1. Problem description & scope of application

This diagnostic guide addresses the critical issue of erratic or jerky movements of hydraulic actuators in industrial manufacturing plants. Such malfunctions lead to loss of precision, increased cycle time, premature component wear and, in extreme cases, can lead to production downtime and safety risks. The area of application includes hydraulically controlled machines such as presses, injection molding machines, machine tools and handling systems that are equipped with proportional directional, pressure or flow valves for sensitive control.

1.1 Symptom classification

Critical: Uncontrollable or sudden movements, complete standstill, immediate danger to personnel or machine. Immediate shutdown required.
Major: Clearly noticeable jerks or vibrations, positioning errors > ±5% of the setpoint, increased noise. May lead to quality defects and reduced productivity. Immediate diagnosis and resolution recommended.
Minor: Occasional, slight jerking, minimal deviations from the target value (e.g. ±1-2%), which do not directly affect the product quality, but indicate an incipient error. Proactive investigation recommended.

2. Safety precautions

ATTENTION: Hydraulic systems work under high pressure and store significant amounts of energy. Improper handling can result in serious injury, including crushing, injection of hydraulic fluid into the skin (up to 600 bar and more), and death. Strictly follow the following safety protocols and the applicable occupational health and safety regulations (e.g. DGUV regulation 3, DIN EN ISO 4413)..

Lockout/Tagout (LOTO): Before any work on the hydraulic system begins, the machine must be switched off from energy in accordance with operational regulations and secured against being switched on again. This includes switching off the electrical supply to the hydraulic unit, securing the main switches and attaching warning signs.

Pressure Relief: Ensure all system pressures are fully relieved BEFORE opening lines or dismantling components. Use pressure gauges and, if available, relief valves for this purpose. Accumulated pressure accumulators must also be completely emptied.

Personal protective equipment (PPE): ALWAYS wear appropriate PPE: safety glasses or visor (DIN EN 166), cut-resistant gloves (DIN EN 388), safety shoes (DIN EN ISO 20345) and, if necessary, protective clothing. When working on hot systems, also wear heat protection gloves.

Fluid temperature: Hydraulic fluid can reach high temperatures. Be careful when handling to avoid burns.

Leaks: Never use your bare hand to check for hydraulic leaks. Use cardboard or other tools. Injection injuries are extremely dangerous and require immediate medical attention.

Environmental protection: Make sure that leaking hydraulic fluid is properly collected and disposed of in accordance with environmental regulations (e.g. Water Resources Act - WHG).

3. Required diagnostic tools

Effective diagnosis requires specialized measuring and testing devices. Make sure that all tools are calibrated and in perfect condition (according to DIN EN ISO 9001 requirements).

Tool	Specification/Model	Measuring range	Purpose
Digital Multimeter (DMM)	True RMS, CAT III/IV, e.g. Fluke 179	Voltage: up to 1000 V AC/DC; Current: up to 10 A AC/DC; Resistance: up to 50 MΩ	Measuring control voltages/currents of proportional valves, resistance of coils, checking signal integrity.
Oscilloscope	2-4 channels, bandwidth > 100 MHz, e.g. Rohde & Schwarz R&S RTB2000	Voltage: 1mV/Div to 10V/Div; Time base: 1 µs/div to 1 s/div	Visualization of PWM signals, feedback signals, detection of noise or signal distortion.
Pressure gauge/manometer case	Class 1.0 or better, glycerin or silicone dampened, e.g. WIKA 233.50	0-600 bar (for system pressure), 0-60 bar (for control pressure)	Measuring working, control, tank and leakage pressures; Confirmation of pressure relief.
Flowmeter	Turbine or gear principle, suitable for hydraulic oil, e.g. Parker Sensocontrol	0-400 l/min, temperature range 0-100 °C	Measuring leakage amounts on proportional valves or actuators; Checking pump performance.
Particle counting device	Online or offline, according to ISO 4406, e.g. Hydac FCD	ISO 4406 Purity classes 4 to 24	Hydraulic fluid purity analysis; Detection of contamination.
Temperature measuring device (IR thermometer / thermal camera)	Measuring range -20 to 400 °C, e.g. Flir E5-XT	Accuracy ±2°C or 2% of reading	Detection of hot spots on valves, cylinders or lines that indicate internal leaks or friction.
Hydraulic oil analysis kit	For viscosity, water content (Karl-Fischer), acid number, additive status	N/A	Comprehensive laboratory analysis of the hydraulic fluid.

