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Introduction to Hydraulic Circuit (System)

Hydraulic System:

The controlled movement of parts or a controlled application of force is a common requirement in the industries. These operations are performed mainly by using electrical machines or diesel, petrol, and steam engines as prime movers. These prime movers can provide various movements to the objects by using some mechanical attachments like screw jack, lever, rack and pinions etc. However, these are not the only prime movers. The enclosed fluids (liquids and gases) can also be used as prime movers to provide controlled motion and force to the objects or substances. The specially designed enclosed fluid systems can provide both linear as well as rotary motion. The high magnitude controlled force can also be applied by using these systems. This kind of enclosed fluid-based systems using pressurized incompressible liquids as transmission media are called as hydraulic systems. The hydraulic system works on the principle of Pascal’s law which says that the pressure in an enclosed fluid is uniform in all the directions.

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The hydraulic systems consists a number of parts for its proper functioning. These include storage tank, filter, hydraulic pump, pressure regulator, control valve, hydraulic cylinder, piston, and leak-proof fluid flow pipelines. The schematic of a simple hydraulic system is shown in figure.

It consists of:

a movable piston connected to the output shaft in an enclosed cylinder
storage tank
filter
electric pump
pressure regulator
control valve
leak proof closed-loop piping.

The output shaft transfers the motion or force however all other parts help to control the system. The storage/fluid tank is a reservoir for the liquid used as a transmission media. The liquid used is generally high-density incompressible oil. It is filtered to remove dust or any other unwanted particles and then pumped by the hydraulic pump. The capacity of the pump depends on the hydraulic system design. These pumps generally deliver constant volume in each revolution of the pump shaft. Therefore, the fluid pressure can increase indefinitely at the dead-end of the piston until the system fails. The pressure regulator is used to avoid such circumstances which redirect the excess fluid back to the storage tank. The movement of the piston is controlled by changing the liquid flow from port A and port B. The cylinder movement is controlled by using a control valve which directs the fluid flow. The fluid pressure line is connected to the port B to raise the piston and it is connected to port A to lower down the piston. The valve can also stop the fluid flow in any of the ports. The leak-proof piping is also important due to safety, environmental hazards, and economic aspects. Some accessories such as flow control system, travel limit control, electric motor starter, and overload protection may also be used in the hydraulic systems which are not shown in the figure .

Hydraulic Circuit – Hydraulic Layout Diagram

Components of Hydraulic System

Basic hydraulic system has the following components:
1) Oil reservoir

2) Rotary pump
3) Pressure relief valve

4) Direction control valve
5) Flow control valve

6) Double-acting cylinder
7) Pressure gauge

8) Filter

1) Oil Reservoir

The main function of the “oil reservoir” is to store a sufficient amount of hydraulic oil in the system.

Apart from this, it has other important functions such as:
(a) To cool the hot return oil.
(b) To settle down the contaminants
(c) To remove air bubbles.
(d) To separate water from the oil etc.

2) Rotary pump

The function of the rotary pump is to pump hydraulic oil to the hydraulic circuit’
It converts the mechanical energy (rotation of shaft) into hydraulic energy
A rotary pump is a positive displacement pump. It can deliver constant flow even at high pressure

3) Pressure relief valve

It is an important component which is required for every positive displacement pump
This valve is connected to the outlet of the pump. Its main function is to release the oil back tank when the pressure increases beyond pre-set value.

4) Direction control valve

It controls the direction of flow of oil, by which it performs extension and retraction of the actuator

5) Flow control valve

It Controls the rate of flow of oil by which the speed of extension or retraction of the actuator is controlled.

6) Actuator

Actuator produces work. There are two types, linear actuator and rotary actuator
A linear actuator is called a cylinder, a rotary actuator is called a motor.
A double-acting cylinder develops force and motion. It converts hydraulic energy into mechanical energy

Force developed = Pressure of oil x Area of piston

7) Pressure gauge

It is an important component of the hydraulic system.
It shows the pressure reading.
Pressure settings are made by looking to the pressure gauge.
Without pressure gauge, it is not possible to make the pressure relief valve setting, unloading valve settings etc.

