TECHNICAL REPORT

Development of Bearings with Low Temperature-riseand High-speed Performance for Machine Tool Main Spindles
K. SUZUKI

S. MATSUE

K. ICHIKAWA

We have developed an angular contact ball bearing and a cylindrical roller bearing which are superior in low
temperature-rise and high-speed performance for machine tool main spindles.
By optimizing the internal design, temperature rise of both bearings has been reduced to approximately half
compared with normal types.
These bearings can reduce power consumption for cooling machine tools, which is effective for energy saving.
And also, the developed bearings are expected to improve machining accuracy by reducing thermal expansion andimprove machining efficiency by increasing high-speed performance of spindles.
Key Words: machine tool, rolling bearing, temperature rise, high-speed capability

1. Foreword

2. Development Target

In recent exhibitions, best represented by The Japan
International Machine Tool Fair (JIMTOF), many
machine tools are put out on display which, in addition
to high accuracy, high speed, high rigidity and highreliability, are also focused on being energy-saving,environmentally-orientated and user-friendly, with aparticular emphasis on being easy for anyone to use.
Moreover, regarding the moving parts of machine tools
such as the main spindle, table, column and so on, theperformance of these moving parts depends greatly on theperformance of rolling bearings.
Out of all the moving parts on machine tools, the main
spindle is one of the most important elemental parts.
Often angular contact ball bearings or cylindrical roller
bearings are used for the main spindle. As for lubrication
methods, oil-and-air lubrication is adopted for high speed
main spindles, while grease lubrication is adopted for low
speed main spindles.
JTEKT has commercialized our High Ability Bearing
series as bearings for machine tool main spindles. We also
have many other product series available to achieve theperformances demanded of various machines.
In addition to these products, JTEKT has developed
angular contact ball bearings and cylindrical roller
bearings with low temperature-rise and high-speed
capability in an effort to save energy. This paper
introduces the results of such developments.

The main cause of temperature rise in machine tool
main spindles is the heat generated from rolling bearings
and the motor. The main spindle undergoes thermal
expansion in the axial direction when it heats up which
impacts upon the accuracy of machined workpieces. As
such, jacket cooling is generally performed to control
main spindle temperature.
Figure 1 shows the percentage that various factors
influence machining center power consumption.
This figure shows that while the coolant unit is thegreatest percentage behind power consumption inmachining centers, main spindle cooling is also a major
contributing factor at around 16%.
Therefore, by developing low temperature-risebearings, it is possible to reduce power consumption
necessitated by cooling and contribute to saving energy.
Figure 2 shows the relationship between features ofthe developed bearings and that of machine tools.
This development aims to suppress temperature rise inrolling bearings and improve speed in order to enhancemachine tool performance. Specifically speaking, as well
as saving energy, the authors aimed to improve machining
accuracy by reducing thermal expansion of the main
spindle and shortening machining time through highspeed rotation of the main spindle.
This development was undertaken with the goal of
reducing temperature rise of a high speed rolling bearing
using oil-and-air lubrication by half that of a normal
bearing.

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)57

−Development of Bearings with Low Temperature-rise and High-speed Performance for Machine Tool Main Spindles−

Feed spindle
4%

OthersHydraulic
Coolant unit
pump
40%
11%
Main spindle
cooling
16% Main spindle
drive unit
16%

Fig. 1 Example of typical power consumption in machining
center by factors
Machine features: Energy-saving
Reduced coolingenergyHigh accuracy machining

Shorter machining time

Reduced thermal expansion Improved machining efficiency
during light cuttingof main spindle

Developed bearing
Reduced bearing temperature-risefeatures:

Improved speed

Fig. 2 Features of developed bearings and machine tools

3. Angular Contact Ball Bearings
3. 1 Characteristics of the Developed Bearing
Table 1 shows JTEKT's High Ability Series of angularcontact ball bearings for machine tool main spindles.
The High Ability Series includes the high-stiffness
Type R, high loading capacity Type C and ultrahigh-speed
Type D & F. One of these, Type F has the advantage ofhigh speed due to the two characteristics of two-direction
oil-and-air supply (axial and outer ring port) and a rolling
contact area design optimal for high speed use. The new
type of angular contact ball bearing developed this time
was based on this Type F.
Figure 3 shows the appearance and heat generation
factors of the developed angular contact ball bearing. The
heat generation factors of angular contact ball bearings
can be broadly grouped into the heat generated in therolling contact areas of the inner ring to balls and outerring to balls, and the heat generated in the sliding contact
areas of the cage to outer ring and cage to balls. During
the development, each factor was investigated.
It is necessary to consider heat generation reduction
and high speed performance to achieve the optimal designof rolling contact areas. Regarding a design that reduces
heat generation, because spin sliding occurs in the area
where the inner ring contacts the balls during high speed
rotation of the angular contact ball bearing, assuming thatthe product of pressure P and sliding speed V (P･V value)at the contact areas affect its heat generation significantly,
therefore the authors aimed to minimize the product of
P･V value. Meanwhile, regarding a design that improves
high speed performance, in order to raise the critical
speed of the main spindle, the bearing that supports themain spindle must be made more rigid. Moreover, it isnecessary to avoid the decreasing of the bearing internal

58

clearance due to centrifugal expansion of the innerring. The rolling contact area of the developed angularcontact ball bearing was optimally designed taking into
consideration these points.
Furthermore, regarding one other heat generating factor,
the sliding area of the cage, using the flexible multibody
dynamics (FMBD) with cage structure supposed elastic,by estimating the amount of heat which would be
generated by each sliding area, the optimal cage design
was achieved. In FMBD, the mutual contacting force ofbearing components and sliding speed under constantspeed rotation were determined and heat generation was
estimated by considering the heat generation amount tobe equal to the power of the contact points between thecage and outer ring and that between cage and balls.
Figure 4 shows an example of estimated results of heatgeneration in the cage sliding area. The X axis shows
the d mn value which is the value of rotational speed
multiplied by the bearing pitch circle diameter (P.C.D).
The figure also shows the amount of heat generated due
to sliding between the cage and outer ring as well as
sliding between the cage and balls (total amount of eachgenerated for each ball area). Figure 4 gives calculation
results under specific load conditions however it wasobserved that the amount of heat generated between
the cage and outer ring was greater than that of heatgenerated between the cage and balls and tended toincrease dramatically with increase in rotational speed.
For the developed bearing, the amount of heatgenerated was calculated as shown above using the cage
design values as parameters, and the cage was designed ina way to reduce heat generation from its sliding area.
Furthermore, the cage material was reviewed for thedeveloped bearing. Seizure of the cage is one of thefactors that cause damage during the high speed rotationof angular contact ball bearings. Therefore, PEEK(Polyetheretherketone) was chosen as a resin with theproperties required in cages under high speed rotation;
namely, high rigidity, heat resistance and a low linear
expansion coefficient.
Table 1 "High Ability Bearing Series" angular contact ballbearings for main spindles of machine tools
Type

Type R

Type CType D

Type F

Application

Highstiﬀness

Highloading ‒capacity

Ultra high-speed

Lubrication
methodGrease/Oil-and-air

SectionproﬁlesOil-and-air

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)

−Development of Bearings with Low Temperature-rise and High-speed Performance for Machine Tool Main Spindles−

Outer ring

Cage
Balls
Inner ring

Factor 1
Heat generation of the rolling contact area

Factor 2

Heat generation due to sliding friction

Heat generation amountFig. 3 Appearance and heat generation factors of developed
angular contact ball bearing
fc-b

Qc-o: cage X outerQc-bring under each rotation speed compared with the normal
Type F bearing, proving there had been improvement.
Also, in contrast to the rotational speed limit for thenormal Type F bearing being 3 300 000 d mn and seizure
occurring at any speed above this, high speed rotationof up to 4 000 000 d mn was possible on the developed
bearing, confirming that high speed performance had
improved.
Next, Fig. 7 shows the results of an evaluation on theinfluence of preload on outer ring temperature rise.
From the results, temperature rise of the outer ring at
higher preload of 0.83 kN was 3 to 8℃ higher than that at
lower preload of 0.42 kN, it was proven that the preload
influence to the temperature rise, however even under
the high preload condition of 0.83 kN, rotation was still
possible without any faults up to a d mn of 4 000 000.

Qc-b: cage X ball
Rotating main spindle

Qc-o
fc-o
2

3

4

dmn value, ×106

Fig. 4 Example of estimated results of heat generation in thesliding area
Fig. 5 Test apparatus for angular contact ball bearing

This section gives the method used to evaluate
performance of the developed angular contact ball bearing
and the obtained results.
Figure 5 shows the test apparatus used. In order tomake conditions in the test apparatus as close to those of
the actual main spindle as possible, a structure imitating
the actual machine driven by a high speed built-in motor
was used and a jacket cooling device was built. One
bearing was used as a sample while a second bearing witha smaller diameter was used as a support bearing, andthey were installed with constant preload.
Table 2 shows the test conditions used. The oil-andair lubrication method was used. Moreover, the developed
bearing used the same style of lubrication as High Ability
Type F, supplying oil-and-air from both the axial direction
and outer ring outside diameter.
During the test, every 1 to 2 hours the rotational speed
was increased step-by-step and the saturation temperatureof the bearing outer ring at each rotational speed wasmeasured.
Figure 6 shows a comparison of results in temperaturerise for the developed bearing and the normal Type Fbearing under each rotational speed.
From Fig. 6 it can be seen that the developed bearing
had approximately 45% less temperature rise in its outer

