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Basic hydraulic system (Excavator)

Understanding hydraulic basic system

Jan., 24th,2007Overseas P/S TeamDocu. No : DXT HB 07 06 - Rev03

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1. Hydraulic principles

2. Basic hydraulic system

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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.


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1) Liquids comport to shaft

Liquids will conform to the shape of any container.

Liquids will also flow in any direction through lines and hoses of various sizes and shapes.


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2) Practically incompressible

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


50Kg

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2) Practically incompressible

Gas is compressible. When gas is compressed, it takes up less space and it’s displacement becomes less. The space previously occupied by the gas may be occupied by another object. Therefore, a liquid is best suited for the hydraulic system because it continually occupies the same volume or displacement.


50Kg

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3) Hydraulics doing work

According to Pascal’s Law, "Pressure exerted on a confined liquid is transmitted undiminished in all directions and acts with equal force on all equal areas." Therefore, a force exerted on any part of an enclosed hydraulic oil system transmits equal pressure in all directions throughout the system.

In the above example, a 50 Kg force acting upon a piston with a 2 Cm. radius creates a pressure of approximately 4 Kg per square centimeter in a confined liquid. The same 4Kg/Cm² acting upon a piston with a 3 Cm. radius supports a 112Kg force weight.

50 Kg.f

2 Cm Radius

112 Kg.f

3 Cm Radius

4Kg.f/cm²


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3) Hydraulics doing work

Force (F) = Pressure (P) X Area (A)

Pressure (P) = Force (F) / Area (A)

Area (A) = Force (F) / Pressure (P)

50 Kg.f

2 Cm Radius

112 Kg.f

3 Cm Radius


4Kg.f/cm²

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4) Orifice effect

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

In right figure, there is an orifice in the pipe between the two gauges.

The gauge up stream of the orifice shows that a pressure of 30 bar is needed to send a flow of 1 LPM through the orifice. There is no restriction to flow after the orifice. The gauge down stream of the orifice shows 0 pressure.

Flow

1 LPM

Flow

1 LPM


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4) Orifice effect

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

The positive displacement pump continues pumping at 1 LPM and fills the pipe. When the pipe is filled, the resistance to any additional flow into the pipe produces pressure. The pressure reaction is the same as Pascal’s Law which states that "pressure exerted on a confined liquid is transmitted undiminished in all directions and acts with equal force on all equal areas." The two gauge readings are the same.

From

PumpFrom

Pump


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4) Orifice effect

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

In Figure, a pressure of 90 bar is required to send 1 LPM through either circuit.

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

Flow

1 LPM

Flow

1 LPM

Bar Bar Bar Bar Bar Bar

30 bar 30 bar 30 bar


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4) Orifice effect

In a system with parallel circuits, pump oil follows the path of least resistance. In figure , the pump supplies oil to three parallel circuits. Circuit three has the lowest priority and circuit one has the highest priority.

When the pump oil flow fills the passage to the left of the three valves, pump oil pressure increases to 30 bar. The pump oil pressure opens the valve to circuit one and oil flows into the circuit. When circuit one is filled, the pump oil pressure begins to increase. The pump oil pressure increases to 60 bar and opens the valve to circuit two. The pump oil pressure can not continue to increase until circuit two is filled. The pump oil pressure must exceed 90 bar to open the valve to circuit three.

There must be a system relief valve in one of the circuits or at the pump to limit the maximum pressure in the system.

30 bar

60 bar

90 bar


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4) Orifice effect


Please write down system pressure on pressure gauge

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1) Hydraulic tank

The main function of the hydraulic oil tank is to store oil. The tank also removes heat and air from the oil. Tanks must have sufficient strength, adequate capacity and keep dirt out.

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1) Hydraulic tankFill cover - Keeps contaminants out of the opening that's used to fill and add oil to the tank and seals pressurized tanks.

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

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

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

Fill Cover

Sight Glass

Suction line

Return line

Drain


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1) Hydraulic tankPressurized TankThe two main types of hydraulic tanks are pressurized and vented (un pressurized).

The pressurized tank is completely sealed. Atmospheric pressure does not effect the pressure in the tank. However, when the oil is sent through the system, it absorbs heat and expands.

The expanding oil compresses the air in the tank. The compressed air forces the oil out of the tank and into the system.

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

Air breather


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2) Hydraulic fluids

The primary functions of hydraulic fluids are

• Power transmission

• Lubrication

• Sealing

• Cooling


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2) Hydraulic fluids

Power Transmission

Because hydraulic fluids are virtually incompressible, once the hydraulic system is filled with fluid it can instantly transmit power from one area to another. However, this does not mean that all hydraulic fluids are equal and will transmit power with the same efficiency. Choosing the correct hydraulic fluid depends on the application and the operating conditions.

Lubrication

Hydraulic fluid must lubricate the moving parts of the hydraulic system. The rotating or sliding components must be able to function without touching other surfaces. The hydraulic fluid must maintain a thin film between the two surfaces to prevent friction, heat and wear.


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Sealing

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

Cooling

The hydraulic system develops heat as it transfers mechanical energy to hydraulic energy and hydraulic energy back to mechanical energy.

As the fluid moves throughout the system, heat flows from the warmer components to the cooler fluid. The fluid gives up the heat to the reservoir or to coolers that are designed to maintain fluid temperatures within design limits.

Other properties expected of the hydraulic fluid are the prevention of rust and corrosion on metal parts, the resistance to foaming and oxidation, the ability to separate air, water and other contaminates from the fluid, and the ability to maintain stability over a wide range of temperatures

2) Hydraulic fluids


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3) Hydraulic pump

The hydraulic pump transfers mechanical energy into hydraulic energy. 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 it into a hydraulic system as flow.

All pumps produce oil flow in the same way. A vacuum is created at the pump inlet. The higher atmospheric or tank pressure pushes the oil through the inlet passage and into the pump inlet chambers. The pump carry the oil to the pump outlet chamber. The volume of the chamber decreases as the chamber approaches the outlet. This decrease in chamber size pushes the oil out the outlet. Pumps produce only the flow (i.e. gallons per minute, liters per minute, cubic centimeters per revolution, etc.) used in the hydraulic system. Pumps DO NOT produce or cause "pressure". Pressure is caused by the resistance to the flow. Resistance can be caused by flow through hoses, orifices, fittings, cylinders, motors, or anything in the system that hinders free flow to the tank.


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3) Hydraulic pump

Tank

PUMP

Engine


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Gear pump Axial propeller pump

Vain pump Internal gear pump

3) Hydraulic pump


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3) Hydraulic pump (Making flow)

Axial piston pump Vent axis piston pump


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3) Hydraulic pump symbol


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4) Control valve

The control valve (Hydraulic system) has three control features

1) Pressure control

2) Directional control

3) Flow control


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4) Control valve

1) Pressure control - Relief valve

Hydraulic systems are designed to operate within a certain pressure range.

Exceeding this range can damage the system components or become dangerous to personnel. The relief valve maintains the pressure within the designed limit by opening and allowing excessive oil to flow either to another circuit or back to the tank.

Closed condition Open condition


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4) Control valve

1) Pressure control - Relief valve symbol

Closed condition Open condition


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4) Control valve


1) Pressure control – Mail relief valve

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4) Control valve


1) Pressure control - Over load relief valve

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4) Control valve

Closed condition

Open condition

C: Orifice

D: Chamber

E: Sleeve

F: Poppet

G: Drain hole


1) Pressure control - Main relief valve

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4) Control valve

Normal pressure (330 bar) Pressure up (350 bar)


1) Pressure control – Main relief valve (2 stage)

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4) Control valve


1) Pressure control – Overload relief valve

Make up function?

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4) Control valve

Closed conditionOpen condition

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


1) Pressure control – Check valve

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4) Control valve


1) Pressure control – Check valve symbol

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4) Control valve


1) Pressure control – Load check valve

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4) Control valve


1) Pressure control – Load check valve

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4) Control valve

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

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1) Pressure control – Pilot control check valve

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4) Control valve


1) Pressure control – Pilot control check valve

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4) Control valve


1) Pressure control – Pilot control check valve ( Holding valve)

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4) Control valve

The make-up valve, looks similar to the check valve. The makeup valve is normally positioned in the circuit between the implement and the tank. During normal operations, the pump or cylinder oil fills the area behind the make-up valve. The pressure in the cylinder keeps the valve CLOSED. When the cylinder pressure is lower than the tank pressure, the makeup valve will OPEN. The tank oil bypasses the pump and flows directly through the make-up valve to the cylinder.

The make-up valve is used to prevent cavitations.


1) Pressure control – Make up valve

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4) Control valve

Directional control valve

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


2) Directional control - Directional control spool

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4) Control valve


2) Directional control - Directional control spool

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4) Control valve


2) Directional control - Directional control spool (symbol)

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4) Control valve



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4) Control valve



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4) Control valve

Two position, 4-way pilot valve Three position, 4-way pilot valve



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4) Control valve

Neutral position Switched position


2) Directional control

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4) Control valve

Tandem circuit Parallel circuit


2) Directional control – Parallel line

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4) Control valve


2) Directional control – Parallel line

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4) Control valve

Flow control valve

Orifice

An orifice is a small opening in the oil flow path. Flow through an orifice is affected by several factors.

Three of the most common are:

1. The temperature of the oil.

2. The size of the orifice.

3. The pressure differential across the orifice.


3) Flow control

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4) Control valve

• Temperature

The oil viscosity changes with changes in temperature. Viscosity is a measurement of the oil's resistance to flow at a specific temperature.

Hydraulic oil becomes thinner and flows more readily as the temperature increases.

• Orifice Size

The size of the orifice controls the flow rate through the orifice. A common example is a hole in a garden hose. A small pin hole will leak in the form of a drip or a fine spray. A larger hole will leak in the form of a stream. The hole, whether small or large, meters a flow of water to the outside of the hose. The amount of water metered depend on the size of the hole (orifice).


3) Flow control

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4) Control valve

Fixed orifice Variable orifice


3) Flow control - Orifice

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4) Control valve

3) Flow control - Orifice

• Pressure Differential

Flow through an orifice is affected by the pressure differential across the orifice. The greater the pressure differential across the orifice, the greater the flow through the orifice.

Below figure, pressure differential is illustrated using the two tubes of tooth paste. When the tube of toothpaste is gently squeezed as in A, the pressure difference between the inside of the tube and the outside of the tube is small. Therefore, only a small amount of tooth paste is forced out.

When the tube is squeezed with greater force as in B, the pressure difference between the inside of the tube and the outside of the tube increases and a larger amount of toothpaste is forced out.


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4) Control valve


3) Flow control – Orifice symbol

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5) Cylinder

Cylinder

Single acting cylinder Double acting cylinder


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Cylinder

Effective area

5) Cylinder


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Cylinder (Cushion damping)

5) Cylinder

Head side damping Rod side damping


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Cylinder symbol

5) Cylinder


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Axial piston motor Bent axis piston motor

6) Motor


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6) Motor

Fixed displacement HYD. motor Variable displacement HYD. motor


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6) Motor


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6) Motor (Swing motor)


Rotating Stop

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6) Motor (Travel motor)


Rotating Stop

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6) Motor (Travel motor- Wheel type)


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7) Remote control valve


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7) Remote control valve


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8) Center joint

Upper hydraulic power from hydraulic pump transmits to the lower hydraulic actuator without any hydraulic hose connection


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9) Cooler

• Transfer heat into air

• Keep machine operating within a desiredtemperature range


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10) Accumulator

Spring loaded type Gas loaded type

• Save Hydraulic energy.

• Compensate system pressure

• Absorption of hydraulic pick pressure on system line

• Remove pump pulsating pressure


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11) Solenoid valve


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12) Proportional valve

The out put pressure is depend on current value

(Variable out put pressure)

Out put


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13) Filter

Filter Strainer

To remove contaminants from hydraulic fluid in system


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14) Restriction valve


Remain return line pressure (Back pressure) for making up

Control valve tank line

Swing motor make up port

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15) Steering unit


One of control valve for steering,

Steering power by operator control is connected to steering cylinder.

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16) Service brake valve


Brake supply power is connected to break actuator.

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17) Shuttle valve

One of special connector.

S1 port is connected anytime if B1 or B2 pressure is high.

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