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MEM341: FLUID POWER TECHNOLOGYMEM341: FLUID POWER TECHNOLOGYMEM341: FLUID POWER TECHNOLOGYMEM341: FLUID POWER TECHNOLOGY

EP ChiangUiTMPP FKM

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1. INTRODUCTION

What is Fluid Power Technology that deals with the generation, control and transmission of power

using pressurized fluids, usually oil or air. Hydraulic systems oil (petroleum or synthetic) or water based fluids. Pneumatic systems air. Can be used to push, pull, regulate, drive virtually all types of modern industries

machines.

3 basic methods to transmitted power: electrical, mechanical and fluid power. Fluid power is widely used because its versatility and manageability.

Advantages of Fluid power: Ease and accuracy of control. Multiplication of force. High horsepower, low weight ratio Low speed torque Constant force or torque. Simplicity, safety, economy. Established standards and engineering

Multiplication and variation of forceLinear or rotary force can be multipliedfrom a fraction of an ounce to several hundred tons of output.

Easy, accurate controlYou can start, stop, accelerate, decelerate, reverse or

position large forces with great accuracy. Analog (infinitely variable) and digital(on/off) control are possible. Instantly reversible motionwithin less than half arevolutioncan be achieved.

Multi-function controlA single hydraulic pump or air compressor can providepower and control for numerous machines or machine functions when combinedwith fluid power manifolds and valves.

High horsepower, low weight ratioPneumatic components are compact andlightweight. You can hold a five horsepower hydraulic motor in the palm of yourhand.

Low speed torqueUnlike electric motors, air or hydraulic motors can producelarge amounts of torque (twisting force) while operating at low speeds. Somehydraulic and air motors can even maintain torque at zero speed without

overheating. Constant force or torqueThis is a unique fluid power attribute. Safety in hazardous environmentsFluid power can be used in mines,

chemical plants, near explosives and in paint applications because it is inherentlyspark-free and can tolerate high temperatures.

Established standards and engineeringThe fluid power industry hasestablished design and performance standards for hydraulic and pneumaticproducts through NFPA, the National Fluid Power Association; ANSI, the

American National Standards Institute; and ISO, the International Organizationfor Standardization.

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Difference between Hydraulics and Pneumatics System

Engineering science pertainingto liquid pressure and flow

Use liquids such as petroleumoils, water, synthetic oils andmolten metals.

Liquids provide a very rigidmedium for transmitting powerand utmost accuracy andprecision of motion.

Engineering sciencepertaining to gaseouspressure and flow.

Used air as the gas medium. Exhibit spongy characteristics

due to compressibility of air.

Less expensive to build andoperate.

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Example of Fluid Power Machinary

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2. Principle of Hydraulics

Energy and Power in Hydraulic System

Introduction

Prime

Mover

Hydraulic

Pump

Hydraulic

Circuit

Hydraulic

Actuator

External

Load

Mechanical

Energy in

Mechanical

Energy out

Heat energy out

Hydraulic System

Follow the conservation of Energy Law:

INPUT ME LOST HE = OUTPUT ME

FRICTION

Analysis of hydraulic circuit The Basic Principle

Pascal Law Extended Bernoulli Equation / Energy Equation Considering All The Losses Hydraulic Pressure, Fluid Leakage, Mechanical

Inefficiency, etc.. Applying Mechanical Principals to find the available/required power, torque,

velocity, etc..

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Example of a simple hydraulic circuit. (Rexroth-Bosh)

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3. Hydraulic Fluid

Main function of hydraulic fluid is to transfer forces and movements (Transmit Power).Other functions are:

Lubricate Moving PartsSeal Clearances Between Mating PartsDissipate Heat

There are diverse range of applications and installations of hydraulic devices.There is no one magic fluid that can do all the things, there for we must select the righthydraulic fluid for the type and conditions of the application.

Hydraulics Fluid Requirements/Characteristics

Lubrication and Anti-Wear characteristics Be able to covered all moving parts. Why? High pressure, insufficient oil delivery, low viscosity, slow or fast

sliding movements. Viscosity

Most important parameter. Given in viscosity index; show haw thick or thin a hydraulic fluid is at a

given temperature. Kinematics Viscosity measured in SI in mm2/s and changes with

temperature. How to determine? Look at the manufactures documentation about the

minimum and maximum permissible viscosities. Viscosity index is to DIN ISO 2909.

