ACCESSORY DRIVE BELT TENSIONER

Abstract

The present invention relates to an accessory drive belt tensioner for use in an engine for motor vehicles, comprising a base, a tensioner arm and a pulley mounted on the tensioner arm, wherein the base and the tensioner arm are arranged in a pivotal arrangement, the base and the tensioner arm are tension loaded and the pivotal arrangement confines a space being aimed for receiving a generator pulley, wherein the tensioner arm is made of a specific thermally conductive plastic composition; comprises cooling fins, the cooling fins being positioned in the vicinity of, or at least partially inside the space confined by the pivotal arrangement; and comprises a bearing surface part, for sliding contact with the base (A) within the pivotal arrangement, consisting of metal.

Claims

1. Accessory drive belt tensioner, comprising a base, a tensioner arm and a pulley mounted on the tensioner arm, wherein the base and the tensioner arm are arranged in a pivotal arrangement, the base and the tensioner arm are tension loaded and the pivotal arrangement confines a space being aimed for receiving a generator pulley, wherein the tensioner arm a) is made of a plastic composition comprising: a thermoplastic polymer; a fibrous reinforcing agent and a thermally conductive filler; and having a tensile modulus at 140 C., measured by the method according to ISO 527-1/2, of at least 8 GPa; and a thermal conductivity () at 23 C., measured in plane in parallel direction by the method according to ASTM E-1461-01, of at least 1.0 W/mK; b) comprises cooling fins, the cooling fins being positioned in the vicinity of, or at least partially inside the space confined by the pivotal arrangement; and c) comprises a bearing surface part, for sliding contact with the base within the pivotal arrangement, consisting of metal.

2. Drive belt tensioner according to claim 1, wherein the plastic composition comprises: 30-70 wt. % of a fibrous reinforcing agent, wherein the fibrous reinforcing agent comprises carbon fibers in an amount of X*(30-60 wt. %) and/or glass fibers in an amount of Y*(40-70 wt. %), relative to the total weight of the composition, wherein X is the weight fraction of carbon fibers and Y is the weight fraction of glass fibers, relative to the total weight of fibrous reinforcing agent; and 1-10 wt. % of a thermally conductive filler, relative to the total weight of the composition.

3. Drive belt tensioner according to claim 1, wherein the plastic composition has a volumetric electrical resistivity (P) of at most 10.sup.8 Ohm*m, measured at 23 C. by the standardized test method according to ICE 60093.

4. Drive belt tensioner according to claim 1, wherein the thermally conductive filler comprises expanded graphite.

5. Drive belt tensioner according to claim 1, wherein the plastic composition has creep modulus (Ec) at 140 C., measured by the method according to ISO 899-1 at a load at 50 MPa and a loading time of 1000 hours, of at least 6 GPa.

6. Drive belt tensioner according to claim 1, wherein the cooling fins are at least partly positioned inside the space formed by the pivotal arrangement and aimed for receiving a generator pulley, while leaving a clearance between the fins and the generator pulley.

7. Drive belt tensioner according to claim 1, wherein the mathematical product of Z*W.sup.1/2, wherein W is the weight of the tensioner arm in grams and Z is the surface area of the cooling fins in cm.sup.2, is at least 500 (g.sup.1/2cm.sup.2).

8. Drive belt tensioner according to claim 1, wherein the tensioner arm comprises an inner bearing surface part consisting of metal, and wherein the inner bearing surface part is at least partly overmolded with the plastic composition.

9. Drive belt tensioner according to claim 8, wherein the inner bearing surface part consisting of metal has an average thickness of at least 1 mm.

10. Drive belt tensioner according to claim 1, wherein the base has an outer bearing surface part made of a plastic material.

11. Drive belt tensioner according to claim 1, wherein the base is made of a plastic composition different than the plastic composition of the tensioner arm.

12. Drive belt tensioner according to claim 10, wherein a part of the plastic composition of the base is overmolded by a layer of plastic material.

13. Drive belt tensioner according to claim 9, wherein the material of the inner bearing surface part has a thermal conductivity (.sub.1), the material of the outer bearing surface part has a thermal conductivity (.sub.2), and the material of the base has a thermal conductivity (.sub.3), and wherein (.sub.1)>(.sub.2) and (.sub.2)>(.sub.3).

14. Drive belt tensioner according to claim 13, wherein the thermal conductivity (.sub.1) of the material of the inner bearing surface part amounts to more than 50 W/mK, wherein the thermal conductivity (.sub.2) of the material of the outer bearing surface part amounts to less than 1.0 W/mK, and wherein the thermal conductivity (3) of the plastic composition of the base is less than 2.0 W/mK and more than 0.5 W/mK.

15. Accessory belt drive system, comprising at least one drive belt tensioner according to claim 1, at least one accessory with a drive pulley as well as a drive belt for driving the drive pulley.