8) Filter

Its main function is to remove suspended solid contaminants from the oil and to provide clean hydraulic oil to the system

Applications of hydraulic systems

1. Machine tools: CNC (computerized numerical control) machines, hydraulic presses, hydraulic shapers, etc.
2. Material handling equipment: Elevators, forklifts, cranes, lifts and hoists etc.
3. Construction field: Earthmoving machines such as excavators, cranes, dozers, loaders, dumpers, tippers, trucks, tractors, etc.
4. Automobiles: Hydraulic brakes, hydraulic steering, hydraulic suspension, hydraulic
clutch, hydraulic power transmission, hydraulic coupling,
5. Material testing laboratory: UTM (universal testing machine) and other destructive testing Machines, BP (burst pressure) testing machine etc.
6. Aerospace: Landing gear, brakes, flight controls (such as), cargo loading door, rudder, elevator, flap, aileron, etc.
7. Railways: Hydraulic brakes, hydraulic steering, hydraulic suspension, hydraulic clutch, hydraulic power transmission, hydraulic coupling hydraulic torque converter, etc.

8. Marine field: Ship steering system, ship-yards, ship-building.
9. Medical equipment: Medical chairs and operating tables.
10. Agricultural equipment: Harvesters, tractors, field sprayers, seeding machine, fertilizer machine etc.

Advantages and Disadvantages of Hydraulic system

Advantages of Hydraulic System :

The basic advantages offered by a hydraulic system are as follows:
l. Hydraulic power is easy to produce, transmit, store, regulate and control, maintain and transform
2. Weight to power ratio of a hydraulic system is comparatively less than that for an electromechanical system. (About 8.5 kg/kw for electrical motors and 0.g5 kg/kw for a hydro system).
3. It is possible to generate high gain in force and power amplification.
4. Hydraulic systems are uniform and smooth, generate step-less motion and variable speed and force to a greater accuracy.
5. The division and distribution of hydraulic power is simpler and easier than other forms of energy.
6. Limiting and balancing of hydraulic forces are easily performed.
7. Frictional resistance is much less in a hydraulic system as compared to a mechanical movement.
8. Hydraulic elements can be located at any place and controlled reversely.
9. The noise and vibration produced by hydraulic pumps is minimal.
10. Hydraulic systems are cheaper if one considers the high efficiency -of power transmission.
I l. Easy maintenance of hydraulic system is another advantage.
12. Hydraulics is mechanically safe, compact, and is adaptable to other forms of power and can be easily controlled.
13. Hydraulic output can be both linear, rotational, and angular. The use of flexible connections in the hydraulic system permits the generation of compound motion without gears etc.

14. Hydraulics is a better over-load safe power system. This can be easily achieved by using a pressure relief valve.
15. Absolutely accurate feedback of load, position, etc. can be achieved in a hydraulic system as in electro-hydraulic and digital electronic servo system. Because of high power and accurate control possibility, in modem engineering language hydraulics is termed as the muscle of the system and electronics its nerves.

Disadvantages of Hydraulic System :

In spite of all the above advantages, hydraulic systems have some drawbacks which are mentioned below.

The disadvantages are:
1. Hydraulic elements have to be machined to a high degree of precision which increases the manufacturing cost of the system.
2. Certain hydraulic systems are exposed to unfriendly climate and dirty atmosphere as the in case of mobile hydraulics like dumpers, loaders, etc.
3. The leakage of hydraulic oil poses problems to hydraulic users.
4. Hydraulic elements have to be specially treated to protect them against rust, corrosion, dirt, etc.
5. Hydraulic oil may pose problems if it disintegrates due to aging and chemical deterioration.
6. Petroleum-based hydraulic oil may pose fire hazards thus limiting the upper level of working temperature. However, due to the availability of synthetic fire-resistant oils this problem is of academic interest nowadays. To combat the environmental effects of petroleum and chemical-based oils, efforts are on to use biodegradable oils now.

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