COMPOSITE FLYWHEEL

Abstract

A method of manufacturing a flywheel comprising: forming a first hollow cylinder from glass fibre composite with magnetic particles dispersed through at least part of the cylinder; curing said first cylinder in a first curing step; forming a second hollow cylinder from carbon fibre composite; and curing said second hollow cylinder in a second curing step.

Claims

1. A method of manufacturing a flywheel comprising: forming a first hollow cylinder from glass fibre composite with magnetic particles dispersed through at least part of the cylinder; curing said first cylinder in a first curing step; forming a second hollow cylinder from carbon fibre composite; and curing said second hollow cylinder in a second curing step.

2. A method as claimed in claim 1, wherein the step of forming said glass fibre composite cylinder comprises: forming a first section of glass fibre composite with magnetic particles dispersed therein; and forming a second section of glass fibre composite around said first section without magnetic particles dispersed therein.

3. A method as claimed in claim 2, wherein the magnetic particles are dispersed within the first section by applying a resin containing magnetic particles to the glass fibres, and wherein the second section is formed by towing glass fibres through a resin bath without magnetic particles.

4. A method as claimed in claim 1, wherein forming the second hollow cylinder comprises winding carbon fibre round the cured first hollow cylinder.

5. A method as claimed in claim 4, further comprising: applying an adhesive layer to the outside surface of the cured first hollow cylinder before forming the second hollow cylinder on top of the adhesive layer.

6. A method as claimed in claim 4, wherein the step of forming the first hollow cylinder comprises forming a removable section on the outer surface of the first hollow cylinder and wherein after curing the first hollow cylinder, but before forming the second hollow cylinder, the removable section is removed.

7. A method as claimed in claim 1, further comprising: inserting the first hollow cylinder inside the second hollow cylinder such that they form an interference fit between an outer surface of the first hollow cylinder and an inner surface of the second hollow cylinder.

8. A method as claimed in claim 7, further comprising a step of removing material from the outer surface of the glass fibre composite cylinder to smooth the surface prior to the inserting step.

9. A method as claimed in claim 7, wherein the glass fibre composite cylinder is formed with an outer diameter slightly larger than the inner diameter of the carbon fibre composite cylinder to ensure a strong interference fit between the two.

10. A method as claimed in claim 7, further comprising applying a lubricant between the glass fibre composite cylinder and the carbon fibre composite cylinder to facilitate the inserting step.

11. A method as claimed in claim 10, wherein the lubricant is an adhesive heated to a temperature sufficient to cause it to act as a lubricant.

12. A flywheel comprising: a first hollow cylinder of glass fibre composite with magnetic particles dispersed through at least part of the cylinder; and a second hollow cylinder of carbon fibre composite surrounding said cylinder of glass fibre composite and being substantially not chemically bonded thereto by polymeric cross-linking.

13. A flywheel as claimed in claim 12, wherein the hollow cylinder of glass fibre composite comprises an inner section containing magnetic particles and an outer section containing substantially no magnetic particles.

14. A flywheel as claimed in claim 13, wherein said inner section and outer section are chemically bonded by polymer crosslinks.

15. A flywheel as claimed in claim 12, further comprising a layer of adhesive between the first cylinder and the second cylinder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] One or more non-limiting examples will now be described, with reference to the accompanying drawings, in which:

[0038] FIG. 1a illustrates a press-fit assembly of a first example;

[0039] FIG. 1b shows the final product of FIG. 1a;

[0040] FIG. 2a shows an intermediate product of a first stage of a second example;

[0041] FIG. 2b illustrates a second stage of the second example;

[0042] FIG. 2c shows the various layers in the second example;

[0043] FIG. 3 illustrates a manufacturing process for the first example; and

[0044] FIG. 4 illustrates a manufacturing process for the second example.

DETAILED DESCRIPTION

[0045] FIG. 1a shows a first example of a method of manufacturing a flywheel. A first cylinder 20 and a second cylinder 21 are combined together to form a flywheel 1 as shown in FIG. 1b.

