1

Hydrodynamic Plain Bearings for the Main Bearing Arrange-

ment of a 6 MW Offshore Wind Turbine

Azadeh Kasiri*1

Andreas Blumberg1, Ralf Schelenz1, Georg Jacobs1

1Center for Wind Power Drives

Campus-Boulevard 61, 52074 Aachen, Deutschland

Hydrodynamic Plain Bearings for the Main Bearing Arrangement of a 6 MW Offshore Wind

Turbine

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Contents

1 Abstract ............................................................................................................... 1

2 Introduction ......................................................................................................... 1

3 Test rig description ............................................................................................. 2

4 Experiments and results .................................................................................... 4

5 Conclusion .......................................................................................................... 8

6 Bibliography ........................................................................................................ 8

Hydrodynamic Plain Bearings for the Main Bearing Arrangement of a 6 MW Offshore Wind

Turbine

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1 Abstract

Roller bearings are used in conventional designs of main bearing arrangements in wind

tur-bines [1]. Manufacturing costs and repair costs for main bearings become increas-

ingly important especially with respect to multi megawatt offshore wind turbines and

growing rotor diameters. Any damage of the main bearing requires at least disassembly

of the rotor to carry out the exchange of the bearing at appropriate locations. This leads

to high costs. In addition there are limitations regarding manufacturing feasibility when

considering larger diameters of roller bearings due scaling up the turbine size [2]. In

comparison to roller bearings, plain bearings have advantages regarding vibration

damping, lower material stresses and reduced installation space. Further benefits are a

theoretical unlimited lifetime under the assumption of a correct design. Moreover using

plain bearings there are design possibilities to avoid disassembling the rotor in case of

bearing damage and carry out repair with on-board equipment. Thus a hydro-dynamic

plain bearing at the rotor shaft can serve as a suitable solution to overcome mentioned

problems when using roller bearings. Moreover it can overcome the restrictions regard-

ing to upscaling and roller bearing dimensions for multi megawatt class wind turbines.

On the other hand, using plain bearings in wind turbines is considered as a challenge at

low sliding speeds operation scenarios since they induce high levels of friction. However,

Witter et al. [3, 4] showed, that under certain conditions hydrodynamic plain bearings

can be operated almost wear-free even at high bearing loads and low sliding speeds.

2 Introduction

Plain bearings for planetary wheels in gearboxes of wind turbine (WT) have been re-

searched for several years. In this scope of application plain bearings become state-of-

the-art. In contrast to roller bearings, plain bearings are characterized by good damping

properties and a simple design. Further investigations have shown that plain bearings

compared to conventional design rules have high technical potential during mixed friction

conditions [3], especially using typical WT gearbox oils. With increasing power class of

wind turbine the dimension of every single component increases due to higher loads.

Plain bearings provide advantages of scalability in comparison to roller bearings de-

signed as main bearing.

Based on these and the advantages compared to roller bearings new investigations try

to find out whether plain bearings are feasible for WT main bearing as well.

Because of the constant position of the tooth mesh relative to the planetary pin the load

zone within the planet bearing is clearly defined. In contrast to that main bearing loads

depending on wind force and direction, result in no clear load zone (figure 1). The de-

flection of the planet shaft is not that high compared to the main shaft. Depending on

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load direction the deflection of the bearing housing is another aspect which is not negli-

gible (figure 2). In critical operation conditions e.g. idling or standstill, the planet bearing

loads are close to zero, as loads depend on the drive torque. For these conditions there

are remaining loads for the main bearing due to wind loads and rotor weight.

Figure 1: load distribution for main bearing (left: rotor side; right: generator side)

Figure 2: bearing housing deflection depending on different load directions - 6

(left), 3 / 9 (middle) an 12 (right) o’clock

These different boundary conditions require a divergent plain bearing design. The de-

veloped main bearing is based on a segmented plain bearing. Executed as a four-point

bearing arrangement with two separate radial bearings and one thrust bearing. The seg-

mented tilting pads adapt to the deflection of the main shaft and the bearing housing. In

case of high wear every single pad can be exchanged easily.

The aim of this paper is to verify the feasibility and reliability of plain bearing as main

bearing for a 6 MW offshore wind turbine.

3 Test rig description

The feasibility and applicability of plain bearings as the main bearing should be demon-

strated via two demonstrators, 1 MW and 3 MW class, in different test phases.

Hydrodynamic Plain Bearings for the Main Bearing Arrangement of a 6 MW Offshore Wind

Turbine

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Figure 3: 1 MW demonstrator with integrated plain bearing

Fehler! Verweisquelle konnte nicht gefunden werden. shows the 1 MW class de-

monstrator mounted on a V52 wind turbine base frame with integrated slide bearing from

the first test phase at center for wind power drives (CWD). The simulation of wind loads

in the experiment is carried out via a wind load simulator unit (WLS), which consists of

three horizontal hydraulic actuators each with 160 kN in axial direction and one hydraulic

actuator with 160 kN in vertical direction. The actuators are joined to the main shaft via

a nonrotating load triangle (red) and a double row taper roller bearing. With the drive the

setup has at least 5 degrees of freedom to apply thrust, tilt and yaw bending and rotor

weight. The plain bearings are supplied with oil via a hydraulic aggregate, which is

placed under the bearings unit. A separate inlet pipe supplies each bearing with oil. The

inlet volume of oil is controlled by the oil pumps. The drive unit is placed on the backside

of the downwind bearing and is joined to the shaft via a coupling.

The applied bearing concept has approximately an technology readiness level TRL of 5

to 6 and considers separated functional surfaces in form of two radial bearings (upwind

and downwind) and one thrust bearing (upwind).Thus a simple construction and low

production costs are ensured. According to the load distributions both the axial bearing

and the radial bearings are designed as segmented tilting pads, whereby both the weight

of the bearing and the material costs can be reduced.

Each radial pad is equipped with temperature sensors. The most loaded pads have a

pressure sensor. The radial clearance can be measured via eddy current sensors which

are mounted in the shaft in the same position as the radial bearings. The housing defor-

mation is measured via strain gauge technology (DMS) mounted in main load directions

on the radial bearing housing.

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4 Experiments and results

The first experimental phase is carried out with a focus on the functionality of the plain

bearings under critical operating points. Thus in this phase no dynamic loads are applied

to the bearings. The most critical operating points such as start stop, idling and opera-

tional limit are checked after run in phase. In following some gained results from idling,

start stop, and operational limit tests are presented.

Operating limit:

To determine the operating limit of the radial plain bearings, they are loaded at rated

speed with a relative mean surface pressing of 1 up to the relative mean surface design

pressing. The friction torque is measured by a torque measuring shaft mounted between

downwind plain bearing and coupling.

Figure 4: Stribeck curves as the result of operating limit test series

Fehler! Verweisquelle konnte nicht gefunden werden. shows the safe mixed lubrica-

tion region for the radial bearings when reducing circumferential speed and rising up to

design mean pressing.

Hydrodynamic Plain Bearings for the Main Bearing Arrangement of a 6 MW Offshore Wind

Turbine

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Figure 5: Friction coefficient of transition points of the stribeck curves versus mean sur-

face pressing.

Idling:

The stribeck tests shows small torque and constant temperature by reducing the circum-

ferential speed under pressing, which are equal to the own weight of rotor and shaft for

standstill. As a result of this gained knowledge, Instead of performance a conventional

Idling test, the pressing on the front bearing is raised to the design pressing value. Under

this condition the rotational speed is reduced to 0.0155 m/s (1 rpm) (Fehler! Verweis-

quelle konnte nicht gefunden werden.).

Figure 6: Reducing circumferential velocity under design pressing

Even for this very low speed the measured torque and the temperatures of the pads

remain in safe region (Fehler! Verweisquelle konnte nicht gefunden werden.). It can

be inferred that even if at very small circumferential speed a stable hydrodynamic oil

layer is built up, which avoid a contact between the shaft and the bearing so that the

temperature and the torque don’t rise extraordinary.

