Kon HYDRAULICS AND PNEUMATICS Typical exam questions and problems in Hydraulics

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1 Kon HYDRULICS ND NEUMICS ypical exam questions and problems in Hydraulics asic hydraulic phenomena, fluid, flow, pressure, etc. X. List the advantages and drawbacks of hydraulics. X. ipe flow. Describe the characteristics of fluid flow in pipe (flow types and losses) and the factors that affect to these characteristics. X. nswer to the following questions (short and compact answers are preferred) a) How does the flow type affect on the pressure or power losses appearing in fluid power systems? b) What is meant by the term viscosity and what effects does the viscosity have on the operation of a fluid power system? X. What is meant with the term viscosity and what effects does the viscosity have on the operation of a hydraulic system? X. Which are the effects of fluid viscosity on the characteristics and operation of a hydraulic system? Use graphs to illustrate the effects. X. What are the functions of hydraulic fluid in hydraulic system? What kind of fluids (types) are used in hydraulic systems? X. Describe the following phenomena (what is meant by these terms, what the phenomena depend on and what kind of effects they have on the fluid power system) a) cavitation b) pressure shock (aka pressure surge or water hammer) Cylinder, pump, motor X. What is a differential coupling for a cylinder? What is it used for and which are its advantages and disadvantages? Use illustrations to clarify your statement. X. Describe the factors that affect the total efficiency of hydraulic pump/motor. Describe also on which parameters these factors are dependent. Use diagrams to support your answer. Valves X. What are the advantages of pilot operated valves over direct controlled valves? Describe the principle of pilot operation and the achieved advantages by using either pressure control valve or directional control valve as an example case. X. For what purposes are the pressure valves used for? Describe the functions and applications of pressure valves in brief and in addition represent one valve (of your choice) and its application more deeply (draw illustration of valve and of hydraulic circuit). X. Explain the differences between a direct operated and a pilot operated pressure relief valve and how the differences occur in a fluid power system s operation. X. Describe the operation of a pilot operated relief valve. Use illustrations (cross-section of the valve and valve symbol) to clarify your statement. X. Describe the structure and operation of pilot operated pressure reducing valve. Draw the symbol of the valve and a schematic picture of the valve structure to support your answer. X. Describe the operation of a pilot operated unloading valve. Use illustrations (cross-section of the valve and valve symbol) to clarify your statement 1

2 X. Describe the structure and operation of pilot operated pressure reducing valve. Draw the symbol of the valve and a schematic picture of the valve structure to support your answer. X. Explain the function principle of a pilot operated unloading valve. lso draw the symbol and a simplified structure graph of the valve. X. Flow valves. Describe the operational characteristics of flow valves of different types (description of internal structures are not required). lso represent all the possible ways to control the speed of a double acting cylinder in one direction (draw circuit diagrams) and describe the characteristics of these in different load conditions. X. Describe the structure of flow control valve (draw a principal figure), describe how the valve operates and to which functions/purposes it is used for. What types of control valves exists? X. Describe the operation of a 3-way flow control valve (aka bypass flow control valve). Use illustrations (crosssection of the valve and valve symbol) to clarify your statement. X. Describe what are the means for controlling the speed of a hydraulic cylinder with a single flow control valve. How does the loading affect to the implementation? Draw a basic hydraulic circuit for each solution that you can think of. X. For what purposes are the pressure valves used for? Describe the functions and applications of pressure valves in brief and in addition represent one valve (of your choice) and its application more deeply (draw illustration of valve and of hydraulic circuit). Filtering X. Describe what are the sources of impurities in hydraulic systems and what aspects should be taken into account when a filtration system is designed into hydraulic system and the filtration rating is selected. X. From where do the impurities or contaminants of hydraulic systems originate? What kind of impurities does the system contain and what effects different kinds of impurities have on the system? X. nswer to the following questions a) From where do the impurities or contaminants of hydraulic systems originate? b) What is meant by the term Cleanliness class (what does it represent?) and how it is defined? c) List at least four different filter types X. Describe the use of filters in hydraulic systems. Mention at least three different filters and describe where to place these filters in hydraulic system (draw an example circuit where to place the filters). X. Enumerate the filter types used in hydraulic systems and describe the meaning of filtration to the operation of a hydraulic system. X. Filtration. Why filtration is needed, how the placement (location) of filters will affect the performance (efficiency) of filtration and how the filtration capacity is expressed (3 ways)? ressure accumulator X. Enumerate the operations that a hydraulic accumulator can be used to in a hydraulic system. X. Describe the structures of pressure accumulators and their applications in fluid power systems (draw three application examples in form of fluid power circuits with accompanying description of operations). 2