Behavior with respect to pressure. Changes of viscosity of the fluid.

Compatibility with different materials. Stability against shearing.

Occurred at metering notches and on the opening and closing of valveseats.

What happen is that viscosity can become lower then the original value. Stability against thermal loads

Fluid temperature can increase during operation (usually

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Low intake of air and good release of air. Good thermal conductivity. Good di-electric (non-conducting) characteristics.

Non-hygroscopic Fire resistant (non flammable) Non toxic as a fluid, vapor and after decomposition. Good protection against corrosion. No formation of sticky substances Good filtration capability Compatibility and exchangeability with other hydraulic fluids. Formation of silt. User friendly servicing. Ecologically acceptable. Costs and availability.

Viscosity of Hydraulic Fluid

Viscosity is the measure of a fluids resistance to flow. High viscosity will results in:

High resistance to flow, cause sluggish operation. Increased power consumption, friction loss up. Increased pressure drop through valves and lines. High temperatures, because friction.

Low viscosity will results in: Increased oil leakage past seals. Excessive wear to moving parts.

Absolute viscosity:

Unit is often expressed in Pa.s or kg/m.s. (SI unit) But also in the unit centi-poise (cP). 1 cP = 0.001 Pa.s

Kinematics viscosity: Unit is expressed in m2/s. (SI unit) But also in the unit called centi-stoke (cS). 1 cS = 1 mm2/s or 1 stokes = 0.0001 m2/s

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Primary functions of Hydraulic Fluid

1. Transmit power

2. Lubricate moving parts

3. Seal clearances between mating parts

4. Dissipate heat

Desired properties of hydraulic fluid1. Good lubricity

2. Ideal viscosity

3. Chemical stability

4. Compatibility with system

materials

5. High degree of incompressibility

6. Fire resistance

7. Good heat transfer capability

8. Low density

9. Foam resistance

10. Non-toxicity

11. Low volatility

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4. HYDRAULIC PUMPS

Hydraulic pump is a devices that convert mechanical energy into hydraulic energy (fluidpower) in the form of PRESSURE and VOLUME FLOW-RATE. The mechanical energyis supply by prime mover usually electric motor or internal combustion engine).

There are two category of pumps: dynamic pumps/nonpositive displacement pumps andPositive displacement pump.

NPDpumps generally used for low pressure, high volume flow applications. Function tomove fluid from one location to another. (centrifugal and axial flow propeller pump).

PD pumps, will ejects a fixes amount of fluid into hydraulic system per rev. canovercome the pressure resulting from the mechanical loads and resistance to fluid flow.High pressure, small, high volumetric efficiency, efficiency of the pump is stable for alarge range. (piston, gear, vane types of various design).

Pumping theory

Pump operate by creating a partial vacuum in a confine space. The atmosphericpressure will push the fluid out of the oil tank into the pump intake. Pump will thenmechanically push the fluid out into the discharge line.

Type of pump1. Gear pump external gear pump, internal gear pump, lobe pump, screw pump.2. vane pump unbalanced vane pump, balanced vane pump.3. piston pump axial design, radial design.

Pump Performance Usually rated according to their volumetric output and pressure. Volumetric output (delivery rate or capacity) is the amount of liquid that a pump

can deliver at its outlet port per unit of time at a given drive speed, usuallyexpressed in GPM or cubic inches per minute.

Sometimes rated according to displacement, that is the amount of liquid thatthey can deliver per cycle or cubic inches per revolution.

Slippage is a measure of a pump's efficiency and usually is expressed in percent.Some pumps have greater internal slippage than others; some pumps are ratedin terms of volumetric output at a given pressure.

DisplacementThe amount of liquid transferred from a pump's inlet to its outlet in one revolution orcycle. Either in cubic meter per rev or meter per cycle. Also know as the volumetricdisplacement, VD.

