HYDRAULIC FLUIDS


Hydraulic fluids for construction machinery


Rüdiger Krethe (Oildoc), Stephan Baumgärtel (VSI)


Construction machinery and changing demands Construction machinery is indispensable in our modern world. Whether a house, factory, a road, or a bridge – old must make way for new, and existing infrastructure is repaired, modernised, or expanded.


Construction machinery itself is also subject to change: cost pressure, digitalisation, demands for high energy efficiency, availability, low emissions, and all of this as sustainably as possible. Hydraulic systems, long an integral part of construction machinery, cannot escape these conditions – nor can the hydraulic fluids used within them.


This article will first look at the general requirements for hydraulic fluids in construction machinery. It will then highlight the consequences of technological advancements for these fluids and how to handle them smartly. These requirements and conditions are also transferable to other mobile machines and work equipment, such as agricultural and forestry machinery.


General requirements Hydraulic fluids primarily serve for power transmission 30 LUBE MAGAZINE NO.187 JUNE 2025


and are therefore an integral part of the system— like the bearings and gears in a gearbox. They also support system control and serve as lubricants, reducing friction and wear, providing corrosion protection, dissipating heat, and transporting particles. The technical specifications, components, and operating conditions of a hydraulic system or machine can lead to very different requirements for the oil. OEMs define these, commonly based on a standardised hydraulic oil type, a viscosity range, and sometimes stipulating an OEM approval.


Viscosity


The most important physical property of hydraulic oil is its viscosity. Oil viscosity influences power transmission efficiency and lubrication. High viscosity causes internal friction and flow losses, reducing efficiency. Low viscosity oil leads to lower efficiency and poor lubrication. Density also affects energy efficiency – the lower the density, the less energy is needed to move the oil and overcome internal friction. Due to viscosity’s importance, international standards categorise it into viscosity classes. For hydraulic fluids, the ISO VG system is widely used (Table 1).


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