HYDRAULICS IT IS THE SCIENCE OF CONVEYANCE OF LIQUID THROUGH PIPES
IT IS AN APPLIED SCIENCE .
DEFINED AS TRANSMISSION & CONTROL OF FORCES AND MOVEMENT BY MEANS OF FLUIDS.
FLUID MECHANICS • IT IS SCIENCE OF BEHAVIOUR OF ANY NON-COMPRESSIBLE SUBSTANCES, • IN MOTION OR AT REST. • IT CAN BE GASES OR LIQUIDS.
HYDRAULICS HYDROSTATICS
HYDROSTATICS :
HYDRODYNAMICS
SCIENCE IS USED WHILE CONSTRUCTING A DAM. DEALS WITH VARIOUS FORCES COMING DUE TO STORAGE OF WATER.
• PASCALS LAW :
IT TELLS BASIC
PRINCIPLE OF HYDRAULICS CONNECTED HYDROSTATICS.
WITH
“WHEN CERTAIN PRESSURE IS APPLIED AT ANY POINT IN A FLUID AT REST, THE PRESSURE IS EQUALLY TRANSMITTED IN ALL THE DIRECTIONS WITH EQUAL FORCE ON EQUAL AREAS AND AT RIGHT ANGLES TO THEM”
• IN 1650,BLAISE PASCAL MADE THIS IMPORTANT DISCOVERY. THIS MEANS FOR EXAMPLE WATER AT GIVEN DEPTH IN A CONTAINER IS PUSHING NOT ONLY DOWNWARDS BUT ALSO SUCH A WAY THAT THE PRESSURE IN ALL DIRECTIONS ARE EQUAL. THIS DISCOVERY LED TO THE INVENTION OF NUMEROUS HYDRAULIC DEVICES INCLUDING HYDRAULIC PRESS
HYDRAULIC PRESS • VARIOUS USES : STARTING FROM THE BRAKE OF A CAR TO MOVEMENT OF WING FLAPS OF AN AEROPLANE
WHEN CRITICAL APPLICATIONS ARE INVOLVED STANDBY PRESSES USED WITH A COMPLETE STANDBY PUMP,PIPE LINES,CYLINDER ETC. IN CASE THE PRIMARY SYSTEM FAILS DUE TO LEAKAGE OR ANY PROBLEM THE SYSTEM SWITCHES OVER TO STANDBY.
HYDRODYNAMICS
• THIS DEALS WITH MOVING FLUIDS. • MOST IMPORTANT THEORY IS BERNOULLI’S THEOREM.
• DANIEL BERNOULLI STATED THAT • ANY POINT IN A PIPE THOROUGH WHICH
WATER IS FLOWING,THE SUM OF PRESSURE ENERGY, KINETIC ENERGY AND POTENTIAL ENERGY IS CONSTANT.
• p/w + v² / 2g + z = constant w
= SP. WEIGHT, IN KG/CUB.M,
w
= ρg
FLOW CONGFIGURATION
LAMINAR
TURBULENT
THE EXISTENCE OF TWO TYPES OF FLOW WAS FIRST DEMONSTRATEDBY OSBORNE REYNOLDS IN 1883 WITH FOLLOWING EXPERIMENT
X
LAMINAR FLOW
X
TURBULENT FLOW
X
TURBULENT FLOW
IN INDUSTRY MOSTLY THE TERM HYDRAULICS MEANS SOMETHING WHICH DEALS WITH OIL
ADVANTAGES OF HYDRAULIC SYSTEMS
• SYSTEM IS AUTOMATICALLY LUBRICATED UNLIKE MECHANICAL SYSTEM WHERE EXTERNAL LUBRICATION IS REQUIRED.
• DEVELOPS HIGH POWER WITH LESS
AMOUNT OF OIL ALSO.POWER/WEIGHT RATIOS HIGH
• SIMPLE DESIGN WITH FEW PARTS.LIKE
HYDRAULIC CYLINDER OR FLUID COUPLING.
• EASE OF CONTROL / SMOOTH CONTROL / PRECISE CONTROL / BETTER SPEED CONTROL FAST CONTROL.
• FORCE MULTIPLICATION WITHOUT LEVERS /
GEARS / PULLEYS.ONLY WITH INCREASE IN OIL PRESSURE AUTOMATICALLY THE FORCE INCREASES.
f
F
a= 2 sq.cm. , A= 500 sq.cm. , F= 2000 kgf f = P X a = F / A X a= 2000/500 X 2 = 8 kgf M.A = 250
LEVER
f
F PULLEYS
GEARS
• LOSSES ARE LESS COMPARED TO OTHER MEANS OF CONTROL. LIKE IN CASE OF A FLUID COUPLING SLIP IS MAXIMUM 1% TO 2%
• INBUILT SAFETY CAN BE PROVIDED. LIKE CONNECTING SUCTION AND DISCHARGE INCASE OF HIGH PRESSURE,FUSIBLE PLUG IN CASE OF A FLUID COUPLING
• JERKS CAN BE AVOIDED (BY PROVIDING DAMPING DEVICE).
• CONTROL ACTION OR FULL CLOSE/FULL OPEN ACTION CAN BE PROVIDED AS PER REQUIREMENT.
• CAN BE USED IN HOT, HAZARDOUS,POISONOUS AND UNDERWATER AREAS.
• IT CAN BE USED WHERE VERY FAST ACTION IS REQUIRED.
