1 ACADEMIC TEXTBOOK FLUID POWER CONTROL SYSTEMS The lecture: 15 hours Kielce University of Technology Faculty of Mechatronics and Machine Design Author: Ryszard Dindorf Kielce, 2011/2012
2 1. Fluid power basic 2. Hydraulic control system 2.1. Introduction 2.2. Hydraulic components Hydraulic pumps Hydraulic motors Hydraulic accumulators Hydraulic cylinders Hydraulic valves Direction control valves Flow control valves Pressures control valves Check valves 2.3. Hydraulic fluids 2.4. Hydraulic circuits 3. Pneumatic control system 3.1. Introduction 3.2. Air compressors 3.3. Pneumatic actuators Single acting cylinders Double acting cylinders Magnetic cylinders Rodless cylinders Semi-rotary actuators Bellows Pneumatic muscles 3.4. Pneumatic valves Directional control valve Shut-off valve Flow control valve Pressure valve 3.5. Pneumatic circuits References CONTENTS
3 1. FLUID POWER BASIC The term fluid power refers to energy that is transmitted via a fluid under pressure . With hydraulics, that fluid is a liquid such as oil or water. With pneumatics, the fluid is typically compressed air or inert gas. The basis of fluid power systems is pressurized fluids. The fluid power systems are generally grouped under the two broad classifications of hydraulic power systems and pneumatic power systems (Fig.1.1). Figure 1.1. Classifications of fluid power systems Hydraulic systems use liquids (oil), while pneumatic systems use gas (air). In the hydrostatic power systems the power is transmitted by increasing mainly the pressure energy of a liquid. Pneumatic systems are power systems using compressed air as a working medium for the power transmission. Other fluids (water, water-based fluids, synthetic fluids) are often used in special applications. Fluid power is one of the three types of power transfer systems, namely fluid power systems, mechanical systems and electrical systems, commonly used today. Each of the systems transfer power from a prime mover (source energy) to an actuators that complete the task (work) required of the system. Figure 1.2 shows the power transmission in a fluid power system. Figure 1.2. Power transmission in a fluid power system Although hydraulic and pneumatic systems share the characteristics of energy transfer by means of fluid pressure and flow, differences affect how and where they are applied . These differences include: accuracy of actuator movement, operating pressure, actuator speed, component weight and cost. Fluid compressibility is the inherent characteristic that produces the difference between hydraulic and pneumatic systems. A gas (air) is compressible, while a liquid can be compressed only slightly. Compressibility produces a more spongy operation in pneumatic systems, which is not suitable where highly accurate movement is required. Hydraulic
4 systems can operate at much higher pressures than pneumatic systems. There are five functions that are basic to system operation of any fluid power system, Table 1.1: energy conversion, fluid distribution, fluid control, work performance and fluid conditioning. Table 1.1. The basic functions of a fluid power system  Energy conversion Fluid distribution Fluid control Fluid power systems do not generate energy, but transform it into a form that can be used to complete a task. The process begins with a prime mover pressurizing a fluid. It ends with an actuator using the energy stored in the pressurized fluid to perform work. The operation of fluid power systems requires the distribution of fluid to the components in the system. Various types of lines are involved in this function. Valves and other components also serve to assist in fluid distribution. Fluid power systems require the control and regulation of the fluid in the system to perform the tasks desired by the system designer. A number of different components are used to control fluid flow rate, direction, and pressure in a system. Control of these three elements allows the system to provide the desired operating characteristics. Work performance Using the energy stored in the pressurized fluid of the system is the primary function of a fluid power system. This process involves actuators that convert the energy stored in the fluid to linear or rotary motion to perform the desired work. Fluid conditioning Fluid power system performance and service life require a fluid that is clean and provides lubrication to system components. This function involves storing fluid, removing dirt and other contaminants, and maintaining proper system operating temperature. Hydraulic power systems use the prime mover (electric motor, diesel engine) to drive a pump that pressurizes a fluid, which is then transferred through valves, pipes and hoses to an actuators (motors, cylinders). Hydraulic power systems have gained wide scale use and applicability in technologically driven industrial manufacturing process; manipulators, robots; transport, road vehicles, rail, shipping, aircraft; public services, road cleaning, health, maintenance, elevators; military vehicles, aerospace etc. Hydraulic system operating pressure ranges from a few 10 MPa to several 50 MPa. Pressures of more than 60 MPa are used in special situations. Industrial systems commonly operate in the low (below 7 MPa) to moderate (below 21 MPa) pressure range. In hydraulic systems in low-pressure region (p<10 MPa) the influence of entrained gas on the bulk modulus is substantial. Hydraulic systems operate at higher pressures, requiring the use of stronger materials and more-massive designs to withstand the pressure. Hydraulic systems applications tend to involve equipment that handles heavier weights, requiring both higher system operating pressure and physical strength of machine parts. Pneumatic power systems normally operate between 0.5 MPa to 0.85 MPa ( bar). The air compressor converts the mechanical energy of the prime mover (electric motor, diesel engine) into pressure energy of the compressed air. This transformation facilitates the transmission and the control of power. An air preparation process is needed to prepare the compressed air for use. The air preparation includes filtration, drying, and the adding of lubricating oil mist. The compressed air is stored in the compressed air reservoirs and transmitted through rigid and/or flexible lines. The pneumatic power is controlled by means of a set of pressure, flow, and directional control valves. Pneumatic systems are commonly used when high-speed movement is required in an application. A speed for a pneumatic rotation motor of over 20,000 revolutions per minute (rpm) is possible. Rapid-response pneumatic cylinder operation is also possible with pneumatic systems. These designs are generally found in situations involving lighter loads and lower accuracy requirements. System operating pressure affects the structure of components. Pneumatic systems operate at much lower pressures and, therefore, can
