TECHNICAL REPORTApproaches for Bearings with Low Temperature-risefor Machine Tool Main Spindles
Y. HAYASHI

R. ONISHI

Low temperature-rise is an important technology in bearings for machine tool main spindles.
Bearings with low temperature-rise enable energy saving by reducing power consumption cooling system formachine tools. This is effective in improving machining accuracy as it reduces thermal extension. It is also effectivein improving machining efficiency as it increases the high-speed capability of the spindle.
Although we are creating a lineup of the angular contact ball bearing and cylindrical roller bearing, which had
low temperature-rise and were excellent in high velocity rotation performance when used by machine tool main
spindles, we would like to incorporate further low temperature rise technology in order to improve product appeal.
Key Words: machine tool, rolling bearing, temperature rising, thermography, fluid simulation

1. Introduction
Improving the performance of the units and individualcomponents which a machine tool is configured from isessential to improving the performance of the machine
tool itself. The main spindle is of particular importance,
as it directly affects machining accuracy and efficiency,
therefore the standard required from the market has grownincreasingly higher in recent years. The performancerequirements of machine tools include the reduction ofprocesses and man-hours as well as a higher capability
to machine difficult-to-machine material. Therefore theperformance of the main spindle in regards to high-speed,
low temperature and so on, has been raised as a key item.
Up until the year 2000, bearings used in the main spindle
were required to have a maximum rotation performanceof around 3 million dmn but in recent years this has risen
to 3.5 million dmn or higher. Low temperature-rise isanother important technology that a main spindle bearing
is required to possess. The less heat a bearing generates,
the less thermal expansion occurs on the main spindle;
which is extremely effective in improving machining
accuracy, the most important feature of a machine tool.
This also contributes to energy-saving as it is possible toreduce power consumption for cooling. In other words,
high performance bearings are essential to achieve highperformance main spindles.
JTEKT offers a product series featuring highspeed angular contact ball bearings and cylindricalroller bearings with an excellent low temperaturerise characteristic for application in the main spindlesof machine tools. In order to achieve even higher

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

performance, we are engaging in the development oflow temperature-rise technology. This paper introduces
examples of approaches for the bearing internal
temperature measurement technology which uses
thermography and a simulation of the airflow inside thebearing under oil/air lubrication.
* dmn: Pitch circle diameter (P.C.D.) of the bearing’s−1rolling element (mm) × rotational speed (min )2. Approaches for achieving low
temperature-rise of main spindle bearings
2. 1 Visualization of bearing internal temperature
Despite low temperature-rise being a key technology
of not only bearings for machine tool main spindles, butbearings in general, not much was known regarding theinternal temperature of bearings during rotation.
The areas of the bearing which generate heat are eachsliding contact portion between the inner ring and rolling
element, cage and outer ring (in the case of outer ringguide cage type bearings) and cage and rolling element, as
well as each sliding contact portion of each rolling contact
portion between the outer ring and rolling element. All ofthese areas are extremely difficult to measure.
Table 1 shows the measuring apparatus generally used tomeasure the temperature of rotating bodies such as bearings.
This method uses a slip ring and transmits a thermocouple
signal through brush contact therefore is limited in regards
to the rotational speed it can be used at. Moreover, while theinner ring temperature can be measured, it was unable tomeasure the individual temperatures of internal components
such as the cage and rolling element.

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−Approaches for Bearings with Low Temperature-rise for Machine Tool Main Spindles−

When measuring using an infrared thermometer, it isdifficult to set measurement points in narrow areas suchas the cage and rolling element, therefore measuring
temperature proved to be a challenge.
This is why JTEKT has used high-sensitivity
thermography and built a system which can measure theinternal temperature of bearings. This system has helped
to clarify the heat-generating factors and parts within
the bearing and through combination with the FEM heatgeneration forecast method, made it possible to reduce1)heat generation within the bearing .
In general, angular contact ball bearings and cylindricalroller bearings are used in the main spindle of machine
tools therefore measurements of internal temperature
were conducted on these bearings as per shown in thebelow table and low temperature-rise was accomplishedbased on the results.

