Laminated bearings with dual stock layers

A frusto-conical laminated elastomeric bearing is provided wherein at least some of the layers are graded in composition so that they have a first relatively high modulus of elasticity at the inner circumference, and a second relatively low modulus of elasticity at the outer circumference, with the grading being arranged so as to provide an optimum balance of strain distribution and spring rates consistent with bearing lifetime and reduced-torque considerations.

This invention relates to laminated bearings and in particular to bearings 
of the type comprising alternating bonded layers of a resilient material 
such as an elastomer and a nonextensible material such as a metal. 
It is well known that the load carrying capacity of a layer of resilient 
material may be increased many times by subdividing it into a plurality of 
layers and separating the layers by intervening layers of a non-extensible 
material. At the same time, however, the ability of the resilient material 
to yield in shear in a direction parallel to the layers is substantially 
unaffected. This concept has been utilized in the design of a wide 
assortment of bearings, as exemplified by the following U.S. patents: 
Finney, U.S. Pat. No. 4,105,266; Finney, U.S. Pat. No. 4,040,690; Johnson, 
U.S. Pat. No. 3,807,896, Peterson, U.S. Pat. No. 3,792,711; Dolling, U.S. 
Pat. No. 3,941,433; Schmidt, U.S. Pat. No. 3,679,197; Lee et al, U.S. Pat. 
No. 3,429,622; Boggs, U.S. Pat. No. 3,377,110; Orain, U.S. Pat. No. 
2,995,907; Krotz, U.S. Pat. No. 3,179,400; Hinks, U.S. Pat. No. 2,900,182; 
and Wildhaber, U.S. Pat. No. 2,752,766; and the prior art cited in each of 
those patents. 
A significant commercial variety of laminated bearings is characterized by 
the alternating bonded lamellae being disposed concentrically about a 
common center, i.e., so that successive alternating layers of resilient 
and non-extensible materials are disposed at successively greater radial 
distances from the common center. This variety of bearings includes a 
number of different configurations, notably bearings which are cylindrical 
or conical (usually, frusto-conical) in shape or which are essentially 
sectors of cylinders, cones and spheres. 
The behavorial characteristics of laminated bearings have resulted in their 
commercial acceptance in different forms for a variety of applications, 
notably in helicopters. By way of example, conically shaped bearings are 
used as main rotor feathering bearings. In such application the conically 
shaped bearings are required to accommodate cyclic torsional motion about 
the center axis thereof while simultaneously carrying a large compressive 
load along that same axis. With such bearings greater compressive stresses 
and compression induced shear strains are established in (a) the resilient 
layers which are closest to the common center and (b) those portions of 
the resilient layers which are closest to the common center, with the 
result that failure from fatigue encountered in accommodating the combined 
situation of loading and torsional motion tends to occur at the innermost 
resilient layer and especially the inner edge portions of the resilient 
layers. In this connection it is to be noted that the edges of the 
elastomer layers tend to bulge from between the adjacent non-extensible 
laminations under compressive loading, thereby adversely effecting bearing 
fatigue. The extent of bulging depends on the shape factor but in any 
event the bulging and hence the wear problem tends to be more severe at 
the apex (inner circumference) edges than at the base (outer 
circumference) edges of conically-shaped bearings due to the higher strain 
levels in the apex region. Accordingly the bearing failure usually is the 
result of extrusion and erosion or fatigue of the elastomer layers at 
their inner edges. 
