Abstract:
A snubber inner member has a doubly convex outer surface portion having a central longitudinal axis. A shim/spacer stack secured to the outer surface portion couples the snubber inner member to the second member. A retainer having one or more engagement surfaces cooperates with one or more engagement surfaces of the snubber inner member to constrain lateral movement of the snubber inner member relative to the retainer while permitting longitudinal movement of the snubber inner member away from the retainer. An elastomer secures the retainer to the first member.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    This invention relates to elastomeric bearings, and more particularly to elastomeric bearings used in helicopter main rotor and tail rotor assemblies.  
           [0003]    (2) Description of the Related Art  
           [0004]    Snubber/damper units are well known in the field of helicopter rotor technology.  
           [0005]    Exemplary snubber/damper units are shown in U.S. Pat. Nos. 5,188,513, 5,092,738, 4,244,677, and 4,105,266. An exemplary snubber/damper is used in a bearingless rotor application to accommodate relative movement and orientation changes of an inner blade member to an outer blade or mounting member. In a common implementation, two snubber/damper subunits are mounted on opposite sides of the inner member. The subunits include a spherical stack of bonded metallic shims and elastomeric spacers to accommodate pitch and flap motions and a flat stack to accommodate lead-lag and, if present, longitudinal motions. The spherical stack is commonly identified as the snubber and the flat stack is commonly identified as the damper. In a common configuration, the spherical layers of the two subunits are concentric.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    Accordingly, one aspect of the invention involves a snubber system for permitting relative rotation of a first member and a second member. A snubber inner member has a doubly convex outer surface portion having a central longitudinal axis. A shim/spacer stack secured to the outer surface portion couples the snubber inner member to the second member. A retainer having one or more engagement surfaces cooperates with one or more engagement surfaces of the snubber inner member to constrain lateral movement of the snubber inner member relative to the retainer while permitting longitudinal movement of the snubber inner member away from the retainer. An elastomer secures the retainer to the first member.  
           [0007]    In various implementations, the shim/spacer stack may be outwardly doubly convex and the snubber system may further have a flat shim/spacer stack coupling the outwardly doubly convex shim/spacer stack to the second member. The snubber system may include a second such inner member, shim/spacer stack, retainer, and elastomer, opposite the first. The shim/spacer stack may consist essentially of a number of metallic shims and a number of elastomeric spacers secured to each other as a unit. The snubber inner member may have a flange extending radially outward beyond an inboard portion of the doubly convex outer surface portion. The snubber inner member one or more engagement surfaces may comprise a perimeter portion of a socket and the retainer one or more engagement surfaces may comprise an outwardly projecting projection.  
           [0008]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a sectional view of a snubber/damper in a neutral condition.  
         [0010]    [0010]FIG. 2 is a sectional view of an alternate snubber/damper in a neutral condition.  
         [0011]    [0011]FIG. 3 is a sectional view of the snubber/damper of FIG. 1 in a pitched condition. 
     
    
       [0012]    Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0013]    [0013]FIG. 1 shows a snubber/damper system  20  which couples an inner member  22  to an outer member  24 . In use, the inner member  22  is a root portion of a helicopter rotor blade flexbeam. The exemplary outer member  24  is a torque tube portion of the blade extending outward and secured to a distal portion of the flexbeam (e.g., via bolts). The exemplary inner member  22  has an elongate cross-section transverse to its central longitudinal axis  500 . The inner member  22  and this cross-section have a top  26 A and a bottom  26 B and relatively shorter sides  28 A and  28 B. In the exemplary implementation, the snubber/damper system  20  holds the inner member  22  in a neutral orientation relative to the outer member  24  wherein the central top/bottom surface normal  502  of the flexbeam is pitched at an angle θ 0  relative to an axis  504  of the snubber/damper system.  
