US3760917A

US3760917A - Clutches

Info

Publication number: US3760917A
Application number: US00279096A
Authority: US
Inventors: B Pagdin
Original assignee: GKN Transmissions Ltd
Current assignee: GKN Automotive Ltd
Priority date: 1972-08-09
Filing date: 1972-08-09
Publication date: 1973-09-25
Anticipated expiration: 1990-09-25

Abstract

The invention provides an electro-magnetic clutch in which axially spaced clutch plates and an intervening clutch plate form part of a magnetic circuit containing a series permanent magnet for engergizing the circuit and establishing clutch engagement, this engagement being effected by passing current through an electrical winding associated inductively with the magnetic circuit while leaving contact faces of the clutch plates in light rubbing engagement or closely spaced relation.

Description

United States Patent Pagdin 1 Sept. 25, 1973 CLUTCHES 2,914,714 11/1959 Pierce et a]. 192/84 PM x 2,956,658 10/1960 Jacschke 12/84 PM [75] Inventor: Brian Colin Pagdin, Sutton 2 962 144 H 960 H t al 192 84 PM coldfield, England einemann e Assignee: GKN T n i i Li it d, Primary Examiner-Allan D. Hermann [22] F1led: Aug. 9, 1972 [57] ABSTRACT [21] Appl' 279,096 The invention provides an electro-magnetic clutch in which axially spaced clutch plates and an intervening 52 US. Cl. 192/84 0, 192/84 PM clutch Plate form P of a magnetic circuit containing 5 1 1m. (:1. F1611 27/01, Fl6d 27/10 a series P magnet for engergizing the circuit [58] Field of Search 192/84 R, 84 c, 84 PM and establishing clutch engagement, this engagement being effected by passing current through an electrical 5 R f ren e Ci winding associated inductively with the magnetic cir- UNITED STATES PATENTS cuit while leaving contact faces of the clutch plates in 2,275,839 3 1942 Boehne 192/84 PM light mbbmg engagement or closely spaced 2,832,918 4/1958 Pierce 192/84 PM X 11 Claims, 4 Drawing Figures as w 28 1 44 2.7 45' 51 2 g 21 29 b 2.4 4/ 15' r38 I? 39 2.5 7 Fa 5e 12 21 184 Z 7 18p 51; 5- 55 Birmingham, England Attorney-Charles J, Merriam et al.

PATENTEU 3.760.917

" sum 1 o 4 Eel PATENTEDSEP25|913 31760 917 SHEET 2 [IF 4 PATENTEB SEP25 I975 SHEET t F 4 R E CULATOR THEORETICAL FICA.

m uzz i z 3355 TEMPERATURE CLUTCI-IES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to clutches comprising rotary driving and driven elements, one of which includes axially spaced clutch plates, and the other of which includes an intervening clutch plate interposed therebetween, the clutch plates forming part of a magnetic circuit which is energised and de-energised magnetically respectively to establish frictional engagement and release of the clutch plates with respect to each other.

2. Description of the Prior Art In a previously proposed form of such clutch, energisation of the magnetic circuit has been established by passing current through an electrical winding interlinked inductively with the magnetic circuit, and release of the clutch has been effected by switching. off the current and separating the clutch plates from each other by spring means.

Reliable release of the clutch cannot be attained merely by switching off the current because of residual magnetism in the magnetic circuit, and it is, therefore, necessary to avoid the effects of residual magnetism that an appreciable gap be opened between the clutch plates by movement of the axially spaced pair of clutch plates away from each other by the spring means.

Consequently, when it is required to re-engage the clutch, the current which, has to be passed through'the winding must be high enough to set up magnetic flux of a level which will pull the axially spaced clutchplates together against'the spring means despite the highreluctance in the magnetic circuit created by the relatively wide gaps.

This appreciably increases the quantity of electrically conductive metal, for example copper, incorporated in the winding, and hence leads to an increase in-the size and cost of the clutch for a giventorque transmitting capacity.

This increases both the bulk andthe cost of the construction.

