Solid Particle Cleanliness

In my previous articles, I have discussed how to achieve cleanliness within gearboxes and pumps, examining the entire aspect of solid particulate contamination ingress.

However, I was recently asked by an engineer following the above articles what a good target for cleanliness in these systems would be.

Cart Before the Horse

I probably should have written this article first, since the standard approach to improving reliability in terms of contamination control is as follows:

1. Set targets for the contamination levels within the system.
2. Undertake the steps to achieve the targets.
3. Measure to ensure the targets are met.

Many programs initially focus on the 3rd step, believing that simply performing oil analysis will prevent failure.  However, as I mentioned earlier in previous articles, this is akin to standing on the bathroom scales each morning and wondering why the hoped-for weight loss is not occurring.  The simple act of measuring is not going to achieve the desired reliability, and as with weight watching, the oil analysis reflects the lubrication lifestyle of the plant.

How Much Should I Weigh?

As with any proactive lifestyle change, we need a target weight.  Many years of medical research have shown that for a given height and gender, there are appropriate target weights.  Though the effort put in now will not be apparent until later in life, and more to the point, it is not a simple matter of the more weight we lose, the longer we will live.  Add variables to account for body shape and structure, etc., and it is not a simple calculation to set a target weight.

So What’s Involved in Setting a Target Cleanliness Level?

Probably the first aspect is the asset type, followed by the operating parameters, and finally, in my humble opinion, the desire to achieve the desired level of reliability.

Machine Type Influences

If we were to put the list of machine types in order from most to least tolerant of the solid particulate, then it would be:

Note that I have provided a generalized guide to the targets above. For those familiar with the ISO 4406:1999 Cleanliness Coding system for solid particles, hydraulics typically require a cleanliness level of around eight times cleaner than gearboxes.

There is a reason for this, and that is that when examining the clearances between components, complex hydraulic systems often feature valving with clearances of less than 5µm. In many cases, the high pressure and flow rate can lead to significant damage from solid particulates.

With gearboxes, however, gear teeth are generally hardened, and along with higher viscosity grades of lubricant, the solid particulate has a less significant impact on the wear rate.

Is a Cleanliness Target Necessary for a Gearbox?

Of course, it will still benefit from a reduction in wear rate and hence an increase in service life.  That said, though, the implementation, or Step 2, must not come at a price greater than the financial gains incurred by the cleanliness control.

As with any reliability initiative, there must be a financial incentive to justify any technical improvements.

But Surely the OEM Will Advise What This Should Be?

Indeed, the Original Equipment Manufacturer may well give a guideline value.  As mentioned, a reader contacted me, and he had approached several OEMs for clarification or had looked up the value in their documentation, which ranged from “not stated” to -/20/15 to 20/18/15 (ISO 4406:1999 solid particle reporting).

In my experience, OEM values are sufficient to avoid short-term and mid-term issues, but are inadequate concerning long-term reliability.  The cynical may suggest that the OEM wants the user to replace parts, regardless.  That said, there is a balance as an OEM does not want a reputation for poor reliability, either. 

More specifically, a value stated in the OEM documentation may not account for the worst-case scenarios in terms of environmental conditions and operating conditions, either geographically or in terms of the business’s nature.

What About the Nature of the Business Or the Environment?

Let’s consider the geography; some locations will be more prone to dust ingress, such as those in or near desert environments. Conversely, the issue is less prevalent in damp environments, such as at sea.  Concerning the industry, cement plants and mining/quarrying will again have higher levels of solid particulate than other industries.

Are There Other Factors to Consider When Adjusting the Target Cleanliness Level?

The cost justification has to be addressed, and therefore, other financial impacts include:

1. Capital cost
2. Repair costs
3. Downtime costs
4. Health and Safety risks
5. Potential energy loss costs

The greater these are, the cleaner the oil needs to be. 

Of course, to undertake a cost-benefit analysis, one must consider the additional costs of improved contamination control to achieve the target cleanliness levels, as well as the costs associated with monitoring these levels.  Once that is known, we can then verify it against the potential savings. Worked from the above expenses.

To establish the possible savings, however, it is essential to look at the existing history and any recorded “Mean Time Between Repairs/Rebuilds/Failures”.  Based on the research, we can then determine the potential life extension and calculate the resulting savings.

As with medical research, the Life Extension Tables serve as a useful guide to conservatively estimate potential gains.  In the image below, for every 1 Range Number improvement in the ISO 4406:1999, we have an incremental increase in life.  For gearboxes, this is not as great as with engines and hydraulics, but from around the 5 Range Numbers improvement (32 times cleaner), we start to see a significant gain.

Is It Worth It?

In my experience with gearboxes, few laboratories ever measure the solid particulate levels, with some suggesting that Ferrous Density is a better guide.  There are various reasons for this, not least the risk of choking the Automatic Particle Counter with large gear wear debris. 

Add in the fact that gear oil samples are often “wet,” leading to counting errors due to the thicker oil, and it is easy to see why most labs shy away.  However, Ferrous Density analysis, such as PQ (Particle Quantifier) or WPC (Wear Particle Concentration), does not offer a predictive view of potential failure, as Ferrous Debris is the end result of the wear process, not the cause.

Consequently, using particle-trending techniques such as a mesh obscuration instrument or a microscope to establish the level will reveal that gearboxes are often significantly dirty.  I have regularly seen ISO 26/23/21 on gear oils in some plants using mesh obscuration trending tools.  Yet, with little effort, this has been reduced to as low as ISO 14/11/9.

That would result in a 12-range number gain, putting the potential life extension factor at around 7 times.

Even being conservative, a more than 6 Range Number improvement is more than double the life currently experienced.

In summary, a Cost-Benefit Analysis is essential to setting a target.  Once the target is set, implement the improvements in contamination control and continually monitor to ensure the targets are being achieved.

• Determine the additional costs of filtration & Contamination Control
• Determine the Savings relating to the Life Extension
• Implement the 5-Year Cost-Benefit Analysis
• Seek Approvals

Targets must be optimized, feasible, and justifiable.

The question of lubricant miscibility usually arises in the context of a product change or the need for a refill. The very asking of it demonstrates operational awareness and knowledge. However, the key to success is the right question and a precise and reliable answer.

The question of lubricant miscibility most often arises when deciding to change the product type or needing a top-up. Just asking it shows awareness and operational knowledge. However, the key to success is a good question and a clear and reliable answer.

In such situations, we usually turn to the manufacturers or suppliers of lubricants or check the available technical sheets. In these documents, we often find that the lubricant in question is fully miscible and can be used interchangeably with competitors’ products meeting certain specifications.

Limited Confidence in the Technical Sheet

It is worth cautioning against taking the information on the technical data sheet alone for granted. For safety and peace of mind, it is always advisable to ask your distributor or manufacturer’s representative to confirm that there are no contraindications to mixing oils X and Y.

