Abstract:
A rotor assembly for a wind turbine including at least one blade adapted to automatically adjust its shape as a function of rotational speed to create an efficient fluid dynamic profile over a wide range of wind conditions and rotational speeds. The rotor assembly includes at least one blade configured to respond to rotation induced forces to automatically bend in a manner to optimize its wind profile.

Description:
RELATED APPLICATIONS 
     This application claims priority based on U.S. provisional application 61/005,595 filed on 5 Dec. 2007. 
    
    
     FIELD OF THE INVENTION 
     The invention described herein relates generally to wind turbines and more particularly to a rotor assembly particularly suited for use in a wind turbine system. 
     BACKGROUND OF THE INVENTION 
     A wind turbine is a machine for converting the kinetic energy of an air flow into mechanical energy. The mechanical energy is most frequently used to drive an electric generator but can alternatively be used to drive a variety of other loads such as a pump, a grinding stone, etc. 
     The prior art is replete with various wind turbine designs. They typically include a rotor assembly including two or more blades, a shaft for transferring rotational energy from the rotor assembly to an electric generator, and a housing generally including a gear box, the electric generator, and an electronic controller. The rotor assembly and housing are typically mounted on a tower to provide sufficient elevation to best expose the blades to the local wind and to assure proper ground clearance. A yaw controller is generally provided to optimally align the rotor assembly relative to the wind direction. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a rotor assembly including at least one blade adapted to automatically adjust its shape as a function of rotational speed to create an efficient fluid dynamic profile over a wide range of wind conditions and rotational speeds. More particularly, a rotor assembly in accordance with the invention includes at least one blade configured to respond to rotation induced forces to automatically bend in a manner to optimize its wind profile. 
     A blade in accordance with the invention is characterized by at least one substantially noncompliant region and at least one substantially compliant region. The blade is mounted with its noncompliant region positioned close to a rotor shaft primary axis and its compliant region positioned radially outward from the noncompliant region and displaced axially therefrom. As a consequence, rotation induced forces act to bend the blade in the compliant region to reduce the axial displacement. The blade is preferably constructed so that the bending occurs along predefined channels to adjust the shape of the blade to create a profile appropriate to the speed of rotation. 
     A preferred rotor assembly in accordance with the invention includes a rotor shaft member carrying a hub structure defining a hub plane oriented substantially perpendicular to the shaft primary axis. The hub structure supports two or more blades which are preferably distributed uniformly around the primary axis. Each blade includes a proximal region which is mounted proximate to the hub structure and a distal region which is spaced radially outward from the proximal region. Moreover, each blade is mounted at an angle relative to the primary axis so as to axially space the proximal and distal regions when the blade is at rest. Moreover, each blade is mounted so as to define a pitch angle appropriate to divert an axial wind component in a direction to rotate the blade around said primary axis. 
     Each blade is constructed so that its proximal region is relatively stiff, or noncompliant, and its distal region is relatively flexible, or compliant. This enables the distal region to automatically bend as the blade rotates to create and maintain an efficient blade profile over a wide range of wind conditions and rotational speeds. 
     A blade embodiment in accordance with the invention includes a noncompliant region adjacent the blade proximal end and one or more regions of progressively greater compliance, or lesser stiffness, approaching a compliant region at the blade distal end. The compliant region extends laterally and longitudinally beyond an outer edge of the noncompliant region. The transitions between regions form bend channels around which the blade will bend as it rotates. The outer transition preferably defines lateral and longitudinal portions to facilitate compound bending by the compliant region. 
     More particularly, a blade embodiment in accordance with the invention comprises a substantially planar, i.e., sheetlike, member having a proximal region, or inner end, and a distal region, or outer end. The blade defines a periphery which includes a first, or leading, edge and a second, or trailing edge. The leading edge of the preferred blade embodiment includes a portion extending substantially longitudinally and a portion extending substantially laterally. The trailing edge similarly includes a substantially longitudinal portion and a substantially lateral portion. The blade is mounted on the hub structure with an appropriate pitch angle so that an axially directed wind will produce a force component to rotate the blade in a direction so that the air is cut first by the leading edge. As a consequence of the rotation and radial acceleration, the blade distal end will move radially outward and axially toward the hub plane, i.e., in a direction opposite to the incoming wind. This action automatically shapes the blade&#39;s compliant region, e.g., draft, tension, and pitch, to define a profile which efficiently converts the wind energy to rotational mechanical energy over a wide range of wind conditions and rotational speeds. 
     The preferred rotor assembly described herein includes at least one substantially planar, i.e., sheetlike, blade having a proximal region which is substantially rigid, a slightly compliant intermediate region radially outward from the proximal region, and a moderately compliant distal region radially outward of the intermediate region. The proximal region defines a substantially circumferential outer edge which serves as a bend channel for the intermediate and distal regions. The intermediate region defines a substantially circumferential outer edge which serves as a bend channel for the distal region. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1   a . is a side elevational view of an exemplary two blade rotor assembly in accordance with the invention and  FIG. 1   b . is an enlarged edge view taken substantially along the plane  1   b - 1   b  of  FIG. 1   a.;    
         FIG. 2  is a front elevation view of the rotor assembly of  FIG. 2 ; 
         FIG. 3  is a rear elevation view of the rotor assembly of  FIG. 1 ; 
         FIG. 4  is a matrix of schematic representations respectively depicting the assembly of  FIG. 1  at 0, (column  1 ), low (column  2 ), and high (column  3 ) rotational speeds; and 
         FIG. 5  is a side elevational view of an exemplary four blade rotor assembly in accordance with the invention oriented with the primary shaft axis substantially coincident with the plane of the paper; 
         FIG. 6  shows the four blade embodiment of  FIG. 5  oriented to show the primary shaft axis extending at about 45° relative to the plane of the paper; and 
         FIG. 7  is an enlarged plan view of a portion of  FIG. 6  showing how the proximal end of a blade is mounted to the hub structure. 
     
