MRO Magazine

Oil Filters, types, ratings, and applications

Effective lubricant filtration is one of the misunderstood and misapplied of maintenance practices.

July 26, 2023 | By L. (Tex) Leugner 

Photo: surasak petchang/Getty Images Plus/Getty Images

Photo: surasak petchang/Getty Images Plus/Getty Images

Lubrication related failures dramatically increase downtime, cause unnecessary high operating costs, and reduce overall equipment life. Oil contamination in recirculating systems like turbines and hydraulics contributes to 70 per cent of lubrication related failures. Unresolved friction and wear problems cost Canadian industry over six billion dollars annually.

Q | Does the maintenance group understand how contaminants enter a lubricated system?
LOGIC: contamination of lubricated systems occurs in four ways, generated by the system itself (by normal wear, poor system or component design, surface fatigue and temperature related chemical reactions), implanted (by welding slag), induced (by careless maintenance practises), and escaped contamination (that enters a lubricated system through poor quality filters or poor filtration system design).

Q | Is the maintenance group familiar with oil filter types and their standards of quality as they apply to the equipment they maintain?
LOGIC: there are two basic types of oil filter elements. Surface elements in which the oil passes through one layer of filter media constructed of pleated paper or fibreglass with a specific pore size that determines the micron size rating, usually a range of 10 to 40 micrometres. Surface type filters are mounted in full flow applications, where all the lubricant in the circuit passes through the filter, but not necessarily through the media if the pressure regulating valve opens during cold start-ups or pressure surges.

The second filter type is depth filters whose elements are constructed of one of two groups of materials. One group of materials is referred to as absorbent and these consist of inactive materials such as cotton waste, waste or wound paper, cloth, and wood pulp. These filters depend upon the absorption of contaminants as the oil flows through the media in tortuous paths. Depth type absorbent filters will not remove oil additives unless the additive is a solid lubricant, such a graphite, and the particle sizes are in the size range which may prevent them from moving through the filter.


Another group of materials used in depth type filters are referred to as adsorbent and consist of chemically active materials such as Fullers earth, active clays, charcoal, or chemically treated paper. These filters remove contaminants through a chemical reaction with the lubricant, and as a result, may remove some oil additives.

Q | Does the maintenance group understand when and where depth type filters should be considered?
LOGIC: depth type filters should be considered for use when oil must be kept extremely clean, and the full flow surface filter may not satisfy this need. Depth type absorbent filters are often installed in parallel to the surface filter, referred to as a side stream system and will remove contamination in the one to 10 micrometre range. They are particularly effective in hydraulic systems and turbines using control valves susceptible to valve sticking and binding due to contaminants in the three to five micrometre size. These combined circuits permit approximately 10 to 15 per cent of the system’s oil to flow through the depth filter.

Poorly constructed absorbent depth filters are susceptible to channelling, a condition whereby the oil flow through the media creates a path of least resistance. Once channelling occurs, effective depth filtration ceases. The best depth type absorbent filters contain finely wound paper and are particularly effective in removing contaminants including water. Some equipment manufacturers provide secondary side stream filtration circuits using depth type filters as original equipment.

Q | Does the maintenance group understand oil filter cleanliness ratings?
LOGIC: to understand the size relationship of a contamination particle of one micrometre, visualize a forty (40) micron particle. It is the largest particle that can be seen with the naked eye and is 0.0016 of an inch in size. A one micrometre particle on the other hand, is one millionth of a metre or approximately 0.0000394 of an inch in size.
Nominal filter ratings refer to a filter that will remove 96 per cent by weight of contamination of a specified size.

Tests have shown that particles as large as 200 micrometres can pass through a nominally rated 10 micrometre filter. An absolute rating indicates the size above which no particles of any size will pass. It does not specify the size of the smallest opening in the media, so that a filter rated at 10 micrometres absolute will do little to reduce silt particles measuring less than 10 micrometres. These filters will be ineffective every time the pressure regulating valve opens (regardless of their absolute rating) when they are used in full flow applications.

Q | Does the maintenance group understand the factors that affect oil filtration?
LOGIC: these factors include temperature changes, pressure drops resulting when the pressure regulating valve opens, and flow rate changes when cold starts and pressure surges occur. Any differential pressure across the element can cause deformation or separation of the filter media pleats, if the pleats are not properly designed and supported resulting in ineffective filtration.

Hydraulic systems are subjected to high pressure and compression of the oil occurs at the rate of about two per cent per 1000 PSI. If the oil volume in the connecting line is 100 cubic inches and the pressure is 1,000 PSI, the liquid compression can reach .005 X 100 or .5 cubic inch. When an operational valve is opened downstream under these pressures, the sudden increased flow rate can be dramatic. This shock flow can be several times pump output, when large bore or long stroke cylinders operating at high pressure decompress quickly.

When pressure line filters are located some distance from the pump outlet or mounted in the return line, these shock flows can cause filter media bunching or total collapse, particularly in poorly constructed filters. Pump pulsations and mechanical vibrations can dislodge fine abrasive particles from the filter media and contamination may re-enter the fluid stream.
L. (Tex) Leugner, author of Practical Handbook of Machinery Lubrication, is a 15-year veteran of the Royal Canadian Electrical Mechanical Engineers, where he served as a technical specialist. He was the founder and operations manager of Maintenance Technology International Inc. for 30 years. Tex holds an STLE lubricant specialist certification and is a millwright and heavy-duty mechanic. He can be reached at


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