It has also been proposed to employ a permanent magnet to establish the necessary flux in the magnetic circuit to bring a single pair of plates into contact with each other for engagement of a clutch. However, the objection to this arrangement is that, since contact takes place between two plates only, the contact pressure which it is necessary to establishfor'a given diameterof clutch and to transmit a given torque is much higher than in the case of a clutch which incorporates axially spaced plates and an intervening plate. Further,

in such, arrangements the magnetic. circuit configuration has been relatively complex incorporating'a plurality'of subordinate circuits designed to avoid or minimise any demagnetisation effect on the permanent SUMMARY OF THE INVENTION The present invention has for its object to overcome or reduce these problems and is based upon appreciation of the fact that when disengagement of a clutch of the kind specified is effected by neutralisation of the magnetic flux by which engagement is established a stable floating relation is created between the driving and driven plates in which these are just out of contact or are in such light rubbing contact as will prevent transmission of torque. Further, the invention is based upon appreciation of the fact that given this floating relationship the number of plates (and hence the number of faces in contact) over which torque can be transmitted without presenting a very high reluctance path when the clutch is disengaged because at each such pair of faces a floating relation will automatically be established and hence also only a minimum gap of stable dimensions will be opened between the plates. Thus problems resulting from a requirement for high magnetisation level to re-establish clutch engagement are avoided.

According to the present invention there is provided a clutch comprising magnetically conductive axially spaced clutch plates having opposed axially spaced contact faces presented towards each other, an intervening magnetically conductive clutch plate axially interposed between said axially spaced clutch plates and having contactfaces presented towards respective ones of said contact faces of said'axially spacedplates, magnetic circuit means defining, incombination with said clutch plates, a magnetic circuit having outer and inner limbs at different radial distances from the axis of said clutch andextending through said contact faces of said clutch plates in a direction transverse to said contact faces, permanent magnet means in said magnetic circuit'for establishing a flux therein to cause said plates to be engagedin frictional torque transmitting relation with each otherat-said contact faces, electrical winding means in inductive relation with said magnetic circuit for setting up a magnetic field opposing said flux in response topassage of an electrical current through said winding means for releasing said'clut'ch.

Preferably said inner and outer limbs of said magnetic circuit are connected by end limbs to define a magnetic circuit of singleloop configuration, said contact facesof said intervening clutch plate having light rubbing contact with respectively associated ones of said contact faces of said axially spaced clutch plates or are spaced therefrom by only a small gap in response to passage through said winding of clutch releasing current.

The invention will now be, described, by way of exampie, with reference to the accompanying drawings wherein:

FIG. 1 is a view in sideelevation and in section on the line 1--l of FIG. 2 of one embodiment of clutch in ac-. cordance with the invention; FIG. 2 is a view in rear elevation with part of the upper left-hand quadrant in-section on the line 2a-2a of FIG. 1, and part of the lower left-hand quadrant in section on the line 2b-2b of FIG. 1.

FIG. 3 is a circuit diagram of a control circuit in which the electrical winding is connected;

FIG. 4 is a graph illustrating the temperature compensation characteristics of the circuit.

The clutch illustrated is of a form intended to be used in conjunction with an internal combustion engine, for

example of a motor road vehicle, for transmitting torque therefrom to the driving wheels of the vehicle, the component parts of the clutch being accommodated in a bell housing which is adapted for secure ment to a body casting of the engine by a flange I] having holes (not shown) for the passage of securing bolts. It will be understood, however, that the clutch may be employed for other purposes and in conjunction with other prime movers.

The clutch comprises rotary driving and driven elements, the former of which includes two axially spaced plates, one of which 12 constituted by the flywheel of the engine itself carried on the end of the driving shaft 13. The other axially spaced clutch plate 14 is of channel shape in cross-section and is adapted for connection to the flywheel 12 and can be driven therefrom while being movable for floating axially relatively to the flywheel by strap elements 15 riveted to lugs 16 on the plate 14 and having holes at their opposite ends through which bolts 17 can screw into threaded sockets in the rim of the flywheel 12.

The driven element of the clutch comprises an intervening plate 18 interposed between the flywheel l2 and the plate 14, such plate having a hub 19 splined internally for engagement with external splines on a driven shaft 20.

The flywheel 12, intervening plate 18, and the plate 14, form parts of a magnetic circuit of simple loop con figuration indicated in broken lines 21. This circuit has an inner limb 22 and an outer limb 23 both extending in a generally axial direction at right angles to the planes of contact faces presented at opposite sides of the plate 18, as indicated generally at 18a and 18b, and contact faces of the plates 12 and 14, as indicated generally and respectively at 12a and 14a.

