This Article takes an In-depth look at Hydraulics and Pneumatics
You will learn more about topics such as:
The Importance of Hydraulics and Pneumatics
Similarities and Differences Between Hydraulics and Pneumatics
Hydraulics
Pneumatics
Deciding Between a Hydraulic or Pneumatic System
The Importance of Hydraulics and Pneumatics
When examining hydraulics and pneumatics, it is important to understand the mechanical differences between them. Both are essential parts of various industries and are critical to the performance of several types of tasks. Which of them is chosen for a particular task is dependent on what is to be accomplished and the setting. In most cases, they are applicable in multiple environments and conditions but can be restricted by climatic conditions as well as the type of terrain.
The difference between hydraulics and pneumatics is in the substances used to operate them. Hydraulics have a pressurized fluid to initiate a mechanical process. Force that is applied at one point is sent to another point using incompressible fluid, which is necessary since it does not change volume when pressure is applied. In the majority of cases, the fluid is oil. The applied force is multiplied as the liquid travels through the system to the point that it can create nine times more force than the initial applied force. Most hydraulic systems involve a network of pistons, where a simple one may have two pistons. The greater the number of pistons or mechanisms the more complex the system, and the greater the amount of force created.
The one similarity between pneumatic and hydraulic processes is the use of pressure. In the case of a pneumatic system, a gas is used. Equipment using a pneumatic system has a set of connected parts. Compressed air is injected using a compressor, which is sent through a series of hoses. The flow of air is regulated by valves. The gas, much like a hydraulic system, under pressure, is transformed by a pneumatic cylinder into mechanical energy. Some forms of pneumatics inject inert gases, ones that do not undergo a chemical change, into the process for specialized tasks and operations.
There are benefits to using a hydraulic system. An important one is the amount of work or force it can create. The four listed below are some of the primary benefits.
Easy to control - Using a simple set of levers and buttons, an operator can start, stop, speed up or slow down a procedure without making major changes.
Force - Force is created without the use of gears, pulleys, or levers and is multiplied from a few pounds into hundreds of pounds.
Constant and consistent – The fluid power of a hydraulic system provides constant torque or force regardless of speed changes.
Economical – Since hydraulic systems have fewer moving parts, they are easy to maintain and simple to use.
Safe – Since hydraulic systems are enclosed and have limited moving parts, they are safer than mechanical or electrical systems.
There are several applications for hydraulics that include hydraulic lifts, brakes, and many parts of aircraft. Since their invention in 1882, the uses of hydraulic systems have rapidly advanced over the years to be a substantial part of many industries.
As with hydraulic systems, there are many advantages to pneumatic ones that include:
Cost – Pneumatic systems cost half as much as hydraulic ones.
Efficient - Air in pneumatic devices is free of moisture, which eliminates any complications with internal parts.
Water treatment – When used to open and close underwater valves, pneumatic systems are ideal since they can sustain overload pressure conditions.
Low maintenance - Pneumatic actuators have a long life and perform well with limited maintenance.
Distance – When power needs to be relayed over long distances, pneumatic systems can do it easily and economically.
Uncomplicated devices - Pneumatic devices are air based with less complicated designs and made from inexpensive materials.
The first use of pneumatic systems was as a message system where notes were placed in a container and moved by air through a system of tubes to another location. Today, there are hundreds of uses of pneumatics from brakes and dental drills to vacuum pumps and sewer systems. The use and application of these devices is limitless mainly because of the low cost of manufacturing them.
Similarities and Differences Between Hydraulics and Pneumatics
It is possible to get confused when examining hydraulics and pneumatics since they both use pressure and another medium to perform their operations. Also, both are economical and safe as well as being self-contained with few working parts. One of the interesting aspects of these processes is how adamantly their supporters advocate for them.
Hydraulics and pneumatics work as an actuator using a pump, which are controlled by valves used to convert pressure into mechanical motion. The amount of force created by the process is greater than that which is initially applied. Though both processes use pressure, the medium to create the pressure is different where hydraulics use oil or water and pneumatics use a gas, which is mainly air.
