"We have large circulating systems that service industrial gear sets with heavy gear oils (ISO 460-680). We tried using 25-micron filtration, but this was as fine as we could go due to the high fluid viscosities. We have since abandoned the filters because of the high pressure drop. What can we do to remedy this?"

It is true that high-viscosity oils are not as easily filtered. However, all of this is influenced by several variables that need to be considered:


Some oils are unfilterable for many reasons that have nothing to do with viscosity. The filterability of your gear oil may need to be evaluated. There are different procedures for doing this. Besides the effects of viscosity and the presence of solid particles, an oil’s filterability is influenced by additives (e.g., VI improvers and precipitants), oxide insolubles, carbon insolubles, wax insolubles and poorly blended oils.

Oil Temperature

Make sure the oil is at operating temperature when the filter pressure drop is evaluated. Gear oils at operating temperature may have considerably lower viscosity. It depends on the exact temperature and the viscosity index of the oil. In addition, most filters have bypass valves that are viscosity sensitive (i.e., pressure drop sensitive). Therefore, during cold startups or other cold operating temperature conditions, all or a considerable portion of the flow bypasses the filter. It is true you won’t be filtering at this time, but as the oil heats up, a higher percentage of the flow (eventually all) will pass through the filter. During bypass, a well-designed filter housing will not permit particles from being pulled off the front side of the filter.

Flow Rate

Many offline filters for gear lubricants run at very low flow rates. Depending on the ingression rate and system size, flows as low as 1 gallon per minute can often maintain the required target cleanliness level. At low flow rates, the pressure drop through the filter is reduced considerably.

Filter Size

High surface-area pleated elements such as those used to filter bearing oils on paper machines can handle relatively high viscosity and high flow rates. These are rather expensive at initial installation but in the long run can be a big cost saver.

Media Type

High pressure-drop depth media filters are not suitable for high-viscosity gear oils. The best filter media has high pore density and hyper-fine fiber diameters. Hydraulic filters sold by leading manufacturers are generally of this type. Filter manufacturers' published PQ curves on their elements show the relationship between flow/viscosity and pressure differential.

Another major consideration is ingression. Every effort needs to be made to minimize particle ingression into the gear case. Once ingression is under control, the job of filtering the oil is simplified. Clean oil is vital to machine reliability. It is rarely a good financial or business decision to accept dirty oil in critical lubricating applications.