4. Checklist for initial assessment

Before starting the detailed diagnosis, a careful assessment of the current situation is essential. This information helps isolate the error and ensure safety.

Point	Description	Status/value	Remark
Machine status	Is the machine at a safe standstill or can it be moved under observation?	[Standstill / emergency stop / observation mode]	Important note for further steps.
Symptom details	Detailed description of irregular movement: When does it occur? In which phase of the cycle? At what frequency? Is it reproducible?	[Detailed description]	Helps narrow down to specific components or operating conditions.
Actuator type	Cylinder (double/single acting), hydraulic motor.	[Cylinder / Engine]	Affects possible error causes and diagnostic paths.
Proportional valve type	Directional, pressure and flow control valve? Manufacturer, model, nominal diameter.	[manufacturer, type, nominal diameter]	For searching specific circuit diagrams and data sheets.
Operating parameters	System pressure (bar), oil temperature (°C), ambient conditions.	[Current value]	Abnormalities may indicate overload or fluid problems.
Alarm history	Are there current or past error messages in the control system (PLC)?	[Error Messages / None]	Error codes (e.g. valve error, temperature alarm) are valuable information.
Last maintenance	When was the hydraulic oil last changed/filtered? When were filters replaced?	[Date/Hours]	Indication of potential contamination problems or worn filters.
Control system	PLC type, valve amplifier/controller used.	[Type/Model]	Important for checking the electrical control.
Visual inspection	Visible leaks, damaged lines/cables, loose connections, unusual noises/vibrations.	[Observations]	Simple visual inspections often provide initial information.

5. Systematic diagnostic flowchart

The following flowchart provides a structured approach to locating the cause of errors in irregular actuator movement. Work through the steps sequentially.

