TECHNICAL FIELD

The present invention relates to a tensioner for an accessory drive of an internal combustion engine, and in particular to a tensioner designed to be mounted directly on a reversible electric machine or starter/generator associated with an engine with "start-stop" functionality. BACKGROUND ART

In accessory drives, the various accessories, including the generator, are operated by a common drive belt driven by the drive shaft. In traditional drives, an alternator that is always driven by the engine is used as a generator; thus, the taut side and the slack side of the belt are determined unequivocally and a tensioner acting on the slack side is used to keep the latter at a predetermined minimum tension level.

In order to reduce consumption and emissions, a reversible electric machine or starter/generator, which has both the function of a generator and of an electric motor, is used instead of a conventional alternator. This enables providing new functions such as automatically switching off the engine when stationary (start-stop) and/or an increase in driving torque (boost) .

In this case, the slack side of the belt varies according to whether the starter/generator behaves as a motor or as a generator. A tensioner is thus needed that is able to act on both sides of the belt, according to the working phases of the starter/generator .

The tensioners, especially in the above-described application, are subjected to high-amplitude oscillations under certain engine operating conditions, such as when starting up for example .

In these conditions, it is necessary that the oscillations are dampened and for this purpose it is necessary to use devices such as, for example, friction bushes or hydraulic dampers, capable of providing high damping torques.

However, the damping characteristics of these devices are not optimized for engine running conditions.

DISCLOSURE OF INVENTION

The object of the present invention is a tensioner comprising an improved damping device capable of solving the aforesaid problem in an inexpensive and compact manner.

The aforesaid object is achieved by a tensioner comprising an improved damping device according to claim 1. BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, a preferred embodiment is described below, by way of non- limitative example and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a tensioner with two degrees of freedom comprising a damping device according to the invention;

Figure 2 is a partial axial section of the tensioner in Figure 1;

Figure 3 is a section along line III-III of the tensioner in Figure 2 ;

Figure 4 is a perspective view of the damping device according to the invention;

Figure 5 is a graph showing the damping torque as a function of the relative rotation of elements of the tensioner in Figure 1 ;

Figure 6 is a perspective view of a tensioner with one degree of freedom comprising a damping device according to the invention;

Figure 7 is a partial axial section of the tensioner in Figure 6 ; and

Figure 8 is a section along line VIII-VIII of the tensioner in Figure 7.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to Figures 1 to 3, a starter/generator 1 is shown that is fastened to an engine block (not shown) and comprises a casing 2 housing a stator and a rotor (not shown) and a drive pulley 3 fitted on a shaft integral with the rotor (also not shown) . The pulley 3 is designed to be coupled to a drive belt 4 (not shown) .

A tensioner 6 with two degrees of freedom is mounted on a support structure 5 that is fastened to the casing 2. The structure 5 has two arms 7 and 8, designed to be fastened to the casing 2 of the starter/generator and downwardly converging to each other, and an L-shaped bracket 9 carried in a cantilevered manner by the arms 7 and 8. More specifically, the two arms 7 and 8 have respective proximal ends connected to the casing 2 of the starter/generator 1 by screws, and respective distal ends joined together. The bracket 9 has a wall 11 parallel to the axis A of the pulley 3 and connected to the distal ends of the arms 7 and 8, and a wall 12 perpendicular to wall 11 and extending upwards from the latter in a position facing the pulley 3 and axially spaced apart therefrom. The tensioner 6 is mounted in a rotationally free manner, by means of a bearing 14, on a pin 13 that projects from the wall 12 towards the pulley 3 and is coaxial therewith. The tensioner 6 comprises a body 15 formed by an annular portion 16 rotationally coupled to the pin 13 by means of the bearing 14 and an elongated support 17, arranged tangentially with respect to the annular portion 16 and integrally fastened thereto at its ends 18 and 19 by a pair of arms 21 and 22, substantially radial with respect to the annular portion 16.

The tensioner 6 also comprises a first pulley 23 and a second pulley 24 carried by the support 17 as described hereinafter. The first pulley 23 is hinged in a fixed position to end 18 of the support 17; the second pulley 24 is hinged to a block 25 movable along a guide portion 26 of the support 17 adjacent to end 19. Pulleys 23 and 24 cooperate from opposite ends with respective portions of the belt 4 and are respectively arranged downstream and upstream of pulley 3.

The support 17 also comprises a cavity 27 adjacent to end 18 and separated from the guide portion 26 by an intermediate transverse separator 28. The guide portion 26 and the cavity 27 are closed at the front by a cover 29 (shown with a broken line in Figure 1) .

In one of the possible constructional variants, the block 25 is slidingly mounted on bars 31 fixed between end 19 and the separator 28 of the support 17. The separator 28 has an opening 32 in which a quadrangular shaped bush 33 is inserted.

A piston 34 is slidingly housed with a predetermined clearance in the cavity 27 and connected to one end of a square- sectioned rod 35 mounted passing through the bush 33 and prismatically coupled thereto. At its opposite end, the rod 35 is fastened to the block 25. Springs 36 designed to push the piston 34 towards the end 18 of the support 17 are interposed between the piston 34 and the separator 28 so as to pull pulley 24 towards pulley 23 and in this way exert the tensioning force on the belt 4.

The cavity 27 contains a viscous fluid. Thus, with the walls of the cavity 27, the piston 34 defines a viscous dynamic damper.

