Water makes a comeback : Page 2 of 4

applications. Chief among them: steel mills, aluminum mills, mining, and oil drilling. Those industrial users have employed water because of fire-safety concerns. Until recently, these users were among the few willing to pay more for water hydraulics equipment.

Now, however, the tide is turning. Engineers are looking at water hydraulics with renewed interest, partially as a result of the environmental movement. Instead of viewing it with a jaundiced eye, many now see water hydraulics as environmentally friendly, non-flammable, inexpensive, clean, readily available, and easily disposable. Meanwhile, hydraulic oil leakage, merely an annoyance a decade ago, has become a potentially costly problem. Users of oil systems can no longer wash oil leaks down the drain. Instead, they must be collected and disposed of in an environmentally friendly manner.

"The oil-hydraulic industry presented a seemingly satisfactory solution to the problems of water hydraulics by offering new designs intended to eliminate leakage," says Ken Kirk, general manager for Schrupp, Inc., Bethel Park, PA. "However, these problems have persisted."

What's more, new noise legislation may lead some machine builders, already concerned about oil leakage and pneumatic noise, to seriously consider water hydraulics for the first time.

Dealing with leaks. Still, engineers have had to tackle some tough technical problems before water hydraulics could take a more prominent position in industry. Prime among those: leakage.

Engineers estimate that water's viscosity is about one-thirtieth that of hydraulic oil. In other words, its acceleration rate is higher, flow velocities are faster, and energy is greater.

Taken together, all of these characteristics translate to a greater potential for destruction. As a result, water systems have traditionally had problems with leakage. That's particularly true for pure tap-water systems, as opposed to those that employ a mixture of 95% tap water and 5% oil.

But manufacturers are dealing with the leakage and erosion problems through the use of new ceramic materials, such as aluminum oxides and zirconias. Hauhinco, for example, has introduced pilot valves-both ball and spool types-for leak-tight applications. The firm's ball valve uses a ceramic ball and a metal seat; its spool-style valve employs a ceramic spool. Spool leak-ages are said to fall into a range between 3 ml/min and 12 ml/min.

The advantage of the ceramic materials: They stand up better to high pressures, exhibit less permanent deformation, corrode less, and wear better under non-lubricated conditions. The result: less valve leakage. "You just do not have the wear on ceramics that you do on metals," notes Ladislaus Stromps of Hauhinco.

Danfoss' NESSIE system, designed for pure tap water, has dealt with leakage by employing a self-lubricating polymer on the moving surfaces of its pumps. Water, flowing over the moving surfaces, creates heat, which activates the polymer. NESSIE's axial piston pump uses the self-lubricating feature on all major contacting surfaces: between piston and cylinder bores, "slippers" and swash plates, and valve plates and the top of the cylinder block, for example.

The self-lubricating polymer also plays a major role in NESSIE's motor design. Danfoss engineers employed the polymer on all the motor's moving parts, as well. It paid dividends. The

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