It is important to remember that lubricant manufacturers are constantly improving additive packages and oil bases to meet increasing demands. This process results from the drive to miniaturize machinery, increase its performance and durability, and simultaneously lead to more intensive loading.

This situation brings challenges and questions. Although the miscibility of oil bases is well understood, experience shows that additives used in lubricants can react with each other in different ways, which are not always beneficial for machines.

In this article, we focus not only on miscibility but also on compatibility, which should be a key issue. Two lubricants can easily mix, but their additive packages can prove incompatible, which can adversely affect the operation of machinery.

Description of the Study

A miscibility test of eight fresh oils used in wind turbine main gearboxes was carried out at the Ecol Oil Analysis Laboratory. The test was carried out in an ‘each-against-all’ arrangement, resulting in 28 50:50 mixed oil samples and eight fresh oil samples.

The study included the determination of key physicochemical parameters that may indicate compatibility or lack thereof in each sample, such as:

1. physical appearance,
2. elemental content,
3. kinematic viscosity and viscosity index,
4. propensity to foam,
5. MPC indicator,
6. Acid Number (AN),
7. IR spectrum.

Physical Appearance

Appearance is the first indicator of whether the mixing process has gone well. Turbidity, or the appearance of suspended solids, indicates problems, while high clarity and a color close to the original color of the blended oils are expected results.

Analysis of Elemental Content

Analyzing the elemental content of fresh oil allows the composition of the additives in the oil to be determined. The most common elements in gear oils are phosphorus, sulphur, zinc, calcium, magnesium, boron, molybdenum and silicon. They are components of additives such as anti-wear and anti-seize agents, corrosion inhibitors, antioxidants, and anti-foaming agents.

Kinematic Viscosity

Kinematic viscosity is a key physicochemical parameter of an oil that largely determines the strength of the lubricating film. Excessive changes in viscosity can lead to serious consequences for the lubricated gearbox, such as reduced performance or accelerated wear.

Propensity to Foam

Foaming propensity indicates whether the aerated oil will generate foam and how intense this will be. The test result is expressed in milliliters of foam obtained in each of the three sequences – the more foam, the worse. In the case of wind turbine gear oils, a minimum foaming tendency is expected (0 ml), as the specific lubrication regime in these systems favors aeration.

MPC Indicator

The MPC (Membrane Patch Colorimetry) indicator determines the risk of oil-generating deposits – both aging and those resulting from additive incompatibility. A high MPC indicates the potential risk of deposits in the system, mainly in the form of very fine, insoluble particles less than 1 µm in size. Operating an oil with a high MPC can lead to problems such as:

• faster filter wear
• reduced radiator efficiency,
• clogging of oil passages,
• increased temperature of friction nodes due to deposits.

The MPC is determined by the coloration of the filter membrane (0.45 µm) after filtering a suitably prepared oil sample.

Acid Number (AN)

The acid number is the number of milligrams of potassium hydroxide (KOH) required to neutralize an oil sample’s acidic components. The initial acid number is oil-specific and depends on the oil base and additives used. As the oil ages and degrades, the acid number increases due to the formation of acidic products. An excessive increase in the acid number indicates the need for an oil change.

IR Spectrum

An IR spectrum is produced by ‘shining infrared light’ on a sample. The chemical molecules absorb the light and vibrate in a way characteristic of their structure, resulting in a unique spectrum – a kind of ‘fingerprint’ of the lubricant. The IR spectrum enables:

• identification of the oil base and additives,
• detection of mixing with other oils,
• evaluation of oil degradation
• identification of chemical contaminants.

It is one of the basic diagnostic tools for analyzing lubricant composition and quality.

Fresh Oils

All the oils analyzed are synthetic oils with an ISO VG 320 viscosity grade, meeting the requirements for CLP-type gear oils. They also have approvals and authorizations from reputable gear manufacturers.

I wanted to make this more specific because, as I mentioned earlier – although ‘on paper’ all these oils are very similar to each other, the results I will present next will show us that, in practice, the technology contained in them can be completely different – hence all the confusion regarding the compatibility of the individual products.

The following tables show sequentially the additive element content (ppm) of the analyzed fresh oils before blending and the typical physicochemical properties.

The IR spectra of the analyzed fresh oils are presented below to complete the picture of differences in the chemistry of the analyzed products. If the previous arguments were insufficient to convince the reader that the differences between the oils can be significant, I think the image below will dispel any doubts.

The differences in the composition of the oils are most clearly visible in the so-called dactyloscopic region; it is in the wavenumber range 1500cm-1 – 750cm-1 that the characteristic ‘fingerprints’ of the product are discernible, making it possible to identify it.

Hydrocarbon bonds, as is to be expected, are present in every oil because, as mentioned, these are synthetic hydrocarbon-based oils. In the case of the D oil described earlier (dark blue line in the graph), there is a noticeable absence on the peak spectrum in the area of the presence of ester additives.

Tests on Mixed Samples

Once the initial values for each oil had been established, the analysis of the results obtained after mixing the fresh products proceeded. The blended samples were subjected to annealing for 5 days at 60°C, followed by measurements.

The MPC index and propensity to foam are the most important parameters that may indicate potential operational problems. The results are presented in graphs showing the cumulative foaming propensity (the sum of the amount of foam in the three sequences) and the MPC. This way of presenting the data ensures readability.

When analyzing the foaming propensity, it was noted that eight samples exhibit this type of property. The 20 ml and 10 ml values for mixtures F + B and E + G can be considered marginal. However, for the remaining samples, the apparent increase in foam generation suggests that the mixing process has significantly increased the foaming potential of the oil.

This is an unfavorable and potentially dangerous phenomenon in the context of the specific gear lubrication regime. Excessive oil foaming should be taken as a signal of possible irregularities in the system. As a result, the six mixtures with high foaming levels can be regarded with high probability as incompatible based on this single parameter alone.

As for the MPC index, the value considered to be of concern is 20 or higher. In the study, eight cases were observed where the index exceeded this limit. Importantly, not all cases correlate with excessive foaming. The most critical values occurred in mixtures:

– F + D,

– E + D,

– D + G,

– D + C,

– D + A,

– H + D.

It is noteworthy that D oil was present in all problematic samples. Already in its fresh state, this oil showed an increased tendency to generate deposits. When mixed with other products, chemical reactions further increased this risk. This situation indicates a critical incompatibility of the additives contained in these products, which can significantly reduce their effectiveness and accelerate degradation.

The study’s results clearly emphasize the need to be careful when mixing lubricants and to carefully check their compatibility before using them in a system.

By adopting criteria based on foaming propensity, MPC index, and the appearance of the sample after mixing, a miscibility matrix of the different oil pairs was created.

