Full type ball bearing for turbochargers

A full type ball bearing for turbochargers comprising an inner ring and an outer ring both made of steel and balls made of a ceramic.

BACKGROUND OF THE INVENTION 
The present invention relates to a full type ball bearing for turbochargers 
for motor vehicle engines, etc. 
With turbochargers, for example, for motor vehicle engines, investigations 
are under way for a change-over from the floating bearing employing 
floating metal and presently in use to the ball bearing (antifriction 
bearing) mainly for the purpose of improving the responsiveness in the 
range of low speeds. 
Since turbochargers are used at a high temperature, under a great load and 
at a high speed of rotation, common ball bearings comprising inner and 
outer rings, balls and a retainer, if incorporated therein, encounter the 
problem of a shortened life. During the rise to a high speed, especially, 
lubrication of the retainer portion poses a problem, leading to a failure 
of the bearing in a short period of time. To overcome the problem 
encountered with the retainer portion, there is a need to finish the 
retainer with high precision or to subject the retainer to a special 
treatment such as silver plating. Accordingly, full type ball bearings 
appear useful for turbochargers, whereas with the conventional full type 
ball bearing wherein the balls are made of steel, the contact between the 
balls involves slippage because the balls move in directions totally 
opposite to each other, entailing depletion of the lubricant and leading 
to an early failure of the bearing. For this reason, the full type ball 
bearing is not in use although the bearing is thought favorable for use in 
turbochargers. 
SUMMARY OF THE INVENTION 
The main object of the present invention is to overcome the foregoing 
problems and to provide a full type ball bearing which is highly 
responsive and has a prolonged life for use in turbochargers. 
The full type ball bearing of the invention for turbochargers is 
characterized in that the bearing comprises an inner ring and an outer 
ring both made of steel, and ceramic balls. 
Being a full type ball bearing, the bearing of the invention is free of the 
problem of a shortened life due to the presence of the retainer 
incorporated in common ball bearings. Moreover, the present bearing has a 
20 to 30% greater number of balls than such common bearings and therefore 
has a greater load rating and prolonged fatigue life. The balls, which are 
made of a ceramic, can be in contact with one another with reduced 
friction, which is favorable in the event of depletion of lubricant, 
consequently rendering the bearing operable free of seizure and giving a 
prolonged life to the bearing. The ceramic having a smaller density than 
steel decreases the moment of inertia involved, reduces the centrifugal 
force acting on the outer ring, and therefore provides a smaller contact 
angle on the inner ring which results in diminished slippage, giving 
improved responsiveness to the turbocharger. 
Thus, the full type ball bearing of the present invention is highly 
responsive and free of seizure and has a prolonged life.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIGS. 1 and 2 show a full type ball bearing embodying the invention for use 
in turbochargers. 
The bearing is a single-row angular-contact ball bearing comprising an 
inner ring 1 and an outer ring 2 both made of steel, such as AISI M50 
(high speed steel), and balls 3 made of a ceramic such as silicon nitride 
(3.2 in density). The outer ring 2 has a counterbore 4. The inner and 
outer rings have inner and outer raceway grooves, respectively, formed on 
them. 
The bearing can be dimensioned suitably. When the bearing is of the JIS 
7001CA type, the main portions have, for example, the following 
dimensions. The bearing is 12 mm in inside diameter, 28 mm in outside 
diameter and 8 mm in width. The shoulder portion of the inner ring 1 has a 
wall thickness of 2.6 mm, which is larger than the wall thickness, 2.0 mm, 
of the shoulder portion of the outer ring 2. The wall thickness of inner 
ring 1 outside of the inner raceway groove is even relative to the inner 
ring's longitudinal centerline. The balls 3 are 13 in number and 4.7625 mm 
in diameter. The balls 3 sit within the inner and outer raceway grooves 
when fitted between the inner and outer rings. The circumferential 
clearance of the bearing is 0.2 to 0.8 times the diameter of the ball 3. 
