Solid anti-friction devices – materials therefor – lubricant or se – Solid anti-friction device – article or material therefor
Reexamination Certificate
2003-02-27
2004-02-10
Howard, Jacqueline V. (Department: 1764)
Solid anti-friction devices, materials therefor, lubricant or se
Solid anti-friction device, article or material therefor
C508S109000, C508S126000, C508S130000
Reexamination Certificate
active
06689721
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a conductive grease, and in particular to such a conductive grease less to reduce conductivity as time passes.
The invention further relates to a conductive grease having excellent conductivity and being less to reduce the conductivity as time passes, and specially to such a conductive grease to be suitably applied to various kinds of rolling bearings, decreasing electrical resistance between inner and outer races.
The invention still further relates to a rolling bearing such as a ball bearing, and particularly to such a rolling bearing employing a grease having conductivity as a lubricant.
The invention yet further relates to a rolling bearing under a conductive condition between the outer race and the inner race, and more particularly to such a rolling bearing to be favorably used to parts at high temperatures in office machinery or information equipment such as copiers, laser beam printers and others (sensitive drum (fixing part), heat roller supporters).
BACKGROUND OF THE INVENTION
In general information equipment, for example, in copiers, movable parts use lots of rolling bearings. Between a raceway surface and rolling elements of the rolling bearing, an oil film is formed during rotation to provide a non-contact between both. In the rolling bearing, since static electricity occurs accompanying with the rotation, there probably arises inconveniences that radiation noises thereby give bad influences as distortion and others to copied images of the copier.
For preventing such inconveniences, a prior art has taken a measure that the conductive grease is packed into the rolling bearing to make the inner and outer bearing races and the rolling elements conductive, and one of the inner and outer bearing races is grounded for removing the static electricity from the rolling bearing.
This matter will be explained with reference to
FIG. 24
of the attached drawings.
A ball bearing
121
of
FIG. 24
comprises an outer race
122
, an inner race
123
, a plurality balls
124
rotatably arranged between the outer race
122
and the inner race
123
, a cage (holder)
125
holding the plural balls
124
, and contacting seals
126
,
126
furnished in sealing grooves
122
b
of the outer race
122
. A space defined by the outer race
122
, the inner race
123
, and the seals
126
,
126
encircling is charged with the conductive grease
127
, and is sealed within the ball bearing
121
by the seals
126
.
Contacting faces between raceway surfaces
122
a
,
123
a
of both races
122
,
123
and balls
124
are lubricated by the conductive grease
127
, while the outer race
122
, the inner race
123
and the balls
124
are made conductive. Further, the outer race
122
or the inner race
123
is grounded (not shown) through the information equipment, for example, the copiers employing the ball bearing
121
so as to cancel static electricity generated by rotation of the ball bearing
121
.
A popular conductive grease is, for example, that carbon black is added as a thickener and a conductive additive (disclosed in JP-B-63-24038), and such a conductive grease displays an excellent conductivity at a beginning period of service. (The term “JP-B” as used herein means an “examined Japanese patent publication”)
However, the conventional conductive grease using the carbon black has a problem that it shows the excellent conductivity at the beginning period, but the conductivity goes down as a time passes.
In short, although the rolling bearing packed with the conductive grease shows the excellent conductivity at the beginning period (the inner and outer raceway surfaces and the rolling elements are conductive), the conductivity declines as the time passes so that resistance value between the inner and outer races of the rolling bearing becomes larger (called as “bearing resistance value” hereafter).
Causes for these phenomena have hitherto been considered as follows. The conductive grease enough exists at the beginning in the contacting faces between the rolling elements and the raceway surfaces of the bearing races of the rolling bearing, and the carbon black in the conductive grease maintains the conductivity between the raceway surfaces and the rolling elements, but owing to relative movements between the raceway surfaces and the rolling elements, the conductive grease is expelled from the contacting face, otherwise chain structures of the carbon black particles are broken. Thus, there occurs a problem that the conductivity declines and the bearing resistance value becomes large as the time passes.
Further, it has been assumed that the conductive grease expelled from the contacting face is difficult to again enter the contacting face, because a worked penetration of this kind of greases is low, and the conductive additive is a fine particle insoluble in the base oil.
It has also been assumed that the conductive grease expelled from the contacting face is difficult to again enter the contacting face, because the conductive additive is a fine particle insoluble in the base oil.
It is disclosed in JP-A-1-307516 that a measure for preventing the time-passing decline in the conductivity of the grease is to limit the worked penetration and soften the conductive grease for avoiding the grease from hardening. (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) The conductive fine powder as the carbon black is, if being sole, shorter in capacity as the thickener than generally known thickener as metallic soaps or urea compound, and oil separation degree of the resulting grease exceeds 2%.
However, inventors made earnest studies and have come to an assumption that the time-passing changes of the conductivity (resistance value) are generated by factors as mentioned below.
The rolling bearing (the ball bearing having the inner diameter: 8 mm, the outer diameter: 22 mm, the width: 7 mm) packed with the conventional conductive grease containing the carbon black was offered to a rotation test (radial load (Fr) 19.6N, rotational speed: 150 rpm (min
−1
), rotating time: 500 hours, test temperature: 25° C.), and the raceway surface of the rolling bearing after the rotation test was investigated by a scanning type electron microscope (SEM) and an energy dispersive spectrometer (EDS). As examples,
FIG. 13
shows an SEM image of the raceway surface of the inner race, and
FIG. 14
shows an EDS measuring chart.
From the SEM image of
FIG. 13
, it is seen that a ground face in the raceway surface of the inner race disappears at a beginning period and wear appear, from which the abrasion is recognized in the raceway surface. Peaks of oxygen are seen in the EDS measuring chart of
FIG. 14
, from which an oxide film is recognized in the raceway surface.
From them, it may be assumed that the cause for the time-passing change in the conductivity depends on formation of the oxide film in the raceway surface owing to shortage in the lubricating capacity of the conductive grease rather than deterioration in performance of the conductive grease.
That is, fine injuries appear in the raceway surface by metal contact between the surface of the rolling elements and the raceway surface. In the injured part, a new face is exposed, and since the new face has a high activity, it is instantly oxidized by oxygen in an air to form the oxide film. This oxide film deteriorates the conductivity to result in heightening the time-passing resistance value.
From the peaks of the EDS measuring chart, it is seen that components of fine amounts contained in the carbon black form films on the raceway surface by the influence of rotation of bearing. The film of the fine amount component decline the conductivity similarly to the oxide film, and consequently, the time-passing resistance value goes up.
Such phenomenon is a problem common to conductive greases serving between relatively acting members.
As a measure against the phenomenon, it may be considered to use a base oil of high viscosity for securing an oil film and prevent the metal contact, bu
Denpo Katsuaki
Hachiya Koichi
Koizumi Hideki
Naka Michiharu
Shouda Tooru
Howard Jacqueline V.
NSK Ltd.
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