Abstract:
A tandem set of angular contact ball bearings each having an inner ring, an outer ring and balls therebetween, wherein each bearing contains balls that are spaced from each other by slug ball separators. A rotary wing aircraft rotor head assembly includes a rotor head member, a plurality of spindles attached to the head member at equal interval around the center of the head member, and a stack of ball bearings mounted on each spindle. Each bearing has an inner ring, an outer ring, and a plurality of balls between the inner ring and the outer ring, and slug ball separators between adjacent balls, and there is a mounting collar on the ball bearings.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/782,311, filed Mar. 13, 2006 and U.S. Provisional Patent Application Ser. No. 60/782,308, filed Mar. 13, 2006, and is a continuation-in-part of U.S. patent application Ser. No. 11/704,762, filed Feb. 8, 2007, which issued as U.S. Pat. No. 8,021,053 on Sep. 20, 2011, all of which are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to ball bearings, and in particular, to ball bearings in rotary wing aircraft. 
     BACKGROUND 
     Rotary wing aircraft, such as helicopters, provide unique environments for the use of ball bearings, particularly in their rotor systems. For example, the bearings in rotor blade mounts must be specially designed to provide reliable ongoing use under the type of load and speed conditions that are unique to helicopters. The use of bearings in other types of machines is nonanalogous to rotary wing aircraft bearings in general and to helicopter bearings in particular. For this reason, bearing designs that are useful in other kinds of machines are not assumed by those of ordinary skill in the art to be suitable for helicopter swashplates, rotor blade mounts, etc. 
     One example of a conventional rotary wing aircraft bearing is in the tail rotor blade mount of a helicopter such as a Sikorsky CH53A/D helicopter. The blade mount in a Sikorsky CH53A/D helicopter includes a 5-bearing stack of ball bearings. Each bearing in the set is of metric size 70 millimeter (mm)-bore, 110 mm-outer diameter (OD) and 18 mm-width, and has a cross-section of 20 millimeter (mm) [(110-70 mm)/2], which corresponds to a basic 114 ball bearing size that is normally fitted with ½ inch balls. A one-piece, open-ended (one open circular segment) molded nylon cage is used to separate the balls in this bearing. To achieve a minimal cage integrity or strength, to improve cage molding process, and to facilitate cage assembly into bearing, the bearing rings and balls had to be compromised in two respects. First, the bearing ball size of 15/32 inch had to be used instead of balls sized at ½ inch, which would nominally be used in bearings of this size, as noted above. Second, the outer ring face had to be chamfered heavily to accommodate installation of the cage. The chamfer is currently dimensioned as 110° Max by 53° Max, which raises concerns over its adverse effect on the strength of the outer ring, which is under heavy thrust loads in application. 
     Based on the foregoing, it is the general object of this invention to provide a bearing for a tail rotor assembly that improves upon prior art bearings. 
     SUMMARY 
     The present invention resides in one aspect in a tandem set of angular contact ball bearings each having an inner ring, an outer ring and balls therebetween, wherein each bearing contains balls that are spaced from each other by slug ball separators. 
     The present invention resides in another aspect in a rotary wing aircraft rotor head assembly comprises a rotor head member having a center, a plurality of spindles attached to the head member at equal intervals around the center of the head member, and a stack of ball bearings mounted on each spindle. Each bearing has an inner ring, an outer ring, and a plurality of balls between the inner ring and the outer ring, and slug ball separators between adjacent balls, and there is a mounting collar on the ball bearings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of a slug ball separator; 
         FIG. 2  is a cross-sectional view of the slug ball separator of  FIG. 1 ; 
         FIG. 3  is a partly cross-sectional view of two balls separated by the slug ball separator of  FIG. 1 ; 
         FIG. 4  is a schematic, partly cross-sectional view of a ball bearing for use in a rotary wing system as described herein; 
         FIG. 5A  is a partly cross-sectional view of the outer ring of the bearing of  FIG. 4 ; 
         FIG. 5B  is a partly cross-sectional view of the inner ring of the bearing of  FIG. 4 ; 
         FIG. 6  is a partial cross-sectional view of a ball bearing stack as described herein for use in a tail rotor mount; 
         FIG. 7  is an exploded perspective view of a tail rotor head assembly comprising a bearing stack as described herein according to an illustrative embodiment of the invention; and 
         FIG. 8  is a schematic, partly cross-sectional, partly broken-away view of a swashplate assembly comprising the bearing of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention provides an improvement to rotor blade mount bearings by providing ball bearings that comprise slug ball separators between balls in the bearing, rather than a bearing cage. As a result, a ball bearing meeting the same design constraints as a prior art caged ball bearing can employ larger balls and obviates the need to chamfer either of the races. In addition, the resulting bearing has a surprisingly increased dynamic load rating and fatigue life. 
