Hydraulic Basics.doc 2

7/21/2019 Hydraulic Basics.doc 2

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Hydraulic Basics

Hydraulics is the science of transmitting force and/or motion through the medium of a

confined liquid. In a hydraulic device, power is transmitted by pushing on a confinedliquid. Figure 1-1 shows a simple hydraulic device. he transfer of energy ta!es place

because a quantity of liquid is sub"ect to pressure. o operate liquid-powered systems, theoperator should have !nowledge of the basic nature of liquids. his chapter covers theproperties of liquids and how they act under different conditions.


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Pressure and Force

#ressure is force e$erted against a specific area %force per unit area& e$pressed in pounds

per square inch %psi&.#ressure can cause an e$pansion, or resistance to compression, of afluid that is being squee'ed. ( fluid is any liquid or gas %vapor&. Force is anything that

tends to produce or modify %push or pull& motion and is e$pressed inpounds.

a. Pressure.(n e$ample of pressure is the air %gas& that fills an automobile tire.

(s a tire is inflated, more air is squee'ed into it than it can hold. he air inside atire resists the squee'ing by pushing outward on the casing of the tire. he

outward push of the air is pressure. )qual pressure throughout a confined area is a

characteristic of any pressuri'ed fluid. For e$ample, in an inflated tire, theoutward push of the air is uniform throughout. If it were not, a tire would be

pushed into odd shapes because of its elasticity.

here is a ma"or difference between a gasand a liquid.*iquids are slightly

compressible%Figure 1-+&. hen a confined liquid is pushed on, pressure builds

up. he pressure is still transmitted equally throughout the container. he fluidsbehavior ma!es it possible to transmit a pushthrough pipes, around corners, and

up and down. ( hydraulic system uses a liquid because its near incompressibility

ma!es the action instantaneous as long as the system is full of liquid.


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#ressure can be created bysquee'ingorpushingon a confined fluidonly if there is a

resistance to flow. he twoways to push on a fluid are by the action of a mechanicalpumpor by the weight of the fluid. (n e$ample of pressure due to a fluids weight would

be in an oceans depths. he waters weight creates the pressure, which increases ordecreases, depending on the depth.

y !nowing the weight of a cubic foot of water, you can calculate the pressure at anydepth. Figure 1- shows a column of water 1 foot square and 10 feet high, which equates

to 10 cubic feet. %ne cubic foot of water weighs 2+.3 pounds.& he total weight of water

in this column is 4+3 pounds. he weight at the bottom covers 1,332 square inches %1square foot&. )ach square inch of the bottom is sub"ect to 1/133 of the total weight, or

3. pounds. hus, the pressure at this depth is 3. psi. 5ou can also create an equal

pressure of 3. psi in a liquid using the pump and figures shown in Figure 1-3.

efore pressure, head was the only way to e$press pressure measurement. It wase$pressed as feet of water. oday, head is still the vertical distance between two levels in

a fluid. In Figure 1-, the head between the top and bottom of the water is 10 feet, which

is equivalent to 3. psi. herefore, each foot of water is equal to 0.3 psi.

he earth has an atmosphere of air e$tending 20 miles up, and this air has weight. his aircreates a head of pressure that is called atmospheric pressure.


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( column of air 1 square inch in cross section and the height of the atmosphere would

weigh 13.6 pounds at sea level. hus, the earths atmospheric pressure is 13.6 psi at sealevel. he role of atmospheric pressure in most hydraulic systems is significant. Figure 1-

2 shows the interaction of hydraulic and atmospheric pressures under the three sets ofconditions listed below7

(1) Diagram A. In the diagram, the tube is open at both ends. hen it is placed in aliquid, the liquid will rise, inside and outside, in proportion to the amount of liquid

displacedby the submerged tube wall.

(2) Diagram B. In the diagram, ends of the tube are closed. hen placed in a liquid, the

liquid level in the tube is forced down because the air in the tube must occupy a space.herefore, the liquid is displaced. he liquid level outside the tube rises in proportion to

the volume of the cylinder wall and the volume of the trapped airbelow the original

liquid level. he atmospheric pressure %13.6 psi& on the liquid outside the tube is notheavy enough to force the liquid inside the tube upward against the pressure of the

trapped air, which is more than 13.6 psi.

(3) Diagram C. In the diagram, the upper end of the tube is closed, but some of the air

has been removed from this tube so that the pressure within the tube is less than 13.6 psi%a partial vacuum&.( perfect vacuum would e$ist if all pressure within the tube could be

eliminated, a condition that never happens. ecause the liquid outside the tube is sub"ect

to full atmospheric pressure, the liquid is forced up into the tube to satisfy the vacuum.

How far the liquid rises depends on the difference in air pressurebetween the trapped airand the atmosphere.


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b. Force.he relationship of force, pressure, and area is as follows7

F = PA

where-

F = force, in pounds

P = pressure, in psi

A = area, in suare inches

Example:

Figure 1-4 shows a pressure of 20 psi being applied to an area of 100 square

inches. he total force on the area is-

F = PA

F = !" # 1"" = !,""" pounds


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Pascal's Law

laise #ascal formulated the basic law of hydraulics in the mid 16th century. He

discovered that pressure e$erted on a fluid acts equally in all directions. His law statesthat pressure in a confined fluid is transmitted undiminished in every direction and acts

with equal force on equal areas and at right angles to a containers walls.

Figure 1-6 shows the apparatus that #ascal used to develop his law. It consisted of two

connected cylinders of different diameters with a liquid trapped between them. #ascalfound that the weight of a small piston will balance the weight of a larger piston as long

as the pistons areas are in proportion to the weights. In the small cylinder, a force of 100

pounds on a 1-square-inch piston creates a pressure of 100 psi. (ccording to #ascals

*aw, this pressure is transmitted undiminished in every direction. In the larger cylinder,the 100 psi of pressure from the small cylinder is transmitted to an area of 2 square

inches, which results in a force of 200 pounds on the second piston. he force has been

multiplied 2 times-a mechanical advantage of 2 to 1. 8sing the same factors, you candetermine the distance the pistons move. For e$ample, if the small piston moves down 10

inches, the larger piston will move up + inches. 8se the following to determine the

distance7

Example: 9etermine 9+

F1= force of $he sma%% pis$on, in pounds

D1= dis$ance $he sma%% pis$on mo&es, in inchesD2= dis$ance $he %arger pis$on mo&es, in inches

F2= force of $he %arger pis$on, in pounds


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Flow

Flow is the movement of a hydraulic fluid caused by a difference in the pressure at two

points. In a hydraulic system, flow is usually produced by the action of a hydraulic pump-a device used to continuously push on a hydraulic fluid. he two ways of measuring flow

are velocity and flow rate.

a. Velocity. :elocity is the average speed at which a fluids particles move past a given

point, measured in feet per second %fps&. :elocity is an important consideration in si'ingthe hydraulic lines that carry a fluid between the components.

b. Flow Rate'Flow rate is the measure of how much volume of a liquid passes a point in

a given time, measured in gallons per minute %;#


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c. %eat Enery and Friction.Heat energy is the energy a body possesses because of its

heat. >inetic energy and heat energy are dynamic factors. #ascals *aw dealt with staticpressure and did not include the friction factor. Friction is the resistance to relative

motion between two bodies. hen liquid flows in a hydraulic circuit, friction producesheat. his causes some of the !inetic energy to be lost in the form of heat energy.

(lthough friction cannot be eliminated entirely, it can be controlled to somee$tent. he three main causes of e$cessive friction in hydraulic systems are7

)$tremely long lines.

?umerous bends and fittings or improper bends.

)$cessive velocity from using undersi'ed lines.

In a liquid flowing through straight piping at a low speed, the particles of the liquid move

in straight lines parallel to the flow direction. Heat loss from friction is minimal. his!ind of flow is called laminar flow. Figure 1-@, diagram (, shows laminar flow. If thespeed increases beyond a given point, turbulent flow develops. Figure 1-@, diagram ,

shows turbulent flow.

Figure 1-9 shows the difference in head because of pressure drop due to friction. Point Bshows no flow resistance (free-flow condition); the pressure at point B is zero. Thepressure at point is at its !a"i!u! because of the head at point #. #s the li$uid flowsfro! point to point B% friction causes a pressure drop fro! !a"i!u! pressure to zeropressure. This is reflected in a succeeding decreased head at points &% '% and F.


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d.Relationship between Velocity and Pressure.Figure 1-10 e$plains ernouillis#rinciple, which states that the static pressure of a moving liquid varies inversely with its

velocity= that is, as velocity increases, static pressure decreases. In the figure, the force onpiston A is sufficient to create a pressure of 100 psi on chamber (. (s piston A moves

down, the liquid that is forced out of chamber ( must pass through passage B to reach

chamber . he velocity increases as it passes through B because the same quantity ofliquid must pass through a narrower area in the same time. Come of the 100 psi static

pressure in chamber ( is converted into velocity energy in passage B so that a pressure

gauge at this point registers D0 psi. (s the liquid passes through B and reaches chamber, velocity decreases to its former rate, as indicated by the static pressure reading of 100

psi, and some of the !inetic energy is converted to potential energy.

