Diagnostics and elimination of overheating of the hydraulic system: thermal imaging, flow/pressure diagnostics and cooling circuit inspection

1. Description of the problem and its scale

Overheating of the hydraulic system is a critical malfunction that significantly affects the performance, reliability and service life of the equipment. The normal operating temperature of the hydraulic fluid is usually 40-60°C. Temperatures above 70°C are considered excessive and can lead to accelerated hydraulic fluid degradation, seal damage, increased component wear, and reduced system efficiency.

This manual covers the diagnosis of hydraulic overheating in a wide range of industrial equipment, including presses, casting machines, mobile hydraulics, hydraulic power units and CNC machine tools. It focuses on systematically identifying root causes of failure to prevent catastrophic failures and optimize uptime.

• Severity Classification:
• Critical: Fluid temperature exceeds 85°C. Requires immediate equipment shutdown to prevent serious component damage.
• Significant: Fluid temperature in the range of 70-85°C. Indicates reduced performance and potential impending failure. Needs immediate diagnosis and elimination.
• Minor: Fluid temperature is consistently above 60°C but below 70°C. Indicates the initial stages of a problem that may worsen. Needs scheduled inspection.

2. Safety measures

⚠ SAFETY WARNING ⚠
All safety procedures must be strictly followed before starting any diagnostic or repair work on the hydraulic system. Failure to do so could result in serious injury or death.

• Lockout and Tagout (LOTO): Always apply Lockout/Tagout procedures to electrical power sources and hydraulic units before any intervention. Make sure the power source is disconnected and the energy is dissipated.
• High pressure: Hydraulic systems work under extremely high pressure (up to 350 bar and more). Never loosen connections, disassemble components, or place body parts near possible pressure leak points. Hydraulic fluid escaping under pressure can penetrate the skin, causing serious injury.
• Hot fluid: Superheated hydraulic systems contain fluid with a high temperature (up to 100°C and above). Use heat-resistant personal protective equipment (PPE) to avoid burns.
• Stored Energy: Accumulators can store significant amounts of energy under pressure, even when the pump is turned off. Make sure all batteries are properly discharged before servicing.
• Moving parts: Beware of moving parts of the equipment that can suddenly activate.
• Personal Protective Equipment (PPE): Always use appropriate PPE, including safety glasses, heat-resistant gloves, protective clothing and safety footwear in accordance with DSTU EN 166 (eye protection), DSTU EN 388 (hand protection) and DSTU EN ISO 20345 (safety footwear).

3. Necessary diagnostic tools

The following set of tools is required for accurate diagnosis of the root cause of hydraulic system overheating:

4. Initial evaluation checklist

Before starting a detailed diagnosis, perform an initial assessment to gather important information about the health of the system. This will help narrow down the range of potential malfunctions.

5. Systematic diagnostic block diagram

Follow this flowchart to systematically locate the root cause of overheating:

