Propeller blade retention system

The invention concerns the mounting of propeller blades to a ring-shaped rotor. The blades are of the variable pitch type, and the shank of each blade extends through a respective hole in the rotor. Each hole contains an annular shelf which is fastened to the wall of the hole and surrounds each shank. Each shank bears a pair of bearing races which sandwich the annular shelf in order to connect the blade to the rotor. Bearing rollers are positioned between the annular shelf and the bearing races.

The invention relates to the mounting of propeller blades in aircraft 
engines and, more specifically, to a mounting system in a contra-rotating 
propeller pair in which propeller blades are supported by a ring. The 
centrifugal load of the propeller blades is distributed as hoop stress in 
the ring, and the ring is supported by a turbine which the ring surrounds. 
The invention concerns mounting the blades to the ring such that they can 
change in pitch. 
BACKGROUND OF THE INVENTION 
FIG. 1 illustrates an aircraft engine 3 of the unducted fan type, in which 
the invention can be used. Region 6 of the engine is shown in 
cross-sectional schematic form in FIG. 2, wherein contra-rotating turbines 
9 (decorated with wide hatching) and 12 (narrow hatching) are driven by a 
hot gas stream 15 provided by a gas generator (not shown). The turbines 9 
and 12, in turn, drive contra-rotating fan blades 18 and 21 shown in FIGS. 
1 and 2. (The term "contra-rotating" means that turbines 9 and 12, as well 
as blades 18 and 21 to which they are attached, rotate in opposite 
directions, as shown by arrows 24 and 27 in FIG. 1.) A view of sub-region 
6A in FIG. 2 is shown in perspective form in FIG. 4. In FIG. 4, annulus 29 
represents turbine blades 30 in FIG. 2. 
The fan blades 18 in FIG. 2 are supported by a polygonal ring 22 in FIG. 4. 
One type of polygonal ring is described in the U.S. Pat. No. 4,863,352 
entitled "Blade Carrying Means", filed by Hauser, Strock, Morris and 
Wakeman on Nov. 2, 1984. This patent is hereby incorporated by reference. 
A cross section 23A of the ring is shown in FIG. 2. The ring 22 is 
connected to the turbine casing 9A in FIGS. 2 and 4 by schematic brackets 
24A in FIG. 2. The ring 22 supports a rotating cowling 28, also shown in 
FIG. 1, by schematic brackets 25. 
The polygonal ring 22 in FIG. 4 includes two types of sections: one type is 
a blade support section 22B, also shown in FIG. 3, which includes bearings 
22D which facilitate pitch change, indicated by arrow 39, of the fan 
blades 18. The other type of section is a connector section 22A in FIG. 4, 
including a pair of slender beams 23, which connects neighboring blade 
support sections 22B. 
The fan blades 18 are fastened to the polygonal ring 22 rather than 
directly to the casing 9A for three principal reasons. One, it is doubtful 
that a turbine casing 9A of customary design could withstand the 
centrifugal force applied by the fan blades 18 during operation. Two, 
different design considerations govern the size and shape of the fan 
system 33 in FIG. 2 as compared with the turbine system 34. Consequently, 
it is not expected that the turbine casing 9A would be of a proper shape 
and location for mounting of the fan blades 18. Three, the casing 9A 
experiences wide temperature excursions, and it is preferable to avoid 
mounting the fan blades to a structure of widely variable temperature. 
In addition, the engine 3 shown in FIG. 1 can be in the thrust class of 
30,000 pounds, which causes a high loading in the fan blades 18. For 
example, assuming that a total of sixteen fan blades are used on the 
engine (eight forward blades 18 and eight aft blades 21), then, as a rough 
approximation, the thrust force of 30,000 pounds, indicated by arrow 35, 
is shared equally by these sixteen blades: each blade accounts for about 
1875 pounds of thrust. If it is assumed that each blade in FIG. 4 is four 
feet long (dimension 37), and if it is further assumed that the thrust 
load is concentrated at the midpoint 40 of each blade, then a moment of 
1875.times.2, or 3750 foot-pounds must be reacted by each mounting 
apparatus shown in FIG. 3. Further, this moment is not static, but changes 
as pitch changes, which is indicated by curved arrow 39 in FIG. 3. 
OBJECTS OF THE INVENTION 
It is an object of the invention to provide a rotatable mount for mounting 
an aircraft fan blade to a rotor. 
