Cage for roller bearings

Abstract

A cage for a bearing comprises an annular body formed from a composite material and having a centerline. The body is dimensioned to be positioned between the inner and outer rings of the bearing and has a first axial end, a second axial end, an inner circumferential surface, and an outer circumferential surface. Several pockets are spaced circumferentially around the centerline, are positioned between the first and second axial ends, and extend radially between the inner and outer circumferential surfaces. Each pocket is dimensioned to accommodate one of the rolling elements. The composite material contains a mixture of a polymer base, reinforcing fibers, and a lubricant.Preferably the polymer base is an ultra-high molecular weight polyethylene or polyetherimide, the reinforcing fibers are short-strand carbon fibers and are between ten and thirty percent by weight, and the lubricant is molybdenum disulfide and is between half a percent and ten percent by weight.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to bearings and, in particular, to cages for rolling bearings.

[0002] Cages for rolling bearings are well known and typically consist of an annular body with several circumferentially spaced pockets, each of which accommodates a separate rolling element of the bearing. The cage maintains a desired spacing between adjacent rolling elements and guides the rolling elements as they traverse a pitch circle defined between the inner and outer rings of the bearing.

[0003] In some applications that require a largely sterile environment, such as semiconductor manufacturing or the food processing industry, bearing cages are often made of a polymer material, such as polyetheretherketone ("PEEK"). However, the cost of manufacturing a cage from PEEK and similar materials is relatively high and requires significant production volumes to be relatively affordable. When using an injection molding process, tooling costs alone can exceed one hundred thousand dollars, and even when manufactured from a sheet of material using machining, PEEK cages are relatively expensive to produce due to the price of the material and manufacturing costs. SUMMARY OF THE INVENTION

[0004] In one aspect, the present invention is a cage for a bearing, the bearing having inner and outer rings and a plurality of rolling elements disposed between the inner and outer rings. The cage includes an annular body formed from a composite material and having a centerline, the body sized to be disposed between the inner and outer rings, and having a first axial end, a second axial end, an inner peripheral surface, and an outer peripheral surface. A plurality of pockets are circumferentially spaced about the centerline, are disposed between the first and second axial ends, and extend radially between the inner and outer peripheral surfaces. Each pocket is sized to receive a separate rolling element. The composite material includes a mixture of a polymer base, reinforcing fibers, and a lubricant.Preferably, the polymer base is ultra-high molecular weight polyethylene or polyetherimide, the reinforcing fibers are short-stranded carbon fibers in an amount between ten percent by weight (10 wt%) and thirty percent by weight (30 wt%), and the lubricant is molybdenum disulfide in an amount between half a percent by weight (0.5 wt%) and ten percent by weight (10 wt%).

[0005] In another aspect, the present invention is a method of forming a cage for a bearing having an inner ring, an outer ring, and a plurality of rolling elements disposed between the inner and outer rings.The method comprises the following steps: providing a tube made of a composite material, the composite material comprising a mixture of a base polymer, reinforcing fibers, and lubricant; machining the outer surface of the tube to a desired outer diameter of the cage and the inner surface of the tube to a desired inner diameter of the cage; cutting the tube to provide at least one annular body having a desired axial length of the cage; and machining a plurality of pockets in the annular body circumferentially spaced about a centerline of the body, each pocket extending radially between the inner surface of the body and the outer surface of the body.The method may further comprise the step of machining at least one cavity and / or at least one groove in an enclosed inner surface of at least one of the pockets, wherein the cavity and / or the groove are configured to contain lubricant. BRIEF DESCRIPTION OF THE DIFFERENT VIEWS OF THE DRAWINGS

