This article analyses systems to treat spent machining fluids, which improve productivity by cutting downtime and extending performance life. Machine tool operators are extending the life of their grinding wheels as much as 10 times by processing spent machining coolants with Hydroflow's Star Filter system. Hydroflow Inc., Salem, NH, markets a vacuum-based Star Filter system that provides a large filter area in a relatively small space for industrial applications requiring high flow rate filtration. Hydroflow's engineers designed the Star Filter to compete with rotary drum vacuum filters, which also provide large filter areas in compact space. Machining line operators use the Star Filter as a standalone, central filtration system. Either Hydroflow or the local contractor connects the system to machine tools so the used fluid will pass through baffles in the Star Filter that reduce the fluid's velocity. The successful inaugural installation of the small-scale HTF system at Twin City has opened opportunities for the filtration system in other die casting facilities.
MANUFACTURERS AIU RAISING THE PRODUCTIVITY of their cutting, drilling, and grinding tools, and extending the service life of their associated coolants, by filtering spent machining oils. The concept has spread to die casting operations, where the oil used to transfer heat to the dies is treated by hot oil filtration systems to prevent carbon buildup. Parts manufacturers and die casters that filter their industrial fluids are reducing maintenance downtime and costs, as well as reducing their purchases of new oils.
Hydroflow Inc. of Salem, N.H., markets a vacuum-based Star Filter system that provides a large filter area in a relatively small space for industrial applications requiring high flow rate filtration-as, for instance, in machining Machine tool operators are extending the life of their grinding wheels as much as 10 times by processing spent machining coolants with Hydroflow's Star Filter system. aluminum and cast iron parts for aerospace or automotive manufacturers. At flows of more than 1,000 gallons per minute the filter will catch particles down to about 5 microns in size. At slower flow rates, the Star Filter can remove the smaller particles, of2 to 5 microns, found in glass and ceramic grinding.
Hydroflow's engineers designed the Star Filter to compete with rotary drum vacuum filters, which also provide large filter areas in compact space. Rotary drum filters induce a vacuum to draw spent machine coolant through a slowly rotating, cylindrical mesh that removes particles from the fluid . The filter mesh, typically stainless steel, is cleaned automatically by pressure jets that force contaminants into the dirty part of the system.
The Star Filter uses filter panels arranged in an eight pointed star, a more compact arrangement than either flat beds or rotary drums. "The Star Filter system is about one-half the length of rotary drum filters for 2,000 to 5,000 gallon-per-minute filtration applications," said Thomas Cassesse, executive vice president of sales and marketing at Hydro flow, which also makes rotary drum vacuum. filters.
Machining line operators use the Star Filter as a standalone, central filtration system. Either Hydroflow or the local contractor connects the system to machine tools so the used fluid will pass through baffles in the Star Filter that reduce the fluid's velocity. This permits larger contaminants- over 30 microns-to settle on the drag conveyor. This is a standard piece of equipment consisting of an angle iron chain equipped with drag flights to remove solids for disposal.
A Star In Filtration
A pump draws the fluid through the Star Filter's filtering element, which is made of eight fuse-bonded, stainless steel microscreen panels, set in steel frames, that rotate around a central post. Each filtering element is customized to serve specific applications. For example, the filter panels can be precoated with cellulose fibers that can filter particles down to one micron in size.
As particles accumulate on the filter panels, they form a cake that is compressed by suction. A vacuum switch will sense when the filter cake has grown to the point that it causes pressure resistance, and begins the automatic regeneration cycle.
This step in the process involves opening air-operated equalizer valves located below the raised clean tank. The system pum.ps supply fluid from a clean tank to the machine tools through the equalizer valves. At the same time, a reverse flow of clean fluid is introduced to hydraulically dislodge the filter cake from the panels, a task facilitated by the vertical angle of the filter panels.
By removing filter cake hydraulically, the Hydroflow engineers eliminated the need for wipers or blades typically used to clear rotary drum filters. That's a plus because these mechanical devices tend to push contaminants through the filter surface into the clean coolant and clog the mesh later. In addition, the rigid filter panels are easier to remove for cleaning than the circular mesh in rotary drum filters.