5 be manufactured using lightweight materials and designs that minimize the amount of material. Pneumatic systems tend to involve applications where the ease of handling and lightweight is critical for effective operation of the tool or system. Fluid power systems are made up of component groups containing parts designed to perform specific tasks . These component groups are in operation together to perform the work desired by the system designer. The work may involve simple or complex tasks, but the component groups perform specific system functions that are basic to all fluid power systems. The structure of typical hydraulic power systems involve five component groups, Table 1.2: power unit (pump, accumulator), actuators (motors, cylinders), conductors (pipe line, hoses), control valves, fluid conditioning (filter, heat exchanger). Table 1.2. The structure of hydraulic power system  Power unit Actuators Conductors Control valves Fluid conditioning The power unit group of components deals primarily with the energy-conversion function of the system. The unit consists of a prime mover, pump, and reservoir. The prime mover is the source of energy for the system. The energy produced by the prime mover turns the pump, which produces fluid flow that transmits energy through the system. The reservoir serves as a storage unit for system fluid. It also performs fluid maintenance functions. The actuators group of components performs the work done by the system. These components convert the energy in the system fluid to linear or rotary motion. The basic actuators are cylinders for linear motion and motors for rotary motion. A variety of cylinder and motor designs are used to produce the specific motion needed to complete the work required of the system. Fluid distribution is the primary function of the conductors group of components. Pipes, hoses, and tubes serve as the conductors that confine and carry system fluid between the pump and other system components. Conductors perform such tasks as the intake of fluid for the pump, distribution of fluid to and from control valves and actuators, transmission of sensing and control pressures, and the draining of liquids that have accumulated in components. Three different types of valves are required to perform the fluid control function in a fluid power system. The valves in the control valves group are: directional control valves, pressure control valves and flow control valves. Directional control valves provide control over fluid flow direction in sections of a system to start, stop, and change the direction of actuator movement. Pressure control valves are used to limit the maximum pressure of the system or in a section of the system. Flow control valves provide control over fluid flow rate in a section of a system to control the rate of movement of an actuator. The fluid conditioning group involves maintaining and conditioning system fluid. This requires removal of dirt and moisture from the fluid and assuring proper operating temperature. Specially designed components are available to perform these tasks. However, basic systems often maintain the fluid using other system components that perform the task as a secondary function. Filters are used to remove dirt and moisture from systems, although a properly designed reservoir can perform this task under certain conditions. Maintaining the proper system operating temperature can require the use of a heat exchanger. However, this task is usually performed by dispensing heat through the reservoir, system lines, and other components. Summing up, the pneumatic system is: clean, fast, intrinsically safe, overload safe, inexpensive for individual components; hydraulic system is: easy controllable, produces extremely large forces, requires high pressures, requires heavy duty components. The dangers of the use of
6 compressed air include: air embolism, hose/pipe whipping, noise, crushing/cutting. The dangers of working with high pressure oil can be infinitely more drastic: high pressure oil injection, oil burns, crushing/cutting, carcinogens. All main hydraulic and pneumatic components can be represented by simple symbols. Each symbol shows only the function of the component it represents, but not its structure. Symbols can be combined to form hydraulic or pneumatic diagrams. A diagram describes the relations between each component, that is, the design of the system. Notice the use of standard, internationally recognized symbols, see ISO , ISO Just a few of the hydraulic and pneumatic symbols are compared in Table 1.3. Table 1.3. Comparison of hydraulic and pneumatic symbols Differences Hydraulic symbol Pneumatic symbol Pumps and compressor differ only by filling in the direction arrow. Hydraulic pump Compressor Cylinders and other linear actuators also differ with respect to supply and direction arrows. Motors and other rotary actuators also differ with respect to supply and direction arrows. Hydraulic double acting cylinder Hydraulic motor Pneumatic double acting cylinder Pneumatic motor Supply and pilot arrows are filled right black or left white. Hydraulically actuated and supplied 3/2 pilot spring Pneumatically actuated and supplied 3/2 pilot spring Hydraulic valves have a crossover to tank. Pneumatic valves tend to have two exhaust outlets to atmosphere. Hydraulic circuit Pneumatic circuit
7 2.1. INTRODUCTION 2. HYDRAULIC CONTROL SYSTEM The hydraulic system transmits the hydraulic power by the controlled circulation of pressurized fluid, usually oil, water or water-basis fluids, to actuators (cylinders, motors) that convert it into a mechanical output capable of doing work on a load . Hydraulic power systems have greater flexibility than mechanical and electrical and can produce more power than such systems of equal size. They also provide rapid and accurate responses to controls. As a result, hydraulic power systems are extensively used in modern aircraft, automobiles, heavy industrial machinery, manipulators and robots, and many kinds of machine tools. Any device operated by a hydraulic system may be called a hydraulic device, but a distinction has to be made between the devices which utilize the impact or momentum of a moving fluid and those operated by a thrust on a confined fluid i.e. by pressure . This leads us to the subsequent categorization of the field of hydraulic systems into: hydrodynamic system and hydrostatics system. Hydrodynamic system deals with the characteristics of a liquid in motion, especially when the liquid impacts on an object and releases a part of its energy to do some useful work. Hydrostatic system deals with the potential energy available when a liquid is confined and pressurized. This potential energy, also known as hydrostatic energy, is applied in most of the hydraulic systems. This field of hydraulics is governed by Pascal's law. It can thus be concluded that pressure energy is converted into mechanical motion in a hydrostatic device whereas kinetic energy is converted into mechanical energy in a hydrodynamic device. Hydrostatic drives, typically rated from 7.5 to 220 kw, incorporate positive displacement pumps and positive displacement motors. Mechanical energy is transmitted from the prime mover to the pump. The pump imparts energy through a fluid to a hydraulic motor. The prime mover can be an electric motor, a gasoline or diesel engine, or a take-off from the main machine drive. Hydrostatic drives offer several features that make them particularly adaptable to many adjustable speed applications: infinitely adjustable stepless change of speed, torque, and power; no damage even if stalled at full load; operation in both directions of rotation at controlled speeds; set speeds held accurately against driving or braking loads; small size and low weight per power output. The hydraulic power can be divided up into the power supply section, the power control section and the drive section (working section). The power supply section is made up of the energy conversion part and the pressure medium conditioning part. In this part of the hydraulic system, the hydraulic power is generated and the pressure medium conditioned. The following components are used for energy conversion converting electrical energy into mechanical and then into hydraulic energy: electric motor, internal combustion engine, coupling, pump, pressure indicator (manometer), protective circuitry (relief valve). The power supply unit provides the necessary hydraulic power by converting the mechanical power from the drive motor. The most important component in the power supply unit is the hydraulic pump. This draws in the hydraulic fluid from a reservoir (tank) and delivers it via a system of lines in the hydraulic installation against the opposing resistances. Pressure does not build up until the flowing liquids encounter a resistance. The power is supplied to the drive section by the power control section in accordance with the control problem. The following components perform this task: directional control valves, flow control valves, pressure valves, check valves (non-return valves). The drive section of a hydraulic system is the part of the system which executes the various working movements of a machine or manufacturing system. Practically, every hydraulic system of any arbitrary hydrostatic drive system consists of pressure energy generators (pumps, accumulators), pressure energy actuators (motors, cylinders), resistance elements (valves, conduits), auxiliary elements (filters, heat exchangers, tanks), and control elements (control valves).
8 2.2. HYDRAULIC COMPONENTS Hydraulic pumps Hydraulic pumps are used to convert mechanical energy (torque, speed) into hydraulic energy (flow, pressure). The basic hydraulic pumps are shown in Table 2.1. Table 2.1. Hydraulic pumps External gear pump Internal gear pump Gerotor pump Screw pump Unbalanced vane pump Balanced vane pump Swashplate axial piston pump Bent axial piston pump Inside impinged radial piston pump Outside impinged radial piston pump
9 The hydrostatic pumps work on the positive displacement principle . Positive displacement pumps have an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pumps as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant given each cycle of operation. The positive displacement pumps can be divided in two main classes: reciprocating, rotary. Displacement is by means of pistons, vanes and gears. In addition, a hydraulic pump will either have a constant or variable displacement. The four most common types of hydraulic pump are the gear, screw, vane and reciprocating: Gear pumps are displacement machines. Depending on their construction they are sub-divided into external gear pumps, internal gear pumps and gerotor gear pump. External gear pumps are widely used, especially in the mobile hydraulic sector. As the gears rotate, the hydraulic fluid is carried via the external spaces between the teeth from the suction side to the pressure side. These pumps are of simple design, cost-effective and robust and operate within a wide speed range (500 to 6000 rpm) at relatively high pressures (up to 30 MPa). Internal gear pump is a pump generally with two gears, an external internally-toothed hollow wheel with an internally rotating pinion. By rotating both wheels the oil is carried in the spaces between the gear teeth. Due to the long arc over which gear meshing takes place, the pump works quietly up to pressures of 30 MPa. The gerotor pump operates in accordance with the planetary principle. The rotor has one gear less than the internally geared stator. As a result of the way the internal and external gears mesh, a good seal is obtained without an additional sickle seal. The pump operates quietly up to pressures of 16 MPa. Screw pumps are similar to internal gear pumps in their main characteristic of possessing an extremely low operating noise level. Screw pumps contain 2 or 3 worm gears within a housing. The worm gear connected to the drive has a clockwise thread and transmits the rotary movement to further worm gears, each of which has anti-clockwise threads. An enclosed chamber is formed between the threads of the worm gears. This chamber moves from the suction port to the pressure port of the pump without a change in volume. This produces a constant, uniform and smooth flow and hence operation tends to be very quiet. There are two types of vane pump in a common use: single stroke (unbalance) and double stroke (balance). With both designs the displacement chamber is formed between the circularshaped stator, the rotor and the vanes. The vanes may be moved radially inside the rotor. What differs them is the form of the ring which limits the stroke movement of the vanes. In these pumps, a number of vanes slide in slots in a rotor which rotates in a housing or a ring. The housing may be eccentric with the center of the rotor, or its shape may be oval. In some designs, centrifugal force holds the vanes in contact with the housing, while the vanes are forced in and out of the slots by the eccentricity of the housing. In one vane pump, light springs hold the vanes against the housing; in another pump design, pressurized pins urge the vanes outward. During rotation, as the space or chamber enclosed by vanes, rotor, and housing increases, a vacuum is created, and atmospheric pressure forces oil into this space, which is the inlet side of the pump. As the space or volume enclosed reduces, the liquid is forced out through the discharge ports. In the reciprocating pump it is this back-and-forth motion of pistons inside of cylinders that provides the flow of fluid. Reciprocating pumps, like rotary pumps, operate on the positive principle - that is, each stroke delivers a definite volume of a liquid to the system. Axial piston pumps convert rotary motion of an input shaft to an axial reciprocating motion, occurring at the pistons. This is accomplished by a swashplate that is either fixed or variable in its degree of angle. As the piston barrel assembly rotates, the pistons rotate around the shaft with the piston slippers in contact with and sliding along the swashplate surface. With the swashplate vertical, no displacement occurs because there is no reciprocating motion. As the swashplate increases in angle, the piston moves in and out of the barrel as it follows the angle of the swashplate surface. With the swashplate vertical, no displacement occurs because there is no
