Diagnosing and Resolving Overheating in Hydraulic Systems: A Technical Guide

1. Problem description & scope of application

This technical diagnostic guide is intended for service technicians and maintenance professionals who are faced with overheating problems in industrial hydraulic systems. Elevated hydraulic oil operating temperature beyond manufacturer-specified limits (typically above 60-70°C for continuous use) results in accelerated oil degradation, reduced efficiency, premature component wear and can result in catastrophic system failure. The symptoms and diagnostic procedures described here are applicable to a variety of hydraulically powered machines, including presses, injection molding machines, machining centers, lifting devices and mobile work machines.

Symptoms of Hydraulic System Overheating:

• Increased oil temperature in the tank and on components (measurement with IR thermometer or integrated sensors).
• Discoloration of the hydraulic oil (dark, brownish tone) and unusual smell (burnt).
• Reduced system performance, slower cycles, irregular movements of actuators.
• Frequent activation of temperature alarms or emergency stop switches.
• Increased pump noise level (cavitation).
• Leaks in seals and hoses due to material aging and hardening.

Severity classification:

• Critical: Oil temperature exceeds the maximum permissible value (e.g. > 85°C) or system switches off repeatedly. Immediate action required to avoid system damage and safety risks.
• Severe: Oil temperature constantly above the recommended optimal range (e.g. 70-80°C), performance losses noticeable. Planning for immediate troubleshooting is necessary.
• Minor: Oil temperature slightly increased (e.g. 60-70°C), no immediate impact on performance. Early diagnosis to prevent escalating problems.

2. Safety instructions

ATTENTION! Working on hydraulic systems involves considerable danger. Hot oil can cause serious burns. Oil under pressure can escape at high speed and cause serious injuries (danger of injection). Unloaded energy storage (hydraulic accumulators) can lead to uncontrolled movements. Strictly follow the safety regulations in accordance with DIN EN ISO 4413 and the manufacturer's instructions.

PERSONAL PROTECTIVE EQUIPMENT (PPE): Always wear safety glasses (DIN EN 166), cut-resistant protective gloves (DIN EN 388), safety shoes (DIN EN ISO 20345 S3) and suitable protective clothing during all diagnostic and maintenance work.

LOCKOUT/TAGOUT (LOTO): Before starting any work on the hydraulic system, the machine must be completely disconnected from the power supply and secured against being switched on again (DIN VDE 0105-100). Pressure accumulators must be professionally relieved. Make sure there are no residual energies.

HOT SURFACES AND LIQUIDS: Hydraulic oil and system components can reach temperatures over 80°C. Before dismantling, allow the system to cool or take appropriate precautions against burns.

PRESSURE RELIEF: Make sure all system pressures are completely relieved before disconnecting any lines or components. Use the relief devices provided for this.

3. Required diagnostic tools

Specific measuring devices are essential for precise error analysis:

4. Checklist for initial assessment

Before the detailed diagnosis, the following points must be checked and documented:

5. Systematic diagnostic flowchart

Troubleshooting is structured, starting with the most likely and easiest causes to check:

1. Überhitzung festgestellt (> X°C über Sollwert):
   1. Sichtprüfung und Erstbewertung (Referenz 4. Checkliste):
      • Ölstand, Farbe, Geruch prüfen.
      • Observe the noise of the pump.
      • Check filter status indicators.
      • Check all ventilation openings and radiators for contamination.

      → If there are any abnormalities: Try to correct the problem directly (e.g. refill oil, change filter, clean cooler).

      → If there are no abnormalities: continue with point 2.

2. Check cooling of the system:
   1. Thermal image analysis of the cooler:
      • Check the temperature distribution on the cooler with a thermal imaging camera.
      • Measure radiator inlet and outlet temperatures (Delta T).
      • Reference values: Air-cooled cooler Delta T (oil on/off) should be 10-20°C. Water cooler Delta T (oil on/off) 5-15°C.

      → If Delta T is too low (< 10°C for air, < 5°C for water): cooler performance insufficient.

      • Cause A1: Radiator fins clogged (dust, dirt with air cooling).
      • Cause A2: Cooling water flow insufficient (valve closed, pump defective, blockage in water cooling).
      • Cause A3: Radiator fan defective or blocked (with air cooling).
      • Cause A4: Bypass valve on the cooler is stuck open (oil bypasses the cooler).

