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 anomalous and excessive rise in oil temperature beyond the expected operating limits (typically above 60-70°C). This severely compromises the efficiency, life and reliability of system components, including pumps, valves, actuators and seals, accelerating oil degradation and potentially leading to catastrophic failure. This diagnostic guide is aimed at maintenance technicians, reliability engineers and plant managers in the machine tool industry, focusing on systemic root cause identification and recovery procedures.

Typical symptoms:

• Increase in oil temperature beyond the operating limit (e.g. > 65°C).
• Abnormal pump and valve noise.
• Slowing of actuator movements or operational irregularities.
• Frequent failures of seals and elastomeric components.
• Dark color and burning smell of hydraulic oil.
• Temperature alarms on the machine control panel.

Gravity Rating:

• Critical: Oil temperature > 80°C. Requires immediate machine shutdown to prevent permanent damage.
• Major: Oil temperature between 70°C and 80°C. Operation is compromised and the risk of failure is high. Immediate diagnosis needed.
• Minor: Oil temperature between 60°C and 70°C. Indicates reduced efficiency of the cooling system or the beginning of a problem. Requires monitoring and diagnostic planning.

2. Safety Precautions

ATTENTION: Before any intervention on a hydraulic system, it is CRITICAL to adopt the following precautions for the safety of personnel and equipment.

• Deactivation and Lockout: Perform the complete LOTO (Lockout/Tagout) procedure on the machine tool and on the hydraulic power unit. Make sure all energy sources (electrical, hydraulic, pneumatic) are isolated and locked in a safe position.
• Accumulated Energy: Hydraulic systems can maintain high pressures even when the machine is off. ALWAYS RELIEF RESIDUAL PRESSURE through the relief valves or by manually cycling the actuators, if possible and safe. Check with pressure gauges.
• Hot Fluid and Pressure: Hot hydraulic oil can cause severe burns. Oil under pressure can inject into the skin. Always wear adequate PPE: safety glasses, oil and heat resistant gloves, long-sleeved protective clothing.
• Hot Surfaces: Components such as heat exchangers, tanks and pipes can reach high temperatures. Use heat-resistant gloves and use caution during inspections.
• Contamination: Avoid prolonged contact with hydraulic oil, which can be irritating to the skin. Use appropriate containers for collecting fluids.
• Confined Spaces: In case of interventions in confined spaces, make sure to respect the specific safety procedures and the presence of support personnel.

3. Diagnostic Tools Required

An effective diagnosis of thermal overload requires the use of specific instrumentation to acquire precise and reliable data.

4. Initial Assessment Checklist

Before starting any invasive diagnostic procedure, it is essential to gather preliminary information about the implant and the problem. This allows the diagnosis to be addressed more efficiently.

5. Flow Chart for Systemic Diagnosis

1. Initial Symptom: Excessive Oil Temperature
   1. Perform Initial Checklist (Sect. 4)
   2. Check Oil Level
      • IF low level: top up with specified oil. Monitor.
      • SE correct level: continue.
   3. Cooling Circuit Inspection
      1. Heat Exchanger (air/oil or water/oil)
         1. SE air/oil exchanger: check cleanliness of fins and fan operation.
            • IF fan does not work: check power supply, fan motor.
            • SE dirty fins: clean thoroughly.
         2. SE water/oil exchanger: check cooling water flow, inlet/outlet water temperature, presence of encrustations (high delta T indicates obstruction).
            • SE insufficient water flow: check water pump, valves, water circuit filters.
            • IF internal encrustations: proceed with chemical washing.
      2. Cooling Circuit Thermostat
         • SE present: check correct operation (opening/closing at the expected temperatures). Use thermal imager for ΔT at the ends.
   4. Diagnosis with Thermal Camera (Sect. 3)
      1. Scan the entire hydraulic system (tank, pump, valves, actuators, pipes) in operating conditions.
      2. Identify areas with abnormal temperatures (hot spots).
      3. IF the pump is significantly hotter (>15°C compared to the tank): Probable internal pump leak or inefficiency.
      4. IF maximum pressure or reducing valves are abnormally hot: Probable incorrect adjustment or internal blockage/clogging.
      5. IF pipes or restrictions are excessively hot: Probable excessive pressure drop or clogging.
   5. Measure Pressures and Flow Rate (Sect. 3)
      1. Install pressure gauges and flowmeters at critical points (pump outlet, before/after valves, actuator inlet).
      2. Discharge Pressure (Pump): Check that the working pressure and the pressure set by the relief valve comply.
         • SE constantly high pressure without load: Probable relief valve blocked/poorly calibrated or blockage on the return.
         • IF unstable pressure: Probable pump cavitation or regulation problem.
      3. Actual Pump Flow Rate: Measure the actual flow rate and compare it with the nominal one.
         • SE significantly lower flow rate (>10%): Probable pump wear or blocked suction.
      4. Pressure Drops: Measure the ΔP through filters, valves, long pipes.
         • SE High ΔP (>5 bar) through a filter: Filter clogged.
         • SE High ΔP on line without apparent obstruction: Undersized pipe or internal restriction.
   6. Oil Analysis (Sect. 3)
      1. Take an oil sample for laboratory analysis.
      2. Evaluate the parameters: viscosity, contamination level (ISO 4406), presence of water, total acid number (TAN).
      3. IF viscosity out of specification or high TAN: Oil degraded, need for replacement.
      4. SE high contamination: Inefficient filters, wear of components, need for additional filtration and verification of causes.
      5. IF presence of water: Leakage into the tank, condensation, need for dehydration/replacement.