[0046] The first cylinder 20 is formed from an inner section 4 of glass fibre with magnetic particles dispersed therein and an outer section 3 of glass fibre with no magnetic particles dispersed therein. This inner section 4 may be referred to as a magnetically loaded composite (MLC) section 4. The outer section 3 is referred to herein as the glass fibre (GF) section 3. These inner section 4 and outer section 3 are formed from substantially hoop wound (i.e. tight helix or high helix angle) glass fibre impregnated in a resin matrix, wound around a mandrel and then cured (e.g. at high temperature in the case of a thermosetting matrix, although it will be appreciated that other matrix materials and other curing methods such as photocuring or microwave curing may be used instead).

[0047] The second cylinder 21 is formed from a section 2 of carbon fibre in a matrix (i.e. it is a carbon fibre reinforced polymer, CFRP section) and is separately formed, e.g. on a separate mandrel, and is separately cured (again, typically cured at high temperature, but other curing methods may apply).

[0048] The first cylinder 20 is formed with an outer diameter that matches (or is slightly larger than) the inner diameter of the second cylinder 21. After both the first cylinder 20 and second cylinder 21 have been separately cured, they are press fit together by inserting the first cylinder 20 inside the second cylinder 21. The two cylinders 20, 21 are engaged with one another by a friction or interference fit. If the inner cylinder 20 was formed and machined to an outer diameter greater than the inner diameter of the second cylinder 21 then a greater force will be required to press fit the two parts 20, 21 together, but the interference fit will be much stronger. Purely by way of example, a typical size difference is an outer diameter of the first cylinder 20 around 0.1 mm greater than the inner diameter of the second cylinder 21.

[0049] As the two cylinders 20, 21 are cured separately and press fit together, the interface 10 that is formed between them in the final product 1 is not a chemical bond, but a purely mechanical bond (interference fit). The stresses that form in each of the two cylinders 20, 21 during the curing process therefore do not transfer to the other cylinder 21, 20 which reduces the risk of crack formation and delamination. In turn, this reduces the failure rate of the product 1 and increases the yield of the manufacturing process.

[0050] The interface 11 between the magnetically loaded composite (MLC) section 4 and the glass fibre (GF) section 3 is a chemical bond that is formed by polymeric crosslinking between the resin matrices of the two sections 3, 4. This chemical bond forms during the curing process as these two sections 3, 4 are cured together. As the properties of the inner and outer sections 3, 4 are sufficiently similar, no significant stress gradient develops across this interface during the curing process.

[0051] FIGS. 2a and 2b illustrate a second example of a method for manufacturing a flywheel 1. In the first stage, illustrated in FIG. 2a, the GF section 3 and MLC section 4 are formed in the same way as described in relation to FIG. 1a above, except that before curing, a third removable section 5 is applied to the outside of the GF section 3. The removable section 5 is a peel ply layer that is used to ensure a good outer surface of the GF section 3 ready for the second stage of manufacture. The three sections 3, 4 and 5 are all cured together and after curing the removable section 5 is removed. FIG. 2a shows the removable section 5 in the process of being removed to expose the underlying surface of the GF section 3. FIGS. 2a and 2b also show a shaft 22 having a larger diameter mandrel section 6 onto which the fibres are wound.

[0052] After the peel ply section 5 has been removed, an adhesive layer 7 is applied to the outer surface of the GF section 3. Then resin impregnated carbon fibre 8 is wound onto the adhesive layer 7 to a desired thickness. A second curing step then cures the carbon fibre section 2 (and in the case of a curable adhesive 7, this is also cured in this step). As the underlying section 3 has already been cured, no chemical bond (i.e. no polymeric crosslinking) is formed between the adhesive 7 and the GF section 3. Accordingly, the stresses that build in the carbon fibre section 2 during the second curing process do not transfer to the glass fibre section 3 and therefore there is no large stress gradient that risks crack formation and delamination. There will still be a difference in modulus and a difference in coefficient of thermal expansion between the CF section 2 and the adhesive layer 7, but as there is no high modulus material in the adhesive layer 7, to resist shrinkage, the adhesive layer 7 is able to accommodate the induced stresses more easily. The sections of the final product are shown in FIG. 2c. It will be appreciated that the sections depicted here are not shown to scale. The adhesive layer 7 and the peel ply layer 5 are in real examples much thinner than is depicted in FIGS. 2a to 2c.