The type of medium used in a hydraulic system is incompressible. The volume of it remains the same regardless of the amount of pressure applied. Hydraulic systems tend to be large and complex. They need room to store their reservoir of fluid, and the components of a system are large and bulky to create the necessary pressure, which makes them more expensive than pneumatic systems. They have the advantage of being able to take on larger loads than pneumatic systems due to their bulk. Since hydraulic systems use oil, which is very viscous, they require time to begin operation and operate slowly. Also, the mechanism to move the oil through the pipes requires more energy, which further slows its operation.
Pneumatic systems are powered by air or inert gases. Since air is the main medium in a pneumatic system, they operate cleaner than hydraulic systems and can be proposed for tasks where food is present or in antiseptic environments. This makes them available to the pharmaceutical and biotech industries. Most pneumatic systems are very simple and require only an on and off switch to activate them, a feature that makes them less expensive than hydraulic systems. They are easy to store and take up less room since they do not have the large reservoir of a hydraulic system. Another benefit of a pneumatic system is the capability of expelling the gas from their operation into the environment. Pneumatic systems operate faster with smaller cylinders since air flows quickly with little resistance or viscosity.
Each system, hydraulic or pneumatic, has specific uses. In most cases, there isn’t any crossover between the two systems due to design, size, and type of processes. Below is a basic summary of the differences between the two systems.
Summary
Medium
Hydraulics – some form of liquid, usually oil
Pneumatics – inert gas or air
Speed
Hydraulics – very slow to operate due to their complex and complicated mechanism
Pneumatics – operate much faster since air or gas is their medium
Cost
Hydraulics – Maintenance and installation is expensive and time consuming
Pneumatics – Easy to install and maintain with a very simple operating system
Pressure
Hydraulics – 3000 to 5000 psi
Pneumatics – 100 psi up to 150 psi
Hydraulics
Uses of Hydraulics
The scientific definition of hydraulics is the mechanical study of fluids, which have the ability to perform complicated work. When you apply pressure to a liquid in a confined space, the pressure is applied equally to all parts of the space. A hydraulic system is capable of multiplying the force created by the pressure. This simple principle enables people to lift thousands of pounds by using a very small amount of force.
The most common and every day use of hydraulics is in a braking system that works through the use of fluids. The fluid in the system is activated by applying pressure to the brake pedal that sends a message to the hydraulic fluid that engages the drum shoes or disc calipers. The mechanical force from the brake pedal is converted into hydraulic pressure in the master cylinder.
These same principles are used in several types of devices such as airplanes where they slow the craft when it lands or trash compactors that are seen on trash trucks. The space program uses hydraulics since they are able to work in a zero G environment. Since their discovery, hydraulics have become a vital part of many industries and their uses have greatly multiplied.
Types of Hydraulics
Part of what defines a hydraulic is the type of fluid it uses, which is determined by its viscosity. In most hydraulic systems, the fluid has a low viscosity that ensures an easy flow to perform the needed work.
Three of the types of hydraulic systems are gear pumps, screw pumps, and fixed placement vane pumps. Each has a rotary type mechanism designed for low tolerances between moving parts. The majority of hydraulic devices have a low rotational speed.
Gear pumps are a common type of hydraulic pump that have a pair of meshed gears inside a casing. The gears in the pump apply the pressure to the fluid. The gears transfer the fluid from one part of the pump to another to apply force. They are reliable since they provide a fixed flow rate.
Rotary vane pumps have adjustable vanes in an enclosed housing. As a rotor moves around the housing, the vanes adjust the tips of the vanes touching the inner surface of the housing. The fluid enters the housing and is discharged as the vane and rotor turn. Rotary vane pumps operate best with a low viscosity fluid. It is the opinion of some engineers that rotary hydraulic pumps are more efficient than gear hydraulic ones.
Screw hydraulic pumps have meshing spiral gears in a cylinder. It operates on the same principles as the Archimedes' screw where oil is forced along the cylinder between the teeth of the gears.
The gear, rotary vane, and screw hydraulic pumps are only three of the many varieties of this useful device. Other types are radial piston, axial piston, and gerotor hydraulic pumps. Each type is designed to perform a specific function.
What Are Hydraulic Pumps Made Of?