The fundamental design criterion of all elastomeric bearings is that they 
carry loads in compression and accommodate motions in shear. In addition 
to concern for size and cost considerations, it is desirable to maximize 
the compressive load capacity of such bearings without adversely affecting 
the torsional spring rate and in particular the ability of the bearing to 
undergo torsional displacement under different magnitudes of alternating 
motion. At the same time, it is desirable to avoid or reduce any 
undesirable impact on strain distribution. Attainment of an optimum 
combination of load-carrying capability, spring rate and torsional shear 
strain distribution is complicated by a number of factors. One such factor 
is that elastomeric materials exhibit changes in modulus of elasticity 
over a range of strain magnitudes and, depending upon the composition of 
the material, the strain distribution in a layer of elastomeric material 
may vary greatly over a given spectrum of input conditions i.e., different 
static and dynamic loads and motions. Another factor is that the strain 
distribution may tend to change within a particular layer and also on a 
layer-to-layer basis under different input conditions. In this connection 
it should be noted, by way of example, that a conical bearing employed in 
a helicopter main rotor retention system is required to undergo dynamic or 
static torsional deflection simultaneously with dynamic or static 
compressive loading. The shear strain produced by torsional deflection is 
not uniformly distributed and will vary in distribution as a function of 
the magnitude of torsional deflection. Additionally shear strains are 
induced by application of compressive loads (either axial or radial) and 
such induced shear strains are maximum at the edges of the elastomer 
layers located along the inner circumference of the bearing and also tend 
to vary with the magnitude of the compressive load. 
As a consequence laminated bearings tend to exhibit changes in spring rate 
or stiffness under different static loads or varying magnitudes of 
alternating motion. While the strain behavior and ultimate wear may be 
modified by simply replacing a given elastomer with another having a 
different modulus of elasticity, that approach is not preferred since it 
may disadvantageously affect either the torsional spring rate or the 
torsional strain distribution. In the typical helicopter application, the 
spring rate of a bearing operating in shear usually is required to be 
within specified limits. Thus, increasing the spring rate may not be 
acceptable since it may result in having to concomittantly increase the 
power capability of an actuator or other device which is coupled to the 
bearing, and/or a decrease in the useful life of the actuator. 
Furthermore, the bulging phenomenon at the outer circumference of a 
conical bearing may not be sufficiently severe as to require any change in 
modulus. On the other hand a change in modulus may increase the torsional 
spring rate at the outer circumference beyond acceptable limits. In this 
connection it should be noted that, on the basis of computer finite 
element analysis of the elastomer layers of a conical bearing, increasing 
the modulus of an elastomer layer will produce a greater contribution to 
the torsional spring rate of an element of the layer located at its base 
end than an element of the same length located at its apex end, due to the 
difference between the effective radii of such elements. Hence merely 
changing the modulus of each layer to reduce the compression-induced 
strains at the apex side of the bearing usually is not a practical 
solution since it makes it difficult to achieve an optimum combination of 
compression-induced edge shear strain, torsional shear strain distribution 
and lowest possible torsional spring rate consistant with the cost, 
lifetime and operating requirements of the system in which the bearing is 
mounted. 
It has been recognized also that absolute uniformity of compression induced 
shear strains within an elastomer layer of a conical bearing is impossible 
to achieve because the strains decrease from a maximum finite value at 
each of its exposed edges to a minimum value at some point intermediate 
those edges. Nevertheless the more uniform the compression induced shear 
strains become between apex and base edges, the less likely that one layer 
will fail a substantial time before the other layers. The same is true if 
the torsional shear strains in adjacent layers are made more nearly the 
same at corresponding points. In this connection it is to be noted that 
because of differences between the average radius of the layers of a 
conical bearing, the compressionally-induced and torsional shear strains 
may tend to vary substantially on a layer-to-layer basis where all of the 
elastomer layers have the same modulus of elasticity and thickness. 
Schmidt, supra, proposed to improve the fatigue life of bearings by 
progressively increasing the thicknesses of successive layers of resilient 
material with increasing radius and simultaneously to progressively 
decrease the modulus of elasticity of those same layers with increasing 
radius. However, the Schmidt technique is expensive in that it requires 
that each elastomer layer be made of a different material. Thus, an 
elastomeric bearing consisting of fifteen resilient layers necessitates 
provision of fifteen different elastomer materials. Even though this may 
be achieved by subdividing a basic elastomer feedstock into fifteen lots 
and modifying each lot with a different amount or type of additive, the 
fact remains that it is costly, time consuming and inconvenient to provide 
a different material for each resilient layer. Furthermore, care must be 
taken to assure that the materials are properly identified so that they 
will be correctly arranged with modulus of elasticity decreasing with 
increasing radius as prescribed by Schmidt. Using a relatively large 
number of elastomer materials as suggested by Schmidt also is 
disadvantageous where the bearings are to be used at relatively low 
temperatures, e.g., -45.degree. to 0.degree. F. Since different elastomer 
stocks will behave differently as the temperature is lowered, only some of 
the elastomer layers in a bearing made according to Schmidt may work 
effectively while the bearing is cold, thereby inhibiting proper bearing 
performance and accelerating bearing deterioration due to uneven strain on 
a layer-to-layer basis. 