The pulleys 23 and 24, of which one is carried by the body 15 and the other is movable with respect to the first along the guide 26, transmit tensioning forces to the belt 4 via the springs 36. Variations in tension in the belt 4 become relative linear movements between the pulleys 23 and 24. Any oscillations are dampened by the interaction between the piston 34 and the fluid in the chamber 27. The use of a linear guide enables providing this damping between the two pulleys 23 and 24 in a simple and reliable manner.

The tensioner 6 comprises a damping device 60 designed to dampen the relative oscillations between the body 15 and the pin 13. The damping device 60 comprises a first damper 61 with constant damping and a second damper 62 with variable damping.

The first damper 61 comprises a Belleville spring 63 axially interposed between wall 12 and a friction ring 64 carried by the body 15.

The second damper 62 is housed on a radial protrusion 65 of the pin 13 and substantially comprises a C-shaped bush 66 frictionally cooperating with a radial protrusion 65 and an open metal ring 67 radially force-fitted on the bush 66 and rotationally coupled thereto by means of a pair of radial projections 68 that engage corresponding holes in the ring 69 (Figure 7B) .

At its ends, the ring 67 comprises respective radially external lugs 70 housed inside a circumferential seat 71 of the body 15 angularly delimited by respective walls 72 and 73, with which the lugs 70 are designed to cooperate as described hereinafter .

The damping between the body 15 and the pin 13 in the above- described case is schematized in Figure 5. A first damping Dl, given by damper 61, acts independently of the angle and direction of relative rotation of the body 15 with respect to the pin 13. This damping is sized so as to optimize the oscillation filtering function under engine running conditions.

Only when an angle Gl equal to the angular clearance between the lugs 70 and the respective walls 72 and 73 is reached, does the second damping D2 due to the second damper 62 come into action, thereby achieving an overall damping D1+D2 sized for the operating conditions that cause large angular displacements of the body 15, and in particular start-stop, boosting and regeneration conditions. Damping D2 is due to the increase in the radial load exerted by the ring 67 on the bush 66 resulting from the contact of one of the lugs 70 with the respective stop wall 72 or 73 and the consequent radial contraction of the bush 66. With reference to Figures 6 to 8, a tensioner 78 with one degree of freedom is shown that comprises an improved damping device according to the invention. As in the previous embodiment, the tensioner 78 comprises a body 15 connected to an alternator/generator, not shown here, by means of a support structure 5.

In this case, the body 15 comprises a circular-shaped central part 79, supported to freely rotate on the pin 13 and coaxial with axis A. The tensioner 78 also comprises two curved arms 80 and 81 extending from opposite ends of the central part 79 on which they have a common origin. The distal ends of the arms 80 and 81 with respect to the central part 79 carry, in a rotationally free manner, respective pulleys 23 and 24 cooperating with the belt 4. In this case, the tensioner therefore has only one degree of freedom, given by the rotation of the body 15 about the pin 13, while the pulleys 23 and 24 have fixed axes with respect to the body 15. As in the previous embodiment, the tensioner 78 comprises a first and a second damper 61 and 62 respectively providing a fixed damping and a variable damping between the body 15 and the support structure 5. As in the previous embodiment, the damper 62 is arranged on a radial protrusion 65 of the pin 13 and the protrusions 70 of the ring 67 cooperate with the lateral walls of an opening 71 made in the central part 79 of the body 15 (Fig. 8) . The damper 61 comprises a Belleville spring 63 axially interposed between the central body 79 of the tensioner 78 and a friction ring 64 carried by a cover 82 integral with the hub 13 (Fig. 7) . In this case, the pulleys 23 and 24 have fixed axes with respect to the body 15 and so any variations in tension on the belt 4 instantaneously turn into rotations of the body 15 on the hub 13. As no damper is present between the pulleys 23 and 24, the only damping force provided is that of damper 60. The damping acting between the pin 13 and the body 15 can again be described by referring to the diagram in Figure 5. A first damping Dl, given by damper 61, acts independently of the angle and direction of relative rotation of the body 15 with respect to the pin 13. This damping is sized so as to optimize the oscillation filtering function under engine running conditions .

Only when an angle Gl equal to the angular play between the lugs 70 and the respective walls 72 and 73 is reached, does the second damping D2 due to the second damper 62 come into action, thereby achieving an overall damping D1+D2 sized for the operating conditions that cause large angular displacements of the tensioner 78, and in particular start- stop, boosting and regeneration conditions.

Damping D2 is due to the increase in the radial load exerted by the ring 67 on the bush 66 resulting from the contact of one of the lugs 70 with the respective stop wall 72 or 73 and the consequent radial contraction of the bush 66.

The advantages that can be achieved are evident from examination of the characteristics of the tensioners 6 and 78 according to the present invention.

The use of a damper 62 that is only activated upon exceeding predetermined angles of rotation of the tensioner 6 with respect to the hub 13 enables achieving a damping torque of varying intensity according to the operating conditions, and therefore an optimized damping value under all conditions.

Furthermore, the damper is compact, simple and inexpensive.

Finally, it is clear that modifications and variants can be made to the tensioner 6 without departing from the scope defined in the claims.

For example, the damping system acting between the pulleys 23 and 24 of the tensioner 6 could be modified.