3 Reservoir X. What are the functions of a hydraulic tank? Describe these functions and the features that affect the capability of a tank to perform these functions. Calculations X. he hydraulic system shown in Fig. 1 is used to power up the newest twirler of a amusement park. he shaft of the twirler is connected to the shaft of hydraulic motor M2. Calculate a) the maximum achievable rotational speed of motor M2 b) to which level the setting pressure of the pressure relief valve (RV) should be adjusted in order to achieve the highest possible value of rotational speed of M2 when the torque load of the motor M2 is 2 Nm c) what is the total efficiency of the system (calculated with the shaft power needed at the pump and the shaft power gained at the motor M2) ump 1: V k,p1 = 1 cm 3 /r, hm,p1 =.93, v,p1 =.97. ump 2: V k,p2 = 2 cm 3 /r, hm,p2 =.94, v,p2 =.95. Hydraulic motor M1: V k,m1 = 75 cm 3 /r, hm,m1 =.96, v,m1 =.92. Hydraulic motor M2: V k,m2 = 5 cm 3 /r, hm,m2 =.98, v,m2 =.9. he leakage of motor M1 is totally external, the leakages of other components are totally internal. he speed of the electric motor that drives the pump is n p1 = 15 r/min. Characteristic curves for the directional valve are shown in ppendix 1. = 86 kg/m 3, C q =,7. M1 2 M2 M 1 RV 3

4 Kon Fluid power and pneumatics ppendix 1 p [bar] Flow Q / [l/min] ressure loss 4

5 X. In a system shown in Fig. 1 a hydraulic motor drives a condensation water pump, whose flow is regulated with a throttle valve. he throttle is subcircular in cross-section and the orifice diameter is adjustable over range d klv = 5-3 mm (corresponding condensation water flow is q lv,min - q lv,max ). (Lauhdutin = Condenser) Define to which level the setting pressure of the pressure relief valve is to be adjusted in order to achieve the highest possible value of q lv,max. System pump: V k,p = 2 cm 3 /r, v,p =.9 and hm,p =.85, the speed of the electric motor that drives the pump is n sm = 15 r/min. Hydraulic motor: V k,m = 125 cm 3 /r, v,m =.8 and hm,m =.95, the leakage of the motor is totally external. Condensation water pump: V k,lv = 12 cm 3 /r, v,lv =.87 and hm,lv =.7. Characteristic curves for the directional valve are shown in ppendix 1. he pressure loss of the condenser is insignificant. C q =.7 ja water = 1 kg/m 3 d klv Lauhdutin L C 5

