Abstract:
A flexible, resilient seal protects a bearing in a pitch joint of a helicopter blade assembly from contaminants. The seal includes a flexible, resilient body having a first end bonded to a stationary portion of the joint, and a second end biased into sealing engagement with a displaceable potion of the joint. The body includes an intermediate bellows section surrounding the bearing that expands and contracts to accommodate displacement between the stationary and movable portions of the joint. The second end of the seal includes a lip that rubs on a sacrificial wear layer formed on the second portion of the joint.

Description:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    This invention was made with government support under contract number DAAH01-99-3-R001 awarded by the U.S. Army Aviation and Missile Command (AMCOM). The government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    This invention generally relates to rotor blade assemblies for aircraft such as helicopters, and deals more particularly with a seal for protecting bearings in pitch joints of the blade assembly from contaminants. 
         [0004]    2. Description of the Related Art 
         [0005]    Helicopter aircraft employ a pitch joint that allows a main rotor blade to change its angle of attack or pitch. The ability to change the pitch on a rotor blade is essential to the operation of a helicopter. Pitch joints operate in a demanding mechanical environment due to high loads and rapid movement of the blades. Moreover, the loading on the pitch joint can reverse direction with every rotation of the rotor blade, depending upon whether the blade is advancing or retreating with respect to the movement of the helicopter. 
         [0006]    The bearings in pitch joints often operate under hostile environmental conditions, and require lubrication in order to reduce wear. For maintenance and other reasons, dry lubricated bearing technologies are sometimes used to lubricate pitch bearings. Dry lubrication bearing technologies employ a low friction coating applied to a bearing metallic substrate that is one of two moving surfaces forming the bearing. The movements between the two surfaces wear away the coating on the bearing. During the wear, some of the low friction coating is transferred from the bearing to the mating part, which is often a sleeve. This transfer of the coating is essential for the correct operation of a dry lubricated bearing. In effect, the two moving surfaces of the bearing both become covered with low friction material during normal operation of the bearing system. Once the transfer of wear debris is established, the wear rate is dramatically reduced, and the life of the bearing is increased. 
         [0007]    The reversing loading nature of the pitch joint places high demands on dry lubricated bearing technologies, especially when repeated gaps occur between the bearing surfaces as a result of loading reversal. Gaps between the bearing surfaces allow the wear debris to be flushed from the bearing joint, thus eliminating the transfer coating that has been transferred to the non-bearing member. Each time the pitch joint is flushed of wear debris, the system must redeposit a transfer coating to the non-bearing surface, thereby diminishing the amount of the low friction coating that remains on the bearing surface. As the low friction coating on the bearing wears, the gap that is created during load reversal increases over time, exacerbating the problem. 
         [0008]    The problem of rapid wear of the low friction coating is further amplified where environmental contaminants such as water, sand or dust enter the joint and flush out the transfer debris at a faster rate than that caused by simple movement of air through the joint. In addition, debris such as sand and dust may become embedded in the relatively soft, low friction coating, thereby increasing friction which reduces the bearings&#39; operating life. 
         [0009]    Previous pitch joint designs have incorporated a seal directly on surfaces of the pitch joints. While this arrangement may be effective in some applications, it is difficult or impossible to add this type of seal construction after the pitch joint has been assembled, such as in a retrofit application. 
         [0010]    Accordingly, there is a need for a bearing seal which effectively protects the bearing against environmental contaminants, yet which has mechanical properties and a physical configuration that does not interfere with the normal movement and operation of the pitch joint. The present invention is directed toward satisfying this need. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    In accordance with one aspect of the invention, a contaminant seal is provided for a pitch bearing for a pitch joint in a helicopter rotor blade assembly. The contaminant seal comprises a unitary body protectively covering the pitch bearing. The body includes a first end, a second end and an intermediate section between the first and second ends. The first end is secured to a stationary part of the pitch joint while the second end has a lip that slideably engages a moveable portion of the joint. The intermediate section is axially expandable between the stationary and moveable portions of the pitch joint, and urges the second end of the seal into constant sealing engagement with the moveable portion of the joint. The intermediate section of the seal includes a flexible bellows that allows the body to expand in a direction toward the moveable portion of the joint. The second end of the seal includes a lip having a rub surface that rubs against a wear coating deposited on the moveable portion of the joint. The intermediate section of the seal comprises a resilient material held in compression between the stationary and moveable portions of the joint so that constant force is imposed on the lip to maintain the seal. 
         [0012]    In accordance with another aspect of the invention, a seal is provided for protectively sealing a bearing in a pitch control joint used in a helicopter rotor blade assembly. The seal includes a first and second ends, and a resilient, intermediate section compressed between the first and second ends. The first end is secured to a stationary portion of the joint and forms a generally annular closure at the first end. The flexible second end engages and seals against a displaceable portion of the joint, forming a generally annular closure at the second end. The intermediate section of the seal protectively surrounds the bearing and biases the second end into continuous sealing engagement with the displaceable portion of the joint. 
