Patent Publication Number: US-2003235499-A1

Title: Multi focus hemi-spherical elastic bearing

Description:
BACKGROUND OF THE INVENTION  
       [0001] The elastomeric bearing system of the present invention relates to an elastomeric bearing system, and more particularly to a multi-focus hemi-spherical elastic bearing having a series of hemi-spherical bearings each rotating about a different focal point.  
       [0002] Bearingless or “flexbeam” rotor systems require resilient load carrying members between the flexbeam and its surrounding torque tube. The load carrying members position the flexbeam and the attached rotor blade spar for pitch change, flapping and lead/lag motion about the intersection of the pitch change and flapping axes.  
       [0003] The load carrying members are typically elastomeric bearings known as snubber/dampers which include vertically stacked arrangements of elastomeric laminates to center the torque tube about the flexbeam while allowing flapping, pitch and lead/lag motions. Spherical bearings or “snubbers” accommodate pitch change and flapping rotation (as well as a small amount of lead/lag rotation) while flat layers accommodate lead/lag linear motions and some radial (spanwise) motion.  
       [0004] The snubber/dampers are located between the flexbeam spar and the torque tube under a preload so that the elastomer laminates thereof remain in compression throughout the full range of articulation as the elastomeric laminates may fail under tension. The snubber/dampers are commonly mounted through a clearance opening in the torque tube and attached through an opening in the flexbeam spar. The snubber/dampers are axially preloaded by a shimming procedure. Preloading reduces the free height of the elastomeric stack while pre-stressing the torque tube. Although highly effective, difficulties arise with conventional bearingless rotor systems.  
       [0005] As the blade lead/lags, the preload leads/lags which generates high bending load moments. The bending load moments may overcome the compressive preload and produce tension in the elastomeric bearing arrangement. Tension is detrimental to elastomeric laminates as tension operates to delaminate the elastomeric bearing arrangement. As lead/lag motion increases, the preload is further reduced which thereby further compounds this effect.  
       [0006] Consideration must also be provided for the size of the elastomeric bearing in relation to the accommodation of loads and motions involved in flight as designs which meet desired flight envelope capabilities may not be readily contained within the torque tube. Simply increasing the torque tube size would undesirably increase rotor system weight and drag.  
       [0007] Accordingly, it is desirable to provide a bearingless rotor system which overcomes these difficulties while improving the fatigue life of the elastomeric snubber/damper bearing.  
       SUMMARY OF THE INVENTION  
       [0008] The multi-focus elastomeric bearing system according to the present invention provides a plurality of hemi-spherical bearing elements arranged in series. The hemi-spherical bearing elements each rotate about a respective focal point.  
       [0009] Snubber bearings allow a bearingless rotor torque tube to pitch, flap, and lead/lag rotate about a fixed point on a flexbeam. Such a snubber is often used in conjunction with a lead/lag damper, and they provide a reaction path to the flexbeam for pitch link forces, rotor flap shears, and damper forces. The pitch motions for a main rotor application are typically 10+/−20 degrees; flap motions are typically 4+/−8 degrees, and lead/lag motions 1+/−3 degrees.  
       [0010] For the outermost hemi-spherical bearing elements, pitch link load, flap shear load, and snubber preload act normal to the elastomer surface (i.e. axial load), and the damper load acts perpendicular to this normal (i.e. radial load). As the bearing rotates, these forces rotate as well, maintaining their direction of action on the outer rubber layer. The outer hemi-spherical bearing element experiences these loads and it is necessary that the compression-induced shear stress due to the axial component of load exceeds the tension-induced shear stress due to the radial component of the load. For a given layer radius, this requirement defines the minimum wrap-around angle required to ensure that the elastomer layer does not go into tension.  
       [0011] For the relatively fixed inner hemi-spherical bearing elements, the pitch link load, flap shear load, snubber preload, and damper load rotate with the bearing outer race, changing the direction of action of these forces, producing a much higher component of radial load relative to axial load. This requires that the inner hemi-spherical bearing elements have a larger wrap-around angle to carry the radial load without the tension induced shear stress due to the radial load overcoming the compression induced shear stress due to the axial load. It is also advantageous for the inner hemi-spherical bearing elements to have a minimum radius.  
