Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
HYDRAULIC CIRCUITS 1.
The figure below shows a regenerative system in which a 25 hp electric motor drives a 90% efficient pump. Determine the external load F and the extension speed V that the hydraulic cylinder can sustain in the regenerative mode of operation. Given that: pump pressure = 72 bar, cylinder piston diameter = 20 cm, piston rod diameter = 10 cm, pressure loss in piping and DCV = 2 bar.
2.
Available are: one vented oil reservoir, one suction line filter, one single fixed displacement pump, one relief valve, one tandem spring-centred manual (leveroperated) 4/3 DCV, two adjustable pressure compensated flow control valves (with checks), piping & fittings and one fixed displacement reversible hydraulic motor. Employing these elements only, use standard symbols and neat drawing to construct a hydraulic circuit that accomplishes speed control of the hydraulic motor with METER-IN effect in both directions of rotation and pump being unloaded when motor stopped. In the normal full load operation of the hydraulic motor of the circuit above, the following is given:
Torque required from hyd. Motor = 250 Nm at a speed of 480 rpm
Motor mech. Eff. = 90% and vol. Eff = 94%.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
Pump pressure = 230 bar and rotates at 1500 rpm. Pump mech. Eff. = 88% and vol. Eff. = 92 %.
Pressure loss in flow control valve (throttle path) = 12 bar and in check pathway is neglected.
Pressure loss in DCV = 3 bar/ way.
Neglecting both external leak and pressure loss in piping, filter and fittings, determine:
3.
i.
Geometrical displacement for both pump and motor.
ii.
Electric motor shaft horsepower to drive the pump.
iii.
A suggested value of pressure setting for the relief valve.
Determine the suitable volume for the oil reservoir of an aircraft hydraulic control system, given that: the
reservoir
is
to
supply oil
to 4 double acting cylinders working in
parallel switching sequence. Piston/ Piston Rod diameters are 75/ 52 mm. Piston stroke is 120 mm. Full extension takes 3 seconds. 4.
The regenerative circuit shown in the figure below is used for closing and opening a steel furnace gate. Closing is done during the extension stroke of the cylinder, against, a total thrust of 5 metric tons. The total thrust required from cylinder during gate opening (retraction stroke) is 500 Kg. Piston diameter = 150 mm. Piston rod diam. = 105 mm. Full extension of 30 cm is to be effected in 15 seconds. a) Redraw the circuit during gate closing. Use arrows to indicate flow and piston rod movement directions. b) Estimate suitable specifications for the pump (type, Q, p) (reasonable values of pressure drop in piping, fittings and valves are to be assumed). c) Determine the maximum speed of gate during opening. Compare this speed to the closing speed. d) Determine how much should be the reading of the pressure gauge (6) during gate opening? e) Put complete specifications for the valve (7). This should include: and P).
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(Name, Size: Q
Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year) f)
Assuming a reasonable value for pump efficiency estimate the necessary motor horsepower.
g) What are the major advantages and disadvantages of using this circuit?
5.
The hydraulic circuit shown in the figure below represents a dentist chair system. Cylinder (1) is for raising and lowering the chair; while cylinder (2) is for tilting and lifting the back of the chair (only one cylinder operates at a time). The specifications of the system are:
Stroke of the main cylinder: 50 cm is to be completed in 5 seconds.
Stroke of back cylinder: 30 cm is to be completed in 3 seconds.
Load on main cylinder: 240 kg.
Main cylinder piston/rod diameters: 6/2.5 cm.
Load on back cylinder: 80 kg.
Back cylinder piston/rod diameters: 3/1.5 cm.
Now, allowing 5% of the load as viscous and mechanical friction, and considering pump overall efficiency of 85% and vol. efficiency of 95 % at a speed of 3000 rpm, determine: a)
Suitable pressure setting of the relief valve in bar.
b)
Suitable pump geometric displacement in cubic cm and shaft horsepower.
c)
Suitable reservoir volume in litre. Page 3 of 19
Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
6.
