Table 1: ISO VG Classes
Single- or multigrade oils Hydraulic fluid viscosity is temperature dependent: it decreases as temperature rises and increases as temperature drops. Mineral oils are significantly temperature sensitive and thus limited in usable temperature range. These are known as single-grade oils. Synthetic oils or mineral oils enriched with viscosity index improvers (VI) change their viscosity with temperature to a lesser extent than conventional mineral oils and are referred to as multigrade oils, which cover a wider temperature range.
Standardised hydraulic oil types Various systems require different oil types, leading to DIN and ISO-standardised hydraulic fluids (Table 2). Modern systems can’t rely on base oils alone; additive- enhanced fluids are essential. Type HL oils, despite improved corrosion and oxidation resistance, are rarely used due to insufficient lubrication. Type HLP oils have become the quasi-standard, meeting the minimum DIN 51524 requirements. However, these are just minimums. Performance often exceeds this and is defined by OEM specs.
Bio-hydraulic fluids For decades, environmentally friendly hydraulic fluids, often referred to as bio-oils, have been used in mobile machinery. These must meet both technical and environmental requirements – such as biodegradability, eco-toxicity, and sustainability – outlined in ISO 15380. Four categories have been defined (Table 3). Modern bio-oils are typically zinc free. HETG (triglyceride-based) and HEES (ester-based) fluids must contain at least 25% biogenic hydrocarbons. HETG oils are based on vegetable oils like rapeseed or sunflower and can be improved via hydrogenation. HEES fluids, especially saturated esters, offer superior oxidation stability and low-temperature performance. HEPG (polyglycol based) fluids are true multigrade oils with excellent aging resistance but are only used in special applications due to compatibility issues.
Table 3: Bio-hydraulc oils according to ISO 15380
Challenges in mobile applications Construction machines often run less than 1,000 hours a year under tough conditions – under cold and hot temperatures, dust, humidity and dirt conditions. Compared to stationary systems, they operate under a wider temperature range and need to provide improved corrosion protection. Multigrade oils enable operation in various temperatures. Typical viscosity ranges for mobile hydraulic fluids are ISO VG 32 to 68. Fluctuating temperatures require detergents to keep water in suspension and avoid corrosion. However, even these should not exceed 0.1% water content. Sometimes engine oils, which also contain detergents, are used in hydraulics, with viscosities classified in SAE grades (e.g., 10W, 20W-20, 30).
Mining machines differ drastically. They usually operate 23 hours a day with one hour for maintenance, requiring longer service life and reliability.
Table 2: System oil requirements
Electrification challenge Electrified construction machinery is becoming more common. Hydraulic systems in these machines differ. Traditional systems use load-sensing (LS) principles to Continued on page 32
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