TECHNICAL REPORT

Initiatives for the High Performance of Machine Tool Spindle Bearings
H. HONJO K. HASAMA

M. HAMANOUE

R. ONISHI

The demands regarding machine tool performance requirements are growing more stringent, and in order tosatisfy such demands, improving the performance of the bearings used in machine tool spindles is an important
issue that must be addressed. This report presents JTEKT’s initiatives for the high functionality of machine tool
spindle products as an example of spindle bearing and unit product development.
Key Words: machine tools, rolling bearing, angular contact ball bearing, cylindrical roller bearing

1. Introduction
In order to further improve machine tool performance,
improving the performance of the spindle unit, one of
the major units affecting machine tool performance, isessential and the spindle bearing plays a particularly
crucial role. Figure 1 shows machine tool and bearingneeds. Due to the growing need for high-accuracy
machining, bearings are being required to offer
high-speed, high-rigidity and low-temperature riseperformance. Moreover, there are intensifying needs for1)
combined processing machines from the perspective ofhigh-efficiency machining. Bearings are being required
to offer both high-speed and high-rigidity performance inorder to cover a wide scope of machining, from precision
machining of molds, etc. to machining of hard-to-cut
titanium alloy, etc. used in aircraft engines, in just a single
machine.
Another issue is the reduction of environmental
burden. Figure 2 shows the relationship between"lubrication method" and the "high-speed and rigidity"of spindle bearings. Widely-adopted lubrication methods
for spindle bearings are grease lubrication and oil-air
lubrication. Grease lubrication is adopted generally due
to the fact it is easy to handle and enables simplification2)of the spindle structure , however in high-speed zones*1
exceeding a d mn value of 1 million, and in zones whereimportance is placed on high-rigidity, oil-air lubrication is
the preferred method. However, due to requirements forworkplace environment improvements and energy-saving
stemming from changes in the environment surrounding
machine tools, it is predicted that the popularity of greaselubrication will grow due to its minimal oil scatter andlower running costs.
*1 d mn value: Bearing P.C.D. (mm) × rotational speed−1(min )

48

Moreover, efforts to support IoT are advancing even in
regards to machine tools, and these include the monitoring
and compensating of thermal displacement, measurement
3)and compensation of geometric error, etc . Furthermore,
there is a need to promote monitoring of the spindle unit
status through visualization of the bearing status during
operation in order to prevent chatter vibration, perform
spindle abnormality diagnoses, predict remaining life andso on.
This report will introduce JTEKT’s development ofa grease-lubricating, high-rigidity, low-temperaturerise, double-row cylindrical roller bearing, a greaselubricating, low-temperature rise angular contact ballbearing as bearings supporting the upcoming needs ofmachine tools, as well as a dynamic rigidity measurement
system.
Machine tool needs

Bearing needs
Higher speed

High-accuracy machining

Higher rigidity
Lower temperature rise

High-efficiency machining

Balancing high-speed
and high-rigidity

Reduced environmental
burdenExtending the scope ofgrease lubricant application

Status monitoring

Visualization of the bearing
status during operation

Fig. 1 Machine tool and bearing needs

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Rigidity

High−Initiatives for the High Performance of Machine Tool Spindle Bearings−

Oil-air

d mn value 1 million

GreaseHigh-speed performanceHigh

Fig. 2 Relationship between "lubrication method" and"high-speed and rigidity" of spindle bearings

2. Development of a Grease-lubricating,High-rigidity, Low-temperature Rise,
Double-row Cylindrical Roller Bearing
2. 1 Issues with Grease-lubricating, Double-row
Cylindrical Roller Bearings
Lathes and machining centers are the typical machine
tools and while the lathe generally places importance onhigh rigidity, as described in the preceding paragraph,
low-temperature rise and high-speed performances are
now also being required. Figure 3 shows a typical bearing
array of a lathe spindle unit. On a lathe spindle, a doublerow cylindrical roller bearing is used as the front bearing
in order to secure radial rigidity and, commonly, greaselubrication is used here. JTEKT’s double-row cylindrical4)
roller bearing adopts a brass retainer and sometimes
contact with the roller during operation can generate
metal chips which lower grease performance, therefore
it was necessary to use resin cage-type retainers in somehigh-speed operation applications. In the beginning, weused polyamide resin for this, however, with increasingly
tougher spindle operating conditions, the issue of retainer
breakages arose. Figure 4 shows the damaged condition
and breakage mechanism of a conventional polyamide
resin cage-type retainer.
Double-row cylindrical roller bearing
Angular contact ball bearing for thrust load
Double-row cylindrical roller bearing