It is a further object of the invention to provide a rotatable mount for 
mounting a highly loaded aircraft fan blade to a rotor. 
SUMMARY OF THE INVENTION 
In one form of the invention, a plurality of propeller blades are carried 
by a ring. Each blade is supported by a trunnion which rides in a hole in 
the ring. The trunnion rides on two sets of bearings, one of which reacts 
centrifugal load, and both of which react moment loads, such as 
aerodynamic loads.

DETAILED DESCRIPTION OF THE INVENTION 
A simplified form of the invention is shown in FIG. 4A, wherein a hollow 
shaft 35A extends through a hole 35B in a polygonal ring 22. The shaft 
connects to a propeller blade 18. A collar 36, which is larger in diameter 
than the hole 35B, prevents centrifugal force from driving the shaft 35A 
out of the hole. The collar 36 acts as an anchor. Alignment bearings 70 
prevent the shaft from skewing into phantom position 36B under the 
influence of moments applied to the blade 18. Such moments can arise from 
the aerodynamic forces applied to the blade. Thrust bearings 75 react 
centrifugal force and allow pitch change of the blade, as indicated by 
arrow 37A. 
A more detailed form of the invention is shown in FIG. 6. The fan blades 18 
in FIG. 1 are mounted to the polygonal ring 22 by trunnions 40 as shown in 
cross-section FIG. 6, and in exploded view in FIG. 5. The trunnions 40 are 
constructed in two parts, namely, a radially inner part 45 in FIG. 5 and a 
radially outer part 50. The two parts are threaded together by threads 55 
in order to capture an annular shelf 58 between the two parts when 
assembled. The annular shelf 58 is fastened to the blade mounting section 
22B of the polygonal ring 22. The trunnion can be disassembled by 
unthreading threads 55 in order to disassemble the trunnion, thereby 
releasing the annular flange and allowing removal of the propeller blade 
from the polygonal ring. 
The threads 55 are of the buttress type, meaning that the angle A1 of one 
thread surface in FIG. 6A is different from the angle A2 of the other 
thread surface (A1 is 7 degrees, while A2 is 30 degrees), making the 
included angle, A3, 37 degrees. Further, the surfaces 60 making the seven 
degree angle with the pitch line 63 are those which abut each other when 
the trunnion 40 is assembled. The thread pitch is 12 threads per inch. The 
pitch diameter 61 in FIG. 6 is three inches. 
Alignment bearings 70 and thrust bearings 75, shown in FIGS. 5 and 6, 
separate the trunnion from the blade mounting section 22B, and allow 
rotation for pitch change. The bearings ride in hardened races 80. (The 
inner trunnion part 45 contains one of the races 80 integrally formed 
therein, but such construction is not strictly necessary.) 
During assembly, the two trunnion parts are threaded together until a 
predetermined amount of loading is applied to the alignment bearings 70 
and the thrust bearings 75. The two trunnion parts are tightened together 
until the upper edge 77 in FIGS. 5 and 12 seats against abutment surface 
78 in FIG. 12. However, random irregularities in size and shape of the 
components in FIG. 6 can cause improper loading. For example, if dimension 
79 in FIG. 5 is too great, the inner and outer trunnion parts will not be 
drawn sufficiently close, causing the bearing pre-load to be inadequate. 
In order to alter this situation, the components are measured, and shims 82 
in FIG. 5 in the form of rings are placed between edge 77 and surface 78 
in FIG. 12, that is, at the location indicated by arrow 84 in FIG. 12. A 
simplified measurement example will be given with reference to FIG. 12A. 
Distances 85 and 86 are measured as shown. When the parts are assembled, 
distance 85 will nearly equal distance 86. The shim is constructed such 
that (distance 85 minus distance 86) plus the shim thickness equals about 
0.005 inches. That is, the shim takes up all but about 0.005 inches of the 
clearance between surfaces 77 and 78 in FIG. 12. Of course, it may occur 
that no shims are necessary. 
Shims 82, in effect, decrease the loading on the bearings 70 and 75 when 
the edge 77 in FIG. 12 contacts the shim 82 and presses the shim against 
the surface 78. Restated, if no shim were installed when the relative 
distances 85 and 86 called for shims, then the pre-load on the bearings 70 
and 75 would be too great when edge 77 met surface 78. 
The trunnion parts are 45 and 50 are threaded together until a proper 
torque is attained. This torque serves to pre-load the bearings 70 and 75 
in order to prevent separation of both the bearings from their respective 
races, and also to prevent separation of edge 77 from surface 78 in FIG. 
