This article highlights that growing environmental and safety concerns, abetted by technology changes, are fueling a resurgence of water as a power transfer medium in hydraulics, where for years oil has been the fluid of choice. The shift has been gradual, but steady, after starting five years ago or more in Europe. In North America, progress has been slower. The traditional water hydraulics markets, steel and aluminum mills and auto assembly plants, have seen a little or no change. Water hydraulics is very attractive because it can be used for more than one purpose. It can turn a wheel, lift a load, or push something and, at the same time, provide fire suppression for an individual machine or an entire space. It can provide humidification to eliminate static electricity in a printing plant, or it can run a food processing plant during the day and become part of the clean-in-place system at night.
Growing environmental and safety concerns, abetted by technology changes, are fueling a resurgence of water as a power transfer medium in hydraulics, where for years oil has been the fluid of choice. The shift has been gradual, but steady, after starting five years ago or more in Europe.
In North America, progress has been slower. The traditional water hydraulics markets, steel and aluminum mills and auto assembly plants, have seen little or no change. Bu t new nurkets, such as food processing and nuclear power, are stirring.
"Five years ago, water hydraulics wasn't a blip on most engineers' radar screens," said Lyle Ruble, North American manager for high-pressure water technologies at Danfoss Graham in Milwaukee. "Eventually, I think water will be mandated in many areas where oil has been used."
Plant safety has been the major incentive fueling the choice of water over oil. Even with its higher implementation costs, water is still cheap fire insurance. Now product safety is joining plant safety as a factor in certain industries.
"Historically, food and pharmaceuticals have been driven by the need for product purity," said Chuck Johnson, general manager of the water hydraulics group at Elwood Corp. in Oak Creek, Wis. "By eliminating oil, we eliminate a potential source of contamination."
Water offers a slew of advantages. It is plentiful and cheap, and disposal is no problem. Leaks evaporate without leaving a sticky, slippery residue.
"Water hydraulics is very attractive because it can be used for more than one purpose," Ruble pointed out. "It can turn a wheel, lift a load, or push something and, at the same time, provide fire suppression for an individual machine or an entire space. It can provide humidification to eliminate static electricity in a printing plant, or it can run a food processing plant during the day and become part of the clean-in-place system at night."
More than just Vanilla
Water systems come in many flavors-natural (tap, processed, or seawater), treated (de-mineralized, deionized, de-salted), and water with additives (oil in water, water in oil or invert emulsion, water in glycol, or synthetic solutions). The amount of water in these variants can range from 40 to 100 percent, and these mixes account for perhaps 75 percent of the applications today.
Some feel the future is tap water, but that issue is still being debated. Pure water systems lack the lubricity of mixes. They require rust-resistant materials that drive up costs and, typically, need some level of bacterial control, such as a biocide, ultraviolet light, pasteurization, or a sealed system. Untreated water is also a relatively poor lubricant and additives are often enlisted to alleviate this problem and keep down hardware costs.
"When freezing protection is required, water glycol is a good choice," declared Gary Bahner, vice president of engineering at Milwaukee's The Oilgear Co. "Water glycol, which is typically 40 percent water, retards corrosion and is compatible with conventional materials. The hardware may run 10 to 50 percent higher than a comparable oil system, but that's considerably less than a plain tap water system, which requires stainless steel, special plastics, ceramics, or other costlier materials that can raise costs 400 percent or more."
During much of the past century, water was no match for oil because of the erosion and corrosion problems water creates. It has a high vapor pressure, 107 times greater than oil, so that water-based systems are highly susceptible to cavitation, which leads to erosion of internal surfaces. Water also is much less viscous than oil and can seep through smaller cracks, making it harder to control leakage.
"With oil valves, leakage is normal and is often designed into the product," explained Elwood's Johnson. "But with water, internal leakage is not normal; it causes erosion and degradation. At thousands of psi, it can destroy parts and seals. To prevent this, water valves must be 'drop tight'- no internal leakage paths."
Water-resistant materials-plastics, ceramics, stainless steel, and coatings-together with tighter tolerances and changes in component design have been successfully tackling erosion problems. A carbon-impregnated plastic known as PEEK, or polyetheretherketone, which forms under pressure, has been adapted by many manufacturers in an effort to reduce clearances between moving parts.
Coatings, self-lubricating plastics, and additives have been enlisted to compensate for water's low lubricity. And roller bearings have been eliminated from pumps and motors and replaced with hydrodynamic bearings.
Still the Higher-Price Spread
While water is cheap, water hydraulics components are not. A typical water hydraulics system costs at least three times as much as an oil system of equivalent performance. Higher costs have kept water hydraulics out of mobile equipment and many food applications, except areas that cannot afford any oil spillage.
