Rolling bearing unit

In order to prevent a reduction in fatigue life and to improve the endurance of a rolling bearing unit which is made up by assembling a pair of angular type ball bearings 4, 5 so as to rotatably support an axial load applied in a substantially constant direction, provided that a radial load F.sub.r and a leftwise axial load F.sub.a are applied to a rotating shaft 2 during running, the contact angle .alpha..sub.1 of the right hand side bearing 4 which takes the axial load F.sub.a is made large, while the contact angle .alpha..sub.2 of the left hand side bearing 5 which takes no axial load is made small, and the left hand side bearing 5 has a slight positive bearing gap during running.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a rolling bearing unit used for example 
for rotatably supporting a rotating shaft of a screw compressor. 
2. Description of the Related Art 
In order to support the rotating shaft of a screw compressor and the like, 
a rolling bearing unit such as shown in FIG. 2 has heretofore been used. 
This bearing unit is provided between an outer peripheral face of a 
rotating shaft 2 to which is secured a rotor 1 of a screw compressor, and 
an inner peripheral face of a housing 3, and is made up by assembling 
together a pair of first and second ball bearings 4, 5 of the angular 
type. With the present example, the so-called back-to-back or double back 
face assembly (DS) is adopted. 
The directions of the contact angles .alpha..sub.1 the first and second 
ball bearings 4, 5 are made opposite to each other. Therefore, when the 
rotating shaft 2 is displaced towards the left in FIG. 2, the first ball 
bearing 4 on the right hand side in FIG. 2 supports the load in the axial 
direction (axial load), while when displaced to the right, the second ball 
bearing 5 on the left hand side supports the axial load, thus preventing 
displacement of the rotating shaft 2 and the rotor 1 relative to the 
housing 3. 
With the rotating shaft 2 of the screw compressor in FIG. 2 however, an 
axial load F.sub.a is applied mainly in a direction as indicated by an 
arrow (left direction of FIG. 2) while a small axial load is only 
occasionally applied an the opposite direction. Consequently, if a pair of 
first and second ball bearings 4, 5 produced according to the same 
specification with respect to contact angle etc. are used with only the 
direction of the contact angles .alpha..sub.1 opposite to each other, then 
the life of the overall ball bearing unit is inadequate. The reason for 
this is as follows. 
Due to the axial load F.sub.a applied in the one direction as mentioned 
above, the rigidity of the first ball bearing 4 on the side for Supporting 
the axial load F.sub.a (the right hand side in FIG. 2) is increased, while 
the rigidity of the second ball bearing 5 on the side which does not 
support the axial load F.sub.a (the left hand side in FIG. 2) is reduced. 
Accordingly, the result of applying the axial load F.sub.a to the first 
ball bearing 4, is to increase the Hertzian contact ellipse occurring at 
the contact regions between the rolling faces of the balls 6 of the first 
ball hearing 4 and the inner ring raceway on outer peripheral face of the 
inner ring 7 and between the rolling faces of the bells 6 of the first 
ball bearing 4 and the outer ring raceway on the inner peripheral face of 
the outer ring 8. The result of this larger contact ellipse being produced 
in the first ball bearing 4 on the side which carries the axial load 
F.sub.a (referred to hereunder sometimes as the axially loaded ball 
bearing 4), is to increase the rigidity of the first ball beating 4. FIG. 
3 shows a general tendency of the relation between the size of an axial 
load F.sub.a acting on an angular type ball bearing, and the radial 
rigidity of the ball bearing. As is clear from FIG. 3, the radial rigidity 
of the first ball bearing 4 is expected to increase during operation of 
the screw compressor. 
On the other hand, in the second ball bearing 5 which does not support the 
axial load F.sub.a, a contact ellipse is not produced in the case of no 
preload applied, or even if produced in the case of a preload applied, to 
is only small. As a result, the radial rigidity of the ball bearing 5 
which does not support the axial load F.sub.a (referred to hereunder 
sometimes as the non axially loaded ball bearing 5) is less. Consequently, 
the majority of the radial load F.sub.r applied between the rotating shaft 
2 and the housing 3 is supported by the first ball bearing or axially 
loaded ball bearing 4, while the second ball bearing 5 not only does not 
support the axial load F.sub.a but also provides only minimal support for 
the radial load F.sub.r. As a result, the load on the axially loaded ball 
bearing 4 is much larger than that on the non axially loaded ball bearing 
5. 
