The present invention relates to a bearing unit provided with an encoder for detecting relative rotation between the rings of the bearing.

A roller bearing usually includes an outer ring, and an inner ring mounted within the outer ring, separated from the outer ring by rolling elements contacting each ring on a raceway. To measure relative rotation between the two rings, an encoder can be fixedly assembled to a first ring, and a sensor can be assembled either to the second ring, or to a larger structure holding the second ring. Frequently, the outer ring of the bearing is stationary, that is, fixedly assembled to a reference structure (which can be a fixed structure or a wheeled carriage body). The inner, rotatable ring, then bears an encoder, e.g. a flange or a ring with regularly spaced angular sectors, magnetized each in opposite direction to the magnetic direction of their direct neighbour. A detector sensitive to the magnetic field variation is placed in the vicinity of the encoder. The detector can be made integral with the outer ring, or it can be assembled directly to the reference structure.

European Patent Application EP 1 571 453 discloses such an instrumented bearing, with an annular magnet anchored to the inner ring, and a detection winding anchored to the outer ring. Both magnet and winding are anchored by engaging snap fixing element into grooves of each ring. Nevertheless, the axial length of the snap fixing elements leaves no space for a lateral seal, so that magnet and winding are directly exposed to the lubricant contained in the bearing, and they do not necessarily prevent the lubricant from escaping the bearing. To partly overcome this drawback, US Patent 5 293 124 discloses a multipolar ring made integral with the rotating ring, a sensor made integral with the non rotating ring, the sensor being coupled with a flux concentrating annular metal plate, the circular edge of which carries a circumference seal lip. Still, the speed measuring system is located within the bearing, considering its position relatively to the seal, so that the system is exposed to lubricant as well as to possible mixtures of lubricant and metal wear debris. Moreover, inner and outer rings have to provide specific grooves or bearing surfaces to be able to accommodate the multipolar ring on one ring, and to accommodate a snap-in bloc carrying the sensor, plate and seal, on the other ring.

The present invention aims at proposing a bearing, instrumented in order to make its rotational speed measurable, and which will resolve the above described disadvantages in a simple and cost effective fashion.

In one embodiment, a bearing unit comprises:

-a bearing outer ring and a bearing inner ring, centred on a common axis, both rings being axially comprised within axial coordinates defining the width of the bearing, the inner ring having at least a first circular groove not serving as a raceway on its external cylindrical surface,

-a tachometric ring, comprising a snap portion assembled within the width of the bearing, with fitting means so sized as to fit into the groove and lock the tachometric ring to the inner ring, said tachometric ring also comprising a support portion protruding outside the width of the bearing, either including, or able to be attached to, a magnetic encoder or a sensor.

The maximum diameter of the snap portion is no larger than the maximum diameter of the inner ring.

Preferably, the outer ring has at least a second circular groove on its internal cylindrical surface, which faces the first circular groove. Preferably, both grooves are designed so as to be able to accommodate the inner and outer ring portions of a seal. In a preferred embodiment, the first circular groove is o f roughly "V" shape section, with rounded bottom, one branch of the "V" intercepting the outer cylindrical surface of the inner ring, and the other branch of the "V" intercepting the adjacent radial surface of the inner ring. In an advantageous manner, the fitting means comprises a fitting annulus of same or lesser circumference as an annulus defined by the first groove, such that the diameter of the fitting annulus can be elastically extended. In a preferred embodiment, the fitting means comprises a plurality of axial elastic tabs that can be locked elastically or by snap action into the first circular groove.

In one embodiment, the tachometric ring includes a magnetized or magnetizable material. In another embodiment, the support portion has an overall shape of a hollow cylinder, with at least a bulge pointing radially out of its external cylindrical surface, so as to facilitate axial positioning or locking of an annular element onto this external surface.

In this embodiment, the support portion may have an overall shape of a hollow cylinder, with two staggered circumferential rows of bulges pointing out of its external cylindrical surface.

In the latter two embodiments, a magnetic encoder or a magnetic sensor may be locked onto the support portion.

Preferably, the bearing unit also comprises a seal or a shield extending between the outer ring and the snap portion of the tachometric ring.

