Basic Hydraulics2

Basic Hydraulics

1 of 1

Tractor & Equipment AIMS & OBJECTIVES

Course title Caterpillar Basic Hydraulic systems Proposed date (s) Max No of Students Course duration 5 Days Audience Work shop engineers and technicians Prerequisites None Course Aims and Objectives At the completion of the course, the Field Service Engineers will be able to

1. Give the meaning of 10 Hydraulic symbols 2. List the available Caterpillar test tools for measuring pressure and flow 3. State Pascals law 4. Describe to the Instructors satisfaction the relationship between flow and pressure 5. Identify 10 given components on a Caterpillar hydraulic schematic 6. Describe to the Instructors satisfaction the purpose and operation of a Pilot hydraulic circuit 7. List the types of Caterpillar Hydraulic Pumps 8. List the types of Hydraulic motors

Subjects covered include: a) Hydraulic nomenclature e.g. pressure, flow, Closed loop system, Open centre system, Load

Sensing b) Hydraulic Pumps and motors types and operation c) Hydraulic Valves d) ISO Symbols

Comments Suggested Handouts / reference material Think Big CDs Cat Basics Library SEEV0529 Fluid Power Symbols SENR3981 Fluid Power Symbols users guide

A&O Caterpillar Basic Hydraulic systems Revision 1.0 14-11-01

UNIT 2Hydraulic Fundamentals - Hydraulic Principles

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Upon completion of this unit, the student will be able to:

1. Demonstrate an understanding of the basic hydraulic principles.

Introduction

Hydraulic systems are extremely important to the operation of heavyequipment. Basic hydraulic principles are used when designinghydraulic implement systems, steering systems, brake systems andpower train systems. An understanding of the basic hydraulicprinciples must be accomplished before continuing into machinesystems.

Lesson 1: Hydraulic Principles Hyd

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Introduction

We all know that hydraulic principles are demonstrated when using aliquid under controlled pressure to do work. There are laws thatstate the action of liquids under conditions of changing flows andincreasing and decreasing pressures. The student must be able tostate and understand these laws to become successful as a heavyequipment technician.

Objectives

Upon completion of this lesson, the student will be able to:

1. State why liquid is use in hydraulic systems.

2. Identify Pascal's Law as applied to hydraulic principles.

3. State the characteristics of oil flow through an orifice.

4. Demonstrate an understanding of the basic hydraulicprinciples.

HYDRAULIC PRINCIPLES

BASIC HYDRAULIC SYSTEMS

Fig. 2.1.1 Liquid Containers

LIQUID

FORCE

WEIGHT50 lbs.

Fig. 2.1.2 Liquid Under Pressure

Practically Incompressible

A liquid is practically incompressible. When a substance iscompressed, it takes up less space. A liquid occupies the sameamount of space or volume even when under pressure. The space orvolume that any substance occupies is called "displacement."

Using a Liquid

There are several advantages for using a liquid.

1. Liquids conforms to the shape of the container.

2. Liquids are practically incompressible.

3. Liquids apply pressure in all directions.

Unit 2 2-1-4 Hydraulic FundamentalsLesson 1

Liquids Conform to Shape

Liquids will conform to the shape of any container. Liquids will alsoflow in any direction through lines and hoses of various sizes andshapes.

FORCE

GAS

WEIGHT50 lbs.

Fig. 2.1.3 Gas is Compressible

3 in. radius 2 in. radius

1130 lbs

FORC

E

500 lbs

40 psi FORC

E

Fig. 2.1.4 Hydraulics Doing Work

Hydraulics Doing Work

According to Pascals Law, "Pressure exerted on a confined liquid istransmitted undiminished in all directions and acts with equal forceon all equal areas." Therefore, a force exerted on any part of anenclosed hydraulic oil system transmits equal pressure in alldirections throughout the system.

In the above example, a 500 pound force acting upon a piston with a2 in. radius creates a pressure of approximately 40 pounds per squareinch (psi) in a confined liquid. The same 40 psi acting upon a pistonwith a 3 in. radius supports a 1130 pound weight.

At this time, perform Lab 2.1.1

Gas is compressible

Gas is compressible. When gas is compressed, it takes up less spaceand its displacement becomes less. The space previously occupiedby the gas may be occupied by another object. Therefore, a liquid isbest suited for the hydraulic system because it continually occupiesthe same volume or displacement.


Force = Pressure x AreaF

P APressure = Force Area

Area = Force Pressure

Fig. 2.1.5 Pascal's Law

A simple formula allows us to determine the Force, the Pressure, andthe Area when two of the three are known. Understanding theseterms are necessary to understand the fundamentals of hydraulics.

Force is the push or pull acting upon a body. Force is usuallyexpressed in pounds (lbs.). Force is equal to the pressure times thearea (F = P x A).

Pressure is the force of a fluid per unit area, usually expressed inpounds per square inch (psi).

Area is a measurement of surface space. The area is calculated insquare inches. Sometime the surface area is referred to as effectivearea. The effective area is the total surface that is used to create aforce in the desired direction.

The surface area of a circle is calculated with the formula:

Area = Pi (3.14) times radius-squaredIf the radius of the circle is 2 inches, Fig. 2.1.4,A = Pi x r squareA = 3.14 x (2" x 2")A = 12.5 sq. in.

With the knowledge of the surface area, it is possible to determinehow much system pressure it will take to lift a given weight.Pressure is the force per unit and is expressed in pounds per squareinch (psi).

If a force of 500 pounds was acting upon an area of 12.5 sq. in., thepressure created would be 40 psi.

The pressure is calculated with the formula:

Pressure = Force divided by Area P = 500 lbs./12.5 sq. in.P = 40 psi


Mechanical Advantage

Figure 2.1.6 demonstrates how liquid in a hydraulic system provides amechanical advantage.

Since all cylinders are connected, all areas must be filled before thesystem pressurizes.

Use the hydraulic formula and calculate the items in question.Cylinders are counted from left to right.

When calculating the pressure in the system, we use the two knownvalues of the second cylinder from the left. The formula used is"pressure equals force divided by area."

Pressure = Force Pressure = 50 lbs Pressure = 50 psiArea 1 sq. in.

Now that we know the pressure in the system, we can calculate theforce of the load for cylinders one and three and the piston area forcontainer four.

Calculate cylinders one and three loads using the formula, forceequals pressure times area (Force = Pressure x Area).

Calculate cylinder four piston area using the formula, area equalsforce divided by pressure (Area = Force / Pressure).

The correct answers are: cylinder one load is 250 lbs, cylinderthree load is 150 lbs and cylinder four piston area is 2 sq. in.


50 lbs

FORC

E

5 sq. in.

?

FORC

E

100 lbs

3 sq. in.

FROMPUMP

?

? ?

1 sq. in. ?

FORC

E

FORC

E

Fig. 2.1.6 Mechanical Advantage

Solving for the large piston we find:

Pressure x Area = Force40 x (3x3) x 3.14 = Force.40 x 28.26 = 1130 lbs.


60 60

12000 120

FLOW1 GPM

Fig. 2.1.7 No Restriction

60 60

12000 120

30 90

FLOW1 GPM

Fig. 2.1.8 Orifice Offers Restriction

Orifice Offers Restriction

An orifice offers a restriction to the pump flow. When oil flowsthrough an orifice, pressure is produced on the upstream side of theorifice.

In figure 2.1.8, there is an orifice in the pipe between the two gauges.The gauge up stream of the orifice shows that a pressure of 207 kPa(30 psi) is needed to send a flow of 1 gpm through the orifice. Thereis no restriction to flow after the orifice. The gauge down stream ofthe orifice shows 0 pressure.

ORIFICE EFFECT

When discussing hydraulics, it is a common practice to use the term"pump pressure." However, the pump does not produce pressure.The pump produces flow. When flow is restricted, pressure isproduced.

In Figures 2.1.7 and 2.1.8, the pump flow through the pipe is 1 gpm.

In Figure 2.1.7, there is no restriction to the flow through the pipe.Therefore, the pressure reading is zero for both gauges.


60

0 120

30 90

60

0 120

30 90

FROMPUMP

60

0 120

30 90

60

0 120

30 90

FROMPUMP

Fig. 2.1.9 Blocked Flow

Oil Flow to Tank Blocked

When the end of either pipe is plugged, oil flow to the tank isblocked.

The positive displacement pump continues pumping at 1 gpm andfills the pipe. When the pipe is filled, the resistance to any additionalflow into the pipe produces pressure. The pressure reaction is thesame as Pascals Law which states that "pressure exerted on aconfined liquid is transmitted undiminished in all directions and actswith equal force on all equal areas." The two gauge readings are thesame.

The pressure will increase until the pump flow is diverted from thepipe to another circuit or to the tank. This is usually done with arelief valve.

If total pump flow was not diverted from the pipe, pressure in thepipe would continue to rise and cause an eruption of the circuit.


60

0 120

30 90

PSI

60

0 120

30 90

60

0 120

30 90

207 kPa (30 psi)

PSI PSI

207 kPa (30 psi) 207 kPa (30 psi)

FLOW1 GPM

60

0 120

FLOW1 GPM

30 9060

0 120

30 90

60

0 120

30 90

PSI PSIPSI

Fig. 2.1.10 Restrictions in Series

207 kPa (30 PSI)

414 kPa (60 PSI)

620 kPa (90 PSI)

FROMPUMP

CIRCUITONE

CIRCUITTWO

CIRCUITTHREE

Fig. 2.1.11 Restrictions In Parallel

Restrictions In Parallel

In a system with parallel circuits, pump oil follows the path of leastresistances. In figure 2.1.11, the pump supplies oil to three parallelcircuits. Circuit three has the lowest priority and circuit one has thehighest priority.

