Visibly clean! Is that clean enough?
June 11, 2024
Technical articles
The cleanliness of hydraulic fluids in heavily loaded hydraulic and hydrostatic systems of mobile and stationary plant, machinery and vehicles has been a perennial topic for decades and has been addressed in countless specialist articles. In practice, however, we regularly encounter uncertainty among users when it comes to correctly maintaining and storing hydraulic oils, or filling machines and plant with them to ensure trouble-free operation in the long term.
The visual assessment of the cleanliness of a hydraulic fluid is a common and even standardised method for defining the cleanliness class (e.g. ISO 4406). The cleanliness of the lubricant is assessed visually, i.e. with the naked eye. Does this therefore mean that if it looks clean, it is clean enough? Far from it! The visual assessment is, as the name suggests, carried out by eye, but it is essential to use a microscope that magnifies the sample many times over. What appears clean without instruments therefore falls far short of the applicable minimum requirements for hydraulic fluids or the machine- and system-specific, individually defined requirements set by OEMs.
In the routine analysis of hydraulic oils in service, automatic particle counters have long been relied upon; these replace visual assessment and lead to more reliable, reproducible and comparable results. In the laboratory at the LTC LAEMMLE Tec Centre, we analyse hydraulic fluids in accordance with ISO 4406 and apply the limit values shown in Figure 2; or rather, we go one step further and filter selected hydraulic fluids during filling to meet standard-specific requirements, for example in accordance with DIN 51524. However, it should be noted that hydraulic oil from an unopened, freshly delivered container does not always meet the required purity class specified by the plant or machine manufacturer. In certain cases, filling the system using a suitable filter system may be necessary to ensure trouble-free operation and to avoid placing an unnecessary load on the system’s internal filters.
To check the condition of the hydraulic fluid, we recommend regular oil analyses at the LTC LAEMMLE Tec Centre laboratory; alongside many other parameters, the measurement of the ISO 4406 cleanliness class forms part of the standard scope of testing and is an important assessment criterion.
So, should you come across an LTC test report in future containing the following comment, you will understand the basis for the assessment and the technical background: “The oil exhibits excessive particle contamination (ISO 4406 cleanliness class) compared with our limit value (21/17/13). The lubricant must be filtered, or the system filters must be replaced. If filtration is not possible or advisable, the oil must be changed.”
Author: Martin Ruch, Head of Technical Support
Figure 1: Close-up of the laemmle-chemicals ROXOR drum
Figure 2: LTC LAEMMLE Tec Centre used oil analysis