Bearings and mechanical seals enhanced with microstructures

Details Bearings and mechanical seals enhanced with microstructures Bearings and mechanical seals enhanced with microstructures

2000-09-14

2001-08-28

Hannon, Thomas R. (Department: 3682)

Bearings

Rotary bearing

Plain bearing

C384S278000, C384S476000

Reexamination Certificate

active

06280090

ABSTRACT:

This invention pertains to bearings and seals with improved mechanical properties, and to improved cooling or heating jackets for heat transfer in bearings, seals, and other devices.
Seals and bearings are among the most widely used components in industry. There is a continuing need for seals and bearings that can operate under higher loads, higher speeds, or higher temperatures. There is a particular need for an inexpensive, “dry-running” mechanical seal, one that does not require periodic re-application of lubricant. Previous dry-running mechanical seals have used a buffer gas instead of a liquid to lubricate the seal faces. However, existing dry-running gas seals are either very expensive, or are designed only for temporary, “backup” uses; and in either case are not usable in some applications.
Three more-or-less distinct lubrication regimes are encountered in typical load-bearing applications: hydrodynamic lubrication, clasto-hydrodynamic lubrication, and boundary lubrication. A given load-bearing application may involve one or more of these regimes.
Hydrodynamic lubrication is the best understood and most commonly encountered regime. In this regime the sliding surfaces are large conforming areas that produce a converging wedge of fluid as they move past one another. The sliding surfaces remain separated by the pressure within the converging wedge. However, because the sliding surfaces are conforming areas, the fluid pressure is sufficiently low that the surfaces do not deform substantially under the load caused by this pressure. Typically, fluid pressures are less than 5 Mpa and film thicknesses are greater than 1 &mgr;m. FIGS.
1
(a) and
1
(b) illustrate two of the many types of load bearing applications that rely on hydrodynamic lubrication, a stator-journal bearing comprising stator
2
and journal
4
, and a shaft-thrust bearing comprising shaft
6
and thrust bearing
8
, respectively. A few of the many other examples of load bearing applications that rely on hydrodynamic lubrication include squeeze film, journal, slider, and hydrostatic bearings; and dry-running “extended life” gas seals (the last item as distinguished from backup-use-only, dry-running gas seals that are not hydrodynamically lubricated, and that have typical life spans of a few hours).
Elasto-hydrodynamic lubrication occurs when the mating bearing surfaces are non-conforming areas that produce very high fluid film pressures. This lubrication regime is similar to hydrodynamic lubrication, except that the fluid pressures are sufficiently high to deform the mating bearing surfaces. The surfaces typically experience some rolling contact and some sliding contact with one another. This contact, coupled with the fluid viscosity and geometry, can generate very high fluid pressures. These high pressures increase the viscosity of the lubricant to a point at which it behaves almost as a solid. Typically, fluid pressures are in the range 0.5-3.0 Gpa, and film thicknesses are greater than 0.1 &mgr;m. FIGS.
2
(a) and
2
(b) illustrate a roller bearing
10
and a ball bearing
12
, respectively, two of the many load bearing applications that rely on elasto-hydrodynamic lubrication.
Boundary lubrication occurs when the lubricant film between the two bearing surfaces is very small, and significant solid-to-solid contact results. Boundary lubrication is produced by very thin surface lubricant films (typically of molecular size). Frictional resistance and wear (bearing life) are determined by the chemical and physical properties of the solids and lubricant at the interface. Boundary lubrication typically occurs in applications where sliding speeds are low and loads are very high. The maximum film thicknesses are 1-10 nm.
Under any of these lubrication regimes, the parameters that determine the performance of the load bearing surface include the coefficient of friction, wear, load bearing capacity, and temperature of the surfaces. Bearing and seal designs have historically focused upon enhancing these properties. For example, porous metal bearings and porous sintered carbide seals faces have been formed by etching processes to create pores that can store lubricant via capillary action. See, e.g., C. Cusano, “Lubrication of Porous Journal Bearings,”
Journal of Lubrication Technology
, vol. 94, pp. 69-73 (1972); R. Divikar, “Sintcred Silicon Carbidcs with Controlled Porosity for Mechanical Face Seals Applications,”
Lubrication Engineering
, vol. 50, pp. 75-80 (1993). The lubricant is then released when the temperature increases, creating a self-lubricating surface.
Triangular, low aspect ratio asperities have been etched into surfaces to store and pump lubricant to different regions of certain types of bearings and seals. See U.S. Pat. Nos. 3,572,730 and 3,586,340. The aspect ratios of these triangular asperities were very low: the minimum horizontal dimension was said to be 0.005 inches, and the maximum vertical dimension 500 microinches. Thus the maximum aspect ratio of an asperity consistent with the teachings of these patents is: maximum height/minimum width=0.0005/0.005=0.1. (In fact, the patents teach that the maximum depth of 500 microinches is undesirable, because excessive amounts of lubricant leak through until the asperities wear down to 100 microinches. Thus these patents affirmatively suggest that superior results are obtained with aspect ratios significantly lower than 0.1.)
Other research has investigated the potential use of micro-structures etched into the surface of hydrodynamic bearings to improve load capacity, stiffness and damping of the bearing. See I. Busch-Vishniac, I. et al., “Smart Hydrodynamic Bearings with Embedded MEMS Devices,” Internet reference, http://www.me.utexas.edu/~microbot/smartbearings.html (1997).
It has been discovered that the properties of mechanical bearings and mechanical seals can be significantly improved by covering the load-bearing surfaces with large fields of high aspect ratio microstructures (HARMs), such as microchannels or microposts. The HARMs can substantially enhance heat transfer capability and lubricant flow. Benefits include reduced operating temperatures, precise metering of lubricant flow to all affected surfaces, increased reliability, increased life, higher maximum rotational speed, and the ability of the seal or bearing to run “dry,” that is, without periodic re-application of lubricant.
The invention may be used with a variety of bearings and mechanical seals, including for example ball bearings, roller bearings, journal bearings, air bearings, magnetic bearings, single mechanical seals, double mechanical seals, tandem mechanical seals, bellows, pusher mechanical seals, and all types of rotating and reciprocating machines. The invention may be used in regimes of hydrodynamic lubrication, elasto-hydrodynamic lubrication, and boundary lubrication.
A modification of these structures results in improved cooling jackets for use with bearings, seals, or other objects where improved heat transfer for cooling (or heating) is needed.
Appropriate aspect ratios for the microstructures, that is, the ratio of their height to their diameter, depends on the application for which they are being used. For example, for mechanical seals the aspect ratio should be between about 0.5 and about 50, preferably between about 1 and about 10. For fluid bearings (i.e., rolling element or journal bearings with fluid lubricant), or for cooling jackets for fluid bearings, the aspect ratio should be between about 0.5 and about 75, preferably between about 1 and about 15. For rolling element bearings, the aspect ratio should be between about 0.2 and about 1.0, preferably between about 0.25 and about 0.5. For fault-tolerant rolling element bearings with solid lubricants, the aspect ratio should be between about 0.2 and about 5, preferably between about 0.25 and about 0.5. For journal or thrust bearings with solid lubricants, the aspect ratio should be between about 0.2 and about 20, preferably between about 0.25 and about 1.0.


REFERENCES:
patent: 3572730 (1971-03-01)

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