WO2006110272A1

WO2006110272A1 - Low torque wheel bearing

Info

Publication number: WO2006110272A1
Application number: PCT/US2006/010244
Authority: WO
Inventors: Mark A. Joki; David Pierman
Original assignee: The Timken Company
Priority date: 2005-04-08
Filing date: 2006-03-20
Publication date: 2006-10-19

Abstract

The drag torque of grease lubricated tapered roller wheel bearing assemblies (10) is reduced using a combination of an increased mobility grease and (1) an improved finish on the rib and roller end contact and/or (2) a reduced rib height (H) which keeps the 'contact height' between the rib surface (18a) and the roller (16) end to about 1 mm. It has been found that the use of either of these two in conjunction with an increased mobility grease creates a synergistic effect which reduces frictional forces in the bearing assembly.

Description

LOW TORQUE WHEEL BEARING Cross-Reference To Related Applications

This application claims priority to US Provisional Patent Application Number 60/669,499 filed April 8, 2005 and which is incorporated herein by reference.

Statement Regarding Federally Sponsored Research Or Development

Not Applicable. Technical Field

Vehicle fuel consumption is a significant operating cost and emissions concern. While not the primary source of parasitic drag in a vehicle because of their slow speed, wheel bearings nonetheless, consume significant power. Greases, including polyurea greases, have been used in taper roller wheel bearings but have not been advantageous for parasitic drag losses and have been inferior to lithium greases in high temperature survivability.

Summary Of The Invention

This invention allows the reduction of drag torque of grease lubricated tapered roller wheel bearings with a combination of an increased mobility grease and (1) an improved finish on the rib and roller end contact and/or (2) a reduced rib contact height which keeps the "contact height" between the rib and the roller end to about 1 mm. It has been found that the use of either of these two in conjunction with an increased mobility grease creates a synergistic effect which reduces frictional forces in the bearing assembly. Brief Description Of The Drawings

FIG. 1 is a cross section of a packaged wheel bearing made in accordance with the present invention; FIG. 2 is an enlarged view of the bearing assembly showing the contact between a roller bearing and rib face of the packaged wheel bearing;

FIG. 3 is a chart graphing torque test data comparing different greases with two different finishes; and

FIG. 4 is a chart graphing the effect of rib/roller contact height on high temperature survivability.

Corresponding reference numerals will be used throughout the several figures of the drawings. Description Of The Invention

The following detailed description illustrates the invention by way of example and not by way of limitation. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what we presently believe is the best mode of carrying out the invention. Additionally, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

Referring to FIG. 1 , a packaged wheel bearing 10 comprises a non- rotating double outer race 12 and two rotating inner races 14 defining outer and inner raceways 12a and 14a, respectively, with two sets of taper rollers 16 interposed between each combination of inner and outer races 12,14. Guide ribs 18 are associated with the inner races 14. The guide ribs 18 have inner faces 18a which define a conical surface. The guide ribs 18 resist the tendency of the tapered rollers 16 to move along the roller axis as load is exerted on the rollers' tapered or conical body surface by race contact. Typically, a grease with a metallic soap thickener such as lithium is injected into the area of the rollers as the bearing is assembled. Elastomeric lip seals 20 are mounted between the races near outer ends of the races to prevent the loss the lubricating grease and prevent the ingress of contaminants into the bearing.

The roller ends are generally spherically profiled. That is, the large end faces of the rollers define a segment of a sphere having a radius slightly less than the apex length for the bearing assembly. The spherically profiled roller ends have a relative sliding velocity against the typically conical contact face of the rib 18. At slow vehicle speeds this contact has insufficient hydrodynamic pressure to avoid metallic contact. At typical highway speeds there is greater hydrodynamic pressure but only if there is an adequate supply of lubricant.

It has been found that the typical NLGI #2 metal soap thickener grease has insufficient mobility to insure that there is an adequate supply of lubricant to the rib area. A polyurea thickened grease improves the mobility of the grease providing adequate grease to the rib. However, with this polyurea grease alone, film formation does not occur except for the synergistic effect of the polyurea grease and one or both of low surface roughness of the mating surfaces (i.e., the roller end face and the rib face 18a) and a reduced rib contact height. This combination of features allows a lubricant film to separate the surface asperities of the roller end face and rib face 18a, dramatically reducing drag.

The typical sum of surface roughness of roller end and rib face is 10 to 20 microinches (μ") arithmetic average. However, for hydrodynamic film separation at highway speeds, the sum must be less than 10 microinches and preferably 5 microinches. Hence, the surface roughness needs to be substantially less than the typical surface roughness, and can be less than one-half or one-quarter of the typical surface roughness. The grease should be thickened to an ASTM D217 penetration of 260-300 and contain oil with an effective viscosity of 110-14OcSt (and preferably

115 cSt) at 4O⁰C. The grease can be thickened in manners well known to those skilled in the art

