Layered flywheel with stress reducing construction
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to a flywheel. More particularly, this invention relates to a flywheel for ultra-high speed operation having a rim and a hub including radially extending spokes coupling the rim to a shaft. 
     Flywheels have long been recognized as convenient devices for the storage of mechanical energy. Energy is stored in a flywheel by causing it to rotate at a high speed about an axis of rotation defined by a shaft. By mounting the shaft in low-friction bearings and the flywheel in an evacuated chamber, frictional energy losses are minimized. Thus, the flywheel has come to be recognized as a convenient device for the relatively long-term storage of energy. Particular attention has been directed to the flywheel as a device for energy storage in mass-transportation vehicles operating under stop-and-go conditions. For example, the flywheel may be charged with energy by bringing its rotational speed to a high level while the vehicle is stopped. Energy is then drawn from the flywheel to accelerate the vehicle and power it toward its next stop. By using regenerative braking energy which would conventionally be dissipated as heat is returned to the flywheel for later use. Thus, the flywheel provides a conceptually simple means of storing energy for vehicular and other uses. 
     However, attempts to construct and utilize such a flywheel have been fraught with difficulties and failures. For example, because the energy stored in a flywheel varies directly with its moment of inertia and as the square of its rotational speed, very high operating speeds for the flywheel are desired. Further, in order to obtain best performance from a vehicle, the weight of the flywheel must be kept to a minimum to reduce vehicle weight. Therefore, flywheels having a rim have been recognized as offering the highest moment of inertia for a given weight. When the rim is made of a multitude of concentric annular shells made from circumferentially extending unidirectional filamentary material in a matrix it is well adapted to withstand the high stresses imposed by centrifugal force at high rotational speeds. Such flywheels conventionally have a hub including spokes coupling the rim to the shaft. U.S. Pat. Nos. 860,336; 3,724,288; 3,964,341; 4,036,080; 4,176,563; 4,183,259 and 4,186,623 illustrate such flywheels. 
     However, even though the stress imposed by centrifugal force varies with the square of the radius from the axis of rotation so that the rim is most highly stressed, at the ultrahigh rotational speeds desired the spokes are also highly stressed. Thus, the desirability of also making the spokes of high-strength, low-weight unidirectional filamentary material in a matrix has been recognized. U.S. Pat. No. 4,286,475 illustrates such a flywheel. 
     Unfortunately, at the ultra-high rotational speeds desired, the rim and spokes of a flywheel stretch and distort to expand radially. Even the unidirectional filamentary material embedded in a matrix is elastic and deforms when exposed to the high centrifugal forces created by ultra-high speed operation of a flywheel. Thus, the flywheel designer is left with the difficult problem of how to unite matrix material spokes to form of hub for the flywheel. U.S. Pat. No. 4,286,475 illustrates one solution to this problem. 
     In view of the many deficiencies of the flywheel art, it is an object for this invention to provide a flywheel having a rim and spokes coupled to a shaft by hub portions which distort in reponse to centrifugal force to match the distortion of the spokes. 
     Another object for this invention is to provide a hub for a flywheel having spokes substantially avoiding stress concentrations between the spokes and the remainder of the hub. 
     Still another object for this invention is to provide a hub for a flywheel with spokes of unidirectional filamentary material embedded in a matrix. 
     In summary, one embodiment of this invention provides a flywheel having a rim and spokes of filamentary material embedded in a matrix. The spokes engage the rim and extend radially inwardly toward but short of the axis of rotation. A hub includes portions axially coextensive with each spoke over a radially extending segment of the latter. The portions are adhesively bonded to the spokes and decrease in transverse cross sectional area with increasing radius throughout the radially extending segment. During operation of the flywheel, the portions distort in response to centrifugal force to expand radially substantially in unison with the spokes. 
