In general, rolling bearings are roughly classified into ball bearings and roller bearings in accordance with the types of rolling elements, and they are further subclassified. That is to say, ball bearings are typically classified into general type ball bearings, crown type ball bearings, thrust ball bearings for angular contact bearings, etc. There are ball bearing cages corresponding to the classified respective ball bearings described above. On the other hand, roller bearings are classified into tapered roller bearings, barrel shaped roller bearings, cylindrical roller bearings, thrust roller bearings, thrust barrel shaped roller bearings, etc., and there are roller bearing cages corresponding to the classified respective roller bearings described above.
Plastics-made rolling bearing cages (abbreviated to cages hereinafter) are ordinarily prepared from polyamide resins (so-called nylon) due to their excellent moldability, excellent mechanical characteristics such as flexibility and excellent productivity such as low material cost, and the nylon cages are widely used. For example, crown type cages, major portions of which are shown in FIG. 1, have a tooth portion 2, and can be freely rotatably held by incorporating a rolling element 4 into a pocket portion 3 having a narrow inlet and exit.
Conventional nylon cages 1, however, deteriorate their characteristics as nylon when continuously used at temperatures no less than 120.degree. C. or used under conditions wherein the nylon cages are contacted with (1) oil containing an extreme pressure additive or other additives, or (2) other acidic chemicals, and therefore they cannot be used in a good condition.
There are polyphenylene sulfide resins (abbreviated to PPS resins hereinafter) which can be used at high temperatures in place of nylon and which can be prepared at relatively low cost. The PPS resins are excellent in chemical resistance, moldability, etc., as well as thermal resistance. There are two types of PPS resins, that is, one is a crosslinked PPS resin (also designated as a branched PPS resin) and the other is a straight-chain PPS resin. The crosslinked PPS resin is prepared, for example, by heating in the air a PPS resin having a melt viscosity just after polymerization as low as about 20 to 100 poise and a low molecular weight at a temperature of not higher than its melting point to oxidize and crosslink to obtain a crosslinked PPS resin having an increased melt viscosity. The crosslinked PPS resin is also prepared by adding a crosslinking agent or a branching agent thereto to introduce a crosslinked or a branched structure and to increase the melt viscosity.
The crosslinked PPS resin thus obtained becomes, however, markedly brittle and lacks in flexibility, though it has a high melt viscosity, that is, improved thermal resistance. Accordingly, said PPS resin is not suited for rolling bearing cages which are required to have flexibility when the rolling element 4 is incorporated into the pocket portion 3.
On the other hand, the straight-chain PPS resin is a resin prepared by linearly growing its molecular chain so that the resin has a high molecular weight in the polymerization step, and the resin is characterized in that it is very flexible and highly tough. Japanese Patent Publication No. 79419/1989 discloses that the straight-chain PPS resin is useful for rolling bearing cages. Though the bearing cages composed of the straight-chain PPS resin is excellent in easiness of construction during assembling, they have low thermal stability and inferior mechanical characteristics such as creep resistance characteristics at high temperatures (not less than 150.degree. C). Accordingly, bearing cages prepared from the PPS resin are deformed by centrifugal force, and come into contact with the outer race to generate extraordinary friction heat. As a result, the lubricant used in the bearing is deteriorated, and the quality of the bearings is lowered.
Examples of plastics used for rolling bearing cages which are used at high temperatures exceeding 150.degree. C. include so-called superengineering plastics such as polyether sulfone (abbreviated to PES hereinafter), polyether imide (abbreviated to PEI hereinafter), polyamide-imide (abbreviated to PAI hereinafter) and polyether ether ketone (abbreviated to PEEK hereinafter).
These plastics are, however, very expensive, and have problems with regard to physical properties when used as bearing cages, for example, appropriate flexibility and fatigue resistance necessary for molding or assembling for the manufacture of the bearing cages, though they are excellent in thermal and chemical resistance. Accordingly, they have not come to be used widely.
Conventional polyamide resins have, as described above, low thermal resistance, and cannot be used as rolling bearing cages when continuously used at temperatures not less than 120.degree. C. Other engineering plastics have low flexibility, moldability and oil resistance, though they have sufficient thermal resistance. They are, therefore, not preferable for rolling bearing cages.