Prospects

Development of Sensor Integrated Bearing Unit for
Machine Tool Spindles

Shohei HASHIZUME*
Daichi KONDO**
Yohei YAMAMOTO***

Yudai NAKANO*
Yoji OHGUCHI**

Machine tools are sometimes called the “Mother Machine” or “Motherof Industry” and have been supporting manufacturing all over the world.
They are required for not only fundamental features like high speed, highrigidity, and super precision, but also condition monitoring and “ConnectedIndustries” related technology. In 2018, NTN developed the “Sensor
Integrated Bearing Unit for Machine Tool Spindles”, and additionally,applied the load detection function and wireless system to the unit in 2020.
NTN has recently added the data receiver unit which collects the data from the “Sensor Integrated Bearing
Unit” in machine tool spindles and the software for data communication. This report introduces thefeatures, structure, and performance of the unit, and offers communication examples between the unit andmachine tools.

1. Introduction
Amid significant changes in the global environmentand social structure, the industrial world has beentackling various social challenges, such as achievingcarbon neutrality to slow down global warming,achieving sustainable development goals (SDGs),and improving production to further enrich peopleʼslives, by integrating new technology created in the
4th industrial revolution (Industry 4.0) into coretechnologies developed over many years.
Among these technologies, machine tools1) 2) that
support manufacturing in various industries, including
the automotive, aircraft, medical and IT sectors, arebeing promoted to further improve basic performance interms of “high speed, high durability and high accuracy”
as a response to energy loss reduction and work force
reduction. Productivity improvements and manpower
reductions using IoT technology are underway.
NTN has developed the “Sensor Integrated Bearing
Unit” for Machine Tool Spindles (hereafter, this bearingunit)3), which has sensors built into the spacer, acomponent of a bearing unit. This enables sensing of thetemperature and vibration around the bearing raceway
surface, making it possible for advanced condition
monitoring of machine tools. NTN showcased areference exhibit of this at the 29th Japan International
Machine Tool Fair (JIMTOF 2018) held in 2018. A loaddetection function was also added to this bearing unit
together with wireless operation4) in 2020.
We have also added a data receiving unit that
acquires data detected by the sensors on thisbearing unit at the exterior of the spindle as well as

communication software to send and receive the datawith the aim of further improving the usefulness of
this bearing unit. This bearing unitʼs characteristics,
structure, performance test results for load and othersensors, and how to connect it to machine tools aredescribed below.

2. Functions and purpose of the sensor
integrated bearing unit
Table 1 shows the functions and purpose of thisbearing unit. When a bearing that supports therotation of the spindle (hereafter, spindle bearing),
a key component on machine tools, is damaged,
operation at the machine tool stops and the spindle
must be replaced; this significantly lowers productivity.
Due to this fact, there is a growing need to detect thisdamage early on. Furthermore, it will be even morenecessary in the future to increase the productivityof machine tools and to minimize the dependence
on individual expertise in machining monitoring in
response to the aging of skilled personnel and thedecline in the working-age population ratio. This
bearing unit is expected to contribute towards theseneeds while also contributing towards achieving
sustainable development goals (SDGs).
For example, if we detect the machine load appliedon the spindle, and use this information for machining
monitoring, we can optimize the machine conditions
such as cutting depth, feed, and rotational speed.
This is expected to provide increased machining
quality and productivity. Furthermore, this bearing
unit detects tool and workpiece collision to reduce* Robotics & Sensing Engineering Dept., Industrial Business Headquarters ** New Product Development R&D Center*** Product Design Dept., Industrial Business Headquarters

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Development of Sensor Integrated Bearing Unit for Machine Tool Spindles
Table 1 Functions & purpose of the sensor integrated bearing unit

Function(1)

Detects external loads
applied to the spindle(3)

Monitors temperature changes
in the spindle bearing

Connection to SDGs

• To see sudden rises in preload thatoccurs before bearing seizure, and todetect signs of bearing seizure early on• To make it easier to manage preload
on bearings after the spindle has been
installed, and to reduce assembly
manhours for the spindle

Detects preload for spindle
bearing(While spindle is rotating
and after spindle has been
installed)(2)

