Special Issue; Products for Industrial Machinery - NTN

High-Speed Tapered Roller Bearing for Machine Tool Main Spindles
The resultant measurements are graphically plotted in
Fig. 4.
In Fig. 4 the difference between the total torque and
rib torque equals the sum of 1), 3) and 4) described
above. Since the effect of 4) is sufficiently small in the
relatively low speed range of less than 4000 min -1 and
1) is predominant over 3) with roller bearings, 3) the
difference between the total torque and rib torque is
essentially equivalent to 1), that is, the EHL rolling
viscosity friction on the raceway surface. As can be
understood from expression (1), when there is no
effect of starvation and heat generation, the rolling
viscosity resistance increases in proportion of the
0.65th power of the rotation speed. However, the total
torque starts to decrease at around 300 min -1 when
the effect of heat generation seems to be minimal.
This appears to be due to the significant effect of
starvation.
Also, it will be understood that as the amount of oil
is reduced, the effect of starvation is more apparent
and the rolling viscosity resistance decreases. At the
Bearing
Table 1 Specifications of torque test bearing
and test conditions
Test
conditions
Description: 32020XUP4
Size: ID 100 x OD 150 x 32 mm total width
Contact angle: 17˚
Rollers: 29 steel rollers
Cage: Pressed steel cage, rolling element guide
Bearing load: 1 kN axial load
Bearing lubrication: Air-oil
Oil supply from both sides of bearing
Nozzle dia. 1.21 (per side)
ISO VG32 oil used
Jacket cooling: None
same time, because the rib is under fluid lubrication
conditions, the rib torque is proportional to the bearing
speed, and increases with a decreased amount of
lubricating oil. This fact further supports the analysis
results described above.
In summary, in order for a tapered roller bearing to
be able to run at a higher speed, it is necessary to
invent a mechanism that minimizes the supply of
lubricating oil to the raceway surface while supplying
plenty of lubricating oil to the rib.
3. Structure and Performance of the
Authors’ Bearing
3.1 Basic structure
Based on the approach described above, the
authors developed and evaluated various prototype
bearings with unique lubrication schemes and internal
constructions. Consequently, the authors finally
developed the unique tapered roller bearing with the
construction illustrated in Fig. 5. Now, referring to Fig.
5, let us describe its features. First, the authors’
tapered roller bearing lacks an ordinary inner-ring rib,
and instead, the rib is situated on the outer-ring side.
This arrangement prevents the lubricating oil from
flying away due to the centrifugal force of the rotating
inner-ring as well as resultant insufficiency in the
amount of lubricating oil. At the same time, to
effectively lubricate the rib, multiple direct-injection
nozzles are provided on the rib surface that supply airoil
lubricating oil. Incidentally, the usual oil supply from
the back side is not provided. Instead, the lubricating
oil from the direct-injection nozzles is directed to the
raceway surface through the spaces between the
rollers. Because the pockets and rollers interfere with
each other at higher speeds on roller guiding cages
such as the standard type, the authors have adopted
Torque Nm
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
21.6cc/h total torque
7.2cc/h total torque
3.6cc/h total torque
1.8cc/h total torque
0.9cc/h total torque
Total torque
Rib torque
21.6cc/h rib torque
7.2cc/h rib torque
3.6cc/h rib torque
1.8cc/h rib torque
0.9cc/h rib torque
Decrease in
oil amount
Decrease in
oil amount
500 1000 1500 2000 2500 3000 3500 4000
Running speed min -1
Fig. 4 Measured torque values of standard tapered roller
bearing 32020
Enlarged view
Outer-ring spacer
guided resin cage
Direct-injection
nozzle
Rib
Ceramic roller
Fig. 5 Section view of developed tapered roller bearing
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