6 Kon Fluid power and pneumatics ppendix 1 p [bar] Flow Q / [l/min] 4 ressure loss 6

7 X. he circuit in the figure 1 drives two hydraulic motors which rotates cutting blades. he loading torque of the motor 2 M 2 = 7 Nm is constant, but the loading torque of the motor 1 M 1 varies according the diagram below. he rotational speeds of the motors should be equal in the time period t 1 - t 2. Calculate how big the diameter of the orifice (throttle) must be so that the speed requirement (demand) will be carried out. he orifice cross-section is circular. t the time t 3 the loading torque of the motor 1 will achieve its maximum value. Calculate the rotational speeds of the motors in the time period t 3 - t 4, when the adjustment (setting) of the orifice will be held at the above calculated value. Calculate also the power needed at the pump shaft in the system, when the load is at its maximum. Hydraulic motors: V k,m1 = 14 cm 3 /r, hm,m1 =.95, v,m1 =.92. ump: V k,p = 1 cm 3 /r, hm,p =.94, v,p =.91. V k,m2 = 6 cm 3 /r, hm,m2 =.97, v,m2 =.9. Rotational speed of the electric motor driving the pump n p = 15 r/min. = 86 kg/m 3, C q =.7. V km1 V km2 M 1 [Nm] V kp t t 1 t 2 t 3 t 4 t 5 X. he cylinder in the system shown in Figure 1 should extend at a speed of.5 m/s (in the direction of the arrow marked with v). Calculate the force F that can be produced during the movement phase. he pump produces constant flow of 5 l/min. he cylinder is assumed to be leak free and frictionless. 1 =.3 m 2, 2 =.1 m 2, d k = 2 mm, C q =.7 and = 86 kg/m 3. 1 v 2 F d k 7

8 X. he circuit in Figure 1 is used for lifting three separate masses. Working cycle of the system is as follows: t the outset all masses are in down positions and the directional control valves are in their center positions. he mass m 1 is lifted first with cylinder S1 by switching the directional control valve SV1 into corresponding position. When the mass m 1 has reached its top position the valve SV1 is returned to center position. hen the masses m 2 and m 3 are lifted to their top positions with cylinders S2 and S3 by switching the directional control valve SV2 into corresponding position. fter this the valve SV2 is returned to center position. Calculate the minimum setting pressure of the pressure relief valve that is needed for these operations to be fulfilled. he losses that are to be taken into account in calculations are the pressure losses in directional control valves and the losses of cylinders. Masses: m 1 = m 2 = 8 kg and m 3 = 13 kg. Cylinders: piston d 1 = 1 mm, piston rod d 2 = 7 mm, total efficiency t =.95, cylinders can be considered leak free. Characteristic curves for the directional valve are shown in Liite 1 (= ppendix 1, ilavuusvirta = Flow, ainehäviö = ressure loss) [Use ppendix shown at page 1, ignore --curve] S1 S2 m 2 m 3 S3 m 1 SV1 SV2 L C 6 l/min X. he system in Fig. 1 is used in studying the division of flows in hydraulics systems. ll the throttles are subcircular in cross-section and all their characteristics are identical except the diameters. he throttles 1, 2, 4 are of fixed size: d k1 = 4 mm, d k2 = 8 mm ja d k4 = 5 mm. Define what the diameter setting of throttle 3 should be to achieve the situation where the pump flow is divided into two pipe flows of equal size at the pipe branching (marked in figure as utkiristeys )? d k2 utkiristeys d k1 d k3 d k4 8

9 X. he system shown in Figure 1 is used to drive two conveyors. Usually the speeds of these conveyors are allowed to be different, but in this case their speeds have to be identical. he directional control valve is in this situation switched in the position. Calculate: a) Into which diameter value d k the adjustable throttle valve should be adjusted in order to achieve the acquired state (he orifice of the throttle is subcircular in cross-section.) b) he speed of the motors at this state c) he required shaft power of the pump at this state Hydraulic motor 1: V k,m1 = 125 cm 3 /r, efficiencies hm,m1 =.96 and v,1 =.88, load torque M m1 = 19 Nm. Hydraulic motor 2: V km2 = 5 cm 3 /r, efficiencies hm,m2 =.91 and v,m2 =.92, load torque M m2 = 6 Nm. ump: V k,p = 7 cm 3 /r, efficiencies hm,p =.94 and v,p =.95. Speed of the electric motor driving the pump n sm = 15 r/min. he opening pressure of the pressure relief valve is adjusted to value of 15 bar. For the throttle valve C q =.7. Density of fluid = 86 kg/m 3. he characteristic curves for the directional control valve are in ppendix