Table 2 shows the samples measured and test
conditions used. These are general usage conditions formachine tools.
Figure 2 shows an example of the actual measurement
results for a standard bearing. The results show that thetemperature of the rolling element is the highest of all
the components within the bearing. The red dots in Fig. 3
indicate the points where temperature was measured
on the outer ring, cage and rolling element. These
measurement results were established as representativevalues and the rotational speeds were set as parameters.
Figure 4 gives the results of temperature measurements at
various portions inside the bearing. Across all rotational
speeds, the temperature of the rolling element was thehighest and rose as rotational speed increased.
Sample bearing

Table 1 Measuring method for rotating body temperature
Measurement
deviceMeasured portion
Problems

Innerring

Rollingelement

Cage

Thermocouple,
slip ring○××

Has limitation
regarding maximum
rotational speed

Infraredthermometer○×△

Diﬃcult to measurenarrow portions

Fig. 1 Test apparatus for angular contact ball bearing

Highly-sensitive
thermography

○

○

○−

Table 2 Test conditions of angular contact ball bearing○…Measurement possible△…Measurement diﬃcult
×…Measurement impossible

Thermography

Sampleconditions

2. 2 Reduction of heat generated inside angularcontact ball bearings
In order to clarify the factors behind internal heatgeneration of angular contact ball bearings which use
ceramic rolling elements, we used a system which utilizes
the high-sensitivity thermography mentioned earlier andmeasured the bearing internal temperature. Figure 1
shows the apparatus used in measurement. This apparatus
adopts DDS motor drive and a constant preload method
and is of a structure which resembles the main spindle ofmachine tools. Moreover, a large opening has been madeat the end of the spindle in order to observe the inside of
the bearing.
Thermography measures the infrared strength (infrareddosage) and converts this to temperature therefore if
there is disturbance such as reflection, it is difficult toobtain an accurate temperature measurement. As such, wehave devised various ways to remove disturbance frombearings and enable accurate temperature measurement.

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Testconditions

Type

Angular contact ball bearing

Main dimensions, mm

u70×u110×20

Rolling element pitchdiameter, mm90

Rolling element material

Si3N4 (ceramic)

Preload method

Constant-pressure
preloading−1

Rotational speed, minMax. 30 000dmn

Max. 270×104

Lubrication method

Oil/air lubrication
70˚C25˚C

Fig. 2 Bearing temperature (standard, 30 000 min−1)

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−Approaches for Bearings with Low Temperature-rise for Machine Tool Main Spindles−

As it was possible to confirm the internal temperatureof the standard bearing, we then performed the heatgeneration forecast mentioned previously and Fig. 5shows the results of a measurement after the bearing wasoptimized by improving the cage design and respective
clearances. Compared with the results of Fig. 2, thereis less warm color, confirming that the bearing internal
temperature has dropped. In other words, throughoptimization of the bearing based on heat generation
forecast, we confirmed a reduction in bearing temperaturerise and verified the validity of our technique foranalyzing heat generation forecast.

