Elastomeric member for energy storage device

An energy storage device (10) is disclosed consisting of a stretched elongated elastomeric member (16), disposed within a tubular housing (14), which elastomeric member (16) is adapted to be torsionally stressed to store energy. The elastomeric member (16) is configured in the relaxed state with a uniform diameter body section, transition end sections, and is attached to rigid end piece assemblies (22, 24) of a lesser diameter. The profile and deflection characteristic of the transition sections (76, 78) are such that upon stretching of the member, a substantially uniform diameter assembly results to minimize the required volume of the surrounding housing (14). During manufacture, woven wire mesh sleeves (26, 28) are forced against a forming surface and bonded to the associated transition section (76, 78) to provide the correct profile and helix angle. Each sleeve (26, 28) contracts with the contraction of the associated transition section to maintain the bond therebetween.

CROSS REFERENCE TO RELATED APPLICATION 
Cross reference is made to a commonly assigned, related application Ser. 
No. 469,617, filed Feb. 25, 1983, now U.S. Pat. No. 4,478,777, entitled 
"Method For Making An Elastomeric Member With End Pieces." 
BACKGROUND OF THE INVENTION 
This invention concerns energy storage devices such as are used in 
regenerative braking systems for storing and releasing the energy normally 
dissipated in the braking of vehicles so that this energy may be utilized 
in vehicle propulsion. 
This invention is specifically concerned with providing an elastomeric 
member and device such as may be usable in energy regenerative systems 
utilizing a torsionally stressed elongated elastomeric member. 
The present invention is related to inventions described in U.S. Pat. Nos. 
4,246,988; 4,305,489; 4,310,079; 4,319,655; 4,333,553, describing 
regenerative braking systems. 
In these patents, a regenerative braking system and energy storage device 
is disclosed including such an elongated elastomeric member which is 
torsionally stressed in order to absorb braking energy. As described in 
U.S. Pat. No. 4,333,553, it is advantageous to axially prestress the 
elastomeric member since the member has a tendency to knot at 
predetermined torsional stress levels, and the tendency to form a knot is 
decreased by applying an axial or stretching tension on the elastomeric 
member. 
The knotting tendency is a disadvantage since the fatigue life of the 
elastomeric member is greatly affected by knotting during the stressing 
cycles. 
It is desirable to enclose such an elastomeric member by mounting it within 
a confining housing in order to protect the member from the environment, 
and to provide a safety shield. 
The axial prestressing is achieved by stretching and results in a very 
substantial difference in diameter of the elastomeric member in its 
relaxed and stretched conditions. 
In the interest of conserving space, it is desirable that the housing be of 
a diameter no larger than is necessary to receive the elastomeric member 
in its stretched condition. 
It is also highly desirable that a sure and reliable mechanical connection 
to the ends of the elastomeric member be provided while allowing a 
constant diameter assembly. 
DISCLOSURE OF THE INVENTION 
The present invention seeks to provide an elastomeric member assembly for 
use as an energy storage device of the general type described, which is 
formed with a main body section and transition sections at each end in 
turn connected to rigid end pieces of a diameter corresponding to the 
final stretched diameter of the elastomeric member main body section. 
Reinforcement means for each of the transition sections is provided which 
absorbs a portion of the tensile forces in order to produce a controlled 
axial elongation of each region of the transition sections. This precludes 
necking down of the transition sections resulting from excessive 
elongation and establishes an overall uniform diameter of the stretched 
member. 
The reinforcement takes the form of a plurality of helically wound strands, 
which preferably are formed into woven wire mesh sleeves, molded into the 
surface of the transition sections. The windings contract radially and 
extend axially during stretching of the elastomeric member in 
correspondence with the transition sections themselves and provide the 
appropriate reinforcement of the elastomeric material composing the 
transition sections.

DETAILED DESCRIPTION 
Referring to FIG. 1, an energy storage device 10 is shown, which includes 
an elastomeric member assembly 12 mounted within a housing 14. The 
elastomeric assembly 12 includes an elastomeric member 16 which in its 
stretched, axially-elongated condition is reduced in diameter, allowing it 
to be fit within an interior space 18 within the housing. As can be seen 
in FIG. 1, the elastomeric member assembly 12 is generally cylindrical in 
shape and of substantially uniform diameter, in its stretched condition, 
so that the housing interior 18 may also be substantially uniform in 
diameter, with a minimum radial clearance space 20 provided therebetween. 
The elastomeric member assembly 12 includes an elastomeric member 16 which 
is secured to rigid end piece assemblies 22 and 24 by virtue of wire 
meshes 26 and 28. This securement is achieved by means of helically wound 
or woven reinforcing strands, here formed into wire mesh sleeves 26 and 
28, which are secured as by bonding onto the transition section of the 
elastomeric member 16 so as to be securely joined thereto. 
