CH617499A5 - Support for electromagnetic brake or clutch - Google Patents

Abstract

The support (80) of an electromagnetic brake or clutch comprises two annular projections (86a, 86b) formed at the surface. A first layer (88) of a magnetic material covers the entire surface and the annular projections. A second layer (94) of a friction material is laid down in the hollow between the two projections (86a, 86b). The whole assembly forms a plane surface which comprises two flush annular parts (92a, 92b) of the magnetic material and an intermediate flush part (94) of the friction material. The latter has a hexagonal structure. The major part of the crystals of the friction material have an orientation on their base planes 0001 parallel to the surface of the coating. <IMAGE>

Description

The invention relates to an electromagnetic brake or clutch armature comprising a friction coating.

There have been described in patent No. 512023 electromagnetic brake or clutch armatures bearing coatings of hexagonal structure with a reticular ratio c / a of the mesh of their crystal lattice close to 1.633. The ratio c / a is the ratio to the distance c which separates two neighboring planes parallel to the base plane 0001 in the lattice at the distance a between two adjacent atoms in the same plane of this type. These coatings are, moreover, endowed with magnetic properties making them suitable for producing electromagnetic brakes and clutches. The ferromagnetic properties of these coatings are however most often inferior to those, for example, of certain conventional alloys with iron, cobalt and nickel, which have different crystalline structures.

The object of the invention is therefore to constitute an electromagnetic brake or clutch armature having both,

on the one hand, friction characteristics as satisfactory as possible, during the engagement of the armatures and, on the other hand, an effective contribution to the closing of the magnetic circuit which tends to clamp the complementary armatures one against the other.

The frame according to the invention is characterized in that the frame comprises at least one annular projection formed on its surface, a first layer of magnetic material covering the entire surface and of the annular projection and flush with the surface. plumb thereof, and that the friction coating is formed on the parts of this surface other than that occupied by the projection, the exterior surfaces of the friction coating and the exterior surfaces of the outcrops of the magnetic material being all substantially contained in the same plane, and the friction coating being made of a material of which substantially all of the phases belong to the hexagonal crystal system, the base planes 0001 of the majority of the crystals of the coating being oriented substantially parallel to the surface of the coating .

Preferably, the magnetic material consists of a material such as glass or Fe-Co-Ni alloys, which have the desired ferromagnetic properties.

Preferably, the crystals of the material with a hexagonal structure have, in an amount of more than 75% of them, an orientation on their base planes 0001 substantially parallel to the surface parts on which this coating is formed.

Metals, oxides or metal carbides which are particularly suitable for constituting the coatings are shown in Table I. The c / a reticular ratio of the mesh of their crystal lattice is also shown there.

Table I

Material c / a

Cobalt 1,624

1.623 mg

Neodymium 1,612

Titanium 1,587

Tungsten (hexagonal shape) 1.61

Yttrium 1,572

Nickel (hexagonal shape) ... 1.59

MO2C 0.969

MO2C 1,574

NbC 0.861

Nb2C

Ta2C 1.59

WC 0.976

W2C 1,578

V2C 1.59

Cr2Oa 2,761

Ti02 1,246

Friction coatings can consist of these materials, taken either in their pure state or alloyed with one another. They can also be combined or alloyed with materials, themselves non-hexagonal, these being taken, in the phases obtained,

in proportions compatible with the preservation of the initial hexagonal structure. Such non-hexagonal materials are, for example, made up of metals such as molybdenum, chromium, aluminum, copper, iron, nickel (non-hexagonal form), niobium; carbides such as B4C, TaC, TiC, Cr3C2, VC, ZrC; or oxides such as Zr02, AI2O3, etc.

It has been found that the hexagonal bodies which have the best friction qualities, that is to say the lowest risk of seizure, are those whose reticular ratio c / a is of the order of or close to 1.633.

In the description which follows, by way of example, an embodiment of an installation for manufacturing a friction coating is described, this description referring in particular to the drawings in which:

fig. 1 shows, schematically, such an installation, some of the parts of this installation being seen in section;

fig. 2 shows, in perspective, a detail of this installation;

fig. 3 shows, on an enlarged scale, a front view of part of the installation of FIG. 1;

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fig. 4 is a point graph showing the relationship between the c / a reticular ratio and the coefficient of friction of a number of hexagonal materials;

fig. 5 is a graph showing the variations in the coefficient of friction, as a function of the temperature for certain coating materials;

fig. 6 to 9 represent a part of an armature on which deposits are made schematically represented at different stages of their development.