[0006] The above summary and detailed description of the preferred embodiments of the present invention will be better understood when read in conjunction with the accompanying drawings. For the purpose of illustrating the invention, the drawings, which are diagrammatic, show embodiments that are presently preferred. It should be understood, however, that the present invention is not limited to the arrangements and instrumentalities in the exact form shown. In the drawings, the following can be seen: Fig. Figure 1 is a side view through a bearing incorporating a cage of the present invention, the bearing being shown with a shaft coupled to an outer member of a machine; Fig. 2 is a perspective view of the cage; Fig. Figure 3 is a broken away enlarged axial cross-sectional view of the cage; Fig. 4 is a detailed view of a section of Fig. 3; Fig. 5 is a fragmentary, more magnified axial cross-sectional view of the cage shown with multiple lubricant cavities on an inner surface of a pocket; Fig. Figure 6 is another fragmentary, more enlarged axial cross-sectional view of the cage shown with a lubricant groove on the inner surface of a pocket; Fig. 7 is a perspective view of a composite tube used to fabricate a plurality of cages; Fig. 8 are several cage bodies that are made of the Fig. 7 shown tube; and Fig. Figure 9 is an illustration of a pocket machining step in one of the Fig. Cage body shown in Figure 8. DETAILED DESCRIPTION OF THE INVENTION

[0007] In the following description, specific terminology is used for convenience only and is not limiting. The words "inner," "inward," and "outer," "outward," refer to directions toward or from a designated centerline or geometric center of a described element, with the specific meaning readily apparent from the context of the description. This terminology includes the words expressly mentioned above, their derivatives, and words of similar meaning.

[0008] We now turn in detail to the drawings, in which the same reference symbols are always used to designate the same elements. Fig. 1-9, a cage 10 for a bearing 1 is shown, wherein the bearing 1 comprises an inner ring 2, an outer ring 3 arranged around the inner ring 2, and a plurality of rolling elements 4 arranged between the inner and outer rings 2, 3. The bearing 1 has the function of rotatably coupling an inner element 5, such as a shaft, to an outer element 6, for example a housing or a hub, such that an element 5 or 6 rotates about a central axis AC when the rolling elements 1 traverse a pitch circle PC defined between the rings 2, 3 ( Fig. 1). Preferably, the inner and outer elements 5, 6 are components or structural elements of a machine or equipment component E used in a sterile environment, such as in semiconductor manufacturing, food processing, etc. However, they can also be used in any other suitable application.

[0009] The cage 10 comprises an annular body 12 formed from a composite material M and having a centerline LC, the body 12 being sized to be disposed between the inner and outer rings 2, 3. The annular cage body 12 has a first axial end 12a, a second axial end 12b, an inner peripheral surface 13A, and an outer peripheral surface 13B. Furthermore, a plurality of pockets 14 are circumferentially spaced about the centerline LC, are disposed between the first and second axial ends 12a, 12b, and extend radially between the inner and outer peripheral surfaces 13A, 13B. Each pocket 14 is sized to receive a separate one of the rolling elements 4 such that the rolling element 4 is freely rotatable within the pocket 14 as it traverses the bearing pitch circle PC.

[0010] More specifically, each pocket 14 is defined by a separate one of a plurality of enclosed surfaces 16 formed in the annular body 12 and having an inner diameter or dimension (neither of which is illustrated) slightly larger than a diameter and / or length of each rolling element 4. In a presently preferred and illustrated application, the rolling elements 4 are balls 7, and each pocket 14 is circular. However, the rolling elements 4 may alternatively be cylinders, needles, tapered rollers, etc. (none of which are illustrated), such that each pocket 4 may be rectangular, frusto-conical, etc. (none of which are illustrated).

[0011] In addition, the composite material M contains a mixture of a polymer base B, reinforcing fibers F and a lubricant L, as in Fig. 4. Preferably, the amount of reinforcing fibers F in the composite material M is between ten weight percent (10 wt%) and thirty weight percent (30 wt%), and the amount of lubricant L in the composite material M is between half a weight percent (0.5 wt%) and ten weight percent (10 wt%). Most preferably, the amount of reinforcing fibers F is about thirty weight percent (30 wt%), and the amount of lubricant L is about five weight percent (5 wt%). However, the composite material M may alternatively contain any other desired amount of reinforcing fibers F and / or lubricant L, as appropriate for a particular application.