1. Symptom: Hydraulic fluid temperature exceeds 70°C.
   1. Check 1: Cooling system
      • Action: Visually inspect the cooler (radiator, heat exchanger). Perform thermal imaging.
      • Question: Is the radiator dirty? Is the coolant fan/pump working? Is there sufficient air/water flow?
      • If NO (the radiator is dirty, the fan is not working or the heat exchanger is cold):
        • Problem node: The cooling system is faulty.
        • Go to section: "8. Step-by-step troubleshooting procedures" → "Cleaning/Repair of cooling system".
      • If YES (cooler clean, working): Continue to "Check 2".
   2. Check 2: System Pressure
      • Action: Connect pressure gauges to the pump discharge line and to the relief valve drain line (if applicable).
      • Question: Is the discharge pressure higher than rated for the duty cycle? Чи є незвичайний перепад тиску через клапани? Is the safety valve significantly hot using the thermal imager?
      • If YES (excessive pressure or overheated relief valve):
        • Problem node: Excessive system pressure / Valve adjustment problems.
        • Action: Check the relief valve settings (Chapter 8). Measure the pressure required to perform the work.
        • If relief valve pressure is significantly lower than discharge pressure or relief valve is permanently open:
          • Problem assembly: Blocked line, defective relief valve (sticking/stuck).
          • Go to section: "8. Step-by-step troubleshooting procedures" → "Valve diagnosis and repair".
        • If the pressure at the relief valve is about equal to the discharge pressure, and it is high, but the system is working:
          • Problem node: Excessive load on the system / Improper adjustment of the relief valve.
          • Go to section: "8. Step-by-step troubleshooting procedures" → "Setting the relief valve".
      • If NO (pressure is normal): Continue to "Check 3".
   3. Check 3: Internal Leakage (Pump)
      • Action: Use a flow meter to measure the flow at the outlet of the pump at maximum pressure (unloaded mode, then under load).
      • Question: Is the actual flow significantly less than the rated flow of the pump at a certain pressure (eg >15% drop)? Is there excessive noise/vibration from the pump?
      • If YES (reduced flow, noise):
        • Problem node: Internal pump leakage (wear).
        • Go to section: "8. Step-by-step troubleshooting procedures" → "Pump repair/replacement".
      • If NO (flow normal, pump quiet): Continue to "Check 4".
   4. Check 4: Internal Leakage (Valves / Actuators)
      • Action: Perform thermal imaging on all valves (manifolds, pressure regulators, check valves) and actuators (cylinders, hydraulic motors) under load and standby.
      • Question: Are there significant localized hot spots (>75°C) on certain valves or actuators?
      • If YES (hot spots):
        • Problem node: Internal valve or actuator leak.
        • Action: Isolate the suspicious component. If the valve is overheating, this may indicate an internal leak due to worn seals or a stuck spool. If the cylinder overheats without movement, this may indicate a piston seal leak.
        • Go to section: "8. Step-by-step troubleshooting procedures" → "Diagnosis and repair of valves/actuators".
      • If NO (no hotspots): Continue to "Check 5".
   5. Check 5: Hydraulic Fluid Condition
      • Action: Collect a sample of fluid for visual inspection and analysis.
      • Question: Is the liquid cloudy, foamy, discolored? Do you smell burning? Does laboratory analysis confirm inappropriate viscosity, particle or water contamination (ISO 4406:2017 for fluid purity)?
      • If YES (poor fluid quality):
        • Problem node: Incorrect fluid viscosity or contamination.
        • Go to section: "8. Step-by-step troubleshooting procedures" → "Hydraulic Fluid Replacement/Filtration".
      • If NO (fluid quality is normal): Continue to "Check 6".
   6. Check 6: Tank Size / Return Loop
      • Action: Check that the hydraulic tank volume is sufficient (usually 3-5 times the pump flow per minute). Review return lines for restrictions.
      • Question: Is the tank too small for the volume of circulating liquid? Is there a restriction in the return line causing excessive back pressure?
      • If YES (tank too small / restriction):
        • Problem node: Insufficient tank volume or restriction in the return circuit.
        • Go to section: "8. Step-by-step Troubleshooting Procedures" → "Optimizing Tank/Return Loop".
      • If NO (tank compatible, no restrictions): Revisit all previous steps, there may be a combination of factors or an obscure malfunction.

6. Malfunction-cause matrix

7. Root cause analysis for each malfunction

7.1. Malfunction of the cooling system

Explanation: The cooling system (radiator, air-oil or water-oil heat exchanger, fan, circulation pump) is designed to remove excess heat generated by the hydraulic system. If the cooling efficiency decreases, heat accumulates, leading to overheating.

