Chap.2 - Hydraulic Pumps

8/4/2019 Chap.2 - Hydraulic Pumps

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A basic hydraulic system contains the following main components:

1. A reservoir (tank) to hold and cool the hydraulic fluid

2. A power source (prime mover), such as an electric motor or aninternal combustion Engine, to drive the pump

3. A pump to move the fluid through the circuit 4. Valves to control the direction, pressure, and flow rate of the fluid

5. An actuator that converts the moving fluid into some sort of useful work.(Hydraulic fluid can turn motors or extend hydraulic cylinders to convert thefluid Energy into useful work)

6. Piping to move the fluid from one location to another.


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Hydraulic System

Reservoir Power source Pump Valves

Direction

Control

Valve

Pressure

Control

Valve

Flow

Control

Valve

Actuator

Linear(Eg. Single

acting

cylinder)

Rotary

(eg. Hyd.Motor)

Accessories

Pipes

Hoses

Fittings


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A reservoir is used in fluid power circuits to hold and supply an appropriatevolume of fluid for the circuit. It may contain a strainer, a filter, an oil levelgage, an air breather, and baffles. As hydraulic fluid moves through thecircuit, friction losses in the pipes, valves, and joints heat the hydraulic fluid.

The bafflesin the reservoir are designed to remove as much heat asnecessary from the circuit fluid.

A due consideration is given to the reservoir while designing the hydraulic

circuit as it is necessary to hold sufficient amount of hydraulic oil for efficientworking of the system.

In this chapter, we are going to deal with all these components one by one.To begin with, let us start with Reservoir


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Fig. 3.2 a Reservoir


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Though each component of the system isequally important, the pump plays asignificant role and has unique position inthe system.

The main purpose of the pump is to createthe flow of oil through the systemand thus

assist transfer of power and motion.


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All pumps operate on the principle that a partialvacuum is created in the inlet of the pump due tointernal operation of the pump.

This allows atmospheric pressure to push the fluidout of the reservoir and into the pump intake.

The pump then mechanically pushes the fluid out

into the discharge line.


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Basically pumps can be classified as positivedisplacement (PD) or non positive displacement(NPD) pumps.

The detail classification of the pumps can berepresented diagrammatically as depicted in thefigure below


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Pumps

Positive

Displacement

RotaryGear Pump

External

Gear Pump

Internal

Gear Pump

GerotorVane Pump

Fixed

Displacement

Variable

Displacement

Screw Pump

Reciprocating

(Fixed and Variable)Axial Piston

Radial Piston

Non Positive

Displacement

CentrifugalPump

Axial Pump

Radial Pump


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Nonpositive-Displacement Pumps.With this pump, the volume of liquid delivered foreach cycle depends on the resistance offered to flow. A pump produces a force on theliquid that is constant for each particular speed of the pump. Resistance in a dischargeline produces a force in the opposite direction. When these forces are equal, a liquid isin a state of equilibrium and does not flow.

If the outlet of a nonpositive-displacement pump is completely closed, the dischargepressure will rise to the maximum for a pump operating at a maximum speed. A pump

will churn a liquid and produce heat. Figure 3-1 shows a nonpositive-displacementpump. A water wheel picks up the fluid and moves it.

Positive-Displacement Pumps. With this pump, a definite volume of liquid is deliveredfor each cycle of pump operation, regardless of resistance, as long as the capacity of thepower unit driving a pump is not exceeded. If an outlet is completely closed, either theunit driving a pump will stall or something will break. Therefore, a positive-

displacement-type pump requires a pressure regulator or pressure-relief valve in thesystem. Figure 3-2 shows a reciprocating-type, positive-displacement pump.


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The three contrasting characteristics in the operation of positive-and nonpositive-displacement pumps are as follows:

Nonpositive-displacement pumps provide a smooth, continuousflow; positive- displacement pumps have a pulse with each stroke oreach time a pumping chamber opens to an outlet port.

Pressure can reduce a nonpositive pump's delivery. High outletpressure can stop any output; the liquid simply recirculates insidethe pump. In a positive-displacement pump, pressure affects theoutput only to the extent that it increases internal leakage.

Nonpositive-displacement pumps, with the inlets and outlets

connected hydraulically, cannot create a vacuum sufficient for self-priming; they must be started with the inlet line full of liquid and freeof air. Positive-displacement pumps often are self-priming whenstarted properly.


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Displacement is the amount of liquid transferred from a pump's inlet to

its outlet in one revolution or cycle. In a rotary pump, displacement isexpressed in cubic inches per revolution and in a reciprocating pump incubic inches per cycle. If a pump has more than one pumping chamber,its displacement is equal to the displacement of one chamber multipliedby the number of chambers. Displacement is either fixed or variable.

a. Fixed-Displacement Pump. In this pump, the GPM output can be

changed only by varying the drive speed. The pump can be used in anopen-center system-a pump's output has a free-flow path back to areservoir in the neutral condition of a circuit.

b. Variable-Displacement Pump. In this pump, pumping-chambersizes can be changed. The GPM delivery can be changed by movingthe displacement control, changing the drive speed, or doing both. The

pump can be used in a closed-center system-a pump continues tooperate against a load in the neutral condition.


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Gear pumps as the name suggests make use of the

principle of two gears in mesh in order to generatepumping action. They are compact, relatively inexpensiveand have few moving parts. Gear pumps are furtherclassified as:

External Gear Pumps

Internal Gear Pumps

Lobe Pumps

Gerotor Pumps


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External. Figure shows theoperating principle of an external

gear pump. It consists of a drivinggear and a driven gear enclosed ina closely fitted housing. The gearsrotate in opposite directions andmesh at a point in the housingbetween the inlet and outlet ports.

Both sets of teeth project outwardfrom the center of the gears. As theteeth of the two gears separate, apartial vacuum forms and drawsliquid through an inlet port intochamber A. Liquid in chamber A is

trapped between the teeth of thetwo gears and the housing so that itis carried through two separatepaths around to chamber B. As theteeth again mesh, they produce aforce that drives a liquid through anoutlet port.


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Gear Pump


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b. Internal.Figure shows aninternal gear pump. The teeth of onegear project outward, while the teethof the other gear project inwardtoward the center of the pump. Onegear wheel stands inside the other.

This type of gear can rotate, or berotated by, a suitably constructedcompanion gear. An external gear isdirectly attached to the drive shaft ofa pump and is placed off-center inrelation to an internal gear. The two

gears mesh on one side of a pumpchamber, between an inlet and thedischarge. On the opposite side of thechamber, a crescent-shaped formstands in the space between the twogears to provide a close tolerance.

Internal Gear Pumps


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Internal Gear Pumps
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Figure shows a lobe pump.

It differs from other gear pumpsbecause it uses lobed elementsinstead of gears.The element drive also differs in alobe pump.In a gear pump, one gear drivesthe other. In a lobe pump, bothelements are driven throughsuitable external gearing

Lobe Pumps
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Although gerotors come in a variety of sizes andconfigurations, they share the same basic principle of

operation: The inner drive element (rotor) has one lesstooth than the scroll section. Both rotate within thepumps housing. Because the inner rotor has one lesstooth, it revolves at a slightly faster rate than the scrollsection.

As the inner rotor revolves, vacuum causes oil to bedrawn into a chamber, which continues to enlargeas the pump shaft turns

Gerotor Pumps


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The chamber reaches maximum volume when the tips

and lobes seal the chamber from both the inlet side (lowpressure) and outlet side (high pressure).

Further rotation causes the chamber tobecome connected to the discharge port,eventually forcing all of the oil out as the

chambers volume becomes smaller

Gerotor Pumps


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In a vane-type pump, a slotted rotor splined to a drive shaft rotates between closelyfitted side plates that are inside of an elliptical- or circular-shaped ring.

Vane Pump

Polished, hardened vanes slide in and out of the rotor slots and follow the ring contourby centrifugal force.

Pumping chambers are formed between succeeding vanes, carrying oil from the inlet tothe outlet.

A partial vacuum is created at the inlet as the space between vanes increases. The oil issqueezed out at the outlet as the pumping chamber's size decreases.
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Vane Pump


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Characteristics of Vane PumpDisplacement of a vane-type pump depends on the width of the ring and rotor and the

throw of the cam ring

Vane pumps have good efficiency and durability if operated in a clean system using thecorrect oil.

They cover the low to medium-high pressure, capacity, and speed ranges

A vane pump is generally quiet, but will whine at high speeds.


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Radial Piston Pumps

In a radial piston pump, the pistons are arranged like wheel spokes in a shortcylindrical block.A drive shaft, which is inside a circular housing, rotates a cylinder block.The block turns on a stationary pintle that contains the inlet and outlet ports.As a cylinder block turns, centrifugal force slings the pistons, which follow a circular

housing.A housing's centerline is offset from a cylinder block's centerline.The amount of eccentricity between the two determines a piston stroke and,therefore, a pump's displacement.Controls can be applied to change a housing's location and thereby vary a pump'sdelivery from zero to maximum.

Piston Pump


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Piston Pump


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Axial Piston Pumps.

In axial piston pumps, the pistons stroke in the same direction on a cylinderblock's centerline (axially).

Axial piston pumps may be an in-line or angle design.

In capacity, piston pumps range from low to very high.

Pressures are as high as 5,000 psi, and drive speeds are medium to high.

Efficiency is high, and pumps generally have excellent durability.

Petroleum oil fluids are usually required.

Pulsations in delivery are small and of medium frequency. The pumps are quiet

in operation but may have a growl or whine, depending on condition.

Except for in-line pumps, which are compact in size, piston pumps are heavyand bulky.

In-Line Pump In an in-line piston pump a drive shaft and cylinder block are on the


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In Line Pump. In an in line piston pump, a drive shaft and cylinder block are on thesame centerline.

Reciprocation of the pistons is caused by a swash plate that the pistons run against as acylinder block rotates.

A drive shaft turns a cylinder block, which carries the pistons around a shaft. The pistonshoes slide against a swash plateand are held against it by a shoe plate.

A swash plate's angle causes the cylinders to reciprocate in their bores.

At the point where a piston begins to retract, an opening in the end of a bore slides overan inlet slot in a valve plate, and oil is drawn into a bore through somewhat less thanhalf a revolution.

There is a solid area in a valve plate as a piston becomes fully retracted. As a pistonbegins to extend, an opening in a cylinder barrel moves over an outlet slot, and oil is

forced out a pressure port.


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Swash Plate Piston Pump

Bent-Axis Axial Piston Pump.


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Bent Axis Axial PistonPump.

In an angle- or a bent-axis-type piston pump, the piston rods are attached by ball jointsto a drive shaft's flange.

A universal link keys a cylinder block to a shaft so that they rotate together but at anoffset angle.

A cylinder barrel turns against a slotted valve plate to which the ports connect. Pumpingaction is the same as an in-line pump.

The angle of offset determines a pump's displacement, just as the swash plate's angledetermines an in-line pump's displacement.

In fixed-delivery pumps, the angle is constant. In variable models, a yoke mounted onpintles swings a cylinder block to vary displacement.

Flow direction can be reversed with appropriate controls.


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Bent Axis Piston Pump

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Chap.2 - Hydraulic Pumps