The circuit shown in the figure below is used as simple ways to make two cylinders (1&2) extend in synchronization. a) Redraw the circuit, neatly, during the extension stroke. (Use arrows to indicate directions of flow and motions). Also, write on the drawing the precise name of every element. b) The two cylinders are to be extended in synchronization so that the speed of cylinder (1) is 0.3 m/s and that of cylinder (2) is 0.25 m/s. Now, if standard piston-rod/piston area ratio for cylinders is considered. The pressure in the return line to the reservoir is atmospheric, load F1 = 50 ton and F2 = 30 ton during extension, the maximum pressure in the circuit is 270 bar, the pressure loss in the line from pump to the blind end of cylinder (1) is 10 bar and pump efficiency is 0.75, determine: i.
Diameter of piston and piston rod for the two cylinders.
ii.
Pump shaft power.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
7.
Assuming that the 4 double-rod cylinders of figure below are identical, what special feature does this circuit possess?
Determine the shaft horsepower of the pump if the loads during power stroke of cylinders are 2 metric tons each at a speed of 0.15 m/s. Given that:
Piston diameter = 10 cm (standard rod/piston diam. ratio), pump efficiency = 80%.
Consider 5 bars to compensate for viscous and mechanical friction in every cylinder and 3 bars as pressure drop in piping and valve.
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Assume any missed data.
Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year) 8.
For the hydraulic circuit shown in the figure below, the geometrical and operational characteristics of cylinders 8, 9 and 10 are listed in the table below: Cylinder 8 Cylinder 9 Cylinder 10 Piston diameter (mm)
100
125
150
Rod diameter (mm)
Standard
Standard
Standard
Load during extension stroke (ton)
20
25
35
Speed required during extension stroke (cm/s)
20
60
50
Load during retraction stroke (ton)
10
15
0
Speed required during retraction stroke (cm/s)
40
60
-
400
400
Friction and viscous resistance in either stroke (N) 400
The stiffness of the spring of cylinder 10 is 2×104 N/m.
The pump efficiency is 75%.
The losses in piping and valves can be ignored.
Find the maximum pump shaft power considering all modes of operation.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year) 9.
For the circuit shown below; write on the drawing name of valves (1), (2) and (3). State clearly the function of each valve. Modify on the drawing the position of valve (4) and use arrows to show the directions of flow and motion during load lowering. Knowing that the load = 1 metric ton, cylinder is 10 cm bore with a standard piston / piston rod size and considering 5 bar friction and other losses in any stroke, determine the pump pressure. a) During load lowering (downward motion) if the cracking pressure of valve (2) is set at 10% above the weight pressure of the load held and valve (1) is set to open when valve (2) cracks. b) During load lifting (upward motion). Also determine Pump shaft power during this stroke if the motion is made at a speed of 0.2 m/s. Pump ηm = 90% and ηvol = 95%.
10. The hydraulic circuit of a woodwork press is shown in the figure below. a.
Give full name description for the numbered items in the circuit.
b.
Redraw the system, with valve (6) actuated, during actual pressing interval. Show on drawing (using arrows): flow directions, valves setting, actuators movement.
c.
Knowing that the press has a number off 6 of cylinder (1) and a number off 3 of cylinder (2), determine:
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year) i.
Pressing force in metric tons.
ii.
Pressure setting of valve (7).
iii.
Rated pump discharge.
iv.
Rated discharge of the single valve (4).
v.
Electric motor shaft horsepower.
Givens:
Press beam dimensions (working): 1 m × 1.8 m
Woodwork pressing stress: 2800 kN/m2
Cylinder (1): standard 10 cm piston diameter and 40 cm stroke.
Cylinder (2): 30 cm bore and 25 cm piston-rod diameter.
Press rising time: 16 seconds.