     One embodiment of a slug ball separator useful in the present invention is shown in  FIG. 1  and  FIG. 2 . Slug ball separator  10  has a generally annular configuration about a central axis A, two ends and a passage therethrough. The slug ball separator  10  further has a generally cylindrical exterior surface  12  having an external diameter d o , axial end faces  14  and  16 , and conical chamfered surfaces  18  and  20  that converge from the end faces  14 ,  16  towards a generally cylindrical interior surface  22  having an internal diameter d i . Chamfered surfaces  18  and  20  may conform to a conical angle C of about 75° to about 120°, for example, about 90°. Interior surface  22  extends for a distance W i  from the narrow end of chamfered surface  18  to the narrow end of chamfered surface  20 . 
     Exterior surface  12  may be contoured so that its diameter is at a maximum between the end faces; for example, exterior surface  12  may define an angle β of about 3° relative to a tangent line t o  thereon that is parallel to axis A. The diameter d o  of surface  12  from axis A thus decreases moving from the tangent point, which is preferably midway between the end faces, towards either end face. Similarly, interior surface  22  may be contoured to define an angle γ of about 3° relative to a tangent line b l  thereon that is parallel to axis A. Accordingly, the diameter d i  of interior surface  22 , measured from axis A, increases moving towards either end face from the tangent point, which is preferably midway between the end faces. The contoured surfaces provided by angles facilitate removal of the slug ball separator  10  from the mold in which it is formed. 
     Slug ball separator  10  has an axial length W f  measured from end face  14  to end face  16 . In a particular embodiment, slug ball separator  10  is designed to be substantially symmetric about a radial centerline CL. 
     Slug ball separator  10  may be formed from a synthetic polymeric material such as bearing grade PEEK (poly ether ether ketone) or other material e.g., PTFE (polytetrafluoroethylene)(such as TEFLON®), polyimide (such as Dupont&#39;s VESPEL®), etc. In particular embodiment, the material is compliant with U.S. military specification MIL-P-46183 as amended 1 Jul. 1999. Preferably, the material will conform to Society of Automotive Engineers, Inc. (SAE) Aerospace Material Specification AMS 3656E issued 15 Jan. 1960, revised 1 Jul. 1993 or AMS 3660C issued March 1966, revised February 1994. 
     Typically, a slug ball separator  10  is used between two like-sized balls that are sized to engage the conical chamfered surfaces  18  and  20 . As seen in  FIG. 3 , the diameter d(spher) of each ball  24  is larger than the outside diameter d o  of slug ball separator  10 . In the illustrated embodiment, the ratio of diameter d o  to the ball diameter d(spher) is about 0.85:1. In addition, the slug ball separator  10  is configured to provide a separation between the balls that is equal to about 3.2% to about 64% of a ball diameter, optionally about 3.2 to about 9.6% or, in a specific example, about 6% of a ball diameter. Thus, in a particular embodiment, the center-to-center distance d(csc) of balls in contact with, but separated by, the slug ball separator  10  is about 1.06 times a ball diameter. 
     A ball bearing  30  comprising slug ball separators is shown in the partial schematic view of  FIG. 4 , which shows balls  24  between inner race  32  and outer race  34  and separated from each other by slug ball separators  10 . As indicated above, due to the use of slug ball separators  10 , ball bearing  30  provides a surprising improvement over a prior art caged ball bearing for the same rotary wing aircraft because it allows the use of a larger ball. For example, a bearing of metric size 70 mm-bore, 110 mm-OD (outside diameter) and 18 mm-width with slug ball separators between the balls can employ a ball of 12.7 mm (½ in.) diameter where a comparative bearing that comprises a nylon separator cage for the balls employs balls of 11.9 mm ( 15/32 in.) diameter. In addition, the races (rings) are stronger than in the prior art bearing because there is no need to chamfer either race to accommodate a cage. In contrast to a caged bearing, the slug ball separators orbit and flow with minimal resistance to lead-and-lag motions of balls  24  as bearing  30  rotates. These advantages are achieved without impact on bearing features such as contact angle, pitch diameter and the number of balls in the bearing. 
     The outer ring  34  of bearing  30  is shown in cross-section in  FIG. 5A . Outer ring  34  is annular about a central axis (not shown). Outer ring  34  has an annular outside surface  34   a  that defines the outside diameter of bearing  30 , a front face  34   b  and a back face  34   c , both of which are annular and perpendicular to the central axis. The interior surface of outer ring  34  defines an inner raceway  34   d . The interior surface of outer ring  34  also includes an annular lead-in surface  34   e  that is substantially parallel to the central axis and is between the inner raceway  34   d  and the front face  34   b . There is also an annular shoulder surface  34   f  that is substantially parallel to the central axis and is between inner raceway  34   d  and back face  34   c . In contrast to a comparative bearing made using a cage for the balls, there is no chamfer surface between the inner raceway  34   d  and the front face  34   b . Lead-in surface  34   e  conforms to a conical lead-in angle of about 1° to about 3° relative to the central axis (and convergent towards the inner raceway  34   d ), to allow balls to be snapped into the bearing raceway when the inner ring  32  is situated within the outer ring  34 , but this is not a chamfer as would be required to accommodate the insertion of a cage after the balls are inserted into the bearing. 