     Other objects and advantages of the invention will appear in light of the following detailed description of a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an axial plan view of a flywheel embodying the invention; 
     FIG. 2 is an elevation view, partly in cross section, taken along line 2--2 of FIG. 1; 
     FIG. 3 is an enlarged fragmentary plan view, partly in cross section, taken along line 3--3 of FIG. 2; 
     FIG. 4 is a fragmentary cross sectional view taken along line 4--4 of FIG. 3; 
     FIG. 5 is a fragmentary cross sectional view similar to FIG. 4 and illustrating an alternative embodiment of the invention; 
     FIG. 6 is an isolated perspective view of a component part of the flywheel illustrated in FIG. 5; 
     FIG. 7 is a fragmentary axial plan view of a flywheel according to another alternative embodiment of the invention; 
     FIG. 8 is a fragmentary elevation view, partly in cross section, taken along line 8--8 of FIG. 7; and 
     FIG. 9 is a fragmentary cross section view similar to FIGS. 4 and 5 and illustrating yet another alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a flywheel 10 having a rim 12 and a hub generally referenced by the numeral 14. The rim 12 is preferably of a conventional construction providing a high ratio of moment of inertia to weight and also a high ratio of elastic modulus to density. For example, the rim 12 may be constructed in accordance with the teachings of U.S. Pat. Nos. 4,036,080 or 4,186,623. The hub 14 is of cruciform shape in plan view and includes a multitude of radially extending spokes 16. The spokes 16 comprise generally flat-sided bars of unidirectional filamentary material embedded in a matrix. The filamentary material extends longitudinally in the bars so that the filaments extended radially in the spokes 16. The spokes 16 may be made in accordance with the teaching of U.S. Pat. No. 4,286,475, the disclosure of which is hereby incorporated herein to the extent necessary for a full understanding of this invention. The hub 14 also includes a central section 18 which is also of cruciform shape in plan view. The central section 18 includes a multitude of radially extending portions 20 each axially aligning with one of the multitude of spokes 16. 
     FIG. 2 illustrates that the central section 18 of hub 14 includes a pair of substantially identical cruciform end plates 22 and 24 each of which defines a pintle shaft 26 and 28, respectively, extending axially therefrom. The pintle shafts 26 and 28 cooperate to define an axis of rotation for the flywheel 10. FIG. 2 also illustrates that the spokes 16 are arranged in five axially spaced radial arrays, or axial levels, of four spokes each. The five radial arrays of spokes are axially spaced apart by four substantially identical cruciform spacer plates 30, 32, 33 and 34. The spokes 16 extend radially inwardly from the rim 12 toward but short of the axis of rotation of the flywheel 10. Each of the end plates 22, 24 and each of the spacer plates 30, 32, 33 and 34 define four of the multitude of radially extending portions 20 of the central section 18. The radially extending portions 20 are axially aligning with and contiguous to the spokes 16. Further, the portions 20 are radially coextensive with the spokes 16 over a region of each spoke extending from a radially inner end 36 of each spoke toward but short of the radially outer end of each spoke. 
     FIGS. 1 and 2 also illustrate that the hub 14 includes four axially extending cap members 21 interposed radially between the radially outer ends of the spokes 16 and the rim 12. The cap members 21 are composed of multidirectional filamentary material embedded in a matrix. For example, the cap members 21 may include a woven cloth of filamentary material embedded in a matrix or a mat of randomly oriented filamentary material embedded in a matrix. In either case, the material from which the cap members 21 are formed has substantially isotropic physical properties. In fact, the cap members 21 may be made of a truly isotropic material such as a metal. For example, the cap members 21 may be made of aluminum alloy material. The cap members 21 extend axially to bridge the axial spaces between the spokes 16. Further, the cap members 21 are adhesively bonded to the outer ends of the spokes 16. Thus, the cap members 21 serve to tie together the radially outer ends of the axially congruent spokes 16 in each of the five levels of the hub 14. Consequently, the cap members 21 increase the rigidity of the hub 14. Still further, the cap members 21 distribute radial loads between the spokes 16 and the rim 12 to avoid stress concentrations in the latter. 
     FIG. 3 illustrates that each of the radial arrays, or levels, of spokes 16 includes an eight-sided center piece 38. The inner ends 36 of the spokes 16 confront but do not contact the center pieces 38. Thus, the inner ends 36 each define a gap &#34;g&#34; with the center pieces 38. The gaps &#34;g&#34; are filled with a scrim-controlled adhesive bonding the inner ends 36 of the spokes 16 to the center pieces 38. The scrim-controlled adhesive is essentially a fabric cloth impregnated with adhesive. When subjected to pressure during curing of the adhesive, the scrim cloth prevents the adhesive from being squeezed out of the joint. Thus, the gaps &#34;g&#34; may be controlled to a high degree of accuracy to insure concentricity of the spokes 16 and rim 12 with the axis of rotation of the flywheel 10. 
     FIG. 3 also illustrates that the spacer plates 30, 32, 33, and 34 each have a cruciform shape in plan view which is substantially identical to that of the end plates 22 and 24. The portions 20 of the end plates 22 and 24 and of the spacer plates 30, 32, 33 and 34 are each axially contiguous to one of the radial arrays of spokes 16. Further, the portions 20 each extend radially outwardly to terminate in a radially outer end 40. The end 40 of each portion 20 is radially outward of the inner end 36 of each spoke 16. Thus, the portions 20 are radially coextensive with the spokes 16 over a region 42 extending from the end 36 to the end 40. Within the region 42, the portions 20 each decrease in transverse cross sectional area with increasing radius. For example, by comparing the transverse width of the portion 20 at the plane defined by the line &#34;a&#34;, viewing FIG. 3, with the width of the portion at the line &#34;b&#34; and noting that the portion has a substantially constant axial thickness, it will easily be seen that the cross sectional area of the portion decreases with increasing radius within the region 42. 
