Hydraulics actuation system


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Hydraulics actuation system

  1. 1. TERM PAPER HYDRAULICS ACTUATION SYSTEMCaterpillar 797B mining truck. Source: CaterpillarSubmitted By:RAJESH KUMAR P2009ME1100LALIT AGGARWAL P2009ME 1088 1
  2. 2. AbstractTable Of ContentsList of FiguresList of TablesObjective 2
  3. 3. Fluid PowerFluid power is the transmission of forces and motions using a confined pressurized fluid. Inhydraulic fluid power systems the fluid is oil or water. Fluid power is ideal for high speed,high force, and high power applications. Compared to all other actuation technologies,including electric motors, fluid power is unsurpassed for force and power density and iscapable of generating extremely high forces with relatively lightweight cylinder actuators.Fluid power systems have a higher bandwidth than electric motors and can be used inapplications that require fast starts, stops and reversals, or that require high frequencyoscillations. Because oil has a high bulk modulus, hydraulic systems can be finely controlledfor precision motion applications.Major advantage of fluid power lies in its compactness and flexibility. Fluid power cylindersare relatively small and light for their weight and flexible hoses allows power to be snakedaround corners, over joints and through tubes leading to compact packaging withoutsacrificing high force and high power. A good example of this compact packaging is Jaws ofLife rescue tools for ripping open automobile bodies to extract those trapped within.But there are some disadvantages also. 1. Hydraulic systems can leak oil at connections and seals. 2. Hydraulic power is not as easy to generate as electric power and requires a heavy, noisy pump. 3. Hydraulic fluids can cavitate and retain air resulting in spongy performance and loss of precision. Hydraulic can become contaminated with particles and require careful filtering. 4. The physics of fluid power is more complex than that of electric motors which makes modelling and control more challenging.Research is going on not only to overcome these challenges but also to open fluid power tonew applications, for example tiny robots and wearable power-assist tools. 3
  4. 4. Some main Applications Of Fluid Power (Hydraulics)Fluid power is extensively used throughout industry and throughout the world because of itsmajor advantages and here are some examples.  Earth moving machines such as excavators  Winches on cranes and boats  Rams in forging and extrusion processes  Automated production lines  Aeroplane controls  Automated assembly units  Machine tools  Braking system  Roller coaster  Earthquake simulatorsFigures showing application of fluid powerCaterpillar 797B mining truck. Source: Caterpillar 4
  5. 5. 40,000 ton forging press. Source: Shultz SteelMAST Laboratory for earthquake simulation. Source: MAST Lab. 5
  6. 6. Caterpillar 345C L excavator. Source: Caterpillar.Hypersonic XLC roller coaster with hydraulic lanuch assist. Source:Wikipedia image 6
  7. 7. Introduction to HydraulicsHydraulics refers to the means and mechanisms of transmitting power through liquids.Hydraulic Actuators, as used in industrial process control, employ hydraulic pressure to drivean output member. These are used where high speed and large forces are required. Thefluid used in hydraulic actuator is highly incompressible so that pressure applied can betransmitted instantaneously to the member attached to it.In fluid power, hydraulics is used for the generation, control, and transmission of power bythe use of pressurized liquids. Hydraulic topics range through most science and engineeringdisciplines, and cover concepts such as pipe flow, dam design, fluidics and fluid controlcircuitry, pumps, turbines, hydropower, computational fluid dynamics, flow measurement,river channel behaviour and erosion. thIt was not, however, until the 17 century that the branch of hydraulics with which we areto be concerned first came into use. Based upon a principle discovered by the Frenchscientist Pascal, it relates to the use of confined fluids in transmitting power, multiplyingforce and modifying motions.Then, in the early stages of the industrial revolution, a British mechanic named JosephBramah utilized Pascal’s discovery in developing a hydraulic press.Principle BehindPascal’s Law“Pressure applied to a confined fluid at any point is transmitted undiminished and equallythroughout the fluid in all directions and acts upon every part of the confining vessel at rightangles to its interior surfaces.” 7
  8. 8. Amplification of Force or Hydraulic “Leverage”As the pressure in the system is the same, the force that the fluid gives to the surroundingsis therefore equal to pressure multiplies by area. In such a way, a small piston feels a smallforce and a large piston feels a large force.The same principle applies for a hydraulic pump with a small swept volume that asks for asmall torque, combined with a hydraulic motor with a large swept volume that gives a largetorque. In such a way a transmission with a certain ratio can be built.Note : Pressure remains same everywhere only force changes due to change in area andwork done in the process remains same i.e. input = output. 8
  9. 9. The illustration given below shows that 1 lb. of force exerted on a 1 sq. in. piston, moved 10in. will lift 10 lbs. a distance of 1 in. with a 10 sq. in. piston. The larger piston will move ashorter distance, but provides the mechanical advantage to lift a much heavier load. Thismechanical workforce advantage is hydraulic leverage.Where hydraulic actuation should be used?Hydraulic actuation should be used for any installations where there are a number ofsystems that can be operated from a single system; installations where the gates, valves,and actuators must be submerged; and installations where the system must operate in apower failure or other emergency. Hydraulic actuators are particularly desirable whereequipment is to be operated frequently, where loads are high, where the speed ofoperation is high or must be varied during operation, and where they are located in ahazardous area requiring explosion-proof and intrinsically safe equipment. 9
  10. 10. Components of Hydraulic Actuation Systems1. Hydraulic Fluid Hydraulic fluid must be essentially incompressible to be able to transmit power instantaneously from one part of the system to another. At the same time, it should lubricate the moving parts to reduce friction loss and cool the components so that the heat generated does not lead to fire hazards. It also helps in removing the contaminants to filter. Figure below shows the role played by hydraulic fluid films in lubrication and sealing.2. The Fluid Delivery Subsystem It consists of the components that hold and carry the fluid from the pump to the actuator. It is made up of the following components. 10
  11. 11. 3. Reservoir It holds the hydraulic fluid to be circulated and allows air entrapped in the fluid to escape. This is an important feature as the bulk modulus of the oil, which determines the stiffness of hydraulic system, deteriorates considerably in the presence of entrapped air bubbles. It also helps in dissipating heat.4. Filter The hydraulic fluid is kept clean in the system with the help of filters and strainers. Metal particles are continually produced by mechanical components and need to be removed along with other contaminants, which can cause blocking of the orifices of servo-valves or cause jamming of spools. The graphical symbol for Reservoirs and Filters 11
  12. 12. 5. Line Pipe, tubes and hoses, along with the fittings or connectors, constitute the conducting lines that carry hydraulic fluid between components. Lines convey the fluid and also dissipate heat. There are various kinds of lines in a hydraulic system. The working lines carry the fluid that delivers the main pump power to the load. The pilot lines carry fluid that transmits controlling pressures to various directional and relief valves for remote operation or safety. Lastly there are drain lines that carry the fluid that inevitably leaks out, to the tank. The various kinds of lines in a hydraulic system Connection Arrangement of Filter and Lines with a Reservoir 12
  13. 13. 6. Fittings and Seals Various additional components are needed to join pipe or tube sections, create bends and also to prevent internal and external leakage in hydraulic systems. Although some amount of internal leakage is built-in, to provide lubrication, excessive internal leakage causes loss of pump power since high pressure fluid returns to the tank, without doing useful work. External leakage, on the other hand, causes loss of fluid and can create fire hazards, as well as fluid contamination. Various kinds of sealing components are employed in hydraulic systems to prevent leakage. A typical such component, known as the O-ring is shown below in Figure. Sealing by O-rings7. Hydraulic Pumps The pump converts the mechanical energy of its prime-mover to hydraulic energy by delivering a given quantity of hydraulic fluid at high pressure into the system. Generally, all pumps are divided into two categories, namely, hydrodynamic or non-positive displacement and hydrostatic or positive displacement. Hydraulic systems generally employ positive displacement pumps only. The graphical symbol for Pumps 13
