WO2008141466A1

WO2008141466A1 - Center-gated molded pulley and method of manufacturing such a pulley

Info

Publication number: WO2008141466A1
Application number: PCT/CA2008/001035
Authority: WO
Inventors: Christiaan Vander Ploeg
Original assignee: Litens Automotive Partnership
Priority date: 2007-05-23
Filing date: 2008-05-23
Publication date: 2008-11-27

Abstract

A novel method of molding a pulley is taught which achieves a degree of roundness which was not generally previously available. The pulley is molded with a center gated mold and the pulley can be fabricated from plastic materials reinforced with glass fiber, rather than glass bead, and these materials result in a generally more robust pulley. Also, by placing a dust shield on the nail used to mold the pulley, while ejecting the nail from the molded pulley, the dust shield can be mounted to the pulley over a face of the bearing.

Description

Center-Gated Molded Pullev and Method of Manufacturing Such A Pullev

FIELD OF THE INVENTION

[0001] The present invention relates to a molded pulley. More specifically, the present invention relates to a pulley molded with a center-gated mold and the method of molding it.

BACKGROUND OF THE INVENTION

[0002] Pulleys molded from plastic materials, such as polyamide or various engineering plastics, which can be combined with glass or other strengthening materials, are well known and are widely used. However, it is difficult to obtain a molded pulley whose roundness is within necessary tolerances for some applications such as tensioners for Front Engine Accessory Drives (FEADs) in automotive uses. [0003] Plastic pulleys with too large a variation in their roundness can create undesired operating noise when they are used and/or can lead to premature aging of components in the systems in which they are employed.

[0004] Stresses introduced into the pulley during the molding process, volumetric shrinkage of the plastic material of the pulley as it cools, plastic flow patterns through the mold resulting in knit lines where flows meet and various other factors contribute to the deformation of the pulley from the desired round shape. In particular, each knit line, which result from the meeting of molten plastic material from different gates in the pulley mold, results in an area of the pulley which is stressed and which is structurally different from the surrounding areas and which will generally result in an out of round portion of the pulley.

[0005] Such out of round, or flat, areas are inevitable with multiple gate molding techniques and various approaches, such as that described in U.S. Patent no. 5,728,343 to Ueno and published U.S. Patent Application 2007/0108839 to Sauer et al. have been made in attempts to mitigate such areas.

[0006] In some prior art attempts to mold a pulley with an acceptable level of roundness, the pulleys have been molded in a gate with thirty or more gates, to reduce the size of the knit lines, and with a plastic combined with a glass bead filler, rather than a glass fiber filler, as the use of beads can reduce the internal stresses in the pulley which could otherwise result in the formation of flat areas. While this molding process can result in pulleys with a reasonable level of roundness (e.g. - no more than seventy microns of diametric dimensional variation on a pulley with a seventy millimeter diameter), the resulting pulley is less than desirable as the use of glass bead filler results in a pulley which is brittle and can be chipped or otherwise damaged and the tooling expense, to manufacture the mold with the high number of gates, is quite high.

[0007] It is desired to have a method for molding a plastic pulley which achieves a desired level of roundness of the pulley and the pulley produced by such a method.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to provide a novel method of molding a plastic pulley which obviates or mitigates at least one disadvantage of the prior art.

[0009] According to a first aspect of the present invention, there is provided a method for molding a molded plastic pulley comprising a bearing, a hub over-molded onto the bearing, a web extending radially from the hub to a peripheral outer surface which can engage a flexible drive means, and a series of ribs, upstanding from the web and extending radially from the hub to the peripheral outer surface. The pulley is molded in a center gated mold such that different flows of molten material are inhibited to inhibit the formation of flat spots in the peripheral outer surface. [0010] Preferably, the pulley is molded from polyamide reinforced with glass fibers, although any other suitable plastic material and/or strengthening material can be employed.

[0011] According to another aspect of the present invention, there is provided a method of molding a plastic pulley comprising the steps of: placing a bearing onto a nail; loading the nail and bearing into a portion of a mold cavity; closing the mold cavity; injecting a shot of molten material into the mold cavity to over-mold the bearing and form a pulley, the molten material entering the mold cavity through a single centrally located gate adjacent an end face of the nail such that the molten material fills the cavity in a substantially single flow to inhibit areas of joins between different flows; opening the mold and removing the nail and molded pulley; and separating the molded pulley from the nail while removing the gate sprue left on the molded pulley after molding.

[0012] The present invention provides a novel method of molding a pulley, which achieves a degree of roundness of the outer peripheral surface of the pulley which was not generally previously available. The pulley is molded with a center gated mold and the pulley can be fabricated from plastic materials reinforced with glass fiber, rather than glass bead, and these materials result in a generally more robust pulley. Also, by placing a dust shield on the nail used to mold the pulley, prior to ejecting the nail from the molded pulley, the dust shield can be mounted to the pulley over a face of the bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached Figures, wherein:

Figure 1 shows front view of a pulley in accordance with the present invention;

Figure 2 shows a section taken through line 2-2 of Figure 1 ;

Figure 3 shows a cross section of a mold used to mold a pulley in accordance with the present invention;

Figure 4 shows a nail, used with the mold of Figure 3, with a bearing loaded onto it;

Figure 5 shows a nail with a molded pulley on it in accordance with another embodiment of the invention; and

Figure 6 shows the nail ejected from the pulley of Figure 5 installing a dust shield.

DETAILED DESCRIPTION OF THE INVENTION

[0014] A pulley, molded in accordance with the method of the present invention, is indicated generally at 20 in Figures 1 and 2. Pulley 20 includes a roller bearing 24 about which a hub 28 is over-molded.

[0015] Pulley 20 further includes an outer peripheral surface 32, which can engage a belt or other flexible drive means, a web 36 extending radially from bearing 24 to peripheral surface 32 and a series of stiffening ribs 40, which also extend radially from hub 30 to peripheral surface 32 and which are upstanding from web 36. [0016] A mold assembly for fabricating pulley 20 in accordance with the method of the present invention is indicated generally at 100 in Figure 3. Mold assembly 100 generally includes an upper die member 104, a lower die member 108 and a mold block 112 which define mold cavity 116. [0017] In a manner similar to that taught in U.S. Patent 6,537,053 to Watkins, mold assembly 100 includes a single, center gate 120 through which molten material is introduced into cavity 116. The use of such a center-gated molding technique reduces or prevents the formation of knit lines which would otherwise result from the meeting of different flows of molten plastic material in the mold. [0018] Specifically, to mold a pulley 20, a bearing 24 is loaded onto a core or nail 124 (as best seen in Figure 4) and the loaded nail 124 and bearing 24 are placed in cavity 1 16 on locator or ejector 128.

[0019] Nail 124 is generally nail-shaped having a shank portion 125 and a head portion 127. The outer edge of head portion 127 is provided with a circumferential longitudinally extending sharp edge 136. Nail 124 overlies the inner race of the bearing 24 such that the outer edge of the head sealingly engages the outer race of bearing 24.

[0020] The nail 124, the outer race of bearing 24 cooperate with the die cavity 1 16 to define a pulley shape that is to be overmolded onto bearing 24. [0021] Mold assembly 100 is then closed and a shot of molten material 132 is injected into cavity 1 16 through center gate 120. In one presently preferred embodiment of the invention, molten material 132 is polyamide with glass fibers. While good results have been obtained with polyamide plastic, the present invention is not limited to the use of this particular material and any other suitable material can be employed as will be apparent to those of skill in the art. Further, the present invention is not limited to the use of glass fiber in the molten material to strengthen the molded pulley and any other suitable strengthening material can be employed or, depending on the plastic material employed, a strengthening material can be omitted altogether. In one embodiment of the present invention, the molten material is polyamide and has from about thirty percent to about fifty percent, by weight, of glass fiber added.

[0022] The use of mold assembly 100 with the single center gate 120 results in the molten material moving through cavity 1 16 as a single flow, avoiding multiple flows of molten material and the knit lines which would otherwise result. Thus, knit lines or other areas where multiple flows would meet, as in conventionally molded pulleys, are avoided as are the stressed areas that would result and lead to out of round areas. Further, the glass fibers in molten material 132 are subjected to less convoluted flow patterns, which results in a desired dispersion of the fibers in the molten material, with a corresponding increase in the stiffness and strength of the resulting pulley 20.

[0023] Also, with prior art plastic molded pulleys formed in molds with multiple gates, knit lines and their related stress areas are formed when the hub is over- molded onto the bearing. These knit lines and stress areas can result in a reduced mechanical attachment between the outer race of the bearing and the hub. Such reduced mechanical attachment can lead to failure of the pulley when the hub separates from and/or rotates with respect to the outer race of the bearing. Such failures are especially troublesome if the pulley is subjected to a wide range of operating temperatures where the different expansion rates of the plastic material and the bearing material can lead to such separations.

[0024] Further, these knit lines and related stress areas in prior art pulleys can result in unequal forces being exerted on the outer race of the bearing by the hub which can result in deformation of the bearing, leading to increased friction within the bearing and/or premature failure of the pulley.