Increased sliding velocity at the rib-roller contact is generally favorable to hydrodynamic film formation, and therefore, low torque behavior. However, the bearing is subjected at times to increased operating temperature due to braking heat. Because the oil in the grease is relatively low in effective viscosity for the benefit at lower temperatures, under these severe conditions, it is not possible to maintain lubricant film separation between the roller ends and the rib. Because of this condition, it is necessary to have reduced sliding velocities. For low sliding velocity, the rib and roller should contact each other a minimum distance away from the closest race contact, i.e., "contact height". When a small sphere, such as the end face 16a of a roller 16, contacts a large cone, such as the rib face 18a, at zero load, it contacts at a point, which when loaded, becomes a contact area A. The design point is the unloaded contact point. The height C of this point above the raceway is the contact height. Rib height H is typically greater than this contact height and is not directly related to the contact height. Rather, the contact height is influenced by the spherical roller end radius and conical rib angle The contact height C is shown diagrammatically in FIG. 2. The nominal contact height should be about 1 mm. Without this feature, it is not possible to take advantage of the torque reductions available from the improved grease and finishes. During use, there is a synergistic affect caused by the combination of the grease and the finish, as noted above. There is also a synergistic effect caused by the combination of the grease and the reduced rib contact height. There the two synergistic effects are, at a minimum, additive, and, there may even be a third synergistic effect (greater than the two synergistic effects individually), when all three factors (grease, finish, and rib contact height) are combined.

The chart of FIG. 3 graphs the effect of lithium and polyurea thickened greases; the lithium grease contains an oil having an effective viscosity of about 150 cSt, one of the polyurea greases contains an oil having an effective viscosity of about 115 cSt, and the other polyurea grease contains an oil having an effective viscosity of about 150 cSt. Each of the three greases was run in three bearings having a 12 microinch Ra composite (sum) finish rib and roller contact and three bearings with a 4 microinch Ra composite (sum) finish rib and roller contact. The three greases had approximate average parasitic drag torques for the two finishes as set forth in Table I below. The data of Table I is shown in FIG. 3.

TABLE I

As seen in the graph of FIG. 3 and Table I above, there was not a significant difference between the average drag torques for the polyurea thickened greases, however, the change in finish from the rough (12 μ") finish to the smooth (4 μ") finish provided a significant advantage.

The graph of FIG. 4 shows bearings lubricated with a polyurea grease containing an oil having an effective viscosity of about 115 cSt run in a high temperature environment (about 275°F) to test survivability of bearings with two different contact heights. The test was conducted on six bearings having a contact height of about 2mm and six bearings having a contact height of about 1mm. The bearings were run for 70 hours. Only two of the six bearing having the high (2mm) contact height survived the full 70 hours, whereas five of the six bearings with the low

(1mm) contact height survived the test. Stated differently, the high contact height bearings had a 33% survivability rate, whereas the low contact height bearing had an 83% survivability rate. Hence, the low rib/roller contact height was found to substantially increase survivability in high temperature environments.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

Claims:

1. A tapered roller wheel bearing assembly (10) comprising an outer race (12), an inner race (14), a plurality of tapered rollers (16) positioned between the inner and outer races, a guide rib (18) extending from the outer end of one of the inner and outer races towards the other of the inner and outer races, a seal (20) between said guide rib (18) and the race (12, 14) opposite said guide rib, and lubricating grease within bearing assembly (10), said lubricating grease having a polyurea thickening agent; said tapered roller having a substantially conical body and a substantially spherical end, said guide rib having a contact face; and wherein: a. the arithmetic average roughness of said guide rib face and roller spherical end having a sum of less than about 10 microinches; and/or b. the contact height between the rib face and the end face of the roller is about 1mm.

2. The tapered roller wheel bearing assembly according to claim 0 wherein average roughness of said guide rib face and roller spherical end having a sum of less than about 5 microinches.

3. The tapered roller wheel bearing assembly according to claim 0 wherein the grease is thickened to an ASTM penetration of 260-300.

4. The tapered roller wheel bearing assembly according to claim 0 where the lubricating grease contains an oil with an effective viscosity of less than 14OcSt at 40⁰C.

5. The tapered roller wheel bearing assembly according to claim 0 where the lubricating grease contains an oil with an effective viscosity of about 115cSt at 4O⁰C.

6. A tapered roller wheel bearing assembly comprising: a. an outer race and an inner race, a guide rib extending from the outer end of one of the inner and outer races towards the other of the inner and outer races, and a seal between said guide rib and the race opposite said guide rib; b. a plurality of tapered rollers positioned between the inner and outer races, said tapered roller having a substantially conical body and a substantially spherical end; and, c. a lubricating grease within the bearing assembly, said grease having a polyurea thickening agent; d. wherein, the arithmetic average roughness of said guide rib face and roller spherical end having a sum of less than 10 microinches.

7. A tapered roller wheel bearing assembly comprising: a. an outer race and an inner race, a guide rib extending from the outer end of one of the inner and outer races towards the other of the inner and outer races, and a seal between said guide rib and the race opposite said guide rib; b. a plurality of tapered rollers positioned between the inner and outer races, said tapered roller having a substantially conical body and substantially spherical end; said and, c. a lubricating grease within bearing assembly, said grease having a polyurea thickening agent; d. wherein, the contact height between the rib face and the end face of the roller is about 1 mm.