The present invention relates to an antifriction bearing, which can be used for any rotating machine requiring rotational guidance for rotating parts by antifriction bearing. It is particularly applicable to antifriction bearings for electric motors. These may be, for example, guided-vehicle traction engines, such as for a railcar.
The antifriction bearings (hereinafter also designated "rolling bearings" or "bearings" in the interests of conciseness) are subject to degradations due to the passage of leakage electric currents. These leakage currents may result, in particular, from the dissymmetry in the structure of the magnetic circuits of the motor or from the motor's inverter power supply. When the currents are generated by dissymmetries of the magnetic circuit, the currents that pass through the rolling bearings have low frequencies, of a few kHz. When the motors are powered by inverters, the leakage currents have higher frequencies, potentially reaching 100 kHz or even a few MHz. These two types of leakage current may be present simultaneously in the motors.
It is necessary to protect the rolling bearings from these leakage currents, because these currents create electric arcs between the rolling elements and the races of the rolling bearing rings. These arcs result in a premature degradation of the bearing, according to a process similar to electric arc welding, by generating high localized temperature rises: they thus damage the lubricant of the bearing and/or the areas of contact between rotating elements and races.
One known solution for protecting the rolling bearings from these leakage currents consists in electrically insulating the inner or outer ring of the rolling bearing by the addition of a dielectric layer of ceramic type, of alumina for example. This solution is, however, more effective for low-frequency currents than for high-frequency currents. Furthermore, because of its relative fragility, this ceramic layer may make the mounting of the antifriction bearings more difficult.
The aim of the invention is therefore to remedy at least partly the drawbacks of this known solution. Its aim is, in particular, to avoid or slow down the premature wear of the antifriction bearings, due in particular to the leakage electric currents.
The subject of the invention is, firstly, a protection device, in particular providing thermal protection and/or protection against leakage electric currents, for an antifriction bearing intended to be mounted on a shaft of an electric motor. This protection device comprises a ring which is interposed between the shaft and the antifriction bearing and which is insulated from the shaft by a dielectric coating.
This additional ring electrically insulates the bearing from the shaft, which preserves the rolling bearing from the leakage currents originating from the shaft without having to modify the design of the standard rolling bearings. It is equally
effective against high-frequency currents and low-frequency currents, because it can be adjusted, independently of the rolling bearing which remains unchanged, to ensure a sufficient insulation against both types of leakage current normally encountered. Furthermore, this electrical insulation is generally backed up with a thermal insulation, which helps to reduce any risk of overheating of the bearing by the transmission of heat via the shaft, which is also a source of premature wear.
Preferably, the ring is based on titanium or a titanium alloy. It has in fact been observed that this type of metal exhibited properties different from those, for example, of steel (which is generally what the shafts are made of), which provided advantages on at least two fronts:
On the one hand, the titanium has a thermal conductivity coefficient at least three times lower than that of steel (approximately 21.6 W/m.K for titanium versus approximately 71 W/m.K for steel), which further contributes to reducing the transmission of heat from the shaft to the rolling bearing.
On the other hand, the choice of titanium (or one of its alloys mostly comprising titanium) enhances the quality of the shrank-on link between the shaft and the ring by taking into account speed and overheating, because the coefficient of thermal expansion of titanium is significantly lower than that of steel (approximately 8.5 x 10-6 K-1 for titanium versus 12.2 × 10-6 K-1 for steel) and its density is much lower (the density of titanium is approximately 60% lower than that of steel).
Finally, it is preferable for the material retained for the ring to have a mechanical resistance that is greater than or equal to that of steel, which is also the case with titanium.
According to a first variant of the invention, the ring is electrically insulated from the shaft by a coating of insulating (dielectric) material covering at least the surface of the ring intended to be in contact with the shaft.
According to a second variant of the invention, which can possibly be combined with the first, the ring is electrically insulated from the shaft by an insulating (dielectric) coating arranged on the outer surface of the shaft at least in its area intended to be in contact with the ring.
The insulating coating may thus be located on the ring, on the shaft or on both at the same time in their area of mutual contact.
Preferably, the or each insulating coating is a ceramic coating, in particular of metallic oxide, metallic nitride or silicon nitride. For example, it may be aluminium oxide, titanium oxide, silicon nitride or a mixture of at least two of these compounds.
It is possible, for example, to use a plasma spraying deposition method to
deposit the insulating coating or coatings.
The invention provides relative securing and positioning means between the ring and the shaft and between the ring and the rolling bearing.
