Basics of Hydraulic Circuits
48
BASICS .OF HYDRAULIC CIRCUITS Authors : P.K. MUKHERJEE S. ILANGO B.E. (Mech) B.E., PGDIT., MBA., f FLOWLINES ENGINEERING PVT. LTD. W-124, 3rd Avenue Anna Nagar Madras - 600 040 INDIA
Transcript of Basics of Hydraulic Circuits
BASICS .OFHYDRAULIC CIRCUITS
Authors :
P.K. MUKHERJEE S. ILANGOB.E. (Mech) B.E., PGDIT., MBA.,
f
FLOWLINES ENGINEERING PVT. LTD.W-124, 3rd Avenue
Anna NagarMadras - 600 040
INDIA
Second Edition : 1996
© Copyright : Authors
Price : Rs. 102
THANKS
We thank M/s Flowlines Engineering Pvt. Ltd., Madras,for having sponsored this book
- Authors
Lasertypesetting at:
Sri Maruthy Laser Printers, 174, Peters Road, Royapettah, Madras-600 014.
PREFACE
Oil Hydraulics is a fascinating field in engineering. Many ofus take this specialised subject for granted. Indian Universitiesgenerally do not offer specialisation in oil hydraulics in graduatelevel or in post graduate level, even.
Generally, it is the international companies involved in oilhydraulics like Rexroth and Vickers who have brought outpublications to spread knowledge relating to the basics of oilhydraulics as well as constructional details of hydraulic elements.
These books cover the principles behind the constructionaldetails of the hydraulic elements and other components extremelywell. We however find that the basics of hydraulic circuits have notbeen given adequate coverage.
The information / knowledge given in this book is preparedand pretested by the experience, we have gained in making oilhydraulic systems at our factory for varied applications.
Further, the experience gained in training our own engineersis also used in the book i.e. in making our engineers capable ofdesigning the hydraulic circuits to meet different applications ofcustomers.
The book starts with a set of questions that an applicationengineer in oil hydraulics should be aware of, before the design ofhydraulic circuit is even considered.
Step by step procedure is laid out from thereon about circuitdesign, chapters are then devoted to designing the circuits for newapplications.
At the end, we have tried to appraise the different problemsthat can come up in hydraulic power units so designed. And a chartis given for trouble shooting.
We sincerely hope that the book serves everyone interestedin learning the basics of hydraulic circuits.
- Authors
V
Chapter 1
BASICS OF HYDRAULIC CIRCUITS
Knowing the Questions
2. Basic Block-Reservoir and Accesories
3. A simple Hydraulic Circuit -1
4. A simple Hydraulic Circuit -2
5. Machine Tool Hydraulics
Hydraulics in Simple Plastic Injection Moulding Machines
7. A Simple Press Circuit
. Few more Applications
9. This way to Hydraulic Circuits
10. Common Problems.
11. Standard Graphical Symbols
KNOWING THE QUESTIONS
Understanding hvdra^lic circuits and hydraulic powerunAs
starts with the end. In hydraulic circuits design, we must first
understand the actuators, that are normally the end points of a
hydraulic circuit.
Actuators are nothing but hydraulic cylinders (linearactuators) or hydraulic motors (rotary actuators).
We must know what the actuator does and we must know
what are the specifications of actuators from the customers, before
starting to design a hydraulic circuit. Designers can study little more
and find out where the actuators are used (i.e.) in what kind of
machine.
A standard list of questions to be asked by a hydraulic circuitdesigner to end user is presented here. Once we have the answers,the hydraulic circuit can be designed.
For simplicity sake, let us assume only hydraulic cylindersare used and not hydraulic motors;
Most of the users of hydraulic power unit will be in a positionto answer the questions listed here.
List of questions to be asked
1. Is the cylinder single acting or double acting?
2. How many cylinders are used?
3. What is the sequence of cylinder movement-one after otheror almost together?
4. What is the function of each cylinder?
5. What machine all these cylinders go to make?
6. What is the bore size of cylinder?
7. What is the ram size (rod of a hydraulic cylinder) ? Customersmay not know this end hence answer is not an essential one.
8. What is the stroke length of the cylinder?
9. Does the customer require manual/solenoid operated(electrically operated or automated) movement?
10. What is the force acting on the cylinder?
11. What is the speed of movement required?
12. Do they require single speed/double speed or multiple speedfor the same cylinder, if so what are these speeds:
Suppose the customer is better informed and can answeryour questions as below:
Question Answer
1. Single acting / Double acting Double acting
2. How many cylinders 2 cylinders
3. Sequence one after other
4. Function of each cylinder 1. Clamping2. Drilling
5. What machine all these Drilling Machinecylinders go to make?
6. Bore size of cylinders 1. Clamping 100 mm dia2. Drilling 63 mm dia
7. Ram size of cylinders I don't know. Standard
size given by anyreputed manufacturer
8. Stroke length of cylinders 1. Clamp 20 mm2. Drilling - 120 mm
9. Manual / Solenoid operated Manual
10. What is the force acting on the 1. Clamping - 500 kgcylinder? 2. Drilling - 300 kg
11. What is the speed of 1. Clamp - 2 meters/mtmovement required 2. Drilling -100mm/min
Armed with above information, you have to start working;
One step at a time
1. We must know how much or what quantity of fluid isrequired to do the work. Hence the first step is to find,-ihepump capacity. The capacity is expressed in litres per minuteand can be calculated from the following formula.
Q Capacity in lit/min = Area of cylinderin sq.cms XVelocity of movementof cylinder in cms/minute
From the above example
i) Q Capacity in lit/min = 78.5 sq.cm (Clamping
cylinder area) X 200
cms/min clamping
speed
15700 cubic cms/min.
15.7 lit/min
ii) Q Capacity in lit/min = 31.15 sq.cm X l0cros/minincase of drilling along area of cylinder X
drilling speed= 31.15 cubic cms/min
0.31pm
1. Since the cylinders have to actuate one after the other, wecan consider a pump having a capacity of 15.71pm or nearabout. This will take care of both clamping as well as drilling.
2 3
2.
2.1
2.2
We know the pump size , what about the working pressure?
We go back to the age old formula (i.e)
Pressure =
Clamping pressure
Clamping pressure
Drilling Pressure Drilling Forcedrill cyl. diameter
300 Kg
31.14 cm2
Forcearea
Clamping ForceClamp Cylinder area
500K g = 6.36 Kg/cm2
78.5cm2
9.6 Kg/cm2
Generally hydraulic pumps are driven by motors of 1440rpm. Thereis a thumb rule forcalculating the HP of the motorrequired, given the flow capacity and pressure.
ower in KWWhere P = o mg pressure in bar
Kg/cm2Q = Flow capacity of pump
in lpmAs per the example,
P600
HP
Note 1 :
9.6 Kg/ 2 x 15.7 lpm
0.25KW600
0.250-346 0.335.0.5 HP
,,Z ,Aar = 1.03 kg/cm2. For sake of simplicity in calculationswe take I bar to be equal to 1 kg/cm.
The maximum working pressure is the higher of the abovetwo. (i.e) 9.6 Kg/sq.cm
So far we have found the
1. Capacity of the pump (i.e) 15.71pm ; 16 ^P'►n'
2. Max working pressure (i.e) 9.6 Kg/sq.cm
3. We can decide about the reservoir size of power unit. TheThumb rule is that it should be about 4 times the capacity ofthe pump (i.e) the pump capacity say 161pm . The reservoirsize should be 16X4=64lit.
As standard reservoir come in 50 lits, 75 .1its, and 100 fits.We can choose 75 lit capacity reservoir.
4. What about the Motor HP rating?
Note 2:
Generally the above formula of-EQ600
for KW is only a thumb
rule and will not be accurate, especially closer to extremes of thecharacteristic curves of the pump. It is better to refer to selectioncurves to decide the exact HP/KW required.
The above type of calculation, tells us following
1. Capacity of pump to be chosen
2. H.P. of the motor to be coupled to the pump.
3. Working pressure
4. Size of reservoir
This is only the beginning, in the design of hydraulic circuit.
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Chapter 2
BASIC BLOCK - RESERVOIR &ACCESSORIES
In the last chapter, we have defined the four parametersrequired for designing the Hydraulic Circuit. (i.e) PumpCapacity/Horse Power required to drive the pump/workingpressure and size of the reservoir. The reservoir serves as a storefor hydraulic oil. The reservoir has certain accessories and theseare explained in this chapter.
The Reservoir in a hydraulic circuit is illustrated in Fig 2 (1)
A breather filler is indicated as 1. and an oil level gauge as
2. The function of the breather fill er is to allow the resen oir tobreathe - (i.e) when the oil from the reservoir is drawn out, itrushes in to fill in the Qap, vacated by oil and as the oil returnsthrough return line filter, air is let t.
Breather filler also help us to fill/refill or empty the Reservoir
with oil.