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)

Table 2 Test conditions for angular contact ball bearing
Item

Conditions

Sample main dimensions

u70×u110×20

Rotational
speed

Max. 45 000 min

Operatingconditions

−1

Load

Constant pressure preload 420 N

Lubrication conditions

Oil-and-air lubrication VG32 equivalent

Cooling conditions

Jacket cooling

Outer ring temperature rise, ℃

3. 2 Performance of the Developed Bearing

60
Normal bearing
50

45% reduction

Developed bearing

40
30
20
10
0
15

20

25

30

35
3

Rotational speed, ×10 min
2

3
6
dmn value, ×10

40

45

−1

4

Fig. 6 Comparison results of temperature rise between developed
angular contact ball bearing and a normal bearing59

−Development of Bearings with Low Temperature-rise and High-speed Performance for Machine Tool Main Spindles−

Outer ring temperature rise, ℃

60
Preload 0.83 kN
50

Preload 0.42 kN

40
30
20
10
0
15

20

25

30

35
3

Rotational speed, ×10 min
2

40

45

−1

4

3
6

dmn value, ×10

Fig. 7 Temperature rise of developed angular contact ballbearing under two preload conditions

4. Cylindrical Roller Bearings
4. 1 Characteristics of the Developed Bearing
There are two types of using cylindrical roller bearings
for machine tool main spindles.
One is using them on the rear of the main spindle
and selecting on the basis of allowing for main spindle
elongation caused by thermal expansion. In this case, theN-type single row cylindrical roller bearing with a ribon the inner ring side is used as the radial load is small
because of just the weight of the main spindle. However,high speed performance is required.
The other is to use a cylindrical roller bearing on thefront side of the main spindle in order to support highcutting load. In this case, high speed performance is not
required however load capacity and rigidity are required;
therefore the NN-type double row cylindrical roller
bearing is used.
The developed bearing this time targets the first methodof usage at the rear of the machining center main spindle.
Figure 8 shows the appearance and heat generation
factors of the developed cylindrical roller bearing.
The heat generation factors of this cylindrical roller
bearing can be broadly grouped into (1) the heatgenerated in the rolling contacting areas of the inner ringto roller and outer ring to roller, (2) the heat generated
due to sliding of the inner ring rib and roller, and (3) theheat generated from the sliding areas of the cage.
For the developed bearing, improvement activities
focused on the main factor of (3), the heat generated fromthe sliding areas of the cage, with particular consideration
taken towards the design of the cage guide method, cage
material and lubrication supply ability.
Normal JTEKT cylindrical roller bearings use the roller
guide method for cages, however when this method isused there is contact between the pocket rib and the roller
in a wedge shape and it is predicted that this increases theheat generated in ultra high-speed rotation. Consequently,
the cage guide method was changed to the outer ring60

guide type.
Each area of the cage was designed using the Taguchi
Method. An evaluation sample was created using fivecontrol factors as parameters, and experiments were
performed using the oil amount as a fluctuating factor andthe outer ring temperature rise as an evaluation property
value in order to determine the optimal level.
It is believed this makes the cage more robust.
For the same reasons as the angular contact ballbearing, PEEK was selected as the cage material.
Next, to suppress temperature rise during high speed
operation, the lubrication supply performance is one
important factor. Therefore, a high speed camera wasused to observe the oil-and-air supplying state.
Figure 9 shows the oil-and-air supplying condition
using a high speed camera. By increasing the frame rate,
the supply interval, supply location and so forth wereclearly observed. In this development, the knowledge
gained through these observations was helpful indetermining the internal design of the bearing from theperspective of improving oil-and-air supply performance.
Heat generation caused by cage sliding friction
Factor 3

Outer ringCage
RollerInner ring
Factor 1
Heat generation of the rolling contact area

Factor 2

Heat generation of the inner ring rib and roller sliding area

Fig. 8 Appearance and heat generation factors of developed
cylindrical roller bearing
10mm

Frame speed: 3 000 fps

2mm

Frame speed: 75 000 fps

Fig. 9 Observation of oil-and-air supplying conditions

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)

−Development of Bearings with Low Temperature-rise and High-speed Performance for Machine Tool Main Spindles−

Rotating main spindle

4. 2 Performance of the Developed Bearing

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)