         [0014]    The exemplary snubber/damper system  20  includes similar components above and below the inner member  22 . The core of the upper and lower halves of the snubber/damper system is a respective upper and lower snubber inner member  30 A,  30 B. Each inner member is coupled to the associated top or bottom surface by a retainer  32 A,  32 B and pad  34 A,  34 B (both described in further detail below). Each inner member  30 A,  30 B has an outboard spherical surface portion  36 A,  36 B. The respective spherical surface portions  36 A,  36 B have centers of curvature  506 A and  506 B which, in the neutral condition, fall along the axis  504  spaced outward from its intersection with the flexbeam central axis  500 . A shim/spacer stack  40 A,  40 B is secured to each inner member  30 A,  30 B. Each stack  40 A,  40 B has alternating elastomeric spacers  42  and metal shims  44  secured to each other. The spacers and shims each have inboard and outboard surfaces with centers of curvature at the associated center  506 A,  506 B. The exemplary spacers and shims are annular, leaving each stack  40 A,  40 B with a central cavity  46  provided by the shim/spacer central apertures. The outboard surface of each stack is secured to an inboard surface  50 A,  50 B of a metallic adapter or transition shim  52 A,  52 B. The surfaces  50 A,  50 B are concentric with their respective shim stacks and inner member outer surfaces. The adaptors  52 A,  52 B each have a flat outer surface  54 A,  54 B. An outer (damper) shim/spacer stack  56 A,  56 B has spacers  57  and shims  58 . Each stack  56 A,  56 B has an inboard surface secured to the outer surface  54 A,  54 B and an outboard surface secured to a plate  60 A,  60 B. Each plate  60 A,  60 B is captured within an associated cap  62 A,  62 B secured to the outer member  24  such as by bolts (not shown) to hold the shim/spacer stacks in compression. The compression provides a desired precompression of the elastomeric spacers.  
         [0015]    The retainers  32 A,  32 B serve to support and retain the associated inner members  30 A,  30 B. The elastomeric pads  34 A and  34 B provide strain isolation between the inner member  22  and the retainers  32 A and  32 B. Accordingly, the pads may advantageously be relatively thick. An exemplary uncompressed pad thickness is 0.025 inch. An exemplary range of uncompressed thicknesses is 0.010-0.040 inch, more narrowly, 0.015-0.035 inch. An exemplary pad material is natural rubber (e.g., per ASTM D 2000). An exemplary retainer material is chopped glass fiber in resin (e.g., per MIL-M-46069) and an exemplary retainer thickness is 0.065 inch. The exemplary snubber inner members have central depending projections  70  and outboard radially-extending flanges  72 . The projections  70  are received within apertures  74  and  76  in the associated retainer and pad, respectively. The flanges  72  serve to provide angular stability to the snubber inner members. In an exemplary embodiment, the flanges  72  are not captured beneath a retainer clip.  
         [0016]    [0016]FIG. 2 shows an alternate inner member mounting configuration in which the cooperating interengaged mounting features of the inner member and retainer are reversed from those of FIG. 1. In the illustrated embodiment, each inner member  130 A,  130 B has a socket  174  extending toward the surface  136 A,  136 B from the underside of the inner member. A complementary projection  170  extends from the outboard surface of the associated retainer  132 A,  132 B. The retainer may advantageously be metallic (e.g., Al or Ti alloys). In the exemplary embodiment, each pad  134 A,  134 B lacks the central aperture of the FIG. 1 pads. In the exemplary embodiment, the projection  170  and socket  174  are largely right circular cylinders. The projection  170  has a principal radius and a flat distal end surface  175  a given height above the remaining outboard surface of the retainer. The socket has a principal diameter slightly greater than the local projection diameter to provide a light friction fit. The root of the projection may be radiused slightly for strength and the mouth of the socket may have a complementary bevel. The depth of the socket may be slightly greater than the height of the projection. Such a configuration permits a relatively greater engaged height between projection and socket than do the features of FIG. 1 for a given retainer/pad thickness. The height is advantageously effective to maintain engagement during peak loads and motions in view of expected reduction in pre-compression due to creep in the spacers over time. The projection transverse dimension is advantageously sufficient to control wear of the projection. The circular projection and socket are easy to manufacture. They are of effective cross-section to transfer the lead-lag forces. An exemplary range of projection diameter is 0.4 inch-1.2 inch and an exemplary range of projection height is 0.1 inch-0.3 inch is 0.050 inch-0.150 inch.  
         [0017]    To assemble the snubber, the undersides of the retainers may be bonded to the outboard surfaces of the pads and the undersides of the pads then bonded to the flexbeam. The flexbeam may be positioned in the outer member  24 . The two bonded stacks, extending from the respective snubber inner members  30 A and  30 B to the plates  60 A and  60 B, are inserted through respective apertures  80 A and  80 B in the outer member. The caps  62 A and  62 B are then secured to the outer member  24  to precompress the snubber (FIG. 2). The preload is such that, given the snubber geometry and the radius of curvature of the surface  36 A,  36 B, during normal operation the snubber inner members will remain compressively engaged to the associated retainers.  