Proposing to make a deposit of a friction material on a support 2, we can have recourse to the installation shown schematically in FIG. 1; this installation comprises a generator 4 producing a plasma jet 6, associated with a device (shown diagrammatically at 8), for example of the vibrating screen type, for injecting a powder of the material intended to constitute the coating, into the jet of plasma 6, at or near the output 10 of the generator.

The plasma generator can be constituted in any known manner. It can, in particular, be constituted as shown in FIG. 1. It comprises a central block 12 ensuring, by means of an insulating spacer 14, the electrical isolation between the anode 16 and cathode parts 18. This central block comprises a diffuser 20 supplied with plasma gas by a pipe 22, this diffuser channeling the gases towards the arc chamber 24. It also includes an anode electrical supply channel 25.

The generator further comprises a rear block 26 made integral with the central block 12 by means of screws made of insulating material 28. The cathode 18, advantageously constituted by a ribbed tungsten bar, is fixed to this rear block by the through a part 30 and a clamp holder block 32 allowing relative longitudinal displacements of the cathode and the anode. A nut 34 ensures, by taking up the play in the thread, a good electrical contact once the position of the cathode has been adjusted. Finally, the rear unit receives the cathode duct 36 for electrical supply.

This generator finally comprises a front unit 38 made integral with the central unit by means of metal screws 40. The anode 16, advantageously made of red copper or cuprotellure, is immobilized in the front unit, by means of a flange 42.

A cooling circuit 44, supplied by a conduit contained in the anode pipe 25, allows a circulation of water, first in contact with the anode 16, the water then passing through the central block, then the rear block where it ensures the cooling of the cathode, before being evacuated by a conduit housed in the cathode duct 36.

The anode 16 advantageously constitutes the plasma ejection nozzle and has, in known manner, an internal profile which is a function of the nature of the plasma which it is desired to use, and of the speed and temperature conditions which the 'we want to achieve. One or more injectors 46 forming part of the powder injection device 8 open, as will be described below in more detail, into the plasma jet near the outlet of the nozzle, that is to say further in it or outside thereof, the powder being entrained in this or these injectors by means of a gas flow, advantageously of the same composition as the plasma gas. The particles of the pulverulent material, injected by the injector (s) 46 into the plasma jet, are then entrained by the latter in the direction of emission of this plasma jet.

The support 2 can be interposed, by any suitable means, across the path of the particles entrained by the plasma jet.

The support 2 constitutes a brake or clutch member, intended to come into rubbing contact with a similar element. In the case where these elements are intended to comprise annular friction coatings 48 (FIG. 1), the means for retaining the support 2, in the path of the particles entrained by the plasma jet, advantageously have an axis diagrammatically crossed by 50 the support 2 at its center and likely to cause the latter to rotate. The parts of the support, which normally do not have to be covered by the coating material, can, during the deposition operation, be covered by a cover 51 shown diagrammatically in phantom.

The support 2 is subjected prior to the production of a deposit of molten material, to a sandblasting or to a similar treatment, in order to give it a surface, having hollows and asperities, cooperating in the subsequent mechanical anchoring of the friction coating on its surface.

The plasma ejection speed 6 is correlatively adjusted, on the one hand, the size and the flow rate of the particles of the material introduced into this plasma jet, on the other hand, to values such that, taking into account the nature of the plasma gas and of the power of the generator, the particles can be melted in their mass during the duration of their stay in the plasma jet, more particularly in the plasma dart, and that a gas barrier is also produced 52 transverse to the direction of the plasma jet, between the output 10 of the generator and the support 2, at a flow rate and a velocity capable of breaking up and deflecting the hot gases of the plasma jet charged with the molten particles and projected onto the support.

Advantageously, this gas barrier is formed substantially at the end 54 of the plasma dart.

The means 56 capable of producing this gas barrier can be constituted by any member having a gas ejection slot 56a, extending over a certain length. These means can, for example as shown in FIG. 2, be constituted by a longitudinal slot tube element 56a, supplied by a source (not shown) of gas, in particular of pressurized air, this element being placed so that the ejection slot 56a produces a gas barrier 52 , located approximately in a plane perpendicular to the direction of the plasma jet 6, and passing substantially at the end of the flame formed by the plasma.

The effect of this gas barrier is then to break the gas flow of the plasma jet and to deflect the hot gases, which makes it possible to reduce the heating of the support 2 during the deposition operation. The gas barrier also deflects the non-molten particles entrained tangentially to the plasma jet and which have a relatively low kinetic energy. On the other hand, the molten particles, which are contained in the plasma jet, are animated by a considerable kinetic energy and cross the gas barrier practically without deviation.