Restrictions In Series

There are two basic types of circuits, series and parallel.

In Fig. 2.1.10, a pressure of 620 kPa (90 psi) is required to send 1gpm through either circuit.

Orifices or relief valves in series in a hydraulic circuit offer aresistance that is similar to resistors in series in an electrical circuit inthat the oil must flow through each resistance. The total resistanceequals to the sum of each individual resistance.

At this time, perform Lab 2-1-3


UNIT 3Hydraulic Fundamentals - Hydraulic System Components

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Objectives

Upon completion of this unit, the students will be able to:

1. State how basic hydraulic principles are used in the operation ofhydraulic system components.

2. State the function of hydraulic tanks, fluids, pumps and motors,various valves and cylinders.

3. Identify the different hydraulic tanks, pumps and motors, fluids,valves and cylinders.

4. Identify the ISO symbol for the hydraulic tank, the pump and/ormotor, the various valves and the cylinders.

Introduction

Mobile construction machines are designed using various hydrauliccomponents (tanks, fluids, pumps and motors, valves and cylinders).Some components when used in different parts of the circuit performdifferent functions. Although these components may look alike, theymay be given different names. The ability to identify the component,state the component's function and describe the component'soperation will allow the serviceman to reduce complex circuits toseveral simple circuits that may be more easily understood.

Lesson 1: Hydraulic Tank

Hyd

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ank

Introduction

When construction machines and equipment are in the design stage,considerable thought is given to the type, size and location of thehydraulic oil tank. Once the machine or equipment is in operation,the hydraulic tank functions as a storage place for the hydraulic oil, adevice to remove heat from the oil and a seperator to remove airfrom the oil. This unit will discuss some of the characteristics of thehydraulic tank.

Objectives


1. Identify the common components of the hydraulic tank andstate the component function.

2. State the characteristics of the vented and the pressurizedhydraulic tank.

Basic Hydraulic Systems

Hydraulic Fluids Hydraulic Tank

HydraulicPumps and Motors Pressure Control Valves Directional Control Valves Flow Control Valves Cylinders

FILL CAP

SIGHT GLASS

SUPPLY ANDRETURN LINES

DRAIN

Fig. 3.1.1 Hydraulic Tank

Hydraulic Tank

The hydraulic oil tank main function is to store oil, however, it hassome other functions as well. The tank must remove heat andseparate air from the oil.

Tanks must have sufficient strength, adequate capacity and keep dirtout. Hydraulic tanks are usually but not always sealed.

Tank components seen in figure 3.1.1 are:

Fill Cap - Keeps contaminants out of the opening that's used to filland add oil to the tank and seals pressurized tanks.

Sight Glass - Used to check the oil level. The oil level should bechecked when the oil is cold. The oil level is usually correct whenthe oil is in the middle of the sight glass.

Supply and Return Lines - The supply line allows oil to flow fromthe tank to the system. The return line allows oil to flow from thesystem to the tank.

Drain - Located at the lowest point in the tank, the drain is used toremove old oil from the tank. The drain also allows for the removalof water and sediment from the oil.


Pressurized Tank

The two main types of hydraulic tanks are pressurized and vented(unpressurized).

The pressurized tank is completely sealed. Atmospheric pressuredoes not effect the pressure in the tank. However, when the oil issent through the system, it absorbs heat and expands. The expandingoil compresses the air in the tank. The compressed air forces the oilout of the tank and into the system.

The vacuum relief valve serves two purposes. It prevents a vacuumand limits the maximum pressure in the tank.

The vacuum relief valve prevents a vacuum by opening and allowingair to enter the tank when the tank pressure drops to 3.45 kPa (.5 psi).

When pressure in the tank reaches the vacuum relief valve pressuresetting, the valve opens and vents compressed air to the atmosphere.The vacuum relief valve pressure setting may vary from 70 kPa (10psi) to 207 kPa (30 psi).

Other tank components are:

Filler screen - keeps large contaminants from entering the tank whenthe fill cap is removed.

Filler tube - allows the tank to be filled to the correct level, but notover filled.

Baffles - prevents the return oil from flowing directly to the tankoutlet, allowing time for bubbles in the return oil to rise to the top.Also, prevents the oil from sloshing which helps reduce forming ofthe oil.

Ecology Drain - used to prevent accidental spills when removingwater and sediment from the tank.

Return screen - prevents larger particles from entering the tank, butdoes not provide fine filtering,

TO PUMP

RETURN

RETURNSCREEN

FILLER SCREEN

VACUUMRELIEF VALVE

PRESSURIZED TANK

FILL CAP

FILLER TUBE

BAFFLES

ECOLOGYDRAIN

Fig. 3.1.2 Pressurized Tank


VENTED TANK

RETURN

BREATHER

TO PUMP

Fig. 3.1.3 Vented Tank

VENTEDTANK

PRESSURIZED

Fig. 3.1.4 Hydraulic Tank ISO Symbols

ISO Symbol

Figure 3.1.4 shows the ISO symbol for the vented and the pressurizedhydraulic tanks.

The vented hydraulic tank symbol is merely an open-topped box orrectangle. The pressurized tank symbol is drawn as a completelyclosed box or rectangle. Tanks are shown with hydraulic lines toenhance understanding.

Vented Tank

The vented or un-pressurized tank differs from the pressurized tank inthat the vented tank has a breather. The breather allows air to enterand exit freely. Atmospheric pressure on the top of the oil forces theoil out of the tank and into the system. The breather has a screen thatprevents dirt from entering the tank.


Lesson 2: Hydraulic Fluids

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Introduction

The selection and care of the hydraulic fluid will have an importanteffect on the life of the system. Just like the hardware componentsof a hydraulic system, the hydraulic fluid must be selected on thebasics of its characteristics and properties to accomplish the designedtask.

Objectives

Upon completion of this lesson the student will:

1. State the functions of hydraulic fluids.

2. Measure the viscosity of fluids.

3. State the meaning of viscosity index.

4. List the types of fire resistant hydraulic fluids.




FORCE

5,000lbs

Fig. 3.2.1 Hydraulic Fluids

Functions of Hydraulic Fluids

Fluids are virtually incompressible. Therefore, fluids can transmitpower instantaneously in a hydraulic system. For example, petroleumoil compresses approximately 1% for every 2000 psi. Therefore,petroleum oil can maintain a constant volume under high pressure.Petroleum oil is the primary fluid used in developing most hydraulicoils.

The primary functions of hydraulic fluids are:

Power transmission Lubrication Sealing Cooling

Power Transmission

Because hydraulic fluids are virtually incompressible, once thehydraulic system is filled with fluid it can instantly transmit powerfrom one area to another. However, this does not mean that allhydraulic fluids are equal and will transmit power with the sameefficiency. Choosing the correct hydraulic fluid depends on theapplication and the operating conditions.

Lubrication

Hydraulic fluid must lubricate the moving parts of the hydraulicsystem. The rotating or sliding components must be able to functionwithout touching other surfaces. The hydraulic fluid must maintain athin film between the two surfaces to prevent friction, heat and wear.

Sealing

Many hydraulic components are designed to use hydraulic fluidinstead of mechanical seals within the component. The viscosity ofthe fluid helps to determine its ability to function as a seal.


Cooling

The hydraulic system develops heat as it transfers mechanical energyto hydraulic energy and hydraulic energy back to mechanical energy.As the fluid moves throughout the system, heat flows from thewarmer components to the cooler fluid. The fluid gives up the heatto the reservoir or to coolers that are designed to maintain fluidtemperatures within design limits.

Other properties expected of the hydraulic fluid are the prevention ofrust and corrosion on metal parts, the resistance to foaming andoxidation, the ability to separate air, water and other contaminatesfrom the fluid, and the ability to maintain stability over a wide rangeof temperatures.

Viscosity

Viscosity is the measurement of a fluid's resistance to flow at aspecific temperature. A fluid which flows easily has a low viscosity.A fluid which does not flow easily has a high viscosity.

A fluid's viscosity is affected by temperature. When a fluid becomeswarmer, the fluid's viscosity becomes lower. Likewise, when a fluidcools, the viscosity increases. Vegetable oil is a very good exampleof how viscosity changes with a change in temperature. Whenvegetable oil is very cold, vegetable oil thickens and is very slow topour. As vegetable oil is heated, vegetable oil becomes thinner andpours more readily.


Saybolt Viscosimeter

The most common tool of measuring viscosity is the SayboltViscosimeter (Figure 3.2.2). The Saybolt Viscosimeter was inventedby and named after George Saybolt.

The Saybolt Viscosimeter unit of measurement is the SayboltUniversal Second (SUS). In the original viscosimeter a container offluid was heated to a specific temperature. When the temperaturewas reached, a stopcock (orifice) was opened and the fluid flowed outof the container and into a 60 ml. flask. A stopwatch was used tomeasure the time it took to fill the flask. The viscosity was recordedas the number of seconds the flask took to fill at a given temperature.If a fluid, when heated to a temperature of 75F, took 115 seconds tofill the flask, it's viscosity was 115 SUS @ 75F. If the same fluidwas heated to 100F and took 90 seconds to fill the flask, it'sviscosity would be 90 SUS @ 100F.

Viscosity Index

Viscosity index (VI) is a measure of a fluid's change in thickness withrespect to changes in temperature. If a fluid's consistency remainsrelatively the same over varying temperatures, the fluid has a highVI. If a fluid becomes thick at low temperatures and very thin athigh temperatures,the fluid has a low VI. In most hydraulic systems,fluids with a high VI is desirable over fluids with a low VI.

Petroleum Oil

All petroleum oil becomes thin as the temperature goes up andthickens as the temperature goes down. If the viscosity is too low,there may be excessive leakage past seals and from joints. If theviscosity is too high, sluggish operation may be the results and extrapower is needed to push the oil through the system. Viscosity ofpetroleum oil is expressed by the Society of Automotive Engineers(SAE) numbers: 5W, 10W, 20W, 30W, 40W, etc. The lower the

THERMOMETER

HEATER

ORIFICE SAYBOLTVISCOSIMETER

60 ml. FLASK

Fig. 3.2.2 Saybolt Viscosimeter


number, the better the oil will flow at low temperatures. The higherthe number, the more viscous the oil and the more suited to hightemperatures.

Synthetic Oils

Synthetic oils are formed by processes which chemically reactmaterials of a specific composition to produce a compound withplanned and predictable properties. Synthetic oils are specificallyblended for extreme service at both high and low temperatures.

Fire Resistant Fluids

There are three basic types of fire resistant fluids: water-glycols,water-oil emulsions and synthetics.

Water-glycol fluids contains 35% to 50% water (water inhibitsburning), glycol (synthetic chemical similar to some anti-freeze) anda water thickener. Additives are added to improve lubrication and toprevent rust, corrosion and foaming. Water-glycol fluids are heavierthan oil and may cause pump cavitation at high speeds. These fluidsmay react with certain metals and seals and cannot be used with sometypes of paints.

Water-oil emulsion are the least expensive of the fire resistant fluids.A similar amount (40%) of water is used as in water-glycol fluids toinhibit burning. Water-oil can be used in typical hydraulic oilsystems. Additive may be added to prevent rust and foaming.

Certain conditions may require that synthetic fluids be used to meetspecific requirements. The fire resistive synthetic fluids are lessflammable than oil and more suitable for used in areas of highpressure and high temperature.

Many times fire resistant fluids react to polyurethane seals and mayrequire that special seals be used.

Oil Life

The hydraulic oil never wears out. The use of filters to remove solidparticles and some chemicals add to the useful life of the oil.However, eventually the oil will become so contaminated that it willhave to be replaced. In construction machines, the oil is replaced atregular time intervals.

The contaminates in the oil may also be used as indicators of highwear and prospective problem areas. One such program that uses oilcontaminates as its source of information is the Caterpillar ScheduleOil Sampling Program (SOS).

At this time do Lab 3-2-1 and 3-2-2


Lesson 2: Hydraulic Fluids

Hyd

rau

lic F

luid

s

Introduction

The selection and care of the hydraulic fluid will have an importanteffect on the life of the system. Just like the hardware componentsof a hydraulic system, the hydraulic fluid must be selected on thebasics of its characteristics and properties to accomplish the designedtask.

Objectives


1. State the functions of hydraulic fluids.

2. Measure the viscosity of fluids.

3. State the meaning of viscosity index.

4. List the types of fire resistant hydraulic fluids.




FORCE

5,000lbs

Fig. 3.2.1 Hydraulic Fluids

Functions of Hydraulic Fluids

Fluids are virtually incompressible. Therefore, fluids can transmitpower instantaneously in a hydraulic system. For example, petroleumoil compresses approximately 1% for every 2000 psi. Therefore,petroleum oil can maintain a constant volume under high pressure.Petroleum oil is the primary fluid used in developing most hydraulicoils.

The primary functions of hydraulic fluids are:

Power transmission Lubrication Sealing Cooling

Power Transmission

Because hydraulic fluids are virtually incompressible, once thehydraulic system is filled with fluid it can instantly transmit powerfrom one area to another. However, this does not mean that allhydraulic fluids are equal and will transmit power with the sameefficiency. Choosing the correct hydraulic fluid depends on theapplication and the operating conditions.

Lubrication

Hydraulic fluid must lubricate the moving parts of the hydraulicsystem. The rotating or sliding components must be able to functionwithout touching other surfaces. The hydraulic fluid must maintain athin film between the two surfaces to prevent friction, heat and wear.

Sealing

Many hydraulic components are designed to use hydraulic fluidinstead of mechanical seals within the component. The viscosity ofthe fluid helps to determine its ability to function as a seal.


Cooling

The hydraulic system develops heat as it transfers mechanical energyto hydraulic energy and hydraulic energy back to mechanical energy.As the fluid moves throughout the system, heat flows from thewarmer components to the cooler fluid. The fluid gives up the heatto the reservoir or to coolers that are designed to maintain fluidtemperatures within design limits.

Other properties expected of the hydraulic fluid are the prevention ofrust and corrosion on metal parts, the resistance to foaming andoxidation, the ability to separate air, water and other contaminatesfrom the fluid, and the ability to maintain stability over a wide rangeof temperatures.

Viscosity

Viscosity is the measurement of a fluid's resistance to flow at aspecific temperature. A fluid which flows easily has a low viscosity.A fluid which does not flow easily has a high viscosity.

A fluid's viscosity is affected by temperature. When a fluid becomeswarmer, the fluid's viscosity becomes lower. Likewise, when a fluidcools, the viscosity increases. Vegetable oil is a very good exampleof how viscosity changes with a change in temperature. Whenvegetable oil is very cold, vegetable oil thickens and is very slow topour. As vegetable oil is heated, vegetable oil becomes thinner andpours more readily.


Saybolt Viscosimeter

The most common tool of measuring viscosity is the SayboltViscosimeter (Figure 3.2.2). The Saybolt Viscosimeter was inventedby and named after George Saybolt.

The Saybolt Viscosimeter unit of measurement is the SayboltUniversal Second (SUS). In the original viscosimeter a container offluid was heated to a specific temperature. When the temperaturewas reached, a stopcock (orifice) was opened and the fluid flowed outof the container and into a 60 ml. flask. A stopwatch was used tomeasure the time it took to fill the flask. The viscosity was recordedas the number of seconds the flask took to fill at a given temperature.If a fluid, when heated to a temperature of 75F, took 115 seconds tofill the flask, it's viscosity was 115 SUS @ 75F. If the same fluidwas heated to 100F and took 90 seconds to fill the flask, it'sviscosity would be 90 SUS @ 100F.

Viscosity Index

Viscosity index (VI) is a measure of a fluid's change in thickness withrespect to changes in temperature. If a fluid's consistency remainsrelatively the same over varying temperatures, the fluid has a highVI. If a fluid becomes thick at low temperatures and very thin athigh temperatures,the fluid has a low VI. In most hydraulic systems,fluids with a high VI is desirable over fluids with a low VI.

Petroleum Oil

All petroleum oil becomes thin as the temperature goes up andthickens as the temperature goes down. If the viscosity is too low,there may be excessive leakage past seals and from joints. If theviscosity is too high, sluggish operation may be the results and extrapower is needed to push the oil through the system. Viscosity ofpetroleum oil is expressed by the Society of Automotive Engineers(SAE) numbers: 5W, 10W, 20W, 30W, 40W, etc. The lower the

THERMOMETER

HEATER

ORIFICE SAYBOLTVISCOSIMETER

60 ml. FLASK

Fig. 3.2.2 Saybolt Viscosimeter


number, the better the oil will flow at low temperatures. The higherthe number, the more viscous the oil and the more suited to hightemperatures.

Synthetic Oils

Synthetic oils are formed by processes which chemically reactmaterials of a specific composition to produce a compound withplanned and predictable properties. Synthetic oils are specificallyblended for extreme service at both high and low temperatures.

Fire Resistant Fluids

There are three basic types of fire resistant fluids: water-glycols,water-oil emulsions and synthetics.

Water-glycol fluids contains 35% to 50% water (water inhibitsburning), glycol (synthetic chemical similar to some anti-freeze) anda water thickener. Additives are added to improve lubrication and toprevent rust, corrosion and foaming. Water-glycol fluids are heavierthan oil and may cause pump cavitation at high speeds. These fluidsmay react with certain metals and seals and cannot be used with sometypes of paints.

Water-oil emulsion are the least expensive of the fire resistant fluids.A similar amount (40%) of water is used as in water-glycol fluids toinhibit burning. Water-oil can be used in typical hydraulic oilsystems. Additive may be added to prevent rust and foaming.

Certain conditions may require that synthetic fluids be used to meetspecific requirements. The fire resistive synthetic fluids are lessflammable than oil and more suitable for used in areas of highpressure and high temperature.

Many times fire resistant fluids react to polyurethane seals and mayrequire that special seals be used.

Oil Life

The hydraulic oil never wears out. The use of filters to remove solidparticles and some chemicals add to the useful life of the oil.However, eventually the oil will become so contaminated that it willhave to be replaced. In construction machines, the oil is replaced atregular time intervals.

The contaminates in the oil may also be used as indicators of highwear and prospective problem areas. One such program that uses oilcontaminates as its source of information is the Caterpillar ScheduleOil Sampling Program (SOS).