Choose a breather filler of 5 cfm airflow capacity when theresevoir is less than501its . For over 50lits, 25 cfm airflow capacitybreather filler can be chosen
The oil level indicator (2) indicates the oil level in thereservoir . Normally the level indicator comes in sizes indicating thelength - (i.e). 3 inches , or 5 inches or 10 inches. It is fixed close tothe top edge of the reservoir or (say) 100 mm below the top plateof the reservoir.
BRE AT HER /FILLER
FIGURE- 2 (1 )
FIGURE _2(2)
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6
The hydraulic circuit starts with the reservoir and withaccessories . Please note further addition in building up the circuit-Figure 2(2)
Please note the addition of pump and the suction strainer. Ifthe pump is say 61pm the , suction strainer size should be atleast 4to 5 times the size of the pump . This will reduce cavitationpossibilities of the pump . Further the suction strainer filtration levelis generally 149 microns.
We have added two more to the circuit building up. Theseare the relief valve and the return 'line filter.
The relief valve is selected based on the required workingpressure & flow. We must be in a position to set the workingpressure with the relief valve and the range has to be chosenaccordingly.
The return line filter is selected on the basis of
jThe volume of flow back to the reservoir. This can be 4 timesthe capacity of the pump
Type of mounting on the reservoir (i.e) Tank top or Inlineor tank immersed types
The filtration level is generally 25 microns or 10 microns.
Please refer figure 2(4).
We have added a gauge isolator and pressure gauge in thisfigure.
Gauge isolator is to isolate the pressure gauge and only whenthe pressure is to be read , we press it (in the case of push to readgauge isolator) to read the pressure . This helps to prolong the lifeof pressure gauge.
The pressure gauge is chosen^for twice the working nressnr_erange. Suppose the working pressure is 30 k sq .cm - It is better tochoose 0 -60 kg/cm .
PR RELIEF VALVE
SUCTION STRAINER
FIGURE. 2 (3)
P T
BASIC BLOCK OF CIRCUIT
FIGURE-2(4)
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For industrial hydraulic power units, it is preferable to
choose 4" dial panel mounted pressure gauge.
In most of the hydraulic circuits the basic block almostremains same.
So, till we come to circuits including double pumps, thisbasic only will be common. Hence to simplify illustrations in thechapters to follow, we shall just put a rectangular block to indicatethe basic block with its features.
To Summarise
Reservoir size - 4 to 5 times pump capacity
suction Strainer - 4 to 5 times pump Flow capacity
iii. The Basic block of hydraulic power unit comprises of
a. Reservoir
b. Suction Strainer
c. Pump coupled to electric motor
d. Pr. relief valve
e. Return line filter
f. Pr. gauge with gauge isolator
g.
h. Level gauge
Breather filler
Chapter 3
A SIMPLE HYDRAULIC CIRCUIT - 1
Before drawing a simple circuit, we must know that thecircuits can be drawn in conventional manner or using modularvalves. conventional manner means - involving more piping work(i.e) connecting the individual valve with properly sized tubing. Thebetter way of doing is using modular valves with manifolds.
Our first attempt here is to make you understand theconventional way.
We shall illustrate subsequently the same circuit - withmodular valves.
Let us consider a system - where the hydraulic cylinder (sayone number) first pushes a load into the furnance.
The basic block, we have understood in the previous chapter.From the basic block, we have a P line (pump line) coming out. Wehave another end of the circuit (i.e) the T line that gets back to the
reservoir.
Refer Figure 3(1)
The P line is connected to a direction Control (DC) Valve -a three position DC valve where in its centre position all ports areblocked . From the service ports of the DC valve, connection is takento A 1 and B 1 ports of a horizontal double acting cylinder.
When the pump is working and solenoid X is energised, oilgets to the Al port of the cylinder. This would push the piston inthe direction indicated.
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A B
(V) CCV_ DIRECTION CC NTROL VALVEP
P T
When solenoid Y is energised, the oil from P line gets throto B and to B1 of the hydraulic cylinder and the piston moves inthe opposite direction.
At any point in time, if neither X solenoid nor Y solenoid isenergised (i.e.) at the neutral position of the DC valve, the pistondoes not move.
This means, wherever or whichever position you want, youcan position the piston by not energising either of the solenoids.(i.e.) at the neutral position of the DC valve.
At this neutral position all ports are blocked and hence the
oil does not get into the cylinder.
FIGURE - 3 (1)
TROL /ALVE WITH
REVERSE CHECK
P T - A,B_SERVICE PORTS OF DC VALVE
FIGURE- 3 (2)At Bt_SERVICE PORTS OF THE CYLINDER
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Hope you have understood completely fig 3 (1)
1. If P and T ports are blocked in neutral position, and the pump,keep running, what is happening to the pressure in the `P'line ?
2. A and B ports of the valve is also blocked in the neutralposition would this mean that where ever the piston isstopped, it cannot be moved by manual force?
The pressure in the P line keeps building up as long as themotor is running which is coupled to the pump. As thepressure keeps building up, a stage can come as the oil hasno place to go, the pipe line can burst or the motor can stall-unless relieved of the pressure.
So we must have the relief valve in the system.In the begenning before starting the motor/pump, the DCvalve is not energised. This means, in the begenning, itself,as the motor/pump is started, (Before energising the valve)the `P' line is pressurised. So, when the pump /motor is on, itstarts on load. Is this desirable? It is not:
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2
In the neutral position, once the oil is in the pipe lines, thecylinder cannot be just pushed manually ; is this required forthe application in mind?
Any designer would like the motor (prime mover) to start onno load.
A B
P T
So in such a case, you must choose a DC valve that has aroute (spool configuration) which will allow the oil flow back intothe reservoir in the neutral position.
FIGURE - 3( 3)
FLOW CONTROL WITH
REVERSE CHECK VALVE
FIGURE _3(5)
FIGURE-3 (4)
Al
B
T
BASIC BLOCK
FIGURE_3(5)
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Look at the DC valve , spool configuations 3 (3) and 3 (4).
In these types of DC valves, the oil can get back to ` T' linein neutral position. Hence no load starting of motor is possible justby choosing a right DC valve configuration . So far we havediscussed oil route that can go to the cylinder and that comes outthrough the DC valve.
Please refer Figure 3 (2)
We introduced another valve here. This valve as it is shownwill allow the oil to go through a restriction which is adjustable.
Now consider the implications of this flow restrictor (Flowcontrol valve with reverse check ) you can reduce the quantity of oilthat will flow through this valve. Result?
The speed of the actuator .during the direction of movementindicated comes down - depending on how much is the restriction.But in the opposite direction it is as free as possible. Why? Becauseit can lift the check valve and flow freely and move the piston inthe opposite direction.
But, the flow control valve allows only limited volume of oilto flow through . What happens to the oil volume that gets pumpedinto the system which is much more.
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Well. This excess oil is relieved/blown through the pressurerelief valve.
Remember - all this is applicable only when you consider adouble acting cylinder movement is to be controlled.
Suppose the application requires only two positions of thecylinder without any intennediate stop.
In this case, we can choose a DC valve of two position, asillustrated in Figure 3(6).
We shall consider a slightly different application in the nextchapter.
Chapter 4
A SIMPLE HYDRAULIC CIRCUIT - 2
Consider an application where a load is to be lifted up (say)
a stacker.
What are the pit falls-
1. The load should not drop down but stay in desired positions.
2. If the load is lifted and has to be brought down slowly, itshould not come down with a 'thud.' (i.e.) without anycontrol-if this happens it means that the load is driving the
system.
Now look at the hydraulic circuit in figure 4(1).
Take figure 4(1). A three position, solenoid operated DCvalve with all ports connected (in neutral) is in use.
So, when X is energised, the load is brought down - andbrought down with a thud. (i.e.) The oil takes the path A to Al andpushed the piston down. Already the load is also acting on the pistonand it comes down fast as the outgoing oil through port B I has a
free passage.
While going up - energise Y and load is lifted up and in the
neutral position, again , the load will drop down - as all ports are
connected.
If a dc valve, where in neutral the P and T lines are connectedin Figure 4(1.1) (A and B blocked), for short duration, the load canbe held in any position (i.e.) when the solenoid valve is not
energised.
16 17
a
FIGURE _4(77)
BASIC BLOCK
FIGURE -4(;)
DCV _ D^RECTON CONTROL _A-E
FIGURE-4(2)
1 8
However, we must remember that all conventional slidingspool valves allow a leakage (quantity of leakage depends on thesize and pressure differential) and so the load can creep down.
Please refer figure 4(2). The additional valve shown is pilotoperated check valve designated as POC in the figure. This valveis a poppet design and hence gives zero leak characteristics.
The POC valve is located on the B line. The advantage is thatthe load can be held in any mid position without the `creeping'mentioned earlier.
One problem remains:
When solenoid X is energised the cylinder can still comedown more driven by the load and can cause uncontrolled dissent.
Please refer figure 4(3). We have added one more element -a pressure control (i.e) Counter Balance valve designated as CBVin the figure.
The idea is to control the dissent. This is done by setting theCBV at a particular pressure. This pressure is set at a valueequivalent to the load divided by the bottom area of the cylinder.
