wOrLdwIdE R. David Whitby
Efficient hydraulic fluids Compressibility is still one of the most critically important factors.
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while phosphate and vegetable-oil esters have compressibilities ydraulic fluids transmit power as a hydraulic system consimilar to that of water. Polyalphaolefins are more compressible verts mechanical energy into fluid energy and subsequently than mineral oils. Water-glycol fluids are intermediate between to mechanical work. To achieve this conversion efficiently, hydraumineral oils and water. lic fluids need to be relatively incompressible. Hydraulic fluids Mineral oils are relatively incompressible, but volume reducmust also minimize wear, reduce friction, provide cooling and pretions can be approximately 0.5% for pressures ranging from 6,900 vent rust and corrosion, be compatible with system components kPa (1,000 psi) up to 27,600 kPa (4,000 psi). Compressibility inand help keep the system free of deposits. creases with pressure and temperature Compressibility measures the relaand has significant effects on high-prestive change in volume of a fluid or solid sure fluid systems. Hydraulic oils typias a response to a change in pressure and cally contain 6% to 10% of dissolved air, is the reciprocal of the volume-elastic which has no measurable effect on bulk modulus or bulk modulus of elasticity. modulus provided it stays in solution. Bulk modulus defines the pressure inBulk modulus is an inherent property crease needed to cause a given relative of the hydraulic fluid and, therefore, is decrease in volume. an inherent inefficiency of the hydraulic The bulk modulus of a fluid is nonlinsystem. Because compression of the fluid ear. When the change in volume against requires energy at the input, which canpressure is plotted on a graph, the result not be converted to useful work at the is a curve rather than a straight line. TanThe size and weight of a hydraulic system output, it is lost energy. The larger the gent bulk modulus relates to the slope of is critical when the importance of a low system actuator and the faster the rethe curve at any specific point and is, bulk modulus becomes greater. quired response time, the higher the entherefore, the true rate of change in volergy inefficiency due to bulk modulus. ume at that specific pressure. There are hydraulic system problems caused by low compressBulk modulus is further defined as isothermal, where the heat ibility, including: associated with compression is dissipated (constant temperature) or isentropic. When the heat associated with compression is not • Servo-valves fail to maintain static rigidity and experience addissipated, both pressure and thermal expansion are considered. verse effects in system amplification. Isentropic can be viewed as dynamic bulk modulus and isothermal as static bulk modulus. The former is typically appropriate to mod• Loss in efficiency because volume reduction due to compressern, high-response hydraulic systems. ibility cannot be recovered. Hydraulic fluids need to have a very low compressibility. A low 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. Hyand stress corrosion. draulic fluids with a high compressibility or low bulk modulus act It is normally desirable that compressibility should be as low as dampening fluids. As a result, hydraulic fluids that contain enas possible, although an extremely high resistance to compresstrained air, due either to poor air release or excessive foaming, are ibility in a hydraulic fluid could actually be disadvantageous in much less effective in transmitting power. readily transmitting shock forces. However, the inherent elasticity The size and weight of a hydraulic system is critical when the of the mechanical components comprising hydraulic systems has importance of a low bulk modulus becomes greater. Poor comthe effect of increasing the absolute compressibility of the fluid to pressibility requires increased line sizes and actuator cross-seca rather higher apparent value. tional areas to compensate for the lower stiffness of the fluid. 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 parafMarketing 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
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