    
    
     DETAILED DESCRIPTION 
     Attention is now directed to  FIGS. 1-3  which illustrate a preferred exemplary embodiment of a rotor assembly  10  in accordance with the present invention. The assembly  10  includes a shaft member  12 , e.g., a collar, intended to be mounted for rotation around a primary shaft axis  14 . The shaft member  12  preferably carries a hub structure  16  configured to mount one or more rotor blades  18 ,  20 . As will be discussed in greater detail hereinafter, the blades are mounted to define a pitch which causes them to rotate counter clockwise, as seen in  FIG. 2 , in response to a wind component  22  directed against the front faces  24  of the blades  18 ,  20 . The hub structure  16  includes a rigid plate  26  defining a hub plane oriented substantially perpendicular to the primary axis  14  of shaft member  12 . The plate  26  is formed to define terminal mounting ears  28 ,  30 , each located radially outward from axis  14  and oriented at an angle, e.g., 30°-35°, relative to the hub plane defined by plate  26 . 
     Inasmuch as the rotor blades  18 ,  20  are preferably identically constructed, the detailed discussion herein will primarily refer to blade  18  but, unless otherwise stated, should be understood as also applying to blade  20 . The blade  18  includes a flexible substantially planar, or sheetlike, member  32  which may be considered as having an elongate somewhat trapezoidal shape having a radially inward proximal region  34  and a radially outward distal region  35 . The planar member  32  defines a periphery including (1) a leading edge  36  having a substantially longitudinally oriented portion  38  and a substantially laterally oriented portion  40  and (2) a trailing edge  42  having a substantially longitudinally oriented portion  44  and a substantially laterally oriented portion  46 . 
     A relatively stiff substantially planar member  50  overlays a portion of the proximal region  34  of the flexible planar member  32 . The stiff planar member  50  is preferably affixed to rigid mounting ear  28  by fasteners  52 , e.g., rivets or bolts, installed adjacent to the outer edge so as to sandwich the flexible member  32  therebetween, as best seen in  FIG. 1   b . Note that a portion of flexible member  32  extends outwardly of the edge  53  of stiff member  50  to define a moderately compliant blade area  54  near the distal region  35 . Note also that a portion of stiff member  50  extends outwardly of the edge  56  of rigid mounting ear  28  to define a slightly compliant area  58  near the proximal region  34  which is less compliant than the area  54 . The rigid mounting ear  28  defines a substantially noncompliant region  60 . More particularly, note that the perimeter of the stiff member  50  is comprised of the outer edge  53 , a substantially longitudinally oriented edge  62  which is preferably aligned with the trailing edge  42  longitudinally oriented portion  44  and a substantially laterally oriented edge  64  which is preferably aligned with the leading edge  36  laterally oriented portion  40 . Also note that the outer edge  53  includes a substantially laterally oriented edge portion  66 , a substantially longitudinally oriented edge portion  68 , and a connecting diagonal edge portion  70 . 
     Note that the trailing edge portion  42  extends beyond the outer edge  53  of stiff member  50 , and that the stiff member  50  edges  62 ,  64  each extend beyond outer edge  56  of the rigid mounting ear  28 . As will be discussed hereinafter, the stiff member outer edge  53  (including  68  &amp;  66 ) substantially defines a fulcrum, around which the compliant blade area  54  can bend, both longitudinally and laterally, in response to rotation induced forces, to form a first bend channel. Fasteners  52  installed adjacent to ear outer edge  56  also substantially define a fulcrum around which compliant regions  54  and  58  can bend to form a second bend channel. 