Now the load is counterbalanced and cannot come down dueits own weight unless solenoid X is energised and the set value ofCBV is exceeded on the ram side of the cylinder.
The CBV has a built in reverse check valve. The advantageis that when the cylinder is to be lifted up just energise solenoid Y.The oil flows into the port B, of the cylinderlifting the reverse checkof the CBV. In furnace gate lifting application, this circuit discussedcan also be employed.
In effect, it means that for such high runaway loads, it isbetter to have a CBV in addition to POC - so that control is possiblein the runaway direction. One more illustration is in the case of avertical drill where at the opening out of the material , the drill triesto break out of control as the load is not there - suddenly. Even in
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a
IL
ROC
LOAD
81
I CBV
FIGURE _4(3)
FIGURE _ 4 (4)
LOAD
Al
Roc
20
,BOTTOM AREA
such applications we must have a CBV in the circuit - to reducesuch undesirable and uncontrollable movements of the drill or thecylinder.
Please refer figure 4(4). Suppose further speed control isrequired during descent of the cylinder, then we can have a flowcontrol valve, designated as FCV in the figure 4(4). This applicationjust discussed is also possible with a pump driven by an engine andwith manually operated dc valve. For example, in any lift platformused by airlines for cargo loading.
To summarise, in this chapter, we have seen practical usageof a pilot operated check valve (POC), counter balance valve (CBV)and Flow control Valve (FCV).
All these valves are also available in modular form but theidea is that we understand by illustrating the tine diagram.
For beginners, (For whom this book is meant), they mustimagine the consequence of not using anyone of the valves in thecircuits discussed.
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Chapter 5
MACHINE TOOL HYDRAULICS
Machine tools are generally differentiated into two classes.(i.e.) special purpose machines (SPM) and general purposemachines. (GPM)
General purpose machines are those which are standardisedand common like lathes, drilling, milling etc., which are availableoff the shelf with standard specifications. The SPM's are tailormade to suit particular component in large volumes. For example,an engine manufacturer, may decide to have a special drillingmachine which can gang drill all the holes on the engine head at atime.
Let us discuss a few circuits generally used in machine toolhydraulics.
Clamping Circuit
In machine tools, the job or the tool or the fixture is to beclamped or held during the machining process.
Please refer fig 5(1)
Here we have two clamping cylinders with one directioncontrol valve (Two position , solenoid operated , spring offset dcv)and one pilot operated check valve . The DCV has only onesolenoid . In the de energised position (X side of the valve), thecylinders are in clamped position . The solenoid Y is to be energisedfor declamping operation . What will happen?
Incase the solenoid is energised , the clamp opens (orreleases), and (say ) the job is either removed or being put back.Imagine in this position , the job is being placed for clamping.
BASIC BLOCK
FIGURE-5(1
BASIC BLOCK
FIGURE-5(2)23
22
Suppose there is a sudden power failure - then immediately, thespring offset position of the dcv, takes place and the job getsclamped . Perhaps even before the job is in its place . In all possibilitythe hand holding the job also gets clamped , instead of the job - ifthe power fails . It is because the pump continues to deliver for fewmore moments even after power failure due to inertia.
Please refer figure 5 (2). We have a different type DCV inplace instead of a spring offset solenoid operated dcv. This valveis known as two position solenoid operated detent type DirectionControl Valve.
The advantage is that this kind of DCV has a mechanicalmemory and even in case of power failure, the position of the spoolof dcv does not change-because of a mechanical detent which keepsthe position of a dcv unchanged incase of power failure.
In figure 5(2), you will find wehave two additional elements-a pressure switch (PS) and a pilot operated check valve (POC)
The function of the POC valve is not to allow leakage of oilin the clamped position of the cylinder. this means slackening ofthe clamp cylinder does not take place.
The function of the pressure switch is that - once theclamping is done, it is quite likely the function of some otheroperation has to take place. We can get an electrical signal from thepressure switch once it is incorporated as shown. The pressureswitch converts a pressure reading into an electrical signal - Thiselectrical signal can be used to trigger some other operation likeenergising some other solenoid.
The pressure switch is normally set for a particular limit -(say)the clamping pressure; Once the pressure switch line reachesthe set clamping pressure limit, a microswitch in the pressureswitch, makes a contact and sends an electrical signal to start thenext operation.
FS
BASIC BLOCK
FIGURE-5(3)
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24
Please refer figure 5(3). We have changed the POC valve toa double pilot operated check valve- what is the significance?
Well. The significance is that by installing a double pilotoperated check valve, we are ensuring the locking of fluid (i.e.)hermitical sealing on both lines of the actuator. Hence even if thereis an internal leakage (in the cylinder across the piston), the cylinderremains rock steady.
Please refer figure 5(4).We have one more member to thefamily of valves of this circuit. A pressure reducing valve (PRV) isin place. This PRV reduces the pressure to the clamping circuit asvery often, the job or the tool has to be held positively, firmly andalso delicately. For this purpose, it is not necessary to go upto thesystem pressure limits. If we do not have a PRV in place, then thepressure in the clamping circuit will reach the level of the systempressure which can distort or deform the job.
Feed circuits in machine tool applications
We must first understand what is usually defined as feedcircuit in hydraulics relating to machine tool applications
Very often, once the job is clamped and the tool is held (saya drill), the tool is moved rapidly towards the job till tool reachesthe job then (say) the drilling starts (feed), After drilling, the tool isretracted rapidly.
The above is common for any machining job,, like turning,milling etc.,
The process as explained involves two speeds; rapidapproach, and a feed speed (lower speed compared to rapid speedand rapid return)
Dual speed ( feed) with meter in flow control valve
Please refer fig. 5(5).
BASIC BLOCK
FIGURE-5 (4)
27
26
n
With this kind of circuit, it is possible to get two (dual) speedsin one direction and one rapid speed in the opposite direction.
Situation 1 : Solenoid X is energised; Flow is through FlowControl valve FCV to port A2 of the cylinder. As the oil has to gothrough the FCV (free flow is not allowed by the check valve ofFCV) the rate of oil flow through part A2 is controlled and hencedesired reduced feed speed can be obtained in the directionindicated. Oil from port B2 flows freely to the tank. This situation(1) happens when DCV 2 is not energised.
Situation 2 : Solenoid X is energised and DCV2 is alsoenergised. In DCV2, you will observe that port P1 and B I areplugged. The purpose of plugging is to make this single solenoidDCV to function like and on/off valve (or two way valve). Insituation (2) oil flows thro' Ti to Al to A2, thus by passing theFCV. This means there is no restriction to the rate of flow (as in thecase of situation 1) and hence rapid (fast) speed can be obtained inthe same direction, indicated. On the other part B2, oil flows freelyto the tank.
/ Thus in one direction indicated we are in position to get two
s'eds. One for rapid approach and the other slow speed for feed
(machining) purpose.
Situation 3: Now solenoid Y is energised and DCV2 is not
energised. The FCV is by passed and rapid return is possible in the-
opposite direction. This is because oil flow through part A2 comes
through the check valve lifting the check valve and to the tank.
Situation 4 : Solenoid Y is energised and DCV2 is also
energised. This situation is superfluous as the oil returning from
part A2 can return freely thro' the check valve of FCV and we are
creating an additional free path for the oil from A2 by energising
DCV2. However, if FCV is not having a built in check valve, then
to obtain rapid return, it is necessary to energise DCV2.
28
3
Al
A 1 1 e
DCV.1 P T
FIGURE - 5 (6)
Al I 81 CCV2
BASIC BLOCK
FIGURE-5(5)
29
T
Dual speed (feed ) with meter out now control valve
Please refer figure 5(6). compared to fig. 5(5), the differencehere is the location of FCV. It is now fixed on the outlet of port B2.The check valve position is such that the flow coming out of portB2 has to go through the control orifice of FCV.
Situation 1: Solenoid X of DCV 1 is energised and DCV2 isnot energised. Oil flows freely to port A2. The flow of oil from portB2 has to come through the control orifice of FCV before going tothe tank line. Hence feed speed is obtained in situation (1) indirection indicated.
Situation 2: Solenoid X is energised DCV2 is also
energised. Oil flows from P line to Port A2 and oil from B2 flowfreely from B2 to Al to T1. Hence rapid speed in the directionindicated is possible.
Situation 3 : Solenoid Y is energised and DCV 2 is notenergised. Oil flows for movement of ram in the opposite direction.Oil flow is from P to B. Since port PI and B I are blocked, the oilflow takes the path through the FCV. It flows lifting the check valveand onto B2. As the oil flow is not restricted rapid speed is obtainedin the opposite direction to the arrow indicated. The return oil flowfrom A2 is free flow to the tank.
Situation 4 : Solenoid Y is energised and DCV2 is also
energised we are creating and additional free path for the oil flow
as in situation 4 of meter in circuit.
The above meter out circuit is especially useful whenrunaway loads are to be controlled. For example while themachining takes place and ends suddenly, the tool may jumpforward. The meter out circuit is useful in minimising such jumps.
