Hydraulic fluid selection is very important because today's hydraulic systems operate at harsh conditions consisting of high pressure, high temperature and high speed. An effective fluid should improve the hydraulic system's operating performance, increase the safety in operation, reduce wear in parts, resist degradation and be cheap. In a hydraulic system, hydraulic oil performs more than all other components. To meet such a role, the fluid must have sufficient chemical and physical properties, even under severe operating conditions and in a negative operating environment. In addition to satisfactory lubrication between parts in a hydraulic system, the fluid must also be successful against foreign particles and contamination, should conform to the sealing elements and should be inflammable during high temperature applications.
Effective transmission of power in a hydraulic system is accomplished with a liquid having the following characteristics:*
- Low Compressibility allows pressure and power to be delivered immediately and efficiently,
- Air Release property prevents any increase in compressibility due to the air bubbles in the liquid,
- Anti-Foaming property prevents the foam’s access to the hydraulic system,
- Proper viscosity makes it is viscous so that it can move smoothly in the system while lubricating it at the same time.
The characteristics that are effective in the selection of hydraulic oil can be listed as follows: Viscosity, specific density, evaporation pressure, toxicity, thermal stability, anti-wear, anti-foam, corrosion inhibitor, dispersion properties, shear degradation, cubic elastic modulus, surface tension, inflammation capability, oxidation, rust inhibitor, lubrication, hygroscopic properties, pour point, detergent properties, viscosity index, air release, etc.
In order to enhance the natural properties or add new properties, or to achieve the predetermined performance characteristics of base oils, additives are usually added in the final formulation of the hydraulic fluid. The use of additives in hydraulic fluids was rather limited before the 1950s. Interest increased after the operating life was increased by oxidation inhibitors. In order to use complex valve units in hydraulic systems operating at high temperatures and under high pressure, there was a great need for high quality hydraulic fluids. The solubility characteristics of the base fluid are generally very important in terms of the efficiency of many additives. Excessive solubility inhibits adsorption of the desired active surface particles on metal surfaces, while insufficient solubility may prevent the desired function of the additive. For hydraulic fluids generally considered for low- or high-temperature applications, generally synthetic oils containing different additives are more compatible, compared to those with mineral base.
In addition to the benefits of the additives, they may also have harmful effects. Such effects occur particularly when an excessive amount of additive is used, or when the additive reacts with other additive substances. While putting additives into the oil, it is important to avoid any degradation in the oil and other additive components. Additives added to hydraulic oils can either have negative affect (antagonism) or positive effect (synergism) in the oil. Main additives used in hydraulic oils, their chemical composition, functions and adding rates are summarized in Table 1, and the hydraulic fluids according to ISO and DIN standards are summarized in Table 2.
Table 1. Hydraulic oil additives**
Table 2. Classification of hydraulic fluids in accordance with ISO 6743 and DIN 51502***
* (Güven, A., II. Ulusal Hidrolik Pnömatik Kongresi ve Sergisi, 133-146, 2001)
** (Hodges, P., “Hyraulic Fluids”, John Willey & Sons, Inc. 1996)
*** (Hodges, P., “Hyraulic Fluids”, John Willey & Sons, Inc. 1996)