         [0013]    In accordance with still another aspect of the invention, a seal is provided for protecting a pitch control bearing from environmental contaminants. The seal comprises an annularly shaped body protectively surrounding the bearing and having first and second ends respectively forming first and second closures between the body and the joint. The body includes a flexible, resilient portion held in compression within the joint, which is expandable with the joint to maintain the first and second closures when the joint expands. The seal may be formed from a hydrogenated rubber. 
         [0014]    The bearing seal of the present invention advantageously seals a dry lubricated bearing against intrusion by environmental contaminants, without interfering with the normal operation of the pitch joint. The use of resilient bellows in the seal produces a spring-like force that causes the seal to remain in tight sealing engagement with a moveable portion of the joint. The material properties of the seal are highly compatible with sacrificial wear coatings applied to the joint that create wear debris which contribute to joint lubrication. 
         [0015]    These and further features, aspects and advantages of the embodiments will become better understood with reference to the following illustrations, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0016]      FIG. 1  is a fragmentary, plan view of a rotor blade assembly for a helicopter, having pitch joints employing a bearing contaminant seal in accordance with the present invention. 
           [0017]      FIG. 2  is a perspective view of the main rotor hub forming part of the assembly shown in  FIG. 1 , and depicting a single pitch housing. 
           [0018]      FIG. 3  is an enlarged, perspective view of the area designated as “C” in  FIG. 2 . 
           [0019]      FIG. 4  is a sectional view taken along the line  4 - 4  in  FIG. 1 . 
           [0020]      FIG. 5  is an enlarged view of the area designated as “D” in  FIG. 4 . 
           [0021]      FIG. 6  is a side elevational view of the seal. 
           [0022]      FIG. 7  is a view of one end of the seal. 
           [0023]      FIG. 8  is a view similar to  FIG. 7 , but showing the areas of the seal that have similar cross section geometries. 
           [0024]      FIG. 9  is a sectional view taken along the line  9 - 9  at  FIG. 8 . 
           [0025]      FIG. 10  is a sectional view taken along the line  10 - 10  in  FIG. 8 . 
           [0026]      FIG. 11  is a perspective view of one side of the seal. 
           [0027]      FIG. 12  is a perspective view showing the opposite side of the seal. 
           [0028]      FIGS. 13 and 14  are enlarged views of the sections respectively designated as “E” and “F” in FIG.  5  when the pitch joint is in a static state, the view in  FIG. 14  having been rotated away from the fastener to show a different portion of the seal. 
           [0029]      FIGS. 15 and 16  are similar to  FIGS. 13 and 14 , respectively, but showing the pitch joint in a dynamic state. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Referring to  FIGS. 1-5 , a main rotor blade assembly generally indicated by the numeral  20  in  FIG. 1  may be used in rotating wing aircraft, such as a helicopter. The rotor blade assembly  20  includes a main rotor hub  24 , three pitch housings  22 , three lag dampers  23  and three blades  28 . The components shown in  FIG. 1  comprise a fully articulated rotor system in which the rotor blades  28  are permitted pitching, flapping and lead-lag movement about respective pitch, flap and lead-lag axes. Lead-lag blade movement is produced using lead-lag hinges  30 , and flap movement is produced using flap hinges  25 . 
         [0031]    A tie bar  24  is connected at its opposite ends to the hinges  25 ,  30  by connecting pins  44 ,  46 . A pitch shaft  34  is contained within the pitch housing  22 . The pitch housing  22  and pitch shaft  34  each have generally circular cross sections and are connected together by means of an inboard pitch bearing  38  and an outboard pitch bearing  39 . As best shown in  FIG. 5 , the inboard pitch bearing  38  is generally L shaped in cross section and is secured to the pitch shaft  34  by means of a fastener  32 . A sleeve  40  of suitable material that compliments the low friction pitch bearing  38  is received within one end of the pitch housing  22  surrounding the pitch bearing  38 . A low friction coating  42  deposited on the pitch bearing  38  reduces friction between the pitch bearing  38  and the sleeve  40 . The interface between the low friction coating  42  on bearing  38  and the inside face of the sleeve  40  form a bearing surface between the pitch shaft  34  and the pitch housing  22  which is lubricated by means of the low friction coating  42 . 