      → If Delta T is normal, but system overheats: Cooling works, but too much heat input. Continue with point 3.

3. Check pump function and system pressure:
   1. Pressure measurement:
      • Measure system pressure at idle and under load. Comparison with target values.
      • Check minimum pressure (control pressure) on the pump.

      → If system pressure is too high: Continue with point 3b.

      → If system pressure fluctuates greatly under load or is too low: Continue with point 3c.

   2. Flow measurement of the pump:
      • Measure the volume flow of the pump (e.g. with a flowmeter after the pressure relief valve).
      • Comparison with manufacturer specifications. (e.g. nominal flow rate - 5%).

      → If delivery volume is greatly reduced (> 10% below nominal value) at normal speed and pressure:

      • Cause B1: Internal leaks in the pump (wear).
      • Cause B2: Blockage of the suction line (cavitation).

      → If delivery volume is normal but pressure is excessive: Continue with point 4.

4. Checking the valves and actuators for internal leaks:
   1. Differential pressure measurement across valves:
      • Measure differential pressure across main directional valves and pressure relief valves using pressure gauges.
   2. Thermal image analysis on valves and actuators:
      • Hotspots on valves or cylinders can indicate internal leaks.

      → If excessive heat generation is detected on valves or actuators:

      • Cause C1: Pressure relief valves open too early/are set incorrectly (permanent conversion of pressure energy into heat).
      • Cause C2: Directional valves or check valves are blocked/jammed (no complete closure, permanent leakage current).
      • Cause C3: Cylinder seals or engine seals are leaking internally (leaking oil generates heat).

5. Check the condition of the hydraulic oil:
   1. Sample removal for laboratory analysis (according to ISO 4021):
      • Determination of viscosity at 40°C and 100°C (DIN 51550).
      • Particle counting (ISO 4406:1999, e.g. 18/16/13).
      • Water content (ASTM D6304).
      • Acid number (TAN, ASTM D664).

      → If viscosity outside manufacturer tolerances (± 10% of nominal value), particle count above alarm level, high water content (> 100 ppm) or high TAN:

      • Cause D1: Incorrect oil type or mixture.
      • Cause D2: Oil aging and degradation (accelerated by high temperatures).
      • Cause D3: Contamination by water or solids (increased friction and wear).

6. Error-cause matrix

7. Root cause analysis for each error

A. Poor cooling performance

• Cause: Radiator fins clogged / cooling water flow insufficient / fan defective:
  • Why it happens: Deposits of dust, dirt or other particles on the fins of an air-cooled cooler drastically reduce the effective heat dissipation area. In water coolers, limescale or corrosion in the cooling water circuit can hinder heat exchange. A defective fan motor or blocked fan blades prevent the necessary air circulation.
  • How to confirm: Visual inspection of the radiator fins. Measurement of the volume flow and pressure in the cooling water circuit in water coolers. Checking the electrical function of the fan (voltage, current) on air coolers. Thermal imaging analysis shows cold areas on the radiator where no heat exchange occurs.
  • Consequential damage: Long-term overheating leads to accelerated oil degradation and shortened service life of seals, pumps, valves and actuators. Increased energy consumption due to reduced system efficiency.
• Cause: Bypass valve is stuck open:
  • Why it happens: A defective or incorrectly set bypass valve directs some or all of the hydraulic oil past the cooler, which massively reduces the cooling performance.
  • How to confirm: Temperature measurement before and after the radiator and at the bypass valve. A small temperature difference across the radiator while the bypass valve is hot at the same time indicates this. If necessary, carry out a functional test of the valve or dismantle and check it.
  • Consequential damage: Permanent overheating with all the associated problems of oil and component degradation.

