Hydraulic Systems Maintenance and Common Faults

Hydraulic systems generate power in many machines, from industrial presses to agricultural equipment. Hydraulic system maintenance must not be neglected to ensure these systems operate smoothly. If proper maintenance is not performed, faults such as hydraulic oil contamination, leaks, cavitation, overheating, and noise can occur, leading to high repair costs. This article addresses the basic maintenance requirements of hydraulic systems, details common fault symptoms and how to prevent them. We also provide step-by-step troubleshooting methods.

Hydraulic Oil Contamination and Types

The cleanliness of the oil in hydraulic systems directly affects the system’s lifespan. Oil contamination may occur in several forms: particulate matter, water, air or chemical degradation. Dirty hydraulic oil can clog the system and cause premature failures . Below are the main types of contamination and their effects.

Particle Contamination

Particle contamination comprises solid materials such as metal shavings, rubber pieces, dust, sand or paint debris . These particles clog narrow channels in the system and wear out orifices and valves. Particle contamination can lead to:

Reduced component lifespan and frequent failure .
Loss of efficiency and production errors .
Pressure fluctuations and overheating .

To prevent particle contamination, use a clean filter unit when filling the hydraulic system and remove protective caps just before assembly . New or repaired circuits should also be flushed and rinsed before being put into service .

Water Contamination

Water contamination occurs when water enters the hydraulic fluid, reducing its lubricating properties. According to a Cylinders Inc article, water contamination causes corrosion, reduces the oil’s film strength and triggers cavitation . The easiest way to tell if there is water contamination is when the oil appears milky like milk. To prevent water contamination, ensure reservoir caps are tightly closed and that there are no leaks in the oil coolers .

Air Contamination (Aeration)

Aeration occurs when air bubbles are carried within the hydraulic flow. This condition arises when air enters the pump’s suction line or when the oil level is low. The McPherson Oil maintenance guide states that aeration is caused by low pressure at the pump inlet and appears as foaming and small vortices in the reservoir . Low oil level, air leaks in the suction line, low oil temperature or using high-viscosity oil can cause aeration . Aeration produces sounds like a “marble gargling noise” in the pump . To identify air leaks, smear oil on potential leak points and observe changes in noise .

Chemical Degradation

Hydraulic oil oxidizes over time, forming gum-like deposits and sludge . High temperature, exposure to air and water accelerate this degradation. Mixing different types of oil can also cause chemical reactions . Oxidized and degraded oil results in increased viscosity and sticky deposits, causing valves to clog and movement to slow . Therefore, it is important to have periodic oil analysis and determine oil change intervals based on the manufacturer’s recommendations .

Ingressed Dirt and External Contamination

Ingressed dirt consists of dust, mud and sand particles that enter during system assembly or repair . This type of contamination also enters the system through sealing failures and cracked hoses. To minimize external contamination, protect hose and connection ends with caps, keep the repair area clean and regularly check air filters .

Leaks, Cavitation and Noise

Leakage in hydraulic systems leads to a loss of efficiency and environmental problems. The McPherson Oil article recommends routinely checking hoses and connections and replacing worn or damaged parts. Missing oil causes the system to overheat and reduces efficiency . Increasing leak volume stresses the pump and results in higher operating pressures and overheating.

Cavitation occurs when the liquid vaporizes at the pump inlet and the vapor bubbles collapse under pressure, striking metal surfaces. This phenomenon is heard as a high-frequency “sharp squeal” or loud hum . The main causes of cavitation include insufficient filling at the pump inlet, a clogged suction filter, high oil viscosity or low fluid level . Continuous cavitation creates pitting on pump surfaces, shortening pump life.

Noise is usually associated with mechanical issues such as cavitation, aeration or worn bearings. To identify the source of noise, pumps, motors and valves should be listened to separately. Where possible, attach a vacuum gauge to the suction line to measure inlet pressure and compare with manufacturer values .

Overheating and Cooling Problems

The ideal operating temperature for hydraulic oil is between 43°C and 60°C (110°F and 140°F) . When temperature increases, the oil’s viscosity decreases, the lubrication film thins and components wear out. The main causes of overheating include:

Insufficient cooling: The oil cooler or fan may be dirty or faulty. Air trapped in the cooling circuit also reduces cooling efficiency.
High rotational speed and pressure: If the pump is constantly operating at maximum flow and pressure, it consumes excessive power and generates heat. For fixed-displacement pumps, ensure the relief valve is properly adjusted; if set too low, the pump by-passes unnecessarily and generates heat .
Leaks and pressure drops: Leakage flow converts into heat. It is important to ensure hose and connection points are tight and seals are intact.

System temperature should be regularly measured and the cooling system checked when high temperatures are detected. If necessary, change the oil viscosity to another class suitable for the application (for example, switch from ISO VG 46 to ISO VG 32).

Filter and Oil Change Intervals

The Vickers manual notes that oil change intervals vary depending on operating conditions and that it is difficult to give a single fixed period . However, some general principles are:

Filter elements: Replace immediately when pressure or vacuum gauges indicate the filter is clogged . In general, replace the filter element when the contamination indicator turns red.
Oil changes: Regularly check the oil’s color, smell and viscosity . Routine oil analysis (particle count, water content, acid number) helps determine the right time for change. When the tank is drained, remove all oil from the system and, if necessary, clean it with a low-viscosity flushing oil . Then fill the system with fresh oil filtered through a 25-micron filter .