The life of the overall rolling bearing unit made up by assembling together 
the pair of first and second ball bearings 4, 5, is predominantly 
influenced by the shorter life one out of the pair of first and second 
ball bearings 4, 5. In the case where, as shown in FIG. 2, only the first 
ball bearing 4 is subjected to a large load while the load applied to the 
second ball bearing 5 is extremely small provided that the first and 
second ball bearings 4, 5 are made according to the same specification, 
then the life for the overall ball bearing unit is predominantly 
influenced by the life of the first ball bearing 4. Moreover, since the 
life of the first ball bearing 4 will not be sufficiently long, then the 
life for the overall rolling bearing unit becomes inadequate. 
In view of this situation, there have heretofore been various attempts to 
extend the life of the rolling bearing unit fitted for example to a screw 
compressor. A first arrangement has been carried out wherein a positive 
gap or actual gap (in contrast to a negative gap under preload conditions) 
is provided rather than a preload being applied to the pair of ball 
bearings of the rolling bearing unit. When a positive or actual gap is 
provided in this way, then the contact pressure on the rolling faces of 
the balls and on the inner and outer raceways of the respective ball 
bearings is smaller than that for the case of a preload applied, so that 
the fatigue life of the rolling faces, as well as that of the inner and 
outer ring raceways is improved. 
In the case of a rolling bearing unit assembled for example into a screw 
compressor and the like, which differs from a bearing unit where a preload 
is required to meet the demand for high rotational accuracy as with 
bearing units used for the shaft of a machine tool, a positive gap is 
applied between the respective ball bearings to thereby reduce the amount 
of heating during operation and thus improve the fatigue life. 
With the construction example shown An FIG. 4, a portion of the housing 3 
opposite to the outer ring 8 of the axially loaded ball bearing 4 is 
formed with a larger diameter so that a gap 9 exists between the outer 
peripheral face of the outer ring 8 and the inner peripheral face of the 
housing 3. Therefore, with this construction example, the radial load 
F.sub.r applied between the rotating shaft 2 and the housing 3 is 
supported only by the non axially loaded ball bearing 5. 
With the construction example shown in FIG. 5 which is disclosed in 
Japanese Patent First Publication KOKAI No. S58160621, there is a change 
in addition to the construction shown in FIG. 4, Specifically, the contact 
angle .alpha..sub.2 of the non axially loaded ball bearing 5 is made 
smaller than the contact angle .alpha..sub.1 of the axially loaded ball 
bearing 4 (.alpha..sub.1 &gt;.alpha..sub.2). By making the contact angle 
.alpha..sub.2 of the non axially loaded ball bearing 5 smaller as with 
this construction example, the load capacity with respect to the radial 
load F.sub.r of the non axially loaded ball bearing 5 is increased. 
With the abovementioned conventional constructions, however, a sufficient 
improvement in life is not always possible. 
At first, in the case of the construction shown in FIG. 2 wherein the first 
and second ball bearings 4, 5 made according to the same specification are 
engagingly supported in the same manner between the outer peripheral face 
of the rotating shaft 2 and the inner peripheral face of the housing 3, 
then as mentioned before, the load on the first ball bearing or axially 
loaded ball bearing 4 is much greater than that on the second ball bearing 
or non axially loaded ball bearing 5, so that the life is shortened. 
Moreover in the case wherein a positive gap is provided inside the 
respective ball bearings 4 and 5, the life is improved compared to the 
case where a preload or a negative gap is provided. However from the 
viewpoint of the increase in load on the axially loaded ball bearing 4, 
this case is basically the same as for the preload case, and hence 
obtaining sufficient life improvement is difficult. 
