Patent Application
20140226928
The present disclosure relates to angular contact ball bearings and, in particular, to angular contact ball bearings that may be employed in relatively high speed applications including, for example, high speed dental handpieces working at speeds of up to 500,000 rpm.
Angular contact ball bearings are generally selected for applications wherein loading occurs in both the axial and radial directions. These bearings may be used in relatively high speed, precision applications including, for example, dental applications, turbo chargers, electric motors, automotive applications, pumps and compressors. However, problems remain due to bearing retainer wear, which is generally understood to be the one of the leading causes of a high speed bearing failure.
For example, retainers used in high speed applications, such as high speed dental handpieces, may be piloted (guided) by the inner or outer bearing ring land surfaces, requiring physical contact between the retainer, formed of a relatively softer material, such as a polymer resin, and the bearing ring land surface, formed of a relatively harder material. Guiding is specifically necessary in the bearings working at extremely high speeds, for example in dental handpieces rotating with the speeds up to 500,000 RPM, to minimize and limit uncontrollable otherwise retainer vibrations. Guiding requires a physical contact between the retainer and ring causing the retainer to wear. Therefore, a need still remains in the bearing industry to avoid or minimize retainer wear that could ultimately lead to premature failure.
An aspect of the present disclosure relates to an angular contact ball bearing. The angular contact ball bearing includes an inner ring having an outer diameter surface and a raceway defined in the outer diameter surface and an outer ring including an inner diameter surface and a raceway defined in the inner diameter surface. Further, the angular contact ball bearing includes a plurality of balls in rolling contact with the raceways. In addition, the bearing includes a retainer having a first side and a second side opposing the first side. The retainer has a width being the distance between said first side and said second side. The retainer also has an outer surface extending between the first side and the second side and an inner surface opposing the outer surface extending between the first side and the second side. The outer surface and inner surface of the retainer define a central portion having a first thickness T1, and tapered portions each having a variable second thickness T2. Each tapered portion (i) extends from one of the sides to the central portion up to 28% of the length of said retainer width, (ii) tapers towards the inner surface of the retainer at an angle of 20° or less relative to said central portion, and (iii) tapers away from the inner diameter surface of the outer ring. Further, the second thickness T2 of the retainer is less than the first thickness T1, wherein T2<T1. In addition, the retainer includes a plurality of through holes, commonly known as ball pockets, defined in the retainer, wherein one ball is positioned in each of the ball pockets in a rotatable manner.
In another aspect, the present disclosure relates to an angular contact bearing. The bearing includes an inner ring including an outer diameter surface, and a raceway defined in the outer diameter surface, and an outer ring including an inner diameter surface, and a raceway defined in the inner diameter surface. The bearing also includes a plurality of balls in rolling contact with the raceways and a retainer. The retainer includes a first side and a second side opposing the first side, the retainer having a width being the distance between the first side and the second side. The retainer also includes an outer surface extending between the first side and the second side and an inner surface opposing the outer surface extending between the first side and the second side. The outer surface and the inner surface define a central portion having a first thickness T1, and tapered portions each having a second thickness T2. Further, each tapered portion: (i) extends from one of the sides to the central portion at least 11% of the length of the retainer width or more; (ii) tapers towards the inner surface of the retainer; (iii) tapers away from the inner diameter surface of the outer ring, and the second thickness T2 is less than the first thickness T1 wherein T2<T1. Finally, a plurality of through holes are defined in the retainer, wherein one of the balls is positioned in each of the through holes in a rotatable manner.
In a further aspect, the present disclosure relates to the incorporation of the angular contact bearing in a dental handpiece.
The above-mentioned and other features of this disclosure, and the manner of attaining them, may become more apparent and better understood by reference to the following description of embodiments described herein taken in conjunction with the accompanying drawings, wherein:
The present disclosure relates to angular contact ball bearings and, in particular, to angular contact ball bearings that may be employed at relatively high speed in dental handpieces operating at speeds up to 500,000 revolutions per minute. Although reference is made to the use of the angular contact ball bearing herein in connection with such application, the invention herein is more broadly directed at reducing retainer-outer ring contact and overall retainer friction and wear.
