BACKGROUND ART A drive, for example a drive for accessories, generally comprises a drive pulley operated by the engine shaft and at least one driven pulley which drives an auxiliary member, for example an alternator, said pulleys being connected to one another via a belt, for example of the poly-V type.

There is known the use of a clutch wheel for controlling a second auxiliary member, for example a water pump, situated in a lateral position with respect to the drive pulley. The clutch wheel co-operates by friction with the back of the belt and is thus drawn along by the drive pulley for actuating the water pump, for example by means of a second clutch wheel permanently connected to the latter.

In the configuration of the type described above, a drive for accessories presents a number of drawbacks.

In particular, in unfavourable climatic conditions, such as in the case of rain, particles of water can deposit on the surfaces in contact, with consequent stopping or malfunctioning of the water pump.

Furthermore, should there be provided a device for bringing the clutch wheel closer to activate selectively the water pump, the clutch wheels and the belts commonly used cause vibrations and noise during the transients of engagement and disengagement.

DISCLOSURE OF INVENTION The purpose of the present invention is to provide a clutch wheel which will be free from the drawbacks described above.

The above purpose is achieved by a clutch wheel, as defined in Claim 1.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the present invention, there is now described a preferred embodiment, purely by way of non-limiting example and with reference to the attached plate of drawings, in which Figure 1 shows a drive for accessories according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION In Figure 1, designated as a whole by 1 is a drive for accessories set on a wall 2 of a casing 3 of an internal-combustion engine 4.

The drive 1 comprises: a drive pulley 5, driven by an engine shaft 6 with axis A coming out from the wall 2; a first driven pulley 7 of an alternator (not illustrated), which can turn about a second axis B parallel to the axis A; and a tensioning pulley 8, which can turn about a third axis C parallel to the axis A.

The pulleys 5, 7 and the tensioning pulley 8 are connected to one another by a belt 9 comprising integrally a back 10, and are arranged at the vertices of a triangular path, in which the side that joins the axes A and B identifies a branch 11 of the drive 1.

According to the invention, the back 10 is embossed.

Embossing is a surface-finishing process that enables a succession of reliefs and recesses to be obtained, which may even be smaller than one millimetre, on the surface of compliant materials, such as, for example, polymeric rubber, and is performed generally according to two different modalities.

In fact, it is possible to obtain the reliefs and recesses by machining away using cutting tools that act on the edge of the belt after the vulcanizing operation, or else by moulding during the vulcanizing operation itself, using an expansion box coated with a fabric, the surface structure of which is'impressed'and'copied' in the elastomeric material of the belt.

Driven by the embossed back 10 of the belt 9 is a second accessory (not illustrated), for example a water pump.

For this purpose, the drive 1 moreover comprises a clutch wheel 12, which can turn about a fourth axis D parallel to the axis A and is set on the opposite side of the tensioning pulley 8 with respect to the vertical branch 11, substantially in a position corresponding to the drive pulley 5.

Set between the clutch wheel 12 and the pulley 5, co- operates with both the clutch wheel 12 and the pulley 5 an intermediate clutch wheel 13 that turns about a fifth axis E and transfers the torque from the engine shaft 6 to the water pump.

In particular, the clutch wheel 13 co-operates permanently with the back 10 of the belt 9 in a position corresponding to the arc thereof that runs over the drive pulley 5. The axis E of the clutch wheel 13 is

preferably mobile under the action of a control device (known and not illustrated), by means of which the clutch wheel 13 can be brought up to and moved away from the clutch wheel 12 for activating and deactivating the water pump.

The clutch wheel 13 has a circumferential clutch surface 14, which preferably has a circumferential development that is not a submultiple of the length of the back 10 and has a multiplicity of substantially circumferential grooves 15 such as to drain away any humidity or water that may be present on the back 10 of the belt 9 in an area corresponding to the clutch wheel 13.

The grooves 15 of the clutch wheel 13 follow a substantially undulated and periodic circumferential path having the same period along the clutch surface 14.

In particular (Figure 2), the grooves 15 are in equal number, and two adjacent grooves are arranged specularly with respect to one another so as to be out of phase by one half-period.

According to another embodiment (Figure 4), the grooves are set alongside one another and spaced at a constant distance point by point so as to be in phase.

Operation of the drive for accessories is described in what follows.

The clutch wheel 13 transfers the torque from the engine shaft 6 to the clutch wheel 12 by friction.

When the drive 1 is in motion, the combined action of the embossed back 10 and the grooves 15 enables draining of any water that may be present between the surfaces in contact, so favouring adherence.

The succession of surface reliefs and recesses obtained by embossing affords characteristics of stiffness different from those of a smooth back, forming a compliant layer, and consequently enables a gradual distribution of the contact pressure.

Furthermore, since the grooves 15 are not circular, their trace on the plane of contact between the clutch wheel 13 and the belt 9 is displaced in a direction parallel to the axis E, instant by instant.

From an examination of the characteristics of the drive 1 made according to the present invention, the advantages that it enables emerge clearly.

In particular, the joint action of the embossed back 10 and of the grooves 15 enables draining of any water that

may be present between the surfaces in contact and, consequently, prevention of phenomena of"aquaplaning" that can jeopardize correct driving of the auxiliary member, in particular the water pump.

A further advantage of the present invention lies in the fact that a back presenting a compliant layer obtained by embossing enables a gradual distribution of the contact pressure tending to dampen the vibrations and the noise.

According to the preferred embodiments described, grooves having a non-circular path enable prevention of localized wear of the back of the belt.

Finally, it is clear that modifications and variations can be made to the drive described and illustrated herein, without thereby departing from the sphere of protection of the present invention, as defined in the annexed claims.

In particular, the present invention can be adapted to any type of belt drive, for example the one for the distribution system.

Furthermore, the reliefs and the recesses obtained by embossing can have any shape whatsoever, for example

that of a truncated pyramid if obtained by moulding, or else can reproduce the profile of the cutting tool if obtained by machining.

The axis E of the clutch wheel 13 can be either fixed or mobile, causing, in the first case, a continuous driving of the accessory driven by the clutch wheel 12 and, in the second case, a selective actuation governed by a remote control.

Alternatively, the clutch wheel 13 can be carried by the shaft of the water pump, and the drive can comprise a coupling set between the shaft of the pump and the clutch wheel to enable selective driving of the pump itself.

The path of the grooves 15 can be closed or open, or a combination of the two, and the grooves 15 can be the same as one another or different from one another.

Furthermore, two grooves set alongside one another can be brought closer together and be substantially tangential to a circumference belonging to the'clutch surface 14.