Guide to the Diagnosis and Resolution of Thermal Overload in Hydraulic Systems

1. Description of the Problem and Purpose

Thermal overload in a hydraulic system manifests itself as an excessive increase in the temperature of the hydraulic oil above the recommended operating limits (typically > 60°C for systems with conventional mineral oil, UNI EN ISO 11158). This critical phenomenon not only reduces the overall efficiency of the machine tool, but also accelerates the degradation of the oil, compromising its lubricating and anti-wear properties. As a result, there is premature wear of components (pumps, valves, actuators, seals) and an increase in unplanned maintenance costs.

This diagnostic guide is designed for experienced maintenance technicians, reliability engineers and production managers, providing a systematic path to identify the root cause of overheating and implement targeted solutions. The goal is to restore optimal operating conditions and extend the useful life of the system.

Severity Classification:

• Critical: Oil temperature > 75°C. Immediate risk of catastrophic component failure, fire, or unplanned production shutdown. Requires immediate intervention with machine downtime.
• Major: Oil temperature between 65°C and 75°C. Accelerated degradation of oil and components, reduction in machine performance. Requires short-term intervention planning.
• Minor: Oil temperature between 60°C and 65°C. A sign of a potential problem, the oil begins to oxidize more rapidly. It requires proactive monitoring and investigation.

2. Safety Precautions

WARNING: Operations on hydraulic systems present significant risks. Before any diagnostic or maintenance work, it is ESSENTIAL to take the following precautions to prevent serious or fatal injuries:

• Lockout/Tagout Device (LOTO): Make sure that the machine is completely de-energized, depressurized and that all energy sources (electrical, hydraulic, pneumatic) are isolated and blocked according to the company procedure (UNI EN ISO 14118 standard).
• Stored Energy: Hydraulic systems can retain energy under pressure even after the pump is turned off. Check for complete pressure relief through relief valves or accumulators. Use pressure gauges to confirm no residual pressure.
• Hot Fluids Under Pressure: Hot hydraulic oil can cause severe burns. Injecting fluid under the skin, even from small leaks, can be lethal. Always wear adequate Personal Protective Equipment (PPE): safety glasses, hydrocarbon resistant gloves (UNI EN 374), long-sleeved protective clothing.
• Hot Surfaces: Components such as tanks, heat exchangers and engines can reach high temperatures. Use thermal gloves if necessary.
• Work Environment: Keep the work area clean, dry and well lit. Prevent slipping caused by oil spills.
• Heavy Lifting: Some components (pumps, motors) can be heavy. Use appropriate lifting equipment and follow safety procedures for handling loads.

3. Necessary Diagnostic Tools

The effective diagnosis of thermal overload requires specific and calibrated instrumentation.

4. Initial Assessment Checklist

Before starting any invasive diagnostic procedure, it is essential to gather information about the machine's operating context.

5. Systematic Diagnosis Flow

This decision diagram guides the technician through a logical path to isolate the root cause of the thermal overload.