Additional objects, advantages, and novel features of this invention shall be set forth in part in the description that follows. A preferred but non limiting form of embodiment will now be described, with reference to the attached drawings, wherein :

-Figure 1 is a view of an axial section of a roller bearing unit according to an embodiment of the invention.

-Figure 2 is a partial perspective view of an axial section of a roller bearing unit according to an embodiment of the invention. As can be seen in FIG. 1 , a rolling bearing 1 includes an outer ring 2 and an inner ring 3 , between which are housed rolling elements 4, balls in this instance, which are held regularly spaced in the circumferencial direction, by a cage 5. The rolling elements 4 contact the inner ring on a toroidal raceway 24, and contact the outer ring on a toroidal raceway 25. The outer ring 2 and the inner ring 3 have a common revolution axis x-x. Along this axis x-x, considered as an abscissa axis, one can define a left extremity A of the bearing, defined as the position of the minimal abscissa of any point pertaining to any of rings 2 and 3 of the bearing. Along this axis x-x, considered as an abscissa axis, one can define a right extremity B of the bearing, defined as the position of the maximal abscissa of any point pertaining to any of rings 2 and 3 of the bearing. The width of the bearing is defined by axial positions comprised between A and B. In the example illustrated on FIG 1 and Fig 2, on each of the two lateral sides 6, 7 of the bearing, a circumferential groove 9, 10, 1 1 , 12, is formed in each ring 2, 3.

On a first side of the bearing, that is left hand side on FIG. 1 , groove 10 on the inner face of the outer ring 2, holds the outer circumference of an HSL (Hammer seal lip) seal 13. The Hammer Seal

Lip seal is named for the claw hammer cross-sectional shape of the seal lip where it rides on the inner ring. The seal has a radial, flexible lip that rides on a special seal groove machined into the external surface of the inner ring. The groove 9, on the outer face of the inner ring 3 , accommodates the inner circumference lip of the HSL seal 13.

The groove 1 1 is symmetrical to the groove 10, and the groove 12, which will be designated as a snap groove, is symmetrical to the groove 9. Grooves 9, 10, 1 1 , 12 can be seen with a closer view on Fig 2. A tachometric ring 14, with an overall shape of a hollow cylinder, moulded in synthetic material, is assembled to the inner ring 3. In alternative embodiments, the tachometric ring could be manufactured out of other elastically deformable materials, such as metal. Geometrically speaking, the tachometric ring 14 can be axially divided into a snap portion 15 and a support portion 16, the snap portion 15 being defined as the portion axially interfering with either of the two rings 2, 3 (that is, axially comprised between points A and B defined above). The support portion 16 is defined as the portion axially protruding further than the lateral sides of both inner and outer rings (that is, protruding further to the right on Fig 1 than point B defined above). On its left hand side extremity, the snap portion 15 ends by a circumferential bulge 17 which extends from the inside o f groove 12 to the outer diameter of the inner ring 3. The maximum outer diameter of the snap portion 15 is substantially equal to the maximum outer diameter of the inner ring 3. The circumferential bulge 17 forms an annulus which can be elastically extended so as to snap-fit the snap portion 15 into the groove 12. In alternative embodiments, the snap portion 15 , instead of forming a continuous annulus, can end with a circumferential line of elastic axial tabs designed to lock elastically into the groove 12. On the support section 16, an annular encoder ring 18 is fitted, which is made of, or contains, magnetized material, or magnetizable material, such as overmolded sintered ferrite or moulded plastoferrite compound. The magnetizable material can be magnetized after the bearing unit has been assembled. Once magnetized, the encoder ring 18 offers a periodic pattern of "North" pole and of " South" pole pointing either axially or radially, at regular intervals along the circumference of the encoder ring 18. In alternative embodiments, the tachometric ring 14 itself can be magnetized, so as to ensure the function of the encoder ring, and the "snap-in" function, with a single part. The encoder ring 18 is axially maintained between one or several stop bulges 19, and one or several snap bulges 20, provided on the surface of the tachometric ring 14, the stop bulges 19 and the snap bulges 20 being circumferentially oriented along the outer cylindrical surface of the support portion 16. In the embodiment depicted on Fig 1 and Fig 2, in order to facilitate elastic accommodation of snap bulges 20 on one end of encoder ring 18 , and of stop bulges 19 on the other end of the encoder ring 18, the snap bulges 20 and the stop bulges 19 are disposed in two staggered rows. In alternative embodiments, a single stop bulge 19 may run around the whole circumference of the support portion 16. An additional protruding element can also be provided on the support portion 16 to prevent the encoder ring 18 from rotating around the support portion 16. Figure 2 is a perspective view of a detail of an axial section of a bearing unit according to the invention. Features common to Figure 1 can be seen, which are then designated by same reference numbers as on Figure 1. As can be seen on figure 2, the encoder ring 18 radially faces an associated magnetic/electrical sensor or transducer 23 , able to convert magnetic flux variations into an electrical signal. The transducer can be for example a magnetoresistor or a Hall Effect probe. The transducer 23 can be either assembled to the outer ring 2, (by means not represented here), or it can be assembled to an element (shaft, or larger structure) which has itself been made integral with the outer ring 2. In alternative embodiments, the transducer 23 can face the encoder 18 or the tachometric ring 14 axially.