  14. 14. Different types of pumps Hydrostatic or Positive Displacement Pumps These pumps deliver a given amount of fluid for each cycle of motion, that is, stroke or revolution. Their output in terms of the volume flow rate is solely dependent on the speed of the prime-mover and is independent of outlet pressure notwithstanding leakage. These pumps are generally rated by their volume flow rate output at a given drive speed and by their maximum operating pressure capability which is specified based on factors of safety and operating life considerations. In theory, a pump delivers an amount of fluid equal to its displacement each cycle or revolution. In reality, the actual output is reduced because of internal leakage or slippage which increases with operating pressure. There are various types of pumps used in hydraulic systems as described below. Gear Pumps The construction of a Gear Pump A gear pump develops flow by carrying fluid between the teeth of two meshed gears. One gear is driven by the drive shaft and turns the other, which is free. The pumping chambers formed between the gear teeth are enclosed by the pump housing and the side plates. A low pressure region is created at the inlet as the gear teeth separate. As a result, fluid flows in and is carried around by the gears. As the teeth mesh again at the outlet, high pressure is created and the fluid is forced out. Figure shows the construction of a typical internal gears pump; Most gear type pumps are fixed displacement. They range in output from very low to high volume. They usually operate at comparatively low pressure. 14
  15. 15. Vane PumpsIn a vane pump a rotor is coupled to the drive shaft and turns inside a cam ring. Vanes are fittedto the rotor slots and follow the inner surface of the ring as the rotor turns. Centrifugal forceand pressure under the vanes keep them pressed against the ring. Pumping chambers areformed between the vanes and are enclosed by the rotor, ring and two side plates. At the pumpinlet, a low pressure region is created as the space between the rotor and ring increases. Oilentering here is trapped in the pumping chambers and then is pushed into the outlet as thespace decreases.Principle of Operation of Vane PumpsPiston PumpsIn a piston pumps, a piston reciprocating in a bore draws in fluid as it is retracted and expels iton the forward stroke. Two basic types of piston pumps are radial and axial.A radial pump has the pistons arranged radially in a cylinder block and in an axial pump thepistons are parallel to the axis of the cylinder block. The latter may be further divided into in-line(swash plate or wobble plate) and bent axis types. 15
  16. 16. 8. Motors Motors work exactly on the reverse principle of pumps. In motors fluid is forced into the motor from pump outlets at high pressure. This fluid pressure creates the motion of the motor, shaft and finally goes out through the motor outlet port and return to tank. All three variants of motors, already described for pumps, namely Gear Motors, Vane Motors and Piston motors are in use. The graphical symbol for Motors9. Accumulators Unlike gases the fluids used in hydraulic systems cannot be compressed and stored to cater to sudden demands of high flow rates that cannot be supplied by the pump. An accumulator in a hydraulic system provides a means of storing these incompressible fluids under pressure created either by a spring or compressed gas. Any tendency for pressure to drop at the inlet causes the spring or the gas to force the fluid back out, supplying the demand for flow rate. Accumulator 16
  17. 17. Types of Accumulators Spring-Loaded Accumulators In a spring loaded accumulator, pressure is applied to the fluid by a coil spring behind the accumulator piston. The pressure is equal to the instantaneous spring force divided by the piston area. The pressure therefore is not constant since the spring force increases as fluid enters the chamber and decreases as it is discharged. Spring loaded accumulators can be mounted in any position. The spring force, i.e., the pressure range is not easily adjusted, and where large quantities of fluid are required spring size has to be very large. A spring-loaded accumulator 17
  18. 18.  Gas Charged Accumulator The most commonly used accumulator is one in which the chamber is pre-charged with an inert gas, such as dry nitrogen. A gas charged accumulator should be pre-charged while empty of hydraulic fluid. Accumulator pressure varies in proportion to the compression of the gas, increasing as pumped in and decreasing as it is expelled. A gas-charged accumulator10. Cylinders Cylinders are linear actuators, that is, they produce straight-line motion and/or force. Cylinders are classified as single-or double-acting as illustrated in Figures with the graphical symbol for each type. Single Acting Cylinder: It has only one fluid chamber and exerts force in only one direction. When mounted vertically, they often retract by the force of gravity on the load. Ram type cylinders are used in elevators, hydraulic jacks and hoists. 18
  19. 19. Cross Sectional View of Single-acting Cylinder 19