Initial assessment & safety:
1. Complete the Initial assessment checklist.
2. Ensure safe power off (LOTO) of the machine. ATTENTION: Pressure relief before any disassembly!
Visual inspection & background noise:
1. Check for obvious mechanical damage to the actuator, lines and proportional valve.
2. Listen for unusual noises (whistling, knocking, cavitation) from the unit, lines or valve.
3. Check the hydraulic oil level in the tank.
4. Result:
  - Anomalies found: Go to step 7 (Mechanical errors).
  - No abnormalities: Continue with step 3.
Hydraulic fluid check:
1. Check the oil in the tank for discoloration, cloudiness, or an unusual smell.
2. Take an oil sample for a quick on-site check (e.g. water test, sedimentation test).
3. Result:
  - Visible contamination (turbidity, deposits) or strong odor: Go to Step 6 (Contamination Analysis).
  - No visible contamination: Continue to step 4.
Electrical signal test of the proportional valve (under voltage, if safe):
1. ATTENTION: Work under voltage only by trained specialist personnel and with suitable PPE (DIN VDE 0105-100).
2. Measure the supply voltage of the valve amplifier/controller with DMM. (Setpoint: e.g. 24 V DC ±5%).
3. Measure the current/voltage signal at the input of the valve coil with DMM or oscilloscope.
4. If PWM control: Measure frequency, duty cycle, current amplitude. (Setpoints from data sheet, e.g. 200 Hz, 0-100% duty cycle, 0-800 mA).
5. Check the resistance of the valve coil (voltage-free!) with DMM. (Setpoint from data sheet, e.g. 2.5-4.5 ohms).
6. Result:
  - Significant deviations from setpoints, unstable signals, high coil resistance: Go to Step 8 (Electrical/Electronic Faults).
  - Signals in target range: Continue with step 5.
Hydraulic function test (without load, if possible):
1. With the valve switched off: Check the zero point of the actuator, is there unwanted creep?
2. Control the valve manually (if available) or via the controller in small steps. Observe the reaction of the actuator.
3. Measure the pressure before and after the proportional valve during movement with pressure gauges. Capture pressure peaks and fluctuations.
4. Result:
  - Irregular response despite correct input signal, pressure fluctuations: Go to step 9 (proportional valve internal errors).
  - Response appears to be mechanically blocked or extremely sluggish: Go to step 7 (Mechanical Errors).
  - System pressure drops significantly: Go to step 10 (System pressure / pump error).
Contamination analysis (in-depth):
1. Take a representative oil sample for particle counting (ISO 4406 purity class) and laboratory analysis (water content, acid number, viscosity).
2. Check the degree of contamination of the return filter and pressure filter.
3. Result:
  - Cleanliness class worse than OEM specification (e.g. > ISO 18/16/13): Go to step 6.1 (Particulate contamination).
  - Water content above 100-200 ppm, increased acid number: Go to step 6.2 (fluid degradation/water).
  - Filter very dirty: Go to step 6.3 (Clogged filters).
1. 6.1 Particulate contamination:
  1. Cause: Wear particles, penetration of dirt.
  2. Measure: Oil change and system flushing, replacing all filters. Use of fine filters.
2. 6.2 Fluid degradation/water:
  1. Cause: aging of the oil, condensation, leakage of cooling water.
  2. Measure: oil change, system drainage, leak detection.
3. 6.3 Clogged filters:
  1. Cause: Inadequate maintenance, high contamination rate.
  2. Measure: filter replacement.
Mechanical errors (actuator / mechanics):
1. Check actuator for freedom of movement (stopped machine, move manually).
2. Check the mechanical guidance of the actuator for stiffness, damage (impact points), and contamination.
3. Check bearings and seals for leaks or damage.
4. Result:
  - Sluggishness / blockage: Repair or replace actuator/machine mechanics.
Electrical / electronic errors (signal path):
1. Check cables and plug connections to the proportional valve for damage, corrosion, loose contacts (visual inspection, resistance measurement with no voltage).
2. Check the shielding and grounding of the signal lines (e.g. to VDE 0100).
3. Measure the output signal of the valve amplifier/controller with an oscilloscope. Look for noise, interference (EMI), or irregular pulses.
4. Check the power supply of the valve amplifier/controller (setpoint: 24V DC ±5%).
5. Result:
  - Cable break, corrosion, poor grounding: Repair/replace cable/plug, check grounding.
  - Noise, interference in the signal: Check shielding, use filter if necessary, correct EMC problems.
  - Defective amplifier/controller: Replace valve amplifier/controller.
  - Defective valve coil (resistance outside tolerance): Replace valve coil.
Proportional valve internal errors:
1. ATTENTION: Disassemble only by qualified personnel! Pay attention to residual pressure!
2. Check the mechanical clearance of the control slide in the valve (if dismantled and cleaned). Look for scratches, burrs, jams.
3. Check the valve spring(s) for breakage or fatigue.
4. Measure the leakage volumes of the valve with a flowmeter (operate system in bypass, leakage connection on the valve to the tank). Compare with manufacturer's information. (> 50% of the nominal leakage as an alarm value).
5. Result:
  - Sluggish/damaged slide: Valve cleaning, overhaul or replacement if necessary.
  - Broken/tired spring: Valve overhaul or replacement.
  - Increased internal leakage: Valve overhaul or replacement.
System pressure / pump error:
1. Measure the main system pressure of the hydraulic unit. Are the setpoints stable? (e.g. 160 bar ±5%).
2. Measure the pressure directly at the pump outlet.
3. Listen for unusual noises from the pump (cavitation, bearing noises).
4. Check the delivery rate of the pump (with a flow meter).
5. Result:
  - Fluctuating/too low system pressure: Check pressure control valves, relief valves, repair/replace pump if necessary.
  - Cavitation noises: Check intake filter, suction line, check oil level.

6. Error-cause matrix

This matrix summarizes the most common symptoms, likely causes and required diagnostic tests to enable rapid fault identification. The causes are ordered by probability (from high to low) within each category.

Symptom	Probable causes (by probability)	Diagnostic test	Expected result if cause is confirmed
Actuator jerks/vibrates when moving	1. Contamination of the hydraulic fluid (particles)	Particle counter, visual inspection of oil filter	ISO 4406 purity class worse than 18/16/13; Filter heavily dirty.
	2. Stuck/sluggish proportional valve spool	Oscilloscope (signal on the valve), pressure measurement before/after the valve, dismantling/visual inspection of the slide	PWM signal on valve correct, but pressure or actuator response irregular; Slider shows scratches/burrs.
	3. Electrical signal interference (noise, EMI)	Oscilloscope on the PWM input of the valve/amplifier	Visible noise peaks, irregular pulse shapes, disturbances in the signal image.
Actuator creeps/drifts when stationary	1. Internal leakage in the proportional valve (zero point not tight)	Leakage measurement at the valve (bypass), actuator pressure measurement at standstill	Leakage quantity > manufacturer's information; Pressure drops over time even though valve is closed.
	2. Internal leakage in the actuator (piston seal defective)	Lockout test on the actuator (shut off lines), leakage measurement on the cylinder ventilation connection	Pressure drops despite valve lock; Oil leaks from breather.
	3. Valve amplifier offset error	DMM/oscilloscope at the valve input (zero position)	Measurement shows small but permanent control signal (current/voltage), even though the setpoint is zero.
Actuator reacts sluggishly/too slowly	1. Increased viscosity of the hydraulic fluid (too cold, wrong oil)	Oil temperature measurement, oil sample for viscosity analysis	Oil temperature below operating range; Viscosity out of specification.
	2. Partial blockage of filters or pipes	Pressure difference measurement via filter, visual inspection of filter	pressure drop across filter increased; Filter heavily dirty.
	3. Partial failure of the hydraulic pump (lack of flow)	Flow measurement at the pump outlet, pressure measurement system pressure	Flow rate below target value; System pressure fluctuates or is too low.
Actuator passes over target position	1. Incorrect setting of the proportional valve controller (P component too high)	Checking controller parameters in the control system	P-Gain value set too high leads to overshoot.
	2. Defective/not calibrated position feedback (position measuring system)	Comparison of target/actual position, checking position measuring system with external measuring device	Deviation between actual position and measured feedback signal.
	3. Mechanical play in kinematics	Game testing of the actuator connection and mechanics	Significant play in joints, bearings or fastenings.

7. Root cause analysis for major defects

A deep understanding of the causes is crucial for sustainable solutions and avoiding repeat errors. The following sections explain the primary root causes.

7.1 Contamination of hydraulic fluid

7.1.1 Particulate contamination

Explanation: Fine particles such as metal abrasion, weld beads, seal residues or ambient dirt (dust) circulate in the hydraulic system. These particles build up in the fine control edges and gaps of proportional valves, blocking or obstructing the slide and causing friction. They can also restrict or clog nozzles and control holes. This leads to imprecise slider positioning and consequently jerky or irregular actuator movement. Purity classes are defined according to DIN ISO 4406, which describe the maximum permissible particle content. Exceeding these classes is a sure sign of contamination.

Confirmation: Particle counting of the hydraulic fluid (online or laboratory analysis) shows a purity class that is significantly worse than the class specified by the component manufacturer (e.g. valve manufacturer Bosch Rexroth, Parker, Eaton) or the machine specification (e.g. ISO 18/16/13). Visual inspection of removed filters shows heavy contamination with dark, muddy abrasion.

Damage if not repaired: Persistent particle contamination leads to abrasive wear on the high-precision fits of valve slides and housings, pumps and actuators. This increases internal leakage, reduces system efficiency and leads to irreversible component wear, requiring complete replacement of affected components.

7.1.2 Water and fluid degradation

Explanation: Water in hydraulic oil (>100-200 ppm) can lead to hydrolysis of additives, reducing the lubricity of the oil and causing corrosion. It forms emulsions that stress the filter systems and promote cavitation in pumps and valves. Oxidized or thermally degraded hydraulic fluid (high acid number) loses its lubricating properties, forms resins and sludge that can stick valve slides and cause stiffness. Oil viscosity that does not conform to the standard (e.g. too high when cold or too low when overheated) also affects the response time and control precision of proportional valves.

Confirmation: Laboratory analysis of the hydraulic fluid (Karl Fischer test for water, acid number according to DIN 51558, viscosity according to DIN 51562) shows values outside the manufacturer's specifications. The smell of the oil is burnt or sour. Oil is cloudy or milky.

Damage if not repaired: Corrosion and cavitation damage to pumps, valves and actuators. Formation of sticky residue that blocks moving parts. Dramatic loss of service life of hydraulic components and increased risk of total failure.

7.2 Stuck/sluggish proportional valve spool

Explanation: The control slide in the proportional valve must be able to move freely and precisely in order to convert the desired signal into a volume flow. Reasons for sluggishness are:

Particles: As described under 7.1.1, particles can jam the slider.
Resin/sludge formation: Degraded hydraulic fluid leaves behind sticky residue.
Mechanical damage: Manufacturing defects, corrosion, deformation of the slide or housing due to overload or incorrect assembly.
Foreign bodies: Foreign materials accidentally penetrated.

The result is an inaccurate or delayed positioning of the slide, which is directly reflected in an irregular movement of the actuator.

Confirmation: After removing the valve from the system and carefully relieving pressure, the slide can be manually checked for freedom of movement (if provided). Visually inspect the slider and housing for scratches, burrs or debris. Pressure measurements before and after the valve show irregular pressure curves despite correct electrical control.

Damage if not rectified: Continued sluggishness can lead to overloading of the valve coil (due to increased current), increased wear on the slide and housing and ultimately total failure of the valve. Accurate control is no longer possible, which significantly affects machine precision.

7.3 Electrical signal interference

Explanation: Proportional valves are controlled via electrical signals (current or voltage, often as PWM). Disturbances in this signal path lead directly to valve malfunction. Possible causes are:

Cable break/contact problems: Interruptions or loose connections in the supply line to the valve coil.
Noise/EMI (Electromagnetic Interference): Inductive or capacitive coupling of interference signals into the control line, e.g. through parallel power cables or inadequate shielding. This results in a "dirty" control signal.
Defective valve controller/amplifier: A malfunction in the amplifier's electronics can lead to an unclean or unstable output signal.
Defective valve coil: A damaged coil (short turns, increased resistance) no longer responds correctly to the input signal.

Confirmation: Oscilloscope measurements at the valve input show a signal that does not correspond to the setpoint (e.g. noisy, unstable, incorrect amplitude/frequency). Resistance measurement of the valve coil deviates from the target value. Visual inspection of the wiring and connectors shows corrosion or damage. Checking the grounding (potential equalization according to VDE 0100).

Damage if not rectified: Incorrect control of the valve leads to uncontrolled movements, overheating of the valve coil and can ultimately destroy the valve controller or the coil. In the long term, this affects machine control and safety.

7.4 Internal leakage in the proportional valve

Explanation: A proportional valve is never completely tight. A small amount of internal leakage is normal and indicated in the datasheet. However, if this leakage becomes too large due to wear (e.g. due to particle contamination), damage to the fits or fatigue of the springs, the valve can no longer hold the actuator when stationary (creep/drift) or can no longer control it precisely during movement. The control signal is no longer fully converted into effective volume flow.

Confirmation: Leakage measurement at the valve (in the bypass to the tank) shows a volume flow rate that is significantly above the maximum specified by the manufacturer (e.g. > 50% deviation). Pressure measurements on the actuator show a slow pressure drop even though the valve should be closed. A thermal camera can reveal local heating at the valve due to increased internal friction and leakage flows.

