BASIC HYDRAULICS
Prepared by: Teh Ewe Thong
Definition of hydraulics • Gene Genera rati tion on of forc forces es and and mot motio ion n usin using g hydraulic fluids • Hydra Hydraul ulic ic flui fluid d rep repres resen ents ts the the med mediu ium m of power transmission
(note pg: 7)
Hydro-mechanics 1. Hydrostatics
2. Hydrodynamics
(note pg: 13)
APPLICATIONS Stationary hydraulics •
Production an and as assembly machines of all types
•
Transfer lines
•
Lifting an and co conveying de devices
•
Presses
•
Injection moulding machines
•
Rolling lines
•
Lifts
(note pg: 8-10)
APPLICATIONS Mobile hydraulics •
Construction ma machinery
•
Tipp Tipper ers, s, exca xcavato vators rs,, elev elevat atiing platforms
•
Lifti ifting ng and conv convey eyin ing g devic evices es
•
Agricultural ma machinery
(note pg: 8-9)
Advantages of hydraulics • Tran Transm smis issi sion on of of larg large e forc forces es usi using ng sma smallll components, i.e. great power intensity • Precise positioning • Star tart-up un under he heavy lo load • Even Even move moveme ment nts s inde indepe pend nden entt of load load,, since since liquids are scarcely compressible and flow control valves can be used • Smooth ooth oper operat atio ion n and and reve revers rsal al • Good ood cont contro roll and and regu regula lati tio on • Favourable heat dissipation
Disadvantages of hydraulics • Poll Pollut utio ion n of of the the env envir iron onme ment nt by by wast waste e oil oil (danger of fire or accidents) • Sensitivity to dirt • Dang Danger er resu result ltin ing g from from exces excessi sive ve pres pressur sures es (severed lines) • Temp Temper erat atur ure e dep depen ende denc nce e (ch (chan ange ge in visco viscosi sity) ty) • Unfavourable efficiency fac factor
Comparisions Hydraulics
Pneumatics
Leakage
Contamination
No disadvantages apart from energy loss
Environmental influences
Sensitive in case of temperature fluctuation, risk of fire in case of leakage.
Explosion-proof, insensitive to temperature.
Energy storage
Limited, with the help of gases.
Easy
Energy transmission
Up to 100 m, m, flow rate v = 2 – 6 m/s, signal speed up to 1000 m/s.
Up to 1000 1000 m, flow rate v = 20 20 – 40 m/s, signal speed 20 20 – 40 m/s. m/s.
Operating speed
v = 0.5 m/s
v = 1.5 m/s
Power supply costs
High (1)
Very high (2.5)
Linear motion
Simple using cylinders, good speed control, very large forces.
Simple using cylinders, limited forces, speed extremely, loaddependent.
Rotary motion
Simple, high turning moment, low speed.
Simple, inefficient, high speed.
Positioning accuracy
Precision of up to ±1 µm can be achieved depending on expenditure.
Without load change precision of 1/10 mm possible.
Stability
High, since oil is almost incompressible, in addition, the pressure level is considerably higher than for pneumatics.
Low, air is compressible.