  20. 20. Double-Acting Cylinder:The double-acting cylinder is operated by hydraulic fluid in both directions and is capable ofa power stroke either way. In single rod double-acting cylinder there are unequal areasexposed to pressure during the forward and return movements due to the cross-sectionalarea of the rod. The extending stroke is slower, but capable of exerting a greater force thanwhen the piston and rod are being retracted.Double-rod double-acting cylinders are used where it is advantageous to couple a load toeach end, or where equal displacement is needed on each end. With identical areas oneither side of the piston, they can provide equal speeds and/or equal forces in eitherdirection. Any double-acting cylinder may be used as a single-acting unit by draining theinactive end to tank.Cross Sectional View of Double-acting Cylinder 20
  21. 21. 11. Control Valve Control valves are essential and appear in all fluid power systems. Valves are sometimes categorized by function, which includes directional control valves for directing fluid flow to one or the other side of a cylinder or motor, pressure control valves for controling the fluid pressure at a point and flow control valves for limiting the fluid flow rate in a line, which in turn limits the extension or retraction velocities of a piston. On/off valves can only be in the states defined by their positions while proportional valves are continuously variable and can take on any position in their working range. A servo valve is a proportional valve with an internal closed-loop feedback mechanism to maintin precise control over the valve behaviour. Types of control valves Left to right: hand-operated directional valve for a log splitter, On-off miniature, solenoid actuated pneumatic valve, Precision proportional pneumatic valves, High precision, flapper-nozzle hydraulic servo valve. Valve actuation symbols Left to right: push-button, lever, springreturn, solenoid, pilot-line. 21
  22. 22. 12. Hydraulic Circuit Drawings Accurate diagrams of hydraulic circuits are essential to the technician who must diagnose and repair possible problems. The diagram shows how the components will interact. It shows the technician how it works, what each component should be doing and where the oil should be going, so that he can diagnose and repair the system. There are two types of circuit diagrams. 1. Cutaway Circuit Diagrams show the internal construction of the components as well as the oil flow paths. By using colors, shades or various patterns in the lines and passages, they are able to show many different conditions of pressure and flow. 2. Schematic Circuit Diagrams are usually preferred for troubleshooting because of their ability to show current and potential system functions. A schematic diagram is made up of consistent geometric symbols for the components and their controls and connections. Schematic symbol systems: I.S.O. = International Standards Organization. A.N.S.I. = American National Standards Institute A.S.A = American Standards Association J.I.C. = Joint Industry Conference 22
  23. 23. A Complete Hydraulic Schematic 23
  24. 24. Different Kinds of Hydraulic ActuatorsA hydraulic actuation system is a drive or transmission system that uses pressurizedhydraulic fluid to drive hydraulic machinery. The term hydrostatic refers to the transfer ofenergy from flow and pressure, not from the kinetic energy of the flow.All hydraulic systems are essentially the same regardless of the application. There are fourbasic components required; a reservoir to hold the fluid; a pump to force the fluid throughthe system; valves to control the flow; and an actuator (motor) to convert the fluid energyinto mechanical force to do the work. 1. Hydraulic JackThe principle behind most hydraulic systems is similar to that of the basic hydraulic jack. Oilfrom the reservoir is drawn past a check ball into the piston type pump during the pistonsup-stroke.When the piston in the pump is pushed downward, oil will be directed past a second checkball into the cylinder. As the pump is actuated up and down, the incoming oil will cause thecylinder ram to extend. The lift cylinder will hold its extended position because the checkball is being seated by the pressure against it from the load side of the cylinder. More liquid 24
  25. 25. is pumped under a large piston to raise it. To lower a load, a third valve (needle valve) opens, which opens an area under a large piston to the reservoir. The load then pushes the piston down and forces the liquid into the reservoir. Because the pump displacement is usually much smaller than the cylinder, each stroke of the pump will move the cylinder a very small amount. If the cylinder is required to move at a faster rate, the surface area of the pump piston must be increased and/or the rate which the pump is actuated must be increased. Oil FLOW gives the cylinder ram its SPEED of movement and oil PRESSURE is the work force that lifts the load.2. Hydraulic brake The hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle. Construction The