       [0012] In one bearing system according to the present invention, the inner hemi-spherical bearing rotates about a focal point which is X distance above a pitch change axis while the outer hemi-spherical bearing rotates about a focal point which is X distance below the pitch change axis. Because both bearings have the same stiffness, each bearing will rotate the same amount and in series. The bearing system thus has an effective rotational center along the pitch change axis. The inner hemi-spherical bearing has a greatly reduced radius and high wrap-around angle, while the outer hemi-spherical bearing has an increased radius and reduced wraparound which provides a more effectively tailored bearing system.  
       [0013] In one bearing system according to the present invention, a third hemi-spherical bearing is provided in series between the inner and outer hemi-spherical bearing. This bearing system provides a more gradual transition between the inner and outer hemi-spherical bearings and also provides the transition from small radius/large wraparound to larger radius/reduced wraparound that is consistent with the loads that are typically applied to a hemi-spherical bearing.  
       [0014] The present invention therefore overcomes difficulties associated with conventional elastomeric bearings while providing an increase in the elastomeric bearing fatigue life.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows:  
     [0016]FIG. 1 is a general perspective view a flexbeam rotor system having a elastomeric bearing system according to the present invention;  
     [0017]FIG. 2 is a side view of the flexbeam rotor system;  
     [0018]FIG. 3 is a is a sectional view of the rotor blade of FIG. 2 taken along the line  3 - 3 ;  
     [0019]FIG. 4 is a general perspective view of the elastomeric bearing system;  
     [0020]FIG. 5 is a schematic view of the elastomeric bearing system illustrating an articulated position;  
     [0021]FIG. 6 is a schematic view comparing elastomeric bearing system radius relative to a focal point location;  
     [0022]FIG. 7 is a sectional view of a multi-focus elastomeric bearing system according to the present invention; and  
     [0023]FIG. 8 is a sectional view of another multi-focus elastomeric bearing system according to the present invention; and 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0024]FIG. 1 illustrates a general perspective view of a flexbeam rotor system  10  which includes a drive shaft  12  which is driven in conventional fashion by an engine  14 , typically through reduction gearing (not shown), for rotation about an axis of rotation  16  (FIG. 2). A rotor hub  18  is mounted on the drive shaft  12  for rotation therewith about axis  16  and supports therefrom a series of blade assemblies  20 . It should be understood that although a particular rotor system  10  is illustrated in the disclosed embodiment, other main and tail rotor systems will benefit from the present invention.  
     [0025] Each blade assembly  20  includes a flexbeam  22  integrally connected to the rotor hub  18  by fasteners  23  (FIG. 2) so as to be flexible about a pitch change axis  26 . Other attachment devices and methods will also benefit from the present invention. An intermediate tube  24  and a torque tube  28  envelopes flexbeam  22  in spaced relation thereto. The torque tube  28  is connected to the flexbeam  22  at its radially outer end by connecting fasteners  30  and is articulately connected thereto through the intermediate tube  24  and snubber-vibration damper system  32 . Torque tube  28  is connected or preferably integral with an aerodynamic rotor blade member  34 . It should be understood that although the description will make reference to but a single blade assembly  20 , such description is applicable to each blade assembly  20 .  
     [0026] Referring to FIG. 2, pitch change loads are imparted to each blade assembly  20  by pitch control rods  36  which are articulatably connected at one end to the outer periphery of the intermediate tube  24  at a pitch horn  38 . The opposite end of the pitch control rod  36  is articulately connected to a swashplate  42 . The swashplate  42  is connected by a scissors arrangement  44  to the rotor hub  18  for rotation therewith. The swashplate  42  receives control inputs from control rods  46 ,  50 .  
     [0027] Pitch control commands imparted by swashplate control rods  46  cause tilting of swashplate  42  about point  48 . Tilting of the swashplate  42  imparts pitch change loads to the intermediate tube  24  through pitch control rod  36 . Pitch change loads to the intermediate tube  24  are imparted to the torque tube  28  and flexbeam  22  through the snubber-vibration damper system  32 . Interaction of the snubber-vibration damper system  32  with the torque tube  28  causes the torque tube  28 , flexbeam  22  and blade member  34  to pitch about pitch change axis  26 . Inputs from control rods  50  cause the swashplate  42  to axially translate along axis of rotation  16  to impart pitch control loads to the intermediate tube  28  and, hence, blade member  34 . When swashplate  42  translates along axis  16 , it imparts collective pitch change to blade assemblies  20 , and when it tilts about point  48 , it imparts cyclic pitch change.  