     When at rest, the nominal plane of blade  18  will extend at an angle or pitch of approximately 30°-35° relative to the hub plane, i.e., 60°-55° relative to the primary axis  14 . This orientation will place the blade distal region  35  radially and axially remote from the proximal region  34  adjacent to mounting ear  28 . 
     Attention is now directed to  FIG. 4  which comprises a matrix of schematic representations depicting different views of the rotor assembly  10  at different rotational speeds. More particularly, rows  1  through  4  of  FIG. 4  respectively show front, top, right side, and left side views whereas columns  1  through  3  respectively show the assembly  10  at rest, at low speed, and at high speed. 
     Initially note that row  1 , column  1 , shows a front view of assembly  10  similar to the showing in  FIG. 2 . Note that row  2 , column  1  is a top view showing that the blades  18 ,  20  are oriented at 30°-35° relative to the hub plane. Now contrast row  2 , column  2  with row  2 , column  1  which demonstrates that as the blades transition from rest to a low rotational speed, the blade distal regions  35  move in a direction toward the hub plane  26  and opposite to the wind direction  22 . This action causes the compliant region  54  to bend along stiff member outer edge  53  and slightly compliant region  58  to bend around non compliant region outer edge  56 . This action modifies the blade&#39;s shape causing its longitudinal leading edge  38  to lift more on average than the longitudinal trailing edge  44  thus increasing its lateral pitch and draft and decreasing its longitudinal pitch and draft resulting in a wind profile which is optimized for the rotational speed. As the speed increases further to column  3 , the blade distal regions  35  move even further axially toward the hub plane  26  decreasing the bending around edge  53  and increasing the bending around edge  56 . This action further modifies the blade&#39;s profile to decrease both longitudinal and lateral pitches, flatten the draft, and move the draft location closer to the lateral leading edge, to optimize it for the increased rotational speed. 
     From the foregoing, it should now be appreciated that a rotor assembly has been disclosed herein utilizing a blade configured to automatically bend as a function of rotational speed to present an optimum wind profile over a wide range of conditions. The blade is characterized by a stiffness which diminishes progressively from the proximal blade end to the distal blade end. Stated otherwise, the blade is characterized by a compliance which increases from the proximal to the distal end. Although the preferred blade described herein achieves this progressive compliance characteristic by using multiple overlapping members, e.g., ear  28  and stiff member  50 , it is recognized that the blade can be alternatively formed, for example, by using a single planar member whose thickness, and thus stiffness, is reduced from the blade&#39;s proximal end to its distal end. Alternatively, stiffening rods or cables can be selectively used to achieve the desired progressive compliance. 
     Embodiments of the present invention can be fabricated in a variety of manners utilizing a wide range of materials and dimensions. The specifications of one exemplary two blade embodiment are summarized in Table A hereinafter. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE A 
               
             
             
               
                   
               
               
                 2 Blade Rotor, 35 Degree Pitch, 44″ Outer Diameter Unflexed, 50″ O.D. Fully Flexed 
               
             
          
           
               
                 Item 
                 Material 
                 Dimension 
               
               
                   
               
               
                 shaft member 12 
                 Steel 
                 1.75″ O.D. × .625 I.D. × .900″ Long, 1.125″ O.D. × .625 I.D. × .125″ long 
               
               
                   
                   
                 (with a stiffening disk 3″O.D. × 1.125″ I.D. by .125″ thick) 
               
               
                   
                   
                 ⅝-18 threaded I.D. 
               