Conclusions

The analysis showed that compatibility – even for apparently similar lubricants – can be a major challenge. The research clearly indicates the potential dangers of mixing incompatible, fresh products. It is worth noting that mixing an in-service oil that has already partially degraded and is operating under more demanding conditions than laboratory conditions can lead to even more serious consequences.

Based on the results obtained and experience to date, we recommend preventive compatibility testing before mixing lubricants. In this way, the risk of negative chemical reactions and associated operational problems can be reduced. However, it is important to remember that laboratory tests cannot fully reflect system conditions such as contaminants or deposits. They can, however, eliminate the most incompatible products and prevent extreme reactions.

Even agents deemed compatible in laboratory tests may, when mixed in the actual system, initially cause, for example, a reduction in filter life due to the leaching of contaminants. It is, therefore, advisable to thoroughly clean and flush out the system before changing to a different or fresh oil, which should be confirmed by the analysis results.

Only then can filling the system with the target lubricant provide the greatest assurance of avoiding operational problems and extending the service life of the oil.

The research presented here is a prelude to further projects. Compatibility analyses are planned not only for fresh agents but also for those already in use, as well as attempts to determine safe mixing ratios that minimize negative effects. We encourage you to follow our publications to keep up to date with the results of further studies.

Summary

1. A conscious approach: Lubricant miscibility is a topic that requires both knowledge and a responsible approach. It is crucial to consider both the miscibility and compatibility of chemical additives.
2. Do not trust technical data sheets alone: Information contained in technical documentation should be treated with limited confidence and always verified with the manufacturer or supplier.
3. Laboratory tests as a basis: An analysis of the miscibility and compatibility of lubricants has shown that not all oils are fully compatible, which can lead to negative effects such as foaming, generation of deposits, or accelerated degradation of lubrication systems.
4. Risk of incompatibility: Despite their apparent compatibility, some lubricants may enter into chemical reactions that impair their properties. The study’s results emphasize particular caution with oils that have a high MPC and a tendency to foam.
5. Importance of system cleaning: Before changing the oil, it is recommended that the system is thoroughly cleaned and flushed to prevent negative reactions and extend the life of the new lubricant.
6. Continuing research: Further research is planned on the compatibility of in-service agents and the determination of safe mixing ratios. We encourage you to keep an eye on future publications to keep up to date with the results and recommendations.
7. Taking care of lubricant compatibility is an investment in the performance and safety of machinery, which translates into longer and trouble-free operation.

Additional authors: Wojciech Majka CEO Ecol, MLE CLS, Jakub Chłodek Oil Analysis Department Director, MLA II, and  Wojciech Jewuła Oil Diagnostic Specialist, MLAII

Why Water in Oil Is So Dangerous

From an early age, we learn that oil and water do not mix. While that is not exactly true in the world of lubrication, it is true to say that any degree of water in a lubricant can cause irreparable harm to both the lubricated components and the oil. 

When water enters an oil sump or reservoir, it exists in one of three distinct phases: free water, emulsified water, or dissolved water.  Each poses different challenges for lubricating oils, but all can significantly reduce the life of the equipment, resulting in excessive maintenance costs and unscheduled downtime.

How Water Exists in Oil: Dissolved, Emulsified, and Free

As the name implies, dissolved water means that the water and oil mix to form a solution, just like dissolving salt in warm water.  While this may seem counterintuitive, since water is polar while oil is largely non-polar, the presence of polar components, such as oil additives, degradation by-products, or certain contaminants, can cause oil and water to mix. 

The amount of water that can be dissolved in a new oil is dependent on the type of base oil, the amount and type of additives contained in the oil, and the temperature. 

For mineral oils or hydrocarbon-based synthetic oils, the base oil’s affinity for water is very low, meaning little water will dissolve in the oil. However, due to the polar nature of some base oils, particularly API Group V base oils, significant quantities of water can be dissolved in the oil.

Likewise, additives affect the solubility of water in oil.  While lightly additized oils, such as turbine oils, contain less than 5% additives and therefore cannot help water become soluble in the oil, more heavily additized oils, such as gear oils, hydraulic fluids, and engine oil, can hold much higher concentrations of water in the dissolved phase.  

In fact, a heavy-duty engine oil that contains as much as 30% by volume of additives can hold more than 2000 ppm (0.2% v/v) of water in oil.

Additives and temperature can make oil hold far more water than you expect.

Similarly, temperature affects the amount of water that can be dissolved in oil.  At room temperature, a conventional R&O oil may hold up to 120-150 ppm (0.012-0/015% v/v) of water in solution.  However, as the oil cools, the solubility of water in oil decreases to the point where at 40°F, the oil may only hold 20-40 ppm of water in solution (Figure 1).

The maximum amount of water that an oil can hold in solution at a given temperature is referred to as the saturation point.  Much like the dew point of air, which is the temperature at which the air becomes saturated with moisture and starts to precipitate water in the form of fog or dew, the saturation point of the oil is the point where no more water can be dissolved in the oil, forcing any additional moisture to come out of solution.

When water comes out of solution, it will coexist with the oil in one of two phases: free or emulsified.  Free water refers to water that has completely separated from the oil and settled to the bottom of the tank or oil sump.  By contrast, emulsified oil refers to a suspension of small water droplets (the dispersed phase) in the oil (the continuous phase). 

Whether mixed oil and water exist in the free or emulsified phase depends largely on a property of the oil known as demulsibility.  The demulsibility of a lubricant can be defined as the ease with which water and oil separate.  If an oil has a high degree of demulsibility, it will very rapidly shed water into the free phase (Figure 2).  By contrast, poor or low demulsibility means that the water will remain mixed with the oil, resulting in a cloudy or hazy appearance (Figure 3).

Like solubility, demulsibility is dependent on the type of oil, the degree of oil degradation, and the presence of specific contaminants.  Light additive oils will separate (demulsify) quickly, typically within 5-10 minutes.  However, more heavily additized and severely degraded oils will take much longer to separate. 

Some oils, particularly those that contain detergents, are severely degraded or have become contaminated with certain contaminants, such as soaps or other process fluids, may lose the ability to shed water completely, forming a stable emulsion (Figure 3).

When it comes to a lubricant’s ability to perform its job, the presence of free or emulsified water is of greatest concern. This is particularly true for wet-sump applications, such as small process pumps or splash-lubricated gearboxes, since the lubricated components operate directly in the oil sump. 

Free and emulsified water in wet-sump applications not only causes rust and corrosion to occur but can also significantly affect film strength, leading to an increase in the rate of oil degradation and the formation of sludge and varnish.

Moisture leads to rust, weak film strength, and rapid oil degradation.

