Bearings – Rotary bearing – Fluid bearing
Reexamination Certificate
1998-10-19
2001-02-20
Noland, Thomas P. (Department: 2856)
Bearings
Rotary bearing
Fluid bearing
C029S898020, C029S898041, C073S430000, C073S739000, C116S300000, C324S1540PB
Reexamination Certificate
active
06190049
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a fluid thrust bearing apparatus wherein a rotatable element is prevented from moving axially out of a close-fitting cavity by cohesive and adhesive forces of a liquid disposed between the rotatable element and the cavity walls and negative (vacuum) pressure created when the rotatable element is axially stressed.
1. Field of the Invention
The present invention relates to the field of thrust bearings used to prevent axial movement of a shaft disposed in a cavity.
2. Description of Related Art
Most indicating instruments (e.g., ammeters, Bourdon tube pressure gauges, etc.) include a shaft, which rotates between radial and thrust bearings. The radial and thrust bearings prevent radial and axial movement of the shaft. The shaft is connected to a pointer that is juxtaposed against a dial, and the amount of shaft rotation correlates to the value of the parameter being measured (e.g., electrical current, pressure, etc.). In this way, the pointer indicates on the dial the value of the parameter.
Most indicating instruments are inexpensive. It is therefore critical that the number of parts are kept to a minimum. Further, it is preferred that manufacturing procedures are simplified to minimize labor costs. The elimination of even a single thrust bearing can provide a significant advantage to indicating instrument manufacturers.
The mechanism employed to create shaft rotation in a particular indicating instrument depends on the parameter being measured. For example, moving-magnet indicating instruments are well known for measuring electrical current. A movingmagnet indicating instrument of the cross-coil type is described in U.S. Pat. No. 5,095,266, which was issued to Yukio Ohike, et al. on Mar. 10, 1992. That patent discloses a pair of mutually perpendicular coils surrounding a magnetized rotor. The rotor is disposed in a cavity and connected to a shaft and associated pointer. Currents that vary with the momentary magnitude of the parameter being measured flow through the coils creating a correspondingly varying magnetic field. The magnetic field created by the current causes proportional rotation by the rotor and, in turn, the shaft and pointer.
By contrast, a “Bourdon tube” apparatus can be used to measure pressure. A Bourdon tube is a curved tube that is closed on one end and connected at its open end to the pressurized cavity to be analyzed. Pressure in the cavity deforms the tube, and the deformation of the tube is mechanically translated into rotation of a gear shaft. The gear shaft is coupled to a pointer, which indicates the measured pressure on a dial.
Most indicating instruments are characterized by low inertia and low friction, thus necessitating damping means. Typically, damping is provided by fluid, such as silicone oil, disposed between the rotating shaft or rotor and the walls of the cavity in which it sits. Unfortunately, most suitable damping fluids tend to migrate through the clearance between the pointer shaft and the instrument housing. This can cause the damping fluid to escape the instrument entirely or migrate into areas of the instrument where it is not desired, such as the pointer, the dial face, or the lens of the instrument. Applicant's U.S. patent application Ser. No. 09/146,741, filed Sep. 3, 1998, which is herein incorporated by reference, describes preferred “migration barriers” that can be used to prevent unwanted migration of damping fluid.
Another common characteristic of electrical and mechanical indicating instruments is that the shaft connected to the pointer must be prevented from moving axially. In most indicating instruments, axial movement is prevented by two physical thrust bearings--mechanical barriers to axial movement of the shaft in either direction. Alternatively, some prior art instruments discourage axial movement magnetically. For example, U.S. Pat. No. 4,710,706, which issued to Robert Krupa on Dec. 1, 1987; U.S. Pat. No. 3,013,210, which issued to Peter Wargo on Dec. 12, 1961; and U.S. Pat. No. 2,867,768, which issued to Austin E. Fibrance, et al., on Jan. 19, 1953, describe the use of a holding magnet to hold a magnetized shaft or rotor against a single thrust bearing, thereby preventing axial movement.
The holding magnets disclosed in these patents also bias control the magnetized shaft and return the pointer to the zero position after electrical input to the measuring device is cut off. This combination of a magnetic thrust bearing and a return-to-zero mechanism reduces the number of necessary parts for the instrument. However, a holding magnet is an inappropriate return-to-zero mechanism for some indicating instruments. For example, the shaft used in a mechanical indicating instrument, such as a Bourdon tube pressure gauge, is usually not magnetized and cannot be controlled with a magnet. Moreover, a variety of other return-to-zero mechanisms (e.g., hairsprings) are available that may be advantageous in certain applications.
What is needed is an apparatus for preventing axial movement of a rotatable element that reduces the number of necessary parts, e.g., by eliminating the need for a holding magnet or one of the two separate mechanical thrust bearings employed in the prior art.
What is needed is a thrust bearing that damps rotational movement of a rotatable element while also preventing axial movement.
What is needed is an apparatus for preventing axial movement of a rotatable element that can be employed in any indicating instrument, including electrical and mechanical indicating instruments.
What is needed is an apparatus for preventing axial movement of a rotatable element that can be employed in an indicating instrument with any return-to-zero mechanism.
SUMMARY OF THE INVENTION
The present invention provides a novel fluid thrust bearing by utilizing the cohesive and adhesive forces of fluid on a shaft in a close-fitting cavity. When used in an indicating instrument, the fluid thrust bearing of the present invention eliminates the need for a holding magnet or one of the two physical thrust bearings of the prior art. In addition, the fluid thrust bearing of the present invention damps rotational movement of the rotatable element while also preventing axial movement. Thus, in applications such as indicating instruments, where cost and ease of manufacture are critical, the present invention permits a reduction in parts and simplified assembly over prior art.
In accordance with one embodiment of the present invention, a rotatable element is disposed in a close-fitting cavity having an open end and a closed end. The closed end of the cavity provides a mechanical thrust bearing to prevent axial movement in one direction. Axial movement towards the open end of the cavity is prevented by disposing a fluid in a small gap between the rotatable element and the walls of the close-fitting cavity. The rotatable element is axially restrained from moving out of the cavity by (1) adhesion forces between and among the rotatable element, the liquid, and the walls of the cavity and (2) negative (vacuum) pressure created by the substantial exclusion of air in the cavity. This novel arrangement provides a reliable, low-cost thrust bearing apparatus employing fewer parts and providing for easier assembly than prior art arrangements.
In accordance with the present invention, the gap between the rotatable element and its associated cavity walls is so small that when the gap is filled with a viscous fluid the rotatable element is maintained in the cavity by molecular adhesion of the fluid to the rotatable element and cavity surfaces, and cohesive forces within the fluid. In addition, if a moderate axial separating force is applied between the rotatable element and the cavity walls, resultant tensile stress of the fluid causes a slight stretching of the fluid and negative (vacuum) pressure, tending to maintain the axial position of the rotatable element in the cavity and eliminating the need for one thrust bearing.
In accordance with a preferred feature of the present invention, a rese
Dorsey & Whitney LLP
Noland Thomas P.
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