Hydraulic System

1. Advantage of Hydraulics

How does a hydraulic crane compare with a mechanical crane that has the same output?

Small and Light

Hydraulic equipment such as hydraulic pumps, cylinders, motors, etc., are able to provide a huge amount of power from a very small machine by using high hydraulic pressure.

For example, the boom elevation cylinder of a 50 ton capacity hydraulic crane has an inner diameter of only 21 cm, but at a high pressure of 240 kg/cm2, it has about 80 tons of output. If we put two cylinders together, it can output a huge 160 tons of power to lift the boom.

Easily Remote Controlled

Because mechanical cranes use a lever, rod, link and chain for the control system, the controls must be placed close to the mechanism. For hydraulic cranes, it is only necessary to connect the control valves to the mechanism using pipes, so the controls can be placed far away.

Easiy to Change Speed Steplessly

Because mechanical cranes use a lever, rod, link and chain for the control system, the controls must be placed close to the mechanism. For hydraulic cranes, it is only necessary to connect the control valves to the mechanism using pipes, so the controls can be placed far away.

2. The Principle of Hydraulics (Pascal's Principle)

A force applied to one section of an enclosed liquid at rest will be transferred to the entire liquid with the same amount of force.

If you press a syringe (area = 1cm2) with a force of 2kg, the hydraulic pressure generated is 2kg/cm2. If this pressure flows through a tube into a cylinder with a 30 cm2 inner diameter, the pressure on all of the internal surfaces of the cylinder(including the piston) will be 2 kg/cm2. Therefore, the movable piston can support a 60 kg object.

3. Flow

Flow is the volume of hydraulic oil that flows in a given unit of time.Generally, it is measured as the amount of hydraulic oil that flows in one minute, in l / min.

When moving cylinders of the same diameter, the one with higher flow will rise faster.

We can compare the speeds at which the two cylinders in figures A and B moved in one minute:

In the human body, the great pumps the blood, the lung keeps the blood clean, and the arms and legs move accoring to the commands of the brain. To look at this as a hydraulic system,

*The heart is the pump ... it pumps the hydraulic oil.*The lungs are filters ... they maintain the purity of the hydraulic oil.*The arms and legs are actuators such as cylinders and mortors ... they respond faithfully to the manipulation of the control valve.

Aside from these, just as the human body is composed of organs and tissues too numerous to count, hydraulic systems are also composed of a large number of parts, each with different functions.

In order to perform various movements such as raising and lowering the boom, winding in the wire rope, swinging and extending or retracting the boom, a hydraulic crane uses a hydraulic circuit that is made up of the

following devices: a hydraulic pump to generete force for motion, hydraulic cylinders and mortors as drive devices, and various valves as control devices.

It is a device that uses the rotational energy provided by an engine or electrical motor to suck hydraulic oil out of the tank and pump it out at pressure into the hydraulic circuit.

If the pump is driven directly by an engine, whenever the engine is running, the pump is working.

When the pump is driven with a PTO, the pump only works when the PTO is connected.

*PTO is an abbreviation for Power Take Off device. It transfers the engine's mechanical force by attaching a gear to the transmisission.

1. Gear PumpThis is used most often in hydraulic cranes.

[Features]* Construction is simple but strong.* It is small and light-weight.* It does not break down often, and is easy to maintain.* It is low in price.* Many models from low pressure to high pressure are available.

External type pumps are used in hydraulic cranes. When the gears inside the casing rotate, a vacuum is produced at the point where the teeth disengage each other, and fluid is drawn in through the intake. This fluid fills the spaces between the teeth and is carried around the circumference of the casing by the rotation of the gears. It is then ejected through the outlet. The maximum output pressure is about 210 kg/cm².

2. Piston Pump(Plunger Pump)[Features]* It is highly efficient because there is little internal leakage.* Some pumps are of variable volume type.   With the same speed of rotation, different flows can be obtained.* It is appropriate for high pressure. * It is high in price.* The hydraulic pressure with less pulse can be provided.

Axial fixed cam plate pumps are often used in hydraulic cranes.In an axial pump, the piston (plunger) causes oil to be sucked in and then discharged. In piton pumps, the sliding portions of the cylinder and piston are long. Because of this, there is little oil leakage, and these pumps are well suited for high pressure pumping.A fixed cam plate works by rotating the drive axis which is incorporated into the cylinder itself. This rotating motion causes the piston to move away from and closer to the cam plate, creating a pumping action.

3. Vane Pump[Features]* It operates quickly.* It is appropriate for low to middle pressure.* It is low in price.

As the rotor rotates, the attached vanes enter and exit, and the volume of the spaces between the vanes changes, creating a pumping action. Because vane pumps are quiet, they are eventually used in steering circuits. There are fixed volume and variable volume models for vane pumps as well.

An actuator is a device that converts the energy of hydraulic oil pressure into straight line or rotational motion. They are like the muscles of the arms and legs in the human body. Hydraulic cylinders and hydraulic motors are actuators.

Hydraulic CylinderThis converts the energy of pressure into straight line motion.

Hydraulic MotorThis converts the energy of pressure into rotational motion.

1) Hydraulic CylindersThese are broken down into two types depending on what kind of motion they can perform. The two types are "single-acting cylinders" and "double-acting cylinders".

Single-acting Cylinders

These cylinders have only one intake/outlet hole. and are moved in one direction by hydraulic pressure, returning by their own weight or by a spring.

Double-acting Cylinders

These cylinders have two intake/outlet holes, and are extended and retracted using hydraulic pressure.

The components of these cylinders include the cylinder, piston rod, oil seal, packing, o-ring and dust seal.

Output

The output value is equal to the product of the hydraulic pressure multiplied by the area of the rod on which the pressure is acting.

The output of a cylinder F is proportional to the pressure P and the pressurized area A.

2) Hydraulic MotorsHydraulic motors are actuators that convert hydraulic pressure into continuous rotational movement. Their continuous is very similar to the construction of hydraulic pumps.The only difference between the two devices is that a hydraulic pump converts mechanical energy, while a hydraulic motor converts fluid energy into mechanical (rotational) energy.

Types of Hydraulic Motors

Gear motor

Piston motors(plunger motors)

Axial piston motor

Features of Hydraulic Motors

These are the same as for hydraulic pumps.

The oil tank is also sometimes called the oil reservoir.

1 The Job of the Oil Tank

1) It stores hydraulic oil, and, when necessary, it supplies the oil to the hydraulic circuit.2) It removes dirt or air bubbles from the hydraulic oil.3) It separates water from the hydraulic from the hydraulic oil. The water that is separated falls to the bottom of the tank, and it must be removed at regular intervals.4) It dissipates the heat generated by the hydraulic devices.

2 Checking the Amount of Oil in the Tank for a Hydraulic Crane

The oil level in the oil tank decreases when the cylinder expands, and increases when the cylinder retracts. When checking the amount of oil in the oil tank of a hydraulic crane, you must be careful to check it while the crane is in traveling mode.

*The standard oil level for the oil tank of a hydraulic crane in traveling mode is indicated on the oil level meter.

3 Air Breather

The air breather allows air to enter and leave the tank as the oil level rises and falls, but it prevents any contaminants from entering the tank with the air. If the air breather becomes clogged, air cannot enter and leave the tank, so the resistance acting on the pump increases. In some cases, the tank ruptures. Therefore, it is necessary to replace the element with a new one regularly.

The air breather also functions as an oil fill hole.

The oil filter removes foreign matter (metal particles, iron filings) from the hydraulic oil.The oil filter protects the various devices in the hydraulic system from contamination, to which they are extremely susuceptible.There are two types: those installed in the tank, and those installed in the hydraulic circuit.

1 Suction Filters

These Filters remove contaminants on the pump intake side.

2 Line Filters

These are used in the primary pressure circuit. Unlike suction filters, there filters mostly are extreamly fine-mesh filter elements(5-20 microns).These filters are always inserted in the brake circuit, clutch circuit, and automatic cutout circuit.

3 Return Filters

These filter remove contaminants at the return side of the oil tank.Some are installed inside the tank, while others are installed in the return line.

Hydraulic oil performs many different functions in order to help the hydraulic devices work smoothly. Aside from acting as a medium to transmit the force of pressure and motion, it also lubricates parts that undergo friction, it seals, cools and prevents rust.

1 Climate and Design Conditions

Generally speaking, the type of hydraulic oil to be used is selected based on two conditions; the climate of the location where the equipment will be used, and the design of the various devices in the hydraulic system.

Note that sometimes the synthetic materials used in the packing seals, etc. will chemically react with the hydraulic oil, so always use the recommended brand names.

2 Viscosity

Once you have chosen the type of hydraulic oil, in order to set the temperature range in which it can be used, you must select a viscosity (ISO viscosity grade).

* The ISO viscosity grade indicates the kinematic viscosity at 40°C.

For example, hydraulic  oil VG46 has a kinematic viscosity of 46 cst (centistokes) at 40°C (oil temperature).

These are many quality requirements for hydraulic for hydraulic oil, but the most important of these is that the oil has an appropriate viscosity not changing much with changes in temperature.

If the viscosity is too high???

International friction is increased, heat is generated, and energy loss is great.

Resistance within the circuit increases, and pressure loss increases.

If the viscosity is too low???

Leakage within the pump increases, volume efficiency is diminished, and the required pressure cannot be achieved.

Due to internal leakage in the control valve actuator, the cylinder is retracted by the load, and the motor cannot produce the required output.

3 Standard Hydraulic Oils Depending on Destinations

The standard hydraulic oil is decided based on the part of the world to which the crane is being shipped, as shown in the figure. For extremely cold areas, a specially selected hydraulic oil is used.

VG46

Temperate Zones - Tropical Zones

VG32

Temperate Zones - Arctic Zones

VG22

Arctic Zones

* VG68 is only used where air temperature is too high for continuous operation.

4. Manufacturer Recommended Properties and Brand Names for Crane Hydraulic Oil

Standard or Class Antiwear Hydraulic Oil

Ambient Temperature Range above 0 C° above -10C° above -15C°

above -25C°

Permissible OilTemperature Range

0 C°to 80 C°

-10C°to 80C°-15C°to 70C°

-25C°to 60C°

Viscosity Grade VG68 VG46 VG32 VG22 or VG15

RecommendedCharacteristics

Specific Gravity15/4 C°

0.86 to 0.88 0.86 to 0.88 0.86 to 0.88 0.85 to 0.88

Flashing Point  C° over 220 over 220 over 220 over 160

Kinematic Viscosity(cst)

40C° 68 46 32 22 or 15

100C° above 9 above 7 above 5 above 3

Viscosity Index above 100 above 100 above 100 above 100

Pour Point below -25 below -25 below -25 below -40

RecommendedBrand Names

IDEMITSU

Daphne SuperHydraulic Fluid 68

Daphne SuperHydraulic Fluid 46

Daphne SuperHydraulic Fluid 32

Daphne SuperHydro 22WR

KYOSEKI Hydlux 68 Hydlux 46 Hydlux 32

NISSEKISuper Hydrando 68

Super Hydrando 46Super Hydrando 32

Super Hydrando 15

ESSO Nuto H68 Nuto H32 Nuto H15

SHELLTellus Oil 68

Tellus Oil 46Tellus Oil 32

MOBIL DTE26 DTE25 DTE24 DTE11

5. Managing Viscosity and TemperatureThe degree to which the temperature of the hydraulic oil increases depends on the length of time the system is in operation, and the type of work that is being done.If the operation is going to last a long time, or if the work is extremely strenuous, the temperature will rise, so you must either pause the operation or change the hydraulic oil to one with a higher viscosity grade.

1. Valves Used in Hydraulic Circuits

Valves are used in hydraulic circuit in order to control hydraulic pressure, direction of flow and amount of flow.1) Relief Valves

If there is no escape route for the hydraulic oil that is continuously pumped out of the hydraulic circuit can burst.A relief valve is a valve that allows the oil in the circuit to escape back into the tank when the pressure exceeds a certain limit in order to prevent the kind of malfunction described above.

2) Pressure Compensated Flow Control ValveThese are flow control valves that maintain a constant pressure difference between the intake and the outlet, so that even if the pressure changes, the flow is constant.These are used in winches, and make more precise inching operation possible.

3) Directional Control Valve

In order to expand the cylinder in the figure, oil is pumped into A; to contract it, oil is pumped into B. The valve that changes the direction of this oil is called a directional control valve.

A directional control valve is a valve that changes the direction in which the oil flows by moving a spool. By changing the direction of oil flow, the direction of a hydraulic cylinder's motion can be changed, or the direction of rotation of a hydraulic motor can be changed.

In a crane, the operator uses a control lever to manipulate the valve. There is also a type of directional control valve called a solenoid valve that is controlled with a magnet(solenoid).

Operation of the crane is performed by using the directional control valve to control the flow of oil. The speed of operation of the actuators changes with the flow of oil.

4) Check Valves

This is a value that allows oil to flow freely in one direction, but stops it from flowing in the opposite direction  (directional control valve). A check valve that has the function of disengaging the return flow prevention is called a pilot check valve. These are installed in the jack cylinder of the outrigger.

When oil flows from left to right, the spring is contracted, the poppet opens and the oil flows freely. The oil cannot flow from right to left.

[Pilot Check Valve]

This valve can be opened and closed as necessary. When there is no pilot pressure, it works just like a regular check valve, allowing flow only in one direction, from the intake to the outlet. When pilot pressure is applied, the spool is raised, the poppet opens, and oil can then flow from the outlet to the intake.

If there is no pilot check valve...The hydraulic oil in the outrigger would flow out and the outrigger would retract.

When a pilot check valve is installed..The check function keeps the hydraulic oil from flowing out. The outrigger remains extended as it is. Even if there is a leak in the circuit, the crane will be prevented from lowering itself due to its own weight.

5) Counterbalance Valve

The counterbalance valve is a flow control valve that controls the fluid returning from an actuator such as a cylinder, so that the actuator does not make, due to the load, any undesired motions exceeding what corresponds to the oil flow from the directional control valve.

Just like the pilot check valve, this valve prevents the crane from lowering itself due to its own weight. In order to stress this function, this valve is also sometimes called a "holding valve".

If there is no counterbalance valve, when lowering a load, the load will accelerate due to its weight, and the speed cannot be controlled.

The counterbalance valves are also used in circuits for boom elevation, boom telescoping and winch operation.

6) Solenoid ValveWhen an electrical signal is input to a solenoid valve(magnetic changeover valve), the drawing force of the solenoid moves the spool, changing the direction of flow. Because the electrical signal is switched at the valve, remote control and automatic control are simple.

The pipes of the circuit carry hydraulic oil from the hydraulic pump to the hydraulic devices. The pipes that carry pressurized oil are as important as arteries and veins are to the human body.

These are many different kinds of, pipes such as steel pipes and high pressure rubber hoses.

Pipes and rubber hoses that can withstand the maximum hydraulic pressures in the relative circuit are used.

(1) Composition of the Hydraulic Hoses Tube Rubber This prevents leakage of the fluid during use.

Wire-reinforced Layer

This prevents expansion and bursting of the inner surface rubber due to pressure.

Inter-layer RubberThis prevents friction between the wires and strengthens the adhensiveness of the wire layer.

String BraidThis prevents damage to the wire layer from outside, and also prevents rusting.

(2) Joints

There are flare joints and flareless joints.

Flare JointsThe tip of pipe is expanded in a lap shape, creating a flare. Then the tapered end of the joint is inserted into the flare and tightened.

Flareless JointThe sleeve that is installed on the pipe is pulled into the other pipe and tightened shut.

If the temperature of the hydraulic oil becomes too hot, many different problems can arise. In order to prevent these problems, when the temperature exceeds a set point, the fluid is cooled by the oil cooler.

Examples of Problems Caused by High Temperature

The precision of hydraulic operations ins diminished due to oil leakage in the control valves and

actuators.

1) Internal leakage in hydraulic devices (especially pumps and motors) increases, and volume efficiency is diminished.

2)Sealing materials used in the packing and oil seals can change their properties.

3)The hydraulic oil will degenerate more quickly.

In case of operation in a hot climate or continuous operation for a long time, the oil cooler prevents temperature increase and mitigating power loss.An air-cooled oil cooler is used in hydraulic cranes.

Generally, the maximum permissible temperature for hydraulic oil is 80C°.When equipment is used in tropical areas, an appropriate hydraulic oil must be chosen.

If the temperature of the hydraulic oil gets too high, the packing can also be adversely affected.TADANO uses materials that are safe up to 110C° for its packing and seals.

Seals are an important part of hydraulic devices.They prevent leakage of oil and contamination by water and foreign matter. Types of seals include o-rings, oil seals, gaskets and packing.

1 Packing Packing prevents oil form leaking out or foreign matter from coming in through small openings that occur as a result of heat expansion or small manufacturing inaccuracies in parts that move against each other. Types of Packing

Packing composed of cotton cloth or asbestos cloth with rubber added

Packing composed of synthetic rubber or leatherSection Shape

U shaped, V shaped and L shaped

Seal [packing] An important part that prevents oil leakage. 

2 O-Rings

These are pieces of synthetic rubber in a ring shape. O-rings with a round section are widely used. They are used in fixed and sliding parts.

Type Shape Features / Uses

U-ShapedPacking

1) Used in internal and external sliding surfaces2) Used for low and high pressure3) Low resistance to sliding motion

V-ShapedPacking

1) Used in internal and external sliding surfaces2) Used for low and high pressure(Depending on pressure, several are applied together.)3) High resistance to sliding motion

L-ShapedPacking

1) Used in external sliding surfaces2) Used for low pressure3) Low resistance to sliding motion

O-ring1) Used in internal and external sliding surfaces2) Used for low and high pressure3) Low resistance to sliding motion

3 Gaskets

Gaskets are seals shaped like flat sheets. They are inserted into the gaps between non-moving parts to maintain air-tightness. They are often used in pipe joints. They can be made of asbestos, synthetic rubber, metal,etc.

The accumulator absorbs the surge pressure that occurs within the hydraulic circuit, decreases the pulsing of the hydraulic pressure, and acts as a temporary source of pressure when the hydraulic pumps is shut off.