Ball bearing for use in vacuum and turbo-molecular pump incorporating same

A ball ball bearing for use in a vacuum is a sealed full-type ball bearing comprising an inner ring and an outer ring both made of stainless steel, balls made of a ceramic composed primarily of silicon nitride, and stainless steel shield plates. The bearing has enclosed therein a grease of low-volatility composition comprising a diester synthetic oil serving as a base oil, an organic condensation agent and an ethylene tetrafluoride resin additive.

BACKGROUND OF THE INVENTION 
The present invention relates to a ball bearing for use in a vacuum and a 
turbo-molecular pump having the bearing incorporated therein. 
Grease-lubricated bearings having a cage are already known as ball bearings 
for use in turbo-molecular pumps. 
Grease-lubricated ball bearings for turbo-molecular pumps are rotated at a 
high speed, for example, of about 75,000 r.p.m. during use, so that grease 
is not fully supplied to the pocket portions of the cage and the guide 
face of the bearing ring opposed to the cage. Further because the bearing 
is used in a vacuum, the frictional heat generated by the rotation of the 
bearing is not transferred to the outside effectively but remains in the 
interior of the bearing. Accordingly, the grease becomes degraded early, 
or early seizure occurs between the ball and the pocket portion of the 
cage or between the outer periphery of the cage and the inner peripheral 
guide face of the outer ring. Consequently, the life of the bearing is 
limited more greatly in a vacuum than in the atmosphere in view of the 
allowable temperature and becomes shorter. This leads to the problem of 
shortening the life of the turbo-molecular pump. 
SUMMARY OF THE INVENTION 
The main object of the present invention is to provide a ball bearing free 
of the above problem and having a prolonged life for use in a vacuum and a 
turbo-molecular pump having the bearing incorporated therein. 
The ball bearing of the present invention for use in a vacuum is a sealed 
full-type ball bearing which comprises an inner ring and an outer ring 
both made of stainless steel, balls made of a ceramic composed primarily 
of silicon nitride, and stainless steel shield plates, the bearing having 
enclosed therein a grease made of a low-volatility composition. 
Preferably, the grease comprises a diester synthetic oil serving as a base 
oil, an organic condensation agent and an ethylene tetrafluoride resin 
additive. 
The turbo-molecular pump of the present invention comprises a multiplicity 
of stationary blades provided inside a casing and arranged axially 
thereof, a rotary member supported inside the casing by ball bearings and 
rotatable by an electric motor, and a multiplicity of rotary blades 
mounted on the rotary member and facing the respective stationary blades 
axially of the member. The turbo-molecular pump is characterized in that 
each of the ball bearings supporting the rotary member is a sealed 
full-type ball bearing which comprises an inner ring and an outer ring 
both made of stainless steel, balls made of a ceramic composed primarily 
of silicon nitride, and stainless steel shield plates, the bearing having 
enclosed therein a grease comprising a diester synthetic oil serving as a 
base oil, an organic condensation agent and an ethylene tetrafluoride 
resin additive. 
The bearing of the present invention is a sealed full-type ball bearing 
having no cage and is therefore free of the problem of seizure of the cage 
portion which is encountered in the prior art. The absence of the cage 
gives an increased agitation-free volume inside the bearing for the 
bearing to enclose an increased amount of grease therein. The absence of 
the cage permits an increased quantity of grease to adhere to the inner 
periphery of the outer ring, so that the grease can be supplied to the 
spaces between the bearing ring and the balls more effectively. The 
present bearing is free of the slippage that would occur in conventional 
bearings between the cage and the balls and between the cage and the inner 
peripheral guide face of the outer ring. This reduces the torque, 
diminishes the heat evolution of the bearing itself and heat evolution of 
the electric motor and therefore decreases the amount of evaporation of 
the grease. Moreover, the grease, which is made of a low-volatile 
composition, evaporates only in a very small amount. 
Since the balls are made of a ceramic, no seizure occurs between the balls. 
The ceramic balls are lightweight, therefore serve to diminish gyroscopic 
slide and reduces the friction torque. This results in the same advantages 
as above, i.e., diminished heat evolution of the bearing itself and the 
motor and inhibited evaporation of the grease to give a longer life to the 
grease. Furthermore, the bearing rings and the shield plates are made of 
corrosion-resistant stainless steel, and the ceramic balls are also 
resistant to corrosion. This feature eliminates the problem heretofore 
encountered with steel materials in the case where apparatus such as 
turbo-molecular pumps are used in a vacuum, i.e., the contamination of the 
interior of the apparatus due to the evaporation of corrosion inhibitor 
oil. 