It must be noted meter out circuits are well accepted andmeter in is not popular. Only for the purpose of theory we haveexplained the' meter in' concept.
FfGURE.5 (7)
FIGURE _5(8)
31
30
Tank line feed control
Please refer figure 5(7). In both meter in and meter outcontrols, speed/feed is obtainable only in one direction. But withtank line feed control, this dual speed is possible in both directions.
To have uniformity of speed, in both directions, in figure5(7), we have drawn a cylinder with ram on both sides of the piston.
The idea is that the equal area becomes available on bothsides and hence same speed is possible in both directions.
Situation I : Solenoid X of DCVI is energised oil flowsthrough to A 1. DCV2 is not energised. Oil flows from B 1 to tankline but has to go through the FCV. Hence feed speed is obtained.
Situation 2 : Solenoid X is energised oil flows through toAl. DCV2 is energised. Oil flow from B 1 to tank line and goesthroughTI to T4. Unrestricted flow and hence full speed is obtainedin this direction.
Section 3 : Solenoid Y is energised and DCV2 is notenergised. Oil flows from P to B I and Al to T. Since DCV2 portT3 is blocked, oil flows through the FCV and feed speed is obtained.
Situation 4 : Solenoid Y is energised and DCV2 is alsoenergised. As explained above, free flow through to tank line ispossible and hence rapid speed indirection of line arrow is obtained.
Standard feed block
We have discussed three standard ways of obtaining feedspeeds. But there is a very popular standard way.
Please refer figure 5(8).
This is a standard block manifolded to take on two DCV's,one FCV and one back pressure valve (pressure control valve) Thefunction of the back pressure valve is essentially to take care ofvarying load conditions, jumping of tools or runaway loadconditions. It can be adjusted to reduce the `jump.
FULL AREA
AVAILABLE
FIGURE _5(9)
33
AREA AVAILABLE
REDUCED AREA
32
0
One advantage of this standard block is that the FCV isconnected to P line directly unlike in other feed circuit . This meansthe response time (of hydrostat in the FCV) of the FCV is muchbetter . This standard feed block is a meter in circuit and you canget feed speed in both the directions.
But one point to note:-
The area difference on either side of the piston in caseordinary double acting cylinder is used in the place of double endedrod. This area difference results in speed difference in eitherdirection.
Please look at the figure 5(9) for better understanding.
Another feature of the standard feed block is the backpressure valve. This back pressure valve allows us to fix the back
pressure at different values. For instance, in a light turning
operation a back pressure of 5 bar is sufficient. But in heavy
intermittant milling operation, an increase in back pressure limit
will help rigidity and to have more control over the movement.
Chucking
The emphasis is on rotary chucks which are used in highproduction machines like CNC lathes.
Following points to be kept in mind while developing thehydraulic circuit.
1. The construction of rotary chuck is such that there will be a
continuous leakage of oil thro rotary joints. Hence there will
be a clamping pressure drop when there is electrical power
failure.
This has to be taken care of by adding an accumulator backupin the circuit.
34
BASIC BLOCK
FIGURE -5 (10),
35
Further because of continuous leakage pilot operated checkvalves are not of any use in rotary chuck applications.
2. The chuck sometimes will have to hold slender jobs. This
m eans reduced pressure adjustment is required.
3. Internal and External chucking is to be taken care of by twoposition double solenoid detented valves (mechanicalmemory type)
4. Once chucking is achieved a pressure switch sends a signalfor starting the next operation.
Please refer figure 5 (10)
On the basic block, we have an accumulator back up, apressure reducing valve. A Direction control valve for selection ofinternal/external clamp (DCVI) (Detent, two position), anotherdirection control valve for champing/declamping and the pressureswitches.
In solenoid operated DC valve, (DCV2) the clampingposition is in non energised condition of solenoid. the reason forchoosing such position is explained in the earlier pages. Pleaseremember what would happen if there is a power failure whileclamping. Hence DCV 2 can also be of detented type.
Even if proper DCV position is chosen (i.e.) the clamping isdone in non energised condition ofthe DCV, there will be a problem.with the operation of rotary chuck because of leakage.
While the chuck is on (clamp is on and running) and if thereis power failure, there is a pressure loss in the clamping line - dueto drain line of rotary chuck. this can lead to a situation where thejob held gets released from the chuck. If it is an eccentric job thatis held or it is a high speed spindle the sudden release of job canhave the effect of a flying saucer hurled at the unsuspecting personnearby. As per the circuit in 5(10) the DCV2 position hold the job
P7 ; CURE 5',! I1
FIGURE - 5(12)
37
C ',ING_'EA-
36
and the accumulator backs up the pressure loss till the rotationceases - depending on accumulator sizing.
The pressure switches serve the function of sending outelectrical signals for commencing next operations. These electricalsignals are sent once the pressure switch set limits for internal/external clampings pressure are reached.
Counter Balancing
Let us understand Counter Balancing. In a conventionalmachine, (say) vertical boring machine, the boring head weight iscounter balanced with mechanical counter weight running onpulley or chain as illustrated in figure 5 (11)
This increases the size and weight of the machine.
In figure 5(12) a compact hydraulic cylinder (single acting)replaces the counter weight. Immediate advantage is the reductionin size/weight of the machine. The circuit for the same is in 5(13)
For the sake of simplicity and to save paper space, we haveshown the circuit starting from P line. This goes thro a pressurereducing valve (PRV), a check valve, a relief valve and then to theinput port of the counter balance cylinder.
The pressure selection for reducing valve is computed by theweight (of the moving head) divided by counter balance cylinderbore area.
The pressure relief valve setting should be above that ofpressure reducing valve setting to avoid draining of system oil.
The relief valve comes into play only during descent of themoving head.
The valve/pump sizing should take care of the rapid ascentof the moving head (and the ram upward movements)
FIGURE- 5(14
MOVING HEAD
FIGURE_5(13)
39
38
The combination of reducing, check and relief valves areavailable as one valve form reputed hydraulic componentmanufacturers. (Refer sketch enclosed)
Please refer figure 5 (14)
When the moving head stroke is less as in the case of slantbed lathe cross slide or the wheel head of a surface grinder, anaccumulator in closed loop is used as a counter balancing device.
By close loop, here we mean that there is no external supplyof oil to the counter balance cylinder. This cylinder is directlyconnected to an accumulator as shown. The sizing of accumulatorand the cylinder are done in such a way, that at the mid point of thestroke, the precharge pressure of the accumulator and the counterbalance oil pressure are equal.
Indexing
Indexing can be linear or rotary. In a linear indexing, therecan be two or more defenite positions. The circuit can be as shownbelow for a two position indexig table.
The two linear positions are at two ends of the ram travel.
For machining requirement, the end positions must be rigidly held
which is achieved by this circuit - By continuous pressurisation of
indexing cylinder (with the aid of two position DC valve).
Please refer figure 5(15)
This circuit is similar to tank line control of speed discussed
earlier.
We can get dual speeds in both directions by allowing thenow to the tank to go through the FCV. When fast speed is requiredDCV2 is also energised.
A two speed control is provided to soften or cushion the end
approach. Alternatively a cushioned cylinder can also be used in
place of duel speed control.
-P
T
FIGURE - 5(15)
FIGURE 5 (16)
41
40
V
Howeverthe advantage of the secondary speed control is that
we can fix the cushioning as per our requirement , whereas in acushioned cylinder the cushioning action is obtained only at theends of stroke.
Rotary indexing table in special purpose machines.
!1LJ
When machining has to be done in more than 3 or4 locations,a rotary indexing table machanism is used.
A plan sketch is shown for understanding.
Please refer figure 5(16)
The rotary indexing table is shown for eight station
I-for loading/unloading
2 to 8 - for various operations such as drilling, tapping, spotfacing, milling, reaming etc.,
In principle the rotation is by a hydraulic motor with a pinion
and an internal bull gear as shown in the sketch.
Please refer figure 5(17) and fig 5(18)
The precise indexing is achieved by face gear mechanism
which also helps in locating and clamping of the rotary table.
Initially the whole indexing table is lifted (declamped),
indexed (rotary movement of hydraulic motor) and lowered(clamped). Once lowered the face gear mechanism assures preciselocation and clamping.
Generally the indexing table rotates little more than the nextlocation (see fig 5 (18) ) repidly and reverses the direction andcomes back slowly to the intended state.
FIGURE - 508)
42
This is achieved by ahydraulic circuit as shown in fig 5 (18).A suggested hydraulic circuit combining the table clamp andidexing is illustrated in fig . attached 5 (19)A.
43
CLAMPING
Al B1
PILOT OPTD
CHECK VALVE
oCV
PR GAUGE
CON NECrON/
P
BASIC BLOCK
LINE DIAGRAM OF CLAMPING CIRCUIT
FIGURE-5(19)
44
T
I
CETOP_ 3
FIGCFE 5(20)
CETOP_5
FIGUFE - 5 (21)
45
MOUNTING HOLES
Hydraulic circuits - in modular form
CONVENTIONAL LINE MOUNTED VALVE
DPOC
DCV
MODULAR STACKABLE VALVE. We have so far illustrated hydraulic circuits drawn inconventional manner and perhaps also executed the same way.