Figure 4 shows the impact that water can have on equipment life.  In this seminal study, researchers at Timken Bearing were able to derive an empirical relationship between the amount of water in an oil (specifically, an R&O ISO VG 68 fluid) and the life expectancy of a rolling-element bearing. 

This would mirror exactly what might happen in a small centrifugal pump.  As the graph illustrates, bearings that operate with water above the saturation point (approximately 100-150 ppm in this case) will have a significantly reduced life expectancy, often as low as 50% of the bearing’s anticipated life.

Preventing Moisture Ingress: Techniques That Work

In process industries such as pulp and paper, petrochemical refining, food and beverage, and wastewater treatment, water is a constant presence, as are pumps and gearboxes.  Because of this, progressive companies seek to control water using improved seals, pre-filtration of new oils, desiccant breathers, and specialized filtration such as super-absorbent polymers and vacuum dehydrators.  Each of these can be effective, but no one method alone can solve all the potential problems.

Water can enter an oil in several ways, but one of the most common is when equipment operates in a high-humidity environment, either inside or outside.  Even with the highest degree of care and attention, humid air can still enter equipment if it is not properly protected.

When water and oil coexist, the degree of saturation of the oil always matches the relative humidity of the air above the oil, and vice versa.  So, if the relative humidity of the air is 80%, the oil in contact with the air will be 80% saturated, assuming the oil and air are at the same temperature.  This is due to Henry’s Law, first discovered in 1803, which states:

“At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.”

 While this concept may seem somewhat abstract based on its definition, Henry’s Law explains several everyday observations, such as why the US Gulf Coast is so humid or why soda goes flat relatively quickly after the can is opened. 

In the case of soda, when the can is sealed at the bottling plant, a small gap, known as the headspace, is left at the top of the can.  Since the soda is saturated with carbon dioxide, the headspace inside the can is also saturated with carbon dioxide due to Henry’s Law. 

However, once the can is opened, the headspace becomes the entire atmosphere.  Recognizing this imbalance between CO₂ content in the air versus the soda, the soda will “fizz” until the concentration of CO₂ in the soda matches the (low) concentration of CO₂ in the air.

Why Headspace Humidity Control Is Essential

In oil, the same holds.  If oil is in contact with water vapor due to high humidity, it will become saturated very quickly.  However, if the headspace humidity is lowered, for example through the use of a desiccant breather, the oil will yield water due to Henry’s Law, effectively dehydrating the oil (Figure 5).

In industrial equipment, a similar effect can be observed.  Figure 6 shows the impact of headspace humidity on two identical centrifugal pumps operating outside in relatively high humidity (74%) and high temperature (93°F).  Both pumps were filled with pre-filtered oil to remove as much moisture as possible and were equipped with bearing isolators to prevent the migration of contaminants through the shaft-seal interface. 

In addition, both pumps were completely sealed from the outside environment by plugging the breather/fill port; one was sealed with a simple pipe plug, and the second was sealed with a “smart” desiccant breather equipped with a humidity sensor. 

As shown in Figure 6, the pump without headspace humidity protection exhibited relatively high internal humidity, which fluctuated between 55% and 65% due to daily temperature fluctuations.  Despite being nominally sealed, humid air exchange is occurring between the outside and inside, likely through the bearing isolator. 

By contrast, the pump with headspace protection maintained a very low internal humidity due to the silica gel actively dehumidifying the headspace.  Based on the graph shown in Figure 4, it can be anticipated that the bearing life of the pump with dry headspace would be 20-40% longer than that of the pump with no protection, indicating that both bearing isolators and desiccant breathers are necessary for achieving maximum bearing life.

Headspace humidity control can increase bearing life by up to 40%.

Similar effects can be seen in most, if not all, wet-sump applications.  Table 1 presents a comparison of water contamination levels in two different sets of splash-lubricated gearboxes: the first with active headspace humidity protection, and the second with no active headspace protection. 

Both sets of gearboxes were operating in high-humidity environments outside.  As can be seen from the data, protecting gearboxes with active headspace protection reduces the amount of water in the oil by up to 80%.  This is an important consideration since many oils are readily hydroscopic and will absorb moisture into the free or dissolved phase if left unchecked.

Mush is made of solid particle contamination, and rightly so – particles are the leading cause of lubrication-related failure.  However, do not overlook the importance of controlling moisture.  For wet-sump applications, such as pumps and gearboxes, moisture is just as likely to induce failure. 

Whether it’s corrosion, loss of film strength, or cavitation, water can and does have a significant impact on rotating equipment. Therefore, ensure that you take every step possible to protect your oil and equipment from the impact of free and emulsified water.

At any given point, the overall contamination in a gearbox is BIG!

Remember that contamination ingression is the overall amount of contaminant in the oil with:

B – Built-in contamination from the unit’s manufacture.

I – Ingested is the “sucked in” contamination during operation, whether as the unit cools or other activity such as top-ups and oil changes.

G – Generated during operation, or in other words, wear and fatigue creating debris, whether from the lubricant, ingested contamination, or other root causes.

Simply put:

Ingression = Built-in Contaminants + Ingested Contaminants + Generated Debris

 Generated debris (or wear) is the result of several root causes, namely;

1. Tightness – fasteners torqued correctly to eliminate vibration from movement
2. Rotating set-up – alignment and balance set-up to avoid overload on the bearings and teeth
3. Lubricant quality means the right specification and the right oil used during lubrication tasks
4. Lubricant contamination levels
5. Other root causes

Based on the Pareto Principle or the 80:20 Rule, the other root causes are the 80% causing 20% of the problems, while the first four listed are the 20% of the root causes creating 80% of the problems.

Because tightness, rotating set-up, and lubricant quality are in control, contamination levels or ingested contaminant levels are left. 

Sadly, contamination control is overlooked on gearboxes, so the ingested levels are usually high, resulting in day-to-day damage. 

Identifying the Sources of Contamination – Built-in and Ingested

Having made sure that the right lubricant is in use on the gearbox to meet the demand of the application and that any overloading or operational abuse has been dealt with, the final stage in a proactive gearbox focus should be to deal with the ingression points of the contamination. 

But, first, it is essential to identify contamination as it can be in one of many forms.  

Contamination is any matter or body that enters the system and causes physical or chemical harm to either the lubricant or the machine, which reduces the effectiveness and life of the lubricant or the asset and potentially impacts the effectiveness of the operation.

Such examples include:

• hard particles from external sources or generated within (wear),
• water, or moisture, from rain or wash-down sprays,
• high temperature from overloading or poor heat transfer,
• aeration from incorrect levels of oil or poor specification in terms of the defoamant,
• UV radiation
• or process matter (chemical or physical) from the environment.