With the construction as shown in FIG. 4 wherein the radial load F.sub.r is 
not supported by the first ball bearing or axially loaded ball beating 4, 
since all of the radial load F.sub.r applied to the second ball bearing or 
non axially loaded ball bearing 5, then the fatigue life of the second 
ball bearing 5 tends to be inadequate. Furthermore, as a result of 
applying all the radial load F.sub.r to the second ball bearing 5, an 
internal axial load in the opposite direction to the axial load F.sub.a is 
produced in the second ball bearing 5. This internal axial load is applied 
together with the axial load F.sub.a to the first ball bearing 4, thus 
further increasing the axial load applied thereto, so that the fatigue 
life of the first ball bearing 4 also tends to be inadequate. 
With the construction example as shown in FIG. 5 wherein the contact angle 
.alpha..sub.2 of the second ball bearing or non axially loaded ball 
bearing 5 is reduced, the load capacity with respect to the radial load of 
the second bell bearing 5 is increased so that the fatigue life of the 
second ball bearing 5 is improved to some degree. However considering that 
the radial load F.sub.r is supported by only the second ball bearing 5, 
then as with the construction example shown in FIG. 4, the fatigue life of 
the second ball bearing 5 tends to be inadequate. Moreover, although the 
internal axial load produced inside the second ball beating 5 is reduced 
with a reduction in the contact angle .alpha..sub.2, this internal axial 
load is still produced so that the fatigue life of the first ball bearing 
or axially loaded ball bearing 4 still tends to be inadequate. 
Therefore, in order to ensure sufficient life of the rolling bearing unit, 
countermeasures such as increasing the size of the first and second 
bearings 4, 5, or using a high quality material for the bearings have 
heretofore been devised. However, with such countermeasures, the cost of 
the rolling bearing unit is increased, resulting in an increase in the 
cost of mechanical equipment such as screw compressors to which the 
rolling bearing unit is fitted. 
Furthermore, in Japanese Patent First Publication KOKAI No. H5-280482 there 
is disclosed a rolling bearing unit where, in order to suppress the 
increase in internal axial load due to centrifugal forces, the contact 
angle of the first ball bearing which supports the axial load is made 30 
to 40 degrees, while the contact angle of the second ball bearing which 
does not support the axial load is made 15 to 25 degrees. 
However, with the construction disclosed in this publication, there has 
been no consideration with regards to the internal gaps and to the support 
of the radial load. Hence the fatigue life extension effect is not really 
adequate. 
SUMMARY OF THE INVENTION 
The rolling bearing unit according to the present invention has been 
developed in view of the above situation. 
An object of the present invention is to provide a rolling bearing unit 
comprising a shaft having an outer peripheral face, a housing having an 
inner peripheral face and a pair of first and second, angular type ball 
bearings provided between the outer peripheral face of the shaft end the 
inner peripheral face of the housing end having inner and outer rings 
respectively, the first ball bearing having a first contact angle, the 
second ball bearing having a second contact angle which is different in 
direction and amount from the first contact angle, the first ball 
supporting an axial load applied from outside in a predetermined direction 
between the shaft and the housing during operation, the inner rings 
securely fitted onto the outer peripheral face of the shaft, the outer 
rings securely fitted into the inner peripheral fade of the housing, the 
contact angle of the first ball bearing being larger than the contact 
angle of the second ball bearing, and the second ball bearing sized to 
have a small amount of positive gap produced inside during operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The rolling bearing unit according to the present invention, as with the 
beforementioned conventional rolling bearing units, has at least one pair 
of first and second ball bearings of the angular type provided between an 
outer peripheral face of a shaft and an inner peripheral face of a 
housing. The contact angles of the fist and second ball bearings are made 
different in direction from each other, so that during use an axial load 
applied in an approximately constant direction from outside is supported 
by the first ball bearing. 
In particular, with the rolling bearing unit according to the present 
invention, the inner rings in the first and second ball bearings ere 
fixedly engaged with sufficient engagement strength to the outer 
peripheral face of the shaft, and similarly the outer rings in the first 
and second ball bearings are fixedly engaged with sufficient engagement 
strength to the inner peripheral face of the housing. Moreover, the 
contact angle of the first ball bearing is larger than the contact angle 
of the second ball bearing. 
