What is the function of hydraulic filter?

Functions of Hydraulic Filters

Hydraulic filters are placed at various locations throughout a system to catch contaminants created at different points in the process. For ideal filtration, filters would be before and after the pump, in the return lines, off-line and within the reservoir. However, considerations for cost and space impact where designers choose to place filters. There are a few primary locations, which determine the types of hydraulic filters used:

Return lines: These filters are usually low pressure. They collect contaminants from the fluid as it returns to the reservoir and can be positioned along the return line or within the tank. See the Chase 10, 11, & 51 series filters.
Suction filters: Another example of a low-pressure filter, suction filters are placed at the pump inlet to ensure no contaminants enter the pump. See the chase 14 & 16 series filters.
Pressure lines: These filters are placed after the pump to keep expensive downstream components safe. Pressure filters have a high-pressure capability to remove internal contamination created within the system. This allows for individual component protection. Saving on downtime, maintenance and component repair or replacement costs.
Off-lines: Also called kidney loops, these filters continue working even while the central hydraulic system is not working. An off-line filter draws the fluid from the reservoir and returns it after filtration. These filters are usually easier to maintain because of their placement within reach. Some popular types of off-line filters are the drum topper and portable filter cart, which can service multiple hydraulic systems. Another common kidney loop system is the Filter Panel. It can be mounted directly to the hydraulic system and provide 24/7 filtration if necessary.
The function of a hydraulic filter is to remove contaminants at a specific stage of system operation. There are several ways contaminants can enter a system:

Environmental: Some contaminants accumulate during the manufacturing process of the hydraulic system. Debris like paint chips, rubber particles and metal shavings can collect in the system. An air hose may blow these contaminants away, but others are too small to be seen and must be filtered out. A large amount of contamination also enter the system in new fluid. Fluid already in a system is typically cleaner than any new fluid that you will add; new oil is not necessarily clean, typically 22/20/18. Therefore, you should follow good filtration practice and use a filter cart or pre-filtered fluid whenever you add fluid to keep it within cleanliness level specification. It would help to filter fluid before adding it to a hydraulic system and multiple times throughout operation.
System-generated: The internal parts of a system, like piston rods and pumps, create contaminants during routine processes. Return-line filters are adept at collecting this kind of contaminant. Castings also wear down due to shock pressures and continual flexing and can then enter the fluid stream.
External: Air can sometimes enter the system through breather caps, imperfect seals or other openings, condensing on the system’s walls and leading to rust. It is crucial to clean breather caps, as dirt eventually gets back into the fluid. Maintenance specialists also introduce new particles when replacing parts.