[0012] Preferably, the reinforcing fibers F contain short-stranded carbon fibers, the lubricant L contains molybdenum disulfide (MoS2), and the polymer base B contains either ultra-high molecular weight polyethylene (hereinafter "UHWPE") or polyetherimide (hereinafter "PEI"). For high-temperature applications, the polymer base B preferably contains polyetherimide / PEI, such that the cage 10 is designed to operate at a temperature, i.e., the "working temperature," of up to two hundred degrees Celsius (200°C). Thus, the cage 10 is designed to function or operate without softening the composite material M, so that the cage 10 remains dimensionally stable and does not bond with the rolling elements 4 or with the inner and outer rings 2, 3.

[0013] Alternatively, the reinforcing fibers F may also contain glass strands, glass beads, Kevlar, or other suitable reinforcing materials. The lubricant L within the composite material M may alternatively contain graphite, polytetrafluoroethylene ("PTFE"), or another suitable lubricant material capable of blending with a polymeric base material.

[0014] With reference to Fig. 5 and Fig. 6, the annular body 12 preferably has at least one cavity 18 and / or groove 20 formed in each closed surface 16 and defining a pocket 14. Each cavity 18 and each groove 20 are designed to contain a quantity of lubricant to reduce friction between the rolling element 4 and the pocket 14, a portion of which can be transferred by the rolling elements 14 to the raceways (not illustrated) of the inner and / or outer rings 2, 3. Preferably, the lubricant contained in the cavities 18 or grooves 20 is a lubricant designed for use in a vacuum environment, such as a PTFE-thickened multi-alkylated cyclopentane ("MAC")-based grease (e.g., NyeTorr® 6200) or a PTFE-thickened perfluoropolyether grease (e.g., NyeTorr® 6300).However, the lubricant contained in the cavities 18 and / or grooves 20 may also be any other suitable lubricant suitable for the particular application of the bearing 1.

[0015] Furthermore, the bearing cage 10 discussed here, in the case of a composite material M having a polymer base B of UHMWPE, has a stiffness of about five hundred megapascals (500 MPa), a glass transition temperature of about minus eighty degrees Celsius (-80 °C), a maximum working temperature of about one hundred degrees (100 °C), a percentage total mass loss due to outgassing in a vacuum of about 0.06, and a coefficient of friction (µ) against the rolling elements 4 of about 0.2.Alternatively, in the case of a composite material M having a polymer base B of PEI, the bearing cage 10 discussed here has a stiffness of approximately six thousand seven hundred megapascals (6700 MPa), a glass transition temperature of approximately two hundred and twenty degrees Celsius (220 °C), a maximum working temperature of approximately two hundred degrees (200 °C), a percentage total mass loss due to outgassing in a vacuum of approximately 0.58 and a coefficient of friction (µ) with respect to the rolling elements 4 between approximately 0.35 and 0.4.

[0016] In comparison, a bearing cage formed from PEEK has a stiffness of approximately 7,500 megapascals (7,500 MPa), a glass transition temperature of approximately 135 degrees Celsius (135 °C), a maximum operating temperature of approximately 120 degrees Celsius (120 °C), a percentage total mass loss due to outgassing in a vacuum of approximately 0.2, and a coefficient of friction (µ) against the rolling elements 4 of approximately 0.3. Thus, the cage 10 discussed here exhibits lower friction and greater dimensional stability than a PEEK cage when the base polymer B is UHMWPE, although the stiffness and maximum operating temperature are lower. Furthermore, the cage 10 with a base polymer B made of PEI has a much higher operating temperature and similar stiffness compared to a PEEK cage, but higher friction and lower dimensional stability.However, the cage 10 discussed here, with both the one and the other base polymer B, has significantly lower manufacturing costs compared to a PEEK cage, as will be discussed in more detail below.

