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 .

Tensioners of the above-mentioned are known, for example, from EP 2128489 or EP 2487295 or WO 2014100894. In these implementations, the tensioner comprises a support rotatably mounted on the body of the starter/generator and two arms (or rotating elements with the equivalent function) carried by the support, which carry pulleys that cooperate with the belt and provide the tensioning force necessary in both phases of operation of the starter/generator.

To avoid dynamic problems, the tensioners must normally be provided with oscillation damping means. A problem related to the above-described solutions is the difficulty in providing this damping in a controlled and reliable manner.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a tensioner for an accessory drive that does not have the above-mentioned problem.

This object is achieved with a linear tensioner 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 1A is a perspective view of a first embodiment of a linear tensioner according to the invention, mounted on a starter/generator ;

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

Figure 2 is a perspective view of a second embodiment of a linear tensioner according to the invention;

Figure 3 is a perspective view of a third embodiment of a linear tensioner according to the invention;

Figure 4 is a perspective view of a fourth embodiment of a linear tensioner according to the invention;

Figure 5 is a perspective view of a fifth embodiment of a linear tensioner according to the invention; Figure 6A is a perspective view of a sixth embodiment of a linear tensioner according to the invention, mounted on a starter/generator ;

Figure 6B is a partial section of the tensioner in Figure 6A; Figure 7A is a longitudinal sectional view of a seventh embodiment of a linear tensioner according to the invention; Figure 7B is a section along line VII-VII of the tensioner in Figure 7A.

Figure 8 is a functional diagram of the tensioner in Figures 7A-7B.

Figure 9A is a functional diagram of the tensioner in Figure 1;

Figure 9B illustrates a first variant of the diagram in Figure 9A;

Figure 9C illustrates a second variant of the diagram in Figure 9A; and

Figure 9D illustrates a third variant of the diagram in Figure 9A. BEST MODE FOR CARRYING OUT THE INVENTION

With reference to Figures 1A and IB, 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 (schematically shown Figures 9A - 9D) .

A linearly guided tensioner 6 according to the present invention 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 with reference to the direction of travel of the belt (Figure 9a) . 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 1A) .

In a first constructional variant (Figures 1A-1B) , 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 .

In a second constructional variant (Figure 2), the block 25 is connected to the piston 34 by two bars 31 mounted to pass through respective bushes 33 housed in the separator 28. In this case, the springs 36 are arranged about the respective bars 31. A third constructional variant (Figure 3) differs from that of Figures 1A and IB in that the separator 28 has two appendages 37 projecting parallel to the rod 35 in the chamber 24 and supporting, at their free ends, a further bush 38 in which the rod 35 slides and defining a further support thereof.

A fourth constructional variant (Figure 4) differs from that of Figure 2 in that a single centrally located spring 36 is used. In a similar manner to the constructional variant in Figure 3, additional support bushes 38 are provided for the bars 31. These bushes 38 are each supported by an appendage 37 axially projecting from the separator 28 and by a respective lateral wall 39 of the cavity 27.

In a fifth constructional variant (Figure 5), the viscous damper is replaced by a pneumatic cylinder 41 having a barrel 42 constrained to end 18 of the support 17 and a rod 43 constrained to the block 25. In this case, the springs 36 are interposed between the block 25 and end 19 of the support 17, and are coaxial with the guide bars 31 on which the block 25 slides .

The functioning of the above-described embodiments is as follows .

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 (Figures 1A-4) or by the pneumatic cylinder 41 (Figure 5) . The use of a linear guide enables providing this damping in a simple and reliable manner . This damping could also be implemented by means of friction damping, always in the linear guide between the pulleys 23 and 24.

In a sixth constructional variant (Figures 6A and 6B) of the tensioner 6, the body 15 is a single plate integrally forming the annular portion 16 and the support 17.

The annular portion 16 is designed to be fitted with the interposition of a first friction bearing 45 on an annular portion 46 of a flange 47 fastened to the casing 2 of the starter/generator 1. A ring nut 48 locks annular portion 16 onto annular portion 46 of the flange 47. A further friction bearing 50 is interposed between the ring nut 48 and annular portion 46 so as to provide, in combination with bearing 45, an axially fixed and angularly free support for the body 15 avoiding direct contact and consequent rubbing between the latter, the flange 47 and the ring nut 48.

With respect to the previous embodiments, the embodiment in Figures 6A-6B introduces damping in the relative rotation of the body 15 with respect to the pin 13 by means of bushes 45 and 50. This damping is constant for any value of relative angular rotation between pin 13 and body 15 and does not depend on the direction of rotation. A seventh variant of the tensioner 6 is shown in Figures 7A- 7B, which 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 acting between the body 15 and the pin 13 in the above-described case is schematized in Figure 8. A first damping Tl, 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 T2 due to the second damper 62 come into action, thereby achieving an overall damping T1+T2 sized for the operating conditions that cause large angular displacements of the body 15, and in particular start-stop, boosting and regeneration conditions. Damping T2 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 operation of the tensioner 6 in Figure 1A, in which pulleys 23 and 24 are respectively arranged downstream and upstream of pulley 3 along the path of the belt, is schematized in Figure 9A. Pulley 23 is integral with the body 15 and pulley 24 is connected to it by a "damper-spring" system of rigidity K in series with a damping C.

In the variant in Figure 9B, the damping C is still in series with the rigidity K between pulleys 23 and 24, but pulley 24 is fixed with respect to the body 15 while pulley 23 is movable. Compared to the variant in Figure 9A, in the variant in Figure 9C, the damping C and the rigidity K are placed in parallel between pulleys 23 and 24. In the variant in Figure 9D, pulley 24 is integral with the body 15 as in the variant in Figure 9D, but the damping C and the rigidity K are arranged parallel to each other, as in the variant in Figure 9C. In the embodiment in Figure 7A-7B, this damping also has a variable component that is only active in the event of large oscillations of the body 15 with respect to the pin 13.

The advantages that can be achieved are evident from examination of the described characteristics of the tensioner 6.

The body 15 designed to be rotatably mounted on a starter/generator 1 and configured so as to support pulleys 23 and 24 enables a relative linear movement between these pulleys, making the use of a hydraulic/pneumatic type of damper both possible and effective. This enables controlling the damping in a simple, reliable and constant manner over time .

The constructional variants shown can be easily mounted on the starter-generator 1 as preassembled units.

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