By 'Conventional Manner' we mean the piping between hevalves or the manifolds. But with modular, stackable. sandwichtype double interface valves, we can reduce the piping or plumbinginvolved to a great extent.
The advantage is that it looks neat and reduces leakage,labour and assembly time.
The limitation is that all valves are not available in modularform.
P
BASIC BLOCK
CLAMPING CIRCUIT IN MODULAR CONVENTIONAL FORM
FIGURE _ 5(21) A
46
The best way to understand the reading of modular hydrauliccircuit is to see how we can convert a conventionally drawn circuitto a modular circuit.
Please refer figure 5(20) and 5(21).
The modular valves, to fecilitate stacking should have twointerface surfaces. One with '0' rings and the other to receive '0'rings.
Every modular valve willhave four passages(i.e.)P,T,AandB.' Normally the top valve in the stack will be a direction controlvalve otherwise a cross over plate or a blanking plate takes the topposition.
A typical hydralic circuit is shown using modular way ofrepresentation.
Please refer figure 5(21A) which is an equivalent modularcircuit of 5 (19) - clamping circuit.
47
CHAPTER 6
HYDRAULICS IN SIMPLE PLASTICINJECTION MOULDING MACHINES
The subject of hydraulics in plastic machine application is avast one and perhaps a separate book can be written for that pu rpose.The idea here is to introduce the subject and cover the basicmachines with conventional valves. Further the emphasis will beon hydraulic circuits and not on the machines.
We shall see simple plastic injection moulding circuits.
The function of hydraulics in these machines are (1) injection(2) clamping.
Please refercircuit 6(1). We have two cylinders one each forclamping and for injection. These are operated manually and oneafter another. Hence the connection shown is in tandem.
(i.e.) Tank line of one DC valves connected to the pressureline of the second DC valve. The DC valves used are tandem valves(i.e.) P&T are connected in neutral position.
The pilot operated check valve (POC) takes care of retainingclamping pressure while the valve is in neutral position. Since bothDCV's are connected in tandem, in neutral position of both DCV'sthe pump output is freely vented to tank.
The circuit has an additional relief valve on the A line ofinjection cylinder. This helps to set the injection pressure ofinjection cylinder.
While this circuit is a basic one, further sophistication can beadded by having solenoid operated DC valves and a hydraulicmotor drive for injection screw drive etc.
CLAMPING INJECTION
FIGURE -6(1)
49
48
CLAMPING INJECTION
POCWOR CLOSED.
CLOSING WILLPRESS VALVE TOTHLS PC6ITION
M
I
L
P
PLASTIC INJECTION MOULDING M/C.
FIGURE - 6( 2)
50
SCREW DRIVE
w
FOI
U
' For illustration phase see circuit fig 6(2).
:The features of the Circuit are
1. Safety provision for clamping cylinder - unless knob A ofthe DC Valve is pushed by a sliding door that will close themachine, oil, cannot flow thro' this DC valve to the clampingcylinder. This is for the safety of the operators.
2. Independent pump lines foreach DCVS to injection cylinder,clamp cylinder and screw drive motor.
3. To reduce heating, an unloding relief valve is provided. Thiswill help the pump to unload during starting, curing etc.,
51
CHAPTER 7
A SIMPLE PRESS CIRCUITAPPLICATION OF DOUBLE PUMPS
We are considering here a hydraulic press. The applicationsof hydraulic press can be many - like deep drawing, forming,shearing, bending, notching, baling, rubber curing etc.,
For the sake of simplicity let us take an example of a curingpress having
a. Single cylinder application - single acting - upstroking (i.e.)gravity return
b. For curing application (i.e) pressure holding for a particularduration of time.
1. Tonnage or force : 100 tons
2. Day light : 1 meter (The height available betweenmoving and fixed plates (i.e.) at the fully retractedposition of the cylinder.
3. Speeds : Rapid appraoch and return 2 meters/mt.Pressing is 0.2 meters/minute -
4. Curing time : 20 minutes (Steam curing)
5. Operation : Manual
Generally above specifications are sufficient to work out thehydraulic circuit and select the components/elements of the circuit.Perhaps the designer on his own can take more interest to know theother details of the press such as the platen weight (for gravityreturn), duty cycle etc.,
M
POC
--LINE OF PILO'.CEC'!R VALVE
FIGURE_7(1)
53
Q
52
a
The steps involved:
1. Find out the working pressure:
At present many of the valves are limited to a workingpressure of 250 bar. Hence we can keep the max workingpressure to 210 bar. (3000psi) and select other parameters.
2. Work out the area of the cylinder:
Generally the press manufacturer will give this information.However the method of working is to be understood.
Working pressure
210 kg/cm2 =
LoadFull bore area of Hydraulic cylinder
100TFull bore area of hyd. cylinder
Area = 476 cm2
This is the full bore area of a cylinder that we can choose.
The cylinder manufacturers follow ISO standards in respectof bore sizes of hydraulic cylinders (e.g.). The preferred bore sizecan be 40,50,63,100,125, 150, 160, 200, 250 and 300 mm.
The full bore area closest to our requirement of 476 cm2 isby using 250 mm bore dia cylinder whose full bore area works outto 490 Sq.cm.
So we choose 250 mm bore dia cylinder. The rod size can beleft to the choice of the manufacturer of hydraulic cylinders.
3. Work back actual working pressure.
Working pressureLoad
area
l00T x 1000 kgs
490 cm 2
= 204 Kg/cm2
54
P
FIGURE _ 7 (2)
FIGURE _7(3)
HP INDICATED BY THE
AREA HATCHED
ztoKI/cm
55
POWER REQLU ED WH'LE
RAPIDLY MOVING THE
PLq'EN AT 10 KG'Cm
BUT USING FULL VOLO'-'E
OF 110 LPM_---
POWER REQUIRE WILEPRESSING AT 210BARBUT USING ONLY LPM
4. Calculate the flow of the pump required:
Q Flow rate =cylinder bore area x velocity
490 cm 2 x 200 ems/m1000
= 981pm
The pump which can deliver 100 1pm will be the mostappropriate. (at 1440rpm & on no load). However it is better toconsult the hydraulic product catalogue of standard manufacturerand choose a pump that meets this requirement.
We have not considered the pressing speed 0.2 meters/minin the above calculation, as such as calculation would result in alower pump capacity and in turn speed.
5. Calculating the electric horse power required:
Standard Industrial drive speed 1440 rpm is assumed here.
The actual horse power required to drive a pump delivering100 lpm at 1440 and at 210 bar can be obtained from theperformance curve of any hydraulic pump manufacuturescatalogue. However for theoretical purpose , the thumb rulecalculation is:
PQ _ 210 k?/cm 2 x 100 lpmH.P. = 600 600
35 KW
= 50 H.P. say (Nearer)
6. Now on to the Circuit,
The circuit is simple and can be understood by the reader.But the importance is the selection of the pump capacity/motorHP/Flow and pressure capacity of elements.
Please refer figure 7(1)
56
100.T UPSTROKING CURING PRESS
SINGLE ACTIN(-, CV-NDER
25O
cE-cN-ARV RELEF TO PEGPC'NER CONSUMPTIGN
PR CON?RCL MODU-E
11G0AR 10 BAR
FIGURE - 7 (4)
57
The DC valve chosen is three position spring centre with allports connected in neutral position. By this we can start the electricmotor on no load and also the POC can normally be connected onlywith such DC valves. these DCV configuration helps the pilot lineof POC to drain in neutral position. As covered earlier the POCvalve hermetically seals so that the pressure is held throughout thecuring process.
In this case the motor HP theoretically works out to about50HP. This is nothing but the power absorbed and can be illustratedin the PQ diagram as follows.
Please refer figure 7(2)
This can be interpreted that throughout the cycle or theoperation we are considering the full pressure and the full volume.The question is, whether this is required?
In case we consider an option where the platon is movedrapidly using higher volume of oil but at low pressure and once thepressing commences, use a higher pressure but with lesser volumeof oil. This is possible with a hi-low circuit using a double pump.Accordingly the PQ diagram changes as in figure 7(3). When weconsider, a double pump, with high volume and low pressure forfaster approach and for pressing, the claculation is as follows :
Assume the weight of platon and ram as 2 T
The pressure required for lifting = LoadArea
2T x 1000 kg
490 cm 2(Bore Area of Cylinder)
= 4.08 Kg/cm2(shown as 10 Kg/cm2 in figure 7 (3)
The theoritical HP
rapid closing =l0KK/cm2 X 1001pm
600PQ
600
1.6 KW
3 HP
For pressing, volume of oil required at 0.2 m/mt
Q = AxV
= Area of Cylinder x Velocity
490 cm2 x 20 cms/mt
10 1pm (say)
The HP requi red for pressing and locking will be ; P here600
will be 210 Kg/cm2 and Q will be 10 lpm.
