Hydraulic System and its Components
Hydraulic System and its Components
A hydraulic system is one of the drive systems which are being used for the control of machinery and equipment. The other two commonly employed drive systems are based on pneumatics and electric power. The word ‘hydraulics’ came from the Greek word ‘hydraulikos’ which means water organ which in turn means water and pipe. It was in early 1900s when the first practical application of hydraulics was done. The hydraulic systems originated from ‘water hydraulics’ which was being practiced since a hundred year before the fluid power systems emerged.
Hydraulics is a branch of science and engineering concerned with the use of fluids to perform mechanical tasks. It is part of the more general discipline of fluid power. Typically, the fluid used in a hydraulic system is an incompressible liquid such as mineral based hydraulic oil. Pressure is applied by a piston to fluid in a cylinder, causing the fluid to press on another piston which delivers energy to a load. If the areas of the two pistons are different, then the force applied to the first piston is different from the force exerted by the second piston. This creates a mechanical advantage.
A controlled application of force is a common requirement for a production process. These operations in a production process are performed mainly by using a prime mover. The prime mover can provide various movements to the objects by using some mechanical attachment. The enclosed fluids (liquids and gases) can also be used as prime movers to provide controlled motion and force to the objects or substances. The specially designed enclosed fluid systems can provide both linear as well as rotary motion. The high magnitude controlled force can also be applied by using these systems. This kind of enclosed fluid based systems using pressurized incompressible liquids as transmission media are called as hydraulic systems. The hydraulic system works on the principle of Pascal’s law which says that the pressure in an enclosed fluid is uniform in all the directions. The force given by fluid is given by the multiplication of pressure and area of cross section (Fig 1). As the pressure is same in all the direction, the smaller piston feels a smaller force and a large piston feels a large force. Hence, a large force can be generated with smaller force input by using hydraulic systems.
Fig 1 Principle of hydraulic system
Hydraulic oil also known as hydraulic fluid is the medium by which power is transferred in the equipments of the hydraulic system. It has the main purpose of transferring potential or kinetic energy (pressure and movements), create volume flow between pump and the motor, and reduce the wear of parts which rub against each other. In addition, it protects the system from corrosion and helps carry away the heat produced during energy the transformation. Important properties of the hydraulic oil include (i) viscosity, (ii) viscosity index, (iii) shear stability, (iv) pour point, (v) sealing compatibility, (vi) density, (vii) foaming characteristic, (viii) bulk modulus / compressibility, (ix) cleanliness, and (x) water content. Free air in the hydraulic oil is considered as contamination. Air typically enters the circuit through the suction line if the seals and fittings are not tight. This free air then can be dissolved in the hydraulic oil. Air release is a measure and there is a time needed to release air bubbles (free air) contained in the oil to the surfaces.
The schematics of a hydraulic system along with a simple hydraulic system are shown in Fig 2. As shown in the schematics of a hydraulic system, the output shaft transfers the motion or force while all other parts help to control the system. The storage / fluid tank is a reservoir for the hydraulic fluid which is used as a transmission media. The fluid used is generally high density incompressible oil. It is filtered to remove dust or any other unwanted particles and then pumped by the hydraulic pump. The capacity of pump depends on the hydraulic system design. The pump normally delivers constant volume in each revolution of the pump shaft. Hence, the fluid pressure can increase indefinitely at the dead end of the piston until the system fails. The pressure regulator is used to avoid such situations which redirect the excess fluid back to the storage tank. The movement of piston is controlled by changing liquid flow from port A and port B. The cylinder movement is controlled by using control valve which directs the fluid flow. The fluid pressure line is connected to the port B to raise the piston and it is connected to port A to lower down the piston. The valve can also stop the fluid flow in any of the port. The leak proof piping is important from the aspects of safety, environmental hazards, and economy. Some accessories such as flow control system, travel limit control, electric motor starter, and overload protection etc. are also used in the hydraulic systems.
