The Source of Hydraulic Power

8/12/2019 The Source of Hydraulic Power

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HHHYYYDDDRRRAAAUUULLLIIICCCSSSAAANNNDDDPPPNNNEEEUUUMMMAAATTTIIICCCSSSChapter: 2

THE SOURCE OF HYDRAULIC POWER

Introduction:

A pump is the heart of a hydraulic system, which converts mechanical energy into

hydraulic energy.

The mechanical energy is delivered to the pump via prime mover such as an electric motor.Due to mechanical action, the pump creates a partial vacuum at its inlet. This permits atmospheric

pressure to force the fluid through the inlet line and into the pump. The pump then pushes the

fluid into the hydraulic system.

Fig 2.1 The hydraulic pump

A Hydraulic pump (fig 2.1 a) takes oil from a tank and delivers it to the rest of thehydraulic circuit. In doing so it raises oil pressure to the required level. Fig 2.1b shows the

symbol of a pump with the arrowhead showing the direction of flow. Fig 2.1c shows pump

associated components for the working of the pump.

Pumping Theory:

Pumps operate on the principle whereby a partial vacuum is created at the pump inlet due

to the internal operation of the pump. This allows atmospheric pressure to push the fluid out of the

oil tank (reservoir) and into the pump intake. The pump then mechanically pushes the fluid out tothe discharge line.


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Fig 2.2 Pumping action of a simple piston pump.

In the above fig 2.2, pump contains two ball check valves, one is connected to the pumpinlet line and allows fluid to enter the pump only, and second one is connected to the pump

discharge line and allows fluid to leave the pump.

When the piston is pulled to the left, a partial vacuum is generated in pump cavity 3because of the close tolerance between the piston and cylinder prevents air inside cavity 4 from

traveling into cavity 3. This vacuum holds the ball of check valve 2 against its seat (lower

position) and allows atmospheric pressure to push fluid from the reservoir into the pump via check

valve 1. This inlet flow occurs because the force of the fluid pushes the ball of check valve 1 offits seat.

When the piston is pushed to the right, the fluid movement closes inlet valve 1 and opens

outlet valve 2. The fluid displaced by the piston is ejected out the discharge line leading to thehydraulic system.

Classification of pump:

1. Dynamic (nonpositive displacement) pumps/ Hydrodynamic pump: This type is generallyused for low-pressure, high-volume flow applications. Because they are not capable of

with-standing high pressures, they are of little use in the fluid power field (250 300psi).

In this clearance b/w the rotation impeller or propeller and the stationary housing

should be less. Thus as the resistance of the external system starts to increase, some of thefluid slips back into the clearance spaces, causing a reduction in the discharge flow-rate.

This is due to the fact that the fluid follows the path of least resistance. When the

resistance of the external system becomes infinitely large, the pump will produce no flow.

This type of pump is used for transporting fluids from one location to another.

There are two types of Dynamic pumps:a) Centrifugal pumps (impeller)

b) Axial flow (propeller) pumps


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a) Centrifugal pump:

Fig 2.3 Centrifugal pump (a) Construction features. (b) Pressure versus flow curve.

The above fig shows the construction features of a centrifugal pump. The fluidenters at the center of the impeller and is picked up by the rotating impeller. As the fluid

rotates with the impeller, the centrifugal force causes the fluid to move radially outward.This causes the fluid to flow through the outlet discharge port of the housing.

The dynamic pumps provide smooth continuous flow (when a demand exists); their

output flow rate is reduced as resistance to flow is increased. This is shown for centrifugalpumps in fig 2.3 b, where pump pressure is plotted versus pump flow. The maximum

pressure is called the shutoff headbecause an external circuit valve is closed which shuts

off the flow. As the external resistance decreases due to the valve being opened, the flow

increases at the expense of reduced pressure.

b) Axial pump:

Fig 2.4, Axial propeller type.

Here the fluid is pumped by the dynamic action of the propeller. Here the fluid

flows parallel to the axis of the shaft. The figure 2.4 shows the axial pump.