1. Initial Check (Machine Running, Normal Conditions):
   1. Oil Temperature > 60°C and rising?
      • NO: No active overheating problems. Monitor.
      • YES: Proceed to Step 2.
2. Cooling Circuit Inspection:
   1. Is the Heat Exchanger (air/oil or water/oil radiator) efficient?
      • Perform Thermographic Test: Point the thermal imager at the exchanger. The temperature difference between oil inlet and outlet should be 5-10°C. The outer surface of the air cooler should show a homogeneous temperature distribution.
      • Check Air/Water Flow:
        • Air/oil radiator: Check fan (speed with tachometer, power with multimeter), clean fins (visual).
        • Water/oil radiator: Check water flow (with flow meter if installed), water regulation valve (operation, calibration), presence of dirt on the water side (differential pressure between water inlet and outlet).
      • Differential Pressure (plate or tube exchangers only, oil side): Install pressure gauges before and after the exchanger. Pressure difference > 1.5 bar indicates internal obstruction.
      • If Cooling Circuit INEFFICIENT:
        • Probable Cause: Obstructed exchanger, faulty cooling fan/pump, blocked thermostat, insufficient flow of the cooling medium.
        • Resolution: Clean exchanger, repair/replace fan/pump, replace thermostat, restore medium flow.
        • Finish Cooling Diagnosis. If the problem persists, proceed to Step 3.
      • If Cooling Circuit EFFICIENT: Proceed to Step 3.
3. Check Pump Efficiency and Internal Leaks:
   1. Check Pump Flow:
      • Install a flow meter on the pump delivery circuit (or on the return to the tank if the machine allows it for an empty cycle).
      • Cycle the machine at rated pressure and under no load.
      • Compare the measured flow rate with the technical data of the pump at different pressures.
      • Significant drop in flow rate (e.g. > 10% below rated pressure): Indicates internal wear of the pump.
      • Probable Cause: Wear of the pump (gears, vanes, pistons, sealing plate).
      • Resolution: Overhaul or replacement of the pump.
      • If the pump flow rate is within limits: Proceed to Step 4.
4. Pressure Drop and Obstruction Analysis:
   1. Evaluation of Differential Pressure on Filters:
      • Install pressure gauges before and after the pressure filter and/or the return filter.
      • Pressure difference > filter limit (e.g. > 2 bar for pressure filter, > 0.5 bar for return filter): Indicates clogged filter.
      • Probable Cause: Filters clogged.
      • Resolution: Replace filter elements.
      • If filters are clean: Proceed to Step 5.
   2. Evaluation of Pressure Drops in Pipes and Valves:
      • Use test pressure gauges at different points of the circuit to identify sections with excessive pressure drops.
      • Hot Spots with Thermal Camera: Detect spots with high temperatures on pipes or valve bodies, indicative of turbulence or throttling.
      • Probable Cause: Undersized pipes, restrictions, valves partially blocked or with high internal pressure drop, discharged accumulator (which does not absorb pressure peaks).
      • Resolution: Redesign pipelines, clean/overhaul valves, recharge accumulator.
      • If no abnormal pressure drops are identified: Proceed to Step 6.
5. Check the Maximum Pressure Valve Adjustment:
   1. Calibrate the Maximum Pressure (or Safety) Valve:
      • With a calibrated pressure gauge, check the valve calibration value.
      • If the valve opens too frequently or is set at too much pressure, it generates heat.
      • Probable Cause: Incorrectly calibrated or faulty pressure relief valve (stuck in partially open position, stem blocked).
      • Resolution: Recalibrate the valve to the value specified by the manufacturer (or overhaul/replace).
      • If the valve is calibrated correctly: Proceed to Step 7.
6. Hydraulic Oil Analysis and Viscosity:
   1. Oil Sampling and Analysis:
      • Take a sample of oil from the tank (following ISO 4021) and send it to a laboratory for full analysis (viscosity, ISO 4406, water, TAN, wear metals).
      • Viscosity out of specification (e.g. > 10% deviation from nominal): Degraded oil or incorrect type.
      • High Contamination (ISO 4406 > 20/18/15): Abrasive particles increase wear and heat generation.
      • Water content > 500 ppm: Reduces lubrication, promotes oxidation and corrosion.
      • Probable Cause: Degraded oil (oxidation, aging), wrong type of oil for operating conditions, contamination (water, particles).
      • Resolution: Replace oil with correct specification, improve filtration, identify and eliminate source of contamination.

6. Fault-Cause Matrix

The following table presents the most common causes of thermal overload, ranked by probability.

7. Root Cause Analysis for Each Failure

7.1. Inefficient Heat Exchanger

Explanation: The heat exchanger is responsible for dissipating the heat generated by the system. Its inefficiency can result from external obstructions (dust, debris on air radiators) or internal obstructions (mud, limescale on water/oil exchangers). Insufficient flow of the cooling fluid (air or water) or a faulty thermostat that does not open the bypass correctly complete the picture.

How to Confirm: Use a thermal imager to detect too low a temperature difference between oil inlet and outlet or uneven heat distribution on the surface. Measure the coolant flow. Check the continuity and operation of the thermostat with a multimeter.