12 under all conditions of engine operation as required to prevent thread 
failure. 
That is, if the bearings 70 and 75 had an improper pre-loading, then, when 
a moment is applied to the trunnion 40, as from the aerodynamic forces 
applied to the blade 18 in FIG. 3, the trunnion 40 can rotate and assume 
the skewed position shown in FIG. 10. This skewing separates bearings from 
their races, as shown by separated bearing 75A in dashed circle 87, which 
transfers the pressure formerly borne by the separated bearing 75A to the 
other bearings, which is undesirable. Further, the separation allows the 
bearings to chatter under some conditions of propeller operation, which is 
also undesirable. The pre-loading prevents this separation. Viewed another 
way, the pre-loading prevents axial movement, along pitch axis 130 in FIG. 
6, of the trunnion 40 with respect to the blade mounting region 22B of the 
polygonal ring 22. 
A dust cap 90 in FIGS. 5 and 6 fits onto the inner trunnion part 45 and 
inhibits entry of debris, as well as preventing airflow through the spaces 
99 between the races 80. Airflow prevention can be desirable in cases when 
region 105 in FIGS. 2 and 6 is kept at a different pressure than region 
109, as can occur when pressurized air is used to purge region 105 of 
volatile gases, such as lubricant vapors. 
Several important features of the invention are the following: 
1. The radially outer row of alignment bearings 70 in FIGS. 5 and 6 are of 
smaller diameter than the radially inner row of thrust bearings 75 because 
the inner row 75 reacts the thrust load imposed by centrifugal force 
acting on the blades. The centrifugal force is greater than the moment 
forces which the alignment bearings 70 react. For example, if each blade 
18 and trunnion assembly 40 in FIG. 4 is assumed to behave as a point mass 
located at midpoint 40, weighing 50 pounds, and rotating in a circle 41 of 
three feet in radius (dimension 92), then the centrifugal load applied to 
each trunnion 40 is at least 50,000 pounds, computed as follows. 
Centrifugal acceleration is equal to w.sup.2 r, wherein w is angular 
velocity, in radians per second, and r is radius. If propeller speed is 
1200 rpm, which corresponds to 20 revolutions per second, then w equals 20 
rev/sec.times.2.times.pi, or about 126 radians per second. Consequently, 
centrifugal acceleration is about 47,000 feet/second.sup.2 (126.sup.2 
.times.3). Dividing this acceleration by the acceleration due to gravity, 
namely, 32.2 feet per second.sup.2, gives the centrifugal acceleration in 
G's, which is about 1460 G's. Therefore, each blade and trunnion assembly, 
which was assumed to weigh 50 pounds when at rest, now applies a radially 
outward (i.e., in the direction of arrow 145 in FIGS. 4 and 12) force of 
about 73,000 pounds (i.e., 1460.times.50) to the thrust bearings 75 in the 
trunnion 40 because of centrifugal force. The force applied by the 
alignment bearings 70, in the outer row, is significantly less. Therefore, 
the outer bearings are smaller than the inner bearings because the load 
which they bear is smaller. 
Both the outer bearings 70 and the inner bearings 75 are tapered roller 
bearings, as shown in FIG. 11. For the alignment bearings 70 which were 
tested by the inventors, the large diameter 110 is 0.205 inches, the small 
diameter 115 is 0.20 inches, and the length 120 is 0.35 inches. There are 
70 bearings in the outer row, which is approximately 4.6 inches in 
diameter. 
For the inner bearings 75 which were tested by the inventors, the large 
diameter 110 is 0.30 inches, the small diameter 115 is 0.22 inches, and 
the length 120 is 0.65 inches. There are 52 bearings in the inner row, 
which is approximately 4.6 inches in diameter. 
2. The angles which each bearing row 70 and 75 make with the pitch axis 130 
in FIG. 6 are different. As FIG. 9 shows, the axis 135 of each bearing in 
each row lies upon a cone. The axes 135A of the bearings 70 in the outer 
row lie on a first cone, while the axes 135B of those in the inner row 75 
lie on a second cone. The first cone can be viewed as pointing radially 
inward, namely, in the direction of arrow 140, which is also shown in FIG. 
5. 
