Hydraulics and Pneumatics

Preventive Maintenance for Fluid Systems 261

Preventive Maintenance for Fluid Systems provides an overview of the benefits of a preventive maintenance program for fluid systems. Contamination in hydraulic or pneumatic fluid is the most common cause of malfunction for hydraulic and pneumatic systems. Preventive maintenance involves using filters or strainers to prevent contamination in the hydraulic fluid. A preventive maintenance program requires system operators to follow routine maintenance schedules regarding seals, conductors, and other system components.

A successful preventive maintenance program can help a manufacturing facility reduce downtime, lessen the need for costly repairs, and increase productivity. After taking this class, users will understand the benefits of a preventive maintenance approach for fluid systems.

  • Difficulty Intermediate

  • Format Online

  • Number of Lessons 19

  • Language English


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Course Outline
  • Benefits of Preventive Maintenance
  • Fluid System Personnel
  • Computerized Maintenance Management System
  • Preventive Maintenance Tasks for Fluid Systems
  • Preventive Maintenance Review
  • Types of Contaminants in a Fluid System
  • Effects of Contamination on a Fluid System
  • Hydraulic System Filtration
  • Pneumatic System Filtration
  • Temperature Control
  • Contamination Control Review
  • Hydraulic Fluid Viscosity and Temperature
  • Leakage in Fluid Systems
  • Adequate Fluid Levels
  • Fluid Maintenance Review
  • Seals for Fluid Systems
  • Seal Installation and Maintenance
  • Monitoring Pressure
  • Final Review
  • Describe the benefits of preventive maintenance.
  • Describe fluid system personnel.
  • Describe computerized maintenance management systems.
  • Describe preventive maintenance tasks for fluid systems.
  • Describe types of contamination in a fluid system.
  • Describe how contamination affects a fluid system.
  • Describe filters and strainers for hydraulic systems.
  • Describe air filtration in a pneumatic system.
  • Describe heat exchangers used in fluid systems.
  • Describe the relationship between hydraulic fluid viscosity and temperature in a hydraulic system.
  • Describe internal and external leakage in fluid systems.
  • Describe adequate fluid levels and associated preventive maintenance tasks.
  • Describe the purpose of seals for fluid systems.
  • Describe seal installation and maintenance.
  • Describe the effects of pressure on fluid systems.
Vocabulary Term

abrasive particles

A small, hard grain that wears down or rubs away material. Abrasive particles in a fluid system are a type of material contaminant.


A type of filter element used to trap very fine particulate matter in a hydraulic system. Absorbent filters consist of porous materials and may include resin for increased strength.


A substance added to another substance or material to improve its chemical and physical properties. Additives in hydraulic fluids are necessary to maintain certain fluid properties.


A type of filter element often used in air breathers to trap various sizes of particulate matter. Adsorbent filter elements often consist of charcoal and chemically treated paper.


A type of heat exchanger that cools compressed air coming from the air compressor in a pneumatic system. An aftercooler also removes moisture from the compressed air.

air breathers

A device that filters air moving into a container or component. Air breathers are used to filter ambient air let into a reservoir.

air compressors

The power source of a pneumatic system that pressurizes ambient air. An air compressor directs pressurized air into the system in order to produce mechanical power.

air receiver

An air storage tank used with pneumatic systems that balances the air compressor capacity and airflow demand. An air receiver can help to cool compressed air.

air-cooled heat exchanger

A component in a hydraulic system that uses cold air and a grid of metal pipes to cool hydraulic fluid. An air-cooled heat exchanger transfers heat from the hydraulic fluid to the air.


A device on a hydraulic reservoir that permits air to move in and out to maintain atmospheric pressure in the reservoir. Breathers are typically outfitted with air breathers to filter the air coming in.


A material that is likely to fracture when a force acts on it. Brittle hydraulic system components may be a result of low operating temperatures.


A condition caused by excess air in flowing hydraulic fluid. Cavitation in flowing hydraulic fluid can affect various system components, but it commonly affects the hydraulic pump.

clean-out plates

A removable transparent plate located at both ends of the reservoir in a hydraulic system. Clean-out plates allow access to the inside of the reservoir.


The small space between components that allows for movement and lubrication. Clearances are necessary for the successful operation of many fluid system components.

computerized maintenance management system

CMMS. A software program that maintains a detailed database about an organization's maintenance operations for various system components. Computerized maintenance management systems allow personnel to handle multiple maintenance schedules at once.


A passive fluid system component that conveys pressurized liquid or gas through a fluid power system. Conductors include pipes, tubes, and hoses.


Any foreign substance in a hydraulic or pneumatic fluid that causes a loss of efficiency or a breakdown in a fluid system. Contaminants include solids, liquids, gases, and forms of energy.


Any foreign material in a hydraulic or pneumatic fluid that has a negative effect on the fluid system. Contamination can be solid particles, liquids, or gases.

control valves

A mechanical device that regulates the flow of liquid or gas by opening, closing, or obstructing one or more passageways. Control valves contain movable parts that can malfunction if subjected to contaminants.