How to confirm:

• Contaminated radiator: A visual inspection will show a significant accumulation of dust, dirt or oil film on the fins of the radiator. The thermal imager will detect that the inlet temperature of the liquid in the cooler is high, and the outlet temperature does not decrease to the required values ​​(the temperature drop is less than normal, for example, less than 10°C).
• Defective fan/pump: Cooler fan does not spin or spins slowly. Cooler circulation pump (for water-oil heat exchangers) does not supply liquid. A multimeter can show a lack of voltage on the fan motor or an open winding.
• Insufficient coolant/air flow: Restriction in air duct or cooling water circuit.

Damage if Ignored: Constantly overheating the fluid leads to accelerated degradation, deposit formation, seal damage, causing internal leaks and rapid wear of pumps and valves. The service life of components is reduced by up to 50% when the temperature rises for every 10°C above normal.

7.2. Excessive pressure in the system

Explanation: Hydraulic energy not used to do work (for example, when the actuator has reached the end position and the pump continues to supply fluid) is converted into heat. This occurs when fluid passes through the relief valve at high pressure or when there are excessive flow restrictions.

How to confirm:

• Incorrectly adjusted relief valve: Pressure gauges show that system pressure is consistently high (>20% of normal), even during no-load cycles, or that the relief valve is constantly open. The thermal imager will show significant heating of the relief valve (>80°C).
• Stick or blocked safety valve: The valve does not operate properly, maintaining high pressure or constantly passing fluid.
• Flow restrictions: Partially blocked filters, constricted lines, improperly selected piping or fittings. Measuring the pressure drop before and after the suspected area will show an excessive drop (eg >5 bar at the area).
• Overload: The equipment is operated with a load that exceeds its design parameters, forcing the pump to work continuously at high pressure.

Damage if ignored: Accelerated wear of pump, valves, energy loss, increased electricity consumption, premature fluid aging. Possible rupture of hydraulic lines or seals.

7.3. Internal leakage of the pump (wear)

Explanation: Over time, the internal components of the pump (pistons, vanes, gears, bearings) wear out, which increases clearances. This allows fluid to flow inside the pump from outlet to inlet without doing any useful work. This internal leakage is converted into heat.

How to confirm:

• Decreased flow: A flow meter measurement will show a significant decrease in actual flow (by 15-20% or more) compared to the pump's nominal value at a given pressure.
• Increased noise and vibration: The pump makes unusual sounds (hum, screeching), and the vibrations increase.
• Casing heating: The thermal imager will show an increased temperature on the pump casing (>80°C), especially in the area of ​​the leak.

Damage if ignored: Loss of system performance, slow actuators, increased energy consumption, complete pump failure with possible damage to other components due to metal shavings.

7.4. Internal leakage of valves/actuators

Explanation: Valves and hydraulic actuators (cylinders, hydraulic motors) contain seals and precision clearances. Worn seals, scratches on spools or sleeves, or misalignment lead to internal leakage of fluid that does not do useful work, but turns into heat.

How to confirm:

• Localized heating: The thermal imager will detect localized hot spots (>75°C) on valve bodies (especially manifolds, pressure regulators, check valves) or hydraulic cylinders/motors, even when they are not running or under light load.
• Cylinder leakage: The cylinder does not hold the load, smoothly descends. Leakage through piston seal.
• Hydraulic motor leakage: Reduced torque, increased body temperature.

Damage if ignored: Reduced positioning accuracy of actuators, loss of power and speed, increased energy consumption, rapid wear of other components due to increased fluid temperature.

7.5. Incorrect hydraulic fluid viscosity or degradation

Explanation: Hydraulic fluid is the blood of the system. Fluid viscosity is critically important for lubrication, cooling, and energy transfer. If the fluid is too viscous, it creates excessive resistance to flow, generating heat. If the fluid is too thin, it does not provide adequate lubrication, increasing internal leakage and friction. Liquid degradation (oxidation, pyrolysis) also reduces its lubricating and cooling properties.

How to confirm:

• Visual inspection: The liquid may be cloudy, have a dark color, smell of burning, or contain foam.
• Laboratory analysis: Will confirm incorrect viscosity, increased acid value, presence of water, metal particles or oxidation products. Normal viscosity for most systems - ISO VG 32, 46, 68 (at 40°C), tolerance ±10%.

