1. Problem Description and Area of Application
Unstable, jerky or oscillating movement of hydraulic actuating mechanisms (hydraulic cylinders and rotary hydraulic motors) in proportional control systems is a critical failure, which leads to reduced product quality, premature wear of mechanical components and emergency shutdowns of production equipment. This manual covers industrial hydraulic systems of metalworking machines, presses, casting machines and lifting and transport equipment of Ukrainian industrial enterprises operating with mineral oils (viscosity classes ISO VG 32, 46, 68) and synthetic fluids.
Hazard Level Classification: Critical (Class 1). Further operation of equipment with uncontrolled oscillations of the actuating mechanism leads to piston rod damage of hydraulic cylinders, seal damage, cavitation wear of distributor working surfaces and damage to mounting flanges.
2. Precautionary Measures and Safety
WARNING: Hydraulic systems are under high pressure (up to 350 bar) and store potential energy in hydraulic accumulators. Before starting any diagnostic or maintenance work, perform the following:
- Apply the lockout/tagout (LOTO) procedure according to ISO 14118.
- Release the remaining pressure in the system, ensuring that the pressure drop is reduced to 0 bar as indicated by the pressure gauge.
- Discharge the hydraulic accumulators by closing the shut-off valves and slowly opening the drain lines to the tank.
- Use personal protective equipment (PPE): safety goggles, heat-resistant gloves (working fluid may reach a temperature up to +80 °C), protective footwear with a steel toe cap.
- Remember the danger of fluid injection under the skin under high pressure — never check leaks by hand, use a piece of cardboard or wood.
3. Required diagnostic tools
| Tool Name | Specification / Model | Measurement range | Purpose |
|---|---|---|---|
| Digital multimeter | True RMS (for example, Fluke 87V) | 0–1000 V AC/DC, 0–10 A, 0–50 MΩ | Voltage check, coil resistance, command signal. |
| Digital oscilloscope | Dual-channel portable (for example, Fluke 123B) | 0–20 MHz, 0–50 V DC | Analysis of the control signal shape for a proportional valve, search for noises and pulsations. |
| Portable vibration analyzer | ISO 20816 compatible | 10–1000 Hz, vibration velocity 0–50 mm/s | Detection of cavitation, hydraulic shock and mechanical resonances. |
| Thermal imager | Range -20 °C ... +250 °C (for example, Flir E4 / E8) | Sensitivity < 0.05 °C | Definition of internal leaks in valves and overheating of throttling elements. |
| Digital Pressure Gauge / Hydrotester | Class accuracy 0.5, with data recording | 0–400 bar | Pressure measurement on the suction line and in the cylinder cavities. |
| Oil sampling kit and particle counter | ISO 4406 / NAS 1638 compatible | Cleanliness class 0–25 | Evaluation of the degree of contamination of the working fluid. |
4. Initial Assessment Checklist
Fill out this check list DO before disassembly or deep diagnostics of the system:
| Parameter / Observation | Normal value | Actual value | Status (OK / Alarm) |
|---|---|---|---|
| Working oil temperature in the tank | +40 °C ... +55 °C | ||
| Viscosity of the working fluid (at operating temperature) | According to OEM recommendations (22–46 °C) | ||
| Cleanliness class of the working fluid (ISO 4406) | No worse than 18/16/13 (for systems with proportional valves) | ||
| Feed pressure / pump discharge pressure | Nominal pressure of the system ± 5% (e.g., 210 bar) | ||
| Pressure pulsation level on the suction main line | < 3% of nominal pressure | ||
| Presence of air in the viewing glass / lubricant | Fully transparent oil, without bubbles | ||
| Condition of valve electrical connectors | Tight fit, absence of moisture and oxidation traces |
5. Systematic Diagnostic Block Diagram
- Step 1: Symptom — The actuator moves jerkily or oscillates in the neutral position.
- Step 1.1: Check for electrical pulsations at the input of the proportional valve control using an oscilloscope.
- Result A (Stable signal, but movement is unstable): Proceed to Step 2 (Hydraulic part).
- Result B (Signal contains noise or fluctuates): Proceed to Step 3 (Electrical part and signal).
- Step 1.1: Check for electrical pulsations at the input of the proportional valve control using an oscilloscope.
- Крок 2: Діагностика гідравлічної частини та стану робочої рідини.
- Крок 2.1: Виміряйте рівень забруднення масла та наявність розчиненого/вільного повітря.
