Dynamic multi-pass test

Constant test conditions are not realistic

RT Filtertechnik as a specialist for filtration in mobile applications determined at a very early stage in cooperation with our customers of mobile hydraulic systems that the common procedures used today to determine the capacity of hydraulic filters only provide limited information in terms of real applications in excavators, wheel loaders or other mobile machines. The standardised test procedures for determining the performance data of a filter element are for making it easier for the end consumer to compare the filter elements.

RT Filtertechnik GmbH naturally also tests your filters according to the standard but has also developed a large number of customer-specific test scenarios. At the beginning, they were still tested in the field. Since 2008, they have also been simulated in RT's own test facility.

Hydraulic systems are usually filtered in the return line and, for mobile work machines, in the tank, due to the space conditions. On mobile work machines, all flow rates of the cylinders of the booms, dredger buckets, pushing shovels and other accessory equipment are bundled at this point by the rotor drive and possibly also by the drive system. There are no constant flow rate conditions at the filter element, as the amount of oil that is pumped in the individual circuits differs, due to the valve connections of the individual circuits. The following diagram clearly shows that the flow rate at the filter element is anything but constant.

Common test procedures do not cover this issue, however. In addition, the end consumer is given false information, as the filter media usually separate better at a constant flow rate that under dynamic load.

Normally, particles can be separated in the filter element, which are smaller than the pores in the filter material. This is done via what are called van der Waals forces. However, under dynamic load, these forces are frequently not sufficient to retain the particles, which are transferred back into the system.

For this reason, the HLCT (Hydraulic Load Cycle Test) procedure has been developed by RT Filtertechnik in Kromsdorf over the past years and a unique test bench has been set up for this purpose. The test bench can be used to simulate load profiles from any type of application that are applied to the filter.

Typical load profiles:

The analyses revealed that the load profiles from the machines can be split up into 4 main categories

Symmetrically distributed load profile with:

B: identical rise time of positive and negative flow rate ramp
C: shorter rise time of positive flow rate ramp compared to negative flow rate ramp
D: longer rise time of positive flow rate ramp compared to negative flow rate ramp
E: identical rise time of positive and negative flow rate ramps and high repeat rate

The test bench:

The test bench is generally designed in such a way that it passes static multi-pass tests as per ISO 16889 between 30 and 740 l/min. A 2-circuit system was installed for this purpose. The first circuit can be operated between 30 and 150 l/min and the second circuit between 120 and 750 l/min. Dynamic cycles can also be applied to both circuits. Changes in volumetric flow from minimum flow rate to maximum flow rate are possible with up to 4 Hz.

In addition, a dynamic cycle can be recorded directly from the field test and entered at the test bench control. The evaluation of the results is based on the withdrawn standard for dynamic multi-pass tests, formerly ISO 23369.

Test procedure

The boundary conditions such as temperatures, flow rates and ramp times must be entered in the first (example here: wheel loader). They can either be provided by our customers or they are measured during common field tests.

A load profile recorded over a specific period is applied to select a representative machine working cycle (cycle that is repeated constantly during normal operation). Then, it is abstracted to program the test bench.


If test contamination is now applied to the filter no longer under constant flow rate conditions, but instead in realistic, changing conditions, the differences between the filter design and the quality of separation are revealed much more clearly. Note that the higher the β value, the poorer the filtration.

The following diagram shows how the different load profiles affect the separation capacity: