point  64  by conduit  74 . Lubrication manifold  52  is connected to a second lubrication point  66  by conduit  76 . A third lubrication point  68  is connected to lubrication manifold by conduit  78 . Finally, lubrication manifold  52  is connected to a fourth lubrication point  70  by conduit  80 . It is understood that while the specific embodiment shown in FIG. 5 for purposes of illustration includes four lubrication points  64 ,  66 ,  68  and  70 , more or less lubrication points could be utilized. 
     Referring to FIG. 5, there is shown an expanded cut away view of pivot joint assembly (of FIG. 4)  20  showing the internal lubrication passages leading to critical site  22  (of FIG.  4 ). Also shown are the internal lubrication points ( 64 ,  66 ,  68 ,  70  ) in fluid communication with the lubricant supply distribution system conduits ( 74 ,  76 ,  78 ,  80 ). 
     Referring to FIG. 6B, a pump actuation control circuit  54  for energizing the pump motor  50  is shown. The control circuit includes a control portion and a test portion. The control portion includes a power supply, an air/ground sensing switch S 1 , a Landing Gear Down sensing switch S 2 , and a lubrication cycle duration timer T 1 , all connected in series from the power supply to the pump motor  50 . The test portion of the circuit includes a switch S 3  that is connected in parallel to the control portion, from the power supply to the pump motor  50 . In normal mode operation, commanding landing gear down while the airplane is airborne energizes lubrication pump motor  50  for a period of time controlled by timer T 1 . S 1  is enabled when the aircraft is airborne &amp; provides logic that protects from un-commanded pump activation during spurious aircraft power transients characteristic of engine starts and power bus transfers. S 2  is enabled when the landing gear experiences a gear up to gear down transition. Depending on the airplane, there are many convenient sources for S 1  and S 2 . Further, it is apparent that the control circuit could be configured to energize the pump motor  50  anytime the landing gear is in an unloaded state, without departing from the spirit of the invention. In test mode operation, S 3  bypasses the control portion of the circuit providing power directly from the power supply to the pump motor. S 3  is provided for routine maintenance testing and trouble shooting. The duration of the period should be of sufficient time to lubricate the pivot joint without the production of excess lubricant. Other factors such as the relative pumping power of the pump, the viscosity of the lubricant, the expected operational temperatures and the relative fluidic resistance of the desired lubrication site will influence the duration of the lubrication cycle. It is understood by those skilled in the art that conventional petroleum lubricants have a wider temperature dependant viscosity index change than modern synthetic lubricants. Therefore, a characteristically more viscous lubricant will require a longer duration than a less viscous lubricant. 
     Referring to FIG. 7, the pump  48  and reservoir  56  may be mounted in the aircraft wheel well or at any other convenient location. The pump  48  and reservoir  56  may be integrated into one unitized package or separate components. Reservoir  56  may be located remote from other elements of the lubrication system to provide easy maintenance access or to provide a relatively warmer environment for the lubricant in consideration of lubricant viscosity and extreme temperatures characteristic of aircraft operating environments. When pump motor  50  is energized by control circuit  54 , a lubricant is forced from the reservoir  56  through the pump  48  into the lubrication manifold  52  for distribution to the various lubrication sites. Reservoir  56  may be a pressurized tank or accumulator to provide “boot strapping” the system, and may be sized to have sufficient capacity to require filling only at normal maintenance intervals. 