It's a bit of a Catch-22. Parts are expensive because they haven't been widely used, and they aren't as widely used, in part, because they are expensive. Many of the materials required to handle erosion and corrosion are also on the pricey side.
Then there is the educational issue. Water is a relatively new medium for most users, especially in the United States.
"You have to walk almost every user through his first application, and that adds cost," Ruble explained.
The Milwaukee-based National Fluid Power Association, a trade organization of more than 235 manufacturers of hydraulic and pneumatic products, has been actively working the educational issue with educational materials, conferences, and seminars. The organization has also been instrumental in the establishment of industry standards.
Some components, long available in oil systems, do not exist for water-for example, large, high-pressure, variable- displacement pumps. But the gap is closing. The Oilgear Co. has developed a series of high-pressure, variable- displacement pumps, suitable for tap water, water/glycol, and 95/ 5 or 98/ 2, capable of impressive volumes and pressures. Pumps delivering 200 gpm and 400 gpm at 3,500 psi are in operation, with perhaps larger units looming in the future.
"With a variable pump, you can shut off flow if you don't need it," said Keith Dell, sales manager at Oilgear's British office, Oilgear Towler, in Leeds, England. "It can replace a multistage centrifugal pump that wastes a lot of energy during idle times."
A few manufacturers offer proportional valves and this year Elwood Corp. will introduce the first cartridge-type proportional pressure and flow control valves that can be used with tap water. The series, which ranges in size from 3/ 4-inch to 4 inches, will handle up to 420 bar.
Currently, Elwood Corp. is the only manufacturer of a water servo valve, but later this year, Danfoss will introduce its version, one that the company claims will offer control equivalent to an electric stepper servo.
Under a joint effort with a Finnish firm, the company is also developing a water filled gear reducer. The reducer will use no oils or grease, making it an ideal option for food applications.
"There are two products we want desperately," Ruble said. "One is a true adjustable flow divider that will provide constant flow regardless of demand. The other is a counterbalance or load-holding valve that will lock in p lace when you raise a load on a crane or cylinder. It could also be used to brake a small tractor going downhill. Currently, we are looking for partners at universities that are willing to do basic research on these product lines."
At Purdue University in West Lafayette, Ind., water hydraulics research work is being supported by Danfoss, which has contributed hydraulic parts, and by Swagelok of Solon, Ohio, a manufacturer that markets filters, fittings, and other fluid systems components.
One of the projects at Purdue is a water-based transmission that the lead researcher, Gary Krutz, hopes will find its way eventually into the automotive industry.
A water-based transmission could yield significant energy savings. Water is 30 times less viscous than oil, so it is easier to pump and pressure losses are much lower. As a result, smaller pumps and motors, as well as less power, are needed to push fluid through the lines. Krutz estimates that a water-based transmission will use 10 to 30 percent less energy than a conventional transmission.
Water viscosity also is relatively insensitive to temperature so a water-based transmission would be more stable than oil over a wider temperature range, and water's higher conductivity makes it easier to dissipate heat
"You can make flow in an oil system laminar," explained Krutz, "but when you move water in any kind of confinement, you have turbulent flow because of the low viscosity: 1 centistoke versus 40 centistokes for oil. If you don't design the boundaries and materials to handle that turbulence, you have erosion . Using techniques, such as CFD, you can design a unit in which the eddies are forced to occur in the middle of the stream, and the turbulence never hits any boundary surface."
To be successful, a water transmission will have to be inexpensive. Krutz feels some of the newer powder metallurgy techniques may hold the answer.
"You can make parts with multiple hits," he said. "The first hit creates the rough shape, then you add a second material to the die and form it again, producing a very hard surface on the outside."
The design uses plain tap water, so algae control is a problem. Krutz believes the solution may lie in filtration.
"Most bacteria is greater than 1 micron diameter and cola, beer, and bottled water processors have been filtering to that level for years," Krutz noted. "However, there is no long-term continuous usage data from these applications."
Krutz admits that boosting reliability will not be easy. "The car transmission has been developed over a long time, so it lasts for 100,000 miles or more," he explained. "The initial water transmissions would be best suited for a vehicle that doesn't accumulate as many miles as a car, such as lawn or golf machinery or a fuel cell-powered vehicle." The makeover will use Danfoss water hydraulic components, Swagelok fittings, gears from Fairfield Manufacturing in Lafayette, Ind., and cylinders made by Parker Hannifin of Cleveland.
Last year Krutz demonstrated the viability of the basic concept and this year is supervising a student project that will modify a Jacobsen commercial lawn mower, donated by Textron Golf, Turf & Specialty Products in Racine, Wis.
It will be tested just about the time this issue goes to press. The steering and the transmission of power to the wheels and mowing reels will operate on plain tap water. Oil will be used only inside the engine.