Forces
Protected against overload, with high system pressure of up to 600 bar, very
Protected against overload, forces limited by pneumatic pressure and
BASIC PHYSICAL PRINCIPLES • Hydros rostatic pre pres ssure – Open ve vessel ps = h.ρ.g ps h ρ g
= hydrostatic pressure (gravitational pressure) = level of the column of liquid = density of the liquid = acceleration due to gravity
[Pa] [m] [kg/m3] [m/s2]
(note pg:14)
Examples:
Column
Reservoir
Elevated tank
(note pg:15)
BASIC PHYSICAL PRINCIPLES Hydros Hydrostat tatic ic press pressure ure – closed closed vessel vessel • Pasca Pascal’s l’s law: law: pres pressur sure e exis exists ts when when a force force F is is imp impose osed d on on an enclosed fluid with a surface A, The pressure exerts an equal effect on all points of the surfaces. F P= A
N/m2
F
P
A
Example: A cylinder is supplied with 100 bar pressure, its effective piston surface is equal to 7.85 cm2. Find the maximum force which can c an be attained. Given that: p = 100 bar = 1000 N/cm 2 A = 7.85 cm2
F P
Example:
F=15000N
D π
A
D
2
=
2
4 4(20)
=
π
D
4(20) =
=
π
5.05cm
P
BASIC PHYSICAL PRINCIPLES Power transmission: • The The sam same e pre press ssur ure e appl applie ies s at at eve every ry poi point nt in a clos closed ed system
Example:
BASIC PHYSICAL PRINCIPLES Displacement transmission:
Example: Calculate S2 Given: A1 = 40 cm2 A2 = 1200 cm2 S1 = 15 cm
BASIC PHYSICAL PRINCIPLES FLOW RATE: • Flow Flow rate rate is the term term used used to to desc describ ribe e the the volum volume e of of liqui liquid d flowing through a pipe in a specific period of time. For example, approximately approximately one minute is required to fill a 10 litre bucket bucket from a tap. tap. Thus, Thus, the flow flow rate amount amounts s to 10 l/min.
BASIC PHYSICAL PRINCIPLES Flow measuring instruments:
BASIC PHYSICAL PRINCIPLES CONTINIUTY EQUATION • If the the tim time e t is is rep repla lace ced d by by s/v s/v (v = s/t s/t)) in in the the form formul ula a for for the flow rate (Q = V/t) and it is taken into account that the volume V can be replaced by A ·s, the following equation is produced: Q=A·v Q = Flow rate v = Flow velocity A = Pipe cross-section
[m3/s] [m/s] [m2]
Example: Calculate the oil flow velocity in a pipeline Given that: Q = 4.2 l/min = A = 0.28
cm2
4.2dm 3 60s
= 0.28
= 0.07·10-3m3/s
·10-4m2
Q
v
Example: Calculate the flow rate needed for the following movement Given that: A = 8 cm2 s = 10 cm t = 1 min
Q
BASIC PHYSICAL PRINCIPLES CONTINIUTY EQUATION • The flow rate of a liquid in terms of volume per unit of of time which flows through a pipe with several changes in cross-section is the same at all points in the pipe (see diagram). This means that the liquid flows through small cross-sections faster than through large cross-sections. The following equation applies: Q1 = A1 v1
Q2 = A2 v2
Q3 = A3 v3
etc.…
• As within one line the value value for Q is always the same, the following equation of continuity applies:
Q1 = Q2 = Q3 A1 v1 = A2 v2 = A3 v3 = etc...
PRESSURE MEASUREMENT
TYPE OF FLOW Two types of flow •
Laminar, Re < 2300
•
Turbulent, Re > 2300
Re = v x d /
v
v is flow velocity in m/s D is pipe diameter in m v is
kinet kinetic ic visco viscocit city y in m2/s
Energy Loss By Turbulent Flow
Hydraulic fluid Types: • Mineral based – For For low low risk risk of of fire fire • Phos Phosph phat atee-es este terr base based d (Syn (Synth thet etic ic oil oil)) – For For high high risk risk of fir fire e
Hydraulic fluid Tasks for hydraulic fluids • pressure transfer, • lubr lubric icati ation on of the the movin moving g part parts s of of dev device ices, s, • cool coolin ing, g, i.e. i.e. div diver ersi sion on of the the hea heatt pro produ duce ced d by by energy conversion (pressure losses), • cush cushio ioni ning ng of osci oscillllat atio ions ns caus caused ed by pres pressu sure re jerks, • corrosion protec tection, • scuff removal, • signal tr transmission.
Hydraulic fluid Hydraulic oil classes (DIN 51524 and 51525): •
Hydraulic oil HL
•
Hydraulic oil HLP
•
Hydraulic oil HV.