Thus, according to one embodiment, the ring has an end face which can abut on a shoulder of the shaft, which makes it possible to lock the ring longitudinally relative to the shaft around which it is arranged. Also, the ring can be immobilized in position on the shaft (once locked/positioned correctly) by force-fitting (or any other mechanical means).
According to a variant, the ring is also electrically insulated from the metallic (electrically and/or thermally conductive) part or parts in contact with the shaft A by the insulating coating in its area intended to be in contact with said mechanical part or parts.
Also the subject of the invention is a shaft of an electric motor on which is mounted an antifriction bearing equipped with the protection device described previously.
Also the subject of the invention is an electric motor comprising a shaft on which is mounted an antifriction bearing equipped with the protection device described previously.
The ring interposed between the shaft and the antifriction bearing can be insulated from the shaft by an insulating coating on the outer surface of the shaft at least in its area intended to be in contact with the ring.
In this case, the insulating coating also preferably covers
- a shoulder of the shaft against which an end face of the ring can abut, and/or
- the outer surface of the shaft intended to be in contact with one or more metallic parts of the bearing.
It should be noted that all the spatial arrangement indications, of the "upper", "lower", "lateral", "horizontal" or "vertical" type should be understood by considering a shaft arbitrarily represented in a horizontal arrangement, but without limiting the invention to this configuration.
Other features and advantages of the invention will become more clearly apparent in light of the following description and the appended drawings, concerning a particular, nonlimiting embodiment, with reference to the following figures which represent:
- Figure 1: an exploded isometric view of an exemplary antifriction bearing to which the invention can be applied,
- Figure 2: the antifriction bearing according to Figure 1 mounted around a shaft and equipped with the protection device according to the invention (seen in longitudinal
cross section of the shaft, only the upper part is represented in the area of mounting of the bearing on the shaft).
These figures are highly schematic and do not necessarily observe the scale between the various components represented in order to make them easier to read. The same components are given the same references in all figures. All the components are not necessarily represented or described in detail, only those that are important to the invention are.
Figure 1 represents an example of a known standard antifriction bearing P,
which comprises a bearing housing 2 comprising a cylindrical body 3, a rolling bearing 4 arranged inside the cylindrical body 3 and a cover 5 comprising a cylindrical body 6 that is inserted into the cylindrical body 3 of the bearing housing and bears on the rolling bearing 4. The rolling bearing 4 is here of the ball type but may be of another type (rollers for example). The inner ring 7 is attached to a rotating shaft A (represented in Figure 2), whereas the outer ring 8 is attached to the bearing housing 2, which is fixed. The housing 2 comprises an abutment collar 9 in which smooth holes 10 are axially drilled. It may be fixed to a fixed part of a rotating machine by means of fixing screws and washers 11, passing through the bores 10 and complementary bores produced on the fixed part of the rotating machine. The cylindrical body 3 of the housing also includes axial tapped holes 12. The cover 5 comprises an abutment collar 13, drilled with axial bores 14. When the cover and the housing are assembled, the abutment collar 13 of the cover 5 abuts against the end of the cylindrical body 3 of the housing 2. The cover 5 is fixed to the housing 2 by means of fixing screws 15 passing through the respective bores 14, 12 of the cover 5 and of the housing 2. A different assembly of the cover and housing may be provided.
Figure 2 represents this type of bearing P, once mounted on the shaft A, therefore only the top half is represented (made of steel), rotating about its longitudinal axis X, associated with a flange 18 and actuated by an electric motor which is not represented. The components 16, 16' on the one hand and 17 on the other hand are commonly called deflectors, and are used, as is known, to ensure the seal-tightness of the rolling bearing and that the lubricant is kept inside the latter, by a labyrinth system. Here, and according to the invention, the bearing P is mounted on the shaft A via a ring B. This ring has a constant cylindrical inner surface 20 matched to the diameter Dl of the shaft A. One of its end faces 21 cooperates with a shoulder 22 present on the outer
surface of the shaft, causing the latter to change from a diameter Dl to a greater diameter D2. The face 21 of the ring B is at least as high as this shoulder 22 (here, it is higher than the difference D2 - Dl). It is locked thereon, the shoulder 22 serving as abutment in order to correctly place the ring longitudinally on the shaft A. The ring B is immobilized in position by force-fitting around the shaft A.
The nut 23 immobilizes the rolling bearing against the face 26 of the ring B, without ever being in direct contact with the shaft, in order to avoid creating electrical (and thermal) bridges between the nut 23 (usually metallic) and the shaft A. It should also be noted that any electrical contact is avoided between the deflector 17 and the nut 23.
