The present invention relates to a contact washer. More particularly, but not exclusively, the present invention relates to a contact washer comprising a metal sheet denned by a peripheral edge, the sheet having an aperture extending therethrough, the metal sheet comprising a plurality of slots extending from the peripheral edge part way to the aperture, or from the aperture part way to the peripheral edge, or mMiires thereof. The present invention also relates to a device for the transfer of electric current. MOre particularly, bur not exclusively, the present Invention relates to a device for the transfer of electric current comprising first and second spaced apart electrically conductive members and a contact washer sandwiched therebetween.

It ¼ often desired to transfer current from one electrically conductive member tp another, where one of the members rotates relative to the other. For small angles of rotation a flexible connector can be used. This is difficult for large angles of rotation and impossible for continuous rotation.

In the case of continuous rotation it is known to sandwich an electrically conductive washer between the two electrically conductive members. For high current applications, typically for currents over around 10,000 Amps) this arrangement has a number of drawbacks. Whilst the washer is substantially flat it tends to contact each member at only a very few discreet contact points. This results in a high joint resistance which in turn causes overheating Further, current constriction at these contact points causes a repulsive force between the washer and the member. A large spring force is required _: to maintain the washer in contact with the member. This is typically achieved by compressing the washer. However; known washers exert a maximum restoring spring force ih the region of the mid compression point which drops away sharply from this point. The Spring force exerted by the washer Can therefore be difficult to control and can change if the Washer is compressed inuse.

The present invention seeks to overcome the problems of the prior art.

Accordingly, in a first aspect, the present invention provides a Contact washer comprising a metal plate defined by first and second feces and a peripheral edge, the metal plate having an aperture extending therethrough defined by an aperture edge of the plate; the washer further comprising at least one of-

(a) a plurality of inner slots, each inner slot extending through the plate from the first fece to the second face and from the aperture edge part way to the peripheral edge; and

(b) a plurality of outer slots, each piiterslpt extending through the plate from the first face to the second face and from the peripheral edge part way to the aperture edge.

The slots in the washer divide the washer into a number of fingers, in use each finger independently contacts the adjacent member, this forces the current flowing through the washer from the central aperture to the peripheral edge flow in a number of parallel radial paths: This significantly reduces the heating at the contact between washer and member; It also significantly reduces the repulsive force due to current oonstiictibn, Further, as the Washer is compressed the fingers tend to twist. This produces a restoring Spring forte which is approximately linearly dependent on the degree of compression of tfie washer.

Preferably the contact; washer comprises a plurality of timer slots.

Preferably the inner slots are equally spaced apart around the aperture edge.

Preferably the contact washer comprises a plurality of outer slots.:

Preferably the outer slots are equally spaced apart around the peripheral edge.

Preferably the contact washer comprises a plurality of inner slots and a plurality of outer slots, the inner slots and outer stots being ihterdigitated. Preferably the Inner slots are equally spaced apart around the aperture edge and the outer slots are equally spaced apart around the peripheral edge, each inner slot being equally spaced apart from the adjacent outer slots and vice versa.

Preferably the metal plate is frustoconical.

Preferably the contact waiter further comprises an electrically conductive coating layer coating the metal plate, the electrical conductivity of the coating layer being higher than that of the metai plate.

In a further aspect of the invention there is provided a device for the transfer of electric current comprising a first electrically conductive member; a second electrically conductive member spaced apartfrom the first; the second member being adapted to rotate with respect to the first about a rotation axis; and, a contact washer as claimed in any one of claims 1 to 9 sandwiched between the first and second members with the rotation axis passing through the aperture, the peripheral edge abutting one member and the aperture edge abutting the other member.

Preferably the device further comprises a third electrically conductive member arranged on the opposite side of the second electrically conductive member to the first; and, a further contact washer as claimed in any one of claims 1 to 9 sandwiched between the Second and third members with the rotation axis passing through the aperture» the: peripheral edge Of the further contact washer abuttihg one of the second and third members and the aperture edge of the further contact Washer abutting the other of the second and third members.

Preferably the device further comprises a rotation axle extending along the rotation axis from the first member, through an aperture ih the second member to the third member Preferably the second member is a wheel.

Preferably the device further comprises a current source connected to one of the first and second members.

Preferably the device further comprises a toad connected to the other of the first and second members.

The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which

Figure ! shows, In perspective view, a contact washer according to the invention;

Figure 2 shows the contact washer of figure tin plan View;

Figure 3 shdws a device for the transfer of electric current according to the invention; and

Figure 4 shows a further embodiment of a device for the transfer of electric current according to the invention In cross section.