The circle through the centers of the balls 3, i.e. the pitch circle, has 
a diameter (PCD) which is so adjusted that the circumferential clearance 
has a value in the above range. Since the inner ring 1 has a larger wall 
thickness than the outer ring 2 as stated above, the PCD is greater than 
the average of the bearing inside diameter and outside diameter. 
The inner ring 1 of increased wall thickness has the advantage that the 
expansion of the inner ring 1 due to the centrifugal force can be smaller. 
The increased PCD is likely to permit the use of one more ball, resulting 
in a corresponding increase in the load rating to lengthen the fatigue 
life. Since the bearing is a full type ball bearing without any retainer, 
lubricant can be supplied to the raceways easily, while the counterbore 4 
formed in the outer ring 2 readily permits escape of the lubricant, 
assuring smooth circulation of the lubricant to inhibit the rise of 
temperature. 
FIGS. 3 to 5 show the results of first comparative performance tests 
conducted for the full type ball bearing of the invention shown in FIGS. 1 
and 2 (example) and a conventional full type ball bearing which is 
identical with the example in configuration and in which the inner and 
outer rings and the balls are all made of steel (comparative example). 
FIG. 8 illustrates the structural relationship between the bearings and 
the oil jet device used for providing lubrication. The bearings are of the 
7001CA type. The inner and outer rings of the example bearing are made of 
AISI M50, and the balls thereof are made of silicon nitride. The 
comparative example bearing is wholly made of AISI M50. The test 
conditions are as follows. 
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Number of revolutions: 
0 to 140,000 r.p.m. 
Axial load: 12 to 40 kgf 
Temperature: room temperature 
Lubrication 
Method: oil mist (1 cc/mm, 4 kg/cm.sup.2) 
Lubricant: Velocity No. 6 (brand name) 
Kinematic viscosity: 
30 m.sup.2 /s (20.degree. C.) 
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FIG. 3 is a graph showing the result obtained by testing the bearings for 
the relationship between the air pressure supplied to a turbocharger and 
the number of revolutions of the bearing. The example is represented by a 
solid line, and the comparative example by a broken line. It is seen that 
the bearing of the example can be rotated at high speeds at a lower 
pressure than the bearing of the comparative example. 
FIG. 4 is a graph showing the result obtained by testing the bearings for 
rapid acceleration characteristics, i.e. for the relationship between the 
elapsed time and the number of revolutions of the bearing during rapid 
acceleration. The graph reveals that the bearing of the comparative 
example behaves unstably during rapid acceleration unlike the bearing of 
the example and is lower in speed. 
FIG. 5 is a graph showing the result obtained by testing the bearings for 
repeated rapid acceleration, i.e. by repeatedly stopping and rapidly 
accelerating the bearings. The bearing samples of the comparative example 
became inoperative due to seizure when subjected to the 
stopping-acceleration cycle several times, whereas those of the example 
remained free of abnormalities even when the cycle was repeated 100 times 
continually. 
FIGS. 6 and 7 show the results of second performance tests conducted for 
the example and the comparative example. The second tests are different 
from the first only in the test conditions as to the axial load and the 
method of lubrication as shown below. FIG. 9 illustrates the structural 
relationship between the bearings and the oil mist device used for 
providing lubrication. 
______________________________________ 
Axial load: 80 kgf 
Lubrication method: 
oil jet 
example: 0.05 liter/min 
comp. ex.: 2.5 liters/min 
______________________________________ 
FIG. 6 corresponds to FIG. 3, and FIG. 7 to FIG. 4. 
The results given in FIGS. 6 and 7 reveal the following. First, the bearing 
of the example is usable free of troubles even if the amount of lubricant 
applied thereto is as small as about 1/50 of the amount used for the 
comparative bearing. Despite the diminished lubrication, the example 
bearing is rotatable at high speeds at a lower supply pressure than the 
comparative bearing and can be rapidly accelerated to a higher speed than 
the comparative bearing. 
Beside silicon nitride mentioned, other ceramics of low density (up to 4) 
are advantageously usable for the balls of the full type ball bearing for 
turbochargers. Such ceramics include sialon (3.2 in density), silicon 
carbide (3.2 in density) and alumina (3.9 in density).