LUBRICATION SYSTEM AND METHOD

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of Provisional U.S. Patent Application No. 62/269,719, filed December 18, 2015, which is hereby incorporated by reference for all purposes in its entirety.

BACKGROUND

[0002] The disclosure relates generally to lubrication of mating surfaces subject to relative motion and loading. More specifically, the disclosure relates to hydrodynamic lubrication of mating surfaces subject to reciprocating motion under load.

[0003] When two solid surfaces in mechanical contact slide relative to each other, the solid surfaces are subject to wear due to dry friction. When the solid surfaces are bearing surfaces, it is desirable to minimize the dry friction between the surfaces in order to prevent premature failure of the bearing due to surface wear. Lubrication is a common technique for reducing dry friction between two mating surfaces subject to relative motion. Fluid film bearings rely on a thin film of lubricant interposed between the mating surfaces to create clearance between the mating surfaces. In these bearings, dry friction between the mating surfaces is essentially replaced with fluid friction between layers of the lubricating film, and the load applied to the bearing will be supported by the viscous forces in the lubricating film.

[0004] Fluid film bearings may be hydrostatic bearings or hydrodynamic bearings.

Hydrostatic bearings use an external source of pressurized fluid to force lubricant between the mating surfaces. Hydrostatic bearings have a load capacity and direct stiffness that do not depend on relative motion between the mating surfaces. In contrast, hydrodynamic bearings use relative motion between the mating surfaces to maintain a lubricating fluid film between the mating surfaces. The most basic hydrodynamic bearing is a journal bearing in which a journal pulls a converging wedge of lubricant between the journal and the bearing. The wedge forms between the journal and bearing due to the journal being eccentric with the bearing as the journal rotates. As the journal gains velocity, liquid flows between the two surfaces at a greater rate. The lubricant, because of its velocity, produces a liquid pressure in the lubricant wedge that is sufficient to keep the two surfaces separated while supporting the applied load. [0005] True hydrodynamic lubrication is difficult to achieve between mating surfaces operating under reciprocating motion and load. The reciprocating motion will prevent formation of a coherent fluid film in the interface between the mating surfaces. Without a coherent fluid film between the mating surfaces, boundary lubrication will prevail over hydrodynamic lubrication during normal operation. In the boundary lubrication mode, the mating surfaces will be partially separated by fluid and partially in mechanical contact. Such mechanical contact, when coupled with high contact pressure between the mating surfaces, will accelerate wear of the mating surfaces compared to the case where full-film

hydrodynamic lubrication develops and is maintained during normal operation.

SUMMARY

[0006] In one aspect, a hydrodynamic lubrication system includes a pair of mating surfaces subject to relative motion. A source of lubricant is in communication with the mating surfaces. A microfluidic channel system including at least one diffuser element is disposed in an interface between the mating surfaces to create a coherent fluid film at the interface in response to the relative motion.

[0007] In another aspect, a method of forming a hydrodynamic lubrication between a pair of mating surfaces includes providing a microfluidic channel system including at least one diffuser element at an interface between the mating surfaces. The method includes connecting the mating surfaces to a source of lubricant. The method further includes subjecting the mating surfaces to relative motion, wherein the microfluidic channel system creates a coherent fluid film at the interface in response to the relative motion.

[0008] It is to be understood that both the foregoing general description and the following detailed description are exemplary of the invention and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The following is a description of the figures in the accompanying drawings. The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. [0010] FIG. 1A shows a general structure of a diffuser element.

[0011] FIG. IB shows a conical diffuser element.

[0012] FIG. 1C shows a flat diffuser element with curved channel walls.

[0013] FIG. 2 is a schematic of a micropump module.

[0014] FIG. 3 A is a schematic of a microfluidic channel system including micropump modules.

[0015] FIG. 3B shows the microfluidic channel system of FIG. 3 A embedded in a planar surface.

[0016] FIG. 3C shows a generalized structure of a microflui die-assisted lubrication system,

[0017] FIG. 4A shows a prior art torque transfer key.

