Troubleshooting Hydraulic System Overheating: Root Cause Analysis, Diagnosis Using Thermal Imaging, Flow/Pressure Control, and Cooling Circuit Inspection

1. Description of the problem and scope of application

Overheating of the hydraulic system is one of the critical faults that significantly shortens the service life of components, reduces equipment efficiency and can lead to unplanned production stoppages. This manual is intended for systematic diagnosis and elimination of root causes of increased hydraulic fluid temperature. Symptoms of overheating include: abnormally high fluid temperature (above 60-65°C), slow or erratic operation of hydraulic actuators, increased cycle times, discoloration of hydraulic fluid, damage to seals and hoses, and increased noise levels.

Scope: The manual covers stationary and mobile hydraulic systems used in industry: hydraulic presses, casting machines, extruders, mobile construction and agricultural machinery, as well as hydraulic power units. The problem of overheating is classified as:

• Critical: Fluid temperature exceeds 80°C. Immediate equipment shutdown. High risk of fire, destruction of seals, complete loss of lubricating properties of the liquid.
• Significant: Temperature in the range of 65-80°C. Urgent diagnosis and elimination is necessary. Leads to rapid wear of components, reduced performance, frequent failures.
• Minor: The temperature consistently exceeds the operating range (usually 40-55°C) by 5-10°C. Needs planned diagnostics. Contributes to the gradual aging of the fluid and components.

Applicable standards: Diagnostics and troubleshooting must meet the requirements of DSTU EN ISO 4413:2018 "Hydraulic drive systems. General rules and safety requirements" and DSTU ISO 15384:2016 "Hydraulic drive systems. Liquids based on vegetable oils" (for relevant systems).

2. Precautions

⚠ SAFETY WARNING ⚠

• LOCKOUT/TAGOUT (LOTO): Before performing any diagnostic or repair work on the hydraulic system, be sure to apply Lockout/Tagout procedures in accordance with internal plant instructions and DSTU requirements ISO 14118. Make sure that all sources of energy (electrical, hydraulic, pneumatic) are isolated and blocked.
• RESIDUAL ENERGY: Hydraulic systems contain fluid under high pressure that can remain even after the pump is turned off. Before disassembling or disconnecting any components, ensure that the pressure is completely relieved. Use pressure gauges to confirm zero pressure. Hot hydraulic fluid can cause serious burns.
• PROTECTION OF PERSONAL EQUIPMENT (PPE): Always use appropriate PPE: safety glasses (DSTU EN 166), heat-resistant protective gloves (DSTU EN 407), oil-resistant work clothes, protective shoes.
• HOT FLUID UNDER PRESSURE: Hydraulic fluid can reach high temperatures (up to 80°C or more) when overheating. Avoid skin contact with hot liquid. When working with hot components, use a thermal imager or contact thermometer to estimate the temperature before touching. Fluid escaping under pressure can penetrate the skin, causing serious injury.
• AUTHORIZED PERSONNEL PRESENCE: Diagnostics and repairs should only be performed by qualified personnel trained in hydraulic systems and familiar with all safety procedures.

3. Necessary diagnostic tools

The following set of tools is required for effective diagnostics of hydraulic system overheating:

4. Initial evaluation checklist

Before starting in-depth diagnostics, it is necessary to conduct a visual inspection and collect information about the operation of the system. This will help narrow down the potential causes.

5. Systematic diagnostic route (Block diagram)

Perform diagnostics following this algorithm to consistently identify the root cause of overheating.

1. Initial rating:
   1. Check the operating temperature of the hydraulic fluid.
   2. If > 65°C: Continue diagnostics.
   3. If 40-65°C, but above the norm: Go to point 2.
   4. If normal (40-55°C): There is no overheating, look for other causes of malfunction.
2. Check the cooling system:
   1. Visual inspection of the cooler:
      • Clogging of the cooler fins (dust, dirt, oil):
        • Yes: Go to → Section 8, item 1 (Cleaning the cooler).
        • No: Continue.
   2. Cooling fan/pump operation:
      • Fan/pump not working or working inefficiently (low RPM, weak flow):
        • Yes: Go to → Section 8, Item 2 (Fan/Pump Check/Repair).
        • No (works fine): Continue.
   3. Thermal imaging of the cooler:
      • Evenness of temperature on the surface of the radiator:
        • Uneven (cold spots): → Possible internal clogging, flow bypass or insufficient coolant flow. Go to → Section 7 (Undercooling), Section 8, Item 1.
        • Smooth, but the cooler is hot: → The load on the cooling system exceeds its capacity. Check coolant pressure/flow (for water systems). Go to point 3.
3. Diagnostics of pressure and flow:
   1. Measurement of pressure at main points:
      • Pump discharge line pressure:
        • Higher than normal (idle or under load): → Blocked return flow, too high relief valve settings, clogged valves. Go to → Section 7 (Excessive Pressure Drop), Section 8, Item 3.
        • Lower than normal: → Pump wear, internal leaks. Go to point 3b.
      • Difference in pressure on the filters:
        • Higher than normal (> 0.5-1.0 bar): → Clogged filter. Go to → Chapter 8, Item 4 (Replacing the filter).
        • Normal: Continue.
      • Pump/motor drain line pressure:
        • Higher than normal (> 0.5 bar): → Internal leak in pump/motor. Go to point 3b.
        • Normal: Continue.
   2. Flow measurement using a flow meter:
      • Direct flow measurement at the pump outlet:
        • Flow much lower than nominal under load (reduction by 15-20%): → Pump wear, internal leaks in the pump. Go to → Section 7 (Pump Wear), Section 8, Item 5.
        • Flow is normal: Continue.
      • Flow through control valves or cylinders:
        • Lower than expected: → Internal leaks in valves/cylinders, flow restriction. Go to → Chapter 7 (Internal leaks), Chapter 8, paragraph 6.
        • Normal: Continue.
4. Thermal imaging identification of internal leaks:
   1. Scanning of components under load (pump, valves, hydraulic motors, cylinders):
      • Local hot spots (difference > 10°C from neighboring areas): → Possible internal leakage or excessive friction in this component. Go to → Chapter 7 (Internal leaks), Chapter 8, paragraph 6.
      • Even temperature distribution: Continue.
5. Hydraulic fluid analysis:
   1. Visual inspection:
      • Opacity, discoloration (dark, milky), burning smell: → Fluid degradation, water/particulate contamination. Go to → Chapter 7 (Fluid contamination/degradation), Chapter 8, paragraph 7.
      • Normal: Continue.
   2. Express analysis (set):
      • High water content (> 0.1%), high solids (above class ISO 4406), reduced viscosity: → Fluid contamination/degradation. Go to → Chapter 7 (Fluid contamination/degradation), Chapter 8, paragraph 7.
      • Parameters are normal: → Consider other, less likely causes (incorrect fluid type, incorrect safety valve setting).

6. Matrix "Failure-Cause"

This chart will help you identify likely causes of overheating based on observed symptoms and diagnostic test results.

7. Analysis of the root causes of each malfunction

Understanding why a failure occurs is critical to effectively resolving it and preventing recurrence.

7.1. Insufficient efficiency of the cooling system

Explanation: This is the most common cause of overheating. The cooler (heat exchanger) is designed to remove excess heat from the hydraulic fluid. If its efficiency decreases, the fluid cannot give off heat fast enough.

Probable factors:

• External contamination: Dust, dirt, oil deposits, fibers that accumulate on the radiator fins form an insulating layer that prevents heat transfer.
• Internal Contamination: Sludge, fluid oxidation products, wear particles can accumulate inside the cooler tubes, reducing the heat transfer area and restricting flow.
• Fan/Cooling Pump Malfunction: Malfunctioning electric motor, damaged fan blades, low RPM or pump failure (for water coolers) results in insufficient airflow or cooling water.
• Incorrect Cooler Size: A cooler that was adequately sized for initial conditions may become inadequate as system load increases, ambient temperature increases, or hydraulic fluid changes.
• Coolant/Air Flow Restriction: Clogged air passages, low pressure, or clogged water circuit.

How to confirm: Thermal imaging will reveal uneven temperature distribution on the surface of the cooler (cold areas due to internal clogging) or general overheating of the cooler with insufficient capacity. Measuring the fan motor or cooling pump current will show the deviation. Checking the pressure before and after the cooler (for water) will reveal internal clogging.

Possible damage: Without elimination will lead to accelerated degradation of hydraulic fluid, damage to all seals, wear of precision components (pumps, valves) and overall reduction in system efficiency.

7.2. Internal leaks in the system

Explanation: Internal leaks occur when hydraulic fluid leaks through gaps or damaged seals inside components without doing useful work. This leads to the conversion of hydraulic energy into thermal energy due to friction and throttling.

Probable factors:

• Pump wear: Increasing clearances between moving parts (rotor-stator, piston-sleeve) leads to a loss of volumetric efficiency and overheating.
• Valve wear: Worn spools, seats, seals in distributors, pressure regulators, check valves allow fluid to flow uncontrollably.
• Wearing of hydraulic cylinders/hydromotors: Damaged piston seals or increased gaps between the piston and the sleeve lead to fluid leakage inside the cylinder.
• High pressure/overload: Prolonged operation at high pressures accelerates wear.