The second cone can be viewed as pointing radially outward, in the 
direction of arrow 145, which is opposite. The apex angle 150 of the first 
cone, upon which the axes 135A of the alignment bearings 70 lie, is less 
than the apex angle 155 of the second cone, upon which the axes 135B of 
the thrust bearings 75 lie. This difference in apex angle results because 
the thrust bearings 75 are closer to being aligned normal (i.e., 
perpendicular) with the centrifugal force vector (which is parallel with 
arrow 145) than are the alignment bearings 70. 
These different orientations of the bearings have an effect on the force 
distribution applied to the polygonal ring 22. For example, even though 
the alignment bearings 70 are almost directly radially outward of the 
thrust bearings 75 in FIG. 6, as indicated by radius line 170, the forces 
applied to the ring by each type differ significantly. 
The thrust load applied by the thrust bearings 75 in FIG. 12 is indicated 
by arrow 190, and it places the annular shelf 58 into shear: the thrust 
load tends to shear off the annular shelf 58 along dashed line 195. In 
contrast, the load of the alignment bearings 70 is indicated by arrow 200, 
and this load is borne predominantly as hoop stress by the region in 
phantom circle 205. This load-bearing region is annular about the pitch 
axis 130, as indicated dotted circle 220 in FIG. 5. The alignment bearings 
70 cause primarily a hoop stress in the material at the periphery of the 
hole 225, while the thrust bearings cause primarily a shear load in the 
annular shelf 58. 
3. A gear sector 230 in FIG. 5, which extends along only a sector of the 
trunnion 40, such as between points 240 and 245, is fastened to the 
trunnion 40 and is driven by a bevel gear 235 in order to change pitch. It 
is preferred that all blades 18 and 21 in FIG. 1 have identical pitch 
angles. However, it sometimes happens that minute manufacturing 
irregularities occur in the blades, giving neighboring blades different 
aerodynamic characteristics, even when they are driven to the same pitch. 
Further, gear lash and other small deviations from theoretical perfection 
in the mechanism which changes pitch can cause neighboring blades to 
acquire small deviations from identical pitch. These and other factors, 
which cause the pitch of the blades to differ from blade to blade, is 
called "pitch rigging error." 
Pitch rigging error refers to the fact that the mechanism which positions 
the trunnions may not perfectly position all of them identically. It also 
refers to the fact that, even if all trunnions are positioned identically, 
there may be factors which cause different blades to be mounted 
differently on different trunnions. And it further refers to the fact that 
apparently identical blades can have minute differences which affect their 
aerodynamic performance. 
Pitch rigging error causes different angles of attack to exist on different 
blades on the same propeller, thus causing the blades to produce different 
amounts of lift, which introduces vibration. The present invention reduces 
pitch rigging error by using a key resembling key 270 shown in FIG. 13. 
That figure shows trunnion part 50 and gear sector 230. Not only does the 
key 270 prevent relative movement between the trunnion part 50 and the 
gear sector 230, but the particular configuration of the key 270 allows 
one to select the relative position between the gear sector and the 
trunnion, thus affecting pitch angle, as will now be explained. 
The actual shape of the key 270 is not necessarily that shown in FIG. 13, 
but may be closer to that shown in FIG. 14, wherein trunnion 40 and gear 
sector 230 are schematically shown. A bolt (not shown) fastens the key 270 
to a hole 271 in the trunnion 40 through hole 273 in FIGS. 13 and 14, and 
the bolt is accessible through a hole 271A in gear sector 230. 
The arrangement of FIG. 14 allows one to control the relative position of 
gear sector 230 with respect to trunnion part 50 by replacing the key 270 
with another key of a different shape. For example, key 270 can be 
visualized as containing two components, 270A and 270B in FIG. 15. By 
first cutting the key 270 along line 274 in order to separate the two 
components 270A and 270B, and then sliding component 270A to the right 
with respect to component 270B, one can obtain the configuration of FIG. 
16. It is preferred that the hole part 273C remain in its former position, 
that is, aligned with hole 271 in the trunnion 40 in FIG. 14. Otherwise, a 
new hole 271 (not shown) in the trunnion would be necessary. 
When installed, the key 270 of FIG. 16 aligns the trunnion and gear sector 
as shown in FIG. 17, wherein the trunnion and sector are now displaced as 
compared with the situation of FIG. 14, as indicated by the non-alignment 
of reference marks 280 in FIG. 17 as compared with those marks in FIG. 14. 