Fig. 3 Typical bearing array of a lathe spindle unitmachine hard-to-cut materials. Retainer breakage isbelieved to be caused due to roller lead-lag created fromrapid acceleration/deceleration and high load leading totensile force/compressive load acting on the peripheral
direction of the retainer bar.
Due to the above, there was a need to develop a
retainer able to withstand rapid acceleration/deceleration
operation and high load in order to respond to highefficiency machining needs.

Shape: Cage-type
Material: Polyamide resinBar

Bar breakage

Peripheral direction load

<Breakage condition of conventional retainer>
High-efficiency machining

Machining of hard-to-cut material

Rapid acceleration/
deceleration operation

High load

Orbital speed difference of each roller
Roller lead-lag occurs
Tensile force/compressive load on
the peripheral direction of the retainer bar
Retainer breakage<Breakage mechanism>

Fig. 4 Damaged condition and breakage mechanism of
a conventional cage

2. 2 Features of the Developed Product⑴Overview of the developed productFigure 5 is a comparison of the conventional anddeveloped products. In order to respond to rapid
acceleration/deceleration operation and high load, wechanged the shape and material of the retainer for the
newly developed bearing. In regards to the shape, theconventional bearing adopted a cage-type retainer,
therefore we used a comb-type on the developed product.
Moreover, regarding material, whilst the conventional
bearing used polyamide resin, the developed bearing
uses super engineering plastic. The roller guide method
is adopted in the retainer of both the conventional anddeveloped bearings.

In order to support the high-efficiency machining
demanded for machine tools, rapid acceleration/
deceleration of the spindle unit is necessary to shorten
non-cutting time. Moreover, there is also a need tosupport cutting at high loads in order to efficiently

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−Initiatives for the High Performance of Machine Tool Spindle Bearings−

Conventional productDeveloped product

Polyamide resin
Cage-type

Super engineering plastic
Comb-type

Roller guideRoller guidecompressive load in the peripheral direction act on the bar
portion.
⑵FEM analysis
In order to confirm the superiority of the developed
product related to its shape, we performed an FEM
analysis of the maximum principal stress generated on
the retainer. Figure 6 shows the results of a comparison
between the conventional and developed products.
Compared to the conventional product, the maximum
principal stress is around one-third for the developed
product when there is peripheral direction load due toroller lead-lag.
⑶Low-temperature rise design
For the developed product, we clarified the heating
mechanism in order to achieve low temperature rise.
Figure 7 shows the heating mechanism of the roller guidecomb-type retainer, and Fig. 8 shows the pocket part of
the retainer during bearing rotation. For the roller guidecomb-type retainer, the rib is distorted (expanded) in theradial direction due to centrifugal force working on it
during bearing rotation and retainer movement increases
due to the retainer guide clearance becoming larger during
rotation. As a result, the contact force of the roller andretainer increases, causing heat generation. Consequently,
we believed suppressing the distortion of the retainer rib
would be an effective means of achieving low temperaturerise.

Bearing andretainer shape
Material
Shape
Retainer guide
methodFig. 5 Comparison of conventional and developed productdesignFirstly, in regards to retainer material, Table 1
provides a comparison of retainer property values for theconventional and developed products. Compared to thepolyamide resin used for the conventional product, thesuper engineering plastic used for the developed producthas superior mechanical strength; namely 1.6 times
greater tensile strength and 3.1 times higher Young’s
modulus, therefore was adopted on the developed productto countermeasure retainer breakage.
Table 1 Comparison of cage property values
Conventional
product

Developed
product

Polyamide resin

Super engineering
plastic

Tensilestrength, MPa

165

260

Centrifugal force works on
the retainer due to bearing rotation

Youngʼsmodulus, MPa

7 400

23 000

Rib distorts (expands) inradial direction

Speciﬁc gravity,
g/cm3

1.33

1.4

Retainer guide clearance grows larger(= greater retainer movement)