The ring B is made of titanium (or of titanium alloy), and is covered over its entire inner surface 20 in contact with the shaft A and on its end face 21 (shaft side) with a coating 19 based on aluminium oxide with a thickness of, for example, 100 to 500 micrometers. This coating 19, arranged at the bearing/shaft interface and eliminating any direct contact between the two components, thus electrically insulates the bearing from the shaft, and thus preserves it from the leakage currents passing through the shaft. It should be noted that the coating is also present on the surface of the ring in contact with the deflector 17, that is to say, on the surface of the rear face of the ring (unlike its front face 21), which makes it possible to avoid the passage of current from the shaft A to the bearing P via this deflector 17.
The titanium or titanium alloy of which the ring is made helps to reduce the transmissions of thermal energy between shaft and bearing, and therefore also helps to increase the life of the bearing. In the example represented in Figure 2, the ring B also blocks the deflector 16 (which is positioned between, on the one hand, the end face 21 of the ring in its upper part protruding above the shoulder 22 of the shaft or of any other part fitted on the shaft and serving as axial abutment for the ring (B), and, on the other hand, a second shoulder 25 of the shaft A, causing its outer diameter to change from the diameter D2 to the diameter D3).
No detail will be given here concerning the mounting of the bearing P on the shaft A, and on the flange 18, which, apart from the interposition of the ring B, remains standard.
Provision may be made to attach the ring B to the shaft A, either definitively or removably. It is thus possible to replace the antifriction bearing P when it is worn, while keeping the ring in place around the shaft A, if the latter has a longer life than the bearing. A standard rolling bearing is thus kept, which can be interchanged without difficulties.
Other embodiments of the invention are possible. Thus, it is possible to envisage that the coating 19 is deposited on the shaft A rather than on the ring B, in all the area intended to be in contact with the bearing P (therefore, here, also at the level of the shoulder of the shaft used to lock the ring and the area in contact with all the metallic compounds forming part of the rolling bearing or its environment - deflector or other -which are themselves in contact with the ring.

CLAIMS
1. Protection device, in particular providing thermal protection and/or protection against leakage electric currents, for an antifriction bearing (P) intended to be mounted on a shaft (A) of an electric motor, characterized in that said protection device comprises a ring (B) which is interposed between the shaft and the antifriction bearing and which is insulated from the shaft by a dielectric coating (19).
2. Protection device according to the preceding claim, characterized in that the ring (B) is based on titanium or a titanium alloy.
3. Protection device according to one of the preceding claims, characterized in that the dielectric coating (19) covers at least the surface (20, 21) of the ring (B) intended to be in contact with the shaft (A).
4. Device according to one of the preceding claims, characterized in that the dielectric coating (19) covers at least the outer surface of the shaft at least in its area intended to be in contact with the ring.
5. Device according to the preceding claim, characterized in that the ring (B) is electrically insulated from the metallic part or parts (17) in contact with the shaft A by the insulating coating in its area intended to be contact with said mechanical part or parts (17).
6. Protection device according to one of Claims 3 or 4, characterized in that the or each dielectric coating (19) is a ceramic coating, in particular of aluminium oxide, of titanium oxide, of silicon nitride or of a mixture of at least two of these compounds.
7. Protection device according to one of the preceding claims, characterized in that the ring (B) has an end face (21) which can abut on a shoulder (22) of the shaft (A) or any other part fitted onto the shaft and serving as an axial abutment for the ring (B).
8. Protection device according to one of the preceding claims,
characterized in that the ring (B) is intended to be immobilized in position on the shaft (A) by force-fitting.
9. Antifriction bearing (P) mounted on a shaft (A) of an electric motor,
characterized in that the bearing is equipped with the protection device according to
one of the preceding claims.
10. Antifriction bearing (P) mounted on a shaft (A) of an electric motor
according to the preceding claim, characterized in that the ring (B) interposed between
the shaft and the antifriction bearing is insulated from the shaft by an insulating coating
on the outer surface of the shaft at least in its area intended to be in contact with the
ring.
11. Antifriction bearing (P) mounted on a shaft (A) of an electric motor
according to the preceding claim, characterized in that the insulating coating covers a
shoulder (22) of the shaft (A) against which an end face (21) of the ring (B) can abut.
12. Antifriction bearing (P) mounted on a shaft (A) of an electric motor
according to one of Claims 10 or 11, characterized in that the insulating coating
covers the outer surface of the shaft (A) intended to be in contact with the metallic part
or parts (17) of the bearing.
13. Electric motor, characterized in that it comprises at least one antifriction
bearing (P) according to one of claims 9 to 12.