Manufacturers of hydraulic pumps select the basic materials for construction based on the type of pump they produce. An important part of the process is to select a metal with sufficient thickness to be able handle the pressure that is produced. In the majority of cases, it is some form of steel or iron where the gauge is a significant factor.
The mechanism and fluid for a hydraulic pump is enclosed in a tube or housing, which has to be completely sealed. This is confirmed through testing before installation of the components. Basic hydraulic pumps are enclosed in cylinders, which is found in screw and rotary vane hydraulic pumps. Gear type hydraulic pumps consist of a housing. The size of this type depends on the work to be completed and the size of the gears.
Pneumatics
Uses of Pneumatics
Pneumatic systems use compressed air to move solid objects through a system of tubes. They initially came into use at the end of the 19th Century and became very popular during the 20th Century. In the middle of the 20th Century they were a method of making purchases at retail outlets where the customer’s bill and money were sent to a central location. This form of use can still be seen at drive up banks.
Since the 1950’s, as technology has advanced, pneumatic messaging systems have lost their appeal. They still have some scientific applications and are used for unique specific types of testing. In the 21st Century, experimentation is being conducted in the possible use of a pneumatic system of trains that uses magnetic levitation in an enclosed tube.
Pneumatic systems use compressed air to create controlled energy and are used in a variety of industries. These systems rely on compressed air provided by an air compressor, which sucks in air from the environment and stores it under high pressure in a receiver. The air is sent to the system through pipes and valves normally at a pressure of 6 kg/sq. mm to 8 kg/sq. mm where maximum force can be 50 kN. The work of the compressed air is performed by a piston or vane.
In recent years, a variety of uses have been discovered for pneumatics. In this era of environmental concern, pneumatics have become a popular replacement for hydraulics since the waste product from its operation is air.
Air brakes are a form of pneumatic using compressed air for a form of friction brake, which begins by the activation of a piston that applies pressure on brake pads.
Exercise machines use the resistant factor of a pneumatic cylinder that can be adjusted to fit the user.
Pneumatic motors where compressed air is converted to mechanical energy by a linear motion.
Pressure regulators designed to stop the flow of a liquid or gas at a specific pressure.
Pipe organs use pressurized air through the pipes to produce musical notes.
Cable-jetting used to put cables into ducts through the use of compressed air.
Pneumatic mail systems deliver mail through pressurized air tubes, which was one of the original uses of a pneumatic system from the 19th Century.
Rodless Pneumatic Cylinders
Rodless pneumatic cylinders are unlike traditional pneumatic cylinders in that they move a load alongside the piston. They have the same stroke length but use less space than a pneumatic cylinder with a rod and have the same force in both directions of their stroke. The stroke of a rodless pneumatic cylinder is completely in the body of the cylinder, which gives it a smaller footprint.
As with other pneumatic processes, sizing is a critical factor with rodless pneumatic cylinders since too large or too small can be detrimental and expensive. To determine the correct rodless cylinder for an application, it is essential to determine the size of the load, working stroke length, cycle rate,orientation of the actuator to the carrier, and velocity of the mass. With these factors in mind, the actuators thrust capacity can be determined based on air pressure.
The three rodless pneumatic cylinder types are magnetic, cable, and slotted, which refer to how the piston is connected to the carrier with the most secure connection being slotted. On each end of a rodless pneumatic cylinder are cushions to prevent hard stops.
Magnetic: The piston and the carrier are connected by magnet, which creates a leak free cylinder.
Cable: A cable connects to the piston. A pulley at each end of the cylinder is connected to the carrier and moves it along the barrel. It is a very simple and uncomplicated structure.
Slotted: A slot is located in the cylinder’s length that has an inner and outer metal band that makes a direct mechanical connection to the piston. The slotted version of rodless pneumatic cylinders is the most common type.
Rodless pneumatic cylinders do not cause deflection due to the position of the rod, which is unlike traditional pneumatic cylinders with a rod that have deflection due to the lateral load. This aspect of rodless pneumatic cylinders makes it possible to produce cylinders with a very long stroke.
Additionally, rodless pneumatic cylinders are environmentally friendly since they are sealed to prevent leakage of lubricant or moisture from the compressed air. With traditional pneumatic cylinders, moisture and lubricant are expelled when the rod is extended.
