15FMCE220 Pneumatic and Hydraulic Systems Part 2 PDF

15FMCE220: Hydraulic systems 1 2  What is fluid power ? Fluid power is the technology that deals with the generation , control ,and transmission of power using pressurized fluids. It can be said that fluid power is the muscle that moves industry....

15FMCE220: Hydraulic systems 1 2  What is fluid power ? Fluid power is the technology that deals with the generation , control ,and transmission of power using pressurized fluids. It can be said that fluid power is the muscle that moves industry. This is because fluid power is used to push , pull ,regulate or drive virtually all the machines of modern industry. Fluid power is subdivided into hydraulics using a liquid such as mineral oil or water, and pneumatics using a gas such as air or other gases 3 On this hydraulic excavator, not only all working movements (linear drives) but also the propulsion of the vehicle (rotary drive) are hydraulically powered. 4 5  Advantages of fluid power 1-Ease and accuracy of control By the use of simple levers and push buttons, the operator of a fluid power system can readily start , stop speed up or slow down and position forces that provides any desired horsepower with displacement tolerance as precise as one ten thousandth of an inch (0.0001 inches) Ref: fluid power with applications 4th edition, Anthony esposito 6  2-Multiplication of force A fluid power system (without using cumbersome gears, pulleys , and levers) can multiply forces simply and efficiently from a fraction of an ounce (28.3495 grams) to several hundreds tons of output (1 ton = 1000 kg). 7  3-Constant force or torque Only fluid power systems are capable of providing constant force or torque at any speeds. The speed can be as small as few inches per hours. 𝑃𝑜𝑤𝑒𝑟 = 𝐹𝑜𝑟𝑐𝑒 × 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑃 = 𝐹. 𝑉 𝑃𝑜𝑤𝑒𝑟 = 𝑇𝑜𝑟𝑞𝑢𝑒 × 𝐴𝑛𝑔𝑢𝑙𝑎𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑃 = 𝑇. 𝜔 8  4-Simplicity, safety, economy In general , fluid power systems use fewer moving parts than comparable mechanical or electrical systems. Thus , they are simpler to maintain and operate. This, in turn, maximize safety, compactness, and reliability. 9  The choice of using electric, hydraulic, or pneumatic motion systems is a fundamental decision that affects performance, costs, maintainability, safety, ease of use, flexibility, and reliability. The needs of the application and the capabilities of the technology must be carefully evaluated—and then balanced with a cost-benefit analysis. 10  Small note before the comparison As systems, neither one is better than the other. There is no "Better", only "More suitable for a specific purpose." Car tyres are Pneumatic... they're full of air. Air is compressible, which is why it's used. If you filled your car tyres with Hydraulic fluid (which is incompressible) you would not only increase the weight of the tyres dramatically (and hence increase the relative tread-wear), but also make them hard, decreasing their ability to absorb shock. 11  Power transmission If a force F1 is applied to an area A1 of a liquid, a pressure p=p1 (=p2) results. If, as in this case, the pressure acts on a larger surface A2, then a larger counter-force F2 must be maintained. If A2 is three times as large as A1, then F2 will also be three times as large as F1. Hydraulic power transmission is comparable to the mechanical law of 12 levers.  Power transmission 𝐹1. 𝑠1 = 𝐹2. 𝑠2 13 Check calculation in classroom note Components for the hydraulic system Pump Actuator: linear or angular Lines Pressure relief valve Directional valves Flow control valve Pressure control valve Tank Reservoir Filters Heat exchanger 14  Structure of a hydraulic system This simplified block diagram shows the division of hydraulic systems into a signal control section and a hydraulic power section. This signal control section is used to activate the valves in the power control section. 15  Hydraulic power section The diagram of the hydraulic power section is complemented in this case by a circuit diagram to allow correlation of the various function groups; the power supply section contains the hydraulic pump and drive motor and the components for the preparation of the hydraulic fluid. The power control section consists of the various valves used to provide control and regulate the flow rate, pressure and direction of the hydraulic fluid. The drive section consists of cylinders or hydraulic motors, depending on the application in question. 16 Hydraulic Actuators 17  End position cushioning (Animation) The illustration shows first the advance of the piston rod from a mid-position to the forward end position, with cushioning at the end of the advance movement. The non-return valve is open during the return stroke. The animation shows the opening of the pressure limiter after a certain pressure has been built up on the outlet side by the cushioning piston. 18  End position cushioning The piston is a short distance before its end position; the hydraulic fluid on the piston-rod side must escape via the adjustable flow control valve above the piston rod This type of end position cushioning is used for stroke speed between 6 m/min and 20 m/min. At higher speed, additional cushioning or braking devices must be used. 19 Telescopic cylinder This type contains multiple cylinders that slides inside each other. They are used where long work strokes are required. 20 Hydraulic motors 1- Continues rotation 2- Limited rotation 21  Hydraulic Motor Single acting Double acting cylinders cylinders  Hydraulic power unit The hydraulic power unit (power supply unit) provides the energy required for the hydraulic installation. Its most important components are: ✓ the reservoir (tank) ✓ drive (electric motor) ✓ hydraulic pump ✓ pressure relief valve (safety valve) ✓ filter and cooler. 