Hydraulic System – Introduction
Hydraulic systems are power-transmitting assemblies employing pressurized liquid as a fluid for transmitting energy from an energy-generating source to an energy-using point to accomplish useful work. The figure shows a simple circuit of a hydraulic system with basic components. Hydraulic systems are used for transmission of power through the medium of hydraulic oil. The hydraulic system works on the principle of Pascal’s law which says that “ the pressure in a fluid at rest is transmitted uniformly in all directions”.
The fluid medium used is hydraulic oil, which may be mineral oil or water or combinations. This area is also known as oil hydraulics.
Functions of the components
1. The hydraulic actuator
- It is a device used to convert fluid power into mechanical power to do useful work. The actuator may be of the linear type (e.g., hydraulic cylinder) or rotary type(e.g., hydraulic motor) to provide linear or rotary motion, respectively.
- The pressurized hydraulic fluid delivered by the hydraulic pump is supplied to the actuators, which converts the energy of the fluid into mechanical energy. This mechanical energy is used to get the work done.
- TYPES OF ACTUATORS:
1. Linear Actuators (Hydraulic cylinders)
2. Rotary Actuators (Hydraulic motors)
a. Continuous rotary actuators
b. Semi rotary actuators
- Functions Of Actuators :
1) To produce motion in one line
2) To produce continuous rotary motion
3) To produce rotary or oscillatory motion less than 3600
4) To apply a force and clamp the job.
2. The hydraulic Pump
- It is used to force the fluid from the reservoir to the rest of the hydraulic circuit by converting mechanical energy into hydraulic energy.
- A pump which is the heart of a hydraulic system converts mechanical energy into hydraulic energy. The mechanical energy is delivered to the pump via prime mover such as the electric motor. Due to the mechanical action the pump creates a partial vacuum at its inlet. This permits atmospheric pressure to force the fluid through the inlet line and into the pump. The pump then pushes the fluid into the hydraulic system.
Importance of Pump :
1. They convert mechanical energy into hydraulic energy.
2. The Volumetric efficiency of the pump is relatively high
3.They have high-performance characteristics under varying speed and pressure requirements
4.Pumps used to generate high pressure in the hydraulic system
- Valves are used to control the direction, pressure, and flow rate of a fluid flowing through the circuit.
Motor 1 – Off 2 – Forward 3– Return 3 2 1 Load Direction control valve Pump Oil tank Filter Actuator Pressure regulator.
- A fluid power system can be broken down into three segments. The power input segment consisting of the prime mover and the pump. The control segment consisting of valves that control the direction, pressure, and flow rate. The power output segment, consisting of the actuators and the load. This unit is devoted to each of the following categories of control valves.
1. Directional control valves
2. Pressure control valves
3. Flow control valves
- DCVs control the direction of flow in a circuit, which among other things; can control the direction of the actuator. PCVs control the pressure level, which controls the output force of a cylinder or the output torque of a motor. FCVs control the flow rate of the fluid which controls the speed of the actuators.
Different types of valves and their functions :
- Pressure relief valves – Relief valve opens and bypasses fluid when pressure exceeds its setting. These are used mostly in all circuits.
- Pressure-Reducing Valve – This type of valve (which is normally open) is used to maintain reduced pressures in specified locations of hydraulic systems.
- Unloading Valves – high-low pump circuits where two pumps move an actuator at a high speed and low pressure, punching press.
- Counterbalance valves – They are used to prevent a load from accelerating uncontrollably. This situation can occur in vertical cylinders in which the load is a weight. This can damage the load or even the cylinder itself when the load is stopped quickly at the end of the travel.
4. External power supply (motor) is required to drive the pump.
5. Oil Tank or Reservoir:
- This is an oil storage tank in which hydraulic oil is stored. The oil passes through various pipelines and after doing useful work in actuator; the oil returns to the oil tank. In the regions of low temperature, oil heaters are attached to air tanks.
- Reservoir is used to hold the hydraulic liquid, usually hydraulic oil.
- Pipelines (Fluid Conducting elements): It is the functional connection for oil flow in the hydraulic system. The efficiency of oil flow is greatly influenced by the physical characteristics of piping systems.
- There are two pipes:
a) The pipe which carries pressurized oil is called pressure pipelines
b) Pipes that carry low pressurized oil or used oil (are called as return pipelines).
- Hoses, pipes, pipe fitting are the parts of the fluid power pipeline.
- It is used to remove any foreign particles so as keep the fluid system clean and efficient, as well as avoid damage to the actuator and valves.
- When hydraulic fluids are contaminated, hydraulic systems may get damaged and malfunction due to clogging and internal wear. They require filtration to remove contaminants.
