CAGE FOR ROLLING ELEMENT BEARINGS

A cage for a bearing includes an annular body formed of a composite material and having a centerline, the body being sized to be disposed between the inner and outer rings of the bearing and having a first axial end, a second axial end, an inner circumferential surface and an outer circumferential surface. A plurality of pockets are spaced circumferentially about the centerline, disposed between the first and second axial ends and extend radially between the inner and outer circumferential surfaces. Each pocket is sized to receive one of the rolling elements. The composite material includes a mixture of a polymer base, reinforcing fibers and a lubricant. Preferably, the polymer base is ultrahigh 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 one half percent to ten percent by weight.

BACKGROUND OF THE INVENTION

The present invention relates to bearings, and more particularly to cages for rolling element bearings.

Cages for rolling element bearings are well known and typically include an annular body with a plurality of circumferentially-spaced pockets, each pocket receiving a separate one of the rolling elements of the bearing. The cage establishes a desired spacing between adjacent rolling elements and guides the elements as the elements traverse a pitch circle defined between the bearing inner and outer rings.

In certain applications which require a substantially sterile environment, such as in semiconductor manufacturing or in the food processing industry, the bearing cages are often formed of a polymeric material, such as polyether ether ketone (“PEEK”). However, the cost of fabricating a cage out of PEEK and similar materials is relatively expensive and require substantial production volumes in order to be relatively affordable. If formed by injection molding, the tooling cost alone may exceed one hundred thousand dollars and even if fabricated by machining from a disk of material, the material price and manufacturing costs result in the PEEK cages being relatively expensive to produce.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a cage for a bearing, the bearing including inner and outer rings and a plurality of rolling elements disposed between the inner and outer rings. The cage comprises an annular body formed of 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 circumferential surface and an outer circumferential surface. A plurality of pockets are spaced circumferentially about the centerline, are disposed between the first and second axial ends and extend radially between the inner and outer circumferential surfaces. Each pocket is sized to receive a separate one of the rolling elements. The composite material including a mixture of a polymer base, reinforcing fibers and a lubricant. Preferably, the polymer base is ultrahigh weight polyethylene or polyetherimide, the reinforcing fibers are short strand carbon fibers in an amount of between ten percent (10%) by weight and thirty percent (30%) by weight, and the lubricant is molybdenum disulfide in an amount of between one half percent (0.5%) by weight and ten percent (10%) by weight.

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 steps of: providing a tube of a composite material, the composite material including a mix of a base polymer, reinforcing fibers and lubricant; machining the outer surface of the tube to a desired outside diameter of the cage and the inner surface of the tube to a desired inside 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 spaced circumferentially 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, the cavity and/or groove being configured to contain lubricant.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used in the following description for convenience only and is not limiting. The words “inner”, “inwardly” and “outer”, “outwardly” refer to directions toward and away from, respectively, a designated centerline or a geometric center of an element being described, the particular meaning being readily apparent from the context of the description. The terminology includes the words specifically mentioned above, derivatives thereof, and words of similar import.

Referring now to the drawings in detail, wherein like numbers are used to indicate like elements throughout, there is shown inFIGS.1-9a cage10for a bearing1, the bearing1including an inner ring2, an outer ring3disposed about the inner ring2and a plurality of rolling elements4disposed between the inner and outer rings2,3. The bearing1functions to rotatably couple an inner member5, such as a shaft, with an outer member6, for example a housing or hub, so that one member5or6rotates about a central axis Ac as the rolling elements1traverse a pitch circle PC (FIG.1) defined between the rings2,3. Preferably, the inner and outer members5,6are components or structural members of a machine or item of equipment E used in a sterile environment, such as semiconductor manufacturing, food processing, etc., but may be used in any other appropriate application.

The cage10comprises an annular body12formed of a composite material M and having a centerline LC, the body12being sized to be disposed between the inner and outer rings2,3. The cage annular body12has a first axial end12a, a second axial end12b, an inner circumferential surface13A and an outer circumferential surface13B. Further, a plurality of pockets14are spaced circumferentially about the centerline LC, are disposed between the first and second axial ends12a,12band extend radially between the inner and outer circumferential surfaces13A,13B. Each pocket14is sized to receive a separate one of the rolling elements4such that the rolling element4is free to rotate within the pocket14as the element4traverses the bearing pitch circle PC.

More specifically, each pocket14is defined by a separate one of a plurality of enclosed surfaces16formed in the annular body12and having an inside diameter or inside dimension (neither indicated) that is slightly greater than a diameter and/or length of each rolling element4. In a presently preferred and depicted application, the rolling elements4are balls7and each pocket14is circular. However, the rolling elements4may alternatively be cylinders, needles, tapered rollers, etc. (none depicted), such that each pocket4may be formed rectangular, frustoconical, etc. (none depicted).

Furthermore, the composite material M includes a mixture of a polymer base B, reinforcing fibers F and a lubricant L, as depicted inFIG.4. Preferably, the amount of the reinforcing fibers F in the composite material M is between ten percent by weight (10%) and thirty percent (30%) by weight and an amount of the lubricant L in the composite material M is between one-half percent (0.5%) by weight and ten percent (10%) by weight. Most preferably, the amount of the reinforcing fibers F is about thirty percent (30%) by weight and the amount of the lubricant L is about five percent (5%) by weight. However, the composite material M may alternatively include any other desired amount of reinforcing fibers F and/or lubricant L as suitable for a particular application.