     Viewing FIG. 4, it will be seen that within the region 42, the spokes 16 do not actually contact the spacer plates 30-34 or end plates 22, 24. Moreover, the center pieces 38 have a greater axial dimension than do the spokes 16. Thus, the spacer plates 30-34 and end plates 22, 24 are spaced apart by the center pieces 38 to define a gap &#34;g&#34; with the spokes 16. The gap &#34;g&#34; is filled with a scrim-controlled adhesive bonding the spokes 16 to the plates 22, 24 and 30-34. 
     FIG. 4 also illustrates that the spacer plates 30-34 define axially extending apertures 44 circumscribing and concentric with the axis of rotation of flywheel 10. The center pieces 38 include reduced-diameter bosses 46 extending axially into the apertures 44 to insure concentricity of the center pieces 38 with the spacer plates 30-34. The bosses 46 cooperate with the remainder of each spacers 38 to define annular shoulders 48 circumscribing the bosses 46. The shoulders 48 engage the spacer plates 30-34 to define radially extending annular bonding areas 50. Because the plates 22, 24 and 30-34 and center pieces 38 are made of metal, they may be brazed together at the bonding areas 50 to form a unitary central section 18 for the hub 14. Alternatively, the plates and center pieces may be adhesively bonded together by a suitable metal-to-metal adhesive. Such an adhesive is made by Minnesota Mining and Manufacturing and sold under the name Scotch Weld 2214. Viewing FIG. 4, it will be seen that the bosses 46 of adjacent center pieces 38 cooperate to define chambers 52 within the apertures 44. When the central section 18 is assembled with adhesive, the chambers 52 form convenient reservoirs for the adhesive so that each of the adjacent center pieces 38 are also bonded together. It will be understood in light of the above that the end plates 22, 24 each define a recess (not shown) for receiving the boss of the adjacent center piece 38 so that the end plates 22, 24 also define annular bonding areas 50 and chambers 52. 
     FIGS. 5 and 6 illustrate an alternative embodiment of the invention wherein a flywheel hub 54 includes spokes 16 arranged as in the embodiment illustrated by FIGS. 1-4. However, the embodiment illustrated by FIGS. 5 and 6 includes cruciform spacer plates 56,58 and end plates (not shown) each defining axially extending bosses 60 which are received in axially extending recesses 62 defined by center pieces 64. Viewing FIG. 6, it will be seen that the center pieces 64 are eight-sided and define four axially extending surfaces 66 (only two of which are visible in FIG. 6) for bonding to the inner ends 36 of the spokes 16 via scrim-controlled adhesive. The center pieces 64 also define a pair of radially extending end surfaces 68 (only one of which is visible) for bonding to the adjacent spacer plates or end plates. The center pieces also define a number of radially extending grooves 70 extending radially outwardly from the recesses 62 and open at their outer ends. Thus, where an adhesive is used to bond the plates 56 and center pieces 64 together, the grooves 70 allow the escape of trapped air and excess adhesive from chambers 72 defined within the recesses 62. 
     FIGS. 7 and 8 illustrate another alternative embodiment of the invention wherein a cruciform flywheel hub 74 includes two axially spaced radial arrays, or axial levels, of spokes 16 which are axially sandwiched with a pair of end plates 76 and 78 and with a single spacer plate 80. The hub 74 includes a pair of center pieces 82. One of the center pieces 82 is received in each one of the radial array of spokes 16. Similarly to the embodiments of FIGS. 1-6, the embodiment illustrated in FIGS. 7 and 8 has the spokes 16 bonded to the center pieces 82 and to the plates 76-80 by scrim-controlled adhesive. However, each of the plates 76-80 defines four holes (not visible in the Figures) axially aligning with similar holes in the other two plates. Four tie bolts 84 pass axially through the holes of the plates 76-80 to apply an axially directed clamping force holding the plates 76-80 together. During assembly of the hub 74, the tie bolts serve to hold the various parts in place while the adhesives cure. Further, the tie bolts assist in holding the component parts of the hub 74 in proper alignment during manufacture to help insure concentricity of the flywheel rim (not shown) and dynamic balance of the flywheel. During use of the hub 74, the tie bolts 84 may remain in place or they may be removed before the hub is used. Removal of the tie bolts insures that they do not interfere with the radial expansion of, and the smooth distribution of stresses within, the hub 74 during operation. Further, removal of the tie bolts 84 obviates any need to provide lashings or other structure preventing the tie bolts from bowing radially outwardly during operation of the flywheel. 