In actual practice, two components of the key 270 are not slid along each 
other as shown in FIGS. 15 and 16, but a group of different keys is made 
as shown in FIG. 18. Preferably, the keys are manufactured such that the 
distance 283, first, is not the same in any two keys and, second, distance 
283 changes in increments which change distance 283A in FIG. 17 in 
increments of 1/4 degree. That is, for example, twelve keys can be made 
such that any selected displacement (i.e., distance 283A in FIG. 17) from 
the following sequence can be selected: 0 degrees, 1/4 degree, 1/2 degree, 
. . . 23/4 degrees. 
In another embodiment, the hole 271 is positioned in the trunnion in FIG. 
14 such that the key 270 can be inverted, as shown in FIG. 19, in order to 
provide displacement of the gear sector 230 in the opposite direction. In 
FIG. 19, reference mark 280A is on the other side of mark 280B, as 
compared with the case of FIG. 17. Further, in still another embodiment, 
it may be desirable to remove material from the key 270 as indicated by 
dashed lines 290 in FIG. 18 in order to reduce weight. The edges of the 
key 270 may need to be chamfered, as indicated by cut 270G in FIG. 18, in 
order to accommodate fillets (not shown) existing in the trunnion or the 
gear sector. If chamfering is needed, and if the inversion feature just 
described is desired, then chamfering of all relevant edges must be done. 
It is noted that surfaces 295 and 296 of the key 270 in FIG. 14 separate 
surfaces 297 and 298 of the trunnion and gear sector, respectively. That 
is, the key 270 acts to maintain surfaces 297 and 298 at selected 
positions with respect to a reference, which is the bolt hole 271 in the 
trunnion. 
Viewed another way, the surfaces on the trunnion and the gear sector which 
contact the key, such as surfaces 295 and 296, act as anchor points in the 
sense that, once a given key has been selected and installed, these 
surfaces anchor the trunnion and the gear sector in the relative positions 
determined by the key. For example, if surface 297 were moved to phantom 
surface 297A, then the gear sector could slide in the direction of arrow 
299 by a distance 299A. Gear sector 230 is thus not anchored in this 
example. 
4. When the aircraft in FIG. 1 is parked on the ground, the wind can cause 
the propeller blades 18 and 21 to rotate, or "windmill." Windmilling 
causes the blades to rock, or "clank" in their dovetail mounts, because 
the fit is loose. Clanking can damage the blades. (The looseness causes no 
problem at operational speed because centrifugal force tightly jams the 
dovetail 250 in FIG. 6 into the dovetail slot 253, thus eliminating the 
loose fit.) 
An anti-clank spring 307 in FIGS. 20A, 20B, and 20C is inserted into a slot 
308 in FIG. 6 in the dovetail 250. The spring 307, in pushing the dovetail 
radially outward from the trunnion, in the direction of arrow 290, 
partially simulates the centrifugal load and locks the dovetail 250 into 
the slot 253. The spring 307, in FIG. 20B, is arched, in that ends 305 and 
310 lie on the same line 315, but the midpoint 312 on the bottom surface 
314 is separated from the line by a space 316. The spring 307 in FIG. 20A 
contains flanges 320 which are defined by cut-out regions 325, which have 
been removed in order to increase the flexibility of the spring. The 
flanges serve to align the spring 307 within the slot 308. 
The spring is constructed such that a force of 450 pounds, indicated by 
arrow 329 in FIG. 20B, is applied to the dovetail 250 in FIG. 6. 
In order to allow withdrawal of the spring 307, which is bound in the slot 
308 in FIG. 6 by the 450-pound force, the spring contains a tapped, 
threaded hole 331 in a leg 333. The threaded hole 331 accepts a jacking 
screw 335 which can be driven against the dovetail 250 in order to 
withdraw the spring. 
5. The blade 18 is fastened to the trunnion 40 by means of a dovetail 250 
in FIG. 6. 
A blade retainer 355, shown hatched in FIG. 7, and also shown in FIGS. 5, 
21A, and 21B, is used to fasten the dovetail 250 in place, in order to 
prevent the dovetail 250 from sliding out of the the slot 253 in FIG. 6, 
in the direction of arrow 260 in FIG. 7. Two bolts 370 and 375, shown in 
FIGS. 6, 7, 21A, and 21B, fasten the retainer 355 in place. The retainer 
355 also locks the leg 333 of the spring 307 against the dovetail and 
prevents the spring 307 from emerging from its slot 308 under the 
influence of vibration. 