This process therefore makes it possible to limit, in a significant proportion, heating of the support under the effect of the non-deviated hot gases which may still have, when they arrive on the support, a temperature of several thousand degrees. In addition, it allows the formation, on the support, of a homogeneous deposit from a homogeneous material essentially melted in its mass, this deposit assuming an annular shape 48 when the support is driven in rotation.

It has in fact been found that the fusion of the particles in their mass represents an essential condition for obtaining a deposit having the desired cohesion. To achieve this, it is therefore necessary to act on the various parameters which are available for regulating the operating conditions of the generator, in particular the temperature of the plasma jet.

The temperature of the plasma jet depends, in known manner, on several parameters, in particular on the composition of the plasma gas, on the supply flow rate of the plasma gas generator, the profile of the ejection anode-nozzle 16, the electrical power dissipated between the electrodes, etc.

It is practical - to determine the values of these parameters experimentally - to take into consideration the plasma ejection speed, which is a function, among other things, of

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the power dissipated in the generator, given that this ejection speed influences the entrainment speed of the particles injected into the plasma, and therefore their residence time within the latter. Indeed, a plasma of too low speed would be too cold and would not allow the complete fusion of the particles. However, there is also an upper limit plasma ejection speed. Beyond this limit, the plasma would obviously be very hot. The residence time of the particles in this jet would however be too short for the particles to be melted. It will therefore be necessary, in each case, to determine the lower and upper limits of the plasma ejection speed. These limits obviously depend also on the melting point of the material used, since this melting point determines the amount of heat which must be supplied to these particles to melt them.

Another important parameter which must be taken into account obviously resides also in the average dimensions of the particles of the powder. If these dimensions are too small, the particles melt and volatilize before arriving on the support 2. If their dimensions are too large, they cannot be completely melted during the duration of their stay in the plasma.

Correlatively, it is necessary to regulate the injection rate of the powders in the plasma jet. The temperature of the plasma jet is, in fact, also a function of this flow rate.

As an indication, we can therefore at this stage indicate that for a plasma consisting of 20% hydrogen and 80% argon, and with a melting material between about 1500 and about 3000 ° C, the particle ejection speed should be between about 100 and about 500 m / s, the powders injected into the plasma jet, especially when they have a very high melting point, should be at a flow rate of around 500 g / h at 1 kg / h and have average particle sizes of about 20-40 | x.

It may also be necessary to take account of the melting points of the materials to be melted, in particular to determine their injection points in the plasma jet. This injection can be done either still in the nozzle 16, by means of an injec-tor 46 opening out inside the nozzle, in particular as shown in FIG. 1, especially in the case of materials with high melting points, either by means of injectors 46a (shown in broken lines in FIG. 1) opening into the plasma dart, downstream from its exit from the nozzle 16, at a distance which may be all the greater the lower the melting point of the material to be melted. It is also possible to have recourse to a plurality of these injectors, in particular when it is desired to inject several of these materials simultaneously into the plasma jet.

Supposing, for example, that we want to deposit a coating based on tungsten carbide W2C, cobalt and iron (which melt at 2850, 1495 and 1100 ° C respectively), we can inject tungsten carbide at inside the nozzle, the cobalt via an injector 46a placed at 1 mm, and the iron via an injector 46b (fig. 3) placed 3 mm from the outlet of the nozzle.

It will also be noted in this regard that, to form an alloy coating, it is possible: a) to resort to the introduction, into the nozzle or close to the outlet thereof, of a mixture of constituents, either of a preformed alloy of these constituents, or b) proceed with the introduction of the constituents by separate injectors opening, into the nozzle and the plasma dart, at distances from the outlet of the nozzle according to their melting points respective.

In case b), the proportions of each of the constituents which should be used to obtain a coating of given composition are determined experimentally each time, composition which can for example be determined by analysis with the Castaing microprobe. The proportions of the constituents injected can vary quite significantly depending, in particular, on the constitution of the plasma gas used and the electrical power dissipated in the torch.

For example, to obtain a coating with 70% of chromium oxide CGO3 and 30% of an alloy of tungsten carbide WC or W2C with 20% of cobalt, the proportions of the various constituents injected into the plasma jet may vary from 50 to 90%

of chromium oxide, 10 to 50% of tungsten carbide WC or W2C and 10 to 30% of cobalt.