         [0018]    [0018]FIG. 3 shows the system  20  in a strained condition having a pitch angle θ 1  for a net pitch from neutral of θ 1 −θ 0 . This movement produces shearing of the spacers in both the inner and outer shim/spacer stacks. In the exemplary embodiment, R 1  designates the radius of curvature of the inner member spherical surface portions, H designates the height of contact between the spherical surface portion and the adjacent spacer, and S indicates the separation between the centers of curvature. The presence of a positive S provides increased H at a given R 1 . The non-zero S causes a shear displacement D accommodated by the outer stacks.  
         [0019]    D is substantially sin(θ 1 −θ 0 )*S/2. For a 25° net pitch with a value S of 1.0 inch, the resulting D is 0.211 inch. Because there are typically phase differences between lead/lag and pitch, this increased motion may typically be accommodated with little change to the dampers (e.g., with slight increase in elastomer spacer thickness). If the lead lag motion is simultaneously present, the presence of two opposed dampers means that the pitch-induced increased damping of one damper is largely compensated for by the opposite pitch induced decreased damping of the other damper. The increased height also permits a relatively greater wrap angle β 1 . Where β 1  is the half wrap angle of the spacer along the inner member outboard surface. For any given spacer layer, that spacer has a wrap angle β i  (FIG. 1) and a central aperture half angle α i .  
         [0020]    The axial and side load pressures P A  and P S  within a snubber spacer layer is:  
           P   A =(Preload/π R   i  sin Φ i )(cos((θ 1 −θ 0 )( R   O   −R   i )/( R   O   −R   1 ))/2 cosΦ i )  
           P   S =(Preload/π R   i  sin Φ i )(sin((θ 1 −θ 0 )( R   O   −R   1 )/( R   O   −R   1 ))/sin Φ i )  
         [0021]    where: (θ 1 −θ 0 ) is the bearing pitch angle; R O  (FIG. 1) and R 1  (FIG. 3) are respective outer and inner radii of the outboardmost and inboardmost spacers; the subscript i designates the particular layer from inboardmost to outboardmost; Φ i  is (β i +α i /2.  
         [0022]    For the spacer to remain in compression, PA-PS&gt;0 therefore:  
         (cos((θ 1 −θ 0 )( R   O   −R   i )/( R   O   −R   1 ))/2cos Φ i )&gt;(sin((θ 1 −θ 0 )( R   O   −R   1 )/( R   O   −R   1 ))/sin Φ i )  
         [0023]    The stress will typically be greatest at the inboardmost layer at which R i  is approximately R 1  and thus (R O -R i )/(R O -R 1 )=1  
         [0024]    Solving for P A -P S =0:  
         (sin Φ i )/(2 cosΦ i )=tan(θ 1 −θ 0 )  
         [0025]    For the outboardmost spacer, R i  is approximately R O  and thus (R I −R i )/(R I −R 1 )=0. Accordingly the required wrap angle is not pitch driven at this layer.  
         [0026]    The required Φ i  for the inboardmost layer can thus be determined for a desired maximum pitch angle from:  
         (sin Φ i )/(2 cos Φ i )=tan(θ 1max −θ 0 )  
         [0027]    The bearing shear stress γ due to pitch is:  
         γ≈(θ 1 −θ 0 )(( R   O   +R   1 )/2)/τ 
         [0028]    where τ is the overall elastomer thickness which may be approximately R O -R 1  if the shim thickness is very small.  
         [0029]    The foregoing may be utilized to engineer snubber geometry. For example, the engineering considerations may include a need to accommodate a given max. bearing pitch angle. The combined flexbeam and retainer thickness may be a given (e.g., a minimum thickness dictated by strength considerations). It may be desired to minimize overall snubber/damper height. The required Φ i  for the inboardmost layer can be calculated as described above. The bearing shear stress γ at the max. bearing pitch angle may also be calculated as described above. The effective snubber height may substantially be related to the sum of R O  and S/2. The available R 1  to achieve a given inboardmost layer wrap angle will depend upon S and the combined thickness of the flexbeam, the retainers, and the inner member flange. A thickness-minimizing configuration may be iteratively solved based upon these parameters and stress analyses conducted to verify that, at the maximum pitch angle stresses are within acceptable limits.  
         [0030]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, when applied as a reengineering of an existing snubber/damper, details of the existing snubber/damper and its environment may particularly influence details of the implementation. Accordingly, other embodiments are within the scope of the following claims.