     FIG. 9 illustrates yet another alternative embodiment of the invention which is generally similar to the embodiment illustrated by FIGS. 5 and 6. The hub 86 illustrated in FIG. 9 includes a multitude of spokes 16 which are arranged in radial arrays with center pieces 88 and sandwiched with spacer plates 90. The center pieces 88 define axially extending recess 92 receiving axially extending bosses 94 defined by the spacer plates 90. The spokes 16 are bonded to the center pieces 88 and to the spacer plates 90 by scrim-controlled adhesive, as with the embodiment of FIGS. 5 and 6. However, the hub 86 includes cruciform spacer plates 90 which each include a pair of component parts 90a and 90b. The parts 90a and 90b are substantially identical to each other and are secured together back-to-back by a scrim-controlled adhesive. The parts 90a and 90b are each stamped outwardly at 96 to define the bosses 94. Because of the stampings 96, the parts 90a and 90b cooperate to define a cavity 98 therebetween. Each of the parts 90a and 90b defines a radially outer end 100 adjacent one of the spokes 16 and tapers axially and radially from the end 100 toward the interface of the two parts of the spacer plate 90. Because of the taper of the parts 90a and 90b adjacent the ends 100, the parts cooperate to define a circumferentially extending V-shaped notch 102 therebetween. As mentioned supra, the spacer plates 90 are cruciform shaped in plan view so that the tranverse cross sectional area of the spacer plates 90 decreases with increasing radius between the inner ends 36 of the spokes 16 and the outer ends 100 of the plates 90. However, the plates 90 also decrease in cross sectional area with increasing radius near their outer ends because of the notches 102. Thus, the spacer plates define a cross sectional area decreasing to zero substantially steplessly with increasing radius. Consequently, a stress concentration at the radially outer end 100 is substantially avoided. 
     During operation of a flywheel according to this invention, centrifugal force causes the rim and spokes to expand radially outwardly. The rim 12 is arranged to continuously exert a radially inwardly directed force on the outer ends of the spokes 16 despite the radial expansion of the rim during operation of the flywheel. Despite the inwardly directed force exerted by the rim at the outer end of each spoke, centrifugal force causes the net force at the inner end of the spokes 16 to be directed radially outwardly. As set out supra, the spokes 16 are secured to the central sections of the hubs 14, 54, 74 and 86 only by adhesive bonding. Thus, the outwardly directed force at the inner ends of the spokes 16 tends to pull the spokes out of the central section; causing stresses in and straining of the adhesive bonds. However, the spacer plates 30-34, 56, 58, 80 and 90 and end plates 22 29, 76, 78 also expand radially outwardly in response to centrifugal force. Further, the radially outwardly directed forces on the spokes 16 are transferred to the central sections of the hubs via the spacer plates, end plates and center pieces, causing further radial expansion of the spacer plates and end plates. Because the spacer plates and end plates decrease in cross sectional area with increasing radius between the inner ends 36 of the spokes 16 and the outer ends of the spacer plates and end plates, the plates expand radially substantially in unison with the radial expansion of the spokes 16. Thus, stress concentrations in the adhesive bonds are substantially avoided. Such stress concentrations could cause localized failure of the bonds and a &#34;zipper effect&#34; leading to failure of the entire bond and destruction of the flywheel. 
     Those skilled in the pertinent art will recognize that the spokes 16 also transfer torque to and from the rim 12. Thus, the adhesive bonding of the spokes 16 to the hubs must not only resist centrifugal forces but also torque-induced forces as well. Such torque-induced forces tend to move the outer ends of the spokes 16 circumferentially with respect to their normal positions. Thus, such forces tend to cause rotational freedom of the spokes relative to their normal positions. Examination of the hub constructions provided by this invention will show that they are well adapted to resist such torque-induced forces because the spokes 16 are bonded to the plates throughout bonding areas which extend radially for a considerable distance as well as circumferentially. The adhesive in these bonding areas is subjected primarily to shear stresses which are well distributed without stress concentrations. Thus, the spokes 16 are secured to the central sections of the hubs substantially without rotational freedom despite the fact that the central sections of the hubs are yieldable radially in response to centrifugal force. That is, the union between the spokes and the central sections of the hubs is substantially rigid circumferentially. 
     Further, those skilled in the pertinent art will recognize that this invention is not limited to flywheels having multiples of four radial spokes. For example, the flywheel could have two or three radial spokes or a number greater than four. It will be apparent in light of the above that this invention provides a flywheel as well as a method of making a flywheel. While this invention has been described by reference to preferred embodiment thereof, no limitation should be implied because of such reference. The spirit and scope of this invention is set forth by the appended claims which alone define the invention.