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In a leveling machine, the hydraulic system is central to the application of leveling force. It does not simply “provide pressure”; rather, through rapid and precise responses, it ensures that high-precision leveling machines can consistently produce sheets with uniform flatness.
Sources of Leveling Force: A Comparison of Hydraulic and Mechanical Systems
Leveling machines apply process forces to sheet metal using two primary drive methods:
Mechanical screw type: A motor drives a worm gear or lead screw to raise or lower the upper roller assembly. This system features a simple structure and low cost, but it has slow response times and limited precision, making it suitable for general leveling applications with modest precision requirements.
Hydraulic servo system: A hydraulic power unit supplies high-pressure oil, and a servo valve precisely controls the pressure in the hydraulic cylinder. This system features fast response times (typically within 100 ms), high pressure control accuracy (within ±1% of the set value), and the ability to perform dynamic closed-loop regulation. This is the standard configuration for modern high-precision leveling machines because it meets the requirement for “dynamic reduction compensation”—that is, it can instantly adjust the roll gap and pressure when the thickness or hardness of the steel strip changes, something that mechanical screws cannot achieve.
Core Components of a Hydraulic Servo System
A complete hydraulic servo system for a leveling machine primarily consists of the following five modules:
Hydraulic Power Source (Pump Station)
As the “heart” of the system, the hydraulic pump station typically includes:
Main Pump: Generally a plunger pump or a variable-displacement vane pump, with its displacement and pressure rating selected based on the maximum leveling force. For example, for a leveling machine with a rated leveling force of 5,000 kN, the system pressure is typically designed to be between 25 and 31.5 MPa.
Accumulator: Provides instantaneous high flow to handle flow spikes caused by the instantaneous opening of servo valves, thereby reducing pump start-stop frequency and system pressure fluctuations.
Cooling and Filtration Circuit: Maintains hydraulic oil temperature within the optimal range of 40–55°C; excessively high temperatures reduce oil viscosity, affecting the dynamic characteristics of the servo valves.
Servo Valves and Proportional Valves
These are the most precise components in the system, responsible for converting weak electrical signals into precise hydraulic flow output.
Servo Valves: Extremely high precision (hysteresis < 0.1%) and high frequency response (up to 100 Hz or higher), but they are expensive and highly sensitive to fluid cleanliness. They are typically used in the main lifting mechanism of the upper roll stand.
Proportional Valves: Slightly lower precision (hysteresis approximately 1%–3%) and lower frequency response (approximately 20–40 Hz), but they are less expensive and more resistant to contamination. They are commonly used for auxiliary functions, such as roll cage locking and side plate adjustment.
Hydraulic Cylinders
As the final force-output components, their design involves specific requirements:
Bore Diameter and Stroke: The bore diameter is determined by the leveling force, while the stroke must accommodate the maximum sheet thickness for both entry and exit (typically 5 to 8 times the maximum sheet thickness).
Low-Friction Seals: To eliminate the “stick-slip effect,” precision leveling machines typically employ low-friction PTFE composite seals to ensure linear response even to minute commands.
Synchronization Requirements: The hydraulic cylinders on the drive side and the operating side must be strictly synchronized. This is typically ensured through a hydraulic synchronization circuit or a closed-loop feedback system using displacement sensors to prevent the upper roller seat from tilting.
Displacement and Pressure Sensors
These sensors serve as the “eyes” of the system:
Magnetostrictive displacement sensors: Built into the hydraulic cylinders, they offer a resolution of 0.5–2 μm and provide real-time feedback on the roll gap position.
Pressure Sensors: Monitor the pressure in the hydraulic cylinder’s working chamber; they can be used to calculate the actual leveling force and to detect abnormalities such as material jams.
Temperature Sensors: Feed oil temperature signals back to the cooling system to achieve closed-loop temperature control.
Controller
Modern CNC leveling machines come standard with a PLC or dedicated motion controller, whose functions include:
Receiving target roll gap and pressure commands set by the host computer.
Reading sensor feedback signals, executing PID control algorithms, and outputting analog signals to drive servo valves.
Implementing off-center load compensation and protective logic (such as over-travel, overpressure, and excessive oil temperature).
Control Modes of Hydraulic Servo Systems
The hydraulic servo system of a leveling machine primarily has three control modes:
Target Mode: The target roll gap serves as the control objective, with feedback provided by a displacement sensor.
Features: The controller drives the servo valve to move the upper roll seat precisely to the set position; the error is typically controlled within ±0.01 to 0.02 mm.
Applications: Normal leveling operations where the material thickness is known and stable.
Force Control Mode
Objective: The target leveling force serves as the control objective, with feedback provided by a pressure sensor.
Features: The controller maintains the hydraulic cylinder pressure at a set value, and the roll gap position automatically adapts to slight fluctuations in sheet thickness.
Applications: Conditions where sheet thickness is uneven (e.g., hot-rolled sheets) or material hardness varies significantly.
Force/Position Hybrid Control and Adaptive Switching
Strategy: High-end leveling machines employ hybrid control. For example, position control is used during the feeding stage to rapidly open the roll gap; during the leveling stage, the system switches to force-based control while preventing overload. The system can automatically switch strategies based on online thickness measurement results.
Common Hydraulic Faults and Diagnostics
Although hydraulic servo systems are highly precise, they are also prone to malfunctions. The following are four common issues and their diagnostic methods:
Unstable Roll Gap Control (Drift or Oscillation)
Symptom: After setting the roll gap, the actual roll gap drifts by more than ±0.05 mm or continues to oscillate slightly.
Causes: Wear or sticking of the servo valve spool; interference with the displacement sensor signal; or suboptimal PID parameters.
Diagnosis: Apply a constant command to the servo valve and observe the displacement feedback curve. If the curve is smooth but the position deviates, the issue is likely with the sensor; if the curve exhibits significant jitter, the problem may lie with the servo valve or the PID parameters.
Asynchronous Operation of Hydraulic Cylinders on Both Sides (Roll Stand Skew)
Symptom: The stroke lengths of the hydraulic cylinders on the operating side and the drive side are inconsistent, causing the upper roll stand to tilt noticeably.
Causes: Inconsistent zero drift in the servo valves on both sides; internal leakage in a hydraulic cylinder on one side; inconsistent clearance in the mechanical guideways or columns.
Diagnosis: Lower the upper roll stand from the upper limit stop to the lower limit stop, and record the displacement difference throughout the entire stroke. If the difference increases as the stroke lengthens, prioritize checking the guide rail clearance and hydraulic cylinder seals.
Slow Pressure Build-up or No Pressure in the Hydraulic System
Symptom: After startup, the pump operates normally, but the system pressure fails to reach the set value quickly or remains consistently low.
Causes: The relief valve is stuck in the normally open position; the accumulator bladder has failed; internal wear in the pump; or a large amount of air in the system.
Abnormal Rise in Hydraulic Oil Temperature
Symptoms: After half an hour of normal operation, the oil temperature exceeds 60°C, and the system experiences pressure fluctuations.
Causes: The relief valve set point is lower than the operating pressure; reduced heat exchange efficiency of the cooler; or significant internal system leakage.
Action: If the oil temperature exceeds 65°C, shut down the machine to allow it to cool. Resume operation only after the temperature has dropped below 50°C.