Preferably the contact angle of the first ball bearing within a range from 
40 degrees to 55 degrees, while the contact angle of the second ball 
bearing is within a range from 15 degrees to 20 degrees. 
Furthermore, in the second ball bearing, a slight positive gap exists 
inside during operation. 
With the rolling bearing unit according to the present invention 
constructed as described above, the load capacity the first ball bearing 
which supports the axial load applied from outside during use is increased 
with the increase in contact angle of the bearing, so that a drop in 
fatigue life of the first ball bearing due to the axial load can be 
prevented. Moreover, with the second ball bearing having a smaller contact 
angle, the increase in internal axial load due to the radial load is 
suppressed. Consequently, the drop in the fatigue life of the first ball 
bearing due to the axial load can be kept small. 
Furthermore, since the first ball bearing not only carries the axial load 
but also carries the radial load, then an excessive radial load is not 
applied to the second bell bearing, so that a drop in fatigue life of the 
second ball bearing can also be prevented. That is to say, with the 
reduction in the contact angle of the second ball bearing, the bearing 
rigidity in the radial direction becomes higher than that of the first 
ball bearing. 
Therefore, in the case where the first and second ball bearings are used 
under the same conditions with regards to the radial direction as in the 
conventional constructions, then the second ball bearing supports the 
majority of the radial load, so that it is difficult due to the radial 
load to maintain the fatigue life of this second ball bearing. 
It should be noted that in the case of the rolling bearing unit according 
to the present invention, by forming a slight positive gap inside the 
second ball bearing, then the first bell bearing also supports part of the 
radial load. More specifically, if the size of the positive gap is 
appropriately controlled according to the relationship with the contact 
angle of the first and second ball bearings as well as the axial load 
applied during operation, then the first and second ball bearings can 
uniformly carry the radial load, so that the drop in fatigue life of the 
second ball bearing due to the radial load can be minimized. 
In this way, with the present invention, the fatigue life of both of the 
angular type ball bearings in the pair can be sufficiently maintained. 
Hence a drop in the rolling fatigue life of the overall rolling bearing 
unit made up by assembling the pair of first and second ball bearings, can 
be suppressed. 
Now, FIG. 1 shows an embodiment of the present invention, where rolling 
bearing unit according to the present invention is assembled into a screw 
compressor. The rolling bearing unit of the present embodiment is 
constructed with a pair of first and second ball bearings 4, 5 of the 
angular type positioned back-to-back between an outer peripheral face of a 
rotating shaft 2 fixed to a rotor 1 of a screw compressor, and an inner 
peripheral face of a housing 3. 
The inner rings 7 of the first and second ball bearings 4, 5 are externally 
secured with an interference fit to the rotating shaft 2, while the outer 
rings 8 are similarly internally secured with an interference fit to the 
housing 3. 
During use of the screw compressor, a radial load F.sub.r, as well as an 
axial load F.sub.a (to the left in FIG. 1) are applied to the rolling 
bearing unit from the rotating shaft 2. Consequently, the first ball 
bearing 4 (the right one in FIG. 1) of the rolling bearing unit, supports 
the axial load F.sub.a, while the second ball bearing 5 (the left one in 
FIG. 5) does not support the axial load F.sub.a. 
Furthermore, the radial load F.sub.r is supported by both of the first and 
second ball bearings 4, 5. 
The contact angle .alpha..sub.1 of the first ball bearing 4 is made larger 
than the contact angle .alpha..sub.2 of the second ball bearing 8 
(.alpha..sub.1 &gt;.alpha..sub.2). For example, in the case of the present 
embodiment, the contact angle .alpha..sub.1 of the first ball bearing 4 is 
set to 40 degrees, while the contact angle .alpha..sub.2 of the second 
ball bearing 5 is set to 15 degrees. Furthermore, with the second ball 
bearing 5, a slight positive gap exists inside during operation. 
That is to say, when the axial load F.sub.a is applied with operation of 
the screw compressor, the balls 6 of the first ball bearing 4 are pressed 
strongly in the axial direction, while the balls 6 of the second ball 
bearing 5 are not pressed in the axial direction. However, when the radial 
load F.sub.r is applied, the balls 6 of the second ball bearing 5 are 
pressed in the radial direction, together with the balls 6 of the first 
ball bearing 4, to thereby support the radial load F.sub.r. 
