HYDRAULIC FLUIDS Hydraulic efficiency Dr Andrew Gelder, Technical Product and Sales Development Manager – Lubricants, IMCD Group
With global issues raising prices, efficiency improvements are warmly welcomed. In the world of lubricants, it is accepted practice to look for opportunities to maximise efficiency. In the automotive sector, and specifically in engine oil and transmission oil developments, efficiency has long been the word on the street.
But, how about efficiency in the world of hydraulic fluids? This is not a traditional area of consideration when it comes to focusing on efficiency, but efficient hydraulic fluids are something that could and should be considered in our cost-conscious world. However, with hydraulic fluids, efficiencies are not as straightforward to achieve as with more traditional fluids.
With engine and transmission oils, drag factors and friction reduction are the main considerations. By focusing on viscosity reduction and lowering the friction coefficient, efficiency reduction can be effectively achieved.
In a hydraulic system, there are two competing mechanisms that should be considered: Firstly, the traditional efficiency mechanism – where the aim is to move the fluid through the pump with minimum effort. This is known as mechanical efficiency.
The second mechanism to address is the actual work the hydraulic fluid does. The fluid starts in a sump and is then pumped around the system, operating hydraulic cylinders under pressure as it goes, before returning to the sump. The hydraulic circuit is not perfect, there are flow losses incurred within any hydraulic component and leakage will occur, reducing the amount of working fluid that actually returns directly to the sump. For every 10 litres of fluid pumped at the start of the circuit, maybe only 9 litres will return. This is known as Volumetric Efficiency and in this example, the system is only 90% efficient.
Both of these mechanisms need to be taken into account and a major issue is that they are
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contradictory. The lower the viscosity of the hydraulic fluid, the more mechanically efficient it is. However, low viscosity fluids will easily leak out of the system, whereas high viscosity fluids are more robust in this area. This is best illustrated in Figure 1, where it can be seen that the viscosity of the fluid acts in a contradictory fashion in the two efficiency mechanisms.
Figure 1: Viscosity to Efficiency Relationship Graph.
The dashed line that can be seen in the graph is the overall efficiency of the system. This can be determined through the product of the Volumetric Efficiency and the Mechanical Efficiency.
Overall Efficiency = Mechanical Efficiency * Volumetric Efficiency
The white zone on the graph highlights the optimum viscosity for the fluid to give the maximum efficiency.
Ideally, maximising the white zone will give a more efficient system and will allow owners and operators to reduce the cost of their operations. Traditional hydraulic fluids are monograde formulations and will have a Viscosity Index, VI, of around 95. Increasing the VI of a formulation will increase the operating range of the fluid and ultimately increase the optimum operating zone in terms of the efficiency. Increasing the VI will also reduce the viscosity change over a temperature range and is demonstrated in Figure 2.
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