Abstract:
A rotor for a propulsive thrust device comprises a hub having a peripheral surface; a plurality of rotor blades received at the peripheral surface of the hub; a first bearing assembly located in the hub and around a shank of a respective rotor blade to support the rotor blade in the hub under centrifugal loading and to allow the rotor blade to rotate about a longitudinal axis; and a second bearing assembly located in the hub and around a shank of a respective rotor blade and inward of the first bearing assembly to preload the first bearing assembly. A preload bearing assembly comprises an outer race; a plurality of rolling elements located therein; and a plurality of studs located in communication with the outer race. Adjustment of the studs distributes a tensile load around the outer race to exert a preload force on a bearing assembly supporting the rotor blade.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application No. 61/301,800, filed Feb. 5, 2010, the contents of which are incorporated herein by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates in general to rotor blades and, more particularly, to rotor blades as propulsive thrust devices in aircraft and boat propellers, helicopters, and aircraft engines in which the rotor blades can be varied in pitch angle to control thrust-producing and/or power-absorbing capacities of such devices. Similar rotor blades with blade pitch angle change capability can also be used in green energy capturing devices such as wind turbines and water turbines. 
       BACKGROUND 
       [0003]    Standard configurations of rotor blades used in aircraft propellers and such that allow for variable pitch operation typically include a retention system having a rotatable root attachment means with the rotatability being effected by a mechanism such as a ball/roller bearing and/or a flexible member. Such a system allows for pitch change of the rotor blade with relatively low friction between components. To provide sufficient structural integrity (e.g., to accommodate the substantial centrifugal and/or bending forces exerted on the mechanisms during operation), the root attachment means and the mechanisms effecting the rotatability are often fabricated in such a way so as to be extremely heavy. 
         [0004]    Some rotor blades, on the other hand, are constructed of lighter, high strength materials, which help to reduce centrifugal loads normally generated during rotation, particularly with regard to metal and/or metal/composite hybrid blade designs, thus resulting in reduced loading of the bearings. This may be of benefit in reducing the overall weight of the components used to support rotor blade loads. However, lighter bearing loads also reduce the ability of the mechanisms involved to support bending forces. Because the rotor blade retention system also affects the foundation stiffness of the rotor blade, rotor blade resonant frequencies are also influenced, which if reduced too much can lead to vibration and/or amplification of forced or self-induced vibration during operation. Thus, a reasonably high degree of stiffness in rotor blade retention systems is desired. 
         [0005]    It is also desirable for rotor blade retention systems, especially with regard to rotor blades in heavy use applications such as those on commuter and military aircraft, to include a relatively quick and simple means of removing and replacing a single damaged rotor blade during the limited access times available for servicing the aircraft. This feature becomes more of a concern with the recent trend towards an increased number of rotor blades used per device. 
       SUMMARY 
       [0006]    In a first aspect, the present invention resides in a rotor for a propulsive thrust device. Such a rotor comprises a hub having a peripheral surface; a plurality of rotor blades received at the peripheral surface of the hub; a first bearing assembly located in the hub and around a shank of a respective rotor blade to support the rotor blade in the hub under centrifugal loading and to allow the rotor blade to rotate about a longitudinal axis; and a second bearing assembly located in the hub and around a shank of a respective rotor blade and inward of the first bearing assembly to preload the first bearing assembly. 
         [0007]    In a second aspect, the present invention also resides in a rotor for a propulsive thrust device. Such a rotor comprises a hub having a plurality of hub arm bores located about a peripheral surface of the hub; a plurality of rotor blades mounted in the respective hub arm bores, each rotor blade being rotatable about an axis extending longitudinally through the rotor blade; a first bearing assembly located between a surface of the hub arm bore and a respective rotor blade to support the rotor blade in the hub arm bore under centrifugal loading; a second bearing assembly located between a surface of the hub arm bore and the respective rotor blade and inward of the first bearing assembly; and a stud in communication with the second bearing assembly, the adjustment of which preloads the first bearing assembly. 
         [0008]    In a third aspect, the present invention resides in a preload bearing assembly for a rotor blade. Such a preload bearing assembly comprises an outer race; a plurality of rolling elements located in the outer race; and a plurality of studs located in communication with the outer race. Adjustment of the studs distributes a tensile load around a circumference of the outer race. The rolling elements are in rolling communication with the rotor blade, and the outer race is in communication with a hub in which the preload bearing is mounted. Distribution of the tensile load around the circumference of the outer race exerts a preload force on a bearing assembly supporting the rotor blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a rotor hub and rotor blade stub assembly for a propulsive thrust device. 
           [0010]      FIG. 2  is a perspective cutaway view of the rotor hub and rotor blade stub assembly of  FIG. 1 . 
           [0011]      FIG. 3A  is a schematic representation of one rotor blade stub in a hub arm bore of the rotor hub and rotor blade assembly of  FIG. 1  before a preload force is applied. 
           [0012]      FIG. 3B  is a schematic representation of the rotor blade stub in the hub arm bore of  FIG. 1  during application of a preload force. 
