Chapter 8: Maintenance of Hydraulic Systems
Common Causes of Hydraulic System Problems
The most frequent issues in hydraulic systems include:
Clogged or dirty oil filters
Inadequate supply oil in the reservoir
Leaking seals
Loose inlet lines causing air intake
Incorrect type of oil
Excessive oil temperature
Excessive oil pressure
Approximately 50% of hydraulic system problems can be traced directly to the oil and can be eliminated
with a proper preventive maintenance program.
Fluid Degradation Issues
Oxidation and Corrosion
Oxidation is caused by chemical reaction between oxygen and oil particles, which can seriously reduce
hydraulic fluid service life.
Effects of oxidation:
Formation of acidic compounds that cause corrosion
Formation of insoluble gums, sludge, and varnish
Increased fluid viscosity
Oxidation is dramatically affected by temperature:
Slower below 60°C
Doubles for every 12°C temperature rise
Rust is the chemical reaction between iron/steel and oxygen, while corrosion is the reaction between
metal and acid. Both cause excessive leakage past sealing surfaces.
Additives can be incorporated to inhibit oxidation, rust, and corrosion, though they increase cost.
Fire-Resistant Fluids
A fire-resistant fluid can be ignited but won't support combustion when the ignition source is removed.
Flammability characteristics:
Flash point: Temperature at which oil gives off sufficient vapors to ignite when exposed to flame
Fire point: Temperature at which oil releases sufficient vapor to support combustion for 5 seconds
Autogenous ignition temperature (AIT): Temperature at which ignition occurs spontaneously
Types of fire-resistant fluids:
1. Water-Glycol Solutions
40% water, 60% glycol
High viscosity index values
Operating range: -20°C to 80°C
Incompatible with zinc, cadmium, and magnesium
2. Water-in-Oil Emulsions
40% water dispersed in special oil base
Good coolant properties but more corrosive
Operating range: -30°C to 80°C
Requires water replenishment due to evaporation
Compatible with most rubber seals
3. Synthetics
Formulated to inhibit combustion
Highest fire resistance
Includes phosphate esters or chlorinated hydrocarbons
Low viscosity index
Incompatible with most rubber seals
Higher cost
4. High-Water-Content Fluids
90% water, 10% additives
Excellent fire resistance and cooling capacity
Low cost
Maximum operating temperature 50°C
Compatible with most rubber seals
Entrained Air
Air can enter hydraulic systems through:
Return lines discharging above oil level
Leaks in conductors
Air pockets in system high points
Effects of entrained air:
Causes cavitation
Reduces fluid bulk modulus
Creates erratic operation
Increases fluid compressibility
Accelerates fluid degradation
Generates noise
A properly designed reservoir helps reduce these problems.
Fluid Neutralization Number
The neutralization number measures the relative acidity or alkalinity of hydraulic fluid, specified by pH
factor. A fluid with a small neutralization number is recommended because high acidity or alkalinity
causes:
Corrosion of metal parts
Deterioration of seals and packing glands
For acidic fluid, the neutralization number equals the milligrams of potassium hydroxide needed to
neutralize acid in a 1-gram sample.
Proper Fluid Management
Usage and Disposal
Best practices for hydraulic fluid management:
1. Select optimal fluid for the application (considering pressure, temperature, and desired properties)
2. Use well-designed filtration system to reduce contamination
3. Follow proper storage procedures
4. Use care when transferring fluids to minimize contamination
5. Regularly check fluid properties (viscosity, acidity, water content, etc.)
6. Maintain system according to manufacturer specifications
7. Reduce or eliminate leakage
8. Properly dispose of used fluid through certified contractors
Contamination Control
Three main filtering methods:
1. Mechanical: Blocks particles by filtering material; traps coarse particles
2. Absorbent: Captures particles within filtering material; traps water and smaller particles
3. Adsorbent: Captures particles on filter surface; traps gases and microscopic particles
Strainers use wire screens wrapped around a metal frame and are suitable for larger particles. They're
typically installed in pump suction lines due to low pressure drop.
Filters use additional materials beyond screens and remove smaller particles measured in microns.
Filter Placement
Proportional flow filtration: Filters only a percentage of system flow
No positive protection for specific components
No way to know when filter is dirty
Full flow filtration: Accepts all pump flow
Should be placed where dirt enters system
Good systems have multiple filters
Should include inlet line filter and high-pressure pump discharge filter
Beta Ratio
The beta ratio of a filter is the ratio of upstream particles to downstream particles of a specific size:
Beta ratio = (number of upstream particles > N μm) / (number of downstream particles > N μm)
Beta efficiency:
Beta efficiency = 1 - (1 / Beta ratio)
Wear Due to Contaminants
Solid contaminants can be classified by size relative to clearance between moving parts:
1. Smaller than clearance: Collect inside clearance when component isn't operating, blocking lubricant
flow
2. Equal to clearance: Rub against mating surfaces, breaking down lubricating film
3. Larger than clearance: Interfere with lubrication by collecting at clearance entrance
Troubleshooting Hydraulic Systems
Noisy Pump
Potential causes:
Air entering pump inlet
Misalignment of pump and drive unit
Excessive oil viscosity
Dirty inlet strainer
Chattering relief valve
Damaged pump
Excessive pump speed
Loose or damaged inlet line
Low or Erratic Pressure
Potential causes:
Air in fluid
Pressure relief valve set too low
Pressure relief valve not properly seated
Leak in hydraulic line
Defective or worn pump
Defective or worn actuator
Actuator Fails to Move
Potential causes:
Faulty pump
Directional control valve fails to shift
Defective actuator
System pressure too low
Pressure relief valve stuck open
Excessive actuator load
Check valve installed backwards
Slow or Erratic Actuator Motion
Potential causes:
Air in system
Viscosity too high
Worn or damaged pump
Pump speed too slow
Excessive leakage through actuators or valves
Faulty or dirty flow control valves
Blocked air breather in reservoir
Low fluid level
Faulty check valve
Defective pressure relief valve
No Pressure
Potential causes:
Pump turning in wrong direction
Ruptured hydraulic line
Low oil level
Pressure relief valve stuck open
Broken pump shaft
Full pump flow bypassed to tank
Overheating
Potential causes:
Heat exchanger turned off or faulty
Undersized components or piping
Incorrect fluid
Continuous operation of pressure relief valve
Overloaded system
Dirty fluid
Reservoir too small
Inadequate oil supply
Excessive pump speed
Clogged air breather