Press lowering time until beam touches work-piece (95 % of the full stroke): 20 seconds. Actual pressing time (5% of the full press stroke): 30 seconds, assume at full pressing pressure.
Allow for 2% volumetric losses in the system, 15 bars pressure loss in pipes, fittings and valves, 0.9 relative density of oil and 78% pump efficiency.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
11. The figure below shows a clamp and drill hydraulic circuit in which a sequence is made between the clamping and drilling processes. It is required to improve this circuit so that the feed speed can be varied and controlled by adding only one valve. a) What type of valve would you add? Draw its symbol. b) Redraw the circuit with this valve fitted and the DCV actuated (during the drilling action). In your drawing, draw all valves in their operating mode. Use arrows to show directions of flow and motion. Also write the name and function of valves S1 and S2. Knowing that:
Clamping cylinder is 40 mm diam. with a pistonrod 20 mm diameter.
Drilling cylinder is 50 mm diam. with a standard piston/ piston rod.
Minimum clamping force = 500 N.
Fast forward speed for clamping = 70 mm/s.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
Max. drilling force (axial) = 4800 N.
Max. drilling feed speed = 2 mm/s.
Estimated mechanical and viscous friction forces in drilling cylinder = 200 N.
Pressure loss:
in DCV = 0.7 bar/way.
in valve S1 = 1.5 bar in either ways.
in valve S2 = 1.2 bar in either ways.
in the valve added for controlling the drilling feed speed = 2.8 bar.
in piping and connections during drilling =1.3 bar in the cylinder supply line and 0.8 bar in the return to tank line.
Determine: i.
Pressure setting of valve S1.
ii.
Relief valve pressure setting and size (discharge capacity).
iii.
Rated pressure, discharge and shaft power of the pump if ηm = 85% and ηvol = 90%.
iv.
%age of shaft power lost in relief valve during drilling.
12. Redraw the shown two-step speed control pneumatic circuit with valves set in position for the low speed step of cylinder extension, knowing that V4 is set to allow a flow rate higher than that supplied with valve V3.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
13. For the circuit shown in the figure below, give the sequence of operation of cylinders 1 and 2 when the pump is turned on. Assume both cylinders are initially fully retracted.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year) 14. In order to control a press cylinder, a double-pump hydraulic system, shown in the figure, is used. i. Describe the operation of the hydraulic circuit. ii. The press cylinder having a piston of 140-mm diameter and a 100-mm diameter rod is to have an initial approach (extended high-speed) of 5 m/min and final pressing (extended low-speed) of 0.5 m/min. the system pressure for rapid approach (high-speed part) is 40 bar and for the final pressing (low-speed part) is 350 bar. The drive speed of both pumps is 1720 rpm. Both pumps have volumetric and overall efficiencies of 0.95 and 0.85 respectively. Determine: 1. The flow rate to the cylinder for rapid approach and final pressing. 2. The pressing force and power. 3. The displacement volume of each pump. 4. The input-power required during rapid approach and during final pressing for each pump. 5. The input-power consumed when the press cylinder is locked. How can you reduce this power? (The relieve-valve setting is 400 bar). 6. The retract speed if the pressure required for retraction is 25 bar maximum.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
VALVES 15. What type of valve is that shown in the figure below? Draw its symbol. Also, determine its pilot plunger diameter and full opening pressure, knowing that:
The valve cracks at 200 bar.
The spool is to move 8 mm for cracking and moves additional 3 mm for full opening.
Spring stiffness = 785 N/cm, spring free length =9.5 cm and assembly length = 8.3 cm.
Why relief valves are not usually used in i.
Pneumatic circuits?
ii.
Hydraulic circuits utilizing pressure compensated variable displacement pump?