         [0032]    In accordance with the present invention, a bearing contaminant seal  26  protectively encloses the pitch bearing  38 , and surrounds an annular gap between the end of the pitch housing  22  and the pitch shaft  34 . As will be described below, the seal  26  is formed of a flexible, resilient material and is held in a compressed state between the end of the pitch housing  22  and the pitch shaft  34 . By virtue of its resiliency and geometry, the seal  26  expands in the direction of its length when the gap between the pitch housing  22  and pitch shaft  34  increases when the rotor assembly  20  is in a dynamic state, i.e., rotating at high speed. 
         [0033]    Referring now also to  FIGS. 6-12 , seal  26  includes a unitary body, formed, as by molding, of a resilient, synthetic flexible material preferably hydrogenated nitrial butadiene rubber (HNBR). HNBR has been found to exhibit desirable properties in aircraft applications and is particularly resistant to degrading by environmental contaminants. HNBR is highly impervious, both at high and low temperatures to virtually all potential contaminants found in aircraft applications, including sunlight, humidity, sand, jet fuel, etc. 
         [0034]    The seal  26  includes a first end  48 , a second, opposite end  50 , and an intermediate section  52  between the ends  48 ,  55 . The first end  48  is bonded around its entire periphery, using a suitable adhesive, to the pitch bearing  38 , as best seen in  FIG. 5 . The first end  48  includes two sections  54 ,  56  which are radially enlarged in order to cover the fasteners  32 . The second end  50  includes a lip having a rub surface  58  which extends around the entire periphery of the second end  50 . The intermediate section  52  of the seal  26  contains a bellows-like fold whose function will be described below. As a result of the geometry of the seal  26  described above, the cross section of the seal  26  varies between the configuration shown in  FIG. 9 , and the configuration shown in  FIG. 10 . The cross sectional geometry shown in  FIG. 9  extends circumferentially in a 60 degree and 30 degree sections designated as “A” in  FIG. 8 . Similarly, the remaining periphery of the seal  26  as shown in  FIG. 8 , designated by “B” extends around the remaining 270 degrees of the seal  26 . 
         [0035]    Referring now also to  FIGS. 13 and 14 , the seal  26  is mounted in a compressed state between the pitch bearing  38  and the pitch housing  22 . More specifically, the first end  48  of seal  26  includes a cut-out  56  in which one end of the pitch bearing  38  is received. A layer of adhesive (not shown) is used to bond the first end  48  to the pitch bearing  38 , thereby forming a closure around the entire periphery of the first end  48  of the seal  26 . 
         [0036]    The intermediate section  52  of the seal  26  floats in the open space between the end of the pitch housing  22  and the pitch bearing  38 . The second end  50  of seal  26  tightly engages and seals against the end of the pitch housing  22 . More particularly, the rub surface  54  ( FIGS. 9 and 10 ) seats against a sacrificial layer  36  of low friction material such as Teflon®, which extends around the entire periphery of the pitch housing  22 . As a result of the bellows-like intermediate section  52  of the seal  26 , and the resilient nature of the seal  26  material, the rub surface  54  is biased into continuous, sealing engagement with the sacrificial coating  36  so as to form a continuous seal around one end of the bearing  38 . The normal, expanded position of the second end  50  of seal  26  is designated by the numeral  50   a.    
         [0037]      FIGS. 13 and 14  depict the pitch joint in a static state when the blade assembly  20  is not rotating at high speed. The dynamic state of the pitch joint is shown in  FIGS. 15 and 16  in which the blade assembly  20  is rotating rapidly. Centrifugal force causes the pitch housing  22  to be displaced radially a short distance, thereby increasing the gap between the pitch housing  22  and the end of the pitch bearing  38 . As the pitch housing  22  is radially displaced, the spring-like nature of seal  26  causes the second end  50  to move outwardly so as to maintain sealing engagement with the pitch housing  22 . Conversely, when the blade assembly  20  slows, reaching a near static state, the radially inward movement of pitch housing  22  causes the seal  26  to compress, as shown in  FIGS. 13 and 14 . 
         [0038]    The seal  26  is installed in the pitch joint in a compressed condition and remains compressed even after centrifugal force due to blade rotation attempts to open the pitch joint. The shape of the cross section of the seal  26  transitions from the shape of the pitch bearing&#39;s periphery to a completely circular shape on the outboard end. It should be recognized that other shapes may be used on the static, inboard side of the pitch joint in order to accommodate differing geometries used to fasten the pitch bearing  38  to the pitch shaft  34 . 
         [0039]    The seal of the present invention may be installed in pitch joints at the time of original manufacture, or as a retrofit on existing rotor blade assemblies. Where the seal  26  is used in retrofit applications, it is possible to apply the sacrificial layer  36  of wear material by cutting strips the material and attaching them to the pitch housing  22  by means of adhesive. It should also be noted that the seal  26  may be installed over pre-existing seals covering the pitch bearings, either at the time of original manufacture or on a retrofit basis. 
         [0040]    Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.