POROUS PLAIN BEARING AND METHOD OF MANUFACTURING SAME

Abstract: A porous plain bearing (1)of the invention is one formed by impregnating a porous sinter (5) with oil or a lubricant (6) containing less than 3 % by weight of a thickener and further impregnating the outer surface of the sinter (5) with a lubricant (7) containing at least 3% by weight of a thickener. In such a bearing,the outflow of the lubricant is prevented even when the temperature varies. A method of manufacturing a porous plain bearing of the invention comprises the steps of homogenizing semi-solid grease so as to atomize the crystals contained in the grease and homogeneously disperse the atomized crystals into the grease,and vacuum -impregnating a porous plain bearing with the resultant grease. This method improves the penetration of the grease into the bearing, reduces the apparent viscosity of the grease, and enhances the efficiency of impregnation.

Description: [Technical Field] This invention relates to a porous slide bearing made of a porous sintered compact impregnated with a lubricant and a method for manufacturing the same. [Background Art] Porous slide bearings are mainly used in motors for household electric appliances, automotive motors and the like. A bearing of this type is made of a sintered compact having pores impregnated with a lubricating oil which oozes out onto the sliding surface during use, thus extending its working life. A porous slide bearing is normally made of a sintered compact having a homogeneous porous structure obtained by subjecting to such treatments as mixing, press molding, calcining or forming finely powdered grains of iron, copper, zinc, tin, graphite, nickel or an alloy thereof. Such a sintered compact has usually pores 50 microns or less in diameter (many of them are 10 microns or less). In order to permit use of such a porous slide bearing for a prolonged period, its pores have to be impregnated with a sufficient amount of lubricating oil. But since it is kept inside the pores of the sintered compact only by the action of surface tension and viscosity, decrease in viscosity might happen together with increase in volume due to frictional heat in the bearing or in a high-temperature environment, causing the lubricating oil to flow out of the bearing. Thus, the range of operating temperature is restricted and the durability of the bearing drops. In order to solve these problems, with a bearing which is required to have a long life, it is necessary to provide an oil tank made of felt or wick and keep it in contact with the outer surface of the bearing. But if such means is employed in applications where oil stain is undesirable, it is necessary to provide a seal to prevent oil leakage. This will restrict the freedom of design and increase the cost. There is disclosed in laid-open German Patent Application No. 2165076 a porous slide bearing which comprises a sintered compact having a lubricant in the form of an oil impregnated into it, whereby the outer surface is impregnated to a predetermined thickness with a sealing material (molten glass). It is an object of this invention to provide an oil-containing bearing which can prevent leakage of lubricant by use of simple means without using an oil tank. Summary of the Invention According to the present invention a porous slide bearing comprises: a porous sintered compact; a first lubricant impregnated into said sintered compact in the form of an oil or a grease containing a thickening agent at a concentration of less than 3 per cent by weight; and a sealing material impregnated into said sintered compact, whereby the outer surface of said sintered compact is impregnated to a predetermined thickness with said sealing material, which is a second lubricant in the form of a grease containing a thickening agent at a concentration of at least 3 per cent by weight. Further according to the present invention there is provided a method for producing a porous slide bearing according to the previous paragraph comprising the steps of: homogenizing a lubricant to make crystals contained therein into fine particles and to disperse them uniformly, and impregnating under vacuum said second lubricant into pores in the porous slide bearing. How the invention may be carried out will now be described by way of example only and with reference to the accompanying drawings in which: Fig. 1 is a vertical sectional front view of the pillow block in use; Fig. 2 is a vertical sectional side view of the same; Fig. 3 is an enlarged sectional view of the oil-containing bearing; Fig. 4 is a graph showing the relation between the shear rate and the apparent viscosity of grease; and Fig. 5 is a flow chart showing the order of the manufacturing steps according to the present invention. [Best Mode for Embodying the Invention] Figs. 1 and 2 show a pillow block having a porous slide bearing 1 supported by a steel housing 2. The bearing 1 has a bore 4 through which a shaft 3 is rotatably mounted. The inner peripheral surface of the bore 4 serves as a sliding surface. The bearing 1 is made of a conventional porous sintered compact 5. The inside of its pores is impregnated with a soft lubricant 6 which is an oil or a grease containing a thickening agent having a concentration of less than 3 per cent, whereas their outer surface is impregnated to a predetermined thickness