16. What type of valves is that shown in the figure below? Explain in a few lines its operation. The shown valve is used in a hydraulic circuit as a meter-out device to maintain constant speed for upward extension of the piston rod of a standard 7 cm diameter hydraulic cylinder. Construct a simple hydraulic circuit that provides this action. If the spigot of the valve is set at an angular position so that the orifice connecting the control chamber to the inlet passage of the spigot cylinder is equivalent to a 2 cm diameter circle. Determine the extension speed of the piston-rod, knowing that:
Pressure drop between the sides of the orifice = 620 kN/m2,
Oil sp.gr. = 0.9 and Orifice discharge coefficient is assumed 0.5.
If this type of valves is not available, can it be replaced by another two valves? If yes, what type would you recommend for each of them?
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
17. What type of valves the following drawing represents? Draw its symbol. If length of Notch in the spool of this valve is 20 mm, determine the percentage increase in controlled flow discharge due to a movement of the spool of 1 mm (to the right, from shown drawing position). Consider laminar flow through notch.
18. Draw the symbol and write the name of the valves shown in the figure below. Use symbolic representation to construct a simple circuit in which two units of valve (ii) are used in connection with other suitable elements to operate a reversible fixed displacement motor so that motor shaft can held in any angular position with no possibility of rotation by external torques.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
19. What type of valves is shown in the figure below? Draw its symbol. Explain using equations its principle of operation in the controlled flow direction, considering laminar flow through the covered length of notch. Given that: spool is 2 cm diameter with semi-circular notch of radius 1.27 mm and length 1.2 cm. spring free length is 8 cm and assembly length (for the position shown in figure) is 6.4 cm, oil viscosity is 1.25 poise and sp. gr. 0.9. a) For free flow operation, determine the valve spring stuffiness so that the spool top (at angle side) becomes aligned with the surface "C" when oil pressure of 3 bar is applied in the free flow direction. b) Considering controlled flow operation, determine the discharge through the valve when the covered length of the notch is set equal to 6.2 mm and pressure difference between inlet and outlet of valve is 7 bar. c) Use four valves of this type in addition to the elements listed below to construct a hydraulic circuit of an overhead hoist and trolley crane. List of elements available:
One fixed displacement pump with proper relief valve.
2 manual operated (tandem-centre) 4/3 direction control valves.
2 reversible fixed displacement hydraulic motors.
Proper piping, tank and oil filter.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
20. What type of valve is that shown in the figure below? Draw its symbol. Also, determine its pilot plunger diameter and full opening pressure, knowing that: The valve cracks at 200 bar. The spool is to move 8 mm for cracking and moves additional 3 mm for full opening. Spring stiffness =785 N/cm, spring free length =9.5 cm and assembly length = 8.3 cm.
21. What type of valves is that shown in the figure below? Draw its symbol. State 3 advantages of this valve design over the basic design of the same valve type. Determine the spring stiffness to insure full opening of the valve when the spool moves 10 mm to the left. Available data:
pilot plunger is 6 mm diameter,
assembly spring deflection is 15 mm and full opening pressure is 70 bar.
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
ACUMULATORS 22. A 400 in3 gas-charged accumulator is precharged to 1500 lbf/in2 Fluid is then pumped into the accumulator until a relief valve setting of 3000 lbf/in2 is reached. If a system operating pressure of 2000 lbf/in2 is assumed, how much fluid will be available from the accumulator for useful purposes? (Clue: draw the figure for the problem.) 23. An accumulator is to be sized to supplement pump delivery during peak demand with 10 liters of fluid between the pressures of 14 MPa and 10 MPa. To be sure that fluid is available from the accumulator at 10 MPa, it is precharged to 8 MPa. What size should the accumulator be if atmospheric pressure equals 101 kPa and: a) the process is considered to be isothermal? b) the process is considered to be adiabatic?
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Alexandria University Faculty of Engineering Mechanical Engineering Department Hydraulic Circuits (4th Year)
ELECTROHYDRAULICS 24. Define a common function for each of the following electrical devices used in controlling fluid power systems: a) Manually operated switch b) Limit switch c) Pressure switch d) Relays e) Timers f)
Solenoids
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