Damage if not repaired: Energy loss due to increased leakage current (heat development in the system), imprecise actuator positioning, increased wear of other components due to overheating of the oil. The actuator can no longer hold a load, which can lead to safety risks.

8. Step-by-step fix procedure

The following instructions are general in nature. ALWAYS follow the machine and component manufacturer's specific instructions.

8.1 Resolving Contamination Issues

SAFETY: Energy release (LOTO) and pressure relief before any work on the system!
Oil change: All hydraulic oil must be drained and disposed of properly (according to WHG).
System cleaning/flushing:
- In case of heavy contamination: Use a separate flushing pump with a fine filter system and flushing oil. The system should be flushed at elevated temperature to loosen deposits.
- Alternatively: Change the filter several times with new oil after a short period of operation.
Filter replacement: ALL hydraulic filters (suction, pressure, return, ventilation filters) must be replaced with new filter elements that correspond to the system. Check the specification (filter fineness in µm, e.g. 10 µm for pressure filters).
Tank cleaning: Open the hydraulic tank, clean it and remove deposits.
Refill: Fill the system with new, clean hydraulic oil that meets the purity class required by the manufacturer (often available as pre-cleaned oil).
Measurement & documentation: After filling and a few hours of operation, take another oil sample and check the purity class. Document the results.

8.2 Repair/replacement of the proportional valve

SAFETY: Energy release (LOTO) and pressure relief of the valve circuit!
Disassembly: Disconnect electrical connections (label!) and hydraulic lines. Catch any escaping oil.
Visual inspection/cleaning (if dismantled):
- Carefully disassemble the valve according to the manufacturer's instructions.
- Clean all components (sliders, housings) with special cleaning fluids for hydraulic components.
- Check the slider for burrs, scratches, discoloration. Light burrs can be carefully removed with a fine honing stone (only by experienced personnel!).
- Check springs for damage or fatigue.
Spare Parts: If worn or damaged, replace the coil, springs or seal kits.
Assembly: Assemble the valve carefully, pay attention to the correct torques for screws (e.g. according to DIN EN ISO 898-1) and the correct positioning of seals.
Replacing the valve (in case of internal leaks or irreversible damage): If repair is not possible or uneconomical, replace the entire proportional valve with a new or reconditioned valve of identical specification.
Commissioning & functional test:
- Ventilate the system.
- First activation at reduced pressure.
- Calibration of the valve (zero point, gain) on the amplifier/controller or via the PLC.
- Fine adjustment of the control parameters.
- Functional test under operating conditions, observation of the actuator movement, measurement of control quality.

8.3 Elimination of electrical signal problems

SAFETY: Energy isolation (LOTO) of the electrical system when working on cables and contacts. Work under voltage only with appropriate training (VDE 0105-100) and PPE!
Cable inspection:
- Visual inspection of the entire cable length for damage, crushing, insulation faults.
- Resistance measurement on cables and contacts (voltage-free) for continuity and contact resistance.
- Check the plug connections for tightness, corrosion and cleanliness. If necessary, clean and treat with contact spray.
Shielding & grounding: Check the correct and continuous grounding of the cable shielding on both sides (control and valve side) in accordance with EMC guidelines (DIN EN 61000). Broken or faulty shielding is a common cause of noise.
Signal filter: If there is persistent noise, using a suitable signal filter (ferrite core, RC element) in the signal path can help.
Valve coil: Replace a defective valve coil. Ensure correct alignment and tight fit.
Valve controller/amplifier: If the amplifier/controller is suspected of being defective, replace it. A previous cross-swap test with a known working device can confirm the diagnosis.
Functional test: After rectification, carefully check the electrical signals (oscilloscope) and the actuator movement.

9. Preventive measures

Prevention is better than cure. By implementing robust preventive measures, the life of hydraulic components can be extended and unexpected failures can be minimized.

