Abstract:
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 (74), and transition end sections (76, 78), 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 elastomeric member (16), a substantially uniform diameter assembly results, to minimize the required volume of the surrounding housing (14). 
     Each of the transition sections (76, 78) are received within and bonded to a woven wire mesh sleeve (26, 28) having helical windings at a particular helix angle to control the deflection of the transition section. Each sleeve (26, 28) also contracts with the contraction of the associated transition section to maintain the bond therebetween. During manufacture, the 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.

Description:
GOVERNMENT RIGHTS STATEMENT 
     The United States government has rights to this invention pursuant to Contract W-7405-ENG-48 awarded by the U.S. Department of Energy and Subcontract 2617509 awarded by the University of California, Lawrence Livermore Laboratory. 
    
    
     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 the 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; 4,304,489; 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. 
     The transition sections are formed with a particular radial profile and a deflection characteristic of each segment of the transition section such that a substantially uniform stretched diameter results. 
     Reinforcement means for each of the transition sections is provided which absorbs a portion of the tensile forces, to produce, together with the transition section profile, the desired deflection characteristic of the transition sections. 
     The reinforcement takes the form of a plurality of helically wound stands which preferrably form woven wire mesh sleeves, each bonded onto or into the surface of a respective one of the transition sections. 
     According to one aspect of the present invention, the helix angle of the windings is related to the other parameters such that each segment of the windings constracts radially while extending axialy during stretching in correspondence with radial contraction of each elastomeric segment of the transition sections so as to not excessibly strain the bond between the the elastomer and the individual strands in the wire mesh sleeves. The sleeves also provide tensile reinforcement of the elastomeric material composing the transition sections to create the appropriate deflection characteristic. In the final stretched state, the wire mesh windings assume a constant helix angle. 
     To manufacture the elastomeric member assembly, wire mesh sleeves of appropriate calculated helix angle and diameter for a particular design are trimmed to the stretched length of the transition sections and attached at one end to a respective end piee. Forming surfaces are provided, each shaped to the calculated profile of the transition regions, and each sleeve and transition section is formed thereagainst by expanding the sleeves by contact with the end of an elastomeric member, causing bonding of the sleeves to the ends, and conforming them to the profile of the forming surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an energy storage device incorporating an elastomeric member assembly according to the present invention. 
     FIG. 2 is a fragmentary view of the end piece and a method of connection thereto to an input/output shaft. 
     FIG. 3 is a fragmentary view of the end piece connected to the fixed end of the elastomeric member assembly shown in FIG. 1. 
     FIG. 4 is a diagrammatical representation of the elastomeric member assembly in its relaxed state. 
     FIG. 5 is a diagrammatical representation of the elastomeric member assembly in its stretched condition. 
     FIG. 6 is a graphical plot depicting the location of surface points on one of the transition sections of the elastomeric member as stretching elongation of the elastomeric member assembly takes place. 
     FIG. 7 is a sectional view of an elastomeric member according to an alternate embodiment of the present invention. 
     FIG. 8 is a sectional view of an elastomeric member according to another alternate embodiment of the present invention. 
     FIG. 9 is a sectional view of an elastomeric member of yet another alternate embodiment of the present invention, shown in the relaxed state. 
     FIG. 10 is a sectional view of the elastomeric member shown in FIG. 9, in the stretched condition. 
     FIG. 11 is a sectional view of an elastomeric member according to another alternate embodiment according to the present invention, shown in the relaxed state. 
     FIG. 12 is a sectional view of the elastomeric member depicted in FIG. 11, in the stretched condition. 
    
    
     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 member 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 mesh sleeves 26 and 28. This securement is achieved by means of helically wound reinforcing strands, here formed into woven wire mesh sleevs 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 woven 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 are welded as indicated at 34 and 36. 
     The left-hand end of the elastomeric member assembly 12 as viewed in FIG. 1 is fixed 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-end, 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 the housing member 14 spaced within the 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 pin 46 and 48 provide a means for maintaining the elastomeric member 16 in an elongated stretched condition such as to provide a prestressing thereof for the purposes 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 formed by the housing member 14 with a substantially constant clearance space therebetween. This allows a relatively compact volume of the energy storage device 10, comprised of the elastomeric member assembly 12 and surrounding housing 14. 
     The elastomeric member 16 is of a molded &#34;elastomeric&#34; material, which term, for definitional purposes is here deemed to include natural rubber compounds. The member 16 includes a main body section 74 of substantially uniform 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 elastomeric 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 74 remains the same, 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 selected 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 size to the final radius r 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 sections 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 one aspect of the concept according to 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 provide a means by which a torque applied in either direction can be supported in the otherwise weak transition sections. 
     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 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. 
     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 i , of each region of the transition sections 76, 78 is a funciton of its axial position, z i , and this is written r i  (z 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 λ; this requires that the initial to final radius ratio be λ1/2. 
     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 i , whereas after elongation they are separated by the amount dz f . Initially, the radius of the element of rubber is r i  (z i ) and after elongation it is the desired value, r f . In order that the volume of this element of rubber be preserved, it is necessary that 
     