In Finney, U.S. Pat. No. 4,105,266 relating to a non-conical bearing, it is 
suggested that by grading the elastomer layers so that they have radially 
varying modulii of elasticity, it is possible to minimize variations 
between the compression-induced shear strains at the inner and outer 
circumferences of the layers. Finney recommends that each layer have at 
least three modulii of elasticity, with the modulii decreasing 
progressively from zone to zone in a radial direction in each layer away 
from the circumferential edge of the layer which would experience the 
greater compression-induced shear strain if the layer had a constant 
nominal modulus of elasticity. Finney specifically suggests that each 
layer may have an inner and outer circumferential portions with modulii X 
and Z respectively, and center portion with a modulus Y, where X&gt;Y&gt;Z. 
However, Finney offers no simple solution to the problem of equalizing 
torsional shear strains on a layer-to-layer basis while at the same time 
reducing non-uniformity of compression-induced shear strains in each layer 
and maintaining the overall torsional spring rate of the bearing at a low 
level. Unlike Schmidt, supra, or Krotz U.S. Pat. No. 3,179,400, but like 
Dolling U.S. Pat. No. 3,941,433, Finney does not require (but does 
consider) variations in the relative thickness of the elastomer layers. 
Unlike Dolling his gradations of modulus are not always the same on a 
layer-to-layer basis--some layers are graded oppositely to others. Finney 
also suggests the use of progressively stiffer elastomers in the layers 
adjacent the upper end plate of his bearing and progressively softer 
elastomers in the layers adjacent the lower bearing end plate, so as to 
substantially equalize compression induced strains throughout the bearing 
without any substantial affect on the bearing's torsional spring rate. 
However, Finney's solutions appear to be limited as to spring rate control 
and load capacity on a layer-to-layer basis since the variations 
contemplated for the elastomer layers involve changes in (a) modulus 
grading by changing the materials used, (b) the relative position of the 
different modulii sections in each layer, and (c) the thickness of each 
layer. In all cases Finney requires three different elastomer stocks to 
achieve grading which will provide improved compression-induced shear 
strain distribution. 
SUMMARY OF THE INVENTION 
The primary object of this invention is to provide a method of making 
laminated bearings of the type described whereby a relatively large number 
of resilient layers each exhibiting a different effective modulus of 
elasticity may be provided using two different resilient stocks. 
Another object is to improve the fatigue life of laminated bearings and in 
particular to provide an elastomeric bearing which not only combines a 
high compressive load to allowable shear ratio but also has an improved 
fatigue life at cold temperatures. 
A further object is to provide a bearing in which at least some elastomeric 
layers have radially-varying modulii of elasticity that tend to minimize 
variations between compression induced shear strains at the inner and 
outer circumferences of each layer. 
Still another object is to provide a bearing having elastomer layers of 
constant thickness which are formed so as to equalize strains due to 
torsion from layer-to-layer, thus assuring a more uniform deterioration of 
the layers under cyclic torsional motion. 
Other more specific objects are to reduce the problem of uneven 
compression-induced shear strain distribution in each layer of a laminated 
conically-shaped bearing while allowing low values of torsional spring 
rate and high loading capacity to be obtained, permit optimization of 
strain distributions produced by torsional deflections about the bearing 
center axis, minimize the impact on torsional strain distribution produced 
from changing the torsional dynamic strain input, and avoid having to use 
(as required by Schmidt, supra) a large number of different elastomer 
stocks to optimize strain distribution. 