               
                 hub structure 16 
                 Steel 
               
               
                 rigid plate 26 
                 Steel (Type 4130) 
                 .125″ thick plate, 4″ × 5″ parallelogram, with two ears (4″ × 3″ × 4″radius) 
               
               
                   
                   
                 bent down 35 degrees. 
               
               
                 noncompliant region 60 
                 Various 
                 .125″ thick steel, .091″ thick HDPE, .040″ aluminum = .256″ total thickness 
               
               
                 outer edge, rigid ear 56 
                 Steel (Type 4130) 
                 5.75″ length (approx. radius 4″), .125″ thick 
               
               
                 terminal mounting ear 28, 30 
                 Steel (Type 4130) 
                 4.0″ lateral edge × 3.0″ longitudinal edge × 4″ radius × .125″ thick 
               
               
                 flexible sheet member 32 
                 High Density 
                 polygon 14″ × 22.78″ × 4.20″ × 12.27″ × 11.00″ × .090″ thick (22.25″ between 
               
               
                   
                 Polyethylene (HDPE) 
                 lateral leading edge and lateral trailing edge). 
               
               
                 leading edge 36 
                   
                 36.78″ 
               
               
                 longitudinally oriented portion 38 
                   
                 22.78″ 
               
               
                 laterally oriented portion 40 
                   
                 14.00″ 
               
               
                 trailing edge 42 
                   
                 27.47″ 
               
               
                 longitudinally oriented portion 44 
                   
                 23.27″ 
               
               
                 laterally oriented portion 46 
                   
                 4.20″ 
               
               
                 fasteners/screws, bolts, or rivets 52 
                 Steel 
                 10-24 × ½ long, .190″ from outer edge, 
               
               
                   
                   
                 spaced approx. equidistant from each other. 
               
               
                 compliant blade region 54 
                 High Density 
                 .091″ thick 
               
               
                   
                 Polyethylene (HDPE) 
               
               
                 slightly compliant region 58 
                 Various 
                 .091 thick HDPE + .040 thick 7075 Aluminum = .131″ total thickness 
               
               
                 stiff planar member 50 
                 Aluminum (Type 7075) 
                 10.75″ × 1.75″ × 13″ (approx. 14″ radius) × 1.75″ × 10.25″ × .040″ thick 
               
               
                 stiff member outer edge, 
                   
                 10.75″ 
               
               
                 longitudinal portion 62 
               
               
                 aligned with trailing 
               
               
                 edge portion 42 
               
               
                 stiff member outer edge, 
                   
                 10.75″ 
               
               
                 lateral portion 64 
               
               
                 aligned with leading edge, 
               
               
                 portion 40 
               
               
                 outer edge, stiff 
                 Aluminum (Type 7075) 
                 1.75″ × 13″ (approx. 14″ radius) × 1.75″ × .040″ thick 
               
               
                 member 53 
               
               
                 stiff member outer edge, lateral portion 66 
                   
                 1.75″ 
               
               
                 stiff member outer edge, 
                   
                 1.75″ 
               
               
                 longitudinal portion 68 
               
               
                 stiff member outer edge, 
                   
                 13″ length (approx. 14.00″ Radius) 
               
               
                 arc portion 70 
               
               
                   
               
             
          
         
       
     