The dynamic pumps are used for low-pressure, high-volume flow applications. Also

since there is a great deal of clearance between the rotation and stationary elements,


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dynamic pumps are not self-priming unlike positive displacement pumps. This is because

the large clearance space does not permit a suction pressure to occur at the inlet port when

the pump is first turned on. Priming is required to pump the fluid from the reservoir.(Priming is the prefilling of the pump housing and inlet pipe with fluid so that the pump

can initially draw in the fluid and pump it efficiently.)

2) Positive Displacement Pumps: This type of pump ejects a fixed quantity of fluid perrevolution of the pump shaft. As a result, pump output flow, neglecting changes in the

small internal leakage, is constant and not dependent on system pressure. This makes them

particularly well suited for fluid power systems. Usually a pressure relief valve is used toprotect the pump against overpressure by diverting pump flow back to the hydraulic tank.

(The over pressure happens if a valve is completely closed and there is no physical place

for the fluid to go.)...

Fig 2.5, Positive displacement pump.

Fig 2.5 shows a simple piston pump called a positive displacement or hyraostaticpump. As the piston is driven down, the inlet valve opens and a volume of fluid is

drawn into the cylinder. Next, the piston is driven up with the inlet valve closedand the outlet valve open, driving the same volume of fluid to the pump outlet.

Classification of Positive displacement pumps:

1. Gear pumps (fixed displacement only by geometrical necessity)

a) External gear pumps

b) Internal gear pumpsc) Lobe pumps

d) Screw pumps

2. Vane pumps

a) Unbalanced vane pumps (fixed or variable displacement)

b) Balanced vane pumps (fixed displacement only)

3. Piston pumps (fixed or variable displacement).


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a) Axial design

b) Radial design

1. a) External gear pump:

Fig 2.6, External gear pump.

Figure 2.6, illustrates the operation of an external gear pump, which develops flow by

carrying fluid between the teeth of two meshing gears. One of the gears is connected to a drive

shaft connected to the prime mover. The second gear is driven as it meshes with the driver gear.Oil chambers are formed between the gear teeth, the pump housing, and the side wear plates. The

suction side is where teeth come out of mesh, and it is here that the volume expands, bringing

about a reduction in pressure to below atmospheric pressure. Fluid is pushed into this void by

atmospheric pressure because the oil supply tank is vented to the atmosphere. The discharge sideis where teeth go into mesh, and it is here that the volume decreases between mating teeth. Since

the pump has a positive internal seal against leakage the oil is positively ejected into the outlet

port.Volumetric displacement and Theoretical flow rate:

V d= (D20 D

2i) L

D

Where,D0= outside diameter of gear teeth (m)

D i = inside diameter of gear teeth (m)

L = width of gear teeth (m)V d = displacement volume of pump (m3/rev)

N = rpm of pump

Q T = theoretical pump flow-rate

Q T= V dx N m3/min .. (1)


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QT= V dx N gpm, since 1 gal = 231 in3

.... (2)

231

Fig 2.7. Positive displacement pump Q vs N and P vs Q curves

From the equation 1 and 2 it can be seen that pump flow varies directly with speed.And it is graphically illustrated in the above figure 2.7.

Volumetric efficiency:

v= Q AQT

Where, QA= actual flow rate

QT= theoretical flow rateThe higher the discharge pressure, the lower the volumetric efficiency because

internal leakage increases with pressure. This is shown by the dashed line in fig.

1. b) Internal Gear Pump:

Fig 2.8. Internal gear pump.

Internal gear pump consists of an internal gear, a regular spur gear, a crescent-shaped seal,and an external housing. As power is applied to either gear, the motion of the gears draws

fluid from the reservoir and forces it around both sides of the crescent seal, which acts as a

seal between the suction and discharge ports. When the teeth mesh on the side opposite tothe crescent sea, the fluid is forced to enter the discharge port of the pump.


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1. c) Lobe pump:

Fig 2.9 Lobe pump.

This pump operates in a fashion similar to the external gear pump. But unlike the external

gear pump, both lobes are driven externally so that they do not actually contact each other.

Thus they are quieter than other types of gear pumps. Due to the smaller number of matingelements, the lobe pump output will have a somewhat greater amount of pulsation, although its

volumetric displacement is generally greater than that for other types of gear pumps.

