Bearings – Rotary bearing – Antifriction bearing
Reexamination Certificate
1999-12-14
2001-11-20
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Antifriction bearing
C384S513000
Reexamination Certificate
active
06318899
ABSTRACT:
TECHNICAL FIELD
The present invention is in the field of rolling element ball bearings. Particularly, the present invention is directed to all-ceramic rolling element ball bearings that require no lubrication.
BACKGROUND OF THE INVENTION
A long recognized need in rolling element ball bearing applications is a bearing that requires no lubrication, can operate at elevated temperatures, is lightweight and, has the ability to operate in a corrosive environment. All metal rolling element ball bearings require some form of lubrication due to the relative motion that is inherent between the balls and the balls raceways (inner and outer races). The relative motion is the result of two radii of different dimensions operating in contact with each other. The ball has a slightly smaller radius than the raceways thereby creating a line-contact arc curve region. The center point of the line-contact arc curve receives the greatest radial load between the ball and the raceways and has no relative motion during bearing operation. However, all points along the line-contact arc curve away from the center point are sliding with respect to the center point of the curve. The design intent of the line-contact arc curve is to displace the bearing loads imposed on the bearing members over an arc length sufficient to prevent permanent damage to the metallic bearing components. This life limitation can be in the form of brinelling (or indenting) of the metallic bearing surfaces, low cycle metal fatigue, or simply frictional wear of the bearing surfaces. Bearing life is extended through prudent design of the line-contact arc curve interface between the balls and the raceways such that the metallic material properties are not exceeded in normal bearing operation. In addition, lubricating fluids (oil, grease, or solid film lubricants) are used in the bearings to lower the friction between the balls and the races thereby extending bearing fatigue and wear life while reducing the bearing drive torque requirement.
Rolling element ball bearings containing metallic components (balls or raceways) are also limited to relatively low operating temperatures. In part, the temperature limitations are due to the degradation in strength of the metallic bearing steels with elevated temperature but also, temperature limitations are imposed because of the operating temperature limitations of the bearing lubricant. The use of dry film lubricants in place of oils and greases is an attempt to increase the operating temperature capability of some bearings without lubricating oil or grease temperature restrictions while minimizing the complexity of the lubrication system.
State-of-the art ceramic balls are being used in hybrid bearings (ceramic balls/metallic raceways) due to the lower dynamic friction coefficient between the ceramic balls and the metallic races versus that of metallic balls to metallic races. The high Young's Modulus and low density of ceramic balls also helps reduce ball deformation during high speed bearing operation for the former, and reduces ball skidding and outer race stresses for the latter. The weak link in hybrid bearings is still the metallic raceways due to metallic material limitations (i.e., compressive strength and hardness). Additionally, the contact region between the ceramic balls and metallic raceways still requires a line-contact arc curve and hence the requirement for some form of lubricant. The hybrid bearing lubrication requirements are less than those of an all-metallic bearing but nonetheless are required (in some amount) for normal bearing operation.
While most all-metallic bearings utilize the line-contact arc curve interface design, some metallic bearings utilize a concept known as a four-point contact bearing configuration. This concept creates point-contact between the ball and the raceway via a curved ball outer surface against a flat (or conical) raceway surface and therefore does not adequately distribute the imposed bearing loads along the contact surface of the metallic raceways. These point-contact (hertzian) loads result in very short bearing life and very low bearing load capacity and are generally used as stop-gap bearings only. Bearing steels have a relatively low compressive strength and hardness and will brinell quickly in point-load contact. The bearing surface of the balls and the raceways in a four-point contact bearing quickly degrade as brinelling occurs increasing interfacial friction and causing micro-welding and material pull-out between the metallic bearing components. The point contact zone immediately degrades to a relatively rough line-contact arc curve as the bearing surfaces rapidly degrade to failure. Continued bearing operation under these conditions always results in catastrophic bearing failure. The best metallic materials therefore will not support point load contact due to insufficient compressive strength and material hardness.
SUMMARY OF THE INVENTION
In view of the deficiencies of the conventional all-metallic and hybrid ceramic/metallic rolling element ball bearings as a result of the metallic material property limitations of compressive strength and hardness, it is the primary object of this invention to provide a unique bearing design with ceramic materials capable of being operated in a four-point (hertzian) contact configuration for extended periods of bearing life without the need for any type of lubricating medium for use between the balls and the raceways.
Accordingly, the present invention provides a ceramic/ceramic bearing structure in which both the balls and the bearing races are designed and fabricated from a state-of-the-art ceramic material such as a silicon nitride (Si
3
N
4
), Alumina (Al
2
O
3
), Zirconia (ZrO), etc. The bearing is designed such that the inner diameter of the bearing inner race is fabricated from a candidate state-of-the art ceramic material which circumscribes (or is otherwise attached to) a metallic rotating shaft member and is anti-rotated and axially and radially positioned via an interference fit, braze joint, coupling, or coupled shaft segment. Said inner race may be split (or monolithic) radially along its axial length such that ceramic balls and a ball separator (ball set) can be installed. The ceramic ball(s) set then engages one half of the ceramic inner race and contacts a single-piece (monolithic or radially split) outer ceramic race. The inner and outer races may have inwardly V-shaped (or conical) surfaces such that the open end of the V-shape on both races (inner and outer) are facing the ceramic balls. Another means of achieving the desired point contact between the ceramic ball and the ceramic raceways is by having the ceramic balls contact the open end of a radius in the raceways. When the second half (or monolith) of the inner (and/or outer race) is installed, the all-ceramic bearing assembly is complete and the ceramic balls are contacting the inner and outer races at 4 points (or 3 points when an axial bearing pre-load is applied). Some dimensional tolerances may be applied to allow for thermal expansion of the bearing elements with respect to themselves and the adjacent shaft and structures. A means of dynamically isolating the bearing may also be applied between the outer race O.D. and the stationary structure in which it is mounted, (i.e., hydraulic mount, spring mount, resilient mount, etc.). Axially pre-loading the bearing such that the ceramic ball is driven through one (point contact location) side of inner race can be achieved through the use of axial springs.
An advantage of the present invention is that it provides a rolling element ball bearing that does not require lubrication between the ceramic ball and the ceramic raceways, vastly simplifying the bearing design and equipment design in which the bearing is installed. A further advantage of the present invention is that the all-ceramic bearing can operate to significantly higher temperatures than conventional all-metallic or hybrid ceramic/metallic rolling element ball bearings providing a strategic advantage in applicatio
Ceramic Engineering Consulting, Inc.
Hannon Thomas R.
Holland & Knight LLP
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