In addition, the bolts 370 and 375 react the impact load occurring when a 
bird strikes the propeller blade. A bird strike applies a force which is 
generally in the direction of arrow 270 in FIG. 7. 
6. An attachment of blade 18 to trunnion 40 by means of a dovetail is not 
considered essential. Other types of fastening are available, such as a 
pinned root arrangement, as shown in FIG. 22. A pin 380 fastens two 
clevises 382. A pinned root can allow one to better control vibrational 
modes of the blade and reduce bending moments imparted to the trunnion 40. 
7. The blade retention system of FIG. 5 can be viewed as comprising an 
annular flange captured within an annular groove. For example, the annular 
flange is the annular shelf 58 in FIG. 10, while the annular groove is 
indicated by bold dashed line 301. The bearings 70 and 75 (not all shown 
in FIG. 10) separate the annular flange from the annular groove. 
8. The loadings applied to bearings 70 and 75 in FIG. 6 as the trunnion 
parts 45 and 50 are threaded together are not identical, partly because of 
the different angles 150 and 155 in FIG. 9 which each bearing axis makes 
with the pitch axis 130. The force applied by the threads 55 is generally 
parallel with the pitch axis 130, but the bearings 70 and 75 do not make 
the same angle with the pitch axis 130, and so the components of the force 
which are normal (i.e., perpendicular) to the bearings are not equal. 
For example, if the bearings were parallel (i.e., angles 150 and 155 in 
FIG. 9 are equal at 180 degrees) as shown in FIG. 23, then threading the 
inner trunnion part 45 onto the outer trunnion part 50 will apply equal 
loads to the bearings. That is, the force 400 applied by the bearing 75 to 
the shelf 58 is opposed by an equal force 401 applied by bearing 70. 
Further, in applying the forces 400 and 401, each bearing 75 and 70 is 
subject to a compressive load of the same size as the respective forces. 
When the apex angles are unequal, as in FIG. 9, the forces corresponding to 
forces 400 and 401 in FIG. 23 become forces 405 and 406, respectively, in 
FIG. 9. Force 405 is equal to the normal force, or loading, on the bearing 
75. Force 405 equals force 400 divided by the sine of angle K1. Angle K1 
equals one-half of angle 155 of the outward pointing cone. 
Force 406, on alignment bearings 70, equals force 401 divided by the sine 
of angle K2. Angle K2 equals one-half of the apex angle 150 of the inward 
pointing cone. The pre-loading on the alignment bearings 70 will be 
greater than that on the thrust bearings 75 because the sine of angle K2 
is less than that of angle K1. Of course, the relative pre-loadings also 
depends upon geometric factors, such as the relative distances 85 and 86 
in FIG. 12. Further, relative pre-loading is not to be confused with 
relative centrifugal loading: the alignment bearings 70 in FIG. 9 
experience virtually no centrifugal loading. 
9. Bearings 70 and 75 in FIG. 9 can be viewed as analogous to a pre-loaded 
thrust bearing pair 70B and 75B, as shown in FIG. 8. However, unlike the 
pair shown in FIG. 8, the axes of bearings 70 and 75 in FIG. 9 make 
unequal angles with the pitch axis 130. In contrast, the corresponding 
angles (not shown) in FIG. 8 are equal. 
1O. The invention has been described in connection with a ring 22 in FIG. 4 
which surrounds a turbine 29. The turbine acts as a source of motive power 
for rotating the ring and the blades 18 attached to the ring. However, it 
is not necessary that the source of motive power be a turbine. Instead, a 
gearbox, or a type of rotor, may be the source of motive power for the 
ring. 
11. The invention can be used when propeller blades, which are sometimes 
called fan blades, depending upon their aerodynamic characteristics, are 
supported by a ring, such as ring 22 in FIG. 4. 
12. The alignment bearings 70 in FIGS. 5 and 6 function to maintain the 
trunnion 50 in a predetermined alignment with the pitch axis 130. The 
alignment bearings prevent wobble, or skew, of the trunnion 50. In some 
situations, the pitch axis 130 is an extension of a radius, similar to 
radius 401, of the polygonal ring 22 in FIG. 4. The pitch axis would then 
coincide with a radius, or extended radius, of the ring. 
13. In the art, an array of bearings, such as bearings 70, is frequently 
called a row of bearings. 
Numerous substitutions and modifications can be undertaken with regard to 
the embodiments disclosed herein without departing from the true spirit 
and scope of the invention as defined in the following claims.