an ignition source, and there is a risk of fluid catching fire in the event of a leak. Fire-resistant fluids usually comprise water-polyalkylene glycol (PAG) mixtures or phosphate esters. When there is a risk of incidental spill into the environment, environmentally friendly biodegradable hydraulic fluids are getting increasingly common in agricultural and construction equipment. For example, in Sweden, hydraulic fluids are expected to meet the environmental requirements of the Swedish standard SS 15 54 34. Such fluids and are formulated using vegetable oils, esters, PAGs, and Polyalphaolefins and can be recognized by the prefix “HE”, e.g. HEES 46. Different hydraulic fluid classes can overlap. For instance, there are products that are both environmentally friendly (HE) and fire-resistant (HF).
When selecting a hydraulic fluid, picking the right viscosity is often the single most important factor to consider. The recommended viscosity grade for each piece of hydraulic equipment and machinery is always specified by the manufacturer, with consideration for ambient conditions (Figure 2).
Since oil viscosity changes with temperature, different viscosity grades will be optimal depending if it is cold or hot outside. Choosing the right grade for the season helps improve performance and longevity of the equipment. For instance, in a Hitachi EX200-5 excavator used in an area with a continental climate, it is advisable to use ISO VG 32 grade in winter and VG 68 grade in summer.
Besides that, due to different thin film rheology, UHVI fluids less prone to leakage, improving volumetric efficiency of hydraulic motors and pumps. For a given viscosity grade, fluid formulation also has an effect on efficiency and reliability of hydraulic equipment (Miller
et.al. 2014).
Last but not least, filtration equipment should be mentioned. The most common cause of hydraulic system failure is fluid contamination. Hence, proper maintenance remains a critical factor, where cleanliness of hydraulic fluid is of paramount importance. ISO 4406 cleanliness levels of 16/13 is the minimum requirement for most systems, with high-pressure systems calling for 15/12 or better, and servo valves calling for 13/10 or better. Hence, the cleanliness requirements for hydraulic fluids are much higher than, for example, for motor oils. Even though different hydraulic systems have different sensitivity to fluid contamination – for instance gear pumps are less sensitive than piston pumps – as a rule of thumb, the service life of a hydraulic system using a clean fluid with 15/12 cleanliness level is estimated to be 10 times longer than when using a contaminated fluid with 24/21 cleanliness level. A wide variety of hydraulic filters with ratings from 1 to 20 um is available to match individual OEM requirements.
Figure 2: Example of manufacturer recommendations: the recommended viscosity grades of HLP-type hydraulic fluids to be used in Hitachi excavators EX 200-5 / 220-5.
To minimise the hit of ambient temperature fluctuations on the performance, hydraulic fluids with ultra-high viscosity index (UHVI) are preferred. Technically, it is feasible to reach VI upto 400 with oil-based formulations and upto 1000 with water-based formulations. Such fluids have very flat viscosity-temperature curve and work well over a wide range of ambient temperatures.
Progress in hydraulics is ultimately related to development of new materials, manufacturing and quality control methods that allow precise engineering at an affordable price. To improve the efficiency of hydraulic systems, tighter dimensional clearances are introduced. Advanced surface finishing and coating methods are used to improve the tribological performance. For example, hydraulic cylinder piston rods are usually hard chrome plated. Tribonex is currently working to develop a chrome-free alternative to hard chrome for this application. In axial piston pumps and motors, the interface between the cylinder drum and the port plate is one of the most challenging design conundrums. Conventionally, leaded brass (CuPb15Sn5) is used right against steel (38CrMoAl or 42CrMo), where the lead not only aids the manufacturing process but also provides good tribological properties, such as low friction over a wide range of speeds and pressures. However, since lead is a RoHS restricted substance, alternatives are being sought to replace the original hard-soft tribological pairing by a hard-hard pairing with hard coatings such as DLC, WC/C and TiN on the port plate. Laser
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