[0018] FIG. 4B shows a prior art recess in an adapter arm for receiving the prior art torque transfer key of FIG. 4A.

[0019] FIG. 4C shows the prior art torque transfer key of FIG. 4A disposed in the prior art recess of FIG. 4B.

[0020] FIG. 4D shows pivoting motion of the torque transfer key of FIG. 4A.

[0021] FIG. 5A shows a microfluidic channel system embedded in a curved surface of a torque transfer key.

[0022] FIG. 5B shows a microfluidic channel system embedded in a planar surface of a torque transfer key.

[0023] FIG. 5C shows a microfluidic channel system embedded in a curved surface of an adapter arm recess.

DETAILED DESCRIPTION

[0024] A method of lubricating two mating surfaces subject to relative motion and loading includes disposing a microfluidic channel system at an interface between the mating surfaces. The microfluidic channel system creates a coherent fluid film between the mating surfaces in response to relative motion between the mating surfaces. In one embodiment, the

microfluidic channel system includes one or more micropump modules, and each micropump module includes a diffuser element. In one embodiment, the microfluidic channel system is disposed between the mating surfaces by embedding the micropump module(s) in one of the mating surfaces. In another embodiment, the microfluidic channel system is disposed at an interface between the mating surfaces by forming the micropump module(s) in a thin plate and attaching the thin plate to one of the mating surfaces. The thin plate may be made to conform to the shape of the respective mating surface. Each diffuser element in a micropump module is configured such that relative motion between the mating surfaces creates a net flow of lubricant through the diffuser element. The microfluidic channel system uses the pumping action of the diffuser element(s) to create the coherent fluid film at the interface between the mating surfaces. The coherent fluid film in turn facilities development of full-film

hydrodynamic lubrication between the mating surfaces. The microfluidic channel system can be used between any pair of mating surfaces where hydrodynamic lubrication is desired, even between mating surfaces subject to reciprocating motion under load or high contact pressure.

[0025] A diffuser element is a flow channel with gradually expanding cross-section. FIG. 1A shows a general structure of a diffuser element 10 having a throat (small opening) 12 with a width Wi at one end and an exit (large opening) 14 with a width W2 at another end, where Wi < W ₂ . Channel walls 16, 18, 21 extend between the throat 12 and exit 14. The diffuser element 10 is shown as a flat diffuser having a rectangular cross-section. However, other diffuser shapes are possible. For example, FIG. IB shows a conical diffuser element 10' having circular throat 12' and exit 14' and a cylindrical channel wall 16' extending between the throat 12' and exit 14'. FIG. 1C shows a flat diffuser element 10" having curved channel walls 16", 18" extending between the rectangular throat 12" and exit 14". Returning to FIG. 1A, the throat 12 and exit 14 are separated by a channel length L. The diffuser element 10 has an opening angle 2Θ at the throat 12 and a channel depth D. The widths W _1; W ₂ , length L, opening angle 2Θ, and depth D are design parameters whose values can be selected to achieve desired flow rates through the diffuser element 10 for a given lubricant viscosity.

[0026] In FIGS. 1A-1C, the dashed lines indicate the open side of the diffuser elements 10, 10', 10". The open side presents an opportunity to impart motion to a boundary of flow inside the diffuser element. The positive direction of the diffuser element 10 is from the throat 12 to the exit 14, as indicated by arrow 17 in FIG. 1A. The negative direction is from the exit 14 to the throat 12, as indicated by arrow 19. The diffuser element 10 has a higher flow resistance in the negative direction 19 than in the positive direction 17 for the same flow velocity. When the diffuser element 10, or variations thereof, is embedded in one of a pair of mating surfaces, relative motion, such as sliding motion or reciprocating motion, between the mating surfaces will cause fluid to move through the diffuser element 10. A net flow will be pumped through the diffuser element 10 due to the different flow resistances in the positive and negative directions 17, 19 of the diffuser element. The diffuser element 10 will essentially function as a micropump providing a positive volume of lubricant between the mating surfaces. [0027] In one embodiment, to achieve the pumping effect of the diffuser element 10, the ratio of W ₂ to Wi (i.e., W ₂ AV ₁ ) may be in a range from 1.2 to 2.5. W ₂ AV ₁ ratio above 2.5 may also create a pumping effect, but the pumping effect may not be very high. In one

