Performance Regulation and Fault Diagnosis of Machine Tool Hydraulic Systems

I. Core Composition and Working Principles of Hydraulic Systems

1. Basic Component Units

• Power components: Hydraulic pumps (gear pumps, vane pumps, piston pumps) convert electrical mechanical energy into hydraulic energy, with an output pressure typically ranging from 3 to 31.5 MPa and a flow rate of 5 to 200 L/min. Their performance directly determines the upper limit of the system's power output.

• Execution components: Hydraulic cylinders (for linear motion) and hydraulic motors (for rotational motion) convert hydraulic energy into mechanical energy. For example, the tailstock 顶紧 of horizontal lathes and spindle tool release in machining centers are both driven by hydraulic cylinders.

• Control components: Include pressure valves (relief valves, pressure-reducing valves), flow valves (throttle valves, speed control valves), and directional valves (directional control valves, check valves). They achieve precise control of execution components by regulating pressure, flow, and direction.

• Auxiliary components: Oil tanks, filters, coolers, pipelines, etc., responsible for storing, purifying, dissipating heat, and transmitting hydraulic oil. The filtration accuracy of filters is usually 10-20 μm to prevent component wear caused by oil contamination.

2. Working Characteristics

• Pressure is positively correlated with load (P=F/A). The maximum working pressure of the system is set by a relief valve to avoid overloading;

• Flow determines the movement speed of execution components (v=Q/A). Stepless speed regulation can be achieved by adjusting the flow through a speed control valve;

• The oil temperature must be controlled within 30-55°C. Exceeding 60°C will cause a decrease in oil viscosity and accelerated aging of seals.

II. Performance Regulation Technology of Hydraulic Systems

1. Pressure and Flow Matching

• Pressure regulation: According to the load requirements of different execution actions, multi-stage pressure control is achieved through pressure-reducing valve groups (e.g., 8 MPa for tool change actions and 16 MPa for clamping actions) to reduce energy loss. System pressure fluctuations should be controlled within ±0.5 MPa; otherwise, action accuracy will decrease.

• Flow optimization: Proportional flow valves are used to achieve continuous flow regulation, matching the movement speed of execution components with the machining rhythm. For example, the flow rate can be adjusted to 50 L/min during rapid movement of the tool magazine and reduced to 10 L/min during positioning to reduce impact and energy consumption.

• Power matching: Pressure-compensated variable displacement pumps are used to achieve "on-demand oil supply". The pump output flow automatically adjusts with changes in system pressure (flow decreases as pressure increases), saving 30%-40% more energy than fixed-displacement pump systems.

2. Dynamic Performance Improvement

• Response speed control: By shortening the length of control oil circuits and selecting large-diameter directional valves (10-20 mm diameter) to reduce hydraulic resistance, the commutation response time of execution components is controlled within 0.1-0.3 seconds, meeting the requirements of high-speed tool changes.

• Buffering and unloading: Buffering devices (such as throttle buffers and overflow buffers) are installed at both ends of hydraulic cylinders to reduce impact pressure at the end of movement; during non-working states, unloading valves are used to reduce system pressure to 1-2 MPa, reducing power loss and heat generation.

• Synchronization control: For multi-cylinder linkage mechanisms (such as dual-drive hydraulic cylinders of gantry milling machines), synchronous valves or electro-hydraulic proportional control are used to achieve position synchronization, with synchronization errors controlled within 0.5 mm to avoid uneven force on the mechanism.

III. Common Fault Types and Diagnosis Methods

1. Typical Fault Manifestations

• Abnormal pressure: Failure to build up pressure (below the set value) may be due to pump wear, stuck relief valves, or pipeline leaks; excessive pressure (exceeding the set value) is mostly caused by relief valve failure or abnormal loads, which may lead to pipeline bursting or motor overload.

• Abnormal actions: Slow or stagnant movement of execution components may be caused by insufficient flow (filter blockage, decreased pump displacement), stuck directional valves, or abnormal oil viscosity; excessive action impact is related to failure of buffering devices or excessive commutation speed.

• Oil contamination and deterioration: Particulate impurities mixed in oil will cause wear and sticking of valve components; excessive moisture (>0.1%) will cause rust and emulsification; oxidative deterioration (acid value >0.5 mgKOH/g) will increase oil viscosity and reduce lubrication performance.

2. System Diagnosis Technology

• Pressure testing: Pressure at key nodes is detected through pressure gauges or pressure sensors, and fault locations are judged by comparing with standard values. For example, if the pump outlet pressure is normal but there is no pressure at the execution component, it is mostly due to pipeline blockage or directional valve failure.

• Oil analysis: Regularly detect oil contamination level (within NAS 8), viscosity (15-40 mm²/s at 40°C), moisture, and acid value. When exceeding standards, the oil must be replaced and the pollution source identified.

• Vibration and noise monitoring: Abnormal noise of the pump (exceeding 85 dB) may be due to cavitation, bearing wear, or misaligned couplings; pipeline vibration is mostly caused by pressure pulsation or loose pipeline fixing.

IV. Maintenance and Management Strategies

1. Daily Maintenance Points

• Oil management: New oil must be filtered (precision ≤10 μm) before injection. The first oil change cycle is 200 hours of operation, and then every 2000-3000 hours; the oil level in the tank must be maintained at 2/3 of the liquid level gauge to avoid cavitation caused by suction.

• Cleaning maintenance: Check the tank liquid level and oil temperature daily, clean the filter element weekly (or replace as needed), and check the tightness of pipeline joints monthly to prevent leakage and contamination.

• Operation parameter monitoring: Record daily data of system pressure, flow, and oil temperature, establish a baseline for normal fluctuation ranges, and promptly investigate the cause when deviating from the baseline by ±10%.

2. Regular Overhaul Items

• Component performance testing: Check the volumetric efficiency of hydraulic pumps (should be ≥85%), response time and leakage of valves every six months, and replace spare parts when wear exceeds limits.

• Seal replacement: Vulnerable parts such as O-rings and oil seals are replaced every 1-2 years, and materials compatible with the oil are selected (e.g., nitrile rubber for mineral oil, fluororubber for high temperature or synthetic oil).

• System flushing: After oil replacement or overhaul, pipelines need to be flushed. Low-viscosity flushing oil is used for circulation for 2-4 hours until the contamination level meets the standard (NAS 7).