Rotor blade shaft integrity monitoring system

A helicopter rotor blade to hub arm attachment configured to react centrifugal loads from the blade through a blade cuff, spindle, and elastomeric bearings utilizes a bolt in the bore of the spindle for secondary load path operation and a spindle integrity monitoring indicator to provide a visual showing of a deviation from the normal load path.

DESCRIPTION 
1. Technical Field 
This invention relates to helicopter rotors and more particularly to a 
redundant tension member for a rotor blade retention spindle in 
combination with an indicator system to reveal the load carrying status of 
the tension member. 
2. Background Art 
Current production helicopters use elastomeric bearings to support rotor 
blades from rotor hubs for full articulated motion. Such elastomeric 
bearings comprise stacks of thin alternating rubber and metal laminates in 
various geometric shapes. Blade motions in lead-lag, flapping, and pitch 
change are accommodated by incremental shear of the rubber laminates, and 
devices are incorporated to prevent tension loads from being introduced 
into these bearings. The need for lubrication of bearings is pre-empted. 
One configuration for a helicopter rotor using elastomeric bearings is 
shown in U.S. Pat. No. 4,203,708 to Rybicki. Each rotor blade is attached 
to its associated rotor hub arm by a root end blade shank or shaft portion 
called a spindle. The spindle extends from the blade attachment joint 
through the center of the elastomeric support bearing and is attached to 
the inner race of that bearing by a large nut on the innermost end of the 
spindle. The outer race of the bearing is in turn attached to the hub arm, 
and the spindle is thus in position to react in tension th=full 
centrifugal blade load. In addition to its threaded inner end portion, the 
spindle also includes an external spline to engage the bearing race to 
preclude relative motion therebetween, and at an intermediate station 
includes a journal type shear bearing to react bearing side loads. Toward 
the outboard end of the spindle, bifurcated flanges provide connecting 
means for the pitch control horn, and a second flange member provides 
attachment means for an inplane blade damper. To this extent the blade 
spindle is similar to that illustrated in U.S. Pat. No. 4,203,708 to 
Rybicki, which is used on the Sikorsky S-76 helicopter. 
Recent development efforts have addressed the desirability of redundancy in 
components to provide alternate or backup load paths in the event of 
concern for the structural integrity of the primary member. A challenge to 
the designer is to integrate a redundancy feature with an indicator system 
to alert pilots or maintenance personnel to the occurrence of an incident 
wherein the secondary system has assumed the role of primary load carrier. 
This integration must be performed with a minimum of negative impact on 
weight, cost, drag, and maintenance requirements. A very recent embodiment 
providing redundancy and inspection to the spindle illustrated in the 
aforementioned U.S. Pat. No. 4,203,708 is shown and claimed in U.S. Patent 
Application Ser. No. 315,125 filed on Oct. 26, 1981 in the name of Donald 
L. Ferris et al and entitled Rotor Blade Shaft Integrity Monitoring 
System, now U.S. Pat. No. 4,373,862. 
DISCLOSURE OF INVENTION 
The aforementioned patent application presents combination of a redundancy 
feature and an inspection system in a specific embodiment using a 
pre-loaded bolt as a redundant member and a pressurized reservoir 
containing red dye adapted to drive the tell-tale free flowing dye through 
any crack or opening in the primary spindle member. The displacement of 
the bellows type reservoir as its pressure is reduced, together with the 
brilliant color of the dye on external surfaces of the spindle, provides a 
dual indication of the diversion of the primary load path to its secondary 
path. 
A second embodiment of the combination of a redundant number and an 
inspection or integrity monitoring system is presented herein as an 
alternate to that presented in the aforementioned patent application. 
While this alternate embodiment also uses a redundant bolt in the hollow 
interior of the spindle, the bolt is not pre-loaded to share the primary 
load but is purposely held in reserve. Similarly, the reservoir containing 
red dye is not pressurized under primary load path operating conditions 
but becomes pressurized upon loading of the redundant bolt. This 
embodiment eliminates the need for seals to form a pressurized chamber 
encapsulating the bolt and internal spindle cavity, thus eliminating any 
environmental adverse effects inherent in the use of seals. Further, the 
requirement in the initial embodiment for the dye to escape through a 
crack or fracture in the spindle is eliminated, thus avoiding the 
possibility of dye flow failure due to crack closure upon load decrease as 
due to a landing. Still further, while the dual indication system depends 
on the visible appearance of the red dye staining the exterior surfaces of 
components, the second indication comprising displacement of the bellows 
as it collapses upon pressure loss is eliminated in this second embodiment 
in favor of a pressure relief cap associated with the dye reservoir. Since 
there is no need for visibly measured axial motion of the reservoir, its 
deflection prior to its pressure relief can be minimal, and the reservoir 
may be quite small. This size advantage is one of the attractive features 
of this alternate embodiment over that presented in the earlier patent 
application. In the case of selection of a system for the SH-60B 
"Seahawk"helicopter, this packaging advantage was appreciated, since 
internal spindle space was limited due to the feature of automatic rotor 
blade folding and the need for attendant components. The interior of the 
spindle, especially the enlarged outer end where the blade is attached, is 
a convenient and close location in which to locate the blade fold 
actuator. Because of this space limitation and the attractiveness of the 
features enumerated above, this alternate embodiment was chosen.