At this time do Lab 3-2-1 and 3-2-2


Lesson 3: Hydraulic Pumps and Motors

Les

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Hyd

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ps

and

Mo

tors

Introduction

Pumps and motors are similar in construction but different inoperational characteristics. Therefore, most of the material in thislesson will concentrate on the nomenclature and operation of pumps.

Objectives


1. State the differences between non-positive and positivedisplacement pumps;

2. State the differences between fixed displacement and variabledisplacement pumps;

3. State the operation of different types of pumps;

4. State the similarity and differences between pumps andmotors; and

5. State how pumps are rated.



Hydraulic Pumps and Motors Pressure Control Valves Directional Control Valves Flow Control Valves Cylinders

Fig. 3.3.0

Hydraulic Pump

The hydraulic pump transfers mechanical energy into hydraulicenergy. It is a device that takes energy from one source (i.e. engine,electric motor, etc.) and transfers that energy into a hydraulic form.The pump takes oil from a storage container (i.e. tank) and pushes itinto a hydraulic system as flow.

All pumps produce oil flow in the same way. A vacuum is created atthe pump inlet. The higher atmospheric pressure pushes the oilthrough the inlet passage and into the pump inlet chambers. Thepump gears carry the oil to the pump outlet chamber. The volume ofthe chamber decreases as the chamber approaches the outlet. Thisdecrease in chamber size pushes the oil out the outlet.

Pumps produce only the flow (i.e. gallons per minute, liters perminute, cubic centimeters per revolution, etc.) used in the hydraulicsystem. Pumps DO NOT produce or cause "pressure". Pressure iscaused by the resistance to the flow. Resistance can be caused byflow through hoses, orifices, fittings, cylinders, motors, or anythingin the system that hinders free flow to the tank.

Pumps can be classified into two types: Non-positive displacementand positive displacement.


INLET OILOUTLET OIL

HOUSING

DRIVE GEAR

IDLER GEAR

Fig. 3.3.1 Gear Pump

Hydraulic Motor

The hydraulic motor transfers hydraulic energy into mechanicalenergy. It uses the oil flow being pushed into the hydraulic system bya pump and transfers it into a rotary motion to drive another device(i.e. final drives, differential, transmission, wheel, fan, another pump,etc.).


PUMP OIL TANK OIL

HOUSING

DRIVE GEAR

IDLER GEAR

Fig. 3.3.2 Gear Motor


Non-Positive Displacement Pumps

Non-positive displacement pumps have more clearances between themoving and stationary parts than positive displacement pumps. Theextra clearance allows more oil to be pushed back between the partsas the outlet pressure (resistance to flow) increases. Non-positivedisplacement pumps are less efficient than positive displacementpumps because the output flow of the pump decreases greatly as theoutlet pressure increases. Non-positive displacement pumps aregenerally either centrifugal impeller type or axial propeller type.These are used in low pressure applications such as automotive waterpumps or charge pumps for piston pumps in high pressure hydraulicsystems.

4

32

1

5

Fig. 3.3.3 Centrifugal Pump

Centrifugal Impeller Pump

The centrifugal impeller pump consists of two basic parts; theimpeller (2) that is mounted on an input shaft (4) and the housing (3).The impeller has a solid disc back with curved blades (1) molded onthe input side.

Oil enters the center of the housing (5) near the input shaft and flowsinto the impeller. The curved impeller blades propel the oil outwardagainst the housing. The housing is shaped to direct the oil to theoutlet port.

Axial Propeller Pump

The axial propeller type pump is shaped like an electric air fan. It ismounted in a straight tube and has an open bladed propeller. Oil ispropelled down the tube by the rotation of the angled blades.

Positive Displacement Pumps

There are three basic types of positive displacement pumps: gear,vane and piston. Positive displacement pumps have much smallerclearances between components. This reduces leakage and providesa much higher efficiency when used in a high pressure hydraulicsystem. The output flow in a positive displacement pump is basicallythe same for each pump revolution. Positive displacement pumps areclassified by both the control of their output flow and theconstruction of the pump.

Positive displacement pumps are rated two ways. One is by themaximum system pressure (i.e. 21,000 kPa or 3000 psi) at which thepump is designed to operate. The second is by the specific outputdelivered either per revolution or at a given speed against a specifiedpressure. The pumps are rated either by lpm @ rpm@ kPa or bygpm @ rpm @ psi (i.e. 380 lpm @ 2000 rpm @ 690 kPa or 100 gpm@ 2000 rpm @ 100 psi).

When expressed in output per revolution, the flow rate can be easilyconverted by multiplying by the speed in rpm (i.e.: 2000 rpm) anddividing by a constant. For example, we will calculate the flow of apump that rotates 2000 rpm and has a flow of 11.55 in3/rev or 190cc/rev.

GPM = in3/rev X rpm LPM = cc/rev X rpm231 1000

GPM = 11.55 X 2000 LPM = 190 X 2000 231 1000

GPM = 100 LPM = 380

PROPELLER

INLET

FLOW

FLOW

INLET

Fig. 3.3.4 Axial Propeller Pump


Volumetric Efficiency

As pressure increases, the close clearances between the parts in apositive displacement pump do not produce the same output flow asinput flow. Some oil will be forced back through the clearancesbetween the high pressure chamber and the low pressure chamber.The resultant output flow, when compared to the input flow, is called"volumetric efficiency". (Input flow is generally defined as the"output flow @ 100 psi".) "Volumetric efficiency" changes aspressure changes and must be specified for a given pressure. When apump that is rated at 100 gpm @ 2000 rpm @ 100 psi is operatedagainst 1000 psi, its output may drop to 97 gpm. This pump wouldhave a "volumetric efficiency" of 97% (97/100) @ 1000 psi.

Volumetric efficiency @ 1000 psi = output flow input flow

Volumetric efficiency @ 1000 psi = 97 100

Volumetric efficiency @ 1000 psi = .97 or 97% efficient at 1000 psi

When the pressure increases to 2000 psi, the output may drop to 95gpm. It would then have a "volumetric efficiency" of .95 or 95% @ 2000 psi. The rpm must remain constant when measuring"volumetric efficiency".


INTAKE

EXHAUST

FIXED DISPLACEMENT

INTAKE

EXHAUST

VARIABLE DISPLACEMENT

CONTROL LEVER

RETAININGPLATE

SWASHPLATE SWASH

PLATESLIPPER PISTON

BARRELASSEMBLY

DRIVESHAFT

Fig. 3.3.5 Piston Pumps


Fixed Displacement Versus Variable Displacement

The output flow of a fixed displacement pump is only changed byvarying the speed of the pump rotation. It must be rotated faster toincrease the flow or rotated slower to decrease the flow. The geartype pump is a fixed displacement pump.

The vane type and piston type pumps may be fixed or variable. Theoutput flow from a variable displacement pump may be increased ordecreased independent of the speed of rotation. The output flow maybe manual controlled, automatic controlled or a combination ofmanual and automatic controlled.


11109

531

6

2 4

7

8

Fig. 3.3.6 Gear Pump

Gear Pumps

The gear pump consists of seal retainers (1), seals (2), seal back-ups(3), isolation plates (4), spacers (5), a drive gear (6), an idler gear (7),a housing (8), a mounting flange (9), a flange seal (10) and pressurebalance plates (11) on either side of the gears. Bearings are mountedin the housing and mounting flange on the sides of the gears tosupport the gear shafts during rotation.

Gear pumps are positive displacement pumps. They deliver the sameamount of oil for each revolution of the input shaft. The pump outputis controlled by changing the speed of rotation. The maximumoperating pressure for gear pumps is limited to 4000 psi. Thispressure limitation is due to the hydraulic unbalance that is inherentin the gear pump design. The hydraulic unbalance produces a sideload on the shafts that is resisted by the bearings and the gear teeth tohousing contact. The gear pump maintains a "volumetric efficiency"above 90% when pressure is kept within the designed operatingpressure range.

Gear Pump Flow

The output flow of the gear pump is determined by the tooth depthand gear width. Most gear pump manufacturers standardized on atooth depth and profile that is determined by the centerline distance(1.6", 2.0", 2.5", 3.0", etc.) between gear shafts. With standardizedtooth depths and profiles, the flow differences within each centerlineclassification of pump is totally determined by the tooth width.

As the pump rotates, the oil is carried between the gear teeth and thehousing from the inlet side to the outlet side of the pump. Thedirection of rotation of the drive gear shaft is determined by thelocation of the inlet and outlet ports. The direction of rotation of thedrive gear will always be to move the oil around the outside of thegears from the inlet port to the outlet port. This is true on both gearpumps and gear motors. On most gear pumps the inlet port is largerin diameter than the outlet port. On bi-directional pumps and motors,the inlet port and outlet port will be the same size.


INLET OILOUTLET OIL

HOUSING

DRIVE GEAR

IDLER GEAR

Fig. 3.3.7 Gear Pump Flow


INLET OILOUTLET OIL

HOUSING

DRIVE GEAR

IDLER GEAR

FORCE

MESHING GEAR TEETH

Fig. 3.3.8 Gear Pump Forces

Gear Pump Forces

The outlet flow from a gear pump is created by pushing the oil out ofthe gear teeth as they come into mesh on the outlet side. Theresistance to oil flow creates the outlet pressure. The unbalance of thegear pump is due to outlet port pressure being higher than inlet portpressure. The higher pressure oil pushes the gears toward the inletport side of the housing. The shaft bearings carry the majority of theside load to prevent excessive wear between the tooth tips and thehousing. On the higher pressure pumps, the gear shafts are slightlytapered from the outboard end of the bearings to the gear. Thisallows full contact between the shaft and bearing as the shaft bendsslightly under the unbalanced pressure.