The wire mesh sleeves 26 and 28 are also secured to the respective end 
piece assemblies 22 and 24 by being received over respective tapered 
forward ends of inner end pieces 30 and 32 and may be bonded by brazing as 
indicated at 34 and 36. 
The left-hand end of the elastomeric member assembly 12 as viewed in FIG. 1 
is fixed up to the housing 14 by means of an end plug 42 secured to the 
housing 14 by means of bolts 44. 
As best seen in FIG. 3, a first cross pin 46 passes transversely through 
the inner end piece 32 and a pin 48 passes through the large diameter 
protruding section of the end plug 42, to thus provide a means for 
anchoring one end of the elongated member assembly 12 to the housing 14. 
The right-hand, opposite end of the elastomeric member assembly 12 is 
affixed to an input/output shaft 50 rotatably mounted in an endcap 
assembly 52 by means of a thrust roller bearing 54 and needle bearing 56. 
The right-hand end of the housing member 14 is fixed by means of machine 
screws 60 to the end cap assembly 52. Seals 62 and 64 seal the interior 58 
of the housing 52. The left-hand end 66 of the input/output shaft 50 
protrudes into the interior of housing member 14, and is received with a 
bore 68 formed in inner piece 30. A drive pin 70 establishes a rotative 
driving connection therebetween as best seen in FIG. 2. 
A thrust bearing 54 and the cross pins 46 and 48 provide means for 
maintaining the elastomeric member 16 in an elongated stretched condition 
such as to provide a prestressing thereof for the purpose as described 
above. 
It can be seen from an inspection of FIG. 1 that the elastomeric member 
assembly 12 is of substantially uniform diameter such as to be housed 
within the interior space 18 within housing member 14 with substantially 
constant clearance space therebetween. This allows a relatively compact 
volume of the energy storage device 10, comprised of the elastomeric 
member assembly 12, surrounding housing 14, and mechanical connections and 
supports described. 
The elastomeric member 16 is of a molded "elastomeric" material, which 
term, for definitional purposes, here is deemed to include natural rubber 
compounds. The member 16 includes a main body section 74 of substantially 
constant diameter in both the initial unstretched condition as shown in 
FIG. 4, and in the stretched axially elongated condition shown in FIG. 5. 
There is a corresponding reduction in diameter due to the separation of 
the end pieces indicated diagrammatically as 30 and 32, since the material 
is not significantly compressible, and its volume is the same relaxed or 
stretched. 
Stretching is achieved by axial separation of the end pieces 30 and 32 
causing length-wise elongation of the body section 74, and, as noted since 
the volume of material contained in the body section 74 remains constant, 
a corresponding reduction in diameter occurs. Such reduction is 
substantially uniform since the axial elongation is substantially uniform 
for each segment of the body section 74. 
Each of the end piece assemblies 22 and 24 are preselected to be of a 
diameter corresponding to the designed for, final, stretched-down diameter 
of the body section 74. 
The elastomeric member 16 also has integral homogeneous transition sections 
76 and 78 of the same elastomeric material, which transition from the 
larger diameter of the body section 74 to the smaller diameter of the 
substantially rigid end piece assemblies 22 and 24. 
The diameter of each segment of the transition sections 76 and 78 must be 
in correspondence with the extent of axial elongation at each segment such 
that each segment will reduce in diameter to the final diameter r.sub.f. 
However, it can also be understood that the separating force is exerted 
throughout the length of the elastomeric member 16. Since the cross 
sectional area of each segment of the transition section 76, 78 is lesser 
than the main body section 74, the transition sections would be stressed 
at higher levels. If the modulus of the transition sections is the same as 
the body section 74, the elongation would inevitably be greater, such that 
neck-down of the material would occur in the transition zone, excessively 
stressing the material in these regions and leading to a potential early 
fatigue failure. 
According to the concept of the present invention, the transition sections 
76, 78 are reinforced by helical windings of reinforcing strands wound or 
woven about, and bonded to, the surface of the transitional sections 76 
and 78. The helical windings provide a tensile reinforcing of the 
transitional sections 76, 78 to control the axial elongation of each 
segment of the transition sections 76, 78. This controlled elongation 
produces a tensile "deflection characteristic" related to the profile of 
the transition section such that upon axial separation of the end pieces 
22, 24 the transition sections 76, 78 will be contracted radially to a 
substantially uniform diameter of the same diameter as that of the 
stretched main body section. At the same time, the helically wound 
reinforcing strands accommodate the axial elongation. 
