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How to Ensure Proper Oil Cleanliness in Lubrication Systems


Maintaining proper oil cleanliness is critical to the success of any industrial operation. Oil contamination from particulate matter accelerates the rate of component wear and can lead to premature component failure. In fact, oil contamination is linked to more than 75 percent of all industrial equipment failures. 

Ensuring oil cleanliness can help mitigate these issues and lead to longer equipment life, less unscheduled downtime and reduced maintenance costs. 

What is oil cleanliness?

Oil cleanliness is a measure of the level of particle contaminants in the oil, including both insoluble and hard particles. Acceptable oil cleanliness levels are often determined by Original Equipment Manufacturer (OEM) recommendations and can be controlled through proactive maintenance methods. 

It is particularly important to maintain oil cleanliness in applications with tight clearances (such as equipment with servo valves) or harsh operating conditions (such as extreme temperatures, pressures of speed.)

Factors that contribute to oil contamination

There are two main sources of oil contamination:

External sources: these include foreign particles such as dirt, dust, and other particulate matter that enter the system. 

Internal sources: these include wear particles that contaminate the oil as a result of mechanical wear. 

There are four main types of mechanical wear: abrasive, fatigue, adhesive, and erosive. 

Wear Type

The information below provides more detail on each type of wear and outlines their effect on equipment performance. 


Particles in the clearance between moving surfaces remove material from the surface.

Direct effects

  • - Dimensional change
  • - Leakage
  • - Lowering efficiency


Two metals rub together ­­– leading to an instantaneous welding on the surface – and the continuous motion leads to the break of the welded points, causing generated wear metals. 

Direct effects

  • - Metal-to-metal contact points
  • - Cold welding
  • - Adhesion and sheering


Repeated stress caused by the clearance of surface particles trapped by the moving surfaces.

Direct effect

  • - Deterioration of finished surface
  • - Leakage
  • - Cracks


Particles impinge on a component surface of edge and remove material due to momentum effects. 

Direct Effects

  • - Slow response
  • - Spool Jamming
  • - Solenoid burnout

How do you measure oil cleanliness?

Oil cleanliness is measured against ISO Cleanliness Code 4406. This code quantifies particulate contamination levels per milliliter of oil at 4, 6, and 14 μm. 

The first step in measuring oil cleanliness is counting the particulate matter using one of several particle counting methods. 

These include: 

ISO 4407: In this method, the oil sample is passed through a very fine filter patch to capture particles. An optical microscope is then used to count the particles that are between 5 and 15 μm. This method was one of the original methods used for particle counting, but it is extremely time-consuming and is rarely used today. 

ISO 11500: This method, which is the most widely deployed method today, uses an optical particle counter. A laser or white light is focused on a capillary detection zone. As the oil sample passes through the detection zone, the particles create a shadow on a photocell detector. The drop in voltage produced by the photocell then helps determine the size of particles passing through. Any particles greater than 4, 6, and 14μm are counted. Because the test is fully automated, results are processed quickly. 

ASTM D7647-10: This method uses the same laser equipment as the ISO 11500 method, but the test sample is pre-treated with solvent dilution techniques. These techniques can eliminate some of the "soft particles," enabling operators to instead count hard particles that have the most significant impact on equipment wear. However, this test is more time-consuming than the ISO 11500 method. 

Regardless of the test method used, it's critical to take a relevant and representative sample when measuring particle counts to properly determine the contamination level. Incorrect sampling can adversely affect the cleanliness level in the sample bottle, which will skew results and lead to incorrect insights. 


Determining the level of contamination

Once operators have accurately measured the particle count, they should then use the ISO 4406 classification to determine the contamination level. 

As mentioned previously, the ISO Cleanliness Code quantifies particulate contamination levels per milliliter of fluid at three sizes: 4, 6, and 14μm. The code is expressed in three numbers, which represent the respective contaminant level code for the correlating particle size. The code includes all particles of the specified size and larger. 

How do you determine proper oil cleanliness code target?

The best way to determine an Oil Cleanliness Code target for a given piece of equipment is to reference the Table of recommended target ISO Celanliness Codes and selection of media for systems using petroleum based fluids per ISO 4406. 

How do you achieve and maintain proper oil cleanliness?

After selecting the appropriate target ISO Cleanliness Code for the application, there are several considerations to achieve and maintain the desired cleanliness:

  • Filter micron rating
  • Filter efficiency (Beta Ratio)
  • Filter size
  • Full flow or side stream
  • Filtration placement
    • Filtration when the lubricant is received
    • Filtration as the lubricant is dispensed
    • Filtration at set interval and based on oil analysis with portable filtration units
    • Permanent filtration installed directly on the equipment

Storage and handling

Lubricants become vulnerable to contamination during storage, and especially when handled or moved. This makes receiving certified clean oil the least efficient manner in achieving desired cleanliness unless it is being delivered directly to the application. 

A sealed container is still susceptible to ingress contamination of particles and water as air temperatures around the container change. 

In order to reduce contamination during storage:

  • Store lubricants in sealed containers that reside in a sheltered room or building.
  • Equip containers with air and water filtration. 
  • Drums should be stored horizontally with large and small bungs in 3 and 9 o'clock positions. 
  • Close bungs promptly after use. 

Dispensing and in-service

Lubricant dispensing and in-service in the equipment is the most critical step in achieving and maintaining desired oil cleanliness. These are the last points before the lubricant comes into contact with the critical wear components. 

In order to achieve and maintain proper oil cleanliness, operators should follow these dispensing and servicing best practices:

  • Always use dedicated pumps, carts, and hoses to dispense the lubricant. 
  • The dispensing pump/cart should be equipped with the appropriate filtering media size and efficiency when filling and topping up oil charges. 
  • Always clean the filling cap before removing it prior to filling the reservoir. 
  • Change the media filter when the OEM recommended maximum pressure differential has been reached. 
  • Ensure that the oil reservoir is closed (no open cover plate) and the filler cap should always be in place. 
  • Always equip the breather on the reservoir with an air filter. Depending on the sensitivity of the application, equip the reservoir with a media that removes water as well. 
  • Implement a used oil analysis program to monitor the in-service oil condition and ensure it meets the target ISO Cleanliness Code. 
  • Control oil leakage, as more frequent oil top-offs expose the application to greater risk of contaminant ingression.  


Maintaining proper oil cleanliness is about more than just using the right lubricants – it's also about taking the right steps to ensure a clean operating environment that helps minimize the risk of particulate contamination. 

More specifically, operators should implement a programmatic lubrication approach built around regular assessments. 

These programs should:
  • Establish limits: Operators should use the techniques outlined in this article to determine the acceptable cleanliness limits required for the application. They should then use proper filtration to further reduce the risk of contamination. 
  • Identify and address ingress sources: Operators must be vigilant about maintaining a clean operator that minimizes lubricant exposure to contaminants. This includes ensuring proper storage and handling practices as well as following the right protocols during lubricant dispensing and servicing equipment. 
  • Implement regular used oil analysis: By implementing a regular used oil analysis program, operators can better monitor lubricant cleanliness levels and quickly identify any potential warning signs of contamination. 

By following these guidelines, operators can help maintain proper oil cleanliness, helping extend equipment life and enhance operational productivity. 

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