Hydraulic fluid Hydraulic fluids with low inflammability (HF liquids):
Hydraulic fluid Viscosity: • The wo word “v “viscosity” can be be de define ined as as “resistance to flow”. The viscosity of a liquid indicates its internal friction, • The The inte interna rnati tion onal al syst system em of stan standa dard rds s define defines s viscosi viscosity ty as “kinem “kinemati atic c viscos viscosity ity”” (unit: mm2 /s or Cst).
Hydraulic fluid ISO standard for Viscosity Grade:
Hydraulic fluid VG selection: • If viscos viscosity ity is too low (very (very fluid) fluid),, more more leakag leakages es occur. occur. The lubricating film is thin and, thus, able to break away more easily resulting in reduced protection against wear. • High High vis visco cosi sity ty res resul ults ts in incr increa ease sed d fric fricti tion on lea leadi ding ng to to excessive pressure losses and heating particularly at throttle points. This makes cold start and the separation of air bubbles more difficult and, thus, leads to cavitation.
Hydraulic fluid VG selection:
Hydraulic system
Hydraulic system
Power supply section The power supply section provides the energy required by the hydraulic system. The most important components in this section are: • drive • pump • pressu pressure re relief relief valve valve • coupling • reser eserv voir oir • filter • cooler • heater
Power supply unit (Power Pack) Example:
Hydraulic Pump The pump converts the mechanical energy in a drive unit into hydraulic energy (pressure energy). •Types:
Gear pump • Gear pumps are fixed displacement displacement pumps since the displaced volume which is determined by the tooth gap is not adjustable.
Axial Piston Pump
Characteristic values for the most common constant pumps
Practical:
Pump characteristic
Reservoir / Tank The tank in a hydraulic system fulfils several tasks. It: • acts as as intake intake and and storage storage rese reservo rvoir ir for for the hydraulic fluid required for operation of the system; • diss dissip ipat ates es heat heat;; • separa separates tes air, air, water water and and solid solid materi materials als;; • suppor supports ts a built-in built-in or or built-o built-on n pump and and drive drive motor and other hydraulic components, such as valves, accumulators, etc.
Reservoir / Tank
Filters •
Filter Filters s are are of of great great signif significa icance nce in hydr hydraul aulic ic syst systems ems for the reliab reliable le functioning and long service life of the components.
•
The effects of of po polluted oi oil:
Filter arrangement
Filter Grades
Filter Grades
Filter designs
Valve Symbols Directional Control Valves Switching position
Flow path
Flow path blocked
Connection ports
2
valv ve 2 - Way val Number of switching positions
Number of ports
3
lve 2 - Way valve
Connection ports P
; Pressure supply port
T
; Return port (Tank)
A,B
; Power/Output/working ports
L
; Leakage port A
B
4 P
T
valvee 2 - Way valv
Methods of actuation:
Hydraulic actuators Linear actuators: • sing single le-a -act ctin ing g and and • doub double le-a -acti cting ng cylind cylinder ers. s. Rotary actuators: • Hydr Hydrau aulilic c mot motor ors s
Single acting cylinder • In single-acting cylinders, only the piston side is supplied supplied with hydraulic fluid. Consequently, the cylinder is only able to carry out work in one direction.
Single acting cylinder • Types:
Double-acting cylinder •
In the the cas case e of dou doubl blee-ac acti ting ng cyl cylin inde ders, rs, both both pis pisto ton n surf surfac aces es can can be be pressurized. Therefore, it is possible to perform a working movement in both directions.
Double-acting cylinder
Double acting cylinder • Types:
Double acting cylinder • End posit sition cus cushioning
Hydraulic motors •
They They con conve vert rt hyd hydra raul ulic ic ene energ rgy y into into mec mecha hani nica call ener energy gy and and generate rotary movements (rotary actuator). If the rotary movement only covers a certain angular range, the actuator is referred to as a swivel drive.
Hydraulic motors •Types:
Valves Nominal sizes:
Valves Design: • Poppet valves • slide valves
Valves Poppet valves:
Valves slide valves
Valves Comparison of valve constructions:
Valves Control edges:
Valves Annular grooves: • With With the the gro groov oves es,, the the pist piston on of valv valve e spo spool ol is supported on a film of oil. On actuation, only the fluid friction needs to be overcome.
