Differences in Hydraulic and Lubrication System Filters

Noria Corporation

"What are the differences between hydraulic and lubrication system filters? Can you use hydraulic filters for gravity flow of lube oil?"

Although the basic principles of filtration are the same in each case, there are slight differences in filters based on the application.

In most force-fed lubrication systems, the filter is after the pump and before the critical machine components. This location poses several problems for the filter. The first issue is the flow rate. Often a filter located at this point in the system will have to deal with 100 percent of the flow coming from the pump.

If the machine requires large amounts of oil to operate reliably, then the filter must be large enough to deal with the flow. This can be achieved by increasing the surface area of the media or changing the media type to one that utilizes a smaller fiber.

The filter also must be able to handle pressure. The pressure from the pump pushing fluid through the small pores at a high rate of speed will cause weak points in the filter. Collapsed center tubes, blown-through media and seams, and failed end-cap adhesive are just some of the potential problems. The solution is a more robust filter, which usually means a higher cost.

When compared to a typical hydraulic filter on the return line, lubrication system filters on average are slightly more robust. When installed on a low-pressure return line, these filters don’t have to be made using reinforced center tubes, sandwiched media or thicker housings.

The basic principles of filtration include the relationship between pore size, flow rate, surface area and the differential pressure. The pore size is a major contributor to the performance of the filter to remove smaller particles. The smaller the pore sizes found in the media, the smaller the particles it can reliably remove.

The pore size can’t be made too small with respect to the flow rate and surface area or the differential pressure will increase to a point where it opens the bypass valve and allows solid contaminants to completely avoid the filter.

These basic principles are the main difference between the many types of filters. Remember, it is driven by the application more so than the machine component type.

There are filters on the market today that could interchange from a hydraulic system into a gravity-flow lube oil filter as long as the expected requirements are relatively the same. However, the flow rate, pressure, surface area and pore size must all be accounted for to achieve the expected results. 

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