10 reciprocating motion. As the swashplate increases in angle, the pistons move in and out of the barrel as it follows the angle of the swashplate. During one half of the circle of rotation, the piston moves out of the cylinder barrel and generates an increasing volume. In the other half of the rotation the piston moves into the cylinder barrel and generates a decreasing volume. This reciprocating motion draws fluid in and pumps it out. Bent axis piston pump consists of a drive shaft which rotates the pistons, a cylinder block, and a stationary valving surface facing the cylinder block bores which ports the inlet and outlet flow. The drive shaft axis is angular in relation to the cylinder block axis. Rotation of the drive shaft causes rotation of the pistons and the cylinder block. In radial piston pumps, the pistons are arranged radially in a cylinder block, they move perpendicularly to the shaft centerline. Two basic types are available: one uses cylindrically shaped pistons, the other ball pistons. They may also be classified according to the porting arrangement: check valve or pintle valve. They are available in fixed and variable displacement, and variable reversible (over-center) displacement. When filling the workspace of the pumping pistons from "inside" (e. g. over a hollow shaft) it is called an inside impinged (but outside braced) radial piston pump. If the workspace is filled from "outside" it is called an outside impinged radial piston pump (but inside braced). The pumping volume may be fixed or variable Hydraulic motors Hydraulic motors convert hydraulic energy into mechanical energy. In order to fulfil exactly the wide range of requirements (speed, torque, power) different design principles need to be applied. The basic hydraulic motors are shown in Table 2.2. Table 2.2. Hydraulic motors Gear motor Gerotor motor Vane motor Axial piston motors Bent axis piston motor Radial piston motor
11 Hydraulic motors are classified as rotary actuators . But strictly speaking, the term rotary actuator is reserved for a particular type of the unit whose rotation is limited to less than 360. Hydraulic motors are used to transmit fluid power through linear or rotary motion. They resemble pumps very closely in construction. However, as already understood, pumps perform the function of adding energy to a hydraulic system for transmission to some remote point, while motors do precisely the opposite. They extract energy from a fluid and convert it to a mechanical output to perform useful work. To put it more simply, instead of pushing on the fluid as the pump does, the fluid pushes on the internal surface area of the motor, developing torque. Since both the inlet and outlet ports in a motor may be pressurized, most hydraulic motors are externally drained. The three most common types of hydraulic motors are the gear, vane and piston: Gear motors are very similar in design to gear pumps. They are differ in the axial pressure field is different and gear motors have a drain case port, as they are designed for changing directions of rotation. The fluid flowing to the hydraulic motor acts on the gears. A torque is produced, which is output via the motor shaft. Gear motors are often used in mobile hydraulics. Annular gear motor, e.g. planetary motor, which works on the planetary principle. Gear motors are normally limited to operating pressures of around 15 MPa and operating speeds of 2,400 rpm. They are available with a maximum flow capacity of 550 dcm 3 /min. Hydraulic motors can also be of the internal gear type. The internal gear type motors can operate at higher speeds and pressures. They also have greater displacements than the external motors. Screw-type motors also form part of gear motors. As in the case of pumps, screw-type hydraulic motors use three meshing screws. The rolling screw set results in an extremely quiet operation. Screw type motors can operate at pressures up to 21 MPa and can have displacement volumes up to dcm 3. The internal construction of the vane motors is similar to that of a vane pump, however the principle of operation differs. Vane motors develop torque by virtue of the hydraulic pressure acting on the exposed surfaces of the vanes, which slide in and out of the rotor connected to the drive shaft. As the rotor revolves, the vanes follow the surface of the cam ring because springs are used to force the vanes radially outward. Vane motors are universally of the balanced design type. Since vane motors are hydraulically balanced, they are fixed displacement units. These motors can operate at pressures of up to 17 MPa and speeds up to 4,000 rpm. The maximum flow usually delivered by these motors is in the range of 950 dcm 3 /min. The vane-type motors have more internal leakage as compared with the piston type and are therefore not recommended for the use in servo control systems. Piston motors are also similar in construction to that of piston pumps. Piston motors can be either fixed or variable displacement units. They generate torque through pressure acting at the ends of pistons, reciprocating inside a cylinder block. To put it rather simply, piston-type hydraulic motors use single-acting pistons that extend by virtue of fluid pressure acting on them and discharge the fluid as they retract. The piston motion is translated into circular shaft motion by different means such as an eccentric ring, bent axis or with the help of a swashplate. The piston motor design usually involves incorporation of an odd number of pistons. This arrangement results in the same number of pistons receiving the fluid as the ones discharging the fluid, although one cylinder may get blocked by the valve crossover. On the contrary, with the number of pistons being even and one getting blocked, there would be one more piston either receiving or discharging the fluid leading to speed and torque pulsations. Piston motors are the most efficient of all motors. They are capable of operating at very high speeds of 12,000 rpm and also pressures up to 35 MPa. Large piston motors are capable of delivering flows up to 1,500 dcm 3 /min. Piston motors are further categorized as radial and axial piston motors. In an axial piston motor, the rotor rotates on the same axis as the pistons. There are basically two types of axial piston motor design. They are in-line piston motor (swashplate type) and bent-axis type.
