Troubleshooting Hydraulic Systems Overheating: Diagnosis, Root Cause Analysis and Recovery Methods

1. Description of the Problem and Scope of Application

Overheating of hydraulic systems is a critical indicator of potential malfunctions, which can lead to significant reductions in equipment efficiency, accelerated component wear, unplanned downtime and, in some cases, catastrophic failures. This manual is designed for maintenance and repair technicians, reliability engineers and plant managers who work with industrial hydraulic systems at enterprises in the Ukrainian manufacturing sector.

Typical Symptoms of Overheating:

• Increased oil operating temperature (higher than recommended by the manufacturer, usually >55-60°C).
• Decrease in performance and speed of executive mechanisms.
• Increased noise level from the pump or valves.
• Accelerated aging and degradation of hydraulic fluid (darkening, burning smell).
• Frequent triggering of temperature sensors and emergency alarms.
• Damage to seals, hoses and gaskets.
• Increased system power consumption.

Types of Equipment Prone to Overheating:

• Hydraulic presses (metalworking, forming).
• Thermoplastic machines (production of plastics).
• CNC machines.
• Mobile hydraulic equipment (construction, agricultural).
• Industrial hydropower stations and power units.
• Hydraulic systems of lifting and transport equipment.

Classification of Severity:

• Critical: Oil temperature exceeds 80°C. There is a high risk of sudden failure of components, water hammer, liquid ignition. Immediate shutdown of equipment is critical.
• Significant: Oil temperature in the range of 65-80°C. Leads to accelerated degradation of liquid, intensive wear of seals and pumps, reduction of efficiency. Requires immediate diagnosis and elimination.
• Minor: Oil temperature in the range of 55-65°C. Indicates increased energy consumption and gradual degradation of the liquid. Needs planned diagnostics and optimization.

2. Security measures

WARNING!

Performing any diagnostic or repair work on hydraulic systems requires strict adherence to safety rules. Failure to follow these instructions could result in serious injury or death.

• Lockout/Tagout (LOTO): Before beginning any work to open the system, remove covers, or replace components, be sure to perform the LOTO procedure on all equipment that may affect the hydraulic system. This includes electrical power to pumps and control systems.
• Personal Protective Equipment (PPE): Always use appropriate PPE: safety glasses or shield, liquid-resistant gloves, protective clothing, protective footwear. When working with noise - hearing protection means.
• Stored Energy: Hydraulic accumulators and lines can contain significant amounts of stored energy under high pressure. Before disassembling or disconnecting components, always ensure that the system is de-energized and all lines are depressurized. Follow the equipment manufacturer's instructions for pressure relief.
• Hot Surfaces and Fluids: Hydraulic fluid and system components when overheated can be extremely hot, causing severe burns. Be careful when handling the system. Use a thermographic camera or an infrared thermometer to safely assess the temperature.
• High Pressure: Injection of hydraulic fluid under high pressure into the skin can cause serious injury requiring immediate medical attention. Never check for leaks with your hands. Use cardboard or other suitable means.
• Fluid Spills: Hydraulic oil spills create a slip hazard. Clean up spills immediately and use absorbent materials.

3. Necessary Diagnostic Tools

A set of specialized tools is required for accurate and effective diagnostics of hydraulic system overheating. The use of certified tools and adherence to measurement techniques is critical for the reliability of the results.

4. Initial Evaluation Checklist

Before starting detailed diagnostics it is critically important to collect as much information as possible about the current condition of the equipment and its operating conditions. This will allow you to localize possible problems and avoid unnecessary steps.

5. Systematic Diagnostic Algorithm

This algorithm provides a step-by-step approach to identifying the root cause of overheating. Follow the sequence of steps to effectively localize the fault.

