Although the hydraulic fluid moves through a relatively closed system, hydraulic oil filters are extremely important. The nature of most hydraulic machinery entails the regular creation of damaging metal chips and filings, and the hydraulic filter is responsible for removing these objects. Other internal contaminants include plastic and rubber particles generated by abraded seals and bearings. Hydraulic filters will also remove external contaminants, such as dust and dirt, that make their way into the hydraulic circuit. These functions are integral to the consistent operation and longevity of any hydraulic-powered device, and unfiltered hydraulic fluid will lead to increased leakage and system inefficiency.
In a typical hydraulic circuit, the hydraulic filter is placed between the reservoir and the pump. Some designs place the filter after the pump, which helps prevent damage to the control valve in the case of pump failure. The design necessities of this layout can be more costly because of the filters relationship to other parts.
The nature of the filter media depends largely on the nature of the contaminants that will be encountered by the hydraulic system. Some systems will also require filters that remove air and water, although particulate is often the primary concern. To that degree, hydraulic filters can remove extremely small particles from the hydraulic fluid; this can include contaminants as small as mere micrometers in size.
Common materials used in hydraulic filters are micro-fiberglass, phenolic-impregnated cellulose and polyester. Many systems simply require specific replacement filters, which can be ordered through filter manufacturers and suppliers, or through the machine system or device manufacturer. When designing a hydraulic system, it is important to consider the contaminant tolerance level of the device, as well as the acceptable drop in pressure caused by the filter. These factors will help you in determining the proper filter media, positioning and design.