Patent Description:
The accessory drive of an internal combustion engine generally comprises a pulley connected to the drive shaft, a pulley connected to the shaft of the electric machine and can comprise one or more pulleys for driving other accessories such as, for example, the conditioning system compressor. The accessory drive further comprises a belt for transmission of the movement between the above-mentioned pulleys and a tensioner configured to ensure a correct minimum tension level of the belt and prevent slipping between the belt and the pulleys.

In conventional accessory drives, in which the electric machine is an alternator driven by the engine, the tensioner acts on the slack span of the belt, namely the span located downstream of the engine and upstream of the alternator with reference to the belt movement direction.

In motor vehicles, a reversible electric machine is increasingly frequently used in place of the conventional alternator; said reversible electric machine can operate not only in the conventional generator mode, but also according to further modes, for example as a regenerative brake (recuperation condition), or as an additional motor operating in combination with the internal combustion engine (boost condition).

With the use of a reversible electric machine, the span of the belt which is taut in the operating conditions in which the electric machine is driven by the engine becomes the slack span when the torque is delivered by the electric machine.

Various solutions have therefore been devised which ensure correct tensioning of both spans of the belt.

One solution consists, for example, in using a tensioner with two arms hinged on a common pin and bearing respective pulleys. The arms are subject to the elastic force of a spring which tends to move them close to each other so as to maintain the pulleys in contact with respective spans of the belt. An example of this solution is described in <CIT>. The common axis of the two arms is arranged within the path of the belt.

The overall dimensions of the base on which the arms pivot and of the spring arranged around the common articulation axis of the arms are such as to make this solution unsuitable for applications in which space constraints exist within the path of the belt such as, for example, in the case of the drive having only two pulleys. Furthermore, the arrangement of the arms with respect to the resultant forces acting on the pulleys is not optimal.

Another solution consists in mounting the tensioner on the electric machine.

According to a known solution, the tensioner comprises a base configured to be fixed to the electric machine, a first annular element rotating with respect to the base around the axis of the electric machine and bearing a first pulley, and a second annular element rotating with respect to the base around the axis of the electric machine and bearing a second pulley.

A spring acts between the two annular elements configured to exert an elastic force between said elements in order to maintain the first and the second pulleys in contact with respective spans of the belt.

A drawback connected with the above-mentioned solution is the need to operate at a relatively high belt tension to allow optimal functioning in the recuperation and boost conditions.

<CIT> discloses a tensioner comprising a base configured to be fixed to the electric machine, an annular element rotatable with respect to the base around the axis of the electric machine and bearing a first pulley, and an arm hinged to the annular element and bearing a second pulley.

A problem connected with this solution is the difficulty of obtaining symmetrical characteristics in the positive and negative torque conditions of the electric machine.

The object of the present invention is to produce a tensioner for an accessory drive which is without the drawbacks connected with the known tensioners specified above.

The above-mentioned object is achieved by a tensioner for an accessory drive according to claim <NUM>.

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

With reference to <FIG>, the number <NUM> indicates an accessory drive of an internal combustion engine <NUM>.

The accessory drive <NUM> comprises a first pulley <NUM> connected to a drive shaft <NUM> of the engine E, with axis EA, a second pulley <NUM> connected to a shaft <NUM> of an electric machine <NUM>, with axis MA, and a belt <NUM> that connects the first pulley <NUM> and the second pulley <NUM> to each other. The accessory drive can comprise other pulleys, not illustrated, for driving other accessories of the engine <NUM> such as, for example, a conditioning system compressor.

The accessory drive <NUM> further comprises a tensioner <NUM> mounted on the electric machine <NUM> and comprising (<FIG>):.

The base <NUM> and the rings <NUM>, <NUM> have an inner diameter greater than the diameter of the second pulley <NUM> so as to allow assembly of the tensioner <NUM> on the electric machine <NUM> in the presence of the second pulley <NUM> (see <FIG>, in which the overall dimension of the pulley <NUM> is illustrated schematically by a dot-dash line).

The first ring <NUM> comprises a radial outer appendage <NUM> supporting in a rotating manner a first pulley <NUM> of the tensioner <NUM>, with axis PA1, by means of a pin <NUM> and a bearing <NUM>. The second ring <NUM> comprises a tubular axial appendage <NUM>, extending in a cantilever fashion from the opposite side of the flange <NUM> of the base <NUM>, on which a second pulley <NUM> of the tensioner <NUM>, with axis PA2, is rotatably mounted by means of a pin <NUM> and a bearing <NUM>.

The first and the second pulleys <NUM>, <NUM> are configured to cooperate with respective sections 8a, 8b of the belt arranged upstream and downstream respectively of the second pulley <NUM> according to the feed direction of the belt (clockwise, with reference to <FIG>).

According to the present invention, the second ring <NUM> rotates with respect to the first ring <NUM> around an axis A2 parallel to the axis A1 and distinct from it. The axis A2 is arranged inside the first ring <NUM> and orbits around the axis A1 when the first ring <NUM> rotates. For this purpose, the first ring <NUM> has a cylindrical inner surface <NUM> with axis A1, which rotates around the cylindrical portion <NUM> of the bushing <NUM>, and an eccentric cylindrical outer surface <NUM> with axis A2, which radially supports the bushing <NUM>.

The first ring <NUM> and the second ring <NUM> define respective housings <NUM>, <NUM> for a spring <NUM> having the purpose of generating an elastic force tending to maintain the pulleys <NUM>, <NUM> in contact with the belt <NUM> and therefore maintain, in use, a predefined tension level in said belt <NUM>.

The spring <NUM> (<FIG>) is an arc-shaped helical compression spring arranged circumferentially with respect to the rings <NUM>, <NUM>. The housings <NUM>, <NUM> consist of radial appendages of the respective rings <NUM>, <NUM> and house respective end portions 34a, 34b of the spring <NUM>. The housings <NUM>, <NUM> define respective circumferential channels <NUM> having U-shaped section and closed, on circumferentially opposite sides from each other, by respective radial walls <NUM> defining respective shoulders for the opposite ends of the spring <NUM>. Respective projections <NUM> for centring the spring <NUM> extend from the walls <NUM>. Inside the channels <NUM>, half-shells <NUM> made of plastic are housed; said half-shells house the spring <NUM>, so as to prevent direct contact between the spring and the housings <NUM>, <NUM>.

The first ring <NUM> is axially locked on the flange <NUM> of the base <NUM> by a disc spring <NUM> (<FIG>) axially compressed between an annular end edge <NUM> of the collar <NUM> of the base <NUM> and the first ring <NUM>. To avoid direct contact between the disc spring <NUM> and the first ring <NUM>, the spring <NUM> is provided with a coating made of plastic material <NUM> which covers the outer edge thereof.

The tubular appendage <NUM> of the second ring <NUM> is arranged inside a recess <NUM> (<FIG>) obtained on a periphery of the first ring <NUM> in order to limit the relative rotation between the rings <NUM>, <NUM> between a free arm position corresponding to the maximum longitudinal expansion of the spring <NUM> and a load stop position corresponding to a position of maximum compression of the spring <NUM>.

The first ring <NUM> has at the bottom a protrusion <NUM> (<FIG>) configured to slidingly engage an arched groove <NUM> of the flange <NUM> of the base <NUM>, so as to limit the angle of rotation of the first ring <NUM> with respect to the base <NUM>.

In the absence of reaction forces from the belt <NUM>, the spring <NUM> tends to maintain the rings <NUM>, <NUM> in the free arm position. In order to allow easy assembly of the belt <NUM>, prior to installation the rings <NUM>, <NUM> are locked to each other in a relative angular installation position by a locking pin <NUM> (<FIG>) which engages respective holes <NUM>, <NUM> thereof. The installation position is expediently near to the load stop position.

Once the belt has been installed, the pin <NUM> is removed and, under the action of the spring <NUM>, the tensioner goes to the nominal position illustrated schematically in <FIG>, in which the two pulleys <NUM>, <NUM> are in a symmetrical position with respect to the bisector line H of the winding angle θ of the belt <NUM> on the pulley <NUM>, coinciding with the direction of the resultant of the pull of the belt <NUM> on the pulley <NUM> in nominal conditions.

Operation of the tensioner <NUM> is as follows.

In normal operating conditions, the engine <NUM> delivers torque and the electric machine <NUM> is driven and operates as an alternator. In this condition, the span 8b of the belt is the taut span and the span 8a is the slack span.

With respect to the nominal position illustrated in <FIG>, the tensioner <NUM> rotates clockwise around the axis A1 as a result of the hubload transmitted by the taut span 8b to the pulley <NUM>. Under the thrust of the spring <NUM>, which tends to move the pulleys <NUM> and <NUM> close to each other, the pulley <NUM> acts on the slack span 8a maintaining in the same a pre-set minimum tension value as the torque varies.

In boost mode, the electric machine <NUM> delivers motive power (positive torque) which is added to that of the engine <NUM>. This tends to reduce the tension in the span 8b and to increase the tension in the span 8a of the belt. In the recuperation mode, on the other hand, the electric machine <NUM> absorbs mechanical power (negative torque), and therefore the tension in the span 8a of the belt <NUM> tends to decrease.

The use of a rotation axis A2 of the second ring <NUM> distinct from the axis A1 of the first ring (coincident in use, as said, with the axis MA of the electric machine <NUM>) allows for reduction of the installation tension of the belt <NUM>, with the torque transmission capacity in the slack span (understood as the span which is slack each time according to the operating conditions).