Slippage Slippage occur when there are oil that leaking from a pressure outlet to a low-

pressure area or back to an inlet. A little slippage is usually design for lubrication purpose. When pressure increase, leakage will increase. Increase slippage is loss of efficiency.

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NPD pump

The volume of liquid delivered for each cycle depends on the resistance offered to flow.

A pump produces a force on the liquid that is constant for each particular speed of thepump.Resistance in a discharge line produces a force in the opposite direction.When these forces are equal, a liquid is in a state of equilibrium and does not flow.If the outlet of a nonpositive-displacement pump is completely closed, the dischargepressure will rise to the maximum for a pump operating at a maximum speed. A pumpwill churn a liquid and produce heat.

PD Pump

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Gear pump One of the simplest type of positive displacement pump construction. Seal chamber is created by the space between the gears teeth.

Using two matching gears as the key to the design.

External type

External gear pump

The Volumetric Displacement,

( )LDDV ioD 224

=

Where Dois the outside diameter of the gear teeth

Diis the inside diameter of the gear teethL is the width of ear teeth

The theoretical flow rate

NVQ DT =

where N is the rpm of pump, how fast the pump is rotating.

D0Di

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Internal type

Internal gear pump

Lobe pump

Vane Pump A cam ring's shape is a true circle that is on a different centerline from a rotor's. Pump displacement depends on how far a rotor and ring are eccentric. The advantage of a true-circle ring is that control can be applied to vary the

eccentricity and thus vary the displacement. A disadvantage is that an unbalanced pressure at the outlet is effective against a

small area of the rotor's edge, imposing side loads on the shaft. Thus there is a limit on a pump's size unless very large hearings and heavy

supports are used.

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The volumetric displacement,

( )eLDDV RCD +=2

Where DC is the diameter of the cam ringDR is the diameter of the rotorL is the width of rotore is the eccentricity between the rotor and cam ring

When the eccentricity of the rotor and cam ring is at i t maximum value, this will alsocoincide with the maximum volumetric displacement.

When eccentricity is equal zero, then volumetric displacement is also zero.


NVQ DT =

Balance vane pump

Astationary, elliptical cam ring and twosets of internal ports.

A pumping chamber is formed between

any two vanes twice in each revolution. The two inlets and outlets are 180

degrees apart. most universal in the mobile-equipment

field

Cam ring

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Piston Pumps

Two types of design axial or radial.

Radial design

In a radial piston pump the pistons are arranged like wheel spokes in a shortcylindrical block..

A drive shaft, which is inside a circular housing, rotates a cylinder block. The block turns on a stationary pintle that contains the inlet and outlet ports. As a cylinder block turns, centrifugal force slings the pistons, which follow a

circular housing. A housing's centerline is offset from a cylinder block's centerline. The amount of eccentricity between the two determines a piston stroke and,

therefore, a pump's displacement. Controls can be applied to change a housing's location and thereby vary a

pump's delivery from zero to maximum.

pintleis a round bar that serves as a stationary shaft around which acylinder block turns.

A pintle shaft has four holes bored from one end lengthwise through part of itslength.

Two holes serve as an intake and two as a discharge.

Two slots are cut in a side of the shaft so that each slot connects two of thelengthwise holes.

The slots are in-line with the pistons when a cylinder block is assembled on apintle.

One of these slots provides a path for a liquid to pass from the pistons to thedischarge holes bored in a pintle.

Another slot connects the two inlet holes to the pistons when they are drawing inliquid.

The discharge holes are connected through appropriate fittings to a dischargeline so that a liquid can be directed into a system.

The other pair of holes is connected to an inlet line Cylinder Block house pintle and pistons.

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Rotors./ reaction ring Drive Shaft. A drive shaft is connected to a cylinder block and is driven by an

outside force such as an electric motor

Axial design Piston and cylinder block are parallel to each others. Can be bent axis configuration or swash plate design. Pump displacement depends on the bore and stroke of a piston and the number

of pistons.

Swash plate design

Cylinder block and drive shaft is on the same centerline. Pistons are connected to a shoe plate and bears against an angled swash plate. The angle of swash plate determines the length of the piston stroke. The inlet ant outlet ports are located in the valve plate.

