fluid power - (me353)- lec4

8/17/2019 Fluid Power - (ME353)- Lec4

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Fluid Power Systems (ME353)

Fall 2012

Lecture 4


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Power Units and Pumps

Source of Hydraulic Power


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Power Unit


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The power unit:

– Provides energy for the operation of the hydraulic system

– Moves fluid through the system

– Provides a safe maximum system operating pressure

– Assists in maintaining correct system operating temperature

and fluid cleanliness

Power units are often supplied by manufacturers as a

package


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A basic power unit

consists of:

– Prime mover to power thesystem

– Pump to move fluid

– Reservoir to store fluid

– Relief valve or pump

compensator to control

maximum system pressure

– Filter to clean the fluid

– Plumbing to transport

fluid to components


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A coupler connects the prime mover to the pump


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The power unit includes

a pressure control valveto limit pressure

Filters and strainers are

included in a powerunit to remove debris

from the fluid


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Basic System Pump


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The basic operating principle that moves fluid through a pump is similar in

all pumps

– Enlarging the volume of a chamber allows fluid to enter the pump

– Reducing the chamber volume moves fluid to the system

– Inlet and discharge valves or ports control fluid movement through the

pump


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The maximum pressure developed in a

hydraulic system is determined by: – Resistance to fluid flow in the system (losses)

– Force the prime mover can exert

The output flow rate of a hydraulic pump isdetermined by:

– Volume of the pumping chamber (Displacement

Volume)

– Operating speed of the prime mover (rpm)


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Basic Pump Classifications

Hydraulic pumps can be classified according to:

–

Displacement (Positive & Non-positive) – Pumping motion (Rotary & Reciprocating)

– Fluid delivery characteristics (Fixed & Variable)


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Displacement relates to how the output of the

pump reacts to system loads

Positive-displacement pumps produce a constant output percycle (no clearance for the fluid to escape)

Non-positive-displacement pumps produce flow variations due

to internal slippage (has large internal clearances

– Allows fluid slippage in the pump

– Results in varying flow output as system load varies


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The basic pumping motions used in hydraulic pumps are:

Rotary pump, the pumping action is produced by revolving

components Reciprocating pump, the rotating motion of the pump input

shaft is changed to reciprocating motion, which then produces

the pumping action


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The delivery of the hydraulic pumps are classified as either

fixed or variable delivery

– Fixed-delivery pumps have pumping chambers with a

volume that cannot be changed; the output is the same

during each cycle

– In variable-delivery designs, chamber geometry may be

changed to allow varying flow from the pump

Gear pumps are fixed-delivery pumps Piston pumps may be designed asvariable-delivery pumps


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When selecting a pump for a circuit, factors

that must be considered are:

– System operating pressure

– Flow rate

– Cycle rate

– Expected length of service

– Environmental conditions

– Cost


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Pump Types


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Positive Displacement Pumps

a- Rotary Type

1- Gear Pump:

Up to 30 m3/hr , 200 bar

Gear pumps are commonly used

Gear pumps are positive-displacement, fixed-delivery, rotary units

Gear pumps are produced with either external or internal gear teeth configurations


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Pumping action of gear pumps results from unmeshing and

meshing of the gears

– As the gears unmesh in the inlet area, low pressure causes

fluid to enter the pump

– As the pump rotates, fluid is carried to the pump discharge

area

– When the gears mesh in the discharge area, fluid is forced

out of the pump into the system


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The gerotor pump design is an internal-gear pump

– Uses two rotating, gear-shaped elements that form sealed chambers

– The chambers vary in volume as the elements rotate

– Fluid comes into the chambers as they are enlarging and is forced out as

they decrease in size

Normally the inner gear is attached to a drive shaft and the outer gear is

turned by the inner gear at the point of contact


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Balanced Gear Pump This is a modification of a regular Gear Pump.

The tubing around the edges of this pump returns pressure -- not liquid –

toopposite sides of the shafts. Very little fluid is transferred anywhere by these

extra tubes.

By equalizing pressure on each side of the shafts, friction and wear are both

reduced in comparison to the standard gear pump.


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Reversing Gear Pump This type of rotary gear pump moves liquid

in the same direction regardless of thedirection the gears turn.

The valves (yellow) are forced open and

closed by the difference in pressure on the

input and output sides.

Fluid in the higher-pressure output side isshown in light blue while fluid in the lower-

pressure input side is dark blue.

Notice that when the green gear starts

moving clockwise, the fluid in the outer

circular tubes changes pressure but stopsflowing, and fluid flows more or less straight

up.

When the red gear is moving clockwise, fluid

travels in a sideways 'S' shape through the

outer circular tubes.


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Lobe Pump The lobe pump is a close relative of the

gear pump

– Two three-lobed, gear-shaped units

are often used to form the pumping

element

– Output flow is larger than a gear pump

of comparable physical size because

of pumping chamber geometry

– Lower pressure rating than gear

pumps

– Tend to have a pulsating output flow


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Screw pumps have pumping

elements that consist of one,

two, or three rotating screws

As the screws rotate, fluid is

trapped and carried along to

the discharge of the pump

The design of screw pumps

allows them to operate at a

very low noise level

Positive Displacement Pumps a- Rotary Type

2- Quimby Screw Pump:Up to 900 m3/hr , 75 bar


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3- Vane Pump:

Up to 20 m3/hr , 130 bar

Vane pumps are positive-displacement, fixed or variable delivery, rotary units,

balanced or unbalanced.


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Vane pump consists of a slotted rotor,

fitted with moveable vanes, thatrotates within a cam ring in the pump

housing

– Rotor is off center in the ring,

which creates pumping chambers

that vary in volume as the pumprotates

– As chamber volume increases,

pressure decreases, bringing fluid

into the pump

– As volume decreases, fluid isforced out into the system


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Parts of a typical vane pump


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Piston pumps are positive-displacement, fixed- or variable-

delivery, reciprocating units

– Several variations

– Many provide high volumetric efficiency (90%), high

operating pressure (600 bar or higher), and high-speed

operation

Piston pump classification is based on the relationship between

the axes of the power input shaft and piston motion

– Radial

– Axial

–

Reciprocating


4- Piston Pumps:


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4-1 Radial (Rotary) Piston Pump:

Up to 20 m3/hr , up to 700 bar

Radial piston pumps have the highest continuous operating pressure capability

of any of the pumps regularly used in hydraulic systems



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Two variations of radial piston pumps:

– Stationary-cylinder design

uses springs to hold

pistons against a cam that rotates with the main

shaft of the pump

– Rotating-cylinder design uses centrifugal forceto hold pistons against a reaction ring


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4-2 Axial Piston (Parallel Cylinder) Pump:


Axial piston pumps use two design variations:

– Inline

– Bent axis


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Inline has the cylinder block and pistons located on the same axis as the

pump input shaft

– Pistons reciprocate against a swash plate

– Very popular design used in many applications

Bent axis has the cylinder block and pistons set at an angle to the input

shaft

– Geometry of the axis angle creates piston movement

–

Considered a more rugged pump than inline – Manufactured in high flow rates and maximum operating pressures


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An inline swash plate & Bent axis axial-piston pumps


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Positive Displacement Pumps b- Reciprocating Type

1- Piston / Plunger Pump:


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Large, reciprocating-plunger pump designs

were widely used when factories had a centralhydraulic power source

Today, plunger pumps are typically found in

special applications requiring high-pressure performance


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2- Diaphragm Pump:Relatively low flow rate and pressure

Cars often use a Diaphragm Pump to move gasoline from the gas tank to the carburettor or

fuel injection plugs.

Positive Displacement Pumps b- Reciprocating Type


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Centrifugal pumps are non-positive-displacement units

– Use centrifugal force generated by a rotating impeller to move fluid

– Large clearances between the impeller and the pump housing allow

internal pump slippage when resistance to fluid flow is encountered in the

system

– Typically used in hydraulic systems as auxiliary fluid transfer pumps

Non-Positive Displacement Pumps


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For high flow rate


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For high flow rate

Centrifugal Pump

Impeller Casing


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Impellers are classified according to the

major direction of flow

Centrifugal pumps may have:

1. Radial-flow impellers

2. Axial-flow impellers

3. Mixed-flow impellers, which combine

radial- and axial-flow principles

radial single-suction closed impeller Open mixed-flow impeller Axial-Flow impeller


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Axial Flow Pump

http://www.gouldspumps.com/cat_pumps.ihtml?pid=236&lastcatid=80&step=4


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Jet pumps are non-positive-displacement pumps

– Sometimes used to transfer fluid within hydraulic systems

– Jet pump creates flow by pumping fluid through a nozzle concentrically

located within a venturi

Jet Pump


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Performance Curve H-Q Curve:

Positive Displacement Pumps

• The flow rate is (constant) not depending

on the head (pressure).

• H and Q depends on r.p.m but not on

each other.

Non-Positive displacement Pumps (Centrifugal)

• The flow rate is depending on the head

(pressure).

• H and Q depends on each other and also on

the r.p.m.

Additi l D i F t f


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Additional Design Features ofPumps

Some basic pump designs can be modified to

allow the unit to produce variable flow outputs

at constant operating speeds

This is achieved by including pump elements

that allow the volume of the pumping chamber

to be changed, either manually or by pressure-

sensing devices


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Unbalanced-vane pumps can be designed to produce variable flow outputs

– A cam ring moves in relation to the center of the rotor

– No flow output is produced when the centers of the ring and rotor are

concentric

– Flow increases as the centers move apart


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Dual pumps are two pumps housed in a single case and driven by a single

prime mover

– Some units include integral valving to provide relief and unloadingcapabilities

– Typical application is a high-low circuit where a large cylinder quickly

extends followed by high-pressure clamping

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