B. Excessive heat generation by pump

• Cause: Internal pump leaks (wear):
  • Why it happens: Wear on pistons, cylinder bores, gears or vanes of a hydraulic pump leads to an increased leakage oil flow within the pump. This leakage oil is put under pressure and then expanded without pressure, whereby the energy is converted into heat.
  • How to confirm: Measurement of pump delivery volume under load. A significantly reduced volume flow (e.g. > 10% below nominal value) at the correct speed and system pressure is a strong indication. Unusual noises (rattling, grinding) from the pump. Thermal imaging analysis can show hotspots on the pump.
  • Consequential damage: Reduced system efficiency, slower machine cycle, increased oil temperature, increased wear of other system components due to contaminated oil.
• Cause: Cavitation in the pump:
  • Why it happens: Excessive negative pressure in the suction line of the pump (e.g. due to a clogged suction filter, suction line that is too thin, oil viscosity that is too high or oil level that is too low) leads to the formation and sudden collapse of steam bubbles. This creates extremely high local temperatures and pressure spikes that overheat the oil and damage the pump.
  • How to confirm: Clear rattling noises from the pump. Measurement of the vacuum in the suction line (typically should not exceed -0.1 to -0.3 bar). Checking the suction filter and the oil level.
  • Consequential damage: Rapid wear of the pump (pitting, erosion), noise emissions, oil aging due to local heat peaks.

C. Excessive heat generation from valves and system pressure

• Cause: Pressure relief valve (DBV) incorrectly set / stuck:
  • Why it happens: If a DBV opens too early or is permanently partially open, part of the pump flow is constantly diverted into the tank via the valve. Here, all of the pressure energy is converted into heat without any mechanical work being done. This is one of the most common causes of overheating.
  • How to confirm: Measurement of system pressure. If the DBV opens without load at a pressure significantly lower than the operating pressure, or if it is constantly open when it should not. Thermal image analysis shows high temperatures at the DBV.
  • Consequential damage: Very rapid overheating of the oil, waste of energy, rapid oil degradation.
• Cause: Internal leaks in valves (e.g. directional control valves, check valves):
  • Why it happens: Wear on slides or seals in valves leads to internal leakage currents. Oil flows from high pressure to low pressure areas without doing any work, and the pressure energy is converted into heat.
  • How to confirm: Thermal imaging analysis on all valves, especially on stationary actuators. Leakage oil measurement on the tank returns of the valves, if possible. With directional control valves, a “soft” holding of the actuator or a release of pressure can be an indication.
  • Consequential damage: Overheating, loss of holding forces in cylinders, reduced efficiency.
• Cause: Too high operating pressure in the system:
  • Why it happens: If the machine is continuously operated at a higher pressure than necessary for the work to be done, an unnecessary amount of energy is converted into heat. This can be caused by incorrect settings or improper sizing of the actuators.
  • How to confirm: Check the machine's print settings and compare them to the application requirements. Measurement of actual working pressure.
  • Consequential damage: Overloading of components, increased wear, wasted energy and overheating.

D. Oil-Related Causes

• Cause: Incorrect oil type / viscosity or oil aging:
  • Why it happens: Too high an oil viscosity leads to increased friction and pressure losses, which generates heat. Too low a viscosity reduces lubricity and leads to increased friction and leaks. Both lead to heat generation. Oil aging through thermal degradation, oxidation and loss of additives changes the original properties of the oil and can also contribute to overheating.
  • How to confirm: Oil analysis. Comparison of the measured viscosity at 40°C and 100°C with the manufacturer's specifications (data sheet). Checking the oil filling log. Determination of acid number (TAN) and particle counting.
  • Consequential damage: Wear of pumps and valves, reduced service life of all components, increased operating costs.
• Cause: Contamination of the hydraulic oil (water, particles):
  • Why it happens: Water in the oil leads to corrosion, accelerates oil degradation and can reduce the lubricant film carrying capacity. Particles (solid contaminants) act like abrasives, increasing friction and causing wear on pumps, valves and cylinders, which in turn leads to increased heat generation.
  • How to confirm: Oil analysis (water content, particle counting according to ISO 4406). Cloudiness of the oil may indicate water.
  • Consequential damage: Massive wear and failure of components, cavitation, filter blockages, shortened oil life.

8. Step-by-step troubleshooting procedures

The following procedures are tailored to the identified causes. Before any maintenance work, the system must be secured in accordance with LOTO.