With the rolling bearing unit according to the present invention 
constructed as described above, the contact angle .alpha..sub.1 of the 
first ball bearing 4 which supports the axial load F.sub.a applied from 
outside during use, is the larger at 40 degrees, so that the load capacity 
of the first ball bearing 4 is larger. Hence the drop in fatigue life of 
the first ball bearing 4 due to the axial load F.sub.a can be prevented. 
Moreover, the second ball bearing 5 has the smaller contact angle of 15 
degrees, with a positive gap existing inside. Hence the increase in the 
internal axial load due to the radial load F.sub.r is suppressed. That is 
to say, due to the application of the radial load to the second bearing 5 
of the angular type ball, an internal axial load in the opposite direction 
to the axial load F.sub.a is produced between the inner ring 7 and the 
outer ring 8 of the second ball bearing 5. However, with the smaller 
contact angle .alpha..sub.2 of 15 degrees, this internal axial load is 
minimal. Consequently, the drop in the fatigue life of the first ball 
bearing 4 due Go the axial load can be kept small. 
Furthermore, since the first ball bearing 4 not only carries the axial load 
F.sub.a but also carries the radial load F.sub.r, then an excessive radial 
load is not applied to the second ball bearing 5, so that the drop in 
fatigue life of the second ball bearing 5 can also be prevented. That is 
to say, with the smaller contact angle .alpha..sub.2 of 15 degrees in the 
second ball bearing 5, the bearing rigidity in the radial direction is 
higher than that of the first ball bearing 4, 
Therefore, in the conventional case where the pair of first and second ball 
bearings 4, 5 are used under the same conditions with regards to the 
radial direction, then the second ball bearing 8 supports the majority of 
the radial load. Under this condition, the contact pressure operational on 
the contact areas between the rolling faces of the balls 6 of the second 
ball bearing 5, and the inner ring raceway on the outer peripheral face of 
the inner ring 7 and between the rolling faces of the balls 6 of the 
second ball bearing 5 and the outer ring raceway on the inner peripheral 
face of the outer ring 8 is increased. Hence it becomes difficult to 
maintain the fatigue life of the second ball beating 5 due to the 
existence of the radial load F.sub.r. 
It should be noted that in the case of the rolling bearing unit according 
to the present invention, by forming a slight positive gap inside the 
second ball bearing 5, then the radial load F.sub.r is supported jointly 
by the first end second ball bearings 4, 5. That is to say, if the size of 
the positive gap is appropriately controlled according to the relationship 
between the contact angles .alpha..sub.1, .alpha..sub.2 of the bearings 4, 
5 as well as the axial load F.sub.a applied during operation, then the 
first and second ball bearings 4, 5 can uniformly carry the radial load, 
so that the extent of the drop in fatigue life of the second ball bearing 
5 due to the radial load F.sub.r can be minimized. In this way, with the 
present invention, the fatigue life of the pair of the first and second 
ball bearings 4, 5 of the angular type constituting the rolling bearing 
unit can be sufficiently maintained. Hence a drop in the rolling fatigue 
life of the overall rolling bearing unit made up by assembling the pair of 
ball bearings 4, 5, can be suppressed. 
Next is a description of the results of life measurements carried out by 
the present inventor in order to verify the effects of the present 
invention. 
For the first ball bearing 4 and second ball bearing 5, as a prerequisite 
for the measurements, two angular type ball bearings made by NSK LTD. and 
referred to as type 7306, with an inner diameter of 30 mm, an outer 
diameter of 72 mm, a width of 19 mm, and having 10 balls, were positioned 
in a back-to-back arrangement and run with the inner rings rotated at 3000 
rpm. Both the axial load F.sub.a and the radial load F.sub.r were 200 kgf 
(F.sub.a =F.sub.r =200 kgf). The results are shown in the Table 1. 