In particular, for given proportions of the constituents in the powders injected into the plasma jet, we find in the coating formed:

the same amount of tungsten carbide, less chromium and more cobalt, using a plasma with 70% argon and 30% nitrogen,

the same amounts of chromium and tungsten carbide, but less cobalt, using a plasma containing 70% argon and 30% hydrogen,

more chromium and less tungsten carbide and cobalt, using a plasma with 70% nitrogen and 30% hydrogen.

It will also advantageously use a sheathing 57 of the plasma jet by a flow of neutral gas, such as argon. This gaseous sheath allows, in particular, the elongation of the flame or the sting of the plasma, whereby the duration of the stay of the particles injected into the plasma jet in the hottest zone of the same is increased. this. Furthermore, this cladding has the effect of protecting the plasma jet from the ambient atmosphere, which results in a reduction of turbulence within the plasma and makes it possible to avoid to a large extent the reactions, possible at high temperature, of the particles entrained by the plasma jet with the ambient air, in particular with the oxygen in the air.

This gaseous sheath 57 can be produced by any appropriate means, in particular using a gas ejection system 58 disposed near the outlet 10 of the generator (or of the nozzle 16). This system, supplied with neutral gas intended to carry out the cladding, advantageously comprises an annular ejection orifice or a plurality of ejection orifices 59 (fig. 3), arranged on an annular ring surrounding the outlet of the plasma jet .

The protection of the plasma jet is further greatly improved, by using a chamber 60 surrounding the plasma jet and comprising an opening 60a at its end opposite to the outlet of the nozzle, this chamber being provided with cooling means (shown diagrammatically in 61), taking advantage in particular of a forced circulation of water.

Alternatively, one can operate in a sealed box in which, after making the vacuum, an inert gas such as argon would have been introduced.

According to yet another variant, the deposits are made in a vacuum chamber. The average free path of the molecules is thus increased.

The degree of fusion of the particles of the powder injected into the plasma can be checked in a fairly simple manner - and consequently adjust the operating conditions of the generator - especially if these molten particles are received in a tank containing the water in which they are soaked. It is observed that, if they have been melted in their mass, they solidify in the form of spherules.

X-ray examination of the deposits obtained by the process further reveals, when the material of the deposit has a hexagonal crystal structure, a preferential orientation of the crystals on their base planes 0001, parallel to the surface on which is produced the deposit.

It has been observed that there exists, at least as a first approximation, for hexagonal materials, a relationship between the c / a reticular ratios of the latter and their coefficients of friction. This relation appears in the table of fig. 4, in which the values of the friction coefficients F (ordinate axis) of various hexagonal materials have been reported as a function of their reticular relationships (abscissa axis).

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The measurements were made in ultra-vacuum (10-11 torr) using two discs, provided with coatings deposited under the conditions described above, and engaged in friction under a load of 5 kg / cm2 and at a speed relative 1 m / s. From this fig. 4 results that the materials with a c / a reticular ratio, situated around 1.61 to 1.63, have a particularly low coefficient of friction, hence a minimum risk of seizure during the friction of the corresponding coatings, one on the other.

In order to obtain coatings which have the lowest possible coefficient of friction, it is naturally also possible to use constituents which, in the pure state, have a hexagonal crystal structure whose reticular ratio is quite far from the value 1.633, in particular by combining them with other constituents in proportions such that alloys or solid solutions are obtained having a reticular ratio closer to the ideal value 1.633 than are the reticular ratios of the hexagonal constituents taken in isolation.

It will also be possible, when constituting these alloys or solid solutions, to take into account other factors, the respect of which may be essential, when it is desired to obtain an effective friction coating. In this case, one of the constituents may have a high hardness, the other a lesser hardness.

The hard component in fact increases the contact rigidity, in other words, the elastic deformations of the roughness of the friction surfaces. The frequency of vibrations linked to friction, while a friction operation is taking place, is higher the higher the hardness of the friction material. It is therefore possible, by the presence of a hard constituent in the coating, to shift the frequency of the vibrations linked to friction towards a range of values sufficiently distant from the natural frequency of all the bodies entering into friction, to avoid any confusion of the frequencies which would cause self-excitation of this natural frequency of the whole.

The soft component may play, to a certain extent, the role of shock absorber, even of lubricant.

For example, an alloy of chromium oxide Cr2C3 (c / a = 2.761), cobalt (c / a = 1.624) and tungsten carbide W2C (c / a —1.578), in the proportions given in the table given below presents a c / a ratio close to 1.633. In addition, in these alloys, chromium oxide, which is hard, increases the contact stiffness while the cobalt, softer, absorbs vibrations and acts as a lubricant.