The foregoing objects are obtained by providing as a preferred embodiment 
of the invention a frusto-conical laminated bearing of the type described 
wherein at least some of the elastomeric layers are graded in composition 
so that they have a first high modulus of elasticity at the apex (inner 
circumference) side of the bearing to restrain bulging, and a second lower 
modulus of elasticity at the base (outer circumference) side of the 
bearing in accordance with the less pronounced bulging at that side, with 
the grading in composition being arranged so that an optimum combination 
of compression-induced shear strain, torsional shear strain distribution, 
and lowest possible spring rates consistent with life-time consideration 
is obtained. 
This grading is achieved by forming the elastomeric-layers so that they 
consist of two sections of elastomeric material laid up side by side, with 
the relative sizes of the two sections varying on a layer-to-layer basis 
so as to provide an effective or composite modulus of elasticity of 
selected value calculated to keep the torsional shear strain distribution 
constant under all input conditions with changing torsional displacement 
magnitude, while at the same time permitting a high compressive stiffness. 
Other objects and features of the invention are described or rendered 
obvious by the following detailed description of a preferred embodiment of 
the invention and the accompanying drawing.

SPECIFIC DESCRIPTION OF THE INVENTION 
In its broadest sense the invention utilizes two elastomeric stocks, the 
first of relatively high modulus and the second of relatively low modulus, 
suitably arranged and proportioned within most or all of the elastomeric 
layers of a laminated bearing so as to more nearly equalize strain 
distribution on a layer-to-layer basis as well as providing in each layer 
an optimum balance of strain distribution, bulging and spring rate. The 
number of layers employing two different elastomer stocks and the relative 
widths of the elastomer stocks in each multi-stock layer, may vary 
according to the size and shape of the bearing, the modulus of elasticity 
of the elastomeric-materials used, and the loads to which the bearing is 
subjected. It also is contemplated that some of the layers may have 
sections of relatively high modulus elastomer stock (and/or of relatively 
low modulus stock) of identical or nearly identical widths, particularly 
in the outermost layers where the effective modulii of the layers may be 
more nearly the same since the torsional shear strain distribution is less 
troublesome on a layer-to-layer basis at the outer circumference of the 
bearing. In this connection it is to be appreciated that the modulii of 
elasticity of different stocks of elastomer or of different portions of an 
individual elastomeric layer may be affected by the use of elastomeric 
filler material in the fabrication of a bearing. For example, in one 
method of fabricating a laminated elastomeric bearing, the elastomeric 
layers are cut from sheets of elastomer and stacked up by hand with the 
layers of nonextensible materials. As heat and pressure are applied to 
bond the elastomeric and nonextensible laminations together, elastomer 
that has a modulus of elasticity equal to or different than the modulii of 
the elastomers in the layers may be introduced into the mold for the 
bearing to develop molding pressure, to fill in gaps, and to bring the 
elastomeric layers to full size. This transfer or filler elastomeric 
material may mix with the basic elastomer stocks in the layers, thereby 
altering their moduli of elasticity, or the filler material may form a 
thin layer along one or both circumferential surfaces of an annular 
bearing. To distinguish between (a) the modulus of elasticity of a basic 
elastomeric stock in a layer of elastomer, or the effective combined 
modulus of two different modulii basic stocks in a layer, and (b) the 
modulus of elasticity of the same basic stock(s) when mixed with small 
amounts of filler material or of the filler material itself when forming a 
relatively thin or narrow surface coating on the layer of basic elastomer 
stock(s), the modulus of elasticity of a basic stock of elastomer used in 
a layer of elastomer and the effective combined modulus of two different 
modulii basic stocks in a layer, is termed herein the "nominal" modulus of 
elasticity of the stock(s). The addition of small amounts of filler 
material is presumed not to affect the nominal modulus of elasticity to a 
significant extent (i.e. in a way that would defeat the purpose of the 
present invention). Similarly the presence of a thin coating of a filler 
elastomer on a layer of a basic elastomeric stock is to be ignored. 
Typically, filler or transfer material will compose about 15% or less of 
the volume of an elastomeric layer in a laminated elastomeric bearing. 