Preferably, the reinforcing fibers F includes short strand carbon fibers, the lubricant L includes molybdenum disulfide (MoS2) and the polymer base B includes either ultrahigh molecular weight polyethylene (hereinafter “UHWPE”) or polyetherimide (hereinafter “PEI”). In high temperature applications, the polymer base B preferably includes polyetherimide/PEI such that the cage10is configured to operate at a temperature, i.e., the “working temperature”, of up to two hundred degrees Celsius (200° C.). As such, the cage10is configured to function or operate without softening of the composite material M, such that the cage10remains dimensionally stable and does not bind with the rolling elements4or with the inner and outer rings2,3.

Alternatively, the reinforcing fibers F may include glass strands, glass spheres, Kevlar or any other appropriate reinforcing materials. The lubricant L within the composite material M may alternatively include graphite, polytetrafluorethylene (“PTFE”) or another appropriate lubricant material capable of mixing with a polymer base material.

Referring toFIGS.5and6, the annular body12preferably includes at least one cavity18and/or groove20formed in each enclosed surface16defining a pocket14. Each cavity18and each groove20is configured to contain a quantity of lubricant so as to reduce friction between the rolling element4and the pocket14, a portion of which may be transferred by the rolling elements14to the raceways (not indicated) of the inner and/or outer rings2,3. Preferably, the lubricant disposed within the cavities18or grooves20is a lubricant adapted for use in a vacuum environment, such as for example, a multiply alkylated cyclopentane (“MAC”) based grease thickened with PTFE (e.g., NyeTorr® 6200) or a PTFE thickened perfluoropolyether grease (e.g., NyeTorr® 6300). However, the lubricant contained within the cavities18and/or grooves20may be any other appropriate lubricant suitable for the particular application of the bearing1.

Furthermore, with the composite material M having a polymer base B of UHMWPE, the present bearing cage10has a stiffness of about five hundred megapascals (500 MPA), a glass transition temperature about negative eighty degrees Celsius (−80° C.), a maximum working temperature of about one hundred degrees (100° C.), a percent total mass loss due to outgassing in a vacuum of about 0.06 and a coefficient of friction (μ) against the rolling elements4of about 0.2. Alternatively, when the composite material M has a polymer base B of PEI, the present bearing cage10has a stiffness of about six thousand, seven hundred megapascals (6700 MPA), a glass transition temperature about two hundred twenty degrees Celsius (220° C.), a maximum working temperature of about two hundred degrees (200° C.), a percent total mass loss due to outgassing in a vacuum of about 0.58 and a coefficient of friction (μ) against the rolling elements4of between about 0.35 and 0.4.

In comparison, a bearing cage formed of PEEK has a stiffness of about seven thousand, five hundred megapascals (7500 MPA), a glass transition temperature about one hundred thirty-five degrees Celsius (135° C.), a maximum working temperature of about one hundred twenty degrees (120° C.), a percent total mass loss due to outgassing in a vacuum of about 0.2 and a coefficient of friction (μ) against the rolling elements4of about 0.3. Thus, the present cage10has lower friction and greater dimensional stability than a PEEK cage when the base polymer B is UHMWPE, although the stiffness and maximum working temperature are lower. Further, the cage10with a base polymer B of PEI has a much higher working temperature and similar stiffness compared to a PEEK cage, but greater friction and less dimensional stability. However, with either base polymers B, the present cage10has a significantly lower manufacturing cost in comparison with a PEEK cage, as discussed in detail below.

Referring now toFIGS.7-9, the present bearing cage10is preferably formed in accordance with the following fabrication method. First, a tube30formed of the desired composite material M. i.e., having a specific polymer base B, desired percentage and type of the reinforcing fibers F and a desired percentage and type of the lubricant L is provided. Next, an outer surface32of the tube30is machined to a desired outside diameter ODCof the cage10and an inner surface34of the tube30is machined to a desired inside diameter IDCof the cage10as indicated inFIG.7. Then, as shown inFIG.8, the tube30is cut to provide at least one annular portion having a desired axial length AL, the portion forming the annular body12of the cage10. Further, as depicted inFIG.9, a plurality of pockets14are machined in the annular body12so as to be spaced circumferentially about a centerline LCof the body12, such that each pocket14extends radially between the inner surface13A of the body12and the outer surface13B of the body12. Finally, if cavities18or grooves20are desired for retaining lubricant, these cavities18and/or grooves20are machined in at least one and preferably all of the enclosed inner surface of the pockets14. However, the present cage10may be formed in any other desired manner or method, such as by injection molding, machining entirely from a circular disc, etc.

Due to both the preferred manufacturing method and material composition as described above, with either polymer base B, the present bearing cage10has a substantially reduced manufacturing cost in comparison with a cage formed of PEEK. Specifically, the cost to manufacture a cage10from the composite material M including UHMWPE as the polymer base B is about five percent (5%) of the cost to manufacture a similar cage from PEEK, in other words, a ninety-five percent (95%) reduction in cost. Also, the cost to manufacture a cage10from the composite material M including PEI as the polymer base B is about thirty percent (30%) of the cost to manufacture a similar cage from PEEK or a seventy percent (70%) cost reduction.

Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention.

All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. The invention is not restricted to the above-described embodiments, and may be varied within the scope of the following claims.