1.

d) Gerotor pump:

Fig 2.10 Gerotor pump.

It operates very much like the internal gear pump. The inner gear rotor (Gerotor

element) is power-driven and draws the outer gear rotor around as they mesh together.

This forms inlet and discharge pumping chambers between the rotor lobes. The tips of theinner and outer rotors make contact to seal the pumping chambers from each other. The

inner gear has one tooth less than the outer gear, and the volumetric displacement is

determined by the space formed by the extra tooth in the outer rotor.

1. e) Screw pump:

Fig 2.11 Screw pump.


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The screw pump is an axial flow positive displacement unit. Three precision

ground screws, meshing within a close-fitting housing, deliver no pulsating flow quietlyand efficiently. The two symmetrically opposed idler rotors act as rotating seals, confining

the fluid in a succession of closures or stages. The idler rotors are in rolling contact with

the central power rotor and are free to float in their respective housing bores on a

hydrodynamic oil film. There are no radial bending loads. Axial hydraulic forces on therotor set are balanced, eliminating any need for thrust bearings.

2. Vane Pumps:

Fig 2.12 Vane pump operation.

Figure 2.12, shows the operation of a vane pump. The rotor, which contains radial slots, is

splined to the drive shaft and rotates inside a cam ring. Each slot contains a vane designed to matewith the surface of the cam ring as the rotor turns. Centrifugal force keeps the vanes out against

the surface of the cam ring. During one-half revolution of rotor rotation, the volume increases

between the rotor and cam ring. The resulting volume expansion causes a reduction of pressure.This is the suction process, which causes fluid to flow through the inlet port and fill the void. As

the rotor rotates through the second half revolution, the surface of the cam ring pushes the vanes

back into their slots, and the trapped volume is reduced. This positively ejects the trapped fluidthrough the discharge port.

Analysis of volumetric Displacement:

Dc = diameter of cam ring (m)

Dr = diameter of rotor (m)

L = width of rotor (m)

Vd= pump volumetric displacement (m3)e = eccentricity (m)

emax= maximum possible eccentricity (m)v dmax= maximum possible volumetric displacement (m3)

we have from geometry,e max = Dc Dr

2


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V dmax = (D2c D2r) L = (Dc + Dr) (Dc Dr) L = (Dc + Dr) 2 e maxL

4 4 4Actual displacement occurs when e max = e

2 a) Pressure compensated vane pump:

Fig 2.13 Variable displacement, pressure compensated vane pump

Pressure compensated vane pump is one in which system pressure acts directly on the cam

ring via a hydraulic piston on the right side. This forces the cam ring against the compensator

spring-loaded piston on the left side of the cam ring. If the discharge pressure is large enough, itovercomes the compensator spring force and shifts the cam ring to the left. This reduces the

eccentricity, which is maximum when discharge pressure is zero. As the discharge pressure

continues to increase, zero eccentricity is finally achieved, and the pump flow becomes zero.

Fig 2.14 Pressure versus flow for pressure compensated vane pump.

As shown in fig 2.14, when the pressure reaches a value called P cutoff, the compensator

spring force equals the hydraulic piston force. As the pressure continues to increase, the

compensator spring is compressed until zero eccentricity is achieve. The maximum pressureachieved is called P deadhead, at which point the pump is protected because it produces no more

flow. As a result there is no power wasted and fluid heating is reduced.


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2 b) BalancedVane Pump:

Fig 2.15 Balanced vane pump principles

A balanced vane pump is one that has two intake and two outlet ports diametricallyopposite each other. Thus, pressure ports are opposite each other, and a complete hydraulic

balance is achieved. One disadvantage of a balanced vane pump is that it cannot be designed as avariable displacement unit. Instead of having a circular cam ring, a balanced design vane pump

has an elliptical housing, which forms two separate pumping chambers on opposite sides of the

rotor. This eliminates the bearing side loads and thus permits higher operating pressure.

Types of Vanes:

1. Dual vanes

2. Pin vanes

3. Angled vanes4. Intra vanes5. Spring loaded vanes.

Fig 2.16 Types of vanes: (a) Dual vane (b) Intra vane (c) Pin vane

(d) spring loaded vane (e) Angular vane(1-System pressure, 2-Vane, 3-Chamfer, 4-Intra vane, 5-Pin, 6-Spring}


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A dual vane consists of two vanes in each slot of the rotor. With this arrangement, the vane

is considerably balanced with a possibility of positive sealing.

