For long service life, safety reasons, and reliable operation of hydraulic circuits, it is very important to use the correct fluid for the application. The most common fluid is based on mineral oil, but some systems require fire resistance because of their proximity to a heat source or other fire hazard. (Water is also making its return to some hydraulic systems because it is inexpensive, fireproof, and does not harm the environment.

Transmit energy.
The main purpose of the fluid in any system is to transmit energy. Electric, internal combustion, steam powered, or other prime movers drive a pump that sends oil through lines to valves that control actuators. The fluid in these lines must transmit the prime movers energy to the actuator so it can perform work. The fluid must flow easily to reduce power losses and make the circuit respond quickly.

In most hydraulic systems, the fluid must have good lubrication qualities. Pumps, motors, and cylinders need ample lubrication to make them efficient and extend their service life. Mineral oils with anti-wear additives work well and are available from most suppliers. Some fluids may need special considerations in component design to overcome their lack of lubricity.

Fluid thickness can be important also because one of its requirements is for sealing. Almost all pumps and many valves have metal to metal sealing fits that have minimal clearance but can leak at elevated pressures. Thin watery fluid can flow through these clearances, reducing efficiency and eroding the mating surfaces. Thicker fluids keep leakage to a minimum and efficiency high.

There are several areas that apply to specifying fluids for a hydraulic circuit. Viscosity is the measure of the fluids thickness. Hydraulic oils thickness is specified by a SUS or SSU designation, similar to the SAE designation used for automotive fluids. SUS stands for Saybolt Universal Seconds (or as some put it, Saybolt Seconds Universal). It is a measuring system set up by a man named Saybolt. Simply stated, the system takes a sample of fluid, heats it to 100° F, and them measures how much fluid passes through a specific orifice in a certain number of seconds.

Viscosity is most important as it applies to pumps. Most manufacturers specify viscosity limits for their pumps and it is best to stay within the limits they suggest. The prime reason for specifying a maximum viscosity is that pressure drop in the pump suction line typically is low and if the oil is too thick, the pump will be damaged due to cavitation. A pump can move fluid of any viscosity if the inlet is amply supplied. On the other end, if fluids are too thin, pump bypass wastes energy and generates extra heat. All other components in the circuit could operate on any viscosity fluid because they only use what is fed to them. However, thicker fluids waste energy because they are hard to move. Thin fluids waste energy because they allow too much bypass.

Viscosity index (or VI) is a measure of viscosity change from one temperature to another. It is common knowledge that heating any oil makes it thinner. A normal industrial hydraulic circuit runs at temperatures between 100° and 130° F. Cold starts could be as low as 40° to 50° F. Using an oil with a low VI number might start well but wind up with excessive leakage and wear or cause cavitation damage at startup and run well at temperature. Most industrial hydraulic oils run in the 90- to 105-VI range and are satisfactory for most applications.

Pour point is the lowest temperature at which a fluid still flows. It should be at least lower than the lowest temperature to which the system will be exposed so the pump can always have some lubrication. Consider installing a reservoir heater and a circulation loop on circuits that start or operate below 60° F.

Refined mineral oil does not have enough lubricating qualities to meet the needs of modern day hydraulic systems. Several lubricity additives to enhance that property are added to mineral oil as a specific manufacturers package. These additives are formulated to work together and should not be mixed with others additives because some components may be incompatible.

Refined mineral oil also is very much affected by temperature change. In its raw state it not only has low lubricity but also would thin out noticeably with only a small increase in temperature. Viscosity modifiers enhance the oils ability to remain at a workable viscosity through a broad temperature range.

There are several causes of hydraulic oil oxidation. These include contamination, air, and heat. The interaction of these outside influences cause sludge and acids to form. Oxidation inhibitors slow or stop the fluids degradation and allow it to perform as intended.

Wear inhibitors are additives that bond with metal parts inside a hydraulic system and leave a thin film that reduces metal-to-metal contact. When these additives are working, they extend part life by reducing wear.

In most hydraulic systems, fast and turbulent fluid flow can lead to foaming. Anti-foaming agents make the fluid less likely to form bubbles and allow those that do form to dissipate more rapidly.

Moisture in the air can condense in a hydraulic reservoir and mix with the fluid. Rust inhibitors negate the effect of this unwanted water and protect the surfaces of the systems metal components. All of these additives are necessary to extend system life and improve reliability.

Overheating the fluid can counteract the additives and decrease system efficiency. Overheating also thins the oil and reduces efficiency because of internal bypassing. Clearances in pump and valve spools let fluid pass as pressure increases, causing more heating until the fluid breaks down. External leaks through fittings and seals also increase as fluid temperatures rise. Another problem caused by overheating is a breakdown of some seal materials. Most rubber compounds are cured by controlled heat over a specific period of time. Continued heating inside the hydraulic system over long periods keeps the curing process going until the seals lose their resiliency and their ability to seal. It is best if hydraulic oil never exceeds 130° F for any extended period. Installing heat exchangers is the most common cure for overheating but designing heat out of a circuit is the better way.

Cold oil is not a problem as far as the oil is concerned but cooling does increase viscosity. When viscosity gets too high, it can cause a pump to cavitate and damage itself internally. Thermostatically controlled reservoir heaters easily eliminate this problem in most cases.