There are several easily identifiable ingression points on a gearbox, namely:

• the seals,
• damaged gaskets,
• the breathers,
• contaminated new oil
• other maintenance activity, such as top-ups from contaminated handling units, or complete drain and refill of the unit not following best practices.

These all allow the introduction of contaminants. Bear in mind that the rate at which these enter the unit will depend to some extent on ambient conditions and, depending on the contaminant type, will be subject to such issues as whether the equipment is located inside or outside, whether it is raining, or whether it is windy. 

For example, in wet conditions, the likelihood of moisture ingress significantly increases; however, the ingress of hard dust particulates from the environment is reduced accordingly. 

On the other hand, given a hot, dry, and windy day, while the risk of moisture ingress is minimized, the risk of ingesting hard atmospheric silica-based particulates is greatly increased.  Of course, depending on the nature of the organization, some contaminants may be unique, such as coal dust or iron ore dust. 

Process chemicals in a nylon spinning or paper mill environment.  Another aspect of increased risks of contamination is nearby activity, for example, the risk of cement dust when construction work is taking place, and again, the risk will be greater in windy or dry conditions.

Of course, the above is a controllable situation on-site and can be dealt with accordingly.  What must be dealt with is the manufacturing debris in the gearbox during commissioning. 

This requires that either the OEM or the installer take the appropriate actions to ensure a clean unit before start-up; otherwise, this debris will immediately impact the system’s reliability.

Dealing With the Contamination

 Rectifying the situation involves upgrading existing fittings and implementing some new components to bring the necessary improvements. 

 However, as a gearbox manufacturer customer, you have two choices.  How you move forward will depend on your Original Equipment Manufacturer’s (OEM) cooperation in these matters.

Unfortunately, owing to certain design criteria, as an end-user, it is difficult to walk away from an uncooperative OEM since they may be the sole source of gearboxes to meet the design constraints of the machine train. 

 It is necessary to discuss your contamination control program with your OEM and have them implement your requirements at the build phase. It is also necessary to ensure compliance through a regular supplier audit. Remember, it is always much cheaper in the long term to pay more upfront at the supply stage than to retrofit the better-quality hardware later.

 A further aspect is to ensure the supplier follows best practices for storing the new or rebuilt units before shipping to the site.

 Whether you choose to implement yourself or set up an ongoing improvement program, you will need to consider the following:

 Seals

 Standard lip seals are a low-cost item but require frequent replacement, and their performance at sealing against oil leakage and contaminant ingress is poor compared to labyrinth seals. 

 Although labyrinth seals are at a greater cost initially, their superior performance will ensure minimal risk from water or dirt ingress and minimize lubricant loss and potential process/environmental problems. 

 In addition, many of the gearboxes I see fitted with labyrinth seals also have a greasing point, as fresh grease should regularly be applied to aid the sealing action yet are often overlooked. 

 Of course, training the maintenance staff to avoid using high-pressure wash-down sprays directly on the seals is a must, although, in food and beverage-related environments, this cannot always be avoided.  In this instance, a seal guard can prove beneficial.

 Breathers

 In many cases, older units still have an open tube for breathing, although newer units now incorporate a vent plug into the fill plug.  When stopping large bodies from falling into the gearbox, these serve their purpose but are ineffective at stopping a destructive smaller than 20µm particle. 

 Essentially, upgrading the breather should minimize or eradicate the ingestion of hard particulates and moisture.  There are several ways to achieve this.  The first would be to fit a good quality breather, such as a 1µm rated unit that will remove as much airborne particulate as possible. 

A standard ß10>200 spin-on filter will perform up to ten times more effectively in air than oil, making these possible in a dusty but dry location. 

 If in a moist environment, then the use of desiccating breathers is advisable.  Please beware of the desiccant in some industries; ensure that the appropriate handling guidelines are adhered to and that they comply with food and other related industries. 

 Also, remember to set them up to minimize damage, so provide a protective housing such as in the photo example of a desiccant breather used on a crusher gearbox where rocks may fall over the side.  Consider mounting the breather away from the unit in a wet environment to avoid wash-down sprays or process water falling on it directly.

However, on splash-lubricated gearboxes, there is little actual need for breathers. Generally, they are there to allow for changes in volume because of top-ups, leakages, and temperature-related volume changes. 

For applications with minimal volume changes, the ideal form of breather is a bladder type.  This effectively seals the internal of the gearbox from the atmosphere, but a small bladder allows for expansion and contraction of the air within because of temperature changes. 

These are especially ideal where high levels of particulate or moisture occur in the environment. In addition, creating a vacuum as levels fall may also reduce leakages.

Breathers/Fillers/Samplers

 It is helpful to combine the functions where regular sampling or using a filter cart occurs, particularly where cost and space constraints dictate. 

 When filling or topping up, it is imperative to ensure that the oil is delivered clean to the gearbox.  This may mean dispensing through a filter cart or using the sealable oil canisters that have become the norm in the industry.

Apart from the supply of clean, new oil, the fill port must be clean before use, so any type of protection that can be added to the fill area is beneficial, mainly where process debris could fall into the fill port when in use. 

Using quick connectors is ideal as, like sampling ports, they minimize the risk of ingested contamination. My preference is to use the sealable type containers with a hand or battery-operated pump piped direct to a quick or snap-on connector on the gearbox to ensure clean delivery of the oil and to minimize any risk of spillage, while also minimizing any longer-term injury resulting from lifting and pouring from heavy containers.

Portable Off-line Filtration

 While some gear units may incorporate a small pump and perhaps even a filter, many gearboxes are not equipped with a pump for circulating the oil, nor are they filtered. This can be easily overcome, and a filter cart can make significant gains quickly. 

 Sometimes, it is impossible to make the necessary upgrades or modifications without a major shut-down, so as mentioned, always specify these upgrades as part of the new order or during the gearbox rebuild. However, filter carts can be applied by quickly changing the fill and drain plugs.

Portable Off-line Filtration

 While some gear units may incorporate a small pump and perhaps even a filter, many gearboxes are not equipped with a pump for circulating the oil, nor are they filtered. This can be easily overcome, and a filter cart can make significant gains quickly. 

 Sometimes, it is impossible to make the necessary upgrades or modifications without a major shut-down, so as mentioned, always specify these upgrades as part of the new order or during the gearbox rebuild. However, filter carts can be applied by quickly changing the fill and drain plugs.

Permanent Off-line Filtration

 A permanent off-line circuit could be employed on larger units, particularly where large volumes of oil and high cleanliness levels must be maintained. The benefit of the permanent mount is that it can continue to operate while the gearbox is not in use. However, the optimum filtering time is during the higher operating temperatures. 

 If high running temperatures are an issue, a cooling circuit could be designed with the offline circuit to reduce the oil temperature and increase the oil life and performance.