Purpose

Implementation
methodLoadSensor• To detect machining load applied
to the spindle, and contributetowards improved machining quality
and productivity using machining
monitoring• To detect collisions between the tool
and workpiece, reduce damage tothe spindle bearing, and use it toinvestigate the cause of the damage

TemperatureSensor• To monitor the bearing raceway
surface and lubrication conditions(4)

Monitors vibration changes inthe spindle bearing

Vibration
Sensor(5)

No need for an externallyconnected cable or wiring
space for power supply

Independent
Power supply(6)

External connection for datatransmission
No need for cable or wiring
space

WirelessModule• Improve productivity,
and reduce energy
loss throughoptimized production• Reduce defective
workpieces (waste)• Contribute towards
automation andmanpower reduction• To monitor the bearing raceway surface• To detect collisions between the tooland workpiece, reduce damage tothe spindle bearing, and use it toinvestigate the cause of the damage• To reduce man-hours to assemble
the spindle• To eliminate the need to change thestructure of the spindle• Reduce energy loss
with independent
power supply andimproved productivity

Workpiece

Workpiece

Workpiece

Workpiece

Fig. 1 Illustration of sensor integrated bearing unit application
Example of improving machining quality by linking machining data to each product of a machine tool48

NTN TECHNICAL REVIEW No.89

damage to the spindle bearing, uses real-timeinformation to determine the cause of the damage,and as shown in Fig. 1, links machining data for eachmanufactured product to both improve machining
quality and promptly investigate the reason why adefect occurred.

Sensor Integrated Bearing Unit

Load applied to tool
when machining

Housing
SpindleTool

3. Structure and specificationsof the sensor integrated bearing unit

BearingOuter ring spacer

Load sensor, vibration sensor,
temperature sensor

Wireless module Inner ring Processing
spacercircuitRotor and stator
Independent power
supplyFig. 3 Example of application for the sensor integrated
bearing unit on a machine tool spindle
3.2 Specifications
Table 2 shows the specifications for this bearing
unit. It lists load, temperature, and vibration
acceleration for sensing items. Bluetooth Low
Energy (2.4 GHz) is used for the telecommunication
standards to allow for a lower power consumption at
the wireless module and to make it more compact.
The operating temperature range is set to -20 to 70 ℃
with consideration for the allowable temperature ofelectronic components mounted on the module.

SpacerBearing

Table 2 Specifications for the sensor integrated
bearing unit

Sensing3.1 Structure
Fig. 2 shows a structural illustration of this bearing
unit. This bearing unit comprises a set of angularcontact ball bearings arranged back-to-back withan outer ring spacer and inner ring spacer provided
between the angular contact ball bearings. The outerring spacer and inner ring spacer are equipped withsensors, circuits to process data detected by thesensors, a wireless module, and an independent
power supply.
The independent power supply uses anelectromagnetic induction generator. A stator
consisting of a coil and yoke is built into the outerring spacer, and a rotor with alternately magnetized
N and S-poles is attached to the inner ring spacer.
Electromagnetic induction is generated by the relativerotation of the outer ring spacer and inner ring spacer
which generates the power required to run thesensors and circuits.
Fig. 3 shows an example of application for thisbearing unit on a machine tool spindle. Factors suchas the internal clearance and the difference between
the width of the inner ring spacer and outer ring
spacer are adjusted to apply preload on the bearing.
This maintains high rigidity at the bearing which
ultimately improves machining quality. With thisstructure, bearing preload and the load applied to thetool during machining act on the outer ring spacer soa load sensor is provided on the outer ring spacer ofthis bearing unit.
The sensors, circuits, and an independent power
supply are built into this bearing unit in a compact
way;this enables it to achieve the same dimensions as
a conventional outer ring spacer without sensors.

Type

Ultra-high speed angular contact
ball bearings with ceramic balls(Equivalent to 5S-2LA-HSE014)
Double row back-to-backarrangement

Bore diameter ×
outer diameter ×
width

φ70 ×φ110 × 20 (per row) mm

Inner ring spacerbore diameter ×
outer ring spacer
outer diameter ×
width

φ70 ×φ110 × 40 mm

Load

Maximum detection load: 45 kN

Temperature

Detection range: - 40 to 125 ℃

Vibrational
acceleration

Detection range: ± 50 G
Response frequency band:
up to 11 kHz

Independent power
supplyElectromagnetic induction
generator

Communication
standard

Bluetooth Low Energy（2.4 GHz）

Operating temperature
range- 20 to 70 ℃

Fig. 2 Structural illustration of sensor
integrated bearing unit

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Development of Sensor Integrated Bearing Unit for Machine Tool Spindles
4. Connecting to equipment

5. Evaluation testFig. 4 shows an schematic example of using thedata communication capabilities of this bearing unit.
Data detected by the sensors is transmitted wirelessly
from the wireless module built into the outer ring
spacer to the data receiving unit provided with a USB
antenna for data reception. The data receiving unit,
the customerʼs machine tool and server are connectedby a LAN cable so that data can be verified.
Data transmitted from the wireless module consistsof 3 types of data; these are load, temperature, andvibration detected by each sensor. Since each data
type is constantly transmitted to the connected
equipment, it is expected that it will be used tocontrol, monitor conditions as the spindle bearing, andprovide predictive maintenance to machine tools that
require a high level of response.
A separate connection program is required
to connect the data receiving unit to dedicatedequipment. However, if the system uses an IoT
platform, it has the advantage of making the systemmore versatile and easier to connect. NTN provides“Bearing Diagnostic Edge Application” for Industrial
IoT Platforms5) to support efforts in this area.
This bearing unit has an independent power
supply to feed power to the unit and can transmit
data wirelessly, which makes it easy to use in an IoTenvironment.

This section introduces the test results for the load,temperature, and vibration sensor built into thisbearing unit.
Fig. 5 shows the evaluated spindle that simulated
the machine tool spindle. This bearing unit wasassembled onto this evaluated spindle and then theevaluation test was carried out. This bearing unit
wirelessly transmitted data detected by the sensors toa receiver installed externally on the spindle using thewireless module.

Data receiving
USB antenna

Data receiving unit
(Single-board computer)

Machining center spindlelessly)ion (wirensmiss
Data traWirelessModuleSensor

Power supply

Independentpower supply

Data transmission(Wired, LAN cable, etc.)

LoadVibration
Temperature

Sensor Integrated Bearing Unit
ServerMachine tool (machining center)Fig. 4 Example of using the sensor integrated bearing unit50

NTN TECHNICAL REVIEW No.89

Load parameters

600

Drivemotor

18 000

Unit incorporated sensor (load)
Rotational speed

16 000400

14 000200

12 000010 000

-200

8 000

-400
0:00

0:05

0:10

0:15

0:20

0:25

0:30

Rotational speed min-1

800

6 000

Operating time min

Area where bearing unit ismounted

Fig. 5 Structure of evaluated spindle
5.1 Verifying load and temperature responsivenessduring an acceleration/deceleration test
An acceleration/deceleration test was carried out
in which the rotational speed was changed, assuming
the actual operation conditions of a machine tool.
Table 3 shows the test conditions and Fig 6 shows
the test results for load and temperature. It wasconfirmed that this bearing unit can detect changes
in bearing preload and temperature for fluctuationsin the rotational speed. It was also confirmed that it
has a higher temperature sensitivity than an externalsensor.
Table 3 Acceleration/deceleration test conditions

Test bearing

φ70 ×φ110 × 20
Equivalent to 5S-2LA-HSE014(Ultra-high speed angular contact ballbearing with ceramic balls)

Preloadmethod

Fixed position preload (preload of 750 N
after spindle installation)

Rotational
speed

6 000 → 10 000 → 6 000 → 8 000 min-1

Lubrication
methodAir-oil lubrication

Lubricating
amount0.03 mL/10 min

Lubricating oil

ISO VG32

Lubricating
airflow rate

30 NL/min

Fluid coolingchannelYes, room temperature tuning

Axis position

Horizontal axis

Temperature °C

25

Housing outer circumferential surface(thermocouple)
Unit incorporated sensor (temperature)
Rotational speed