23  Hydraulic power unit: Reservoir The hydraulic reservoir contains the hydraulic fluid required the operate the installation. The size of the reservoir will depend on the practical application involved; for stationary systems, the volume of fluid delivered by the pump in 3 to 5 minutes can be taken as a guide. 24  Externally toothed gear pump Develops flow by carrying fluid between the teeth of two meshing gears. One of the gears is connected to a drive shaft connected to the prime mover. The second gear is driven as it meshes with the drive gear. 25  Internally toothed gear pump The inner gear is driven by a motor. The teeth of the inner wheel drive the outer gear wheel. The rotary motion creates a vacuum in the gaps between the teeth, causing hydraulic fluid to be sucked in. On the other side, the teeth engage once more and oil is displaced from the tooth chambers. The design can deliver pressures of up to approx. 175 bar. 26 27  Circuit diagram: Return flow filter An oil filter situated in the return line to the tank has the advantage that the filter is thus easy to maintain. A disadvantage, however, is that contamination is removed from the hydraulic fluid only after it has passed through the hydraulic components. This configuration is often used. 28  Circuit diagram : Pump inlet filter With this configuration, the pump is protected from contamination. The filter is, on the other hand, less easily accessible. If these filters have a too fine mesh, suction problems and cavitation effects may occur. Additional coarse filters upstream of the pump are recommended. 29 Circuit diagram: Pressure line filter Pressure filters can be installed selectively upstream of valves which are sensitive to contamination; this also enables smaller mesh sizes to be used. A pressure-resistant housing is required, which makes this configuration more expensive. 30  Circuit diagram: Contamination indicator It is important that the effectiveness of a filter can be checked by a contamination indicator. The contamination of a filter is measured by the pressure drop; as the contamination increases, the pressure upstream of the filter increases. The pressure acts on a spring- loaded piston. As the pressure increases, the piston is pushed against a spring. There are a number of different display methods. Either the piston movement is directly visible or it is converted into an electrical or visual indication by electrical contacts. 31  Circuit diagram: Hydraulic power unit The illustration shows the detailed circuit symbol for a hydraulic power unit. Since this is a combination unit, a dot/dash line is placed around the symbols representing the individual units. 32 33  Properties of hydraulic oil The single most important material in a hydraulic fluid itself. Hydraulic fluid characteristics have a crucial effect on equipment performance and life. Essentially, hydraulic fluid has four primary functions: 1) To transmit power 2) To lubricate moving parts 3) To seal clearness between mating parts 4) To dissipate heat 34 To accomplish properly these primary functions and be practical from a safety and cost point of view, a hydraulic fluid should have the following properties: ✓ Good lubricity ✓ Ideal viscosity ✓ Chemical and environmental stability ✓ Compatibility with system materials This is a challenging list ✓ Large bulk modulus , and no single hydraulic ✓ Fire resistance fluid possesses all of ✓ Good heat transfer capability these desirable characteristics. The fluid ✓ Low density power designer must ✓ Nontoxic select the fluid that ✓ Foam resistance comes the closest to ✓ Inexpensive being ideal overall for a ✓ Readily available particulate application 35  Pressure relief valve In this design incorporating a poppet valve, a seal is pressed against the inlet port P by a pressure spring when the valve is in its normal position. In this situation, for example, an unloaded piston rod is executing an advance stroke and the entire pump delivery is Check Design pressure flowing to the cylinder. calculation note for PRV in classroom 36  PRV used to limit system pressure This illustration shows a pressure relief valve within a basic hydraulic circuit (used to control a double acting cylinder). 37  Two-way pressure regulator This valve is normally open. The outlet pressure (A) acts via a pilot line on the left- hand surface of the pilot piston against an adjustable spring force. Pressure regulators reduce the inlet pressure to an adjustable outlet pressure. It is appropriate to use these in hydraulic installations only if different pressures are required. 38  Two way pressure regulator In the circuit illustrated, the piston rod of the cylinder is executing an advance stroke. The pressure at the outlet A of the pressure regulator is less than the system pressure at P and constant. 39 Directional Control Valves 40  2/2-way valve The 2/2-way valve has a working port A, a supply port P and a leakage-oil port L. In the case of the valve shown here, of slide design, flow from P to A is closed in the normal position. A relief line leading to the leakage-oil port is provided to prevent a build-up of pressure in the spring and piston chambers. 41  2/2-way valve as by-pass valve This example shows a 2/2-way valve used as a by-pass valve; when the 2/2-way valve is actuated, the flow control valve 0V3 is by- passed, causing the piston rod of the cylinder to advance at maximum speed. 42  Circuit diagram: 2/2-way valve as final control element In its initial position, the cylinder is advanced. If the 2/2- way valve 0V1 is actuated, the entire volumetric flow passes to the tank and piston rod of the cylinder is reset by the external load m. If 0V1 is not actuated, the system pressure set on the pressure limiter 0V2 builds up and the piston rod advances. In the initial position, the pump operates against the preset system pressure, which has an unfavorable effect on the power balance of the circuit shown. 