Fig 2 Schematics of a hydraulic system along with a simple hydraulic system
Types of hydraulic system
There are two types of hydraulic systems namely (i) open centre system, and (ii) closed centre system. An open centre system is one which has fluid flow, but no pressure in the system when the actuating mechanisms are idle. The pump circulates the fluid from the reservoir, through the selector valves, and back to the reservoir. The open centre system can employ any number of subsystems, with a selector valve for each subsystem. The selector valves of the open centre system are always connected in series with each other. In this arrangement, the system pressure line goes through each selector valve. Fluid is always allowed free passage through each selector valve and back to the reservoir until one of the selector valves is positioned to operate a mechanism. When one of the selector valves is positioned to operate an actuating device, fluid is directed from the pump through one of the working lines to the actuator. With the selector valve in this position, the flow of fluid through the valve to the reservoir is blocked. The pressure builds up in the system to overcome the resistance and moves the piston of the actuating cylinder and the fluid from the opposite end of the actuator returns to the selector valve and flows back to the reservoir. Operation of the system following actuation of the component depends on the type of selector valve being used.
In the closed centre system, the fluid is under pressure whenever the power pump is operating. There are a number of actuators arranged in parallel out of which some of the actuating units are operating at the same time, while some other actuating units are not operating. This system differs from the open centre system in that the selector or directional control valves are arranged in parallel and not in series. The means of controlling pump pressure varies in the closed centre system. If a constant delivery pump is used, the system pressure is regulated by a pressure regulator. A relief valve acts as a backup safety device in case the regulator fails. If a variable displacement pump is used, system pressure is controlled by the integral pressure mechanism compensator of the pump. The compensator automatically varies the volume output. When pressure approaches normal system pressure, the compensator begins to reduce the flow output of the pump. The pump is fully compensated (near zero flow) when normal system pressure is attained. When the pump is in this fully compensated condition, its internal bypass mechanism provides fluid circulation through the pump for cooling and lubrication. A relief valve is installed in the system as a safety backup.
An advantage of the open centre system over the closed-centre system is that the continuous pressurization of the system is eliminated. Since the pressure is built up gradually after the selector valve is moved to an operating position, there is very little shock from pressure surges. This action provides a smoother operation of the actuating mechanisms. The operation is slower than the closed centre system, in which the pressure is available the moment the selector valve is positioned.
Components of a hydraulic system
A hydraulic system consists of a number of parts for its proper functioning. Regardless of its function and design, a hydraulic system has a minimum number of basic components. The main components of a hydraulic system are (i) hydraulic pump, (ii) reservoir for hydraulic fluid, (iii) filter, (iv) actuator, (v) accumulator, (vi) directional control valve, (vii) flow control valve, (viii) pressure relief valve, and (ix) pipes and fittings.
Hydraulic pump – Hydraulic pump converts mechanical energy from a prime mover (electric motor) into hydraulic (pressure) energy. The pressure energy is used then to operate an actuator. Pump pushes on a hydraulic fluid and create flow. The combined pumping and driving motor unit is known as hydraulic pump. The hydraulic pump takes hydraulic fluid from the reservoir and delivers it to the rest of the hydraulic circuit. In general, the speed of the pump is constant and the pump delivers an equal volume of fluid in each revolution. The amount and direction of fluid flow is controlled by some external mechanisms. In some cases, the hydraulic pump itself is operated by a servo controlled motor but it makes the system complex. The hydraulic pump is characterized by its flow rate capacity, power consumption, drive speed, pressure delivered at the outlet, and efficiency of the pump. The pump is normally not 100 % efficient. The efficiency of a pump can be specified by two ways. One is the volumetric efficiency which is the ratio of actual volume of fluid delivered to the maximum theoretical volume possible. Second is power efficiency which is the ratio of output hydraulic power to the input mechanical / electrical power. The typical efficiency of a hydraulic pump varies from 90 % to 98 %. The hydraulic pumps are generally of two types, namely (i) centrifugal pump, and (ii) reciprocating pump.
Centrifugal pump uses rotational kinetic energy to deliver the fluid. The rotational energy generally comes from an electric motor. The fluid enters the pump impeller along or near to the rotating axis, accelerates in the propeller and flung out to the periphery by centrifugal force. In centrifugal pump the delivery is not constant and varies according to the outlet pressure. These pumps are not suitable for high pressure applications and are generally used for low-pressure and high-volume flow applications. Most of the centrifugal pumps are not self-priming and the pump casing needs to be filled with liquid before the pump is started.
The reciprocating pump is a positive plunger pump. It is also known as positive displacement pump or piston pump. It is frequently used where relatively small quantity is to be handled and the delivery pressure is quite large. The construction of these pumps is similar to the four stroke engine. The crank is driven by some external rotating motor. The piston of the pump reciprocates due to crank rotation. The piston moves down in one half of crank rotation, the inlet valve opens and fluid enters into the cylinder. In second half crank rotation the piston moves up, the outlet valve opens and the fluid moves out from the outlet. At a time, only one valve is opened and another is closed so there is no fluid leakage. Depending on the area of cylinder the pump delivers constant volume of fluid in each cycle independent to the pressure at the output port.
Reservoir for hydraulic fluid – The reservoir for hydraulic fuel is a tank for holding the fluid required to supply the system, including a reserve to cover any losses from minor leakage and evaporation. The reservoir is generally designed to provide space for fluid expansion, permit air entrained in the fluid to escape, and to help cool the fluid. The reservoir tank is either vented to the atmosphere or closed to the atmosphere and pressurized. Hydraulic oil flows from the reservoir tank to the pump, where it is forced through the system and eventually returned to the reservoir tank. The reservoir tank not only supplies the operating needs of the system, but it also replenishes fluid lost through leakage. Furthermore, the reservoir serves as an overflow basin for excess fluid forced out of the system by thermal expansion (the increase of fluid volume caused by temperature changes), the accumulators, and by piston and rod displacement. The reservoir also furnishes a place for the fluid to purge itself of air bubbles which can enter the system. Foreign matter picked up in the system can also be separated from the hydraulic fluid in the reservoir or as it flows through line filters. Reservoir tank is either pressurized or non-pressurized. Baffles and/or fins are incorporated in most of the reservoir tanks to keep the hydraulic oil within the reservoir from having random movement, such as vortexing (swirling) and surging. These conditions can cause the oil to foam and air to enter the pump along with the oil. For the purpose of the hydraulic components performing correctly, the hydraulic oil is to be kept as clean as possible. Contamination of the oil is one of the common causes of hydraulic system troubles.
Filter – Foreign matter and tiny metal particles from normal wear of valves, pumps, and other components usually enter the hydraulic system. Strainers, filters, and magnetic plugs are used to remove foreign particles from hydraulic oil and are effective as safeguards against contamination. Magnetic plugs, located in a reservoir tank, are used to remove the iron or steel particles from the hydraulic oil. Strainer is the primary filtering system which removes large particles of foreign matter from the hydraulic oil. Even though its screening action is not as good as a filter’s, a strainer offers less resistance to flow. Strainers are used to pump inlet lines where pressure drop is to be kept to a minimum. Filter removes small foreign particles from hydraulic oil and is most effective as a safeguard against contaminants. Filters are generally located in a reservoir tank, a pressure line, a return line, or in any other location wherever necessary. They are classified as full flow or proportional flow. A bypass relief valve in a body allows a liquid to bypass the filter element and pass directly through an outlet port when the element becomes clogged. Filters which do not have a bypass relief valve have a contamination indicator. This indicator works on the principle of the difference in pressure of the hydraulic oil as it enters a filter and after it leaves an element.
Actuator – Hydraulic actuator receives pressure energy and converts it to mechanical force and motion. An actuator can be linear or rotary. A linear actuator gives force and motion outputs in a straight line. It is more commonly called a cylinder but is also referred to as a ram, reciprocating motor, or linear motor. A rotary actuator produces torque and rotating motion. It is more commonly called a hydraulic motor or motor. Valves are used in hydraulic systems to control the operation of the actuators.
Accumulator – Accumulators are like an electrical storage battery. A hydraulic accumulator stores potential power, in this case hydraulic fluid under pressure for future conversion into useful work. This work can include operating cylinders and fluid motors, maintaining the required system pressure in case of pump or power failure, and compensating for pressure loss due to leakage. Accumulators can be employed as fluid dispensers and fluid barriers and can provide a shock absorbing (cushioning) action. Accumulators can be spring loaded, bag type or piston type.
