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INTRODUCTION AND BACKGROUND 1.What are the main functions of the fluid in Hydraulic Power Transmission Systems? Mainly, hydraulic fluids are used in hydrostatic power systems to transmit power. In addition to the power transmission, the hydraulic fluids also serve to lubricate the contact surfaces, cool different elements, and clean the system. 2.Identify the main requirements for fluids used in Hydraulic Power Transmission Systems. -Satisfactory flow properties throughout the entire range of operating temperatures.-A high viscosity index that ensures moderate viscosity variation in relation to the temperature fluctuations.-Good lubricating properties to reduce the wear and increase the service life of the system.-Low vapour pressure to avoid cavitation.-Compatibility with system materials since the fluid should not react chemically with any of the used materials or deteriorate their physical properties.-Chemical stability to increase the service life of liquid and avoid performance deterioration.-Corrosion protection by adding effective corrosion inhibitors.-Rapid de-aeration and air separation.-Good thermal conductivity to rapidly dissipate the heat generated due to friction between elements and due to hydraulic losses.-Fire resistance is essential in some applications.-Electrically insulating properties can be significant in a number of modern designs.-Environmental acceptability. 3.Make a list with the main properties of fluids -Viscosity-Oil density-Oil compressibility-Vapour pressure-Lubrication and anti-wear characteristics-Compatibility-Chemical stability-Oxidation stability. -Foaming. -Cleanliness. -Thermal properties. -Acidity. -Toxicity. -Environmental acceptability 4.Analyze and describe briefly the influence that viscosity and compressibility have on the performance of hydraulic power transmission systems? -Viscosity:-Hydraulic losses in transmission lines. -Resistance to fluid flow in narrow conduits. -Viscous friction forces and damping effect. -Compressibility:-Hydraulic capacitance 5.Identify the typically used fluids in Hydraulic Power Transmission systems. What advantages/disadvantages has each type of fluid? Mineral oils. Advantages: Inexpensive, widely available, great range of viscosity, good lubricity, non-corrosive, compatible with most sealing materials, chemically stable for reasonable operating temperatures. Disadvantages: Non-compatible with butyl rubber sealings, chemical breakdown at high temperatures, flammability, increase of viscosity at high pressures. Oil-in-Water emulsion Advantages: Fire resistant, highly incompressible, good cooling properties. Disadvantages: Poor lubricity and low viscosity. Water-in-Oil emulsion Advantages: Fire resistant. Disadvantages: Reduced lubrication properties, reduced fire-resistant properties at high temperatures. Water-glycol fluidAdvantages: Very low flammability, very stable respect to shear, good anti-freeze properties. Disadvantages: Cannot be used at high temperatures, they attack most paints.Synthetic oils Advantages: Compared to mineral oils, they are better in thermal stability, oxidation stability, viscosity-temperature properties, low temperature fluidity, operational temperature limits and fire resistance. Disadvantages: Compared to mineral oils, they are worse in hydrolytic stability, corrosion protection, toxicity, compatibility with elastomers and construction materials, the solubility of additives, frictional characteristics, cost and availability. 6.Which basic properties of fluids can be improved by additives? -Chemical stability. -Oxidation stability. -Foaming. -Lubrication and wear. -Viscosity index. Cleanliness

HYDRAULIC ACTUATORS 1.Describe the function of hydraulic actuators in Power Transmission Systems. Hydraulic actuators are used to drive loads by converting the hydraulic power into mechanical power. The mechanical power delivered to the load is managed by controlling the fluid pressure and flow rate, by using various hydraulic control valves. 2.What criteria you can use to classify the hydraulic actuators? The hydraulic actuators are classified depending on the motion type. 3.Draw a sketch showing all types of actuators used in hydraulic systems. These are the different types of actuators used in hydraulic systems: –Hydraulic cylinders, which perform a linear motion. –Hydraulic motors, which perform a continuous rotary motion. –Hydraulic rotary actuators, which perform a limited angular displacement. 4.Which are the main parts of hydraulic cylinders? A hydraulic cylinder consists mainly of a piston, a piston rod, cylinder barrel, cylinder head, and cylinder cap. 5.What specifications are needed to define the size of hydraulic cylinder? The size of a hydraulic cylinder is determined considering the force that is needed and the pressure available. The following relations are used: (1, 2) 6.What is a cushioning system in hydraulic cylinders? In what kind of application do you require it? A cushioning arrangement is used to reduce the piston speed to a limiting value before reaching its end position. The cushion dissipates the kinetic energy of the moving parts. This arrangement is necessary in high speed and/or great inertia applications, because the sudden stopping of the piston can result in a severe impact force (which is proportional to the mass and the square of the velocity of the moving parts). This impact affects both the cylinder and the driven mechanism, and in order to avoid it, a cushioning arrangement is used.7.Identify and describe briefly the process and criteria of sizing hydraulic cylinders. In order to size the hydraulic cylinder, the load to be moved and the hydraulic operating system pressure that is available have to be taken into account. Dividing the force by the pressure, the area is obtained, and with the area, the size of the rod can be determined. System losses and inefficiencies have also to be considered.(3)In high speed or great inertia operations, a cushioning system is needed. The cushioning should produce a controlled deceleration of the cylinder, near one or both end positions. This is done by creating a decelerating pressure force. For an ideal operation, the piston will be fully stopped at the end of the cushioning stroke. When the cylinder is installed horizontally, the decelerating (cushioning) force can be calculated: (4)Apart from that, the maximum axial load acting on the hydraulic cylinder must not exceed the limit at which buckling takes place. The limiting load, for buckling, is calculated as follows: (5)Finally, the minimum length of the hydraulic cylinder can be calculated this way: (6, 7) 8.What types of rotary actuators are commonly used in hydraulic systems? –Rotary Actuator with Rack and Pinion Drive–Parallel Piston Rotary Actuator–Vane-Type Rotary Actuators 9.What is the difference between, rotary actuators and hydraulic motors? Hydraulic motors perform continuous rotary motion, while rotary actuators perform a limited angular displacement. 10.What types of hydraulic motors are used in hydraulic systems? –Bent-Axis Axial Piston Motors–Swash Plate Axial Piston Motors–Vane Motors–Gear Motors Which specifications are needed to define a hydraulic motor? The motor speed depends on the flow rate, while the supply pressure depends mainly on the motor loading torque. In the case of an ideal motor with no leakage and no friction, the following relations are used: (8,9)