Material

Contact force of roller andretainer increases

Maximum primary stress, MPa

Furthermore, regarding the retainer’s shape, while
the conventional product featured a cage-type retainer
whereby each side of the bar was annular, the comb-type
is characterized by having only one annular-shaped side
as per Fig. 5. Through adopting this shape, it is possible
to reduce the stress generated when tensile force andHeat generation initiated from retainer

Fig. 7 Heating mechanism of roller guide comb-type cage

500

Conventional product (cage-type)
Developed product (comb-type)

400
300
200
100
0

0

100

200

300

Peripheral load from the roller, N

Fig. 6 Analysis result of maximum principal stress

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2. 3 Performance of the Developed Product

Rib⑴Durability test
We performed a durability test to confirm the effects
of changing the material and shape of the developed
product. Table 2 shows the conditions, and Fig. 10 shows
the results of this durability test. While the conventional
product broke after 300 hours, the developed productwas capable of over 930 hours of continuous operation,
confirming a durability of more than 3 times higher.

RollerPre-distortion
Rib distorts in radial direction

Table 2 Durability test conditions

Post-distortion

Fig. 8 Pocket part of cage during bearing rotation(conceptual diagram)

Guide clearance during
bearing rotation
Small guide
Large guide
clearance
clearance

In order to suppress rib distortion due to centrifugal
force, it would be effective to select a material withsuperior mechanical strength, and reduce rib weight.
Regarding the material, the super engineering plastic
selected as a countermeasure to breakage is also effective
as a countermeasure to rib distortion. Moreover, in orderto reduce rib weight, we shortened the length of the rib.
Figure 9 shows the relationship between rotational speed
and guide clearance as a parameter of rib length throughthe result of FEM analysis. As Fig. 9 shows, shortening
rib length reduces the guide clearance during bearing
rotation.

0Long rib length
Short rib length

2 000

4 000

6 000

8 000

10 000−1

Rotational speed, minFig. 9 Relationship between rotational speed andguide clearance (FEM analysis result)

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

Item

Test conditions

Bearing(I.D. × O.D. × Width)

NN3018(u90mm × u140mm × 37mm)

Rotational speed

Rapid acceleration/deceleration
between 0 ⇔ 6 000 min−1

Lubrication method

Grease lubrication(Filled to 10% of spatial volume)

Radial clearance uponassembly−30 μmSpindle orientation

Horizontal

CoolingNatural heat dissipation

Conventional
product

300h

Developed
product

Retainer breakage

Truncation(no retainer breakage)Over 930h
0

200

400

600

800

1 000

Test time, hFig. 10 Durability test result⑵Temperature rise test
In order to compare the temperature rising tendenciesof the conventional and developed products, as wellas confirm the effects of a low-temperature rise design
with the parameter of rib length, a rotational test was
carried out. Table 3 shows the conditions, and Fig. 11
shows the results of this test. Developed product 2 (witha short rib length) has 46% lower temperature rise thanthe conventional product. This is believed to be because,
compared to the cage-type retainer of the conventional
product, the comb-type retainer of the developed product2 has good grease discharging efficiency from the race
portion, which may have reduced the mixing resistance ofgrease.
We also observed that, developed product 2, which isdesigned specifically to have low-temperature rise, has
24% lower temperature rise than developed product 1(which has a long rib length), confirming that shortening
rib length is effective in lowering temperature rise.

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Table 3 Temperature rise test conditions
Item

Test conditions

Bearing(I.D. × O.D. × Width)

NN3018(u90mm × u140mm × 37mm)

Rotational speed

MAX: 9 000 min−1(d mn value: 1.04 million)

Lubrication method

Grease lubrication(Filled to 10% of spatial volume)

Radial clearance
upon assembly−2 μm
Horizontal

CoolingNatural heat dissipation

Outer ring temperature rise, ℃

Spindle orientation

30

20
15

3. 1 Overview of the developed productAs the temperature of a machine tool spindle rises,
thermal expansion occurs in the axial direction, which inturn affects the accuracy of the machined parts. As such,in order to respond to the need for machine tools to offer
high-accuracy machining, as mentioned in Section 1
above, there is a need to lower the temperature rise of
the bearing, which is the cause of heat generation in thespindle. In regards to this, JTEKT was already selling aseries of angular contact ball bearings for machine tool
spindles with excellent low-temperature rise and high5)
speed performance , but we undertook the challenge oflowering temperature rise even further.
To achieve low temperature rise, there is a need toreduce grease mixing resistance and the contact resistance
between the retainer and balls, as these cause heatgeneration, therefore retainer design has a significant
influence. As a part of a revised retainer design on thedeveloped product, we optimized the clearance between
the retainer and inner ring, as well as the contact points
with the balls inside the retainer pocket. Figure 12
shows a comparison of the conventional and developed
products. The clearance between the retainer and innerring was enlarged compared to the conventional productto improve grease fluidity and reduce mixing resistance.
Moreover, compared to three contact points with the balls
inside the retainer pocket for the conventional product,
the developed product was made to have just one contact
point.

46% lower

Conventional productDeveloped product 1(Long rib length)
Developed product 2(Short rib length)25

3. Development of a Grease-lubricating,Low-temperature Rise Angular
Contact Ball Bearing

24% lower

10
5
0

0

2 000

4 000

6 000

Rotational speed, min

8 000

10 000−1

Fig. 11 Temperature rise test results

From the above results, we believe that developed
product 2 can contribute to high-speed, low-temperaturerise lathe spindles which are generally required to havehigh-rigidity. We also believe that this product canfurther contribute to higher rigidity through adoption onmachining center spindles, for which importance is placedon high-speed performance.

Conventional productDeveloped product

A
Cross-section of
the assembly
axial direction

GreaseA

Clearance: small

Clearance: large
GreaseSmall heat generation
due to grease mixing
resistance

Bearing andretainer shape
Retainer pocket
top view(direction of the
A arrow)

Rotational
direction

3 contact points

Rotational
direction

1 contact point
Low heat generation due
to minimal sliding
contact points

Fig. 12 Comparison of conventional and developed product design

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3. 2 Temperature Rise Test
Table 4 shows the conditions and Fig. 13 shows
the results of a temperature rise test. Compared to theconventional product, we confirmed that the developed
product had 15% lower temperature rise. Moreover, there
is also an expectation that grease life will improve as it
is majorly affected by temperature, and we calculated an
improvement of over 10%.
Table 4 Temperature rise test conditions
Test conditions
HAR014CA-5DBB(u70mm × u110mm × 20mm)×4 rows

Rotational speed

MAX: 10 000min−1(d mn value: 900 000)

Preload method

Fixed position preload

Lubrication
methodGrease lubrication

CoolingNatural heat dissipation

Outer ring temperature rise, ℃

Item
Bearing(I.D. × O.D. × Width)20
1515% lower

the spindle is rotating, therefore preload changes as it
is affected by centrifugal force and heat. Moreover, inmachine tools where the spindle orientation can change,
preload can vary due to the weight of the spindle acting
on the bearing depending on spindle orientation. Despite
these points, there are practically no cases of measuring
preload during spindle operation.
The next section will introduce JTEKT’s efforts
to develop a dynamic rigidity measurement system
6)by adopting magnetic bearing technology , to enablemeasurement of spindle performance in an arbitrary
operating status.

4. 2 Features of the Developed ProductFigure 14 is an outline of the dynamic rigidity
measurement system. This system comprises of a radial
exciter and an axial exciter, and is mounted on the spindle
housing. In terms of rigidity measurement, an arbor using
the suction force of an electromagnet is mounted to thespindle tip using a non-contact method then vibrated at
different frequencies to detect the displacement at suchtimes with a sensor.
SpindleRadial exciter

10

Axial exciterArbor

5
Dynamic rigidity
measurement system

0Conventional productDeveloped productFig. 13 Temperature rise test results

4. Development of a Dynamic Rigidity
Measurement System for Machine Tool Spindles
4. 1 Issues in Conventional Spindle Performance
Management
A major factor influencing spindle performance isbearing preload. A higher preload is advantageous forrigidity, however disadvantageous for high-speed, heatgeneration and life properties. Meanwhile, a smaller
preload is advantageous for high-speed, heat generationand life properties, however disadvantageous for rigidity.
Moreover, variation of preload creates inconsistency inspindle performance, which in turn impacts workpiece
accuracy. As such, one potential approach to obtaining
stable spindle performance is appropriate preload
management.
Bearing preload is often managed in accordance witheither spindle natural frequency, axial static rigidity
or start-up torque, and these are generally measured
when the spindle is static. However, during machining

