Definition Of Hydraulic & Pneumatic Systems

Hydraulic and pneumatic devices are all around us. They're used in manufacturing, transportation, earthmoving equipment and common vehicles we see every day.

The brakes on your car are hydraulically operated; the garbage truck that passes weekly by your house uses hydraulic power to compact trash. Your mechanic uses a hydraulic lift when working on the underside of your car.

Pneumatic systems are equally widespread. Trucks and buses use air-actuated brakes. Spray painters use compressed air to spread paint. Ever been irritated in the morning by the sound of a jackhammer? That's a pneumatic machine hard at work using compressed air.

In 1647, French mathematician Blaise Pascal developed a principle of fluid mechanics known as Pascal's law. It states that when pressure is applied at any point in a confined fluid, the pressure will increase equally at every point in the container. As convoluted as this principle may sound, it's the basis for the operation of a hydraulic system.

Suppose you have a hollow cylinder that has a piston with an area of 2 square inches and it receives an input force of 100 pounds. This results in a pressure of 50 pounds per square inch (100 pounds/2 square inches).

This pressure gets passed by the hydraulic transmission system to another cylinder, known as an actuator, which has a piston with an area of 6 square inches. At 50 psi, this cylinder now has an output force of 300 pounds (50 psi X 6 square inches).

Pascal's Law gives hydraulic systems their advantage. A minimal input to a small device can result in a larger force output in a bigger actuator. It's a simple way of multiplying the output force sufficient to handle heavy workloads.

Since hydraulic systems can operate at pressures up to several thousand psi, the output force at the actuator can be huge. With this higher force output, the mechanical actuator now has the power to perform heavy lifting, pushing and moving tasks, such as earthmoving.

A hydraulic system uses a transmission network to carry a pressurized fluid that drives hydraulic actuators. The hydraulic fluid gets its pressure from a pump driven by a prime mover, such as an electric motor or a gas/diesel engine. The pressurized oil is filtered, measured and pushed out through the transmission system to an actuator to perform some action. Afterward, the fluid returns under low pressure to a reservoir where it's cleaned and filtered before returning to the pump.

Hydraulic systems are used in manufacturing and production plants, such as the steel and automobile industries, to operate all types of mechanical equipment. They're used to move, push and lift materials in industries like mining, earthmoving and construction.

Hydraulic oil – Hydraulic fluids are non-compressible and have low flash points.

A reservoir – The reservoir holds the fluid for the system. It has space for fluid expansion, lets air entrained in the liquid escape and helps the liquid to cool. Fluid flows from the reservoir to the pump, which forces it out through a piping network and, ultimately, back to the reservoir.

Filtering devices – Small metal particles and other foreign matter usually find their way into the fluid. The hydraulic system uses several filters and strainers to remove these foreign particles. Fluid contamination is one of the most common sources of problems in a hydraulic system.

A prime mover – Electric motors or gas-powered diesel engines are used to drive the fluid pump.

A pump – The pump draws the fluid from the reservoir and forces it through a pressure-regulating valve and out the transmission network to the actuators.

Connectors – A network consisting of pipes, tubing and flexible hoses transports the fluid to the mechanical actuators.

Valves – Various valves control the amount of fluid flow, its pressure and direction.

Actuators – Actuators are the devices that perform work motions. They can be rotary, such as a hydraulic motor, or linear, like a cylinder.

A hydraulic system has numerous advantages over pneumatic and other types of mechanical drive systems because it:

• Uses small components to transfer large forces with consistent power output.
• Has actuators that are capable of precise positioning.
• Is able to start up under heavy initial loads.
• Produces even and smooth movements under varying loads since the fluids aren't compressible and flow rates can be accurately controlled with valves.
• Delivers consistent power at moderate speeds compared to pneumatic systems.
• Is easy to control and regulate with pressure, directional and flow control valves.
• Dissipates heat easily and quickly.
• Performs well in hot environments.

• Pumps, valves, transmission networks and actuators are expensive.
• They can pollute the workplace with leaks, which may cause accidents or fires.
• They're not suitable for cycling at high speeds.
• Hydraulic fluids are sensitive to dirt contamination and must be tested regularly.
• Ruptures of high-pressure lines can cause injuries.
• Performance of hydraulic fluids is a function of changes in temperature, which can cause changes in viscosity.

The most common hydraulic fluids are based on mineral oils, polyalphaolefins and phosphate esters because of their low compressibility. Water isn't suitable because it can freeze in cold temperatures and boil in high-temperature environments. Water can also cause corrosion and rusting.