The profile of the transition sections 76, 78 and the helix angle of the 
reinforcing strands are such that the change in position of each portion 
of the helical reinforcing windings resulting from the axial elongation of 
the member 16 is in correspondence with the change in position of the 
corresponding points on the surface of the transition sections 76, 78. 
Thus, excessive stressing of the bond between the helical reinforcing 
windings and the transition sections 76, 78, does not occur which would 
otherwise tend to weaken or destroy the connection therebetween. 
Such a reinforcement is advantageously provided by the wire mesh sleeves 26 
and 28, each consisting of sets of oppositely wound and woven individual 
strands bonded or molded into a respective transitional section 76 or 78 
with one end of the sleeves 26 and 28 welded or otherwise secured as 
described above to a respective end piece 22 or 24. Such woven and 
oppositely wound wire strands simultaneously provide a means by which a 
torque applied in either direction can be supported in the otherwise weak 
transition sections. 
As can be understood from the above description, it is firstly required 
that the sleeves follow the surface of the elastomer as it is elongated so 
that the sleeves remain an integral part of the surface. Secondly, it is 
required that neither the mesh nor the elastomer undergo significant 
elongation in the vicinity of the end pieces since the diameter is the 
desired final value prior to elongation. Consequently, the elastomer 
adjacent the end piece is virtually free of tensile stress, whereas the 
sleeve adjacent the end piece accommodates virtually all of the tensile 
load. Thirdly, it is required that the mesh and elastomer undergo the full 
desired elongation and corresponding reduction in diameter at the ends 
opposite the end pieces. Thus the elastomer there accommodates virtually 
all of the tensile load whereas the sleeves are virtually free of tensile 
stress. Hence, there is a smooth transfer of tensile load from the 
elastomer to the mesh and thence to the end pieces. As can be appreciated, 
the profile of the transition section and the orientation of the wire 
strands cannot be arbitraily chosen if these requirements are to be 
satisfied. 
The following analysis provides a mathematical solution to determining the 
various required parameters for a particular application: 
FIG. 4 shows the elastomeric member 16 and sleeves of wire mesh 26, 28 in 
the initial (unstretched) condition. In the initial relaxed state, the 
radius of the surface, r.sub.i, of each region of the transition sections 
76, 78 is a function of its axial position, z.sub.i, and this is written 
r.sub.i (z.sub.i). 
FIG. 5 shows the elastomeric member 16 and lengths of wire mesh 26, 28 in 
the final (stretched) condition. The goal is to have a uniform radius in 
the final condition and a uniform pitch angle of the reinforcing strands 
in the final condition. 
The material in the body section 74 is to be stretched such that its final 
to initial length ratio is .lambda.; this requires that the initial to 
final radius ratio be .lambda.1/2. 
Consider the small element of rubber and the small element of the wire mesh 
sleeve on the surface of this rubber indicated by the dashed lines in 
transition section 78. Initially the dashed lines are separated by the 
small distance dz.sub.i, whereas after elongation they are separated by 
the amount dz.sub.f. Initially, the radius of the element of rubber is 
r.sub.i (z.sub.i) and after elongation it is the desired value, r.sub.f. 
In order that the volume of this element of rubber be preserved, it is 
necessary that 
EQU r.sub.i.sup.2 (z.sub.i)dz.sub.i =r.sub.f.sup.2 dz.sub.f (1) 
It should be clear that as the length and the radius of the element of 
rubber change according to the above equation, the orientation of each of 
the small wires comprising the small element of the wire mesh sleeve must 
change if it is to remain an integral part of the surface. That is, both 
the pitch angle and the angle of the profile relative to the axis of the 
elastomer must change in a prescribed manner which can mathematically be 
expressed as 
EQU (r.sub.i (z.sub.i)d.phi.).sup.2 +(dr.sub.i).sup.2 +(dz.sub.i).sup.2 
=(r.sub.f d.phi.).sup.2 +(dz.sub.f).sup.2 (2) 
where d.phi. is the small angle defined by the ends of the small segment of 
wire mesh in question. 
Let the ratio of r.sub.i (z.sub.i) to r.sub.f be called f(z.sub.i), i.e., 
##EQU1## 
Note that at z.sub.i =0, f=1, and at z.sub.i =l.sub.i, f=.lambda.1/2 where 
l.sub.i is the initial length of the transition section. 