Table II indicates certain alloys which are advantageously applicable to the production of coatings having all the characteristics described above.

We indicate in each column the percentage of each of the constituents in alloys whose complete constitutions are deduced from the horizontal rows of the table.

Table II

Co

W2C

CraOä

CrsC2

Mo

Or

(%)

(or WC) (%)

(%)

(or Cr) (%)

(%)

(%)

75

25

65

35

75

25

15

70

15

70

15

15

70

17

13

45

20

35

50

20

10

20

6

24

70

14

56

30

10

40

50

Preferably, the coating materials have a sufficiently high allotropic transformation point, so that they are not liable to undergo allotropic transformations during friction. If necessary, the allotropic transformation point of the friction coating material is raised, by combining it with a second material, the role of which is then to introduce into the crystal lattice of the first a certain proportion of substitution atoms which, all by preserving the desired hexagonal crystal structure, leads to an elevation of this allotropic transformation point. For example, an addition to cobalt, the allotropic transformation point of which is 417 ° C., of molybdenum makes it possible to obtain an alloy having a markedly higher allotropic transformation point. For example, an alloy containing 75% cobalt and 25% molybdenum has an allotropic transformation point of the order of 980 °.

This effect on the allotropic transformation point of a cobalt-based alloy is still clearly demonstrated by the curves in FIG. 5 which reflect the variation of the friction coefficient F (ordinate axis) as a function of the temperature (expressed in degrees Celsius on the abscissa axis) for the coatings of two discs engaged in friction, one of which is fixed, under a load of 5 kg / cm2, and at a relative speed of 1 m / s under a vacuum of 10 ~ 11 torr. The temperature is measured by means of a thermocouple brought in the immediate vicinity of the friction surface of the fixed disc.

Under these conditions, curve A relates to variations in the coefficient of friction of a coating containing 70% by weight of cobalt and 30% by weight of tungsten carbide W2C.

Curve B relates to the friction characteristics of a coating made of an alloy containing 60% by weight of cobalt, 20% by weight of W2C and 20% by weight of molybdenum.

These two alloys have a normally hexagonal structure. As can be seen in the figure, the coefficient of friction undergoes a sudden change at a temperature of around 470 ° C for the CO / W2C alloy (curve A) and at a temperature slightly below 800 ° C for the alloy further containing molybdenum (curve B). These changes in the coefficient of friction coincide with the allotropic transformation of the hexagonal structure into a cubic structure. It will be noted that the coatings in question again exhibit their initial low coefficient of friction after cooling.

The curves in fig. 5 therefore demonstrate the effect of molybdenum on the allotropic transformation point of an alloy of cobalt and tungsten carbide W2C. They also make it possible to demonstrate the much higher coefficients of friction of the alloys corresponding to cubic structures, which do not allow regular and progressive engagement, without seizing, of the surfaces engaged in friction.

As already indicated above, these friction coatings with a hexagonal structure lead to particularly advantageous results when they rub on other coatings having the same structural characteristics. The materials constituting two friction coatings will however preferably be different from the point of view of their chemical nature, in particular will not exhibit mutual solubility in order to avoid any possible phenomenon of adhesion of one coating to the other. Thus, preferably carbide-based coatings will be rubbed against oxide-based coatings.

In general, the use of refractory oxides and carbides with a hexagonal crystal structure is very advantageous, because:

- the compact nature of their crystal structures,

especially when their c / a reticular ratio is close to 1.633,

- their hardness which results from the compact nature of their crystal structures and from the interatomic forces binding the atoms to each other,

- reduced work of plastic deformation of their surface layers, thanks to the preferential orientation of their planes

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base 0001, especially when their c / a reticular ratio is close to 1.633, and

- their low contact adhesion.

The values of the various parameters are given below, by way of example, for which a coating of excellent quality is obtained on a support, having undergone a prior sandblasting operation, when using the alloys defined in the above table.

The constituents of the alloy are injected into the plasma jet in one of the three modes considered above, namely:

- either the injection of a mixture of powders,

- either separate injections of the constituents of the alloy,

- either injection of the alloy prepared beforehand.

Diameter of the ejection orifice of the anode-nozzle 16 of the plasma generator: 6 to 10 mm.

Distance from the support to the end of the dart: about 5 cm.

Constitution of the plasma gas: argon / hydrogen or argon / nitrogen or nitrogen / hydrogen mixtures: for example A / H2 mixture at the rate of 751 of argon and 161 of hydrogen.