1. Transmit power and force through conductor lines to actuators to perform a work motion.
2. Lubricate the components, devices, valves and actuators in the circuit.
3. Act as a coolant by transferring heat away from any hot spots in the system.
4. Seal clearances between moving parts to increase efficiency and lessen the heat from excess leakages.

Some of the properties and characteristics of a hydraulic fluid are as follows:

Viscosity – Viscosity is the internal resistance of a fluid to flow. It increases as the temperature goes up. An acceptable hydraulic fluid must be able to provide a good seal at piston, valves and pumps but not be so thick that it impedes liquid flow.

Fluids with high viscosities can lead to power loss and higher operating temperatures. A fluid that's too thin can cause excessive wear of any moving parts.

Chemical stability – A hydraulic fluid must be chemically stable. It has to resist oxidation and be stable under severe operating conditions, such as high temperatures. Operating for long periods of time at high temperatures can shorten the useful life of the fluid.

Flash point – A flash point is the temperature when a fluid turns into a vapor in a sufficient volume to ignite or flash in contact with a flame. Hydraulic fluids need a high flash point to resist combustion and exhibit a low degree of evaporation at normal temperatures.

Fire point – Fire point is the temperature where a fluid vaporizes in a sufficient volume to ignite when exposed to a flame and continue to burn. As with the flash point, an acceptable hydraulic fluid must have a high fire point.

Pneumatic systems are like hydraulic systems, but they use compressed air instead of a fluid to transmit power. They rely on a constant source of compressed air to control energy and actuate motion devices.

Manufacturing plants use compressed air to drive pneumatic drills and presses and to lift objects and move materials. Fabrication shops use a pneumatic machine to hold unfinished products for welding, brazing and forming operations.

Air compressor - The air compressor draws air from the atmosphere, pressurizes it and stores the compressed air in a tank for release to the transmission system.

Prime driver – A prime driver, such as an electric motor or a gas-powered engine, provides the power to an air compressor.

Control devices – Valves regulate the pressure and control flow and direction.

Air tank – A tank holds compressed air for delivery to mechanical devices.

Actuators – These are devices that take the energy from compressed air and convert it into mechanical movements.

Transmission system – A network of pipes and tubing transports the compressed air to actuators.

Efficiency - The supply of air is free and unlimited. Compressed air is easy to store, transport and can be released to the environment without costly treatments.

Simple Design – The configuration and components of a pneumatic system have a simple design and are easy to maintain. They're more durable and aren't easily damaged.

The ability to operate at higher speeds – Pneumatic systems can operate actuators at more rapid cycles, such as in packaging production lines. Linear and oscillating movements are easy to adjust by using a pressure-regulating valve to control flow rate and pressure.

Cleanliness – No risk of leaking hydraulic fluids that pollute the environment. Pneumatic systems are preferred in workplaces that need high levels of cleanliness. Exhaust air devices clean up the air being released back into the atmosphere.

Less Costly – Pneumatic components are less expensive, and compressed air is widely available in manufacturing areas. Maintenance costs are lower compared to hydraulic systems.

Safer to operate – Pneumatic systems are safe to use in inflammable environments without dangers from fire or explosions. Pneumatic components don't overheat or catch on fire when overloaded.

Able to function in harsh environments – Dust, high temperatures and corrosive environments have less effect on pneumatic systems compared to hydraulics.

Reduced power – Pneumatic systems typically operate at less than 150 psi and provide less total force at actuators. Pneumatic cylinders are usually small and don't have the power to handle heavy loads.

Noisy – Air compressors generate more noise, and compressed air is noisy when it's released from the actuators.

Rough movement – Because air is compressible, the motion of pneumatic actuators can be rough, which reduces the accuracy of the movements of the system. Piston speeds are uneven. Hydraulics movements are smoother.

Need pre-treatment of air – Before use, air needs processing to remove water and dust particles. If this isn't done, the increased friction between the control devices and moving components will wear out the part and require premature repair or replacement.

Hydraulic actuators are more suitable for operations that need high force. They're rugged and can produce forces up to 25 times greater than a pneumatic actuator with the same size piston. Hydraulic systems can also operate up to 4,000 psi. Pneumatic actuators are usually less than 150 psi.

The compressibility of air and pressure losses reduce the efficiency of pneumatic systems. The compressor must run continuously to maintain pressure in the lines even when the actuators aren't moving; hydraulic systems can hold constant pressure without the pump running.