Types of Pneumatic Systems
Pneumatic systems create force through the use of contained air systems where energy is stored under pressure and released by valves. The air continues to expand until it gets to atmospheric pressure. The implementation of a pneumatic system is best in conditions that do not need a lot of power in a limited space. Four types of pneumatic systems are dilute phase, dense phase, vacuum based, and pressure based.
Dilute-phase pneumatics is used to move powders or particles at a high gas velocity. Perfect calibration is not required unlike a dense-phase system. It is ideal for moving dry bulk materials as well as being cost effective. Feeds or grain are dumped into a high velocity gas stream using a rotary airlock valve where it is carried through pipes. This method is widely used as a conveying system for grain and cereal production.
Dense-phase pneumatics has a line pressure calibrated to match the characteristics of the material being moved allowing solid material to transform into a liquid state when being moved at a slower velocity. It can be used to transport abrasive materials without damaging system components. To assist the movement of materials and avoid clogs, busters are placed along the pipe to keep the material moving.
Vacuum based pneumatic systems pull materials to them. This form of pneumatic system is used when materials are being sent to one central location and can lift items from open containers. Since vacuum systems seldom have leak problems, they can be used for the movement of hazardous materials.
Pressure based pneumatic systems can send objects to multiple locations by using diverter valves, which open and close to control the flow of air in the system. This type of system can be controlled by the operator who has the ability to raise and lower the pressure. Pressurized systems can carry heavier materials over long distances. Pressure in the system is controlled by a rotary airlock valve.
The Parts of a Pneumatic System
Cylinders, rotary actuators, and air motors create the force and energy of a pneumatic system for the movement and processing of materials. Other components required to control and operate the system include air service units for the air compressor and valves to monitor air pressure as well as the direction and movement of the actuators. The system can be broken into two components – air pressure production and a consuming system.
Essential parts of the system include a compressed air transportation and distribution system consisting of an air compressor, electric motor, pressure switch, check valve, storage tank, pressure gauge, auto drain, air dryer, filters, air lubricator, pipelines, and different types of valves. The air consuming system has an intake filter, compressor, air take off valve, auto drain, air service unit, directional valve, actuators, and speed controllers.
When the system is activated, an intake filter removes contaminants from the surrounding air as the compressor draws in the air. The compressor converts mechanical energy into the potential energy of compressed air. Since there are several types of compressors, the type chosen depends on the needed delivery pressure and air flow. The air that is compressed is stored in a receiver tank to smooth the pulsating flow from the compressor and cool and condense any moisture. The receiver has the added function of removing the heat created by the compressed air.
A very important part of the operation of a pneumatic system is to keep it maintained with regular lubrication of the cylinders and valves. Most systems put a fine mist of oil into the compressed air, which helps to lubricate the moving parts. Oil viscosity is between 20 to 50 centistokes.
Deciding Between a Hydraulic or Pneumatic System
Hydraulic and pneumatic systems are a part of engineering that deals with fluid power or fluid mechanics, a branch of science that deals with how fluids react. Hydraulics create, control, and produce power through the use of pressurized liquids or fluids, which are a form of matter that include liquids, gases, and plasmas.
When making the decision to use a hydraulic or pneumatic system, it is important to examine the advantages of each form since each type of system is engineered to perform specific functions.
Advantages of Hydraulics
Safe and easy to maintain with fewer moving parts
Responsive and supplies more power
Liquid does not absorb supplied energy
Easily controlled
Advantages of Pneumatics
Uses air
The compressibility of air.
Simple in construction and easy to handle.
Ability to control pressure and force
Low Maintenance
Explosion-proof
Low cost
Fire-proof
Manufacturing of a Hydraulic System
The theory behind a hydraulic system was established by the use of Pascal’s law, the fluid in a system is equal to its area. Most systems use some form of piston and cylinder apparatus though there are gear forms as well. The convenience of a cylinder type system is where a small amount of torque can produce great force.
By adjusting the volume of the fluid in the system, the amount of force that can be controlled through the efficiency of the adjustments is very limited. In cases where motors are used, there is greater control over the system. Presently, motor driven systems are more popular than rotating ones since motor systems offer the operator greater control.