As noted above, bearing retainer (i.e., cage, separator) wear is understood to be one of the single leading causes of bearing failure. Since the bearing works at the extremely high speed, retainer dynamic stability is desirably maintained to avoid retainer uncontrollable vibration causing undesirable handpiece noise and accelerated wear. One of the ways to minimize retainer vibration is piloting or guiding it by means of designing it with dimensions resulting in the retainer rotating in the close proximity to the piloting land. The retainer can be piloted either by the inner ring land or outer ring land. Piloting the retainer by an outer ring land is preferable due to the fact that inner ring rotational speed is near 2.5 times faster than the retainer speed in the bearing size typical for the most high speed dental handpieces. Therefore, less wear is seen on a retainer surface when it is piloted by an outer ring land. However, some wear still exists.
The outer surface 134 and inner surface 136 define a central portion 138 having a first thickness T1, and tapered portions 140a, 140b each having a second thickness T2. The outer surface 134 of the central portion 138 of the retainer may be cylindrical in shape. The tapered portions 140a, 140b extend from each side to the central portion 138. In addition, the tapered portions extend along up to 56% of the length L of the width of the retainer between the retainer sides 130, 132 including all values and ranges from 11% to 56%, 11% to 40% along the length L of the outer surface 134, i.e., is the width of the retainer between the retainer sides 130, 132. For example, each tapered portion may extend from each side 130, 132 along up to 28% of the length L of the width being the distance between the first side and the second side, including all values and ranges from 5.5% to 28% of the length L, such as 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%, 9.0%, 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5%, 15.0%, 15.5%, 16.0%, 16.5%, 17.0%, 17.5%, 18.0%, 18.5%, 19.0%, 19.5%, 20.0%, 20.5%, 21.0%, 21.5%, 22.0%, 22.5%, 23.0%, 23.5%, 24.0%, 24.5%, 25.0%, 25.5%, 26.0%, 26.5%, 27.0%, 27.5%, and 28.0%, and preferably from 5.5% to 28%, 5.5% to 20%, 6.0% to 20%, 10% to 20%, etc. Furthermore, referring to
In embodiments, the tapered portions 140a, 140b are symmetrical relative to an axis A-A which axis is orthogonal to the outer surface 134 of the retainer 108. The axis A-A is also parallel to the sides 130, 132 of the bearing 100. In other embodiments, the tapered portions are asymmetric, yet still taper away from the inner diameter surface of the outer ring and towards the inner surface of the retainer. In other words, the taper on either side may taper at different angles or extend along different lengths.
In embodiments, the angle α of the taper is in the range of 20° or less, including all values and ranges therein, and preferably 10°, 11°, 12°, 13°, 14°, 15°, 16°, 17°, 18°, 19°, 20°, 10° to 15° and more preferably 13° to 14°, relative to a plane defined by the central portion 138 of the retainer as illustrated in
In addition, the tapering of the retainer creates an extra space 160 (see
In embodiments, the retainer 108 is formed from a polymer material. Polymer materials include, for example, those exhibiting antifriction characteristics and may be inherently lubricious or various additives may be blended in the materials such as oils, molybdenum disulfide, graphite, hexagonal boron nitride, PTFE powder, or tungsten disulfide. Preferably, the polymer materials, include for example, polyimide (PI), polyamide imide (PAI), polyether ether ketone (PEEK), polysulfones, or epoxy resins. Such polymer materials may therefore include those that have glass transition temperature of 150° C. or higher, such as 150° C. to 350° C. Other materials may be utilized, such as a ceramic material or other inorganic materials. The use of such anti-friction materials reduces the overall wear of the bearing systems. Referring to
Being that the first annular gap 144 is smaller than the second annular gap, the retainer 108 is piloted, or guided by, the outer ring, wherein the inner diameter surface 120 provides piloting lands 150a, 150b on either side of the raceway 116. With the first annular gap dimensions and the wobbling, non-concentric, nature of high speed retainer motion, a cycling direct physical contact between the outer surface of the retainer and the outer ring land is present. This contact serves the purpose of limiting retainer vibration, but as may be appreciated in return, it prompts retainer wear.
Referring again to
In embodiments, the plurality of ball pockets 110 are defined in the central portion 138 of the retainer 108 and preferably, are centered around the central axis A-A. Alternatively, the plurality of ball pockets are offset from the central axis A-A and are defined in both the central portion and part of the tapered portion. Further, when the ball pockets 110 are fully defined in the central portion 138, the outer surface 134 of the central portion 138 of the retainer 108 forms a land or lands 154 around and completely surrounding the plurality of ball pockets 110.
