FLUID POWER HANDBOOK
WWW.MOBILEHYDRAULICTIPS.COM
HYDRAULIC
FLUIDS
HYDRAULIC
fluid is the medium used to provide consistent and powerful transmission of energy throughout a hydraulic system. That, in turn, allows actuators and drives to generate linear or rotary motions and perform work.
This simple energy-transfer 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 that functions safely, repeatedly, reliably and efficiently. 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. Heat transfer. 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 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.
24
FLUID POWER WORLD
7 • 2020
As oil returns to the reservoir, 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 a warmed tank or, via a special warming circuit, into a cold system to help minimize the possibility of damage during cold starts. Closed-loop hydraulic systems require a special bleed-off circuit connected to tank, to ensure 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. Contaminant removal. 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, and returning clean fluid 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. Sealing. 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