BEST MODE FOR CARRYING OUT THE INVENTION 
A typical Sikorsky type elastomeric bearing rotor head is depicted in FIG. 
1 wherein hub 10 includes four hub arms 12 and which hub 10 is mounted for 
rotation about axis 13 of drive shaft 14. A rotor blade 16 is mounted to 
each hub arm 12 by a spindle assembly 18 to provide full articulated rotor 
blade motion in pitch, droop, flap, lead, and lag. The main component of 
the spindle assembly 18 is the spindle shaft 20, as best shown in FIG. 1a, 
a generally cylindrical member extending generally radially from the rotor 
axis 13. The inner end 22 of the spindle 20 is threaded and engages 
retention nut 24 (see FIG. 2). The nut 24 bears against the inner race 26 
of a bearing array 28 having an outer race 30 or flange that bolts to the 
outer flange 32 (see FIG. 1) of the hub arm 12. The bearing array 28 
comprises spherical and thrust bearings 33 and 34, respectively, more 
specifically described with reference to U.S. Pat. No. 3,782,584. Spline 
36 of spindle 20 engages an internal spline (not shown) of inner race 26 
of bearing 34 to prevent relative rotation of the bearing array 28 and 
spindle 20. The further complexity and importance of the spindle assembly 
18 is realized by noting that its components also include shear bearing 
42, droop stop ring 46, pitch control horn 48, and damper mounting lugs 
50. 
The spindle assembly 18 depicted in this application differs from that of 
the aforementioned application on similar subject matter because the 
SH-60B helicopter is equipped for automatic blade folding for storage 
purposes. The mechanism to perform this folding function consists of a 
hinged joint 52, including lock assembly 51, at the point of connection 
between the rotor blade 16 and spindle assembly 18. Although this joint 52 
is not a part of this invention, it does produce an effect, inasmuch as 
the internal space available for locating spindle integrity monitoring 
items is limited due to the need to locate blade fold items in the same 
area. More specifically, FIG. 2 depicts the electrical blade fold actuator 
54 located in the internal chamber 56 of the outer cone shaped end of the 
spindle 20. 
FIG. 2 illustrates the redundant means used with the spindle assembly to 
provide a secondary load path for the centrifugal load imposed in flight 
by the extending rotor blade in the event of damage to the spindle per se. 
Also shown is the specific embodiment conceived for this application of a 
spindle integrity monitoring means to indicate to the operator or service 
personnel that the redundant member has taken over full load carrying 
function and that spindle replacement should be scheduled. Still with 
reference to FIG. 2, we observe that the primary load path for the 
reaction to blade generated centrifugal force travels as a tension load 
the length of spindle 20 from the load connection at joint 52 to the 
threaded connection 22 at nut 24. The load then becomes a compressive load 
from nut 24 into the bearing array 28 and outer bearing race 30. The 
bolted connection between race 30 and hub flange 32 transfers the load to 
hub arm 12. 