Figure 1-11 shows the combined effects of friction and velocity changes. (s in Figure 1-D

pressure drops from ma$imum at B to 'ero at . (t 9, velocity is increased, so thepressure head decreases. (t ), the head increases as most of the !inetic energy is given

up to pressure energy because velocity is decreased. (t F, the head drops as velocity

increases.


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e. Work. o do wor! in a hydraulic system, flow must be present. or!, therefore, e$erts

a force over a definite distance. It is a measure of force multiplied by distance.

&.Power. he standard unit of power is horsepower %hp&. ne hp is equal to 220 ft lb of

wor! every second. 8se the following equation to find power7

P = f # d$

where-

P = power, in hp

f = force, in P*

d = dis$ance, in psi$ = $ime (1,+1)


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Enery! "or#! and Power

)nergy is the ability to do wor! and is e$pressed in foot-pound %ft lb&. he three forms of

energy are potential, !inetic, and heat. or! measures accomplishments= it requiresmotion to ma!e a force do wor!. #ower is the rate of doing wor! or the rate of energy

transfer.

a. Potential Enery. #otential energy is energy due to position. (n ob"ect has potential

energy in proportion to its vertical distance above the earths surface. For e$ample, waterheld bac! by a dam represents potential energy because until it is released, the water does

not wor!. In hydraulics, potential energy is a static factor. hen force is applied to a

confined liquid, as shown in Figure 1-3, potential energy is present because of the staticpressure of the liquid. #otential energy of a moving liquid can be reduced by the heat

energy released. #otential energy can also be reduced in a moving liquid when it

transforms into !inetic energy. ( moving liquid can, therefore, perform wor! as a result

of its static pressure and its momentum.

b. $inetic Enery'>inetic energy is the energy a body possesses because of its

motion. he greater the speed, the greater the !inetic energy. hen water is

released from a dam, it rushes out at a high velocity "et, representing energy ofmotion-!inetic energy. he amount of !inetic energy in a moving liquid is directly

proportional to the square of its velocity. #ressure caused by !inetic energy may

be called velocity pressure.

c. %eat Enery and Friction.Heat energy is the energy a body possesses because of itsheat. >inetic energy and heat energy are dynamic factors. #ascals *aw dealt with static

pressure and did not include the friction factor. Friction is the resistance to relative

motion between two bodies. hen liquid flows in a hydraulic circuit, friction producesheat. his causes some of the !inetic energy to be lost in the form of heat energy.

(lthough friction cannot be eliminated entirely, it can be controlled to some

e$tent. he three main causes of e$cessive friction in hydraulic systems are7

)$tremely long lines.

?umerous bends and fittings or improper bends.

)$cessive velocity from using undersi'ed lines.

In a liquid flowing through straight piping at a low speed, the particles of the liquid move

in straight lines parallel to the flow direction. Heat loss from friction is minimal. his!ind of flow is called laminar flow. Figure 1-@, diagram (, shows laminar flow. If the

speed increases beyond a given point, turbulent flow develops. Figure 1-@, diagram ,

shows turbulent flow.


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Figure 1-D shows the difference in head because of pressure drop due to friction.#oint shows no flow resistance %free-flow condition&= the pressure at point is'ero. he pressure at point B is at its ma$imum because of the head at point (. (s

the liquid flows from point B to point , friction causes a pressure drop from

ma$imum pressure to 'ero pressure. his is reflected in a succeedingly decreasedhead at points 9, ), and F.


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d.Relationship Between Velocity and Pressure.Figure 1-10 e$plains ernouillis

#rinciple, which states that the static pressure of a moving liquid varies inversely with itsvelocity= that is, as velocity increases, static pressure decreases. In the figure, the force on

piston A is sufficient to create a pressure of 100 psi on chamber (. (s piston A movesdown, the liquid that is forced out of chamber ( must pass through passage B to reach

chamber .

he velocity increases as it passes through B because the same quantity of liquid must

pass through a narrower area in the same time. Come of the 100 psi static pressure in

chamber ( is converted into velocity energy in passage B so that a pressure gauge at thispoint registers D0 psi. (s the liquid passes through B and reaches chamber , velocity

decreases to its former rate, as indicated by the static pressure reading of 100 psi, and

some of the !inetic energy is converted to potential energy.

Figure 1-11 shows the combined effects of friction and velocity changes. (s in Figure 1-Dpressure drops from ma$imum at B to 'ero at . (t 9, velocity is increased, so the

pressure head decreases. (t ), the head increases as most of the !inetic energy is given

up to pressure energy because velocity is decreased. (t F, the head drops as velocityincreases.


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e. Work. o do wor! in a hydraulic system, flow must be present. or!, therefore, e$erts

a force over a definite distance. It is a measure of force multiplied by distance.

&.Power. he standard unit of power is horsepower %hp&. ne hp is equal to 220 ft lb ofwor! every second. 8se the following equation to find power7

P = f # d$

P = power, in hp

f = force, in P*d = dis$ance, in psi

$ = $ime (1,+1)


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%ydraulic ystems

( hydraulic system contains and confines a liquid in such a way that it uses the laws

governing liquids to transmit power and do wor!. his chapter describes some basicsystems and discusses components of a hydraulic system that store and condition the

fluid. he oil reservoir %sump or tan!& usually serves as a storehouse and a fluid

conditioner. Filters, strainers, and magnetic plugs condition the fluid by removingharmful impurities that could clog passages and damage parts. Heat e$changes or coolers

often are used to !eep the oil temperature within safe limits and prevent deterioration of

the oil. (ccumulators, though technically sources of stored energy, act as fluidstorehouses.

-1. (asic ystems

he advantages of hydraulic systems over other methods of power transmission are-

Cimpler design. In most cases, a few pre-engineered components will replace

complicated mechanical lin!ages.

Fle$ibility. Hydraulic components can be located with considerable fle$ibility.

#ipes and hoses instead of mechanical elements virtually eliminate location

problems.

Cmoothness. Hydraulic systems are smooth and quiet in operation. :ibration is

!ept to a minimum.

Bontrol. Bontrol of a wide range of speed and forces is easily possible.

Bost. High efficiency with minimum friction loss !eeps the cost of a power

transmission at a minimum. verload protection. (utomatic valves guard the system against a brea!down

from overloading.

he main disadvantage of a hydraulic system is maintaining the precision parts when

they are e$posed to bad climates and dirty atmospheres. #rotection against rust,corrosion, dirt, oil deterioration, and other adverse environmental conditions is very

important. he following paragraphs discuss several basic hydraulic systems.

a. %ydraulic )ac#.In this system %Figure +-1&, a reservoir and a system of valves has

been added to #ascals hydraulic lever to stro!e a small cylinder or pump continuously

and raise a large piston or an actuator a notch with each stro!e. 9iagram ( shows aninta!e stro!e. (n outlet chec! valve closes by pressure under a load, and an inlet chec!

valve opens so that liquid from the reservoir fills the pumping chamber. 9iagram

shows the pump stro!ing downward. (n inlet chec! valve closes by pressure and anoutlet valve opens.


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b. *otor-Re+ersin ystem.Figure +-+ shows a power-driven pump operating a

reversible rotary motor. ( reversing valve directs fluid to either side of the motor and

bac! to the reservoir. ( relief valve protects the system against e$cess pressure and can

bypass pump output to the reservoir, if pressure rises too high.


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c. ,pen-enter ystem. In this system, a control-valve spool must be open in the center

to allow pump flow to pass through the valve and return to the reservoir. Figure +-shows this system in the neutral position. o operate several functions simultaneously, an

open-center system must have the correct connections, which are discussed below. (n

open-center system is efficient on single functions but is limited with multiple functions.


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(1) eries Connec$ion. Figure +-3 shows an open-center system with a series connection.

il from a pump is routed to the three control valves in series. he return from the first

valve is routed to the inlet of the second, and so on. In neutral, the oil passes through the

valves in series and returns to the reservoir, as the arrows indicate. hen a control valveis operated, the incoming oil is diverted to the cylinder that the valve serves. Eeturn

liquid from the cylinder is directed through the return line and on to the ne$t valve.

his system is satisfactory as long as only one valve is operating at a time. hen this

happens, the full output of the pump at full system pressure is available to that function.


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However, if more than one valve is operating, the total of the pressures required for each

function cannot e$ceed the systems relief setting.

(2) eriesPara%%e% Connec$ion. Figure +-2 shows a variation on the series connection. ilfrom the pump is routed through the control valves in series, as well as in parallel. he

valves are sometimes stac!ed to allow for e$tra passages. In neutral, a liquid passesthrough the valves in series, as the arrows indicate. However, when any valve is

operating, the return is closed and the oil is available to all the valves through the parallelconnection.

hen two or more valves are operated at once, the cylinder that needs the least pressure

will operate first, then the cylinder with the ne$t least, and so on. his ability to operate

two or more valves simultaneously is an advantage over the series connection.

