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Reconditioning and durable wear protection for gear boxes and bearings in wind turbines

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Figure 6. Optimising the metal surfaces of tribologic systems will increase the parts life time. If the Wind Turbines life is only prolonged with about six days the product costs are already paid. Furthermore there is a targeted exchange and maintenance of the gear box and bearings possible.Use of Wind Energy – Future Development The share of renewable energy sources in total energy consumption is to be increased to 20% in 2020.To achieve this extremely ambitious target, the share of renewable energy sources in electricity generation in the European Union has to be increased to proximately 34% by 2020. The continuing expansion of renewables in Europe and also in Germany will be characterised mainly by the use of wind energy. 20,301 wind turbines with a total capacity of 23,903 MW have been installed in Germany at the end of 2008.

 40.2 TWh of wind electricity was generated during this year. At this time the installed capacity in Europe amounted to 65,946 MW and worldwide to 120,798 MW. Maintenance of Wind Turbines The most expensive components of a wind turbine, beside tower and blades, are c gearbox and bearings requiring about 13% of the total costs. The average lifetime of these important components is between five to seven years in Western Europe and about two to three years in Asian countries.

To replace the main gear multiplier of a 1.5 MW wind turbine after an unexpected damage costs about 250,000 Euros including crane costs. It also has to be mentioned that availability of gears and bearings is limited. The operation and maintenance of a wind turbine and especially of the abovementioned components can be supported by various measures, such as integrating a condition monitoring system or developing appropriate maintenance strategies.The lifetime of the bearings and gears in new as well as in existing wind turbines can be extended by REWITEC’s Nanocoating system even when the first signs of decay appear.

REWITEC was founded in 2003 with the goal of reducing the problem of wear and tear in tribologic systems.REWITEC Technology – Nanocoating for
Tribosystems The coating technology is not based on modification of the oil film, but on modification of the surface texture of frictional metal parts and the creation of a new very smooth metal silicate coating. This metal silicate coating reduces friction and increases efficiency, especially in older
engines and gearboxes.
Figure 1. Friction and pressure in engines and gearboxes generates high temperatures on metal surfaces that trigger the chemical bonding process of REWITEC.Figure 2. The soft nanoparticles remove dirt from the rubbing metal surfaces and react with the metal atoms to form a smooth and protective layer on the surface.Figure 3. The final result is a new and very smooth metal surface structure. These properties, in turn, give the machine more power, higher efficiency and longer life.
Due to optimisation of compression pressure in engines, oil and fuel consumption will be noticeably reduced (Figures 1 to 3).
The synthesised metal silicate compound only has to be added to the original lubricant and is carried to the affected areas of the engine or gear box. In
the rubbing area, the metal silicates react due to friction, temperature and high partial pressure and bond into the metal surfaces. They fill the smallest corrugations pores or micro cracks in the metal surface and build an extremely smooth and solid metal silicate layer. The treated metal
silicate surfaces have a much higher resistance to abrasion and wear, have a lower friction coefficient and allow for better and faster build-up of a lubricating film.

 A complete lubricant analysis proved that REWITEC is not an additive but a surface enhancement treatment. The important oil characteristics such as total acid number TAN and air and water release capabilities will not be altered.
Figure 4. a – This ball bearing shows damage due to wear and tear. The running surface of the balls clearly shows pitting. Bearing type: Nr. 6205 (d = 25 mm, D = 52 mm) Lubricant: Special bearing grease Operating time: 50 hours Rotational speed: 500 min-1 Nominal load: 5.000 N radial (range of fatigue) b – This ball bearing does not show any wear. The running surface of the balls clearly shows a wear resistant coating. Bearing type: Nr. 6205 (d = 25 mm, D = 52 mm) Lubricant: Special bearing grease treated with REWITEC Operating time: 50 hours Rotational speed: 500 min-1 Nominal load: 5.000 N radial (range of fatigue)
 Appearance and colour are changed due to the coating concentrate. Viscosity and foam test show only a slight change in the run-in phase,but within tolerances.Beside the Nanocoating concentrate that is applied through the gear and engine oils, REWITEC also produces formulated bearing and gear greases.

A comparison of a standard bearing grease and the same bearing grease formulated with the Nanocoating concentrate is illustrated in Figure 4.Condition Monitoring Different methods for proving the successful outcome of the Nanocoating are useful: Oil analysis for measuring the metal particles in the oil, silicon imprints of the metal surfaces for microscopic analysis of the metal texture, electrical resistance analysis of the metal surfaces and the condition monitoring by noise and vibration harshness analysis before and after REWITEC treatment.