embodiment, the ratio of Wi to L may be 0.15 or higher. In one embodiment, each of W _1; W ₂ , and L may be in a range from 1 mm to 2 mm. In general, the parameters of the diffuser element 10 will also depend on the viscosity of the lubricant. The above parameter values have been used with water, which has a viscosity of 0.001 Pa.s, and a lubricating oil having a viscosity of 0.045 Pa.s at 100°C and 0.300 at 40°C.

[0028] FIG. 2 shows a micropump module 30 having a main flow axis M according to one illustrative embodiment. The micropump module 30 includes a diffuser element 40 having a throat 42 and exit 44. The diffuser element 40 may have the characteristics described above for the diffuser element 10. The diffuser element 40 is aligned along the main flow axis M such that flow through the diffuser element 40 is generally along the main flow axis M. The micropump module 30 may include other flow channels 32, 34 connected to the throat 42 and exit 44, respectively, for continuity of flow along the micropump module 30. The

configuration of the other flow channels 32, 34 may vary between micropump modules. For example, the flow channel 34 is shown as having flow branches 34A, 34B. In some micropump modules, these flow branches may be omitted.

[0029] FIG. 3A shows a microfluidic channel system 50 including micropump modules 30, 30A, 30B according to one illustrative embodiment. The main flow axes M, MA, MB of the micropump modules 30, 30A, 30C are aligned and determine the main flow direction of the microfluidic channel system. As previously noted, a microfluidic channel system 50 may have one or more micropump modules. Therefore, the number of micropump modules shown in FIG. 3A are for illustration purposes only.

[0030] FIG. 3B shows a general representation of a microfluidic channel system 50 embedded, or formed, in a surface 46. The surface 46 is shown as a planar surface, but it could be a curved surface in other examples. The surface 46 may be one of a pair of mating surfaces, or the surface 46 may be a thin plate to be adhered to one of a pair of mating surfaces.

[0031] FIG. 3C shows a general representation of a microfluidic-assisted lubrication system. In FIG. 3C, mating surfaces 52A, 54A of members 52, 54 are in opposing relation. The members 52, 54 generally represent bearing components or other two objects subject to relative motion and loading. One or both of the mating members 52, 54 may move to provide a relative motion between the mating surfaces 52A, 54A. In one embodiment, the relative motion is a sliding motion or a reciprocating motion. In another embodiment, the relative motion may be rotational motion. A microfluidic channel system 50 is disposed in an interface between the mating surfaces 52A, 54A. For example, in one embodiment, this may involve embedding the microfluidic channel system 50 in one of the mating surfaces, e.g., mating surface 52A. The mating surfaces 52A, 54A are in communication with a source of lubricant 56. The source 56 can be a chamber filled with lubricant. The source 56 may be connected to a flow passage 57 formed in one of the members 52, 54. Alternatively, the members 52, 54 may be encapsulated in the source 56 so that the peripheries of the mating surfaces 52A, 54A are exposed to the lubricant. For hydrodynamic lubrication, a pump is not required to force the lubricant from the source 56 to the mating surfaces 52A, 54A. Instead, as the mating surfaces 52A, 54A move relative to each other, lubricant will be pulled onto the mating surfaces 52A, 54A to form a lubricant film 58. Development of the hydrodynamic mode of the lubricant film 58 will be aided by the microfluidic channel system 50 that provides a net flow of lubricant and coherent fluid film between the mating surfaces 52A, 54A as the mating surfaces move relative to each other.