Shown in figure ! is a contact washer 1 aocordingto the invention. The contact washer 1 comprises a frustoconical metal plate 2. The metal plate 2 is typically a copper al!oy> for example CW106 chromium, copper, zirconium. The metal plate 2 Is optionally coated in a metal film of a high electrical conductivity than the metal piatd; The film can for example be a silver film which is preferred due to Its hardness arid few oxidation. The metal plate 2 is defined first and second faces 3,4 and a peripheral edge S, An aperture 6 extends through the center of the plate 2 and is defined by an aperture edge 7 of the plate 2.

Figure 2 shows the contact washer 1 of figure 1 in plan view, A set of inner slots S extends through the plate 2 from the first face 3 to the second fate 4 and from the aperture edge ? part way to the peripheral edge 5. The itiner slots 8 are equally spaced around the aperture edge 7 and extend radially outwards.

A set of outer slots 9 extends through the plate 2 from the first face 3 to tire second face 4 and from the peripheral edge 5 part way to the aperture 6; The outer slots 9 are equally spaced around: the peripheral edge 5 and extend radially inwards.

The inner slots 8 and outer slots 9 are interdigitated as shown With each inner slot 8 spaced equally from the adjacent outer slots 9 and Wce versa, The slots 8, 9 divide the plate into a number of radially extending fingers 10

Shown in figure 3 is a device 11 for the transfer of electric current according to the invention. The device 11 comprises first and second electrically tendUCtive members 12,13. The first member 12 i«$ spaced apart from the second member 13, The Second member 13 is adapted to rotate about a rotation axis 14 with respect to the first member 12. An electrically insulating rotation axle IS extends along the rotation axis 14 between the first and second members 12,13.

Sandwiched between the first and second members 12,13 Is a Contact Waiher 1 according to the invention. The rotation axle IS extends through the central aperture 6. The first member 12 is connected to earth 16. The second member 13 is connected to a current source 17 adapted to provide a current to the second member 13. In use a current flows fiom foe current source 17 to the second member 13. From there it flows through the contact washer 1 to the find member 12 and then to earth 16; As previously described, each of the fingers 10 of the washer 1 defined by the inner and outer slots 8,9 abuts either the first member 12 or second member 13, This forces tfie current to flow along a series of substantially parallel paths radially outwards as shown. The current flowing through each point where the washer 1 contacts the first or second member 12,13 is therefore reduced. This reduces the heating at the contact points. More significantly, the repulsive force due to current constriction at each contact point is reduced. The repulsive force is a function Of the current density. Having inner slots and outer slots 8,9 which divide the washer l into around ten fingers for example will typically reduce this repulsive force by a factor of greater than ten for a constant current flowing through the washer 1.

Shown in figure 4 is a further embodiment of a device 11 for the transfer of electric current according to the invention. The device 11 comprises first and third spaced apart electrically conductive members 12,18 in the form of support fingers. A second electriealty conductive member 13 is arrayed between the first and third support fingers 12,18 as shown. The second member 13 is typically a wheel. The wheel 13 is free to rotate about a rotation axis; 14 with respect to the first and third members 12,18. A rotation axle 15 extends alOhg the rotation axis 14 from the first member 12, through an aperture 6 in the center of the wheel 13 to the third inember 18. A first contact washer 1 according to the invention is sandwiched the wheel 13 and the first member 12. A second contact washer l is sandwiched between the wheel 13 and the third member 18.

lh use a current Is provided to the edge of the wheel 13 from a Current source 17. The current flows through fire Wheel 13 then through the washers 1 and support fingers 12,18 to earth 16, typically via a load 19 as shown. As before, the design of the washers 1 reduces the heating at the point of contact between the washers 1 and the members 12,18; It also reduces the repulsive force due to Current Constriction at the contact points»

A significant consideration in the design of the device of figure 4 is the lorentz force. Currents flow through the two support fingers 12,18 iri the same direction and accordingly the: two support fingers 12,18 are attracted to each Other due to the Lorentz force. If the current is large the lorentz force is correspondingly large and this may cause the support fingers 12*18 to deform towards each other slightly, so compressing the washers 1. As the wheel 13 turns then friction between the wheel 13 and the washers 1 produces a frictional force on foe peripheral edge 5 of ea ch ; washer 1, This twists the fingers of each washer 3L slightly. As discussed above, when the fingers of a contact washer 1 according to the invention are twisted then when the washer 1 is compressed ft produces a restoring spring force which only weakly depends on the degree of compression, Accordingly, even though the support fingers 12*18 compress the washers 1 foe wheell3 does hot jam. The resistance to rotation of the wheel 13 is therefore subsfohtially unchanged.