High viscosity can contribute to high fluid temperatures and greater energy consumption. On the other hand, if viscosity is too low then there will be an insufficient fluid film between moving parts to properly protect and lubricate. Low viscosity fluids can also waste energy by allowing too much fluid to bypass at the pump.
Factors that influence viscosity selection can include:
- Pump design
- Cold start temperatures
- Operating temperature ranges
- Viscosity index
- Viscosity shear stability
- Proper additive formulation
In the case of mobile equipment, hydraulic systems used to perform work will use significantly more fuel as the efficiency of the hydraulic systems reduces. Much of the fuel consumed in mobile equipment that relies on hydraulic power to perform the work is used for operating the hydraulic pump, which is the heart of the hydraulic system.
Efficiency in mechanical operating systems is affected by frictional impediment and the energy necessary to overcome the loss of efficiency due to friction. Most frictional resistance in a hydraulic circulating system is from fluid friction, which is influenced by the system design. In hydraulic systems the more circuitous the path, the greater the fluid friction and greater the efficiency losses. For many fluid power applications, the majority of the energy losses occur in valves and fittings in which there is a change in the cross-section of the flow path and a change in the direction of the flow. Properly sized directional valve for the application is important.
The amount of laminar flow versus turbulent in a hydraulic system will also influence efficiency for power transmission. Laminar flow of a fluid is more efficient than turbulent flow. If excessive turbulent flow is allowed to exist higher fluid temperatures will occur due to greater frictional energy losses. Hose or pipe diameters that are too narrow and excessive material roughness in contact with the fluid contribute to turbulent flow.
In the case of stationary plants operating hydraulic systems, pump inlet and reservoir design can influence efficiency. This can affect foaming of the fluid, contamination control, and possible contributors to cavitation.
Even after system design has been optimized and the proper hydraulic fluid viscosity and formulation has been selected, efficiency and power loss can still occur if the fluid condition is not kept clean and dry, and not allowed to age beyond service life. Leaking fittings, as well as contributing to power loss during operation, will also allow contaminants to enter once the machine has been shut down. ALS Tribology has the testing and reporting tools available to provide the information needed to monitor fluid and equipment integrity through oil analysis to compliment other asset reliability practices. For assistance in optimizing your in-service fluid analysis, contact ALS Tribology.
David Doyle, CLS, OMA I, OMA II
Key Accounts and Special Projects