18 000
16 00024

14 00023

12 00022

10 000

21

8 000

20
0:00

0:05

0:10

0:15

0:20

0:25

0:30

Rotational speed min-1

26

6 000

Operating time min

Fig. 6 Acceleration/deceleration test results
5.2 Indentation bearing vibration verification test
To tackle the detection of an abnormality on abearing that uses a vibration sensor, damage (anindentation) was simulated on the bearing raceway
surface. Then a vibration verification test was carriedout on the evaluated spindle in Fig. 5 with the same
condition as for the acceleration/deceleration test.
Table 4 shows the test conditions and Fig. 7 shows a3D illustration of the simulated damage (indentation)
created on the bearing raceway surface. During thetest, the vibration on the outer circumferential surfaceof the evaluated spindle housing was measured at thesame time together with the vibration detected by thevibration sensor built into the outer ring spacer, andboth measurements were compared. For a vibrationof 5 000 min-1 at the test bearing, while checking
components with frequency that matches the innerring damage, feature quantities of the sensor built into
the outer ring spacer were greater than at the housing
outer circumferential surface, showing that the signal
to noise ratio improved (Fig. 8). The vibration sensor
built into the outer ring spacer is considered to beable to detect damage on the bearing with better
accuracy.

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Development of Sensor Integrated Bearing Unit for Machine Tool Spindles
Table 4 Vibration verification test conditions
Test bearing

Equivalent to φ70 ×φ110 × 20 5S-2LA-HSE014(Ultra-high speed angular contact ballbearing with ceramic balls)

PreloadmethodFixed position preload (preload of 750 N
after spindle installation)

Rotational
speed

5 000 min-1Lubrication
methodAir-oil lubrication

Simulateddamage(Indentation)

1 location on inner ring raceway surface
Depth of 6 µm

Simulated damage(indentation)

Comparison of feature quantities
for inner ring damage frequency

Fig. 7 3D illustration of the simulated damage (indentation)
created on the bearing raceway surface

32

10

External vibration
sensor

Built-in vibration
sensor

6. Summary
Trials are underway to use and introduce sensing
technology on machine tools, against the backdropof the rising demand for condition monitoring andmachining monitoring.
To meet this demand, NTN is making progress with
developing the “Sensor Integrated Bearing Unit for
Machine Tool Spindles”, which has a built-in wirelessfunction and can detect load, temperature, and vibration.
NTN has also made proposals for new methods toconnect developed products and machine tools.
We will continue to repeatedly evaluate this bearing
unit through operation tests for its practical use andpromote further improvements and refinements to its
function. Furthermore, we will also work on establishing
more advanced condition monitoring and predictive
maintenance technology by combining this bearing unitwith AI technology. These efforts will support the efficient
operation of machine tools.
We intend to work on developing technology thatwill enrich the lives of people and be ecologicallyfriendly in support of sustainable development goals(SDGs) for the future.
References

１） Naoki Matsumori, Keiichi Ueda, Technical Trendof the Precision Bearings for Machine Tools, NTN
TECHNICAL REVIEW, No.84, (2016) 40.
２） Keiichi Ueda, Technical Trend of the Precision
Bearings for Machine Tools, Bearing & Motion
Tech, September 2016 Edition, No.002, 33.
３） Shohei Hashizume, Yasuyuki Fukushima, YusukeShibuya, Yohei Yamamoto, Development of Sensor
Integrated Bearing Unit for Machine Tool Spindles,
NTN TECHNICAL REVIEW, No.86, (2018) 50.
４） Shohei Hashizume, Yusuke Shibuya, Daichi Kondo,
Yohei Yamamoto, Hiroyuki Iwanaga, Developmentof Sensor Integrated Bearing Unit for MachineTool Spindles, NTN TECHNICAL REVIEW, No.88(2020) 33.
５） Hiroyuki Iwanaga, Development of Edge
Application for Bearing Diagnosis, Inspection
Technology, March 2022 Edition, Vol.27 No.3, 32.

Fig. 8 Indentation bearing vibration verification test results
Photo of authors

52Shohei HASHIZUMEYudai NAKANO

Daichi KONDO

Yoji OHGUCHI

Yohei YAMAMOTO

Robotics & Sensing
Engineering Dept.,
Industrial Business
Headquarters

Robotics & Sensing
Engineering Dept.,
Industrial Business
Headquarters

New ProductDevelopment R&DCenterNew ProductDevelopment R&DCenter

Product Design Dept.,
Industrial Business
Headquarters

NTN TECHNICAL REVIEW No.89