The ‘built-in’ inefficiency of every hydraulic system:
Compression of the oil.
A fluid’s compressibility is defined by its bulk modulus of elasticity – which is the opposite of compressibility. Meaning, as the bulk modulus of elasticity increases, compressibility decreases.
Bulk modulus is an inherent property of the oil and therefore an inherent inefficiency of a hydraulic system.
The fluid in the pipeline and actuator must be pressurized, and consequently compressed, before it will move a load.
Because this compression of the fluid requires work at the input – which cannot be converted to useful work at the output – it is lost work and therefore a contributing factor to the overall inefficiency of the hydraulic system.
The larger the actuator and the faster the response time, the higher the inefficiency attributable to bulk modulus.
And in high-performance, closed-loop electro-hydraulic systems, deforming oil volumes affect dynamic response, causing possible stability problems such as self-oscillation.
Unlike viscosity index, bulk modulus cannot be improved with additives. However,hydraulic equipment users can take steps to minimize the inefficiencies and potential control problems associated with compression of the fluid.
The first is to ensure hydraulic equipment doesn’t run hot.
Compressibility of the fluid increases with temperature. Mineral hydraulic oil is approximately 30 percent more compressible at 100°C than it is at 20°C.
Of course, there are many reasons why you should never allow hydraulic equipment to run hot – most of which we’ve already discussed. Reduced bulk modulus is another one.
The second is to prevent conditions that cause aeration.
Air is 10,000 times more compressible than oil. One percent of entrained air by volume can reduce the bulk modulus of oil by as much as 75 percent.
While controlling aeration is largely a design issue – for example, the amount of dwell time the oil has in the tank – proper maintenance also plays an important role.
Dissolved air comes out of solution as temperature increases, which is another reason to maintain appropriate and stable operating temperatures.
Also, oxidative degradation and water contamination inhibit the oil’s ability to release air, often resulting in an increase in entrained air and thus compressibility.