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A New Test Rig to Study Rolling Element Bearing Thermomechanical Behavior


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Introduction

    The rolling element bearing (REB) is an essential component
in mechanical transmission to reduce friction between rotating
parts. Now, with the development of the electrical motor
in mechanical industry, REBs may work at very high-rotation
speed. It leads to an increase of REB power losses and temperatures.
In literature several approaches are used to estimate
the REB thermomechanical behavior. Hence, for some
applications, several divergent viewpoints can be found on
thermal behavior modeling according to the dissipation
sources taken into account. In order to overcome these discrepancies,
a specific test rig was designed to obtain information
on REB thermomechanical behavior. This test rig allows
for obtaining REB power losses thanks to torque measurement.
To study high-speed applications, the (N.dm) product
expected volume is higher than one million.

    Rolling element bearings are widely used in mechanical
transmission to reduce friction between two rotating parts.
With the further development of the electrical motor in mechanical
industry, REBs operate more and more at high rotational
speed. For these applications, REBs power losses can
be predominant in mechanical transmissions. Several global
models can estimate the REB resistive torque (Refs.1–2).
Hence, for some applications, several divergent viewpoints
can be found between these models.
In order to overcome these discrepancies, some measurements
are required. In the literature, several tests rigs are presented
to measure REB torque loss.
A first set of test rigs measure torque loss on REB outer ring. In this case, the REB outer ring has to be mounted in the
inner ring of a hydrostatic bearing. The torque is measured
via a load sensor located on a beam between the REB outer
ring and hydrostatic bearing housing (Ref.3).
Brecher et al. (Ref.4) used a hydrostatic bearing to measure
the REB torque loss for high-speed application. In this
test rig a telemetry system is used to measure the REB inner
ring temperature.
Neurouth et al. (Ref.5) have also used this design to measure
the REB frictional torque for grease-lubricated thrust
ball bearings.
However the hydrostatic bearing can be complex to use
and modifies the outer ring thermal behavior.
REB torque loss can also be measured with strain gauges
located on the housing. Hannon (Ref.6) developed a test rig
for four types of REBs within a similar size range. A slip ring
allows measuring the REB inner ring temperature. In this test
rig, four identical REBs are mounted on the main shaft. The
global torque loss is divided by four in order to obtain the
REB torque loss. The REBs’ torque loss is measured thanks to
a strain gauged torque table. This test rig has been developed
for low-rotational speed and strong radial load conditions
(up to 260kN).
Pinel et al. (Refs.7–8) developed a test rig for a 35mm bore
diameter angular-contact ball bearing under thrust load and
for very high-speed application; the maximum rotational
speed is equal to 72,000rpm, which corresponds to a (N.dm)
product equals to 3.4 million. The REB torque is measured
with strain gauges located near the end of an arm that prevents
the housing from rotation. However, this measurement
can be complex to realize.
Finally, REB torque loss can be measured on the inner ring
by using a torquemeter. In this case, the torquemeter measures
the global torque of the shaft. However, some components
can affect this measurement (seals, REBs mounting,
etc.).
Takabi et al. (Ref.9) designed an REB test rig to study the
deep-groove thermal behavior in oil bath lubrication. The
test rig is composed of two REB mountings and one test bearing.
A torque sensor measures the torque loss of the system.
        REB tests rigs with vertical shaft are also presented in the
literature. These test rigs allow testing thrust ball bearings
(Ref.10) or cylindrical (Ref.11) and tapered roller bearings
(Refs.12–13) under axial load. The torque measurement is
realized with a torquemeter located on the vertical shaft.
To finish, some test rigs have been realized only for one
application. Ke et al (Ref.14) developed a specific REB test rig to study the thermal characteristics of double-row tapered
roller bearings of a high-speed locomotive. Blake and Truman
(Ref.15) designed a test rig to measure the running torque of
tapered roller bearings.
The abovementioned test rigs are dedicated to one operating
condition or one size of REB. The new test rig developed
in this study is dedicated to a wide range of REB dimensions
and for different operating conditions.
In the first section of this paper a new REB test rig design is
presented. The second part of this paper is dedicated to the
first experimental results.
The New Test Rig Design
Specifications of this test rig. The new test rig has been designed
to be able to study the thermomechanical behavior of
several kinds of REBs and for different lubrication conditions
(grease, oil bath and oil jet). This modularity has been a key
point during the test rig design. The tested REB outer diameter
is between 72mm and 150mm. A lot of REBs can be tested
in this test rig: deep groove ball bearings, angular contact ball
bearings, roller bearings, etc. Radial and axial load can be applied
on the tested REB.
Test rig operation. Thus far, several tests rigs have been
presented. For each test rig, a specific method is used to measure
the REB torque loss. In this new test rig, REB torque loss
measurement is divided into two steps:

? Calibration phase: Four identical REBs are mounted
and work in the same operating condition (rotational
speed, radial load, lubrication). These REBs are jetlubricated
with the same lubricant at a given oil injection
temperature. The torque loss is divided by four in order to
isolate the torque loss of one REB.

? Measure phase: Two REBs (calibration block) are
removed and replaced by the tested REB (Fig.2). The
torque loss of mounting blocks (determined in calibration
phase) is subtracted to the global torque measurement in
order to isolate the contribution of the tested REB.
This architecture requires a specific test rig design. The
main challenge is to connect the blocks while maintaining
a correct concentricity of the main shaft. Labyrinth seals are
used to guarantee oil tightness and to limit power losses.
Deep grove ball bearings are used in in the mounting blocks;
their characteristics are presented in Table 1; they have been
chosen in order to work at high-rotational speed.
Test rig components:
REB lubrication. The REB test rig is composed of two oil
tanks. The first one is dedicated to the lubrication of calibration
and mounting blocks. These REBs are always lubricated
with the same oil at the same temperature (around 70°C).

    The
lubricant properties are presented in Table 2.
The second reservoir is dedicated to the tested REB; it
allows testing different lubricants. Gear pumps impose an
oil flow between 0 and 1.5 L/min on the REB. Each oil tank
is surrounded by hot plates to warm the lubricant. The oil
pipes are thermally isolated to reduce heat exchange with the
ambient air, allowing for an oil injection temperature even
higher than 100°C. The electro spindle maximum rotational
speed is equal to 18,000rpm. A hydraulic jack allows applying
a radial load on REB up to 20kN.

Conclusion

    This research work aims to present a new REB test rig dedicated to the
study of the thermomechanical behavior of this mechanical component.
In the first part of this paper, this new test rig is presented. It has
been designed to study different kinds of REBs and for different operating
conditions.
The second part is dedicated to the first results that have been
obtained. An 85mm pitch diameter deep groove ball bearing was
tested under different operating conditions. Experimental results
have been compared with global models of power losses. This part
underlines that these models can estimate the REB torque loss for lowspeed
application. However, when the (N.dm) product tends toward a
million, the REB torque loss increases suddenly; this increase is not
correctly taken into account in the global models. Moreover, the influence
of the REB thermal behavior and the oil flow rate on the REB
torque loss were highlighted.

2024-07-12