Diagnostic guide: Troubleshooting unstable movement of the hydraulic drive

1. Description of the problem and scope of application

Unstable movement of a hydraulic actuator is a critical fault, which can manifest as jerkiness, uneven speed, delays in response to control commands, or a complete lack of control over positioning. This problem directly affects the accuracy, productivity and reliability of industrial equipment such as presses, injection molding machines, CNC machine tools, mobile hydraulics and packaging machines. Movement instability can lead to product shortages, damage to equipment, and create unsafe working conditions.

Classification of malfunction by degree of importance:

• Critical: Immediate stoppage of production process, high risk of equipment damage or accident. Needs immediate diagnosis and elimination.
• Significant: Decrease in productivity, deterioration of product quality, increased wear of components. Needs urgent intervention.
• Minor: Intermittent, barely noticeable deviations that can develop into a significant or critical malfunction without appropriate intervention. Needs planned diagnostics.

This manual provides a systematic approach to diagnosis, from symptoms to root cause determination and steps to remedy, with an emphasis on proportional valves, contamination analysis, and signal integrity.

2. Safety measures

WARNING! Working with hydraulic systems involves a high risk of injury due to high pressure, hot oil and electric current. Observe the following critical safety precautions:

• Lockout-marking (LOTO): Before starting any work on the hydraulic system, be sure to perform the lockout-markout procedure in accordance with the EN 1037 standard and the company's internal rules.
• Relieving pressure: Make sure all system pressure is relieved. Residual pressures can cause oil to escape under high pressure, which is extremely dangerous.
• Hot oil: Hydraulic oil can heat up to high temperatures during operation. Use personal protective equipment for hands (heat-resistant gloves) and eyes.
• Electrical safety: Check the absence of voltage on electrical components (valves, sensors, controllers) before disassembling or diagnosing them. Follow the electrical safety rules of DSTU EN 50110-1.
• Personal protective equipment (PPE): Always use safety glasses, gloves, overalls and safety shoes.
• Energy storage: Remember that some elements (pressure accumulators) can store energy even after the pump is stopped. Follow procedures for safely depressurizing batteries.

3. Necessary diagnostic tools

The following set of tools is required for effective diagnosis of unstable movement of the hydraulic drive:

4. Initial review checklist

Before starting a detailed diagnosis, perform the following review:

5. Systematic diagnostic algorithm

Follow this step-by-step algorithm to identify the root cause of erratic movement:

1. Symptom confirmation:
   • Action: Start the drive, observe its movement. Determine the exact nature of the instability (jerks, slowness, overshoot).
   • Analysis: Is the movement chaotic or repetitive? Does it depend on the load or the direction of movement?
2. Proportional valve electrical system check:
   1. Control signal from controller (PLC/PAC):
      • Action: Using an oscilloscope, check the shape and amplitude of the control signal (typically 0-10V or 4-20mA) directly at the controller output.
      • Norm: Stable, clear signal without ripples and noise. For proportional valves with PWM control, check the fill factor.
      • Not normal: Unstable, pulsating signal, noises. → Probable cause: Controller failure, control cable failure, electromagnetic interference.
   2. Signal on proportional valve:
      • Action: Check the same signal directly on the proportional valve coil connector.
      • Norm: The signal is identical to the output signal of the controller.
      • Not normal: The signal differs from the controller signal (voltage drop, distortion). → Probable cause: Cable damage, poor contact in the connector, high resistance in the circuit.
   3. Proportional valve coil resistance:
      • Action: Turn off the power, disconnect the connector. Using a multimeter, measure the resistance of the valve coil.
      • Norm: The resistance corresponds to the manufacturer's specification (for example, 2.5 Ohms for 24V DC).
      • Not normal: Open circuit (infinite resistance) or short circuit (zero/very low resistance). → Probable cause: Valve coil failure, inter-turn short circuit.
   4. Coil Drive Current:
      • Action: Use the current clamps of the multimeter to measure the current flowing through the coil during operation.
      • Norm: The current corresponds to the calculated value according to the control signal.
      • Not normal: The current does not correspond to the control signal. → Probable cause: Valve electronics problem, coil malfunction.
3. Hydraulic System Check:
   1. System Pressure:
      • Action: Connect pressure gauges to pump discharge line, valve control line, and actuator inlet. Observe the stability of the pressure.
      • Norm: Stable pressure without significant pulsations. The deviation is no more than ±5% from the set value.
      • Not normal: Sharp pressure fluctuations, pressure drop under load. → Probable cause: Pump wear, safety/control valve failure, air in system, clogged filter.
   2. Hydraulic fluid flow:
      • Action: Using a portable flow meter, measure the flow at the inlet to the actuator and, if possible, at the outlet of the pump.
      • Norm: Flow is stable and responds to control commands.
      • Not normal: Unstable flow, significant flow drop. → Probable cause: Pump wear, clogged filter, internal leaks in valves or actuators.
   3. Hydraulic oil contamination:
      • Action: Take an oil sample from the tank and from the injection line. Analyze the sample with a purity analysis kit or send to a laboratory.
      • Standard: The oil purity class meets the requirements of the equipment manufacturer (for example, ISO 4406: 18/16/13 or better).
      • Not standard: Cleanliness class worse than recommended. → Probable cause: Inefficient filtration, component wear, external contamination. Contamination can lead to jamming of the proportional valve spool.
   4. Presence of air (cavitation):
      • Action: Visually inspect the oil in the tank for foam or bubbles. Listen to the pump for characteristic cavitation noises (cracking).
      • Norma: Oil without bubbles, the pump works quietly.
      • Abnormal: Foam, bubbles, pump noise. → Probable cause: Low oil level, leaks on the suction line of the pump, malfunction of the suction filter.
4. Proportional valve inspection (mechanical part):
   1. Spool mechanical jamming:
      • Action: After safely disassembling the valve, visually inspect the spool for burrs, corrosion, deposits. Try to manually move the spool (if possible).
      • Norm: The spool moves freely, without sticking.
      • Not normal: Spool jams or moves with great resistance. → Probable cause: Pollution, mechanical wear, skew, thermal deformations.
   2. Wearing of internal seals:
      • Action: When disassembling the valve, inspect the seal for damage, pitting.
      • Norm: Seals are whole, elastic.
      • Not normal: Damage, hardness of seals. → Probable cause: Age wear, chemical degradation of oil.
5. Hydraulic Actuator Check (Cylinder/Hydromotor):
   1. Internal Leaks:
      • Action: For cylinder - pressurize one cavity and block the other, measure pressure drop rate or drain line leak. For the hydraulic motor - measure the leakage through the drain line.
      • Norm: Minimum leakage that meets the specification (for example, no more than 5 cm³/min for a cylinder).
      • Not normal: Significant internal leakage. → Probable cause: Wear of piston/rod seals, damage to cylinder mirror/hydraulic motor friction pairs.
   2. Mechanical friction/jamming:
      • Action: Disconnect the drive from the mechanical load. Check the ease of moving it by hand.
      • Norm: The drive moves freely.
      • Not normal: Jamming, excessive friction. → Probable cause: Guide damage, housing deformation, bearing failure (for hydraulic motor).