12 Hydraulic accumulators Hydraulic accumulators have various tasks to fulfil in a hydraulic system: energy storage, fluid reserve, emergency operation, equalising of forces, damping of mechanical and pressure shocks, leakage oil compensation, oscillation damping, pulsation damping, vehicle suspension, recuperation of deceleration energy, maintaining pressure constant and flow compensation (expansion tanks) . There is always an equilibrium between pressure of the hydraulic fluid and the counter pressure generated by the weight (weight loaded accumulator), the spring (spring accumulator) or the gas (gas pressure accumulator). The basic hydraulic accumulators are shown in Figure 2.1. Figure 2.1. Hydraulic accumulators: a) weight loaded, b) spring, c) piston, d) bladder, e) diaphragm Gas pressurized accumulators are catagorized according to their separating element (between the gas and fluid) into bladder accumulators, piston accumulators and diaphragm (membrane) accumulators . With this hydraulic accumulators gas is used as loading medium for the hydraulic fluid. Bladder accumulators contain a fluid and a gas chamber separated by a gastight bladder. The fluid chamber around the bladder is connected to the hydraulic circuit, so that when pressure is increased this chamber fills, squeezing the bladder and compressing the gas. When the pressure drops the compressed gas expands, forcing the accumulated fluid into the circuit. Piston accumulators comprise a fluid and a gas chamber with a piston as a gastight separating element. The gas side is pre-filled with nitrogen. The fluid chamber is connected to the hydraulic circuit, so that when the pressure rises the piston accumulator absorbs fluid and the gas is compressed. When the pressure drops the compressed gas expands, displacing the accumulated pressure fluid in the circuit. Diaphragm accumulators comprise a steel container which is resistant to compression and is usually either spherical or cylindrical in shape. The separating element inside the accumulator is a diaphragm made of an elastic material (elastomer). There are two types of diaphragm accumulator available: In the screwed model the diaphragm is held in position by screwing the top and bottom part to clamping nuts. It is possible to exchange the diaphragm in this model. At the bottom of the diaphragm in the centre there is a valve plate, which prevents the diaphragm being from pulled out when fluid is connected. In welded accumulators the diaphragm is pressed into the lower part before circular seam welding is carried out. By using a suitable welding process and by situating the diaphragm correctly, this ensures that the elastomer material is not damaged when the welding is carried out. It is not possible to exchange the diaphragm Hydraulic cylinders The hydraulic cylinder is the connecting element between the hydraulic circuit and the operating machine. It carries out linear (translatory) movements in order to transmit forces. Cylinders are linear actuators whose output force or motion is in a straight line. Their function is to convert hydraulic power into linear mechanical power. Hydraulic cylinders extend and retract
13 to perform a complete cycle of operation. Their work applications, as earlier discussed, may include pulling, pushing, tilting and pressing. The type of cylinder to be used along with its design is based on a specific application. Hydraulic cylinders are suitable for lifting, lowering and locking operations and for load shifting. Cylinders with a piston are the most common form of hydraulic cylinder. The mechanical force is produced by the action of the hydraulic fluid on the piston. The basic hydraulic cylinders are shown in Table 2.3. Table 2.3. Hydraulic cylinders Single acting cylinder for pushes Single acting cylinder for pulling Single acting plunger cylinder Double acting cylinder, single rod Double acting cylinder, double rod Double acting tandem cylinder Single acting telescope cylinder Double acting telescope cylinder The simple basic hydraulic cylinders : Cylinders with spring return are single acting cylinders. They are used in applications where an external, restoring force does not exist. Return springs may be situated either within the cylinder or mounted onto the cylinder as a separate component. As these springs can only carry out limited strokes and exert limited forces, they are mainly to be found in "small cylinders". The plunger is a single acting piston which is used in plunger cylinder. This is a cylinder model with only one piston area and hence it can only transfer forces due to pressure. Plunger cylinders are used wherever a definite direction of force will ensure a return of the piston to its starting position. The examples of this are upstroke presses, lifting devices, etc. Retraction of the piston can only occur through the weight of the piston or due to an external force being applied.
