Abstract:
An electrically heatable elastomeric bearing according provides an electrically heatable elastomeric bearing shim layer which replace one or more of the metallic shims currently used in non-heatable elastomeric bearing designs. The heatable shim layers facilitate the maintenance of a constant bearing spring rate to prevent an increase to bearing loads and reduced strength that may otherwise occur at cold temperatures.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an elastomeric bearing, and more particularly to a heatable elastomeric bearing with integral heating elements which facilitates cold weather operation. 
         [0002]    Elastomeric bearings are used in many applications. Elastomeric bearings typically include alternating layers of elastomeric material and metallic or composite shims. These elastomeric bearings often replace non-lubricated or self-lubricated bearings such as Teflon fabric lined bearings. Typical aerospace elastomeric bearing applications include spherical rod end bearings for pitch control rods and dampers, spherical blade retention bearings for fully articulated rotors, and cylindrical bearings for semi-articulated rotors and fluid-elastic damper seals. 
         [0003]    Cold temperatures may reduce the service life and static strength of elastomeric bearings as the elastomeric material therein may be relatively sensitive to such cold temperatures. The elastomeric bearings may also stiffen when exposed to cold temperatures. This may result in an increased spring rate that transfers increased loads to the mating components that may reduce fatigue life of the mating components. An increased spring rate may also result in dynamic behavior alteration of systems which utilize the elastomeric bearings. 
         [0004]    In rotary-wing aircraft operations, it is common practice to use a warm-up procedure to soften the elastomeric bearings prior to rotor start-up. This typically involves pilot movements of the collective, cyclic, and yaw controls, with gradually increasing amplitude and frequency. This procedure is time consuming and is difficult due to the bulky cold weather gear often worn by the pilots and/or ground crew. The procedure is also less effective on certain elastomeric bearings that do not move significantly in response to control movement (i.e. damper bearings). 
         [0005]    For rotary wing aircraft which utilizes a servo flap control system, a torsional stiffness constraint is typically imposed on the spherical elastomeric blade retention bearing. Variations in this spring rate due to cold temperature may affect stability and dynamic characteristics of the rotor. Further complicating pre-flight procedures of aircraft with servo flap control, stick movement will not generate movement of the blade retention bearings. For fluid/elastic damper systems, current pre-flight warm-up procedures may be ineffective at warming the elastomeric cylindrical “seals”, resulting in high initial spring rates, with increased loadings on the damper retention hardware. This may result in the aforementioned dynamic and service life concerns. 
         [0006]    Accordingly, it is desirable to provide a system and method to warm-up elastomeric bearings prior to rotor start-up. 
       SUMMARY OF THE INVENTION 
       [0007]    The electrically heatable elastomeric bearing according to the present invention provides an electrically heatable elastomeric bearing shim which replaces one or more shims currently used in non-heatable elastomeric bearing designs. The shims include two shim layers which sandwich a heater mat therebetween. The heater mat includes a wire or heating foil encapsulated between two insulating layers. An electrical jumper connects the heatable shims to each other and to a power supply. The wire jumpers provide slack to permit free relative movement of the electrically heatable elastomeric bearing. 
         [0008]    During operation, the shear deformable elastomeric material layers shear within the constraints of the heatable shim layers. The heatable shim layers guide the elastomeric shear deformation such that the displacement trajectory of a first member relative to a second member is a predefined movement. The heatable shim layers facilitate the maintenance of a constant bearing spring rate to prevent an increase in bearing loads and reduced strength that may otherwise occur at cold temperatures. 
         [0009]    In the illustrated rotary-wing aircraft rotor head embodiment, the elastomeric bearings are heatable using on-board or external power sources prior to rotor start-up. In-flight, the power need only be selectively applied as strain energy will warm the elastomeric material layers. 
         [0010]    The present invention therefore provides a system and method to warm-up elastomeric bearings prior to rotor start-up. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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: 
           [0012]      FIG. 1A  is a general perspective view an exemplary rotary wing aircraft rotor head embodiment for use with the present invention; 
           [0013]      FIG. 1B  is a general perspective view a servo flap rotor blade for use with the present invention; 
           [0014]      FIG. 2A  is a sectional view of heatable elastomeric bearing; 
           [0015]      FIG. 2B  is an expanded sectional view of a heatable shim layer within the heatable elastomeric bearing; 
           [0016]      FIG. 3  is a perspective view of heatable elastomeric bearing; 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]      FIG. 1A  illustrates a general perspective view of a rotor system  10  which includes a rotor shaft  12  driven in conventional fashion by an engine through a reduction gearing for rotation about an axis of rotation A. A rotor hub  14  is mounted to the rotor shaft  12  for rotation therewith about an axis of rotation to support a multiple of rotor blade assemblies  16  (illustrated schematically) therefrom. 
