EDUC6505 Assignment 3 CASE STUDIESAbstract: Concepts relevant to fluid mechanics are introduced. A range of applications of hydraulics in transport and lifting devices are examined. Innovations in braking systems and lifting devices are discussed. Anton Taverne
The Use of Fluid Mechanics in Braking Systems Fluid statics Fluid dynamics At rest In motion Fluid Mechanics - the study of liquids and gases Pressurised air Pressurised liquid Pneumatics Hydraulics
Fluids can be liquids or gasesLiquids take the Gases fill the shape of the container by container diffusion Liquids - Hard to compress Gases - Easy to compress
Liquids display capillary action and surface tension
Pressure in fluids Pressure = Force / Area Fluids exert pressure in all directions, and perpendicular to walls of container.In a static fluid (i.e. gas or liquid), the pressure at anypoint will, as a result of gravity, depend on the depth.The more fluid there is above a point, the greater thepressure
Pascal’s Principle Any change in pressure, from the application of an external force, results in the same change of pressure anywhere in an enclosed static fluid. This is Pascal’s Principle and is the basis for hydraulics and pneumatics.In the hydraulic press shown, the input pressure = the outputpressure, so F1 / A1 = F2 / A2This allows a small force(e.g. a foot on a brake pedal)to become a much larger force(e.g. pushing a brake pad against a brake disc)Basic hydraulic systems include : a fluid (such as oil) a reservoir a means of increasing the pressure (such as pushing a pedal) a valve to control the direction of flow a method for relieving any excess pressure an actuator (e.g. the pistons pushing against a brake shoe) filters to remove dust or metal particles http://hyperphysics.phy-astr.gsu.edu/hbase/pasc.html
A comparison of Hydraulics and Pneumatics Hydraulics (liquids) Pneumatics (gas) All force is directly transferrable. Liquid Gas absorbs excess force, so less damage by does not absorb any supplied energy shock. Less maintenance. Capable of much higher loads Minimum spring action More immediate response when effort Need to bleed off excess pressure is applied / removed Oil used is flammable, but also No fire risk protects parts Compressed gas can be stored. No need for electricity http://en.wikipedia.org/wiki/Pneumatics
Archimedes principleThe buoyant force on a submerged object is equal to the weight of the liquid displaced by the object.A solid object placed on a liquid will push down on it, displacing it.The displaced liquid is also pulled down by gravity and will, by pushing other liquid particles, apply an upwardforce on the solid. The denser of the two will apply the greater force.As a result, a solid object partially or wholly submerged in a liquid will experience an upward force (thebuoyant force) that is equal to the weight of the liquid that the object has displaced. If the object floats, itsmass will equal the mass of the liquid displaced by it.
Braking systems One purpose of any braking system is to prevent something from moving. The second is to reduce a moving object’s kinetic energy (KE = ½ mv2) by reducing its velocity. The work done in achieving this is given by W=Fd. A force F needs to be applied over a distance d (the stopping distance of the object). Often this force is frictional: the kinetic energy is converted to heat by two surfaces rubbing against each other.Alternatives include• electromagnetic braking (the production of eddy currents and hence magnetic fields when metal wheels spin in an applied magnetic field) used in trains• regenerative braking (using the kinetic energy to produce electricity to charge a battery) used in hybrid cars• reversing the direction of spin of propellers or using propellers at the front or sides of ships• firing retro rockets (space crafts)• extending the wing flaps of aeroplanes.
Braking SystemsAs the Kinetic Energy depends on the mass of the vehicle,the braking force for a bicycle will be much less than that of atrain or truck moving at the same speed.Ideally, a braking system will start to respond instantaneouslyto the driver’s input, reduce the velocity smoothly over avery short distance, and require minimal physical effort fromthe driver.A brake can be applied by squeezing hand-grips, pressingpedals or pushing a button.
MechanismsMethods for transferring the force (or signal) from the driver to the braking mechanism include:Mechanical linkagesBowden cablesHydraulic cablesElectrical cables
Air BrakesHeavy vehicles such as trucks and buses typically use air brakes.The high pressures needed to stop such heavy vehicles with hydraulics-based braking can lead to catastrophic leaking. The slide valve controls which way the pressure is directed.
Safer BrakingABSTo brake effectively, the brake should not lock. If the wheels stop turningwhile the vehicle is still in motion, the driver loses control over the steering asthe tyres glide over the road. Anti-lock Braking Systems adjust the pressureon each brake so that the wheels stop quickly but smoothly. Prior to theintroduction of ABS, good drivers knew to “pump the brakes”.ESPTo calculate how much braking force is needed, a vehicle needs to beequipped with an Electronic Stability Program (ESP). The ESP maintains thestability of the car during braking with the aid of different sensors integratedinto a vehicle.An Electronic Stability Program maintains the stability of a car during braking.A range of sensors in the vehicle provide data which is used to adjust thebraking effect of each wheel.