[0025] In contrast, in the present invention, the use of center-gated mold assembly 100 avoids the formation of knit lines and/or stress areas in hub 28 with the result that hub 28 achieves the desired connection between the plastic material of hub 28 and the outer race of bearing 24 and the absence of stress areas in hub 28 prevents undesired deformations of the outer race of bearing 24. [0026] When the contents of cavity 116 have cooled sufficiently, mold assembly 100 is opened (for example by lifting upper die member 104, and nail 124, with pulley 20 still attached, is ejected by locator 128, or by any other suitable means as will occur to those of skill in the art. Nail 124 and pulley 20 are removed from the upper die member 104 with the gate sprue 144 still attached. The nail 124 and pulley 20 with gate sprue 144 are then moved to another station, where nail 124 is pressed out of pulley 24. Since nail 124 includes circumferentially extending sharp edge 136, the removal operation for moving the nail 124 relative to the bearing 24 will also cut the gate sprue 144, which will remove the gate sprue 144 from pulley 20 as nail 124 is separated from pulley 24.

[0027] With the above-described embodiment of the invention, pulleys 20 have been repeatedly fabricated with exceptional levels of roundness (e.g. - less than thirty microns of diametric dimensional variation on a pulley with a seventy millimeter diameter). Further, these pulleys 20 have been fabricated with glass fiber strengthening fillers in the plastic material, rather than the glass bead fillers employed in the prior art, resulting in pulleys 20 having increased resistance to chipping or other failures associated with pulleys formed from glass bead fillers. [0028] Figures 5 and 6 show another inventive feature of the present invention. Specifically, it is often desirable and/or required to provide a dust shield for at least one face of bearing 24 to inhibit dirt, water or other foreign materials from entering bearing 24. It is known to place such dust shields on a bolt used to install pulley 20 in place so that the dust shield is positioned when the pulley is installed. However, providing dust shields in this manner increases manufacturing costs as it requires a separate part to be available on an assembly line and requires another assembly step, which can be inadvertently omitted and/or incorrectly performed. [0029] In other cases, the dust shield can be pressed onto bearing 24 prior to the installation of pulley 24, but this requires an additional manufacturing step for pulley 20 thus increasing the manufacturing costs.

[0030] Unlike the prior art, and as shown in Figures 5 and 6, with the present invention nail 124 has a reduced diameter portion 125 enabling a dust shield 140 to be coaxially placed on nail 124. As nail 124 is pressed out of bearing 24 and pulley 20, removing gate sprue 144, the ram or other device pressing on nail 124 can also press against dust shield 140 to set it in place in pulley 20 over the surface of bearing 24 as shown in Figure 6. Thus, the only additional manufacturing step required to mount dust shield 140 is to load dust shield 140 onto nail 124 before ejecting nail 124 from pulley 20.

[0031] As will now be apparent to those of skill in the art, the present invention provides a novel molded plastic pulley which achieves a degree of roundness which was not generally previously available. Further, the present invention provides for the molding of such a pulley from plastic materials reinforced with glass fiber, rather than glass bead, materials which results in a generally more robust pulley. Also, by placing a dust shield on the nail used to mold the pulley, prior to ejecting the nail from the molded pulley, the dust shield can be mounted to the pulley over a face of the bearing. [0032] The above-described embodiments of the invention are intended to be examples of the present invention and alterations and modifications may be effected thereto, by those of skill in the art, without departing from the scope of the invention which is defined solely by the claims appended hereto.

Claims

We claim:

1. A method of molding a plastic pulley comprising the steps of: providing a mold assembly having a mold cavity; providing a nail having a head and a shank, said head having a circumferentially extending outer sharp edge; placing a bearing onto the shank of the nail; loading the nail and bearing into a portion of mold assembly; closing the mold assembly; injecting a shot of molten material into the mold cavity to over-mold a pulley onto the bearing, the molten material entering the mold cavity through a single centrally located gate adjacent an end face of the nail such that the molten material fills the cavity in a substantially single flow; opening the mold and removing the nail and molded pulley with a gate sprue attached to the molded pulley; and removing the gate sprue from the molded pulley while removing the nail from the bearing.

2. The method of claim 1 wherein the step of removing the gate sprue comprises moving the nail relative to the bearing to remove the gate sprue from the molded pulley.

3. The method of claim 2 wherein the sharp edge of the nail cuts the gate sprue to remove the gate sprue from the molded pulley.

4. The method of claim 1 further comprising the step of loading a dust shield onto the nail after removing the nail and molded pulley from the mold assembly and wherein the step of removing the nail from the bearing also seats the dust shield over one side of the bearing.

5. The method of claim 4 wherein the step of removing the gate sprue comprises moving the nail relative to the bearing to remove the gate sprue from the molded pulley.

6. The method of claim 5 wherein the sharp edge of the nail cuts the gate sprue to remove the gate sprue from the molded pulley.

7. The method of claims 1 or 6 wherein the molten material comprises polyamide.

8. The method of claims 1 or 6 wherein the molten material contains glass fibers.

9. The method of claims 1 or 6 wherein the molten material is an engineering plastic.