1. Overheat Confirmation:
   1. Record current hydraulic fluid temperature using built-in sensor or external IR thermometer.
   2. Compare with recommended operating temperatures (typically 40-55°C). If >60°C, continue diagnosis.
2. Cooling System Check:
   1. For air coolers:
      1. Inspect cooler fins for contamination (dust, dirt, oil).
         • If dirty: Go to item 8.1 (Cleaning the Cooler).
      2. Check the operation of the cooling fan (rotation, direction of air flow).
         • If not working or not working efficiently: Use a multimeter to check the power and the fan motor windings. Go to point 8.1.
      3. Using a thermographic camera, measure the air temperature at the inlet and outlet of the radiator.
         • If the temperature difference is small (less than 5°C): Possible internal blockage or insufficient fluid flow through the cooler.
   2. For water coolers:
      1. Check cooling water flow (pressure, flow).
         • If the flow is insufficient: Check the water filters, valves, water circuit pump.
      2. Measure the water temperature at the inlet and outlet of the heat exchanger.
         • If the water temperature difference is insignificant: Internal clogging of the heat exchanger is possible.
   3. Using a thermographic camera, measure the temperature of the hydraulic fluid at the inlet and outlet of the cooler.
      • Expected difference: 8-15°C (depending on the type and size of the cooler).
      • If the difference is less: The cooler is not working efficiently. Go to point 8.1.
3. Fluid Level and Quality Check:
   1. Check the fluid level in the tank.
      • If the level is below the minimum: Top up the fluid to the required level using the same brand and type of oil (see 8.2).
   2. Take a sample of the fluid for visual assessment and further laboratory analysis (see 3. and 8.3).
      • If the liquid is dark, cloudy, has a burning smell: The liquid has degraded. Go to item 8.3 (Replacing Fluid and Filters).
      • If analysis shows contamination (>ISO 4406: 18/16/13), water content (>0.1%) or high oxidation: Go to 8.3.
4. Diagnostics of Pressure in the System:
   1. Connect a hydraulic manometer to the pump discharge line (in front of the safety valve).
   2. Start the system and record the maximum working pressure.
   3. Check the relief valve setting (pressure gauge after the valve).
      • If the pressure exceeds the operating pressure or the relief valve opens at a lower pressure than set: Possible incorrect adjustment or wear of the relief valve. Go to 8.4 (Adjustment/Replacement of Relief Valve).
      • If the pressure drops unexpectedly under load: Possible internal leaks in the pump or valves. Go to point 5.
5. Finding Internal Leaks:
   1. Pump Internal Leaks:
      1. Use a flow meter to measure pump flow at idle and under load.
         • If the flow decreases significantly under load (more than 10-15% of the nominal): Indicates internal pump leakage due to wear. Go to section 8.5 (Repair/Replacement of Pump).
      2. Use a thermal imaging camera to inspect the pump housing and drain line to the tank.
         • Localized heating of the pump housing or unusual temperature increase in the discharge line (more than 10°C from the tank temperature): Confirms an internal leak.
   2. Internal leaks in manifolds and valves:
      1. Use a thermographic camera to inspect the valve body and manifolds.
         • Localized housing hot spots (15-20°C higher than adjacent components): Indicates internal fluid throttling due to spool wear or seal damage. Go to point 8.5.
      2. Check valve drain lines for abnormal flow when actuators are idle.
   3. Internal leaks in hydraulic cylinders:
      1. Bring the cylinder to the end position and shut off the pressure supply.
         • If the cylinder rod moves involuntarily: Indicates a piston seal leak. Go to point 8.5.
      2. Use a thermal imaging camera to inspect the cylinder body.
         • Temperature difference along the body or at the outlet of the drain line: May indicate an internal leak.
6. Evaluation of the Electric Motor (if a hydraulic pump drive):
   1. Use a multimeter to measure the current consumed by the electric motor.
   2. Compare with rated current.
      • If the current exceeds the rated current: The electric motor is operating with an overload, which can lead to heating of both the motor itself and the hydraulic fluid. Check the mechanical part of the pump for binding or excessive friction. Go to item 8.6 (Diagnosis/Repair of the Electric Motor).
   3. Use an infrared thermometer or thermographic camera to measure the temperature of the motor housing and bearings.
7. Evaluation of Design/Underloading Failures:
   1. If all previous checks show no obvious failures, but overheating persists, consider under-sizing the cooler or general inadequate design of the hydraulic system for the current load conditions.
   2. Review the technical documentation of the equipment, the design loads and the capacity of the chiller.
      • If the system parameters (pressure, flow) have been increased, but the cooler has not been changed: Go to point 8.7 (Optimization of the System).

