This invention relates to a grease composition for high-speed rolling bearings. More particularly, it relates to a grease composition employed for bearings in electrical components and accessory devices for automotive vehicles, such as alternators, electromagnetic clutches, tension pulleys or the like. The rolling bearings are employed for electrical components employed for automotive vehicles, such as alternators, electromagnetic clutches for car air conditioners, idle pulleys or electric fan motors, and accessories such as fluid couplings for cooling fans, tensioners for timing belts or pneumatic pumps. Mainly the greases are employed for lubricating the rolling bearings.
Conventional greases for rolling bearings include lithium soap greases employing less costly mineral oils as the base oil, polyurea greases or greases known as wide-range greases, in which the ester synthetic oils are thickened with lithium soap or sodium terephthalamate. If the service life longer than those of the above greases or high temperature durability are required, greases employing silicone oil, fluorosilicone oil or fluorinated oil (perfluoro polyether oil) are employed as the base oil.
In automotive vehicles, for keeping pace with the propagation of FWD or front wheel-driven cars aimed at reducing the size and weight of the vehicles, and with the demand for an increased living space within the cars, the engine room space is necessarily reduced, while the electrical components and accessories with smaller size and weight are required. In addition, higher performance and higher output are demanded of the electrical components and accessories, such that in the case of an alternator, for example it becomes necessary to increase the rotational speed by special designing artifices to compensate for reduction in output power caused by size reduction. Moreover, in keeping with the demand for quiet operation, the degree of hermetic sealing of the engine room is advanced and hence the engine room tends to be heated so that components capable of withstanding higher temperatures are necessitated. Currently, a longer service life of the bearings, less grease leakage, superior low-temperature properties, superior rust-preventive properties, and superior noiseless properties of the bearings are demanded in connection with the greases for use with sealed bearings employed in electrical components and accessories for automotive vehicles.
Under these conditions, the conventional greases, such as mineral oil-lithium soap grease, mineral oil-polyurea grease or ester synthetic oil-lithium soap grease, are unsatisfactory in connection with the service life of the bearings employed under high temperature and high speed operating conditions. Besides, the conventional greases such as the greases employing silicone oil, fluorosilicone oil or fluorinated oil as the base oil are not fully satisfactory in that they fail to meet such properties as load withstanding properties, wear resistance, rust-preventative properties or noiseless properties, while being extremely expensive.
For the grease for long service life high speed rolling bearings, a grease employing synthetic oils, such as poly-.alpha.-olefin or ester and also employing urea compounds as the thickener has been developed and put to practical application.
However, the recent tendency is towards increasingly higher performance and higher output of the electrical components and accessaries for automotive vehicles, such as alternators, and hence towards an increasingly higher speed and temperatures under which the grease is employed. For example, the demand is raised for increasing the operating temperature and the operating speed from 130.degree. to 140.degree. C. and 10000 to 16000 rpm to 150.degree. to 180.degree. C. and 18000 to 22000 rpm, respectively. With the urea grease employing the aforementioned poly .alpha.-olefin and ester as base oil, satisfactory service life of the grease cannot necessarily be achieved.
On the other hand, with bearings for electromagnetic clutches and intermediate pulleys, to say nothing of the bearing for alternators, not only the problem of reduction in service life of the grease due to elevated operating temperatures, but also the problem of the bearing peel life due to high load conditions caused by the use of poly-V-belts, has been presented. In this consideration, the urea grease employing the aforementioned poly .alpha.-olefin or ester cannot meet the demands raised of the grease.
Besides, the grease employing alkyl diphenyl ether as the base oil and also employing an urea thickener has also been proposed by Japanese Laid-Open Patent Application Nos. 1-259097 and 3-28299. However, the above-mentioned problems have not been solved satisfactorily with this prior art grease so that a further improvement has been desired.