Bearings – Rotary bearing – Antifriction bearing
Reexamination Certificate
2001-07-03
2002-05-21
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Antifriction bearing
Reexamination Certificate
active
06390684
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
This invention relates to devices having rotating shafts supported by end bearings, and in particular to scanning galvanometers and other reciprocating rotor devices wherein rotor acceleration contributes to repetitive asymmetrical loading on the rotor bearings.
2. Background Art
The rotor and load of a galvanometer scanner have mass, and as a result act like a gyroscope rotor when rotating. Because of this, the rotor-load assembly resists any attempt to change the axis of rotation, and also any attempt to change its speed of rotation. This phenomenon is recognized in the motions of a toy top, which stands upright on it's tip when first released, and then begins to tip it's axis of rotation with respect to gravity and to precess that axis as friction with the air and between the tip and the surface upon which it rests gradually slow down the spin.
Galvanometer scanners are not often exposed to forces which attempt to change the tilt of the axis, although they can be in some applications on mobile platforms. On the other hand, galvanometer scanners are almost always used in applications in which the speed of rotation is changed rapidly, and in the limit, reversed in sense. In fact, it is one of the attributes sought in galvanometer scanners that they be capable of extremely rapid changes in rotational speed and direction. For example, it is common to have a beam scanning galvanometer with a rotor motion that is represented as continuous saw-tooth waveform in which one cycle consists of a constant velocity period of movement in one direction for “scanning”, followed by a rapid fly-back period of slowing and reversing of direction, motion in the other direction, slowing and reversing direction again, and re-acceleration to scan speed. Any of these changes results in an attempt by the rotor and its load to precess, and the attendant moments are resisted by the bearings which support the rotor and load.
Examples that may provide context for the reader include the prior art of Montagu's U.S. Pat. No. 5,225,770, which illustrates a conventional rotor and bearing arrangement. The prior art of Chandler's U.S. Pat. No. 5,280,377 provides a contextual explanation of the forces to which a scanning galvanometer rotor of this sort is subjected, including the acceleration forces occurring during the fly back period between constant speed forward scan.
In addition, any imbalance in the rotating parts causes the rotor assembly to attempt to rotate on an axis which passes through the mass center, and to the degree that this axis departs geometrically from the rotor axis, a set of additional moments is imposed periodically on the bearings. As a practical matter, some degree of imbalance is always present in a working galvanometer scanner.
In the limit, these moments can exceed the load rating of the bearings, and cause irreversible damage to the bearing parts, in particular the raceways and balls, which leads to loss of smoothness of operation and eventually to failure of the bearing.
Since these issues are not confined to galvanometer scanners, and are characteristic of all rotating machinery, much effort has been devoted to ameliorating the effects of the gyroscopic moments on bearings. Modern rotating machinery, like galvanometer scanners, is balanced to the highest degree of precision practicable. Bearings have been developed to display very great resistance to what is called in the trade “Brinelling”, a name for the localized dents in the balls and rings which results from exceeding the elastic limit of the materials. Special materials, such as ceramics, which have very high elastic limits in compression, have been used. However, galvanometer designers continue to produce designs which are capable of greater accelerations than the bearings can stand.
Moving for background purposes to an unrelated field of art; rheology is defined as the science of the flow and deformation of matter. Some materials approach the behavior of ideal fluids and are described as being viscous. Other materials approach the behavior of ideal solids and are described as being elastic. Visco-elastic materials may be formed into fabricated useful shapes having desirable rheological properties.
Load shock and vibration from other sources also cause potentially damaging asymmetrical bearing loads. Devices for dampening irregular, externally induced load shock and vibration in rotating assemblies, such as road surface effects in vehicle axle bearing assemblies, have been the subject of patents. By way of example, Pinkos et al's U.S. Pat. No. 5,730,531 discloses a center bearing assembly with rheological fluid for dampening vibrations. An electromagnetic field, varied by an electronic controller in relation to the axle speed, acts on the rheological fluid in the dampener, to increase or decrease the effectiveness of the dampener as between the center bearing and the support structure.
In another vibration dampening application, Duggan's U.S. Pat. No. RE36270, reissue of U.S. Pat. No. 5,452,957, first published in Sep. 26, 1995, discloses a vehicle axle and bearing assembly, with the axle and center bearing supported within a donut-shaped bladder formed of an elastomeric material and filled with a rheological fluid, the assembly being attached by a suitable support bracket to the vehicle. A controllable source of electromagnetic field is located adjacent the bladder, and used to vary the flow or shear characteristics of the rheological fluid in the bladder, thus offering a variable dampening capability.
The rheology art described above has no where been suggested as useful for or applicable to the general art and the particular shortcomings of high speed reciprocating rotor and galvanometer bearings as to the problem of out of balance conditions and asymmetrical bearing loads and damage caused by acceleration.
SUMMARY OF THE INVENTION
As was intimated in background section, in the special case of galvanometers scanners and other similar high speed reciprocating rotor devices, what is often desired is a scan or rotary position versus time profile which might, for example, be a saw-tooth waveform in which there is a constant velocity period followed by a rapid fly-back period, similar to the horizontal scan waveform of a television tube. Of course, in the television tube, it is an electron beam, rather than a rotor of significant mass, that is being manipulated.
Just as in the case of the television tube, the part of the scan which is required to be precise is the constant velocity “forward” portion. The fly back portion is relatively uncontrolled, but is required to take place in the shortest possible time. As a result, a very large acceleration is applied at the end of the forward scan to slow and reverse the direction of rotation, return the rotor to the other end of its angular path, slow and reverse its direction again, and re-accelerate it to scan speed. The degree of difference in the asymmetrical loading between the scan phase and the fly back phase is substantially the result of rotor acceleration. It is the gyroscopic loads induced by these accelerations which have the most potential for damage to the bearings, but which occur during the part of the scan which is not required to be under precise geometrical control.
This observation leads to the possibility of providing a coupling between the bearings and their housing which is stiff during the constant-velocity portion of the scan, assuring adequate geometrical precision, but is resilient during the period of large bearing moments. This coupling would desirably absorb some or most of the force applied between the bearing and it's housing during rotor acceleration by deflecting, but would return to it's initial position when the acceleration forces disappeared.
In pursuit of the goals of the invention, such a coupling has been successfully constructed by placing a visco-elastomeric “O” ring or a multiplicity of “O” rings in a concentric fashion between the outer
Asmus Scott J.
GSI Lumonics Corporation
Hannon Thomas R.
Maine Vernon C.
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