WORLDWIDE R. David Whitby Efficient hydraulic fluids Compressibility is still one of the most critically important factors. ydraulic fluids transmit power as a hydraulic system con- while phosphate and vegetable-oil esters have compressibilities H verts mechanical energy into fluid energy and subsequently similar to that of water. Polyalphaolefins are more compressible to mechanical work. To achieve this conversion efficiently, hydrau- than mineral oils. Water-glycol fluids are intermediate between lic fluids need to be relatively incompressible. Hydraulic fluids mineral oils and water. must also minimize wear, reduce friction, provide cooling and pre- Mineral oils are relatively incompressible, but volume reduc- vent rust and corrosion, be compatible with system components tions can be approximately 0.5% for pressures ranging from 6,900 and help keep the system free of deposits. kPa (1,000 psi) up to 27,600 kPa (4,000 psi). Compressibility in- Compressibility measures the rela- creases with pressure and temperature tive change in volume of a fluid or solid and has significant effects on high-pres- as a response to a change in pressure and sure fluid systems. Hydraulic oils typi- is the reciprocal of the volume-elastic cally contain 6% to 10% of dissolved air, modulus or bulk modulus of elasticity. which has no measurable effect on bulk Bulk modulus defines the pressure in- modulus provided it stays in solution. crease needed to cause a given relative Bulk modulus is an inherent property decrease in volume. of the hydraulic fluid and, therefore, is The bulk modulus of a fluid is nonlin- an inherent inefficiency of the hydraulic ear. When the change in volume against system. Because compression of the fluid pressure is plotted on a graph, the result requires energy at the input, which can- is a curve rather than a straight line. Tan- The size and weight of a hydraulic system not be converted to useful work at the gent bulk modulus relates to the slope of is critical when the importance of a low output, it is lost energy. The larger the the curve at any specific point and is, bulk modulus becomes greater. system actuator and the faster the re- therefore, the true rate of change in vol- quired response time, the higher the en- ume at that specific pressure. ergy inefficiency due to bulk modulus. Bulk modulus is further defined as isothermal, where the heat There are hydraulic system problems caused by low compress- associated with compression is dissipated (constant temperature) ibility, including: or isentropic. When the heat associated with compression is not dissipated, both pressure and thermal expansion are considered. • Servo-valves fail to maintain static rigidity and experience ad- Isentropic can be viewed as dynamic bulk modulus and isothermal verse effects in system amplification. as static bulk modulus. The former is typically appropriate to mod- ern, high-response hydraulic systems. • Loss in efficiency because volume reduction due to compress- Hydraulic fluids need to have a very low compressibility. A low ibility cannot be recovered. compressibility translates into a fast response time, resulting in a • Cavitation, which may cause metal fracture, corrosive fatigue high-transmission velocity of pressure and low power loss. Hy- and stress corrosion. draulic fluids with a high compressibility or low bulk modulus act as dampening fluids. As a result, hydraulic fluids that contain en- It is normally desirable that compressibility should be as low trained air, due either to poor air release or excessive foaming, are as possible, although an extremely high resistance to compress- much less effective in transmitting power. ibility in a hydraulic fluid could actually be disadvantageous in The size and weight of a hydraulic system is critical when the readily transmitting shock forces. However, the inherent elasticity importance of a low bulk modulus becomes greater. Poor com- of the mechanical components comprising hydraulic systems has pressibility requires increased line sizes and actuator cross-sec- the effect of increasing the absolute compressibility of the fluid to tional areas to compensate for the lower stiffness of the fluid. a rather higher apparent value. These increases also mean a larger volume of hydraulic fluid and greater hydraulic system weight. Different hydraulic fluids have different compressibilities. For David Whitby is chief executive of Pathmaster example, naphthenic oils have a higher bulk modulus than paraf- Marketing Ltd. in Surrey, England. You can finic oils. Water is slightly less compressible than mineral oil, contact him at [email protected]. 72 • JANUARY 2011 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG.