Bearings – Linear bearing – Lubricating
Reexamination Certificate
1999-03-09
2001-03-06
Wright, Dirk (Department: 3681)
Bearings
Linear bearing
Lubricating
Reexamination Certificate
active
06196720
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a bearing used for a pulley that contacts a timing belt for automobile engines, an auxiliary machine driving belt or the like.
Idler pulleys are installed to increase the wrapping angle of a belt, such as a timing belt driven by the engine of an automobile or an auxiliary machine driving belt, so as to impart a suitable tension to the belt. As for pulleys, there is a pulley of the type (having the so-called “hat type” outer ring) in which a pulley peripheral surface to be contacted by a belt is formed directly on the outer surface of the outer ring of a ball bearing; however, idler pulleys of the type in which a pulley body having a pulley peripheral surface and a ball bearing are fitted together are frequently used.
In this type of pulleys, when the pulley body rotates as it receives the rotating force from the belt, the outer ring of the ball bearing fitted therein rotates together with the pulley body.
In this connection, when such pulley is operated in a cold ambient, an abnormal noise (hoot sound) is sometimes produced depending upon the pulley specifications or operating conditions. This abnormal noise in a cold environment, or the so-called abnormal noise under cold ambient, does not always occur in the market; rather, it depends on atmospheric temperature, etc., and in Japan it occurs only in very limited districts (Hokkaido, etc.). Further, it occurs in a brief time (one minutes at most) subsequent to the start of the engine of an automobile, with so such sound occurring thereafter. Further, it occurs more often when a single-row deep groove ball bearing is used as such ball bearing, while the use of an angular ball bearing of a plural row type results in lesser occurrence.
The abnormal noise under cold ambient has such complicated properties and the cause of occurrence thereof, though not yet having been clarified, is supposed from the results of various experiments to be the self-excited vibration of the rolling elements. More particularly, in a cold ambient, the viscosity of the base oil in a grease increases, tending to cause unevenness in the oil films on the raceway surfaces, which unevenness, in turn, minutely periodically varies the friction coefficient between the rolling bodies and the raceway surface, thereby causing the self-excited vibration of the rolling elements. This self-excited vibration is believed to cause the pulley system to resonate, producing the axial vibration (translation) of the outer ring, leading to the occurrence of abnormal noise under cold ambient.
Conventionally, as a abnormal noise under cold ambient preventive measure, investigations have been made as to the use of a grease which is superior in low temperature properties (a grease which, even in a cold environment, forms oil films uniformly on the areas of contact between the rolling elements and the raceway surfaces of the inner and outer rings). This preventive means is intended to effectively suppress the occurrence of abnormal noise under cold ambient by enhancing the lubricating performance of grease in a cold environment, and a substantial effect can be expected. However, since the viscosity of grease decreases, there is anxiety about the lubricating performance at high temperatures, possibly leading to a lowering of durability. Pulleys used in automobiles are operated at high temperatures and high speeds, and since their durability is one of the important characteristics, a measure that possibly leads to a lowering of durability cannot be employed.
Thus, a grease serving as a abnormal noise under cold ambient preventive measure is required to have two properties: oil film stability at low temperatures and durability at high temperatures. However, it is very difficult to satisfy these two contradictory properties at the same time.
On the other hand, even if a grease is effective for abnormal noise under cold ambient preventive measure, it could, depending upon its composition, cause brittle peeling in the raceway surface or the surface of the rolling elements. This brittle peeling, unlike the ordinary peeling in the raceway surface or the surface layers of the rolling bodies caused by metal fatigue, is a peculiar phenomenon of fracture that suddenly occurs from an inner region that is considerably deep, and it is believed that vibrations due to the trend toward higher speed cause mirror wear in the rolling surfaces, which wear, in turn, causes the formation of a fresh surface, thereby providing a catalytic action to decompose the grease, attended by evolution of hydrogen, the latter entering the steel to brittle the matrix. Therefore, in selecting a grease, it is necessary to consider not only abnormal noise under cold ambient but also the composition thereof from the viewpoint of brittle peeling.
Accordingly, an object of the invention is to provide a bearing for automobile pulleys that is capable of reliably preventing brittle peeling and abnormal noise under cold ambient.
SUMMARY OF THE INVENTION
To achieve said object, a bearing for a pulley used in an automobile according to the invention is a bearing fitting in the inner portion of a pulley body having a peripheral surface to be contacted by a belt driven by an engine of the automobile, and having a grease composition therein. The grease composition comprises a base oil having a mixture of PAO oil and ester oil in a ratio by weight of from 50:50 to 90:10 with a viscosity of not more than 70 mm
2
/S (at 40° C.), a thickener having alicyclic diurea compound, wherein a ratio of said base oil and said thickener is 5-25 wt-%, and an additive having ZnDTC. The “PAO” is &agr;-olefin (&agr;-olefin oligomer) produced by oligomerization of ethylene, while the “ZnDTC” is zinc dithiocarbamate, expressed by the following structural formula.
The grease composition shown above has a long life and a lubricating property excellent under high temperature conditions and has good low temperature characteristics such that its viscosity will never become too high even under low temperature conditions. Therefore, it is capable of forming a uniform sturdy oil film even under low temperature conditions, and suppressing the occurrence of abnormal noise under cold ambient. In addition, the reason for limiting the proportion of the thickener to the above range is that if the amount of the thickener is as small as less than 5% by weight, the resulting grease is a liquid that is lacking in viscosity, not suitable for being sealed in a bearing, while if it exceeds 25% by weight, the consistency is very low to the extent of decreasing its practicability for use as a grease to be sealed in a bearing. Further, the ZnDTC added as an additive is highly effective in suppressing brittle peeling, and capable of greatly increasing the durability of bearings.
As said bearing, if use is made of an arrangement (ball bearing) comprising an inner ring fixed to a shaft, an outer ring fitted in the inner portion of the pulley body and a plurality of balls interposed between the inner and outer rings, wherein among the inner and outer rings, at least the outer ring is contacted at two points by each of said balls with contact angles, then the behavior, particularly axial behavior, of the balls is suppressed by the outer ring. Therefore, the self-excited vibration of the balls is effectively suppressed; thus the occurrence of abnormal noise under cold ambient can be suppressed more reliably.
The term “contact angle” used herein refers to an angle made between the direction of a load imposed on the rolling body and a plane perpendicular to the axis of the bearing. The larger he contact angle, the greater the effect of suppressing self-excited vibration of balls. While a larger contact angle is advantageous in that this makes it less liable for abnormal noise under cold ambient to occur, too large a contact angle could lead to drawbacks including a decrease in bearing life. Thus, the contact angle should be determined within a suitable range by considering these factors.
According to a bearing for a pulley u
Koremoto Takahiro
Nozaki Seiichi
Arent Fox Kintner & Plotkin & Kahn, PLLC
NTN Corporation
Wright Dirk
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