Fig. 10 Test apparatus for cylindrical roller bearing

Table 3 Test conditions for cylindrical roller bearing
Item

Conditions

Sample main dimensions

u60×u95×18

Rotational
speed

Max. 39 000 min

Operatingconditions

−1

Load

no loadOuter ring temperature rise, ℃

Lubrication conditions

Oil-and-air lubrication VG32 equivalent

40
Normal bearing
Developed bearing

30

46% reduction
20
10
0

5

10

15

20

25

30
3

35

Rotational speed, ×10 min
0.5

1.0

1.5

40

−1

2.0

2.5

3.0

6

dmn value, ×10

Fig. 11 Comparison results of temperature rise between
developed cylindrical roller bearing and a normal bearing
Outer ring temperature rise, ℃

Figure 10 shows the test apparatus used to evaluate
performance of the cylindrical roller bearing and Table 3
shows the test conditions.
The test apparatus has a configuration whereby the
sample bearing and a supporting ball bearing witha smaller diameter than the sample bearing, wereassembled and external drive was performed using a belt.
Because a bearing for use on the rear of the main spindle
was targeted in this development, the test apparatus was
configured so as to apply no external load to the sample
bearing. Moreover, the oil-and-air lubrication method
used under high − speed rotation conditions was used forthe test.
When the bearing rotates at high speed, centrifugal
expansion of the inner ring occurs, reducing the radial
clearance. Therefore, the authors believed that the radial
clearance upon assembly had a significant influenceon temperature-rise performance. As such, the radial
clearance upon assembly conditions was made constantacross all tests performed using each sample.
In the same way as 3. 2, test procedures involved
increasing the rotational speed step-by-step every 1 to 2
hours and measuring the saturation temperature of the
bearing outer ring at each rotational speed.
Figure 11 shows a comparison of results in temperaturerise for the developed bearing and the normal bearing(with a polyamide roller guide cage) under each rotational
speed.
From Fig. 11 it can be seen that the developed bearing
had approximately 46% less temperature rise in its outerring under each rotation speed compared with the normal
bearing, proving there had been improvement.
Also, in contrast to the rotational speed limit for
the normal bearing being 1 500 000 d mn and seizure
occurring at any speed above this, high speed rotationof up to 3 000 000 d mn was possible on the developed
bearing, confirming that high speed performance had
improved.
Next, the advantage of changing the roller material on
suppressing temperature rise was confirmed.
Figure 12 shows the results of a comparison madeon outer ring temperature rise for the respective roller
materials.
The ceramic roller type had lower temperature risecompared to the bearing steel roller under specific high-speed rotation conditions, therefore confirming thattemperature rise was suppressed at high speed.
Changing roller material from bearing steel SUJ2
to ceramic Si3N4 is believed to have the advantages ofsuppressing load on bearing rolling elements causedby heat generation due to a small linear expansion
coefficient, as well as suppressing rolling bearing loadcaused by centrifugal force due to small density.

40
Developed bearing (bearing steel roller)
Developed bearing (ceramic roller)

30
20
10
0
5

10

15

20

25

30
3

Rotational speed, ×10 min
0.5

1.0

1.5

2.0

35

40

−1

2.5

3.0

6

dmn value, ×10

Fig. 12 Comparison results of temperature rise depending onroller materials

61−Development of Bearings with Low Temperature-rise and High-speed Performance for Machine Tool Main Spindles−

5. Conclusion

References

The new angular contact ball bearing and cylindricalroller bearing for machine tool main spindles developedby JTEKT have a suppressed temperature rise of
approximately half compared to normal bearings underoil-and-air lubrication conditions. Moreover, high-speed
capability was improved and it was verified that rotationsof 4 000 000 d mn (spindle dia. = u70, RPM = 45 000−1min ) and 3 000 000 d mn (spindle dia. = u60, RPM =−1
39 000 min ) were possible for the angular contact ballbearing and cylindrical roller bearing respectively.
This type of low temperature-rise rolling bearing
is believed to be advantageous for reducing power
consumption necessary for cooling of machine tool main
spindles and saving energy. Moreover, other advantages
can be expected such as better machining accuracy
by suppressing main spindle thermal expansion andimproved machining efficiency through improved highspeed capability.
JTEKT will continue to exert every effort in improving
the performance of rolling bearings to respond to machine
tool needs.

1)
2)

Y. Hayashi: JTEKT ENGINEERING JOURNAL,
no.1005E (2008) 57.
O. Higashimoto: JTEKT ENGINEERING JOURNAL,
no.1001E (2006) 89.

*1 High Ability is a registered trademark of JTEKTCorporation.

K. SUZUKI*
*
**

62

S. MATSUE*

K. ICHIKAWA**

Experiment & Analysis Dept., Bearing Operations
Headquarters
Driveline System Engineering Dept. 1, Automotive
Systems Business Headquarters, Doctor of Mechatronics

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)