6. Matrix "Symptom - Possible cause"

7. Analysis of the root causes of malfunctions

7.1 Contamination of hydraulic oil

Explanation: Solid particles (dust, metal shavings, seal wear products) are the most common cause of malfunctions in hydraulic systems. They enter the system from the outside (due to leaky seals, impure oil filling) or are formed internally as a result of component wear. These particles act as an abrasive, damaging the precision valve and actuator surfaces, and can jam the spools of proportional valves, preventing them from moving smoothly. This leads to a jerky, uneven movement of the drive.

Confirmation: Laboratory analysis of oil samples or use of a portable particle counter (e.g. HYDAC FCU 1000) will show a ISO 4406 purity class that is significantly worse than the manufacturer's recommended (e.g. instead of 18/16/13 it will be 22/20/17). Visually, the oil may be cloudy, have sediment or change color.

Consequences: If not removed, contamination accelerates the wear of all moving parts of the system (pumps, valves, cylinders), causes internal leaks, overheating of the oil, reduced efficiency and, ultimately, failure of expensive components.

7.2 Proportional valve malfunctions (electrical/mechanical)

Explanation: Proportional valves are the "heart" of precision hydraulic control. Their failure can be electrical (failure of the coil, electronics, connector) or mechanical (jamming of the spool, wear of internal parts). Electrical problems lead to a fuzzy or missing control signal, causing instability. Mechanical problems prevent the spool from moving smoothly, which directly affects the flow and pressure that the valve regulates.

Confirmation:

• Electrical: A multimeter will show an open/short circuit in the coil. The oscilloscope will detect an unstable or distorted control signal at the valve connector.
• Mechanical: After disassembly, a visual inspection will reveal burrs, corrosion, dirt on the spool. The spool does not move freely by hand. A test to measure the hysteresis and response time of the valve will show deviations from the passport data.

Consequences: Loss of precise control of the actuator, inability to reach the set position, increased cycle time, damage to other equipment due to incorrect movement, complete system shutdown.

7.3 Wear of the hydraulic pump or failure of the pressure/flow regulator

Explanation: Pump wear leads to reduced performance, internal leaks, and pressure/flow pulsations. Failure of pressure or flow regulators (eg relief valves, pressure relief valves) can cause unstable operating pressure or improper flow distribution, which directly affects the smoothness and controllability of actuator movement.

Confirmation:

• Measurement of pump pressure and flow will show values ​​below nominal.
• Pressure pulsations in the injection line detected by a pressure gauge with data logging or an oscilloscope with a pressure sensor.
• Check the settings and operation of the pressure/flow regulators: they may "stick", "drift" or not respond to changes.