Directional control valves 2/2-way valve:
Directional control valves 3/2-way valve
Directional control valves 4/2-way valve
Directional control valves 4/3-way valve with pump by-pass (re-circulating)
4/3-way valve with pump by-pass (re-circulating)
Directional control valves 4/3-way valve, mid position closed
Pressure valves • Pressure re relief va valves • Pressure re regulator • 2-way pr pressure regulator • 3-way pr pressure regulator
Pressure valves Pressure relief valves
Pressure valves Pressure relief valve, internally controlled, cushioned: • Cushioning pistons and and throttles are often often installed in pressure relief valves to eliminate fluctuations in pressure. The cushioning device shown here causes: • fast fast open openin ing g • slow slow closin closing g of the the valv valve. e.
Pressure valves Pressure relief valve, externally controlled
Pressure valves Pressure relief valves are used as: • Safety valves A pressure relief valve is termed a safety valve when it is attached to the pump, for example, to protect it from overload. The valve setting is fixed at the maximum pump pressure. It only opens in case of emergency. • Counter-pressure val valves These counteract counteract mass mass moments moments of inertia with tractive loads. The valve must be pressure-compensated and the tank connection must be loadable. • Brake valves These prevent pressure peaks, which may arise as a result of mass moments of inertia on sudden closing of the directional control valve. • Sequence valves These open the connection to other consuming devices when the set pressure is exceeded. There are both internally and externally controlled pressure relief valves. Pressure relief valves of poppet or slide design may only be used as sequence valves when the pressure is compensated and loading at the tank connection has no effect on the opening characteristics.
Pressure valves Pressure regulators: •
Pressu Pressure re regula regulator tors s redu reduce ce the input input pres pressur sure e to to a specif specified ied output output pressure. They are only used to good effect in systems where a number of different pressures are required.
2-way pressure regulator
Pressure valves 3-way pressure regulator
Non-return valves / Check valves • NonNon-re retu turn rn val valve ves s bloc block k the the flow flow in in one one dire direct ctio ion n and and permit free flow in the other.
Pump protection
Other applications:
Non-return valves / Check valves Piloted non-return valve
Flow blocked from B to A
Flow from A to B
Flow from B to A with X signal
Exercise: A scissor lift is used to lift heavy loads to the platforms of varying heights. The loaded lift must be able to remain at given height over a long period of time. The lift is powered by a double acting cylinder. Position sketch
Load
Piloted non-return valve:
Flow control valves •
Flow Flow con contr trol ol val valve ves s are are used used to to redu reduce ce the the spe speed ed of of a cyli cylind nder er or or the r.p.m. of a motor.
•
Flow Flow cont contro roll valv valves es are are cla class ssif ifie ied d as eith either er:: • flow flow contr control ol valv valves es or • flow flow regula regulatin ting g valves. valves.
Flow control valves One-way flow control valve
Flow control valves Two-way flow control valve •
To mai maint ntai ain n a con const stan antt spee speed d in the the case case of of a cha chang ngin ing g load load.. the the pressure drop ∆p via the the throttle throttle point point can be be kept consta constant. nt.
Flow control valves Two-way flow control valve
Accumulator Accumulators perform special functions in hydraulic systems: • To act act as an emerge emergency ncy powe powerr source source,, e.g. e.g. to complete a working stroke in case of drive or pump failure. • To comp compens ensate ate for leakag leakage e losse losses. s. • To compens compensate ate for for variation variations s in fluid volume volume due due to changes in temperature. • Absorp Absorption tion of of shock shock waves waves and pressu pressure re peaks peaks due to switching actions and applications.
Accumulator Design:
Diaphragm accumulator
Bladder accumulator
Bladder accumulator Operation:
Accumulator applications Reduce vibration and shock:
Accumulator applications Installation for emergency power source:
Thank you