14 Double acting cylinders have two opposing effective areas which are of the same or different sizes. They are fitted with two pipe ports which are isolated from each other. By feeding fluid via ports "A" or "B" the piston may transfer pulling or pushing forces in both stroke directions. Two types of double acting cylinder exist: single rod cylinder and double rod cylinder. Single rod cylinders have a differential piston which is fixed to a piston rod smaller in diameter than the cylinder diameter. The maximum power that can be transmitted depends on the piston area for outward travel and on the annulus area for return travel. The respective areas to be filled are equal in length because of the stroke, but vary in volume owing to the difference between piston and annulus size. The stroke speed is thus inversely proportional to the area. The differential piston is a double acting piston in a single rod cylinder. As it is fixed on one side to a piston rod with a smaller diameter, it has two effective working areas of different sizes. Double rod cylinders have a piston firmly fixed to two piston rods of a smaller diameter. The maximum force that can be transmitted depends, in both directions, on the same annulus area and also on the maximum permissible operating pressure. This means that at the same operating pressure equal forces act in both directions. As the areas, stroke lengths and therefore also the space to be filled are the same on both sides, it follows that the speeds will also be the same. For special applications there are versions of double rod cylinders with different piston rod diameters. In double acting cylinders operating in tandem, there are two cylinders which are connected together in such a way that the piston rod of one cylinder pushes through the bottom of the other cylinder to its piston area. By using this arrangement the areas are added together and large forces may be transferred for relatively small external diameters without increasing the operating pressure. Telescopic cylinders are a special design of hydraulic cylinder that provide an exceptionally long output travel from a very compact retracted length. If the pistons are placed under pressure via port (A), the sections extend one after another. The pressure is dependent on the size of the load and the effective area. Hence the piston with the largest effective area extends first. At constant pressure and flow the extension begins with the largest force and the lowest speed and finishes with the smallest force and the highest speed. In double acting cylinders the pistons are extended in the same way as in single acting cylinders. The order in which the individual stages are retracted depends on the size of the annulus area and on the external load. The piston with the largest annulus area returns first to its starting position. In double acting cylinders the pistons are extended in the same way as in single acting cylinders. The order in which the individual stages are retracted depends on the size of the annulus area and on the external load. The piston with the largest annulus area returns first to its starting position via pipe port (B) when it is placed under pressure Hydraulic valves A valve is a control device used for adjusting or manipulating the flow rate of a liquid (hydraulic fluid) in a pipeline. The valve essentially consists of a flow passage whose flow area can be varied. The external motion can originate either manually or from an actuator positioned pneumatically, electrically or hydraulically, in response to some external positioning signal. This combination of the valve and actuator is known as a control valve or an automatic control valve. Control is achieved by influencing the start, stop, direction, pressure and flow of the hydraulic fluid operating in the system. Valves may be sub-divided : according to their function into directional valves, pressure control valves, flow control valves and non-return valves, according to their construction into poppet valves, spool valves, or rotary spool valves, according to their spool positions: 4/3-way (3 positions) directional valves, 4/2-way (2 positions) directional valves and other,
15 according to the ports controlled: 3/2-way (3 ports) directional valves, 4/3-way (4 ports) directional valves and other, according to their open and closed loop characteristics: sequence valves, proportional/servo valves and other, according to their type of control into pilot valves and main valves, according to their type of actuation into muscle power (manually or foot actuation), mechanically (roller shaft or roller), electrically (solenoids) or pressure (pneumatic or hydraulic actuated) or a combination of these. Basically there are three types of control valves : 1. Direction control valves: Direction control valves determine the path through which a fluid traverses within a given circuit. In other words, these valves are used to control the direction of flow in a hydraulic circuit. It is that the component of a hydraulic system that starts, stops and changes the direction of the fluid flow. Additionally the direction control valve actually designates the type of hydraulic system design, either open or closed. An example of their application in a hydraulic system is the actuator circuit, where they establish the direction of motion of a hydraulic cylinder or a motor. 3. Flow control valves: The fluid flow rate in a hydraulic system is controlled by flow control valves. Flow control valves regulate the volume of oil supplied to different parts of a hydraulic system. Non-compensated flow control valves are used where precise speed control is not required, since the flow rate varies with the pressure drop across a flow control valve. Pressure-compensated flow control valves are used in order to produce a constant flow rate. These valves have the tendency to automatically adjust to changes in pressure. 2. Pressure control valves: Pressure control valves protect the system against overpressure conditions that may occur either on account of a gradual build up due to decrease in fluid demand or a sudden surge due to opening or closing of the valves. Pressure relief, pressure reducing, sequencing, unloading, brake and counterbalance valves control the gradual buildup of pressure in a hydraulic system. Pressure surges can produce instantaneous increases in pressure as much as four times the normal system pressure and that is the reason why pressure control devices are a must in any hydraulic circuit. Hydraulic devices such as shock absorbers are designed to smoothen out pressure surges and also to dampen hydraulic shock Direction control valves Direction control valves are used to control the direction of flow in a hydraulic circuit. They are primarily designated by their number of possible positions, port connections or ways and the manner in which they are actuated or energized . For example, the number of porting connections is designated as ways or possible flow paths. For example, the number of porting connections are designated as ways or possible flow paths. A four-way valve would have four ports: P (Pump), T (Tank), A (A chamber of actuator) and B (B chamber of actuator). A threeposition valve is indicated by three connected boxes. There are many ways of actuating or shifting the valve. They include push button, hand lever, foot pedal, mechanical, hydraulic pilot, air pilot, solenoid and spring (see Fig.2.2). Figure 2.2. Simple actuating the 4/3-way directional control valve
