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Coolant-management system provides more than filtration.

Coolant-management system provides more than filtration

The coolant-management system at New Hampshire Ball Bearings (NHBB), which was designed and manufactured by Hydroflow Inc, Chelmsford, MA, exemplifies how the latest generation of filter systems can benefit machine-tool users. NHBB is a principal subsidiary of Minebea Co Ltd, and a manufacturer of precision bearings.

With the increase in market demand for large-diameter bearings used in the aircraft-engine and machine-tool industries, NHBB's Peterborough facility has increased its production capabilities of precision ball bearings ranging in size from 1 inch to 6 inches dia. A desire to supply the end user with a better product, with more consistency, and at less cost was the prime motivating force during the shift in production from miniature ball bearings to large bearings. With this change came the realization of a need to address total coolant-management requirements of the grinding area.

Need for coolant management

Precision grinding is necessary to obtain user diameter specifications of within 1 or 2 ten-thousandths of an inch. And, to meet customer needs, surface smoothnesses of 2 CLA or better must be obtained on materials that include 52100 bearing steel, M50 high-temperature tool steel, and BG42 high-temperature stainless tool steel.

As the first consideration, meeting specifications for tolerances and surface characteristics on large workpieces demands careful coolant-temperature control. When just the heat from holding a bearing race in your hand is enough to throw it out of tolerance, having the coolant temperature within 2 degrees of the ambient temperature becomes critical. This is particularly important when the ambient temperatures within the grinding area can vary by up to 30F during a working day.

Originally, most grinders used in production of large-diameter bearings were equipped with settling tanks. Some were outfitted with cartridge filters. Periodically, we replaced the cartridge filter elements. At that time, the settling tanks were cleaned and replenished with fresh coolant. Maintaining coolant temperature in a machine tool's settling tank is extremely difficult. Oil loses its absorbed heat very slowly, causing the tank temperature to rise steadily over the course of the day.

Settling tanks provide filtration levels of only about 50 microns, leading to scratches on work surfaces. Operators of machine tools equipped with settling tanks continually must use trial-and-error methods to obtain precise tolerances and surface-finish requirements. This results in high scrap rates and rework levels.

A significant need for our planned-coolant management system was the ability to remove small solid contaminants from water soluble coolant, synthetic coolant, or grinding oils. While most machine tools were going to be converted from oil to water-soluble coolant, the flexibility to use any coolant was necessary to accommodate future changes in production requirements.

A single, centralized filter system offers overall monitoring and control and can facilitate coolant conditioning such as tramp-oil removal and coolant make-up and mixing. The reliability of a single filter, however, can jeopardize production schedules, and one large unit limits flexibility and expandability. NHBB wanted the benefits of a large coolant filter, without sacrificing flexibility or ensured continuous operation.

Zone concept

The first critical step toward designing a coolant-conditioning system that provided flexibility and control without the drawbacks of a large, single filter was to separate our needs into identifiable, definable groupings. The 70 grinders in the grinding area were categorized into eight groups, or zones. Each zone consisted of machine tools with similar filtration needs, coolant-conditioning needs, and location on the production floor. A relatively small individual filter for each zone could provide the particular type and level of filtration currently necessary, but also allow for structural change to meet future needs without major expense.

Although eight zones were designed as separate entities, they had to be integrated for system-wide monitoring and control. A master control system was necessary to tie operation of each zone into the system and oversee coolant conditioning. For example, if Zone Filter 4 required servicing during production hours, the coolant-management system had to be capable of transferring the filtration of coolant used by the machine tools in Zone 4 to the filter equipment handling Zone 5. Likewise, the system had to be designed to meet dissimilar needs, zone by zone, for coolant-temperature control, tramp-oil removal, and coolant make-up and mixing.

Using the zone concept in the design stage and working closely with the equipment manufacturer, NHBB put the coolant-management system into operation in March 1987. Since then, the benefits of proper filtration, control, and flexibility have become evident throughout the grinding area.

Coolant collection and transfer

The machine tools in the grinding area were rearranged into eight zones and settling tanks on individual filters were replaced with a system of in-floor coolant collection sumps. One initial benefit was that large areas of floor space became available for immediate production use or future expansion.

PVC piping is used to collect contaminated coolant from each grinder and direct it by gravity flow to a common-zone coolant-collecting sump. Each of the eight in-floor coolant-collecting sump stations includes a main and standby submersible sump pump. The pumps and level controls installed in the sump are monitored and controlled by the coolant-management system. From each zone, contaminated coolant is pumped through overhead piping to the filter room, where coolant is directed either to the corresponding zone filter or, by opening and closing two valves, into a manifold to be redirected to a different filter. The capability of directing the flow of coolant from its assigned filter to another zone filter allows equipment maintenance and servicing without interrupting production.

Solids removal

Seven of the eight zones are served by floor-mounted vacuum filters. These filters are sized to handle about 10 gpm/sq ft of filter media. All seven vacuum filters have been sized to handle different zone requirements. They contain between 12 and 24 sq ft of filter media and handle coolant flow rates up to 250 gpm, depending on the type of coolant used.