most common arrangement of hydraulic brakes consists of the following:  Brake pedal or lever  A pushrod (also called an actuating rod)  A master cylinder assembly containing a piston assembly (made up of either one or two pistons, a return spring, a series of gaskets/ O-rings and a fluid reservoir)  Reinforced hydraulic lines  Brake calliper assembly usually consisting of one or two hollow aluminum or chrome-plated steel pistons (called caliper pistons), a set of thermally conductive brake pads and a rotor (also called a brake disc) or drum attached to an axle. The system is usually filled with a glycol-ether based brake fluid (other fluids may also be used). System Operation Within a hydraulic brake system, as the brake pedal is pressed, a pushrod exerts force on the piston(s) in the master cylinder causing fluid from the brake fluid reservoir to flow into a pressure chamber through a compensating port which results in an increase in the pressure of the entire hydraulic system. This forces fluid through the hydraulic lines toward one or more callipers where it acts upon one or two calliper pistons sealed by one or more seated O-rings which prevent the escape of any fluid from around the piston. The brake calliper 25
  26. 26. pistons then apply force to the brake pads. Subsequent release of the brake pedal/ leverallows spring(s) to return the master piston(s) back into position. This relieves the hydraulicpressure on the calliper allowing the brake piston in the calliper assembly to slide back intoits housing and the brake pads to release the rotor. The hydraulic braking system is designedas a closed system: unless there is a leak within the system, none of the brake fluid enters orleaves it, nor does it get consumed through use.Advantages and disadvantages of Hydraulic ActuatorsHydraulic actuators are widely used in many systems such as drives of machine tools, rollingmills, pressing, road and building machines, transport and agricultural machines. There aregreat advantages of hydraulic actuator which differentiate them from mechanical andelectric transfers which explain such their widespread application.Advantages: 1. Power-to-weight ratio: Hydraulic components, because of their high speed and pressure capabilities, can provide high power output with vary small weight and size, say, in comparison to electric system components. It is one of the reasons that hydraulic equipment finds wide usage in aircrafts, where dead-weight must be reduced to a minimum. 2. Infinitely variable control of gear-ratio in a wide range and an opportunity to create the big reduction ratio which can be used in high power appliances for example to open of the gate of a canal or a dam. 26
  27. 27. 3. Tall Condition and Overload Protection: A hydraulic actuator can be stalled without damage when overloaded, and will start up immediately when the load is reduced. The pressure relief valve in a hydraulic system protects it from overload damage. During stall, or when the load pressure exceeds the valve setting, pump delivery is directed to tank with definite limits to torque or force output. The only loss encountered is in terms of pump energy. On the contrary, stalling an electric motor is likely to cause damage. Likewise, engines cannot be stalled without the necessity for restarting. 4. Variable Speed and Direction: Most large electric motors run at adjustable, but constant speeds. It is also the case for engines. The actuator of a hydraulic system, however, can be driven at speeds that vary by large amounts and fast, by varying the pump delivery or using a flow control valve. In addition, a hydraulic actuator can be reversed instantly while in full motion without damage. This is not possible for most other prime moversDisadvantages:It is also necessary to highlight the disadvantages of hydraulic actuators: 1. Efficiency: Efficiency of a volumetric hydraulic actuator is a little bit lower, than efficiency of mechanical and electric transfers, and during regulation it is reduced. 2. Conditions of operation: It’s operational condition influence its characteristics. 3. Hydraulic system is susceptible to contaminations & foreign object damage (FOD). 4. Mishandling and constant exposure to hydraulic fluid and its gas fumes without proper equipment and precautions is a health risk.ConclusionModern robotic systems are difficult. drives are a mechanical part of this systems. Threetypes of drives are basically used now: electric, pneumatic and hydraulic. Each type has itsown advantages and disadvantages.ReferencesArtemieva T.V., Lisenko T.M. Hydraulic, hydromachines and hydropneumoactuator,Moscow, 20054. Majumdar S.R. Oil Hydraulic Systems : Principles and Maintenance, McGraw-Hill, 2001Ian C. Turner, Engineering Applications of Pneumatics and Hydraulics, Butterworth-Heinemann, 1995Appendices. 27
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