firm is currently the only one to offer a water hydraulic motor.
To further address leakage, engineers throughout the industry also have redesigned many water hydraulic components. They typically deal with water's lower viscosity by incorporating tighter part tolerances. Most pumps, for example, must now use clearances of 0.0001 inch, rather than the more common clearances of 0.0005 inch in oil hydraulics. To fight off corrosion, a host of parts, including pistons, cylinder blocks, swash plates, and valve plates, now consist of stainless steel. Housings, too, have been re-designed: Most now employ cast bronze. And some manufacturers use urethanes and reinforced plastic valve seats to reduce leakage.
For the most part, manufacturers have not dramatically changed the design of seals to accommodate water. But even there, subtle changes have surfaced. Many valve seats now use a high-nitrile BUNA-N rubber for sealing plungers and O-rings in valves. "Most erosion problems occur near seals, where you have very high velocities," notes Gary Bahner, vice president of engineering for The Oilgear Co., Milwaukee, WI. "All you need is a pinhole, and water will go through very fast."
Achieving proportional control. System manufacturers realize, however, that they won't break ground in new markets without state-of-the-art controls. That's particularly true in industries such as automotive, where users are accustomed to high-tech features. "To win customers in these areas, we have to produce equipment that's as easy to use as its oil equivalents," one engineer says.
That's one of the reasons why Hauhinco spent five years developing a ceramic spool valve. Traditionally, spool valves have failed to stand up to the rigors of water. But without spool valves, Hauhinco engineers believed they would be unable to build a proportional control system with the same characteristics as an oil-based system. Hence, the decision to introduce the ceramic valve spool.
Across the industry, manufacturers feel they should concentrate on proportional control over the next five years. To achieve gains in that area, engineers have worked to alleviate problems caused by the electrical conductivity of water. Unlike hydraulic oil, which has a low electrical conductivity, water can cause problems when used with servo valves.
Still, some firms have succeeded in developing proportional control systems. Others are close behind. Elwood Corp., Oak Creek, WI, now offers a water hydraulic valve commanded by an electrical pressure feedback signal from a closed-loop controller. Using a proportional/servo solenoid, it supplies fluid to, or relieves it from, the system. This step accurately controls system pressure and, therefore, load force.
Cost cutting the key. Despite these advances, engineers say more work is needed. Before water hydraulics seeps into a wider range of applications, it must:
Cut costs. Systems that use 95% water are roughly 20% more costly than oil hydraulics, but pure tap water systems are three to four times more expensive. Water hydraulics makers expect cost to drop as the market grows.
Eliminate bacterial growth. Bacterial growth inside the water is a problem that manufacturers are only starting to address with biocide programs.
Develop new pumps. Development of inexpensive, pressure-compensated pumps would eliminate the need for