Hydraulic fluid is the energy transfer medium in all hydraulic systems. This simple function is only achieved by a fluid that does not easily trap gasses. Trapped gas and foaming problems would bring a higher level of compressibility to a fluid that is usually relied upon to support a very stiff, fast reacting system.
Where transmitting energy is the core function of hydraulic fluid, it is also useful in four secondary functions—heat transfer, contamination removal, sealing and lubrication.
Hydraulic machines produce a lot of excess heat in normal operation, often caused by inefficiencies within the components themselves. Pumps and motors allow fluid to pass through the fine clearances between internal parts when the system pressures are high. The heating in this situation is caused by large volumes of fluid molecules rubbing against metal surfaces. Without a method to carry thermal energy away from these surfaces, overheating can result with damage to seals, valve plates and other components.
As oil returns to the reservoir where a large volume of fluid is held, it often passes through a cooler to help maintain an optimal temperature range before being pumped out to the system once again.
Hydraulic fluid can also carry heat from warmed tank, or via a special warming circuit, into a cold system—this helps minimize the possibility of damage during cold starts.
Closed-loop hydraulic systems require a special bleed-off circuit connected to tank, in order to make sure that some oil is always being cooled. A charge or replenishing pump brings the cooled oil back into the circuit to help maintain a suitable overall system temperature.
Hydraulic fluid can suspend and carry solid particles and water away from sensitive components. Filters and other conditioning devices perform the critical job of stripping and trapping the contaminants, allowing clean fluid to be returned to the circuit.
Knowing that solid contaminants are suspended in a fast-moving stream of fluid is not a substitute for monitoring cylinder wear or for a suitable filtration program. When a flow valve opens only slightly, and the pressure differential across the valve is high, the same suspended particles may now cause erosive damage to a valve—as though it had been shaped with a file or a grindstone.
While seals and O-rings close the major clearance between some parts, the hydraulic oil finishes the job in the fine clearances where the solid material of an undersized shaft seal might cause damage.
Another example is the spool valve, which has a seal at each end to prevent oil from escaping to atmosphere. Inside the valve, each notch and undercut on the spool is sealed from the next only by the tight tolerance of the spool and valve bore, together with the oil’s surface tension and resistance to shearing.
Critical to this sealing function is the viscosity of the fluid. Viscosity index (VI), which is the change in viscosity over a swing of temperature changes, is also a key factor. A fluid with a high VI number is able to resist changes in viscosity as it heats up, allowing the fluid to maintain a consistent seal.
Lubrication is required in most hydraulic components to protect internal parts from friction wear. Oil provides full-film lubrication between moving parts, such as the slippers and valve plate of a piston pump. Without the lubricating properties of oil, hydraulic systems would be unreliable—yielding a very short life for many components.
The majority of hydraulic machines use refined mineral oil base stock or a synthetic oil. These oils are formulated and manufactured to specific industrial test standards for important properties such as viscosity, viscosity index and pour point. These three properties along with ambient and operating temperatures are often carefully considered when choosing a fluid.
If ambient machine temperatures are low, one would choose an oil with lower rated viscosity and pour point. Pour point is simply the temperature at which oil will still pour. If a machine sees varying temperatures—as happens to an all-weather mobile machine—a high viscosity index is crucial.
It is important to consider the viscosity requirement as specified by component manufacturers. A piston pump, for example, may require a viscosity between 16 and 40 centistokes. Centistokes describe the kinematic (measured while flowing) viscosity of a fluid, regardless of temperature. This data helps the user select a final ISO viscosity that will conform to required kinematic viscosity, at the final operating temperature.
Hydraulic oil has a package of chemical additives designed to improve the performance of both the oil and the components in the hydraulic system. These additives can improve the foaming resistance of the oil or help to quickly release at the tank, any trapped air brought into a hydraulic system via bad cylinder or motor seals, or through poorly sealed hose connections. Rust and oxidation inhibitors are powerful chemicals that can ultimately reduce the internal production of particle contaminants as they trap water and keep it away from ferrous metal surfaces.
Vane pumps are among the most efficient pumps, from a volumetric standpoint. There is little, if any, clearance between the knife-edge of the vane and the cam ring. To help provide a lubricating boost for these pumps, a hydraulic fluid with an antiwear or extreme pressure additive is required. These additives react with metal surfaces creating a thin, sacrificial lubricant film.
The overall additive package often separates high-quality fluids from economy-priced hydraulic oil, where a poor additive mix can actually become corrosive to the yellow metals (brass and bronze) used in hydraulic components.
Hydraulic fluid is the life-blood of many mobile and stationary machines. It is difficult to overemphasize the care that should be taken to maintain this precious medium. It is equally important to remember that while a quality fluid has been engineered and designed to perform challenging tasks, it cannot compensate for a system with an undersized reservoir or a motor with an excessive shaft load. If the hydraulic components are properly specified and the overall system is well designed, a good quality hydraulic fluid will serve the critical function that ties the pump and the actuator together, along with all components in between.