[0017] We now turn to the Fig. 7-9. The bearing cage 10 discussed here is preferably formed according to the following manufacturing process. First, a tube 30 is provided which is formed from the desired composite material M, that is, which has a specific polymer base B, a desired percentage and type of reinforcing fibers F, and a desired percentage and type of lubricant L. Next, an outer surface 32 of the tube 30 is machined to a desired outer diameter ODC of the cage 10, and an inner surface 34 of the tube 30 is machined to a desired inner diameter IDC of the cage 10, as shown in Fig. 7. Then, as in Fig. 8, the tube 30 is cut to provide at least one annular portion having a desired axial length AL, which portion forms the annular body 12 of the cage 10. Furthermore, as shown in Fig.9, a plurality of pockets 14 are machined into the annular body 12 so as to be circumferentially spaced about a centerline LC of the body 12, and such that each pocket 14 extends radially between the inner surface 13A of the body 12 and the outer surface 13B of the body 12. Finally, if cavities 18 or grooves 20 for receiving lubricant are desired, these cavities 18 and / or grooves 20 are machined into the enclosed inner surface of at least one pocket 14, and preferably into the enclosed inner surfaces of all of the pockets 14. However, the cage 10 discussed herein may be formed in any other desired manner or by any other desired process, such as injection molding, complete machining from a circular disk, etc.

[0018] Due to both the preferred manufacturing method and the preferred material composition, as described above, with one or the other polymer base B, the bearing cage 10 discussed herein has significantly lower manufacturing costs compared to a cage formed from PEEK. More specifically, the cost of manufacturing a cage 10 from the composite material M containing UHMWPE as the polymer base B is approximately five percent (5%) of the cost of manufacturing a similar cage from PEEK. In other words, this is a ninety-five percent (95%) reduction in cost. Furthermore, the cost of manufacturing a cage 10 from the composite material M containing PEI as the polymer base B is approximately thirty percent (30%) of the cost of manufacturing a similar cage from PEEK, or a seventy percent (70%) cost reduction.

[0019] Representative, non-limiting examples of the present invention have been described in detail above with reference to the accompanying drawings. This detailed description is merely intended to provide those skilled in the art with further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention.

[0020] Furthermore, combinations of features and steps disclosed in the above detailed description may not be required to practice the invention in its broadest sense, and are instead taught only to particularly describe representative examples of the invention. Furthermore, various features of the representative examples described above, as well as the various independent and dependent claims below, may be combined in ways not specifically and explicitly enumerated to produce additional useful embodiments within the spirit of the present teachings.

[0021] It is intended that all features disclosed in the description and / or claims be disclosed separately and independently of one another, and independently of the combinations of features in the embodiments and / or the claims, for the purpose of original written disclosure and for the purpose of limiting the claimed subject matter. Furthermore, it is intended that all ranges of values ​​or indications of groups of entities disclose any possible intermediate value or entity for the purpose of original written disclosure and for the purpose of limiting the claimed subject matter. The invention is not limited to the embodiments described above and may be varied within the scope of the following claims.