- Результат А (Виявлено повітря або завищені механічні домішки): Ймовірна причина — кавітація або заклинювання золотника через забруднення. Перейдіть до Розділу 7 (Аналіз першопричин).
- Result B (Oil is normal): Check the condition of hydraulic components (play, internal leakage in the cylinder or valve).
- Крок 2.1: Виміряйте рівень забруднення масла та наявність розчиненого/вільного повітря.
- Step 3: Proportional valve (electromagnetically controlled distributor) diagnostics.
- Step 3.1: Check the amplifier card and position feedback (LVDT sensor inside the valve).
- Result A (LVDT signal is distorted or absent): Position sensor failure or feedback cable break.
- Result B (Spool does not respond to control signal change): Contamination of the spool working zone or solenoid failure (open circuit/short circuit in windings).
- Step 3.1: Check the amplifier card and position feedback (LVDT sensor inside the valve).
6. Fault Matrix and Causes
| Symptom | Likely causes (in order of priority) | Diagnostic test | Expected result upon confirmation of the cause |
|---|---|---|---|
| Rod movement in neutral (drift) | 1. Amplifier zero shift 2. Valve spool wear 3. Residual pressure in the drain line |
Voltage check on the coil at zero position (multimeter in DC mV mode). | The displacement voltage differs from the nominal value (e.g., > 50 mV instead of 0 V). |
| Rivets during movement under load | 1. Air presence in the hydraulic cylinder 2. Valve sticking (Stick-Slip effect) 3. Wear of cylinder guide rings |
Thermal imaging inspection of the hydraulic cylinder and valve; pressure analysis with an oscilloscope. | Local temperature rise in the throttling zone or pressure drops (> 15% of nominal). |
| Cyclic auto-oscillations of the system (hunting) | 1. Too high a gain of the loop (Gain) 2. Play in the mechanical feedback 3. Incorrectly tuned PID regulator |
Reduction of the gain parameter in the axis controller (Motion Controller). | Loss of oscillations when Gain is reduced by 15–20% (accompanied by an increase in tracking error). |
7. Root Cause Analysis
7.1. Contamination of the working fluid and valve sticking (Stick-Slip)
Чому це відбувається: Малий радіальний зазор між золотником та гільзою прецизійного пропорційного розподільника (зазвичай 3–5 мікрометрів) створює критичну чутливість до твердих частинок. Якщо клас чистоти масла перевищує ISO 18/16/13, мікрочастинки шлама заклинюють золотник. Це викликає ефект зупинки-ковзання (stick-slip): золотник різко зривається з місця під дією електромагніту, створюючи перерегулювання.
Як підтвердити: Виконайте лабораторний аналіз проб масла на вміст твердих частинок згідно з ISO 4406. Проведіть огляд золотника під збільшувальним склом на предмет поздовжніх задирів.
Damage caused by: Wear of the spool edges, overheating of the solenoid coils due to increased holding current consumption, complete valve failure.
7.2. Signal Degradation (LVDT Sensor)
Чому це відбувається: Пропорційні клапани високої динаміки використовують вбудовані індуктивні датчики переміщення (LVDT) для контролю реального положення золотника. Вібрація обладнання, термічні цикли та проникнення вологи в роз'єм викликають погіршення ізоляції обмоток LVDT або появу високочастотних електромагнітних перешкод (EMC).
How to confirm: Connect the oscilloscope to the feedback signal outputs of the LVDT. Check the amplitude of the reference frequency and the absence of phase distortions.
What damage is caused: The system loses positioning accuracy, uncontrolled oscillations of the actuator mechanism occur around the set point.
7.3. Presence of Undissolved Air in the Hydraulic System
Чому це відбувається: Підсмоктування повітря через сальники всмоктуючого насоса, низький рівень масла в баку або турбулентність потоку створюють вільні бульбашки повітря. Стисливість повітря у багато разів перевищує стисливість мінерального масла, що призводить до пружинного ефекту в гідроциліндрі.
Як підтвердити: Візуальний огляд масла в баку (наявність емульсії або піни), характерний гудячий звук роботи насоса (кавітаційний шум), аналіз коливань тиску за допомогою швидкодіючого манометра.
What damage is caused by: Cavitation damage to pump and hydraulic distributor metals, loss of hydraulic drive stiffness, surges and damage to seals.
8. Step-by-step recovery procedures
- Cleaning and rinsing procedure for the proportional valve:
- Perform LOTO, release the system pressure.
