Snubber-bearing with combined elastomer laminates

A snubber set (30) for a helicopter rotor (10) is comprised of an inner race (44), an intermediate race (45), a preload plate (46), an inner portion (48) having alternating laminates of elastomer (52) and metal shims (54) which are spherically shaped, and an outer portion (50) having alternating laminates of elastomer (58) and metal shims (60) which are flat. The flat laminates (58) are comprised of a center portion (61) made from a high loss elastomer and a peripheral portion (62) made from a high shear strength elastomer.

DESCRIPTION 
1. Technical Field 
This invention relates to snubber-bearings on helicopter rotors, and more 
particularly to the elastomer damping laminates within snubber-bearings. 
2. Background Art 
In flexbeam helicopter rotors, whether of the even bladed crossbeam variety 
shown in U.S. Pat. No. 4,244,677, or of the odd numbered blade variety 
disclosed herein, it is necessary to provide positioning and load carrying 
means between the flexible spar and its enveloping torque tube to perform 
the function of positioning the spar for pitch change and flapping motion 
about the intersection of the pitch change and flapping axes, for 
imparting pitch change control and other loads therebetween, and for 
accommodating relative motion therebetween. 
A snubber-bearing, or snubber, serves as a supporting attachment to react 
control loads between the flexible spar and the rigid torque tube of a 
helicopter rotor and maintain the proper relationship between the spar and 
torque tube. The snubber is typically made of a rubber or elastomer 
material which accommodates the pitch motions of the torque tube, the 
flapwise bending of the spar, and the in-plane, edgewise bending 
(lead-lag) of the blade. 
In U.S. Pat. No. 4,244,677, Noehren et al disclosed the use of a snubber 
with alternating laminates of elastomer and metal shims. This snubber 
takes advantage of the higher shape factor, which is the ratio of 
compression area to bulge area, of a series of thin layers versus a single 
thick layer. The higher shape factor increases the compression strength 
and stiffness of the snubber. The snubber included spherical laminates 
which accommodate the pitch change and flapping motions and flat laminates 
which dampen the lead-lag motion of the blade. 
A limiting factor in the design and use of the laminate snubbers is the 
magnitude of the shear strain encountered at the free edges of the flat 
elastomer laminates, which is where the maximum shear strain occurs. The 
shear strain at the free edge is a combination of static shear strain due 
to compressive pre-loads and dynamic shear strain due to the motion to be 
dampened. For main rotors with high head-moments and corresponding high 
chordwise loading, the shear strain at the free edges can exceed the 
allowable shear strain of elastomers which have acceptable loss factors or 
damping ability. This result is due to the inverse relationship between 
allowable shear strain and loss factor for elastomer materials. 
A common method to increase the shear strength of an elastomer laminate to 
dynamic strain is to `pre-load` the snubber prior to installation by 
subjecting the snubber to a compressive load. Allowable shear strain for 
fatigue (i.e. under dynamic loads) of an elastomer increases under a 
compressive loading and decreases if subjected to tensile loading. In U.S. 
Pat. No. 4,244,677 the precompression was performed by utilizing a bolt to 
draw the inner race towards the outer race. This pre-load bolt was mounted 
between the inner and outer race and therefore required the laminates to 
be apertured to accommodate the bolt. The free surface around the aperture 
created an area of significant stress concentration due to the bulge area 
around the aperture being forced into a small volume. 
U.S. Pat. No. 5,092,738 for "Flexbeam Helicopter Rotor With Improved 
Snubber-Vibration Damper Between the Torque Tube and the Flexible Spar 
Member" by Byrnes et al discloses a method of preloading laminates which 
are continuous, i.e. without apertures. In that invention the preloading 
is performed by positioning preload plates which are bonded to the outer 
laminate and bolt down onto the torque tube. The requirement for a preload 
bolt and aperture, and thereby the free surface and stress concentration 
area, is eliminated. 
Unfortunately, the radial edge of a laminate, whether continuous or not, is 
still a free edge and cannot be compression loaded. The result, for a 
preloaded laminate of elastomer, is that the allowable shear strain for 
fatigue is maximum at the center and decreases radially toward the edge. 
At the edge, the allowable shear strain for fatigue of the laminate is 
minimum (approximately equal to the unloaded allowable shear strain of the 
elastomer). A result of the compressive preload is that the compression 
induced shear strain (i.e. static shear strain) increases radially to a 
maximum at the radially outer edge. Therefore, the shear strain at the 
edge governs the selection of elastomer. 