Once removed, the vacuum-compressed filter cake settles in the dirty tank. There, the drag conveyor removes the solids in the form of sludge for treatment by waste disposal methods . The remaining dirty tank coolant is run through the Star Filter for further cleaning.
A problem in many vacuum filtration systems is the migration of dirty fluid into the clean portion of the system. Hydroflow developed a patented wet seal with highly toleranced machine surfaces that is installed between the suction manifold and filter element to prevent this migration. The positive seal eliminates the need for rotary seals, O- rings, or bearings that wear over time and need replacement.
GKN Automotive Inc. of Sanford, N.C. , manufactures the constant velocity joints used in front-wheel-drive vehicles of Ford, Chrysler, Toyota, and other automakers. The company installed two Star Filter systems in 1997. The first was to remove the metal fines from spent watersoluble coolants used in grinding and turning operations. The second unit serves a unique role for a Star Filter: removing carbon from the water-soluble coolant that GKN Automotive uses to quench induction-hardened parts.
GKN Automotive realized dramatic improvement in tool and coolant performance, according to Don Neeley, maintenance manager at the Sanford plant. "We have seen from 100 percent to 1,000 percent extension of grinding wheels' and diamond dressers' tool life since using GKN Automotive realized dramatic improvement in tool and coolant performance, according to Don Neeley, maintenance manager at the Sanford plant. "We have seen from 100 percent to 1,000 percent extension of grinding wheels' and diamond dressers' tool life since using the Star Filter. In fact, we had a grinding wheel whose body broke before the surface finish was worn down," said Neeley. The longer life of machine tools also boosted throughput because machine operators could run them longer. "Machines that ran 20 parts per cycle were able to run 150 parts per cycle," he said. The lathes experienced a 10 to 20 percent increase in service life after Star Filter processed their coolants. Longer service life has reduced GKN Automotive's maintenance downtime and costs.
The filtered coolants also lasted longer at the parts plant. "We were lucky to get two months of service from coolants used at each grinding and turning machine tool's sump," said Neeley. "Now, those coolants last two years because Star Filter processes them." The watersoluble coolant used to quench induction-hardened parts lasted four weeks prior to the Star Filter installation, which extended performance life to 18 months.
Another benefit of the Star Filter, according to Neeley, is its redundant pump and filtering element. This enables maintenance staff to service the units while tooling and quenching operations continue, thus reducing maintenance downtime.
"We estimate that the total annual savings, including reduced maintenance downtime and costs, are $507,000 for the first Star Filter unit, and $250,000 for the second unit," Neeley said.
GKN Automotive is adding a third Star Filter to provide a common filtration system for every grinder and lathe in the plant, some 165 pieces of equipment, according to Neeley. He and his colleagues built a superstructure to accommodate more than 9,000 feet of overhead piping, ranging from 2 to 18 inches in diameter. It will deliver more than 3,360 gallons per minute of coolant, traveling over 22 feet per second, to the Star Filter.
Scaling Down For Die Casting
Filtration is making an impact on the hot end of metalworking, too, in the heat transfer fluids used to heat the dies that cast metal components.
Die casting companies are tripling the life of their heat transfer pumps, reducing pump maintenance downtime and associated costs, and are doubling the performance life of their hot oils by using a filtration system designed by Liquid Process Systems of Charlotte, N. c., and introduced commercially in 1996.
For years, Liquid Process Systems has designed hot oil filtration systems for the huge presses used to form particleboard out of wood chips coated with glue. These six-to seven- story-high presses contain 6- to 8-inch-diameter pipes that circulate hot oil to add heat to the process. Louisiana Pacific uses Liquid Process Systems' hot oil filtration presses at its wood product plants in Minneapolis and in Roxborough, N.C.