How to confirm: The flow meter will show a significant decrease in the effective flow from the pump under load. The thermal imager identifies hot spots on component housings (pumps, valves, cylinders) where fluid is throttled. Flow measurement in drain lines will reveal excessive leakage.

Possible damage: Decreased equipment performance, slow operation, loss of effort, further wear of components due to increased temperature and fluid degradation.

7.3. Excessive pressure drop

Explanation: Excessive pressure drop occurs when fluid passes through significant resistances, resulting in energy loss as heat. This happens when throttling the flow.

Probable factors:

• Clogged filters: Clogged filter elements create significant flow resistance. A pressure drop of more than 0.5 bar on the filter (for conventional systems) is a sign of clogging.
• Pipelines/hoses that are too small: Incorrect calculation or retrofitting of the system using pipes of a smaller diameter than necessary.
• Partially Closed Valves: Clogged, malfunctioning, or improperly adjusted valves can cause them to partially close and throttle.
• High fluid viscosity: Using fluid with a higher viscosity than recommended or operating at low temperatures without proper warm-up.

How to confirm: Pressure gauges installed before and after a potential restriction point (filter, valve) will show a significant pressure drop. The thermal imager will detect hot spots on these components.

Possible damage: Decreased efficiency, overheating of liquid, damage to components from excessive pressure, increased cavitation.

7.4. Pump/motor wear

Explanation: The wear of the pump leads to a decrease in its volumetric efficiency, that is, part of the liquid that it should supply to the system instead flows back inside the pump through the increased gaps, turning into heat.

Probable factors:

• Long-term use: Natural wear and tear of moving parts over time.
• Fluid contamination: Abrasive particles in the liquid accelerate the wear of precision parts of the pump.
• Cavitation: Insufficient pressure at the pump suction leads to the formation of bubbles, which, when collapsed, cause surface erosion.
• Incorrect installation/centering: Excessive loads on bearings and seals.

How to confirm: Flow measurement with a flowmeter will show a significant decrease in pump volume delivery under load. The thermal imager will detect an increased temperature on the pump housing. The level of noise and vibration will increase. Oil analysis may show an increased content of metal wear particles.

Possible damage: Complete loss of pump performance, destruction of internal components, damage to the entire hydraulic system with metal particles.

7.5. Liquid degradation/contamination

Explanation: Hydraulic fluid performs not only the function of energy transmission, but also lubrication and cooling. Its degradation or pollution worsens all these properties, contributing to overheating.

Probable factors:

• Oxidation: Occurs with prolonged exposure to high temperatures and air, forming sludge, acids and other decomposition products that increase friction and pollute the system.
• Water contamination: Water reduces the lubricating properties of the fluid, causes corrosion and can lead to cavitation.
• Particulate contamination: Dust, dirt, metal wear particles act as an abrasive, accelerating component wear and creating additional friction.
• Wrong type of fluid: Using a fluid with the wrong viscosity or chemical composition can result in increased friction or seal incompatibility.

How to confirm: Visual inspection (discoloration, turbidity, smell of burning). Laboratory or express analysis of oil will show an increased content of water, solid particles, a change in viscosity, and a high acid number.

Possible damage: Accelerated wear of all moving parts, corrosion, clogging of filters and valves, complete system failure.

8. Sequential troubleshooting procedures

Before performing any procedures, ensure that all safety precautions are in place (Section 2).

8.1. Cleaning and checking the cooler

1. BLOCKING/MARKING.
2. Cleaning external surfaces: Use compressed air (from a safe distance, with PPE) or wash the surfaces with water under low pressure with a special cleaner for radiators, removing dust, dirt and oil deposits. Make sure the cooler is dry before restarting.
3. Flow Check (for water coolers): Check the water pressure and flow through the cooler. For typical industrial coolers, the water pressure drop should not exceed 0.5 bar.
4. Check for internal clogging: If external cleaning does not help and the thermal imager shows cold areas, the internal channels of the cooler may need to be chemically flushed or replaced.
5. Verification: Start the system, monitor the fluid temperature. It should stabilize in the working range of 40-55°C. The thermal imager should show a uniform temperature distribution over the cooler.

8.2. Cooling fan/pump inspection and repair

1. BLOCKING/MARKING.
2. Visual inspection: Check fan blades for damage, drive belt integrity (if applicable).
3. Electrical check: Using a multimeter, check the supply voltage and current consumption of the fan/pump motor. Compare with passport data. A current deviation may indicate a motor malfunction or a mechanical jam.
4. Mechanical check: Check for ease of shaft rotation (after power off and unlock). Backlash in the bearings may indicate wear.
5. Replacement of faulty components: Replace damaged blades, faulty motor or pump.
6. Verification: After replacement/repair, run system, check fan/pump operation, monitor fluid temperature.