 In addition, with a permanent offline filtration loop on a gearbox, the circuit can be started separately before starting the gearbox, particularly for vertical shaft gearboxes; this can be beneficial in ensuring a “wet start” with oil flowing to the components before start-up.

 The choice between a portable unit and a permanent mount unit will come down to criticality of production (i.e., the need for reliability), safety, severity/penalty of failure, and if none of the above are important, then the need to achieve a reasonable life extension within a limited budget on contamination improvement.

Absolute cleanliness levels should not be quoted as individual units within the same site may have differing needs, imposing higher or slacker cleanliness limits.  However, in the majority of cases, there are areas for significant improvement from the typical ISO 4406 25/23/20 often seen. 

It is also safe to say that the stringent cleanliness levels required by complex hydraulics are not necessary with gearboxes unless circumstances are exceptional. However, aiming at a cleanliness target of ISO 4406 18/16/13 or better is not unreasonable.  With improvements to that level, life extensions over 3 times are realistic.

Built-in Filtration

As dealt with earlier, the cleanliness of units at the commissioning stage is crucial to ensuring successful infant reliability and increased unit life.  It is common to find manufacturing debris (casting sand, machining swarf, etc) present in a new gearbox. 

At least one OEM told me this is normal and that it will be an extra cost if the client wishes to remove it! This is unacceptable, and as a client, vote with your order book.  Unfortunately, reality sometimes means staying with the supplier, but at least ensure that the best quality breather and seals are fitted as standard when specifying new units. 

Storage

In terms of the gearboxes in storage, ensure that any openings in the castings, etc, are plugged, the shafts and gears are covered with a protective film of grease or oil, and this is thoroughly removed before use. 

Use the portable filter cart to flush the gearbox through before it is turned during installation.  The best way is to use a low-viscosity oil of the specified gear oil that can splash through the box, ensuring that all the dead zones are cleaned and that any debris is dislodged and trapped by the filter cart. 

If you request the OEM to do this before delivery, ensure they flush according to the appropriate standards and show certification or proof of achieving your required levels. 

While all these additional specifications add to the initial purchase cost, the savings incurred in the increased reliability and life of the unit far outweigh the penalty.

Other Opportunities?

 Where several identical gearboxes are in close proximity, a possible upgrade that assists with contamination control is to put in a circulating “dry sump” setup as in the diagram below. Having the tank as central to the lubrication minimizes effort because of:

• The oil is maintained in the ideal state, so it will last much longer and reduce the need for individual oil changes on each gearbox.
• The oil is always being filtered and cooled again, adding to the oil life and reducing the gearbox wear.
• The circuit can be started before the start-up of the gearboxes so “dry starts” are avoided.
• A single oil analysis sample is all that is required on the common return to monitor for oil health, contamination, and wear debris, albeit if wear debris levels change, trouble-shooting samples will need to be taken on each gearbox drain.

Wear monitoring is one of the primary objectives of oil analysis for predictive maintenance. Many oil analysis tests are considered suitable for wear debris analysis, and some, like elemental analysis and Wear Particle Index, are recommended within routine analytical slates.

Others, like Analytical Ferrography, are only requested occasionally and are typically reserved for in-depth investigations whenever required.

Elemental Analysis

Elemental analysis is a part of almost all routine oil analysis test slates across many applications. It is a cornerstone of lubricant analysis and a powerful ally for wear detection at early stages.

Inductively Coupled Plasma (ICP) or Rotating Disc Electrode (RDE) are the most common analysis methods for conducting elemental analysis for used lubricants, and they are performed respectively per ASTM D5185 and ASTM D6595.

Elemental analysis is a standard method used to determine quickly and in one single measurement the concentration of various elements within an oil sample.

It identifies and quantifies at low parts per million (ppm) concentrations for at least 21 elements: Iron (Fe), Copper (Cu), Aluminum (Al), Chromium (Cr), Nickel (Ni), Lead (Pb), Silver (Ag), Tin (Sn), Silicon (Si), Sodium (Na), Potassium (K), Magnesium (Mg), Calcium (Ca), Phosphorus (P), Zinc (Zn), Boron (B), Molybdenum (Mo), Titanium (Ti), Vanadium (V), Barium (Ba), Sulfur (S).

Oil analysis professionals can monitor the levels of wear metals, additives, and some contaminants through elemental analysis. This helps assess the condition of oil and equipment and identify potential issues, such as contamination, additives depletion, or excessive wear.

However, it is essential to acknowledge the inherent limitations of elemental analysis. The ICP technique can detect elements accurately (with high repeatability) if the elements are below five microns in size, with repeatability tailing off for particles approaching and exceeding seven microns.

The RDE technique also provides confidence for detection up to a five µm size range, with detectability tailing off as particles exceed 10 microns. For RDE and ICP techniques, particle size visibility can vary by element type, such as copper and chromium.

While the accuracy of the analysis can be influenced by factors such as sample handling and instrument calibration, these techniques (ICP and RDE) are considered the preferred means for routine wear debris assessment for commercial labs.

Considering these constraints, most laboratories complement elemental analysis with additional test methods that detect particles beyond the incipient wear range (>5 µm size).

In the context of gearbox oil analysis, techniques like Particle Quantification and Ferrographic Analysis are complementary tools that provide a more holistic approach to monitoring gearbox condition.

Ferrous Density Analysis

The most prevalent and affordable method for assessing ferrous density is Particle Quantification Index (PQI), also known as Wear Particle Index (WPI).

This test directly quantifies the magnetic mass of ferrous debris in the oil sample, regardless of particle size and shape. It provides a non-dimensional value representing the total amount of the existing ferrous debris in the sample.

PQI is a powerful tool for wear detection, monitoring ferrous particles larger than the elemental analysis detection range.

In this regard, PQI is an invaluable complementary test to elemental analysis.

Performed per ASTM D8184, Particle Quantifier Index is recommended as a routine test for all types of industrial, mining, shipping, and aviation equipment.

Direct Read Ferrography (DRF) is another cost-effective technique for characterizing the wear concentrations of larger-size particles.

By leveraging the magnetic susceptibility of particles, DRF enables the calculation of the Wear Severity Index based on a direct reading of both large and small particles. Consequently, DRF can serve as a suitable alternative to PQI.

Analytical Ferrography

It is helpful to understand the underlying reason (the wear mode) whenever elemental analysis, PQ Index, or DR Ferrography indicates the presence of an elevated wear metal concentration. In these circumstances, it is helpful to ‘see’ what is occurring inside the machine.

Analytical Ferrography is a powerful and proven qualitative technique that utilizes a microscope to examine wear particle appearance, including the color, size, and shape of particles in the oil sample, to determine the different wear modes and the possible sources of wear debris.