- Remove the proportional valve, and clean the outer surface from contaminants.
- Carefully remove the solenoid covers, pull out the spool from the sleeve (WARNING: the spool and sleeve are a matched pair, never interchange them with other valves).
- Clean the parts in an ultrasonic bath using a certified cleaning solvent for hydraulics for 15 minutes.
- Blow with compressed air free of moisture and oil contamination.
- Assemble the node using new sealing rings (O-rings) made of FKM (Viton) according to ISO 3601.
- Tighten the fastening screws of the valve with a torque wrench in a cross pattern according to the manufacturer's instruction (for example, for bolts M5 — 9 N·m, M6 — 15.5 N·m).
- Electrical signal check and calibration procedure:
- Connect the multimeter to the outputs of the control board (±10 V or 4–20 mA).
- Check the zero position using the zero balance potentiometer (Zero adjustment) on the amplifier card.
- Make sure that the power supply provides a stable 24 V DC with ripple less than 100 mV.
- Protect sensor feedback cables using a grounded metal conduit to eliminate electromagnetic interference.
- Air removal procedure (System purging):
- Fill the hydraulic reservoir with fresh filtered oil via the filtration station (not below ISO 16/14/11).
- Start the pump on no load (without load on the actuating mechanisms) for 10 minutes.
- Slowly move the hydraulic cylinder from one extreme position to the other at the minimum speed for no less than 10 cycles, gradually increasing the load.
- Open the screws for air release on the top covers of the hydraulic cylinders until a continuous stream of oil without bubbles appears.
- Verification after repair:
- Start the production cycle in manual mode (JOG), monitoring the smoothness of movement using linear displacement sensors.
- Check for leaks by means of a pressure test of the connections.
- Ensure the absence of vibrations and abnormal noises using a vibrometer (vibration velocity value must be less than 2.8 mm/s according to ISO 20816-3).
9. Preventive Measures
| Primary Cause | Preventive strategy | Monitoring method | Recommended interval |
|---|---|---|---|
| Oil Contamination | Installation of autonomous filtration stations (offline filters) with a micron rating beta coefficient $\beta_{4}(c) \\ge 200$. | Laboratory analysis of oil samples or online cleanliness sensors. | Every 500 hours of operation |
| Moisture ingress | Application of silica gel desiccants on the hydraulic accumulator cushions. | Control of water content (ppm) in oil. | Monthly |
| Valve seal wear | Use of original fluorocarbon rubber (FKM) repair kits. | Visual inspection for absence of external leaks. | Every 6000 hours / 1 year |
10. Spare Parts and Components
For rapid restoration of hydraulic systems' functionality, UNITEC-D GmbH supplies from its warehouse in Germany and through the European logistics network a wide range of industrial components:
| Description of the part | Specification | When to replace | Category UNITEC |
|---|---|---|---|
| Proportional hydraulic distributor | NG6 / NG10, with electronic board and feedback (analog Bosch Rexroth / Parker) | Wear of the spool, damage to the LVDT or internal leaks > 150 cm³/min | Hydraulics / Proportional Valves |
| Line filter element | Glass fiber, rating 3 microns, collapse pressure 210 bar | When the pressure drop on the filter exceeds 2.5 bar or every 1000 hours | Filtering / Elements |
| Hydraulic Distributor Seal Kit | FKM (Viton), complete set of ring seals and support washers | At each scheduled disassembly or appearance of leakage traces | Seals / Repair Kits |
| Linear displacement sensor (magnetostrictive) | Output 4–20 mA / SSI, length to order (up to 3000 mm) | In case of signal loss or positioning error > 0.05 mm | Sensors / Automation |
Familiarize yourself with the full range and select compatible components for your equipment in our electronic catalog UNITEC-D.
11. References
- ISO 4406: Active Hydraulics — Working Fluids — Method of Coding Contamination Level with Solid Particles.
- ISO 20816-1: Vibration — Measurement and evaluation of machine vibration.
- ISO 14118: Machine Safety — Prevention of Unexpected Start-Up (Lockout/Tagout).
- Технічні посібники OEM: Bosch Rexroth RE 29050, Parker Hannifin Hydraulic Valve Troubleshooting Manual.
- Related Guides UNITEC-D: "Analysis and Filtration of Industrial Lubricants in Heavy Machinery", "Diagnosis of Electromagnetic Actuators of Hydraulic Distributors".