The flywheel 12 is made of a magnetically conductive material, for example cast iron or steel.

The intervening plate 18 comprises an inner ring-like or annular component 24 secured to the hub 19 by means of a thin plate 25 riveted to flanges respectively on the hub and on the annular component 24 as shown. The plate 18 further comprises an outer or ring-like or annular component 26. Both the

components

24 and 26 are made of magnetically conductive material and they are connected to each other by an intervening part 27 formed wholly of a non-metallic magnetically nonconductive material. Thus, whilst the

components

24 and 26 may be formed of steel or cast iron, the component 27 is preferably an asbestos-based composition such as that sold under the registered trade mark FERODO. The

components

24 and 26 have radially projecting flanges 24a and 26a presented respectively outwardly and inwardly and which are of wavy or castellated configuration, as seen particularly in FIG. 2, to effect keying with the insert part 27.

It will be noted that the material of the insert part 27 constitutes a friction pair with respect to the material of the flywheel 12 (and as hereinafter described the material of which the plate is formed), and preferably also a metallic inset 30 hereinafter mentioned.

The insert part 27 does, of course, present high magnetic reluctance between the

components

24 and 26 and prevents leakage between the inner and outer limbs 22, 23 of the magnetic circuit.

Referring now to the plate 14, this also is formed of inner and outer annular or ring-like components indicated generally at 28, 29, each formed of electrically conductive materials such as steel or cast iron, and connected to each other by an intervening part 30 formed of a magnetically non-conductive metal such as aluminium, preferably cast as an insert embracing the

components

28, 29.

Radial flanges

31 and 32 of the inner and

outer components

28, 29 respectively, which collectively present part of the contact face 14, are separated from each other by a wavy slot 33 in which engages a correspondingly wavy rib 34 of the insert part 30, the latter incorporating the lugs 16 which project through apertures in the circumferential wall of the outer components 29.

The insert part 30 thus serves to present a path of high magnetic reluctance between the inner and

outer components

28 and 29 and prevents magnetic leakage between the inner and outer limbs 22 and 23.

The magnetic circuit is completed through a permanent magnet 35 of annular form of high coercivity. A suitable material is a barium ferrite ceramic composition. This magnet is polarised axially with one face, for example 350, consituting the north pole and the opposite face 35b constituting the south pole. The magnet 'is carried by a supporting member formed of axially spaced parts 36 and 37, the former of which is of generally L-shape in cross-section and is supported from the plate 14 through the intermediary of a magnetically conductive bearing in a gap at 38 between a spigot 41 on the inner component 28 of plate 14 and a tubular portion on part 37. The bearing comprises a sleeve 40 predominantly of magnetically conductive material. The sleeve may have an overall thickness of 0.050 inches and be composed of steel with a thin layer, for example 0.008 inches, of vanadium at the interface at which relative movement occurs, e.g., between the sleeve 40 and spigot 41.

When, however, magnetic flux is established in the circuit 21, forces of magnetic attraction exist, firstly across the annular gaps between the fixed plate 12 and the magnetically

conductive components

24 and 26 of the plate 18, and secondly across the annular gaps be tween the

components

24 and 26 of the plate 18 and plate 14. Plates 14 and 18 are axially movable and are thus attracted leftwards towards plate 12 so that plate 18 becomes clamped between plate 12 and plate 14. It will be evident that axial movement of plate 14 does not alter the radial dimensions of the gaps 38 and 44 and consequently magnetic forces existing radially across these gaps do not oppose the axial movement of plate 14 occurring during clutch engagement. As hereinafter explained, this movement is extremely small due to the fact that the annular gaps between plates 12, 18 and 14 have zero values (the plates are in light rubbing engagement) or are of very small values under conditions in which the clutch is disengaged.

The outer component 29 of plate 14 has a circumferential wall 43 which is spaced by an air gap 44 from the circumferential wall 42 of the part 37.

The support member comprising the parts 36 and 37 is of channel shape in diametral cross-section and constitutes a housing containing a winding 45 in which the turns are concentric with the axis 200 of the clutch and are preferably formed of aluminium foil. Such winding can be energised by passage of a direct electric current in a direction such as to set up a magnetic field opposing the flux in the circuit 21 when it is required to bring about disengagement or release of the clutch.

It will be understood that the winding 45 is stationarily supported in the housing formed by the parts 36, 37, this housing being itself non-rotatable.