Consequences: Unstable movement, system power reduction, oil overheating, accelerated wear of other components due to incorrect operating parameters.

7.4 Air in the hydraulic system (cavitation)

Explanation: Air can enter the system due to leaking connections in the pump suction line, low oil level in the tank, or incomplete air removal after service. Air bubbles in the oil are compressed under pressure and expand as it falls, causing a "spring" effect in the hydraulic circuit. This results in jerks, noise (cavitation) and unstable, uncontrolled movement of the actuator.

Confirmation:

• Visual inspection of the oil in the tank: the presence of foam, large bubbles.
• Characteristic cracking or rumbling of the pump (cavitation noise).
• Measuring pressure with an oscilloscope will show sharp and rapid fluctuations.

Consequences: Destruction of component surfaces (especially pumps and valves) due to shock bubble collapse, oil overheating, accelerated wear of the system, complete loss of control.

8. Step-by-step troubleshooting procedures

1. Hydraulic oil contamination removal:
   1. Filters replacement: Install new filters with appropriate filtration fineness (eg 10 µm for drain and 3 µm for pressure filter). Make sure the filters meet the ISO 16889. standard
   2. System flushing: If the level of contamination is high, flush the system using special flushing oil or clean hydraulic oil and additional filters.
   3. Hydraulic oil change: Drain the old oil completely and fill with new oil of the appropriate type and purity class according to ISO 4406 (eg HLP 46, purity class 17/15/12).
   4. Leakage check: Eliminate all potential points of contamination (damaged tank seals, leaky lids, breathers).
2. Proportional valve diagnosis and repair:
   1. Check electrical connections: Clean and check the reliability of the contacts in the valve connectors. Check the integrity of the cable.
   2. Coil replacement: If the resistance of the coil does not meet the norm, replace the coil with an original one (for example, Bosch Rexroth R900012345).
   3. Valve Cleaning/Repair:
      • CAUTION: Proportional valve disassembly is a complex operation that requires cleanliness and skill. Avoid disassembly unless necessary.
      • Disassemble the valve (after depressurizing!), carefully disassemble it in a clean environment.
      • Clean the spool and housing from dirt and deposits using special solvents for hydraulics.
      • Check the spool for burrs or wear. Sand out small burrs or replace spool/valve if wear is significant.
      • Replace all seals and O-rings with new ones that meet the manufacturer's specifications.
      • Assemble the valve to the recommended tightening torques (e.g. 12 Nm for the mounting bolts).
   4. Calibrating the valve: After installing and connecting the valve, calibrate it according to the manufacturer's instructions, using specialized software or the controller's PID controller.
3. Hydraulic pump and regulator repair:
   1. Pump replacement/repair: If diagnostics have confirmed significant wear or malfunction of the pump (drop in performance, excessive noise), it must be replaced or repaired in a specialized workshop.
   2. Checking/replacing regulators: Check the serviceability and settings of safety, reducing valves. Clean or replace them if they stick or don't respond. Adjust the pressure according to the system diagram.
4. Bleed the system:
   1. Add oil: Make sure the oil level in the tank is at the top of the recommended range.
   2. Inspection of the suction line: Check all connections on the pump suction line for leaks, replace damaged hoses or seals.
   3. Bleed the system: Run the system at low rpm, let the oil circulate, repeatedly moving the actuators through the full stroke range to expel air through drain lines or special bleed valves. This can take up to several hours.
5. Hydraulic drive repair:
   1. Replacement of seals: In case of internal leaks in the cylinder or hydraulic motor, replace the piston, rod, and cuff seals.
   2. Mechanical Check: Eliminate mechanical backlash or friction in the actuator guides or hydraulic motor bearings.

9. Preventive measures

Regular maintenance and prevention are critical to prevent unstable hydraulic systems.

10. Spare parts and components

For quick troubleshooting, it is recommended to have the following spare parts available:

For ordering and detailed information on spare parts, please refer to the UNITEC-D e-catalog at: www.unitecd.com/e-catalog/

11. Links

• DSTU EN ISO 12100: Machine safety. General design principles. Risk assessment and risk reduction.
• DSTU EN ISO 13849-1: Machine safety. Elements of control systems related to safety. Part 1: General design principles.
• DSTU EN 50110-1: Operation of electrical installations.
• ISO 4406: Hydraulic fluid - Coding of the level of contamination by solid particles.
• ISO 16889: Hydraulic fluid - Filters - Multipass test method for evaluating filtration efficiency.
• EN ISO 10816: Mechanical vibration. Evaluation of machine vibration based on measurement results on stationary parts.
• Operation and maintenance manuals from equipment manufacturers (OEM).
• Related UNITEC-D Maintenance Guides: www.unitecd.com/maintenance-guides/