<FIG> is a graph that illustrates different positions of the axis A2 with respect to the axis A1 (where the X and Y axes represent the coordinates in mm measured from the axis of the drive shaft), indicated as N1-N10. A1 indicates a comparative example in which the axis A2 coincides with the axis A1 of the electric machine <NUM> (coordinates <NUM>;<NUM> with respect to the axis of the drive shaft).

<FIG> is a graph that represents, for the points of the graph of <FIG>, the trend of the tension in the slack span of the belt <NUM> when the torque of the electric machine <NUM> varies (negative torque values identify the recuperation mode, and positive values identify the boost mode).

The torque value of <NUM> corresponds to the belt installation tension, the same for all the examples (<NUM> N). The line A1 represents also in this case the comparative example in which the axes A1 and A2 coincide.

Given the same installation tension, the examples N1 and N5-N10 determine an increasingly higher tension of the slack span with respect to the comparative example A1, while in examples N2, N3 and N4 the tension of the slack span is lower than the comparative example at least in one of the recuperation and boost conditions. Of the positive examples, N10 is the best as it presents a symmetry of the tension curves in recuperation and boost conditions (as can be seen from the graph of <FIG>, the tension values at the extremes of the curve torque values, equal to +/- <NUM>, are substantially the same).

The increase in tension in the slack span can be exploited to lower the installation tension of the belt.

<FIG> illustrates, for the example N10 of <FIG>, a condition in which the installation tension has been lowered by 50N with respect to the comparative example (<NUM> N instead of <NUM> N). From an examination of the figure, it is easy to see that in a normal operating range, from approximately -<NUM> to +<NUM>, in which there are no risks of slipping, the tension remains lower than the reference example; this entails a reduction in losses due to friction and therefore a reduction in consumption.

In the regions with high torque (><NUM> in module), on the other hand, where problems of slipping can occur, the drive becomes more rigid than when axes A1, A2 coincide, and the torque transmission capacity is improved.

In order to clarify the incidence factors of the position of A2 compared to A1 on the balance of the tension of the slack span in the recuperation and boost conditions, <FIG> shows various further examples N11-N16 corresponding to positions of the axis A2 situated at equal distance from the axis A1. Namely, the points N11-N16 are on a circumference with centre A1 (axis A2 coincident with A1). Different behaviours correspond to these points, given the same distance A1-A2 (<FIG>), from which it is deduced that the key factor is not the distance between A1 and A2.

It has been experimentally verified that the determining factor to obtain symmetric behaviour of the tensioner <NUM> in the positive and negative torque conditions is the angle formed between the plane identified by the axes A1-A2 and the plane containing the axis A1 and the bisector H of the winding angle θ of the belt <NUM> on the pulley <NUM> of the electric machine <NUM> (<FIG>) in nominal conditions. It should be noted that in the symmetric layout of the drive <NUM> with only two pulleys illustrated in <FIG>, the bisector H intersects the axis of the drive shaft EA, but this condition does not generally occur.

The optimal angle α varies as the winding angle θ varies and is expressed by the relation determined experimentally: <MAT> where α and θ are expressed in degrees (°) and c is a variable ranging from +<NUM>° to -<NUM>°.

The value c=<NUM> corresponds to perfect symmetry of the curves of <FIG> or <FIG> (tension as a function of the torque) in the positive and negative torque conditions. The above linear relation is shown in <FIG> for the value c=<NUM>.

The extremes of the variability interval of c are calculated based on accepted dissymmetry values in the above-mentioned curves equal to <NUM>% of the installation tension. In particular, for the example N10, with an installation tension of <NUM> N an imbalance of <NUM> N is obtained for c=+<NUM>° and <NUM> N for c=-<NUM>°, both values being lower than <NUM> (<NUM>% of the installation tension).

Surprisingly, said optimal angle is independent of both the diameter of the pulley <NUM>, and the layout of the drive.

Since the pulley system is symmetric with respect to the bisector line H of the winding angle θ, and since the resulting system of forces is symmetric, the plane P can be positioned indifferently on one side or the other of the line H (namely towards the pulley <NUM> or towards the pulley <NUM>), forming with it in each case an angle α. In other words, two tensioners having respective axes A2 positioned on planes P arranged on opposite sides of the line H but forming with it the same angle α have identical behaviour.

The optimal position of the plane P defined above refers to the nominal position of the tensioner.

