Tensioner for an accessory drive of a motor vehicle and accessory drive including such a tensioner
A tensioner for an accessory drive of an internal combustion engine has a belt on a first pulley connected to a drive shaft of the engine and a second pulley connected to an electric machine. The tensioner has a base configured to be fixed to a casing of the electric machine, a first ring rotating with respect to the base about a first axis, a second ring rotating with respect to the first ring about a second axis distinct from the first axis, a first tensioning pulley carried by the first ring and rotating with respect to it about its own axis, a second tensioning pulley carried by the second ring and rotating with respect to it about its own axis and elastic means acting on the first and on the second ring to push the first and the second pulleys into contact with respective spans of the belt.
1. A tensioner for an accessory drive of an internal combustion engine, the drive including at least a first pulley connected to a drive shaft of the engine, at least a second pulley connected to an electric machine and a belt wound at least on the first and second pulleys, the tensioner comprising:
a base configured to be fixed to a casing of the electric machine;
a first ring rotating with respect to the base about a first axis (A 1 );
a second ring rotating with respect to the first ring about a second axis (A 2 ) distinct from the first axis (A 1 );
a first tensioning pulley carried by the first ring and rotating with respect to the first ring about its own axis (PA 1 );
a second tensioning pulley carried by the second ring and rotating with respect to the second ring about its own axis (PA 2 ); and
an elastic member acting on the first and second rings to push the first and second tensioning pulleys into contact with respective spans of the belt,
wherein under nominal conditions a plane passing through the first and second axes (A 1 ; A 2 ) is inclined with respect to a bisector of a belt winding angle ( 0 ) on the second pulley by an angle (a) that is a function of the belt winding angle ( 0 ) on the second pulley according to the relation:
α=−0.21660+97.267 +c,
where α and θ are expressed in degrees (°) and c is a variable comprised in the range between +10° and −10°.
2. The tensioner as claimed in claim 1 , wherein one of the rings includes an appendage traveling within a recess of another of the rings for limiting the relative rotation between said rings.
3. The tensioner as claimed in claim 1 , wherein one of the rings includes one of a protrusion and a groove, and the base includes another of the protrusion and the groove, wherein the protrusion and the groove slidingly cooperate for limiting the relative rotation between said one of the ring and the base.
4. The tensioner as claimed in claim 3 , further comprising a pin insertable into respective holes of the first and second rings.
5. An accessory drive for an internal combustion engine comprising at least a first pulley connected to a drive shaft of the engine, at least a second pulley connected to an electric machine, a belt wound at least on the first and second pulleys, and a tensioner according to claim 1 .
6. The tensioner as claimed in claim 1 , wherein the base has an annular configuration, and wherein the base and the first and second rings have an internal diameter greater than the diameter of the second pulley so that the tensioner can be mounted on the electric machine without interfering with the second pulley.
7. The tensioner as claimed in claim 1 , wherein said elastic means comprises at least one spring acting between the first and second rings.
8. The tensioner as claimed in claim 7 , wherein the spring is a compression spring having end portions hooked to appendages of the respective first and second rings and arranged tangentially with respect to said rings.
9. The tensioner as claimed in claim 7 , wherein the at least one spring is a compression spring having a first end portion housed in a first housing integral with the first ring and a second end portion housed in a second housing integral with the second ring.
10. The tensioner as claimed in claim 9 , wherein the housings consist of radial appendages of the first and second rings and define circumferential channels closed, at circumferentially opposite ends, by radial walls defining shoulders for the first and second end portions of the spring.
11. The tensioner as claimed in claim 1 , wherein the base includes an annular collar having an axis coincident with the first axis, the first ring being rotatably mounted around the collar by means of a first bushing.
12. The tensioner as claimed in claim 11 , wherein the first ring has a cylindrical inner surface, coaxial to the first axis, which rotates around the first bushing, and an eccentric outer cylindrical surface coaxial to the second axis, said second ring being rotatable around the cylindrical outer surface of the first ring by means of a second bushing.
13. The tensioner as claimed in claim 12 , comprising at least a spring exerting an axial load on at least one of said bushings to dampen the rotation of said rings.
14. The tensioner as claimed in claim 13 , wherein the axial load acts on one of the two rings, the other ring being axially interposed between at least a portion of said ring and said base.
15. The tensioner as claimed in claim 13 , wherein the spring is axially interposed between the first and the second rings and exerts axial loads in opposite directions on said first and second rings, one of said first and second rings being pushed against the base and another of said first and second rings being pushed axially against a shoulder integral to the base.
16. The tensioner as claimed in claim 13 , comprising two springs exerting axial loads on the first and second rings, respectively.