Bent axis

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Drive shaft and cylinder block are set at an offset angle. Piston rods are connected to the drive shaft flange by ball and socket joints. Universal link connects the block to the drive shaft to provide alignment and

positive drive.

The volumetric displacement

)tan(DAYVD=

Where D is the piston circle diameterA is the piston areaY is the number of pistons is the offset angle


NDAYNVQ DT )tan(==

Pump Performance

Volumetric Efficiency of Pump

Efficiency is calculated using the ratio of the output over the initial input.

Volumetric efficiency indicates the amount of leakage that takes place within the pump.There for the volumetric efficiency of any type of pump is given by,

T

Av

Q

Q

rateflowltheoreticathe

rateflowactualthe

input

output=

==

Mechanical Efficiency

Mechanical efficiencies indicates the amount of energy losses that occur for reasons

other than leakage. This includes friction between moving components and also how theoil moving inside the pump.

A

T

A

T

mT

T

NT

pQ

pumptodeliveredpoweractual

leakagenoifoutputpowerpumpthe

input

output====

Where p is the pump discharge pressureTA is the actual torque delivered to the pump.

TTis the theoretical torque required to operate the pump based on design.

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2

pVT DA

=

Overall Efficiency

Take into account all the losses between the inlet of the pump and the outlet of the pump.

NT

pQ

A

Amvo ==

Pump Performance Comparison

Gear pumps in general the least expensive but also provides the lowest level ofperformance.

Gear pumps wear rapidly, but simple in design and compact. Vane pumps performance and efficiency are between gear pumps and piston

pumps. Piston pumps the most expensive and complex in design, but also provides the

highest level of overall performance. Can be driven at high speeds (rpm 5000).Operate at high pressure.

Pump Noise

Pump created noise during operation.

Undesirable. Loud noise is bad to health. OSHA (Occupational Safety and Health Administration) stipulates 90dB(A) is the

max. sound level a person may be exposed to during 8-hr period in theworkplace. Noisy factory or gear pump.

Vane pump vacuum cleaner What cause noise?

Fiction between moving components. Misaligned pump to motor coupling. Improper installation of pump or motor mounting plates. Pump cavitations. Pump operated at excessive speed or pressure.

To reduce noise, can use sound absorption materials. Unusual pump noise can also indicate the performance of a pump. Increase

noise could indicates increased wear.

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Pump Cavitations

Pump cavitations is due to the entrained air bubbles in the hydraulic fluid or vaporizationof the hydraulic fluid.

Prevent cavitations: keep suction line velocity low (1.2 m/s

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5. HYDRAULIC ACTUATOR

Pumps perform the function of adding energy to the fluid of a hydraulic systemfor transmission to some output location.Convert hydraulic energy to mechanical energy is the function of actuator.Linear motion hydraulic cylinders / linear actuator. Provide a push or pull forceto move a load along a straight line.Rotational motion Hydraulic motors / rotary actuator. Rotate a shaft to providetorque to drive a load along a rotary path.

Objective: You should be able to Describe the construction and design features. Identify the various types of mountings and mechanical linkages for

transmitting power. Calculate the load carrying capacity, speed and power.

Types of Hydraulic Actuator.

Hydraulic cylinders

Constructed of a piston or plunger that operate in a cylindrical housing (barrel) bythe action of liquid under pressure.Many types of design to perform the desirable action.

HYDRAULICACTUATOR

LINEAR

ACTUATOR(LINEAR MOTION)

ROTARY

ACTUATOR(ROTATIONAL MOTION)

SINGLEACTING

CYLINDER

DOUBLEACTING

CYLINDER

LIMITEDROTATIONAL

MOTOR

CONTINUOUSROTATIONAL

MOTOR

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Rexroth

Hydraulic cylinder in action.

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A single acting cylinderDesign:a piston house in a barrel with andopening in the dead end that allow pressurizefluid to enter the space between piston andinside the barrel to push the piston outward frominside the barrel. Force can only act in onedirection only, which is to push the piston out. Toretract or extend (depend on design) the pistoninto the barrel, gravity, outside force or spring isused.

A 3D model of a single acting spring return hydraulic cylinder.