     The inner ring  32  of bearing  30  is shown in cross-section in  FIG. 5B . Inner ring  32  is annular about a central axis (not shown). Inner ring  32  has an annular inside surface  32   a  that defines the outside diameter of bearing  30 , a front face  32   b  and a back face  32   c , both of which are annular and substantially perpendicular to the central axis. The outer surface of inner ring  32  defines an outer raceway  32   d . The outer surface of inner ring  32  also includes an annular lead-in surface  32   e  between the outer raceway  32   d  and the front face  32   b . There is also an annular shoulder surface  32   f  that is parallel to the central axis and is between outer raceway  32   d  and back face  32   c.    
     It is readily apparent from  FIGS. 5A and 5B  that bearing  30  is an angular contact bearing that can support a load in a direction parallel to the central axis of the bearing, due to the asymmetric disposition of the raceways on the rings. 
     The use of slug ball separators yields a dynamic load rating increase of about 14.5% and a bearing fatigue life increase of about 50% over a bearing having a nylon cage for the balls, according to formulations established in Anti-Friction Bearing Manufacturer Association, Inc. Standard number 9-1990. 
     In another embodiment, the present invention is utilized in the bearing of a rotor mount. For example,  FIG. 6  provides a cross-sectional view of a bearing stack useful in a rotary wing aircraft tail rotor head assembly for a Sikorsky CH53A/D helicopter. Each of the four rotor blades of the tail rotor assembly is fitted with a bearing stack on a respective spindle attached to the rotor head. Bearing stack  40  comprises five matched ball bearings  30   a - 30   e  all utilizing the same size balls  24  separated by slug ball separators  10  and dispose between inner races  32  and outer races  34  as described herein. Bearing stack  40  is a sub-component of the tail rotor head assembly that permits the blade to rotate in response to rudder control input. The manufacturer material specification for the rings is AMS 6440 or AMS 6441; the specification for the halls is AMS 6440 or SAE51100. In one evaluation, the use of PEEK or PTFE slug ball separators as described herein resulted in an increase of the dynamic load rating of the bearing by about 14% and an increase of the fatigue life by about 50% relative to the use of nylon cage in the bearing. 
     In an illustrative environment of use shown in  FIG. 7 , bearing stack  40  comprises part of a tail rotor head assembly  50  for a Sikorsky CH53A/D aircraft. Assembly  50  comprises a head member  52  that has a center  52   a  and that carries multiple (e.g., four) blade mount assemblies  54  at equal intervals (e.g., of 90°) around the center  52   a . Each blade mount assembly  54  comprises a spindle  56  on which a bearing stack such as bearing stack  40  is mounted. Each blade mount assembly  54  also includes a mounting collar  58  that is secured to the bearing stack, e.g., bearing stack  40 . The mounting collar  58  is thus rotatable in the blade mount assembly  54  and is adapted to have a tail rotor blade mounted thereon. The tail rotor blade is thus rotatable about the spindle  56 . 
     A ball bearing having slug ball separators instead of a nylon cage can also be employed in a rotary wing aircraft swashplate. As is known in the art, a swashplate generally comprises a stationary plate mounted on a mast and a rotating plate mounted on the mast in juxtaposition to the stationary plate. There is a thrust bearing between the stationary plate and the rotating plate to facilitate rotation of the rotating plate. The thrust bearing comprises an inner race and an outer race and a plurality of balls between the inner race and the outer race. In the prior art, the balls were kept in place by a cage. In keeping with the present invention, the bearing comprises slug ball separators between the balls. Thus, the bearing  30  of  FIG. 4  is seen in  FIG. 8  as a thrust bearing portion of a swashplate assembly. The swashplate assembly  35  comprises a stationary inner swashplate member  36  and a rotating outer swashplate member  38 . The inner race  32  of bearing  30  is in contact with the stationary swashplate member  36  and the outer race  34  is in contact with the rotating outer swashplate member  38 . Between the inner race  32  and the outer race  34 , the bearing  30  comprises a plurality of balls  24  that are separated by slugs  10 . The use of slug ball separators yields a dynamic load rating increase of about 14.5% and a bearing fatigue life increase of about 50% over a bearing having a nylon cage for the balls, according to formulations established in Anti-Friction Bearing Manufacturer Association, Inc. Standard number 9-1990 
     Unless otherwise specified, all ranges disclosed herein are inclusive and combinable at the end points and all intermediate points therein. The terms “first,” “second,” and the like, herein d o  not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. All numerals modified by “about” are inclusive of the precise numeric value unless otherwise specified. 
     Although the invention has been described with reference to particular embodiments thereof, it will be understood by one of ordinary skill in the art, upon a reading and understanding of the foregoing disclosure, that numerous variations and alterations to the disclosed embodiments will fall within the spirit and scope of this invention and of the appended claims.