The MT2V NC Profile Milling Machine is a CNC copy milling machine manufactured by OER LIKON and imported by our factory. It was installed and debugged in July 1997 and put into operation shortly after. However, starting from January 1998, after the cold machine started its hydraulic lubrication system, oil leakage began to occur at the lower end of the main spindle housing. As temperatures dropped, the oil leakage became more severe, but it gradually stopped after the spindle ran for about 20 minutes. Throughout this period, all machine tools operated normally without any alarm signals.
The MT2V CNC profile milling machine is a heavy-duty machine tool that uses an independent rail automatic lubrication system for isotropic rail lubrication. Its hydraulic and spindle lubrication systems differ significantly from those of the FB2V and FB3V CNC profile milling machines produced by Oricon. The hydraulic and spindle lubrication system is shown in the diagram below.
The hydraulic and spindle lubrication systems share a main oil tank (using HM68 anti-friction hydraulic oil). Motor M1 drives two coaxial gear pumps, 1P and 2P. Pump 1P provides 8MPa pressure oil through check valve 5, direct relief valve 6, and accumulator 8 to the hydraulic system. Each solenoid valve controls the spindle speed, tool chuck clamping, spindle head loosening, and Z-axis clamping. Pump 2P supplies lubricating oil to the headstock via direct relief valve 2 and chiller 1, then distributes it through oil distribution pipes to lubricate bearings and gears inside the headstock. The oil is collected into the return oil pool at the bottom of the headstock and is drawn back into the main tank by pump 3P.
After the main power switch is turned on, pump 3P automatically starts for 20 seconds to draw oil from the sump. When the hydraulic lubrication system is activated, pump 3P runs; when the system is stopped, pump 3P delays for 45 seconds to remove the remaining oil in the sump. The chiller 1 monitors the temperature of the lubricating oil flowing through it. If the oil temperature rises to a set point, the chiller activates to cool the oil. If the temperature drops below the set value, the chiller stops.
8. Accumulator 9. Hydraulic Selector Switch 10. Oil Pressure Gauge
Upon analyzing the fault phenomenon and the hydraulic lubrication principle, we concluded that the issue was not due to a high-pressure hydraulic system leak, as the machine operated normally without any alarms. The problem was likely in the spindle lubrication system. Since the machine was still under warranty, foreign maintenance personnel replaced pump 3P twice, but the issue persisted. As temperatures rose, the oil leakage gradually decreased and eventually disappeared. However, during the winter of 1998, the problem reappeared.
By reviewing the hydraulic lubrication data, we found that when the hydraulic selector switch 9 is set to "2P", the oil pressure gauge 10 should show 0.5–1 MPa. During a cold start at 6°C, the oil pressure in the "2P" mode was 1.8 MPa, which dropped as the spindle ran and the oil temperature increased. Once the oil reached 25°C, the chiller started, and the pressure dropped to 0.7 MPa. As the pressure decreased further, the oil leakage reduced and eventually stopped.
The oil temperature fluctuated between 18°C and 26°C, with the oil pressure varying between 0.7 MPa and 1.3 MPa accordingly. Based on these observations, we realized that the oil pressure adjustment during installation in summer needed to be adjusted again in winter. Lower ambient temperatures increased the viscosity of the lubricant, reducing the return oil flow from the pump when the system started cold, leading to excessive oil supply and oil leakage.
From the relationship between oil temperature and pressure, we understood that the specified "2P" oil pressure range of 0.5–1 MPa in the data should not be interpreted as a static setting, but rather as a dynamic range that fluctuates with temperature (always above 0.5 MPa). Setting the pressure too low could lead to insufficient lubrication and potential equipment failure, while setting it too high could cause excess oil supply during cold starts, resulting in oil leakage.
After adjusting the oil pressure higher, the relationship between oil temperature and pressure during normal operation improved. When the cold start system was activated, the oil pressure in the "2P" mode was 1.3 MPa. This adjustment ensured that the oil pressure matched the temperature changes, preventing oil leakage.
In summer, adjusting the direct relief valve 2 had little effect on the lower limit of the oil pressure, especially when the spindle was running at high speeds and the oil temperature was maintained at 29°C, causing the chiller to stop. At this point, adjusting the relief valve did not change the oil pressure. After the adjustments were made, the oil leakage problem has not occurred again in the past two years, and the machine operates smoothly.
8. Accumulator 9. Hydraulic Selector Switch 10. Oil Pressure Gauge
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