FIG. 1 illustrates how the shear modulus of elasticity of a number of 
different elastomer compounds can vary in accordance with the torsional 
shear strain experienced by each elastomer. These curves were derived by 
subjecting specimens of each compound to displacement only in shear at 
75.degree. F. while the specimens were free of compressive loading. The 
several compounds were derived from a single basic rubber stock and were 
produced by adding different amounts of carbon to the basic stock. 
As is apparent from a comparison of Curve A with curves G or H, a 
relatively soft (low modulus) material can accommodate varying degrees of 
torsional strain with less affect on its shear modulus than can a 
relatively stiff (high modulus) material. Thus for example, at 50% strain 
the differences between the shear moduli of the compounds represented by 
curves A and G or H is substantially less than it is at 5% strain. Stated 
another way, the higher the strain which it experiences, the more an 
elastomer with a relatively high shear modulus tends to behave like an 
elastomer with a relatively low shear modulus. 
The present invention takes advantage of the behavior represented by the 
representations of FIG. 1 by making an elastomeric-bearing of two 
different elastomeric compounds, one selected for its characteristic of 
exhibiting relatively small changes in shear modulus (both static and 
dynamic) over a wide range of strain magnitudes, and the other selected 
because it exhibits greater changes in shear modulus under strain inputs 
in the same range of magnitudes. By appropriately proportioning the two 
compounds in one or more of the elastomer layers, it is possible to 
provide a bearing in which the distribution of torsionally-induced strain 
can be held nearly constant throughout the bearing for different input 
motions within the range of motions which the bearing is intended to 
accommodate, or at least as constant as in the case where, as taught by 
Schmidt, each resilient layer is made of a different elastomer. In the 
latter case each stock will exhibit a different modulus under each input 
condition so that optimization of strain distribution for all of the 
layers over the entire range of expected strain inputs is very difficult. 
To optimize strain distribution according to this invention, each resilient 
layer of an intended bearing is subjected to computer finite element 
analysis to determine the strains in different portions of each layer for 
two different elastomer stocks, one having a relatively great stiffness 
and the other having a relatively small stiffness. The layer is analyzed 
at the maximum and minimum degrees of strain which the bearing will 
experience in its intended application, e.g., 50% and 5% strain 
respectively. Once the strains experienced by each computer analysis 
element of each layer (typically each layer is subdivided by the computer 
into 15 equal width elements) at maximum and minimum conditions for each 
of the two elastomers has been determined, it is possible to compute what 
proportion of the width of each layer has to be made up of each of the two 
elastomers in order to equalize strain on a layer to layer basis at the 
maximum and minimum strain inputs (as used in this context the term 
"width" refers to the dimension extending parallel to the layers of 
non-extensible material as shown in FIG. 2). The result in a 
conically-shaped or frusto-conical bearing is that in each dual stock 
layer the stiffer stock is located at the inner edge and extends toward 
the outer circumference while the softer stock is located at the outer 
edge and extends toward the inner circumference far enough to abut the 
stiffer stock, and additionally the width of the softer stock usually 
increases with the average radius of the individual layers, being greatest 
at the outermost dual stock layer and smallest at the innermost dual stock 
layer. 
FIG. 2 illustrates a particular embodiment of a frusto-conical laminated 
bearing suitable and intended for use as a bearing in a helicopter main 
rotor retention system. The bearing is made by providing two annular rigid 
metal end members 2 and 4 which have frusto-conical inner and outer 
surfaces 6 and 8 respectively. In the completed bearing alternating bonded 
layers of a resilient material 10 and a non-extensible material 12 are 
disposed between end members 2 and 4, with surfaces 6 and 8 of the latter 
bonded to a layer of resilient material. The bearing has three layers of 
resilient material. The resilient material preferably is an elastomer such 
as a natural or synthetic rubber, but is also may be a suitable plastic 
material of elastomeric character. The nonextensible material may be steel 
or another kind of non-extensible material such as another metal (e.g. 
aluminum or titanium) or sheets of fiberglass or reinforced plastic. As is 
evident from the drawing, the layers 10 and 12 are frusto-conical in shape 
and extend generally parallel to and coaxial with the surfaces 6 and 8 of 
the two rigid metal end members. 