- Filters are classified as
i. Reservoir filters:
ii. Line filters
iii. Off-line filters
iv. Other cleaning equipment
- Functions of Filter:
1) Take care of the cleanliness of the components.
2) Reduce the maintenance.
3) To remove silting.
4) To increase the system reliability.
5) To prevent the entrance of solid contaminants to the system.
8. Pressure regulator
- Pressure regulator regulates (i.e., maintains) the required level of pressure in the hydraulic fluid.
- The piping is shown in Fig. is of closed-loop type with fluid transferred from the storage tank to one side of the piston and returned from the other side of the piston to the tank. Fluid is drawn from the tank by a pump that produces fluid flow at the required level of pressure. If the fluid pressure exceeds the required level, then the excess fluid returns back to the reservoir and remains there until the pressure acquires the required level.
- Accumulators are devices that store hydraulic fluid under pressure. Storing hydraulic fluid under pressure is a way of storing energy for later use. Perhaps the most common application for an accumulator is supplementing the pump flow in a hydraulic system in which a high flow rate is required for a brief period of time.
- Types of Accumulators ;
1. Weight loaded accumulator
2. Spring-loaded accumulator
3. Gas-charged accumulator
4. Piston type
5. Bladder type
6. Diaphragm type
10. Hydraulic Power Pack :
- The hydraulic power unit (power supply unit) provides the energy required for the hydraulic installation.
- The main components of power packs are – The reservoir (tank), Drive (electric motor), Hydraulic pump, Pressure relief valve, filter, and cooler.
- The pump or motor unit may be mounted on the tank or separately a packs are usually available in either horizontal or vertical configurations. The basic unit may be piped to the cylinders or actuators through a suitable control valve.
- The hydraulic power packs consist of a reservoir/tank that house the hydraulic fluid, which is the working medium.
Working of Hydraulic Power Pack :
- The working of a power pack commences when the pump is initialized with the help of an electric motor coupled to it. The oil is pumped from the reservoir along the suction line through a suction strainer with a capability to retain the foreign particles up to 149
- From the suction line the oil is forced into the pressure line through the pump at 35 bars. There is provision to measure the pressure, with the help of a pressure gauge. An isolator is used to measure the pressure immediately in any line.
- When the set of pressure is reached, the fluid moves to the cylinder present at the fixture (clamp). The hydraulic energy of the fluid is converted back to the mechanical energy by the cylinder.
- According to the direction of the energizing of the solenoid valve, the linear movement of the clamps (clamping and unclamping) is controlled. When the solenoid valve is energized in reverse, the unclamping of the workpiece occurs. There is a return line provided so that the used fluid may be utilized again. Due to the friction losses, the total energy is not converted into useful work so a part is converted into the heat. So, a heat exchanger is incorporated. The return line filter has a return capacity of 10 microns.
Cylinder movement is controlled by a three-position change over a control valve.
1. When the piston of the valve is changed to the upper position, the pipe pressure line is connected to port A and thus the load is raised.
2. When the position of the valve is changed to a lower position, the pipe pressure line is connected to port B and thus the load is lowered.
3. When the valve is at the center position, it locks the fluid into the cylinder(thereby holding it in position) and dead-ends the fluid line (causing all the pump output fluid to return to tank via the pressure relief).
Some questions related to hydraulic circuit :
Draw a sketch of a simple oil hydraulic circuit and write down the name and working function of each of the components used in it.
Basic Hydraulic Circuit Diagram :
a) Oil Tank or Reservoir: This is an oil storage tank in which hydraulic oil is stored. The oil passes through various pipelines and after doing useful work in actuator; the oil returns back to the oil tank. In the regions of low temperature, oil heaters are attached to air tanks.
b) Filter: This element filters the oil before going to the next element i.e. pump.
c) Pump: Hydraulic pump is the heart of any hydraulic system. Its main function is to create the flow of oil under pressure through the entire hydraulic system and hence to assist the transfer of power and motion (i.e. useful work).
d) Direction control valves/Flow control valves/ Pressure Relief Valves (Fluid Controlling Elements): These valves are fitted in hydraulic system at particular locations. These valves control the flow of oil in the system. They also direct the flow of oil in the system as also they control the speed of the actuator.
e) Actuators: (Fluid Power utilization elements): These elements are known as actuators (either rotary or linear). The pressurized oil acts on actuator elements. The oil gives or transfers its power to the actuator to create useful work or Mechanical Advantage.
f) Pipelines (Fluid Conducting elements): It is the functional connection for oil flow in the hydraulic system. The efficiency of oil flow is greatly influenced by the physical characteristics of piping systems.
There are two pipes:
a) Pipe which carries pressurized oil is called as pressure pipelines
b) Pipes that carry low pressurized oil or used oil (are called as return pipelines). Hoses, pipes, pipe fitting are the parts of the fluid power pipeline.
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