A Step-by-Step Approach for Fault Detection and Remedy

Because hydraulic systems are complex, troubleshooting must be done systematically. The following steps help find the fault more easily:

Identify the fault symptom – Is the system making loud noise, moving slowly, overheating or losing power? Note the symptoms.
Check oil level and condition – If the oil level is low, replenish it; examine the oil’s color and smell. If the oil is foamy or milky, there may be water or air contamination .
Check filter and pressure indicators – Clogged filters cause pressure drop and cavitation. Replace the element if the filter’s differential pressure indicator is red .
Inspect hose and connection leaks – Check all hoses, fittings and valve seals for leaks . Even small leaks allow air or contaminants to enter and cause pump cavitation.
Listen to the pump – A high-pitched whistle or hum indicates cavitation . A gurgling sound like “marbles gargling” indicates aeration . To localize the noise source, connect a vacuum gauge to the suction line and measure the pressure, comparing with catalog values .
Measure temperature and pressure – If the hydraulic circuit temperature exceeds 60°C, check the cooler, thermostat, valve settings and oil viscosity . If system pressure is lower than expected, the pump may be worn or valves faulty; if it is too high, the relief valve may be incorrectly adjusted .
Pressure testing and circuit diagram review – Trace the flow path on the system schematic to identify pressure test points. Install pressure gauges near the suspected fault and observe pressure fluctuations. If necessary, sequentially isolate components to identify the faulty part.

Common Faults and Their Causes

After the step-by-step troubleshooting guide, it is useful to look at specific fault types and their possible causes. Below, the most common problem sources are listed for situations like hydraulic system not working, low pressure, overheated pump, directional control valve malfunction and electrical faults. This checklist helps technicians narrow down problems quickly and manage the maintenance process effectively.

Hydraulic System Not Working

Hydraulic pump not receiving drive or its driver is faulty: The motor or coupling driving the pump may be broken, cutting power transmission.
Reverse rotation: Pumps – especially gear and vane types – cannot intake if they rotate backwards.
No oil in the hydraulic system or low tank level: Insufficient oil feed draws air and causes pump cavitation.
Oil viscosity too low or excessively fluid: The system does not generate pressure; internal leakage in the pump increases.
Dirty filter and air entry from suction line: A clogged filter prevents adequate intake; air entry causes aeration and loss of efficiency.
Pump worn or damaged: Internal wear decreases flow; worn gears and vanes reduce efficiency.
Oil leaks in the pressure line: Leaks load backpressure on the pump and reduce efficiency.
Relief valve set too low: Oil by-passes before the pump reaches its designed pressure.
Excess load and wrong connections: If the hydraulic circuit is poorly designed or the load is higher than expected, the system may not move.

Low Pressure

Worn or damaged pump: Internal leakage leads to pressure drop.
Oil viscosity too low or cooler setting incorrect: Thin oil increases leakage and decreases pressure; if the cooling system is inadequate, the oil thins more.
Electric motor or coupling failure: The pump may not rotate at the required speed to generate sufficient pressure.
Incorrect pressure settings: A too-low relief valve setting causes oil to by-pass before pressure builds.
Valve contamination and leaks: Contamination prevents valves from fully closing; cylinder bore wear and piston seal damage also cause pressure loss.
Wrong seal or seal material: Using inappropriate materials increases internal leakage.
Small reservoir and aeration: If the tank volume is insufficient, oil heats quickly and viscosity drops; air bubbles in oil prevent pressure buildup.
Faulty relief valve and accumulator: A broken spring or contamination prevents the relief valve from holding pressure; if accumulator gas is lost, pressure fluctuates.

Overheated Pump

Cavitation and air entrainment: Vacuum at the pump inlet, a clogged filter or low oil level cause gas bubbles and cavitation .
Relief valve set too high: The pump operates continuously at high pressure and generates heat.
Excess load and worn pump: Loads higher than expected or internal leakage stress the pump.
Low oil volume and hot oil: Low oil level and high inlet temperature lower the viscosity; the pump cannot lubricate.
Clogged pump suction: A blocked suction line prevents sufficient oil flow, leading to cavitation and overheating.

Directional Control Valve Malfunction

Contaminated or sticking spool: Particles and deposits prevent spool movement.
Mechanical wear and leakage: Worn valve body and seals leak, reducing pressure.
Bolts tightened too tightly: The valve body becomes oval and the spool sticks.
Cold oil: At low temperature, high viscosity slows spool movement.
Coil or electrical energy fault: The solenoid coil may be burnt or the PLC signal may not arrive.

Electrical Faults

No power to solenoid valve coil or coil burnt out: Broken electrical connections or overheating damage the coil.
PLC or control system faults: Software errors or module failures prevent system operation.
Magnetic sensor failure: The cylinder position sensor may be faulty or misaligned, cutting the control signal.

Weekly Maintenance Checklist

In addition to the manufacturer’s instructions, a weekly checklist can be applied to keep hydraulic systems running smoothly. The 10-point quick maintenance check proposed by McPherson Oil summarizes the basic steps operators should take each week :

Check oil level and top it off with the same brand and viscosity if necessary .
Inspect air intake filters and filler screens, and clean if there is clogging .
Check filter indicators, and replace the element if differential pressure is high .
Visually inspect all hose and pipe connections for leaks; leaks cause both environmental contamination and energy loss .
Measure system temperature; if overheating, check the cooler and relief valve settings .
Look for signs of aeration in the reservoir; foaming or vortex indicates aeration .
Listen to the pump; a high-pitched whistle indicates cavitation, while a gargling sound indicates aeration .
Take an oil sample and examine color, odor and particles; plan an oil change if heavily contaminated .
Check servo valve temperature; temperatures above 65°C may mean the valve is sticking .
Measure the electric motor’s bearing and housing temperature; abnormal heating indicates wear or misalignment .

Conclusion: Regular Maintenance for Efficient Hydraulic Systems

Regular maintenance is essential for reliable and long-lived operation of hydraulic systems. Faults such as contamination, leaks, cavitation, overheating and noise reduce system performance and increase costs.

Hydraulic Systems Maintenance and Common Faults