●Measurement points

Direction of photoFig. 3 Photography direction and the point of measurement

80
Rolling element
Cage
Outer ringTemperature, °C

706050403020

2. 3 Reduction of heat generation inside double
row cylindrical roller bearings
It is a known phenomenon that when double row
cylindrical roller bearings are rotated at high speed, theyheat up rapidly from a certain rotational speed onwards.
We investigated this phenomenon using this system.
For this investigation, we used the bearing stand-alone
test apparatus shown in Fig. 6 and made an opening at theend of the spindle in order to measure the bearing internal
temperature, as was done for the abovementioned test
apparatus shown in Fig. 1.
The bearing used was a u100 I.D. double row
cylindrical roller bearing with a brass cage as shown inTable 3.
The left half of Fig. 7 shows the result of measuring
bearing internal temperature when abnormal temperaturerise occurs. Compared with the result of the right halfof Fig. 7, where there is no abnormal temperature riseoccurring, temperature rise of the cage is significant andit is believed that this heat effects the entire bearing,
therefore the phenomenon of increased temperature riseat high-speed rotation is surmised to be caused by thisheat generation of the cage.
As such, we established the design elements of thecage as parameters and performed an investigation using
cage samples with differing lengths as per Table 3.
Figure 8 shows the plotted results of the cage rib
temperature by rotational speed for each sample. Cagetemperature is shown on the Y axis, while rotational
speed is shown on the X axis, and it can be seen that
depending on the differences in cage rib length, there is atendency for the cage temperature to rise rapidly once acertain rotational speed has been reached. In other words,
the phenomenon of rapid temperature rise when a double

10Sample bearing

0010 000

20 00030 00040 000

Rotational speed, min−1

Fig. 4 Bearing temperature (standard)
Thermography

70˚C

Fig. 6 Test apparatus for cylindrical roller bearing
140˚C25˚C

Fig. 5 Bearing temperature (Optimal design, 30 000 min−1)

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

At abnormal
temperature rise

At no abnormal
temperature rise

25˚C

Fig. 7 Cage temperature (8 000 min−1)

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−Approaches for Bearings with Low Temperature-rise for Machine Tool Main Spindles−

row cylindrical roller bearing is rotated at high speed
is believed to be due to heat generation in the cage ribportion.
Rapid temperature rise of the rib portion is thought
to be related to the volume of the rib portion and therotational speed. This is surmised to be due to thecentrifugal force which is applied to the rib portion. We
performed an FEM analysis regarding the deformation of
the rib portion caused by centrifugal force. The top half ofFig. 8 shows the results of a deformation analysis while
Fig. 9 shows an image of such deformation.
As shown in the top half of Fig. 8, the longer the ribs
(bigger the volume), the more deformation occurs on theouter ring due to centrifugal force accompanying the rise
Table 3 Test conditions of cylindrical roller bearing
Type

Double row cylindrical roller bearing

Main dimensions, mm

u100×u150×37

Cage material

BrassSample ##1#2#3
Rib length

Rough shapeCage

Rib lengthLong−1300
280
260
240
220
200
1801601401201008060
40
200Short

3 000 to 12 000Big

Cage contact range

Rib deformationTemperature, °C

Rotational speed, minMedium

Small
#3
#2
#10

2 000

4 000 6 000 8 000 10 000 12 000 14 000
Rotational speed, min−1

Fig. 8 Cage temperature and a cage deformation FEM
analysis results (#1-3 correspond to Table 3)

Deformation

RollerCage

Fig. 9 Deformation of the cage by centrifugal force (FEM)

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in rotational speed. Moreover, the longer the ribs are,the greater the tendency for deformation to begin at lowspeed rotation.
Comparing the top and bottom halves of Fig. 8, thereis good consistency between the rotational speed at whichrib deformation exceeds the cage contact range and therotational speed at which the cage temperature rises
rapidly, as shown in the bottom half of the figure.
From the above, it can be said that by measuring thetemperature of a cage during rotation it is possible toverify the FEM analysis result with actual measurement.
By utilizing FEM analysis and minimizing rib
deformation at the same time as optimizing design toalso consider the impact on other performances, it waspossible to achieve a double row cylindrical roller bearing
with an even better low temperature-rise property.