Damage if ignored: Accelerated wear of all hydraulic system components, especially pumps and valves, due to insufficient lubrication or excessive friction. Clogging of filters and valves with degradation products.

7.6. Contamination of hydraulic fluid

Explanation: Particles, water and air are the main contaminants of hydraulic fluid. The particles cause abrasive wear on components, increasing internal leakage and generating heat. Water reduces lubricating properties, causes corrosion and can form emulsions that make circulation difficult. Air (cavitation) causes micro-explosions that damage surfaces and generate heat.

How to confirm:

• Visual inspection: Liquid may contain visible particles, be cloudy (water) or foamy (air).
• Laboratory analysis: High purity class (eg ISO 4406: 22/18/13 and above), presence of water (>200 ppm), presence of air (strong foam, characteristic cavitation noise).

Damage if ignored: Catastrophic wear of pumps, valves and hydraulic cylinders, corrosion of metal parts, reduction of system efficiency and its complete failure.

8. Step-by-step troubleshooting procedures

8.1. Cleaning/Repair of the cooling system

1. ⚠ SAFETY: Apply LOTO to electrical and hydraulic systems.
2. Cleaning the radiator:
   • Use compressed air (dry, cleaned) or a low-pressure cleaning solution to remove dust, dirt and oil deposits from the radiator fins.
   • Make sure the air flow is not blocked by foreign objects.
3. Fan Inspection:
   • Visually inspect the fan blades for damage.
   • Check the electric fan motor with a multimeter: measure the resistance of the windings, check the supply voltage (according to the ratings). Replace the faulty motor or fan unit (UNITEC Category: Electric motors).
4. Coolant pump inspection (for water-to-oil heat exchangers):
   • Make sure there is a flow of cooling water/fluid.
   • Check the pump for blockages or malfunctions. Replace if necessary (UNITEC Category: Pumps).
5. Verification: Start the system, check the temperature difference on the cooler. It should be 10-15°C.

8.2. Diagnostics and repair of valves

1. ⚠ SECURITY: Apply LOTO. Decompress the system.
2. Relief valve:
   • Setup check: Using a calibrated pressure gauge and flowmeter, check the valve actuation pressure. Adjust according to manufacturer's specifications (tolerance ±3 bar).
   • Repair/Replacement: If the valve sticks, does not operate, or continuously skips, remove it for inspection. Check the spool for wear, burrs, corrosion. Replace the seal. If the damage is significant, replace the valve completely (UNITEC Category: Pressure valves).
3. Distribution valves:
   • Thermal imaging: Detect overheated areas.
   • Repair/Replacement: Disassemble the valve. Check spools for sticking, wear. Replace seals and O-rings. Check the control solenoids with a multimeter (winding resistance: 15-30 ohms, supply voltage). If there is significant wear, replace the valve (UNITEC Category: Distribution valves).
4. Verification: After repairing or replacing valves, check system pressure and temperature. Make sure there are no localized hot spots.

8.3. Pump repair/replacement

1. ⚠ SECURITY: Apply LOTO. Drain the liquid from the tank.
2. Dismantling the pump: Carefully dismantle the pump, drain the remaining liquid.
3. Inspection and repair:
   • Disassemble the pump according to the manufacturer's instructions.
   • Inspect all internal components (pistons, vanes, gears, plates, bearings) for wear, scratches, cavitation damage.
   • Replace worn parts with a repair kit (UNITEC Category: Pump Repair Kits) or individual components (UNITEC Category: Hydraulic Components).
   • Pay special attention to shaft seals and bearings.
4. Pump replacement: If wear is significant or repair is not economically feasible, replace the pump with a new one of identical or equivalent specifications (UNITEC Category: Hydraulic pumps).
5. Installation and start-up: Install the pump, fill the system with liquid, perform the air removal procedure (section 8.6).
6. Verification: Start the system, measure the pump flow under pressure with a flow meter. Compare with nominal characteristics. Control the temperature.