FUNDAMENTAL PRINCIPLES • PASCAL’S LAW • FORCE EXERTED BY A PISTON CAN BE DETERMINED BY F=PXA
• TO DETERMINE VOLUME REQUIRED TO MOVE A PISTON A GIVEN DISTANCE : V=A X L, A= PISTON AREA , L=STROKE LENGTH
• HYDRAULIC OIL IS PRACTICALLY NONCOMPRESSIBLE. 0.4% AT 1000 PSI AT TEMP 40 DEG C 1% AT 2000 PSI 1.1% AT 3000 PSI
• THERE MUST BE A PRESSURE DROP ACROSS AN ORIFICE OR RESTRICTION TO CAUSE A FLOW THROUGH IT OR IF THERE IS NO PRESSURE DROP THERE IS NO FLOW.
• FLOW VELOCITY THROUGH A PIPE VARIES INVERSELY AS THE SQUARE OF INSIDE DIAMETER.REDUCING DIA BY HALF,VELOCITY INCREASES FOUR TIMES.
• FRICTION LOSS (PRESSURE DROP) IN A PIPE IS DIRECTLY PROPORTIONAL TO SQUARE OF VELOCITY.
Hydraulic Principle I : Movement depends on Flow • FOR ANYTHING TO MOVE IN A HYDRAULIC MACHINE, THE ACTUATORS MUST BE SUPPLIED WITH FLUID FLOW. NOTHING MOVES WITHOUT FLOW.
THE CYLINDER SHOWN IN FIG. IS RETRACTED. IT CAN EXTEND ONLY IF THERE IS FLOW INTO PORT A.IF THERE IS NO FLOW INTO PORT A,THE PISTON WILL NOT MOVE
Port A
Port B
Hydraulic Principle II : Rate of Flow Determines Speed • ACTUATOR SPEED DEPENDS UPON RATE OF FLOW.THE FASTER FLUID FILLS THE CYLINDER,THE FASTER THE PISTON WILL MOVE.THE FASTER FLUID FILLS THE EXPANDING SPACES IN A HYDRAULIC MOTOR, THE FASTER THE SHAFT WILL TURN.
WHAT ARE THE POSSIBLE CAUSES OF SLOW ACTUATOR SPEED?ANYTHING WHICH REDUCES FLOW RATE IN OR OUT OF THE CYLINDER. FOR EXAMPLE
SPEED
• PUMP FLOW OUTPUT MAY BE LOW.IF THE PUMP SHOULD BE PRODUCING 10 GALLONS PER MINUTE (0.631 LITRES/SECOND),AND IS PRODUCING ONLY 6 GPM (0.379 L/S),ACTUATOR SPEED WILL BE SLOW.
5 GPM (0.318 l/s)
SPEED
• IN THIS CASE, ACTUATOR SPEED WOULD BE AFFECTED IN BOTH DIRECTIONS.
10 GPM (0.631 l/s)
• THE DIRECTIONAL
CONTROL VALVE SPOOL MAY NOT HAVE SHIFTED ALL THE WAY.INCOMPLETE SHIFTING WILL RESTRICT FLOW THROUGH THE VALVE,REDUCING THE FLOW RATE DELIVERED TO THE ACTUATOR.
T
A
P
B
T
A
P
B
• NOTICE THAT IN THIS
CASE SPEED MAY BE AFFECTED IN ONE DIRECTION ONLY.IF THE VALVE SHIFTS FULLY THE OTHER WAY, CYLINDER SPEED WILL BE NORMAL IN THE OTHER DIRECTION.
HYDRAULIC PRICIPLE III : Load Determines Pressure • THE PRESSURE DEVELOPED IN AN ACTUATOR DEPENDS ON THE LOAD.THAT IS,PRESSURE WILL RISE UNTIL THE FORCE IT EXERTS ON THE PISTON IN A CYLINDER CAN MOVE THE LOAD.THE GREATER THE LOAD,THE HIGHER THE PRESSURE WILL RISE.
PRESSURE IN AN ACTUATOR WILL RISE UNTIL SOMETHING MOVES. NORMALLY THE LOAD WILL MOVE. IF THIS LOAD DOES NOT MOVE,IT IS BECAUSE EITHER
50 PSI (344 KPa)
150 PSI (1.034 KPa)
A) THE LOAD ITSELF
975 LBS (4.337 N)
2925 LBS (13.010 N)
MAY HAVE BECOME TOO GREAT,OR THE CYLINDER MAY BE BINDING OR SIDE LOADED.TOO MUCH FORCE,AND THEREFORE PRESSURE,IS REQUIRED TO MOVE THE LOAD.IN THIS CASE THE SYSTEM RELIEF VALVE WILL OPEN AT ITS SETTING TO PREVENT DAMAGE TO THE SYSTEM.THEN ALL PUMP FLOW GOES THORUGH THE RELIEF VALVE TO TANK INSTEAD OF TO THE ACTUATOR
B) OR,THE LOAD MAY BE NORMAL BUT THERE IS NOT ENOUGH PRESSURE AVAILABLE TO MOVE IT.THE RELIEF VALVE MAY BE LEAKING OR SET TOO LOW.THE PUMP MAY BE SLIPPING ITS TOTAL VOLUME.OTHER LEAKS IN THE SYSTEM MAY BE ABSORBING ALL PUMP OUTPUT AND PREVENTING THE PRESSURE IN THE CYLINDER FROM RISING ENOUGH TO MOVE THE LOAD.