with a lubricant 7 which is a grease containing a thickening agent with a concentration of 3 per cent by weight or more. The hard grease 7 hardens to close the open ends of the pores and thus prevent leakage of the soft grease 6. If the hard grease 7 at the outer surface has too high a viscosity, it would be difficult for it to penetrate into the pores. Thus it will bulge from the outer surface of the sintered compact 5. This is not desirable from the viewpoint of handling of the product. Thus the concentration of the thickening agent should not be more than about 50 per cent and preferably be about 25 per cent. As the thickening agent for the soft grease 6 or the hard grease 7, a soap such as lithium, sodium, magnesium, barium or aluminum soap may be used. The thickening agent is dispersed in the base oil and transforms itself into a micellar structure, thus serving to keep the oil in a semisolid state. As is apparent from Fig. 4, which shows the relationship between the shear rate and the apparent viscosity of the grease, the viscosity is 100 kg/m*s (1000 poise) or more at the outer peripheral surface of the bearing where the shear rate is zero, even if a grease having a concentration of 3 per cent is used, provided the base oil has a viscosity of 0.1 kg/m*s (1 poise.) On the other hand, at the sliding surface where the shear rate is large, the apparent viscosity is sufficiently low. As is apparent from such relationship, on the sliding surface, even a grease which exhibits the same behavior as an oil behaves like a high-viscosity oil at the outer peripheral surface where no shearing force acts. Therefore, it can effectively prevent leakage of the grease. Table 1 shows some greases which can be used as the soft grease 6 to be filled deep inside the pores and the hard grease 7 to be filled at the outer surface thereof. Next, the production method embodying this invention will be described with reference to Fig. 5. Usually, a semisolid grease is made by mixing a thickening agent into a base oil such as a petroleum lubricating oil or a synthetic lubricating oil (e.g. silicone oil and poly-α-olefin), together with various additives (such as an antioxidant, a detergent-dispersant, an extreme pressure additive, an antifriction agent, an oilness improver, a friction conditioner, a viscosity index improver, a flow-point curing agent, an anticorrosive and an antifoamer), melting it into a liquid grease by heating to a phase transition temperature and cooling it. As shown in Fig. 5, the production method according to this invention comprises the grease production step 10 for producing the semisolid grease, homogenizing step 11 in which the semisolid grease is made into a viscous liquid state by shearing, and vacuum impregnating step 12 in which the sintered bearing is impregnated with a liquid grease made by heating the grease in a viscous liquid state. By subjecting the semisolid grease made in the grease production step 10 and cooled thereafter to a homogenizing treatment by shearing, the crystallized thickening agent is finely grained and dispersed uniformly. As a result, the semisolid grease turns into a viscous liquid state and its apparent viscosity lowers. This will improve the penetrability of the grease into the pores in the sintered bearing. Next, the sintered bearing is subjected to vacuum-impregnation with the homogenized viscous grease. In the vacuum impregnation step 12, as with any conventional method, after placing the sintered bearing in a vacuum vessel, the viscous liquid grease is poured into the vessel kept under vacuum. Then the interior of the vessel is heated to impregnate the sintered bearing with the grease now in a liquid state. After impregnation, the remaining grease is recovered from the vessel, which is now put under the atmospheric pressure. Finally the sintered bearing is taken out. Since the grease used in the vacuum impregnation step 12 is a viscous liquid grease made by homogenizing a semisolid grease and thus has a low apparent viscosity, the impregnating temperature necessary to turn it into a liquid grease during the vacuum impregnation step can be reduced as low as 70 - 100°C, less than half the temperature which has heretofore been necessary.

Claims: A porous slide bearing (1) comprising: a porous sintered compact (5); a first lubricant (6) impregnated into said sintered compact (5) in the form of an oil or a grease containing a thickening agent at a concentration of less than 3 per cent by weight; and a sealing material impregnated into said sintered compact (5), whereby the outer surface of said sintered compact (5) is impregnated to a predetermined thickness with said sealing material, which is a second lubricant (7) in the form of a grease containing a thickening agent at a concentration of at least 3 per cent by weight. A method for producing a porous slide bearing according to Claim 1, comprising the steps of: homogenizing a lubricant to make crystals contained therein into fine particles and to disperse them uniformly, and impregnating under vacuum said second lubricant into pores in the porous slide bearing.

Inventors: ITO YOSHINORI, NAKANISHI KIYOSHI, SATO TASUKU, ITO, YOSHINORI, NAKANISHI, KIYOSHI, SATO, TASUKU
Applicants: NTN TOYO BEARING CO LTD, NTN CORPORATION
Application year: 1990
Main classification: F16C33
IPC codes: H02K5, F16C33