Hydraulic principle

ntroduction:

Earlier, weights were lifted using pulleys, levers, block and tackles, etc. Movements for a ship’s rudder or steering a vehicle where achieved by mechanical linkages like cams, levers, couplings, and gears which made the system complicated. These manual or mechanical methods of operation had several limitations. They also involved huge man power and long working hours for a particular job. As the population and technology increased exponentially, the demand for quicker and easier to operate equipment increased. To cater to this need, hydraulic machines were introduced.

Basic Hydraulic Principles

A simple hydraulic system consists of hydraulic fluid, pistons or rams, cylinders, accumulator or oil reservoir, a complete working mechanism, and safety devices. These systems are capable of remotely controlling a wide variety of equipment by transmitting force, carried by the hydraulic fluid, in a confined medium. Modern developments in hydraulics have involved many fields in engineering and transportation. These systems transfer high forces rapidly and accurately even in small pipes of light weight, small size, any shape, and over a long distance. These systems play a vital role from small car's steering to super sonic aircraft’s maneuvering devices. More powerful and accurate systems are also used in maneuvering huge ships.

Pascal’s Law:

Pascal's Law, framed by Blaise Pascal, states that “Pressure applied to any part of a confined fluid transmits to every other part with no loss. The pressure acts with equal force on all equal areas of the confining walls and perpendicular to the walls." This is the basic principle for any hydraulic system.

Hydraulic Pressure & Force:

Pressure can be defined as “the force acting on unit area, applied in a direction perpendicular to the surface of the object”.

Pressure = Force/ Area.

So, hydraulic pressure can be stated as the force exerted by a fluid on unit area, anywhere on the surface within the container.

A Simple Hydraulic System:

A closed toothpaste tube can be considered as an example for a simple hydraulic system. The toothpaste can be considered as a hydraulic fluid working inside the system which is confined. Four or five holes are made on the tube. By keeping the cap of the toothpaste tube closed, apply pressure at a particular point on the tube. This makes the toothpaste to come out from all the holes evenly. This is a simple example to understand Pascal’s law. Thus force applied at a particular point on a fluid in closed system, transfers equal force on all other parts of the system. Further, the force acts perpendicular to the walls of the confined area.

Other example is the hydraulic brakes used in automobiles. The driver applies force at the break pedal (one particular point on fluid in closed system), thus transmitting this force onto the hydraulic fluid under the piston (break pedal), transfers equal force to all other brake shoes, thus stopping the vehicle.

There are several other areas where hydraulics are applied. They are:

1. Automobile garage

2. Petrol pumps

3. Measuring weights of heavy-lift trucks

4. Hydraulic cranes

5. Automobile steering gears

6. Automobile brake, (disc brakes)

7. Ship's steering gear

8. Robotics

9. Aircraft's rudder and other maneuvering systems

10. Industries and power plants

11. Servo machanisms and control systems etc.

Also, this same principle is used in hydraulic lifts and other machineries. In the next article we will discuss how hydraulic presses are used in industries.

Read more: http://www.brighthub.com/engineering/civil/articles/43171.aspx#ixzz1I3pfN5wd

CFL Hazard for information

for information .

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Basic hydraulic fiiting

Lines
	-continuous line - flow line
	-dashed line - pilot, drain
	-envelope - long and short dashes around two or more component symbols.

