Measuring and testing – By abrasion – milling – rubbing – or scuffing
Reexamination Certificate
2003-02-24
2004-04-06
Chapman, John E. (Department: 2856)
Measuring and testing
By abrasion, milling, rubbing, or scuffing
C073S577000
Reexamination Certificate
active
06715336
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to a device and corresponding method for testing materials for structural weakness. In the preferred embodiment, the invention is directed to a device and a method for testing ball and/or roller bearings to determine their tendency to experience fretting.
BACKGROUND OF THE INVENTION
Ball bearings and roller bearings are components that are used in vast array of machine tools and high-performance instruments. Bearings are found in an enormous variety of moving components, ranging in magnitude from heavy earth-moving equipment to delicate instruments such as disc drives, video tape recorders, and high-precision machine instruments. In short, the rolling contact bearing (i.e., ball bearings, roller bearings, and the like) is a fundamental element of machinery that, in many instances, determines the overall performance characteristics of the machine. If a particular bearing breaks or seizes, for example, not only is the individual segment or module wherein the bearing lies compromised, but the entire instrument is likely to cease functioning properly. Further, due to the very tight tolerances inherent in their function, if one bearing begins to break down or deform, it will generate intense friction and result in overheating which will degrade the race or track in which the bearing rides. This in turn will cause other bearings with which it is housed to become deformed, leading to a domino effect that rapidly affects the entire bearing assembly.
It follows from the above paragraph that the physical specifications of bearings are extremely important in manufacturing and maintaining precision instruments and machinery that have moving parts. In order to construct bearings and their races, and to keep them within precise tolerances over a defined period of time, a number of parameters must be monitored. These parameters include the specific dimensions of each bearing and its race, the hardness of those components, and their ability to withstand impact, resist corrosion, and resist distortion under load. Although all bearings become unserviceable over time due to wear, they may also become unserviceable because of seizing, breakage, undue wear or premature aging, false brinelling, flaking, and corrosion. Bearing failure can also be the result of rolling fatigue, incorrect selection, improper handling and/or improper maintenance, excessive load, poor shaft or housing accuracy, peeling, spalling, chipping, cracking, rust, corrosion, and fretting.
Many methods have been described to increase the life of bearings and decrease their incidence of breakdown. These include special alloys from which to fabricate the bearings (see, for example, U.S. Pat. Nos. 6,358,333; 6,422,756; and 6,315,455, all to Tanaka); use of ceramic bearings (see U.S. Pat. No. 6,416,228); use of silicon hybrids (see, for example, NSK Robust Series), as well as specific designs for the bearing structure itself (see U.S. Pat. Nos. 5,700,093 to Hiramatsu and 5,803,614 to Tsuji). Clearly, any method to optimize bearing performance must rely on tests to measure the physical characteristics of the bearing itself, both before and after the bearing has been put to its prescribed use.
Conventional methods to test bearing performance include simple performance verification tests wherein a prototype of the bearing is put through high pressure, high speed heavy duty use, etc., to verify bearing loss, seizure, and service life. (See, for example, Mitsubishi Heavy Industries, Ltd., Technical Rev. 39:26-30 (2002).) Other conventional means of testing bearing endurance include seizure tests wherein machinery is operated using the prototype bearing and the oil supply is curtailed while the machine continues to run. By measuring the oil flow and temperature, the response time required to save the system from breakdown can be determined. See SensIT Newsletter #3, June, 2001. Other means to test bearing breakdown include vibration testing to determine fretting resistance and impact resistance. All of these tests are implemented on bearing assemblies and the test conditions in local contact areas cannot be controlled precisely. Therefore, the failure modes of the bearings can only be investigated statistically.
Methods used to make conventional bearing assessments rely on mechanical mechanisms. Currently, conventional actuators, such as step motors, servo motors, and hydraulic or pneumatic actuators are used to deliver force or pressure in a test environment. A particular disadvantage when using conventional actuators is their poor dynamic response to a quick force change, especially under a heavy load.
More recently, electronic equipment and computer chips have been making use of piezoelectric devices as switches, thereby obviating the need for mechanical switches with their size and inherent error. The piezoelectric effect was first discovered when a pressure was applied to a quartz crystal and an electric charge in the crystal was created. It was later found that by applying an electric charge to the crystal the material would deform in shape in a standard degree. The first use of the piezoelectric effect was in ultrasonic submarine detectors developed during World War I. It was later found that barium titanate ceramics could be made which exhibited the piezoelectric phenomenon.
Because the tolerances of bearings are so precise and their proper action crucial to their operation, there is a need for the development of bearing test mechanisms that can apply force in a way that is controlled to deliver similar magnitude to a much more discrete area and measure the results on a nanoscale range. Further, because bearings are inherently moving parts, they are particularly subject to dynamic stress that can lead to fretting of the bearing.
Fretting is a form of adhesive wear; it occurs as the result of small scale oscillatory movements between the bearing and its housing. Typically fretting appears as highly polished regions on the bearing surface or as pockmarks on the bearing. Fretting is also accompanied by evidence of material movement between the housing and bearing back. Fretting occurs when there is unwanted relative movement between the bearing and housing. This movement can arise from any number of sources, such as an oversized housing, dirt or burrs on mating faces of the housing, insufficient bolt torque, deformation of the bearing under load, etc. Another cause is the flexibility of the bearing assembly itself. If the material from which the bearing and/or bearing race is assembled has inappropriate stiffness, the entire assembly may flex sufficiently under dynamic load to cause a relaxation of the radial interface and allow unwanted relative movement between the bearing and the race.
Fretting can eventually lead to overheating of the bearing material due to poor heat dissipation between the bearing back and the housing. This, in turn, leads ultimately to failure of the bearing entirely or generation of excessive heat that causes other components to fail.
The invention described herein was designed to investigate, measure, and otherwise quantify and/or qualify the fretting resistance of a bearing material. The invention is capable of controlling the load placed on a hearing while simultaneously subjecting the bearing to precisely controlled oscillatory movement. The invention thus yields superior dynamic results, and reduces the time-of-testing cycle significantly.
SUMMARY OF THE INVENTION
A first embodiment of the invention is directed to a device for testing the fretting of a bearing. In this embodiment, the device is dimensioned and configured so that a first object (e.g., a ball bearing) is urged against and dynamically translated, rotated, or otherwise urged against the surface of a second object (e.g., a flat plate of bearing housing material). In this embodiment of the invention, the device comprises a force scanner which comprises a first piezoelectric actuator. A workpiece holder is also optionally included, the workpiece holder being dimensioned and configured to hold
Chapman John E.
DeWitt Ross & Stevens S.C.
Leone, Esq. Joseph T.
nPoint, Inc.
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