[0053] FIG. 3 illustrates the steps of the process that is used to form the flywheel 1 in the first example as shown in FIGS. 1a and 1b.

[0054] In step 30 the magnetically loaded composite (MLC) section 4 is wound onto a mandrel, this section 4 comprising glass fibre impregnated with a matrix with magnetic particles dispersed therein. The matrix may be applied to the glass fibre filaments by any suitable technique. The fibres are hoop wound for maximum circumferential strength. In step 31 the glass fibre (GF) composite section 3 is wound onto the MLC section 4 (without any intermediate curing). The GF section 3 is also hoop wound for maximum circumferential strength, but the fibre tow is impregnated with resin without any dispersed magnetic particles. The resin may be applied to the glass fibre filaments by any suitable technique such as towing the filaments through a resin bath. The same glass fibre tow may be used in both sections 4 and 3. In step 32 the sections 3 and 4 are simultaneously cured, e.g. by heating in an oven using the desired cure cycle.

[0055] In step 33, the carbon fibre (CF) composite section 2 is formed by winding a carbon fibre tow impregnated with resin around a separate mandrel. Again, the CF filaments are hoop wound for maximum circumferential strength. In step 34 the CF composite section 2 is cured using a suitable cure cycle.

[0056] Steps 30, 31 and 32 may be carried out completely separately from steps 33 and 34. It is not important whether the CF section 2 is formed before, after or simultaneously with the GF+MLC sections. After both curing steps 32, 34 have been carried out, the first cylinder 20 (GF+MLC) is press fitted into the second cylinder 21 in step 35 so as to form the final flywheel 1 as shown in FIG. 1b.

[0057] FIG. 4 illustrates the steps of the process that is used to form the flywheel 1 in the second example as shown in FIGS. 2a, 2b and 2c.

[0058] In step 40 the magnetically loaded composite (MLC) section 4 is wound onto a mandrel, this section 4 comprising glass fibre impregnated with a matrix with magnetic particles dispersed therein. The matrix may be applied to the glass fibre filaments by any suitable application or impregnation technique. The fibres are hoop wound for maximum circumferential strength. In step 41 the glass fibre (GF) composite section 3 is wound onto the MLC section 4 (without any intermediate curing). The GF section 3 is also hoop wound for maximum circumferential strength, but the filaments are impregnated with resin without any dispersed magnetic particles. This may be achieved for example by towing the filaments through a resin bath. This may involve switching from a direct application technique to a resin bath application technique between the winding of sections 4 and 3. The same glass fibre tow may be used in both layers 4 and 3. In step 42 a removable, peel ply section 5 is applied to the outer surface of the GF section 3. In step 43 the sections 3, 4 and 5 are simultaneously cured, e. In step 44 the removable, peel ply section 5 is removed by peeling it away, leaving behind the GF and MLC sections 3, 4. The outer surface of GF section 3 that is left after removal of the peel ply section 5 is of uniform diameter, but textured so as to provide a good key for adhesive layer 7 (i.e. it permits a good mechanical bond with adhesive layer 7). In step 45 an adhesive layer 7 is applied to the outer surface of the cured GF section 3. In step 46 the carbon fibre (CF) composite section 2 is formed by winding carbon fibre filaments impregnated with resin around the adhesive layer 7. Again, the CF filaments are hoop wound for maximum circumferential strength. In step 47 the CF composite section 2 (and, possibly adhesive layer 7, depending on the particular adhesive chosen) is cured. The resulting, fully cured flywheel 1 has layered structure as shown in FIG. 2c.

[0059] It will be understood that the description above relates to a non-limiting example and that various changes and modifications may be made from the arrangement shown without departing from the scope of this disclosure, which is set forth in the accompanying claims.