6. Matrix of Malfunctions and Causes

This table summarizes common symptoms of overheating, likely root causes (ranked by frequency of occurrence), diagnostic methods, and expected outcomes.

7. Root Cause Analysis for Each Malfunction

7.1. Clogged or Inefficient Cooler

Explanation: The cooler (radiator or heat exchanger) is designed to remove excess heat from the hydraulic fluid. Clogging of external ribs (dust, dirt, fibers) or internal channels (sludge, liquid oxidation products) significantly reduces its heat dissipation capacity. Fan failure (for air) or insufficient cooling water flow (for water) also leads to inefficiency.

Confirmation: The temperature difference of the hydraulic fluid at the inlet and outlet of the cooler is less than 8°C. The surface temperature of the cooler using a thermographic camera shows uneven heat distribution or a generally high temperature. For air - no or weak air flow. For water - insufficient pressure/flow of cooling water.

Damage: Constant overheating of the liquid, accelerated oxidation and degradation of the oil, which leads to wear of all system components and the formation of deposits.

7.2. Internal Sources

Explanation: Internal leaks occur when hydraulic fluid passes through seals or gaps that should be sealed. This could be pump wear (increased clearances between rotor/gears and housing), wear of spools in valves (distributors, regulators), or damage to piston/rod seals in hydraulic cylinders. When the fluid is throttled through these gaps, the kinetic energy is converted to heat and the fluid heats up.

Confirmation:

• For the pump: Pump flow measurement under load shows a drop in performance by >10-15% of nominal. A thermographic inspection of the pump casing and drain line reveals localized hot spots or abnormal heating of the drain line.
• For valves: Thermographic examination of the valve body shows local hot zones (15-20°C above the temperature of the surrounding components) due to constant fluid throttling. Abnormal flow in valve drain lines during idle time.
• For cylinders: "Sliding" of the cylinder rod under load when the pressure supply is blocked.

Damage: Significant loss of system efficiency, increased energy consumption (pump works harder to compensate for leakage), accelerated wear of other components due to constant fluid heating, potential equipment failure.

7.3. Low Hydraulic Fluid Level

Explanation: Insufficient fluid in the hydraulic tank reduces the volume of fluid available for circulation and cooling. This leads to accelerated circulation of a smaller volume of liquid, which does not have time to give off heat through the surface of the tank or the cooler. A low level can also cause pump cavitation due to air intake.

Confirmation: A visual inspection of the oil level indicator in the tank shows the level below the minimum mark. Possible signs of cavitation are increased pump noise.

Damage: Pump cavitation (intense impeller/rotor wear), increased fluid oxidation due to aeration, seal damage and rapid component wear.

7.4. Degradation or Contamination of Liquids

Explanation: Hydraulic fluid degrades over time due to oxidation (under the influence of oxygen and high temperatures), hydrolysis (with water) and thermal decomposition. This leads to a loss of lubricating properties, a change in viscosity, the formation of acids and deposits. Particulate contamination (ISO 4406) increases friction between moving parts, which generates additional heat.

Confirmation: Visually, the liquid is dark, opaque, and has a burning smell. Laboratory analysis of oil (according to DSTU ISO 4406) shows a high level of pollution (for example, >ISO 4406: 18/16/13), water content >0.1%, high acid number (TAN >0.5 mg KOH/g) or a change in viscosity by >10% from the nominal one.