An Intra vane consists of a smaller vane within a larger vane with a beveled edge. Thedelivery pressure (system pressure) from the pump is directed above the smaller vane resulting in

less vane loading.

In pin vane the pressure is directed underside the pin which forces the vane out against the

cam ring.In a spring loaded vane, a spring is used to force the vane against the cam ring along with

system pressure.

In angled vanes, the vanes are positioned at an angle in the rotor. This reduces the loadingon the vane without any mechanical means.

3 Piston Pumps: A piston pump works on the principle that a reciprocating piston can draw influid when it retracts in a cylinder bore and discharge it when it extends. To explain operation

of piston pump explain pumping theory.

Piston pump classification:

1.

Fixed or constant displacement pump2. Adjustable or variable displacement pump

As per geometrical and physical arrangement of pistons they may be classified as

1. In-line crank shaft driven pump

2. Axial piston pump

3. Radial piston pump

1. In-line crank shaft driven pump:

Fig 2.17 In-line crank shaft driven pump

Above fig 2.17 illustrates a crank shaft driven piston pump. Crank-shaft drives are well

known from automotive and steam engines. They require a parallel arrangement of

cylinders and hence may require more space. This specific factor may limit their use inmodern hydraulic systems which need to be more compact. However, they are still many

applications where one or two-cylinder piston pumps are used, e.g. in certain mobileapplications like small fork-lifters, simple loaders, etc.


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2. Axial Piston pump: Classification axial piston pumps:

1. In-line axial piston pumps (Swash Plate Design)

2. Bent axis piston pumps (Bent-Axis Design)3. Radial Piston Pump

1. In-line piston pump (swash plate design):

Fig 2.18 In-line piston pump

Figure 2.18 shows the swash plate design in-line piston pump. In this type, the

cylinder block and drive shaft are located on the same centerline. The pistons areconnected to a shoe plate which bears against an angled swash plate. As the cylinder

rotates the pistons reciprocate because the piston shoes follow the angled surface of the

swash pate as shown in fig 2.18. The outlet and inlet ports are located in the valve plate so

that the pistons pass they inlet as they are being pulled out and pass the outlet as they are

being forced back in.

Fig 2.19 Variation in pump displacement

To obtain variable displacement capability, the swash plate is mounted in a movableyoke, as shown in fig 2.19. The swash plate angle can be changed by pivoting the yoke on

pintles. Positioning of the yoke can be accomplished by manual operation, servo control,

or a compensator control. The maximum swash plate angle is limited to 17 0

.


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Theoretical volume (q) = n.N.h.A

Where, n = number of bore/cylindersN = speed of the pump

d = diameter of the bore

h = stroke length which depends on the piston angle0

A = area of the piston bore

2. Bent axis piston pumps:

Fig 2.20 Bent axis piston pumpFig 2.20 shows an axial piston pump that contains a cylinder block rotating with the

drive shaft. However, the centerline of the cylinder block is set at an offset angle

relative to the centerline of the drive shaft. The cylinder block contains a number of

pistons arranged along a circular. The piston rods are connected to the drive shaftflange by ball-and-socket joints. The pistons are forced in and out of their bores as the

distance between the drive shaft flange and cylinder block changes. A universal link

connects the block to the drive shaft to provide alignment and positive drive.

Fig 2.21 Volumetric displacement changes with offset.

The volumetric displacement of the pump varies with the offset angle as shown in

fig 2.21. No flow is produced when the cylinder block centerline is parallel to the drive

shaft centerline. Can vary from 0o to 30o offset angles.


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Volumetric Displacement and Theoretical Flow rate:

= offset angle (o)

S = piston stroke (m)

D = piston circle diameter (m)Y = number of pistons

A = piston area (m2)

N = pump speed (rpm)

QT= theoretical flow-rate (m3/min)

From fig. tan () = S, S = D tan ()D

Total displacement volume VD= YAS = YAD tan ()

Theoretical flow-rate QT= N x Vd = DANY tan () m3/min

3.

Radial Piston Pump:

Fig 2.22 Operation of a radial piston pump

Fig 2.22, shows the construction and operation of a radial piston pump. It consists

of a pintle to direct fluid in and out of the cylinders, a cylinder barrel with pistons, and arotor containing a reaction ring. The pistons remain in constant contact with the reaction

ring due to centrifugal force and back pressure on the pistons. For pumping action, the

reaction ring is moved eccentrically with respect to the pintle or shaft axis. As the cylinderbarrel rotates, the pistons on one side travel outward. This draws in fluid as each cylinder

passes the suction ports of the pintle. When a piston passes the point of maximum

eccentricity, it is forced inward by the reaction ring. This forces the fluid to enter the

discharge port of the pintle. In some models, the displacement can be varied by moving thereaction ring to change the piston stroke.


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PUMP PERFORMANCE:

The performance of a pump is primarily a function of the precision of itsmanufacture. Components must be made to close tolerances, which must be maintained

while the pump is operating under design conditions. The maintenance of close tolerances

is accomplished by designs that have mechanical integrity and balance pressures.

Theoretically the ideal pump would be one having zero clearance between allmating parts. Although this is not feasible, working clearances should be as small as

possible while maintaining proper oil films for lubrication between rubbing parts.

1. Pump efficiencies:

1. Volumetric efficiency (v): Volumetric efficiency indicates the amount ofleakage that takes place within the pump. This involves considerations such

as manufacturing tolerances and flexing of the ump casing under design

pressure operating conditions.

v= actual flow-rate produced by pump = QAtheoretical flow-rate pump should produce QT

Gear pumps - 80% to 90%

Vane pumps 82% to 92%Piston pumps 90% to 98%

2. Mechanical efficiency (m): Mechanical efficiency indicates the amount of energylosses that occur for reasons other than leakage. This includes friction in bearings

and between other mating parts. It also includes energy losses due to fluidturbulence.

m = pQTTAN where, p = pump discharge pressure (Pa)

QT = pump theoretical flow-rate (m3/s)

TA = actual torque delivered to pump (N-m)

N = pump speed (rpm)

m = theoretical torque required to operate pump = TTactual torque delivered to pump TA

TT= Vd x p, TA= actual horsepower delivered to pump x 63000

2 N (rpm)

OrTA= actual power delivered to pump (W)

N (rad/s)

N (rad/s) = 2 N (rpm)60


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4. Overall efficiency (0): = actual power delivered by pump

Actual power delivered to pump

0= v x m = p QATAN

5. Pump power: = work / time = (p x A x d) / T= (pressure x flow rate) / 600 kw

PUMP SELECTION:

Pumps are selected by taking into account a number of considerations for a complete

hydraulic system involving a particular application. Among these considerations are flow-raterequirements (gpm), operating speed (rpm), pressure rating (psi), performance, reliability,

maintenance, cost, and noise. The selection of a pump typically entails the following sequence of

operations:

1. Select the actuator (hydraulic cylinder or motor) that is appropriate based on the loads

encountered.

2. Determine the flow-rate requirements. This involves the calculation of the flow ratenecessary to drive the actuator to move the load through a specified distance within a given

time limit.

3. Select the system pressure. These ties in with the actuator size and the magnitude of theresistive force produced by the external load on the system. Also involved here is the total

amount of power to be delivered by the pump.

4.

Determine the pump speed and select the prime mover. This, together with the flow-ratecalculation, determines the pump size (volumetric displacement).

5. Select the pump type based on the application (gear, vane, or piston pump and fixed orvariable displacement).

6. Select the reservoir and associated plumbing, including piping, valving, filters and

strainers, and other miscellaneous components such as accumulators.

7. Consider factors such as noise levels, horsepower loss, need for a heat exchanger due togenerated heat, pump wear, and scheduled maintenance service to provide a desired life of

the total system.

8. Calculate the overall cost of the system.

VARIABLE DISPLACEMENT PUMPS:

Explain any one of the above variable pumps.

Problems can be solved by the above formulas, for particular type.

Documents

The Source of Hydraulic Power