%3) F%ow Di&ider. Figure +-4 shows an open-center system with a flow divider. ( flowdivider ta!es the volume of oil from a pump and divides it between two functions. For

e$ample, a flow divider might be designed to open the left side first in case both control

valves were actuated simultaneously. r, it might divide the oil to both sides, equally orby percentage. ith this system, a pump must be large enough to operate all the functions

simultaneously. It must also supply all the liquid at the ma$imum pressure of the highest

function, meaning large amounts of hp are wasted when operating only one control valve.


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d. losed-enter ystem. In this system, a pump can rest when the oil is not required tooperate a function. his means that a control valve is closed in the center, stopping the

flow of the oil from the pump. Figure +-6 shows a closed-center system. o operate

several functions simultaneously, a closed-center system have the following connections7

(1) Fi#ed-Disp%acemen$ Pump and Accumu%a$or. Figure +-@ shows a closed-center

system. In this system, a pump of small but constant volume charges an accumulator.

hen an accumulator is charged to full pressure, an unloading valve diverts the pumpflow bac! to a reservoir. ( chec! valve traps the pressured oil in the circuit.


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hen a control valve is operated, an accumulator discharges its oil and actuates acylinder. (s pressure begins to drop, an unloading valve directs the pump flow to an

accumulator to recharge the flow. his system, using a small capacity pump, is effective

when operating oil is needed only for a short time. However, when the functions need alot of oil for longer periods, an accumulator system cannot handle it unless the

accumulator is very large.

(2) .aria/%e-Disp%acemen$ Pump. Figure +-D shows a closed-center system with a

variable-displacement pump in the neutral mode. hen in neutral, oil is pumped until the

pressure rises to a predetermined level. ( pressure-regulating valve allows the pump toshut off by itself and maintain this pressure to the valve. hen the control valve is

operating, oil is diverted from the pump to the bottom of a cylinder. he drop in pressurecaused by connecting the pumps pressure line to the bottom of the cylinder causes the

pump to go bac! to wor!, pumping oil to the bottom of the piston and raising the load.


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hen the valve moves, the top of the piston connects to a return line, which allows the

return oil that was forced from the piston to return to the reservoir or pump. hen thevalve returns to neutral, oil is trapped on both sides of the cylinder, and the pressure

passage from the pump is dead-ended. (fter this sequence, the pump rests.


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ecause todays machines need more hydraulic power, a closed-center system is more

advantageous. For e$ample, on a tractor, oil may be required for power steering, power

bra!es, remote cylinders, three-point hitches, loaders, and other mounted equipment. In

most cases, each function requires a different quantity of oil. ith a closed-center system,the quantity of oil to each function can be controlled by line or valve si'e or by orificing

with less heat build up when compared to the flow dividers necessary in a comparable

open-center system. ther advantages of a closed-center system are that-

It does not require relief valves because the pump simply shuts off by itself when

standby pressure is reached. his prevents heat buildup in systems where relief

pressure is frequently reached.

It has lines, valves, and cylinders that can be tailored to the flow requirements of

each function.

It has an available reserve flow to ensure full hydraulic speed at low engine

revolutions per minute %rpm&.


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-. olor odin

In this course, the figures that show oil-flow conditions or paths are prepared with

industrial standardi'ed color codes. able +-1 lists the colors for the hydraulic lines andpassages that are in many of the figures7

Table 2-1. Figure colors

Line/Passage Color

perating pressureRed

'"haustBlue

nta*e or drainGreen

+etered flowYellow

-/. Reser+oirs

( reservoir stores a liquid that is not being used in a hydraulic system. It also allows

gases to e$pel and foreign matter to settle out from a liquid.

a. onstruction. ( properly constructed reservoir should be able to dissipate heat fromthe oil, separate air from the oil, and settle out contaminates that are in it. Eeservoirs

range in construction from small steel stampings to large cast or fabricated units. he

large tan!s should be sandblasted after all the welding is completed and then flushed andsteam cleaned. 9oing so removes welding scale and scale left from hot-rolling the steel.

he inner surface then should be sealed with a paint compatible with the hydraulic fluid.

?onbleeding red engine enamel is suitable for petroleum oil and seals in any residual dirtnot removed by flushing and steam cleaning.

b. 0ape.Figure +-11 shows some of the design features of a reservoir. It should be high

and narrow rather than shallow and broad. he oil level should be as high as possibleabove the opening to a pumps suction line. his prevents the vacuum at the line openingfrom causing a vorte$ or whirlpool effect, which would mean that a system is probably

ta!ing in air. (erated oil will not properly transmit power because air is compressible.

(erated oil has a tendency to brea! down and lose its lubricating ability.


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c. ie. Eeservoir si'es will vary. However, a reservoir must be large enough so that it has

a reserve of oil with all the cylinders in a system fully e$tended. (n oil reserve must be

high enough to prevent a vorte$ at the suction lines opening. ( reservoir must have

sufficient space to hold all the oil when the cylinders are retracted, as well as allow spacefor e$pansion when the oil is hot.

( common-si'e reservoir on a mobile machine is a +0- or 0-gallon tan! used with a 100-

;#< system.


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greatest when there is a large temperature difference. Eeservoirs that are built into front-

end loader arms are very effective in transferring heat.

e. Ventilation and Pressuriation.


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moisture can become a maintenance problem. ( tan! should have a water trap for

moisture removal= it should be placed where it can be inspected daily.

-4. trainers and Filters.

o !eep hydraulic components performing correctly, the hydraulic liquid must be !ept as

clean as possible. Foreign matter and tiny metal particles from normal wear of valves,

pumps, and other components are going to enter a system. Ctrainers, filters, and magneticplugs are used to remove foreign particles from a hydraulic liquid and are effective as

safeguards against contamination.


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b. Filters. ( filter removes small foreign particles from a hydraulic fluid and is most

effective as a safeguard against contaminants. Filters are located in a reservoir, a pressure

line, a return line, or in any other location where necessary. hey are classified as fullflow or proportional flow.

(1) Fu%%-F%ow Fi%$er %Figure +-1&. In a full-flow filter, all the fluid entering a unit passesthrough a filtering element. (lthough a full-flow type provides a more positive filtering

action, it offers greater resistance to flow, particularly when it becomes dirty. ( hydraulic

liquid enters a full-flow filter through an inlet port in the body and flows around an

element inside a bowl. Filtering occurs as a liquid passes through the element and into a

hollow core, leaving the dirt and impurities on the outside of the element. ( filtered liquidthen flows from a hollow core to an outlet port and into the system.

( bypass relief valve in a body allows a liquid to bypass the element and pass directlythrough an outlet port when the element becomes clogged. Filters that do not have a

bypass relief valve have a contamination indicator. his indicator wor!s on the principleof the difference in pressure of a fluid as it enters a filter and after it leaves an element.

hen contaminating particles collect on the element, the differential pressure across it

increases. hen a pressure increase reaches a specific value, an indicator pops out,signifying that the element must be cleaned or replaced.


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(2) Propor$iona%-F%ow Fi%$ers%Figure +-13&. his filter operates on the venturi principle

in which a tube has a narrowing throat %venturi& to increase the velocity of fluid flowing

through it. Flow through a venturi throat causes a pressure drop at the narrowest point.his pressure decrease causes a suc!ing action that draws a portion of a liquid down

around a cartridge through a filter element and up into a venturi throat. Filtering occurs

for either flow direction.

(lthough only a portion of a liquid is filtered during each cycle, constant recirculation

through a system eventually causes all of a liquid to pass through the element. Eeplacethe element according to applicable regulations and by doing the following7

Eelieve the pressure.

Eemove the bowl from the filters body.

Eemove the filter element from the body, using a slight roc!ing motion.

Blean or replace the element, depending on its type.

Eeplace all old -ring pac!ing and bac!up washers.

Eeinstall the bowl on the body assembly. 9o not tighten the bowl e$cessively=

chec! the appropriate regulations for specifications, as some filter elements

require a specific torque.

#ressuri'e the system and chec! the filter assembly for lea!s.


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-2. Filterin *aterial and Elements.

he general classes of filter materials are mechanical, absorbent inactive, and absorbent

active.

*ec0anical &ilterscontain closely woven metal screens or discs. hey generally remove

only fairly coarse particles.

3bsorbent inacti+e &ilters, such as cotton, wood pulp, yarn, cloth, or resin, removemuch smaller particles= some remove water and water-soluble contaminants. he

elements often are treated to ma!e them stic!y to attract the contaminants found in

hydraulic oil.

3bsorbent acti+e materials, such as charcoal and fullers earth %a clayli!e material ofvery fine particles used in the purification of mineral or vegetable-base oils&, are not

recommended for hydraulic systems.

he three basic types of filter elements are surface, edge, and depth.