Directional control valve – Directional control valve is used to control the distribution of energy in a fluid power system. It provides the direction to the fluid and allows the flow in a particular direction. It is used to control the start, stop and change in direction of the fluid flow. It regulates the flow direction in the hydraulic circuit. The directional control valve contains ports which are external openings for the fluid to enter and leave. The term ‘way’ is being used for identifying the number of ports. As an example, a valve with four ports is known as a four-way valve. Directional control valves can be classified in four ways namely (i) type of construction (poppet valves, and spool), (ii) number of ports (two way valves, three way valves, and four way valves), (iii) number of switching position (two position, and three position), and (iv) actuating mechanism (manual actuation. mechanical actuation, solenoid actuation, hydraulic actuation, pneumatic actuation, and indirect actuation).
Flow control valve – The fluid flow rate is responsible for the speed of actuator (motion of the output) and is to be controlled in a hydraulic system. This operation can be performed by using a flow control valve. In practice, the speed of actuator is very important in terms of the desired output and it is required to be controlled. The speed of the actuator can be controlled by regulating the fluid flow. A flow control valve can regulate the flow or pressure of the fluid. The fluid flow is controlled by varying area of the valve opening through which the fluid passes. The fluid flow can be decreased by reducing the area of the valve opening and it can be increased by increasing the area of the valve opening. A very common example to the fluid flow control valve is the household tap. The pressure adjustment screw varies the fluid flow area in the pipe to control the discharge rate.
In general, the hydraulic systems have a pressure compensating pump. The inlet pressure remains almost constant but the outlet pressure keeps on fluctuating depending on the external load. It creates fluctuating pressure drop. Thus, the ordinary flow control valve is not able to maintain a constant fluid flow. A pressure compensated flow control valve maintains the constant flow throughout the movement of a spool, which shifts its position depending on the pressure. Flow control valves can also be affected by temperature changes. It is because the viscosity of the fluid changes with temperature. Hence, the advanced flow control valves often have the temperature compensation. The temperature compensation is achieved by the thermal expansion of a rod, which compensates for the increased coefficient of discharge due to the decreasing viscosity with temperature.
The flow control valves work on applying a variable restriction in the flow path. Based on the construction, there are primarily four types of flow control valves namely (i) plug valve, (ii) butterfly valve, (iii) ball valve, and (iv) balanced valve.
Pressure relief valve – The pressure can increase gradually when the system is under operation. The pressure control valve protects the system by maintaining the system pressure within the desired range. Also, the output force is directly proportional to the pressure and hence, the pressure relief valve ensures the desired force output at the actuator.
The pressure relief valve is used to protect the hydraulic components from excessive pressure. This is one of the most important components of a hydraulic system and is essentially required for safe operation of the system. Its primary function is to limit the system pressure within a specified range. It is normally a closed type and it opens when the pressure exceeds a specified maximum value by diverting pump flow back to the tank. The simplest type valve contains a poppet held in a seat against the spring force. The fluid enters from the opposite side of the poppet. When the system pressure exceeds the pre-set value, the poppet lifts and the fluid is escaped through the orifice to the storage tank directly. It reduces the system pressure and as the pressure reduces to the set limit again the valve closes. This valve does not provide a flat cut-off pressure limit with flow rate since the spring is to be deflected more when the flow rate is higher. There are different types of pressure control valves which are used. These are (i) direct type of relief valve, (ii) unloading valve, (iii) sequence valve, (iv) counter-balance valve, and (v) pressure reducing valve.
Pipes and fittings – Three common types of pipe lines which are being used in the hydraulic systems are pipes, tubing, and flexible hoses. These are sometimes being referred to as rigid, semi-rigid, and flexible lines respectively. The two types of tubing used for hydraulic lines are seamless tubes and electric welded tubes. Both are suitable for hydraulic systems. Knowing the flow, type of fluid, fluid velocity and system pressure help determining the type of tubing which need to be used. Hoses are used when flexibility is necessary. Fittings are used to connect the units of a hydraulic system, including the individual sections of a circulatory system. Many different types of connectors are available for hydraulic systems. The types which are to be used depend on the type of circulatory system (pipe, tubing, or flexible hose), the fluid medium, and the maximum operating pressure of a system. Some of the most common types of connectors are threaded connectors, flared connectors, flexible hose couplings, and reusable fittings.