The project partner in condition monitoring analysis is the company μ-Sen GmbH. Since 2001, μ-Sen has established itself as a solution provider for fault detection systems for wind energy plants (WEP). Along with vibration reports for WEP, this includes a service concept for turbine condition monitoring via the Internet. Since 2001,condition monitoring systems developed by μ-Sen have been installed in more than 700 WEP (21 different types of wind turbines from 11 manufacturers).Condition Monitoring via Body Sound Analysis for Wind Energy Plants Body sound is a very sensitive indicator for changing conditions of moving machines and system components.
Figure 5. Illustration (scheme/left) of wind turbine drive train with acceleration sensors; (right) Gearbox with acceleration sensor (sensitivity: 100 mV/g).
Body sound sensors can be easily retrofitted, because they do not influence the measured object due to the relative small size and mass.The advantage of the acceleration sensors mostly used for recording of the body sound is, beside the big dynamic area, the calculation of acceleration in speed and amplitude. Thanks to these advantages acceleration sensors are ideal for recording
body sound signals.

Due to the fast development in the field of wind energy area since the midnineties and the simultaneous rapid increase in the power output of wind
energy plants, the measuring requirements at these plants are also growing. Beside the monitoring techniques, mainly body sound based diagnosis systems (Condition Monitoring Systems – CMS) are increasing, which allows early failure detection on the main components of the drive train. The increased use of computer based CMS for different manufactures has already reduced the cost of such a system to around 10,000 Euros, cost that is amortised in one to two years.

For a better understanding of the results of the REWITEC Nanocoating via condition monitoring, the measurement data recording and measurement data
analysis is explained below.Measurement Data Recording for Body Sound Analysis Condition Monitoring Systems are used for early failure detection in wind turbines in Germany. These systems are installed in the nacelle of the WEP and the body sound sensors are at the main components (main bearing, gearbox, generator) of the drive train (compare Figure 5/schematic description of drive train).For verifying the effect of REWITEC Nanocoating, the sensor position was above the high speed shaft at the second spur gear stage of the WEP. Besides other points, the trend of the spectral component (amplitude of rotary frequency of the intermediate shaft in envelope spectrum) was also measured. The envelope algorithm is a very successful method for early failure detection of bearings and toothing damage.The spectral components in the envelope arise due to impacts which happen during roll over of chips on the
running trace of rolling bearings. These impulses stimulate structure resonances and allow a very early detection of such damage. The same applies to
corresponding damage on gear teeth.CMS are mostly configured so that on days without special incidents (no violation of the limit) four time series will be recorded.

 Out of these 300 to 400 specific values in the time- and frequency domain, the single specific values will be calculated. In the trends of the supervised specific values we get four new measuring points every day.Practical applications and results of the REWITEC treatment in different wind
turbines are illustrated in the next section.Flender Gearbox of TACKE TW 250 Wind Power Plant  Date: August 2007  Data: TACKE TW 250, Flender gearbox, oil content: 75 litres Mobil oil XMP SHC 320 Target of application: restoration of worn gear surfaces,  reduction of micro pitting,  wear protection and extension of lifetime of the gearboxes Application method:  neutralisation of the oil fine filter, using main filter > 60 μm for approximately 500 running hours,  application of the REWITEC concentrate to the gear oil, reactivation of the standard oil filters approximately 500 running hours after
treatment (Figure 6).Figure 6. Optimising the metal surfaces of tribologic systems will increase the parts life time. If the Wind Turbines life is only prolonged with about six days the product costs are already paid. Furthermore there is a targeted exchange and maintenance of the gear box and bearings possible.

TACKE TW 250 – documentation of the gearbox conditions before and after the coating (Figures 7 to 8).Conditions before Coating
The gears were showing significant wear and tear and fragments from the tooth flanks had broken off. The bevelled edges and tooth tips were sharp edged. Replicas were taken from the marked tooth. The measured values of the resistances of the tooth surfaces were 0 Ohm. Conditions after 1400 Operating Hours
The teeth have a smooth and shiny surface. Replicas were taken from the marked tooth again. They were showing significant smoothing of the surface compared to the replicas that were taken before. Values of about 50 Ohm were measured for the resistances of the tooth surfaces due to the formation of the REWITEC coating. The highest values of resistance were measured at the lower part of the teeth.Results  restoration of worn gear surfaces, reduction of micro pitting,  the wind turbine has been running without any mechanical problems until December 2007.Flender Gearbox of TACKE Wind Power Plant TW 600  Date: February 2008  Data: TACKE TW 600 with 360 litres Mobil oil Castrol Optimol X320 Target of Application  restoration of worn-out gear teeth surfaces, reduction of grey stoking, protection against further wear and prolongation of life time Conditions before Treatment Wear traces and grey stoking on gear teeth surfaces.