The layers 10 and 12 are of uniform thickness, and the nonextensible layers 
12 (which are commonly called shims) are thinner than the resilient 
layers. If desired the shims could be as thick or thicker than the 
resilient layers. Additionally the resilient layers are made so that they 
have greater stiffness at their inner circumference ends than at their 
outer circumference ends and so that a more uniform strain distribution is 
obtained in each layer and also on a layer-to-layer basis. This is 
achieved by making the resilient layers 10 from two different resilient 
stocks arranged in the manner illustrated in FIG. 2. Depending upon the 
required characteristics of the bearing the several elastomer layers 10 
may but need not have sections of identical widths. 
Referring to the drawing, the first or outermost resilient layer 10A is 
made up by laying onto the inner surface 6 of end member 2 two elastomeric 
stocks represented as frusto-conical sections 101 and 102, with section 
101 being an elastomeric material having a selected relatively high 
modulus of elasticity, and section 102 being an elastomeric material 
having a modulus of elasticity which is less than the modulus of section 
101. The sections are applied so that they abut one another as shown. Then 
a shim 12 is placed over this composite layer and the second two-section 
layer 10B is applied over that shim. In the illustrated embodiment, the 
second layer is substantially identical to the first except that the 
section 102 of the second layer is smaller in width than the corresponding 
section of the first layer. The two sections 101 and 102 of the second 
layer may but need not have the same overall width as the combined 
sections of the first layer, and the same is true of the third layer. 
The third elastomeric layer 10C is prepared in the same way by first 
placing a second shim over the second composite layer and then laying 
another two-section composite layer over the second shim. In this layer 
the section 102 is once again smaller than section 102 of the second 
layer. 
After the three resilient layers have been laid up, the other end member 4 
is engaged with resilient layer 10C and then the assembled parts are 
forced together in a mold under suitable heat and pressure so as to cause 
the sections of each dual-section elastomer layer to bond to each other 
and also cause the elastomer layers to bond to the adjacent shims 12 or 
end members 2 or 4, as the case may be. In the completed bearing each 
group of resilient sections 101 and 102 is integrated to form a single 
resilient layer. 
In the molding of the layers, additional elastomer material may be 
introduced into the mold for the bearing for the purpose of developing 
sufficient molding pressure, to fill in whatever gaps may exist between 
the various sections of elastomeric material, and to bring the elastomeric 
layers to full size. This filler or transfer elastomeric material 
preferably has a modulus of elasticity equal to the modulus of elasticity 
of the sections 102, but a larger or smaller modulus elastomeric material 
also may be used. In any event, as noted previously, this filler or 
transfer material will comprise about 15% or less of the volume of each 
elastomeric layer in the bearing. 
The method of manufacturing described above is especially suitable where 
the resilient stock is an elastomer which can be fused and molded under 
heat and pressure. Where the resilient stock is a rubber, the bonding step 
involves vulcanization. Other aspects of the procedure of assembling and 
bonding the array of resilient layers, metal shims and the bearing members 
2 and 4 are well known to persons skilled in the art of making laminated 
elastomeric bearings and are not described herein in detail since they are 
old and form no part of this invention. 
As noted previously, the sections 101 and 102 are suitably arranged and 
proportioned in order to equalize strain distribution and provide an 
optimum balance of strain distribution bulging and spring rate. A 
relatively high modulus stock in the form of sections 101 is used at the 
apex or inner circumference of the bearing in order to restrain bulging at 
the inner edge of the bearing, since wear and shear strains induced from 
compression are historically highest at that edge. On the other hand, the 
need for bulge restriction is not as severe at the base or outer 
circumference of the bearing. Consequently use of a lower modulus stock 
102 in that region of the bearing is feasible. As in the illustrated 
embodiment, the elastomer layers 10 and shims 12 may have the same width. 
Preferably, however, the widths of layers 10 and shims 12 decrease with 
increasing distance from the center axis of the bearing. 