<FIG> are partial schematic sections that illustrate alternative solutions for the axial and radial support of the rings <NUM>, <NUM> on the base <NUM>. Said solutions are illustrated by using the same numbers to indicate parts identical or corresponding to parts already described with reference to <FIG>, and refer to the detail highlighted in <FIG>. For the sake of brevity, the descriptions of the support bushings are omitted, but they must in any case be interposed axially and/or radially whenever there is relative slipping between the rings <NUM>, <NUM> and with respect to the base <NUM> and the spring <NUM>, in order to avoid premature wear and to control damping of the oscillations.

In the solution of <FIG>, the disc spring <NUM> acts between a shoulder <NUM> integral with the first ring <NUM> and the second ring <NUM>, exerting axial loads on them in opposite directions. In this way the first ring <NUM> is pushed axially against a shoulder <NUM> integral with the base <NUM> and the second ring <NUM> against the base <NUM>.

In the solution of <FIG> there are two disc springs 43a, 43b which act between a shoulder <NUM> integral with the base <NUM> and the respective rings <NUM>, <NUM>. In this way it is possible to independently control damping of the rotation of the first ring <NUM> and damping of the rotation of the second ring <NUM>.

In <FIG> the first ring <NUM> is axially supported by the second ring <NUM>, and the disc spring <NUM> acts between the fixed shoulder <NUM> integral with the base <NUM> and the first ring <NUM>. In this case, therefore, the first ring <NUM> and the second ring <NUM> are arranged in series with respect to the axial load of the spring <NUM>.

In the solution of <FIG> there are two disc springs 43a, 43b, one of which acts between the shoulder <NUM> integral with the base <NUM> and the first ring <NUM>, and the other between a shoulder <NUM> integral with the first ring <NUM> and the second ring <NUM>.

Lastly, <FIG> illustrates a solution in which the spring <NUM> acts on the first ring <NUM>, which rests simultaneously on the base <NUM> and on the second ring <NUM>.

The solution adopted has an impact on the possibilities of controlling damping of the rotations of the first and the second rings <NUM>, <NUM>, but does not vary the general operation of the tensioner previously described.

<FIG> and <FIG> illustrate a tensioner <NUM> which is described below only insofar as it differs from the tensioner <NUM> described, using the same reference numbers to distinguish parts identical or corresponding to parts already described.

The tensioner <NUM> differs from the tensioner <NUM> due to the fact that the spring <NUM> is a helical traction spring arranged tangentially with respect to the first and the second rings <NUM>, <NUM> and having respective hook-shaped ends 34a, 34b hooked to respective pegs <NUM>, <NUM> integral with the first ring <NUM> and with the second ring <NUM> respectively and extending axially from respective external radial appendages <NUM>, <NUM> thereof.

The positions of the pulley <NUM> carried by the first ring <NUM> and the pulley <NUM> carried by the second ring are reversed, with respect to the tensioner <NUM>, since the traction spring <NUM> (instead of the compression spring as in the tensioner <NUM>) determines a relative rotation in the opposite direction between the two rings <NUM>, <NUM>. The effect of the spring <NUM> is in any case always that of generating an elastic force tending to maintain the pulleys <NUM>, <NUM> in contact with the belt <NUM> and therefore to maintain, in use, a predefined tension level in the belt <NUM>.

From an examination of the tensioners <NUM>, <NUM> produced according to the invention, the advantages it offers are evident.

In particular, due to the use of tensioner pulleys carried respectively by a first ring rotating around a first axis and by a second ring rotating with respect to the first ring around a second axis eccentric with respect to the first axis, it is possible to reduce the installation tension of the belt, given the same torque transmission capacity, without substantial increases in overall dimensions and cost of the tensioner.

Claim 1:
A tensioner for an accessory drive of an internal combustion engine (<NUM>), the drive (<NUM>) including at least a first pulley (<NUM>) connected to a drive shaft (<NUM>) of the engine (<NUM>), at least a second pulley (<NUM>) connected to an electric machine (<NUM>) and a belt (<NUM>) wound at least on the first and second pulleys (<NUM>, <NUM>), the tensioner (<NUM>) including:
- a base (<NUM>) configured to be fixed to a casing (<NUM>) of the electric machine;
- a first ring (<NUM>) rotating with respect to the base (<NUM>) about a first axis (A1); and
- a first tensioning pulley (<NUM>) carried by the first ring (<NUM>) and rotating with respect to the first ring (<NUM>) about its own axis (PA1);
characterized by comprising
- a second ring (<NUM>) rotating with respect to the first ring (<NUM>) about a second axis (A2) distinct from the first axis (A1);
- a second tensioning pulley (<NUM>) carried by the second ring (<NUM>) and rotating with respect to the second ring (<NUM>) about its own axis (PA2); and
- elastic means (<NUM>) acting on the first and second rings (<NUM>; <NUM>) to push the first and second pulleys (<NUM>, <NUM>) into contact with the respective spans (8a; 8b) of the belt.