8.3. Diagnostics and adjustment of safety/control valves

1. BLOCKING/MARKING.
2. Checking the settings: Using a pressure gauge, check the opening pressure of the relief valve. It must meet the manufacturer's specification (±5%).
3. Check for internal leakage: For pilot valves or valves with drain lines, measure the flow in the drain line. Excessive flow indicates an internal leak.
4. Cleaning/Repair: If the valve is clogged or stuck, disassemble, clean and inspect it. Replace the seal. If the valve is worn, it must be replaced.
5. Verification: After setup/repair, run the system. Check the pressure and temperature of the fluid under load.

8.4. Replacement of filter elements

1. BLOCKING/MARKING.
2. Relieve pressure: Relieve system pressure.
3. Replacement: Open the filter housing, remove the old element. Install the new filter element in the direction of flow. Make sure all seals are installed correctly.
4. Verification: Start the system. The pressure drop across the filter should be within the normal range (usually <0.5 bar for clean filters). Control the temperature of the liquid.

8.5. Repair or replacement of the hydraulic pump

1. BLOCKING/MARKING.
2. Relieve pressure: Relieve system pressure. Drain fluid from pump and lines.
3. Dismantling: Disconnect the piping, electrical connections and remove the pump from the mounting plate.
4. Condition assessment: Detailed inspection of the pump for wear, damage, cavitation.
5. Repair/Replacement: Depending on the degree of damage, carry out repair (replacement of seals, bearings, internal elements) or complete replacement of the pump with a new one, identical in specifications (flow, pressure, working volume).
6. Installation: Install the pump following the recommended tightening torques and centering procedures.
7. Filling and Bleeding: Fill the system with clean hydraulic fluid, bleed to remove air.
8. Verification: Start the system at idle speed, gradually increase the pressure. Check pressure and flow with a flow meter. Monitor the temperature of the pump using a thermal imager.

8.6. Repair or replacement of hydraulic cylinders / motors / valves (elimination of internal leaks)

1. BLOCKING/MARKING.
2. Relieve pressure: Relieve system pressure.
3. Disassembly: Disassemble the suspected component (cylinder, motor, valve) from the system.
4. Disassembly and Inspection: Carefully disassemble the component, inspect the seals, pistons, spools, surfaces for wear, scratches, corrosion.
5. Replacement of seals and worn parts: Replace all seals (cuffs, rings) and any worn internal components.
6. Assembly and assembly: Assemble the component following the manufacturer's instructions. Install it in place in the system.
7. Verification: Start the system, monitor the temperature of the component with a thermal imager. Check its functionality (speed of movement of the cylinder, revolutions of the motor, operation of the valve) and the absence of overheating.

8.7. Changing the hydraulic fluid and cleaning the system

1. BLOCKING/MARKING.
2. Relieve pressure: Relieve system pressure.
3. Fluid Drain: Drain all old hydraulic fluid from tank, lines, pumps and cylinders.
4. System flushing (if necessary): If the fluid is heavily contaminated or degraded, consider flushing the system with a special flushing fluid.
5. Filter replacement: Install new filter elements (section 8.4).
6. Filling with clean fluid: Fill the system with recommended hydraulic fluid to OEM specifications (eg ISO VG 46, purity class ISO 4406 18/16/13).
7. Bleeding: Perform the bleeding procedure to remove air from the system.
8. Verification: Start the system, monitor the temperature of the fluid and the operation of the equipment. It may be necessary to re-analyze the oil after several hours of operation.

9. Preventive measures

Regular maintenance is key to preventing overheating and extending the life of your hydraulic system.

10. Spare parts and components

Critical spare parts must be available for quick troubleshooting and scheduled maintenance. UNITEC-D GmbH offers a wide range of components for hydraulic systems that meet CE and UkrSEPRO standards.

To order and select components, visit our UNITEC-D E-Catalog.

11. References

• DSTU EN ISO 4413:2018. Hydraulic drive systems. General safety rules and requirements for systems and their components.
• DSTU ISO 15384:2016. Hydraulic drive systems. Liquids based on vegetable oils. Requirements
• Original Equipment Manufacturer (OEM) Operation and Maintenance Guidelines.
• Hydraulic fluid specifications from suppliers (eg ExxonMobil, Shell, Fuchs).
• UNITEC-D internal manuals for maintenance of hydraulic systems.