Thus it may be appreciated that in providing the tapered retainer the areas of direct retainer—outer ring contact are reduced and therefore the sliding retainer friction and wear is minimized. The lands 154 forming the relatively narrow cylindrical “belt” remain, in embodiments, immediately on the left and on the right relative to the ball pocket (i.e., through hole), providing for retainer piloting. By tapering the outer surface of the retainer, not only is wear reduced, but an extra space 160 is created allowing for the lubricant to be collected there through centrifugal forces during the bearing operation. To compensate for the retainer mechanical integrity weakening due to the reduced cross section due to the implementation of the double taper, the retainer inner diameter may be reduced as compared to a standard cylindrical retainer and the thickness of the retainer may be increased, increasing the retainer cross section and compensating for the taper. However, as noted above, the gap between the inner surface of the retainer and the outer diameter surface of the inner ring is greater than the gap between the outer surface of the retainer and the inner surface of the outer ring. For example, the outer ring gap (clearance) should be maintained between 0.001″ and 0.002″ for the retainer to be considered an outer ring piloted—for a typical dental bearing. At the same time the inner ring clearance is typically larger, for example, between 0.005″ and 0.007″.
The angular contact bearings so formed exhibit a Weibull Characteristic Life of 60 hours or greater, such as in the range of 60 to 85 hours, including all values and ranges therein, such as 65 hours, 82 hours etc., when tested according to the New Hampshire Ball Bearings accelerated life testing protocol, described further in the example below, wherein the bearings are utilized in a high speed dental handpiece subjected to rotational cycling under external load, at the speeds up to 500,000 rpm, in the conditions of marginal lubrication with repetitive sterilization at 275° F. until bearing failure. The angular contact bearing described herein may be incorporated into various devices and, particularly, in devices that exhibit a rotational speed of greater than 50,000 rpm, including all values and ranges therein, such as 50,000 rpm to 1,000,000 rpm, including 100,000 rpm, 500,000 rpm, etc.
In an embodiment, illustrated in
In other embodiments the angular contact bearing is included turbines, turbo-chargers, high speed electric motors, or other high speed mechanisms, wherein the bearings rotate at speeds of greater than 50,000 rpm.
The bearings disclosed herein were tested in comparison to a standard bearing in a high speed dental handpiece, such as that illustrated in
The retainer material was TORLON 4301 available from Solvay Plastics and the bearing rings and balls were made from 440C stainless steel. For three designs, ten samples each, were tested using Midwest Tradition high speed dental handpieces, available from Dentsply Midwest, Inc., including 1) the design of the present disclosure including a 7° taper, 2) the design of the present disclosure including a 14° taper and 3) bearing of standard design. Testing was performed at speeds of around 400,000 rpm at 30 psi driving air pressure, a cycling side load of 5 oz, wherein the cycling sequence is 10 seconds on, 5 second off until failure. The side load is synchronized with handpiece rotation. The hand piece was also sterilized at 275° F. for 30 minutes after each 1 hour of cycling. Failure is understood to occur when the speed drops over 20% below starting or initial speed. Table 1 below includes the testing data and
As seen in the data provided above, bearings with double tapered retainer implementation demonstrate near 10% life improvement in a 7° retainer taper and up to almost a 50% life improvement in a 14° retainer OD taper compared to similar bearings assembled with the standard retainers having a generally cylindrical OD.
Thus, provided herein is an angular contact high-speed ball bearing assembly using a full ball retainer guided by the outer ring land. The retainer has, in embodiments, a symmetrical tapered outer diameter facing the bearing guiding land, (i.e., the inner diameter surface of the outer ring), which forms a narrow cylindrical “belt” bordering the ball pocket (i.e., through hole) area. This reduces frication and retainer wear as opposed to cylindrical surfaces that extend across substantially the entire outer diameter as tapered retainer geometry provides for less direct contact between the retainer outer diameter (or outer surface) and the piloting surface of the bearing outer ring. Moreover, an additional space is created between the retainer outer surface and guiding surface allowing for more of conventional lubricant to be accommodated therein as compared with traditional cylindrical outer diameter retainer shape.
The foregoing description of several methods and embodiments has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the claims to the precise steps and/or forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.