           [0013]      FIG. 3C  is a schematic representation of the rotor blade stub in the hub arm bore of  FIG. 1  after application of a preload force. 
           [0014]      FIG. 4  is a perspective view of an inner preload bearing assembly and studs for the rotor of  FIG. 1 . 
           [0015]      FIG. 5  is a schematic representation of a hub arm bore and a rotor blade received therein, the rotor blade being supported by an outer bearing assembly and preloaded by an inner preload bearing assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring to  FIG. 1 , a rotor for use with rotor blades for a propulsive thrust device is designated generally by the reference numeral  10  and is hereinafter referred to as “rotor  10 .” Rotor  10  comprises a hub  12  having a plurality of hub arm bores  14  (or sockets or the like) equidistantly located about a peripheral surface of the hub and a plurality of studs  16  associated with each hub arm bore. The studs  16  are preferably received in bosses (shown at  18  in  FIGS. 2 and 3 ) located around each hub arm bore  14 , movable in the bosses, and can be tensioned therein via nuts (shown at  17  in  FIGS. 2 and 3 ). A rotor blade  20  (only stubs of each rotor blade shown) is mounted in each hub arm bore  14 , each rotor blade being rotatable about an axis  22  extending through the hub arm bore to effect the changing of rotor blade pitch. Movement of the studs  16  is along axes parallel to the axis  22 . Tensioning the studs  16  via the nuts  17  (preferably with common drive tools) allows for both the retention of a rotor blade  20  in a respective hub arm bore and the loading of two bearing assemblies (described below as an outer bearing assembly  40  and an inner preload bearing assembly  50 ) associated therewith. The present invention is not limited to the incorporation of eight hub arm bores  14 , as shown, as any suitable number of hub arm bores may be located about the peripheral surface of the hub  12 . Furthermore, the present invention is not limited to the use of four studs  16  associated with each hub arm bore  14 , as shown, as any suitable number of studs  16  may be used. 
         [0017]    As shown in  FIG. 2 , each hub arm bore  14  defines a bore  26  into which a shank portion  30  of a rotor blade  20  is received. The outer bearing assembly  40  is located in each hub arm bore  14  to retain the rotor blade  20  in the bore  26  and to facilitate the rotation thereof about the axis  22  in a rotor blade pitch changing operation. The inner preload bearing assembly  50  is also located in each hub arm bore  14 , each inner preload bearing assembly being adjustable via the studs  16  associated therewith. The studs  16  are elongated pin-type members extending through holes in the bosses  18 , the bosses being located equidistantly around each hub arm bore  14 , each stud being engageable with a tab  58  protruding from an outer race  54  of the inner preload bearing assembly  50 . In the engagement of the stud  16  with the tab  58 , a hexagonal shaped head, splined shank, or the like is received into a correspondingly shaped structure to prevent rotation of the stud during tensioning. The present invention is not so limited, however, as the heads of the studs may be integral with the tabs  58 , or the studs may be threadedly received in the tabs. By tensioning the studs  16 , the inner preload bearing assembly  50  is pulled outwardly along the axis  22 , thereby preloading the outer bearing assembly  40 . 
         [0018]    Preloading of the outer bearing assembly, as shown in  FIGS. 3A ,  3 B, and  3 C, comprises tensioning the studs  16 . Before the initial tensioning of the studs  16  ( FIG. 3A ), the rotor blade  20  is in contact with the inner preload bearing assembly  50 , and a gap G 1  is present between the tabs  58  and receiving surfaces  59  in the hub arm bore  14 . Upon initial tensioning of the studs  16  and pulling the inner preload bearing assembly  50  and rotor blade  20  outwardly ( FIG. 3B ) in the direction of arrow  55 , rolling elements (shown at  42  in  FIG. 5 ) on the inner race (also shown in  FIG. 5  at  46 ) of the outer bearing assembly  40  are urged into initial contact with the outer race (shown in  FIG. 5  at  44 ) formed in the hub arm bore  14  and a gap G 2  is formed. Further tensioning ( FIG. 3C ) causes the elastic deformation of the rolling elements and races of both bearing assemblies (tabs  58  are brought into contact with receiving surfaces  59 ) until the gap G 2  is eliminated, thereby establishing the predetermined amount of static preload between both bearing assemblies. Additional tensioning is imparted to the stud  16  by applying additional torque to inhibit separation of the tabs  58  from the receiving surfaces  59  to provide substantially constant tensile loading on the stud  16 . The amount of static preload is suitably sufficient such that when significant centrifugal loading develops during operation of the rotor  10  (which can reduce the established static preload force to some degree), there is sufficient preloading remaining to inhibit the unloading of the rolling elements  52  in the inner preload bearing assembly  50  when bending loads are combined with centrifugal loading. This establishes the bending capacity of the system in operation. 
         [0019]    As shown in  FIG. 4 , the structural material of the inner preload bearing assembly  50  defines a plurality of arches  65  (or other suitable configuration) extending between each of the four tabs  58 , as shown. The arches  65  facilitate the distribution of point loads around the outer race  54  when the studs  16  pull the outer race in the outward direction into a preloading position. 
         [0020]    As shown in  FIG. 5 , rolling elements  42  of the outer bearing assembly  40  and rolling elements  52  of the inner preload bearing assembly  50  comprise low-friction ball bearing elements that permit, upon rotation of the rotor blade  20  about the axis  22 , the pitch angle of the rotor blade to be altered when an inwardly protruding pin  61  is moved by a timing mechanism (not shown) located in the hub  12 . The present invention is not limited to the use of ball bearing elements, however, as the rolling elements may be tapered roller bearings, or any other suitable type of bearing element. The rolling elements  42  of the outer bearing assembly  40  are captured between an outer race  44  defined by a machined inner surface of the hub arm bore  14  and an inner race  46  defined by a machined surface of the shank  30  of the rotor blade  20  and held therein by a cage  60 . The present invention is not limited to the use of machined surfaces integral to the rotor blade and hub structure, however, as the races may be separate elements. 
         [0021]    The cage  60  holding and supporting the rolling elements is an elongate flexible member (e.g., fabricated of a plastic material or the like) having pockets for the accommodation of the rolling elements and is referred to hereinafter as “necklace  69 .” One or both ends of the necklace  69  include a tab with a hole or loop feature. Engagement of the hole or loop feature may be made with a separate hook-shaped element to withdraw the necklace  69  from the hub arm bore  14 . When the necklace  69  is in the hub arm bore  14 , the ring structure is formed, and the outer bearing assembly  40  is capable of being preloaded. 
         [0022]    By tightening the nuts  17  on the studs  16 , the preload force is established and the rotor  10  is operational. 
         [0023]    When the nuts  17  are loosened to release tension on the studs  16 , the preload force generated by the inner preload bearing assembly  50  is released, and the outer race  54  thereof can move further inward into the hub arm bore  14 , thereby allowing the rotor blade  20  to also move inward. This unloads the outer bearing assembly  40  and provides for sufficient room around the outer bearing assembly (which is the primary bearing providing support to the rotor blade  20  and further retaining the rotor blade in place) to permit removal of the necklace  69 . The necklace  69  can be pulled as an elongate element through a loading hole (shown at  64  in  FIG. 1 ) in the side of the hub arm bore  14  located just inboard of where the outer bearing assembly  40  is located in the hub arm bore. During removal of the rotor blade  20 , the inner preload bearing assembly  50  remains inside the hub arm bore  14 , thereby allowing it to be protected from exposure to potential external contamination. A replacement rotor blade  20  can then be inserted into the hub arm bore  14 , and the necklace  69  can be reinserted into the space defined by the outer race  44  defined by the machined inner surface of the hub arm bore  14  and the inner race  46  defined by the machined surface of the shank  30  of the replacement rotor blade  20 . Once this is done, the inner preload bearing assembly  50  can be pulled back outward by tightening the studs  16  to the preset torque, thus restoring the preload between both bearing assemblies and rendering a propulsive thrust device into which the rotor  10  is incorporated ready for flight. 
         [0024]    Also as shown in  FIG. 5 , angles of contact during operation of the rotor  10  are approximated by lines  70  connecting a first pair of rolling elements  52  in the inner preload bearing assembly  50  and an opposing pair of rolling elements  42  in the outer bearing assembly  40 . These lines  70  define an outer focal point  72  and an inner focal point  74  for all the rolling elements in each bearing assembly. The outer focal point  72  and the inner focal point  74  are coincident with the axis  22 . The distance between the inner focal point  74  and the outer focal point  72  provides a measure of the stability provided by the system of bearings defined herein by the outer bearing assembly  40  and the inner preload bearing assembly  50  in preventing bending and/or “rocking” loads from deflecting the retention of the rotor blade  20 , thus augmenting a foundation stiffness for the attachment of a rotor blade  20  to the hub  12 . This foundation stiffness allows resonant frequencies of the rotor blades  20  to be maintained at higher values to avoid undesirable rotor system vibration issues. 
         [0025]    Referring now to all of the Figures, protective coatings and/or low friction sleeves can be employed to resist metal-to-metal fretting or surface wear caused by fatigue loading. The coatings and/or low friction sleeves can be provided in regions of the rotor  10  where motion under load is apt to occur. One such region is defined by the contact surfaces of the outer diameter of the inner preload bearing assembly  50  and the inner surface of the arm hub bore  14 , where preferably there is a close tolerance slip fit. In this case, the hub arm bore  14  defines a surface where enhanced strength is desirable. This surface can be protected either by use of a coating thereon and/or by use of a coating on the engaging surface of the outer race  54  of the inner preload bearing assembly  50 . Such a coating is preferably a thin layer of a soft metallic or plastic material such as silver plating or other material that can be applied by any suitable means, including, but not limited to, methods such as plasma spraying. 
         [0026]    As stated above, motion between the stud  16  and the hole in the boss  18  is prevented by applying sufficient torque to the stud so that rotor blade  20  loading does not cause separation between the tabs  58  and the receiving surfaces  59 . 
         [0027]    Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the foregoing description.