     Attention is now directed to  FIGS. 5-7  which illustrate an exemplary four blade rotor assembly  74  in accordance with the present invention. The embodiment  74  can be constructed similarly to the two blade embodiment  10  previously discussed with reference to  FIGS. 1   a ,  1   b , and  2 - 4 . 
     The rotor assembly  74  includes a shaft member  76 , e.g., a collar, intended to be mounted for rotation around a primary shaft axis  77 . The shaft member  76  preferably carries a hub structure  78  configured to mount rotor blades  80 ,  82 ,  84 ,  86 . As with the aforediscussed two blade embodiment, the blades are mounted to define a pitch which causes them to rotate counter clockwise, represented by direction arrow  87  in response to a wind component  88  directed against the front faces  89  of the blades  80 ,  82 ,  84 ,  86 . The hub structure  78  includes a rigid plate  90  defining a hub plane oriented substantially perpendicular to the primary axis  77  of shaft member  76 . The plate  90  is preferably formed to define rigid mounting ears  100 ,  102 ,  104 ,  106  each located radially outward from axis  77  and oriented at an angle, e.g., 45°, relative to the hub plane defined by plate  90 . 
     Inasmuch as the rotor blades  80 ,  82 ,  84 ,  86  are preferably identically constructed, the detailed discussion herein will primarily refer to blades  80  and/or  84  but, unless otherwise stated should be understood as also applying to blades  82  and  86 . Each blade includes a compliant substantially flexible planar, or sheetlike, member  107  which may be considered as having an elongate polygonal, or somewhat trapezoidal, shape having a radially inward proximal region  108  and radially outward distal region  109 . The planar member  107  defines a periphery including (1) a leading edge  110  having a substantially longitudinally oriented portion  112  and a substantially laterally oriented portion  114  and (2) a trailing edge  115  having a substantially longitudinally oriented portion  117  (including segments  118  and  119 ) and a substantially laterally oriented portion  120 . 
     With particular reference to  FIG. 7 , note that in addition to the compliant member  107 , blade  84  is comprised of two additional planar layers, i.e., a relatively stiff inner member  122  and a more compliant intermediate member  124 . The stiff inner member  122 , intermediate member  124 , and compliant member  107  are sandwiched together and mounted to the noncompliant rigid ear  102  by fasteners  126 , e.g., screws or rivets. Note in  FIG. 7  that the ear  102 , the inner member  122 , and the intermediate member  124  respectively define outer edges  130  and  132 . Also note that fasteners  133  secure planar members  107 ,  122 , and  124  to each other and that fasteners  134  secure members  107  and  124 . The materials and dimensions of the ear  102  inner member  122  intermediate member  124 , and compliant member  107  are selected to exhibit diminishing stiffness progressing toward the outer distal end of the blade. 
       FIGS. 5-7  also show a central brace  140  which is preferably welded at  142  against the hub structure ears  100 ,  102 ,  104 ,  106  to enhance rigidity, particularly for use in large diameter rotor assemblies. 
     As with the aforedescribed two blade embodiment of  FIGS. 1   a ,  1   b , and  2 - 4 , the embodiment  74  of  FIGS. 5-7  will rotate in response to a wind component  88  striking the front faces  89  of blades  80 ,  82 ,  84 ,  86 . Transitions between regions of different stiffness in each blade define bend channels that follow substantially helical patterns relative to shaft axis  77 . The stiffness of these regions progressively diminish with radial distance from the shaft axis  77 . As the speed of rotation increases, each blade progressively bends around bend channels respectively defined by edge  132 , edge  130 , and fasteners  126 , each of which acts as a fulcrum to allow the blades distal end to move radially outward and toward the hub plane defined by rigid plate  90 . As the speed of rotation increases, the blade bending along the helical bend channels results in the blade forming a helicoid like structure, with a diminishing helical pitch and an expanding rotational diameter around shaft axis  77 , thus reshaping the profile to optimize it for the current rotational speed. 
     As previously noted, embodiments of the invention can be provided in a wide range of materials and dimensions. The specifications of one exemplary four blade embodiment are summarized in Table B hereinafter. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE B 
               
             
             
               
                   
               
               
                 4 Blade Rotor, 45 Degree Pitch, 37″ Outer Diameter Unflexed, 46″ Outer Diameter Fully Flexed - Materials and Dimensions 
               
             
          
           
               
                 Item 
                 Material 
                 Dimension 
               
               
                   
               
               
                 rotor assembly 74 
                   
                   
               
               
                 shaft member 76 
                 steel 
                 1.75″ O.D. × .800″ L, 1.25″ O.D. × .6″ L, &amp; .1″ L, 17 MM I.D. × 1.5″ length 
               
               
                   
                   
                 (.100″ step and .600 step O.D. 1.25″) fastened to plate 90 &amp; brace 140 with screws. 
               
               
                 shaft axis 77 
               
               
                 hub structure 78 
               
               
                 Blade 80 
                 Various 
                 polygon 14.375″ × 20.72″ × 4.5″ × 11.44″ × 9.75″ × .091″ 
               
               
                   
                   
                 to .285″ thick, 20.125″ overall length. 
               
