Bladed rotor assembly
Most blade failures, in bladed rotor assemblies, are attributed to vibratory-induced, alternating stress which fatigues the blade material. Blade vibratory stress is very sensitive to the amount of damping present. Damping usually takes the form of aerodynamic, material and friction damping. Of the three, friction damping is the only one which can be controlled, since material damping is an inherent property of the material, and aerodynamic damping is dependent on blade environment. It is an object of this invention to disclose a blade rotor assembly with novel friction damping. It is also an object of this invention to set forth a method of forming a bladed rotor assembly with such friction damping. 
     It is particularly an object of this invention to set forth a bladed rotor assembly, for a turbine or the like, comprising a wheel; said wheel having a periphery with a recess formed therein; said recess having confronting surfaces, a plurality of blades, each thereof having a root; said blades roots being set, radially, in said recess with at least a minute clearance between said roots and said surfaces; means interposed between at least one of said surfaces and at least a plurality of said roots for effecting damping (a) between said one surface and said plurality of roots, and (b) between said roots of said plurality thereof. 
     It is also an object of this invention to set forth a method of forming a bladed rotor assembly, for a turbine or the like, comprising the steps of: providing a wheel; forming a recess, having confronting surfaces, in the periphery of the wheel; setting at least one, substantially annular element loosely in said recess; providing a plurality of root-ended blades; securing the root ends of said blades in said recess with said element interpositioned between said root ends and at least one of said surfaces; and indexing said blades for a uniform spacing therebetween. 
    
    
     Further objects of this invention as well as the novel features thereof will become more apparent by reference to the following description taken in conjunction with the accompanying figures in which: 
     FIG. 1 is a fragmentary, elevational view, partly in cross-section, of an embodiment of the invention; 
     FIG. 2 is a cross-sectional view taken along section 2--2 of FIG. 1; 
     FIG. 3 is a cross-sectional view taken along section 3--3 of FIG. 1; 
     FIG. 4A is a fragmentary view, partly in crosssection, of the root end of one of two locking-pin confronting blade roots; 
     FIG. 4B is a view, like that of FIG. 4A, of the root end of the other of the two locking-pin confronting blade roots. 
     FIG. 5 is a fragmentary plan view of the periphery of the novel bladed rotor assembly, less the shrouds, and with only two cross-sectioned blades shown in place. 
    
    
     As shown in the figures, a bladed rotor assembly 10, according to the embodiment of the invention, comprises a wheel 12 in the periphery of which is formed a recess 14. The recess has, intermediate the depth thereof, inwardly-extending, keying prominences 16; the walls of the recesses define the confronting surfaces between which blades are fixed. It is a great plurality of root-ended blades 18 which are fixed in the recess, and the root end of each blade 18 has a tapered land 20 which engages the keying prominences to secure the blade in place. A plurality of circular damping wires 22 are loosely placed in the recess 14, between the shanks of the root ends and the confronting surfaces of the recess, prior to blades installation, in order to damp the blades vibration during operation of the rotor assembly 10. The wires 22 effect a frictional, rubbing contact between one or more of the blades 18, should such one or more vibrate out of phase or unison with the others thereof, and upon the blades vibrating in unison, the wires 18 move therewith and effect a damping frictional engagement with the confronting surfaces of the recess 14. 
     The blades 18 are set apart, throughout the periphery of the wheel 12, by means of intervening packers 24. Each packer has a concave and convex surface for defining an interfacing engagement thereof, on each side, with the airfoil-shaped bodies of the blades 18. Immediately adjacent the periphery of the wheel 12 are grooves 26, formed in each of the confronting surfaces, slidably to receive the guide limbs 28 of the packers 24. 
     The root ends of the blades 18 have a width &#34;W&#34; and a thickness &#34;T&#34; both of which are greater than the void obtaining between the prominences 16. Accordingly, to accommodate for the insertion of the root ends, at one point along the circumference of the recess 14, the keying prominences 16 are disrupted by arcuate cut-outs 30. To insert each blade 18 it is necessary only to turn it sideways about its elongate axis, that is, ninety degrees of arc from its normal attitude in the wheel 12, and pass its thickness (&#34;T&#34;) dimension through the cut-outs 30. Then by turning it ninety degrees again, the land 20 will be locked in the lower portion in the recess 14 by the prominences 16. At a point near where the cut-outs 30 are formed, and on the outer periphery of the wheel 12, are a pair of reliefs 32 through which the guide limbs 28 of the packers 24 are passed into the grooves 26. 