     [0028] Referring to FIG. 3, each blade assembly  20  includes a multi-focus elastomeric bearing system  52  within the intermediate tube  24  and/or a torque tube  28 . The elastomeric bearing system  52  is located between the flexbeam  22  and the intermediate tube  24  and/or the torque tube  28 . Each elastomeric bearing system  52  is mounted to the flexbeam  22  through a fixed inner race  53 . Inner race  53  is preferably a rigid hemi-spherical member attached directly to the flexbeam  22 . A snubber bearing is often used in conjunction with a lead/lag damper  54  to provide a reaction path (to the flexbeam) for pitch link forces, rotor flap shears, and damper forces.  
     [0029] It should be understood that various bearingless rotor systems as well as other elastomeric pivots will benefit from the present invention. Preferably, a removable preload cap  56  attached to the intermediate tube  24  through fasteners  58  or the like to provides access and preload to the elastomeric bearing system  52  (also illustrated in FIG. 4).  
     [0030] The elastomeric bearing system  52  includes a plurality of hemi-spherical bearing elements  60   a ,  60   b  and cylindrical bearing elements  62 . The cylindrical bearing elements  62  are axisymmetric shells defined about the pitch change axis  26  to accommodate some of the pitch motion and all of the spanwise linear motion. Although described with regard to hemi-spherical elastomeric bearings such as articulated rotor retention and bearingless rotor snubber bearings i.e., those requiring externally applied precompression, other elastomeric bearings such as pitch link, damper rod ends, hemi-spherical shell type bearings and other elastomeric pivots will also benefit from the present invention.  
     [0031] The elastomeric bearing system  52  allows a bearingless rotor torque tube to pitch, flap, and lead/lag rotate about a fixed point along the pitch change axis  26  of the flexbeam  22 . The pitch change axis  26  is herein illustrated as the center of the flexbeam  22 , however, the present invention should not be so limited. That is, the elastomeric bearing system  52  may define a focal point which is at neither the center of the flex beam nor along the pitch change axis  26 .  
     [0032] Referring to FIG. 5, for the outer hemi-spherical bearing elements  60   b , pitch link load, flap shear load, and preload act normal to the elastomer surface (i.e. axial load), and the damper load acts perpendicular to this normal (i.e. radial load). As the bearing rotates, these forces rotate as well, maintaining their direction of action on the outer rubber layer. For example only, bearing position A schematically illustrates the elastomeric bearing system  52  with a preload of 5000 lb axial load and a 1000 lb radial load. The outermost layer of the outer hemi-spherical bearing element  60   b  experiences these loads (regardless of motion). It is important that the compression-induced shear stress due to the axial component of load exceeds the tension-induced shear stress due to the radial component of the load. For a given layer radius, this requirement defines the minimum wrap-around angle required to ensure that the elastomer layer does not go into tension.  
     [0033] For the relatively fixed inner hemi-spherical bearing elements  60   a , the pitch link load, flap shear load, snubber preload, and damper load rotate with the bearing outer race, changing the direction of action of these forces, producing a much higher component of radial load relative to axial load. As illustrated by position B (in phantom), for a 20 degree pitch angle, the 5000 lb axial load and 1000 lb radial load will load the innermost layer of the inner hemi-spherical bearing element  60   a  with 4,356 lb axial load and 2,650 lb radial load. This requires that the inner hemi-spherical bearing elements  60   a  have a larger wrap-around angle to carry the radial load without the tension induced shear stress due to the radial load overcoming the compression induced shear stress due to the axial load.  
     [0034] It is also advantageous for the inner hemi-spherical bearing elements  60   a  to have a minimum radius, because the motion induced shear stress is related to rθ/t where t defines the required thickness of the elastomer. As the flexbeam geometry is typically fixed, it is often necessary to increase r to achieve the required wrap-around angle. This may result in a relatively large bearing which is impractical for certain applications.  
     [0035]FIG. 6 illustrates two bearings with the same wrap-around angle, i.e. Y degrees. For a flexbeam that is 2 inches thick, the bearing L requires a radius of 2.78 inches to achieve a wraparound which locates the bearing focal point at the center of the flexbeam. If the focal point is located 0.5 inches above the center of the flexbeam, however, bearing U provides the same wrap-around with a bearing of radius 1.60 inch. Motion induced strain is less for the 1.6 inch radius bearing while the compression induced shear stress is greater. However, compression induced shear stress is readily compensated for by reducing the thickness of the shear deformable elastomeric material layers.  