Root cause	Prevention strategy	Monitoring method	Recommended interval
Particulate contamination	Regular filter changes, use of high-quality filters (e.g. beta ratio > 200), ventilation filters with moisture protection, careful cleanliness during maintenance work.	Regular particle counting (ISO 4406), pressure difference measurement via filter, visual inspection of oil in the tank.	Filter change: according to manufacturer's specifications or pressure difference; Particle counting: semi-annually to annually.
Water and fluid degradation	Use of appropriate hydraulic oil according to manufacturer's specifications (DIN 51524), avoidance of overheating (temperature monitoring), regular oil analysis, draining the system if necessary.	Oil analysis (water, acid number, viscosity), temperature monitoring of the hydraulic oil.	Oil analysis: annually or after 2000 operating hours; Temperature: continuous.
Stuck/sluggish proportional valve spool	Maintaining fluid purity (see above), avoiding overheating, regularly checking valve settings.	Monitoring the actuator movement (precision, smoothness), measuring the current/voltage signal on the valve.	Functional test: monthly; Signal testing: annually for precision-critical applications.
Electrical signal interference	Correct installation and grounding of cables (according to VDE 0100), regular checking of cables and plug connections for damage, use of shielded cables.	Visual inspection of cable/plug, resistance measurement (voltage-free), oscilloscope measurement if suspected.	Visual inspection: every six months; detailed electrical test: every 2-3 years.
Internal leakage in the proportional valve	Maintaining fluid purity, avoiding overpressures and pressure peaks, proactively replacing seal sets during major overhauls.	Leakage measurement (if technically feasible), observation of creep/drift at standstill, thermography.	Annual functional test; Leakage measurement: every 2-3 years for precision-critical valves.

10. Spare Parts & Components

Quick troubleshooting requires the availability of the right replacement parts. UNITEC-D offers a wide range of hydraulic components that meet the highest quality standards (CE, TÜV).

Partial description	Specification	When to replace	UNITEC category
Proportional directional control valve	Identical model and manufacturer (e.g. Bosch Rexroth, Parker, Eaton) or compatible CE-certified spare part; Nominal diameter, max. pressure, nominal volume flow.	In the event of irreversible damage, excessive internal leakage, unrecoverable sluggishness or total electrical failure.	Hydraulic valves
Valve coil for proportional valve	Voltage (V DC/AC), Current (mA), Resistance (Ω), Connector Type.	In the event of an electrical defect (cable break, winding short, impermissible resistance) or mechanical damage.	Valve accessories
Filter elements (pressure, return, suction filters)	Filter fineness (µm), beta ratio, length, diameter, sealing material.	According to the maintenance schedule or when the maximum permissible pressure difference is reached.	Hydraulic filter
Hydraulic oil (HLP type)	Viscosity (cSt at 40°C, e.g. ISO VG 46), specifications (DIN 51524 part 2), manufacturer specification.	According to the maintenance schedule or if the oil quality deteriorates (contamination, aging).	Hydraulic fluids
Seal kits for proportional valve/actuator	Material (NBR, FKM, PTFE), dimensions, manufacturer/valve type specific.	In the event of visible leaks or during overhauls.	Seals
Valve amplifier / valve regulator	Identical type or compatible replacement device; Input/output signals, interface (analog, CAN, EtherCAT).	In the event of an electronic defect that cannot be corrected through software/calibration.	Electronic components

For detailed information and to order spare parts, visit our UNITEC-D e-catalogue.

11. References

DIN EN ISO 4413: Hydraulic fluid technology - general rules and safety requirements for hydraulic systems.
DIN ISO 4406: Hydraulic fluid technology - Code for describing particle contamination of solid particles in fluids.
VDE 0105-100: Operation of electrical systems.
DIN EN 61000 (EMC guidelines): Electromagnetic compatibility.
DGUV regulation 3: Electrical systems and equipment (formerly BGV A3).
Manufacturer manuals for specific proportional valves (e.g. Bosch Rexroth, Parker Hannifin, Eaton Vickers).
Internal UNITEC-D maintenance manuals for specific machine types.