A vacuum filter removes solid contaminants down to 10 microns by drawing liquid from the dirty holding tank through a non-woven, synthetic-fabric filter supported on a perforated steel plate. Solids that become trapped on the filter media form a filter cake, which in turn traps additional solids.

As solid contaminants build up on the filter media, the pressure drop across the filter increases. When the pressure drop reaches a predetermined level, the filter automatically goes through a regeneration cycle. During regeneration, the filter/system pump is deactivated, and a dragout conveyor within the filter advances a short distance, bringing fresh media with it. Spent media are automatically discharged from the filter and collected in a self-dumping sludge hopper. While the filter is going through a regeneration cycle, clean coolant is pumped from the clean-coolant holding tank, thereby providing a continuous supply of clean coolant to the machine tools.

Using vacuum filters is a significant improvement over individual machine-tool coolant-settling tanks. Their ability to remove solids down to 10 microns facilitates the meeting of specifications for size and smoothness. Disposal costs have gone down because the nearly dry sludge from the vacuum filter is conveniently handled and allows more solids to be packed into 55-gallon drums for disposal.

The eighth filter in the system is a precoat filter, which has been designed to remove all solids down to one-half micron from 66 SSU oil at a flow up to 150 gpm. Ultra-fine filtration such as this is required for the super finishing machine tools in Zone 8 and is accomplished by pumping the contaminated coolant through diatomaceous earth, which forms a filter cake around a series of perforated stainless steel tubes. As solids are trapped within the filter cake, they enhance the filtration process. When the pressure drop across the filter reaches a predetermined point, the filter automatically enters a regeneration cycle. The backwashed diatomaceous earth and contaminants are removed from the filter in nearly dry form for easy disposal. During the regeneration cycle, clean oil is pumped from the filter's clean-oil reservoir for uninterrupted machine-tool operation.

Following filtration, each zone filter pumps clean coolant directly back to the machine tools through overhead cast-iron piping or, by opening and closing two valves, into a manifold located in the filter room, from which it can be redirected to a different zone. In the event that filter servicing is required, the option of being able to pump clean coolant from one zone filter to another grinding zone allows for continuous production.

Coolant conditioning

The coolant-management system has been designed to monitor and control coolant temperature, to remove tramp oils, and to control coolant concentration levels.

An air-cooled, closed-loop, ethylene glycol chiller is used for cooling. It is located outdoors, adjacent to the filter room. From a single outlet of the chiller, manifold piping is used to supply individual tube-type heat exchangers positioned on top of each zone filter. At each filter, an automatic valve on the inlet of the heat exchanger opens and closes in response to varying zone-cooling requirements. Ambient temperature and clean coolant are monitored individually at each zone. Adjustments to meet cooling requirements are performed automatically.

A high-speed centrifuge removes tramp oils. This extends coolant life by eliminating the major cause of coolant rancidity. Coolant for processing is directed from the clean-coolant header of any one of the zone filters to the centrifuge through a system of pipes and solenoid valves. Because tramp-oil-contamination levels vary from zone to zone, depending on the type of grinder in the zone, processing time varies widely. Several zones need tramp-oil removal-processing only once a week, while other zones require daily processing.

Controlling coolant-concentration levels and adding deionized makeup water is an important function of the coolant-management system. Concentration levels in each zone are monitored carefully, and adjustments can be maintained easily--even though the task is complicated by using two types of coolant. A water-soluble coolant is first added to the coolant, followed by the addition of synthetic or semisynthetic coolant.

A 1500-gallon coolant holding tank is located near the coolant-mixing station and allows for the testing of different types of coolants and concentration levels. The holding tank also allows temporary storage of a zone filter's coolant while maintenance or servicing is performed. This saves the cost of disposing of coolant and replacing it when the servicing is complete.

System control

The operation of the coolant-management system is fully automated by a single computerized control located in the filter room. Every facet of coolant flow, coolant condition, and equipment operation is monitored and controlled, zone by zone, as an integrated system.

A video-display screen on the control panel offers system-status reports in graphic form and aids in troubleshooting, system diagnostics, and servicing. For example, if the liquid level in the contaminated-coolant collecting sump of Zone 6 reaches a high level, an alarm will sound and the standby pump in the collecting sump will begin pumping. Without shutting down the entire system, the operator can query the user-friendly system using a panel-mounted keyboard and menu-driven software. The problem can be quickly identified and the automatic system response confirmed. Also, step-by-step maintenance procedures can be displayed. A printer is connected to the computerized control system, providing a record of all system functions and activities.

PHOTO : Precision ball bearings ranging in size from 1 inch to 4 inches dia produced by machine tools tied into the latest generation of coolant-management systems.

PHOTO : Clean coolant can be pumped directly back to machine tools or into a manifold for distribution to a different grinding zone.

PHOTO : New Hampshire Ball Bearings maintenance supervisor, Robert Smith, reviews a computer printout that provides coolant-management-system operating data.

PHOTO : Filtration room houses vacuum filters (foreground) and precoat filter.
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Bozogan, John W.; Dechene, Brian S.
Publication:Tooling & Production
Date:Apr 1, 1989
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