The gradual chemical attack on a material caused by moisture or atmospheric agents. Corrosion in a fluid system can be caused by factors such as contamination or a small leak.


The metal case enclosing the crankshaft, motor oil, and associated parts in a reciprocating engine. A crankcase requires lubrication in order to function.


An unproductive block of time during which machine operations cease. Downtime is often the result of mechanical problems.

drain lines

A conductor that carries pressurized fluid from the actuator back to the reservoir. A properly connected drain line relieves excess pressure as fluid leaves the actuator.

dynamic seals

A seal used between two connected components in a fluid system that move relative to one another during operation. A dynamic seal experiences faster wear than a static seal.


The amount of power output a system produces compared with the amount of power input. Efficiency ratings are used to determine the operating conditions of system components.


A flexible type of plastic. Elastomers include different varieties of rubber and are used in seal materials for fluid systems.

energy contaminants

A type of contaminant that hinders the normal, effective performance of a hydraulic or pneumatic fluid. Energy contaminants include heat, magnetic fields, and radiation.

external leakage

The unintentional movement of fluid from a fluid system into the environment. External leakage happens in both pneumatic and hydraulic systems.

failure rate

The frequency with which a system or component fails. Failure rates for some fluid system components, such as seals, are predictable.

filter media

An individual layer of material used in a filter to trap particulate matter. Filter media are also referred to as the filter elements.


FRL. A device that conditions air for use in a pneumatic system. A filter-regulator-lubricator helps to prevent moisture from contaminating pneumatic fluid after it leaves the compressor.


A device that prevents particulate matter from contaminating a fluid system. Filters are installed at various places throughout a fluid system.


A fluid component that connects, redirects, extends, or terminates the run of a fluid conductor. Fittings can help prevent external leakage as long as they are installed correctly.


A fluid component that connects, redirects, extends, or terminates the run of a fluid conductor. Fittings should be checked weekly as part of a preventive maintenance program.

fluid flow

The motion of a liquid or a gas subjected to unbalanced forces or stresses. Fluid flow in a fluid system develops pressure as it meets resistance.

fluid system engineer

The person responsible for assembling a fluid system and choosing compatible equipment. A fluid system engineer ensures that all components can work together to produce mechanical power.

fluid system operator

A person trained to run a specific machine on a daily basis. A fluid system operator is often required to perform preventive maintenance.

fluid systems

A power transmission system that uses the force of flowing liquids or gas under pressure to transmit power. Fluid systems include hydraulic systems and pneumatic systems.


A force that resists motion between two components that are in contact with each other. Friction generates heat and increases wear between components.


An enclosed system of assembled gears that transmits mechanical energy from a prime mover to an output device. A gearbox requires lubricants to effectively operate.


Any source of potential damage, harm, or human health risk in the workplace. Hazards can include noise pollution and biological agents.

heat exchangers

A hydraulic component that helps relieve excess heat in a hydraulic system. Heat exchangers may use water or air to cool hydraulic or pneumatic fluid.

heat transfer ratings

The rate at which thermal energy is exchanged between physical systems. The heat transfer rating of a heat exchanger describes its effectiveness in transferring thermal energy.

hydraulic filter

A device that removes contaminants from oil and other hydraulic fluids. Hydraulic filters contain absorbent or adsorbent filter media.

hydraulic fluid

A liquid, typically oil, that flows through the system to transfer power in a hydraulic system. Hydraulic fluid is generally very thick.

hydraulic pumps

A mechanical power source that generates fluid flow in order to move fluid through a hydraulic system. Hydraulic pumps receive power from a prime mover, such as a motor or engine.

hydraulic system

A type of fluid power system that uses oil or another liquid under pressure to transmit mechanical power. A hydraulic system can transmit large amounts of power within a small space.

inlet line

The port on the hydraulic pump that receives fluid from the reservoir. The inlet line typically contains a strainer to trap particulate matter before it enters the hydraulic pump.


A type of heat exchanger that cools air inside an air compressor in a pneumatic system. An intercooler also removes moisture from the compressed air.

internal leakage

The movement of fluid from one component to another within a hydraulic system. Internal leakage is often intentional and necessary to the regular operation of a hydraulic system.


The movement of fluid out of a fluid system or from one fluid system component to another. Leakage in a fluid system may decrease efficiency and lead to system failure.

limit switches

An electrical device on a control valve that is operated by the motion of a machine part or application of force. Limit switches often operate in sequence.


The act of applying a substance, such as grease or oil, to a part in order to reduce friction between two surfaces in relative motion. The lubricating properties of hydraulic fluid can be decreased by contaminants.


The necessary support and repair of machines. Maintenance includes tasks such as lubricating, cleaning, and replacing parts.

maintenance personnel

The group of people responsible for performing basic machine support and repair. Maintenance personnel are required to keep accurate records and may also train fluid system operators.

material contaminants

A type of contaminant that hinders the normal performance of a hydraulic or pneumatic fluid. Material contaminants include solids, liquids, or gases.

metallic particles

A hard, small flake of metal. Metallic particles are a type of material contaminant in a fluid system.