One solution is to select an elastomer which meets the requirements for 
shear and then increase the number of laminates until the desired amount 
of damping is attained. Unfortunately, this solution increases the size 
and weight of the snubber, and the size of the envelope containing the 
snubber, which results in an undesirable increase in aerodynamic drag of 
the rotor. Another solution is to select an elastomer which meets the 
damping requirements and then reduce the thickness of each laminate, 
thereby taking advantage of the increased shape factor in order to stay 
below the allowable shear strain. The drawback to this solution is that 
the increase in the number of laminates requires a corresponding increase 
in the number of shims. This solution also results in an increase in the 
size and weight of the snubber and in aerodynamic drag. 
In U.S. Pat. No. 4,886,419 McCafferty disclosed a spherical bearing for 
fully articulated retention of a tail rotor or fan blade to a hub. The 
spherical bearing was comprised of alternating laminates of elastomer, 
with particularly shaped and oriented apertures formed in the centers, and 
metal shims. Coupons of elastomer, with greater damping characteristics 
than the elastomer used in the laminate, were fitted within the openings 
to provide lead-lag damping for the retention system. Unfortunately, 
having a free edge around the openings distorts the pressure and shear 
strain distributions in the laminates and coupons and requires that a high 
strain allowable material be used for both the laminates and the coupons. 
This shear strain allowable requirement thereby eliminates the possibility 
of using a high damping material for the coupon. 
DISCLOSURE OF INVENTION 
An object of the present invention is an improved flat, continuous 
elastomer damping laminate for a snubber which increases the loss factor 
of the snubber without increasing the size and weight of the snubber. 
Another object is an improved snubber set for a helicopter rotor which 
results in an increase in lead-lag damping without increasing the 
aerodynamic drag of the rotor. 
According to the invention, a flat damping laminate for a snubber is 
comprised of a continuous center portion of high damping, moderate shear 
elastomer and an annular peripheral portion of moderate damping, high 
shear elastomer. The two portions are the same thickness, substantially 
the same shear modulus, and are joined such that there is continuity in 
the laminate. The center portion, since it is in an area of low shear and 
high compression, provides maximum damping while meeting the allowable 
shear strain requirements. The peripheral portion provides shear strength 
at the free edge where the shear strains are maximum and the compressive 
pre-load is minimum. In this way the loss factor of each laminate is 
increased and the overall loss factor of the snubber may be increased 
without having to increase the size of the snubber. In addition, the 
number of laminates required is reduced, and, correspondingly, the number 
of shims required is reduced, which further reduces the size and weight of 
the snubber. 
The foregoing and other objects, features and advantages of the present 
invention will become more apparent in light of the following detailed 
description of the exemplary embodiment thereof, as illustrated in the 
accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1, a helicopter flexbeam rotor 10 is comprised of a 
plurality of blade assemblies 12 (only one of which is shown), disposed on 
a hub member 18 which is mounted on a fuselage (not shown) and connected 
to an engine 15 by a drive shaft 17. Each blade assembly 12 is connected 
to hub member 18 by a radially-extending flexible spar 20. The hub member 
18 reacts centrifugal, torsional, and bending loads of the blade 
assemblies 12. Each blade assembly 12 also includes a blade 14 having an 
airfoil portion 22 at the outer end and an elliptical torque tube portion 
24 at the inner end, which portions are connected directly or indirectly 
to spar 20. The torque tube portion 24 envelops the spar 20 and provides 
clearance to permit twisting of the spar 20 to accommodate blade 14 pitch 
changes. The pitch changes are transmitted from a pitch control rod 26 
which is connected to the torque tube 24. 
Referring to FIG. 2, a snubber set 30, disposed between the spar 20 and the 
torque tube 24, maintains the proper relative alignment of the torque tube 
24 and the spar 20 and accommodates pitch motion, flapwise bending, and 
edgewise bending (lead-lag motion). The snubber set 30 is comprised of an 
upper snubber 40 and an identical lower snubber 42. Since the upper 
snubber 40 and lower snubber 42 are the same, only the upper snubber will 
be described in detail and will be referred to simply as a snubber. The 
snubber 40 is comprised of an inner race 44, an intermediate race 45, a 
pre-load plate 46 which is mounted on the torque tube 24, an inner portion 
48, and an outer portion 50. The snubbers 40 are pre-loaded during 
installation by tightening the preload plate 46 down onto the torque tube 
24, which compresses the snubber 40. The compression increases the 
allowable shear stress of the snubber 40. 