CONTROL VALVES 1.Discuss briefly the control of power in the hydraulic power systems.The control of hydraulic power in hydraulic power systems is carried out by means of control valves. The control requirements are imposed by the function of the system. The parameters of the mechanical power delivered to the load are managed hydraulically by controlling the pressure, flow rate, or the direction of flow.2.Draw in a sketch, the classification of valves that are used in hydraulic power transmission systems. The control valves are classified into the following main categories:-Ordinary switching valves-Proportional valves-Servovalves-Digital valvesWe are going to focus on the ordinary switching valves. These are the different types:-Pressure control valves (PCVs).(Relief valves (direct- and pilot-operated),Pressure-reducing valves (direct- and pilot-operated),Sequence valves (direct- and pilot-operated),Accumulator charging valves) -Directional control valves (DCVs; direct- and pilot-operated)-Flow control valves (FCVs) (Throttle valve,Series pressure-compensated FCV,Parallel pressure-compensated flow control valves,Flow dividers)-Check valves (Direct-operated check valves,Pilot-operated check valves (hydraulically or mechanically piloted)). DIRECTIONAL CONTROL VALVES 1.Explain briefly the function of the directional control valves. Directional control valves (DCVs) are used to start, stop, or change the direction of fluid flow. The control positions determine the way in which the lines are interconnected, and consequently the directions of fluid flow.2.How do we specify the type and the size of directional control valves? These valves are specified by the number of connected lines (ways) and the number of control positions. For example, A 4/3 DCV has four ways and three positions. There are two main types of directional control valves: spool type and poppet type. 3.Draw a schematic of a 4/3 DCV that is direct-operated electrically, and briefly explain its function. There are 3 positions and 4 doors. When the valve is normally in the neutral position. When the solenoid a is activated, the position of cross arrows is working. Finally, when the solenoid b is activated the parallel arrows are working. It will be returned to the neutral position by springs. 4.Why do we need to use pilot-operated directional control valves? The pilot operated directional control valves are used when the flow rate of the system is very high. In those cases, if we use a direct-operated valve, the solenoid will have to make a very high force, and the consumption will be very high. Therefore, we use the pilot operated ones to avoid the high consumption. 5.Draw schematically and explain the function of a 4/3 pilot-operated directional control valve. In order to understand its functioning, we are going to use the next figure:The pilot-operated directional control valve consists of two stages: a pilot valve and a main valve. 

The pilot valve is a direct-operated directional control valve, controlled electrically in this example. The valve is supplied by the high-pressure oil from the main valve high-pressure port (P). The oil is also drained through the main valve return line (T). With the pilot valve spool in the neutral position, the control chambers (C and D) are drained to the tank and the main spool is put in its neutral position. When the solenoid (a) is energized, the pilot valve spool moves to the right. The high-pressure oil reaches the chamber (D) and the main spool moves to the left. In the same way, the connection of electric power to the solenoid (b) results in a motion of the main spool to the right. The supply and return lines of the pilot stage are mostly connected to the main stage supply and return lines. However, optionally, the valve may include an arrangement for switching any of them to external ports. 6.List the different ways of control of directional control valves. There are a lot of different ways to control DCVs: mechanically (by a hand lever, by cam and roller, by a rotary knob), hydraulically, pneumatically, using solenoids… 7.Discuss the pressure losses in directional spool valves. Flow rates through the spool valve restrictions, Q, and the pressure drop across it, ΔP, are related by the following expressions:The loss of power, ΔN, due to oil flow through the valve restriction is given by the following relations:It is important to reduce the pressure and power losses in the valves. Therefore, the restriction area, A_v, should be increased as much as possible. The manufacturers of DCVs offer the characteristic chart, like the one shown in Fig. 5.40. This chart gives the pressure-flow relation for the directional control valves. Generally, the pressure losses are reduced by increasing the spool diameter and stroke. These requirements are in contradiction to the requirements of minimum dimensions and weight. Moreover, the increase in the diameter increases the mass, decreases the valve natural frequency, and increases the response time. Therefore, for small flow rates, the spool dimensions may be minimized, while the increase in flow rate imposes the necessity for dimensions to increase. 8.Cite and discuss the criteria to choose a proper size of directional control valves? In order to choose a correct size of the directional control valves, we have to take into account the pressure losses. In a good design, the pressure drops between P-B and A-T have to be lower than 1.5-2 bar. Therefore, when we choose a valve, we have to check the tables like the one shown in Figure 5.40 and see if the pressure drops are lower than 2 bars for our conditions. If they are higher than 2 bar, we have to choose a bigger valve, and check the tables again, until we find one that fulfils our specification.