Vacuum BoostersBraking can be assisted by making use of the partial vacuum in the exhaust manifold of petrol-driven cars(or a vacuum pump in diesel-driven buses).When the brake pedal is pressed, air at atmospheric pressure is let into the booster unit by a valve. http://auto.howstuffworks.com/auto-parts/brakes/brake-types/power-brake2.htm>
INNOVATIONInnovations in brake technology aim to facilitate one or more of the following: Brake energy recuperation Lighter vehicles Efficient, controlled braking on increasingly crowded roadsOne such innovation combines “brake by wire” (using electronic signals)with Electronic Wedge Braking (EWB)
Electronic Wedge BrakingElectric motors (3, 4)move the brake pad (2)using a series of rollers (5)that move along wedge-shaped surfaces (6).This immediately brakes the rotating disk (1). http://www.zigwheels.com/news-features/auto-insight/brakebywire-technology- the-future-of-automobile-braking-system/10519/1> Every 10 milliseconds a set of sensors measure the speed of the wheel, the forces on the brake and the wedge’s position. Under software control, the electric motors then adjust the position of the brake pad.
Electronic Wedge Braking http://www.zigwheels.com/news-features/auto-insight/brakebywire-technology- the-future-of-automobile-braking-system/10519/1Pressure from the wedge on the disk applies the brake.The disk’s rotation drags the wedge with it, enhancing the braking, so the EWB needs less energythan hydraulic braking systems.A common drawback of vacuum boosted brakes is the ease with which locking can occur.This locking is considered an advantage in EWBs, as the wedge’s position can be so finely adjusted.The wedge is pulled in just enough to achieve the desired braking.It is prevented from being fully dragged in and locking the wheel.
Advantages over conventional hydraulic braking systemsAs the activation of the brake is done within the wheel assembly itself, the system is a bolt-on and plug-in assemblywithout the need for vacuum boosters and brake fluid reservoirs (freeing up space in the engine compartment) or thehydraulic connection between the brake pedal and individual wheel brakes.It allows a quicker response – it can take up to a second for the brake signal to reach a rear trailer in trucks fitted withair brakes. (Aschenbrenner)Conventional ABS typically take around 150 ms to generate full braking power. The EWB only needs around 100 ms.( Aschenbrenner)It is “less expensive and more efficient than conventional hydraulic systems.” (Aschenbrenner)It has smaller dimensions and therefore reduces total vehicle weight.The EWB’s software can help bring the vehicle to a smooth halt by reducing the braking force just before it stops, andcan prevent the vehicle from reversing while pulling away up a hill. The concept of using a wedge is not new. The ability to control its movement to the nearest micrometer (using sophisticated sensors and electric motors precisely controlled by software) is new.
Fluid mechanics in lifting devicesUsually the task of a lifting device is to raise an object with minimal effort.This can result from changing the direction of a forcee.g. single pulley first-order leveror by reducing the force needed by applying it through a greater distancee.g. inclined plane
Car jacks and car hoistsWhen a car has a flat tyre, a simple mechanical car jack is used to raise one corner of a car.Although such jacks have a significant mechanical advantage, there is still significant effortinvolved.They do not allow much working space under the carand are not considered safe for that purpose. (Oten: Lifting Devices) The introduction of hydraulic car hoists, typically electrically operated, provided a more rapid and higher lift of the entire car, and a safer, more comfortable work space for mechanics. (Oten: Lifting Devices)
Elevators As buildings have become taller, there has been a greater need for elevators. Hydraulic elevators such as that show diagrammatically have a number of advantages.There is no need for cables, pulleys or motorsabove the elevator itself and there is no need for acounterweight.The telescopic, hydraulically controlled pistontakes up little space.The pumping unit need not be near the shaft.They are fairly cheap to install and run, but theiruse is limited to low-rise buildings and relatively (OTEN Lifting Devices p. 46)slow speeds
Cherry Pickers Special platforms, lifted hydraulically, provide a safe working space for working with power lines, or changing light bulbs in high places. It reduces the risks associated with setting up and climbing ladders, and, when not in use, take up limited space.Tower CranesTo build modern skyscrapers, building materials need to be lifted high into the air and movedacross floors. It would be very difficult to manoeuvre large cranes safely in our crowdedcities, so the cranes used for this are built on-site.A Derrick crane sits on top of a tower.As construction of the next building levelbegins, the derrick crane is lifted on itshydraulic legs, the crane is used to liftmaterials to build the next layer to supportit, and the legs are retracted.The crane’s tower can be strapped to theexisting building for safety.On completion of the building, the tower isdismantled in a similar fashion.
Innovation: Problems with the use of tower cranes include the wind affecting the operation of the jib, and torsion forces occurring in the tower. These limit the possible size of the jib. www.ggcrane.com/press/crane2-spring2010-lo.pdfCranes with two jibs were first patented in Gaspard Gillis in 1968, and were redesigned in2008 by his son, in response to a greater number of large construction projects.
