HYDRAULICS Large machine tools and universal milling-boring machines use electrohydraulic systems to counterbalance the weight of the saddle and spindle during vertical movements. This counterbalance ensures smooth and precise motion control for the electromechanical driving unit. Two hydraulic approaches are typically used for this purpose: the active circuit with a power unit, and the closed circuit with accumulators. In an active circuit, three-way highperformance pressure-reducing valves equipped with onboard digital electronics manage closed-loop pressure control in counterbalance cylinders. A pressure transducer built into the valve's pressure ports enhances accuracy. This system requires a power unit with a variable displacement piston pump to generate line pressure. The Atos Smart Tuning feature optimises the valve's dynamic performance, ensuring stable pressure regulation, even during variations in headstock load, with response times as fast as 15 milliseconds. The closed-circuit system, often used in high-speed counterbalance operations, leverages accumulators to store and release large volumes of fluid with minimal pressure variation. This lowpower system includes a PED-certified safety control block with integrated functions such as a shut-off valve, pressure gauge, leak-free pressure relief valve, and manual override. Stick-slip in hydraulic cylinders In machine tool counterweight systems, stick-slip refers to irregular movement caused by adhesion between cylinder seals and sliding surfaces. This effect occurs during the transition from static to dynamic friction, resulting in vibration when the cylinder rod first moves. Stick-slip is often linked to the microstructure of cylinder housing surfaces and intermolecular forces between the sliding components. Over time, it also contributes to accelerated seal wear. G9 piston seals The G9 sealing system, was developed by Atos R&D by conducting software simulations, test bench trials, and collaboration with specialised suppliers and European OEMs. These efforts identified a polymer compound that significantly reduces the difference between static and dynamic friction coefficients, addressing the root cause of stick-slip. Eliminating stick-slip not only improves operational smoothness but also extends the lifespan of the hydraulic cylinders. Field tests show that the reduced friction provided by this solution leads to better dynamic performance and improves the efficiency of the milling head’s vertical motor by lowering power consumption. The G9 system incorporates a singleacting seal with a specially designed lip that ensures zero leakage across its full operating range. This is particularly beneficial in closed-circuit systems with accumulators, where continuous leakage would otherwise necessitate frequent oil refilling. The G9 seal also prevents the milling head from descending when electrical power is lost, a problem seen in conventional systems due to leakage. The design of the G9 seal, without an energised O-ring, provides additional stability by preventing the seal from tilting in its groove. The lip design improves low-pressure sealing and contributes to the hydraulic system's overall dynamic stability. Atos hydraulic cylinders with the G9 sealing system, have been widely adopted in the milling machine industry due to their operational consistency and durability, which have been demonstrated through extensive research and field testing. For further information please visit: https://www.atos.com/en-it The performance of machine tools can be compromised by the stick-slip effect, a common issue in hydraulic systems. The G9 sealing system, addresses this problem effectively, eliminating stick-slip in both active hydraulic circuits and closed circuits with accumulators. 18 HYDRAULICS & PNEUMATICS October 2024 www.hpmag.co.uk Preventing stick-slip in hydraulic cylinders: The role of advanced sealing systems Hydraulic cylinder for counterbalance machine tool
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