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Figure 7: Temperature and friction torque trend regarding to Fehler! Verweisquelle

konnte nicht gefunden werden.

Start Stop:

Start stop tests run for more than 12000 cycles. Fehler! Verweisquelle konnte nicht

gefunden werden.shows how each cycle was designed. The radial plain bearings is

first put under load up to the design pressure (Zone I), in the next step the rotational

speed is raised to rated speed (Zone II). The rated speed and the applied load are kept

constant (Zone III). A very quick shut down within 2 seconds follows and the rated speed

falls to zero (Zone IV). After that, the applied load is reduced to zero.

Figure 8: Designed cycle for start stop tests

Fehler! Verweisquelle konnte nicht gefunden werden. shows the surface of one of

the most loaded pads in start stop tests after 12000 cycles. Traces of run in are visible

but there is no wear on the surface of pads. A stable hydrodynamic oil layer covers also

here the pads so that under small rotational speeds the pads do not get any damages

Hydrodynamic Plain Bearings for the Main Bearing Arrangement of a 6 MW Offshore Wind

Turbine

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from wear. The measured radial clearance for start stop tests is around 4 µm. As before

a stable trend for temperature and friction torque could be observed (Fehler! Verweis-

quelle konnte nicht gefunden werden.).

Figure 9: the trend of friction coefficient, temperature and friction torque of start stop

tests

It is remarkable that the oil inlet temperature has a noticeable effect on stability of the oil

layer. The higher the oil inlet temperature the lesser the oil viscosity and therefore the

smaller the radial clearance. Thus the friction coefficient rises with rising inlet tempera-

ture (Fehler! Verweisquelle konnte nicht gefunden werden.)

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Figure 10: The effect of temperature of radial clearance

5 Conclusion

A 1MW test rig based on vestas V52 wind turbine with integrated plain bearing as main

bearing is tested at CWD. The feasibility and functionality of the plain bearing as main

bearing could be proven regarding to performed tests for the most critical operating

points like start stop and idling. A stable oil layer which avoid a hard contact even if at

very less circumferential speeds is identified as main reason for moderate measured

temperatures and less frictional torque. From the modified Idling test can be interfered

that a stable Oil layer exists already for a less circumferential speed around 0.0155 m/s.

Another important gained knowledge from the tests is the dependence of the radial clear-

ance of the plain bearing on the oil temperature. The higher the oil temperature, the

smaller the radial clearance. A 4 µm radial clearance is measured at upwind plain bear-

ing for Start stop tests.

6 Bibliography

[1] E. Hau, Windkraftanlagen; Grundlagen, Technik, Einsatz, Wirt-

schaftlichkeit, Berlin, Heidelberg: Springer Verlag, 4. Auflage,

2008.

[2] N. Weinhold, „Das Riesenrad; (...) Machbarkeitsstudie für eine

20-Megawatt- Turbine (...),“ Neue Energie, Bd. 04/2011, Nr.

04/2011, 2011.

[3] D. Witter, R. Schelenz und G. Jacobs, „Simulation hochbelas-

teter, langsam drehender Radialgleitlager im Mischreibungsbe-

reich unter Berücksichtigung elasti-scher Verformung und Ver-

schleiß,“ in s VDI-Berichte 2147 - Gleit- und Wälzlagerun-gen,

Düsseldorf, VDI Verlag GmbH, 2011, pp. 159-171

Hydrodynamic Plain Bearings for the Main Bearing Arrangement of a 6 MW Offshore Wind

Turbine

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[4] D. Witter, G. Jacobs, R. Schelenz, I. Weiser, „ Hydrodynamic

plain bearings in a main gearbox of a 6 MW wind turbine“ CWD

Conference, Fachtagung, Session 6 „ Plain bearings in WTG

gearboxes“, Aachen, March 2017