16. Accessory belt drive system according to claim 15, wherein the base and the tensioner arm of the drive belt tensioner are arranged in a pivotal arrangement, wherein the drive pulley is centrally positioned inside the pivotal arrangement.

17. Accessory belt drive system according to claim 15, wherein the accessory belt drive system is an alternator start stop belt tensioner system having two drive belt tensioners, and comprising an alternator pulley as the generator pulley centrally positioned inside the pivotal arrangement.

Description

[0066] The invention is illustrated with the following figures.

[0067] FIG. 1 a drive belt tensioner according to the invention in an exploded view in a first perspective from diagonally above;

[0068] FIG. 2 the drive belt tensioner of FIG. 1 in a second perspective view from diagonally below;

[0069] FIG. 3 the drive belt tensioner of FIG. 1 in the mounted condition in a perspective view from diagonally above;

[0070] FIG. 4 the drive belt tensioner of FIG. 1 in the mounted condition in a perspective view from diagonally below;

[0071] FIG. 5 the drive belt tensioner of FIG. 1 in a longitudinal sectional view;

[0072] FIGS. 1 to 5, which are described jointly below, show a drive belt tensioner 2 according to the invention that can also be referred to as belt tensioning device. The drive belt tensioner 2 comprises a base member 3 which can be attached to a stationary component such as an accessory (not shown), a tensioner arm 4 which is pivotably supported relative to the base member 3 by a bearing arrangement 5 around a pivot axis A, and a spring element 6 which resiliently supports the tensioner arm 4 against the base member 3 in circumferential direction. For mounting the base member 3 this has three flange portions 11, projecting radially outwards and having bores, through which screws can be passed for attaching on the stationary component. The tensioner arm 4 carries at a free end portion a tensioning roller 7, which is rotatable around an axis of rotation B arranged parallel to the pivot axis A. The tensioning roller 7 is rotatably supported on a reinforcing element 8 of the tensioner arm 4 and is attached thereto by means of a screw 9. Furthermore, a disc 10 is provided axially next to the tensioning roller 7, which protects the bearing 12 against penetrating dirt. The tensioner arm 4 is axially and radially supported relative to the base member 3 via the bearing arrangement 5 so as to be rotatable around the pivot axis A. The tensioner arm 4 is connected via a connection mechanism 13 to the base body 3. The tensioner arm 4 is arranged at least approximately in a plane with the bearing arrangement 5, so that the axial size of the device is small.

[0073] The spring element 6 is formed as a helical spring, wherein a spring center line or spring axis extends essentially parallel to the pivot axis A. A first spring end 16 of the helical spring 6 is bent radially outward and is supported on a corresponding abutment face 17 of the base member 3 in circumferential direction. The opposite second spring end 23 of the helical spring 6 is also bent radially outward and is supported on a corresponding abutment face 24 of the tensioner arm 4 in circumferential direction. The helical spring 6 effects a spring-loaded tensioning of the tensioner arm 4 relative to the base member 3, so that the belt of the belt drive is pre-tensioned.

[0074] The helical spring 6 is arranged coaxially outside of the bearing arrangement 5 for supporting the tensioner arm 4. The helical spring 6 and the bearing arrangement overlap at least with partial portions in axial direction, to keep the design space small in axial direction. It can be seen especially in FIG. 5, that the helical spring 6 has a proportionally large diameter in relation to the axial length. The number of turns is larger than one and smaller than two. Preferably, the circumferential extension of the helical spring 6 is between 540 and 690. The ratio of nominal diameter D6 of the helical spring 6 to the axial length L6 is, in the mounted condition of the helical spring, in which the helical spring is axially pre-tensioned, between 3.0 and 9.0, especially between 5.0 and 8.0. It is understood, that said values are not limiting and that other values can be used. Within said ranges all intermediate ranges can be considered. In principle, alsodepending on the design space conditionslarger values than 9.0 can be used, whereby the spring would then be in relation to the diameter extremely short in axial direction. Furthermore, it is obvious, that the named ratio of spring diameter to the axial length in the mounted condition is also dependent on the wire diameter of the spring wire. The larger the wire diameter the smaller the axial length of the helical spring 6 can be selected.

[0075] The drive belt tensioner 2, respectively the tensioner arm 4 has a through opening 18, which is arranged coaxially to the longitudinal axis A. In this manner, the base member 3 can be screwed to an accessory in a simple manner, wherein an end of the drive shaft can also extend into the through opening 18, if necessary. Overall, an axially very short design is achieved. At least in a portion of the through opening 18, a smallest inner diameter D18 of the through opening 18 is preferably larger than an outer diameter of the drive shaft (not shown) and is especially also larger than an outer diameter of the belt pulley (not shown) connected to the drive shaft. The base member 3 has an annular portion 25 for supporting the tensioner arm 4. From the annular portion 25 a flange portion extends radially outward, which serves as axial support face 21 for the helical spring 6. Several attachment portions 11 project radially outward from the flange portion, each attachment portion having a respective bore for attaching the base member 3 on the stationary component. The attachment portions 11 are arranged on a larger diameter relative to the flange portion and relative to the helical spring 6. Thus, forces and moments acting on the base member 3 can be supported well and transferred into the stationary component.

[0076] The helical spring 6 is inserted with axial pre-tension between the support face 21 of the base member 3 and an axial opposite support face 22 of the tensioner arm 4. In this manner, the tensioner arm 4 is loaded axially away from the base member 3, wherein said components are axially supported on each other via the connection mechanism. The support face 21 for the spring 6 extends via a circumferential partial portion of the base member 3. At least a partial portion of the support face 21 is arranged in a plane, which has an axial overlap with the drive shaft. The support face 21 of the base member 3 has in circumferential direction the shape of a ramp, which is adapted to the incline of the helical spring 6. It can be seen especially in FIG. 1, that the support face 21 of the base member 3 is formed by circumferentially distributed and radially extending ribs on which the spring 6 is axially supported in the mounted condition. Between the ribs respectively recesses 20 are formed, which prevent a mass accumulation in this area. Incidentally, the ribs affect in an advantageous manner a good transfer of frictional heat, which is produced during the operation of the drive belt tensioner.

[0077] In the embodiment disclosed herein, the drive belt tensioner 2 is formed such, that the bearing arrangement 5 of the tensioner arm 4 is arranged on the base member 3 in view of the accessory 28 behind the belt plane. The belt plane is the plane, which is formed by the belt center in the mounted condition. The bearing arrangement 5 comprises at least one first bearing element 30 which is associated to the base member 3, and a second bearing element 31 which is associated to the tensioner arm 4. The first bearing element 30 can also be referred to as outer bearing surface part, and the second bearing element 31 can also be referred to as inner bearing surface part 31.

[0078] The first bearing elements 30 are, when seen in a half longitudinal section view, approximately C-like and have radially inwards a cylindrical portion 32, from which two flange portions 33, 34 project radially outward. The first bearing elements 30 thus encompass the annular portion 25 of the base body. The first flange portion 33, which is facing the tensioner arm 4, forms an axial bearing face, to support the tensioner arm 4 in a first axial direction, while the second flange portion 34, which is axially distanced to the first flange portion 33, forms an axial bearing face for the tensioner arm 4 in an opposite second axial direction. The cylindrical portions 32 form a radial bearing face for the tensioner arm 4.

[0079] The first bearing elements 30 and the base member 3 are integrally produced, especially by multi component injection molding. In this case, the first bearing elements 30 are made from a different plastic material than the base member 3. The bearing material consists of a low friction plastic material, for example a high strength polyamide with polytetrafluoroethylene (PTFE) components with a strength of for example between 2,000 MPa and 4,000 MPa. Relative thereto, the plastic material of the base member 3 can be a fiber reinforced polyamide with a strength of for example between 15,000 MPa and 22,000 MPa. By means of the multi component injection molding method, the unit of the base member 3 with first bearing elements 30 can be manufactured in a simple manner and cheaply with one tool in one working step.

[0080] The tensioner arm 4 has a sleeve portion 39, onto which the second bearing element 31 is pressed which is formed as a bearing bushing. The bearing bushing 31 is especially a formed sheet metal part and can for example be made from aluminum or an aluminum alloy. A bushing portion 27 of the bearing bushing and the cylindrical portion 32 of the first bearing element 30 form a radial bearing, while a flange portion 28 of the bearing bushing 31 and the flange portions 33 of the first bearing element 30 form an axial bearing.

[0081] For pre-tensioning the drive belt tensioner in a mounting position, the tensioner arm 4 and the base member 3 are rotated relative to each other so far, till the mounting bores 36, 37 align with each other, so that the mounting pin 38 can be inserted into these. In this mounting position, the base member 3 of the drive belt tensioner 2 is mounted on the accessory. After completion of the mounting of the belt drive and of the belt around the drive pulley of the accessory, the mounting pin 38 is pulled and the tensioner arm 4, due to the pre-tensioning force of the spring 6, is loaded against the belt.

[0082] Preferably a high strength fiber reinforced plastic material can be used as base material for the base member 3 and the tensioner arm 4, for example a glass fiber reinforced polyamide. It can be seen especially in FIG. 5, that reinforcing elements 8, 26 are provided in the base member 3 and the tensioner arm 4, which are made from a different material. In particular, the base member 3 has reinforcing elements in the form of bushings 26 provided at the connection portions 11. The bushings 26 are made from a metal material and are over-molded with the plastic material of the base member 3. The tensioner arm 4 has a reinforcing element 8 formed as a metal bearing journal 8, which is over-molded with the basic plastic material and serves as carrier for the bearing 12 of the tensioning roller 7.