16 Although they may be designed as rotary or poppet style, the spool type directional control is the most common. This design consists of a body with internal passages that are connected or sealed by a sliding spool along the lands of the valve. Direction control valves may also be categorized as normally open and normally closed valves. This terminology would normally accompany the direction control valves, as reflected in the examples of two-position valves given below. Normally open and normally closed valves are compared in Table 2.4. The spool type directional control valves in industrial applications are sub-plate or manifold mounted. The porting pattern is industry standard and designed by valve size. Directional control valve sizing is according to flow capacity which is critical to the proper function of the valve. Flow capacity of a valve is determined by the port sizes and the pressure drop across the valve. A direct acting direction control valve can be actuated either manually or with the help of a solenoid. Direct acting indicates that some method of force is applied directly to the spool, causing the spool to shift. Single and double solenoid control valves are available with DC 12 or 24 volts solenoids or AC 50/60 Hz 120 or 230 volt solenoids. For the control of systems requiring high flows pilot operated directional control valves must be used due to the higher force required to shift the spool. The top valve, called the pilot valve, is used to hydraulically shift the bottom valve, or the main valve. Table 2.4. Normally open and normally closed valves Solenoid operated 2/2-way valve, normally closed Solenoid operated 2/2-way valve, normally open Solenoid operated 3/2-way valve, normally closed Solenoid operated 3/2-way valve, normally open The directional control valve actually designates the type of circuit. One can categorize most hydraulic circuits into two basic types: closed center and open center, as shown in Figure 2.3. Figure 2.3. Basic hydraulic circuits: a) closed center, b) open center
17 Open center circuits are defined as circuits whose route pump flow back to the reservoir through the directional control valve during neutral or dwell time . This type of a circuit typically uses a fixed volume pump, such as a gear pump. If flow were to be blocked in neutral or when the directional control valve is centered, it would force flow over the relief valve. This could possibly create an excessive amount of heat and would be an incorrect design. A closed center circuit blocks pump flow at the directional control valve, in neutral or when centered. We must utilize a pressure compensated pump, such as a piston pump, which will de-stroke; or an unloading circuit used with a fixed volume pump. Directional control valve (4/3-way - four ports, three-position, solenoid operated) incorporates a neutral or center position which designates the circuit as open or closed, depending on the interconnection of the P and T ports, and designates the type of work application depending on the configuration of the A and B ports. The four most common types of three-position valves are: closed type, open type, closed type, flow type, and tandem type, shown in Figure 2.4. Figure 2.4. Directional control valves: a) closed type, b) open type, c) float type, d) tandem type Flow control valves Flow control valves are used to regulate the flow rate of oil supplied to different areas of hydraulic systems. One of the most important applications of flow control valves in hydraulic systems is in controlling the flow rate to actuators (cylinders and motors) to regulate their speeds. Any reduction in flow will in turn, result in a speed reduction at the actuator. Flow control valves are used to influence the speed of movement of actuators. By changing the opening to flow, the fluid is either closed loop or open loop controlled at the throttling position. Dependent on their behaviour the flow control valves may be divided into throttling valves and pressure compensated valves. In addition, both types may operate dependent upon pressure difference or independently of pressure difference across them. Some factors, which should be considered during the design stage of a flow control valve are : The maximum and minimum flow rates and the fluid density, which affect the size of the valve. The corrosive property of the fluid, which determines the material of construction of the valve. The pressure drop required across the valve. The allowable leakage limit across the valve in its closed position. The maximum amount of noise from the valve that can be tolerated. The means of connecting the valve to the process i.e. screwed, flanged or butt welded. There are many different designs of valves used for controlling flow. Many of these designs have been developed to meet specific needs. Flow control valves are classified as: fixed or nonadjustable, adjustable or throttling and pressure compensated, they are shown in Figure 2.5. The flow through throttle valves is dependent on the pressure difference at the throttling point i.e. the greater the pressure difference the larger the flow. Throttle valves are used where a constant operating resistance is given or where speed variation with variable loads has no effect or is actually desirable. The amount of flow through an orifice will remain constant as long as the pressure differential across the orifice does not change. Changing load or upstream pressure will change the pressure drop across the valve.
18 Figure 2.5. Flow control valves: a) fixed (non-adjustable), b) adjustable (throttling), c) pressure compensated Pressure control valves The term pressure valve includes all valves which change the operating pressure of a system or part of a system to a pre-determined value . Depending on the seal of the throttle area we differentiate between two types of pressure valve: spool valve and poppet valve. According to the function the pressures valves are sub-divided into relief valves, sequence valves, pressure unloading valves and pressure reducing valves. The two basic pressure control valve design types are: direct-acting pressure control valves and pilot-operated pressure control valves. For accurate control of pressure and force in a hydraulic circuit, five different types of pressure control valves have been developed. These are given below along with their graphical representation, as shown in Figure 2.6. Figure 2.6. Pressure control valves: a) relief valve, b) reducing valve, c) unloading valve, d) counterbalance or brake valve, e) sequence valve The most widely used type of pressure control valve is the pressure relief valve since it is found in practically every hydraulic system (Fig. 2.6a). It is a normally closed valve whose function is to limit the pressure to a specified maximum value by diverting the pump flow back to the tank. The primary port of a relief valve is connected to system pressure and the secondary port connected to the tank. When the poppet in the relief valve is actuated at a predetermined pressure, a connection is established between the primary and secondary ports resulting in the flow getting diverted to the tank. Pressure-reducing valves are normally open pressure control valves that are used to limit pressure in one or two legs of a hydraulic circuit (Fig. 2.6b). Reduced pressure results in a reduced force being generated. Unloading valves are remotely piloted, normally closed pressure control valves, used to direct flow to the tank when pressure at a particular location in a hydraulic circuit reaches a predetermined value (Fig. 2.6c). A counterbalance valve again is a normally closed pressure control valve and is particularly used in cylinder applications for countering a weight or overrunning load. Brake valves are normally closed pressure control valves that are frequently used with hydraulic motors for dynamic braking (Fig. 2.6d). The operation of these valves involves both direct and remote pilots connected simultaneously. During running, the valve is kept open through remote piloting, using system pressure. This results in eliminating any back pressure on the motor that might arise on account of downstream resistance and subsequent load on the motor or cylinder.
