Low friction reciprocating piston assembly
Reference Number: N 01-02
Inventors: Rao, Vemulapalli D. N.; Kabat, Daniel M.; Yeager, David A.
In a first aspect of this invention, a new piston construction is provided with a coating of solid film lubricant (SFL) comprising graphite and molybdenum disulfide in a resin, the construction having one or more of the following features: (i) inducing operation of the lubrication film around the piston to be in the hydrodynamic regime by selectively undercutting the piston skirt broad surface to define oil supply reservoirs and by adjusting the piston-bore wall gap to promote the hydrodynamic regime; (ii) attracting an oil film to fill the adjusted gap by selecting the resin with special characteristics; and (iii) increasing the operating temperature of the solid film lubricant by combining boron nitride and molybdenum disulfide in selected proportions.
More particularly, the low-friction piston construction for use in an oil-fed cylinder bore wall, comprises: (a) means providing a piston crown extending across substantially the entire cross-section of the cylinder bore for sliding movement therein; (b) a circumferentially extending skirt wall depending from the crown; and (c) reciprocating force transmitting means connected to the interior of at least one of said skirt wall and crown, said skirt wall having portions thereof relieved in a radially inward direction, at least the unrelieved portions of the skirt wall having a solid film lubricant coating on its exterior thereof consisting of graphite, molybdenum disulfide, boron nitride, and epoxy resin, said resin comprising a polymer base that provides a supply of at least one of water vapor and hydrocarbon to the graphite component and attracts an oil film thereto.
Another aspect of this invention is a method of making a low-friction piston for use in an oil-fed cylinder bore wall, comprising: (a) providing a cylindrical piston body with a crown, side walls depending from said crown, and structure for supporting at least said crown about a diametrically extending pin axis, said side walls having portions, on opposite sides of said pin axis, conforming to the shape of said bore wall but defining a radial gap therebetween; (b) undercutting the exterior surface of said side walls to define radially exposed lands for sliding along said bore wall; (c) introducing a predetermined pattern of asperities into said lands; (d) uniformly spraying a solid film lubricant coating onto the lands, such coating comprising solid lubricants and a supporting resin, said resin comprising a polymer base that provides a ready supply of at least one of water vapor and hydrocarbon to the solid lubricants and is effective to attract oil film thereto; (e) forming shallow pockets at the mouth of said asperities; and (f) polishing the exposed solid film lubricant crystals to further reduce the height of the coating at said asperities.
Yet still another aspect of this invention is a method of using a low-friction piston in an oil-fed cylinder bore of a reciprocating assembly, said assembly having a central axis of reciprocation, comprising: (a) providing a low-friction piston comprised of a cylindrical body with a crown, depending side walls, and structure for supporting at least said crown about a diametrically extending pin axis, the side walls having portions on opposite sides of said pin axis to conform to the shape of the bore wall but defining a radial gap therebetween, the side walls being relieved on the exterior surface thereof to define radially exposed lands for sliding movement along the bore wall, said lands having a predetermined pattern of asperities therein, and a solid film lubricant coating on said lands, said coating consisting of solid lubricants and a supporting resin, the resin being a polymer base providing a ready supply of at least one of water vapor and hydrocarbon to the solid lubricants and is effective to attract an oil film thereto; (b) mounting the piston on a connecting rod having a pin connecting one end of said rod to said piston for articulating reciprocating movement within said bore, said rod exerting a force on said piston side walls nonaligned with said axis; (c) operating said reciprocating assembly to transfer a portion of said solid film lubricant coating to the cylinder bore wall at least at regions of piston slap, said transferred coating possessing microasperities for trapping oil to retain an oil film during reciprocation in a thickness that produces a lambda of six or greater; and (d) providing reservoirs of oil adjacent said retained oil films for replenishing oil in such films during each reciprocation of said piston.