Oil Cleanliness Class

What is oil cleanliness class?

Oil cleanliness class is a numerical indicator that defines the level of contamination of oil by solid particles of specific sizes. It is one of the most important parameters characterizing the quality of hydraulic, turbine, gear, and other lubricating oils used in precise machine systems. Maintaining the appropriate oil cleanliness class is crucial to ensure the reliability, efficiency, and long life of these systems.

Definition and importance of cleanliness class

The oil cleanliness class informs about the quantity of solid particles (e.g., dust, sand, metal shavings, oil oxidation products) suspended in a unit volume of oil (usually in 1 ml or 100 ml). These particles, even microscopic in size, can cause serious issues in precision machine components, such as pumps, valves, cylinders, and bearings.

Why is oil cleanliness critical?

Solid contaminants in oil are the main cause of:

• Abrasive wear: Hard particles getting between cooperating surfaces act as abrasive material, causing accelerated wear.
• Erosive wear: Particles hitting surfaces at high speeds (e.g., in valves) cause erosion.
• Fatigue wear: Particles can initiate microcracks on surfaces, leading to material fatigue damage.
• Blocking and jamming of precision components: E.g., control valves, sliders.
• Degradation of oil properties: Metal particles can act catalytically, accelerating oil oxidation.
• Shortening of the life of components and entire systems.
• Increased risk of failures and unplanned downtimes.

Many modern hydraulic and lubricating systems operate with very small clearances between elements (on the order of a few micrometers), so even particles invisible to the naked eye can be harmful to them.

Most popular cleanliness classification standards

Various standards are used to determine the oil cleanliness class, the most important of which are:

1. ISO 4406 (current version e.g., ISO 4406:2021):

This is currently the most commonly used international standard. The cleanliness class is expressed using three numbers (e.g., 18/16/13), which correspond to numerical codes defining the number of particles in 1 ml of oil, larger than specific sizes:

• First number: Code for the number of particles ≥ 4 µm (micrometers).
• Second number: Code for the number of particles ≥ 6 µm.
• Third number: Code for the number of particles ≥ 14 µm.

Each numerical code in the ISO scale corresponds to a certain range of particle counts (e.g., code 18 means from 1300 to 2500 particles/ml). The lower the numbers in the ISO code, the cleaner the oil.

2. NAS 1638 (National Aerospace Standard):

This is an older standard, withdrawn but still occasionally found in some specifications. The cleanliness class according to NAS is expressed using one number (e.g., NAS 7), which defines the maximum allowable number of particles in five different size ranges (5-15 µm, 15-25 µm, 25-50 µm, 50-100 µm, >100 µm) in 100 ml of oil. The lower the NAS class, the cleaner the oil.

There are also other standards, e.g., SAE AS4059, which are extensions or modifications of the above.

How to interpret results and typical requirements?

Manufacturers of machines and equipment (OEM) typically specify the required oil cleanliness class for their systems. For example:

• Precision hydraulic servo valve systems: may require ISO class, for example, 16/14/11 or better.
• Standard mobile hydraulic systems: e.g., ISO 19/17/14.
• Gear lubrication systems: e.g., ISO 21/19/16.

Regular examination of oil cleanliness class as part of oil monitoring allows assessing whether the oil meets requirements and whether the filtration system operates correctly.

Main sources of oil contamination

• New oil: Oil “straight from the barrel” is not always clean enough for precision systems. It may contain contamination from the production process, transport, or the packaging itself.
• Built-in contamination: Residues from the machine’s production and assembly process (shavings, sealing remnants).
• Internally generated contamination: Products of abrasive wear of machine parts, oil aging products.
• External contamination entering from the environment: Dust, dirt, moisture entering through leaky seals, breathers.

Methods for controlling and maintaining cleanliness class

The key way to achieve and maintain the desired oil cleanliness class is filtration. This involves using:

• Filters in the system: Installed on the suction, pressure, or return line.
• By-pass (off-line) filtration: An independent filtration loop that continuously cleans a portion of the oil from the main tank.
• Filtration of new oil before filling into the system: A very important practice.
• Proper reservoir breathers (with air filters).
• Attention to cleanliness during handling and oil topping.

Is cleanliness class guaranteed for new oil?

Oil manufacturers usually do not guarantee a specific cleanliness class for new oil in standard packaging (barrels, buckets), because the cleanliness is influenced by the entire logistical chain. Therefore, it is recommended to filter new oil before its first use in the machine, especially in systems with high cleanliness requirements. Exceptions may be special aviation products or oils delivered in specially prepared, clean containers on a custom order.

Mobipol offers oil cleanliness class testing services on-site at the client’s location, using portable particle counters.