210x10(i.e.) 600 = 3.5 KW
= 5 HP (say)
So if we choose a motor of 5 HP, we can run the system andachieve what we require but of course we would require doublepump. This double pump should be in a position to deliver 1001pmat 10 kg/cm2 while lifting and start giving 101pm at 210 kg/cm2.
A double pump circuit is shown in figure 7(4). To fecilitateour requirement of 1001pm at 10 kg/cm2 and later on 101pm at 210kg/cm2 we must have two relief valves set at two pressure ratingsas above. A standard pressure control module (PCM) is available,wherein we get initially 100 lpm at 10 kg/cm2. Once pressing
commences , the relief valve on the large volume pumps startsunloading and the high pressure low volume pumps startsdelivering at a pressure of 210 kg/cm2. So the nett effect is thereduction in HP from 50 to 5 using a double pump.
58 1 59
Chapter 8
FEW MORE APPLICATIONS
In this chapter we shall look at more applications - such astable feed drilling, press brake, rubber moulding, thread rolling andstacker hydraulic circuits. The idea behind choosing the variedapplications is that the reader understands the circuits, and giveshim a feeling that any other circuit similar is easy to understand andto appreciate.
The reader should bear in mind that the more circuits helearns, and puts to use, it becomes easier for him to think of anapplication and try his hand in designing the circuit.
Let us now look at the first application. Please refer fig (8.1)This is for table feed drilling machine.
Table feed drilling machine application :
Here the table with the component is lifted up rapidly. Afterthis rapid lift, the feed for drilling, milling etc., takes place. Oncethe operation is over, the table comes down rapidly. You look atfig 8(1), you will observe that all ingredients of a basic block andsimilarity with fig 5(6) feed / speed control are there. However,physically, the machine tool manufacturers opt for a free standingpumping set and all control values suitably manifolded to bemounted on the column of the drilling machine.
The features here are,
(1) The main DCV has a tandem open centre configurationwhereby pump unloading is achieved. (ie.) the motor stantson no load when DCV is in neutral position.
(2) Two speeds (rapid and feed) are possible in upward directionand rapid descent in opposite direction are possible.
Xi;°CJ
-- - --- -
HYDRAULIC CIRCUIT FOR TABLE FEED DRILLING M/C
FIG. P-1
61
60
Press brake application
Press brake are used for sheet metal working like bending,notching, punching etc.,
Please refer figure 8(2)
The customer for this press brake requires a rapid downwardapproach, and full tonnage for doing the work and thereafter a rapidreturn.
Now look at the two cylinders doing the work. Herehydraulically, synchronising the movements of both cylinder ramsare not considered. This is achieved by mechanical means.
The features of this circuit are
I. High low double pumping system to minimise input power(please refer chapter 7)
2. The main DCV is pilot operated (for handling larger flow)
with tandem open centre configuration (i.e) P&T connectedin neutral position for no load start.
3. Counter balance value (CBV) set for a value just to balancethe. weight. This is to avoid run away load tendencies.
4. A secondary relief value (no 19) limits the maximum liftingtonnage to the desired value. What happens if this valve isnot provided?
If the valve is not provided their the press works at the full
set pressure of the main high pressure relief valve (i.e) at the end
of the lift stroke.
This also helps in not loading the structure of the machine tomax value at the upward end of the stroke.
Thread rolling application
Please refer figure 8.3
Threading on metals can be done in many ways like threadturning, chasing and thread forming. For mass production normallythread forming is preferred using thread rolling machine.
62
M
HV RAU_IC CIRCUIT FOR PRESS BRAKE
FIG.. 9.2
63
I
The principle of the thread rolling is that [tic job is kept
between two rotating full profile thread rolls. These !hread rolls are
driven with electric motors. However, hydraulics is used to form
the thread on the job by plunging in the rolls at low feed rate. These
rolls then return at rapid speed. The features are
1. The pump starts at a low pressure (of about 10kgicm2)keeping the rolls away from each other.
2. The rolls move together synchronised mechanically (not byhydraulics in this circuit)
3. When the DCV is energised the rolls come towards eachother and towards the job at slow speed (and at higherpressure meter in circuit)
4. Generally the thread rolling machine the tonnage can be highcompared to the HP. employed. For instance with a 2 HPmotor and with very slow feed corresponding to 40 bar andwith 300 mm bore cylinder we can get the desired hightonnage.
Rubber moulding and curing application
This hydraulic circuit is employed in a multiple platten,stream curing up stroking press. Please refer fig 8(4)
In this press upstroking is done by large ram type singleacting cylinder. Lowering of the ram is by gravity.
The features are
1. A double pump with hi -low circuit , we have seen earlier isemployed.
2. A special DC value manually operated with decompressionfeature is used.
3. The DC value itself is a load holding zero leak value .
4. With 18 inch ram diameter we are able to get load of 350tons at 220 Bar.
SCR T ^.cA R^__iNT,
F:G -8-3-
65
64
5. The steam curing takes place in pressed (ie) load holdingcondition say for above 20 to 30 minutes This is called asvulcanising . The DCV can hold the pressure and the pumpis usually stopped to save power.
6. Ram type single acting cylinder is used as we do not requireany power for lowering the plattem . This is achieved bygravity.
Stacker application
Please refer figure 8.5 This looks more complicated. Butonce the circuit is understood, the simplicity of the logic can beappreciated. In any large processing plant such as for examplecement plant, paper bags are used for packing the cement.
For easy transportation, these cement bags have to be stacked
in an appropriate manner. This is called Pellatisation. This
hydraulic circuit as in figure (8-5) helps us to achieve this objective.
The features are
1. We have two cylinder one for stacking and other fordestacking
2. Here a double purp is employed (ie) two pumps are drivenby a single motor but two different outputs are taken forindependent operation.
Cylinder 1 is for unloading pallet trays to the cruveyor. It isan independent operation. So an independent pump is used,
Cylinder 2 is for stacker application, where as the load(cement bags) increases the stacker arm is brought down,step by step.
After the job is done, the empty stacker arm is to be lifted uprapidly. To achieve this rapid speed, without load, aregenerative circuit is employed.
66
a
HYDRAULIC CIRCUIT FOR RUBBER CURING M/C
FIG.: 6- 4
67
I
0
ackEC_Y LIP.,F?2_L
For positioning the stacker arm at mid intervals , a POC valve
is used. A counter balance valve preset for maximum load
pressure assures smooth lowering . The proportional valve is
programmed for acceleration and deceleration v ith varying
load conditions.
3. We come across a new feature -- a preportional pressure an('
flow control valve combination is used as a load sensing
system. (9) for smooth acceleration and declaration with
varying load conditions. In addition to proportional control.
PLCs (Program able logic controls) are used for different
programming for different sizes of cement bags . (loads)
With PLC and proportional controls, smooth movement is
taken care of.
The number of hydraulic circuits can be limited only by
imagination acid by actual requirements.
There can he many solutions to one reuuirements and t`ae
near optimum circuit designing Lames b years of oxposure
.nd experience
`1YOPAULIC C'RCUIT FOP ST.;CKER
'IG: 6-5 -
08
b:?
CHAPTER 9
THIS WAY TO HYDRAULIC CIRCUITS
This book is meant to give basic knowledge on designing a
hydraulic circuit. But a hydraulic circuit designer should have solid
information base about the hydraulic elements, their symbols,
functions and certain fundamentals. The reason that this chapter
comes almost at the end of the book is that we have assumed the
readers have a working knowledge of hydraulics principles,
theories and the functional aspects of hydraulic elements. However
our thinking is also to show the way to the building of hydraulic
circuits.
It is possible to write a separate book on the principles,constructions , functions of oil hydraulics and the elements used.But we are going to make an attempt to give a concise account ofthese fewer aspects.
9.1 The symbols :
The Hydraulic circuit designer should be thorough with the
symbols and we have reproduced these symbols in the annexure.
9.2 Pascals Law
This is a simple law and means this :
Pressure applied on a confined fluid is transmittedundiminished in all directions and acts with equal force on equalareas and at right angles to them.
How is this applicable to everyday usage of hydraulics`!
If we apply a small force on a small area we can get ittransmitted to larger area and you will get higher force available to
A .CAD OF 1OCKG
ON1CS0 CM
PISTON
10SO CM
L--- DD7E'_CPES
Qt I
- CONS NEJ
FIG_ 9.1
LONG PIPE LINE
FIG- 9 2
71
'C00 KG
PRES-:PE
yy_ SDP PCPT
7!a .ISTCN
i
70
work foryou (of course it should be a force on confined fluid) pleaserefer figure 9.1
9.3 Positive displacement pumps
We have come across, in cur daily living domestic pump sets- which are centrifugal pumps.
How are the pumps used in hydraulic power units differentfrom this centrifugal pumps. The differences are,
1. Most pumps used in hydraulic systems are positivedisplacement pumps
2. The pump output is normally constant irrespective of thepressure.
3. The outlet is positively sealed from the inlet, so that whatevergets into the pump is forced out through the outlet port.