     [0036] Referring to FIG. 7, the elastomeric bearing system  52  includes hemi-spherical bearing  60   a ,  60   b  arranged in series. The hemi-spherical bearing elements  60   a ,  60   b  each of which rotate about a respective focal point  66   a ,  66   b . Preferably, the hemi-spherical bearing elements  60   a ,  60   b  are tailored in stiffness to insure smooth operation without binding or fore-shortening.  
     [0037] Each bearing  60   a ,  60   b  includes a plurality of layers of shear deformable elastomeric material layers  68  separated by shim layers  70  formed of high-stiffness constraining material such as composite or metallic layers. It should be understood, however, that various materials of differing rigidity will also benefit from the present invention. Relatively rigid transitional members  72  may additionally be located between hemi-spherical bearings  60   a ,  60   b.    
     [0038] The hemi-spherical bearings  60   a ,  60   b  are preferably of equal rotational stiffness. Hemi-spherical bearing  60   a  rotates about its focal point  66   a  which is X distance above the pitch change axis  26 , and hemi-spherical bearing  60   b  rotates about its focal point  66   b  which is X distance below the pitch change axis  26 . Because both bearings  60   a ,  60   b  have the same stiffness, each bearing will rotate the same amount and in series. The bearing system  52  thus has an effective rotational center along the pitch change axis  26 . Hemi-spherical bearing  60   a  has a greatly reduced radius and high wrap-around angle, while hemi-spherical bearing  60   b  has an increased radius and reduced wrap-around angle. The radial component of load is less significant as the bearing extends away from the flexbeam  22 .  
     [0039] Preferably, any number of bearings may be utilized in the series so long as the following relationships are maintained. A total bearing system stiffness is defined by the relationship: 
       Kbrg= 1/(1 /k 1+1 /k 2+1 /k 3+ . . . 1 /kn ) 
     [0040] where  
     [0041] k1, k2, k3 . . . kn is the rotational stiffness of each hemi-spherical elastomeric bearing;  
     [0042] and the motion of each said hemi-spherical elastomeric bearings is defined by the relationship: 
     θ n= (θ* Kbrg )/ kn   
     [0043] where  
     [0044] θ is the total motion of the series of said hemi-spherical elastomeric bearings.  
     [0045] Preferably, the total motion of each bearing sums to zero to prevent binding and ensure smooth operation of the bearing system. That is, the bearing system  52  is defined by the relationship: 
     θ1 *e 1+θ2 *e 2+θ3 *e 3 + . . . θn*en= 0. 
     [0046] where  
     [0047] e1, e2, e3, . . . en defines the individual focal point offsets of each hemi-spherical elastomeric bearing relative to a desired center of rotation.  
     [0048] Referring to FIG. 8, another bearing system  52 ′ is illustrated. Bearing system  52 ′ includes three hemi-spherical bearings  60   a ′,  60   b ′, and  60   c ′. Hemi-spherical bearing  60   a ′ rotates about its focal point  66   a ′ which is X distance above the pitch change axis  26 , and hemi-spherical bearing  60   b ′ rotates about its focal point  66   b ′ which is X distance below the pitch change axis  26 . Hemi-spherical bearing  60   c ′ rotates about its focal point  66   c ′ which is located along the pitch change axis  26 . Bearing system  52 ′ also has an effective rotational center along the pitch change axis  26 . Bearing system  52 ′ provides a more gradual transition between the hemi-spherical bearings and also provides the transition from small radius/large wraparound to larger radius/reduced wraparound that is consistent with the loads that are typically applied to a hemispherical bearing.  
     [0049] It is typically advantageous to match the stiffness of the individual hemi-spherical bearings, however, this need not always be required. Matching the stiffness of a small radius bearing and a large radius bearing is preferably achieved by increasing the wraparound of the smaller radius bearing and reducing the wraparound of the large radius bearing. Additional bearing matching is achieved by tailoring the shear modulus, number of rubber layers, and thickness of the layers as generally known to one skilled in the art of elastomeric bearings in combination with the disclosure of the present invention.  
     [0050] The present invention provides structural benefits without compromising the bearing life and also allows separate pre-compression of the snubber as required. The present invention also increases snubber/damper life by assuring that the bearings always operate in compression.  
     [0051] The foregoing description is exemplary rather than defined by the limitations within. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.