A piston pump, for example, may require between 16 and 40 centistokes, which is a description of kinematic (i.e., measured while flowing) viscosity. It is important to consider the viscosity requirement of the components in your hydraulic system. If your machine sees varying temperature ranges, like an all-weather mobile machine, a high viscosity index is very important, which describes an oil’s ability to maintain its viscosity over a wide temperature range. For example, if ambient temperatures are low, you would choose an oil with lower rated viscosity and a low pour point, which is the temperature at which oil will still pour. Those three properties are often considered when choosing a fluid for a particular application, which is based on maintaining a specific viscosity throughout a particular set of ambient and machine operating conditions. However, the majority of machines use refined or synthetic oil, which are formulated and manufactured to specific test standards for important properties like viscosity, pour point and viscosity index, to name a few. These functions of hydraulic fluid are common to all types, except some water-based fluids, which require special design considerations during engineering. Without the lubricating properties of oil, hydraulic systems would be terribly inefficient and awfully unreliable. Oil provides full-film lubrication between moving parts, such as the slippers and lens plate of a piston pump. Lubrication is required in most hydraulic components to protect internal parts from wearing, or even downright melting, as a result of metal-to-metal friction. A spool valve, for example, has a seal at each end to prevent oil from escaping the valve, but each notch on the spool is sealed from the neighboring cavities by only the tight metal-to-metal tolerances and the oil’s surface tension and resistance to shearing. Oil without impetus would remain within sensitive components, allowing essentially trapped contamination to slowly destroy its surroundings.Īlthough most believe hydraulic oil is what pieces of hardware-such as O-rings or U-cups-seal against, hydraulic fluid (especially oil) actually provides sealing within the internal components of pumps, valves and motors. Heat can be considered a form of contamination, but hydraulic fluid also carries particles and water away from sensitive components through filters or other conditioning devices, where it is cleaned up and returned to the circuit. If closed-loop hydraulic systems didn’t bleed off fluid at a controlled rate, contamination would quickly accumulate to critical and damaging levels. Conversely, hydraulic fluid can carry heat into a system during cold starts, when needed. As oil returns to the reservoir, it often passes through a cooler to help maintain optimal temperature range before it is pumped back out to the system. Without a way to carry heat away from these components, they could easily overheat with resulting damage of seals and internal components, especially as a result of low local viscosity. Hydraulic machines produce a lot of excess heat in normal operation, often caused by inefficiencies of the components themselves, like pumps and motors. Although transmitting hydraulic energy is the core purpose of hydraulic fluid, it is useful in four secondary functions-heat transfer, contamination removal, sealing, and lubrication. However, the job of hydraulic fluid goes beyond simple transmission of power. Hydraulic fluid is the energy transfer medium in all hydraulic systems.
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