Damage: Accelerated wear of all moving parts of the system (pumps, valves, cylinders), clogging of filters and small passages, jamming of valves, corrosion.

7.5. Excessive Pressure in the System / Choking

Explanation: If the system is operating at a higher pressure than necessary, or if there is excessive throttling of the fluid (for example, due to improperly adjusted valves or flow restrictions), this generates a significant amount of heat. Safety valves, which are constantly open due to overload or malfunction, also constantly throttle the fluid, converting energy into heat.

Confirmation: Pressure measurement with a manometer shows an operating pressure higher than the recommended by the equipment manufacturer. Checking the setting of relief valves reveals that they open at a lower pressure than set or are constantly open.

Damage: Increased energy consumption, intensive wear of the pump, damage to seals, general overheating of the system.

7.6. Incorrect Fluid Viscosity

Explanation: Using hydraulic fluid with the wrong viscosity can cause overheating. If the viscosity is too high, the fluid offers excessive resistance to flow, increasing friction and generating heat. If the viscosity is too low, internal leakage increases, which also leads to heating.

Confirmation: Checking the specification of the fluid poured into the system and comparing it with the recommendations of the equipment manufacturer. Laboratory viscosity analysis (according to DSTU ISO 3104) shows values ​​that differ by more than 10% from the recommended operating temperature.

Damage: If the viscosity is too high - wear of pumps, filters, increased resistance. If the viscosity is too low - increased internal leaks, cavitation, reduced lubricity.

7.7. Overloading the Electric Motor of the Pump

Explanation: If the hydraulic pump is overloaded (for example, due to internal leaks, excessive pressure or mechanical faults), the electric motor driving it will be overloaded. This leads to increased current consumption and heating of the electric motor itself, which can be transferred to the hydraulic fluid and cause overall overheating of the system.

Confirmation: Measuring the electric motor current with a multimeter shows a value higher than the nominal . Thermographic examination of the electric motor reveals a significant increase in the temperature of the housing (>80°C) or bearings.

Damage: Overheating and failure of motor windings, damage to bearings, loss of pump drive efficiency.

7.8. Insufficient Cooling System Size or General Design Imperfections

Explanation: In some cases, especially when upgrading equipment or changing technological processes, the existing cooling system may not be sufficient to remove all the heat generated. This may be the result of increased workloads, speeds or cycle times without the chiller being recalculated and upgraded accordingly.

Confirmation: After eliminating all other possible malfunctions, overheating remains. Calculation of the heat balance of the system shows that more heat is generated than can be removed by the existing cooler. The temperature of the fluid constantly exceeds the norm even with nominal loads.

Damage: Chronic overheating, which leads to accelerated wear, fluid degradation and unstable operation of equipment, impossibility of operation at full capacity.

8. Step-by-Step Troubleshooting Procedures

Before performing any procedures, be sure to follow the safety precautions in Section 2.

8.1. Cleaning or Repair of the Cooling System

1. Follow the LOTO procedure.
2. For air coolers:
   • Remove contamination (dust, dirt, oil) from the outer fins with compressed air (pressure <6 бар) or special cleaning agents.
   • Check the electrical supply and condition of the fan motor. If necessary, replace the fan or its motor.
3. For water coolers:
   • Flush the water circuit to remove deposits and sludge. Use special chemical solutions for cleaning if the clogging is significant.
   • Check the functionality of the water pump and water flow control valves.
4. After cleaning/repair, run the system and check the hydraulic fluid temperature difference at the cooler inlet/outlet. It should be 8-15°C.

8.2. Topping up Hydraulic Fluid

1. Follow the LOTO procedure.
2. Use only the same brand, type and viscosity grade of hydraulic oil as recommended by the equipment manufacturer (eg ISO VG 46 or 68).
3. Make sure the fluid is filtered when topping up to avoid additional contamination of the system.
4. Add fluid to the mark recommended by the manufacturer.