Two-Jib CraneA central tower has two jibs, one on each side. Rotation of the jibs around thetower is achieved with propellers (driven by electric motors) on the ends of thejibs. These propellers also provide braking. The force to turn the jibs is notapplied at the tower, and less force is needed, saving energy.Areas in which the two-jib tower crane represent an improvement on existing cranes include: Longer jibs Higher rotation speeds Better braking power Wind has less effect Torsion is minimised Improved work efficiency Energy savings www.ggcrane.com/press/crane2-spring2010-lo.pdfComputer modelling studies conducted at the University of Alberta indicate the two-jibcrane can complete the same task as two separate cranes, but at half the cost.
Hydraulics in the aeronautical industryThe most important use of hydraulics in aircraft is moving the various control surfacesin response to pilot input, to control the plane’s speed, orientation and direction. Ailerons trailing edge sections near the ends of the wing that help control roll (turning about the longitudinal axis). Elevators hinged sections on the horizontal stabiliser (or tail plane) that can lift or lower the tail, thereby controlling the pitch (turning about the lateral axis). A rudder the trailing edge section on the fin at the rear of the plane. It controls yaw, the sideways movement of the tail and nose (turning about the vertical axis). Flaps trailing edge sections of the wings, closer to the fuselage. These help control the lift and also help slow down a plane after landing.
Hydraulics in the aeronautical industryIn large commercial jets, flying at speed, the forces required to move these surfaces are enormous.The hydraulic system’s components must therefore be able to withstand very high pressures.At the same time they should be light weight (to maximize payload) and very reliable, as failure canbe catastrophic. Usually there are redundant systems in case of failure.The constant vibrations in aircraft can lead to fatigue in materials, which must also be able towithstand harsh operating conditions.Hydraulic systems can also be found in: the operation of external doors dropping and raising the landing gear shock absorbing struts
Part of an aircraft’s hydraulic system Available from http://www.gen-aircraft-hardware.com/store.asp
Sources and references Aschenbrenner, N(2005): “Pictures of the Future Fall 2005” retrieved on May 16th, 2012 from http://www.siemens.com/innovation/en/publikationen/publications_pof/pof_fall_2005/auto_electroni cs/braking_systems.htm Copeland, P. L. (2005) Engineering Studies: The definitive Guide Volume 2 . (2nd ed.). Helensburgh, NSW: Anno Domini 2000 Pty Ltd Copeland, P. L. (2000) Engineering Studies: The definitive Guide Volume 1. Helensburgh, NSW: Anno Domini 2000 Pty Lt Course Notes 2011: “1. Engineering Materials and Applications” EDUC6505 Engineering Education Studies 2 University of Newcastle Gillis,P, Al-Hussein, M & Hasan, S (2010): “An Innovative Tower Crane: Tower Crane with Two Jibs” CRANE AND HOIST CANADA, Vol 22, SPRING 2010 retrieved retrieved on May 27th, 2012 from www.ggcrane.com/press/crane2-spring2010-lo.pdf Metcalfe, P. & Metcalfe, R. (2009) Excel Senior High School Engineering Studies. (2nd ed.). Glebe, NSW: Pascal Press
Sources and referencesOTEN “Aeronautical Engineering” (2000) Published by Learning Materials Production, Open Training and Education Network – Distance Education, NSW Department of Education and Training, 2000. 51 Wentworth Rd. Strathfield NSW 2135. Retrieved on May 4th , 2012 from http://www.tale.edu.au/tale/components/includes/trap.html?uid=MzA2NkBUYUxFXzIwMDVfREVUTFJNX1YyOTEN “Braking Systems” (2000) Published by Learning Materials Production, Open Training and Education Network – Distance Education, NSW Department of Education and Training, 2000. 51 Wentworth Rd. Strathfield NSW 2135. Retrieved on May 4th , 2012 from http://www.tale.edu.au/tale/components/includes/trap.html?uid=MjkzNEBUYUxFXzIwMDVfREVUTFJNX1YyOTEN “Lifting Devices” (2000) Published by Learning Materials Production, Open Training and Education Network – Distance Education, NSW Department of Education and Training, 2000. 51 Wentworth Rd. Strathfield NSW 2135. Retrieved on May 4th , 2012 from http://www.tale.edu.au/tale/components/includes/trap.html?uid=MzA1OUBUYUxFXzIwMDVfREVUTFJNX1YyOTEN “Personal and PublicTransport” (2000) Published by Learning Materials Production, Open Training and Education Network – Distance Education, NSW Department of Education and Training, 2000. 51 Wentworth Rd. Strathfield NSW 2135. Retrieved on May 4th , 2012 from http://www.tale.edu.au/tale/components/includes/trap.html?uid=Mjk3MEBUYUxFXzIwMDVfREVUTFJNX1Yyhttp://hyperphysics.phy-astr.gsu.edu/hbase/pbuoy.htmlhttp://en.wikipedia.org/wiki/Pneumatics