On the right hand side of the bearing, groove 1 1 on the inner face of the outer ring 2, holds the outer circumference of a metal shield 22. The inner circumference of shield 22 is bent inwardly towards balls 4 so as to form a cylindrical or a tapered gap 21 . The inner diameter surface of gap 21 is complementary at short distance with the outer cylindrical surface of the snap portion 15. A narrow passage (preferably less than 1/3 mm) is thus left between the two surfaces, avoiding direct friction between the shield 22 and the tachometric ring 14, and constituting a labyrinth seal. The shield 22 limits the exit of lubricant from the bearing, or the ingress o f contaminants into the bearing. In some embodiments, when the stop bulge 19 runs continuously or at least around a major part, of the external circumference of the support portion 16, and when this stop bulge is located axially at a short distance (preferably less than 2 mm) from the shield 22, the space left between the shield 22 and the stop bulge 19 acts as a continuation of the labyrinth seal. The stop bulge 19 thus also contributes to limiting the ingress of contaminants into the bearing.

As can be derived from the previous description, the grooves 1 1 and 12 can also be used to accommodate a second HSL seal symmetrical to seal 13. Consequently, the bearing inner and outer rings 3 and 2 of the present invention, can be used either for assembling a standard non instrumented bearing with two lateral HSL seals, or they can be used for assembling an instrumented bearing as described. This strongly reduces the manufacturing costs of the instrumented bearing, as no extra costs are generated for producing specific bearing rings. Thanks to the shield 22, efficient sealing between inside and outside of the bearing is achieved.

The invention is not limited to the embodiments described and illustrated here, which are to be regarded as mere examples of a wider range of embodiments. Overall shapes of the bearing rings, of their grooves, of seal and shield can vary. The HSL seal may be replaced by another type of seal or by a shield, and the shield itself may be replaced by another shield type, or by a seal which contacts the inner ring 3 through a deformable contacting lip. The geometry of the tachometric ring may also vary, in order to accommodate different groove profiles or different encoder geometries. The support portion may have an outer diameter noticeably larger than the outer diameter of the inner bearing ring. The snap portion may have an outer diameter smaller than the outer diameter of the inner bearing ring. The overall geometry of the tachometric ring can be optimized so as to make the ring radially deformable, and facilitate snapping of the snap portion into the snap groove. Materials of the tachometric ring may vary, e.g. the ring can be made of a rolled sheet of steel, of overmoulded ferrite or of injected plastic, with or without mineral fillers.

In the present description, the tachometric ring 14 is assembled to the inner bearing ring 3 , and is surrounded by an external sensor 23.

Alternative embodiments can be envisaged, with the tachometric ring 14 assembled to the outer bearing ring 2, facing, or surrounding, a sensor made integral with the inner bearing ring 3 , or made integral with the reference structure. The inner diameter of the snap portion would then be no less than the minimum diameter of the outer bearing ring. The tachometric ring 14 can also be assembled to the outer bearing ring 2, and support a magnetic sensor on its inner diameter. The instrument bearing according to the invention ensures good sealing performance with minimal manufacturing costs, due to the fact that it is partly set up with parts already manufactured in large series.