Th,.re is a misconception or shall we say a wrong expressiongzrerally voiced by many users of oil hydraulic systems.
They come with a complaint, at times, that the pump doesnot develop pressure. We must understand very clearly that thepumps here, create only flow; and not pressure. The pressure in thesystem is due to the resistance to the flow. The resistance or thepressure is created when the hydraulic oil has to flow thro' pipes(pipe friction), thro' bends and joints and thro' orifices of thevalves; and from the load of an actuator. You will come acrosspump specification of manufactures that the max. pressure of thepump is say 160 bar. This only means that the pump can withstanda pressure of 160 bar and not that it creates a pressure of 160 bar.
If this complaint is that the pump is net developing pressure,then the user is trying to tell us that with his system. he is net ableto get the work done. If the problem is with the pumps internals, itcan be because the pump is womout and the sealing between suctionand delivery is no forger effective. But the problem can beanywhere along the lire from suction strainer to the cylinder. Weshall discuss this subject separately.
72
-=---1 L-_ 1-- - --^
THE PRESSURE -_-- __ PRESSURE HERE
HERE - --- - _ - `--- -- - THEN NO FLOW
FIGURE-9 3 (1 )
AN WCREASE _ - .
PRESSURE -- - 'IE OIL TO F^_OW -- - TO THIS PLACE
HERE CASES TORO THE ORIFICE
FIGURE-9 3 (2)'HE PRESSURE DROP_-_ 7
A_TND R ARE EGUAL SIZE CYL'.NDERS
LOA[_' CS A_IS_ IOC.-?,___ SAD _N
57C D[I L EST TAKE
THE PATH OP LEAST RESISTANT E(,e )
CIL WILL ALOW TO_A __F_RST. UFT
LIFT ^ IULLY AND THEN WILL
FLOW T0 B
PUMP
FIGURE_9-4
73
e
N
9.4 Pressure drops and orifices :
Any Hydraulic power unit has flow of oil thro' Orifices (Anyvalve has an orifice and the more the valves are in a power unit, thegreater is the emphasis on pressure drops.
Even on a normal flow of fluid through a long pipe line, therewill be a pressure drop. Refer figure 9.2.
The pressure drop is because of the friction of the pipe.
As the oil, flows thro' the pipe, fittings, manifolds andvalves, there is a reduction in the final pressure available to theactuator. It is not possible to avoid the pressure drop. However byproper sizing of the pipe and the valves it is possible to reduce thispressure drop. Energy lost due to pressure drop is converted to heat.To reduce this pressure drop, proper selection of valve and pipingare important.
The heat produced due to energy lost results in viscosityreduction of oil. This reduction of viscosity results in leakages, lossin lubricity and cause wear and tear and inefficiency.
Then, can we use the orifices to adjust speed?
Yes. This is the right question. To tune the system for speeds,we can open and close the restrictions.
9.5 Path of Least Resistance
Fluid flows through the path of least resistance when thereis more than one possible flow path in the system. please refer figure9.4
A and B are equal size cylinders load on A 100 Kg. Load onB is 500 Kg.
Oil will first take the path of least resistance (ie) oil will flowto A first lift it fully, and then will flow to B.
A'.,GE aRESSURE
EKG CIO
"
;.UGE PRESSURE
FIGURE-9 5
GAUGE PRESSURE
G:^1 CRFS A?41 PP
FRICTION REDJCES THE
HEAD AT SUCCEEDINGPOINTS EXCEPT WHERE
LARGE? PIPE REDUCES
VELOOCT-- -, THE P'.-0 "
AND PRESSURE INCREASE
A`vr'SL I, rt
ABSOLUTE PRESSURE
760r 10M!s '1 27MIsIN HG VACUMN SCALE. WATERCCWMN 0IL COLUMN
BAROMETER SCALEVACUMN CAI;GESVACUMN I^N7-'S RANGE
FIGURE 9 6
ATMOSPHERIC PRESSURE = 1.03 AC;CM
BAR
76CmmCF HG'-0 "E'ERS GF WATER COLUMN
1127 METERS CF OIL COLUMN
GAUGE PRESSURE IN BG/CM2+1.03 NO'CM ABSOUJTE PRESSURE IN NGrrCM
75
74
9.b Bernoulli's Principle of oil flow'
Understanding Bernoulli's principle is an important factor inthe design of hydraulic valves. In such valves the oil path sizeschange and Bernoulli's principle tells us, what happens if there isan increase or decrease in the size cf the oil path. We have earlierin the chapter seen the changes in pressure drops across orifices.Here we shall discuss the path ways.
We must first understand that the fluid in a working systemcontains energy (ie;i Kinetic energy by virtue of velocity and itsweight and potential energy in the form of pressure.
Bern-)ulli's principle states that if the flow rate is constant,the sums of the kinetic energy and the pressure energy at variouspoints in a system must be constant. Therefore, if the kinetic energydecreases, it results in an increase in the pressure energy.
Please refer figure 9.5
As in figure 9.5, when the Cross Sectional area of a flow pathincreases, the decrease in Kinetic energy (velocity) iesuits incoirespond-iig increase of pressure energy as shower by a higherheat;
4.7 The effects of pressure
In a hydraulic power unit, pressure is generated by resistanceto the flow of oil and mainly by the load. In other words pressureis proportional to the work load, and a pressure guage readingindicates generally the work load.
In many places, the user of a hydraulic power unlit ignorestLe importance of apressure guage and normally the pressure guagecf a system does not work.
Only when there is a problem in the power unit, theimportance of the pressure guage will be realised.
The pressure guage normally ignores atmospheric pressure.(ie) The standard guage points to zero at atmospheric pressure.
76
TI
FfGURE.97( Ii
GU^F.9 ! 2)
ATMOSPHERIC PRESSURE
PUSHES MERCURY IN TO
THE TUOE_
MERCURY RISES BY 760 mm
ST_MO_SP_HERIC PRESSURE
PUSHES_ODU iN TO THE
PJMP Li IE
A
9.7.1 Atmospheric pressure :
It is the weight of air, atmosphere outside, that exists. We areused to it, so we do not feel it.
This is actually the weight of air in the atmosphere that actson every square centimeter.
In terms of value, this atmospheric pressure works out to 1.03Kg/cm2. Please refer figure 9.6
* One atmospheric Pressure = 1.03 kg/cm2
1. BAR
760 mm of Hg
10 meter of water column
11.27 meter of oil column
* Guage pressure in Kg/cm2 + 1.03 Kg/cm2 = Absolutepressure in kg/cm2
Most of the hydraulic oil pumps used are capable of creating
only a partial vacuum. Air pressure on the oil in the reservoir pushes
oil up the suction line. As the pump cannot suck the oil all the way
through, the height of pump installation above oil level is a matterof concern.
Pumps available today can normally create a partial vacumn•equivalent to about 150 mm of mercury which is one fifth on thevacumn scale. In terms of oil height one fifth of the oil height worksout to about 2.25 metres. This is in ideal conditions. But when wetalk of practicalities, considering the suction pipe and fitting losses,when suction strainer is also in use, we can consider only about onemetre or there about.
Therefore a hydraulic pump with adequately sized inlet pipeand suction strainer should never be mounted higher than one meterabove the oil level.
For information the suction pipes are selected for amaximum oil velocity of 1 meter per second.
Cavitation and aeration in a pump
A pump with adequate suction characteristics (ability toincrease partial vacumn to the extent of one fifth in vacumn scale(ie) six inches of Hg or 150 mm of Hg)-can be installed in withinone meter height of oil level. If the pump does not have suchcharacteristics, the pumps can be mounted below the oil level.
We must remember here that for every foot of oil, oil createsa static head pressure of 0.4 psi.
As we try to locate the pumps below the oil level, for eachfoot, we add a positive pressure of 0.4 psi available to the inlet ofthe pump.
Under these conditions the pressure losses due to strainer,suction line and fittings still to be substracted from the totalpressure available at the inlet conditions.
In a partial Vacumn Condition, when the pump is locatedabove the oil level, then this less than atmospheric pressure acts onthe oil.
The mineral based hydraulic oil, which we use, contain about8 to 9% of dissolved air. Now, when the `less than atmosphericpressure' acts on the oil, the air in the oil expands and becomes ahigher percentage of volume. This means, more air in oil gets intothe pump chamber. At the outlet of the pump is the system pressurewhich is considerably more. The air bubbles in the oil will nowcollapse at considerable pressure in the pumping chamber. Thisrapid collapsing of air bubbles results in rapid energy losses in theform of heat and noise. This heat can result in combustion of oilleading to carbonisation.
When the pressure gets low on the suction side, it can alsoresult in vaporisation of oil, the combined effect of all this is knowas cavitation effects.
78 1 79
Chapter 10
TROUBLE SHOOTING
It is only appropriate that a person who designs the circuits,knows what can go wrong with the power units.
In the pages to follow we are giving charts descrih,ngcommon problems and remedies that can solve these problems.
These charts also will be very useful for maintananceengineers using hydraulic power units in their shops.