A secondary or backup load path is established for the tension load 
referred to above by the use of a long bolt 60 through the internal bore 
62 of spindle 20. The bolt head 64 and a spherical washer 66 are located 
at the radial inner end of spindle 20, and the spherical washer 66 
functions due to its shape to center the bolt 60 and render essentially 
constant tensile loading across its diameter. The threaded end 68 of bolt 
60 receives nut 70, and both are located in internal chamber 56 of the 
outer end of spindle 20. A stackup of first and second washers 72, 74 and 
spacer 76 allows the nut 70 to indirectly bear against shoulder 78 of 
spindle 20 where cavity 56 transitions to bore 62. Thus located, the bolt 
60 stands ready to receive the tension loading of spindle 20 inboard of 
spindle shoulder 78, should damage occur to that spindle 20, rendering it 
unable to sustain the blade load. In any instance wherein a redundant 
member assumes loading due to failure of the primary member, it is 
mandatory that the takeover be known to the operator or maintenance 
personnel so that replacement of the primary member can be scheduled. It 
is tedious and difficult to inspect the entire length of the spindle to 
learn of a possible damaged condition because a large portion of the 
spindle 20 is hidden from external view by the bearing array 28 and hub 
arm 12. Thus, a dual spindle inspection system has been incorporated to 
monitor the integrity of the spindle 20 to provide a readily detectable 
and visual indication of the spindle load carrying function. The 
embodiment of the system presented herein comprises a reservoir assembly 
80 in the form of a metal bellows 81 containing red dye in a 
non-pressurized condition. As shown in detail in FIG. 3, the reservoir 80 
is ring shaped and is sealed at opposite axial ends by end plates 82, 83. 
Extending through the outer end plate 82 of reservoir assembly 80 is a 
single transfer tube 84 leading from the spindle cavity 56 externally 
through an observation port 86 in spindle 20. Cap 88 is attached to the 
outer end of the tube 84 and serves as a relief valve. It is the function 
of the system to pressurize the reservoir 80 as the bolt 60 is loaded, 
with the resultant pressure buildup sufficient to drive the cap 88 from 
the end of the tube 84 and spray the dye contained therein onto adjacent 
and external parts and surfaces. The absence of the indicator cap 88 and 
the stain left from the red dye on rotor components external of the 
observation port 86 thus provides two signals to maintenance personnel of 
load path transfer from the primary to a secondary system. 
The providing of pressurization to the reservoir 80 as a function of bolt 
loading is accomplished as follows with reference to FIG. 3. The tang 
washer 90 that is used to lock spindle 20 to nut 70 includes an ear 91 
that extends outwardly into cavity 56 and secures transfer tube 84 in 
place. The reservoir assembly 80, consisting of bellows 81, end plates 82, 
83, transfer tube 84 and relief cap 88 are thus securely retained against 
an outer shoulder 92 of nut 70. From FIG. 3 it may be noted that the inner 
end plate 83 of the reservoir assembly 80 is set back slightly outboard of 
the inner face 94 of the nut 70 and outer face 95 of spacer 76. Gap "a" is 
established between spacer 76 and reservoir assembly 80 and must exist 
during primary load path operation. Spacer 76, which directly abuts 
shoulder 78 of spindle 20, is cup shaped, and its outer wall 96 overlaps 
the outer diameters of washers 72 and 74 and nut 70. Washer 74 is 
fabricated of a soft, extrudable, or frangible material such as lead or a 
suitable non-metallic, and its width exceeds the dimension of gap "a". The 
gap "a" is predetermined with regard to the thermal characteristics of the 
spindle 20 and bolt 60 as well as load stretch of spindle 20. An annulus 
97 and several radial holes 98 are formed in spacer 76 at locations 
adjacent washer 74. In the event that the spindle inboard portion becomes 
damaged and unable to react tension loading, the bolt 60 will become 
loaded in tension as the load path is transferred to the bolt through 
spacer 76, washers 72 and 74, and nut 70. Under loading and due to its 
soft material, washer 74 will decrease in thickness by extruding into 
annulus 97 and holes 98 of spacer 76. The outboard portion of damaged 
spindle 20 will have outward with spacer 76 and washer 72 along bolt 60 
towards nut 70, thus closing the gap "a" and causing face 95 of spacer 76 
to press against reservoir assembly 80. Sufficient travel of the spacer 76 
along the bolt 60, based on the extrusion of washer 74, is required to 
squeeze the reservoir assembly 80 against nut shoulder 92 to provide the 
pressure buildup necessary to blow off cap 88 and spray the red dye onto 
external surfaces. With the cap 88 blown from the end of the transfer tube 
84, centrifugal blade force will assist the pressure in reservoir 80 in 
forcing the dye out of the tube 84. 