TABLE 1 
__________________________________________________________________________ 
Type 1 2 3 
Non-load side 
Load side 
Non-load side 
Load side 
Non-load side 
Load side 
Contact angle 
(.alpha..sub.2) 
(.alpha..sub.1) 
(.alpha..sub.2) 
(.alpha..sub.1) 
(.alpha..sub.2) 
(.alpha..sub.1) 
(degrees) 40 40 15 40 15 40 
__________________________________________________________________________ 
Load share 
F.sub.r 
14.5 185.5 200 0 50.1 149.9 
(kgf) F.sub.a 
17.0 217.0 53.9 253.9 14.0 214.0 
Each row 
24121 21.4 22.2 77.1 1306 35.1 
(h) 
RFL(*) 
Total (h) 
21.4 18.1 34.5 
LR(**) 
1 0.85 1.6 
__________________________________________________________________________ 
(*)RFL: Rolling Fatigue Life 
(**)LR: Life Ratio 
In Table 1, type 1 is a comparative example with the contact angles 
.alpha..sub.1, .alpha..sub.2 for the pair of ball bearings 4, 5 both set 
at 40 degrees. 
Type 2 is a comparative example having the construction shown in FIG. 5, 
With the contact angle .alpha..sub.1 of the axially loaded ball bearing 4 
set at 40 degrees, and the contact angle .alpha..sub.2 of the non axially 
loaded ball bearing 5 set at 15 degrees, and a gap provided between the 
outer ring of the ball bearing 4 and the housing 3. 
Moreover, type 3 is an example of the present invention, with the contact 
angle .alpha..sub.1 of the axially loaded ball bearing 4 set at 40 
degrees, and the contact angle .alpha..sub.2 of the non axially loaded 
ball bearing 5 set at 15 degrees, and an assembly axial gap set at 0.010 
mm provide a positive gap for the ball bearing 5. 
The axial load (F.sub.a) on the non-load side is an internal axial load. 
The axial load on the load side is the sum of the internal axial load and 
the external axial load. 
As is apparent from the contents of Table 1, with the present invention the 
fatigue life of the first ball bearing 4 which supports the axial load 
F.sub.a applied from outside, and the fatigue life of the second ball 
bearing or non axially loaded ball bearing 5 are balanced, so that the 
life of the overall rolling bearing unit is sufficiently improved. 
For example, with the type 3 construction of the embodiment of the present 
invention, the overall life is extended by 1.6 times that of the general 
conventional type 1 construction. On the other hand, in the case of type 2 
as shown beforehand in FIG. 5, where the contact angles are changed and a 
gap is provided around the periphery of the outer ring Of the axially 
loaded ball bearing 4, the fatigue life of the non axially loaded ball 
bearing 5 is reduced due to the existence of the radial load F.sub.r, so 
that the life of the Overall rolling bearing unit drops. 
Table 2 shows the life of an overall rolling bearing unit for the case 
wherein the radial load F.sub.r is made 50 kgf, 67 kgf, 100 kgf, 200 kgf, 
400 kgf, 600 kgf, and 800 kgf with the axial load F.sub.a set at 200 kgf. 
Also, as well as changing the values for the radial loads F.sub.r, the 
contact angles .alpha..sub.1 the axially loaded ball bearing 4 are changed 
in six steps, namely; 30 degrees, 40 degrees, 45 degrees, 50 degrees, 55 
degrees, and 60 degrees. Furthermore, with the respective cases, the 
proportion (L.sub.0 /L.sub.1) of the life L.sub.0 for the case where for 
the contact angle .alpha..sub.2 of the non axially loaded hall bearing 5 
is made 15 degrees, relative to the life L.sub.1 for the case where the 
contact angle .alpha..sub.2 is made the same as that of the contact angle 
.alpha..sub.1 of the axially loaded ball bearing 4 (.alpha..sub. 
=.alpha..sub.2), is disclosed in Table 2 as a life ratio. The size, 
rotating conditions, and conditions such as the inner gap of the 
respective bell bearings 4, 5 are made the same as for the case of the 
calculations results of Table 1. 
