High-speed Angular Contact Ball Bearings New 9 Series for ... - NTN

NTN TECHNICAL REVIEW No.782010
New Product
High-speed Angular Contact Ball Bearings
New 9 Series for Machine Tool
Futoshi KOSUGI
Many 9 series angular contact ball bearings are used for the main spindles of machine tools in
Europe. However, demand for them has risen also in Japan recently. We improved the current
design and developed new 9 series high-performance angular contact ball bearings.
We introduce the features and the performance test data of these new 9 series high-speed
angular contact ball bearings in this report.
1. Introduction
In order to shorten lead time by addition of process
integration capability, an increasing number of machine
tool models have recently been adopting unique main
spindles whose angle can be varied during machining
operation, and examples of such main spindles include
milling main spindles on 5-axis machine tools and
combined machine tools.
Main spindles on these machines swing to change
their angle, and a sufficiently large space is needed on
these machine tools to accommodate the motion of
main spindles. Therefore, any main spindle in these
machines needs to be compact in size, and the main
spindle bearing also needs to be compact in size. For
this reason, 9 series bearings are used because of
their low cross-sectional heights and smaller width.
This article hereunder describes the features and
performance of the NTN’s newly developed “highspeed
angular contact ball bearing 9 series” products.
2. Advantages of 9 Series bearings
Fig. 1 provides information that helps clarify the
advantages of 9 series bearings in comparison with 0
series bearings. Because of lower cross-sectional
height, 9 series bearings have the following
advantages over 0 series bearings:
• When main spindle diameter (bearing bore
diameter) is same with both series (Fig. 1a) for a
given main spindle diameter, a bearing product of
smaller outside diameter may be used. Thus, a more
compact gearbox design is possible, which leads to
size reduction for main spindle.
• When bearing outside diameter is same with both
series (Fig. 1b)
In an application where there is limitation on the
bearing outside diameter dimension, use of a 9
series bearing can lead to larger main spindle
diameter. This feature will allow a main spindle
design that provides greater shaft rigidity.
0 series 9 series 0 series
9 series
a) Comparison for same shaft diameter (bearing bore diameter) b) Comparison for same bearing outside diameter
Fig. 1 Feature of 9 series angular contact ball bearing
Industrial Business HQ. Industrial Engineering Dept.
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High-speed Angular Contact Ball Bearings New 9 Series for Machine Tool
On the disadvantage side, having balls smaller than
those of 0 series, 9 series bearings are inferior to 0
series bearings in terms of their radial load capacity
and maximum allowable axial load. To eliminate this
problem, NTN has developed 9 series bearing variants
that boast greater load capacity.
3. Improved functions on New 9 Series
products
3.1 Load rating and allowable axial load
Our newly developed high-speed angular contact
ball bearing products—new 9 series—feature improved
internal design that include rolling elements of
increased diameter as shown in Fig. 2, thereby the
load rating has been improved by a maximum of 50%
and the allowable axial load has been increased by
New design
Old design
Fig. 2 Design of new product
20% (Fig. 3). Consequently, the new 9 series bearing
products boast longer life and increased allowance
against axial load that occurs during tool change
process on machine tools; thus, these bearings will be
used in a larger scope of applications.
Fig. 4 provides axial rigidity graph and radial rigidity
graph with high-speed angular contact ball bearings
(new design and old design), internal bore diameter
100 mm, with constant pressure preload (GN preload).
3.2 Result of operation test
The result of operation test with the high-speed
angular contact ball bearings (new design and old
design) is described below.
Fig. 5 schematically illustrates the construction of
the test rig (main spindle type test rig) used for the test.
Fig. 6 summarizes a result of temperature rise test
on the new and old bearing design variants with
ceramic balls, air-oil lubricated, and with fixed position
preload.
The pattern of heat rise on the outer ring of our new
design, up to dmn = 1.4510 6 (12,000 min -1 ) is
identical to that of the old design. However, at higher
speed range, the temperature with the new design is
somewhat higher (1 to 2˚C). A possible cause is a
larger ball size, which leads to greater PV value
(contact surface pressure x slip velocity).