[0032] One practical application of the microfluidic-assisted lubrication system is in a universal joint that is used in downhole drilling motors and described in International Application No. PCT/US2015/040513 titled "Universal Driveshaft Assembly," the content of which is incorporated herein by reference. This universal joint includes a torque transfer assembly having a plurality of torque transfer keys that mate with recesses in the arms of an adapter. FIG. 4A shows one of the torque transfer keys 190 having a D-shape defined by a convex cylindrical surface 196 and parallel planar surfaces 193, 199. FIG. 4B shows one of the adapter recesses 208 shaped to receive a torque transfer key 190. The recess 208 has a planar floor surface 210 and a concave cylindrical surface 212. FIG. 4C shows the torque transfer key 190 disposed in the recess 208. In this position, the convex cylindrical surface 196 of the torque transfer key 190 mates with the concave cylindrical surface 212 of the recess 208, and one of the planar surfaces 193, 199 of the torque transfer key 190 mates with the planar floor 210 of the recess 208.

[0033] The torque transfer key 190 is allowed to pivot relative to the recess 208. FIG. 4D illustrates pivoting motion of the torque transfer key 190 about a center O. As the torque transfer key 190 pivots, there will be relative sliding and reciprocating motion between the torque transfer key convex surface 196 and the recess concave surface 212 and between the torque transfer key planar surface 193 (or 199) and the recess planar floor surface 210. The mating surfaces are also subject to relatively high contact pressure due to the loads imposed on the torque transfer assembly during use of the universal joint.

[0034] FIG. 5 A shows a torque transfer key 190' incorporating a microfluidic channel system 50. In the example shown in FIG. 5A, the microfluidic channel system 50 is embedded in the convex cylindrical surface 196' of the torque transfer key 190'. When the torque transfer key 190' is disposed in the recess 208 as explained above for the torque transfer key 190, the microfluidic channel system 50 will be between the convex cylindrical surface 196' of the torque transfer key 190' and the concave cylindrical surface 212 (see FIGS. 4B and 4C) of the recess 208. The arrangement of the microfluidic channel system 50 on the convex surface 196' may be such that the main flow axis of the microfluidic channel system 50 will be aligned with the direction of relative motion between the torque transfer key and adapter recess, i.e., along the direction of pivoting motion shown in FIG. 4D.

[0035] In use, the universal joint is encapsulated in an oil-filled chamber. The oil in the chamber is pulled in between the mating torque transfer key convex surface 196' having the microfluidic channel system 50 and the recess concave surface 212 by relative motion between the mating surfaces 196', 212, i.e., pivoting of the torque transfer key 190' relative to the mating recess 208. There will be a net flow of oil in each diffuser element of the microfluidic channel system 50 in response to the reciprocating or sliding motion of the mating surfaces 196', 212. The net flow of oil will provide a coherent fluid film in between the mating surfaces 196', 212 that will aid in development of full-film hydrodynamic lubrication between the mating surfaces during normal operation.

[0036] A microfluidic channel system could also be formed in the torque transfer key planar surface that would mate with the recess planar floor surface 210 to facilitate development of hydrodynamic lubrication between these surfaces. FIG. 5B shows an example of embedding a microfluidic channel system 50' in a planar surface 193' of a torque transfer key 190". Another possibility is to form a microfluidic channel system in the adapter arm recess. FIG. 5C shows a microfluidic channel system 50" embedded in the concave surface 212' of an adapter recess 208'. The microfluidic channel system 50" may be used in lieu of, or together with, the microfluidic channel system 50 (see FIG. 5A) in the mating torque transfer key 190' (see FIG. 5A).

[0037] The microfluidic-assisted lubrication system described above can be used wherever there are mating surfaces subject to relative motion and loading. These types of mating surfaces occur in various downhole tools, such as the universal joint described above.

Another example is a rolling cutter drill bit and other such tools including bearings with mating surfaces subject to relative motion and loading. The microfluidic lubrication system may also be useful in biomechanics, such as between bearing surfaces in an artificial hip joint and the like. In general, the microfluidic-assisted lubrication system may be used between plain bearing surfaces, thrust bearing surfaces, radial bearing surfaces, and the like. The microfluidic-assisted lubrication system is especially useful under conditions where it would normally be difficult to develop hydrodynamic lubrication between mating surfaces, such as when the mating surfaces are operating under reciprocating motion and load.

[0038] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.