The housing is itself carried by a bracket 46 to which the housing is secured by bolts 47 preferably cemented into threaded holes in the part 36 of the housing. The bracket 46 projects in an arm 48 which is secured to the bell housing by means of a clamping bolt 49 passing through a slot 50 extending in a direction generally lengthwise of the axis 20a to enable the axial position of the bracket 46 and hence the housing 36, 37 to be adjusted.

It'will be evident that such adjustment can be performed from a position externally of the bell housing since the clamping bolt 49 merely requires to be slackened to permit the bracket to be adjusted axially. In practice an adjustment may be effected by moving the bracket fully to the left to contract the gap between the left-hand end of the housing 36, 37 and the insert 30 to zero, and then moving the bracket to the right by a predetermined clearance dimension, for example 1 millimetre.

The magnitude of the gap is set so that notwithstanding the ability of the plate 14 to float or move axially as permitted by the straps 15, no contact will occur between the insert 30 and the housing or the winding 45 carried thereby. If desired positive stop means may be provided to limit axial movement of float of the plate 14 in a direction to the right as seen in FIG. 1, although by virtue of the forces of the forces of magnetic attraction, as hereinafter described, the plate 14 never moves, during operation, further to the right than a position in which the mutually contacting faces of the plates 12, 18, and 14 are just out of contact or are in light non-torque transmitting rubbing engagement.

The degree of axial overlap between the housing and the inner and

outer components

28, 29 of the plate 14, namely at the gaps at which the sleeve 40 is inserted, and at the gap 44, provides a large area for traversing by flux in the magnetic circuit 21 to reduce reluctance introduced into the magnetic circuit at these positions, bearing in mind particularly the high proportion of the thickness of the sleeve 40 which is magnetically conductive.

Consequently in the released condition of the clutch engagement under the polarisation established by the permanent magnet is assured as soon as current in the winding 45 is reduced to zero or a low value.

It is comtemplated that the clutch may be used at high speeds of revolution, for example up to 6,000 r.p.m.

The value of current in the winding may be controlled as a function of a selected parameter of the driving or driven element of the clutch. Thus a control means providing an output signal varying as a function of the speed of rotation of the driving or driven element of the clutch may control or operate a current regulating means for reducingthe current to zero (or a predetermined value) in accordance with the magnitude time characteristic providing smooth take-up of drive. In a motor road vehicle the ignition system of the vehicle may include a means for providing such a control signal.

In one simple control circuit illustrated in FIG. 3 which may be employed, the circuit comprises a voltage regulator comprising a zener diode ZD1 which acts as a voltage reference in series with a load resistor R1 connected to a voltage source in the form of a low voltage battery Bl which may be the normal rechargeable accumulator provided in a vehicle in which the clutch is employed and which typically has a voltage in the.

Initially, when the clutch is disengaged, the value of 1 R3 is selected to produce the current through winding 45 which will neutralise the magnetic flux due to the permanent magnet 35 and as the value of R3 is reduced the current through the winding 45 is reduced and may become zero so that the whole of the flux due to the permanent magnet is then operative to establish the plates of the clutch in engagement with each other under pressure to transmit torque.

Any fluctuation in the value of the voltage provided from the battery B1 (such as may arise from changes in the speed of operation of the prime mover which drives a dc generator or alternator with appropriate rectifying circuit for charging the battery B1) does not produce any variation in the value of current through the winding 45 because the voltage across zener diode ZD1 is constant. Only the current flow through R1 and ZD1 will change.

The slider or other control operative to vary the value of R3 is coupled mechanically to a 'clutch controlling member such as a clutch pedal 51 pivoted about an axis 52.

The value of the current selected for neutralising the flux due to the permanent magnet will achieve such neutralisation for a selected value of reluctance of the magnetic circuit, that is when the contact faces of the plates have zero gap (are in light rubbing engagement) or a gap of a very small value. The system is stable under these conditions. Any tendency for the gap to widen will alter the reluctance of the magnetic circuit and the flux tending to be set up by the permanent magnet will then predominate over that tending to be set up by the winding 45 thereby causing the gap to revert to its former value.