 Replicas were taken from the marked gear teeth. Measurement of the electrical resistance on gear teeth surface resulted in 0 Ohm.Conditions after Two Years The gear teeth are still much smoother and shinier. Replicas were taken again from the gear teeth. In comparison to the earlier replicas there was an obvious smoothening of the teeth surfaces.Because of the REWITEC Nanocoating the electrical resistance of the gear teeth surface has increased up to 100 Ohm. The highest values were measured on top and on bottom of the gear teeth.TACKE TW 600 – Documentation of Gear
Condition before and after REWITEC Treatment through Replica Analysis (Figures 9 to 10)Microscopic analysis on the replicas of the gear surfaces: The REWITEC coating is clearly visible.

The grooves are smaller and the tooth surface much smoother and grey stoking was nearly completely gone.Results
 restoration of worn out gear teeth surfaces, reduction of grey stoking, the wind power plant is operating since the REWITEC coating without any mechanical problems.AN Bonus 1.0 MW with Flender Gear Box  Date: August 2008  Data: AN Bonus 1.0 MW with Flender gear box PEAS 4355.7, 150 litres Tribol
oil 1710/320 Target of application: restoration of worn out gear teeth surfaces, reduction of grey stoking,  protection against further wear and
prolongation of life time,  analysis of REWITEC Nanocoating with CMS of μ-Sen GmbH Documentation of Gear Condition before and after REWITEC Treatment Conditions before the Treatment Different wear traces, partly due to metal shavings passing through the gear teeth.Imprints were taken from the gear teeth marked, and measurements of the electrical resistance on the teeth surface showed 0 Ohm.Conditions after about Seven Months after the Treatment
Gear teeth surfaces are clearly much smoother and shinier.

 Replicas were taken again from the gear teeth, and compared to the earlier replicas. There was a clear smoothening of the teeth surfaces.Because of the REWITEC coating the electrical resistance of the gear teeth has increased up to 50 Ohm.μ-Sen – Condition Monitoring System (CMS) Results The frequency spectrum after =REWITEC treatment from the μ-Sen Condition Monitoring System (CMS) shows that the
amount and level of the peaks of the rotary frequency of the high speed shaft is reduced by about 20% (Figure 11 ). The acoustic analyses show also that the conditions of the gear box and bearings are clearly improved, even though readings after the REWITEC treatment were taken in the time of the year with more wind and higher load.

 This new report indicates the first, much promising results of the use of REWITEC Nanocoating in wind turbine gearboxes, analysed via the condition monitoring system. The condition monitoring system further validates the positive results measured by other means. The intention of the cooperation between μ-Sen and REWITEC is to increase the lifetime of the existing wind energy plants. The intent is to prove
that it is possible to effectively reduce the initial damage in wind energy gearboxes (grey stoking, micro pitting, micro cracks)and also to prevent or delay larger damage and failures.The targeted combination of REWITEC and CMS can provide the following advantages to the plant manager: optimised use of REWITEC via body sound analysis, increased efficiency due to reduction of friction, minimising of load due to reduction of rotor unbalances and adjustment failures, early failure detection.Conclusions The results from the imprints, electrical resistance measurements and the CMS analyses confirm that there is a clear improvement and smoothening of the worn out gear surfaces and load-carrying areas will be clearly enlarged. These all together
will lead to a prolonged gear box and bearing lifetime. The treatment of generator bearings, pitch bearings and open pitch gears with the REWITEC coating grease will prevent wear in all grease-lubricated wind turbine applications. Increasing wear can be stopped and already existing damage can be “frozen”. Optimising the metal surfaces of tribologic systems will increase the lifetime of wind turbine components. The cost of the REWITEC treatment is recovered within just six days! The increased uptime of a wind power generator will result in substantial savings for operators and greatly diminished maintenance intervals (Table 1).

ABOUT THE AUTHORS Dipl.-Ing. Stefan Bill,
Managing Director REWITEC GmbH Lahnau/Germany
Dipl.-Phys. Holger Fritsch
Managing Director μ-Sen GmbH Rudolstadt/Germany
  Mr Stefan Bill
Rewitec GmbH
Dr. Hans Wilhelmi Weg 1
35633, Lahnau, GERMANY
  T: +49 6441445990 Germany
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  Dermott Reilly, UK & Ireland
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