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Filters Optimize Hydraulic System Function

In many pieces of heavy-duty equipment, hydraulic systems are subsystems that convert power generated by an engine or motor into kinetic energy (force and motion). Hydraulic systems relay that energy and transfer it as pressurized fluid to create motion in equipment components. This often makes hydraulic power the prime force used to accomplish major construction, mining, agricultural and manufacturing projects.

Hydraulic oil is incompressible, which makes it ideal for transferring power. It’s a liquid, so it flows through fluid conveyance (tubes and hoses) and it’s an oil, so it lubricates moving parts at the same time as it transfers fluid power.

Note: there are some specialized hydraulic systems that use glycol rather than oil.

The Benefits of Filtered Hydraulic Fluid

A filter is not required for a hydraulic system to perform its functions. However, a key success factor for hydraulic systems is maintaining fluid cleanliness in order to optimize function. Typical contaminants found in hydraulic systems are sand / silica, silt, metal, oxides, carbon and even air. Water is another major contaminant that brings with it a unique set of potential problems. Because water is more compressible than oil and has no sealing or lubricating ability, it will negatively affect the performance of a hydraulic system. It also promotes the corrosion of metal surfaces, which degrades performance and can serve as an additional source of contamination.

Contaminant capture and retention is the primary goal of all hydraulic filtration systems. Besides their destructive effect on system components, hydraulic oil contaminants can cause systems to lose energy, lubricating capacity and the ability to dissipate heat – plus there is the potential for leakage caused by contamination. In addition to preventing these negative outcomes, effective filtration can also help reduce hydraulic oil replacement and disposal costs.

Clean hydraulic fluid will uphold system performance and reliability. Beyond this, hydraulic fluid filters can help improve system function in ways that stretch beyond the traditional role of “filtration,” in order to support ever-increasing system complexity.


When a machine rolls off the manufacturing line, it works at a very specific performance level to accomplish its intended function. It’s able to push a certain amount of dirt, lift heavy objects, or move into difficult locations. When hydraulic fluid isn’t clean, system components start to wear and performance degrades. That degradation means the equipment may not be able to perform as intended. In addition, contamination can compromise machine stability, which can impact equipment control and/or consistency of operation.

One of the most significant developments in hydraulic fluid filtration is the implementation of a new ISO standard that better reflects dynamic operating conditions, including the ever-changing flow conditions experienced by all hydraulic systems. Rather than measure performance as a steady state multi-pass flow (ISO 16889), the new standard (ISO 23369) acknowledges the cycles and flow changes of hydraulic systems in real-world operating conditions. This change in measurement is also prompting changes to filter technology (media) that can deliver better contaminant retention under these fluctuating pressure and flow conditions. Improved contaminant retention means enhanced protection to help ensure that equipment will continue to perform better, longer and with lower risk for wear, damage and unplanned downtime.


Equipment owners also want to avoid hydraulic system failure that results in loss of the equipment’s primary function. Unplanned downtime must be avoided in order to keep projects on time and on budget.

Filtration plays a critical role in maintaining reliability in two ways:

  1. Protection of the whole system: Filters remove contamination that can create wear or cause catastrophic failure of hydraulic system components.
  2. Reliability of the filter itself: Because hydraulic systems are pressurized, the entire system is only as strong as its weakest link. Therefore, filtration reliability is critical. Any leakage can have a devastating impact on the function and availability of the entire machine.

New hydraulic system designs have requirements for smaller space claims and reservoir size, less fluid use, reduced air in systems and a greater need for real time-monitoring of system performance.

Smaller Reservoir Size and Less Fluid

Environmental regulations require bigger aftertreatment systems, which impose a burden on the available space within equipment for hydraulic systems. In addition to less space, there are also additional environmental and operating cost considerations which create a preference for lower overall oil volume. 

Hydraulic System Contamination

Contamination can enter a hydraulic system at multiple points, but protection should start at the hydraulic reservoir, which can be a site of contamination (e.g., water can form inside the tank as outside air cools). It’s also important to note that new hydraulic oil is not necessarily clean. At an approximate ISO cleanliness standard of 21/19, it often does not meet OEM standards. This has prompted many OEM’s to recommend filtering new oil via an offline kidney loop before pumping the oil into equipment tanks. 

In addition to externally generated contamination which can enter a hydraulic system at a variety of points (e.g., breathers, seals, with new oil, or via maintenance), contamination is also generated internally, often as the result of system components wearing through typical operation. Plus, there’s always the possibility of catastrophic component failure, and if that does happen, filtration systems help to contain the result of that failure and prevent the expansion of damage throughout the system. 

The illustration below represents a simplified hydraulic circuit with potential filtration integration points labeled.

  1. Suction Strainer – Removes large particles or objects built into the system during assembly or introduced during standard maintenance. Prevents catastrophic failure. Also known as a safety filter.
  2. Suction Line Filter – Designed to remove particles in the 5 to 150 micron range. Easy to service and less expensive than other types of filters. Low by-pass valve use is recommended to prevent pump starvation.
  3. Pressure Line Filter – Protects high-pressure side components. Helps prevent component wear or failure brought about by debris in the system.
  4. Return Line Filter – Captures debris from component wear or ingression before it travels into the reservoir.
  5. In-tank Return Line Filter – Space-saving in-tank return and suction line filters.
  6. Kidney Loop Filters – Off-line filtration supplements system cleanliness. Use with stationary and mobile equipment to achieve and maintain proper ISO cleanliness levels.
  7. Reservoir Air Breather – Prevents ingression of airborne contaminants from entering the reservoir tank.

Every hydraulic circuit is different, but our Donaldson hydraulic experts are available to work with you to identify and optimize the filtration systems that work to meet your specific application and budget requirements.  

Questions on how our technologies can benefit your business?