A metric unit of measurement equal to one thousandth of a millimeter (0.000039 inches). Microns are used to measure material contaminants.


Condensed water droplets. Moisture can contaminate a fluid system and form internal rust.


A rubber-like material that resists petroleum or oil compounds. Nitrile is used in many seals for fluid system components.

nonpositive seal

A seal that allows a small amount of fluid to pass between components. Nonpositive seals are used for controlled internal leakage and lubrication.

operating pressure

The pressure at which a system or component normally runs. Operating pressure is often used to specify the capabilities of specific fluid system components, such as control valves and conductors.


A material's chemical reaction with oxygen. Oxidation causes rust to form on metal surfaces and can lead to deterioration.

particulate matter

A solid contaminant that can alter the necessary clearance between components. Particulate matter includes particles of metal, dirt, and sand.

petroleum-based oil

A thick hydraulic fluid developed from petroleum. Petroleum-based oils provide good lubrication and are compatible with most seal materials.


A rod inside a cylinder that is moved by hydraulic or pneumatic pressure. Pistons in a pneumatic piston compressor may generate excess heat during operation.

pneumatic fluid

Compressed gas or air. Pneumatic fluid transfers the power in a pneumatic system.

pneumatic system

A fluid power system that uses the force of flowing gas to transmit mechanical power. A pneumatic system uses a compressor to generate fluid flow.

positive seal

A seal that does not allow any amount of fluid to pass through. A positive seal helps prevent external leakage in a fluid system.

power variables

Any component or characteristic of a fluid system that can be measured and quantified. Power variables include flow rate, operating pressure, and system efficiency.

pressure drop

The difference in pressure between any two points of a system or a component. Pressure drop is also referred to as pressure differential.

pressure line filters

A type of absorbent hydraulic filter that helps to sustain system operating pressures and prevent contamination. Pressure line filters are placed on conductors carrying active fluid after it leaves the pump.

pressure rating

The range of pressure a fluid system component can withstand during normal operation. Pressure ratings apply to various components within a fluid system.

pressure ratings

The range of pressure a fluid system component can withstand during normal operation. Pressure ratings apply to various components within a fluid system.

pressure-regulating valve

A control valve that ensures a consistent supply of lower-pressure air. Pressure-regulating valves may be used in filter-regulator-lubricator units.

preventive maintenance

Regularly scheduled service and upkeep performed while a machine is still in working order. Preventive maintenance can prolong equipment life and increase production.


The physical and mechanical characteristics of a material that distinguish it from other materials. Properties determine how a material will perform in different environments.

reactive maintenance

Service or repair performed only after a machine fails or experiences problems. A reactive maintenance approach typically increases downtime and reduces profits.

relief valves

An adjustable control valve that regulates pressure in a fluid system. The relief valve can shut off fluid flow when a desirable pressure is reached.


The main storage container for fluid in a hydraulic system. A reservoir may be pressurized or nonpressurized.


A raw polymer that has not been formed into its final molded shape. Resin is added to different absorbent filter elements to provide strength.

return line filters

A type of absorbent hydraulic filter that is used for hydraulic systems with small reservoirs and high-performance pumps. A return line filter is placed on conductors carrying fluid back to the reservoir.


A flexible device in a fluid system that controls or prevents fluid leakage between components and blocks contaminants from entering the system. A seal must be compatible with the hydraulic or pneumatic fluid that it comes into contact with.


A flexible device in a fluid system that prevents external fluid leakage between components and blocks contaminants from entering the system. A seal must be compatible with the hydraulic or pneumatic fluid it comes into contact with.

sight glass

A glass tube connected vertically to a hydraulic system reservoir for the purpose of indicating the fluid level. A sight glass shows a maximum and minimum level, indicating if sufficient fluid exists for machine operation.


A form of contamination that can affect a hydraulic system. Sludge is gummy liquid matter that forms in the reservoir.

static seals

A seal used between stationary and connected components in a fluid system. A static seal can withstand a wide range of pressure.


A screen used to trap large and coarse particulate matter in a hydraulic system. A strainer is an absorbent filter installed on the pump inlet line.

synthetic oil

A hydraulic fluid developed without petroleum. Synthetic oils are manufactured using chemicals and additives not found in nature.

synthetic rubber

An elastic material made from latex. Synthetic rubber seals are commonly used in fluid system fittings and components.


A systematic approach to solving problems quickly and efficiently. Troubleshooting often involves a logical process of elimination to identify the source of a problem.


The ability to be both elastic and viscous when undergoing deformation. Viscoelasticity in rubber seals allows them to stretch and then re-form.


A fluid's ability to flow at a given temperature. The viscosity of a fluid decreases as temperature increases.

viscosity index

VI. A measure of a fluid's change in viscosity within a given temperature range. Viscosity index helps determine the temperature range at which a hydraulic fluid can perform.

water vapor

An invisible, gaseous form of water. Water vapor can form during the process of evaporating or boiling water.

water-cooled heat exchanger

A component in a fluid system that relieves excess heat by using cold water-filled pipes or tubes. A water-cooled heat exchanger transfers the heat from hydraulic fluids to cold pipes.