Claims

[1] Cage for a bearing, the bearing having an inner and an outer ring and a plurality of rolling elements arranged between the inner and outer rings, the cage comprising: an annular body formed from a composite material and having a centerline, the body being sized to be disposed between the inner and outer rings, and having a first axial end, a second axial end, an inner peripheral surface, an outer peripheral surface, and a plurality of pockets circumferentially spaced about the centerline, disposed between the first and second axial ends, and extending radially between the inner and outer peripheral surfaces, each pocket sized to receive a single one of the rolling elements, the composite material comprising a mixture of a polymer base, reinforcing fibers, and a lubricant. [2] Bearing cage according to claim 1, wherein: an amount of reinforcing fibers in the composite material is between ten percent by weight and thirty percent by weight; and an amount of lubricant in the composite material is between half a percent and ten percent by weight. [3] The bearing cage of claim 1, wherein an amount of the reinforcing fibers in the composite material is about thirty percent by weight and an amount of the lubricant in the composite material is about five percent by weight. [4] A bearing cage according to claim 3, wherein the reinforcing fibers comprise short-strand carbon fibers and the lubricant comprises molybdenum disulfide. [5] Bearing cage according to claim 4, wherein the polymer base contains ultra-high molecular weight polyethylene or polyetherimide. [6] Bearing cage according to claim 1, wherein: the reinforcing fibers contain short-stranded carbon fibers; and / or the lubricant contains molybdenum disulfide. [7] A bearing cage according to claim 1, wherein the polymer base contains either ultra-high molecular weight polyethylene or polyetherimide. [8] A bearing cage according to claim 1, wherein the polymer base comprises polyetherimide and the cage is designed for operation at a temperature of up to two hundred degrees Celsius. [9] The bearing cage of claim 1, wherein each of the plurality of pockets is defined by a separate one of the plurality of enclosed surfaces formed in the annular body, and the annular body further comprises at least one cavity and / or at least one groove formed in each enclosed surface, each cavity and / or groove being configured to contain a lubricant. [10] The bearing cage of claim 1, wherein the annular body is formed from a tube formed from the composite material, the outer surface of which is machined to a desired outer diameter and the inner surface of which is machined to a desired inner diameter, whereupon a portion of the tube is cut to a desired axial length and the plurality of pockets are machined into the portion of the tube. [11] A bearing cage according to claim 1, wherein the coefficient of friction between the annular body and each of the rolling elements has a value of less than 0.

25. [12] A method of forming a cage for a bearing comprising an inner ring, an outer ring and a plurality of rolling elements arranged between the inner and outer rings, the method comprising the following steps: Providing a pipe made of a composite material, the composite material containing a mixture of a polymer base, reinforcing fibers and lubricant; machining the outer surface of the tube to a desired outer diameter of the cage and the inner surface of the tube to a desired inner diameter of the cage; Cutting the tube to provide at least one annular body having a desired axial length of the cage; and machining a plurality of pockets in the annular body circumferentially spaced about a centerline of the body, each pocket extending radially between the inner surface of the body and the outer surface of the body. [13] The method of claim 12, further comprising the step of machining at least one cavity and / or at least one groove in an inner surface of at least one of the pockets, the cavity and / or the groove being adapted to contain lubricant. [14] The method of claim 12, wherein: an amount of reinforcing fibers in the composite material is between ten percent by weight and thirty percent by weight; and an amount of lubricant in the composite material is between half a percent and ten percent by weight. [15] The bearing cage of claim 12, wherein an amount of the reinforcing fibers in the composite material is about thirty percent by weight and an amount of the lubricant in the composite material is about five percent by weight. [16] A bearing cage according to claim 12, wherein the reinforcing fibers comprise short-strand carbon fibers and the lubricant comprises molybdenum disulfide. [17] A bearing cage according to claim 12, wherein the polymer base contains ultra-high molecular weight polyethylene or polyetherimide. [18] A cage for a bearing, the bearing having an inner and an outer ring and a plurality of rolling elements arranged between the inner and outer rings, the bearing cage comprising: an annular body formed from a composite material and having a centerline, the body being sized to be disposed between the inner and outer rings, and having a first axial end, a second axial end, an inner peripheral surface, an outer peripheral surface, and a plurality of pockets disposed between the first and second axial ends and extending radially between the inner and outer peripheral surfaces, each pocket being sized to receive a single one of the rolling elements, the plurality of pockets being circumferentially spaced about the centerline, the composite material comprising a mixture of a polymer base, reinforcing fibers in an amount between ten percent and thirty percent by weight, and a lubricant in an amount between one-half percent and ten percent by weight. [19] The bearing cage of claim 18, wherein an amount of the reinforcing fibers in the composite material is about thirty percent by weight and an amount of the lubricant in the composite material is about five percent by weight. [20] A bearing cage according to claim 19, wherein the polymer base contains ultra-high molecular weight polyethylene or polyetherimide, the reinforcing fibers contain short-strand carbon fibers and the lubricant contains molybdenum disulfide.

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