The results are provided per ASTM D7684 and are complemented by photos, allowing the comments to be visualized. Given the time and expertise required to perform the diagnosis, its cost is relatively high.

Accordingly, it is conducted as a secondary test when other lower-cost methods have strongly indicated a problem.

With routine testing, the oil sample is drawn from the primary sampling location and represents the whole system. Secondary sampling locations close to the components should be utilized if a machine has multiple branches or subsystems.

These samples will reveal the highest concentration of wear metals and the likely source of the wear and failing components.

Given the above information, a well-designed analysis slate includes elemental analysis and Wear Particle Index as essential elements. When further investigation is needed, conduct Analytical Ferrography.

Please note that using permanently located oil sampling ports will yield the most representative results.

For a deeper investigation, oil analysts may opt for Filter Debris Analysis. This analytical technique involves examining debris and particulate matter captured within oil filters, which are explicitly designed to trap solid particles, wear debris, and sometimes even water contaminants.

Analysts can find critical clues about the types of wear occurring in the equipment by carefully scrutinizing the composition, size, shape, and quantity of particles in the oil filter.

By understanding the origins of these particles, maintenance teams can take proactive measures to rectify issues, prevent further damage, and extend the lifespan of machinery.

Case Study: Gearbox Condition Monitoring through Iron Content, Wear Particle Index, and Analytical Ferrography

The table below provides the oil analysis results of two samples drawn from a gearbox in the port industry. This case study focuses on the wear metals concentration data acquired through ICP and Particle Quantification Index.

Additionally, the Analytical Ferrography analysis was conducted only for the second sample, promptly following the release of the report of the first sample. All the other parameters are satisfactory, including Viscosity at 40°C, Total Acid Number, and Water Content.

A notably elevated PQ Index compared to the iron concentration detected by ICP has led us to deduce that there has been an accumulation of large ferrous debris within the oil sample, reaching notably high levels. This finding prompted consideration of abnormal wear phenomena inside the gearbox.

This could be attributed to various factors like the ingress of abrasive contaminants, suboptimal lubrication, misalignment issues, or excessive mechanical loading. It should be investigated further.

In this situation, maintenance personnel were asked to draw an urgent representative sample to confirm these abnormal results because this was the inaugural sampling for this gearbox.

One oil analysis report should never be used as the basis for a decision involving replacement or immobilizing equipment.

In addition to the routine analysis slate, Analytical Ferrography was requested. The lab was notified of the emergency and was asked for a quick turnaround.

Simultaneously, maintenance personnel checked the operating conditions of the gearbox, verifying the OEM-recommended parameters, the load, the speed, and the operating temperature.

They were also asked to assess alignment and vibration. Considering other non-destructive inspection techniques to confirm findings is recommended in these situations before embarking on any potentially expensive decision or action.

These combined actions are essential to promptly address and rectify any potential issues within the gearbox and provide a supporting finding. They will serve as crucial inputs for Root Cause Analysis once the lab releases the oil analysis report.

When the second sample report was received, the PQ Index result was confirmed high, and the iron content was relatively the same as the previous sample.

The Analytical Ferrography examination revealed the presence of several types of particles, including those attributed to Normal Rubbing Wear, Fatigue Chunks, Laminar Particles, Corrosive wear debris, Dark Metallo-Oxides Particles, Non-Metallic Crystals, Friction Polymers, and Fibers. Below is a compilation of potential sources of these particles:

1. Rubbing wear particles indicative of normal rubbing wear.
2. Friction polymers can be attributed to excessive mechanical load or stress on the lubricant.
3. Fatigue chunks, suggestive of abnormal periodic machine vibration.
4. Laminar particles are suggestive of a rolling contact failure.
5. Dark Metalo-Oxides, potentially resulting from high operating temperature and/or suboptimal lubrication conditions.
6. Fibers that may have entered the gearbox during maintenance or been introduced to the sample during its extraction.

These insights into the origin of the detected particles are invaluable for understanding the underlying factors contributing to the observed wear patterns. This case study is a concrete example of how combining wear debris analysis techniques is essential for gearbox condition monitoring.

By implementing this proactive approach, equipment managers can maintain high levels of equipment uptime and prevent costly breakdowns.

Wind energy has become an increasingly important component of our energy grid as the world moves toward more sustainable power generation sources. Although harnessing wind power to do helpful work extends back to ancient times (think sail ships and windmills), stable electricity generation is a reasonably modern development.

With this development has come a growing appreciation for the role of lubricants in wind turbine reliability and an accompanying evolution in wind turbine gear oil formulations as the operating requirements are better understood.

Early Wind Turbines and Their Lubricants

The modern wind industry begins in the late seventies and early eighties. At this stage, the units were relatively small, with a capacity of no more than 100-200 kilowatts. As with many new technologies, there was a great deal of design experimentation, and many turbine variations co-existed.

Typical blade configurations sported anything from two to six blades, with some vertical-axis turbines thrown in the mix. Eventually, the “Danish” design (three-blades, horizontal axis) would cement its place as the industry standard configuration.

In these early years, wind turbine gear units were generally industrial gearboxes, repurposed. The approach was relatively crude and simple: rather than pushing electricity into an electric motor and extracting power through a gear reducer, engineers put the process in reverse, spinning the gearbox through power derived from the wind to drive a generator. But as Jim Carey (former ExxonMobil wind turbine gear oil formulator) explained in Episode 24 of Lubrication Experts:

“they would put [industrial gearboxes] up the tower and run for 6, 8, 10, 12 months. And they would all fail; broken teeth, bearings that would wipe out, you would have shafts that would snap. Seven to 10 years into the early nineties or so, or through the eighties into the early nineties, people realized, oh, wait a minute, if we want reliable power generation, you must increase the quality [of the gearing]. Not to the degree of, say, aircraft machining quality and whatnot, but we have to get a little bit better at devising machines that have tighter tolerances and are capable of more sustained power throughput from wind events to generate electricity.”

Early wind turbine gear oil formulations were also reasonably crude; formulations resembled “cut back” engine oils, containing the same metal-containing antiwear, antioxidant, and detergent additives seen in the automotive formulating tradition.

The Emergence of Micropitting as a Failure Mode

The subsequent generations of turbines heralded an era of tighter tolerances and more sustained, higher power throughput. This evolution led to the rise of micropitting failures in wind turbine gear sets.

Micropitting is a surface fatigue failure arising with rolling or sliding contacts. It’s frequently seen in gears subjected to intense loads, especially if the loading is variable.

Under these conditions, the roughness of the tooth surfaces and the oil film thickness are on the same scale, so the surface asperities come into contact with one another. The high loads placed through wind turbine gear sets and the variation of load induced by changing wind patterns make the wind turbine gearbox an ideal environment for micropitting.