140
Basic dynamic load rating Basic static load rating Allowable axial load
120
100
New design
Old design
kN
80
60
40
20
0
50
100 150 50 100 150 50 100 150
Fig. 3 Basic load ratings and allowable axial load (steel ball, contact angle 20˚)
15
6
Axial displacement μm
10
5
New design
Old design
Radial displacement μm
5
4
3
2
1
New design
Old design
0
0 500 1000 1500 2000 2500 3000
Axial load N
0
0 500 1000 1500 2000 2500 3000
Radial load N
Fig. 4 Rigidity graph (bearing bore dia. 100 mm, constant pressure preload)
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NTN TECHNICAL REVIEW No.782010
Fig. 7 graphically plots the variation in bearing
preload in this test: the diagram shows that the new
design helps reduce possible increase in preload by
approximately 15% compared with the old design. To
sum up, the temperature on outer ring on our new
design is somewhat higher compared with the old
design: nevertheless, our new design boasts improved
anti-seizure property because temperature difference
across its inner ring and outer ring is smaller,
minimizing the increase in preload.
Fig. 8 illustrates test result with constant pressure
preload type bearing specimens.
The temperature on the outer ring of our new design
is higher by 1˚C compared with the old design:
nevertheless, our new design is capable of stable
operation up to ultrahigh-speed region of dmn = 2.40
10 6 (20,000 min -1 ).
Drive
system
Test bearing
Preload N
Temperature rise on outer ring ˚C
5000
4000
3000
2000
1000
20
18
16
14
12
10
8
6
4
2
New design
Old design
0
0 5000 10000 15000
Bearing running speed min -1
5S-2LA-HSE920UDBP4 (New design)
5S-2LA-HSE920DBP4 (Old design)
Air-oil lubricated
Air supply rate: 40 NL/min
Oil supply rate: 0.03 mL/5 min
Fixed position preload: 1800 N
w/ jacket cooling
New design
Old design
0
0 5000 10000 15000 20000
Bearing running speed min -1
Approx. 15%
reduction
in increase
in preload
Fig. 7 Correlation between speed and preload
Fig. 5 Test spindle
Fig. 8 Test results (Constant pressure preload, Air-oil)
Temperature rise on outer ring ˚C
20
18
16
14
12
10
8
6
4
2
New design
Old design
5S-2LA-HSE920UDBP4 (New design)
5S-2LA-HSE920DBP4 (Old design)
Air-oil lubricated
Air supply rate: 40 NL/min
Oil supply rate: 0.03 mL/5 min
Preload on mounted bearing: 49 N
w/ jacket cooling
0
0 5000 10000 15000 20000
Bearing running speed min -1
Fig. 6 Test results (Fixed position preload, Air-oil)
Temperature rise on outer ring ˚C
20
18
16
14
12
10
8
6
4
2
0
New design
Old design
2LA-HSE920UDBP4 (New design)
2LA-HSE920DBP4 (Old design)
Grease lubrication
Prefilled grease: SE-1
Fixed position preload: 49 N
w/ jacket cooling
0 5000 10000 15000
Bearing running speed min -1
Fig. 9 Test results (Fixed position preload, Grease)
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High-speed Angular Contact Ball Bearings New 9 Series for Machine Tool
Fig. 9 summarizes test result with grease-lubricated
specimens having steel balls, with fixed position
preload setting. Even under these conditions, heat rise
on the outer ring of the new bearing design remains
low up to dmn = 1.4410 6 (12,000 min -1 ).
4. Conclusion
Compared with old products, our newly developed
high-speed angular contact ball bearing 9 series
products boast greater radial load capacity and
allowable axial load, and longer life and increased load
capacity during tool change process on machine tools.
In terms of functions, the bearing temperature on our
new design is 1 to 2˚C higher compared with old
design. Nevertheless, the temperature gradient across
the inner ring and outer ring on our new design is low
and increase in preload in high-speed bearing
operation is also low. Thus, our new design excels in
anti-seizure property.
NTN will add its “high-speed angular contact ball
bearing new 9 series” products, which boast compact
size and enhanced functionality, to its ULTAGE Series
line of products, and will actively market these novel
products.
Photo of author
Futoshi KOSUGI
Industrial Business HQ
Industrial Engineering Dept.
-53-