     As can be appreciated, then, from the foregoing, blades 18 and packers 24 are assembled, in turn, onto the wheel 12. A first blade is set in place, via the cut-outs 30, and slid therefrom along the recess 14, and then a first packer 24 is set in place, via the reliefs 32, and slid therefrom along the grooves 26. In sequence, then, the rest of the complement of blades 18 and packers 24 are installed thusly, with two exceptions, however. The first and last blades installed are unique, to accommodate a final, pinned lock-up of the blade complement. The first to be installed is blade 18a (FIG. 4A); it has a chamfer 34, at the base of the root thereof, on the &#34;suction&#34; (i.e., convex) side of the blade body. The last blade to be installed is blade 18b (FIG. 4B); it has a chamfer 36, at the base of the root thereof, on the &#34;pressure&#34; (i.e., concave) side of the blade body. 
     Blade 18a, following insertion thereof, is slid fully circumferentially (substantially three hundred and sixty degrees of arc) along the recess 14 until it is adjacent to the cut-outs 30, again, by means of which it accessed the recess 14. The full, remaining complement of blades is set in the recess 14, in like manner, to line up, juxtapositionally, behind blade 18a. The last blade 18b, then, is installed, and it too will be immediately adjacent to the cut-outs 30, albeit opposite them from blade 18a. The final packer 24 is then set into the space intervening blades 18a and 18b, via the reliefs 32. Now, all blades and packers are shifted until blades 18a and 18b are astride the cut-outs 30. 
     In traverse of the wheel 12, and opening onto the cut-outs 30, is formed a locking-pin hole 38. A pin 40 is press-fitted into the hole 38 to interface the chamfers 34 and 36. During installation of pin 40, it may be necessary to make minor, adjusting re-orientations of the blade complement, in order to admit the pin between the chamfers. 
     With the pin 40 in place the packer 24 most adjacent to the reliefs 32 is sufficiently displaced from the latter to prevent its access thereto, and the pin 40 blocks access to the cut-outs 30 by blades 18a and 18b. 
     As explained thus far, then, the damping wires 22 are set loosely in the recess 14 of the wheel 12. Thereafter, the blades 18, 18a and 18b and packers 24 are installed in the wheel 12, with the wires 22 interposed between the confronting side surfaces of the recess 14 and the blade roots. As shown in FIGS. 2 and 3, a plurality of wires 22 are employed in this exemplary embodiment. However, in alternative embodiments, single wires 22, on each side of the blade roots may be employed. Too, single &#34;damping&#34; wires 22 of one given gauge may be used, as aforesaid, with one or more &#34;loading&#34; wires 22 of same or different gauge set in thereunder to load the single &#34;damping&#34; wires. 
     The wires 22, of whatever complement, are rolled to define thereof substantially the diameter of the wheel 12; i.e., a three hundred and sixty degree loop, of approximately or substantially the aforesaid diameter, and cut to that dimension. Then they are set into the recess 14. Next, they are cut, once more, to define a small gap between the ends thereof. Where three wires 22 are used, as shown in this embodiment, in the opposite sides of the recess, they are set with the end-gaps spaced apart. Thus, the wires are set with the gaps approximately one hundred and twenty degrees apart. 
     The blades 18, 18a and 18b are loosely set in the recess 14. Following their insertion, and setting apart by means of the packers 24, they can be slightly rocked from side to side, and fore and aft, pivotably on the keying prominences 16; they can also be displaced, axially, if ever so slightly. Simply, they and the recess 14 define a minute clearance therebetween. This design, together with the novel cut-outs 30 and packers 24, accommodates a simple, quick and inexpensive assembly of the bladed rotor without sacrificing efficiency thereof. Notwithstanding the relatively loose fit of the blades 18, 18a and 18b and packers 24 in the wheel 12, they cooperate to substantially close off the recess 14, from the working bodies of the blades. This can be seen in FIGS. 1, 3 and 5. The packers 24 close into near engagement with the pressure and suction sides of the blades, define a land between the blades--bridging across the recess 14--and define a common surface with peripherally outermost surfaces of the wheel 12. There obtains a very small clearance between each blade and packer in the full complement of blades 18, 18a and 18b and the packers 24. This is so that an optimum tolerance build-up will be provided to facilitate assembly of the aforesaid full complement in the wheel 12, otherwise the last blade 18b or the last packer would not be able to be properly fit in the wheel 12, unless the same were undersized or oversized. 
     The packers 24 intervene between the blades, and set the latter apart, but other means are employed uniformly to space apart the blades of the complement thereof. Such spacing means are shrouds. 