Plasma gas flow: 75 1 / min Ar

161 / min H2,

Particle ejection speed: 100-500 m / s.

Power dissipated in the plasma jet: 24 to 28 kW (for example 310 A at 80 V, or 620 A at 45 V, or 800 A at 30 V).

Average particle size of the powder injected into the plasma jet: 20-40 (t.

Hourly quantity by weight of powder introduced into the plasma jet: 500-1000 g / h.

Pressure of the gas supply to the gas barrier ejection member 56: 3 bars.

Gas flow from this gas barrier: 30 to 50 m3 / h.

Gas constituting the gaseous sheath 61: argon.

Flow rate of the organ 60 for injecting the gas sheath: 5 to 151 / h.

The duration of a deposition operation varies from a few seconds to a few minutes, depending on the thicknesses desired for the coating, these thicknesses then varying from a few hundredths to a few tenths of a millimeter.

The method described makes it possible to take advantage of the qualities of the two alloys on the same contact surfaces, in particular by using parts of the kind of that of FIG. 9, in which a section is shown through a part 80 of a rotor (in the case of a clutch) or of an annular inductor (in the case of a brake), the axis of which is shown diagrammatically by the line 81 in phantom, coagulating in a known manner with a coil 82 to attract and drive or brake, when the coil is energized, an armature 84, partially shown schematically in phantom in FIG. 9, the surface of which is provided with a coating of the same type as that of the rotor or of the inductor.

According to this method, one carries out, both on the annular projections 86a, 86b and on the intermediate part 87 of the support frame 80 shown in FIG. 6, a first deposit 88 of the material having the desired magnetic properties in a thickness preferably less than the height of the projections 86a, 86b (FIG. 7). A second deposition 90 is then made of the material having the required friction characteristics, the thickness of this second deposition being such that the sum of the thicknesses of the first and second deposition is greater than the height of the annular projections 86a, 86b ( fig. 8). Finally, using a diamond grinding wheel (not shown), a grinding and polishing of the surface of the coating formed by these two deposits 88 and 90 is carried out until a flat surface is obtained and flush with the projections. 86a, 86b the material of the first deposit 88. The final planar coating (FIG. 9) then comprises annular outcrops 92a, 92b of magnetic material which allow the easy passage of the lines of force of the magnetic field created by the coil 82, therefore an increased tightening torque, and an intermediate outcrop 94 of material of the second deposit which provides for the achievement of the most favorable friction conditions, when the opposite surfaces of the reinforcements are engaged. The friction material of the intermediate outcrop 94 is harder and more resistant to wear than that of the annular outcrops 92a, 92b, to avoid the occurrence of alternating wear processes of the first and second above-mentioned outcrops.

The electromagnetic brake or clutch armature can be used for motor brakes, vehicle brakes, etc.

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Claims (4)

617,499 1. Electromagnetic brake or clutch armature comprising a friction coating, characterized in that the armature comprises at least one annular projection (86a, 86b) formed on its surface, a first layer (88, 92a, 92b) d '' a magnetic material covering the entire surface and the annular projection (86a, 86b) and flush with the latter, and in that the friction coating (94) is formed on the parts of this surface other than that occupied by the projection (86a, 86b), the outer surfaces of the friction coating (94) and the outer surfaces of the outcrops (92a, 92b) of the magnetic material all being substantially contained in the same plane, and the coating of friction consisting of a material of which substantially all the phases belong to the hexagonal crystalline system, the base planes 0001 of the majority of the crystals of the coating being oriented substantially parallel to the surface of the coating. 2. Armature according to claim 1, characterized in that the armature comprises a support (80) on which are arranged concentrically two annular projections (86a, 86b) forming between them a hollow area, the layer of magnetic material (88 , 92a, 92b) flush with the plumb of the two annular projections (86a, 86b) and the friction coating (94) flush with the plumb area. 2 3. Reinforcement according to one of claims 1 or 2, characterized in that the orientation of more than 75% of the crystals of the material of the friction coating (94) on their base planes 0001 is substantially parallel to the surface parts on which the coating (94) is formed. 4. Armature according to any one of claims 1 to 3, characterized in that the material of the magnetic layer (88, 92a, 92b) consists of an alloy of cobalt, iron, nickel, and that the material of the coating friction (94) consists of cobalt alloyed with one or more of the constituents of the material class comprising WC, W2C, Cr203, Cr3C2, Cr and Mo.