Finding the Ideal Wind Turbine Gearbox Oil Analysis Strategy

Developing an oil analysis strategy for any component involves a lot of variables. When we think about Wind Turbine Gearboxes, this also holds true. Before we begin thinking about the strategy for oil analysis, we need to understand the manner in which the fluid and equipment components degrade during service. This must then be matched to appropriate oil analysis tests and frequencies to predict fluid and component failures. Typically, oil analysis can be used as a predictive tool to monitor three key aspects; lubricant health, equipment health and contaminant ingression.

Which oil analysis tests can be used?

Oil analysis tests provide us with a closer look at the functioning of the equipment through the results. However, there are some oil analysis tests which provide multipurpose results while others can only provide single purpose results. If we look at the three key aspects; lubricant health, equipment health and contaminant ingression we can determine the following about multipurpose tests:

Multipurpose Test Aspects
FTIR Lubricant Health & Contaminant Ingression
ICP Spectroscopy Lubricant Health, Contaminant Ingression & Equipment Health
Particle Count Contaminant Ingression & Equipment Health
Analytical Ferrography Contaminant Ingression & Equipment Health

On the other hand, there are single purpose tests which address each of these aspects as noted below:

Aspect Single Purpose Tests
Lubricant Health Viscosity, Linear Sweep Voltammetry (RULER), Foam, Acid Number, Base Number
Contaminant Ingression   Water
Equipment Health WPC, Large Particle Spectroscopy

When we look at the functionalities of wind turbine gearboxes as categorized by the three key aspects, we can group the types of tests into two tiers. The first tier includes the basic tests, these are the tests which can trigger the Tier II tests but still give us an idea of what’s happening within the oil. Let’s take a closer look at these tiers when grouped according to the key aspects.

Monitoring Lubricant Health

Oxidation and Additive Depletion are the two major modes of degradation for in service wind turbine gear oils. These two modes of degradation directly affect the performance of the product, its protection of the equipment and can produce degradation products which cause deposits. We should therefore have some oil tests which can capture information about the onset of these degradation mechanisms.

Tier I Tests

Viscosity – this is one of the most important properties of any lubricant. If the viscosity is too high, this can lead to additional energy expended for the application but if the viscosity is too low, then it can cause wear on the components. In wind turbine gearboxes, it is usual to notice a downward trend in viscosity. This may be due to additive depletion however, a sudden drop in viscosity is typically an indication of the incorrect oil being placed into the system. In most instances, this may be hydraulic oil.

The viscosity index should also be trended. This can be achieved by measuring the viscosity at both 40°C and 100°C and performing the calculation. The viscosity index calculation can provide an indication of how temperature impacts the fluid’s resistance to flow. This viscosity index trend can identify the ingress or formation of degradation products as it is more sensitive when compared to individual viscosities.

Fourier Transform Infrared (FTIR) – this is one of the most powerful tools as it can examine the molecular fingerprint of the lubricant. It can trend the depletion of antiwear and extreme pressure additives or identify whether the oil has become contaminated with another fluid. This helps us to determine the occurrence of additive depletion or oxidation.

ICP Spectroscopy – this is used to measure metallic components in additive systems but is not as effective in trending additive depletion since the concentration may not change even though the function has been depleted. Most wind turbine gear oils are formulated with extreme pressure, antiwear, anti scuff additives and detergents which can be detected by ICP.

Tier II Tests

Linear Sweep Voltammetry (RULER) – this test measures the remaining useful life of antioxidants in the lubricant hence it is ideal for monitoring lubricants which can experience oxidation. Wind turbine gearboxes often experience a wide range of temperatures and have an oxidative environment which makes it the prime candidate for oxidation.

Foam – this can be caused as a result of a mechanical or chemical challenge in the gearbox. Although ASTM D892 is the regular foam test used for gear boxes, the Flenders Foam test subjects the fluid to more conditions representative of a wind turbine gearbox and is better suited for these gearboxes. It is more expensive and should be used when investigating foam issues and not on a regular basis.

Acid Number (AN) – Oxidation may not always produce acids however, some formulations may contain additive components which are acidic in nature. For these formulations, the trend in acid number helps to understand if the acidic additive components are being depleted. On the other hand, acids may form during oxidation which could balance the depleted acidic additive components causing the final number to show little or no change.