The inner portion 48 includes a stack of alternating layers of laminates of 
elastomer 52 and metal shims 54 which are spherically shaped. The 
laminates form a spherical bearing which is concentric about a center 
point 56 and provides control over the pitch and flap motion of the blade 
14. The outer portion 50 also includes a stack of alternating layers of 
laminates of elastomer 58 and metal shims 60, but which are flat in 
construction. The flat laminates motion of the blade. It is these flat 
laminates to which the present invention is directed. 
In prior art snubbers, the laminates are constructed from a single material 
which has to meet both the damping and shear strength requirements of the 
snubber. Since damping and shear strength are inversely related the number 
of laminates has to be increased in order to meet both requirements. 
Increasing the number of laminates, and the corresponding number of shims, 
increases the size of the snubber, which increases the aerodynamic losses 
of the rotor, and weight of the snubber. 
In an exemplary embodiment, flat laminates 58 in accordance with the 
present invention are comprised of a continuous, circular center portion 
61 and an annular peripheral portion 62, as shown in FIGS. 3 and 4, of the 
same thickness t. The center portion 61 is fabricated from a material 
chosen principally for its damping characteristics and the peripheral 
portion 62 is fabricated from a material chosen principally for its shear 
strength characteristics. The center portion 61 and the peripheral portion 
62 have the same, or substantially the same, material shear modulus. The 
shear moduli of the two portions are substantially the same if the two 
portions deform compatibly. Incompatible deformation may cause 
discontinuities and stresses at a junction 64 between the two portions. 
The junction 64 provides a joining between the two portions 61,62 such 
that the laminate 58 is continuous with no free edges other than the 
radially outer edge of the peripheral portion 62. 
By combining a center portion material with high damping ability and a 
peripheral portion material with high shear strength, the snubber overall 
damping ability can be increased without increasing the overall size of 
the snubber. The combined laminate takes advantage of the compression 
pre-load to increase the strength of the center portion to acceptable 
levels and of the inherent shear strength of the peripheral portion to 
meet the shear strain requirements near the radial edges of the laminate. 
Selection of elastomer materials for the laminates and determination of the 
radius of the center portion, or the inner radius of the peripheral 
portion, is dependant upon the shear distribution in a cross section of 
the laminates and follows well known design practices for snubber 
bearings. An elastomeric material which meets the shear strain 
requirements at the outer edge of the laminate is selected for the 
peripheral portion. An elastomeric material which meets the damping 
requirements of the laminate is selected for the center portion. The 
radius of the center portion is then determinable from the distribution of 
shear stress, which is related to shear strain by the shear modulus, 
generated by the compression pre-load. An example of a shear stress 
distribution is shown in FIG. 5. The center portion radius is selected to 
be equal to the radius at which the shear strain equals the allowable 
shear strain of the center portion reduced by an acceptable factor to 
accommodate peak loading conditions during operational use. As with prior 
art damping laminates, the materials selected must also meet fatigue 
requirements for dynamic shear loading. For example, FIG. 5 is a graphic 
presentation of the distribution of shear stress, for a 1.5" radius, 0.10" 
thick laminate under 2250 psi of compression loading. In this example a 
silicone elastomer (loss factor=0.65, allowable shear strain for fatigue 
@36.times.10.sup.6 cycles=8%, allowable static shear strain =65%, =8%, 
shear modulus=200 psi) was selected for the center portion and a 
polybutadiene elastomer (loss factor=0.4, allowable strain for fatigue 
@36.times.10.sup.6 cycles=16%, allowable static shear strain=130%, shear 
modulus=200 psi) was selected for the peripheral portion. For a laminate 
of these dimensions and materials, and using a factor of two (2) to 
accommodate peak loads during operational use, the radius of the center 
portion (high damping, moderate shear strength), or the inner radius of 
the peripheral portion (moderate damping, high shear strength) is 
approximately 1.125". 
Although the invention has been shown and described with respect to 
exemplary embodiments thereof, it should be understood by those skilled in 
the art that various changes, omissions and additions may be made therein 
and thereto, without departing from the spirit and the scope of the 
invention. For example, although the damping laminate of the invention has 
been depicted and described in a circular configuration, other geometries 
would be within the scope of the invention.