43  3/2-way valve The 3/2-way valve has working port A, a supply port P and a tank port T. Volumetric flow can be routed from the supply port to the working port or from the working port to the tank port. The third port in each case is closed. In the normal position shown, P is closed and flow released from A to T. 44  Circuit diagram: 3/2-way valve as final control element The illustration shows the circuit of 3/2 valve final control. 45  3/2-way valves as diverter In addition to their application as final control elements, 3/2- way valves can also be used as diverters. In this case, port T is connected to a further device, to which a switch-over can then be made. The part circuit diagrams show the facility to switch between the flow control valves with different settings and between heating and cooling. The circuit symbol is drawn reversed to simplify the representation of the circuit diagram. 46  4/2-way valve The 4/2-way valve has two working ports A and B, a supply port P and a tank port T. The supply port is always connected to one of the working ports, while the second working port is routed to the tank. In the normal position, there is flow from P to B and from A to T. In contrast to valves with three pistons, 4/2-way valves with two pistons do not require a leakage-oil port (see topic 74). 47  4/2-way valve The 4/2-way valve is actuated, and there is flow from P to A and from B to T. 4/2-way valves are also available which are normally open from P to A and from B to T. 48  Circuit diagram: 4/2-way valve The illustration shows the same circuit as the previous illustration, but with the 4/2-way valve as a circuit symbol. 49  Circuit diagram: 4/3-way valve with pump bypass The illustration shows the same circuit as the previous illustration, but with the 4/3-way valve as a circuit symbol. 50  4/3-way valve with closed mid- position The circuit shows the 4/3-way valve in functional representation as a final control element of a double acting cylinder. The valve is in its mid- position; the pump is operating against the system pressure set on the PRV. 51 Actuating Methods: Manually Operated Mechanically Operated Hydraulically or Pneumatic Operated Electrical (Solenoid) Operated Combined 52 53 54 55  Non-return valve Non-return valves block flow in one direction and allow free flow in the other. In the direction of flow shown, the sealing element is pressed against a seat by a spring and the hydraulic fluid. These valves are also available in designs without springs. Since there must be no leaks in the closed position, these valves are generally of poppet design. 56  Circuit diagram: Pump protection In this circuit, the non- return valve is used to protect the pump. This prevents a load pressure from driving the pump in reverse when the electric motor is switched off. Pressure peaks do not affect the pump but are discharged via the pressure relief valve. 57  Pump Protection using Non-return valve 58 59 Flow Control Valve 60  One way Flow control Valve 61 Control the cylinder speed in both directions 62 It may required high force to actuate the directional control valve. For that a manual switch (similar to the directional control valves) have been used to actuate the directional control valve ,with less force 63  OR / Shuttle Valve Activated when any of its sides activated 64  AND/Dual Pressure Line Activated when both of its sides activated 65 66 Other applications 67  Circuit without brake valve (Animation) One application of pressure relief valves is as brake valves; these prevent pressure peaks which may otherwise occur as the result of mass moments of inertia when a directional control valve is suddenly closed. The animation shows an (incorrect) circuit in schematic form in which the working line on the exhaust side has fractured due to the absence of a brake valve. The next animation shows the correct circuit. 68 Circuit without brake valve (Animation) 69 Circuit without brake valve (Animation) 70 Circuit without brake valve (Animation) 71 Circuit without brake valve (Animation) 72  Circuit diagram: Brake valve This circuit incorporates not only a brake valve on the piston-rod side but also a non-return valve on the inlet side via which oil can be taken in from a reservoir during the vacuum phase following the closure of the directional control valve. The following animation shows the events which occur in the two working lines. 73  Circuit with brake valve (Animation) The necessity of the brake valve can be demonstrated by the preceding animation. 74 Circuit with brake valve (Animation) 75 Circuit with brake valve (Animation) 76 Circuit with brake valve (Both directions) Counter Balance Valve Application 77  Unloading Valve Unloading valves are very similar to pilot operated, balanced relief valves. The only difference is where the pilot pressure comes from. 78  Unloading Valve In unloading valves when external pressure is applied to Port X that is greater than the pilot setting, the pilot poppet will crack open, allowing the main poppet to open. System flow will then unload from Port P to Port T. 79  Unloading Valve The unloading valve is put to good use in a system where a high flow volume is needed at a lower pressure, and then later a low flow volume is required with a higher pressure. Unloading valves are typically used in high- low circuits 80  Unloading Valve At the beginning of the outstroke, the pump needs low pressure to compress the undeformed spring. Once the spring is partially deformed, the pump needs higher pressure to push the piston forward 81  Unloading Valve Typical pump provides High pressure at low flowrate or low pressure at high flowrate. For High pressure high flowrate, the pump becomes very expensive 82 Unloading Valve 83  Unloading Valve Stage 1 Stage 2 Stage 3 84 Sequence Valve 85