10 Kon Introduction to fluid power ppendix 1 p [bar] Flow Q / [l/min] ressure loss 1

11 X. he system in Fig. 1 is used for lifting masses. Lift function is realized with hydraulic motors with wire cable drums attached to them. he lift phase starts when the spool of the directional control valve is shifted to the left position. Describe the function of the system at lifting phase and calculate the lifting speeds of the masses. lso calculate the pressures at pressure gauge and the needed pump shaft powers during lifting phase and also during the phase when the masses are held in upper positions i.e. the spool of the directional control valve is in the center position. he leaks of the motors are fully external. Characteristic curves of directional control valve are shown in ppendix 1 (ppendix 1 = Liite 1; ilavuusvirta = Flow, ainehäviö = ressure loss) echnical data for motors, loads and wire drums: V k,m1 = 16 cm 3 /r, v,m1 =.9 and hm,m1 =.85, m 1 = 5 kg, d 1 =.5 m. V k,m2 = 26 cm 3 /r, v,m2 =.9 and hm,m2 =.85, m 2 = 15 kg, d 2 = 1. m. ump: V k,p = 6 cm 3 /r, v,p =.95 and hm,p =.9. he speed of the electric motor driving the pump n sm = 1 r/min. V km1 d 1 m 1 V km2 d 2 m 2 29 bar 11

12 Kon Hydrauliikka ja pneumatiikka Liite 1 p [bar] ilavuusvirta Q / [l/min] ainehäviö 12

13 X. he cross-sections of the throttle valves in the system of Fig. 1 are subcircular ( d k1, d k2, d k3 and d k4 ). he sizes of d k1 = 4 mm, d k2 = 8 mm and d k4 = 5 mm. Calculate what should the diameter of d k3 to be in order to achieve a situation where the flow in the pipe junction (= utkiristeys) divides into two components of equal size when the directional control valve is switched to another position and the piston of the cylinder travels outwards. he losses of flow meters, directional control valve and cylinder are assumed to be insignificant. he diameter of the piston of the cylinder is d 1 = 63 mm and the diameter of piston rod is d 2 = 3 mm. d k2 utkiristeys d k1 d k3 d k4 X. Calculate the flow through the orifice 3 ( d k3 ) of Figure 1 in a situation where the flow through the pressure relief valve is 25 l/min. he diameters of subcircular orifices are d k1 = 2 mm, d k2 = 3 mm, d k3 = 1 mm, d k4 = 1.5 mm. C q =.6 and = 86 kg/m 3. he characteristic curve of pressure relief valve is shown below. p [bar] 2 d k2 d k1 d k3 d k Q[l/min] 13

14 X. he system in Figure 1 is used to drive a cylinder and a hydraulic motor connected in parallel. When the cylinder is totally extended (in outmost end-position) and stopped, the hydraulic motor that is loaded with constant torque load of 2 Nm should rotate at speed of 5 r/min. he rotational speed is adjusted with adjustable throttle valve. Calculate into which diameter value the adjustable throttle valve should be adjusted in order to achieve the acquired state (he orifice of the throttle is subcircular in cross-section). fter the adjustment of the throttle valve the cylinder is retracted (to inmost end-position). hen the directional valve is switched to position that starts the outward motion of cylinder. Calculate the speed of the cylinder and the speed of the hydraulic motor while the cylinder moves. Hydraulic motor: V k,m = 8 cm 3 /r, hm,m =.96, v,m =.92. ump: V k,p = 45 cm 3 /r, hm,p =.9, v,p =.97. he speed of the electric motor driving the pump n p = 15 r/min. Cylinder: piston d m = 63 mm, piston rod d mv = 45 mm, hm,s =.95, v,s = 1., load mass m = 3 kg. = 86 kg/m 3, C q =.611. Characteristics of directional valve is shown in ppendix 1. [Use ppendix shown at page 1, ignore --curve] m 3 bar 14