         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 
     
         (r.sub.i (z.sub.i)dφ).sup.2 +(dr.sub.i).sup.2 +(dz.sub.i).sup.2 =(r.sub.f dφ).sup.2 +(dz.sub.f).sup.2                 (2) 
    
     where dφ is the small angle defined by the ends of the small segment of wire mesh in question. 
     Let the ratio of r i  (z i ) to r f  be called f(z i ), i.e., ##EQU1## Note that at z i  =0, f=1, and at z i  =l i , f=λ1/2 where l i  is the inital length of the transition section. 
     Equations 1-3 are combined to give the differential equation: ##EQU2## From FIG. 5, the final pitch angle, θ, is seen to be proportional to the ratio of r f  to z f .sbsb.Tot, where z f .sbsb.Tot is the stretched length of the sleeves 26, 28 and r f  is the radius of the stretched sleeves 26, 28. Furthermore, since the final pitch angle is a constant, ##EQU3## 
     But, dz f  =f 2  (z i )dz i , so ##EQU4## and Eq. 4 becomes: ##EQU5## 
     For the transition sections 76, 78 to be smooth and continuous with the body section 74: ##EQU6## 
     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: ##EQU7## Eq. (8) gives the profile of the transition zone. It is found by: (1) Picking values of ##EQU8##  between 1 and λ 1/2  ##EQU9## (2) Find x from Eq. (9) (3) Evaluate the integral of Eq. (8) to find the value of z i  that corresponds to a specific f. 
     In a similar manner, an equation relating the final axial location, z f , of a point on the surface of the transition section 74, 76, can be written in terms of the original f(z i ): 
     where ##EQU10## 
     In particular, the final length of the mesh 26, 28 is found from above with f 2  =λ, i.e., x=π/2: ##EQU11## 
     For example, if λ=4, the value of z i  can be found for any value of f between f=1 and f 2  =λ=4 from: ##EQU12## In terms of the original value of f(z i ), the axial location of a point on the surface after elongation is given by: ##EQU13## The final pitch angle (which will be uniform in the Final Transition Zone) is given by: 
     
         tan θ=5/4=0.559, or θ=29.2° 
    
     FIG. 6 shows several points (1 through 7) on the surface of the relaxed transition sections 76, 78 for the case λ=4. After elongation of the elastomeric member 16, the points 1 through 7 become the points 1&#39; through 7&#39; on the final surface. Note that the total length of the mesh after stretching is: ##EQU14## 
     In order to manufacture an elastomeric member assembly according to the above described concepts, there is initially determined the stretch ratio λ desired for the particular application. 
     For such λ 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 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. 469,618, filed on Feb. 25, 1983, 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 are shown in FIGS. 7-12. In FIG. 7, the elastomeric member assembly 12a is shown in which substantially rigid end pieces 32a are constructed of a very high modulus stiff material with an intervening transition region 78a in which there is a gradual gradation of stiffness of the rubber as may be produced by carefuly controlled curing and varied compositon of the material in the region. 
     In FIG. 8 the same result is produced by a rigid end piece 32b bonded to a transition section 78b, provided by sections 80 of elastomeric material of increasing diameter and decreasing degrees of stiffness such as to provide an incremental change in the stiffness of the segments throughout the transition zones 78b to approximate the arrangement shown in FIG. 7. 
     In FIG. 9, the deflection characteristic may be produced by a conical insert 82 of a relatively stiff material bonded to a transition region 78c constructed of relatively softer material. 
     As shown in FIG. 10, the deflection characteristics produced have the same end result of a constant diameter stretched elastomeric member 12c. 
     In FIG. 11, a complementary dish shaped insert 84 of a relatively stiff material receives a transition section 78d of relatively softer material, and, as seen in FIG. 12, produces the same result upon being stretched. 
     Accordingly, it can be appreciated that the stated objectives 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 produce this result.