Equations (1)-(3) are combined to give the differential equation: 
##EQU2## 
From FIG. 5, the final pitch angle, .theta., is seen to be proportional to 
the ratio of r.sub.f to z.sub.f.sbsb.Tot, where z.sub.f.sbsb.Tot is the 
stretched length of the sleeves 26, 28 and r.sub.f is the radius of the 
stretched sleeves 26, 28. Furthermore, since the final pitch angle is a 
constant, 
##EQU3## 
But, dz.sub.f =f.sup.2 (z.sub.i)dz.sub.i, so r.sub.f 
(d.phi./dz.sub.i)=Cf.sup.2 (z.sub.i) and Eq. 4 becomes: 
##EQU4## 
For the transition sections 76, 78 to be smooth and continuous with the 
body section 74: 
##EQU5## 
This condition results in less criticality in fitting the mesh lengths 26, 
28 to the transition sections 76, 78. 
The solution to Eq. (6) with the value of C given by Eq. (7) is: 
##EQU6## 
Eq. (8) gives the profile of the transition zone. It is found by: 
(1) Picking values of f=r.sub.i (z.sub.i)/r.sub.f between 1 and 
.lambda..sup.1/2 where 
##EQU7## 
(2) Find x from Eq. (9) 
(3) Evaluate the integral of Eq. (8) to find the value of z.sub.i that 
corresponds to a specific f. 
In a similar manner, an equation relating the final axial location, 
z.sub.f, of a point on the surface of the transition section 74, 76, can 
be written in terms of the original f(z.sub.i): 
##EQU8## 
In particular, the final length of the mesh 26, 28 is found from aboue with 
f.sup.2 =.lambda., i.e., x=.pi./2: 
##EQU9## 
For example, if .lambda.=4, the value of z.sub.i can be found for any value 
of f between f=1 and f.sup.2 =.lambda.=4 from: 
##EQU10## 
In terms of the original value of f(z.sub.i), the axial location of a 
point on the surface after elongation is given by: 
##EQU11## 
The final pitch angle (which will be uniform in the Final Transition Zone) 
is give by: 
EQU tan .theta.=5/4=0.559, or .theta.=29.2.degree. 
FIG. 6 shows several points (1 through 7) on the surface of the relaxed 
transition sections 76, 78 for the case .lambda.=4. After elongation of 
the elastomeric member 16, the points 1 through 7 become the points 1' 
through 7' on the final surface. Note that the total length of the mesh 
after stretching is: 
##EQU12## 
In order to manufacture an elastomeric member assembly according to the 
above described concepts, there is initially determined the stretch ratio 
.lambda. desired for the particular application. 
For such .lambda. there is a specific pitch angle of the windings of the 
wire mesh sleeves 26, 28. 
Wire mesh sleeves 26, 28 of appropriate pitch angle may then be constructed 
for the desired final stretched radius with the individual length of wire 
mesh held at the stretched or final radius. The appropriate length of the 
sleeve may be trimmed, such length being calculated from the above 
described equation for z.sub.f.sbsb.Tot. 
The sleeves 26, 28 are then secured to the end piece assemblies 22, 24 as 
described above. A profile defining rigid member is then constructed 
having a profile which is in accordance with the above described 
equations. 
Each wire mesh sleeve 26, 28 is then positioned against a support member, 
and deformed so as to insure contact with the profile of the support 
members, as by contacting the transition sections 76, 78 which may also be 
caused to conform to the correct profile at the same time, as by a molding 
step. The sleeves 26, 28 are then bonded to the elastomer to complete the 
manufacturing process. 
Such bonding may be achieved by a molding-in process in which the support 
member forms a part of the mold cavity as described in co-pending patent 
application, Ser. No. 469,617, filed on 02/25/83, assigned to the same 
assignee as the present application. 
The bonding process may involve precoating or plating of the mesh, or other 
techniques well known to those skilled in that art. The details thereof do 
not form a part of the present invention, and thus are not here included. 
The resulting structure provides a very secure connection between the end 
pieces and the elastomeric member absorbing both tensile and torsional 
loads so as to provide an energy storage device having very extended 
service life, and with a high degree of reliability in field performance. 
At the same time, the components thereof are relatively easily 
manufactured at relatively low cost. 
As described above, the transition sections 76, 78 have an elongation or 
deflection characteristic which corresponds with each of their profiles 
such that deflection occurs in each segment tending to conserve the volume 
of rubber, i.e., a degree of axial elongation will occur under an endwise 
separating force to cause a reduction in diameter to the final diameter. 
Alternative arrangements for producing such deflection characteristic may 
be provided as described in co-pending application, Ser. No. 469,619, 
filed on 02/25/83. 
Accordingly, it can be appreciated that the above objects of the present 
invention provide the means by which a prestressed elastomeric member may 
be housed within the confines of a constant diameter housing of minimum 
cross sectional dimension. 
At the same time, a reliable connection to the elastomeric member for 
inputting of the torsional and stretching loads are also afforded by the 
construction of the embodiments described. 
It may be appreciated that there are many variations in the approach 
whereby the deflection characteristics of the transition sections may be 
controlled in order to product this result.