January/February 2014 | Renewable Energy Focus30

For wind turbine manufacturers,

it’s no longer enough to supply quality

wind turbine gearboxes. These days,

there’s much more emphasis on the

“service” aspect — meaning suppliers

and gearbox overhaul service provid-

ers are taking a more proactive ap-

proach in serving their clients’ main-

tenance needs. These services range

from providing real-time systems

“monitoring” to anticipate when tur-

bines might require servicing before

they break down, to actually building

in periodic maintenance schedules

to avoid costly downtimes that often

come with catastrophic failures.

This strategy is becoming in-

creasingly important when it comes to

THE TREND toward larger wind turbines

and the growth in off -shore wind farms

creates new maintenance challenges. None-

theless, gearbox makers and overhaul service

companies are adapting to meet demand.

Wind turbine gearboxes submitted for an overhaul undergo a rigorous test procedure before release to the customer. (Images courtesy of ZF Services UK Ltd)

off -shore wind farms, where opera-

tors share horrors stories of having

to wait for months on end to receive

replacement parts. To address this

issue, gearbox overhaul companies are

getting more sophisticated in their ap-

proach. “We’re beginning to add new

things such as condition monitoring

and vibration analysis, which allows us

to monitor the customers’ equipment

remotely,” said Dave Morgan, UK

business manager, ZF Wind Services, UK. “This is especially critical with

off -shore installations, where you could

be waiting a few months if the gearbox

goes down.”

Remote condition monitoring lets

service companies track the status

of key drive train elements, gauging

attributes such as wind speeds, shaft

speeds, bearing temperatures, power

output and other critical parameters.

Some advanced monitoring systems

even boast the capability of detecting

cracks in gear teeth! (REF, Sept/Oct

2013 edition).

“With our ability to do condition-

ing monitoring, we can make predic-

tions on when to remove gear boxes,

and we’re working with our customers

to devise planned maintenance sched-

ules to fi gure out the best time to do

that,” Morgan explained. For example,

on off -shore wind farm systems that

entail, say, 30 turbines, ZF Services

may advise proactively overhauling

5-6 turbines per year. “This repre-

sents a considerable cost savings com-

pared to running the turbines until

they fail, then ordering the parts and

scheduling a trip out to the installa-

tion site.”

In markets such as the UK, where

off -shore wind farms are common-

place, this can be particularly chal-

lenging. Not only has there been a

proliferation of off -shore wind farms,

but, as Morgan notes, there has been

a drive to increase the size of the tur-

bines and rotor diameters to increase

overall yields. (Hence the popularity

of 6MW and 8MW turbines.) Morgan

is also seeing more “fl oating” systems

crop up, which pose their own unique

maintenance challenges.

“We’re moving more into the off -

shore fi eld, although they’re not easy

to get to,” Morgan explained. “While

they are built to last a long time,

we’re also looking at doing things

such as taking components of the gear

box apart in sections up the tower,

thereby savings time and costs. We’re

also doing endoscope inspections on

an annual basis.”

The overhaul processWhen ZF Services receives a gear-

box slated for overhaul, the external

condition of the unit and its ancil-

laries is checked and the results are

recorded. During disassembly, the

condition of each internal component

is inspected and noted. Failure mode

and cause are recorded where appli-

cable, helping to minimise the risk of

repeat or premature failure. A com-

prehensive gearbox condition report

Proactive approach to gearbox repairs

Renewable energy • technology update

focus:Technology

REF0114_FocusTech_Wind turbine p30-31 30 13-02-14 15:19:10

31January/February 2014 | Renewable Energy Focus

About: Reg Tucker is the US editor of Renewable Energy Focus

is then produced and discussed with

the customer to approve further work.

Clients are encouraged to inspect the

stripped unit at any time during the

overhaul process, ZF Services says.

During overhaul, bearings and seals

are routinely replaced with original

equipment parts. ZF Services UK

technicians measure gears, shafts and

bores to preestablished standards to

assess whether components are within

tolerance and can be reused safely,

require specialist refurbishment or

need to be replaced altogether. Gears

within tolerance can be reground

using the latest CNC technology and

machinery, whilst bores and shafts

can be returned to original equipment

specifi cations using industry standard

recovery techniques.

Alternatively, gears and shafts

deemed beyond the limits of safe

recovery are replaced by original-

equipment standard components

meeting the rigorous guidelines estab-

lished by the wind turbine industry. In

instances where parts are obsolete or

prove hard to obtain, ZF Services UK

Ltd’s specialist suppliers are capable

of reverse engineering these to OE

standards.

According to Morgan, the scope

for the maintenance and repair of

gearboxes can include items such as

brakes, lubrication and cooling sys-

tems, torque arms, and rotor shaft re-

pairs. Gearboxes can be returned fully

dressed with all necessary ancillaries

fi tted off ering what he calls a “plug-

and-play” unit that reduces on-site

downtime on re-fi t. Ancillary items

are checked, overhauled or replaced,

thereby ensuring reliable, long-term

operation.

Morgan also stresses that every

wind turbine gearbox overhauled at

ZF Services UK’s facility in Notting-

ham, central England, undergoes a

rigorous test procedure before release

to the customer. Here, a purpose-built

validation rig for testing gearboxes

of turbines up to 2MW generating

capacity can analyse oil fl ow, gearbox

temperature, tooth contact pattern,

and noise and vibration profi les. How

it works: the test rig applies a load

through the gearbox via a “slave gear-

box” and output motor and records

the current draw, temperature, vibra-

tion and torque fi gures on the test

rig’s human-machine interface (HMI)

panel. Thermal imaging is used to en-

sure that there are no local hot spots

in the gearbox, and an oil fi ltering

unit checks for correct oil fl ow and fi l-

ters oil down to fi ve microns to ensure

the gearbox is free of any particulate

contaminants.

While overhaul specialists such as

ZF Services are putting more of an

emphasis on proactive maintenance

schedules (what’s that expression

about an ounce of prevention?), they

also acknowledge that average turbine

life spans are on the rise: 10 years on

average for 650kwh units; 12-14 years

on the 1.3 MW; and then 15 years-plus

on the 2MW, Morgan notes. “All these

units will need at least one overhaul in

their lifetime,” he fi gures.

Technology

Planned maintenance represents considerable costs savings

compared to running a turbine to the point of failure.

During disassembly, the condition of each internal component is inspected and recorded; failure mode and cause are noted where applicable.

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