[0083] The tensioner arm 4 is made of a plastic composition comprising: a thermoplastic polymer; a fibrous reinforcing agent and a thermally conductive filler. The plastic composition has a tensile modulus at 140 C., measured by the method according to ISO 527-1/2, of at least 8 GPa; and a thermal conductivity () at 20 C., measured by the method according to ASTM E-1461-01, of at least 1.0 W/mK.

[0084] The base member 3 is made from a (second) plastic composition that is different than the plastic composition of the tensioner arm 4. For a good thermal behavior of the belt tensioner 2 the material of the second bearing element 31 has a higher thermal conductivity (.sub.1) than the material of the first bearing element 30. Furthermore, the material of the first bearing element 30 can have either a higher or a lower thermal conductivity (.sub.2) than the second plastic composition of the base A. For example, the material of the second bearing element 31 can have a thermal conductivity (.sub.1) of more than 50 W/mK; the material of the first bearing element 30 can have a thermal conductivity (.sub.2) of more than 0.5 W/mK and less than 2.0 W/mK; and the material of the base member 3 can have a thermal conductivity (.sub.3) of less than 1.0 W/mK and more than 0.3 W/mK. Alternatively, the material of the second bearing element 31 can have a thermal conductivity (.sub.1) of more than 50 W/mK; the material of the first bearing element 30 can have a thermal conductivity (.sub.2) of less than 1.0 W/mK; the material of the base member 3 can have a thermal conductivity (.sub.3) of more than 1.0 W/mK and less than 2.0 W/mK.

[0085] The through opening 18 of the tensioner arm 4 is formed such, that the drive shaft and/or the belt pulley of an accessory (not shown) can extend thereinto in the mounted condition. The wall of the tensioner arm 4 encasing the through opening 18 is provided with circumferentially distributed cooling fins 19, that can also be referred to as ribs. The cooling fins 19 serve especially for two functions, namely they transfer the frictional heat produced during operation away from the tensioner arm 4. Furthermore, the ribs 19 contribute to a targeted air supply in direction towards the accessory, on which the drive belt tensioner is mounted, to effectively cool it.

[0086] The invention is further illustrated with the following examples and comparative experiments.

Materials

[0087] M-1: polyamide composition comprising 39 wt. % of PA46, 60 wt. % of glass fibers, and 1 wt. % of auxiliary additives; parallel=0.56 W/mK; tensile modulus 9.5 GPa at 140. [0088] M-2 polyamide composition comprising 40.5 wt. % of PA46, 55 wt. % of glass fibers, 3.5 wt. % thermally conductive filler, and 1 wt. % of auxiliary additives; parallel=1.34 W/mK; tensile modulus 9 GPa at 140.

Methods

Thermal Conductivity

[0089] Thermal conductivity measurements were carried out according ASTM E1461-01 and were done on injection-molded plates, dried as molded, at 23 C. in-plane in the direction parallel to the melt flow direction. Herein the thermal conductivity is derived from the thermal diffusivity (D) measured by laser flash technology, the bulk density () and the specific heat (Cp) at 23 C., using the method described in Polymer Testing (2005, 628-634), wherein thermal conductivity is calculated as A=a*p*Cp.

[0090] The samples for the measurement in parallel direction were prepared as follows. First an injection-molded plate with dimensions 80801 mm was injected from a gate at the middle of one of the sides. Than a piece of 10101 mm was cut from the center. From this part 10 samples of 1011 mm were cut, wherein the length of100 was perpendicular to the flow direction. Then the samples was rotated 90 around its length axis and put next to each other to form a new arrangement of 10101 to allow the parallel in-plane measurement instead of the through plane measurement. This is illustrated in FIG. 6.

Electrical Resistivity

[0091] The electrical resistivity measurements were based on the volumetric resistivity measured by the test method according to ICE 60093. The test was done at 23 C. on a plate, dried as molded, and measured in the plate thickness direction.

Tensile Modulus

[0092] The tensile modulus was measured by the method according to ISO 527-1/2, at a temperature of 140 C., on molded test samples, plate, dried as molded.

Tests

[0093] Accessory drive belt tensioners were prepared with the tensioner arm made by injection molding from the above materials, wherein modifications were applied with and without fins, and with and without a bearing surface part consisting of metal. The weight of the molded parts was about 130 grams and the surface areas of the fins was about 85 cm.sup.2.

[0094] The modification with the tensioner arm made of M-2, in combination with the fins and the metal part showed a very good long term performance. All other modifications were insufficient.