Circular -large circle - pump, motor -small circle - Measuring devices -semi-circle - rotary actuator Square -one square - pressure control function -two or three adjacent squares - directional control Diamond -diamond - Fluid conditioner (filter, separator, lubricator, heat exchanger) Miscellaneous Symbols -Spring -Flow Restriction Triangle -solid - Direction of Hydraulic Fluid Flow -open - Direction of Pnematic flow Pumps and Compressors Fixed Displacement hydraulic pump -unidirectional -bidirectional Variable displacement hydraulic pump -unidirectional -bidirectional Compressor Motors Fixed displacement hydraulic motor -unidirectional -bidirectional Variable displacement hydraulic motor -unidirectional -bidirectional Pneumatic motor -unidirectional -bidirectional Rotary Actuator - hydraulic - pneumatic Cylinders Single acting cylinder -returned by external force -returned by spring or extended by spring force Double acting cylinders -single piston rod (fluid required to extend and retract) -double ended piston rod Cylinders with cushions - single fixed cushon - double fixed cushion - single adjustable cushion - double adjustable cushion Directional Control Valves Directional control valve (2 ports / 2 positions) -Normally closed directional control valve with 2 ports and 2 finite positions. -Normally open directional control valve with 2 ports and 2 finite positions. Directional control valve (3 ports / 2 positions) -Normally closed directional control valve with 3 ports and 2 finite positions. -Normally open directional control valve with 3 ports and 2 finite positions. Directional control valve (4 ports / 2 positions) -directional control valve with 4 ports and 2 finite postions Directional control valve (4 ports / 3 positions) -directional control valve with 4 ports and 3 finite postions *-(center position can have various flow paths) Directional control valve (5 ports / 2 positions) Normally a pneumatic valve -directional control valve with 5 ports and 2 finite postions Directional control valve (5 ports / 3 positions) Normally a pneumatic valve -directional control valve with 5 ports and 3 finite postions Proportional directional control valve Electro-hydraulic servo valve -The spool positions on these valves is variable allowing for variable flow conditions. -single-stage direct operation unit which accepts an analog signal and provides a similar analog fluid power output -two-stage with mechnical feedback indirect pilot operation unit which accepts an analog signal and provides a similar analog fluid power output Control Methods Manual Control -general symbol (without showing the control type) -pushbutton -lever -foot pedal Mechanical Control -plunger or tracer -spring -roller -roller(one direction only) Electrical Control -Solenoid (the one winding) Pilot Operation -pneumatic -hydraulic Pilot operated two-stage valve -Pneumatic: Sol first stage -Pneumatic: Air pilot second stage -Hydraulic: Sol first stage -Hydraulic: Hyd pilot second stage Check valves, Shuttle valves, Rapid Exhaust valves -check valve -free flow one direction, blocked flow in other direction -pilot operated check valve, pilot to close -pilot operated check valve, pilot to open Shuttle valve -to isolate one part of a system from an alternate part of circuit. Rapid exhaust valve/Pneumatic -installed close to an actuator for rapid movement of the actuator. Pressure Control Valves Pressure Relief Valve(safety valve) normally closed - line pressure is limited to the setting of the valve, secondary part is directed to tank. Proportional Pressure Relief - line pressure is limited to and proportional to an electronic signal Sequence Valve - when the line pressure reaches the setting of the valve, valve opens permitting flow to the secondary port. The pilot must be externally drained to tank. Pressure Reducing valve - pressure downstream of valve is limited to the setting of the valve Flow Control Valves Throttle valve -adjustable output flow Flow Control valve -with fixed output (variations in inlet pressure do not affect rate of flow) -with fixed output and relief port to reservoir with relief for excess flow (variations in inlet pressure do not affect rate of flow) -with variable output -fixed orifice -metered flow toward right free flow to left -pressure compensated flow control fixed output flow regardless of load -pressure and temperature compensated -with variable output and relief port to reservoir Flow dividing valve -flow is divided equally to two outputs. Shut-Off Valve -Simplified symbol Accumulators Filters, Water Traps, Lubricators and Miscellaneous Apparatus Filter or Strainer Water Trap -with manual drain -with automatic drained Filter with water trap -with manual drain -automatic drain Air Dryer refrigerant, or chemical removal of water from compressed air line Lubricator -oil vapor is indected into air line Conditioning unit -compound symbol of filter, regulator, lubricator unit -Simplified Symbol Heat Exchangers -air or water cooled unit designed to remove heat from oil returning to reservoir