The pressurized oil is also directed between the sealed area of thepressure balance plates and the housing and mounting flange to sealthe ends of the gear teeth. The size of the sealed area between thepressure balanced plates and the housing is what limits the amount offorce that pushes the plates against the ends of the gears.

Gear Pumps with Pockets

Gear pumps with a housing that is machined with pockets for thegears have a radius from the pocket walls to the bottom of thepockets. The isolation plate or the later pressure balanced plate usedin the pocket must have chamfered or curved outer edges to fit fullyagainst the bottom of the pocket. Using a sharp edge isolation plate,a sharp edge seal retainer or a sharp edge pressure balance plate in ahousing pocket will force the pressure balance plates against the endsof the gears and cause a failure.



POCKETPRESSURE BALANCE

PLATES HEAD

SHARP EDGECHAMFERED EDGEFig. 3.3.10 Gear Pumps With Pocket

1

2

Fig. 3.3.9 Pressure Balance Plates

Pressure Balance Plates

There are two different types of pressure balance plates used in gearpumps. The earlier type (1) has a flat back. This type uses anisolation plate, a back-up for the seal, a seal shaped like a 3 and aseal retainer. The later type (2) has a groove shaped like a 3 cut intothe back and is thicker than the earlier type. Two different types ofseals are used with the later type of pressure balance plates.


5 7

8

4 6

321

10

11

12 13

9

9

Fig. 3.3.11 Vane Pump

Both the fixed and variable vane pumps use common partnomenclature. Each pump consists of the housing (1), the cartridge(2), the mounting plate (3), the mounting plate seal (4), the cartridgeseals (5), the cartridge backup rings (6), the snap ring (7) and theinput shaft and bearing (8). The cartridge consists of the supportplates (9) the ring (10), the flex plates (11), the slotted rotor (12) andthe vanes (13).

The slotted rotor is turned by the input shaft. The vanes move in andout of the slots in the rotor and seal on the outer tips against the camring. The inside of the fixed pump displacement ring is elliptical inshape. The inside of the variable pump displacement ring is round inshape. The flex plates seal the sides of the rotor and the ends of thevanes. In some lower pressure designs, the support plates andhousing seal the sides of the rotating rotor and the ends of the vanes.The support plates are used to direct the oil into the proper passagesin the housing. The housing, in addition to providing support for theother parts of the vane pump, directs the flow in and out of the vanepump.

Vane Pumps

Vane pumps are positive displacement pumps. The pump output canbe either fixed or variable.


1

Fig. 3.3.12 Vane Pressurization

Vanes

The vanes are initially held against the cam ring by centrifugal forcecreated by the rotation of the rotor. As flow increases, the resultantpressure that builds from the resistance to that flow is directed intopassages in the rotor beneath the vanes (1). This pressurized oilbeneath the vanes keep the vane tips pushed against the cam ring toform a seal. To prevent the vanes from being pushed too hard againstthe cam ring, the vanes are beveled back (arrow) to permit a balancingpressure across the outer end.

Pressure

PRESSURIZED FLEX PLATES

Pressure

Fig. 3.3.13 Pressurized Flex Plates

Flex Plates

The same pressurized oil is also directed between the flex plates andthe support plates to seal the sides of the rotor and the end of thevanes. The size of the seal area between the flex plate and the supportplates is what controls the force that pushes the flex plates against thesides of the rotor and the end of the vanes. The kidney shaped sealsmust be installed in the support plates with the rounded o-ring sideinto the pocket and the flat plastic side against the flex plate.

INLET PORT

ROTOR

OUTLETPORT

VANES

CAMRING

Fig. 3.3.14 Vane Pump Operation

1 2

Fig. 3.3.15 Balanced Vane Pump

Balanced Vane Pump

The balanced vane pump has an elliptical shaped cam ring. Thisshape results in the distance between the rotor and the cam ringincreasing and decreasing twice for each revolution. The two inlets(1) and two outlets (2) opposite each other balance the forces againstthe rotor. This design does not require large bearings and housings tosupport the rotating parts. The maximum operating pressure for vanepumps is 4000 psi. Vane pumps used in mobile hydraulics have amaximum operating pressure of 3300 psi, or less.

Vane Pump Operation

When the rotor rotates around the inside of the cam ring, the vanesslide in and out of the rotor slots to maintain the seal against the camring. As the vanes move out of the slotted rotor, the volume betweenthe vanes changes. An increase in the distance between the cam ringand the rotor causes an increase in the volume. The increase involume creates a slight vacuum that allows the inlet oil to be pushedinto the space between the vanes by atmospheric or tank pressure. Asthe rotor continues to rotate, a decrease in the distance between thering and the rotor causes a decrease in the volume. The oil is pushedout of that segment of the rotor into the outlet passage of the pump.


INLET PORT

ROTOR

OUTLETPORT

VANES

RING

Fig. 3.3.16 Variable Vane Pump


Variable Vane Pump

Variable output vane pumps are controlled by shifting a round ringback and forth in relation to the rotor centerline. Variable outputvane pumps are seldom, if ever, used in mobile hydraulicapplications.

INSTRUCTOR NOTE: At this time, perform Lab 3.3.2

5 6

7

1 2

3

4

Fig. 3.3.17 Common Parts

Piston Pumps

Most piston pumps and motors have similar or common parts and usethe same nomenclature. The pump parts in Figure 3.3.17 are the head(1), the housing (2), the shaft (3), the pistons (4), the port plate (5),the barrel (6) and the swashplate (7).

The two designs of piston pumps are the axial piston pump and theradial piston pump. Both pumps are highly efficient, positivedisplacement pumps. However, the output of some pumps are fixedand the output of some pumps are variable.


INSTRUCTOR NOTE: Use piston pump demonstrator to showhow oil enters and discharges from the barrel assembly.

INTAKE

EXHAUST

FIXED DISPLACEMENT

INTAKE

EXHAUST

VARIABLE DISPLACEMENT

CONTROL LEVER

RETAININGPLATE

SWASHPLATE SWASH

PLATESLIPPER PISTON

BARRELASSEMBLY

DRIVESHAFT

Axial Piston Pumps and Motors

The fixed displacement axial piston pumps and motors are built in astraight housing or in an angled housing. The basic operation ofpiston pumps and motors are the same.

Stright Housing Axial Piston Pumps and motors

Figure 3.3.18 shows an illustration of the positive displacement fixedoutput axial piston pump and the positive displacement variableoutput axial piston pump. In most publications the fact that bothpumps are positive displacement is considered to be understood andthe pumps are refered to as fixed displacement pumps and variabledisplacement pumps. In the fixed displacement axial piston pumps, the pistons movebackward and forward in a line that is near parallel to the centerlineof the shaft. In the straight housing piston pump shown in the left illustration ofFigure 3.3.18, the pistons are held against a fixed, wedge-shapedswashplate. The angle of the swashplate controls the distance thepistons move in and out of the barrel chambers. The larger the angleof the wedge-shaped swashplate, the greater the distance of pistonmovement and the greater the pump output per revolution. In the variable displacement axial piston pump, either the swashplateor the barrel and port plate may pivot back and forth to change itsangle to the shaft. The changing angle causes the output flow to varybetween the minimum and maximum settings although the shaftspeed is held constant.On either pump, when a piston moves backward, oil flows throughthe intake and displaces the piston. As the pump rotates, the pistonmoves forward, the oil is pushed out through the exhaust and into thesystem.

Most piston pumps used on mobile equipment are axial piston pumps.

Fig. 3.3.18 Common Parts


Angled Housing Axial Piston Pump

In the angled housing piston pump shown in Figure 3.3.19, thepistons are connected to the input shaft by piston links or sphericalpiston ends that fit into sockets in a plate. The plate is an integralpart of the shaft. The angle of the housing to the shaft centerlinecontrols the distance the pistons move in and out of the barrelchambers. The larger the angle of the housing, the greater the pumpoutput per revolution.

The output flow of a fixed displacement piston pump can only bechanged by changing the input shaft speed.

Straight and Angle Housing Piston Motors

In the straight housing fixed displacement piston motor, the angle ofthe wedge-shaped swashplate determines the speed of the motoroutput shaft.

In the angle housing fixed displacement piston motor, the angle of thehousing to the shaft centerline determines the speed of the motoroutput shaft.

In both motors, the output shaft speed can only be changed bychanging the input flow to the motor.

FLUSHING VALVE(INSIDE HEAD)

CASE

SHAFT

RETAINING PLATE LINKPISTON

HEAD

BARREL

PORT PLATE

Fig. 3.3.19 Angled Housing Axial Piston Motor


Radial Piston Pump

In the radial piston pump Figure 3.3.20, the pistons moves outwardand inward in a line that is 90 degrees to the centerline of the shaft.

When the cam follower rolls down the cam ring, the piston movesoutward. Atmospheric pressure or a charge pump pushes oil throughthe valve inlet port and displaces the piston movement. When thecam follower rolls up the cam ring, the piston moves inward. Oil ispushed out of the cylinder and through the outlet port.

CAM RING

CAM FOLLOWER

PISTON

VALVE

Fig. 3.3.20 Radial Piston Pump

Some smaller piston pumps are designed for pressures of 10000 psior more. Piston pumps used in mobile equipment are designed for amaximum pressure of 7000 psi or less.

HOUSING

OUTLET

RING GEAR

CRESCENT

DRIVE GEAR

INLET

Fig. 3.3.21

Internal Gear Pump

The internal gear pump (Figure 3.3.21) has a small drive gear (piniongear) that drives a large ring gear (outer gear). The ring gear isslightly larger in pitch than the drive gear. A stationary crescent islocated below the pinion gear between the drive gear and the ringgear. The inlet and outlet ports are located at either end of thecrescent.

When the pump rotates, the teeth of the drive gear and the ring gearunmesh at the pump inlet port. The void between the teeth increasesand fills with inlet oil. The oil is carried between the drive gear teethand the crescent, and the ring gear teeth and the crescent to the outletport. When the gears pass the outlet port, the void between the teethdecreases and the teeth mash. This action forces the oil out frombetween the teeth and into the outlet port.

The internal gear pump is used as the charging pump in some largepiston pumps.


OUTERGEAR

INNERGEAR

Fig. 3.3.22

Conjugate Curve Pump

The conjugate curve pump (Figure 3.3.22) is also called aGEROTORTM pump. The inner and outer members rotate within thepump housing. Pumping is achieved by the way the lobes on theinner and the outer member contact each other during rotation. Asthe inner and outer members rotate, the inner member walks aroundinside the outside member. The inlet and outlet ports are located onthe end covers of the housing. The fluid entering through the inlet iscarried around to the outlet and squeezed out when the lobes mesh.

A modified conjugate curve pump is used in many steering systemshand metering unit (HMU). When used in the HMU, the outer gearis stationary and only the inner gear rotates.


MONO-DIRECTIONALFIXED DISPLACEMENT

PUMP

MONO-DIRECTIONALVARIABLE DISPLACEMENT

PUMP

BI-DIRECTIONALVARIABLE DISPLACEMENT

PUMP

BI-DIRECTIONALFIXED DISPLACEMENT

PUMP

Fig. 3.3.23 Pump ISO Symbols

MONO-DIRECTIONALFIXED DISPLACEMENT

MOTOR

MONO-DIRECTIONALVARIABLE DISPLACEMENT

MOTOR

BI-DIRECTIONALVARIABLE DISPLACEMENT

MOTOR

BI-DIRECTIONALFIXED DISPLACEMENT

MOTOR

Fig. 3.3.24 Motor ISO Symbols

Motor ISO Symbols

Motor ISO symbols are distinguished by a dark triangle in a circlewith the point of the triangle pointing toward the center of the circle.An arrow across the circle indicates a variable input per revolution.


Pump ISO Symbols

Pump ISO symbols are distinguished by a dark triangle in a circlewith the point of the triangle pointing toward the edge of the circle.An arrow across the circle indicates a variable output per revolution.


Lesson 4: Pressure Control Valves

Pre

ssu

re C

on

tro

l V

alve

s

Introduction

Pressure Control Valves are used to control the pressure in a circuitor in a system. The valve function will remain the same although thedesign may change. Examples of pressure control valves includerelief valves, sequence valves, pressure reducing valves, pressuredifferential valves and unloading valves.

Objectives


1. List the four most common pressure control valves.

2. State the functions of the relief valve, sequence valve,pressure reducing valve and the pressure differential valve.

3. Identify the ISO symbol for the four most common pressurecontrol valves.




Relief Valves

Hydraulic systems are designed to operate within a certain pressurerange. Exceeding this range can damage the system components orbecome dangerous to personnel. The relief valve maintains thepressure within the designed limit by opening and allowing excessiveoil to flow either to another circuit or back to the tank.


Fig. 3.4.1 Cracking Pressure

Simple Pressure Relief Valve, Cracking Pressure

Figure 3.4.1 shows a simple relief valve in the "cracking pressure"position.

The simple relief valve (also called direct acting relief valve) is keptclosed by spring force. The spring tension is set to the "reliefpressure" setting. However, the relief pressure setting is not thepressure at which the valve first begins to open.

When a condition develops that causes a resistance to the normal oilflow in the circuit, excessive oil flow causes the oil pressure toincrease. The increasing oil pressure is sensed at the relief valve.When the force of the increasing oil pressure overcomes the force ofthe relief valve spring, the valve moves against the spring and beginsto open. The pressure required to begin valve opening is called the"cracking pressure." The valve opens just enough to allow excess oilto flow through the valve.


Fig. 3.4.2 Relief Pressure Setting

Simple Pressure Relief Valve, Relief Pressure Setting

An increase in the resistance to oil flow increases the volume ofexcess oil and increases the circuit pressure. The increase in circuitpressure overcomes the new spring tension and further opens therelief valve.

The process is repeated until the maximum volume of oil (full pumpflow) is flowing through the relief valve. This is the "relief pressuresetting" as shown in Figure 3.4.2.

The simple relief valve is commonly used where the volume ofexcess oil flow is low or where there is a need for a quick response.This makes the simple relief valve ideal for relieving shock pressuresor as a safety valve.

Pilot Operated Relief Valve, CLOSE Position

The pilot operated relief valve (Figure 3.4.3) is often used in systemsthat require a large volume of oil and a small differential between thecracking pressure and the full flow pressure.

In the pilot operated relief valve, a pilot valve (simple relief valve) isused to control the unloading valve (main valve).

The pilot valve is much smaller and does not handle large volume oilflow. Therefore, the spring in the pilot valve is much smallerallowing more precise pressure control. The difference between thepilot valve cracking pressure and maximum pressure is held to aminimum.

The unloading valve is large enough to handle the complete pumpflow at the designed maximum relief pressure. The unloading valveuses the system oil pressure to keep the valve closed. Therefore, theunloading valve spring does not need to be strong and heavy. Thisallows the unloading valve to have a more precise opening pressure.

The system oil flows into the relief valve housing, through theunloading valve orifice and fills the unloading valve spring chamber.The oil in the unloading valve spring chamber comes in contact witha small area of the pilot valve. This allows the pilot valve to use asmall spring to control a high pressure. When the oil pressureincreases in the system, the same pressure is in the unloading valvespring chamber. Therefore, the oil pressure is the same on both sidesof the unloading valve. The combined force of the system oilpressure in the unloading valve spring chamber and the spring forceon the top of the unloading valve is greater than the force of thesystem oil pressure against the bottom of the valve. The combinedforce in the spring chamber keeps the unloading valve closed.


PILOT VALVE

UNLOADINGVALVE

PUMP FLOW

TOTANK

TOSYSTEM

UNLOADINGVALVE SPRING

PILOT VALVESPRING

UNLOADINGVALVE ORIFICE

Fig. 3.4.3 System Oil Flow

Pilot Operated Relief Valve, OPEN Position

When the system oil pressure exceeds the pilot valve spring setting(Figure 3.4.4), the pilot valve opens. The open pilot valve allows theoil in the unloading valve spring chamber to flow to the tank. Thepilot valve opening (orifice) is larger than the unloading valve orifice.Therefore, oil flows pass the pilot valve much faster than through theunloading valve orifice. This allows the pressure to decrease in theunloading valve spring chamber. The force of the higher system oilpressure moves the unloading valve against the spring. The excessivepump oil flows through the throttling holes in the unloading valve tothe tank. The throttling holes allow the unloading valve to dump thevolume of oil necessary to maintain the desired relief pressure.

TOTANK

TOSYSTEM

PUMPFLOW

PILOTVALVE

UNLOADINGVALVE


PILOT VALVESPRING

PILOT VALVEORIFICE

UNLOADINGVALVE ORIFICE

Fig. 3.4.4 Pilot Valve Open


Relief Valve ISO Symbol OPEN

The relief valve ISO symbol in Figure 3.4.6 shows a single valveenvelope in the OPEN position.

When the force of the system oil pressure overcomes the spring force,the arrow moves down (valve opens) and connects the oil line fromthe pump with the oil line to the tank. The pump oil flows throughthe valve to the tank.

FROMPUMP

TOTANK

Fig. 3.4.6 Relief Valve ISO Symbol Open to Flow


FROMPUMP

TOTANK

Fig. 3.4.5 Relief Valve ISO Symbol

Relief Valve ISO Symbol CLOSED

The relief valve ISO graphic symbol in Figure 3.4.5 can representeither a simple relief valve or a pilot operated relief valve. The ISOsymbol is the same for all relief valves.

The above relief valve ISO symbol shows a single valve envelope inthe CLOSED position. The system pressure is sensed through thepilot line at the top of the envelope and works to move the valve(arrow) against the spring. During normal operations, the pump flowis blocked at the closed valve.

Variable Relief Valve ISO Symbol

Figure 3.4.7 shows the ISO symbol for a variable relief valve.

The variable relief valve is a single envelope valve with an arrowthrough the spring. The arrow shows that the spring tension can bevaried.


FROMPUMP

TOTANK

Fig. 3.4.7 Variable Relief Valve

TOCIRCUIT 1

OUTPUT TOCIRCUIT 2

TO TANK

FROMPUMP


CHAMBER

UNLOADINGVALVE

PILOTVALVE

Fig. 3.4.8 Sequence Valve CLOSED


Sequence Valve, CLOSE Position

The sequence valve (Figure 3.4.8) is simply a pilot operated reliefvalve in series with the second circuit. The sequence valve is usewhen two circuits are supplied by one pump and one circuit haspriority over the other.

The sequence valve blocks pump oil flow to circuit 2 until circuit 1 issatisfied. When pump oil fills circuit 1, the oil pressure begins toincrease. The increase is sensed throughout the circuit as well as atthe bottom of the unloading valve and in the unloading valve springchamber of the sequence valve.

Sequence Valve, OPEN Position

When the pressure in the unloading valve spring chamber exceeds thesetting of the pilot valve spring, the pilot valve opens. The open pilotvalve allows the oil in the unloading valve spring chamber to flow tothe tank. This allows the pressure to decrease in the unloading valvespring chamber. The force of the higher system oil pressure movesthe unloading valve against the unloading valve spring force andopens the passage to circuit 2. Pump oil flows through the sequencevalve to circuit 2. The sequence valve remains open until thepressure in circuit 1 decreases to less than the pressure setting of thesequence valve.

TOCIRCUIT 1

OUTPUT TOCIRCUIT 2

TO TANK

FROMPUMP


CHAMBER

UNLOADINGVALVE

PILOTVALVE

Fig. 3.4.9 Sequence Valve OPEN

FROMPUMP

TOCIRCUIT 2

Fig. 3.4.10 Sequence Valve ISO SYMBOL


Sequence Valve ISO Symbol

The operation of the sequence valve is the same as the operation ofthe relief valve.

In the relief valve, the spring chamber is normally drained internallyto outlet passage. In the sequence valve, the outlet passage connectsto the second circuit. Because the second circuit is under pressurewhen the sequence valve opens, the pilot valve spring chamber mustbe externally drained to the tank.


Pump Start-up

Figure 3.4.11 shows the pressure reducing valve in the normally openposition.

At pump start-up, the valve spring force holds the valve spool and thepiston to the right. The supply oil flows around the pressure reducingvalve spool to the controlled oil circuit (downstream side of thevalve). The supply oil also flows through the oil passage to thepiston chamber at the right of the valve spool. Any change in thecontrolled oil circuit pressure is sensed in the piston chamber. Atpump start-up, the supply oil pressure and the controlled oil pressureare the same.

SUPPLYOIL

PISTON

VALVESPOOL

CONTROLLEDOIL CIRCUIT

DRAIN DRAIN

PISTONCHAMBERSHIMS

VALVESPRING

Fig. 3.4.11 Pressure Reducing Valve


Pressure Reducing Valve

The pressure reducing valve allows two circuits of different pressuresto be supplied by the same pump. The maximum supply oil pressureis controlled by the system relief valve. The pressure reducing valvecontrols the maximum pressure in the controlled oil circuit.

Normal Operating Condition

Figure 3.4.12 shows the pressure reducing valve in the normaloperating condition.

When the pressure increases in the controlled oil circuit, the increaseis sensed in the piston chamber. The increasing pressure moves thepiston to the left against the valve spool and the force of the spring.When the valve spool moves to the left, the valve spool restricts thesupply oil flow through the valve and reduces the controlled oilcircuit pressure.

The moving valve spool creates a variable orifice between the supplyoil and the controlled oil circuit. The variable orifice allows the oilflow to increase and decrease as needed to control the pressure in thecontrolled oil circuit.

The oil in the spring chamber must be drained to the tank. Anyincrease in the spring chamber oil pressure will cause an increase inthe valve setting.

SUPPLYOIL

VALVESPRING

PISTON

VALVESPOOL

CONTROLLEDOIL CIRCUIT

ORIFICEDRAIN DRAIN

PISTONCHAMBERSHIMS

Fig. 3.4.12 Normal Operating Condition


Pressure Reducing Valve ISO Symbol

Figure 3.4.13 shows the ISO symbol for the pressure reducing valve.

The ISO symbol uses a single envelope to represent the infinitepositioning or metering capability of the pressure reducing valve.

The pump oil flows through the NORMALLY OPEN valve to thecontrolled oil circuit. The controlled oil circuit pressure is sensedthrough the pilot line and moves the valve (arrow) against the spring.When controlled oil circuit pressure overcomes the spring force, thevalve shifts downward and restricts the oil flow to the controlled oilcircuit. The upstream pressure may continue to increase. However,the downstream pressure will not increase beyond the pressurereducing valve setting.

When the controlled oil circuit pressure decreases, the spring forcewill shift the arrow upward to the open position. The valveconstantly meters the oil flow to maintain the controlled oil circuitpressure.


FROMPUMP

TO CONTROLLEDOIL CIRCUIT

Fig. 3.4.13 Pressure Reducing Valve ISO Symbol



Pump Start-up

Figure 3.4.14 shows a pressure differential valve. The pressuredifferential valve maintains a specified difference in pressure betweentwo circuits.

At pump start-up and whenever the pressure in the primary circuit isless than 345 kPa (50 psi), the spring force holds the valve spool tothe right. The oil flow is blocked to the secondary circuit. Anychange in the primary circuit pressure is sensed at the valve spool.

PRIMARYCIRCUIT

SECONDARYCIRCUIT

50 PSISPRING

VALVESPOOL

VALVEBODY

SUPPLY OIL

Fig. 3.4.14 Pump Start-up

Pressure Differential Valve

In figures 3.4.14 and 3.4.15, the spring exerts a 50 pound force on the1 sq. inch valve spool. The supply oil pressure must exceed 345 kPa (50 psi) to overcome the spring force and move the valvespool.

PRIMARYCIRCUIT

SECONDARYCIRCUIT

50 PSISPRING

SUPPLY OIL

VALVEBODY

VALVESPOOL

Fig. 3.4.15


Normal Operating Condition

When the primary circuit is filled, pressure begins to increase. Whenthe primary circuit pressure increases to more than 345 kPa (50 psi),the primary pressure overcomes the 345 kPa (50 psi) differentialvalve spring force and moves the differential valve to the left. Supplyoil flows to the secondary circuit. Supply oil also flows through thepassage to the differential valve spring chamber.

When the secondary circuit is filled, the pressure begins to increase.The same pressure increase is sensed in the differential valve springchamber. The combined oil pressure and spring force move the valvespool to the right and attempts to shut off the flow of oil to thesecondary circuit. However, the increase in pressure in the primarycircuit keeps the valve open. The pressure increases in both theprimary and secondary circuits until the relief valve opens and sendsthe pump flow back to the tank.

The pressure differential valve establishes a position that constantlymaintains a 345 kPa (50 psi) pressure difference between the primaryand the secondary circuits at all pressures above 345 kPa (50) psi.

Pressure Differential Valve ISO Symbol

The pressure differential valve ISO symbol (Figure 3.4.16) is acombination of the pressure relief valve symbol and the pressurereducing valve symbol.

The pressure from the inlet side is sensed by the valve and worksagainst the spring force as in the pressure relief valve. The outletpressure is sensed by the valve and works with the spring force. Thedifference between the inlet and outlet pressures is always equal tothe valve spool spring force pressure regardless of changes inpressure at the inlet port. Example, a spring force pressure of 345kPa (50 psi) will produce a pressure differential between the inlet andoutlet pressure of 345 kPa (50 psi).

The spring is changed to meet any required change in the differentialpressure. Normally, shims are not used to change the pressurerequirements.


INLET OUTLET

Fig. 3.4.16 Pressure Differential Valve ISO Symbol


Lesson 5: Directional Control Valves

Dir

ecti

on

al C

on

tro

l V

alve

s

Introduction

Directional control valves are used to direct oil into separate circuitsof a hydraulic system. The maximum flow capacity and the pressuredrop through the valve are the first considerations. Directionalcontrol valves may be interfaced with manual, hydraulic, pneumaticand electronic controls. These factors are mostly determined duringthe initial system design.

Objectives


1. State the function of the manual spool type control valve, therotary type control valve and the solenoid actuated controlvalve.

2. State the function of the simple check valve, the pilotoperated check valve and the shuttle valve

3. Identify the ISO symbols for the various directional controlvalves.




VALVE BODY

VALVE BORE SPOOL LANDS

SPOOL GROOVE

Fig. 3.5.1 Valve Spool

Directional Control Valve

The directional control valve is use to direct the supply oil to theactuator in a hydraulic system.

The valve body is drilled, honed and sometime the bore is heat treated.The inlet and outlet ports are drilled and threaded. The valve spool ismachined from high grade steel. Some valve spools are heat treated,ground to size and polished. Other valve spools are chrome plated,ground to size and polished. The valve body and valve spool are thenmated in assembly to the design specifications. When assembled, thevalve spool is the only part that moves.


Valve Spool

The valve spool (Figure 3.5.1) consist of lands and groves. The spoollands block the oil flow through the valve body. The spool grovesallow oil to flow around the spool and through the valve body.

The position of the spool when not activated is called the "normal"position.

When an "open center" valve is in the normal position, the supply oilflows through the valve and back to the tank. When a "close center"valve is in the normal position, the supply oil is blocked by the valvespool.

Open Center Directional Control Valve in HOLD Position

Figure 3.5.2 shows a cutaway diagram of a typical open centerdirectional control valve in the HOLD position.

In the HOLD position, the pump oil flows into the valve body,around the valve spool and returns to the tank. The pump oil alsoflows to the load check valve. The passage behind the load check isfilled with blocked oil. The blocked oil and the load check valvespring keep the load check valve closed. The valve spool also blocksthe oil in the line to the rod end and the head end of the cylinder.

TO TANK

TOTANK

TOTANK

FROMCYLINDERROD END

FROMPUMP

VALVEBODY

VALVESPOOL

LOADCHECKVALVE

FROMCYLINDERHEAD END

Fig. 3.5.2 Directional Control Valve in HOLD

TO TANK

TOTANK

TOTANK

FROMCYLINDERROD END

FROMPUMP

VALVEBODY

VALVESPOOL

LOADCHECKVALVE


Fig. 3.5.3 Directional Control Valve RAISED


Open Center Directional Control Valve in RAISE Position

Figure 3.5.3, shows the valve spool at the instant the spool is movedto the RAISE position.

When the valve spool is moved to the RAISE position, the valvespool blocks the pump oil flow to the tank. However, pump oil flowis open to the load check valve. The valve spool also connects thecylinder head end to the oil behind the load check valve and thecylinder rod end to the tank passage. The load check valve prevents .

Open Center Directional Control Valve, RAISE Position

In Figure 3.5.4, the increase in pump oil pressure overcomes thepressure behind the load check valve (unseats the load check valve).The pump oil flows pass the load check valve and around the valvespool to the head end of the cylinder.

The oil in the rod end of the cylinder flows pass the valve spool tothe tank.

TO TANK

TOTANK

TOTANK

FROMCYLINDERROD END

FROMPUMP

VALVEBODY

VALVESPOOL

LOADCHECKVALVE


Fig. 3.5.4 Raise Position


the oil in the head end of the cylinder from flowing into thepump oil passage. The blocked pump oil flow causes anincrease in the oil pressure

ONEPOSITION

TWOPOSITION

THREEPOSITION

Fig. 3.5.5 ISO Symbols

Directional Control Valve ISO Symbols

Basic Envelope

The basic valve ISO symbol in Figure 3.5.5 consists of one or morebasic envelopes. The number of envelopes used represents thenumber of positions that the valve can be shifted.

Flow Path

In Figure 3.5.7, the lines and arrows inside the envelopes are usedbasically to represent the flow paths and directions between ports.

THREE-WAY SIX-WAYTWO-WAY FOUR-WAY

Fig. 3.5.6 Valve Port

CROSSFLOW

FLOW INONE

DIRECTION

FLOW INEITHER

DIRECTION

FLOWBLOCKED

PARALLELFLOW

Fig. 3.5.7 Flow Path

Valve Port

Shown in Figure 3.5.6 are the valve ports for attaching working lines.A valve with two ports is commonly referred to as a two-way valve.This is not to be confused with a two-position valve shown in Figure3.5.5. Valves may have as many positions and ports as needed.However, most valve positions are in the range of one to three andvalve ports in the range of two to six.


Three Position Valve

Figure 3.5.8 shows three ISO symbols of the three position valve. Inthe three position valve, the center position is the NEUTRAL orHOLD position. When the valve is not doing work, the valve isplaced in the HOLD position.

Depending on the design of the spool, the center position servesseveral purposes.

The ISO symbol at the top represents a closed center valve. When inthe HOLD position, the close center spool blocks all oil flow.

The ISO symbol in the middle represents a tandem center valve.When in the HOLD position, the tandem center valve blocks oil flowat A and B but connects the pump to the tank.

The ISO symbol on the bottom represents an open center valve.When in the HOLD position, the open center valve connects all portsto the tank.

CLOSED CENTER

TANDEM CENTER(CATERPILLAROPEN CENTER)

OPEN CENTER

A B

P T

A B

P T

A B

P T

Fig. 3.5.8 Three Position Valve


FROMPUMP

TOTANK

CHECKVALVE

LOWER

RAISEMANUAL CONTROL

TO CYLINDER ROD END

TO TANKTO CYLINDER HEAD EN

Fig. 3.5.9 Six Way Valve

FROMPUMP

TOTANK

CHECKVALVE

LOWER

RAISEPILOT CONTROL

TO CYLINDER ROD END

TO TANKTO CYLINDER HEAD END

PILOT OIL

PILOT OIL

Fig. 3.5.10 Six Way Valve

Six Way Valve

Three Position, Six Way, Close Center, Pilot Controlled Valve

Figure 3.5.10 shows a three position, six way, close center, pilotcontrolled valve. In the HOLD position, all oil flow is blocked at thecontrol valve spool.

Three Position, Six Way, Open Center, Manual Controlled Valve

Figure 3.5.9, shows a three position, six way, open center, manualcontrolled valve in the HOLD position. The pump oil flows aroundthe valve spool to the tank. The oil in the cylinder is blocked at thecontrol valve spool.


SOLENOIDACTUATOR

MANUALACTUATOR

PUSHBUTTONACTUATOR

PEDALACTUATOR

OILACTUATOR

AIRACTUATOR

SPRINGACTUATOR

PUSH-PULL LEVERACTUATOR

MECHANICALACTUATOR

DETENTEDACTUATOR

Fig. 3.5.11 Directional Control Valve Actuator

VALVEBODY

PLUG

HEAD END

ROD END

HEAD ENDFROM PUMP

TO TANK

CHAN

NELS

CHANNELS

PORT

PORT PORT

PORT PORT

PORT

PORT

PORT

VALVEBODY

PLUG

FROM PUMP

ROD END TO TANK

Fig. 3.5.12 Rotary Valve

Rotary Valve

The rotary valve (Figure 3.5.12) consists of a round plug withpassages or channels. The channels in the plug connect with the portsin the valve body. Instead of shifting to the right or to the left, thevalve rotates.

In the diagram on the left, the valve connects the pump to the rod endof the cylinder. The oil in head end flows to the tank. When thevalve is rotated 90 degrees, the pump is connected to the head endand the oil in the rod end flows to the tank.

The rotary valve shown is a four-way valve. However, rotary valvesmay also be two-way or three-way. The rotary valve is used in lowpressure operations.


Directional Control Valve Actuator

Figure 3.5.11 shows the ISO symbols for various directional controlvalve actuator.


Check Valve

The purpose of a check valve is to readily permit oil flow in onedirection, but prevent (check) oil flow in the opposite direction. Thecheck valve is sometimes called a "one way" check valve.

Most check valves consist of a spring and a tapered seat valve as inFigure 3.5.13 above. However, a round ball is sometimes usedinstead of the tapered seat valve. In some circuits, the check valvemay be free floating (has no spring).

In the valve on the left, when the pump oil pressure overcomes theoil pressure in back of the check valve plus the check valve slightspring force, the check valve opens and allows the oil to flow to theimplement.

In the valve on the right, when the pressure of the pump oil is lessthan the oil pressure in the implement, the check valve closes andprevents implement oil flow back through the valve.

TOIMPLEMENT

FROMPUMP

TOPUMP

FROMIMPLEMENT

Fig. 3.5.13 Check Valve


PILOTOIL

FROM CONTROLVALVE

TOCYLINDER

PILOTVALVE

CHECK VALVE

RODPILOT VALVEOIL CHAMBER

Fig. 3.5.14 Forward Flow

PILOTOIL

TO CONTROLVALVE

FROMCYLINDERPILOT

VALVE

CHECK VALVE


Fig. 3.5.15 Flow Blocked

Flow Blocked

When oil flow from the control valve cease, the check valve seats asshown on the right of Figure 3.5.15. The oil flow from the cylinderto the control valve is blocked at the check valve.

The pilot operated check valve is most often used in operations whereload drift is a problem. The pilot operated check valve allows loaddrift to be held to a very close tolerance.

Pilot Operated Check Valve

The pilot operated check valve differs from the simple check valve inthat the pilot operated check valve allows oil flow through the valvein the reverse direction.


Forward Flow

Figure 3.5.14 shows a pilot operated check valve. The pilot operatedcheck valve consist of a check valve, a pilot valve and a rod. Thepilot operated check valve allows free flow from the control valve tothe cylinder.

Reverse Flow

The valve in Figure 3.5.16, shows oil flow from the cylinder to thecontrol valve.

When flow is required, pilot oil is sent to the pilot valve oil chamber.Pilot oil pressure moves the pilot valve and rod to the right andunseats the check valve. The cylinder oil flows through the checkvalve to the control valve and then to the tank.

The pressure ratio between the load pressure and the pilot pressure isdesigned into the valve. The valve used on the Explorer training unithas a pressure ratio of 3:1. The pressure needed to open the checkvalve is equal to one-third of the load pressure. A load pressure of4134 kPa (600 psi) requires a pilot pressure of 1378 kPa (200 psi) toopen the check valve.

PILOTOIL

TO CONTROLVALVE

FROMCYLINDERPILOT

VALVE

CHECK VALVE


Fig. 3.5.16 Reverse Flow


Check Valve ISO Symbols

In Figure 3.5.17, symbols A and B represents the simple check valvein the OPEN and CLOSE positions.

Symbol C represent the shuttle valve. The shuttle (resolver) valveallows two separate circuits to supply oil to a third circuit whilekeeping the two separate circuits isolated from each other.

Symbol D represents the pilot operated check valve.

CHECK VALVE (OPEN)A B

CHECK VALVE (CLOSED)

CIRCUIT 2CIRCUIT 1

CCIRCUIT 3

SHUTTLE VALVE(RESOLVER VALVE)

D

PILOT OPERATEDCHECK VALVE

PILOT

Fig. 3.5.17 Check Valve ISO Symbols


Make-up Valve

The make-up valve in Figure 3.5.18, looks similar to the check valve.The makeup valve is normally positioned in the circuit between theimplement and the tank. During normal o

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