GB2449117A

GB2449117A - High speed flywheel

Info

Publication number: GB2449117A
Application number: GB0709085A
Authority: GB
Inventors: Jonathan James Robert Hilton; Douglas Isaac Lascelles Cross
Original assignee: Flybrid Systems LLP
Current assignee: Flybrid Systems LLP
Priority date: 2007-05-11
Filing date: 2007-05-11
Publication date: 2008-11-12
Anticipated expiration: 2027-05-11
Also published as: GB2449117B; GB0709085D0

Abstract

A high speed flywheel, suitable for use contained in an evacuated housing in a vehicle, comprises a hub formed of two halves, and a rim 6 interference-fitted over an external surface of the periphery of the hub. The periphery of the hub also has an internal surface from which at least one inclined web extends towards the centre of the flywheel. The flywheel may have a pair of inclined webs with outer webs 20,22 extending axially and radially inwardly from points 42,44 on the perimeters 16,18 of the hub halves to a joining point 24 and inner webs 46,48 extending inwardly from the joining point 24. The inner webs 46,48 may be provided with bearings 38,40. The hub may be formed from steel and the rim may be formed from a filament wound carbon fibre composite. The primary failure mode of the flywheel is preferably cracking of the hub; the composite rim would then contain the debris.

Description

High Speed Flywheel This invention relates to flywheels, and

particularly to energy storage flywheels for use in vehicles.

Flywheels typically comprise a relatively heavy mass, mounted on a shaft and arranged to rotate with the shaft. Bearings are provided in the housing to support the shaft. The use of flywheels in vehicles is known, for example as an aid for acceleration or deceleration of the vehicle, It is also known to use a flywheel for energy storage, whereby the kinetic energy of the flywheel is converted into electrical energy. The kinetic energy of a flywheel is directly proportional to the rotational inertia and the square of the angular velocity. A flywheel used for energy storage in a vehicle must achieve an optimum balance of mass, inertia and rotational speed. The faster the flywheel can be made to rotate, the smaller and lighter it will be for a given storage capacity.

Stationary flywheels are capable of high speeds, for example greater than 100,000 revs per minute. However, flywheels used in vehicles typically run at speeds of around 20,000 revs per minute or lower, due to certain constraints.

Firstly, the speed at which a flywheel within a vehicle can operate is limited by susceptibility to vibration caused on operation of the vehicle, which can cause significant strain on the flywheel mounting and bearings, thereby increasing friction and wear and reducing the efficiency of the flywheel, and furthermore constituting a potential safety risk. The susceptibility of the flywheel to vibration can be minimised by providing the flywheel with a high natural frequency of vibration.

I

For the above reasons, flywheels in vehicles must adhere to stringent safety criteria, including provision of a benign, or at least controllable, failure mode.

When the primary failure mode, i.e. the mode which occurs at the lowest rotational speed, is reached, the flywheel and its housing must be designed to contain all debris generated on the breaking up of the flywheel components.

A further design criteria for high speed flywheels in the automotive industry is a good control of the length of the shaft between the bearing locations. When the flywheel is rotated at high speeds, the rim will be subject to radial growth and axial shrinkage. This axial shrinkage can be problematic if transmitted to the centre of the flywheel and the bearings.

The bearings on which the flywheel rotates are of the angular contact variety, which are designed to accommodate significant loads in both the radial and axial directions. To work correctly, it is necessary to control the axial load on this type of bearing to within tight limits; at low flywheel speeds there must be an acceptable level of end load and at high speed it must not drop to zero or the balls will skid and fail very quickly.

It is an aim of the present invention to address the above constraints to overcome or at least mitigate the aforementioned problems, and to provide a high speed flywheel for use in a vehicle.

Accordingly the present invention provides a high speed flywheel as claimed in claim 1 of the appended claims.

An advantage of the present invention is that the flywheel has a low mass and a relatively high inertia, as the provision of webs on the hub minimises the weight within the hub.

A further advantage of the present invention is that the stiffness of the hub is maximised by the shape formed by the webs. The flywheel of the present invention also exhibits a high natural frequency, and is therefore less susceptible to vibration caused on operation of the vehicle. Furthermore, the hub is almost entirely protected from the effect of axial shrinkage between the bearing locations when the flywheel is operated at high speeds.

The interference fit between the hub and the composite ring ensures that the radial load in the composite part is maintained to a very low value. This is particularly advantageous if the material used to form the rim has a relatively low strength in a radial direction, such as unidirectional carbon fibre which has a low strength in the cross fibre direction.

A further advantage of the current invention is that the majority of the strain encountered as the flywheel speeds up is generated between the ring of screws and the rim of the flywheel. The portion of the flywheel inboard of the ring of screws is stiff, has relatively low strain at full speed and consequently does not change length very much (all materials get thinner as you stretch them -an effect defined by the poisons ratio of the material concerned).

Preferably, the outer webs extend from a point on the peripheries which is at a greater axial distance from the centre of the flywheel than the attaching means.

An advantage of this is that the stiffness of the hub is maximised. More preferably, the bearing housings are at a greater axial distance form the centre of the flywheel than the attaching means.

One or more spacers may be provided between the two halves of the flywheel hub. A spacer can advantageously provide a drive method for the flywheel which is independent of the bearing support shaft.

The primary failure mode of the flywheel is preferably failure by cracking of the hub. An advantage of this is that the composite rim acts to contain the debris caused by the breaking up of the hub on failure.