3. Approaches for high-accuracy fluid
simulation technology
Bearings for the main spindles of machine tools
primarily adopt either the oil/air or grease methods oflubrication. High velocity rotation spindles in particular
adopt the oil/air lubrication method. Oil/air lubrication
is a method whereby minute amounts of oil controlled ata fixed amount are supplied and delivered together withcompressed air to each bearing. Compared with greaselubrication, oil/air lubrication has small mixing resistanceand better air-cooling results utilizing compressed air,making it suitable for high velocity rotation. Figure 10gives an overview of the oil/air lubrication system.
However, due to the characteristic of oil/air lubricationthat it supplies oil to the inside of the bearing fromexternally with compressed air, not all of the oil/air
supplied reaches the inside of the bearing. This issurmised to be because, as Fig. 11 shows, an air curtain
forms between the oil/air discharge port and the bearing
during high velocity rotation causing a portion of theoil/air to be pushed back thereby decreasing the ability
to supply to the inside of the bearing. In a separate
confirmation test, we attempted supplying only oil
directly inside the bearing in order not to be effected by
the air curtain and by doing so confirmed that equivalentrotational performance could be obtained with only
1/1 000 of the oil supplied during oil/air lubrication. This
point differs to the grease lubrication method whereby
lubricant is present within the bearing from the beginning.
Based on the above results, in order to clarify theair flow/air curtain behavior within the bearing anddetermine the best grease nozzle shape and location toeffectively supply lubrication inside the bearing, we arecollaborating with Toyota Central R&D Labs., Inc using2)
their high accuracy fluid simulation technology to build
a behavior analysis system for the air flow/air curtain

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−Approaches for Bearings with Low Temperature-rise for Machine Tool Main Spindles−

behavior within bearings. Figure 12 shows the simulation
results of flow velocity distribution inside a bearing. The
flow velocity distribution of Fig. 12 does not take thenozzle into account however we are gradually clarifying
the flow direction and volume of the oil/air ejected fromeach nozzle in bearings which rotate at high velocity.
Through the knowledge obtained using this system, wewill make a proposal for an optimal oil/air supply methodwith better cooling efficiency in the near future andanticipate it will contribute to achieving even better low
temperature-rise bearings.
Air source

Air cleaning unit

Remove water,
oil and contaminants.

Oil/air lubricatormain unit

Operates a pump intermittently
to supply a determinate quantityof oil mixed with air.

OilAirOil/air lubricatormixing valve
Oil/airBearing
outer ring

Outer ring spacer

Bearing
inner ring

Inner ring spacer
A4. Conclusion
This time, by applying high-sensitivity thermography,
we established a technology to accurately measure
bearing internal temperature (rolling elements, cages)
unable to be measured in the past. By combining thismeasurement technology with FEM heat generation
forecasting we made great progress in regards to reducing
bearing heat generation and clarifying the seizing
mechanism, which we believe is useful for the designof bearings used in machine tool main spindles andreduction of heat generation in bearings used across allfields.
Moreover, by establishing a high accuracy fluid
simulation technology, we will continue to strive for evenbetter low temperature-rise performance and improved
product appeal.
By developing low temperature-rise bearings, webelieve we can contribute to better machining efficiency
and improved quality and accuracy through improving themachining accuracy and high-speed potential of machine
tool main spindles. We can also anticipate this will haveenergy-saving benefits due to a reduction in the power
necessary for main spindle cooling.
It is JTEKT’s wish to continue developing bearings
which can be used by our customers with peace-of-mind
and contribute to energy-saving initiatives.

Fig. 10 Oil/air lubrication system

References
Air curtain forms(supposition)

Oil/air1)
2)

NozzleK. Suzuki, S, Matsue, I, Kazuyuki: JTEKTENGINEERING JOURNAL, No. 1010 (2012).
M. Inagaki: R&D Review of Toyota CRDL, vol. 39, No.
1, pp. 29-35 (2004).

Inner ring high velocity rotation

Fig. 11 The air curtain of bearing side (A section)

Rolling elementCage

Inner ring Axial direction

Radial direction

Outer ring

3.0
U (m/s)0.0

Y. HAYASHI****

Fig. 12 Flow velocity distribution inside bearing

JTEKT ENGINEERING JOURNAL English Edition No. 1012E (2015)

R. ONISHI**

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

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