As described above, the present invention diminishes the evaporation of 
grease enclosed in the ball bearing, assures satisfactory lubrication over 
a prolonged period of time, reduces the likelihood of seizure and 
therefore gives an improved life to the ball bearing, consequently 
improving the life of the turbo-molecular pump.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described below in greater detail with 
reference to the accompanying drawings. 
FIG. 2 schematically shows the main portion of a turbo-molecular pump. 
With reference to FIG. 2, a rotary member 2 is provided on its upper 
portion with a multiplicity of rotary blades 1 arranged axially thereof. 
The rotary member 2 has a shaft 3 rotatably supported inside a casing 5 by 
two ball bearings 4. The casing 1 is fixedly provided inside the upper 
portion thereof with a multiplicity of stationary blades 6 concentric with 
the rotary member 2 and the rotary blades 1 and axially facing the 
respective rotary blades 1. The casing 5 has an air intake opening 7 at 
its upper end and an air discharge opening 8 at its lower portion. The 
lower portion of the casing 5 is internally provided with the stator 9, 
for example, of a high-frequency electric motor 15, and a motor rotor 10 
mated therewith is mounted on the lower portion of the shaft 3. Above and 
below the motor rotor 10, the shaft 3 is supported by the bearings 4. 
The turbo-molecular pump described above operates in the following manner. 
A vacuum system comprises a chamber (not shown) communicating with the air 
intake opening 7 of the casing 5, the casing 5 and the air discharge 
opening 8 communicating with a roughing rotary pump (not shown). First, 
the roughing rotary pump is operated to maintain the entire vacuum system 
in a low vacuum. The motor 15 is then started to rotate the shaft 3 and 
produce an interaction between the rotary blades 1 and the stationary 
blades 6, whereby gas molecules in the chamber and the turbo-molecular 
pump are continuously discharged therefrom through the opening 8 to 
realize a high vacuum within the chamber and the pump. 
FIG. 1 shows an example of bearing 4. 
With reference to FIG. 1, the bearing 4 is an angular contact full-type 
ball bearing which comprises a counterbored inner ring 11, an outer ring 
12 not counterbored, balls 13 and shield plates 14 attached to the 
respective ends of the outer ring 12. 
The inner ring 11, the outer ring 12 and the shield plates 14 are made of 
stainless steel. The balls 13 are made of a ceramic composed primarily of 
silicon nitride. Enclosed in the space between the inner ring 11 and the 
outer ring 12 is a grease made of a low-volatility composition (vapor 
pressure: 7.5.times.10.sup.-6 torr at 20.degree. C.). Preferably, the 
grease comprises a diester synthetic oil serving as a base oil and more 
excellent in high-temperature high-speed lubricity than conventional 
fluorine-containing base oils, 1% of organic condensation agent such as 
Bentonite, and 29% of ethylene tetrafluoride resin additive having good 
stability and high lubricity at high temperatures. 
To substantiate the advantage of the present invention, a full-type ball 
bearing (example of the invention) was fabricated from an inner ring and 
an outer ring both made of JIS SUJ2 and ceramic balls, with the above 
grease of low-volatility composition enclosed in the bearing. The bearing 
was subjected to a life test under the following conditions. 
______________________________________ 
Number of revolutions 
75,000 r.p.m. 
(inner ring): 
Degree of vacuum: 
10.sup.-1 to 10.sup.-2 torr 
Temperature: Atmosphere: room temperature 
Saturation temperature during 
rotation (inner ring): 80.degree. C. 
Load: Thrust preload: 1.5 kgf 
______________________________________ 
The test revealed that the full-type bearing of the example had a life of 
7,500 to 10,000 hours, which was much longer than the life (2,000 to 5,000 
hours) of a bearing wherein the inner ring, outer ring, balls and grease 
were made of the same material as those of the example and which included 
a cage of bakelite, and the life (2,000 to 4,000 hours) of a conventional 
full-type bearing wherein the inner ring, outer ring and balls were made 
of the same material as those of the example and which had enclosed 
therein a grease comprising a fluorine-containing base oil.