Graphical representation of hydraulic elements
The hydraulic and pneumatic elements such as cylinders and valves are connected through pipelines to form a hydraulic circuit. It is difficult to represent the complex functioning of these elements using sketches. Therefore graphical symbols are used to indicate these elements. The symbols only specify the function of the element without indicating the design of the element. Symbols also indicate the actuation method, direction of flow of air and designation of the ports. The symbol used to represent an individual element display various characteristics namely (i) function, (ii) actuation and return actuation methods, (iii) number of connections, (iv) number of switching positions, (v) general operating principle, and (vi) simplified representation of the flow path. The symbol does not represent the characteristics such as (i) size or dimensions of the component, (ii) particular manufacturer, methods of construction, or costs, (iii) operation of the ports, (iv) any physical details of the elements, and (v) any unions or connections other than junctions.
Earlier the ports were designated with letter system such as pressure port P, working port A, and B, exhaust port R, and S, and pilot port Z, and Y. Now as per ISO 5599 the ports are designated based on number system such as pressure port 1, working port 2, and 4, exhaust port 3, and 5, and pilot port 12 and 14. Fig 3 shows some of the graphical symbols with letter designation.
Fig 3 Some of the graphical representation of hydraulic components
Hydraulic-circuit diagrams
Hydraulic-circuit diagrams are complete drawings of a hydraulic circuit. Included in the diagrams is a description, a sequence of operations, notes, and a components list. Accurate diagrams are essential to the designer, the people who build the machine, and the people who maintain the hydraulic system. There are four types of hydraulic-circuit diagrams. They are block, cutaway, pictorial, and graphical. These diagrams show (i) the components and how they interact, (ii) how to connect the components and (iii) how the system works and what each component is doing.
Block diagram shows the components with lines between the blocks, which indicate connections and/or interactions. Cutaway diagram shows the internal construction of the components as well as the flow paths. Because the diagram uses colours, shades, or various patterns in the lines and passages, it can show the many different flow and pressure conditions. Pictorial diagram shows a circuit’s piping arrangement. The components are seen externally and are usually in a close reproduction of their actual shapes and sizes. Graphical diagram is the short-hand system of the industry and is usually preferred for design and troubleshooting. Simple geometric symbols represent the components and their controls and connections.
Advantages and disadvantages of hydraulic systems
The advantages of hydraulic systems are (i) the hydraulic system uses incompressible fluid which results in higher efficiency, (ii) the hydraulic system delivers consistent power output which is difficult in pneumatic or mechanical drive systems, (iii) the hydraulic system employs high density incompressible fluid hence the possibility of leakage is less in hydraulic system as compared to that in pneumatic system also the maintenance cost is less, (iv) the hydraulic system performs well in hot environment conditions, (v) the actuator in a hydraulic system can be driven at different speeds, (vi) the hydraulic actuator can be reversed instantly while in full motion without damage, (vii) the pressure relief valve in a hydraulic system protects the system from overload damage, (viii) hydraulic components, because of their high speed and pressure capabilities, can provide high power output with very small weight and size, and (ix) a hydraulic actuator can be stalled without damage when overloaded, and can start up immediately when the load is reduced.
The disadvantages are (i) the material of storage tank, piping, cylinder and piston can be corroded with the hydraulic fluid, and hence need care for selecting the materials and hydraulic fluid, (ii) the structural weight and size of the system is more which makes it unsuitable for the smaller instruments, (iii) the small impurities in the hydraulic fluid can permanently damage the complete system, hence it is essential to be careful and suitable filter is to be installed, (iv) the leakage of hydraulic fluid is also a critical issue and suitable prevention method and seals are to be adopted, and (v) the hydraulic fluids, if not disposed properly, can be harmful to the environment.
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