Hub-pulley assembly having a rotationally disengageable pulley

1. A pulley-hub assembly comprising a hub adapted to be connected to a rotational member of a motor vehicle and having an axis (A), a pulley adapted to cooperate with a drive belt, at least one relief member radially arranged with respect to said axis (A) and carried by one of said hub and pulley, and a bidirectional disengagement structure which allow a relative rotation between said hub and said pulley in both directions, wherein said bidirectional disengaging structure comprises at least a first abutting area and a second abutting area carried by the other of said hub and pulley and spaced in a circumferential direction to delimit a respective hollow space accommodating with circumferential clearance said at least one relief member, said first abutting area defining a first stop position in which said hub and said pulley are coupled in a first direction of rotation, and said second abutting area defining a second stop position in which said hub and said pulley are coupled in a second direction of rotation opposite said first direction of rotation, said hub-pulley assembly further including a viscous friction material contained inside said hollow space and cooperating between said hub and said pulley.

2. The pulley-hub assembly as in claim 1 further including a bearing provided on each opposing side of the pulley and between the hub and pulley and positioned outboard of and axially blocking radially extending seals provided on opposing sides of the hollow space and between the hub and pulley.

3. The pulley-hub assembly as in claim 1 wherein said at least one relief member comprises a radially extending tooth.

4. The pulley-hub assembly as in claim 3 wherein said radially extending tooth extends from said pulley.

5. The pulley-hub assembly as in claim 1 wherein said at least one relief member engages said first or second abutting area to thereby couple the hub and pulley together in one of said first or second directions of rotation.

6. The pulley-hub assembly as in claim 1 wherein the viscous friction material comprises a paste material.

7. The pulley-hub assembly as in claim 1 wherein the paste material comprises a polytetrafluroethylene based polymer powder, graphite and a liquid or semisolid halogenated polyether based lubricating material.

8. The hub-pulley assembly according to claim 1 , wherein said hollow space is an area formed around a portion of the outer periphery of said hub and is defined between opposing side walls each of which include a portion having an inwardly converging profile forming said first and second abutting areas narrowing the width of said hollow space.

9. The hub-pulley assembly according to claim 8 , wherein said hollow space is annular and the abutting areas interact with the relief member.

10. The hub-pulley assembly according to claim 1 , wherein said hollow space is closed with respect to the external environment by means of said hub, said pulley and at least one closing member positioned there between.

11. The hub-pulley assembly according to claim 1 , wherein said friction material has a viscosity from 10 to 30 according to the Mooney ML (1+4) test method at 50° C.

12. An accessory drive for an internal combustion engine, comprising a first pulley adapted to be connected to a crankshaft of the engine, a second pulley adapted to be connected to a reversible electrical machine and a third pulley adapted to be connected to an accessory, a belt wound about said first, second and third pulleys, and a tensioner cooperating with said belt and comprising a support arm adjustably mounted to the engine at a first end, an end of a first arm is pivotally mounted at a second end of the support arm, and an opposite end of the first arm pivotally supports a rigid arm having first and second idle pulleys mounted at respective opposite ends thereof, the first idle pulley cooperating with a first branch of said belt defined between said first and second pulleys, and the second idle pulley cooperating with a second branch of said belt comprised between said second and third pulleys, wherein at least one of said first, second and third pulleys is a hub-pulley assembly according to claim 1 .

13. The drive according to claim 12 , wherein said tensioner comprises a mobile rigid arm supporting said first and second idle pulleys.