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Modern machine tools are complex, high tech systems. While their chip producing components, including the control system and process software, attract the most interest, the ‘subservient’ components are usually overlooked. These components address duties including conveyance, filtration and conditioning of cooling lubricants (CL). In systems that do this continuously, trouble-free operation is primarily ensured by centrifugal pumps. Solutions reports.
For economical and ecological reasons, today's production plants need to keep their CL resources in good condition. Machining processes such as turning, drilling and milling generate larger chips than those produced by grinding and honing and the kind of material being machined also affects the chip geometry – for example, tough, high alloy steel behaves differently than cast iron or aluminium alloys. Two practice relevant properties are dependent on this influence: a propensity for snarled chips and contamination of the CL, mainly by small and minute particles.
In terms of specific weight and viscosity, cooling lubricants consisting either of water and oil or of water and chemical solutions differ from those of pure oil. Chemical additives also play an important role with regard to conveyance, filtration and cleaning.
For CL as an auxiliary medium and resource, the importance of proper handling is evidenced by the fact that modern machine tools have separate supply systems and that those systems are designed and built by specialised companies. Such systems have an impact on the nature of the chip disposal and CL conditioning process, hence influencing the quality of the machined parts.
Knoll Maschinenbau GmbH in Bad Saulgau, Germany, is well established as a supplier of conveying and filter systems. Since 1970, Knoll has worked in close cooperation with German pump manufacturer KSB AG because these technical factors play an important role in pump design.
Managing director Matthias Knoll describes one of the company's latest developments this way: "Our high performance band filters for cooling lubricants, for example, can save users as much as 60,000kWh of electricity each year. KSB’s centrifugal pumps equipped with frequency controlled motors contribute directly to that achievement.”
Customised filter systems and pumps
The needs and objectives of machine tool users have changed in recent years and modern machine tools are making parts ever more quickly and, therefore, produce more chips per unit of time. Also, as speed and accuracy requirements become more stringent, the nature of the chips also changes. More small particles are produced and different materials are coming into use. As the cooling lubricant picks up most or at least some of the chips as they pass through the machine, its composition changes significantly.
It is important for both the machine operator and the workshop environment to be protected against gaseous and liquid emissions and modern filter systems must address these issues.
"We differentiate between two basic types of system – VRF rotating vacuum filters and VL vacuum filters," explains Michael Pfeiffer, design team leader for screw pumps and centrifugal pumps at Knoll. "VRF filters collect chips from the CL on a stainless steel fabric filter stretched over a drum. A centrifugal vacuum pump transfers the CL through the fabric and into the drum. In an air assisted VL vacuum filter, the contaminated CL is pumped onto a bevelled band filter, below which a filter chamber with an air compressor generates vacuum conditions.
“Together with the hydrostatic pressure of the chip contaminated fluid on the filter, this creates a major difference in pressure between the two sides of the filter surface, which causes the CL to pass through the filter belt,” he continues. “Although VL systems are mainly intended for use in grinding processes, VRF systems are very good for a comparatively large range of machining processes on lathes, drills and milling machines.”
Depending on the application, a filter system is used either for emulsions or oil. In terms of specific weight and viscosity, emulsions – mixtures of mineral oil and water – are similar to a mixture of water and chemicals, while cooling and cutting oil is of higher viscosity and lower weight. The pumps and filters must always be designed specifically for the application.
In the case of the pumps, this applies both to the hydraulic components and the seals. This is especially relevant in selecting seals, as due attention must be paid to several important factors, such as the type of material being machined and, above all, the size and concentration of the chips in the CL to be filtered. The normal chip load range is approximately around 100mg/l and particle sizes are below 50µm. "However," as Mr Pfeiffer explains, "the centrifugal pumps we use are among the most robust components in the entire CL cycle."
That is a positive point for both the engineering contractor and the user, because a single VL filter system includes three or four centrifugal pumps. A self-priming centrifugal pump, such as the KSB Etaprime with a mechanical seal, transfers the chip-laden CL from the machine tool's sludge tank to the filter. At various points around the system, KSB Etanorm pumps handle partial flows of CL and ultimately, the cleansed CL is transferred back to the supply tank by a KSB Etabloc pump. This pump is also ideal for moving the CL from the supply tank to the machine tool.
The potential and relevance of energy efficiency are best explained by the largest central filter system to have been built by Knoll Maschinenbau. It serves around 50 machine tools that together require up to 14,000l/minute of emulsion and four VLX vacuum filter systems remove all the metal particles from the fluid down to a size of 30µm. Each of these four VLX systems has a capacity of 5,000l/minute so one system is redundant and serves as a backup. The same applies to one of the three KSB pumps in the closed circular supply line.
The decisive efficiency aspect is that the pumps have frequency-controlled motors and therefore only consume the exact amount of electricity they need at their respective operating point along the system curve. For each individual VLX filter system, Knoll has calculated annual energy savings of up to 60,000kWh.
The frequency-controlled pumps in the closed circular line account for a major share of the savings on energy. For KSB, it is matter of principle for the pumps to be adjusted to their optimum operating points. In this respect it is essential to take the impeller diameter into account in the energy consumption calculation. The pump specialist must ensure that the impeller diameter exactly matches the operating point required.
In order to account for all relevant circumstances, KSB has developed the EasySelect pump configurator software and database that can be used by its partners. This ensures that such application specific criteria as pumped fluid, operating point, cost of energy, NPSH (Net Positive Suction Head), shutoff head, purchase cost and volume flow to allow for future applications are accounted for in pump selection. For the users, this means that they get exactly the optimal hydraulic system they need for their respective requirements. Motor efficiency also has a positive impact, if only to a linear degree.
“Energy efficiency,” explains Mr Pfeiffer, “is equally relevant to small scale applications. Demand driven control is not just a question of absolute power – it can be worthwhile for pump ratings as low as 4kW. It all depends on the operating conditions such as changes in volume flow or on the size of the tank. Pumps serving small vessels have to cut in and out frequently, so a frequency inverter can control the pump's output as a function of the fill level. That saves a lot of electricity.”
Today's trend toward energy saving solutions is buoyed by a number of factors: frequency inverters have become much less expensive, and manufacturers like KSB have increased the energy efficiency of their pumps. In addition, users are attaching more importance to a holistic approach to purchase and operating costs (total cost of ownership) and life cycle costs. Lastly, cost considerations have been joined by enlightened attitudes to the use of energy and resources in general. All in all, this is reason enough for manufacturers like Knoll and KSB to dedicate even more time and energy to the subject of energy efficiency.