1. Technical Field
The present invention relates to a lubricant composition for chains and to a chain having the lubricant composition for chains adhered to its surface.
2. Description of Related Art
As a power transmission mechanism and a conveyer mechanism, chains such as bushing chains and roller chains are conventionally used. A bushing chain is constructed by alternately connecting a pair of outer link plates, which are connected together with two pins at both ends, and a pair of inner link plates having two bushings at both ends so that the pins on the adjoining sides of two pairs of outer link plates are fitted into the bushings. A roller chain additionally includes rollers fitted on the bushings.
The bushing has a cylindrical shape. There are various types of bushings, such as a roll-type bushing produced by rolling up a rectangular base material cut out from band steel into a cylindrical shape by a forming machine, a seamless-type bushing using a cylindrical member formed by casting and having no seam along a generatrix, and a sintered-type bushing made of a metal sintered body. In the case of roll-type and seamless-type bushings, in order to improve the abrasion resistance property, a lubricant is usually present on the sliding surfaces of the bushings and the pins and on the sliding surfaces of the bushings and the rollers. In the case of a sintered-type bushing, pores of the bushing are impregnated with lubricant oil.
As the lubricant, usually lubricants, which belong to grade No. 10-50 of SAE (Society of Automotive Engineers in the United States of America) viscosity classification (engine oil: equivalent to ISO VG (International Organization for Standardization Viscosity Grades) 22-320) and are liquid at room temperature, are often used.
In the case of a roll-type bushing, a joining section in rolling up the rectangular base material remains as a seam, and therefore when a chain is constructed by inserting pins into the bushings and uses the lubricant, the lubricant flows out through the seam, resulting in a problem that the chain has a short abrasion elongation-resistant life.
In order to reduce an outflow of the lubricant, Japanese Patent Application Laid-Open No. 8-277886 discloses an invention of a bushing having a plurality of blind grooves in the inner circumferential surface. In the case of the bushing of Japanese Patent Application Laid-Open No. 8-277886, however, the lubricant held in the blind grooves tends to flow to the seam because of the influence of centrifugal force applied to the chain, and the lubricant flown out of the blind grooves flows out from the ends in the longitudinal direction of the seam in a short time, resulting in a problem that the lubricant is not held in the bushing for a long period of time, and lubricity and abrasion resistance are not maintained.
Japanese Patent Application Laid-Open No. 2007-218430 discloses a seamless-type bushing having a plurality of blind grooves in the inner circumferential surface. With this bushing, although there is no loss of a lubricant caused by the seam, there is a problem that it is impossible to sufficiently prevent leakage of the lubricant from openings at both ends in the axial direction of the bushing.
In the case of a sintered-type bushing, as described above, by impregnating the pores with a lubricant which is liquid at room temperature and holding the lubricant in the pores, scattering of the lubricant caused by the centrifugal force applied to the chain is prevented. The lubricant is sucked out from the pores of the bushing by a pump function produced by the pin when the pin sways, oozes out because of expansion caused by frictional heat, and forms an oil film in the sliding section between the bushing and the pin. This oil film prevents seizure of the pin. When the swaying motion stops, the lubricant is sucked into the pores again with a lowering in temperature, and therefore the lubricant is required to have fluidity according to the pore diameter so that it is able to enter and leave the pores. Hence, there are problems that the lubricant also tends to flow out and the loss of the lubricant is significant.
As described above, various mechanisms for physically holding lubricants have been studied.
As an invention of a lubricant composition for bearings, Japanese Examined Patent Application Publication No. 63-23239 discloses an invention of a lubricant composition for bearings prepared by mixing ultrahigh molecular weight polyethylene or low molecular weight polyethylene and lubricating grease having a dropping point higher than the melting temperature of the polyethylene. Japanese Patent Application Laid-Open No. 9-268298 discloses an invention of a solid lubricant for bearings prepared by mixing lubricating grease, ultrahigh molecular weight polyolefin, and an oil extravasation inhibitor.