8.4. Diagnostics and repair of drives (cylinders, hydraulic motors)

1. ⚠ SECURITY: Apply LOTO. Decompress the system.
2. Cylinders:
   • Leak test: Disconnect the cylinder rod from the load. Apply pressure to one side of the piston and close the drain line. Watch for stem movement or pressure drop.
   • Repair/Replacement: Disassemble the cylinder. Inspect the sleeve mirror, rod for damage. Replace the piston and rod seals (UNITEC Category: Hydraulic seals). Replace or repair sleeve/stem for significant damage.
3. Hydromotors:
   • Leakage/Efficiency Test: Measure the flow on the drain line of the hydromotor. Excessive flow indicates an internal leak.
   • Repair/Replacement: Disassemble the hydraulic motor. Inspect internal components for wear. Replace worn seals and parts. Replace the hydraulic motor with significant wear (UNITEC Category: Hydraulic motors).
4. Verification: Start the system, check the operation of the actuators, the absence of internal leaks and local overheating.

8.5. Replacement/Filtration of hydraulic fluid

1. ⚠ SECURITY: Apply LOTO. Drain the liquid.
2. Fluid Drain: Completely drain all old hydraulic fluid from the tank, lines, cylinders and other components. Dispose of in accordance with environmental standards.
3. System Flushing: Consider flushing the system with an appropriate flushing fluid if contamination has been significant.
4. Filter replacement: Install new filters (pressure, drain, air) according to the manufacturer's specifications (UNITEC Category: Hydraulic filters).
5. Filling with new fluid: Fill the system with new hydraulic fluid of the appropriate type and viscosity (eg HLP 46, ISO VG 46) as recommended by the equipment manufacturer. Use a clean container and pump for refueling.
6. Filtration (if necessary): If the system is heavily contaminated, or the fluid is new, but there is doubt about its purity, perform an additional fine cleaning with an external filter unit until the required purity class is reached (for example, ISO 4406: 18/16/13).
7. Verification: Start the system. Control the temperature. After 50-100 hours of operation, take a sample for laboratory analysis to ensure that the fluid meets purity standards.

8.6. Tank/Return Loop Optimization

1. ⚠ SECURITY: Apply LOTO. Drain the liquid.
2. Tank Inspection: Make sure the inside of the tank is clean. Check the presence of partitions and their condition - they contribute to cooling and deaeration.
3. Tank size: If the tank volume is less than 3-5 times the pump's minute flow, consider installing a larger tank or an additional external cooler.
4. Return Line:
   • Inspect return lines for kinks, constrictions, corrosion, or blockages that could create excessive back pressure.
   • Make sure the end of the drain pipe in the tank is below the liquid level to prevent aeration.
5. Verification: Start the system, monitor the foam level and tank temperature. Measure the back pressure.

9. Preventive measures

Regular maintenance is key to preventing hydraulic systems from overheating.

10. Spare parts and components

The following spare parts are critical for prompt repair and maintenance of hydraulic systems. Always use original or certified analogues that meet CE and UkrSEPRO standards.

For order and detailed parts catalog visit: www.unitecd.com/e-catalog/

11. Links

• DSTU ISO 4406:2017 – Hydraulic systems. Hydraulic fluid purity code.
• DSTU EN ISO 12100:2017 – Machine safety. General design principles. Risk assessment and risk reduction.
• DSTU EN 166:2017 – Individual eye protection. Requirements
• DSTU EN 388:2017 – Gloves protective against mechanical risks.
• DSTU EN ISO 20345:2017 – Personal protective equipment. Shoes are safe.
• Operation and maintenance manuals of OEM manufacturers of hydraulic equipment.
• UNITEC – Handbook of hydraulics.