Obviously the foregoing arrangement is not the only possible way of 
practicing the invention since the relative proportions of the sections 
101 and 102 as well as the total number of layers and the number of 
two-layers will depend on the size and anticipated operating conditions of 
the bearing and the modulii of the stock used to make the elastomeric 
layers. Thus it is contemplated that a bearing utilizing this invention 
may have a relatively large number of elastomer layers, e.g., fourteen, 
and that some of the layers, (e.g., those closest and/or furthest from the 
center axis of the bearing) may consist of a single elastomer which may be 
either of the elastomers used in the dual stock layers or an additional 
elastomer of different modulus. The relative dimensions of the sections 
101 and 102 of the elastomeric layers required to achieve uniform strain 
distribution over the expected range of strain inputs may be determined by 
computer finite element analysis using a program derived from the finite 
element program TEX-GAP described in U.S. Pat. No. 4,105,206. 
The following example illustrates a preferred form and the advantages of 
the present invention. 
EXAMPLE 
A frusto-conical bearing having three elastomeric layers as shown in the 
drawing may be constructed according to the present invention where the 
elastomer layers and the intervening metal shims have thicknesses of 0.100 
and 0.050 respectively and the sections of the individual elastomer layers 
have widths in inches arranged as follows: 
______________________________________ 
LAYER SECTION 101 SECTION 102 
______________________________________ 
(10A) 0.0 2.18 
(10B) .58 1.60 
(10C) 1.07 1.11 
______________________________________ 
In the foregoing bearing, the sections 101 are made of a material having a 
Young's shear modulus of elasticity of 230 psi and sections 102 having a 
Young's shear modulus of elasticity equal to 85 psi. The foregoing values 
are for nominal modulii of elasticity and each of the materials forming 
the sections 101 and 102 are essentially the same elastomer with the 
differences in modulii being achieved by varying the amount of carbon 
which is added to the elastomeric material. 
The inner surface 6 of outer member 2 of the bearing has a diameter of 
2.382 inch at one end and 4.692 inch at the other end, while the outer 
surface of inner bearing member 4 has a diameter of 1.690 at one end and 
4.000 at the other end. 
A bearing constructed according to the foregoing example will have an 
overall axial spring stiffness of 882,000 lb/in, an overall torsional 
stiffness of 
##EQU1## 
and substantially equalized shear strains from layer-to-layer, thus 
assuring uniform deterioration under cyclic torsional motion. 
In contrast, a bearing of like size made with a different elastomer stock 
in each layer shows substantially the same strain in all three layers at 
50% strain but substantially non-equalized strain distribution at 5% 
strain, due to the elastomer strain sensitivity illustrated by the curves 
shown in FIG. 1. 
The invention may be practiced otherwise than as already described and 
illustrated. Thus, only some of the elastomer layers need be made in two 
sections, the layers could have varying thicknesses, injected filler 
material could be designed to function as a dam or bulge restrainer, and 
the bearing could have a different number of resilient layers. 
The invention also may be applicable to bearings of other shapes. Thus, the 
bearing could be more nearly a full cone or it could be a cylindrical 
bearing wherein portions of the elastomer layers are disposed at an angle 
to the common axis, as in the bearing shown in U.S. Pat. Nos. 4,640,690 
and 4,105,266 (FIG. 1). The bearings or portions thereof also may be 
spherical like, for example, the bearings shown in U.S. Pat. Nos. 
4,105,266 (FIG. 6), 3,429,622, 3,941,433, 2,900,182 (FIG. 8) and 3,790,302 
(FIG. 3, bearing unit 80) and the references cited therein. In each case, 
however, the same advantages may be obtained, e.g., bearings of selected 
compressive and torsional load characteristics can be made using only two 
different stocks. 
Another advantage is that the invention allows the manufacture of bearings 
with more uniform shear strain distribution without loss of adequate 
control, thereby forestalling bearing failure as a result of extrusion and 
fretting erosion as the bearing undergoes repeated changes in loading. 
Additionally and equally important, it is possible to adjust the bearing 
torsional spring rate so as to reduce the power required to be exerted by 
a connected actuator or operator, e.g., a hydraulic piston, thereby 
contributing to the lifetime and reliability of the actuator and/or 
allowing the use of a smaller actuator. Still other advantages and 
modifications will be obvious to persons skilled in the art.