8.3. Replacement of Hydraulic Fluid and Filters

1. Perform the LOTO procedure and provide a container to collect the waste fluid.
2. Drain the used hydraulic fluid from the tank and system.
3. Replace all hydraulic filters (rotary, pressure, suction - in accordance with DSTU ISO 2941-2943).
4. Clean the hydraulic tank of deposits.
5. Pour in new hydraulic fluid of the appropriate brand and purity grade (recommended ISO 4406: 17/15/12 or better) through the special filter cart.
6. Start the system, bleed air and check the fluid level.

8.4. Adjusting or Replacing the Relief Valve

1. Follow the LOTO procedure.
2. Connect a calibrated pressure gauge to the pressure line test point.
3. Start the system and gradually increase the pressure to the required value (according to the manufacturer's specification).
4. Adjust the relief valve until it activates at the required pressure.
5. If the valve does not hold pressure or does not respond to adjustments, it needs repair or replacement.
6. After adjustment, check the temperature of the fluid under load.

8.5. Repair or Replacement of Components with Internal Leaks (Pump, Valves, Cylinders)

1. Perform the LOTO procedure and release all system pressure.
2. Disassemble the faulty component.
3. Inspect:
   • For the pump: Assess the condition of the rotor/gears, bearings, seals. In case of wear >0.05 mm or damage to the working surfaces – replace or repair the pump.
   • For valves: Check the spools for wear, seizing, damaged seals. In case of significant wear or size discrepancy, replace the valve.
   • For cylinders: Inspect cylinder mirror and rod for damage, replace piston and rod seals.
4. Assemble or install the new component following the tightening torques (according to ISO 4017, ISO 4032) and installation procedures.
5. After starting the system, check its operation, pressure, flow and absence of overheating.

8.6. Diagnostics and Repair of the Electric Motor

1. Follow the LOTO procedure.
2. Using a multimeter, check the resistance of the motor windings.
3. Inspect the bearings for wear. Replace bearings if play or noise is present.
4. Check rotor balancing.
5. If damage to the windings or critical wear is detected, send the motor for overhaul or replace it.
6. After repairing/replacing the motor, check the load current consumption. It should be within the nominal range.

8.7. Optimization of the Cooling System

1. If chronic overheating persists after eliminating all malfunctions, it is necessary to recalculate the thermal balance of the system.
2. Consider installing a more powerful cooler or an additional cooling circuit.
3. Consider reducing the operating pressure or flow if it does not affect the process.
4. Consider using hydraulic fluids with improved heat dissipation properties.

9. Preventive Measures

Preventing overheating is critical to ensure the durability and reliability of hydraulic systems.

10. Spare Parts and Components

The availability of high-quality spare parts is a guarantee of quick and effective troubleshooting. UNITEC-D GmbH offers a wide range of hydraulic components that meet CE and UkrSEPRO standards.

To order quality hydraulic components and spare parts, visit our e-catalog: www.unitecd.com/e-catalog/

11. Links

• DSTU ISO 4406: Volumetric hydraulics of hydraulic drives. liquids The method of coding the level of contamination by solid particles.
• DSTU EN 837-1: Manometers. General requirements and tests.
• DSTU EN 13187: Non-destructive testing. Thermographic control. General principles.
• DSTU ISO 11158: Lubricating oils, lubricants and related products. Classification. Group H (hydraulic systems).
• DSTU ISO 2941-2943: Volumetric hydraulics of hydraulic drives. Filters.
• DSTU EN 61010-1: Safety requirements for electrical equipment for measurement, control and laboratory use. Part 1. General requirements.
• Equipment operation and maintenance instructions (OEM manuals).
• Additional materials of UNITEC-D GmbH for the maintenance of hydraulic systems.