6. Cause-Fault Matrix

7. Root Cause Analysis for Each Fault

7.1 Inefficiency or Malfunction of the Cooling Circuit

Why it happens: The heat exchanger (whether air or water) is the key component for dissipating the heat generated in the system. Its inefficiency can arise from:

• External clogging: Dust, dirt, metal shavings or debris accumulate on the fins of the air/oil exchangers, drastically reducing the heat exchange surface.
• Internal clogging: Encrustations, sediments (if water/oil exchanger) or oil sludge can reduce the passage of fluid inside the exchanger, decreasing the effectiveness of the exchange.
• Insufficient cooling flow: Faulty or low-flow cooling water pump, blocked or partially closed valves in the water circuit, or non-working or low-speed fans in the air exchangers.
• Failure thermostat: A thermostat stuck in the closed position prevents the flow of oil through the heat exchanger or, if in the water circuit, limits the flow, preventing heat dissipation.

How to confirm:

• Using a thermal imager to measure the temperature difference (ΔT) between the inlet and outlet of the oil and coolant. A high ΔT of the oil with a low ΔT of the cooling fluid indicates inefficiency of the exchanger.
• Measurement of the flow rate of the cooling fluid (air or water) with a flow meter.
• Visual inspection for external dirt.
• Functional test of the thermostat.

Damage if unresolved: Accelerated oil degradation, premature wear of seals and heat-sensitive components, reduced system performance, machine lockout due to temperature alarm.

7.2 Wear or Malfunction of the Hydraulic Pump

Why it happens: A used or defective hydraulic pump generates excess heat due to internal leaks (volumetric losses) or mechanical inefficiency. These losses require the pump to work longer or harder to maintain the desired pressure and flow rate, converting mechanical energy into heat rather than useful hydraulic energy.

• Wear of internal components: Worn pistons, gears, vanes or cylinder blocks increase internal clearance, causing leakage losses and, consequently, heat generation.
• Cavitation: Insufficient suction (clogged suction filter, low oil level, undersized or blocked suction pipe) leads to the formation and collapse of air bubbles in the oil, generating noise, vibrations and heat.

How to confirm:

• Measurement of the pump flow rate with an in-line flow meter and comparison with the nominal flow rate. A significant reduction in flow rate (>10%) is a clear indicator of internal wear.
• Vibrational analysis on the pump can reveal bearing problems or imbalance.
• Inspection of intake filter and oil level.
• Oil analysis for the presence of metal particles (wear indicator).

Damages if unresolved: Loss of machine efficiency, slow response of actuators, catastrophic pump failures (potentially releasing metal fragments into the circuit), severe oil contamination.

7.3 Incorrect Calibration or Blockage of the Regulation Valves

Why it happens: Pressure relief valves, reducing valves or other control valves can generate excessive heat if poorly calibrated or blocked. If a pressure relief valve is set too low, oil is continuously discharged to the tank at a higher pressure than necessary, converting energy into heat. If partially blocked, it can create a fixed restriction or a continuous flow diversion.

• Incorrect calibration: The relief valve opening pressure is lower than the pressure required by the load, causing constant flow through the valve and dissipation of energy into heat.
• Partial/Total Blockage: Contaminants in the oil can block the valve spool in an abnormal position, creating fixed restrictions or forcing oil to pass through orifices not designed for continuous flow.

How to confirm:

• Pressure measurement upstream and downstream of the valves with digital pressure gauges.
• Using the thermal imaging camera to detect hot spots on specific valves, especially relief or control valves.
• Functional test of the valve (if possible) to verify its response to pressure variations.

Damage if unresolved: Continuous overheating of the oil, reduction in valve life, power losses and system inefficiency.

7.4 Clogged Filters or Incorrect Choice of Oil

Why it happens:

• Clogged filters: A hydraulic filter is designed to remove contaminating particles from the oil. When it becomes clogged, it offers greater resistance to oil flow, causing an excessive pressure drop (ΔP) and generating heat. This is especially true for return or pressure filters.
• Wrong choice of oil: Using an oil with too high a viscosity increases internal friction and resistance to flow, generating heat. An oil with too low viscosity may not guarantee adequate lubrication and increase internal losses of the components, also generating heat due to friction and leaks. An oil without adequate anti-wear or anti-oxidation additives will quickly degrade, losing its lubricating and cooling properties.