               
                 Blade 82 
                 Various 
                 polygon 14.375″ × 20.72″ × 4.5″ × 11.44″ × 9.75″ × .091″ 
               
               
                   
                   
                 to .285″ thick, 20.125″ overall length. 
               
               
                 Blade 84 
                 Various 
                 polygon 14.375″ × 20.72″ × 4.5″ × 11.44″ × 9.75″ × .091″ 
               
               
                   
                   
                 to .285″ thick, 20.125″ overall length. 
               
               
                 Blade 86 
                 Various 
                 polygon 14.375″ × 20.72″ × 4.5″ × 11.44″ × 9.75″ × .091″ 
               
               
                   
                   
                 to .285″ thick, 20.125″ overall length. 
               
               
                 rotational direction 87 
               
               
                 wind component (direction) 88 
               
               
                 face of blades 89 
               
               
                 rigid plate 90 
                 steel (type 4130) 
                 .100 thick plate perpendicular to shaft axis 77, 
               
               
                   
                   
                 approximately 8.9″ diameter with 4 mounting ears 
               
               
                   
                   
                 (approx. 2.5″ × 4″ with arc 5.38″ long), within that dia., 
               
               
                   
                   
                 bent down 45 degrees, spaced 90 degrees apart. 
               
               
                 terminal mounting ear 100, 102, 104, 106 
                 steel (type 4130) 
                 4″ × 2.5″ × 3.1″ × 1.67″ × .6″ 
               
               
                 flexible sheet member 107 
                 high density 
                 polygon 14.375″ × 20.72″ × 4.5″ × 11.44″ × 9.75″ × .091″ 
               
               
                   
                 polyethylene 
                 thick, 20.125″ overall length. 
               
               
                 inward proximal region 108 
               
               
                 outward distal region 109 
               
               
                 leading edge 110 
                   
                 35.125″ 
               
               
                 leading edge longitudinally oriented 
                   
                 20.75″ 
               
               
                 portion 112 
               
               
                 leading edge laterally oriented portion 114 
                   
                 14.375″ 
               
               
                 trailing edge 115 
                   
                 25.75″ 
               
               
                 trailing edge longitudinally oriented 
                   
                 21.25″ 
               
               
                 portion 117 
               
               
                 including longitudinal segments 118 and 119 
                   
                 9.75″ and 11.436″ 
               
               
                 trailing edge laterally oriented portion 120 
                   
                 4.5″ 
               
               
                 inner relatively stiff planar member 122 
                 FR-4 G10 
                 .063″ thick G10 epoxy sheet, 6.38″ × 6.38″ × 7.72″ radius 
               
               
                   
                 epoxy sheet 
               
               
                 intermediate relatively stiff planar member 124 
                 FR-4 G10 
                 .031″ thick G10 epoxy sheet, 7.5″ × 7.5″ × 7.5″ radius 
               
               
                   
                 epoxy sheet 
               
               
                 fasteners/machine screws 126 
                 stainless steel 
                 10-24 × .5 L, for securing members 107, 122, and 124 
               
               
                   
                   
                 to each mounting ear 100-106, 4 ea. per blade. 
               
               
                 outer edge, rigid ear 128 
                 steel (type 4130) 
                 3.11″ at 80.2 degrees, 1.67″ at 134 degrees, .6″ at 
               
               
                   
                   
                 145.77 degrees with fastener 126 adjacent edge 
               
               
                 inner relatively stiff member, outer edge 130 
                   
                 7.72″ radius 
               
               
                 outer relatively stiff member, Outer edge 132 
                   
                 7.75″ radius 
               
               
                 fasteners/rivets 133 &amp; 134 
                 aluminum 
                 tubular truss rivets for securing members 107, 122, and 124 together 4 ea. per blade 
               
               
                 brace plate 140 
                 steel (type 4130) 
                 flat plate approximately 5″ × 5″ × .10″ thick, perpendicular to shaft member 
               
               
                   
                   
                 with 2.5″ edges welded to interior of terminal mounting ears 100, 102, 104, 106 
               
               
                 welded edges 142 
                   
                 4 ea. 2.5″ edges of member 140 welded to mounting ears 100, 102, 104, 106