     The outermost ends of the blades 18, 18a and 18b have tenons 42 extending therefrom which are received in inner and outer shrouds 44 and 46. The inner shroud 44 has a plurality of precisely-spaced apertures 48 which are larger than the cross-sectional dimension of the tenons 42, and the outer shroud 46 has precisely-spaced apertures 48a which are of substantially the same dimension as the tenon cross-sections, albeit slightly larger to aid assembly. The shrouds 44 and 46 are disposed in surmounting relationship, and the tenons 42 are passed through the inner shroud, through the outer shroud 46, defining close fit with said apertures 48a, and are fixed thereto by peening over the ends (of the tenons 42) and creating an interference fit between the tenons and said apertures 48a. 
     The larger apertures 48, in the inner shroud 44, accommodate therewithin a slightly radiused, stress-relieving conformation at the base 50 of the tenons 42. The inner shroud 44 is of approximately half the thickness of the outer shroud 46. This defines a difference in masses therebetween which, in cooperation with the relative spacing between the shanks of the tenons 42 and the surfaces of the apertures 48, gives rise to a relative sliding or damping movement between the shrouds. 
     In this, our disclosure, we cite the inner shroud 44 and the outer shroud 46. In fact, in this embodiment, there are a plurality of inner shrouds and outer shrouds. By way of example, twenty inner and outer shrouds 44 and 46 are used, each thereof subtending an arc of substantially eighteen degrees. Each of the shrouds has seven apertures 48 and 48a, respectively, formed therein, equally spaced apart. A set of seven blades 18, then, with the therewith engaged shrouds, defines a packetted assembly, and twenty of such assemblies define the full complement for the bladed rotor assembly--in this embodiment. 
     The shrouds 44 and 46 are in surmounting relationship, as stated, but the ends of each are displaced, or circumferentially spaced apart. As shown in FIG. 1, for example, the shroud ends are a minimum of four blade positions apart. Thus, the shroud and gaps are bridged across by an inner or an outer shroud. Also, the ends of the shrouds most adjacent to the locking pin 40 are not less than two blade positions away from the pin. 
     By restricing blade end motion through continuous shrouding, vibratory modes with end or tip motion are eliminated, plus all but one of the out-of-phase modes, significantly reducing the number of vibratory modes associated with packetted assemblies. This suppresses the first tangential cantilevered vibratory mode which is the most severe since it is the most easily excited. The overlapping shroud design offers additional damping from friction rubbing along the shroud interfaces and also allows the bladed rotor to be easily assembled. 
     Friction damping, provided by the wires 22, is the result of the interface rubbing along the wires surfaces. The amount of damping present is dependent upon centrifugally induced wire loading reacting against the blades and the surfaces of the recess and its associated coefficient of friction. By varying the mass of the wire or wires 22, the load is easily controlled for a given set of parameters. 
     The damper wire or wires 22, set between the blades 18 and the walls of the recess 14 dissipate vibratory energy independent of relative blade motion. The invention, therefore, operates under two likely conditions created by excitation frequency: (1) When the motion of the blades 18 is in unison, the damper wires 22 will follow the blades, creating slippage along the interface of the wires and the walls of the recess 14; (2) When a difference in relative blade motion occures, i.e., with the blades &#34;out-of-phase&#34; with each other, the slippage occurs along the blades and the damper wires interfaces. 
     The damper wires 22 are capable of deflecting enough to take up a tolerance difference between the blades 18 while still producing the required blade loading. This requirement is met by providing damper wires 22 small enough to deflect into a loading position and, if necessary, by stacking additional wires 22 behind, to create the optimum loading. 
     Key benefits of this invention are: (1) Elimination of several vibratory modes, including the easily excited first tangential cantilevered vibratory mode; (2) Damping control optimization of vibration of blades 18 excited in-phase or out-of-phase, relative to one another; (3) Blade Spacing of blades 18 controlled by shroud tenon apertures 48 and 48a; (4) Submerged damper wires 22 do not interfere with blade airfoil performance or the structural integrity of the blades 18; (5) Damper wires 22 damp tangential, axial and torsional vibratory modes; (6) Damper wires 22 seat themselves properly with blade tolerances; (7) Wires 22 and recess 14 are covered by the packers 24, providing a smooth steam path at airfoil base of the blades 18; (8) Shrouding of the blades seals the steam path at the outer diameter of the wheel 12, hence greater blade performance results; (9) As the shrouds are overlapped, they are not subject to hoop stress; (10) The inner blade shroud 44 is designed to ride on the outer shroud 46 adding damping from shroud rubbing; (11) The arcuate cut-outs 30 allow for a full complement of blades 18 of equal strength (no weak locking blade); (12) Low cost, easily assembled, loose fitting blades 18; and (13) Separate blade packers 24 minimize blade mass for higher speed capability. 
     While we have described our invention in connection with a specific embodiment thereof, it is to be clearly understood that this is done only by way of example, and not as a limitation to the scope of our invention as set forth in the objects thereof and in the appended claims.