The flux due to most permanent magnets varies with temperature and for a typical barium ferrite type magnet as already mentioned decreases at the rate of 0.2 percent per degree centrigrade temperature rise. The. temperature range for a clutch or vehicle operation is typically minus 20 C to plus 150 C, and it is thus important to provide means for compensating for variation in the flux due to the permanent magnet. This is substantially achieved when the winding 45 is made of an electrically conductive metal having a temperature coefficient of resistance such that the resistance of the winding rises by 0.2 percent per degree centigrade and the winding already described formed-from aluminum foil is in conformity with this characteristic.

mediary of plug and socket connector elements indicated diagrammatically at 54a, 54b.

The clutch will fail safe" in the event of an involuntary interruption or discontinuance of current through the winding 45 since the polarisation by the permanent magnet will establish energisation in the magnetic circuit and result in the clutch being maintained in its position of engagement.

Although in the embodiment described the contact faces 12a, 18a, 18b, 140 all lie in planes which are at right angles to the axis 46, it will be understood that the form of the components concerned may be varied to provide contact faces of frusto-conical or other similar form if desired. Further, the number of pairs of axially spaced plates such as 12 and 14 and the number of intervening plates such as 18 may be increased if desired.

Due to the smooth take-up of drive which can be achieved by control of current in the winding 45, it is unnecessary for the annular plate 25 to incorporate shock-absorbing means such as is conventionally provided in the driven plate of clutches which are engaged under spring pressure and consequently a simplification and reduction in cost is achieved in respect of this part of the construction.

I claim:

1. A clutch comprising:

a. magnetically conductive axially spaced clutch plates having opposed axially spaced contact faces presented towards each other,

b. an intervening magnetically conductive clutch plate axially interposed between said axially spaced clutch plates and having contact faces presented towards respective ones of said contact faces of said axially spaced plates,

c. magnetic circuit means defining, in combination with said clutch plates, a magnetic circuit having outer and inner limbs at different radial distances from the axis of said clutch and extending through said contact faces of said clutch plates in a direction transverse to said contact faces,

d. permanent magnet means in said magnetic circuit for establishing a flux therein to cause said plates to be engaged in frictional torque transmitting relations with each other at said contact faces,

e. electrical winding means in inductive relation with said magnetic circuit for setting up a magnetic field opposing said flux in response to passage of an electric current through said winding means for releasing said clutch.

2. A clutch according to claim 1 wherein:

a. said inner and outer limbs of said magnetic circuit are connected by end limbs to define a magnetic circuit of simple loop configuration,

b. said contact faces of said intervening clutch plate have light rubbing contact with respectively associated ones of said contact faces of said axially spaced clutch plates or are spaced therefrom by only a small gap in response to passage through said winding of clutch releasing current.

3. A clutch according to claim 2 wherein:

a. said intervening clutch plate comprises inner and outer rings of metallic magnetically conductive material connected to each other by an intermediate part of high magnetic reluctance formed of nonmetallic material,

b. at least one of said axially spaced clutch plates also comprises inner and outer rings of metallic magnetically conductive material connected with each other by an intermediate part including a slot system of a configuration to extend obliquely across said intermediate part between inner and outer radial boundaries thereof to reduce magnetic leakage between said inner and outer rings of said axially spaced clutch plate with said slot system containing a magnetically non-conductive metal.

4. A clutch according to claim 3 wherein the nonmetallic material of said intermediate part is selected to form a friction pair with at least said metallic material of which said metallic magnetically conductive inner and outer rings of said axially spaced clutch plate is composed.

5. A clutch according to claim 1 wherein:

a. at least said winding is carried by a non-rotating support member,

b. said non-rotating support member is mounted on a magnetically conductive bearing on one of said axially spaced clutch plates.

6. A clutch according to claim 5 wherein:

a. said magnetically conductive bearing comprises a sleeve formed predominantly of magnetically conductive material,

b. said sleeve is carried on a spigot projecting axially from said one axially spaced clutch plate in a central region thereof.

7. A clutch according to claim 5 wherein said nonrotating support member also carries said permanent magnet means.

8. A clutch according to claim 1 wherein:

a. at least said winding is carried by a non-rotating axially adjustable support member,

b. retaining means are provided for retaining said support member at a selected position of axial adjustment with respect to a bell housing enclosing said clutch.

9. A clutch according to claim 8 wherein:

a. said support member comprises a housing affording a chamber for receiving said winding;

b. said retaining means comprises a bracket connecting said housing to said bell housing,

c. said bracket is secured to said bell housing for adjustment axially of said clutch with respect to said bell housing and includes a portion accessible externally of said bell housing for effecting said axial adjustment.

