What is Hydraulics or Explain Hydraulics?
Transmission & control of forces & movements by means of fluids is called hydraulics.Fluids under pressure can be used for Power Transmission. Fluids means gases (air) and liquids (oil or water etc). The system which uses air as working medium is called pneumatics and which uses oil/water is called Hydraulic system.
What is Pressure?
Force is the effort required to do the work. Pressure means force exerted per unit area,
generally measured in psi, or kg/sq cm, or bars*.
What is Atmospheric Pressure?
At sea level the whole column of atmospheric air exerts a weight or force of 14.7 pounds for every square inch i.e. a pressure of 14.7psi or 1.03kg/sqcm. This is called atmospheric pressure.
1 Atmospheric Pressure = 1.03 Bar =14.7 psi
Flow & Pressure are inter-related. Flow is responsible for causing the motion of piston in a cylinder. It is the movement of hydraulic fluid caused by a difference in pressure at two points. When we open the kitchen tap the pressure difference (between the water tank at height and tap) pushes the water out, or causes the water to flow. In a hydraulic system flow is usually produced by the action of the hydraulic pump. If the pressure is not sufficient to take the load on the cylinder, it will not move.
Notice:
1. Oil is most commonly used hydraulic fluid, because it acts as lubricant for all moving parts of hydraulic system.2. Generally the weight of hyd. Oil is around 55-58 pounds/cubic feet. One foot of oil causes a pressure of 0.4 psi. A 10 m column of water causes a pressure of 1 kg/sq cm.
3. There must be a pressure drop across an orifice/restriction to cause flow. If there is no flow, there is no pressure drop and vice versa.
4. Force exerted by a cylinder is dependent on pressure of oil supplied & piston area
5. Speed of the cylinder is dependent on piston area and the rate of fluid flow into it.
6. Fluid velocity through a pipe varies inversely to the square of inside diameter.
7. Friction in pipes results in pressure drop
8. Air is compressible, where as oil is incompressible practically.
9. Pump only transfers the fluid. It is the resistance which develops pressure.
10. It is the atmospheric pressure which is responsible for pushing of oil from tank to the suction chamber of the pump.
What is Pascal’s Law?
PRESSURE APPLIED ON A CONFINED FLUID IS TRANSMITTED UNDIMINISHED IN ALL DIRECTIONS AND ACTS WITH EQUAL FORCE ON EQUAL AREAS AND AT RIGHT ANGLES TO THEM (If a force F is applied on a piston of area A, (over a confined fluid ) then it gives a pressureP = F/A.
This pressure will be uniform in the entire confined fluid at rest.
Explain Hydraulic Press (BRAMAH PRESS):
Since pressure in the confined fluid is uniform throughout and by applying this pressure on large areas large forces can be developed. This is the starting point for development of Hydraulics (see the fig below).If two cylindrical chambers which are connected and fitted with pistons of area A1, A2 and if a Force F1 acts on piston of area A1, it develops a pressure p in the confined fluid. This pressure will be uniform in the entire fluid in double cylinder arrangement, and develops a force F2= P x A2. Hence forces will be proportional to the area of the pistons. There is no energy creation and work done will be same by both the pistons.
What is Bernoulli’s Principle?
This is nothing but law of conservation of energy. If the flow rate is constant, the totalenergy at any point of continuous path of flowing fluid is same as at any other point. (Sum
of motion energy, pressure energy, and potential energy is constant.).To know the pressure
or flow velocity at any point in the circuit, this principle is used widely.
Advantages of Hydraulic Systems:
Due to limitations of other power transmission system such as electrical, electromechanical and pneumatic etc. hydraulic power transmission is preferred. Large forces can be transmitted to long distances with high pressure stability and quick response. There are multiple application possibilities which is suitable for use where large forces with infinitely variable speeds are to be applied in given directions. Hydraulic equipments give smooth operation for longer period with very less maintenance cost. Normally oil contamination control and leakage control may give long life to hydraulic components.Some Other advantages:
1. Highly compact- Power to weight ratio is very high. A hydraulic motor weighs about 1/7 th of an electric motor of same power
2. Precise control- depending on different requirements we can get exact speed, force and position of user,
3. Over load protection- in case there is over load in pipe line or user, there is provision of relief valve set at a certain maximum pressure to take care of it,
4. Suspension of load for long period- by providing a pilot operated non-return valve in pipeline, load may be suspended for a longer period,
5. Flexibility in design- As per needs of production, scheme of hydraulic circuit may be changed easily only with addition of a few components,
6. Easy maintenance- its maintenance is easy. Only oil contamination control will fulfill major portion of maintenance work. For this monitoring of set parameters and inspections of pipe lines, religiously is necessary
7. Variable Speed Controls: - We can get infinitely variable speeds and positions as per need of users.
8. Stalling of loads:- The loads can be stalled to zero speed without any damage to the equipments
SIMPLE HYDRAULIC CIRCUIT (OPEN CIRCUIT)
Here we have a hydraulic system in its most simple form. A pump 1 with fixed flow sucksfluid from a tank 2 and feeds it into the system connected to it. In zero position of the
manually operated direction control valve the hydraulic fluid, circulates almost without
pressure from the pump to the tank 2. The dc valve is spring centered. When the dc valve 4
is operated into its left switching position, (parallel arrows) fluid reaches the piston
chamber of cylinder 5. The piston rod travels outwards. The speed of the outward travel
depends on the pump flow and the cylinder size (piston area). The force available at the
piston rod is dependent on the piston area and the maximum system pressure. The
maximum system pressure and thus the loading of the hydraulic system is set at the
pressure relief valve 3. The actual pressure available, determined by the resistance to be
overcome at the user, can be read at the pressure gauge 6.