Most cooling systems are manufactured using rotating hydraulics. Hydraulic systems tend to take up less room than motor driven ones making them easier to use.
Manufacturing of a Pneumatic System
Pneumatic systems rely on some form of gas to produce force. In most cases, pressurized air is used to drive the system with a spray of lubricant. Most pneumatic systems operate at 100 psi with few systems able to produce 3000 to 5000 psi. Pneumatics are designed to handle small loads.
At the heart of every pneumatic system is an air compressor designed to reduce the volume of air, which increases the pressure. Hoses, controlled by valves, are used to send air to an actuator. A central part of the system is the filter that keeps contaminants out of the system.
Since a pneumatic system depends on an air compressor, it is important to select one that best fits the use of the pneumatic system. Three types of air compressors are reciprocating, rotary screw, and rotary centrifugal. The selection of the air compressor is the first step in the creation of a pneumatic system. In many cases, a form of piston compressor is the most popular and widely used.
When to Choose a Pneumatic System
There are several considerations that have to be taken into account when choosing to use a pneumatic system as a source of power. Hydraulic systems have higher power density with a higher cost while pneumatic systems have higher speed and lower cost. Although these factors can influence a decision to use pneumatics or hydraulics, there are several other factors to consider.
Load - The amount of force to move a load is a determining factor when differentiating between pneumatics and hydraulics. If a load is only a few pounds, pneumatics are a more practical choice since the force of hydraulics is not necessary.
Cost - Pneumatics are capable of operating from a single centralized compressor that can run a whole facility. This configuration drastically reduces costs of repair and maintenance since hydraulic systems have machine by machine pumps and motors.
Durability - Pneumatic systems are highly durable and seldom need repair. Even though they may fail gradually or leak air, this does not deter from their ability to function. In the case of leaks, and unlike hydraulic systems, pneumatic systems are more environmentally friendly since they only leak air and not oil. Any leaks in the system do not require immediate action and do not present a hazard but must be repaired in order to maintain production.
Speed - The wide use of pneumatic systems is found in factory automation, packaging, and applications where speed is essential and loads are low. The speed that pneumatic systems produce enhances production and the efficiency of an operation.
When choosing a pneumatic system, right sizing is essential to ensure efficiency. The determination of the right size in essence means that a pneumatic system must be chosen to perfectly match the needs of an application since undersized or oversized systems can have a negative effect and be less efficient. In the discussion of right sizing, the most minute aspects has to be considered down to the size of the tubing and hoses.
Once the properly sized pneumatic system is installed, it is important to make the proper adjustments to the system to reduce costs. Normally, moving a load does not require the same pressure in both directions. By reducing and adjusting the pressure for the different directions, it is possible to reduce the costs of operating the system. Minor adjustments and modifications greatly enhance the efficiency of a pneumatic system as well as lower the costs of operation.
The Cost of Pneumatic and Hydraulic Systems
The main difference between hydraulic and pneumatic systems is the cost of electricity to operate them. For many years, pneumatic systems seemed to be more cost effective, and traditionally have had an electrical efficiency of 10% compared to other mechanical or motor driven systems. The one drawback is the need for constant maintenance that has a serious influence on efficiency and productivity. An interesting factor in determining the cost is the number of air compressors used to drive the system where the use of one compressor to drive 50 to 100 systems is more cost effective than any form of motor driven system.
The initial cost of a hydraulic system is normally rather high. For a permanent system, it usually includes some form of installation cost. The main factor in purchasing a hydraulic system is the lifespan of the equipment, which can be increased with preventative maintenance and proper upkeep. As with a pneumatic system, hydraulic system costs can be controlled by the use of a central power supply that can operate several pieces of equipment. An increase in the number of machines relying on one power source can significantly lower cost, which also reduces noise, the use of less horsepower, and increased machine performance.
To determine the basic difference between hydraulic and pneumatic systems is the amount of work or force developed. Hydraulic systems have the potential to handle heavier loads and produce more work than pneumatic systems while pneumatic systems are perfect for the movement of smaller items. As with any form of machinery, the final decision depends on the purpose of the equipment to be purchased where cost really isn’t the factor and efficiency is.
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