10. A clutch according to claim 1 wherein:

a. said winding is connected in electrical circuit means,

b. said permanent magnet means provides a flux level in said magnetic circuit means decreasing with increase temperature,

c. said electrical circuit means including said winding provide an electrically conductive path of increasing resistance in response to increase temperature such as to compensate at least approximately for said decreasing flux level.

11. A clutch according to claim 10 wherein said circuit means a. has input terminals for connection to a source of voltage,

b. has a voltage stabilising means for maintaining substantially constant voltage across said winding notwithstanding variations in the voltage level of said source.

Claims

1. A clutch comprising: a. magnetically conductive axially spaced clutch plates having opposed axially spaced contact faces presented towards each other, b. an intervening magnetically conductive clutch plate axially interposed between said axially spaced clutch plates and having contact faces presented towards respective ones of said contact faces of said axially spaced plates, c. magnetic circuit means defining, in combination with said clutch plates, a magnetic circuit having outer and inner limbs at different radial distances from the axis of said clutch and extending through said contact faces of said clutch plates in a direction transverse to said contact faces, d. permanent magnet means in said magnetic circuit for establishing a flux therein to cause said plates to be engaged in frictional torque transmitting relations with each other at said contact faces, e. electrical winding means in inductive relation with said magnetic circuit for setting up a magnetic field opposing said flux in response to passage of an electric current through said winding means for releasing said clutch.

2. A clutch according to claim 1 wherein: A. said inner and outer limbs of said magnetic circuit are connected by end limbs to define a magnetic circuit of simple loop configuration, b. said contact faces of said intervening clutch plate have light rubbing contact with respectively associated ones of said contact faces of said axially spaced clutch plates or are spaced therefrom by only a small gap in response to passage through said winding of clutch releasing current.

3. A clutch according to claim 2 wherein: a. said intervening clutch plate comprises inner and outer rings of metallic magnetically conductive material connected to each other by an intermediate part of high magnetic reluctance formed of non-metallic material, b. at least one of said axially spaced clutch plates also comprises inner and outer rings of metallic magnetically conductive material connected with each other by an intermediate part including a slot system of a configuration to extend obliquely across said intermediate part between inner and outer radial boundaries thereof to reduce magnetic leakage between said inner and outer rings of said axially spaced clutch plate with said slot system containing a magnetically non-conductive metal.

4. A clutch according to claim 3 wherein the non-metallic material of said intermediate part is selected to form a friction pair with at least said metallic material of which said metallic magnetically conductive inner and outer rings of said axially spaced clutch plate is composed.

5. A clutch according to claim 1 wherein: a. at least said winding is carried by a non-rotating support member, b. said non-rotating support member is mounted on a magnetically conductive bearing on one of said axially spaced clutch plates.

6. A clutch according to claim 5 wherein: a. said magnetically conductive bearing comprises a sleeve formed predominantly of magnetically conductive material, b. said sleeve is carried on a spigot projecting axially from said one axially spaced clutch plate in a central region thereof.

7. A clutch according to claim 5 wherein said non-rotating support member also carries said permanent magnet means.

8. A clutch according to claim 1 wherein: a. at least said winding is carried by a non-rotating axially adjustable support member, b. retaining means are provided for retaining said support member at a selected position of axial adjustment with respect to a bell housing enclosing said clutch.

9. A clutch according to claim 8 wherein: a. said support member comprises a housing affording a chamber for receiving said winding; b. said retaining means comprises a bracket connecting said housing to said bell housing, c. said bracket is secured to said bell housing for adjustment axially of said clutch with respect to said bell housing and includes a portion accessible externally of said bell housing for effecting said axial adjustment.

10. A clutch according to claim 1 wherein: a. said winding is connected in electrical circuit means, b. said permanent magnet means provides a flux level in said magnetic circuit means decreasing with increase temperature, c. said electrical circuit means including said winding provide an electrically conductive path of increasing resistance in response to increase temperature such as to compensate at least approximately for said decreasing flux level.

11. A clutch according to claim 10 wherein said circuit means a. has input terminals for connection to a source of voltage, b. has a voltage stabilising means for maintaining substantially constant voltage across said winding notwithstanding variations in the voltage level of said source.