A. Correct inadequate cooling performance

1. Clean radiator fins (air cooler):
   • Safety: Use LOTO.
   • Cleaning: Use compressed air (max. 6 bar, distance > 15 cm) or suitable radiator cleaner. Do not damage the slats.
   • Verification: Put the cooler under load again after cleaning. Measure the temperature difference (Delta T). Target: 10-20°C.
2. Check cooling water flow/clean radiator:
   • Safety: Apply LOTO.
   • Check: Check cooling water pressure and flow (manometer, flow meter).
   • Cleaning: Rinse the water cooler according to the manufacturer's instructions, chemical descaling if necessary.
   • Verification: Check cooling water flow and Delta T after cleaning. Target: Delta T 5-15°C.
3. Repair/Replace Radiator Fan:
   • Safety: Apply LOTO.
   • Repair: Check electrical connections, test fan motor. If necessary, replace the fan motor or complete fan.
   • Verification: Check fan function under load. Check airflow before/after cooler.
4. Check/Adjust/Replace Radiator Bypass Valve:
   • Safety: Apply LOTO.
   • Check: Check valve for free movement, check spring. If necessary, check the pressure setting.
   • Adjustment/replacement: Adjust valve according to manufacturer's specifications or replace defective valve.
   • Verification: System under load, thermal image analysis on the valve. Oil should be completely routed through coolers.

B. Fix heat generation by pump

1. Internal pump leaks:
   • Safety: Apply LOTO.
   • Action: If there are significant internal leaks, repair or replace the pump. Repairs must be carried out by qualified personnel and include the replacement of worn internal parts.
   • Verification: After replacement/repair, measure delivery volume under load. Target: Max. 5% deviation from the nominal value. Observe system temperature.
2. Fix cavitation in the pump:
   • Safety: Apply LOTO.
   • Action: Check and correct oil level. Check/clean/replace suction filter. Check the suction line for blockages or leaks. If necessary, adjust oil viscosity or check suction line dimensions.
   • Verification: Measure vacuum in the suction line (should be -0.1 to -0.3 bar). Observe the noise produced by the pump.

C. Eliminate heat generation from valves and system pressure

1. Check/adjust/replace pressure relief valve (DBV):
   • Safety: Apply LOTO, relieve system pressure.
   • Measure: Set DBV according to manufacturer specifications. Clean or replace stuck valves.
   • Verification: Check pressure setting with precise pressure gauge. Thermal image analysis on the DBV under load.
2. Fix internal leaks in valves:
   • Safety: Apply LOTO, relieve system pressure.
   • Measure: Dismantle affected valves, clean them and check seals/slides. Replace wearing parts or replace the entire valve.
   • Verification: Check system function and thermal image analysis after installation.
3. Adjust operating pressure:
   • Safety: Apply LOTO, relieve system pressure.
   • Action: Correct print settings according to machine and application requirements.
   • Verification: Operate the system under load and carry out pressure measurements.

D. Correct oil-related causes

1. Oil change and system flushing:
   • Safety: Apply LOTO, provide container for used oil.
   • Action: Drain all hydraulic oil, flush system and fill with correct manufacturer-specified oil type and viscosity. Change filter.
   • Verification: Carry out oil analysis after filling, observe system temperature.
2. Remove contamination of the hydraulic oil:
   • Safety: Apply LOTO.
   • Measure: If water enters, find and eliminate the cause (e.g. check tank ventilation filter, seals). In the event of particle contamination, filter change and, if necessary, additional offline filtration. Flush system if necessary.
   • Verification: Repeat oil analysis to confirm oil purity.

9. Preventive measures

10. Spare Parts & Components

The availability of the right spare parts is crucial for quick and efficient troubleshooting. All components listed below can be found in our e-catalog at www.unitecd.com/e-catalog/.

11. References

• DIN EN ISO 4413: Hydraulic fluid technology - general rules and safety requirements for hydraulic systems and their components.
• DIN VDE 0105-100: Operation of electrical systems - Part 100: General specifications.
• VDI 2417 Sheet 1: Testing hydraulic fluids – determining the kinematic viscosity.
• ISO 4406:1999: Hydraulic fluid technology - liquids - coding of the degree of contamination by solid particles.
• ISO 4021: Hydraulic fluid technology – sampling of particle-contaminated fluids from the pressure system.
• Manufacturer maintenance manuals for the respective hydraulic system.
• UNITEC maintenance guides for specific components.