Most of the problems in hydraulic machinary are relates; tooil contamination. The suction strainer can clog du,; tocontamination. This can result in cavitation of pump.Contamination can also make th,, valve sticky. The DC valve 'oilscan bum due to sticky valves.
Anotherpoint is that we should ensure oil and airdo not mix.They are not made for each other.
A loose suction joint can allow system toaerate causingsevere pump noise and jerky movement of actuator.
A word of Caution
We have come across problems that are not associated with
power units but are machine problems or relating to electrical
circuits. Therefore it is essential that before trouble shooting, the
problem is identified to be relating the hydraulics side of he
machine.
80
REMEDY : d
CHART I - REMEDIES
a. Any or all of the following : Replace dirtyfilters - Washstrainers in solvent compatible with system fluid - Cleanclogged inlet line - Clean reservoir breather vent - Changesystem fluid - Change to proper pump drive motor speed -Overhaul or replace superchange pump - Fluid may be toocold
b. Any or all of the following: Tighten leaky inlet connections- Fill reservoir to proper level (with rare exception all returnlines should be below fluid level in reservoir) - Bleed air fromsystem - Replace pump shaft seal (and shalft if worn at sealjournal)
c. Align unit and check condition of seals, bearings andcoupling
d. Install pressure guage and adjust to correct pressure
e. Overhaul or replace
CHART II - REMEDIES
a. Any or all of the following : Replace dirty filters - Cleanclogged inlet line - Clean reservoir breather vent - Changesystem fluid - Change to proper pump drive motor speed -Overhaul or replace supercharge pump
b. Any or all of the following: Tighten leaky inlet connections- Fill reservoir to proper level (with rare exception all returnlines should be below fluid level in reservoir) - Bleed air fromsystem - Replace pump shaft seal (and shaft if worn at sealjournal)
c. Align unit and check condition of seals and bearings Locateand correct mechanical binding - Check for work load inexcess of circuit design.
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d. Install pressure guage and adjust io correct pressure (Keepatleast 125 PSI difference between valve settings)
e. Overhaul replace
f. Change filters and also system fluid if of improper viscosity-Fill reservoir to proper level
g. Clean cooler and/or cooler strainer - Replace cooler controlvalve - Repair or replace cooler.
CHART III - REMEDIES
a. Any or all of the following : Replace dirty filters - Cleanclogged inlet line - Clean reservoir breather vent - Fillreservoir - proper level - Overhaul or replace superchargepump
b. Tighten leaky connections - Bleed air from system
c. Check for damaged pump or pump drive - replace and aligncoupling
d. Check for damaged pump or pump drive - replace and aligncoupling
d. Adjust
e. Overhaul or replace
f. Check position of manually operated controls - checkelectrical circuit on solenoid operated controls - Repair orreplace pilot pressure pump
g. Reverse rotation
h. Replace with correct unit
CHART - III
HNCORF,ECT FLOW
A S
ENO FLOW
PUMP NOT RECEIVINGFLUID I
REMEDY a
PJHP CRIVE MOTORNOT CPERATIN G
REMEDY c
P PDRIVE MOTORP RNING IN WRONG
DIRECTION
REMEDY g
DIRECTION CONTROL
SET IN WRONGPOSITION
REMEDY :f
ENTIRE FLOW PASSINGOVER RELIEF VALVE
REMEDY d
DAMAGED PUMP
REMEDY c
IMPROPERLY
ASSEMBLED PUMF
REMEDY e
LOW FLOW
P FLCV CONTROL SETTOO LOW
REMEDY d_
RELIEF OR UNLOAD:NGV V SET TOO LOW
REMEDY e ar f
EXTERNAL LEAX INSYSTEM
REMEDY
YOKE ACTUATINGDEVICE INOPERATIVE
(VARIABLE DISPLACEMENTPUMPS)
REMEDY
RPM OF PUMPDRIVE MOTORINCORRECT
REMEDY h
WORN. PUMPV V, MOTOR, CYLINDEROR OTHER COMPONENT
REMEDY : e
C
L EXCESSIVE FLOW I
FLOW CONTROL SETTO HIGH
REM E!'Y d
I YOKE ACTUATINGDEVICE INOPERATIVE
(VARWBLF ISPLAC MEN''PUMPS)
REMEDY
IMPROPER SIZEPUMP USED 'ORREPLACEMENT
REMEDY 'I
ERPM OF PUMP DRIV
MOTOR INCORRECT
REMEDY h
i
84
CHART - V
FAULTY OPERATION
in
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K
VfU,
3 w JO ..O
a I- HG' °O W =Z uOf
U
w
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z Q u
n o0 r roz < o O J
z 5 Wcr fe3
UO OU
86
0
A
NOMOVEMENT
NO FOLW ORPRESSURE
REMEDY : SEECHART III COL A
LIMIT ORSEQUENCE DEVICE
(MECHANICALELECTRICAL ORHYDRAULIC)
INOPERATIVE ORMISADJUSTED
REMEDY e
MECHANICALBIND
REMEDY b
NO COMMAND
SIGNAL TO SERVOAMPLIFIER
REMEDY f
INOPERATIVE OR
MISADJUSTEDSERVO AMPLIFIER
REMEDY c
INOPERATIVESERVO V V
REMEDY c
IWORN OR DAMAGED
CYLINDER ORMOTOR
REMEDY e
c
SLOWMOVEMENT
LOW FLOW
REMEDY : SEECHART Ill COL 8
FLUID VISCOSITYTOO HIGH
REMEDY : a
INSUFFICIENTCONTROL PRESSURE
FOR VV
REMEDY : SEECHART IV
NO LUBRICATIONOF MACHINE
WAYS OR LINKAGE
REMEDY g
MISADJUSTED OR
MALFUNCTIONINGSERVO AMPLIFIER
REMEDY : c
STICKING SERVOVALVE
REMEDY : d
WORN OR DAMAGEDCYLINDER OR
MOTOR
REMEDY : e
ERRATICMOVEMENT
ERRATICPRESSURE
REMEDY : SEE
CHART IV COL C
AIR IN FLUID
REMEDY : SEE
CHART 1
D
EXCESSIVE SPEEDOR MOVEMENT
EXCESSIVE FLOW
REMEDY : SEE
CHART III COL C
FEED BACKTRANSDUCER
MALFUNCTIONING
REMEDY e
NO LUBRICATION MISAD^ JUSTED IOF M/C WAYS OR MALFUNCTIONING(
OR LINKAGE I SERVO AMPLIFIER
REMEDY : g REMEDY : ..I
ERRATIC COMMANDSIGNAL
REMEDY f
MISADJUSTED ORMALFUNCTIONINGSERVO AMPLIFIER
REMEDY c
I MALFUNCTIONING(FEEDBACK TRANSDUCER
REMEDY
STICKING SERVO VALVE
REMEDY d
WORK OR DAMAGEDCYLINDER OR MOTOR
87
REMEDY : e
OVER-RIDINGWORK LOAD
REMEDY h
CHART IV - REMEDIES
a. Replace dirty filters and system fluid
b. Tighten leaky connections (fill reservoir to proper love; andbleed air from system)
c. Check gas valve for leakage Charge to correct pressureOverhaul if defective
d. Adjust
C. Overhaul or replacc
CHART V - REMEDIES
a. fluid may be too cold or sl.ou:d be changed to clean fluid of';orrcct viscosity
b. Locate hind and repair
c. Adjust, repair, or replace
d. Clean and adjustor replace - Check condition of system fluidand filters
e. Overhaul or replace
f. Repair command console or interconnecting wires
g. Lubricate
h. Adjust, repair, or replace counterbalance valve.
STANDARD GRAPHICAL S''MBOLS
88
RESERVOIR VENTED
PRESSUR GAUGE
ACCUMULATOR
GAS CHARGED
FALTER OR STRAINER
PRESSURE SWITCH
CHECK VALVE
MANUAL SHUT OFF
VALVE
VALVE , MAXIMUM
PRESSURE (RELIEF)
l-----]
0
4
90
MULTIPLE FLOW PATHS
(ARROW SHOWS FLOWDIRECTION)
UNLOADING VALVE,
INTERNAL DRAIN,
REMOTELY OPERATED
DECELERATION VALVE,
NORMALLY OPEN
SEQUENCE VALVE
DIRECTLY OPERATED
EXTERNAL DRAIN
PR. REDUCING VALVE
COUNTER BALANCE
VALVE WITH INTEGRAL
CHECK
FTFMlEllTURE AND
PR COMPENSATED
FLOW CONTROL WITH
INTEGRAL CHECK
I DIRECTIONAL VALVE .TWO POSITION, THREE
CONNECTION
DIRECTIONAL VALVE,
THREE POSITION,FOUR
CONNECTION
VALVE, INFINITEPOSITIONING (INDICATEDBY HORIZANTAL BARS)
PROPORTIONAL CONTROL
PRESSURE COMPENSATOR,
DETENT
i LEVER
I
SOLENOID
SOLENOID CONTROLLED
PILOT PRESSUREOPERATED
SPRING
--
f1