Damage if Not Resolved: Accelerated degradation of the oil, premature wear of all seals and components sensitive to high temperatures, reduction in machine performance due to reduced viscosity of the oil.

7.2. Excessive Internal Losses (Wear of Components)

Explanation: Wear of pumps, hydraulic motors, valves (especially proportional or directional valves) and cylinders causes an increase in internal losses. This means that some of the pumped oil does not perform useful work but returns to the tank through increased clearances, converting hydraulic energy into frictional heat.

How to Confirm: Carry out a volumetric efficiency test of the pump with a flow meter. A drop in flow rate of 10-15% from ratings at operating pressure indicates significant wear. Thermal imaging can reveal abnormal hot spots on pump casings or hydraulic motors.

Damage if Not Resolved: Further acceleration of component wear, decline in efficiency and speed of actuators, premature failure of the pump or motors, with possible metallic contamination of the entire system.

7.3. Clogged filters

Explanation: A clogged pressure or return filter creates a significant pressure drop, impeding oil flow. This pressure drop is converted into heat.

How to Confirm: Monitor the differential pressure across the filters. Most filters are equipped with clogging indicators. With precision pressure gauges, a differential reading greater than 2 bar for the pressure filter or 0.5 bar for the return filter (generic values, check manufacturer specifications) is a clear indication. The thermal imaging camera may show a localized hot spot on the filter body.

Damage if Not Resolved: In addition to overheating, the filter bypass valve may open, allowing unfiltered oil to flow into the system, causing abrasive wear and catastrophic failure of sensitive components.

7.4. Degraded or Inappropriate Hydraulic Oil

Explanation: Hydraulic oil has a limited useful life. Over time, oxidation (accelerated by high temperatures) degrades additives, reduces viscosity and promotes the formation of acids and sludge. An oil with lower viscosity than required increases internal losses. Water (even in traces > 500 ppm) and solid particles (ISO 4406 > 20/18/15) reduce the lubricating power and increase friction and wear.

How to Confirm: Only laboratory oil analysis can confirm this fault. Key parameters: viscosity (out of range), particle number (high), water content (high), TAN (Total Acid Number, high).

Damage if Not Resolved: Severe wear of all components, corrosion, foaming, reduction in oil and machine life. Loss of energy efficiency.

7.5. Maximum Pressure Valve Incorrectly Calibrated or Faulty

Explanation: The maximum pressure valve protects the system from overpressure. If calibrated too low, or if it does not open and close properly due to dirt or mechanical failure, a significant portion of the pump's flow is continuously discharged to the high-pressure reservoir, generating a large amount of heat.

How to Confirm: Connect a precision pressure gauge upstream of the valve and check the actual opening pressure. Use a thermal imaging camera on the valve; an intense hot spot is indicative of a continuous flow of high-pressure oil passing through it.

Damages if Not Resolved: High power consumption, severe overheating, and long-term, potential failure of the valve itself and the pump due to ongoing stress.

8. Step-by-Step Resolution Procedures

ATTENTION: ALWAYS carry out the LOTO procedures before physically intervening on the machine. Wear appropriate PPE.

8.1. Resolution Inefficient Heat Exchanger

1. External Verification: Visually inspect the air radiator fins. Remove dust, dirt and debris with compressed air (max 6 bar) or washing with specific detergent and water (making sure it does not penetrate the oil circuit).
2. Check Cooling Fan/Pump Operation:
   • Fan (Air/Oil Radiator): Check electrical power supply (230V AC or 24V DC with multimeter), engine operation and integrity of the blades. Replace if faulty.
   • Water Pump (Water/oil radiator): Check rotation, noise, flow. Clean in-line filter. Replace if defective.
3. Internal Cleaning (Water/Oil Exchanger): If the differential pressure on the water side is high, proceed with a chemical washing cycle (using specific products for limescale/corrosion) or dismantle the exchanger for mechanical cleaning of the plates/pipes.
4. Check Thermostat: Check the operation of the exchanger thermostat. Test it on the bench by immersing it in hot water and checking the opening/closing (or the electrical continuity of the contact). Replace if it does not respond correctly (e.g. opens at 65°C, closes at 55°C).
5. Post-Intervention Check: Start the machine and monitor the oil temperature and the temperature difference on the exchanger.

