The purity requirement

Marginal Column

Multi-stage filters satisfy maximum requirements.

Selecting suitable hydraulic filters and using them properly will extend the service life of systems and minimize the total cost of ownership.


They are miniscule, often visible only with the microscope, but can nevertheless bring an entire system to a stop: dirt particles in hydraulic fluid. Some 70 to 80 percent of malfunctions in hydraulic systems can be traced back to contamination of the fluid. That makes filtering out this grime essential to system reliability. In addition to identifying the appropriate filter concept, proper service during ongoing operations is essential. That is because only regular and correct monitoring of the fluid’s purity can guarantee that the system will operate without interruptions, throughout its entire life cycle.

Exactly the right fit

In general, hydraulic fluids have to adhere to a certain minimum purity class, as defined in ISO 4406. The purity class for the system as a whole is oriented on the most sensitive of all the individual components. It is for this reason that, right from the design phase, the filters are dimensioned and positioned to ensure that the degree of purity required by the “weakest link” will be maintained under all operating conditions. But the focus is not only on the purity class.

Filters also have to be able to collect a large volume of contaminant and, when clean, should induce only minimal pressure loss. They should be compatible with the medium being filtered. Good characteristics for volumetric flow fatigue are desirable, as is mechanical stability of the support tube inside the filter element.

Three values that make a difference

  • βx Filtration grade
  • α Dirt absorption
  • Δp Differential pressure

Good filter elements are characterized by ideal ratios among these three factors: filtration grade, dirt absorption capacity, and differential pressure.

Looking for causes and perfect solutions

Typical contaminants in hydraulic fluids and lubricating oils are particles, foreign liquids and air. While particles can increase friction and cause greater component wear, water causes corrosion, affects the viscosity, and reduces lubricating properties. Gases diminish valve response behavior. Foaming in the medium will also cause energy waste and pump damage.

Always watchful

Debris can enter from the outside, but it may also be caused by the ongoing operation of the system itself. To guarantee system reliability, it is important to continuously monitor the hydraulic fluid. Regular inspections of the filter element and the medium, matched to local operating conditions, keep contaminants from affecting the fluid circuit and provide information on the nature and size of the residues. Thoughts on the source of the pollution can be derived from this, and this is the best approach to finding effective solutions.

5 tips for users

  • Even new hydraulic fluids, right from the factory, are often not pure enough. That is why filters of appropriate fineness should be used when filling the system.
  • Hydraulic fluids must correspond to the purity class specified by the system’s manufacturer.
  • To keep the maintenance effort for filter systems as low as possible, it is necessary to pay attention to easy accessibility right from the purchase of the system.
  • Maintenance schedules tailored to prevailing conditions guarantee uninterrupted operation.
  • Contaminants can also enter the system from the outside. That makes it necessary to carefully clean the area around the filter housing before replacing the filter element.

Useful helpers

The amount of water and particles in hydraulic fluids can be precisely measured with a variety of devices made by Rexroth. Online condition monitoring sensors continuously audit the quality of the hydraulic fluid. The purpose is to detect deterioration at an early date and to counteract it. Oil purification devices can also be used to multiply the life of the hydraulic fluid itself, reducing wear to system components and, as a consequence, operating costs.