Edited by Alan L. Hitchcox
If components are engineered for use with water-based fluids, such as the electrohydraulic servocylinder, at right, bottom, and stainless-steel proportional valve, at right, top, water hydraulic systems can achieve performance comparable to that of oil hydraulic systems.
Even though the term hydraulics often refers to water, oil has become the fluid of choice for most industrial and mobile hydraulic systems. This is because the characteristics of water are more aggressive to industrial components.
For example, oil’s higher viscosity makes it a much better lubricant than water. This is because a fluid’s ability to prevent metal-to-metal contact increases with viscosity. Water’s lower viscosity also makes it more prone to leakage than oil. Components for water hydraulic systems, therefore, generally must be manufactured to tighter tolerances and with more aggressive sealing systems to prevent wear and leakage.
Water is also corrosive and can produce erosive wear. To combat these detrimental effects, water hydraulic components must be made of corrosionresistant, hardened materials. Tighter tolerances and expensive materials, such as stainless steels, add considerably to the cost of water hydraulic components and systems.
The other side of the coin
However, water also holds several advantages as a hydraulic fluid. Its low viscosity means it offers lower pressure drop when used in applications comparable to those using oil. It also has a higher bulk modulus than oil, making it (at least potentially) more responsive in closed-loop servo applications. Water is also inherently environmentally friendly, and costs far less per gallon than oil.
But most important of all, water is fireproof. When a hydraulic system must operate in close proximity to flames, hot metal, or other environments with a high potential for fires, the safety aspects of water’s non-flammability far outweigh the additional costs associated with a water hydraulic system.
For most systems, pure water is rarely used. Instead, additives are incorporated with water to impart some degree of lubricity and improve its performance as a hydraulic fluid.
Serving a need
To keep the performance and reliability of water hydraulic systems on par with oil-based hydraulics, ATOS SpA, Sesto Calende, Italy, offers a line of hydraulic components designed especially for water hydraulic systems. The components are designed not only to stand up to the potentially aggressive characteristics of water, but to also take advantage of its many attributes.
For example, directional valves, proportional valves, and cylinders use stainless steel for all internal surfaces that come in direct contact with water. This avoids the rapid corrosion that would result from using carbon steel while costing considerably less than components made entirely of stainless steel. The components are also manufactured to closer tolerances than those used for oil hydraulics.
Internal surfaces are also reinforced to stand up to the potentially erosive effects of high-velocity streams of water. As a result, more than 75% of ATOS’s valves have a rated life exceeding 10 million cycles . The valves also feature ISO 4401 subplate mounting for easy installation and maintenance. Round head stainless steel cylinders and servocylinders conform to ISO 6020/1 and come in multiple configurations and with a wide variety of options configurations .