Basic Hydraulics by Teh

BASIC HYDRAULICS

Prepared by: Teh Ewe Thong

Definition of hydraulics • Generation of forces and motion using hydraulic fluids • Hydraulic fluid represents the medium of power transmission

(note pg: 7)

Hydro-mechanics 1. Hydrostatics

2. Hydrodynamics

(note pg: 13)

APPLICATIONS Stationary hydraulics • • • • • • •

Production and assembly machines of all types Transfer lines Lifting and conveying devices Presses Injection moulding machines Rolling lines Lifts

(note pg: 8-10)

APPLICATIONS Mobile hydraulics • • • •

Construction machinery Tippers, excavators, elevating platforms Lifting and conveying devices Agricultural machinery

(note pg: 8-9)

Advantages of hydraulics • Transmission of large forces using small components, i.e. great power intensity • Precise positioning • Start-up under heavy load • Even movements independent of load, since liquids are scarcely compressible and flow control valves can be used • Smooth operation and reversal • Good control and regulation • Favourable heat dissipation (note pg:10)

Disadvantages of hydraulics • Pollution of the environment by waste oil (danger of fire or accidents) • Sensitivity to dirt • Danger resulting from excessive pressures (severed lines) • Temperature dependence (change in viscosity) • Unfavourable efficiency factor

(note pg:10)

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, flow rate v = 2 – 6 m/s, signal speed up to 1000 m/s.

Up to 1000 m, flow rate v = 20 – 40 m/s, signal speed 20 – 40 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 large forces can be generated F < 3000 kN.

Protected against overload, forces limited by pneumatic pressure and cylinder diameter F < 30 kN at 6 bar.

BASIC PHYSICAL PRINCIPLES • Hydrostatic pressure – Open 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 Hydrostatic pressure – closed vessel • Pascal’s law: pressure exists when a force F is imposed 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 (note pg:17)

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 be attained. Given that: p = 100 bar = 1000 N/cm2 A = 7.85 cm2

F P

Example:

F=15000N

A=

πD 2 4

D2 =

4(20)

D=

4(20)

π π

= 5.05cm

P

BASIC PHYSICAL PRINCIPLES Power transmission: • The same pressure applies at every point in a closed system

(note pg:22)

Example:

(note pg:23)

BASIC PHYSICAL PRINCIPLES Displacement transmission:

(note pg:25)

Example: Calculate S2 Given: A1 = 40 cm2 A2 = 1200 cm2 S1 = 15 cm

(note pg:26)

BASIC PHYSICAL PRINCIPLES FLOW RATE: • Flow rate is the term used to describe the volume of liquid flowing through a pipe in a specific period of time. For example, approximately one minute is required to fill a 10 litre bucket from a tap. Thus, the flow rate amounts to 10 l/min.

(note pg:29)

BASIC PHYSICAL PRINCIPLES Flow measuring instruments:

(note pg:277)

BASIC PHYSICAL PRINCIPLES CONTINIUTY EQUATION • If the time t is replaced by s/v (v = s/t) in the formula 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]

(note pg:31)

Example: Calculate the oil flow velocity in a pipeline Given that: 4.2dm 3 Q = 4.2 l/min = = 0.07·10-3m3/s 60s

A = 0.28

cm2

= 0.28

·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

(note pg:32)

BASIC PHYSICAL PRINCIPLES CONTINIUTY EQUATION • The flow rate of a liquid in terms of volume per unit 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 for Q is always the same, the following equation of continuity applies: Q1 = Q2 = Q3 A1·v1 = A2·v2 = A3·v3 = etc...

(note pg:34)

PRESSURE MEASUREMENT

(note pg:37)

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 kinetic viscocity in m2/s

(note pg:39)

Energy Loss By Turbulent Flow

Hydraulic fluid Types: • Mineral based – For low risk of fire • Phosphate-ester based (Synthetic oil) – For high risk of fire

(note pg:70)

Hydraulic fluid Tasks for hydraulic fluids • pressure transfer, • lubrication of the moving parts of devices, • cooling, i.e. diversion of the heat produced by energy conversion (pressure losses), • cushioning of oscillations caused by pressure jerks, • corrosion protection, • scuff removal, • signal transmission. (note pg:70)

Hydraulic fluid Hydraulic oil classes (DIN 51524 and 51525): • Hydraulic oil HL • Hydraulic oil HLP • Hydraulic oil HV.

(note pg:71)

Hydraulic fluid Hydraulic fluids with low inflammability (HF liquids):

(note pg:72)

Hydraulic fluid Viscosity: • The word “viscosity” can be defined as “resistance to flow”. The viscosity of a liquid indicates its internal friction, • The international system of standards defines viscosity as “kinematic viscosity” (unit: mm2/s or Cst).

Hydraulic fluid ISO standard for Viscosity Grade:

Hydraulic fluid VG selection: • If viscosity is too low (very fluid), more leakages occur. The lubricating film is thin and, thus, able to break away more easily resulting in reduced protection against wear. • High viscosity results in increased friction leading 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 • pressure relief valve • coupling • reservoir • 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 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 intake and storage reservoir for the hydraulic fluid required for operation of the system; • dissipates heat; • separates air, water and solid materials; • supports a built-in or built-on pump and drive motor and other hydraulic components, such as valves, accumulators, etc.

Reservoir / Tank

Filters • •

Filters are of great significance in hydraulic systems for the reliable functioning and long service life of the components. The effects of polluted oil:

Filter arrangement

Filter Grades

Filter Grades

Filter designs

Valve Symbols Directional Control Valves Switching position

Flow path

Flow path blocked

Connection ports (note pg:92)

2

2 - Way valve

Number of switching positions Number of ports

3

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

2 - Way valve

Methods of actuation:

(note pg:93)

Hydraulic actuators Linear actuators: • single-acting and • double-acting cylinders. Rotary actuators: • Hydraulic motors

(note pg:228)

Single acting cylinder •In single-acting cylinders, only the piston side is supplied with hydraulic fluid. Consequently, the cylinder is only able to carry out work in one direction.

(note pg:228)

Single acting cylinder • Types:

(note pg:230)

Double-acting cylinder •

In the case of double-acting cylinders, both piston surfaces can be pressurized. Therefore, it is possible to perform a working movement in both directions.

(note pg:231)

Double-acting cylinder

Double acting cylinder • Types:

(note pg:233)

Double acting cylinder • End position cushioning

(note pg:235)

Hydraulic motors •

They convert hydraulic energy into mechanical energy 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.

(note pg:250)

Hydraulic motors •Types:

(note pg:253)

Valves Nominal sizes:

(note pg:148)

Valves Design: • Poppet valves • slide valves

(note pg:151)

Valves Poppet valves:

(note pg:152)

Valves slide valves

(note pg:154)

Valves Comparison of valve constructions:

(note pg:155)

Valves Control edges:

(note pg:160)

Valves Annular grooves: • With the grooves, the piston of valve spool is supported on a film of oil. On actuation, only the fluid friction needs to be overcome.

(note pg:161)

Directional control valves 2/2-way valve:

(note pg:180/184)

Directional control valves 3/2-way valve

(note pg:188)

Directional control valves 4/2-way valve

(note pg:190)

Directional control valves 4/3-way valve with pump by-pass (re-circulating)

(note pg:195)

4/3-way valve with pump by-pass (re-circulating)

(note pg:191)

Directional control valves 4/3-way valve, mid position closed

(note pg:197)

Pressure valves • Pressure relief valves • Pressure regulator • 2-way pressure regulator • 3-way pressure regulator

(note pg:164)

Pressure valves Pressure relief valves

(note pg:166)

Pressure valves Pressure relief valve, internally controlled, cushioned: • Cushioning pistons and throttles are often installed in pressure relief valves to eliminate fluctuations in pressure. The cushioning device shown here causes: • fast opening • slow closing of the valve.

(note pg:168)

Pressure valves Pressure relief valve, externally controlled

(note pg:170)

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 valves These counteract mass 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. (note pg:168/9)

(note pg:169/171)

Pressure valves Pressure regulators: • Pressure regulators reduce the input pressure to a specified output pressure. They are only used to good effect in systems where a number of different pressures are required.

2-way pressure regulator

(note pg:172/3)

Pressure valves 3-way pressure regulator

(note pg:176)

Non-return valves / Check valves • Non-return valves block the flow in one direction and permit free flow in the other.

(note pg:201)

Pump protection

(note pg:203)

Other applications:

(note pg:204)

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 control valves are used to reduce the speed of a cylinder or the r.p.m. of a motor. Flow control valves are classified as either: • flow control valves or • flow regulating valves.

Flow control valves One-way flow control valve

Flow control valves Two-way flow control valve • To maintain a constant speed in the case of a changing load. the pressure drop ∆p via the throttle point can be kept constant.

Flow control valves Two-way flow control valve

Accumulator Accumulators perform special functions in hydraulic systems: • To act as an emergency power source, e.g. to complete a working stroke in case of drive or pump failure. • To compensate for leakage losses. • To compensate for variations in fluid volume due to changes in temperature. • Absorption of shock waves and pressure 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