Seal for a joint or juncture – Seal between relatively movable parts – Relatively rotatable radially extending sealing face member
Reexamination Certificate
2000-03-03
2002-01-29
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Relatively rotatable radially extending sealing face member
C277S400000, C277S459000
Reexamination Certificate
active
06341782
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to seals with improved frictional behavior and, more particularly, to a seal having a load-bearing surface whose load-carrying capacity is improved by the presence of micropores.
It is well known from the theory of hydrodynamic lubrication that when two parallel surfaces, separated by a lubricating film, slide at some relative speed with respect to each other, no hydrodynamic pressure, and hence no separating force, can be generated in the lubricating film. The mechanism for hydrodynamic pressure buildup requires a converging film thickness in the direction of sliding. In conventional applications, this often is obtained by some form of misalignment or eccentricity between the sliding surfaces, for example, hydrodynamic thrust and journal bearings.
For liquid lubricants, the macrosurface structure, particularly in the form of waviness on the sliding surfaces, has been studied in the past for both parallel face thrust bearings and mechanical seals. The load carrying capacity in these cases is due to an asymmetric hydrodynamic pressure distribution over the wavy surface. The pressure increase in the converging film regions is much larger than the pressure drop in the diverging film regions. This is because the pressure drop is bounded from below by cavitation, whereas the pressure increase has effectively no upper limit. Microsurface structure in the form of protruding microasperities on the sliding surfaces also can be used to generate a locally asymmetric pressure distribution with local cavitation. The integrated effect of these microasperities can be useful in producing separating force between parallel sliding surfaces. Asymmetric pressure distribution also can be obtained by depressed surface structures, and various forms of grooves are used in bearings and mechanical seals. See, for example, T. W. Lai, “Development of Non-Contacting, Non-Leaking Spiral Groove Liquid Face Seals, Lubr. Eng., vol. 50, pp. 625-640 (1994).
U.S. Pat. No. 5,952,080 to Etsion et al. discloses a method for designing bearings, of improved performance, the load-bearing surfaces of which feature micropores. The hydrodynamic pressure distribution of a suite of bearing surfaces with different micropore geometries and densities is modeled numerically. The load-bearing surfaces of the bearings are fabricated with micropores having the optimal density and geometry determined by the numerical modeling. Substantially conical micropores may be created by single laser pulses, with the pore size and shape controlled by controlling the laser beam profile, the laser beam power, and the optical parameters of the focusing system.
A microsurface structure in the form of micropores has several advantages over other microsurface structures, particularly those involving protruding structures, in moving load-bearing surfaces. These advantages include:
1. Ease of manufacturing.
2. The ability to optimize pore size, shape, and distribution using theoretical models.
3. Good sealing capability in stationary (static) conditions.
4. Providing microreservoirs for lubricant under starved lubrication conditions, for example, at startup and after lubricant loss.
Although hydrodynamic gas seals operate on essentially the same principles as hydrodynamic liquid seals, the well-known and well-characterized differences in the physical properties of the lubricants lead to different design and operating principles. The more substantial differences in physical properties include:
1. Pressure Distribution
In an incompressible fluid, a full converging-diverging film has pressures that are greater than ambient and pressures that are less than ambient. In a highly compressible gas film, however, the pressure may always be greater than ambient. The hydrodynamic pressure of an incompressible film is independent of the ambient pressure, hence, the absolute pressure can be determined by summing the pressure rise and the ambient pressure. The hydrodynamic pressure of a compressible film, on the other hand, is dependent on the ambient pressure.
2. Variable Density
Because gases are compressible, density must be treated as a variable to prevent significant error in modeling and performance. This significantly complicates the modeling mathematics and influences the behavior of the seal.
3. Dimensional Accuracy
Films in gas bearings tend to be appreciably thinner than in liquid (incompressible) lubrication, such that the minimum film thickness may be of the same order of magnitude as the surface roughness of the bearing surfaces.
4. Heat Transfer
In gas-lubricated seals, gas, rather than liquid, is used to cool and lubricate the seal faces. Characteristically, the heat capacities of gases are significantly lower than the heat capacities of liquids. Consequently, gas seals are much less suitable for removing heat generated at the seal faces.
5. Viscosity
Unlike incompressible fluids, in which the viscosity decreases with increasing temperature, the viscosity of compressible fluids tends to increase with increasing temperature.
The differences in the physical properties of compressible and incompressible lubricants are so substantial that the development of noncontacting, gas-lubricated seals for pumps, compressors, etc., has been described by Netzel (Lubrication Engineering, pp. 36-41, May 1999) as the most significant development in the field of sealing technology in the 20th century. Moreover, the principles for designing such gas-lubricated seals differ from those of their liquid-lubricated seal counterparts.
The most efficient design element of the prior art is the spiral groove seal face. Upon rotation of the shaft, pressure is built in each spiral groove. The hydrodynamic lift achieved separates the seal faces and allows the passage of gas across the seal face.
It must be emphasized that the spiral groove seal is subject to various operating problems, including low NPSH (net positive suction head) operation and mechanical problems that result in the loss of seal flush and other tribological problems at the seal faces. Moreover, the circumferential region with the spiral grooves substantially enlarges the width of the annular region of the seal relative to the width of comparable seals for liquid systems, thereby increasing the material and fabrication costs.
To date, the use of micropore technology as a means of providing hydrodynamic lift has been limited almost exclusively to liquid-lubricated seals. Although modeling of gas-lubricated hydrodynamic bearings having micropores was indicated by PCT Application No. US97/16764 to Etsion et al., which is incorporated by reference for all purposes as if fully set forth herein, and an air bearing at atmospheric-pressure having improved lift was disclosed, there has been little reason to think that the utilization of micropore hydrodynamic lift technology would provide lift of a magnitude of practical importance for most applications having gas-lubricated hydrodynamic seals, particularly in view of the above-described, marked differences in physical properties of compressible and incompressible lubricants and the resulting requisite differences in design of the seal. Moreover, as described below, the mating seal surfaces of the prior art—including those of PCT Application No. US97/16764—are complicated and costly to fabricate.
It must be emphasized that the hydrodynamic lift provided in liquid systems is based on the incompressibility of the liquid. Whereas the minimum pressure in the diverging region is limited by cavitation, the maximum pressure in the converging region is unlimited. It is this asymmetric behavior of the pressure curve that causes hydrodynamic lift. For this reason, the use of micropores for promoting hydrodynamic lift is most efficient for low-pressure systems. In high-pressure systems, the potential for pressure drop in the diverging region reduces the overall effect of the hydrodynamic lift.
Thus, because the hydrodynamic lift provided in liquid systems is related primarily to the cavitation property of the liqu
Beres John L.
Friedman Mark M.
Knight Anthony
Surface Technologies LTD
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