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

Fig. 14 Schematic of a dynamic rigidity measurement system

4. 3 Measurement Example
Figure 15 shows an example of dynamic rigidity−1
measurement when rotational speed is 5 000 min andthe spindle is vibrated in the radial direction. It shows
the input frequency in the X axis and a dimensionlessquantity relative to rigidity in the Y axis. We observed thatrigidity decreases when frequency is around 900 Hz andwere able to confirm the 1-dimensional natural frequencyof the spindle in the radial direction.
Moreover, Fig. 16 shows the influence of spindleorientation and rotational speed on the measurement of
natural frequency. We confirmed that the spindle’s natural
frequency differed depending on rotational speed, and thatthe result also changed depending on spindle orientation.
This is in good consistency with the theoretical values
considering changes in bearing contact angle during
operation and the influence of spindle orientation, andindicates that spindle rigidity can be measured during
spindle operation.

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1.E+0.1

DSP Unit

1.E+00

Bearing: 7916C-5
Boundary dimensions:
u80mm × u110mm × 16mm
(I.D. × O.D. × Width)

1.E−01

1.E−02

1.E−03
1

10

100

1 000

10 000Frequency, Hz

Fig. 15 Example of dynamic rigidity measurement

Natural frequency, Hz

1 200
Radial direction

1 000

Spindleorientation: Upward-facing vertical: Horizontal: Downward-facing verticalcontinue to diversify, and a need will arise to develop
bearings not customized to offer certain limited
performances, but rather to meet a variety of demands.
Moreover, in order to support the realization of machine
tools with autonomic functions of self-diagnosing
problems and machining at optimal conditions in
response to IoT advancements, JTEKT wishes to promote
development of products relating to the monitoring of
spindle unit status through the visualization of bearing
status during operation.

References
1)
2)

800
600

Axial direction

3)

400
200
0

4)
0

2 000

4 000

6 000

Rotational speed, min

8 000

10 000−1

Fig. 16 Influence of spindle orientation and rotational speed
in natural frequency measurement

Based on these results, we believe adopting the
dynamic rigidity measurement system would make thefollowing possible.
1) Measurement of rigidity and natural frequency when
the spindle is static⇒ Apply to preload management in the same way as
the conventional method
2) Measurement of rigidity and natural frequency when
the spindle is rotating⇒ By ascertaining the natural frequency of thespindle, it is possible to contribute to optimizationof machining conditions, such as chatter vibration
prevention
Moreover, because it would be possible to measurespindle properties in mid-operation, we believe it would
be possible to contribute to monitoring of spindle unit
status in the future, such as through spindle abnormality
diagnosis, and will continue to promote developmentin preparation for establishment of a dynamic rigidity
measurement system.

5)
6)

Journal of the Japan Society for Precision Engineering,
vol. 78, No. 9 (2012) 740 (in Japanese).
JTEKT CORPORATION: Precision Ball & Roller
Bearings for Machine Tools, CAT. No. BA005JA-0DS,
35.
Journal of the Japan Society for Precision Engineering,
vol. 83, No. 1 (2017) 11.
O. HIGASHIMOTO: Machine design, Vol. 61, No. 3(2017) 23 (in Japanese).
Y. HAYASHI: JTEKT ENGINEERING JOURNAL, No.
1005 (2008) 58 (in Japanese).
H. UEYAMA: Koyo Engineering Journal, No. 163E
(2003) 24.

H. HONJO*

K. HASAMA* M. HAMANOUE**

R. ONISHI**

5. Conclusion
Higher functionality of bearings for machine tool
spindles would lead to higher performance of machine
tools. Moreover, it is believed that such needs will

54

*
**

Industrial Machinery Application Engineering Dept.,
Bearing Operations Headquarters
Experiment & Analysis Dept., Bearing Operations
Headquarters

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