How to confirm:

• Measurement of the pressure drop (ΔP) across the filter with pressure gauges. A ΔP > 5 bar indicates a clogged filter and the need for replacement.
• Visual inspection of the state of the filter element (if possible).
• Consultation of the technical documentation of the machine and the oil supplier to verify the correct specification (UNI ISO 3448).
• Laboratory analysis of the oil for viscosity, TAN, TBN and presence of additives.

Damage if unresolved: Increased system contamination (if filter bypass opens), pump cavitation, premature component wear, rapid oil degradation.

8. Step-by-Step Resolution Procedures

8.1 Resolution of Cooling Circuit Inefficiency

1. Air/Oil Exchanger Cleaning:
   1.  SAFETY: LOTO applied.
   2. Use compressed air (max 6 bar, with specific nozzle for cleaning) or wash with water and specific degreasing detergent, making sure not to damage the fins.
   3. Check the operation of the fan: test the electric motor (voltage, current, winding resistance with a multimeter), replace if defective.
2. Water/Oil Exchanger Maintenance:
   1.  SAFETY: LOTO applied. Drain the affected water and oil circuits.
   2. Inspect the water circuit for blockages. Clean or replace filters.
   3. Measure the cooling water flow rate and compare it to the OEM specifications.
   4. In case of internal encrustations: carry out chemical washing of the exchanger with specific products to remove limescale deposits or sludge. Follow the detergent manufacturer's instructions.
3. Check/Replace Thermostat:
   1.  SAFETY: LOTO applied.
   2. Remove the thermostat. Test its operation in a thermostatic bath or check its continuity/resistance with a multimeter if it is electrical.
   3. Replace the thermostat if it does not open or close at the correct temperatures.

8.2 Resolution of Pump Wear or Malfunction

1. Intake Inspection and Cleaning:
   1.  SAFETY: LOTO applied. Release the pressure.
   2. Check and clean the suction filter in the tank.
   3. Check the integrity of the suction pipe to avoid air infiltration.
   4. Top up the oil level if necessary.
2. Replacement of Worn Pump:
   1.  SAFETY: LOTO applied. Relieve pressure, drain oil from pump.
   2. Disassemble the pump.
   3. Install a new pump with the same OEM specifications.
   4. Ensure correct alignment of the pump-motor coupling (maximum misalignment 0.05 mm with dial gauge or laser).
   5. Restart the system and carefully monitor the temperature and pressure.

8.3 Troubleshooting Control Valves

1. Check and Calibration of Maximum Pressure Valve:
   1.  SAFETY: LOTO applied. Release the pressure.
   2. Using a precision pressure gauge, calibrate the valve to the pressure specified by the machine manufacturer, typically 10-20 bar above the maximum working pressure.
   3. Cycling to verify pressure stability.
2. Cleaning or Replacing Blocked/Clogged Valves:
   1.  SAFETY: LOTO applied. Release the pressure.
   2. Disassemble the valve. Inspect the spool and seats for contamination or damage.
   3. Thoroughly clean the internal passages and the spool with specific hydraulic solvent.
   4. Replace the valve if damaged or if cleaning does not restore correct operation.

8.4 Filtration and Oil Problem Solving

1. Clogged Filter Replacement:
   1.  SAFETY: LOTO applied. Release the pressure.
   2. Replace the filter element with a new one, with the same filtration fineness (e.g. 10 µm absolute, according to UNI EN ISO 16889).
   3. Monitor ΔP post-replacement.
2. Replacement of Degraded/Non-Compliant Oil:
   1.  SAFETY: LOTO applied.
   2. Completely drain all used oil from the tank and all accessible lines and components.
   3. Clean the tank internally to remove sludge and contaminants.
   4. Fill with new hydraulic oil compliant with OEM specifications (UNI ISO VG viscosity, AW/EP additives). Check the purity (e.g. ISO 4406 code 18/16/13).

9. Preventive Measures

10. Spare Parts and Essential Components

Having critical spare parts available is fundamental to reduce machine downtime. It is recommended to consult the UNITEC electronic catalog for specific codes and availability.

To purchase and consult on spare parts, visit our online catalogue: www.unitecd.com/e-catalog/

11. References

• UNI EN ISO 4406: Hydraulic oil cleanliness code.
• UNI EN ISO 18434-1: Monitoring and diagnosis conditions with infrared thermography.
• UNI EN ISO 16889: Hydraulic filters – Filtering elements – Multiple pass method for determining the filtration characteristics.
• UNI EN 837-1: Spring pressure gauges – Bourdon tube pressure gauges.
• CEI EN 61010-1: Safety requirements for electrical equipment for measurement, control and laboratory use.
• UNI ISO 2909: Mineral oils for industrial uses – Determination of the viscosity index.
• Operation and maintenance manuals from the machine tool OEM manufacturer.
• Workplace safety regulations (Legislative Decree 81/08 and subsequent amendments) for LOTO procedures and PPE.