         [0018]    Each blade assembly  16  is mounted to the rotor hub  14  so as to be flexible about a pitch change axis P. It should be understood that various attachment systems and rotor blade pitch change systems are usable with the present invention. Pitch change loads are imparted to each rotor blade assembly  16  by pitch control rods  18  which are articulatably connected at one end to the rotor blade assembly  16  at a pitch horn  20 . The opposite end of the pitch control rod  18  is articulately connected to a swash plate assembly  22 . 
         [0019]    As shown, a heatable elastomeric bearing  24  may be utilized in the rotor system  10  at a multitude of locations for a multitude of purposes. Typical applications include spherical rod end elastomeric bearings for pitch control rods and dampers, spherical blade retention elastomeric bearings for fully articulated rotors, and cylindrical bearings for semi-articulated rotors and fluid-elastic damper seals as well as other locations. 
         [0020]    It should be understood that although the elastomeric bearings in the illustrated embodiment are depicted within a rotor system  10 , the elastomeric bearings of the present invention are not limited to just rotor head applications but may be in any location in which elastomeric bearings are utilized such as in substitution for non-lubricated or self-lubricated bearings such as Teflon fabric lined bearings. The heatable elastomeric bearings  24  are most preferably utilized in rotor systems which are of the fully articulated rotor system type with servo flap control  16 S ( FIG. 1B ) as such rotor systems have spherical elastomeric blade retention bearing  24 S which are not readily exercised through pilot control during conventional preflight procedures. 
         [0021]    Referring to  FIG. 2A , the heatable elastomeric bearing  24  generally includes a plurality of shear deformable elastomeric material layers  30  separated by heatable shim layers  32  formed of high-stiffness constraining material such as composite or metallic layers. Each heatable shim layer  32  preferably includes a first outer shim  34  and a second outer shim  36  which are preferably manufactured of a metallic material to sandwich a heater mat layer  38  therebetween to conduct thermal energy therefrom ( FIG. 2B ). Preferably, the insulators electronically insulate the thermal element  40  but conduct thermal energy to the shims  34 ,  36 . The heater mat layer  38  is preferably a wire or heating foil thermal element  40  encapsulated between a first insulator  42  and a second insulator  44  typically manufactured of a non-metallic material such as fiberglass ( FIG. 2B ). Preferably, the heatable shim layers  32  are equally spaced throughout the heatable elastomeric bearings  24  stack-up, but need not be utilized in every shim layer. Most preferably, a heatable shim layer inner endplate  46  and a heatable shim layer outer endplate  48  mount the heatable elastomeric bearing  24  to a first member A and a second member B ( FIG. 3 ). 
         [0022]    Referring to  FIG. 2B , an electrical jumper  50  preferably connects each thermal element  40  within the heatable elastomeric bearing  24  to a power supply P. Each electrical jumper  50  extends from the heatable elastomeric bearing  24  to provide slack which permits free relative movement of the deformable elastomeric material layers  30 . 
         [0023]    It should be understood, however, that various shim materials of differing rigidity will also benefit from the present invention such that an alternate design would integrate the insulating layers into two composite shim layers. 
         [0024]    Referring to  FIG. 3 , the heatable elastomeric bearing  24  operates as a coupler between the first member A and the second member B. During operation, the shear deformable elastomeric material layers  30  shear within the constraints of the shim layers  32 . The shim layers  32  guide the elastomeric shear deformation such that the displacement trajectory of the first member A relative to the grounding member B is a predefined movement. The heatable shim layers  32  facilitate the maintenance of a constant bearing spring rate to prevent an increase to bearing loads and reduced strength that may otherwise occur at cold temperatures. In the illustrated aircraft rotor head embodiment, the heatable elastomeric bearing  24  are heatable using on-board or external power sources prior to rotor start-up. In-flight, the power need only be selectively applied as strain energy will to warm the elastomeric material layers  30 . 
         [0025]    Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present invention. 
         [0026]    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.