19 A sequencing valve again is a normally closed pressure control valve used for ensuring a sequential operation in a hydraulic circuit, based on pressure (Fig. 2.6e). In other words, sequencing valves ensure the occurrence of one operation before the other Check valves A check valve is installed in a hydraulic system to control the direction flow of hydraulic fluid. The check valve allows free flow of fluid in one direction, but no flow or a restricted one - in the other direction. There are two general designs in check valves . One has its own housing and is connected to other components with tubing or hose. Check valves of this design are called in-line check valves. In the other design, the check valve is a part of another component and is called an integral check valve. It will not be covered because its operation is identical to the in-line check valve. The symbol for check valves (non-return valves) is a ball which is pressed against a sealing seat (Fig.2.7). This seat is drawn as an open triangle in which the ball rests. The point of the triangle indicates the blocked direction and not the flow direction. Pilot controlled check valves are shown as a square into which the symbol for the check valve is drawn. The pilot control for the valve is indicated by a control connection shown in the form of a broken line. The pilot port is labelled with the letter X. Figure 2.7. Symbols of check valves: a) unloaded, b) spring loaded, c) pilot-controlled Check valves are a simple but important part of a hydraulic system . Simply stated, these valves are used to maintain the direction that fluid flows through a system. And since check valves are zero leakage devices we can use them to lock hydraulic fluid from the cylinders. This section has been designed to help you understand how the different valves function and the strategy of where they are used in the system. In-line check valves are classified as directional control valves because they dictate the direction the flow can travel in a portion of the circuit. Because of their sealing capability many designs are considered to have zero leakage. The simplest check valve allows free flow in one direction and blocks flow from the opposite direction. This style of check valve is used when flow needs to bypass a pressure valve during return flow, as a bypass around a filter when a filter becomes clogged, or to keep flow from entering a portion of a circuit at an undesirable time HYDRAULIC FLUIDS The hydraulic working fluid is the single most important component of any hydraulic system. It serves as a lubricant, heat transfer medium, sealant and most important of all, a means of energy transfer. Hydraulic fluids are basically non-compressible in nature and can therefore take the shape of any container . This tendency of the fluid makes it exhibit a certain advantage in the transmission of force across a hydraulic system. The use of a clean, high-quality fluid, is an essential prerequisite for achieving efficient operation of the hydraulic system. This has necessitated the development of modern fluids designed specifically for application in hydraulic systems. Although hydraulic fluid types vary according to application, the four common types are: 1. Petroleum-based fluids which are the most common of all fluid types and widely used in applications where fire resistance is not required.
20 2. Water glycol fluids used in applications which require fire resistance fluids. 3. Synthetic fluids used in applications where fire resistance and nonconductivity is required. 4. Environment-friendly fluids that end up causing minimal effect on the environment in the event of a spill. The first major category of hydraulic fluids is the petroleum-based fluid, which is the most widely used type. The crude oil that is quality refined can be used for light services. Additives should be added to these fluids in order to maintain the following characteristics: good lubricity, high viscosity index, oxidation resistance. The primary disadvantage of a petroleum-based fluid is that it is flammable. The hydraulic fluids have the four essential primary functions of power transmission, heat dissipation, lubrication and sealing to accomplish all of which should possess the following properties: ideal viscosity, good lubricity, low volatility, non-toxicity, low density, environmental and chemical stability, high degree of incompressibility, fire resistance, good heat-transfer capability, foam resistance and most importantly, easy availabihty and cost-effectiveness. It is quite obvious that no single fluid can meet all the above requirements and it is therefore essential that only the fluid that comes closest to satisfying most of these requirements be selected for a particular application. The following components are used to condition the hydraulic fluid: filter, cooler, heater, thermometer, pressure gauge, reservoir (tank), filling level indicator. The tank in a hydraulic system fulfils several tasks: acts as an intake and storage reservoir for the hydraulic fluid required for operation of the system; dissipates heat; separates air, water and solid materials; supports a built-in or built-on pump and drive motor and other hydraulic components, such as valves, accumulators, etc. Hydraulic filters can be arranged in various different positions within a system (pressure line filter, pump inlet filter, return flow filter, by-pass flow filtering) HYDRAULIC CIRCUITS A hydraulic diagram is a compilation of hydraulic graphic symbols, interconnected, showing a sequence of operational flow. In short, they explain how a circuit functions. Correct diagram reading is the most important element of hydraulic troubleshooting. Although initially most circuits may appear complicated, recognizing standard symbols and systematic flow tracings simplifies the process. A hydraulic system can be divided into the following sections: the signal control section and the power section. The signal control section is divided into signal input (sensing) and signal processing (processing). Signal input may be carried out: manually, mechanically, contactlessly (electrically). Signals can be processed by the following means: operator, electricity, electronics, pneumatics, mechanics, hydraulics. The basic functional requirements common to all hydraulic systems are as follows: hydraulic source (pumps), the means of distributing the hydraulic power (pipes and flexible hose), the means of controlling the hydraulic power (pressure and flow valves), the means to provide load actuation (cylinders and motors). A hydraulic drive system is a transmission system that uses pressurized hydraulic fluid to drive hydraulic machinery, equipment and devices. A simple hydraulic circuits, for example Figure 2.8, which shows a cylinder and a motor actuated circuits illustrating basic system components . The hydraulic circuit requires a tank with its fluid, a pump, a pressure-relief valve, a directional control valve, and a cylinder (motor) to provide the force (torque) to move the load. Pump (1) draws oil from tank (2), and the pump output line will contain high-pressure filter (3) to prevent dangerous particles from passing into the system and causing damage. Pressure-relief valve (4) is required to the working pressure and also to protect the system from catastrophic failure should the pump output flow not be required by the load. Operation of directional valve (5), usually by means of an electrical signal, allows fluid to flow in either of two directions, as indicated on the valve symbol. Actuators cylinder (6) and motor (7) will move in the appropriate direction, depending on the input signal selected.
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