When a pork processing plant in Denmark recently installed a new hydraulic
cutting machine, no one worried that hydraulic oil would leak into the meat. The
company's seeming lack of concern over contamination wasn't misplaced, however.
Its new machine doesn't use hydraulic oil. Instead, it operates off tap water.
For the machine's designer, the decision to go with water hydraulics, rather than more widely accepted alternatives, is at the cutting edge of a self-renewal trend. Throughout the long history of hydraulics, the oil vs. water debate has intermittently resurrected itself, most recently with the introduction of a raft of effective, new water hydraulics components.
In the past, water-based systems have earned only a grudging respect from the engineering community. During the last half century of its 200-year history, water hydraulics has been viewed mostly as a curiosity-an outdated technology driving creaky machinery.
No pork barrel. But advances in water-based components are changing that. At SFK Meat Systems in Denmark, the decision to go with water hydraulics was one based on new technical criteria, rather than traditional beliefs. The machine's engineers selected Danfoss' NESSIE water hydraulics system over more traditional alternatives, such as pneumatics or electric motors. "We moved away from air-driven machinery because of the cost of running it and the noise," explains Erik Petersen, manager for trade and service at SFK Meat Systems, designer of the new machine.
Pneumatic versions, Petersen explains, typically exhibited noise levels of 88-89 dBA, compared to as little as 80 dBA on the water hydraulic system. Similarly, SFK steered clear of electric motors, because of the large package size needed for the saw's 3-hp requirements.
Experts expect food processing to be one of the biggest new applications for water hydraulics. Unlike oil-based hydraulics, water systems present no potential for contamination of food. As a result, they also hold appeal for the pharmaceuticals industry.
The new hope for water hydraulics extends into a number of other areas. Hauhinco Maschineenfabrik, Sprockhovel, Germany, has sold water-based systems to a variety of new customers, including a German luxury-car manufacturer, which uses it in welding robots. Hytar Oy, Tampere, Finland, has applied its water-based systems to seawater ballast pumps, diesel emission-control systems, and water cutting power packs for the paper industry.
Water hydraulics fell out of favor early this century when engineers developed oil-resistant seals. From that point on, oil hydraulics dominated. Water, meanwhile, came to be thought of as a cost- ly alternative with inherent technical problems.
That was then. That characterization was accurate-at one time. Water presented a host of tough technical challenges for engineers. Among them: low viscosity, poor lubricity, corrosiveness, freezing, and fluid loss at high temperatures. As a result, component manufacturers found it difficult to maintain seal integrity. They also struggled to design state-of-the-art pumps and valves and to prevent erosion of internal surfaces. Though most of those problems were solvable, the cost of the solutions was, in many cases, three to four times as high as those of oil hydraulics.
That's why, for the past 40 years, water hydraulics has centered on a select few 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 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.
Develop new pumps. Development of inexpensive, pressure-compensated pumps would eliminate the need for large and costly central accumulator systems. As a result, water hydraulics could move into applications now reserved for smaller systems.
Of the remaining work, however, the most important may be the reduction of system costs.
Still, some manufacturers report that customers are finding economic advantages in water hydraulics, despite the high initial costs. A German carmaker, after performing a cost analysis, reportedly selected water hydraulics over oil and pneumatics because of the lower overall costs. When considering operational costs, the carmaker said, water hydraulics can be a bargain.
Based on such appraisals, water hydraulics manufacturers are quite optimistic that they can recapture their former prominence.