" All these filtration systems are designed to treat continuous flow of15 to 200 gallons of hot oil per minute. I did not even consider building a filtration system small enough to serve die casting applications, requiring 5-gallon-per-minute treatment, until I was approached by Don Jenkins of Twin City Casting in 1996," recalled Zak Shums, a mechanical engineer and president of Liquid Process Systems.
Jenkins, the corporate equipment manager for Twin City Die Casting Co. in Minneapolis, attended a seminar in Grand Rapids, Minn., where he heard Shums discuss hot oil filtration for the wood industry. "We custom-cast aluminum parts for a variety of clients-for example, Empire Level tools, IBM computer parts, RCA television sets, and Rosemount gas instruments," said Jenkins.
There are 13 separate heating systems that raise the temperature of oil used to heat dies at Twin City's Minneapolis plant. Each system consists of an electrical heater, oil reservoir, pump, and controls to send a continuous stream up to 500°F oil to the casting dies.
Over time, the hydrocarbon oil breaks down, producing carbon concentrations. Carbon would accumulate on the heat transfer surfaces in the form of coke and sludge, fouling the dies , heater elements, pump elements, and seal faces . The carbon deposits made the pumps' relief valves sluggish and caused their poppet valves to stick.
Twin City had to repair its hot oil pumps as often as every two months, at a cost of about $700 per breakdown in parts and labor. Jenkins told Shums that a smaller version of his company's hot oil filtration system could alleviate the pump problems of Twin City and other die casters.
"We purchased a small-scale filtration system and modified it to work with 650°F oil. This involved replacing the brass internal parts with cast iron and replacing its rubber gaskets with a composite gasket," said Shums. Based on this prototype, Liquid Process launched a smaller system, called HTF, at Twin City.
The new filtration system stands 24 inches tall , half the size of its predecessors serving particleboard presses. The HTF housing is made of carbon steel and contains filter elements consisting of fiberglass wound on stainless steel cores that capture particles down to 10 microns in size.
"Any finer micron filtration could remove needed additives in the oil," Shums noted.
Liquid Process installed the inlets of the HTF filters close to the discharge point of the oil pumps they serve at Twin City, rather than before the system pump, to take advantage of the maximum turbulence of the oil and its maximum discharge pressure. "This fluidizes the contaminants in the oil and optimizes filtration," Shums explained.
About 10 percent of the hot oil flow is diverted through the filter, cleaned, and then reintroduced downstream or to the suction side of the recirculation pumping system as the heating system operates. All of the heat transfer oil is filtered between 15 and 20 times per day.
Pressure gauges were installed before and after the HTF system to measure the pressure differential across the filter, indicating when the filter elements must be replaced as they fill up with solids. Maintenance crews remove the elements and dispose of them as they do other contaminated oil filters. The filters last from three to six months, depending on the cleanliness of the oil. Liquid Process also supplied the diaphragm seals and heat sinks that protect the pressure gauges from the hot oil.
Since the installation of the filtration systems in late 1996, the hot oil pumps at Twin City Die Casting last three or four times as long, according to Jenkins. The oil, which costs about $300 per 55-gallon drum, also lasts longer. "However, the HTF system is only as good as the maintenance system that supports it," Jenkins stressed. "You have to keep an eye on the filters and change them when necessary."
The successful inaugural installation of the small-scale HTF system at Twin City has opened opportunities for the filtration system in other die casting facilities, for example, at Briggs & Stratton Inc., a manufacturer of lawn mower engines based in Milwaukee. This plant die casts the aluminum cylinders, heads, rods, and caps for lawn mower engines used by Toro and Murrey.
"We have installed six HTF systems to filter heating oil in our dies. In a year of operation we have had no pump failure due to coking of oil ," said Jim Rummel, a mechanical engineer and manufacturing project engineer at Briggs & Stratton's Milwaukee facility. "The $700-perunit price of the HTF systems is cheap insurance against such failure," Rummel added.
Liquid Process Systems is aiming the HTF system at filtering heat transfer oil in other industrial applications as well. These include filtering oil used to heat plastic injection molds, silk screening rollers, and plastic film rollers.