Base Number (BN) – there are some formulations which use basic additive components rather than acidic. These are typically in the form of detergents. They can neutralize carboxylic acids produced during oxidation. This test helps us understand the reserve alkalinity and the fluid’s ability to resist oxidation. It can indirectly correlate the amount of sludge being produced by looking at the calcium or magnesium losses.

Monitoring Contaminant Ingression

There are four main sources of contaminants to which wind turbine gearboxes are susceptible; moisture, dirt, hydraulic oil commingling or grease ingression. These can increase the rate of wear, accelerate fluid degradation and cause reliability challenges. Here are some of the tests which can be performed to aid us in identifying the presence of contaminants.

Tier I Tests

Water (Karl Fischer) – the ingression of water into any gearbox can prove catastrophic. In wind turbine gearboxes, this ingression of moisture is highly dependent on the location and climate of the wind farm, gearbox manufacturer and maintenance practices. Simple practices such as the use and replacement of desiccant breathers can affect the quantity of moisture ingression.

Particle Count – this measures both the cleanliness of the oil and the effectiveness of the filters. As per AWEA/AGMA 6006-A01 the maximum particle count should be 18/16/13.

ICP Spectroscopy – this can identify the presence of any metallic components which can be assessed as contaminants. These can be potassium, sodium, calcium, zinc, boron or barium caused by grease, hydraulic oil, dirt or water ingression.

Tier II Tests

Fourier Transform Infrared (FTIR) – this can diagnose the presence of foreign fluid ingression, sludge formation chemistry or identify particulate ingress sources.

Analytical Ferrography – this test helps to separate the contaminants from wear particles and can allow for clear identification of contaminants.


Equipment Health

The health of the equipment is of paramount importance. We can understand its health by identifying the presence of wear material.

Tier I Tests

Wear Particle Concentration (Direct Read Ferrography) – ICP can only capture particles less than 8 microns. However, if abnormal wear is present, it can generate particles in excess of 8 microns. Any sharp increase in metals above this size will be a cause for concern.

ICP Spectroscopy – Most of the large particles eventually get broken down into smaller particles which can be detected by ICP. Increasing levels of iron, copper, tin, aluminium and /or chromium can indicate that wear is occurring in the gearbox and it may need to be inspected. These increased rates of metals can also increase the rate of fluid degradation through oxidation and increase the wear rates.

Tier II Tests

Analytical Ferrography – this can characterize the type of wear and differentiate between cutting wear, rubbing wear, erosion and scuffing. This test should be performed once triggered by an abnormal result from the Wear Particle Concentration Test.

Large Particle Spectroscopy – these can be observed by using Scanning Electron Microscopy or Energy Dispersive Spectroscopy (SEM/EDS) which allows one to “look” at the particles to determine its metal-chemical composition. This can be triggered by abnormal Wear Particle Concentration to allow further insight into large non-ferrous wear metals.

Recommended Oil Analysis for Routine Testing

Tier I tests should be the first steps to monitoring Lubricant Health, Contaminant Ingression or Equipment Health. Only when triggered, should Tier II tests be performed to gain further insight into these key aspects. The below is a summary of the conditions for performing Tier II tests.

Tier I Tier II Conditions for Tier II Tests
Viscosity (40°C) Viscosity (100°C) Performed to better understand oil degradation and changes in viscometrics.
ICP Spectroscopy Linear Sweep Voltammetry (RULER) Performed on formulations with antioxidants.
Wear Particle Concentration Analytical Ferrography Triggered by elevated WPC results.
Particle Count Acid Number Performed on fluids with high initial acid numbers.
FTIR Base Number Performed on fluids with low initial base numbers.
Water Foam Performed if there is observable foam or evidence of foam during inspections.

Given that wind turbine gearboxes are in high risk areas, it is critical that an ideal oil analysis strategy should be developed. It is important to identify Tier I & II tests which assess lubricant health, equipment health and contaminant ingression. Tier I tests are designed to monitor the lubricant and allow Tier II tests to be triggered once abnormal findings are present. Oil analysis strategies should also consider the environmental and operational factors of these wind turbine gearboxes.