When the asperities make direct contact, there is a sudden increase in the contact pressure; this intense pressure leads to a plastic deformation on the material’s surface, and this plastically distorted surface is prone to crack under the strain of repeated cyclic loading and unloading. This culminates in the formation of micro-sized pits, hence the term “micropitting.”

Two Ways to Skin the Formulation Cat

To solve the issue of micropitting in wind turbines, two entirely different schools of thought emerged. The first approach leaned heavily into the automotive tradition, continuing to use heavily metal-laden formulations with a high reliance on antiwear tribofilms to reduce energy concentration in the gear tooth asperities.

The second approach was to almost entirely remove metal-containing additives from the formulation and rely instead on synthetic base oils (now mostly metallocene polyalphaolefins) to support the contact pressure and provide sufficient film strength to reduce the cyclic loading on asperities.

These differences may seem trivial at a surface level, but they led to dramatic outcomes – specifically radical lubricant incompatibility issues between lubricants from the two formulation philosophies.

The past decades have seen numerous gear oil foaming issues and additive precipitation when switching from one oil style to another. This could only be eliminated through the most intensive gearbox flushing procedures when a product change was required.

Fork in the Road

Today, most wind turbine gear oil formulations have adopted the metal-free style. This reflects a broader trend in industrial lubricant formulation, driven by the longer lubricant lifecycles.

Whereas a typical engine oil may last 15,000 miles (equivalent to approximately 2,000 hours), an industrial oil could see a drain interval closer to 8,000 hours. Additionally, the operation means a heightened risk of water contamination, an issue compounded by metal-containing formulations due to their hygroscopic nature.

The philosophy behind metal-free industrial lubricants is for the lubricant to withstand environmental contamination but still efficiently eject any infiltrating impurities.

The lubricant absorbs water from its environment with metallic additives, introducing unnecessary moisture to critical machine components such as bearings, gears, and shafts. Metal-free formulations allow contaminants to be dropped out on the first circulation, leaving nothing but clean lube oil in the machine.

This philosophy is concluded in wind turbines, where the latest gear lubricants are often sold with a ten-year warranty. For a lubricant to survive almost 100,000 hours of operation in water-rich offshore and coastal environments, the oil must readily separate from contaminants encountered in service, and the additive package must be robust enough not to deplete.

Future Wind Turbine Gear Oils

Today the limiting factor on gear oil life appears to be the depletion of the additive package. To date, most lubricant manufacturers have avoided commercializing additive packages that extend the life of the turbine oil.

But with the rising costs and logistical challenges of changing wind turbine gear oils in remote locations becoming a drag on wind farm profitability, it may be only a matter of time before these solutions come to market.

Unless OEMs are willing to adopt a radically different formulating strategy (such as the use of PAG base oils), it is unlikely that wind turbine gear oils will change much in the coming decades. Instead, the focus will likely shift to remote condition monitoring of the lubricants to ensure maximum service life.

In this brief video, Rafe Britton explains why open gear lubrication presents unique challenges and how to overcome them. 

Open gears are not enclosed within a housing and are subjected to harsh operating conditions making them more prone to wear and damage. They can experience excessive friction and heat when they are not properly lubricated, leading to premature failure and downtime.

Precision open gear lubrication helps to reduce friction, dissipate heat, and protect against wear and corrosion. It can extend the lifespan of the gears, reduce maintenance costs, and improve the overall efficiency of machinery.

Proper open gear lubrication is essential for industrial machinery’s safe and efficient operation. By selecting suitable lubricants and following best practices for application, you can maximize the lifespan of your open gears and reduce the risk of costly downtime.

Many of us use kidney loop filtration to clean our oil and extend the life of our equipment.  Don’t overlook small gearbox oil filtration! Neglecting to filter small reservoirs may keep you from reaching your machine reliability goals.

Whether to use kidney loop filtration should be based on criticality, maintenance history, and difficulty achieving target cleanliness. The reservoir size can undoubtedly create challenges but has little to do with whether to install kidney loop filtration.

Factors to consider:

• Equipment criticality:  If the equipment being considered is vital to your process, then keeping the oil clean is necessary.  To determine equipment criticality, consider factors like cost of downtime, impact on safety, impact on the environment, spare availability, and the impact on production.

• MTBF: Mean time between failure can also help you prioritize the equipment needing oil filtration improvement.

• Current contamination issues: All oil reservoirs of critical equipment should have cleanliness targets, but the difficulty in maintaining oil cleanliness can vary drastically.  If you have trouble maintaining the cleanliness target and all contamination ingress steps have been completed, offline filtration may be the solution.

• Accessibility to the equipment: Some equipment is difficult to access.  If an offline filtration system is designed correctly, it can provide safer and easier access to the oil for sampling and filtering.

• No amount of filtration can or should replace due diligence in controlling contamination ingress.

Once you decide to proceed with kidney loop filtration, the real work begins. 

• Choose a location for the small gearbox filtration equipment that is out of the way yet provides access for oil sampling, filter changes, and pressure monitoring. Choose a location that prevents damage from occurring to the filtering system.  Be sure to build a robust system and monitor it frequently.  A failed component on kidney loop filtration can quickly pump your oil to the floor instead of returning it to the reservoir.

• Choose the correct filter media and beta rating for the situation. To do this, you must compare current cleanliness levels to the target.  You will also need to consider the type of contamination you are attempting to filter, such as water. 

• Choose the correct flow rate. For small reservoirs, I prefer to err on the low side.  Low flow rates make managing the reservoir level easier and can provide better filtration. 

• Consider what will happen to the oil levels of the reservoir if the filtration unit fails or loses power. We prefer to mount the filtration below the oil reservoir.  Then recheck the oil reservoir after the filter unit is full of oil.  Then we unplug the unit to test that the oil levels stay satisfactory.  If the filtration unit is mounted above the reservoir, loss of power to the filtration unit could result in an overfull gearbox or reservoir.

The payback can be impressive if your equipment choices and the installation are made correctly.  Below is an example of a Warren Thick Stock Pump with two bearing reservoirs. The smaller of the two only has a capacity of about 2.5 quarts.  

For this application, I chose the CC Jensen 15/12 filtration unit with a flow rate of .088 GPM (Yes, 5.3 gallons per hour!).  We also made several changes to the system to lessen the moisture ingress that was occurring during upset conditions. Our efforts lowered the oil particle count from ISO 25/22/15 with very high moisture levels to 16/14/11 with Karl Fisher moisture levels under 100 ppm.

 These improvements have more than doubled the MTBF.  This reduction in maintenance/production costs translates to an estimated $80,000 per year in savings.

Kidney loop filtration on small gearboxes and reservoirs can provide the same benefits and life extension on larger systems. Remember to reduce contaminant ingression as much as possible, build a robust system, and size the flow rate accordingly.