A double acting cylinderDesign:a piston house in a barrel with openingsin the dead end that allow pressurize fluid toenter the space between piston and inside the

barrel to push the piston outward from inside thebarrel and out from another opening. The pistonand rod can extend or retract depend on the flowof the pressurize oil.

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A telescopic hydraulic cylinder and its usage in forklift.

Double-acting hydraulic cylinder construction.

Hydraulic cylinder Analysis

- Cylinder force, velocity & power (Actual and Theoretical).- Cylinder performance (overall efficiency, volumetric efficiency and

mechanical efficiency).- During extension stroke and also retraction stoke they are different.

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areapistoneffectivepressureplyForce = sup

areapistoneffective

rateflowvolume

Velocity =

pressureplyrateflowvolumePower sup=

Extension Retraction

pistonApF = rodpiston AApF =

pistonA

Qv =

rodpiston AA

Qv

=

vFQpH == vFQpH ==

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Hydraulic mill type cylinder Hydraulic tie rod type cylinder

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Tie rod hydraulic cylinder

Double rod end hydraulic cylinder

Hydraulic motors

2 types, limited rotation hydraulic motors (oscillation motors or rotary actuators) &continuous rotation hydraulic motors (hydraulic motors).Hydraulic motors are redesigned pumps.

Types of hydraulic motors constructions: gear, vane or piston.

Example:Hydraulic impact wrench, operates with flow-rate of 15 to 45 Lpm andpressure of 70 to 140 bars.

Limited rotation hydraulic motorsCan rotate clockwise or counter-clockwise but not in a complete circle. Usuallyusing vane/vanes.

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Hydraulic motors

Gear motors

- Develops torque due to hydraulic pressure acting on the surfaces of thegear teeth.- Can be reversed by reversing the direction of fluid flow.- Volumetric displacement is fixed.- Max operating pressure ~ 14 MPa.- Max flow-rate ~ 570 Lpm.- Max rpm ~ 2400 rpm.

Vane motors- develop torque by the hydraulic pressure acting on the exposed surfaces

of the vanes.- Springs/pressure loaded vanes to hold the vanes to the cam ring.- Usually fixed displacement, balanced design.- Max operating pressure ~ 17 MPa.- Max flow-rate ~ 950 Lpm.- Max rpm ~ 4000 rpm.

Piston motorscan be fixed or variable displacement units.- Generated torque by pressure acting on the ends of pistons reciprocating

inside a cylinder block.- In-line piston motor (swash plate design), Axial piston motor (bent-axis

design).- Can operate at highest speeds and pressures.- Max operating pressure ~ 35 MPa.- Max flow-rate ~ 1700 Lpm.- Max rpm ~ 12000 rpm.-

Axial Piston Motor (Rextroth)

D

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Bent axis hydraulic motor.

Swash Plate design Hydraulic Motor with Planetary Gear and BreakAccessory.

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Different designs of hydraulic motor.

Hydraulic motors analysis

- theoretical - torque, power, flow-rate

- actual - torque, power, flow-rate- performance motor efficiencies, volumetric efficiency, mechanicalefficiency, overall efficiency

Theoretical Torque,2

pVT DT = [Nm]

Theoretical Power,

2

pVTH DTT == [Watt]

Theoretical Flow-rate, NVQ DT = [ m3/s]

Actual values can be measured from the device directly.

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Hydrostatic transmissions

A system consisting of a prime mover, a hydraulic pump, a hydraulic motor, andappropriate valves and pipes that can provide adjustable speed drives.Can be found in tractors, rollers, front-end loaders, hoes and lift trucks.

Advantages:- infinitely variable speed and torque in either direction and over the full

ranges.- Extremely high power to weight ratio.- Ability to be stalled without damage.- Low inertia of rating members, fast starting and stopping with smoothness

and precisions.- Flexibility and simplicity of design.

Main component of a hydrostatic transmission.

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Diagram depicting a simplified hydraulic transmission.

pumpPrime

mover motor

ReservoirInput power(torque &rpm)

Pump output(Operating

pressure &oil flow-rate

Motor Output(Torque &rpm)

Oil leakage

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