There are a number of variants that may be desirable to the particular 
embodiments illustrated and described heretofore. One of these is the use 
or non-use of the extrudable/crushable washer 74. Some designers may 
prefer to omit the washer completely in favor of precisely positioning the 
nut 70 axially along the bolt 60 so as to create a predetermined looseness 
for spacer 76 between the nut 70 and the shoulder 78. This may have the 
advantage of weight saving in omitting washer 74, but it introduces the 
possibility of occasional contact between the spacer 76 and the reservoir 
assembly 80 due to aircraft maneuvers or altitude changes. It also 
introduces the possible error in gap measurement. The advantage of use of 
the extrudable washer 74 is that there can be a positive torque exerted on 
nut 70 that will provide assurance of a firm stack up of parts and a 
reliable expectation that bolt loading will trigger pressurization of the 
reservoir 80. A conical spring or Belleville type washer is an alternate 
to the extrudable washer 74. It is very important that the selected member 
provide repeatability at similar levels. Another of the advantages of the 
extruded type member 74 is that the controlled rate of extrusion prevents 
high impact loading of the bolt 60. The extrusion action serves as a 
damper in the gradual buildup of the full tension loading. Another variant 
is the use of dye and the blow-off cap 88. While the spread of the dye, 
especially as assisted by centrifugal force, is used as the preferred 
medium, a designer's choice may be to keep the pressurized fluid 
captivated and merely provide a movable indicator at the exterior end of 
the transfer tube 84. While one transfer tube is depicted, several may be 
utilized, or other means of conducting the dye to the exterior of the 
spindle may be used while accomplishing the objects of this invention. 
Likewise, another form of collapsible container could be used to perform 
the same function as the metal bellows 81 depicted herein. 
Another possibly desired variation in the configuration of our shaft 
monitoring system relates to the length of the redundant bolt and location 
of the reservoir. This variant can also be implemented without departure 
from the scope of the invention and is shown as FIG. 4. The radial inner 
end of the spindle 20, where the thread 22 and spline 36 are located, is 
considered to be the most critical portion of the spindle and the focal 
point of our inspection system. Thus, in order to save weight and yet 
monitor the spindle inner end, it may be desirable to use a shorter bolt 
that does not extend through the full bore of the spindle. A short bolt 61 
is depicted in the bore 62 of spindle 20, with its head 64 and spherical 
washer 66 abutting the spindle inner end. 
The apparatus for securing the bolt to the spindle, and the reservoir 
containing the dye are both located in this alternate embodiment in a 
counterbore 63 of the spindle 20 at a radial station closer to the bolt 
head, where they perform functions similar to the apparatus of FIG. 3. 
Cylindrical nut 100 engages threads 69 of bolt 61, and the stack up of 
first and second washers 102, 104, and spacer 106 allows the nut 100 to 
indirectly bear against shoulder 108 of spindle 20 where the counterbore 
63 transitions to bore 62. Nut 100 is designed to be torqued onto the 
spindle from one end, due to its internal location, and multiple wrench 
holes 110 are available for such torquing. Reservoir assembly 112 (similar 
to reservoir assembly 80 of FIG. 3) consists of bellows 113, end plates 
114 and 115, transfer tube 116 and relief cap 118, as well as cup-member 
120 and screw 122. Bellows 113 is ring shaped and brazed to end plates 114 
and 115, and together with long tube 116 forms a pressure vessel to hold 
the dye. The reservoir assembly 112 is rigidly attached to the end 124 of 
bolt 61 by screw 122 in a manner to leave gap "a" between the inner axial 
end 126 of cup 120 and the outer end 128 of spacer 106, and also to leave 
the bellows unpressurized. The cup member 120 may be restrained from 
dropping away from screw 122 on assembly by a rubber band 130 or other 
temporary retention means necessary until the screw 122 is engaged. 
The providing of pressurization of the reservoir 112 as a function of bolt 
loading is accomplished as follows with reference to FIG. 4, and generally 
following the manner of the configuration of FIG. 3. In the event that the 
spindle inboard portion, as in the area of the spline 36, becomes damaged 
and unable to react tension loading, the bolt 61 will become loaded in 
tension as the load path is transferred to the bolt through spacer 106, 
washers 102 and 104 and nut 100. Washer 104 is soft, and will extrude into 
holes 132 in spacer 106 as the spacer moves toward the nut 100 and closes 
gap "a". Once the gap "a" is closed, spacer 106 will contact cup-member 
120, and continued axial motion outward (to the right in FIG. 4) will 
drive end plate 115 outward toward end plate 114, this pressurizing the 
bellows 113. As described heretofore for FIG. 3, the cap 118 will be blown 
off the end of tube 116 and the red dye will be sprayed onto external 
surfaces. The difference between the two configurations is the longer tube 
and cylindrical nut of FIG. 4, which accomplishes a weight savings. 
We wish it to be understood that we do not desire to be limited to the 
exact details of construction shown and described, for obvious 
modifications will occur to a person skilled in the art.