( sur&ace-type elementis made of closely woven fabric or treated paper. il flowsthrough the pores of the filter material, and the contaminants are stopped.

(n ede-type &ilteris made up of paper or metal discs= oil flows through the spaces

between the discs. he fineness of the filtration is determined by the closeness of the

discs.

( dept0-type elementis made up of thic! layers of cotton, felt, or other fibers.


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-. 3ccumulators.

,i*e an electrical storage batter% a hdraulic accu!ulator stores potential power% in this caseliquid under pressure, for future conversion into useful wor!. his wor! can include

operating cylinders and fluid motors, maintaining the required system pressure in case ofpump or power failure, and compensating for pressure loss due to lea!age. (ccumulators

can be employed as fluid dispensers and fluid barriers and can provide a shoc!-absorbing

%cushioning& action.

(ccumulators are used mainly on the lift equipment to provide positive clamping actionon the heavy loads when a pumps flow is diverted to lifting or other operations. (n

accumulator acts as a safety device to prevent a load from being dropped in case of an

engine or pump failure or fluid lea!. n lifts and other equipment, accumulators absorb

shoc!, which results from a load starting, stopping, or reversal.

a. prin-Loaded 3ccumulator. his accumulator is used in some engineer equipment

hydraulic systems. It uses the energy stored in springs to create a constant force on the

liquid contained in an ad"acent ram assembly. Figure +-12 shows two spring-loadedaccumulators.


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he load characteristics of a spring are such that the energy storage depends on the force

required to compress a spring.

he free %uncompressed& length of a spring represents 'ero energy storage. (s a spring iscompressed to the ma$imum installed length, a minimum pressure value of the liquid in a

ram assembly is established. (s liquid under pressure enters the ram cylinder, causing a

spring to compress, the pressure on the liquid will rise because of the increased loadingrequired to compress the spring.

b. (a-5ype 3ccumulator. his accumulator %Figure +-14& consists of a seamless, high-

pressure shell, cylindrical in shape, with domed ends and a synthetic rubber bag that

separates the liquid and gas %usually nitrogen& within the accumulator. he bag is fullyenclosed in the upper end of a shell. he gas system contains a high-pressure gas valve.

he bottom end of the shell is sealed with a special plug assembly containing a liquid

port and a safety feature that ma!es it impossible to disassemble the accumulator withpressure in the system. he bag is larger at the top and tapers to a smaller diameter at the

bottom.

(s the pump forces liquid into the accumulator shell, the liquid presses against the bag,

reduces its volume, and increases the pressure, which is then available to do wor!.


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c. Piston-5ype 3ccumulator. his accumulator consists of a cylinder assembly, a pistonassembly, and two end-cap assemblies. he cylinder assembly houses a piston assembly

and incorporates provisions for securing the end-cap assemblies. (n accumulator

contains a free-floating piston with liquid on one side of the piston and precharged air or

nitrogen on the other side %Figure +-16&.

(n increase of liquid volume decreases the gas volume and increases gas pressure, which

provides a wor! potential when the liquid is allowed to discharge.


34/164

d. *aintenance.efore removing an accumulator for repairs, relieve the internalpressure7 in a spring-loaded type, relieve the spring tension= in a piston or bag type,

relieve the gas or liquid pressure.

-6. Pressure 7aues and Volume *eters.


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#ressure gauges are used in liquid-powered systems to measure pressure to maintain

efficient and safe operating levels. #ressure is measured in psi. Flow measurement may

be e$pressed in units of rate of flow-;#< or cubic feet per second %cfs&. It may also bee$pressed in terms of total quantity-gallons or cubic feet.

a. Pressure 7aues. Figure +-1@ shows a simple pressure gauge. ;auge readingsindicate the fluid pressure set up by an opposition of forces within a system. (tmospheric

pressure is negligible because its action at one place is balanced by its equal action atanother place in a system.

b. *eters.


36/164

In a nutating-piston-disc flow-meter, liquid passes through a fi$ed-volume measuring

chamber, which is divided into upper and lower compartments by a piston disc %Figure +-

1D&. 9uring operation, one compartment is continually being filled while the other isbeing emptied. (s a liquid passes through these compartments, its pressure causes a

piston disc to roll around in the chamber. he discs movements operate a dial %or

counter& through gearing elements to indicate that a column of fluid that has passedthrough the meter.


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-8. Portable %ydraulic-ircuit 5esters.

Hydraulic power is an efficient method of delivering hp by pumping a fluid through a

closed system. If the amount of flow or the pressure un!nowingly decreases, the amountof hp delivered to a wor!ing unit will be reduced, and a system will not perform as it

should.

a. 5esters. #ortable hydraulic-circuit testers %Figure +-+0& are lightweight units you canuse to chec! or troubleshoot a hydraulic-powered system on the "ob or in a maintenance

shop. Bonnect a tester into a systems circuit to determine its efficiency. Burrently, several

hydraulic-circuit testers are on the mar!et. perating procedures may vary on different

testers. herefore, you must follow the operating directions furnished with a tester tochec! or troubleshoot a circuit accurately.


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b. 9mproper ,peration. hen a hydraulic system does not operate properly, the trouble

could be one of the following7

he pump that propels the fluid may be slipping because of a worn or animproperly set spring in the relief valve.

he fluid may be lea!ing around the control valves or past the cylinder pac!ing.

Cince hydraulic systems are confined, it is difficult to identify which component in a

system is not wor!ing properly.


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a. Tubing. The two tpes of tubing used for hdraulic lines are sea!less and electric-welded.

oth are suitable for hydraulic systems. Ceamless tubing is made in larger si'es thantubing that is electric-welded. Ceamless tubing is flared and fitted with threaded

compression fittings. ubing bends easily, so fewer pieces and fittings are required.

8nli!e pipe, tubing can be cut and flared and fitted in the field. ;enerally, tubing ma!es aneater, less costly, lower-maintenance system with fewer flow restrictions and less

chances of lea!age. Figure +-+1 shows the proper method of installing tubing.

>nowing the flow, type of fluid, fluid velocity, and system pressure will help determine

the type of tubing to use. %?ominal dimensions of tubing are given as fractions in inchesor as dash numbers. ( dash number represents a tubes outside diameter 9G in

si$teenths of an inch.& ( systems pressure determines the thic!ness of the various tubing

walls. ubing above 1/+ inch 9 usually is installed with either flange fittings with metalor pressure seals or with welded "oints. If "oints are welded, they should be stress-

relieved.

b. Pipin.5ou can use piping that is threaded with screwed fittings with diameters up to

1 inches and pressures of up to 1,000 psi. here pressures will e$ceed 1,000 psi andrequired diameters are over 1 inches, piping with welded, flanged connections and

soc!et-welded si'e are specified by nominal inside diameter %I9& dimensions. he thread

remains the same for any given pipe si'e regardless of wall thic!ness. #iping is usedeconomically in larger-si'ed hydraulic systems where large flow is carried. It isparticularly suited for long, permanent straight lines. #iping is taper-threaded on its 9

into a tapped hole or fitting. However, it cannot be bent. Instead, fittings are used

wherever a "oint is required. his results in additional costs and an increased chance oflea!age.


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he minimum bend radius for fle$ible hose varies according to its si'e and construction

and the pressure under which a system will operate. Bonsult the applicable publicationsthat contain the tables and graphs which show the minimum bend radii for the different

types of installations. ends that are too sharp will reduce the bursting pressure offle$ible hose considerably below its rated value.

9o not install fle$ible hose so that it will be sub"ected to a minimum of fle$ing duringoperation. ?ever stretch hose tightly between two fittings. hen under pressure, fle$ible

hose contracts in length and e$pands in diameter.

(2) ef%on*-pe ose' his is a fle$ible hose that is designed to meet the requirements

of higher operating pressures and temperatures in todays fluid-powered systems. hehose consists of a chemical resin that is processed and pulled into a desired-si'e tube

shape. It is covered with stainless-steel wire that is braided over the tube for strength and

protection. eflon-type hose will not absorb moisture and is unaffected by all fluids usedin todays fluid-powered systems. It is nonflammable= however, use an asbestos fire

sleeve where the possibility of an open flame e$ists.

Barefully handle all eflon-type hose during removal or installation. Charp or e$cessive

bending will !in! or damage the hose. (lso, the fle$ible-type hose tends to form itself tothe installed position in a circulatory system.

d. 9nstallation. Flaring and bra'ing are the most common methods of connecting tubing.

#reparing a tube for installation usually involves cutting, flaring, and bending. (fter

cutting a tube to the correct length, cut it squarely and carefully remove any internal or

e$ternal burrs.

If you use flare-type fittings, you must flare the tube. ( flare angle should e$tend 6

degrees on each side of the centerline. he areas outer edge should e$tend beyond the

ma$imum sleeves I9 but not its 9. Flares that are too short are li!ely to be squee'edthin, which could result in lea!s or brea!s. Flares that are too long will stic! or "am

during assembly.

>eep the lines as short and free of bends as possible. However, bends are preferred to

elbows or sharp turns. ry not to assemble the tubing in a straight line because a bendtends to eliminate strain by absorbing vibration and compensating for temperature

e$pansion and contraction.

Install all the lines so tht you can remove them without dismantling a circuits

components or without bending or springing them to a bad angle. (dd supports to thelines at frequent intervals to minimi'e vibration or movement= never weld the lines to the

supports. Cince fle$ible hose has a tendency to shorten when sub"ected to pressure, allow

enough slac! to compensate for this problem.


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>eep all the pipes, tubes, or fittings clean and free from scale and other foreign matter.

Blean iron or steel pipes, tubes, and fittings with a boiler-tube wire brush or with

commercial pipe-cleaning equipment. Eemove rust and scale from short, straight piecesby sandblasting them, as long as no sand particles will remain lodged in blind holes or

poc!ets after you flush a piece. In the case of long pieces or pieces bent to comple$

shapes, remove rust and scale by pic!ling %cleaning metal in a chemical bath&. Bap andplug the open ends of the pipes, tubes, and fittings that will be stored for a long period.

9o not use rags or waste for this purpose because they deposit harmful lint that can cause

severe damage in a hydraulic system.


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2-1. Fi!!ings and Connec!ors.

Fittings are used to connect the units of a fluid-powered system, including the individual

sections of a circulatory system.


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b. Flared onnectors.he common connectors used in circulatory systems consist oftube lines. hese connectors provide safe, strong, dependable connections without having

to thread, weld, or solder the tubing. ( connector consists of a fitting, a sleeve, and a nut

%see Figure +-+2&.


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Fittings are made of steel, aluminum alloy, or bron'e. he fittings should be of a material

that is similar to that of a sleeve, nut, and tubing.

Fittings are made in unions, 32- and D0-degree elbows, s, and various other shapes.Figure +-+4 shows some of the most common fittings used with flared connectors.

Fittings are available in many different thread combinations. 8nions have tube

connections on each end= elbows have tube connections on one end and a male pipe

thread, female pipe thread, or tube connection on the opposite end= crosses and s haveseveral different combinations.

ubing used with flared connectors must be flared before being assembled. ( nut fits over

a sleeve and, when tightened, draws the sleeve and tubing flare tightly against a male

fitting to form a seal. ( male fitting has a cone-shaped surface with the same angle as theinside of a flare. ( sleeve supports the tube so that vibration does not concentrate at the

edge of a flare but that it does distribute the shearing action over a wider area for added

strength. ighten the tubing nuts with a torque wrench to the value specified in applicableregulations.

If an aluminum alloy flared connector lea!s after tightening to the specified torque, do

not tighten it further. 9isassemble the lea!ing connector and correct the fault. If a steel

connector lea!s, you may tighten it 1/4 turn beyond the specified torque in an attempt tostop the lea!. If you are unsuccessful, disassemble it and repair it.

Flared connectors will lea! if-

( flare is distorted into the nut threads.

( sleeve is crac!ed. ( flare is crac!ed or split.

( flare is out-of-round.

( flare is eccentric to the tubes 9.

( flares inside is rough or scratched.

( fitting cone is rough or scratched.

he threads of a fitting or nut are dirty, damaged, or bro!en.


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c. Flexible-%ose ouplins. If a hose assembly is fabricated with field attachable

couplings %Figure +-+6&, use the same couplings when fabricating the replacement

assembly, as long as the failure %lea! or brea!& did not occur at a coupling. If failureoccurred at a coupling, discard it.


47/164


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hen measuring a replacement hose assembly for screw-on couplings, measure from the

edge of a retaining bolt %Figure +-+@&. #lace the hose in hose bloc!s and then in a bench

vice %Figure +-+D&. 8se the front or rear portion of a hac!saw blade for cutting. %If youuse the middle portion of a blade, it could twist and brea!.& For effective cutting, a blade

should have +3 or + teeth per inch. o remove an old coupling on a hose assembly that is

fabricated with permanently attached couplings, you "ust discard the entire assembly %seeFigure +-0&.

d. Reusable Fittins. o use a s!ived fitting %Figure +-1&, you must strip %s!ive& the

hose to a length equal to that from a notch on a fitting to the end of the fitting. %( notchon a female portion of a fitting in Figure +-1 indicates it to be a s!ived fitting.&

o assemble a conductor using s!ived fittings-

1. 9etermine the length of the s!ive.+.


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6. Insert the hose into the female soc!et and turn the hose countercloc!wise until it

bottoms on the shoulder of the female soc!et, then bac! off 1/3 turn.

@. #lace the female soc!et in an upright position %Figure +-3& and insert the malenipple into the female soc!et.

D. urn the male nipple cloc!wise %Figure +-2& until the he$ is within 1/+ inch of

the female soc!et.10. Eepeat the above process on the opposite end of the hose.

hen assembling conductors using nons!ived-type fittings, follow the above procedures.

However, do not s!ive a hose. ?ons!ived fittings do not have a notch on the female

portion of a fitting %Figure +-4&.

Figure +-6, diagram (, shows a female hose coupling. ne end of the hose has a spiralridge %course thread& that provides a gripping action on the hose. he other end %small

end& has machine threads into which the male, fi$ed or swivel, nipple is inserted.

Figure +-6, diagram shows the male adapter, and diagram B shows the male and thefemale swivel body. hese fittings contain a fi$ed or swivel he$-nut connector on one

end. he opposite end is tapered and has machine threads that mate with the threads in a
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female fitting. ith a long taper inserted into a hose and screwed into a female coupling,

the taper tends to e$pand a hose, forcing it against the inside diameter of a female fitting.

Figure +-@ shows the assembly of a clamp-type coupling. If you use this coupling, donot s!ive the hose. *ubricate the I9 of a hose and the 9 of a stem. Blamp a hose stem

in a bench vice and install a hose. urn the hose countercloc!wise until it bottoms againstthe shoulder of the stem %Figure +-@, diagram (&. If you do not have a vice, force the

stem into the hose by pushing or stri!ing the stem with a wooden bloc!. #lace the clamphalves in position %Figure +-@, diagram & and draw them together with a vice or with

e$tra long bolts until the standard bolts protrude far enough to grip the nuts. Eemove the

e$tra long bolts and place retaining bolts through the clamp. ighten the nuts until youget the required torque %Figure +-@, diagram B&.

;,5E:5ou may have to retighten the bolts after the hose assembly has been operating

about 10 to +0 hours. 8se clamp-type couplings on hose assemblies with diameters of 1

inch or greater. 8se reusable screw-type fittings on hose assemblies with diameters lessthan 1 inch.


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-11. Lea"age.

(ny hydraulic system will have a certain amount of lea!age. (ny lea!age will reduce

efficiency and cause power loss. Come lea!age is built in %planned&, some is not. *ea!agemay be internal, e$ternal, or both.

a. 9nternal.his type of lea!age %nonpositive& must be built into hydraulic components

to lubricate valve spools, shafts, pistons, bearings, pumping mechanisms, and other

moving parts. In some hydraulic valves and pump and motor compensator controls,lea!age paths are built in to provide precise control and to avoid hunting %oscillation& of

spools and pistons. il is not lost in internal lea!age= it returns to a reservoir through

return lines or specially provided drain passages.

oo much internal lea!age will slow down actuators. he power loss is accompanied by

the heat generated at a lea!age path. In some instances, e$cess lea!age in a valve couldcause a cylinder to drift or even creep when a valve is supposedly in neutral. In the case

of flow or pressure-control valves, lea!age can often reduce effective control or evencause control to be lost.

?ormal wear increases internal lea!age, which provides larger flow paths for the lea!ing

oil. (n oil that is low in viscosity lea!s more readily than a heavy oil. herefore an oils

viscosity and viscosity inde$ are important considerations in providing or preventinginternal lea!age. Internal lea!age also increases with pressure, "ust as higher pressure

causes a greater flow through an orifice. perating above the recommended pressures

adds the danger of e$cessive internal lea!age and heat generation to other possible

harmful effects.

( blown or ruptured internal seal can open a large enough lea!age path to divert all of a

pumps delivery. hen this happens, everything e$cept the oil flow and heat generation at

a lea!age point can stop.

b. External. )$ternal lea!age can be ha'ardous, e$pensive, and unsightly. Faulty

installation and poor maintenance are the prime causes of e$ternal lea!age. oints may

lea! because they were not put together properly or because shoc! and vibration in the

lines shoo! them loose. (dding supports to the lines prevents this. If assembled andinstalled correctly, components seldom lea!. However, failure to connect drain lines,

e$cessive pressures, or contamination can cause seals to blow or be damaged, resulting ine$ternal lea!age from the components.

c. Pre+ention. #roper installation, control of operating conditions, and proper

maintenance help prevent lea!age.

(1) 4ns$a%%a$ion. Installing piping and tubing according to a manufacturers

recommendations will promote long life of e$ternal seals. :ibration or stresses that result


52/164

from improper installation can sha!e loose connections and create puddles. (void

pinching, coc!ing, or incorrectly installing seals when assembling the units. 8se any

special tools that the manufacturer recommends for installing the seals.

(2) 5pera$ing Condi$ions'o ensure correct seal life, you must control the operating

conditions of the equipment. ( shaft seal or piston-rod seal e$posed to moisture, salt, dirt,or any other abrasive contaminant will have a shortened life span. (lso, operators should

always try to !eep their loads within the recommended limits to prevent lea!age causedby e$cessive pressures.

%&


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Pumps

Hydraulic pumps convert mechanical energy from a prime mover %engine or electric

motor& into hydraulic %pressure& energy. he pressure energy is used then to operate anactuator. #umps push on a hydraulic fluid and create flow.

/-1. Pump lassi&ications

(ll pumps create flow. hey operate on the displacement principle. Fluid is ta!en in and

displaced to another point. #umps that discharge liquid in a continuous flow arenonpositive-displacement type. #umps that discharge volumes of liquid separated by

periods of no discharge are positive-displacement type.

a. ;onpositi+e-


54/164

b.Positive-Displacement Pumps' ith this pump, a definite volume of liquid is

delivered for each cycle of pump operation, regardless of resistance, as long as the

capacity of the power unit driving a pump is not e$ceeded. If an outlet is completelyclosed, either the unit driving a pump will stall or something will brea!.

herefore, a positive-displacement-type pump requires a pressure regulator or pressure-relief valve in the system. Figure -+ shows a reciprocating-type, positive-displacement

pump.

Figure - shows another positive-displacement pump. his pump not only creates flow,

but it also bac!s it up. ( sealed case around the gear traps the fluid and holds it while itmoves. (s the fluid flows out of the other side, it is sealed against bac!up. his sealing isthe positive part of displacement. ithout it, the fluid could never overcome the

resistance of the other parts in a system.


55/164

c. Characteristics.he three contrasting characteristics in the operation of positive- and

nonpositive-displacement pumps are as follows7

?onpositive-displacement pumps provide a smooth, continuous flow= positive-

displacement pumps have a pulse with each stro!e or each time a pumping

chamber opens to an outlet port.

#ressure can reduce a nonpositive pumps delivery. High outlet pressure can stop

any output= the liquid simply recirculates inside the pump. In a positive-

displacement pump, pressure affects the output only to the e$tent that it increasesinternal lea!age.

?onpositive-displacement pumps, with the inlets and outlets connectedhydraulically, cannot create a vacuum sufficient for self-priming= they must be

started with the inlet line full of liquid and free of air. #ositive-displacementpumps often are self-priming when started properly.


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-+. #erformance

#umps are usually rated according to their volumetric output and pressure. :olumetric

output %delivery rate or capacity& is the amount of liquid that a pump can deliver at itsoutlet port per unit of time at a given drive speed, usually e$pressed in ;#< or cubic

inches per minute. ecause changes in pump drive affect volumetric output, pumps are

sometimes rated according to displacement, that is the amount of liquid that they candeliver per cycle or cubic inches per revolution.

#ressure is the force per unit area of a liquid, usually e$pressed in psi. %


57/164

#-$. %li&&age

Clippage is oil lea!ing from a pressure outlet to a low-pressure area or bac! to an inlet. (

drain passage allows lea!ing oil to return to an inlet or a reservoir. Come slippage isdesigned into pumps for lubrication purposes. Clippage will increase with pressure and as

a pump begins to wear. il flow through a given orifice si'e depends on the pressure drip.

(n internal lea!age path is the same as an orifice. herefore, if pressure increases, moreflow will occur through a lea!age path and less from an outlet port. (ny increase in

slippage is a loss of efficiency.

#-'. (esigns

In most rotary hydraulic pumps %Figure -&, the design is such that the pumpingchambers increase in si'e at the inlet, thereby creating a vacuum. he chambers then

decrease in si'e at the outlet to push fluid into a system. he vacuum at the inlet is used

to create a pressure difference so that fluid will flow from a reservoir to a pump.However, in many systems, an inlet is charged or supercharged= that is, a positive

pressure rather than a vacuum is created by a pressuri'ed reservoir, a head of fluid above

the inlet, or even a low-pressure-charging pump. he essentials of any hydraulic pump

are-

( low-pressure inlet port, which carries fluid from the reservoir.

( high-pressure outlet port connected to the pressure line.

#umping chamber%s& to carry a fluid from the inlet to the outlet port. ( mechanical means for activating the pumping chamber%s&.

#umps may be classified according to the specific design used to create the flow of aliquid.


58/164

(2) Diffuser Pump (Figure 3-!). Cimilar to a volute type, a diffuser pump has a series of

stationary blades %the diffuser& that curve in the opposite direction from whirling impellerblades. ( diffuser reduces the velocity of a liquid, decreases slippage, and increases a

pumps ability to develop flow against resistance.

b. Rotary Pump'In this positive-displacement-type pump, a rotary motion carries a

liquid from a pumps inlet to its outlet. ( rotary pump is usually classified according to

the type of element that actually transmits a liquid, that is, a gear-, vane-, or piston-typerotary pump.


59/164

c. Reciprocatin Pump. ( reciprocating pump depends on a reciprocating motion to

transmit a liquid from a pumps inlet to its outlet. Figure -+shows a simplified

reciprocating pump. It consists of a cylinder that houses a reciprocating piston, Figure -+,1= an inlet valve, Figure -+, += and an outlet valve, Figure -+, , which direct fluid to

and from a cylinder. hen a piston moves to the left, a partial vacuum that is created

draws a ball off its seat, allowing a liquid to be drawn through an inlet valve into acylinder. hen a piston moves to the right, a ball reseats and closes an inlet valve.

However, the force of a flow unseats a ball, allowing a fluid to be forced out of a cylinder

through an outlet valve.

-). Gear Pu*&s

;ear pumps are e$ternal, internal, or lobe types.

a.Eternal' Figure -4 shows the operating principle of an e$ternal gear pump. It

consists of a driving gear and a driven gear enclosed in a closely fitted housing. he gears

rotate in opposite directions and mesh at a point in the housing between the inlet andoutlet ports. oth sets of teeth pro"ect outward from the center of the gears. (s the teeth

of the two gears separate, a partial vacuum forms and draws liquid through an inlet port

into chamber (. *iquid in chamber ( is trapped between the teeth of the two gears andthe housing so that it is carried through two separate paths around to chamber . (s the

teeth again mesh, they produce a force that drives a liquid through an outlet port.

b. 9nternal.Figure -6 shows an internal gear pump. he teeth of one gear pro"ectoutward, while the teeth of the other gear pro"ect inward toward the center of the pump.

ne gear wheel stands inside the other. his type of gear can rotate, or be rotated by, a

suitably constructed companion gear. (n e$ternal gear is directly attached to the driveshaft of a pump and is placed off-center in relation to an internal gear.
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60/164

he two gears mesh on one side of a pump chamber, between an inlet and the discharge.

n the opposite side of the chamber, a crescent-shaped form stands in the space betweenthe two gears to provide a close tolerance.

he rotation of the internal gear by a shaft causes the e$ternal gear to rotate, since the two

are in mesh. )verything in the chamber rotates e$cept the crescent, causing a liquid to betrapped in the gear spaces as they pass the crescent. *iquid is carried from an inlet to the

discharge, where it is forced out of a pump by the gears meshing. (s liquid is carried

away from an inlet side of a pump, the pressure is diminished, and liquid is forced infrom the supply source. he si'e of the crescent that separates the internal and e$ternal

gears determines the volume delivery of this pump. ( small crescent allows more volume

of a liquid per revolution than a larger crescent.

c. Lobe.Figure -@ shows a lobe pump. It differs from other gear pumps because it uses

lobed elements instead of gears. he element drive also differs in a lobe pump. In a gearpump, one gear drives the other. In a lobe pump, both elements are driven through

suitable e$ternal gearing.


61/164

-+. ,ane Pu*&s

In a vane-type pump, a slotted rotor splined to a drive shaft rotates between closely fitted

side plates that are inside of an elliptical- or circular-shaped ring. #olished, hardened

vanes slide in and out of the rotor slots and follow the ring contour by centrifugal force.#umping chambers are formed between succeeding vanes, carrying oil from the inlet to

the outlet. ( partial vacuum is created at the inlet as the space between vanes increases.

he oil is squee'ed out at the outlet as the pumping chambers si'e decreases.

ecause the normal wear points in a vane pump are the vane tips and a rings surface, the

vanes and ring are specially hardened and ground. ( vane pump is the only design thathas automatic wear compensation built in. (s wear occurs, the vanes simply slide farther

out of the rotor slots and continue to follow a rings contour. hus efficiency remains highthroughout the life of the pump.

a. 0aracteristics.9isplacement of a vane-type pump depends on the width of the ring

and rotor and the throw of the cam ring. Interchangeable rings are designed so a basic

pump converts to several displacements. alanced design vane pumps all are fi$eddisplacement. (n unbalanced design can be built in either a fi$ed- or variable-

displacement pump. :ane pumps have good efficiency and durability if operated in a

clean system using the correct oil. hey cover the low to medium-high pressure, capacity,

and speed ranges. #ac!age si'e in relation to output is small. ( vane pump is generallyquiet, but will whine at high speeds.

b. =nbalanced Vane Pumps.In the unbalanced design, %Figure -D&, a cam rings shape

is a true circle that is on a different centerline from a rotors. #ump displacement dependson how far a rotor and ring are eccentric. he advantage of a true-circle ring is that

control can be applied to vary the eccentricity and thus vary the displacement.


62/164

( disadvantage is that an unbalanced pressure at the outlet is effective against a small

area of the rotors edge, imposing side loads on the shaft. hus there is a limit on a pumpssi'e unless very large hearings and heavy supports are used.

c.Balanced Vane Pumps' In the balanced design %Figure -10&, a pump has a stationary,

elliptical cam ring and two sets of internal ports. ( pumping chamber is formed between

any two vanes twice in each revolution. he two inlets and outlets are 1@0 degrees apart.

ac! pressures against the edges of a rotor cancel each other. Eecent designimprovements that allow high operating speeds and pressures have made this pump the

most universal in the mobile-equipment field.


63/164

d.


64/164

Bombining pump deliveries does not alter the ma$imum pressure rating of either

cartridge. Ceparate circuits require separate pressure controls to limit ma$imum pressurein each circuit.

Figure -1+, shows an installation in which double pumps are used to provide fluid flow

for operation of a cylinder in rapid advance and feed. In circuit , two relief valves are

used to control pumping operation. In circuit (, one relief valve and one unloading valveare used to control pumping operations. In both circuits, the deliveries of the pump

cartridges are combined after passing through the valves. his combined flow is directed

to a four-way valve and to the rest of the circuit.


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In circuit , an upper relief valve is vented when a cylinder rod reaches and trips a pilot

valve. ( vented relief valve directs the delivery of a shaft-end pump cartridge freely bac!to a tan!. (nother relief valve controls the ma$imum pressure of a circuit. (n unloading

valve and a relief valve in circuit ( do the same operation. he output of both pump

cartridges combines to supply fluid for a rapid advance portion of a cycle. hen theoutput of one circuit returns to the tan!, after reaching a certain point in the cycle, the

other circuit completes the advance portion of a cycle. oth pump outputs are then

combined for rapid return.

e. 5wo-tae Pumps. wo-stage pumps consist of two separate pump assembliescontained in one housing. he pump assemblies are connected so that flow from the

outlet of one is directed internally to the inlet of the other. Cingle inlet and outlet ports are

used for system connections. In construction, the pumps consist of separate pumpingcartridges driven by a common drive shaft contained in one housing. ( dividing valve is

used to equali'e the pressure load on each stage and correct for minor flow differences

from either cartridge.

In operation, developing fluid flow for each cartridge is the same as for single pumps.Figure -1 shows fluid flow in a vane-type, two-stage pump. il from a reservoir enters

a pumps inlet port and passes to the outlets of the first-stage pump cartridge. %#assages in

a pumps body carry the discharge from this stage to an inlet of the second stage.&


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utlet passages in the second stage direct the oil to an outlet port of the pump. #assage 8

connects both chambers on the inlet side of a second-stage pump and assures equalpressure in both chambers. %#ressures are those that are imposed on a pump from e$ternal

sources.&

( dividing valve %see Figure -1& consists of sliding pistons ( and . #iston ( is

e$posed to outlet pressure through passage :. #iston is e$posed to the pressure betweenstages through passage . he pistons respond to maintain a pressure load on a first-stage

pump equal to half the outlet pressure at a second-stage pump. If the discharge from the

first stage e$ceeds the volume that can be accepted at the second stage, a pressure riseoccurs in passage . he unbalanced force acting on piston causes the pistons to move

in such a manner that e$cess oil flows past piston through passage 5 to the inlet

chamber of a first-stage cartridge. Fluid throttling across piston in this manner

maintains pressure in passage :.

If the discharge from a first-stage pump is less than the volume required at a second-stagepump, a reduced pressure occurs at piston . (n unbalanced force acting on piston (

causes the pistons to move so that oil flows past piston ( into passages A and to

replenish a second-stage pump and correct the unbalanced condition. #assages J and 5provide a means for lea!age around the pistons to return to the inlet chamber of a first-

stage pump. #istons ( and always see! a position that equally divides the load between

the two pumping units.


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-. Pis!on Pu*&s

#iston pumps are either radial or a$ial.

a. Radial.In a radial piston pump %Figure -13&, the pistons are arranged li!e wheelspo!es in a short cylindrical bloc!. ( drive shaft, which is inside a circular housing,

rotates a cylinder bloc!. he bloc! turns on a stationary pintle that contains the inlet and

outlet ports. (s a cylinder bloc! turns, centrifugal force slings the pistons, which follow acircular housing. ( housings centerline is offset from a cylinder bloc!s centerline. he

amount of eccentricity between the two determines a piston stro!e and, therefore, a

pumps displacement. Bontrols can be applied to change a housings location and thereby

vary a pumps delivery from 'ero to ma$imum .

Figure -12 shows a nine-piston, radial piston pump. hen a pump has an unevennumber of pistons, no more than one piston is completely bloc!ed by a pintle at one time,

which reduces flow pulsations. ith an even number of pistons spaced around a cylinder

bloc!, two pistons could be bloc!ed by a pintle at the same time. If this happens, three

pistons would discharge at one time and four at another time, and pulsations would occurin the flow. ( pintle, a cylinder bloc!, the pistons, a rotor, and a drive shaft constitute the

main wor!ing parts of a pump.


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%1& #intle. ( pintle is a round bar that serves as a stationary shaft around which a cylinder

bloc! turns. ( pintle shaft %Figure -14& has four holes bored from one end lengthwise

through part of its length. wo holes serve as an inta!e and two as a discharge. wo slotsare cut in a side of the shaft so that each slot connects two of the lengthwise holes. he

slots are in-line with the pistons when a cylinder bloc! is assembled on a pintle. ne of

these slots provides a path for a liquid to pass from the pistons to the discharge holesbored in a pintle. (nother slot connects the two inlet holes to the pistons when they are

drawing in liquid. he discharge holes are connected through appropriate fittings to a

discharge line so that a liquid can be directed into a system. he other pair of holes is

connected to an inlet line.

%+& Bylinder loc!. ( cylinder bloc! %Figure -16& is a bloc! of metal with a hole bored

through its center to fit the pintles and cylinders holes that are bored equal distances

apart around its outside edge. he cylinders holes connect with the hole that receives a

pintle. 9esigns differ= some cylinders appear to be almost solid, while others havespo!eli!e cylinders radiating out from the center. ( cylinders and pintles holes are

accurately machined so that liquid loss around a piston is minimal.


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%& #istons. #istons are manufactured in different designs %see Figure -1@&. 9iagram (shows a piston with small wheels that roll around the inside curve of a rotor. 9iagram

shows a piston in which a conical edge of the top bears directly against a reaction ring of

the rotor. In this design, a piston goes bac! and forth in a cylinder while it rotates aboutits a$is so that the top surface will wear uniformly. 9iagram B shows a piston attached to

curved plates. he curved plates bear against and slide around the inside surface of a

rotor. he pistons sides are accurately machined to fit the cylinders so that there is aminimum loss of liquid between the walls of a piston and cylinder. ?o provision is made

for using piston rings to help seal against piston lea!age.

%3& Eotors. Eotor designs may differ from pump to pump. ( rotor consists of a circular

ring, machine finished on the inside, against which the pistons bear. ( rotor rotates within

a slide bloc!, which can be shifted from side to side to control the pistons length of

stro!e. ( slide bloc! has two pairs of machined surfaces on the e$terior so that it can slidein trac!s in the pump case.

%2& 9rive Chaft. ( drive shaft is connected to a cylinder bloc! and is driven by an outside

force such as an electric motor.


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b.!ial Piston Pumps. In a$ial piston pumps, the pistons stro!e in the same direction on

a cylinder bloc!s centerline %a$ially&. ($ial piston pumps may be an in-line or angledesign. In capacity, piston pumps range from low to very high. #ressures are as high as

2,000 psi, and drive speeds are medium to high. )fficiency is high, and pumps generallyhave e$cellent durability. #etroleum oil fluids are usually required. #ulsations in delivery

are small and of medium frequency. he pumps are quiet in operation but may have agrowl or whine, depending on condition. )$cept for in-line pumps, which are compact in

si'e, piston pumps are heavy and bul!y.

(1) 4n-6ine Pump. In an in-line piston pump %Figure -1D, diagram (&, a drive shaft andcylinder bloc! are on the same centerline. Eeciprocation of the pistons is caused by a

swash plate that the pistons run against as a cylinder bloc! rotates. ( drive shaft turns a

cylinder bloc!, which carries the pistons around a shaft. he piston shoes slide against a

swash plate and are held against it by a shoe plate. ( swash plates angle causes the

cylinders to reciprocate in their bores. (t the point where a piston begins to retract, anopening in the end of a bore slides over an inlet slot in a valve plate, and oil is drawn into

a bore through somewhat less than half a revolution. here is a solid area in a valve plateas a piston becomes fully retracted. (s a piston begins to e$tend, an opening in a cylinder

barrel moves over an outlet slot, and oil is forced out a pressure port.

%a& 9isplacement. #ump displacement depends on the bore and stro!e of a piston and thenumber of pistons. ( swash plates angle %Figure -1D, diagram & determines the stro!e,

which can vary by changing the angle. In a fi$ed angles unit, a swash plate is stationaryin the housing. In a variable units, it is mounted on a yo!e, which can turn on pintles.

9ifferent controls can be attached to the pintles to vary pump delivery from 'ero to the

ma$imum. ith certain controls, the direction of flow can be reversed by swinging ayo!e past center. In the center position, a swash plate is perpendicular to the cylinders,

and there is no piston reciprocation= no oil is pumped.


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(/) Componen$s' he ma"or components of a typical, fi$ed-displacement in-line pump arethe housing, a bearing-supported drive shaft, a rotating group, a shaft seal, and a valveplate. ( valve plate contains an inlet and an outlet port and functions as the bac! cover. (

rotating group consists of a cylinder bloc! that is splined to a drive shaft, a splined

spherical washer, a spring, nine pistons with shoes, a swash plate, and a shoe plate. hena group is assembled, a spring forces a cylinder bloc! against a valve plate and a

spherical washer against a shoe plate. his action holds the piston shoes against a swash

plate, ensuring that the pistons will reciprocate as the cylinder turns. ( swash plate is

stationary in a fi$ed-displacement design.

(c) 5pera$ion. ( variable-displacement in-line pump operates the same as a fi$ed angle

e$cept that a swash plate is mounted on a pivoted yo!e. ( yo!e can be swung to change aplate angle and thus change a pumps displacement. ( yo!e can be positioned manuallywith a screw or lever or by a compensator control, which positions a yo!e automatically

to maintain constant output pressure under variable flow requirements. ( compensator

control consists of a valve that is balanced between a spring and system pressure and a

spring-loaded, yo!e-actuating piston that is controlled by a valve. ( pumps compensatorcontrol thus reduces its output only to the volume required to maintain a preset pressure.


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action is the same as an in-line pump. he angle of offset determines a pumps

displacement, "ust as the swash plates angle determines an in-line pumps displacement.

In fi$ed-delivery pumps, the angle is constant. In variable models, a yo!e mounted onpintles swings a cylinder bloc! to vary displacement. Flow direction can be reversed with

appropriate controls.


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-. Pu*& &era!ion

he following paragraphs address some of the problems that could occur when a pump is

operating7

a. ,+erloadin.ne ris! of overloading is the danger of e$cess torque on a drive shaft.orque is circular force on an ob"ect. (n increase in pressure/pump displacement will

increase the torque on a shaft if pump displacement/pressure remains constant. ften in a

given pac!age si'e, a higher ;#< pump will have a lower pressure rating than a lower;#< pump. Cometimes a field conversion to get more speed out of an actuator will cause

a pump to be overloaded. 5ou may need a larger pump.

b. Excess peed. Eunning a pump at too high a speed causes loss of lubrication, which

can cause early failure. If a needed delivery requires a higher drive speed than a pumpsrating, use a higher displacement pump. )$cess speed also runs a ris! of damage from

cavitation.

c. a+itation' Bavitation occurs where available fluid does not fill an e$isting space. It

often occurs in a pumps inlet when conditions are not right to supply enough oil to !eepan inlet flooded. Bavitation causes the metal in an inlet to erode and the hydraulic oil to

deteriorate quic!er. Bavitation can occur if there is too much resistance in an inlets line,

if a reservoirs oil level is too far below the inlet, or if an oils viscosity is too high. It canalso occur if there is a vacuum or even a slight positive pressure at the inlet. ( badly

cavitating pump has oil bubbles e$ploding in the void. he only way to be sure a pump is

not cavitating is to chec! the inlet with a vacuum gauge.

o prevent cavitation, !eep the inlet clean and free of obstructions by using the correct

length of an inlets line with minimum bends. (nother method is to charge an inlet. heeasiest way to do this is to flood it by locating the reservoir above the pumps inlet. If this

is not possible and you cannot create good inlet conditions, use a pressuri'ed reservoir.5ou can also use an au$iliary pump to maintain a supply of oil to an inlet at low pressure.

5ou could use a centrifugal pump, but it is more common to use a positive-displacement

gear pump with a pressure-relief valve that is set to maintain the desired chargingpressure.

d. ,peratin Problems.#ressure loss, slow operation, no delivery, and noise are

common operating problems in a pump.

(1) Pressure 6oss. #ressure loss means that there is a high lea!age path in a system. (badly worn pump could cause pressure loss. ( pump will lose its efficiency gradually.he actuator speed slows down as a pump wears. However, pressure loss is more often

caused by lea!s somewhere else in a system %relief valve, cylinders, motors&.


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(2) %ow 5pera$ion. his can be caused by a worn pump or by a partial oil lea! in a

system. #ressure will not drop, however, if a load moves at all. herefore, hp is still being

used and is being converted into heat at a lea!age point. o find this point, feel thecomponents for unusual heat.

(3) 8o De%i&er. If oil is not being pumped, a pump-

Bould be assembled incorrectly.

Bould be driven in the wrong direction.

Has not been primed. he reasons for no prime are usually improper start-up, inlet

restrictions, or low oil level in a reservoir.

Has a bro!en drive shaft.

() 8oise. If you hear any unusual noise, shut down a pump immediately.

Bavitation noise is caused by a restriction in an inlet line, a dirty inlet filter, or toohigh a drive speed. (ir in a system also causes noise. (ir will severely damage a

pump because it will not have enough lubrication. his can occur from low oil ina reservoir, a loose connection in an inlet, a lea!ing shaft seal, or no oil in a pumpbefore starting. (lso, noise can be caused by worn or damaged parts, which will

spread harmful particles through a system, causing more damage if an operation

continues.

Hydraulic 0c!ua!ors

( hydraulic actuator receives pressure energy and converts it to mechanical force and

motion. (n actuator can be linear or rotary. ( linear actuator gives force and motionoutputs in a straight line. It is more commonly called a cylinder but is also referred to as aram, reciprocating motor, or linear motor. ( rotary actuator produces torque and rotating

motion. It is more commonly called a hydraulic motor or motor.

-1. ylinders

( cylinder is a hydraulic actuator that is constructed of a piston or plunger that operatesin a cylindrical housing by the action of liquid under pressure. Figure 3-1 shows the basic

parts of a cylinder. ( cylinder housing is a tube in which a plunger %piston& operates. In aram-type cylinder, a ram actuates a load directly. In a piston cylinder, a piston rod isconnected to a piston to actuate a load. (n end of a cylinder from which a rod or plunger

protrudes is a rod end. he opposite end is a head end. he hydraulic connections are a

head-end port and a rod-end port %fluid supply&.


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a. inle-3ctin ylinder. his cylinder %Figure 3-1& only has a head-end port and is

operated hydraulically in one direction. hen oil is pumped into a port, it pushes on a

plunger, thus e$tending it. o return or retract a cylinder, oil must be released to a

reservoir. ( plunger returns either because of the weight of a load or from somemechanical force such as a spring. In mobile equipment, flow to and from a single-acting

cylinder is controlled by a reversing directional valve of a single-acting type.

b.


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c.


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e. Ram-5ype ylinder.( ram-type cylinder is a cylinder in which a cross-sectional area

of a piston rod is more than one-half a cross-sectional area of a piston head. In many

cylinders of this type, the rod and piston heads have equal areas. ( ram-type actuatingcylinder is used mainly for push functions rather than pull.

Figure 3-1shows a single-acting, ram-type cylinder. ( single-acting ram applies force inone direction. his cylinder is often used in a hydraulic "ac!. In a double-acting, ram-type

cylinder, both stro!es of a ram are produced by pressuri'ed fluid. Figure 3-+shows thiscylinder.

Figure 3-3 shows a telescoping, ram-type, actuating cylinder, which can be a single- or

double-acting type. In this cylinder, a series of rams are nested in a telescoping assembly.)$cept for the smallest ram, each ram is hollow and serves as a cylinder housing for the

ne$t smaller ram. ( ram assembly is contained in a main cylinder housing, which also

provides the fluid ports. (lthough an assembly requires a small space with all of the rams

retracted, a telescoping action of an assembly provides a relatively long stro!e when the

rams are e$tended.

&. Piston-5ype ylinder.In this cylinder, a cross-sectional area of a piston head isrefer

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