     Referring to FIG. 11, a preferred pressurized reservoir  56  is shown. The reservoir includes a housing  90  having a stepped chamber defining a piston bore and piston-rod bore. A piston assembly  92  is disposed therein and is free to reciprocate axially within the chamber dividing it into a lubricant reservoir chamber portion  94  and a pressurized chamber portion  96 . The piston has an air exit passage  92   a  extending from the reservoir side piston face, axially through the piston assembly to the piston-rod end. An integral grease relief valve-extend stop  98   b  terminates the piston-rod end of the air exit passage. The passage  92   a  provides fluid communication from the reservoir chamber  94  to the housing&#39;s exterior through the grease relief valve-extend stop  98   b . An endcap  98  terminates one end of the housing defining the reservoir chamber  94  and has a pressure outlet  98   a  and fill inlet  98   b  disposed therein. The reservoir outlet is connected to pump ( 48 ,  148  of FIGS. 6A and 10) via conduit ( 82 A,  182 A). The pressurized chamber portion  96  is connected to the airplanes landing gear hydraulic system. The piston is urged towards the endcap  98 , pressurizing the reservoir  94  in response to the pressurized hydraulic fluid being supplied to the pressurized chamber  96  via the conduit from the aircraft&#39;s landing gear hydraulic system. The position of the piston assembly  92  and therefore the reservoir quantity  94  may be ascertained by visual inspection of the externally exposed piston rod position in relation to the housing  90 . Travel of the piston assembly in either axial direction is limited by the grease relief valve-extend stop  98   b  in one direction and the housing  90  in the other. The reservoir chamber  94  is refilled using the endcap fill inlet  98   b . A check valve within the inlet  98   b  seals the inlet against reservoir pressure during normal operation. A baffle  100  is disposed between the endcap  98  and the housing  90  adjacent to the reservoir chamber  94  to cause the lubricant to enter the reservoir chamber  94  in a level fashion and also allows air to escape out the air exit passage  92   a  through the grease relief valve-extend stop  98   b . The exit of lubricant out the grease relief valve-extend stop  98   b , during the refilling operation, gives a visual indication that the reservoir chamber  94  is full. 
     Various changes ma be made to the present lubrication system invention, for example, electric pump  48  could be replaced by a purely mechanical pump, and advantageously relocated to utilize the relative motion between landing gear components to actuate the pump. One such pump is shown is shown in FIG.  9 A. Referring to FIG. 10, the pump  48  has been replaced by a plunger actuated multicylinder pump  148  and relocated to the landing gear truck beam  18 . The pump actuation control circuit  54  is replaced by a mechanical actuation connecting link  154 . One end of the link is pivotally connected to the bifurcated yoke  16 . The other end is pivotally connected to the pumps  148  external crank arm  148   b . As in a preferred embodiment, lubricant is supplied to the pump from the reservoir using a conduit  182 A. However, check valve  149  is used to prevent the pump from forcing the lubricant back to the reservoir during pump deactivation. 
     In normal operation, when the aircraft is being supported by it&#39;s deployed landing gear  12 , the pump  148  is unpressurized due to the lack of relative motion between the landing gear truck beam  18  and the bifurcated yoke  16 . The lost motion clearance  148   e  between pump driver  148   d  and pump plunger drive plate  148   f  prevents small relative motions between the beam  18  and yoke  16 , that are characteristic of taxiing &amp; takeoff roll, from actuating the pump plungers  148   g  &amp; therefore pressurizing the pump  148 . When the airplane experiences a transition to not being supported by its deployed landing gear  12  such as at liftoff, the landing gear truck beam  18  pivots about pivot joint  20  in response to the influence of gravity and/or a stowage actuator  19 . The relative motion between the truck beam  18  and the bifurcated yoke  16  causes the actuation connecting link  154  to activate the pumps  148  mechanical actuation crank arm assembly ( 154 ,  148   b ,  148   h ,  148   d ), which in turn actuate the multi-cylinder pump plungers  148   g . Lubricant is forced to the desired lubrication sites similar to those of a preferred embodiment of the present invention. 
     Yet another change, for example, may be made to the present in flight autolubrication system wherein the mechanical pump  148  could be replaced by a hydraulically actuated pump. One such pump  248  is shown is shown in FIG.  9 B. In FIG. 9B, a hydraulic piston housing  248   a  and piston  248   b  replaces the mechanical actuation crank arm assembly ( 154 ,  148   b ,  148   h ,  148   d ) shown in FIG. 9A, and is used to actuate the multi-cylinder pump  248  shown in FIG.  9 B. The hydraulic piston housing  248   a  of pump  248  is slaved to a convenient source of hydraulic pressure  254  that is active when the landing gear experiences an extension or retraction transition. A preferred hydraulic source  254  would be the landing gear up actuation system because many landing gear systems use gravity as an extension means to save weight. In either case, it is apparent that the landing gear pivot joint will be advantageously unloaded in either case. 
     Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention.