A
hydraulic or
hydrostatic drive system or
hydraulic power transmission is a drive or transmission system that uses hydraulic fluid under pressure to drive machinery. The term hydrostatic refers to the transfer of energy from flow and pressure, not from the
kineticKinetic may refer to:*Kinetic *Kinetic art*Kinetic, Seiko's trademark for its automatic quartz technology.*Kinetic theory*Kinetic energy*A projectile, which is a type of kinetic weapon....
energy of the flow.
Such a system basically consists of three parts. The generator (e.g. a
hydraulic pumpHydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic.Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more...
, driven by an
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
, a combustion
engineAn engine is a machine that produces mechanical force and motion from another form of energy . It is also referred to as a prime mover. An automobile makes use of several motors to start the car and drive the car's various pumps – but the power plant that propels the car is called an engine...
or a
windmillA windmill is a machine which translates linear motion of wind to rotational motion by means of adjustable vanes called sails. The main use is for a grinding mill powered by the wind, reducing a solid or coarse substance into pulp or minute grains by crushing, grinding, or pressing...
); valves, filters, piping etc.
A
hydraulic or
hydrostatic drive system or
hydraulic power transmission is a drive or transmission system that uses hydraulic fluid under pressure to drive machinery. The term hydrostatic refers to the transfer of energy from flow and pressure, not from the
kineticKinetic may refer to:*Kinetic *Kinetic art*Kinetic, Seiko's trademark for its automatic quartz technology.*Kinetic theory*Kinetic energy*A projectile, which is a type of kinetic weapon....
energy of the flow.
Such a system basically consists of three parts. The generator (e.g. a
hydraulic pumpHydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic.Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more...
, driven by an
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
, a combustion
engineAn engine is a machine that produces mechanical force and motion from another form of energy . It is also referred to as a prime mover. An automobile makes use of several motors to start the car and drive the car's various pumps – but the power plant that propels the car is called an engine...
or a
windmillA windmill is a machine which translates linear motion of wind to rotational motion by means of adjustable vanes called sails. The main use is for a grinding mill powered by the wind, reducing a solid or coarse substance into pulp or minute grains by crushing, grinding, or pressing...
); valves, filters, piping etc. (to guide and control the system); the motor (e.g. a
hydraulic motorA Hydraulic motor is a mechanical actuator that converts hydraulic pressure and flow into torque and angular displacement . The hydraulic motor is the rotary counterpart of the hydraulic cylinder....
or
hydraulic cylinderA Hydraulic cylinder is a mechanical actuator that is used to give a linear force through a linear stroke. It has many applications, notably in engineering vehicles.- Operation :...
) to drive the machinery.
Principle of a hydraulic drive
Pascal's law is the basis of hydraulic drive systems. As the pressure in the system is the same, the force that the fluid gives to the surroundings is therefore equal to pressure x area. In such a way, a small piston feels a small force and a large piston feels a large force.
The same counts for a hydraulic pump with a small swept volume, that asks for a small
torqueTorque, also called moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
, combined with a hydraulic motor with a large sweptvolume, that gives a large torque.
In such a way a transmission with a certain ratio can be built.
Most hydraulic drive systems make use of hydraulic cylinders. Here the same principle is used- a small torque can be transmitted in to a large force.
By throttling the fluid between generator part and motor part, or by using hydraulic pumps and/or motors with adjustable swept volume, the ratio of the transmission can be changed easily. In case throttling is used, the efficiency of the transmission is limited; in case adjustable pumps and motors are used, the efficiency however is very large. In fact, up to around 1980, a hydraulic drive system had hardly any competition from other adjustable (electric) drive systems.
Nowadays electric drive systems using electric servo-motors can be controlled in an excellent way and can easily compete with rotating hydraulic drive systems. Hydraulic cylinders are in fact without competition for linear (high) forces. For these cylinders anyway hydraulic systems will remain of interest and if such a system is available, it is easy and logical to use this system also for the rotating drives of the cooling systems.
Hydraulic cylinder
Hydraulic cylinderA Hydraulic cylinder is a mechanical actuator that is used to give a linear force through a linear stroke. It has many applications, notably in engineering vehicles.- Operation :...
s (also called linear hydraulic motors) are mechanical actuators that are used to give a linear force through a linear stroke. A hydraulic cylinder is without doubt the best known hydraulic component. Hydraulic cylinders are able to give pushing and pulling forces of millions of metric tons, with only a simple hydraulic system. Very simple hydraulic cylinders are used in presses; here the cylinder consists out of a volume in a piece of iron with a plunger pushed in it and sealed with a cover. By pumping hydraulic fluid in the volume, the plunger is pushed out with a force of plunger-area * pressure.
More sophisticated cylinders have a body with end cover, a piston-rod with piston and a cylinder-head. At one side the bottom is for instance connected to a single clevis, whereas at the other side, the piston rod also is foreseen with a single clevis. The cylinder shell normally has hydraulic connections at both sides. A connection at bottom side and one at cylinder head side. If oil is pushed under the piston, the piston-rod is pushed out and oil that was between the piston and the cylinder head is pushed back to the oil-tank again.
The pushing or pulling force of a hydraulic cylinder is:
F = Ab * pb - Ah * ph
F = Pushing Force in N
Ab = (π/4) * (Bottom-diameter)^2 [in m2]
Ah = (π/4) * ((Bottom-diameter)^2-(Piston-rod-diameter)^2)) [in m2]
pb = pressure at bottom side in [N/m2]
ph = pressure at cylinder head side in [N/m2]
Apart from miniature cylinders, in general, the smallest cylinder diameter is 32 mm and the smallest piston rod diameter is 16 mm.
Simple hydraulic cylinders have a maximum working pressure of about 70 bar, the next step is 140 bar, 210 bar, 320/350 bar and further, the cylinders are in general custom build. The stroke of a hydraulic cylinder is limited by the manufacturing process. The majority of hydraulic cylinders have a stroke between 0,3 and 5 metres, whereas 12-15 metre stroke is also possible, but for this length only a limited number of suppliers are on the market.
In case the retracted length of the cylinder is too long for the cylinder to be build in the structure. In this case telescopic cylinders can be used. One has to realize that for simple pushing applications telescopic cylinders might be available easily; for higher forces and/or double acting cylinders, they must be designed especially and are very expensive.
If hydraulic cylinders are only used for pushing and the piston rod is brought in again by other means, one can also use plunger cylinders. Plunger cylinders have no sealing over the piston, or the piston does not exist. This means that only one oil connection is necessary. In general the diameter of the plunger is rather large compared with a normal piston cylinder, because this large area is needed.
Whereas a hydraulic motor will always leak oil, a hydraulic cylinder does not have a leakage over the piston nor over the cylinder head sealing, so that there is no need for a mechanical brake.
Common faults
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Hydraulic motor
The hydraulic motor is the rotary counterpart of the
hydraulic cylinderA Hydraulic cylinder is a mechanical actuator that is used to give a linear force through a linear stroke. It has many applications, notably in engineering vehicles.- Operation :...
.
Conceptually, a hydraulic motor should be interchangeable with
hydraulic pumpHydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic.Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more...
, because it performs the opposite function -- much as the conceptual DC
electric motorAn electric motor uses electrical energy to produce mechanical energy, usually through the interaction of magnetic fields and current-carrying conductors. The reverse process, producing electrical energy from mechanical energy, is accomplished by a generator or dynamo. Traction motors used on...
is interchangeable with a DC
electrical generatorIn electricity generation, an electrical generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities...
. However, most hydraulic pumps cannot be used as hydraulic motors because they cannot be backdriven. Also, a hydraulic motor is usually designed for the working pressure at both sides of the motor. Another difference is that a motor can be reversed by a reversing valve.
Another factor affecting the operation of hydraulic motors is fluid flow rate. Pressure in a hydraulic system is like the voltage in an electical system and fluid flow rate is the equivalent of current. Pressure provides the force and flow rate the speed. The size of the pump decides the flow rate not just the pressure.
Hydraulic valves
These valves are usually very heavy duty to stand up to high pressures. Some special valves can control the direction of the flow of fluid and act as a control unit for a system.
Open and closed systems
A open system is one where the hydraulic fluid is returned into a large unpressurised tank at the end of a cycle through the system. In contrast a closed system is where the hydraulic fluid stays in one closed pressurised loop without returning to a main tank after each cycle. See open and closed systems.
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