8.2. Resolving Excessive Internal Losses

1. Detailed diagnosis: Identify the specific component with internal leaks (pump, valve, cylinder) through volumetric efficiency tests and targeted thermography.
2. Pump Overhaul or Replacement: If the volumetric efficiency of the pump is less than 90% of the nominal at operating pressure, proceed with the overhaul (replacement of repair kit) or replacement of the complete unit.

   ATTENTION: During replacement, make sure that the coupling with the engine is aligned (max misalignment 0.05 mm) and that the cleanliness level (ISO 4406) of the oil is maintained.

3. Valve/Cylinder Overhaul/Replacement: For valves or cylinders showing excessive bypass or leaks, proceed with replacement of the internal seals or the entire component.
4. Post-Intervention Check: Monitor the oil temperature and, if possible, repeat the volumetric efficiency test of the pump.

8.3. Resolving Clogged Filters

1. Replacement of Filter Elements: Replace all filter elements (pressure, return, tank vent) following the manufacturer's specifications. Use original spare parts or certified equivalents.
2. Contamination Check: After replacing the filters, investigate the cause of the clogging. If the filter is heavily contaminated with metal particles, there may be a rapidly wearing component upstream (e.g. pump).
3. Post-Intervention Check: Check that the clogging indicators are at zero and that the differential pressure has returned within the limits (e.g. < 0.2 bar at full speed).

8.4. Degraded or Inappropriate Hydraulic Oil Resolution

1. Oil Drain and Replacement: Completely drain the old oil from the tank and from all low points in the circuit. Clean the tank from sludge and debris.
2. Refill with Correct Oil: Refill the system with new hydraulic oil, of the correct viscosity and specification (e.g. ISO VG 46, HLP) according to the machine manufacturer's recommendations (UNI EN ISO 6743-4).

   WARNING: Make sure the new oil has the required cleanliness level (ISO 4406 < 18/16/13).

3. Contamination Check (Water): If the analysis has detected water, identify and resolve the source of entry (e.g. broken tank vent, cylinder seals, condensation). Install dehumidifier filters if the environment is very humid.
4. Post-Service Check: Monitor the oil temperature and schedule a new analysis after a running-in period.

8.5. Resolution of pressure relief valve incorrectly calibrated or faulty

1. Valve Calibration: With the aid of a precision pressure gauge installed on the maximum pressure line, calibrate the valve to the value specified in the machine manual (e.g. 250 bar +/- 5 bar).
2. Inspection and Cleaning: If the calibration is not stable or the valve generates heat even at rest, disassemble it (after LOTO and depressurization) and inspect the stem, spring and seat for dirt, wear or damage. Thoroughly clean or replace defective components.
3. Valve Replacement: If the valve is damaged or it is not possible to restore correct operation, replace it with an original or equivalent spare part.
4. Post-Intervention Check: Start the machine and monitor the oil temperature, checking that the valve does not drain continuously.

9. Preventive Measures

Prevention is crucial to maintain the efficiency and durability of hydraulic systems.

10. Spare parts and components

The use of quality spare parts is essential for the reliability of the system. UNITEC-D GmbH offers a wide range of certified hydraulic components for your needs.

To purchase original and certified spare parts, visit our e-catalog: www.unitecd.com/e-catalog/

11. References

• UNI EN ISO 11158: Hydraulic oils - Requirements for H, HL, HM, HV, HR, HG, HE oils.
• UNI EN ISO 4406: Hydraulic fluids - Method for coding the level of contamination by solid particles.
• UNI EN ISO 6743-4: Lubricants, industrial oils and related products (Class L) - Classification - Part 4: Family H (Hydraulic systems).
• UNI EN ISO 14118: Machinery safety - Prevention of unexpected starting.
• Technical and Maintenance Manuals of the Manufacturer (OEM) of the Machine.
• UNITEC-D GmbH - Related Maintenance Guides (e.g. 'Hydraulic Pump Wear Diagnosis').