The rim is preferably formed of filament wound carbon fibre. An advantage of forming the rim of filament wound carbon fibre is that is has relatively high Young's Modulus (in a direction parallel to the grain), therefore the radial growth of the flywheel when it is spinning at high speeds is minimised, and the rim and the hub stay connected by the interference fit even at high speeds.

An embodiment of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a side elevation of a flywheel in accordance with the present invention; Figure 2 is a cross sectional view of the flywheel of Figure 1 along the lines Il-Il.

Figure 3 is a detailed cross sectional view of area Ill as indicated in Figure 2.

Referring to Figures 1 and 2, the flywheel 2 comprises a steel hub arrangement 4 and a filament wound carbon fibre composite rim 6. The inner diameter of the rim 6 is selected such that when it is fitted over the outside of the hub arrangement 4, a radial interference fit is achieved, thus ensuring that the rim 6 stays in compression with the hub 4 at high operating speeds.

The flywheel 2 is mounted on a shaft (not shown) and is contained in a housing (not shown) in which a vacuum is maintained. The housing is fixed to, and stationary with respect to, the vehicle in which the flywheel is fitted.

The hub 4 is formed of two opposing halves 8, 10 which meet, but are not joined, at surfaces 12, 14 on their outside perimeters 16, 18. The two halves 8, 10 have outer webs 20, 22 which extend from a point 42, 44 on the outside perimeters 16, 18, to a joining point 24, where they are joined by attaching means comprising screws 26 secured with nuts 28. The screws pass through bores 30, 32 formed in each of the halves 8, 10. The attaching means form a ring around the centre of the flywheel 2. The screws 2 also pass through bores 34 formed in a spacer 36 which is located between the two halves of the hub 4.

The point 42, 44 on the perimeters 16, 18 from which the outer webs 20, 22 extend is at a greater axial distance from the centre of the flywheel 2 than the joining point 24, i.e. the webs extend generally inwardly towards the joining point 24. This construction, combined with the diameter of the ring formed by the attaching means, provides the hub with a high stiffness.

Beyond the joining point 24 and towards the centre of the flywheel 2, inner webs 46, 48 extend generally axially outwardly from the centre of the flywheel 2.

Bearings 38, 40 are provided on each of the inner webs 46, 48 to support the flywheel from the housing. A step 54, 56 on each of the inner webs rests against the inner race 50, 52 of the bearings 38, 40 and provides axial restraint.

The bearings 38, 40 supporting the flywheel 2 are located on their outside diameter in the housing which surrounds the flywheel. The drive shaft passes along the central axis 50 of the flywheel 2 and locates in the centre of the spacer 34 by means of a spline (not shown) located on the inside of the central aperture of the spacer 34.

Because the inner webs 46, 48 extend axially outwardly from the centre of the flywheel, the distance between the bearings 50, 52 is maximised, thus increasing stability of the flywheel 2 on the shaft.

When the flywheel 2 is operated at high speed, the rim 6 encounters radial growth. This radial growth causes the two outer webs 20, 22 to be urged towards one another. Therefore the effect of the radial growth is taken up by the outer webs 20, 22, and the centre of the flywheel 2, within the ring defined by the fastenings, suffers from substantially no axial shrinkage. Therefore the bearings 50, 52 maintain substantially the same distance apart as when the flywheel 2 is stationary.

In an alternative embodiment, the two halves of the hub 4 could be formed of low cost steel forgings. The rim 6 could be formed from low cost carbon fibre tow.

The webs could also be formed into different shapes having the same effect of isolating the centre of the flywheel from the effects of axial shrinkage. For example, more than two webs could be used, or the webs could be oppositely inclined.

Alternative fastener could be used to join the two halves of the hub 4 together, such as rivets or spot welds.

Claims

Claims 1. A high speed flywheel for a vehicle comprising a hub having a

periphery, the periphery having an external surface over which a rim is secured by an interference fit, and an internal surface, wherein at least one inclined web extends from a first point on the internal surface to a second point towards the centre of the flywheel.
2. A high speed flywheel as claimed in claim 1 wherein the first point is at a greater axial distance from the centre of the flywheel than the second point.
3. A high speed flywheel as claimed in claim 1 wherein the second point is at a greater axial distance form the centre of the flywheel than the first point.
4. A high speed flywheel for a vehicle comprising a hub and a rim, the hub comprising two opposing halves, the two halves having peripheries, and outer webs extending from the peripheries towards attaching means, and inner webs extending from the attaching means towards the centre of the flywheel, wherein bearings are provided on the inner webs, and wherein the bearings support the flywheel from the housing, and wherein the rim is fitted with an interference fit over external surfaces of the peripheries of the halves of the hub.
5. A high speed flywheel as claimed in claim 4 wherein the outer webs extend from a point on the peripheries which is at a greater axial distance from the centre of the flywheel than the attaching means.
6. A high speed flywheel as claimed in claim 4 or claim 5 wherein the bearings are at a greater axial distance from the centre of the flywheel than the attaching means.
7. A high speed flywheel as claimed in any of claims 4 to 6 wherein one or more spacers are provided between the two halves of the hub.
8. A high speed flywheel as claimed in any of claims 4 to 6 wherein the primary failure mode of the flywheel is cracking of the hub.
9. A high speed flywheel as claimed in any of claims 4 to 6 wherein the rim is a composite formed of filament wound carbon fibre.
10. A high speed flywheel as claimed in any of claims 4 to 6 wherein the two halves are steel forgings.
11. A high speed flywheel as claimed in any of claims 4 to 8 wherein attaching means are machine screws, rivets or spot welds.
12. A high speed flywheel substantially as hereinbefore described and with reference to the accompanying figures.