TABLE 2 
______________________________________ 
.alpha..sub.1 
Fr/Fa 
(degrees) 
0.25 0.33 0.5 1 2 3 4 
______________________________________ 
30 1.02 1.02 1.03 1.15 1.38 1.4 1.33 
40 1.06 1.06 1.12 1.51 1.68 1.52 1.36 
45 1.08 1.11 1.24 1.75 1.8 1.54 1.36 
50 1.14 1.21 1.46 2.05 1.89 1.55 1.37 
55 1.24 1.4 1.82 2.38 1.99 1.61 1.42 
60 1.45 1.73 2.31 2.75 2.11 1.73 1.52 
______________________________________ 
.alpha..sub.1 : Loadside contact angle 
As is clear from Table 2, when the contact angle .alpha..sub.1 of the 
axially loaded ball bearing 4 exceeds 40 degrees, then by making the 
contact angle .alpha..sub.2 of the non axially loaded ball bearing 5 small 
(15 degrees), then the life extension effect is remarkable, while when 
this contact angle .alpha..sub.1 is 45 degrees, the life extension effect 
is even greeter. 
As is also clear from Table 2, if the contact angle .alpha..sub.1 of the 
axially loaded ball bearing 4 is increased to 60 degrees, then the life 
from a calculation point of view is increased. However adopting such a 
large contact angle is undesirable from another standpoint. That is to 
say, when the rolling bearing unit to which the present invention is 
addressed, is assembled for example into a screw compressor, then due for 
example to poor fit at the time of assembly, or to deflection of the 
rotating shaft 2, the central axis of the inner ring 7 can become inclined 
to the central axis of the outer ring 8. When in such a case a large 
contact angle of 60 degrees is adopted, then there is a tendency for balls 
6 to ride up on the shoulder of the inner ring raceway and the outer ring 
raceway. If this occurs, then the rolling surface of the balls 6 can be 
appreciably damaged, thus Considerably marring the life of the rolling 
bearing unit. Therefore, the upper limit value for the contact angle 
.alpha..sub.1 is preferably kept to around 55 degrees. 
Moreover, the contact angle .alpha..sub.2 of the non axially loaded ball 
bearing 5 is preferably made small (close to zero degrees) so as to 
minimize the internal axial load produced in the second ball bearing 5 
when the radial rigidity of the second ball bearing 5 is increased and the 
second ball bearing 5 takes the radial load F.sub.r. 
However, due to temperature conditions of the part on which the rolling 
bearing unit is mounted, end to the conditions for engagement with the 
rotating shaft 2 and with the housing 3, then if a certain size contact 
angle (above 15 degrees) is not set, it may not be possible to ensure 
stable operation. Therefore, taking these conditions into consideration, 
the contact angle .alpha..sub.2 of the second ball bearing or non axially 
loaded ball bearing 5 is preferably selected to be within the range from 
15 degrees to 20 degrees. 
In the abovementioned embodiment, the present invention is applied to 
constructions wherein the pair of ball bearings 4, 5 ere positioned 
back-to-back. However, the present invention is not limited to the 
back-to-back assembly and can also be effected in relation to a 
front-to-front assembly (double front assembly (DF)). Whether to adopt the 
back-to-back assembly or the front-to-front assembly is determined 
depending on the required functions for the rolling bearing unit. 
That is to say, in cases requiring an increase in rigidity with respect to 
the bending moment of the rotating shaft 2, then the back-to-back assembly 
is adopted. On the other hand, in cases where it is necessary to minimize 
the drop in life of the rolling bearing unit due to the bending moment, 
then the front-to-front assembly is adopted. A tandem arrangement assembly 
is outside of the scope of the present invention. However, in order to 
support a greater load, then the present invention can be freely effected 
in a so called multi-row assembly bearing, with back-to-back assembly or 
front-to-front assembly rolling bearing units assembled in groups in the 
axial direction. 
Due to the construction and operation of the present invention as described 
above, a reduction in internal axial load produced due to radial loading 
can be achieved while sufficiently maintaining load capacity with respect 
to external axial loading. Moreover, it is also possible to sufficiently 
maintain the radial load life of the ball bearing which does not support 
the axial load. As a result, a sufficient improvement in the life of the 
overall rolling bearing unit is achieved.