Diagnosis and elimination of random movements of the hydraulic actuator: analysis of the proportional valve, contamination and signal integrity

Problem description and scope

This manual is designed for technicians who encounter random, unregulated movements of hydraulic actuators in production lines (automotive, aerospace, food, chemical, energy). The problem is characterized by:

• uneven expansion/bending rates;
• unexpected stops or rebounds;
• high level of noise and vibration during operation.

Classification of severity:

• Critical – affects the safety of personnel or may lead to production shutdown.
• Important - reduces productivity, causes additional costs.
• Not significant – slight loss of accuracy, no immediate impact.

Safety measures

WARNING: Perform a lockout/tagout (LOTO) of the hydraulic system before any intervention. Wear safety glasses, gloves, protective clothing and fire retardant clothing. When working with high pressure (>150 bar), use a protective shield. Make sure the system is discharged and residual energy (hydraulic, electrical) is removed.

Diagnostic tools are required

Initial Assessment Checklist

Systematic diagnostic process

1. Definition of symptoms
   • Irregular speed
   • Position bounces >5% from set value
   • Increased noise (>70dB) or vibration (>4.5mm/s RMS)
2. Checking the power supply of the proportional valve
   1. Measure the supply voltage (24VDC ±5%). If deviation >5% – go to point 3.
   2. Measure the control current (4-20mA). A value of less than 4mA or >20mA is a suspicion of a damaged driver.
3. Control signal analysis
   1. Connect the spectrometer to the output of the controller. Check the waveform. If the signal "hisses" or has noise >0.5mA RMS, electromagnetic interference or a damaged cable is suspected.
   2. Check the end of the cable for corrosion, mechanical damage.
4. Proportional valve status evaluation
   1. Measure the pressure at the outlet port of the valve under different commands (0%, 50%, 100%). The indicators must correspond to the characteristics: 0% → <0,5 bar, 100 % → 250 bar (за умовами системи). Відхилення >10% – suspicion of wear of the sprinkler element.
   2. Use a thermal chamber: overheating >5°C above the norm in the area of ​​the electromagnetic drive indicates that the movement is blocked.
5. Hydraulic fluid purity control
   1. Take a fluid sample (100 ml) and pass the analysis according to ISO 8573‑1 (particles >5µm). If the fraction is >10ppm, there is an increased risk of jamming of the valve channels.
   2. Measure the pressure drop through the filter. If ΔP >2bar at 150L/min – the filter is dirty.
6. Mechanical inspection of the actuator
   1. Check the backlash in the bearings using a backlash gauge (0.05mm is allowed). If >0.1mm, worn bearings are suspected.
   2. Measure vibration during operation (vibration analyzer). Threshold: <4.5mm/s RMS. Exceeding - possible rod imbalance.

Symptom-cause matrix

Root cause analysis

1. Damage to the proportional valve

The proportional valve regulates the flow of hydraulic fluid in accordance with the analog signal of 4‑20mA. Wear of the sprinkler element, accumulation of particles or corrosion lead to a non-linear characteristic. At the same time, accuracy is lost, and the actuator receives sudden changes in pressure.

How to confirm: Valve outlet pressure test with step commands. If the graph is not monotonous or has "jumps", the reason is the valve.

Possible consequences: Overloading of mechanical elements, increased wear of bearings, emergency stop of the line.

2. Contamination of hydraulic fluid

Contamination with particles >5 µm (water, metal fragments) reduces the smoothness of the flow through the narrow channels of the valve. This causes random pressure drops that are perceived as "jumps" in the actuator.

How to confirm: Laboratory analysis of liquid sample for ISO 8573‑1 and measurement of pressure drop through the filter with standard flow distribution.

Consequences: Damage to seals, increased noise, possible blocking of valve elements.

3. Violation of the electrical control signal

The 4‑20mA signal can be distorted by electromagnetic interference, bad connections, or a damaged driver. Distortion results in random commands that do not match the given parameters.

How to confirm: Connect the spectrometer, measure the noise and amplitude. If the noise level is >0.5mA RMS or there are pulse interruptions, the signal is damaged.

Consequences: Unpredictable movement of the actuator, risk of collisions of mechanical parts.

4. Mechanical backlash in actuator bearings

Worn bearings create backlash, which manifests itself as a bounce of position under load. This is especially noticeable when changing the direction of movement.

How to confirm: Measure the backlash with a measuring pin (allowable ≤0.05mm). Check vibration under load.

Consequences: Damage to the actuator body, increased noise, reduced service life.

Step-by-step elimination procedures

1. Restoring the serviceability of the proportional valve

1. Lock out power and relieve pressure (LOTO).
2. Remove the valve from the mounting panel, connect the 24VDC test power supply unit.
3. Perform a step test: apply 4mA, 12mA, 20mA and record the output pressure.
4. If the deviation is >10%, replace the sprinkler element (part No. U‑PV‑SP‑01, hole diameter 0.8mm, material – stainless steel AISI304).
5. After replacement, perform calibration according to the OEM instructions (DSTUENISO 4413).
6. Check operation in "loading" mode, compare with technical characteristics.

2. Cleaning and replacing the hydraulic fluid filter

1. Open the filter compartment, remove the filter element.
2. Check the pressure drop ΔP. If >2bar, replace the element.
3. Flush the system with clean hydraulic fluid (ISOV‑G‑46) to a purity of <5 ppm часток >5µm.
4. Install a new filter (part #U‑FL‑H‑02, purity class ISO 5).
5. Start the system, check the absence of increased noise.

3. Restoration of the integrity of the control signal

1. Check the power and signal terminals for corrosion.
2. Replace the control cable if resistance >0.2Ω is measured or insulation damage is detected.
3. Install a shielded cable (type U‑CABLE‑SH‑04, shield - copper, diameter 4mm²).
4. Install a 0.1µF/10µF power supply filter (to smooth out noise).
5. Check signal after replacement: noise <0.3mA RMS, stability ±0.2mA.

4. Replacement of actuator bearings

1. Remove the actuator, disassemble the case.
2. Measure the backlash; if >0.05mm – replace bearings (part #U‑RB‑A‑07, type 6205‑2RS, diameter 25mm).
3. Apply a torque of 12Nm when tightening the fixing bolt (DSTUENISO 6789).
4. After installation, conduct a vibration test: <4.5mm/s RMS.
5. Restore the system, check the positioning.

Prevention measures

Spare parts and components

To order spare parts, use the UNITEC‑D electronic catalog: https://www.unitecd.com/e-catalog/.

Link

• DSTU EN ISO 4413 - Hydraulic systems - Basic safety requirements.
• DSTU EN ISO 8573‑1 - Cleanliness of hydraulic fluids.
• DSTU EN ISO 9001 - Quality Management System.
• XYZ Series Proportional Valve OEM Manual (internal document).
• UNITEC‑D Maintenance Guide #12 - Proportional Hydraulic Valves.